The Asteroid Hazard

SP-1310

Settore Protezione Civile

THE ASTEROID hazard

Protezione Civile Regione Piemonte

Osservatorio Astronomico di Torino

INAF istituto nazionale di astrofisica national institute for astrophysics

An ESA Communications Production Copyright 2009 Š European Space Agency

THE ASTEROID Hazard Evaluating and Avoiding the Threat of Asteroid Impacts

Settore Protezione Civile

THE ASTEROID hazard Evaluating and Avoiding the Threat of Asteroid Impacts FIRST ENGLISH EDITION

An ESA Communications Production Publication The Asteroid Hazard (ESA SP-1310 March 2009; first English edition) ESA Project Leader K. Fletcher Text editor P. Bond Layout Contactivity bv, Leiden, the Netherlands Publisher ESA Communication Production Office

ESTEC, PO Box 299, 2200 AG

Noordwijk, the Netherlands

Tel: +31 71 565 3408 Fax: +31 71 565 5433 www.esa.int Printed in the Netherlands Price €40 ISBN 978-92-9221-403-6 ISSN 0379-6566 Copyright © 2009 European Space Agency

First English edition, 2009 Coordination

G. Ercole, Director, Public Works Department of the Regione Piemonte A. Lazzari, Director, Civil Protection Department of the Regione Piemonte S. Peressin, Civil Protection Department of the Regione Piemonte Authors

M. Di Martino, A. Carbognani, A. Cellino, G. De Sanctis and V. Zappalà, INAF Turin Astronomical Observatory We thank R. Somma of Thales Alenia Space for writing Chapter 11.

Scientific Editors

M. Di Martino, INAF Turin Astronomical Observatory Translated and updated from Il Rischio Asteroidi (2nd. ed., 2005) General Coordination

A. Migliore, Director, Public Works of the Regione Piemonte A. Lazzari, Manager, Civil Protection Department of the Regione Piemonte Scientific Referees and Authors

A. Carbognani, A. Cellino, M. Di Martino, G. De Sanctis and V. Zappalà, INAF Turin Astronomical Observatory Thanks to Alenia Spazio S.p.a. for contributions to Chapter 11.

Editorial Coordination

M. Di Martino, INAF Turin Astronomical Observatory S. Peressin, Civil Protection Department of the Regione Piemonte

Contents

Foreword .

Introduction .

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The Solar System . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

A trip around the planets .

Chapter 2

Asteroids . . . . . . . . . . . . . . .

The asteroid population Observation of asteroids Binary asteroids . .

Physical properties .

Dynamical evolution .

Origin of the asteroids .

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57 66 68 70 79 82

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Historical notes and general characteristics .

Comet orbits . . . . . . . .

Comet structure . . . . . . .

The origin of comets . . . . . .

Comet Shoemaker–Levy 9 . . . . .

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Chapter 3

Comets

Chapter 4

Meteors and bolides .

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Introduction . . . .

Meteor showers . . . .

Bolides . . . . . .

Observing bolides . . . .

Meteors with anomalous trails .

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109 109 110 117 127 133 3

Chapter 5

Meteorites . . . .

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Historical background . . . . .

The structure of meteorites . . . .

The ages of meteorites . . . . .

Classification of meteorites . . .

Meteorites from the Moon and Mars .

The fall and identification of meteorites The origin of meteorites . . . .

The determination of orbits . . .

Tektites . . . . . . . .

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137 137 142 146 149 155 160 164 166 167

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Introduction . . . . . . . . . . .

The origin of NEOs . . . . . . . . . .

Near-Earth Asteroids: a nearly constant population . .

The current NEO population . . . . . . .

Near-Earth Asteroids as a potential economic resource .

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173 173 174 183 187 195

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197 197 201 203 208 212 214

Threats from NEOs and possible countermeasures .

Chapter 6

Dangerous objects – Near-Earth Objects .

Chapter 7

The risk of impact and the consequences .

Intrinsic probability of impact . .

Frequency of collision with NEOs .

Consequences of impacts . . .

Impacting the ground . . . .

Quantifying the destructive effects .

The Tunguska event . . . .

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Chapter 8

Programmes of discovery and physical studies .

The NEO search programmes . . . . .

Possible defence against risk of impact . . .

Strategies to mitigate the threat . . . .

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221 221 222 226 228

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237 237 238 242

Chapter 9

Impact craters .

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Introduction . . . .

Types of impact structures .

The formation of craters .

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Craters on Earth . .

Some of Earth’s craters

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The fall and retrieval of meteorites .

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Chapter 10

Chapter 11

Observing NEOs from space . .

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Introduction . . . . . . . . . . . . . .

The role of spacecraft . . . . . . . . . . . .

The spacecraft as an element of an integrated network for observing NEOs

277 277 279 290

Chapter 12

Mass media relations . . .

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Scientific dissemination . . .

Close encounters with asteroids .

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List of impact craters on Earth .

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Glossary

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Appendix

5

Foreword

Technological development, which is one of the most important features of the northwestern part of Italy, has, for a long time, led the regional government, the Regione Piemonte, to promote and stimulate the development of projects and activities involving local scientific and academic institutions, as well as industries which are active in the field of applied research.

This is part of a more comprehensive effort aimed at planning the future of this region and its people in a world that is rapidly changing and requires new skills and know-how to face the challenges posed by the scientific and technological development of humankind.

In this reference frame, the Civil Protection Department of Regione Piemonte has long been active in the development of editorial activities aimed at providing accurate information on topics of interest. This includes the subject of risks related to some natural phenomena, often not very well known, as is the case with the impact threat from extraterrestrial bodies, extensively covered in this book.

The task of producing a very informative and detailed description of this subject has been assigned to highly qualified specialists from the Astronomical Observatory of Torino, which belongs to the Italian National Institute of Astrophysics (INAF), and from Thales Alenia Space.

This book extensively covers all aspects of the problem posed by the flux of small bodies that collide with Earth and occasionally produce local, or even, global devastation that may have been responsible in the past for mass extinctions in the terrestrial biosphere.

The role played by scientific institutions in the Piemonte region, which are very active in carrying out important studies of these fascinating phenomena, is also shown by the fact that the impact hazard posed by Near-Earth Objects (asteroids and comets which may transit close to our planet) is generally quantified by means of the so-called Torino Scale.

It is a great pleasure for Regione Piemonte to accept the proposal by the European Space Agency (ESA) to extend to a wider European audience the result of this editorial work. The prestige of ESA as one of the world leaders in space missions of extreme complexity and scientific interest is universally appreciated. It 7

is a matter of great satisfaction to see that the strategic planning of ESA activities also includes the establishment of synergies with the activities of education and outreach, carried out by local administrations such as Regione Piemonte.

We are proud of this and we thank ESA for its interest in our editorial project.

Our sincere appreciation also goes to the authors of the book for their diligence and skill in producing a very interesting and informative document.

We hope that the readers will appreciate this work and that they will enjoy the book while gaining an improved understanding of the complex interaction of our beautiful planet with its surrounding celestial environment.

Luigi Sergio Ricca Councillor for the regional department of civil protection

8

Introduction

Since its formation more than four and a half billion years ago, Earth has been subject to impacts from space. Although the frequency of the encounters and the characteristics of the impactors have changed over time, Earth still encounters a significant flux of these objects as it sweeps through interplanetary space.

They range from micron-sized dust particles, of which thousands of tonnes enter the upper atmosphere and burn up each year, to much larger bodies, with diameters measured in kilometres, which strike Earth with globally catastrophic consequences over much larger timescales. In between these extremes are many thousands of moderately sized objects (of the order of 100 m in diameter) that can cause significant regional damage and result in major casualties. The entry of these intermediate objects can result in blast waves, injection of material into the atmosphere, tsunamis (ocean waves), and propagation of electromagnetic effects.

Most asteroids reside in a belt between Mars and Jupiter, and are in relatively stable and well-known orbits. The potentially dangerous ones are in elliptical orbits that cross Earth’s orbit and therefore could impact our planet.

It is estimated that there are about 1100 of these objects greater than 1 km in diameter and perhaps a million in the 100 m class. The orbits of many of the largest asteroids have been determined.

Impacts can also occur from short-period comets in asteroid-like orbits, as well as long-period comets that originate in deep space. The latter are the most dangerous objects because their number and orbits are poorly known, and they appear so infrequently that there are no historical observations of them.

Furthermore, comets impact at much higher velocities than asteroids, so their destructive effects may exceed those from asteroids of the same size.

The most important unknowns for NEOs (Near-Earth Objects) are their bulk mass and density, internal structure, detailed mass distribution and material cohesion. These geophysical parameters can only be accurately determined by in situ space missions, as well as by studies of the surface geology and regolith properties. Spin rate, pole orientation and precession can be, at least in part, determined via remote observations, both ground-based and space-borne.

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An impact by a NEO larger than about a few kilometres in diameter would produce global devastation, not only due to the release of a huge amount of impact energy in a specific region, but also through a number of related effects, such as immense oceanic waves (tsunamis) or the injection of large quantities of debris into the atmosphere, which would result in near total darkness persisting for months to years. Objects between about 100 m and 1 km in diameter would produce massive environmental disturbances that would kill millions of people, but their effects would tend to be regional rather than global. NEOs less than 100 m in diameter would produce large craters but fewer casualties and no global effects.

We are also obliged to examine the potential threat that future NEO impacts can pose to our society, to assess our vulnerability to such events, and to determine whether there are prudent and judicious actions that we should undertake in order to minimise or mitigate their potential effects. In order to address these issues, the Organisation of Economic Cooperation and Development (OECD), under the auspices of the Global Science Forum (GSF), convened a meeting of government representatives and international experts in the fields of NEOs and risk analysis at ESA’s ESRIN establishment in Frascati, Italy, 20–22 January 2003. The workshop identified a need to develop NEO policies at a national level, thereby enabling a response by governments to the NEO issue (coordinated at an international level) that is consistent with the approach adopted for more familiar hazards that nations may encounter, such as earthquakes and extreme weather events.

In considering how to quantify the extent of casualties and property damage caused by a NEO impact, we must consider a number of objective aspects (e.g. wave propagation) and subjective aspects (e.g. vulnerability of society and infrastructure).

In identifying appropriate risk methodologies, we have to consider the issue of the availability of models to represent the characteristics of NEOs, their interaction with the atmosphere and the oceans of Earth, and the transmission of the hazard to properties and persons. Due consideration has also to be given to the availability of digital models representing the physical nature of countries, in particular bathymetric and elevation data at coastlines, and population distributions. These tend to break up the globe into cells of latitude and longitude, offering a choice of resolution in both cell size and elevation. Developing a computational tool capable of performing a detailed analysis (especially for the tsunami risk), even for a single country, is a major undertaking, both in time and resources, and should not be embarked on lightly or without justification.

Protection against the threat posed by NEOs must go through at least 4 phases, which in part can be carried out in parallel: • Phases 1–2: assessment of the threat, including a characterisation of the NEO population; • Phases 3–4: definition and set up of a protection system.

There is, at present, no definite and reliable evaluation of the threat posed by 10

NEOs. The characterisation of the NEO population should comprise, at least, objects with a diameter larger than 1 km, which could cause a global catastrophe, while keeping in mind that a complete survey should ideally include NEOs with diameters down to 50–100 m. Ground-based and space-based facilities are needed to achieve this objective. It is believed that a survey of dangerous NEOs larger than 1 km could be completed by 2015, if an international coordinated effort is initiated.

At present, the European effort in the field of NEO research is insufficient. Yet Europe, including some of the richest countries on Earth, has a moral obligation to give an adequate contribution to the solution of this problem, for which global cooperation is indispensable.

Europe can immediately make major contributions based on its existing knowledge and facilities, such as European observatories in both hemispheres and space missions such as ESA’s GAIA, which will be launched in 2011.

A limited number of observing programmes are currently searching for hazardous objects. Five of them are in the US, one in Japan, and one in Italy.

Globally, they discover about 40 NEOs per month. However, discovery is only the first step in a long process; follow-up observations of the objects must continue for an extended period of time in order to obtain sufficient positional data to compute reliable orbits. Based on such calculations, it is possible to predict the path of the objects in the future and to identify possible collisional events up to about 100 years in advance. The observational data are collected by the Minor Planet Center (MPC) of the International Astronomical Union (IAU), located in Cambridge, USA. The computed orbits are analysed by other centres for risk assessment, the most important and active being the NEODyS service in Pisa (Italy) and the SENTRY system in Pasadena (USA). When a possible impact event is identified, the related probability is computed and the event is classified (ranked) according to two ‘risk scales’ that have been developed in recent years.

All data related to such potential impact events are posted on the web pages of the services and are accessible to everybody.

The follow-up activity is performed by many tens of individuals, groups and teams, both professionals and amateurs. Their work is coordinated by the Spaceguard Central Node of the Spaceguard Foundation hosted at ESRIN, the ESA facility in Frascati (Italy). The Node also organises special campaigns for objects that are particularly difficult to observe, or which deserve specific attention because of the associated possibility of impacts in the future.

In order to have a complete assessment of the nature of the body and the consequences for our planet in case of impact, it is essential that space-borne missions are carried out, with the purpose of acquiring a wealth of data that would otherwise be inaccessible. Hence, this phase should include more than one mission devoted to a better understanding of the physical and geophysical characteristics 11

of a NEO, and at least one additional mission to test some space-borne mitigation techniques.

In January 2004, following the presentation of the preliminary studies of 6 space missions for NEO exploration, ESA established the international NearEarth Object Mission Advisory Panel (NEOMAP), which consisted of six European scientists active in studies of Near-Earth Asteroids, with the task of advising ESA on cost-effective options for participation in a space mission to contribute to our understanding of the terrestrial impact hazard and the physical nature of asteroids.

The results of the NEOMAP studies were published in July 2004 in the report ‘Space Mission Priorities for Near-Earth Object Risk Assessment and Reduction’.

Of all six missions reviewed, the Panel recommended that ESA give highest priority to the Don Quijote concept as the basis for its participation in NEO impact-risk assessment and reduction.

ESA’s Don Quijote mission consists of two spacecraft which are to be launched into separate interplanetary trajectories: • An Orbiter spacecraft, called Sancho. After arriving at the target asteroid and being inserted into an orbit around it, it will measure with great accuracy its position, shape, mass, and gravity field for several months before and after the impact of the second spacecraft.

In addition, the Orbiter will operate as a back-up data relay for transferring all the data collected by the Impactor during approach, it will image the impact from a safe parking position and it will also investigate the surface composition of the asteroid.

• An Impactor spacecraft, named Hidalgo. After following a very different route from that of the Orbiter, the spacecraft will impact an asteroid of approximately 500 m diameter at a relative speed of about 10 km/s. This spacecraft will demonstrate the ability to autonomously hit the target asteroid, based on data provided by the onboard high-resolution camera.

After completion of the data collection with ground-based and space-borne techniques, we should deal with the realistic test of a mitigation mission, then the real ‘Spaceguard’ phase, in which both ground-based and space-borne systems are simultaneously operational and capable of a quick response, should follow.

Waiting for the above to become reality, I strongly encourage the reading of this book which elucidates clearly, but strictly, all the aspects related to the study, the observation and mitigation of the threat from small bodies of the Solar System.

Marcello Coradini Coordinator, Solar System Missions ESA Paris, December 2008 12

This book is dedicated to all our friends and colleagues no longer with us who dedicated their lives to the advancement of asteroid science and contributed to making the defence of Earth’s living species against catastrophic impacts with extraterrestrial objects an achievable goal for the first time in the history of the terrestrial biosphere.

13

Chapter 1

The Solar System

Earth is one of eight planets that orbit the star we call the Sun and it occupies third place in order of increasing distance from it. The orbits of the planets are shaped like ellipses, as Kepler discovered in the 17th century. These ellipses are moderately eccentric, which means that they depart only slightly from a circle.

The value of the semimajor axis of Earth’s orbit – around 150 million kilometres – is called the astronomical unit (AU). It is used as the unit of measure for distances within the Solar System.

The definition of what we should call a ‘planet’ has recently been the subject of a specific study by the International Astronomical Union (IAU). After a fierce debate during its last General Assembly, held in Prague in August 2006, the IAU reached the following conclusions: a ‘planet’ is a body orbiting a star, sufficiently massive to have a general shape determined by its self-gravitation, and to have

Image 1. The major bodies of the Solar System in comparison with the Sun. In August 2006 the International Astronomical Union introduced the definition of 'dwarf planets', that at present include Ceres, in the asteroid main belt, and Pluto and another three trans-neptunian objects in the Edgeworth–Kuiper belt.

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efficiently ‘cleared’ the region surrounding its orbit. In other words, a planet must have removed minor bodies that originally occupied the same region as its orbit.

According to this definition, there are eight planets in our Solar System: Earth plus five known since ancient times, namely Mercury, Venus, Mars, Jupiter and Saturn, and the two large planets subsequently discovered by means of the telescope, Uranus and Neptune. As can be seen, Pluto is no longer included in the list of the Solar System planets, as it was in the past. The reason is that Pluto is now known to be only one of the biggest objects belonging to a large population of minor bodies orbiting in the outer Solar System, and therefore it does not satisfy the requirement of having cleared its orbital neighbourhood. According to the new definition issued by the IAU, Pluto is now classified as a dwarf planet, and it is not a unique case. Ceres, the largest asteroid, is also a dwarf planet, as are Eris, Makemake and Haumea, three recently discovered Trans-Neptunian Objects (TNO) that are larger (Eris) or similar in size to Pluto.

Table 1 shows some of the main physical characteristics of the planets, while Table 2 shows some of the most important planetary satellites. Table 3 shows the fundamental physical parameters that characterise the Sun. The data in these tables are the result of long observational activities that go back to the beginnings of modern astronomy with a major contribution from exploration carried out by space probes over the last 40 years. In particular, close encounters with all of the planets and many of their satellites have taken place since the late 1970s. At the same time, the Sun has also been closely observed through instruments carried on balloons, rockets, orbiting space stations and artificial satellites.

Table 1

The Planets’ Physical Parameters Object

Mean distance from the Sun (AU)

Orbital period (days or years)

Orbital inclination (°)

Orbital eccentricity

Period of rotation (days)

Diameter (km)

Mass (kg)

Density (g/cm3)

Main gases in atmosphere

Mercury

0.387

87.97 d

7.00

0.206

58.650

4878

3.30 · 1023

Venus

0.723

224.70 d

3.39

0.007

5.427

–

243.018

12 104

4.87· 1024

5.250

Earth

1.000

365.26 d

0.00

CO2

0.017

0.997

12 756

5.98 · 1024

5.515

Mars

1.524

686.98 d

N2+O2

1.85

0.093

1.029

6787

6.42 · 1023

3.940

CO2

27

Jupiter

5.203

11.86 y

1.31

0.048

0.415

142 800

1.90 · 10

1.326

H2+He

Saturn

9.537

29.46 y

2.48

0.054

0.445

120 660

5.69 · 1026

0.687

H2+He

Uranus

19.191

84.01 y

0.77

0.047

0.717

51 118

8.68· 1025

1.270

H2+He

Neptune

30.069

164.80 y

1.77

0.009

0.671

49 528

1.02 · 1026

1.638

H2+He

16

Image 2. Ultraviolet image of Venus taken by the Mariner 10 spacecraft. The planet is completely covered by clouds. (NASA/JPL)

Image 3. Image of Mars taken by the Hubble Space Telescope during the opposition of August 2003, its closest approach to Earth in almost 60 000 years. The south polar cap is seen during its summer retreat. (STScI/NASA/JPL)

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Table 2

The physical parameters of the major satellites Name

Distance from the planet (km)

Orbital period (days)

Orbital inclination (°)

Orbital eccentricity

Rotation period (days)

Diameter (km)

Density (g/cm3)

Moon

384 400

27.32

18–29

0.055

27.32

3476

3.34

Io

422 000

1.769

0.04

0.041

1.769

3643

3.53

Europa

671 000

3.552

0.47

0.010

3.552

3130

2.99

Ganymede

1 071 000

7.155

0.19

0.002

7.155

5268

1.94

Callisto

1 883 000

16.689

0.28

0.007

16.689

4806

1.85

Mimas

185 520

0.942

1.53

0.020

0.942

418 x 392 x 382

1.15

Enceladus

238 020

1.370

0.02

0.004

1.370

512 x 494 x 490

1.61

Tethys

294 660

1.888

1.09

0.000

1.888

1060

0.96

Dione

377 400

2.737

0.02

0.002

2.737

1120

1.47

Rhea

527 040

4.518

0.35

0.001

4.518

1528

1.23

Titan

1 221 850

15.945

0.33

0.029

15.945

5150

1.88

Hyperion

1 481 100

21.277

0.43

0.104

irregular

185 x 140 x 113

0.57

Iapetus

3 561 300

79.330

7.52

0.028

79.330

1436

1.09

Miranda

129 800

1.413

4.22

0.003

1.413

240 x 234 x 233

1.20

Ariel

191 200

2.520

0.31

0.003

2.520

581 x 578 x 578

1.67

Umbriel

266 000

4.144

0.36

0.005

4.144

1170

1.40

Titania

435 800

8.706

0.10

0.002

8.706

1580

1.71

Oberon

582 600

13.463

0.10

0.001

13.463

1520

1.63

Triton

354 760

5.877

156.83

–

5.877

2704

2.05

5 513 400

360.136

7.23

0.751

?

340

?

Nereid

Table 3

The Sun’s physical parameters Diameter

1 392 000 km

Mass

1.99 · 1033 g

Average density

1.41 g/cm3

Acceleration of gravity to the photosphere

280 m/s2

Escape velocity

614.4 km/s

Brightness

3.8 · 1033 erg/s

Temperature of the photosphere

5800 K

Temperature of the sunspots

~4000 K

Temperature of the chromosphere

4500 – 10 000 K

Temperature of the corona

Several million K

Period of rotation

Approx. 25 days at the equator

Solar constant

1.36 · 106 erg/cm2/s

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The main aim of solar observations has been to acquire information relating to the star’s external layers, particularly the chromosphere and corona, and to study the phenomena of photospheric activity (the photosphere is the external layer of the Sun that coincides with its visible disc), including its spots, prominences and flare phenomena. The Sun offers a unique opportunity to closely study stellar phenomena that cannot be detected in other stars because

Image 4. Image of Jupiter taken by the Hubble Space Telescope in June 1999. The largest planet in the Solar System has a strongly turbulent atmosphere. (STScI/NASA/JPL)

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they are too far away. Beyond pure scientific interest, studying solar activity is extremely important, as solar flares can cause strong electromagnetic storms that are propagated as far as our planet and beyond. As a result, they can produce disturbances in telecommunications, cause damage to satellites and threaten the health of humans on board space stations. From a less worrying point of view, they also result in the formation of impressive and spectacular aurorae (popularly known as the Northern and Southern Lights), that are sometimes visible even in midlatitudes.

Obviously, as far as planets are concerned, the Solar System is a unique source of information. However, discoveries of planetary objects orbiting other stars have been made recently – usually indirectly, by means of very precise measurements of nearby star motions. More than 300 extrasolar planetary bodies are currently known, with masses generally between one and 10 times that of Jupiter, and following either circular orbits or paths with elevated eccentricity. The great distances of these extrasolar planetary systems and the very weak intrinsic brightness of extrasolar planets makes it extremely difficult to obtain reliable data on the most important physical parameters of these bodies. Despite these limitations, improved observational techniques are expected to allow us to discover planets comparable in size to Earth in the near future. This is a very exciting new branch of modern astrophysics, and many research teams in different countries are actively working in this field, using some of the most advanced observing facilities currently available, both on the ground and in space.

The data in Tables 1 and 2, however, only represent a part of what we know about our Solar System. It is, for example, immediately worth noticing that the Sun and planets with their satellite systems, are not the only objects present in the System, even if they represent over 99% of its total mass. In fact, the tables do not mention what is collectively called the population of small Solar System bodies: asteroids, comets, meteoroids, TNOs, and interplanetary dust. In fact, as will be shown later, this population of small bodies is much more important than their small mass would make us think at first sight. In particular, the small bodies must be taken into account in order to understand the Solar System’s formation processes and its physical state during the early phases of its history.

A look at the data in Table 1 shows that the largest planets can be organised into two major sub-families: the inner or terrestrial planets and the outer planets or giants. In fact, it is easy to see that the fundamental physical properties suddenly change beyond Mars’ orbit. The main characteristics of the inner planets, Mercury, Venus, Earth and Mars, are: they are relatively small (the largest planet in this group is Earth, with a diameter of almost 13 000 km, just larger than Venus); density is relatively high, typical of solid rocky bodies; they have few, if any, natural satellites; and atmospheres that are 20

predominantly made up of carbon dioxide (Mars and Venus), nitrogen and oxygen (Earth), or absent, as in the case of Mercury.

On the other hand, the giant planets, namely Jupiter, Saturn, Uranus and Neptune, are further from the Sun. They are characterised by: very large masses and dimensions; low density that is typical of predominantly gaseous bodies; a high number of natural satellites; and atmospheres that are primarily made up of hydrogen and helium, the most volatile elements in nature. Incidentally, the number of known satellites is still expected to increase, as many have been found by probes transiting in proximity to these planets and by dedicated, ground-based searches. There is reason to believe that a large number of other, quite small, satellites exists, though discovery is difficult from Earth.

As already mentioned, according to the new definition of the International Astronomical Union, Pluto is no longer included in the list of planets in Table 1. Previously, this body was traditionally included in the planet lists, but it represented an obvious anomaly. In fact, despite its location in the outer Solar System, its dimensions are small and its physical properties are more similar in some respects to the inner planets. Pluto is also characterised by a very pronounced orbital eccentricity that causes it to move periodically to heliocentric distances smaller than Neptune’s orbit. Moreover, Pluto has a relatively large satellite, called Charon, that is the only other example – together with the Earth–Moon system – of a system where a satellite’s dimensions are not insignificant compared to the planet. For all these reasons, Pluto clearly represented an anomaly among the planets.

After the discovery of the existence of a very large population of small bodies orbiting beyond Neptune (the so-called objects of the Edgeworth– Kuiper belt, named after the astronomers who were the first to predict the existence of TNOs) it became clear that Pluto, if was it discovered today, would not be considered a planet, but one of the largest bodies in the transNeptunian population. This change of status has now been officially adopted by the IAU, although it instigated an uproar of protests and arguments both for and against. This shows that classifying objects into different categories (planets, asteroids, comets, meteoroids) is not always easy, especially when the results are not in agreement with some historical customs.

The orbital motion of planets and their satellites, as with all the Solar System’s other objects, is described very well by Newton’s law of gravity.

Published in 1686 in the Philosophiaes naturalis principia mathematica, it states that two material points exercise an attraction on each other that is directly proportional to their masses and inversely proportional to the square of their distance. Using the law of gravity, Newton solved the problem of the motion of two material points and was able to confirm the three laws that Kepler had formulated some decades before.

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ORBITS AND THEIR CHARACTERISATION According to Newton’s universal law of gravity, in an ideal case of a system with just two bodies – a star and a planet – the planet traces an elliptical orbit around the star, with the star situated in one of the ellipse’s foci.

An ellipse is a geometric figure defined as the place where points of a plane whose sum of the distances from two fixed points, called foci, are constant. The circle is a degenerate form of an ellipse, where the two foci coincide. Every planetary orbit is defined by a certain number of orbital parameters that define its shape and orientation in space. The first orbital parameter is the semimajor axis (a) that determines the orbit dimensions, while the second parameter is called eccentricity (e), and describes the elongation of the ellipse. Eccentricity can vary between 0, corresponding to a circular orbit, and 1 that corresponds to a parabolic orbit, possible for a body that is not gravitationally linked to a star. Hyperbolic orbits (e>1) are only possible with passing comets that are destined to abandon the Solar System because of planetary perturbations. Parameters a and e determine the minimum and maximum distances from the star along the orbit. With the Solar System, they are indicated by perihelion and aphelion respectively, the smallest and greatest points of distance from the Sun. The third orbital parameter is inclination (i), defined as the angle that the orbit’s plane forms with a plane of reference, normally defined as the plane of Earth’s orbit, otherwise known as the plane of the ecliptic. Obviously, the plane of any orbit will cut the plane of the ecliptic in two points known as nodes. Remember that the plane of the ecliptic is taken as a reference to define an object’s celestial latitude. This is the exact equivalent on the celestial sphere of the normal latitude that we are used to finding indicated on the terrestrial planisphere. The ascending node is defined as the node where the planet transits when – in its orbital motion – it passes from negative to positive values of the celestial latitude.

The fourth orbital parameter is the longitude of the ascending node (Ω), defined as the angular distance along the ecliptic of the ascending node, beginning from a fixed point taken on the ecliptic (defined by the Sun’s position at spring equinox).

The inclination i and Ω then set the orbital plane’s orientation in space.

The fifth orbital parameter is known as the argument of the perihelion (w), and it represents the angular distance of the perihelion from the ascending node, measured on the plane of the orbit. The argument of the perihelion, then, sets the orbit’s orientation on its plane.

Lastly, the sixth parameter is known as the time of perihelion passage (To) and identifies the moment when the perihelion passage occurs. It sets the celestial body’s time position along its orbit.

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The problem of the actual motion of several bodies orbiting the same star, however, is far from simple, since each body’s motion is influenced by the presence of all the others. For this reason, to say that each planet moves on a perfect, elliptical orbit is just an early approximation, valid only if a planet’s motion is solely determined by the presence of the Sun (‘the two body problem’). Due to the presence of all the other bodies, every object’s motion is influenced by gravitational perturbations that must be taken into account when calculating the actual orbital motion. The effect of perturbations is to provoke small and continuous variations in the orbital parameters. In fact, it can be shown that in many cases the resultant motion is chaotic, and cannot be calculated precisely beyond a limited amount of time. Overall though, the planetary system is not wildly chaotic, as the largest planets have probably occupied their current positions since the early history of the Solar System, more than 4 billion years ago.

There is a small, but significant, exception to the statement that the planets’ motion satisfies the universal law of gravity formulated by Newton. There is a minor difference between the value of the period of precession (a motion involving progressive rotation of an orbit over its plane) observed for Mercury’s orbit and the value that would be expected by Newton’s classical law. Although the difference is only in the order of a few arcseconds a year (an arcsecond is the 3600th part of a degree), it is still relevant. The importance of this fact is that it is one of the validity tests of Einstein’s Theory of General Relativity, which predicts that Mercury’s period of precession will have a value that is totally consistent with what has been observed. From this point of view, the motion of Mercury can be used to prove that Newton’s law is wrong, and the correct law of gravity is General Relativity, even if the differences between the two theories are negligible in most practical cases. In fact, in the past, observing the planets has been one of the strongest points of Newton’s law. One of its greatest triumphs was the 19th century discovery of the planet Neptune, whose presence was predicted by analysing some anomalies in Uranus’ orbital motion.

The information we have on the physical parameters that characterise different planets originates from various types of observation. The masses of planets with satellites, for example, can be calculated by measuring their period of revolution and the semimajor axis of the satellite’s orbit, since Kepler’s third law describes the relationship between these two quantities and the mass of the central planet. The orbital parameters (orbital semimajor axis, eccentricity, inclination, etc.) of the planets themselves were determined over the last few centuries through accurate observations of the apparent motion of these bodies on the celestial sphere.

In the past, the linear dimensions (diameters) of the bodies were deduced by measuring the apparent diameter of their small discs, as seen from Earth, 23

KEPLER’S LAWS The three fundamental laws that govern the motion of the planets bear the name Kepler, after the great astronomer who lived in the late 16th and early 17th centuries. A contemporary of Galileo Galilei, he deduced the three laws by studying the enormous amount of data from observations on the apparent motion of planets, especially Mars, collected over several decades by Danish astronomer Tycho Brahe. Those years saw the progressive affirmation of the heliocentric system of the world, contrasted against Aristotle’s traditional geocentric conception, adopted by the Church and imposed as dogma. The very fact that Kepler’s laws derived from observations and comparison with the reality of natural phenomena, testified to a new way of thinking that was the beginning of the modern era. The three laws are as follows: • First Law The planets orbit the Sun along elliptical orbits, with the Sun occupying one of the foci.

• Second Law In its orbital movement around the Sun, a planet moves in such a way that the area swept by the Sun–planet radius is constant for the same unit of time.

• Third Law The cube of the semimajor axis of a planet’s orbit is proportional to the square of the period of time used to complete a whole revolution around the Sun. The meaning of the First Law is clear. The Second Law states that the segment connecting the Sun to the moving planet sweeps out equal areas in equal times. As orbits are elliptical (First Law), and the Sun–planet distance is therefore not constant, it follows that, for the Second Law, a planet moves more quickly when it is near its perihelion (the least distance from the Sun along the orbit), while it moves more slowly close to its aphelion (the furthest point of the orbit from the Sun). The Third Law shows that planets’ periods of revolution around the Sun increase with the size of orbit. This is why the most distant planets take longer to complete an entire orbit around the Sun.

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taking into account the planet’s known distance at the time of observation.

(This distance can be calculated once the orbit is known). However, there are major limitations on what can be discovered about the planets from Earth. In fact, observations involving ground-based telescopes suffer from intrinsic limitations that cannot be avoided. Nevertheless, some information can be obtained about the surface nature of objects like Mars or the Moon, or the top layers of planetary atmospheres, as in the case of Jupiter and Saturn. The rings of Saturn have been observed from Earth for hundreds of years.

Analysis of the absorption bands in the spectrum of reflected solar light also supplied precious information on the composition of planets’ atmospheres and of the solid surfaces of bodies with no atmosphere, like the Moon, Mercury, and the asteroids that mostly orbit in the region between Mars and Jupiter. With comets too, spectroscopic observations help to highlight the presence of complex chemical compounds, whether organic or not, in the comet’s coma.

However, our present knowledge of the structure and properties of all the planets is very advanced. For every object, we have estimates and detailed theoretical models of the internal structure, the density profile (that is, the density variation within the planetary body), the magnetic field and the geological history of the surface. We have enough information to undertake studies of comparative planetology, the new discipline of planetary sciences that studies affinities and differences between planets, in order to infer knowledge of the different objects’ likely histories, and the most important factors that influence the evolution of their structures, surfaces and atmospheres.

All this would have been unthinkable before the introduction of space probes to explore our planetary system. In fact, over the last few decades, space missions have supplied a far greater amount of information about Solar System bodies than was gained in 350 years after the invention of the telescope in the early 17th century. The reasons are obvious, since space probes have been able to travel very close to many planetary bodies, as well as performing orbital surveys or landings on their surfaces. In this way, precious information has been gained on the structure of the gravitational fields of the bodies that are being visited. In turn, precise knowledge of the gravitational field helps obtain very detailed information on a planet’s internal structure. Planetary magnetic fields have been measured on the spot using purposely designed tools.

Planetary surfaces have been observed from close range, obtaining information well beyond the capability of terrestrial telescopes. This is especially true for objects such as Venus, whose dense and impenetrable atmosphere had always prevented us from obtaining direct images of the underlying solid surface (although information had been obtained through 25

radar observations from Earth). In certain cases, probes have landed and sent back impressive images of their landing sites, apart from taking precise measurements of a whole series of physical parameters of the crust and atmosphere. A limited number of seismometers have been placed on the surfaces of the Moon and Mars, and these tools can supply invaluable information on seismic processes, as well as showing the presence or absence of geological activity. This has shown, for example, that the Moon is, from the geophysical point of view, a planetary object that is practically dead.

Additionally, in the case of Earth’s satellite it has been possible to take samples of rock and surface dust and bring them back to Earth, where they have been subsequently analysed in detail. Apart from meteorites, these lunar

Image 5. Above, an image of Saturn, taken by the Hubble Space Telescope on 22 March 2004, that highlights the planet’s cloud bands.

Below, an image of the planet with its system of rings, taken by the Cassini spacecraft on 16 May 2004. The image shows the planet from a perspective that is impossible from Earth and highlights the planetary disc’s shadow on the rings. The rings also project a shadow on the planet’s disc. The main discontinuity between the rings, known as the Cassini Division, is shown clearly in this image. (NASA/JPL)

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samples have long been the only fragments of bodies different from Earth that we have been able to study in labs. More recently, a space mission called Stardust has collected and brought back to Earth particles from the coma of a comet, and these samples are presently being thoroughly analysed in a number of laboratories. In addition, other space missions have already been launched with the assignment of collecting and bringing samples of the surfaces of other minor bodies and the solar wind back to Earth, while missions are being scheduled to bring samples of martian soil back to Earth over the next decades.

A trip around the planets Based on this very general overview, it is clear that every planet in the Solar System is a world in itself and very different from the others. A good way to get to know them better is to go over each planet’s main physical characteristics one by one, visiting them in order of increasing distance from the Sun. Mercury Moving away from the Sun, the first planetary body is Mercury. It is a small planet (two planetary satellites, Jupiter’s Ganymede and Saturn’s Titan are larger), and its solid surface is studded with thousands of impact craters and scarred by large fractures. In 1974, the Mariner 10 spacecraft photographed 45% of Mercury’s surface, and the remainder has recently been imaged at high resolution

by NASA’s MESSENGER spacecraft. A mission of the European Space Agency (ESA), called BepiColombo, is also expected to carry out an extensive in situ investigation of Mercury, with launch currently scheduled for 2013.

Mercury’s craters are all sizes and their morphology depends on the individual diameter. Simple craters less than 10 km

Image 6. Image of planet Mercury, taken by the Mariner 10 spacecraft. The surface is heavily cratered and, at first sight, resembles the Moon.

The presence of craters on all the solid surfaces of planets and satellites in the Solar System testifies to the collision processes that led to their formation, and to a history of collisions with small bodies from their birth right through until today. (NASA/JPL)

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across are in the form of a bowl. Between 10 and about 130 km the craters display a central peak. Between about 130 and 310 km there is also a central ring, in addition to the peak. For craters over 300 km in diameter, multiple rings appear (multi-ring craters). The youngest craters are surrounded by wide rays of ejected material that can extend 1000 km from the crater. However, the blankets of ejecta (or debris) around Mercury’s craters extend only 65% of the distance they would reach on the Moon, probably an effect due to the enhanced surface gravity of Mercury compared to that of the Moon.

There are two large impact basins on Mercury, the ancient Borealis Basin, with a diameter of 1530 km, and the younger, better preserved Caloris Basin, which is 1550 km in diameter. One half of it was photographed by Mariner 10 because the basin was close to the planet’s terminator (the border between the dark part and the part illuminated by the Sun) during the flybys. The remainder was imaged by MESSENGER in 2008. The Caloris Basin is surrounded by circular chains of mountains that are 2 km higher than the surrounding land, while the base is fractured and very uneven. The formation of this basin is due to the impact of an asteroidal body whose diameter must have been around 100 km.

One of Mariner 10’s most unexpected discoveries was Mercury’s magnetic field. The planet has a dipolar field, tilted by about 11° compared to the rotation axis. Although its intensity is 1000 times less than on Earth, it is strong enough to divert charged particles in the solar wind, trap them and create a magnetosphere.

The planet is so near the Sun that the high surface temperature (around 400 °C in the daytime) would cause tin and lead to melt. Mercury is also practically deprived of an atmosphere because its low surface gravity and the violent sweeping of the solar wind let any original atmosphere escape into space in the very remote past.

These environmental conditions do not, however, seem to prevent the presence of ice at the planet’s poles. Between March 1991 and March 1992, radar images of Mercury were obtained with the 305 m diameter antenna at Arecibo, in Puerto Rico, at a wavelength of 12.6 cm. Likewise, between 8 and 23 August 1991, observations at 3.5 cm were made from the 70 m diameter antenna in Goldstone (California), coupled with the 27 antennas of the Very Large Array (VLA) in Socorro (New Mexico). Analysis of the signals reflected from Mercury’s surface emphasised areas of high reflectivity near the north and south poles. Characteristics of radar waves reflected from these areas are similar to those reflected by Mars’ polar caps and the surfaces of Jupiter’s frozen satellites. Based on this analogy of behaviour, the high reflectivity areas at Mercury’s poles were interpreted as deposits of water ice. Images of the polar regions transmitted by Mariner 10 show large craters in which water ice can remain if it is on the crater floor and perpetually in the shade.

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The largest reflecting structure near Mercury’s south pole is the Chao Meng-Fu crater, 150 km in diameter. Other, smaller structures can be found in the surrounding area. In the north polar region, deposits are apparently distributed more evenly over 25 craters. It is estimated that the temperature at the base of the craters in the shade is less than –160°C. In these conditions, ice may have been preserved unaltered for billions of years. If the deposits are really rich in water ice, it is reasonable to think that they might be due to the collision of ice-bearing comets on the planet’s surface.

One of the many unclear aspects of this planet is the dynamical process that has been responsible for the 3:2 resonance between the duration of its orbital period around the Sun (87.97 days) and the rotation period around its own axis (58.65 days). This means that its solar day (from sunrise to sunrise) corresponds to two full revolutions around the Sun. When a body that orbits another has a numerically simple relationship between the period of revolution and the period of rotation, it is said that it is in a ‘spin–orbit resonance’. Most of the largest natural satellites in the Solar System – including our Moon – have a period of revolution that is perfectly synchronised with their orbital period, which means that they always show the same face to the planet they orbit. In this last case, the spin–orbit resonance is 1:1. This simple configuration between orbital motion and period of rotation is easily explained by tidal effects induced by the largest body, which have the tendency to synchronise the two periods over time.

In the case of the Moon, our satellite rotated around its own axis in less than 10 hours during its formation, but this motion gradually slowed to the current 27.32 days. Braking effects induced on the Moon would have been caused by Earth’s tides, eventually reaching stability corresponding to a spin– orbit resonance of 1:1. As for Mercury, the combination of its rotation period and orbital revolution around the Sun means that the interval between two consecutive transits of the Sun to the meridian of a point on its surface (solar day) is equal to about 176 terrestrial days, a period of time that equals two Mercury years.

Because of its proximity to the Sun, observing Mercury from Earth is not easy. Under favourable conditions, Mercury can be seen by the naked eye shortly after sunset or before dawn, but it is not easily observed in the twilight conditions.

In order to complete the study of Mercury, the least well-known planet in the Solar System, the MESSENGER probe was launched in 2004, and it is presently completing three flybys of the planet. Also in preparation is the above-mentioned BepiColombo, one of ESA’s most ambitious planetary missions, which is being undertaken in collaboration with Japan. So we still have to wait a few years before many mysteries that shroud this small and torrid planet, can be revealed.

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Venus The next planet from the Sun is Venus, whose dimensions and mass are very close to those of Earth. The similarities, however, end there. Venus’ rotation axis is practically orthogonal to its orbital plane (so the planet doesn’t have seasons), and the period of rotation is 243 days in the retrograde direction (opposite with respect to the other planets), so it spins clockwise when seen from the north pole of the Sun. The origin of the planet’s retrograde rotation is still a mystery. Together with Mercury, Venus is one of the planets that has the longest period of rotation. Combining the duration of rotation with its revolution around the Sun (224 days), Venus’ day is equal to 117 days on Earth.

Venus has a solid surface and extensive atmosphere of carbon dioxide that causes a ‘greenhouse effect’ that, in turn, maintains the planet’s surface temperature constantly around 460 °C. The atmosphere is pervaded by opaque clouds made of sulphuric acid which prevent direct observation of its surface, and exercises a pressure on the ground that is 90 times higher than on Earth at sea level. These extreme environmental conditions make investigating the surface difficult with automatic probes. For this reason, the global mapping of Venus’ surface has required radar instruments in orbit around the planet that can observe through its blanket of clouds.

Between 1990 and 1994, the Magellan probe completed a detailed mapping of the planet’s surface. Its radar images revealed the presence of numerous shield volcanoes with their calderas and huge lava flows, hundreds of kilometres wide. Impact craters on Venus are less numerous than on the Moon, following crustal rejuvenation (‘resurfacing’) due to widespread volcanism that has eliminated most of the oldest craters. On Venus, our snow (formed by crystals of water ice) could not exist, but, from radar data obtained from Magellan, it was discovered that mountain tops over 4000 m high are covered by a substance that very efficiently reflects radar waves. The nature of this material is unknown.

The most probable theory is that it is a thin layer of pyrite, condensed at height due to the relatively ‘low’ temperature at those heights (around 440°C).

At present, ESA’s Venus Express spacecraft is orbiting the planet, taking a wealth of spectroscopic data, images and temperature profiles. These data will require a long period of analysis and interpretation in the forthcoming years.

In situ investigations by space probes have convincingly shown that, despite it having the same dimensions as Earth, Venus has no magnetic field that protects it from solar wind. This is probably due its long period of rotation around its own axis that doesn’t allow it to develop an efficient ‘dynamo effect’ internally.

Like Mercury, Venus can be observed after sunset or before dawn. Observation, however, is much easier because it is a much brighter body (thanks to the blanket 30

Image 7. Map of one of Venus’ hemispheres obtained with the radar altimeter on the Magellan spacecraft.

The colours show the altitude: blue shows the areas below the average radius of the planet and red shows those above average. The ‘continent’ at the top of the image is Ishtar Terra, where the Maxwell Montes are located. (NASA)

Image 8. Radar image of Venus’ surface taken by the Magellan spacecraft. The image shows some characteristic structures of volcanic origin. Venus’ surface is entirely inaccessible by direct optical observation, even when using probes in orbit around the planet, because of the presence of a thick, hot, carbon dioxide atmosphere. The atmosphere is, however, transparent to radar waves, so Magellan was able to produce a complete and detailed map of Venus’ surface. (NASA/JPL)

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of clouds that reflects solar light very effectively), and because it moves further away from the Sun. At average latitudes, Venus is able to set/rise 3 hours before/ after the Sun, and is the third brightest celestial body, after the Sun and the Moon.

The Moon From Venus, the next stop is the Earth–Moon system. The Moon is Earth’s only natural satellite. Satellites in the Solar System usually have unimportant masses compared to the planets they orbit, but this ‘rule’ does not apply here – the lunar mass is 1/81 of Earth’s. Our satellite is, therefore, a body of planetary size. Seen through a telescope, the Moon’s visible hemisphere has two different types of land: the dark lowlands (maria) and the bright highlands (terrae). The lunar maria’s albedo (or reflectivity) is between 0.05 and 0.1, while that of the terrae is higher, ranging between 0.12 and 0.18. The terrae’s greater albedo is due to greater aluminium content and less iron compared to the maria, which are predominantly basaltic in composition.

For planetary bodies with solid surfaces, there are basically four types of processes that can alter their surface morphology: impact cratering, tectonics, volcanism and erosion (degradation due to the presence of an atmosphere and/ or flowing liquid). For the Moon, the most important process is impact cratering, while the other processes are of minor importance. In the Moon’s visible hemisphere from Earth, 300 000 craters exist with diameters over 1 km, while there are 234 with diameters over 100 km. The terrae are completely saturated by impact craters, including some very large ones, and they are the oldest part of the lunar crust. The maria, however, have fewer impact craters and they appear to be relatively smooth. The interpretation is that the maria have been created by flows of melted lava occurring more recently than the solidification times of the older terrae.

As with all planets similar to Earth, the Moon also displays signs of volcanic activity on its surface – albeit modest compared to Earth. Lunar volcanic structures are more evident in the maria, thanks to the presence of less craters.

The structure of the maria suggests that these lowlands were formed by fluid material coming out from inside the Moon and filling already existing depressions. Looking closely, we can see lobed slopes in the Mare Imbrium, features that are also very common in basaltic lava flows observed on Earth.

This gave rise to the theory that lunar seas are caused by outpourings of huge basaltic lava flows from inside the Moon following the impact of small asteroids on its surface. Rock samples from the maria brought back to Earth by Apollo missions have confirmed that their composition is similar to terrestrial basaltic lava, which has a characteristic dark colour and is formed by iron-rich minerals (where the colour comes from) and magnesium. As on Earth, lunar lava has 32

Image 9. Image of the lunar hemisphere visible from Earth, taken in 1992 by the Galileo spacecraft, during its journey towards Jupiter.

(NASA/JPL)

Image 10. Panoramic view of Reiner Gamma (shown by the red circle), a flat area with an intense magnetic anomaly. (NASA, Lunar Orbiter Photographic Atlas of the Moon)

some small glass beads – a sign that magma was mixed with gas during the eruption. The main characteristic of the lunar basalts is that they are all very old, aged between 3.8 and 3 billion years. By comparison, Earth’s oldest lava goes back just 70 million years.

Several large craters of likely volcanic origin are fractured at the base. In some cases, we can observe along these fracture lines some irregular-shaped small craters surrounded by a dark halo (called DHCs or dark-halo craters).

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The classical example is Alphonsus, with a diameter of 119 km. DHCs are probably of volcanic origin and the dark halo is probably a deposit of ashes emitted during eruptions. A plausible process for forming these structures is the following. Fluid lava escapes at high pressure from a narrow crack in the terrain, making lava shoot out to a considerable height. During this ballistic flight, lava tends to separate into small fluid drops that turn into solid spheres when they cool down. It was probably these drops that fell on the ground and covered it with the dark deposits visible from Earth. The lava’s fluidity and low acceleration of lunar gravity prevented the structure from growing and taking on the characteristic shape of a volcano. DHCs must not be confused with dark-halo impact craters (DHICs) that are normal impact craters, with diameters of 1–3 km, surrounded by dark coloured ejecta. In this case, the dark material was probably formed by lava underneath the lunar surface that was brought to the surface during the crater’s formation.

When lava emission from the interior takes place over longer periods and viscosity is high, it cannot get out of the cracks in the ground, giving rise to a volcanic structure which assumes the shape of a dome with a diameter of 10–20 km and 300–400 m high. Sometimes, on the upper part of the dome, a small crater with a typical diameter of around 1 km can be observed (the volcano’s ‘mouth’). The dome’s walls can be steep, although the average inclination is between 1° and 2°. On Earth, these differences in inclination are due to different compositions of lava. The steepest domes contain lava with more silicon, but less iron and magnesium. The opposite applies for the domes with less inclination.

Domes usually tend to be in groups and isolated structures are relatively less frequent (an example is the Mons Gruithuisen Range in the Mare Imbrium).

Lunar domes were described in detail for the first time by R. Barker in 1932. Dome systems were also discovered on Venus by the Magellan probe (1990–1994), for Image 11. Image of the Mare Orientale, a 620 km diameter structure with concentric rings. These structures are the example in the Alpha Regio area. impact result of the most violent impacts recognisable on the Moon, if Average sizes of these domes are the large basaltic maria are excluded. (STScI/NASA/JPL) 34

20 km wide and 750 m high, with summits that appear fractured with a smaller central crater. Similarity with lunar domes is notable. One of the most notable lunar regions for its number of domes is the Marius Hills (near the 41 km diameter Marius crater), in the Oceanus Procellarum. This area was selected as the landing place for an Apollo mission, but was never used.

Domes are also found near the Hortensius and Milichius craters. The widest complex of domes is the Mons Rümker, a volcanic complex with a diameter of 70 km, also in the Oceanus Procellarum. Their lack of volume means domes are only visible from Earth when highlighted by shadows near the terminator.

Among the most common structures on lunar maria are sinuous valleys (rimae). The best known structure of this type is the 80 km long Rima Hadley, at the borders of the Palus Putredinis. It was, in fact, inside Hadley that Apollo 15 astronauts photographed the layered structure of the lunar maria, as proof that lava effusions alternated over time with periods of inaction. Observations indicate that many lunar valleys originate in irregularly shaped craters (some surrounded by dark material). Before the Apollo missions, there were many theories on the origin of the sinuous valleys, some supposing the presence of liquid water on the lunar surface. Nevertheless, the absence of water on the Moon and the basaltic nature of the maria have caused us to consider these valleys as being channels eroded by lava, even if the formation process still has to be clarified.

On Earth, lava channels are formed when magma leaks out at a moderate rate and then cools at the borders of the active part of the flow, towards the flow’s axis. This axis becomes the sinuous valley. Should the central surface also cool, the channel can become a river of lava, with magma flowing below the surface. With this process, channels of lava are structures built on already existing ground. Should the lava be very fluid, the result will be a channel cut into the surface, more or less like what happens with a flow of water. In this case, the channel is a feature of erosion. On Earth, the first process prevails over the second.

Tectonics is the entire process of breaking and deforming the solid crust of a planet. Tectonics is dynamically complex on Earth because of the presence of a number of different continental plates and the presence of convective motion in the mantle. This steadily produces a drift of the continental plates and a rejuvenation of the overall crust, with the formation of new ocean floor.

The subduction of the material of colliding continental plates leads to the formation of new mountain chains, that are subsequently erased by erosion processes. All of these processes are typical of a geologically active planet like our Earth, still characterised by the presence of active volcanoes and frequent seismic events in the regions of contact between colliding plates.

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On the Moon, on the other hand, the geological processes are much simpler, and they generally reproduce features observed on Earth, where they are much more prominent. Structures of tectonic origin observed on the Moon can derive either from crust compression or extension. In the maria, compression has produced the so-called dorsa, networks of wrinkle ridges that are 10–100 m higher than the surrounding land and therefore visible only when they are near the terminator. The dorsa are zones of compression and look like the folds of a tablecloth when the opposite edges are pushed towards the centre of the table.

The structures produced by the crust stretching following ground movement along the plane of fracture are called faults (rupes). These faults are very numerous in the lunar maria although they have modest inclinations. An example is the Rupes Recta in the Mare Nubium (110 km long, 240–300 m high). When two faults run parallel to each other and the ground on their inside is on a level lower than the surrounding land, they form graben. Lunar grabens are 1–2 km wide and between 10 and 100 km long.

Dorsas can usually be found towards the centre of lunar maria, while grabens occur towards the edges. This distribution can be understood if we consider that a sea is produced by filling an impact basin with lava. As the basin fills up, the weight that the bottom has to bear is greater towards the centre than at the edges. At the centre of the basin then, the mare tends to sink and create a dorsa, while at the edges it tends to dilate and form grabens. An example of this relationship between dorsas and grabens is in the Serenitatis and Humorum maria. The dorsa on the east edge of the Mare Serenitatis is rather spectacular. It is known as Dorsa Serpentine and its northern part is called Dorsa Smirnov. However, not all surface structures observed on the Moon, either in the maria and terrae, have been explained. Images sent to Earth by space probes often show strange domes, holes and irregular depressions that do not appear to be a consequence of an impact or volcanic process. We cannot exclude the possibility that these structures were formed after gas leaked from the lunar subsoil.

During exploration with the Lunar Orbiters, it was realised that the probes’ circumlunar orbits were subject to gravitational perturbations caused by welldefined areas on the Moon’s surface. The disturbing effects of these areas prevent a satellite from staying long in lunar orbit, without having to resort to motion corrections. These regions, where a local increase of gravity acceleration is experienced, are called mascons (from mass concentration). Usually mascons are associated with the lunar maria (an example of a mascon can be found in the Mare Crisium), but the dynamics of their origin have not yet been fully clarified.

We cannot speak about tectonics and volcanism without mentioning seismic activity on Earth’s satellite. Recording lunar earthquakes has been 36

possible thanks to the Apollo Lunar Seismic Network, a network of stations placed on the Moon’s surface by the Apollo astronauts. There were five stations in the network, situated during missions 12, 14, 15, 16 and 17.

Recording of seismic data stopped in September 1977. The stations recorded from 600 to 3000 lunar quakes a year, most of them with magnitude of less than 2. The most intense had a magnitude of 4, which is a very low value compared to Earth’s standards, indicating that the lunar interior is not geologically active. No seismic event has been associated with grabens or other kinds of faults. All the body of evidence collected by detailed lunar studies is in general agreement with the predictions of a general lack of geologic activity in a body that, due to its moderate size, has efficiently dissipated its internal heat over a long period of time. The lunar interior is now mostly in a solid state, and is no longer able to produce the kind of tectonic activity that took place in the early stages of Moon’s history.

Unlike Earth, the Moon does not have a dipolar type of global magnetic field. Our satellite only has some local magnetic fields that are situated in certain areas of its surface. A very interesting surface structure, due to the presence of a strong local magnetic anomaly, is the Reiner Range in the Oceanus Procellarum.

Telescopic observation and direct exploration have shown the absence of any significant lunar atmosphere. In fact, the Moon is surrounded by a very sparse layer of gas or exosphere, with a density of 2·105 molecule/cm3 in the night hemisphere and 104 molecule/cm3 in the day hemisphere. Overall mass of the lunar atmosphere is just 104 kg, 1014 times less than the mass of Earth’s atmosphere. The main component gases are hydrogen (H), helium (He), neon (Ne) and argon (Ar). The H and the Ne are of solar origin, as is 90% of the He. The remaining percentage of He and all of the Ar originate from the decay of radioactive elements in the lunar soil.

Mars Located at a distance of around 1.5 AU from the Sun, Mars is the fourth and last of the so-called terrestrial planets. It is a relatively small body in comparison to Earth: the average equatorial diameter is around half that of Earth, while its mass is 1/10 that of Earth. The period of rotation (24h 37m) is practically identical, and a Mars day is called a sol. The inclination of the rotation axis on the orbital plane is 25.1°, similar to Earth, so Mars’ seasonal cycle is analogous to ours, though with some differences. As the martian year is equal to 687 Earth days, the seasons last around twice as long, whilst the orbit’s greater eccentricity (0.093 against Earth’s 0.017) makes for much greater annual climatic differences.

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Image 12. Panorama of the landing place of the Mars Exploration Rover Spirit. (NASA)

Mars’ surface resembles some of Earth’s deserts; its yellow-reddish colour is due to the presence of iron oxides. In general, the ground appears covered by dust and rocks of various sizes and shapes. Mars’ rocks appear richer in volatile elements than those found on Earth and they also show a greater iron/ magnesium ratio. The dominant characteristic of the planet’s topography is the difference in height (~5 km) between the lowlands in the northern hemisphere and the highlands in the southern hemisphere. The dichotomy between Mars’ two hemispheres is presumably due to two different geological histories. The southern hemisphere’s crust is heavily covered by craters, evidence that it is very old, while the northern hemisphere has relatively few craters, and is therefore younger.

The Tharsis region is a vast volcanic area that interrupts the sharp boundary between the north and south hemispheres. To the northeast of Tharsis is the Solar System’s largest volcano, Olympus Mons, with a diameter of 700 km at the base and a height of 26 km. Mars’ volcanoes are similar to Earth’s shield volcanoes (like those in Hawaii), but are very large. This characteristic is explained by the fact that Mars’ crust, instead of continuously drifting as in the case of the continental plates on Earth, remained mostly at rest, allowing volcanoes to steadily grow over hot spots in the mantle and to reach notable sizes.

Two important formations that still have to be completely understood are the Valles Marineris and the basin of Hellas. The Valles Marineris (given this name in memory of the first missions of the Mariner probes) are a system of canyons situated just below Mars’ equator, in the southern hemisphere. The main canyon is around 4500 km long, with a maximum depth of 7 km and average width of 200 km. The Hellas basin is probably the result of an asteroid impact when Mars was young. Hellas’ diameter is around 2000 km and it is 9 km deep compared to the surrounding land. The basin is surrounded by a ring of mountains with a diameter of around 2300 km, created following the impact.

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Before the space exploration era, estimates of Mars’ atmospheric pressure (based on the diffusion of solar radiation) gave values around 100 mbar, and CO2 (carbon dioxide) seemed a minor component. In situ measurements by the first space probes allowed us to establish that in fact the atmospheric pressure varies between 7 and 10 mbar and that carbon dioxide is the main component. Atmospheric pressure can reach values of 14 mbar at the base of the deepest canyons, falling to 0.3 mbar on the top of Mars’ gigantic volcanoes.

By comparison, Earth’s atmospheric pressure at sea level is 1013 mbar. Winds on Mars’ surface are usually weak, with speeds around a few metres per second for most of the year. Things can change during spring and summer in the southern hemisphere. It is not infrequent that some dust storms develop locally with wind speed over 40 m/s. Sometimes a local dust storm builds up and eventually covers the entire planet, as happened in 1969 (during Mariner 9’s historic mission), and in 2001. In this case, because of the lifted dust, the planetary atmosphere becomes opaque and prevents observation of the surface. The moving dust leads then to remodelling of the planet’s surface characteristics.

The planet’s largest visible reserve of volatile substances is concentrated in the two polar caps. These have two main components: the seasonal cap and the permanent cap. During winter in one hemisphere, when the temperature drops to –120°C, a part of the CO2 of Mars’ atmosphere condenses in the polar regions in the form of dry ice. The condensation process takes place under a blanket of polar clouds made of carbon dioxide, forming a seasonal polar cap. Naturally, when winter is coming in one hemisphere, carbon dioxide ice sublimation is beginning in the other hemisphere, giving rise to an exchange of gas between the planet’s opposite poles. Because of this condensation–sublimation process, Mars’ atmospheric pressure experiences variations of some 30% during its year. During winter, the edge of the north polar cap can actually push itself to 65° latitude, while the southern cap spreads to –55°. In the sublimation phase, during Mars’ spring, the north cap withdraws around 20 km a day, before stabilising at 85° of latitude. The south cap on the other hand withdraws around 15 km a day, but this rate differs at various longitudes.

One recent and important discovery (1999) is that Mars does not have a dipolar type global magnetic field like Earth: space probes have established the existence of a whole series of local magnetic fields that are peppered over the planet’s surface. Most sources of the magnetic field are found in the heavily cratered regions in the southern hemisphere, while the lowlands of the northern hemisphere contain far fewer of them. The fact that the planet’s northern hemisphere contains few magnetic sources supports its surface rejuvenation, in agreement with the lower abundance of craters observed in those regions.

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Approximately every 2 years and 49 days, Mars reaches solar opposition with respect to Earth, and its distance from our planet reaches a relative minimum. At these times, Mars is easily observable during the whole night.

Not all oppositions of Mars, however, are equally favourable for observations.

When opposition occurs with the planet near the perihelion of its orbit (nearer the Sun), then the Earth–Mars distance reaches an absolute minimum, while Mars’ apparent diameter is at its maximum. In this case, we speak of ‘great oppositions’. They occur at 15–17 year intervals. The best great oppositions occur when Earth is also close to its aphelion, because the two planets are even closer. These conditions, however, are much less frequent.

Missions to Mars Mars has been visited by many planetary missions over the last few years.

NASA’s Mars Exploration Rover, Mars Odyssey and Mars Reconnaissance Orbiter missions, together with the ESA’s Mars Express, are currently operational at the red planet. These missions’ main aim is to take highresolution images and study the mineralogical composition of its surface. Mars Express carries a radar (the MARSIS instrument) that ‘sees’ up to 5 km below the red planet’s surface. A very exciting result of this instrument has been the detection of the possible evidence of large masses of frozen water beneath the planet’s surface in the south polar region.

Since January 2004, two NASA rovers – able to move independently on the planet surface and named Spirit and Opportunity – have made a chemical– physical study of rocks and soil in the Gusev impact crater and the Meridiani Planum alluvial plain, two diametrically opposite regions. The two rovers had a precursor in the late 1990s with the Mars Pathfinder mission, the first to land a rover on the red planet. The Mars Global Surveyor mission (1999–2006) produced more than 240 000 images of the planet’s surface, leading to a number of new discoveries.

The Mars Reconnaissance Orbiter, which carries the most powerful camera ever to orbit Mars, is also conceived as a communication link between future probes and Earth. In Image 13. Artist’s impression of a Mars Exploration Rover. (NASA) 40

2008, NASA’s Mars Phoenix landed in the Martian Arctic, obtaining images and analysing samples of ice and soil. According to the plans, 2013 will see the dispatch of a surface laboratory (Mars Science Laboratory) to drill into the surface and analyse rock minerals in a search for evidence of liquid water.

Further into the future, various space agencies are planning to collect and bring back samples of Mars soil to Earth. According to the most optimistic predictions, humankind will not set foot on the red planet before 2030.

Jupiter Beyond Mars we go past the so-called main belt of asteroids (which is mentioned in more detail in following chapters) and reach the largest planet in the Solar System. Jupiter has a mass equal to 318 times that of Earth, a diameter 11 times greater than our planet, and an 11.86 year orbital period. Like all gaseous planets and unlike Earth-type planets, Jupiter does not have a solid surface: the atmospheric gases gradually transition from a gaseous state to liquid and solid states towards the planet’s interior. The period of rotation around its own axis is 9h 55m and its powerful centrifugal force modifies the planet’s shape, making it a distinctly elongated spheroid. Jupiter’s escape velocity is rather high (around 60 km/s), and this maintains a thick atmosphere, whose main component is hydrogen, followed by helium.

Jupiter’s atmosphere is cut across by current flows that divide into bands of dark colour and zones of light colour parallel to the equator. The alternation of bands and zones is also clearly visible from Earth with small telescopes. A unique characteristic of Jupiter’s atmosphere is the presence of a large storm at a latitude of 22° in the southern hemisphere: the Great Red Spot (GRS). This

Image 14. A model of the Voyager 1 spacecraft that, with its twin Voyager 2, has provided unique information on the physical properties of the giant planets, from Jupiter to Neptune, and their satellite systems. (NASA/JPL)

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anticyclone has been present since the first telescopic observations of Jupiter in the 17th century. It has an elliptical shape of 25 000 x 12 000 km and has a typical reddish colour. Its origin and long duration are still not entirely understood.

According to current understanding, based mainly on data collected by space probes, there are three separate types of clouds found at different heights in the atmosphere. At the atmospheric level where the pressure is 1000 mbar (around the value of Earth’s surface atmospheric pressure) the clouds are made of ammonia (NH3).

Between these and the underlying level to 2000 mbar are clouds Image 15. A photomontage of images taken by the Galileo made of ammonium hydrosulphide spacecraft, showing the size of Jupiter and its four main satellites (from the top down: Io, Europa, Ganymede and (NH4SH), while further down, as Callisto). (NASA) far as the level of 5000 mbar, are water vapour clouds (H2O).

Until recently, Jupiter was the only planet besides Earth where lightning had been observed (observations of Saturn from the Cassini probe have now also detected lightning storms). The first images of this phenomenon in Jupiter’s atmosphere were from the probes Voyager 1 and 2, taken during their 1979 flybys: the images sent to Earth showed lightning in the planet’s night hemisphere. Electric activity on Jupiter was also more recently monitored by the Galileo probe that confirmed and elaborated on what we already knew from the Voyagers. On 7 December 1995 a capsule (probe) released by the Galileo orbiter entered Jupiter’s atmosphere close to its equator. During its descent, optical sensors did not reveal lightning in the proximity, while radio sensors (sensitive between 0.1 and 10 kHz) recorded around 50 000 distant discharges between 1000 and 10 000 km from the probe.

The overall picture is that the latitudes between 47° and 49° (in both hemispheres) are most electrically active. A typical storm on Jupiter measures around 1500 km in diameter and produces, on average, 20 flashes a minute.

Lightning is located in the atmospheric layer where pressure is between 2000 42

and 5000 mbar inclusive, in the region occupied by clouds of water. This suggests that the process of generating the lightning is similar on Jupiter and on Earth – that is, electrification following convective movement of cloud.

Jupiter’s system of satellites forms a miniature Solar System: currently more than 60 are known. The planet’s four largest satellites (Io, Europa, Ganymede and Callisto), were discovered by Galileo Galilei in 1610 during his first telescopic observations of the planet. All of these satellites have dimensions that are comparable with those of the Moon, while Ganymede is even bigger than Mercury and is the largest satellite in the Solar System. Each of them has a solid surface, as well as being a world in itself.

The most interesting of the four are, without doubt, Europa and Io. The Solar System’s most active volcanoes are present on the multi-coloured surface of Io, where the mountains can reach 15 km above the surface. It is not yet clear what Io’s multi-coloured lava is made of. It obviously cannot be pure sulphur because this element is not able to create big volcanic structures (they would collapse under their own weight), so it is probably silica with traces of sulphur that are responsible for the colouration. Europa, on the other hand, is characterised by its frozen surface slashed with crevasses, the absence of craters (a sign of surface rejuvenation processes), and fields of ‘icebergs’ discovered by the Galileo orbiter. It is very probable that under the satellite’s crust of ice, there is an ocean of brackish liquid water to be considered as a possible candidate for an extraterrestrial ecological niche.

As a planet external to Earth’s orbit, Jupiter is well visible in the night sky during the months near its opposition to the Sun. The apparent brightness of the planet is second only to Venus.

Saturn Saturn is about twice as far from as Jupiter (about 10 AU), and its period of revolution is 29.5 years. Saturn is also a giant planet, with a composition similar to Jupiter’s. Its mass is 95 times that of Earth and its diameter is 11 times greater. Together, Jupiter and Saturn represent 90% of the mass of all bodies that orbit the Sun. Even when observed superficially, Saturn appears very different from Jupiter, not only because of its smaller apparent size, but also because of the low contrast of its atmospheric clouds. Although it has a system of bands and zones similar to Jupiter’s, Saturn is less spectacular because its atmospheric transparency is less, due to the lower temperature. Occasionally however, Saturn’s atmosphere shows interesting details and rapid evolution. A remarkable event was the appearance, in September 1990, of a large clear spot in the planet’s equatorial region. The White Oval Spot (WOS) was visible even with 43

Image 16. A close-up of Saturn’s rings taken by the Cassini spacecraft on 21 June 2004, nine days before it entered orbit around the planet. (NASA/JPL)

small telescopes, maintaining its oval shape for about three weeks before it began to lengthen in longitude and disappeared completely. Other large spots were observed in 1876, 1903 and 1933, that is at 30 year intervals (the same as Saturn’s orbital period). This temporal coincidence suggests that the large equatorial clear spots are linked to the planet’s seasonal cycle. It is not necessary to wait until 2020 to observe WOS on Saturn though: smaller, light coloured ovals and darker spots sometimes appear in the planet’s atmosphere, as happened fairly frequently at the beginning of the 21st century.

Recently, the Cassini mission has detected radio emissions that are interpreted as being produced by lightning storms, so Saturn is the third planet, in addition to Earth and Jupiter, for which such events in the atmosphere have been detected.

Saturn is well known because of the system of rings situated in its equatorial plane. The other giant planets (Jupiter, Uranus and Neptune) also possess a system of rings, but they are much thinner than Saturn’s, and hardly visible from Earth. Saturn’s rings were discovered on 25 March 1655 by Christian Huygens. That night, the Dutch astronomer aimed his telescope towards Saturn, discovered the planet’s first satellite (Titan), and gave an explanation of the strange ‘handles’ present around the planet that had fascinated astronomers since the time of Galileo. Huygens had realised that they were one, single ring around the planet. At first the explanation was contested, and only completely 44

approved in 1665. 10 years later, Giovanni Domenico Cassini noticed that there was a ring with less material (later called the Cassini Division) between the darker, external A ring and the brighter, inside B ring. Inside ring B is ring C, which is semi-transparent and very different from rings A and B. In turn, inside C, is ring D which extends to the top of Saturn’s clouds. The thickness of the rings is a few hundred metres, while the external diameter of ring A is around 270 000 km: it is essentially a bi-dimensional structure. The rings are made up of ice fragments orbiting the planet.

Today Saturn’s known satellites number more than 60, among which the most interesting is Titan. This satellite, visible even with a small telescope, has a diameter of 5150 km and is the biggest of Saturn’s satellites with a diameter greater than Mercury. Titan orbits at a distance of about 1 222 000 km from Saturn, needing 15.945 days to complete an entire orbit. It has an average density of 1.88 g/cm3, which makes us think of a mixed rock–ice composition. Titan’s escape velocity is 2.6 km/s, and, thanks to its low temperature, this is enough to maintain a rather thick atmosphere. The main component is nitrogen (90%), followed by methane. The high percentage of nitrogen in the atmosphere is similar to Earth, but oxygen is not present. The pressure on the satellite’s surface is 1.5 atmospheres, while the temperature is around –180°C. Under these conditions methane can exist in the solid, liquid and gaseous states. Methane on this giant satellite plays the same role as water on Earth and the existence of methane oceans and icebergs was once thought to be possible. The reality is slightly different, with the discovery by Cassini of methane rain and lakes. Seen from the outside, Titan has a rather uniform orange colour, due to the diffusion of solar radiation by methane and nitrogen compounds present in its atmosphere.

It is not possible to observe the satellite’s surface directly in visible light, but it is possible in the infrared and around 1.6 µm away from methane absorption bands. A series of images taken with the 8.2 m VLT telescope in February 2004 highlighted darker regions than do not change shape over time. They are obviously surface details although their nature is not yet clear. Titan may also make a contribution to the mystery of the origin of life, because its organic chemistry is probably similar to that on the primordial Earth.

The Cassini–Huygens mission Cassini–Huygens is a co-operative mission between NASA, ESA and ASI (Italian Space Agency). The Huygens probe was aimed at clarifying the chemical–physical processes on Titan, while the Cassini orbiter is still studying Saturn and its satellites. Cassini–Huygens was launched on 15 October 1997, and in December 2000 it went beyond Jupiter’s orbit, eventually reaching Saturn and entering orbit around the planet on 1 July 2004.

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Image 17 (left). Image of Titan taken by Cassini during its close flyby on 26 October 2004. The Xanadu ‘continent’ is visible in the centre. The resolution varies from 2 to 4 km per pixel. (NASA/JPL) Image 18 (right). Close up of Titan’s surface taken by the Huygens probe on 14 January 2005, immediately after its landing. (ESA)

The first high-resolution images that Cassini sent to Earth were of Saturn’s satellite Phoebe (220 km in diameter), taken during a flyby on 11 June 2004.

Phoebe was observed during the Voyager missions in the early 1980s, but the images taken were of low quality. Cassini’s images, on the other hand, resolved details only tens of metres across and the satellite’s surface appeared cratered and covered with a layer of dark dust. Spectroscopic analysis revealed the presence of vast quantities of water ice: Phoebe is probably a gigantic comet core captured by Saturn’s gravitational field.

Cassini’s first images of Titan were also interesting. In fact, those infrared views taken on 14 June 2004 at 0.938 nm showed the same surface characteristics as those taken at the VLT in February that we mentioned earlier. Since then, Cassini has been continuing its systematic exploration of the Saturnian system.

The Huygens probe’s objective was to study Titan’s atmosphere and the surface where it landed on 14 January 2005, after being detached from Cassini on 25 December 2004. Following a slow parachute descent through the satellite’s atmosphere, the probe survived around 2.5 hours after impacting with the ground. Huygens sent back images of the visible dark areas in the infrared images that suggested at least some of them are lakes.

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Uranus Uranus, the fourth planet in order of mass in the Solar System, was the first to be discovered after the invention of the telescope. It was found by William Herschel on 13 March 1781, using a small telescope of 15.7 cm in diameter. It was only in May 1781 that the planetary nature of Uranus was recognised. By calculating the orbit, a value of the semimajor axis was found to be around 19 AU from the Sun. Uranus is a planet with an equatorial diameter four times greater than Earth and a mass 14.5 times greater. The planet’s orbital period is 84 years and since its discovery it has completed just 2.7 orbits, which is why little is known about its seasonal cycle. Uranus’ main characteristic is the 82° tilt of its rotation axis compared to its orbital plane. This means that the polar regions are continuously exposed to the Sun for long intervals – around 20 years. The main atmospheric component that can be detected from Earth is molecular hydrogen, followed by helium.

Uranus is the only one of the four gaseous planets which does not emit at thermal infrared wavelengths a quantity of energy greater than it receives from the Sun. In the cases of Jupiter, Saturn and Neptune, the emitted energy is around twice the amount received from the star, so it must include some source of internal energy. At an average distance of 19.2 AU from the Sun, Uranus receives energy equal to 3.7 W/m2. Only 70% of this is absorbed by the planet, i.e. 2.6 W/m2. This absorbed energy is then re-emitted as thermal infrared radiation. The absence of significant emission of energy, besides the solar contribution, doesn’t necessarily mean the absence of an energy source inside Uranus.

According to current theories, the excess of emitted energy from the gaseous giants is at least partly due to a process of gravitational contraction of their cores, or to changes of states of some materials present at large depths (the helium, in the case of Saturn). There are no clear reasons to explain the differences in the emitted energies of Uranus and, in particular, Neptune, which is expected to have a very similar internal structure. This problem is then still open to theoretical investigations, but it is clear that new data from space probes are necessary.

Before Voyager 2 there were only indirect and inconclusive indications of the existence of a magnetic field on Uranus. After the mission the answer was affirmative. The axis of the planet’s magnetic dipole is tilted around 60° compared to the axis of rotation and the centre of the dipole is 0.3 Uranus radii away from the centre of the planet in the direction of the northern hemisphere.

The sidereal period of rotation of the dipole, and therefore of the inside of Uranus, is 17.24 hours.

Uranus has a system of 9 main rings that are thin and very dark, discovered during observations from Earth via the technique of stellar occultations in 47

Image 19. Uranus imaged by the Voyager 2 spacecraft on 17 January 1986, from a distance of 9 million km. The image on the left shows the planet’s natural colours, as it would appear to the human eye. The image on the right has been processed to exaggerate the differences by using false colours. This reveals the general structure of the atmosphere, with different bands that surround the planet’s pole (shown by the area in dark orange). Uranus is unique among the planets, as its rotation axis lies approximately on the orbital plane. With the other planets, the rotation axis is approximately perpendicular to the orbit’s plane. This peculiarity of Uranus is generally explained by it having suffered a violent impact with a large planetesimal in primordial times. This collision would have completely altered the state of the planet’s rotation, tilting its rotation axis to the currently observed value. (STScI/NASA/JPL)

Image 20. Image of Uranus taken by the Hubble Space Telescope, showing its system of rings. The rings’ existence was entirely unknown before 1977, when they were discovered indirectly, through the observation of the planet’s occultation of a star. This false colour image also reveals the atmosphere’s structure. (STScI/NASA/JPL)

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1977. Moving outwards away from the planet, the rings have the following names: 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta and Epsilon. The particles that make up the rings are very dark and this explains their low visibility. In 1986, Voyager 2 confirmed the presence of the 9 rings and discovered two others that are even fainter, as well as showing that the main rings are embedded in clouds of dust.

Like all planets with orbits external to Earth, Uranus is visible in the night sky during opposition. Seen from Earth when at its closest, about 18 AU, it shines like a star at the visible limit for a naked eye. With a telescope, on the other hand, it has a blue–green colour, and shows a disc with an apparent diameter 465 times smaller than the full Moon.

Neptune The eighth planet in the Solar System, Neptune, was discovered as the result of analysis of the planet’s gravitational perturbations on the motion of Uranus. It was found on 23 September 1846 by Johann Galle, based on calculations by French mathematician Urbain Leverrier. English mathematician John Adams had also reached a similar conclusion as Leverrier, but his prediction was not

Image 21. Image of Neptune taken during the 1989 flyby of Voyager 2. (NASA/JPL)

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taken seriously by astronomers in England and so research took place at a slow pace.

The planet is in third place in order of mass in the Solar System (17.1 times Earth), with a slightly smaller diameter than Uranus. Apart from excess infrared emissions, its atmosphere’s composition and dynamics are similar to those of Uranus. We should remember though that, despite its greater distance from the Sun, Neptune’s atmospheric activity appears more evident. One of Voyager 2’s most interesting discoveries (1989) was the Great Dark Spot (GDS) that was an anticyclonic structure similar to the Great Red Spot on Jupiter. The GDS has now disappeared, but it was found at 22°S like Jupiter’s spot, although its shape and dimensions were more varied than the latter’s.

Cloud formations in Neptune’s upper atmosphere were numerous, and similar to Earth’s cirrus. Most of the winds blow in the opposite direction to the planet’s rotation, reaching speeds of 300 m/s, second only to those on Saturn.

As with Uranus, Neptune’s magnetic field is unusual. The magnetic dipole’s axis is tilted 47° compared to the rotation axis, and its centre is at 0.55 planetary radii from Neptune’s physical centre. The field’s intensity, then, varies depending on its position on the surface. Variations in radio waves produced by Neptune’s magnetic field allowed us to determine the planet’s period of rotation at about 16 hours.

Neptune has a system of rings situated in the planet’s equatorial plane, but they are very thin and dark, and contain a number of denser arcs of material.

Among this planet’s satellites is Triton, explored by Voyager 2 in 1989. Triton has a diameter of over 2600 km and is one of the Solar System’s largest satellites. The surface is made up of ices and rocks with a temperature of around –235°C, and a very sparse atmosphere of nitrogen and methane. The satellite has a polar cap and its own seasonal cycle. There are few impact craters, which is a sign that the crust has undergone processes of surface rejuvenation. Traces of internal activity have been identified on the surface in the form of active geysers.

Neptune is not visible to the naked eye: its observation is only possible with a telescope. Observed from Earth, at a minimum distance of 29 AU, it appears like a star of magnitude +8. The average apparent diameter of its disc is just 2.35 arcseconds, so it is extremely difficult to see any detail.

Pluto Although it is no longer classified as a planet, it may be worthwhile to include a few words about Pluto. Discovered in February 1930 by Clyde Tombaugh, it orbits at an average distance from the Sun of 40 AU, taking around 248 years to complete a revolution. In 1978, it was discovered that Pluto has a big 50

Image 22. Images of Pluto taken in 1994 by the Hubble Space Telescope. Despite its small apparent diameter, it clearly has areas of different reflectivity on its surface. The original images, before processing, are shown at top left.

(STScI/NASA/JPL)

satellite, Charon, which revolves around it at a distance of around 20 000 km, in about 6.4 days. From analysing the mutual motion of Pluto and Charon, it has been possible to establish that Pluto has a very low mass (0.002 terrestrial masses), and its diameter is around 2400 km, less than our Moon. Charon, on the other hand, has a mass that is eight times smaller than Pluto’s, with a diameter around half that of Pluto.

The Pluto–Charon system has never been visited by a space probe, so we know very little about it. The surface details are not known. It is probable that their surfaces are made of various ices, mixed with silicates and scarred by impact craters. In the years around its perihelion, when the surface temperature reaches a maximum, Pluto develops a sparse nitrogen atmosphere that has the tendency to disappear and condense on its surface as soon as it moves away from the Sun. Pluto is one of the largest objects belonging to the population of the Edgeworth–Kuiper belt. This is the large belt of small frozen bodies with orbits that lie between Neptune’s (at 30 AU) and 100 AU from the Sun. According to current estimates, the recently discovered object named Eris is slightly larger than Pluto. Other large bodies currently known are Varuna (900 km in diameter), and Quaoar (diameter 1250 km). As discussed previously, Pluto can no longer be considered a planet, and those who do not agree with the official decision of the International Astronomical Union (IAU) do so mostly for historical reasons.

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The Solar System’s Past Our Solar System has an impressive variety of characteristics that make it an extremely fascinating subject and an inexhaustible source of information on the history of the bodies within it, as well as on the formation of the Sun and the objects that orbit it. From analysing the most primitive meteorites, we know that the Solar System has existed for around four and a half billion years.

It is interesting to note that the System’s oldest rocks have not been found on our planet or the Moon. This should not be a surprise, as the rocks that form

Image 23. Images of four proto-planetary discs around stars immersed in the Orion Nebula at a distance of about 1500 light years from Earth. These exceptional images were taken by the Hubble Space Telescope, which can outperform ground-based telescopes by operating above Earth’s atmosphere. The proto-planetary discs are flattened structures of dust and gas that surround very young stars, like those that are currently forming in the Orion Nebula.

According to current theories, these discs are possible locations for the formation of planetary systems through processes of dust and gas accretion from the material present in the discs themselves. (STScI/NASA/JPL)

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the more massive planetary bodies are not primitive samples of the material that existed during the formation of the planets. The reason is that the planets are massive enough to have experienced important phenomena of geologic evolution during their history. In particular, the solid material that forms the terrestrial planets suffered events of complete melting and subsequent resolidification early in their history. All of this caused complete reprocessing of the minerals present, so that every trace of the properties of the original material has been irremediably lost.

Intact samples of more primitive material, can still be found, though, in the population of small bodies, asteroids and comets, which, in many cases, did not suffer any appreciable geological evolution, due to their small masses and dimensions. In fact, these small bodies are the origin of some meteorites whose age is around 4.6 billion years. This is determined by measurements based on the content of isotopes of elements created during spontaneous radioactive decay processes. This age should coincide with that of the entire Solar System.

It is presently believed that the Sun and its planetary system were formed at the same time. The data we have show that the planets were formed through accretion of small pieces of solid material. According to current theories, the formation of the Sun was accompanied by the formation of a protoplanetary disc of dust and gas. Thanks to recent observations – particularly in the thermal infrared – a large number of protoplanetary discs have been discovered around other young stars. Among the best known are those around the stars Vega, Fomalhaut and Beta Pictoris, together with those present around some stars in the Orion Nebula.

Solid particles of increasing dimensions formed in the protoplanetary disc, gently colliding with each other and giving rise to a population of so-called planetesimals. The last phases of planetary formation would have witnessed the presence of massive bodies – the so-called planetary embryos – that swept up the area where they were located, incorporating the planetesimals that were found there, or gravitationally perturbing others, substantially changing their orbits. The result was that the largest planets underwent impacts sufficiently energetic as to melt large fractions of their bodies, while many large planetesimals (perhaps the size of Mars) still wandered through the planetary system in an advanced state of formation, provoking new and impressive collisional phenomena.

The most important effects of this violent phase in the history of the planets can still be observed today. One of these was probably the traumatic interruption to the formation of a planet in the area occupied by the current main asteroid belt. In that region, between Mars and Jupiter, the strong gravitational perturbations induced by giant Jupiter prevented a single planet from being formed. Most of the planetesimals present were perturbed and 53

expelled from the region, while a small percentage remained locally to make up the current asteroid population.

Another important consequence of the final and violent phases of the formation of the planets was – in all probability – the formation of the Moon, which, according to most accredited theories, originated following a gigantic impact on Earth in primordial times, probably from a planetesimal the size of Mars. Other examples of effects of catastrophic impacts are the anomalous obliquity of the planet Uranus, whose axis of rotation lies practically on its orbital plane (unlike the other planets, whose axis of rotation is roughly perpendicular to the orbit’s plane around the Sun), and probably also the planet Venus’ abnormally long period of rotation, including its retrograde rotation, which is opposite to the other planets. In both cases it is thought that an extremely energetic impact altered the direction of the planet’s spin axis.

It can, therefore, be said that many of the Solar System’s current characteristics are the result of violent collisions. From this point of view, it can be concluded that every planetary system should be considered a unique example – if formation processes are similar to what took place in the Solar System. In fact, the probability of producing two systems with the same properties would be very low. This is not only due to the limits imposed by having the right quantity of mass and the same chemical composition of pre-stellar material, but also the fact that it is practically impossible to create two systems which experienced the same collisions between their original planets, during their formation.

Impacts in the present Solar System Apart from the violent phases associated with the initial processes of planetary formation, observations show that sporadic collisions with small bodies have subsequently characterised the individual history of all planets and their satellites. Images of the surfaces of the Moon, Mars, Mercury and Venus, as well as almost all the largest planetary satellites, indicate a great abundance of impact craters. Our Earth also bears scars of dozens of these events, even though geologic evolution and the action of atmospheric agents acting on our planet have erased the traces of many of the oldest events. On the other hand, where the planetary surface has not suffered significant changes since very early times – as with the Moon or Mercury – the number of impact craters is enormous. This testifies to the fact that collision phenomena with bodies up to 10 km in diameter – presumably asteroids and comets – have always happened, and they still characterise the evolution at present on planetary surfaces.

As far as our planet is concerned, the most classic example is the event that is presumed to have led to the extinction of the dinosaurs 65 million years ago.

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Image 24. Image of Saturn’s satellite Mimas, taken by the Voyager 2 spacecraft. This satellite has a diameter of around 400 km and its main surface feature is a large impact crater, named Herschel, which has a diameter of 130 km. The presence of craters like this – with diameters comparable to the bodies that host them – is problematic when studying the surface structures of the small bodies in the Solar System. At the same time, they are witnesses of a long history of collisions that has involved all the bodies in the Solar System since the remote era when they were first formed. (STScI/NASA/JPL)

Much more recently, a widely quoted example took place in Tunguska, Central Siberia, in June 1908, resulting in the destruction of a vast area of forest.

Another classical example of the collision phenomenon in very recent times, which fortunately did not affect our planet, was the impact of comet Shoemaker–Levy 9 with Jupiter in July 1994.

Events like these are characterised by the release of enormous quantities of energy, that – as with the theory about the end of the dinosaurs – is enough to completely justify the phenomena of widespread mass extinction of many living species, in agreement with evidence collected by palaeontologists.

For this reason, governments today, stimulated by the information available on bodies that can strike Earth, are beginning to take some preventive measures aimed at preventing such a catastrophe. In fact, only now in human history do scientific and technological tools exist that can allow us to avoid possible calamities associated with impacts by asteroidal bodies.

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Chapter 2

Asteroids The asteroid population The existence of a planet orbiting between Mars and Jupiter had been suspected as early as the 18th century, when it was noticed that the distribution of distances of the known planets from the Sun satisfied a certain empirical mathematical relationship – the Titius–Bode ‘law’. This relationship suggested the existence of another planet, with its orbital semimajor axis around 2.8 AU. For this reason, many observers had been looking for a possible ‘missing planet’ for some time. The search for this hypothetical object seemed to be crowned by success when, on the night of 1 January 1801, Father Giuseppe Piazzi discovered a new object that moved with respect to the stars from the observatory in Palermo. Based on subsequent observations, he succeeded in proving that it really was a planet, with a semimajor axis very near what had been expected for the missing planet, according to Titius–Bode’s empirical law. What left onlookers perplexed, however, was the object’s lack of brightness, as it could not be seen by the naked eye. This also indicated that it was a body of small proportions, a really minor planet.

To complicate things, in the years immediately after the discovery of this first minor planet, named Ceres, three similar objects were discovered with similar orbits and lack of brightness. They were named Pallas, Juno and Vesta.

Many more were discovered as the 19th century progressed, and the discoveries became even more frequent in the following century, especially towards the end, when dedicated surveys to discover these objects were carried out by different observatories. Today, the number of objects with accurately determined orbits amounts to over 100 000, while a still greater number of other objects (around 180 000), is waiting to be observed more extensively in order to get a more reliable value of their orbital parameters. Our inventory is probably complete down to sizes of the order of 10 km.

Among the population of these minor bodies, there are just 26 objects with diameters greater than 200 km; the others have much smaller dimensions. The smallest objects known have sizes down to just a few metres in diameter. Of course, bodies of this size may be detected only in particular circumstances, namely when 57

THE TITIUS–BODE LAW This empirical law takes its name from the two astronomers who were the first to publicise its existence in the 18th century, basing it on the data available on the distances of known planets at that epoch. The ‘law’ can be expressed like this: let us consider the succession of numbers that is achieved by the following general formula:

dn = 0.4 + 0.3 . 2n where, values are replaced to n, progressively: minus infinite, zero, 1, 2, 3,… up to 6. The values that are achieved this way for dn are, respectively: 0.4, 0.7, 1.0 1.6, 2.8, 5.2, 10, 19.6,…

If these numbers are compared with the values of the planets’ semimajor axes as far as Uranus, measured in AU and shown in Table 1 in the first chapter, it will be seen how they seem to arrange themselves almost exactly according to the Titius–Bode relationship values. The fact that, at the time when the law was identified, no planets with semimajor axis equal to 2.8 AU were known, stimulated different observatories to undertake systematic searches for this ‘missing’ planet. The discovery of the first asteroid, Ceres, seemed, at the time, another confirmation of the accuracy of the Titius–Bode ‘law’. Subsequently, the discovery of Neptune, with its semimajor axis of 30 AU, provided the first major anomaly in the ‘law.’ However, Pluto, discovered in 1930, was fairly close to what was expected for the first planet after Uranus, according to Titius–Bode.

Strictly speaking, there is no physical reason why the planets’ distances should follow a trend described by a relationship like Titius–Bode. For this reason, the close coincidence between the law’s predictions and planet orbits as far out as Uranus has long been considered a rather amazing or even embarrassing coincidence. Today, numerical simulations of planetary accretion processes using supercomputers indicate that the growth of planetary systems like ours tends to occur in such a way that the resultant planets have orbits that follow Titius–Bode-like relationships. From this point of view, therefore, it seems that there are no more mysteries and the Titius–Bode law is considered an important curiosity, especially for science historians.

MAGNITUDES AND NAMES OF THE ASTEROIDS When a celestial body is observed (including asteroids), one of the most important parameters to measure is its apparent magnitude, i.e. the body’s apparent brightness. Considering that an asteroid has no internal source of visible radiation, and can be detected only because it scatters the solar radiation incident on its solid surface, its apparent brightness measured by a groundbased observer mainly depends on its distance from both the Sun and Earth, and on its size and surface reflectivity (albedo). For any asteroid, the apparent magnitude based on the distance from the Sun and Earth is given by:

m = M + 5log ( r ∆ ) + kψ where Μ is a constant known as absolute magnitude. It refers to the apparent magnitude the object would display if observed at zero phase angle, and at unit distance from both the Sun and Earth. The absolute magnitude includes in its definition the role played by the size and albedo of the object. The symbol r in the above equation is the distance from the Sun in AU, Δ is the

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distance from Earth in AU, k is a constant known as the phase coefficient and Ψ the phase angle in degrees (the Sun–asteroid–Earth angle, which describes the illumination conditions of the object seen by the observer). A zero phase angle corresponds to perfect opposition, namely to the object being located in the sky at 180 degrees from the Sun. The absolute magnitude and the phase coefficient can be obtained from photometric observations of the objects at different phase angles, taking into account the fact that the heliocentric and geocentric distances and the phase angle are known orbital parameters that are listed in the ephemerides.

It should be remembered that the definition of magnitude in astronomy is logarithmic, so that greater magnitudes correspond to fainter objects. The brightest star in the sky, Sirius, has an apparent magnitude of –1.5, while stars that are barely visible to the naked eye have an apparent magnitude of +6. Typical NEOs have apparent magnitudes of +16, +17 or fainter, and are visible only through a fairly large telescope.

New asteroids are always assigned temporary designation codes based on the date of discovery. This system has been in use since 1925. The first 4 characters are the year of the discovery, followed by a letter that indicates the first or second part of the month in which the asteroid was identified. Correspondence between letters and dates is the following: A C E G J L N P R T V X

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

1–15 1–15 1–15 1–15 1–15 1–15 1–15 1–15 1–15 1–15 1–15 1–15

B Jan. D Feb. F Mar. H Apr. K May M June O July Q Aug. S Sep. U Oct. W Nov. Y Dec.

16–31 16–29 16–31 16–30 16–31 16–30 16–31 16–31 16–30 16–31 16–30 16–31

A indicates the period 1–15 January, B 16–31 January, C 1–15 February and so on. The letter I is omitted (it goes from H to J), and the Z is not used. Another capital letter (the I is always omitted, but the Z is included), indicates the order of discovery within the period of 15 days identified by the first letter. The correspondence between letters and order of discovery is shown here: A = 1st F = 6th L = 11th Q = 16th V = 21st

B = 2nd G = 7th M = 12th R = 17th W = 22nd

C = 3rd H = 8th N = 13th S = 18th X = 23rd

D = 4th J = 9th O = 14th T = 19th Y = 24th

E = 5th K = 10th P = 15th U = 20th Z = 25th

Using this system, in any half of the month, up to 25 new asteroids can receive a provisional designation. If the number of identified bodies is over 25, the system starts again by recycling the second letter, to which a numerical index is added. The number 1 is used if it is the first time it is recycled, 2 for the second and so on. When the orbit is understood with enough precision, the asteroid receives a numbered designation. This means that it is identified by a progressive number. At this stage, a name may also be proposed by the discoverer (for example, 243 Ida).

The final decision belongs to a special Commission of the International Astronomical Union (IAU).

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they closely approach Earth. All these bodies are known as asteroids (the term minor planets was also used until recently). Every freshly discovered asteroid is identified by a provisional code. After a few years of observation, the code is replaced by an order number, possibly followed by a name chosen by the person who discovered it (see the relevant insert for the rules).

The vast majority of asteroid orbits – like the first objects described earlier – are in the region between the orbits of Mars and Jupiter. More precisely, the so-called main belt is approximately between 2.1 and 3.3 AU. Outside the main belt, a certain number of objects exists that orbit at a greater distance from the Sun, and we will look into this later. Within Jupiter’s orbit, and well away from the main belt, are some asteroids of the Hilda and Thule families, which orbit at typical distances of 3.4 and 3.5 AU respectively. More important, there are also two large groupings of asteroids, called Trojans, with the same orbital semimajor axis as Jupiter.

The existence of the Trojans is an elegant confirmation of some classic results of celestial mechanics – the discipline that studies the dynamical phenomena in planetary sciences and astronomy. In particular, the great mathematician, Lagrange, studied the so-called ‘problem of the three bodies’ in the 19th century, meaning the dynamical behaviour of three bodies that are subject to mutual gravitational interaction. This is a very complicated problem in general terms, and in most cases it cannot be resolved analytically, that is, the motion of the three objects does not satisfy a law which can be written in terms of simple mathematical functions, unlike the motion of one planet around a single star.

Nevertheless, in 1772 Lagrange showed that a certain number of stable configurations of a three-body system exist where the three bodies can remain indefinitely, in the absence of external perturbations. These configurations apply under the following conditions: 1. Two of the three bodies, each called a primary, have a far greater mass than the third body, which has such a small mass that it does not perturb the motion of the other two.

2. The two primaries move on a circular orbit, compared to each other.

3. The third body’s motion takes place in the plane that contains the primaries’ orbits.

A practical application of this theory is given by the example of a system that includes two massive objects, like a star and a planet, and a third object of negligible mass, like an asteroid. With the Sun and a planet – Jupiter, for example – there are five positions in a reference frame rotating with Jupiter (one in which the positions of the Sun and Jupiter are fixed), in which a third body could stay indefinitely. Two of these points (indicated by L4 and L5) are 60

those that form an equilateral triangle with the Sun and the planet. These two points are located in the same orbit as the planet, as they have to be at the same constant distance from both the Sun and the planet. One precedes the planet along its orbit and the other follows it.

The Trojan asteroids are a practical application of the predictions of this theory, as these objects are distributed in two groups that correspond exactly to Lagrange’s L4 and L5 points. Current thinking is that the Trojans number several hundred thousand, so they make up a substantial subset of the whole asteroid population. Lagrange’s remaining three points are on the Sun–planet line: L3 beyond the Sun, L1 between the Sun and planet, L2 beyond the planet.

For the Earth–Sun system, L3 is at 1 AU from the Sun (opposite to Earth), while L1 and L2 are about 1.5 million km from Earth.

Inside the main belt, the asteroids are not distributed homogeneously. In fact, if we look at the distribution of the semimajor axes of the objects’ orbits, we can easily see a certain number of gaps (known as ‘Kirkwood gaps’). Orbits with these semimajor axis values appear to be ‘forbidden’, i.e. they do not correspond to the orbital semimajor axis of any observed asteroid. Later on, we

L4

J

L5

Image 1. Projection of the asteroids’ positions on the ecliptic plane on 22 July 2004. The orbits of the planets, Mercury, Venus, Earth, Mars and Jupiter, are shown in blue. The positions of both numbered and unnumbered asteroids are shown in green. Objects with perihelion distance lower than 1.3 AU are shown in red.

Objects observed during more than one opposition are represented with full symbols, while the bodies seen during just one opposition are represented with empty symbols. The two clouds of objects that follow and precede Jupiter’s position by 60° represent the Trojans, shown in dark blue. Numbered periodic comets are represented by full blue squares, while the other comets are shown by empty blue squares. (Minor Planet Center)

Image 2. This figure shows the relative position of the two Lagrangian points, L4 and L5, in relation to the positions of the Sun and planet Jupiter. There are two populous groups of asteroids, called Trojans, whose orbits correspond to Jupiter’s L4 and L5 Lagrangian points. Mars also has four Trojans, three of which orbit in correspondence with its L5 point and the other one in correspondence with L4, while Neptune has six Trojans, all at the L4 Lagrangian point. No objects of this type have been discovered along the orbits of the other planets.

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will see that these particular values correspond to some of the most important mean motion resonances with Jupiter, which are dynamically unstable areas. If we look at a graph that shows the value for known asteroids of orbital inclination versus the semimajor axis, we can see that another ‘forbidden’ region lies in the inner belt, forming a kind of identifiable edge. This area corresponds to one of the most important secular resonances, as will also be explained later.

Besides the main belt asteroids and the Hilda, Thule and Trojan objects, a further asteroid sub-population exists with some possibility of being involved in possible catastrophic collisions with our planet. These objects’ orbits take them to the innermost region of the Solar System, within Mars’ orbit. They have conventionally been classified into three populations that take the names Aten, Apollo and Amor. About 450 Atens, 2350 Apollos and 2050 Amors were known at the end of 2007. The three groups are distinct due to their orbital parameters. In particular, Atens have an orbital semimajor axis less than 1 AU, but their aphelion distance (the greatest distance from the Sun along the orbit) is greater than Earth’s perihelion distance, equal to 0.983 AU. (The perihelion distance is the shortest distance that a planet travels from the Sun, moving on its elliptical orbit). Atens are, therefore, objects that spend most of their orbital period inside Earth’s orbit, but their aphelion can extend to a distance beyond Earth’s orbit.

On the other hand, the Apollo objects are characterised by a larger semimajor axis than Earth’s, but their distance at perihelion is less than Earth’s aphelion distance, equal to 1.017 AU. They are, therefore, objects that orbit

Image 3. Distribution of known asteroids according to the semimajor axes of their orbits. The numbers below indicate the main mean motion resonances with Jupiter that correspond to a series of ‘forbidden’ values of the semimajor axis, known as Kirkwood gaps. (Minor Planet Center)

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Image 4. Sketch of Earth’s orbit and the orbits of the asteroids that gave their names to the Aten, Apollo and Amor groups.

beyond Earth’s orbit for most of the time, but during every revolution around the Sun, they travel inside our planet’s orbit for a while. Amors are objects whose orbits are characterised by a perihelion distance between 1.017 and 1.3 AU, i.e. between the orbits of Earth and Mars. We could initially conclude that the Amor asteroids are not a problem for Earth, as they cannot intersect its orbit, unlike the Aten and Apollo objects, but the situation is more complex.

Dynamical evolution of all Aten, Apollo and Amor objects (see chapter 6), is eminently chaotic. This means that these asteroids have relatively fast orbital evolutions, and the distinction between the three groups corresponds only to a transitory situation.

Apart from objects belonging to the Aten, Apollo and Amor groups, the first asteroid whose orbit is completely inside Earth’s was recently found. It was discovered on 11 February 2003 by the LINEAR (Lincoln Near Earth Asteroid Research) survey and received the temporary name 2003 CP20. This asteroid has a semimajor axis of 0.76 AU and an eccentricity of 0.29. This means it reaches 0.98 AU from the Sun at aphelion, slightly less than Earth’s perihelion. On 10 May 2004, another asteroid of this type, 2004 JG6s, was discovered with its aphelion at 0.973 AU. Asteroids of this type have theoretically been expected as evolutions of Aten–Apollo–Amor and they are known as Inner Earth Objects (IEOs). Today, only 8 IEO objects are known, but they are probably only a marginal subset of the existing IEO population, which is very hard to detect from the ground since they are always located at small angular elongations from the Sun. Collectively, all objects that can enter the region occupied by the terrestrial planets are now termed NEOs (NearEarth Objects). Some NEOs are also NEAs. Small bodies in the outer regions of the Solar System Over the last 10 years, we have realised that even the Solar System’s peripheral areas are populated by numerous small bodies. For the time being, the most important external population is the Edgeworth–Kuiper belt, made up of small frozen bodies (containing water and ammonia ice), with orbits fairly close to the plane of the ecliptic and distances between 30 and 50 AU from the Sun. Often known simply as the Kuiper belt, it is thought to be the most important source of short-period comets (orbits less than 200 years).

The long-term members of this belt are called Trans-Neptunian Objects (TNOs) or Kuiper Belt Objects (KBOs). Discoveries began in 1992 and now there are more than 1000 known objects. It is estimated that the whole population is around 200 million. Historically, the first TNO to have been discovered was Pluto, which held the status of planet until the General Assembly of the International Astronomical Union in 2006, when it was 63

Image 5. Drawings showing the scale of the Solar System, from the Oort Cloud to the main asteroid belt. Sedna’s orbit is shown in red. (NASA/JPL)

Image 6. Three separate exposures that overlap the same stellar field, taken with a telescope. The telescope has a motor that allows it to stay aimed at the same area of sky and this compensates for Earth’s rotation, so ‘stationary’ objects like stars always appear in the same position in all the exposures. As the image clearly shows, asteroids in the telescope’s field of view move perceptibly between one exposure and the next. (D. di Cicco)

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Image 7. Artist’s impression of Sedna, the most distant small body that has ever been discovered in the Solar System. (NASA/JPL)

officially removed from the list of planets, as discussed in chapter 1. Among the largest bodies currently known in this belt are Haumea and Makemake (about 1500 km in diameter), Quaoar (diameter about 1200 km), and Eris, discovered in 2003, which has an estimated diameter of about 2600 km, and is therefore larger than Pluto.

The outer edge of the Kuiper Belt is not sharp. Beyond 50 AU, and up to some hundreds of AU from the Sun, there are the so-called scattered TNOs. One example is Sedna (around 1500 km in diameter), which was discovered on 14 November 2003. Sedna is the furthest object from the Sun discovered to date (it is currently at 90 AU). It has a semimajor axis of 532 AU and a very high eccentricity of 0.86. The corresponding orbital period around the Sun is 10 500 years. It could not have been discovered for a long time if it had not been not close to perihelion (at 75 AU from the Sun). At aphelion, the distance of Sedna is 988 AU, so it would not be observable. Beyond the Kuiper Belt and the scattered TNOs, there is evidence of the existence of the so-called Oort cloud, which extends up to 100 000 AU from the Sun and is the most likely source of long-period comets (see the chapter on comets). The estimated population is 200 billion. Over the next few years other discoveries of bodies will certainly be made belonging to these remote areas of the Solar System.

There is also a population of more than 150 bodies with orbits between Jupiter and Neptune. The first member to be discovered was Chiron in 1977 (diameter about 200 km), between the orbits of Saturn and Uranus. These bodies are collectively known as Centaurs and are probably in transition from the Kuiper belt to the inner Solar System. Centaurs were considered to be asteroids until it was discovered that some of them exhibited phenomena 65

resembling weak cometary activity. This is the case with Chiron itself, which, initially thought to be an asteroid, is now considered to be a comet because in 1988, during its passage to perihelion, it developed a coma of gas and dust.

KBO designations usually follow the same rules as asteroids, and not the rules for comet designation. At the distance from the Sun where they move – over 30 AU – cometary activity can hardly occur and/or be detectable, since the temperature is extremely low and the surfaces are frozen. As we have seen, interconnections between the different categories of small bodies are numerous, so we will regard asteroids as small bodies in the Solar System with dimensions of at least 10 m and orbits that do not go beyond Jupiter’s orbit, including the Trojans.

Now it is time to study the physical properties of asteroids – how large they are, what they are made of, how many exist and their history. To do this, however, it is first necessary to include how asteroids are observed and the types of information that can be obtained.

Observation of asteroids A recurring question is ‘what does a typical asteroid look like when it is observed with a telescope’? In the overwhelming majority of cases, what is seen is a pointlike object that, at first sight, cannot be distinguished from a star, if it were not for the fact that an asteroid moves slowly across the celestial sphere, compared to stars that are ‘fixed’. Some illustrations in this chapter are made in the classic manner, namely by photographing a region of sky with a telescope that moves in such a way as to perfectly follow the apparent motion of the stars. Asteroids can then be recognised as they move with respect to the stars, leaving a detectable trace in the final image. This is how, historically, known asteroids were discovered – by detecting their apparent motion across the celestial sphere.

The fact that asteroids are generally faint (even the brightest ones are never detectable with the naked eye) and look similar to stars explains why none were discovered before 1801. At the same time, it can be understood why remote observation of asteroids with Earth-based telescopes cannot reveal fine details of the asteroid topography. Even making use of the most powerful telescopes available today, the vast majority of asteroids are still generally point-like objects without appreciable extension. (In this respect, the situation has been evolving in recent times, thanks to the availability of adaptive optics systems fitted to the largest telescopes in the world. These systems correct the blurring of the images due to the atmosphere, and allow the telescopes to obtain images fairly close to their theoretical diffraction limit).

On the other hand, observations still give us an impressive amount of information. First, measuring the objects’ positions at different times allows us 66

to identify the characteristics of their motions and calculate their orbits.

Getting a very precise orbit is a complex assignment that requires observing the object for a long period of time. In particular, for an object to acquire a definitive official name, it is necessary for it to be observed during different apparitions. The meaning of this expression can be understood when it is considered that any given asteroid cannot always be observed at any moment, as the orbital motion of Earth and the asteroid means the object spends some time close to the Sun, so it cannot be observed at night. Every object is visible only during certain periods of time for various weeks, called apparitions, during which it is accessible to night observation. In general then, the best moment for observation is at opposition, or rather when the object is in the opposite direction to the Sun. (Opposition, naturally, does not occur for objects that never travel further from the Sun than Earth).

Other information that can immediately be obtained from observations is the apparent brightness. In general, an asteroid’s brightness varies continually and displays a periodic variation that is characteristic for every object. This is explained by the fact that the shapes of these objects are not perfect spheres, but can be quite irregular. As the objects rotate around their spin axis, therefore, they present to the observers variable areas of illuminated surface.

Image 8. Light curves of asteroids 39 Laetitia and 44 Nysa, whose shapes are very similar to three axis ellipsoids.

The points represent measurements taken with a photometer.

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By analysing the period of brightness variation, it is then easy to determine the period of rotation. When observations of brightness variation are obtained at different apparitions, corresponding to different Sun–Earth–asteroid configurations, it is then possible to derive the direction of the object’s axis of rotation in space. The same trend in brightness variation also gives us rough indications of the object’s general shape.

In the next paragraph we will talk about other information that can be obtained by applying techniques of spectroscopic investigation, that is to say analysis of radiation intensity received at different wavelengths. We will also discuss the technique of radiometric observation that obtains information on asteroids’ dimensions and surface reflectivity. For now it is worth mentioning that asteroids are accessible, not only through optical observation with telescopes, but also using radar techniques, since many of them pass fairly close to Earth. This last type of observation has developed substantially over the last 20 years, thanks to the availability of powerful transmitters and sensitive receivers. These instruments are necessary since the radar echo received from asteroids – as with any other body – quickly decreases with distance (more precisely, with the inverse of the 4th power of the distance). For radar observations, NEOs are obviously more easily observed as they can approach quite near Earth. Radar observations of these asteroids provide detailed information on their orbital motion and physical properties, such as dimensions, structure and surface composition. Additionally, extremely detailed radar images have been obtained, in some cases underlining the binary nature of some asteroids, e.g. 4179 Toutatis and 4769 Castalia, which seem to be made up of two objects in contact.

Binary asteroids Although they are small bodies, even asteroids have their own gravitational field which is able to maintain one or more satellites in orbit (as happens with planets) around the central body. This is true if distances are not excessive, otherwise the Sun’s gravity field takes the upper hand. We talk about asteroids with satellites when one of the two components in the system is much smaller than the other, while, with double asteroids, the components are equally important. The term ‘binary asteroids’ is normally used if relative sizes are not specified.

After a long debate triggered by some inconclusive pieces of observational evidence, we know now with certainty that binary asteroids do exist. Asteroid satellites are important because they allow us to measure the total mass of the system, using Kepler’s third law. These are very important data, since asteroid masses are physical parameters of great significance, but extremely difficult to estimate. Binary asteroids have been discovered among the near-Earth, main 68

belt, Trojan and trans-neptunian populations. The discovery of these systems is the direct result of improvements in radar, optical and space observation techniques (e.g. space probes and the Hubble Space Telescope). The two components of binary asteroids are usually very near to each other so it is very difficult to separate them in optical images. Moreover, the satellite companions are also generally very faint, making the discovery of these systems a real challenge.

The first certain discovery of an asteroid with a satellite was made by NASA’s Galileo probe that passed near the asteroid 243 Ida (31 km in average diameter) on its way towards Jupiter in 1993, and discovered Dactyl, a satellite only 1.4 km in diameter. In 1998, the first discovery from Earth was made with the use of a telescope equipped with an adaptive optics system. This was a 13 km satellite, later named Little Prince, in orbit around asteroid 45 Eugenia (215 km in diameter). In 2000 there was the discovery of 90 Antiope, which turned out to be a real double asteroid, with two components of comparable sizes (diameter 85 km), situated 170 km apart with an orbital period of 16.5 hours. In 2001, it was discovered that 22 Kalliope (previously considered to be metallic in composition due to its spectroscopic properties) had a small satellite. This made it possible to derive a measurement of the density of the main body. It turned out to be 2.3 g/cm3, a surprisingly low value that one would expect for a rocky asteroid, not a metallic one. In 2001, the first binary asteroid was also discovered among the Trojans – 617 Patroclus. This system,

Image 9. Image of asteroid 243 Ida taken by the Galileo spacecraft at a distance of around 10 000 km. The small object on the right is its satellite Dactyl, which is around 100 km from the centre of Ida. It has an approximately spherical shape and a diameter of around 1.4 km. It was the first binary asteroid system to be discovered. (NASA/JPL)

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Image 10. A composite image of the binary trans-Neptunian object 1998 WW31, taken by the Hubble Space Telescope. (STScI/NASA)

like Antiope, has components of the same size so it is an authentic double asteroid. Then, in 2002, again by means of observations from the ground, the first binary trans-neptunian system was discovered, 1998 WW31. Six other binary TNOs have since been discovered. In 2005 there was the discovery of the first TNO, Haumea, accompanied by two small satellites, followed in 2006 by the discovery of the first triple system in the main belt, comprising asteroid 87 Sylvia and two small moons. In February 2008, 2001 SN263 was revealed as the first near-Earth triple asteroid ever found.

To explain the existence of binary or multiple asteroids, current theories invoke the destruction of a progenitor body through collisions with other asteroids. Even tidal interaction of an asteroid with a planet, during a close encounter, could lead to the fragmentation of the body with the creation of a binary or multiple system. The last mechanism is inherently more likely for NEOs, and this might explain the larger abundance of binary systems observed among the near-Earth population.

Physical properties As the name ‘minor planet’ implies, asteroids are modest-sized objects. The largest one, Ceres, has a diameter of around 1000 km. Ceres is a real giant in comparison to the large majority of the asteroid population, however, as only a 70

The first triple asteroid In 1993 the Galileo probe flying towards Jupiter had a close encounter with asteroid 243 Ida, discovering a small satellite about 1.5 km in size. Until then, the only other asteroid observed by a spacecraft was 951 Gaspra. The discovery of Ida's moon seemed to suggest that the number of binary or multiple objects should be a large fraction of the whole asteroid population. Thanks to the capability of the ground-based instruments, more than 50 binary asteroids have been discovered, confirming this hypothesis. About one third of these binaries belong to the group of Near-Earth Asteroids (NEAs).

In 2005, a group of American and French astronomers discovered a second satellite orbiting around an asteroid, 87 Sylvia, making it the first known triple asteroid. Sylvia, discovered in 1866, is one of the major bodies of the main belt, with an irregular shape of 280 x 380 km. The name Sylvia refers to the mythical mother of the two founders of the city of Rome, Romulus and Remus, whose names have been assigned to the larger satellite (discovered in 2001) and to the smaller one, respectively.

The two moons are very small and orbit around Sylvia in the same plane and with trajectories that are practically circular. The smaller of them, Remus, has a diameter of about 7 km, is 710 km from Sylvia and its orbital period is about 33 hours. The second one, Romulus, has a diameter of 18 km, an orbital period of 87.6 hours, and its distance from Sylvia is about 1360 km. The orbital characteristics of the two satellites allowed an accurate measurement of the mass and density of Sylvia (1.2 g/cm3). This low density value implies that this object probably originated from the gravitational reaccumulation of fragments produced by a catastrophic collision that in remote ages destroyed a parent body. The two small satellites are probably fragments produced by this event that, after the formation of the new asteroid, have been captured by its gravitational field.

On the basis of present knowledge, we estimate that about 6% of asteroids have companions, and the astronomical observations and numerical simulations suggest that the ‘rubble-pile’ asteroids, or second generation objects, must be relatively numerous. The probability of discovering more multiple asteroids in the near future is not negligible.

Image of triple asteroid 87 Sylvia, showing two companions. (ESO)

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small minority of these bodies can boast sizes greater than about 50 km. In 2006 the General Assembly of the International Astronomical Union introduced a new category of planetary objects, the so-called dwarf planets, and Ceres is one of the few Solar System objects belonging to this class. In this respect, Ceres is for the asteroid belt what Pluto and Eris are for the population of TNOs, with the difference that the predominance of Ceres with respect to the rest of the asteroid population is even sharper.

Since the number of objects increases as smaller sizes are considered, we can say that the current inventory of asteroids totals almost 440 000 objects, including the numbered ones with a very well determined orbit (around 203 000 in December 2008), as well as the objects whose orbits have not yet been sufficiently well defined (around 237 000). This total represents just a modest fraction of the existing objects, since our inventory is limited by the fact that, below a certain limit of apparent brightness, the objects are too faint to have been detected.

The concept of a total number of asteroids is entirely ambiguous though, since what must first be defined is exactly what is intended by the term ‘asteroid’. This type of definition becomes important when the objects considered are increasingly small and numerous. Logically, any small sized body orbiting the Sun could be defined an asteroid, at least if it didn’t show cometary activity phenomena. This definition would then also include particles

Image 11. The sizes of the three largest asteroids, compared to Italy.

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of interplanetary dust that are normally considered objects of a different nature. However, one difference between an asteroid and a dust particle could be that the dust particle has a motion that is significantly influenced by non-gravitational phenomena, like the pressure exerted by solar radiation. On the other hand, it has been noticed that non-gravitational effects can also act on bodies of some hundred metres, or even some kilometres, in size. Objects in this range are generally considered to be asteroids.

From this discussion it is necessary to conclude that the distinction between the various categories of small bodies in the Solar System (asteroids, comets, meteorites and interplanetary dust) is very often conventional and based on deep-rooted customs. Even the difference between asteroids and comets, which at first sight seem to have very different properties, appears very transitory when we consider that, over time, comets exhaust their supply of volatile elements and are destined to extinguish themselves, becoming asteroidlike objects.

The information we have on the sizes of asteroids is mostly based on indirect techniques. The vast majority of the objects, in fact, is simply too small to study in detail with any telescope. Most data were obtained by the so-called radiometric technique, which is based on simultaneous measuring of solar radiation reflected by asteroids at infrared wavelengths beyond 5 µm. In fact, both the quantity of scattered sunlight that an asteroid emits at visible wavelengths and the quantity of thermal radiation emitted at medium-IR wavelengths ultimately depend on two parameters, namely the object’s surface reflectivity (the term albedo is used by planetary scientists) and its dimensions.

Simultaneous measurement of both the fluxes of radiation allows us to determine size and albedo. As it is extremely difficult to conduct thermal infrared observations from Earth, because of absorption due to the atmosphere, most available radiometric data for asteroids has been obtained via observations from artificial satellites orbiting our planet. We will speak in detail about space observations in chapter 11.

The composition of asteroids appears to be quite heterogeneous. In the absence of samples taken in situ, current knowledge is mostly based on spectroscopic observations – sunlight scattered by the surface of these bodies is analysed at various wavelengths from ultraviolet to near-infrared. In fact, minerals present on the surface give rise to characteristic bands of absorption that can easily be detected. The asteroids’ spectral characteristics allow for a classification into separate taxonomic classes. Among them, the most important are: 1 Class C. Low-albedo asteroids, with mostly flat reflectance spectra, that are thought to be rich in carbon. They represent 60% of all asteroids and are concentrated in the external part of the main belt.

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Image 12. Distribution of the different taxonomic classes of asteroids, based on their heliocentric distance.

Image 13. These two graphs show the spectra of sunlight reflected by some of the brightest asteroids, selected to represent the different spectral classes. All of the spectra are normalised to 0.56 micron (Âľm) and arbitrarily moved in the direction of the vertical axis so that they do not overlap.

Image 14. Reflection spectra of some of the most important minerals present in meteorites. Iron–nickel alloy (a), olivine (b), orthopyroxene (c), feldspar (d) and spinel (e). The different spectra have arbitrarily been moved in the direction of the vertical axis so that they do not overlap.

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2. C lass S. These asteroids are rich in silicates and show the spectral characteristics of rocky bodies containing pyroxene and olivine, besides iron and nickel. They are very abundant among the Apollo–Amor type asteroids and represent 30% of the bodies listed in the main belt.

3. Class M. Objects exhibiting some evidence of an overall metallic composition, including iron and nickel.

There are other classes made up of smaller number of objects: for example the E taxonomic class, characterised by a high albedo; the V class, whose spectrum is very similar to that of the large asteroid 4 Vesta, and is diagnostic of an overall basaltic composition; the P and G classes, that are a subspecies of the large C class; and others.

When interpreting reflectance spectra, the problem is that many different minerals can be present in varying proportions, giving origin to many complex and mutually overlapping absorption bands. For this reason, spectroscopic data supply information that is not unequivocal. We know, though, that asteroid surfaces are rocky and include minerals, including various types of silicon, that often correspond to those found in different classes of meteorites.

This fact strengthens the conclusion accepted by most researchers that the great majority of meteorites have an asteroidal origin. As mentioned above, at least a limited number of asteroids also seems to exhibit spectral behaviour that would indicate the presence of large quantities of metal (iron and nickel), suggesting that these objects are the metallic cores of differentiated parent bodies, like ‘miniature planets.’ The mantle and crust of these parent bodies would have been removed by catastrophic collisions during their history.

Today we know that, over time, asteroids underwent an evolution that was essentially determined by collisions. This conclusion is based on a lot of evidence that goes from theoretical calculation of the probabilities of collision and general distribution of the known physical properties, up to data supplied by images of the heavily cratered surfaces of asteroids 243 Ida, 951 Gaspra, 253 Mathilde, 5535 Annefrank, 433 Eros and 25143 Itokawa, observed in recent years by the space probes Galileo, NEAR-Shoemaker, Stardust and Hayabusa. Moreover, there is other important observational evidence of the fact that catastrophic collisions took place in the asteroid belt. This is the existence of groups of objects that should certainly be considered fragments generated by the destruction of a certain number of parent bodies. These groups take the name dynamic families and are characterised by the fact they share very similar values of semimajor axis, eccentricity and inclination, inherited by the parent body.

The families are extremely important, as they allow us to observe objects that are, in practice, samples of the innermost layers of their progenitor body.

The number of identifiable families in the main asteroid belt supplies 75

Image 15. Image of asteroid 951 Gaspra taken by the Galileo spacecraft at a distance of around 5300 km. The illuminated surface is about 18 km across. The north pole is situated at top left. (NASA/JPL)

Image 16. Different, but typical ways for asteroidal objects to fragment: (a1) longitudinal ‘splitting’; (a2) ‘conical’ destruction of a spherical object during low speed impacts; (b) ‘nucleus’ type destruction during high speed impacts.

The arrows indicate the rotation direction of the fragments. (NASA)

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THE NEAR-SHOEMAKER MISSION: THE EXPLORATION OF 253 MATHILDE AND 433 EROS On 14 February 2000, NASA’s NEAR (Near Earth Asteroid Rendezvous) probe entered orbit around the asteroid 433 Eros. This was an important event in the exploration of the Solar System, because the spacecraft, later named NEAR-Shoemaker as a tribute to the great planetary geologist Eugene Shoemaker, was the first to enter orbit around an asteroid: previous missions towards asteroids were limited to close flybys.

Launched from Cape Canaveral on 17 February 1996, NEAR-Shoemaker had a mass of 805 kg and a volume of few cubic metres, with various experiments on board, including a CCD multi-spectral camera for taking images, infrared and X-ray / gamma ray spectrometers, a magnetometer and a laser rangefinder for measuring distances. On 27 June 1997 the probe flew near the asteroid 253 Mathilde, and then continued on its trajectory towards its primary target, the large Near-Earth Asteroid, 433 Eros.

253 Mathilde was the first C-type asteroid to be visited by a spacecraft. It measured 66 x 48 x 46 km with an average density of just 1.3 g/cm3, which indicated that the asteroid’s structure is not compact but rather porous, with voids inside. The surface was not entirely imaged, but the explored part exhibited huge craters, with 5 of them having sizes that compare to its entire diameter. The presence of these craters indicates that the asteroid’s porous structure succeeded in absorbing the energy from very energetic impacts without being totally disrupted.

A first attempt at orbital insertion around Eros failed in December 1998 but the second attempt was successful. The mission concluded on 12 February 2001, when NEAR landed on Eros and became the first spacecraft to land on an asteroid.

Eros is an S-type asteroid that was discovered on 13 August 1898 by G. Witt, director of the Berlin observatory. After calculating its orbital elements, it was found that Eros moves around the Sun in 1.76 years at a distance that varies between 1.13 and 1.78 AU. So Eros is an Amor type asteroid, external to Earth’s orbit. Under the best conditions, Eros can approach within 0.15 AU of our planet , so for the time being it is not thought to be a danger to Earth.

In 1996, however, a study of Eros’ orbit, published by the late Paolo Farinella and colleagues, concluded that, due to an interaction with Mars, its perihelion is destined to decrease to 1 AU in about 0.55 million years. As a consequence, there is a 50% probability that the asteroid will strike Earth in the next 1.14 million years.

Eros is an elongated asteroid, measuring 35 x 14 x 13 km, with a period of rotation of 5h 16m around its minor axis. Its average density, as measured by NEAR-Shoemaker, is 2.5 g/cm3, similar to asteroid 243 Ida and compatible with its mineralogical classification.

Eros appears to be a solid, consolidated body without large internal hollows. From NEARShoemaker’s extensive observations, there are no satellites around Eros. The surface appears covered by about 10 m of regolith studded by impact craters. These craters help us estimate an age of between 1 and 2 billion years. The largest crater measures 6 km in diameter and is situated in proximity to Eros’ north pole. The crater is crossed by a series of parallel cracks, which also run all along the asteroid’s largest axis, and are similar to structures on Phobos, the largest satellite of Mars. It is probable that they are the effects of an old impact that nearly caused Eros to break apart. Chunks of rock on Eros’ surface are numerous, probably ejecta from crater formation that fell onto the asteroid’s surface.

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Image 17. Close up image of 433 Eros taken by the NEAR-Shoemaker spacecraft. (NASA/JPL)

Image 18. Three images of asteroid 253 Mathilde taken by the NEARShoemaker spacecraft.

The image at bottom left shows a comparison of the three asteroids visited by the Galileo and NEARShoemaker missions.

(NASA/JPL/Caltech)

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information on the intensity of the collisional evolution experienced by the population overall. Lastly, the families’ physical parameters allow us to obtain essential information on the physics that governs the catastrophic destruction of bodies with diameters of tens or hundreds of kilometres, for which selfgravitation becomes important, including those that were progenitor bodies of the families currently identified.

In particular, studying the families allows us to deduce the ejection speeds of the fragments produced by catastrophic collisions. This is of great importance as it allows us to assess the efficiency that the collisional events can have in producing fragments expelled at such a speed in order to end up in some dynamically unstable area of the main belt, namely the resonances. From these regions, objects then tend to migrate towards the Solar System’s innermost regions, in order to resupply the population of Aten, Apollo and Amor objects that, as we saw earlier, have a probability of impacting our planet.

At this point, it would be helpful to address the problem of orbital evolution of asteroids that has been closely analysed over the last few years, benefiting from continuous improvement in the technology necessary to carry out numerical integration of the orbits (i.e. the calculation of orbital motion over time).

Dynamical evolution Before the great importance was recognised of the intrinsic physical properties of small bodies in order to understand the Solar System’s history, these objects aroused interest in researchers for the many dynamical phenomena that they exhibit. In other words, asteroids were seen as a population of practically point-like objects that could be used for verifying the accuracy of celestial mechanics’ models when describing the behaviour of a complex system like the Solar System. In particular, we have already mentioned that bodies in the Solar System, including planets, do not have simple motions as if every object were the only one to orbit the Sun. In fact, the Solar System is a system of many bodies, though the two main ones are the Sun and planet Jupiter, due to their preponderant masses.

In particular, asteroids’ orbits are acutely sensitive to the presence of planets, especially Jupiter. Their orbital parameters are subject to continuous changes due to planetary gravitational perturbations. These variations occur over short periods (‘short period’ in astronomy means some tens of thousands of years), as well as over longer periods (‘secular’ variations, occurring over time scales of hundreds of thousands of years).

For this reason, in order to better characterise an asteroid’s orbit, rather than simply considering the instant values of its orbital parameters, the 79

Image 19. Diagram showing the geometry of the 3:1 mean motion resonance. If the asteroid and planet are in positions P and G1 at the same epoch, the asteroid will be at P again after a complete revolution. At that epoch, the planet will be at G2, while it will be at G3 after the asteroid has made two complete revolutions. At the end of the asteroid’s third revolution, the initial configuration will be repeated.

Image 20. The eccentricity (left) and inclination (right) of the asteroids’ orbits plotted against the semimajor axis (measured in AU). The empty Kirkwood gaps can be seen, as well as the groupings that correspond to the main dynamic families.

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so-called proper elements are used. Their meaning, simplifying things a little, is that they are an average of the orbital elements, when short period variations are neglected and secular variations are averaged over long periods of time. For many asteroids in the main belt, it is possible to calculate proper elements, as these objects, apart from the variations we mentioned before, tend to have quite stable orbits over long periods of time.

The regions of the main belt near the Kirkwood gaps, the ‘forbidden’ values of orbital semimajor axis mentioned before, correspond to hypothetical orbits for which stable orbital elements cannot be calculated, even over relatively short time spans. The reason for this is that these particular orbits have a very chaotic behaviour. The concept of chaos is not new in dynamics, but the study of asteroids has contributed to better specifying its meaning. A chaotic orbit is highly unstable, and does not allow us to accurately predict an object’s position except for limited intervals of time. Today, in fact, we know that stable and predictable orbits over indefinite spans of time in the future don’t exist in the Solar System, even in the case of planets. These bodies’ orbits, however, are only moderately chaotic, and have not varied a lot over the Solar System’s history. Certain asteroids’ orbits, though, can change dramatically in relatively short times. This is true particularly for NEO objects due to their close encounters with terrestrial planets, as will be discussed in detail in another section.

Within the main belt, chaos is strictly associated with certain types of orbits, because of resonance phenomena. Under what conditions can an orbital resonance take place? If two celestial bodies orbit the Sun with periods P1 and P2, their motion is in resonance when there are two small and whole numbers, N1 and N2, such that:

P1 N 2 = P2 N1

This relationship expresses the fact that after N1 orbits of the first body and N2 orbits of the second, the relative positions of the two bodies repeat exactly. In particular, the so-called Kirkwood gaps in the main belt correspond to values of the orbital semimajor axis that lead to a periodic repetition of the relative positions of Sun, Jupiter and the asteroid. This is a consequence of Kepler’s third law that ensures that the semimajor axis of the orbit corresponds to a welldefined value of the orbital period. This means that, for certain values of the orbit’s semimajor axis, a situation occurs where the period of a corresponding revolution is a simple fraction of the period of Jupiter. For example, the important Kirkwood gap at a semimajor axis of 2.5 AU corresponds to orbits around the Sun characterised by a period of revolution equal to one third of Jupiter’s period. A similar situation ensures that a hypothetical resonant 81

asteroid and Jupiter would be in the same relative position every three revolutions of the asteroid, corresponding to one Jupiter year. A situation of this type is known as a 3:1 mean motion resonance. The other major Kirkwood gaps in the main belt correspond to 5:2, 8:3 and 9:4 mean motion resonances with Jupiter. Additionally, the distribution of main belt asteroids in semimajor axis is confined by two important resonances 4:1 and 2:1. The reason these resonant orbits are chaotic is, in fact, complex, and cannot easily be explained without using mathematical concepts of celestial mechanics that are beyond the scope of this book.

On the other hand, a situation of resonance can sometimes correspond to a mechanism of orbital protection inside a generally chaotic area. This is the case with the Hilda and Thule objects that are beyond the main belt, in orbits corresponding to the 3:2 and 4:3 resonances, respectively. In fact, what really makes a given orbit chaotic in the Solar System is the possibility of being affected by the overlapping of different resonances.

Concerning these other possible types of resonance, great importance is given to the so-called secular resonances, that are verified when an object’s precession period corresponds to a major planet’s period of precession. The phenomenon of the orbits’ precession is similar to the process for which a body in rotation ‘precesses’ in comparison to a fixed axis, under the influence of external forces. In particular, precession occurs because bodies in the Solar System are not perfectly spherical and don’t all orbit on the same plane, so some forces result that provoke a slow and steady motion of rotation of the orbital plane. Consequently, the point in which the orbital plane meets some given plane of reference (like the equator in the celestial sphere), describes a circle on a time scale that is typically some tens of thousands of years. The very orientation of the orbit also ‘rotates’ on its plane, in the same time scale. When an asteroid’s period of precession is equal to the period of a planet’s precession, we speak of secular resonance. This type of resonance is also generally associated with phenomena of chaotic behaviour. From this point of view, the most important case in the main belt is the so-called ν6 secular resonance with Saturn, that borders the inner edge of the belt.

The fact that objects found in one of these resonances – mean or secular motion – chaotically evolve on very short time scales, has fundamental importance regarding the origin of the NEOs.

Origin of the asteroids How could an enormous number of small bodies orbit in the region where, according to Titius–Bode’s empirical law, one might expect the presence of just one planet? What is the real meaning of the Titius–Bode law? Based on what 82

has already been mentioned on the formation of the Solar System, planets must have formed through accretion. Numerical models of these processes show that when a planetary system is formed like this, with the embryos of the protoplanets growing at the expense of material orbiting in their area of formation, distribution of planets at the end of the process has the tendency to satisfy a relationship similar to Titius–Bode. From this point of view, there should not be many mysteries.

What is surprising with asteroids in the main belt region, though, is that the growth process was somehow stopped, and a large part of the mass there has been irremediably lost – the total mass of all asteroids today being far less than what would be expected for a planet formed in that region. The reason for the failure in the formation of a planet in this zone is probably the rapid growth of Jupiter in the adjacent region. In particular, it is thought that during the last phases of its formation, Jupiter could have perturbed and expelled numerous massive planetesimals from its vicinity. Ejected into the current asteroid belt area, these Jupiter-scattered planetesimals could have perturbed the bodies orbiting there, causing an increase in the average encounter velocity to the point of making mutual impacts between these bodies destructive, rather than constructive. If this was the case, a planet would never have been formed and most of the mass would finally have been expelled from the area or pulverised following mutual and violent impacts.

According to an alternative theory, at least two large planetesimals of mass equivalent to Mars could have formed in the asteroid belt during the last stages of planetary formation. These two large objects could have reciprocally perturbed each other up to the point of provoking mutual expulsion from the region, expelling most of the mass present at the same time. This theory also appears reasonable and we know that planetesimals comparable in mass to Mars probably wandered around the Solar System during the last phases of planetary formation, provoking violent impacts, as the existence of our Moon and the obliquity of Uranus seem to suggest.

In any case, it can be concluded that asteroids should be ‘living fossils’ of the original population of planetesimals present in their zone at the epoch when the largest planets originated. For this reason, their study can supply key information about the formation of our Solar System as we see it today.

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Chapter 3

Comets Historical notes and general characteristics Out of all the bodies that make up the Solar System, comets are the most spectacular and probably the most beautiful. Although they have been observed for centuries, only over the last few years have we started to obtain a fairly accurate idea of their origin and evolution and understand the chemical– physical processes taking place in them.

Most of our current knowledge has been obtained over the last few decades, mainly thanks to results from various space missions: Giotto, which observed comets Halley and Grigg–Skjellerup at close range; Deep Space 1, which flew near comet Borrelly’s nucleus; Stardust, which crossed comet Wild 2’s coma, collecting samples of comet material to bring back to Earth; and Deep Impact, which sent a copper projectile with a mass of 372 kg at a velocity larger than 10 km/s to impact with the nucleus of comet Tempel 1. A lot more study is still needed before we have a clear picture of the dynamic and physical nature of these heavenly bodies.

The word comet comes from the Greek kometes (i.e. long-haired) and they were already well known in the remote past. Their recurrent appearance is mentioned in historical texts from all former major civilisations and the most ancient information available dates back to the third millennium BC. Some comets were so luminous and appeared so suddenly in the sky with their long tails that they were thought to be inauspicious stars. Fear of comets and the belief that they were bad omens persisted even in modern times. Aristotle and Ptolemy thought comets were inside Earth’s atmosphere and were therefore local phenomena. Seneca (4 BC – 65 AD) first considered them as distinct heavenly bodies. The first measurements of their distance from Earth were carried out by Tycho Brahe, who observed a comet in 1577. He came to the conclusion that the comet must have been at least 230 Earth radii from Earth and therefore well outside the latter’s field of influence. A natural interpretation of such appearances was, however, still wanting and many years passed before we found out more about this celestial phenomenon. The motion of comets was first explained by English astronomer Edmond Halley in 1705. He 85

Image 1. Comet Donati was discovered on 2 June 1858 at the Arcetri Astronomical Observatory (Florence, Italy). This drawing shows it in the sky above Cambridge, USA, where American astronomer H. P. Tuttle tracked it for several months. (Le Ciel)

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Image 2. Halley’s comet represented in the Bayeux tapestry during its apparition of 1066. The tapestry is a strip of cloth 70 m long and 50 cm high, embroidered in coloured wool and attributed to Queen Matilda, wife of William the Conqueror, and their court.

Image 3. The adoration of the Wise Men, painted by Giotto in the Cappella degli Scrovegni, Padua. It shows the first realistic representation of a comet that is known in the western world.

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Image 4. Portrait of Sir Edmond Halley. In 1705 he calculated that a bright comet was periodic and that there would be another apparition in 1758. Halley had already died when the comet reappeared as predicted, and since then it has been called Halley’s comet.

Image 5. Comet Ikeya–Seki, photographed just before dawn in 1965. (U.S. Naval Observatory)

discovered that they followed a parabolic trajectory around the Sun, calculated the orbits of 24 bright comets and discovered that the orbits of comets that appeared in 1531, 1607 and 1682 were very similar. Halley then drew the conclusion that it must be the same object orbiting the Sun along a particularly elongated ellipse and with an orbital period of 76 years. He predicted its return in 1758–1759 and the comet actually reappeared in December 1758, but Halley, who died in 1742, could not witness confirmation of his predictions.

This comet was, therefore, named after him.

After the discovery of the telescope, or, more precisely, after the use of photography was introduced in astronomy in the late 19th century, the number of comet discoveries rose, at first slowly and then increasingly rapidly. M. F. Baldet’s comet catalogue of 1950 reported 1738 objects observed between 2135 BC and 1948 AD, including 132 doubtful ones. If we consider the available information correct, a new comet was identified on average every four years between Christ’s birth and the 17th century. In our times, however, around 20 are discovered every year from Earth, not including the reappearances of a dozen periodic comets that are already known. Comets currently listed in catalogues number just over 2000, but, thanks to systematic research programmes (see chapter 8) aimed at discovering objects that are potentially dangerous for our planet, this number is increasing quite rapidly.

The way to name comets has recently been changed by the International Astronomical Union. Once discovered, the comet is immediately given a provisional identity comprising the year of its discovery, followed by a capital 88

Image 6. Halley’s comet during its most recent apparition in 1986. The negative image was taken with the Schmidt telescope of the La Silla observatory in Chile.

(ESO)

Image 7. Comet West as it appeared in March 1976. (NASA)

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letter and a figure. The letter progressively identifies periods of 15 consecutive days, while the number indicates its order of discovery during that time. For example, comet 1996 B1 was the first discovered in the second half of January 1996. After its perihelion passage, each comet receives a definitive designation that includes the year of perihelion passage, followed by a Roman numeral specifying the order of such passage in that year. Obviously, the year of discovery doesn’t always coincide with the year of perihelion passage.

This relatively recent official designation system has replaced the one that also named comets after their discoverers, which is now unofficial, although still in vogue. If the discovery is made by more discoverers, independently or jointly, the comet is named after up to three of them. Sometimes, instead, as in the case of Halley’s comet, it is named after the astronomer who carried out special studies, although its actual discovery is not attributable to an astronomer in particular. In the case of a periodic comet, the letter P is put before the name.

Comet orbits Like all the bodies in the Solar System, comets are subject to the Sun’s gravitational field and their orbits can be elliptical, hyperbolic or parabolic, unlike those of planets, which all move along ellipses. Cometary elliptical orbits are generally considerably eccentric, i.e. their major axis is much longer than their minor one (Schwassmann–Wachmann 1 is a rare comet whose orbit is almost circular). In order to define a comet’s orbit, it is necessary to adopt the orbital parameters that we mentioned earlier (chapter 1).

Calculating a comet’s orbit is not, however, as straightforward as calculating that of a planet. In fact, such bodies can be perturbed by planets and reaction forces caused by surface activity on the nucleus in the form of vapour and dust jets resulting from the sublimation of cometary ices. This takes place when these objects get close to the Sun. These jets have a rocket-like effect on the nucleus, which tends to divert its trajectory so that the action of these non-gravitational forces makes accurately calculating comets’ orbits very complex.

A comet’s mass is so small compared to those of the planets that a close encounter with one of them, especially Jupiter, can thoroughly change its orbit.

Particularly large changes took place, for example, in the case of comet Brooks 2 whose revolution period around the Sun was shortened from 31.4 to 7.07 years after it passed near Jupiter in 1866. Similarly, until 1767 comet Lexell had a period of 11.4 years and a perihelion distance of 2.96 AU, while in 1770 the period became 5.6 years and the perihelion distance 0.67 AU. In 1779, due to further perturbations, these values became, respectively, 90

174.3 years and 5.35 AU. The comet was then lost in the outer Solar System.

Finally, comet Shoemaker–Levy 9 split up into over twenty pieces as a result of tidal effects caused by its passage close to Jupiter, before slamming into the giant planet in July 1994.

The most remarkable cases concern the perturbations that cause an exceedingly long-period orbit to change orbit after passing near a planet, so that it becomes elliptical with an aphelion situated approximately at the distance of the perturbing planet’s orbit. Comets captured in this way become part of a comet family: Jupiter’s family includes more than 70 comets with revolution periods ranging from around 5 to 8 years. The other giant planets, Saturn, Uranus and Neptune also have their own comet families, although less numerous and less accurately defined.

Comets’ orbits can be divided into three groups: short period comets (P<20 years), which usually have quite low orbital inclinations in relation to the ecliptic; medium period comets (20<P<200 years); and long period comets (P>200 years), whose components have orbital inclinations that can be of any values.

When observed from above the Sun’s north pole, the huge majority of comets, particularly short period ones, move around the Sun in a prograde or anticlockwise direction, like planets. Others, like Halley’s comet, have a retrograde orbital motion. When comparing orbits, we also notice that some comets move along nearly identical trajectories. These are always long-period comets travelling on the same orbital plane and approaching the Sun at intervals of several years or, sometimes, centuries. They show a considerable coincidence of orbital parameters, including an extremely low value of perihelion distance, which leads us to think that there is some particular relationship between these heavenly bodies. This fact can be explained by admitting that various comets in the group originated in the distant past from the successive splitting of a progenitor comet.

In confirmation of such a theory, we can refer to the exceptional examples of comets Biela and Taylor. Both objects were observed before and after their splitting into two bodies. Comet Biela was discovered in 1826 and its division into two parts was observed during its passage of 1845–46. On their successive appearance in 1852, the two cometary fragments were approximately 2.6 million kilometres apart. Over the following years, the comet completely disappeared. However, in 1885, 1892 and 1899 a swarm of meteors, commonly called falling stars, was observed that was clearly produced by debris from the disrupted comet.

Some comets’ perihelia graze the Sun with values under 10 million kilometres, causing them to become very luminous. The Great Comet of 1843, for example, was observed in broad daylight while it was near perihelion, but in many such 91

cases the blinding light of our star makes observations very difficult. Comets of this type are, however, short-lived as, besides burning off rapidly, they risk being disrupted by tidal forces when passing near the Sun. In addition to the two cases of comets Biela and Taylor mentioned above, comets 1883 II and Ikeya–Seki (passing perihelion in 1965) shared a similar fate.

Ikeya–Seki passed less than 500 000 km from the Sun and its nucleus split into two. Today, we know more than 15 comets that have broken up into two or more pieces, or undergone a complete break up, when passing near the Sun. The orbital elements of other Sun-grazing comets – known as the Kreutz Group – are so similar that they lead us to think about one big comet that disintegrated a long time ago, creating a family of fragments. Thanks to a coronagraph (an instrument that occults the blinding disc of the Sun in order to observe around it), the Solar Maximum satellite discovered a dozen new comets grazing our star during its Sun observation mission. The first of them, called Howard–Koomen–Michels, literally disintegrated on its perihelion

Image 8. Image of the solar corona taken by the SOHO (Solar Heliospheric Observatory) spacecraft in June 1998, showing two comets about to collide with the Sun. (ESA/NASA)

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passage. Similar discoveries have been made in far greater numbers by the SOHO satellite that has identified over 1500 comets transiting close to the Sun.

Comet structure According to a scheme that is especially helpful for grouping and classifying various phenomena, a comet is usually divided into three components that are referred to when describing its various aspects. They are: • A nucleus, a small solid body (generally) measuring some kilometres across; • A coma, a wide nebulosity measuring some hundreds of thousands of kilometres in diameter, more or less rounded, and made of gases and dust released from the nucleus; • A tail: a complex structure of ionised gases (plasma) and dust, sometimes several million kilometres long.

The head is composed of the nucleus and coma as a whole.

Nucleus The idea that a comet’s coma has a solid nucleus was put forward by Laplace and Bessel in the 19th century. However, the modern model of a cometary nucleus is often defined as a dirty snowball, first proposed by American astronomer Fred Whipple in 1950. In the Whipple model, a comet’s nucleus is a structure made of water ice, dust and small quantities of other types of ice gravitationally and/or mechanically linked together. This model has been confirmed by numerous studies, including observations of comet Halley’s nucleus, carried out in situ by the Giotto probe in 1986. In addition to Halley’s nucleus, those of comets 19P/Borrelly (Deep Space 1 mission, September 2001), 81P/Wild 2 (Stardust mission, January 2004) and 9P/Tempel 1 (Deep Impact mission, July 2004) were also imaged. These observations show that, in general, nuclei are very dark, with albedos lower than, or equal to, 5%, and tend to have a more or less elongated tri-axial ellipsoid shape. Halley’s nucleus measures 16 x 8 km, Borrelly’s is 8 x 4 km, while Tempel 1’s is 7.6 x 4.9 km. Wild 2 on the other hand has a spherical nucleus with a 5 km diameter, which is probably due to the fact that the comet is still relatively young.

The nucleus is the origin of all cometary phenomena. Dynamic observations and models agree that their density typically ranges between 0.2–0.8 g/cm3, even though some comets can have higher density nuclei. Evidence of such low density comes from samples of cometary dust collected by the Giotto probe near Halley’s comet as well as by aircraft and sounding balloons in Earth’s upper atmosphere. Such low densities show that cometary nuclei contain some inner vacuum, both on a macroscopic (chambers) and microscopic scale 93

(porosity), and this rather friable structure would explain why fragmentations have been observed several times.

So far, axial rotation periods of just over twenty nuclei have been determined. These periods range between about 4 hours and 7 days, with an average slightly under 24 hours. Cometary nuclei have a very dark surface. Halley’s surface albedo (i.e. the percentage of reflected solar light), was about 3% (lower than coal): this means that it absorbs 97% of incident energy. Such a low surface albedo may be due to the presence of intrinsically dark materials on the nucleus’ surface, Image 9. The nucleus of comet Halley seen by the camera on or may be caused by darkening the Giotto spacecraft in March 1986. The nucleus appeared very dark, reflecting only around 4% of the sunlight it received, and of chemical organic compounds measured 16x8x7.5 km. (ESA) as the result of solar irradiation and cosmic rays, by microscopicscale effects of the surface structure, or a combination of all three factors.

Studies of the colours of cometary nuclei have shown that different comets have surface colours that vary from grey to dark red. These colour differences may be due to different dust/ice ratios or a combination of surface composition and history of exposure to cosmic rays.

A cometary nucleus is basically made of water ice, refractory dust, organic solid compounds, carbon monoxide and carbon dioxide, that are each present with a percentage that varies between 5 and 15%. The presence of nitrogen has been noted, as well as argon, ammonia, methane, cyanogen, formaldehyde and a certain number of hydrocarbons. These and other volatile atoms can be trapped in the water ice. Silicate and carbonaceous compounds, oxides and metallic dust are also present. Many of these components are believed to be the remains of the solar proto-planetary nebula.

The structure of a comet’s nucleus is basically an icy interior covered by a dark crust of varying thickness that is made from refractory volatile material – the solid residue left over when the frozen component vaporised. The very dark surface crust effectively absorbs the solar heat and transfers it to the inside thus causing sublimation of the ice. Consequentially, vapour pressure provokes the 94

weakest areas of the surface crust to break up and gas escapes from the fractures.

Naturally, the more passages near the Sun, the thicker the surface crust will be.

As time goes by, the cometary nucleus will lose all its volatile elements (ices) and so will become an inactive body that no longer shows any signs of activity.

Imaging of Halley’s nucleus took place when the comet was near the Sun and so the jets of gas were only visible as they came out of the cracks from the obscure and dark crust. On the other hand, images of the Borrelly, Wild 2 and Tempel 1 nuclei were taken when the comets were relatively far from the Sun, and undergoing minimal nuclear activity. This made it possible to take images that showed the surface details. Borrelly’s nucleus appears ploughed by ridges and cracks, without obvious crater impacts, while the nuclei of Wild 2 and Tempel 1 are covered by circular, flat-based structures that are similar to impact craters. It is probable that these strange circular structures are impact craters that were slightly deformed by sublimation activity of nucleus ice. On Wild 2, 20 weak jets of gas erupted from cracks in the crust and hills up to a hundred metres high – a considerable size when compared with its diameter – were noticed. This implies a compact nucleus able to sustain the weight of significant vertical structures. Borrelly is a very old comet (probably over 1000 passages to perihelion) and so it is reasonable to think that craters have simply been deleted by the sublimation of the surface material. This would explain their absence.

If sublimation of nucleus ices takes place via a spherical symmetry process, the rotation period of the nucleus does not suffer variations. If, however, the nucleus has a very irregular shape and the process of gas emission is concentrated only in certain regions of the surface, the rotation period can vary. If the rotation period increases above a certain limit (which depends on the degree of nucleus cohesion), the nucleus can split into two or more parts.

Image 10. The nucleus of comet Borrelly imaged by the Deep Space 1 spacecraft in September 2001. (NASA)

Image 11. The nucleus of comet Wild 2 imaged by the Stardust spacecraft in January 2004. (NASA)

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Table 1

Composition of ices in a cometary nucleus Molecule

Abundance (%)

H2O – Water

85

CO – Carbon Monoxide

4

CO2 – Carbon Dioxide

3

CH2O – Formaldehyde

2

CH3OH – Methanol

2

N2 – Nitrogen

1

Others

3

This fragmentation process overlaps with tidal action caused by the Sun, and is favoured by the low cohesion of the comets’ nuclei. Both processes reach their maximum intensity when nearing perihelion. The case of comet West (C/1975 V1) is well known: it passed perihelion on 25 February 1976 and its nucleus separated into two pieces on 5 March, then became three pieces on 8 March and four on 11 March. Each of the four nuclei developed a tail, thus giving life to four comets from one original.

The coma A comet’s atmosphere is called a coma, from the Latin coma, meaning head of hair. It is rather ephemeral and it originates from the gaseous mixtures produced by sublimation of the ices that form the nucleus. At great distances from the Sun, the nucleus is cold and inactive and there is no significant atmosphere around it. As it approaches the Sun and as the temperature increases, the nucleus’ surface layers begin to release volatile mixtures that go directly from a solid to a gas phase. The distance at which sublimation begins to become significant depends on the chemical composition of the ice that makes up the nucleus. The most volatile ices, for example, are carbon monoxide and carbon dioxide, which begin to vaporise at distances over 10 AU from the Sun, while water ice starts to sublime significantly around 3 AU. During its 1986 apparition, Halley’s comet showed the first signs of sublimation when it was at a distance of around 6 AU. As soon as the ice begins to vaporise, it immediately spreads out into the space around the nucleus. As it has a relatively small mass, its gravitational field is too weak to prevent gases produced by sublimation from escaping its influence. On average, a comet loses 0.1–1% of its own mass at every perihelion passage. The number of gas molecules per unit of volume decreases with the square of the distance from the nucleus, with the average value around 96

1000 particles per cm3. Dust density is also lower, with just one particle per cm3.

As gas and dust expand, the particle density decreases and at a distance of around 20–30 nuclear diameters the gas is so thin that it can no longer affect the motion of the dust. The coma reaches its maximum size at 1.5–2 AU from the Sun and after that it tends to shrink. This does not mean that gas stops leaking from the nucleus. It is due to the fact that when the coma molecules are ionised by solar ultraviolet radiation, they stop interacting with the visible radiation and become invisible from Earth. For this reason, the coma’s diameter depends on the distance that the molecules cross before being dissociated. This distance clearly decreases as the comet nears the Sun (because the flow of ionising radiation increases), and this explains the apparent reduction in size.

Observations of Halley’s comet by space probes confirmed that gases and dust are almost exclusively produced by the part of the nucleus that is exposed to the Sun. The volatile materials that sublime directly from the nucleus (parent molecules) are subject to destructive processes after they are lost in the cometary coma, due to the interaction with solar radiation that provokes photo-dissociation and photo-ionisation phenomena. They can suffer chemical reactions, break up into simpler fragments (daughter molecules) or become electrically charged (ions). Gases that escape from the nucleus drag with them

Image 12. The inner coma of comet Hyakutake imaged by the Hubble Space Telescope in March 1996. Some jets of gas and dust are visible leaving the nucleus. (ESA/NASA/STScI)

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dust particles that are mixed with ice, radially accelerate them and transport them to the coma.

The nucleus of Halley’s comet released a nearly equal quantity of gas and dust every second. At 1 AU from the Sun, gas was produced at a rate of around 30 tons a second, while around 24 tons a second were emitted in the form of solid particles. Gases and dust are only produced by certain regions of the nucleus’ surface and escape from the interior through cracks in the crust of inactive material covering the cometary nucleus. Recent ultraviolet observations showed that a coma can be surrounded by a gigantic cloud of hydrogen that can have dimensions similar to the Sun (around 1 400 000 km) – as was the case with comet Bennett.

Recently, X-rays have been seen to originate from some comets’ comas, as in the case of comet Hyakutake. The process that gives origin to this emission follows interaction between solar wind heavy ions and atoms of the gases present in the coma. The process is the following. These ions possess a substantial amount of kinetic energy and their collision with atoms from coma gas elements can extract the electrons that are innermost and nearest the atomic nuclei. The electron cloud is then recomposed and leads the external electrons to reoccupy the orbits left free, creating X-ray emissions from the atom.

Active nuclei at great distances from the Sun We have said that a comet nucleus begins to show signs of consistent activity at a distance of around 3 AU from the Sun, when the temperature is such that it allows the sublimation of water ice (a cometary nucleus’ main component). There are some exceptions however: some nuclei are seen to be active at distances well beyond 3 AU. For example, in February 1991, when Halley’s nucleus was at a distance of about 14 AU from the Sun, it increased in brightness and developed a coma with a diameter around 300 000 km.

Spectroscopic measures showed that the radiation emitted by the coma did not contain emission lines, but it was simply a reflection of the solar spectrum, so Halley’s outburst consisted of dust emissions from the nucleus.

Another interesting case is comet 29 P/Schwassmann–Wachmann 1, with a 14.6 year orbital period and a perihelion distance of 5.72 AU from the Sun. Despite it always being beyond Jupiter’s orbit, the nucleus sometimes produces enormous eruptions of gas and dust that increase its brightness. Another comet whose nucleus showed exceptional activity at great distances from the Sun was Hale–Bopp. On 26 September 1995, before reaching perihelion, when it was still at 6.59 AU from the Sun, the Hubble Space Telescope noticed an imposing outburst of dust. An enormous amount of activity followed for several months, so that it was a problem to estimate the nucleus’ diameter.

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Nucleus activity at great distances from the Sun is difficult to explain with the usual process of ice sublimation: temperatures are too low and the ice that sublimes at low temperatures is a minor component, so it is necessary to look for alternative processes. An answer could come by analysing the physical characteristics of the water ice in more detail, especially from an experimental point of view (as has already been partly done).

Common water ice is basically inactive at great distances from the Sun, but there may be other types of ice. If the temperature is very low, around a few dozen degrees Kelvin, the water ice becomes amorphous, without an orderly structure like common ice. Its density is less than normal ice and, if heated above 153K, the water molecules redistribute and release heat with an explosive type of reaction. If a nucleus rotates slowly (Schwassmann–Wachmann 1 has a rotation period of 120 hours, while Halley’s is just 10.3 hours), causing heat accumulation, and if there are regions of amorphous ice near the surface that have survived the passage to the perihelion, when temperatures exceed 153K, the transformation from amorphous to crystalline ice can result in outbursts on the nucleus, even at great distances from the Sun. It is reasonable to think that new comets like Hale–Bopp (those that pass perihelion for the first time) are formed entirely of a mixture of dust and amorphous ice. Low temperatures in the outer Solar System, where nuclei are formed, make this theory reasonable. On approaching the Sun, the amorphous ice transforms into crystalline ice, activating the nucleus when it is still over 3 AU from the Sun, as happened with Hale–Bopp. After a few passages to perihelion, the surface layer of ice is by now common ice and the comet goes back to its inactive state after 3 AU, except for occasional outbursts due to residual amorphous ice under its surface. For comets with diameters over 8 km, heat generated by radioactive elements decaying would be enough to provoke the amorphous–crystalline transition near the nucleus’ centre, but the process is slow and there would not be any explosive activity. There are still quite a lot of issues concerning cometary nuclei physics that have to be understood in detail.

The tail As a comet approaches the Sun, the solar wind, a plasma that is made of electrically charged particles such as electrons and protons that have been freed by the Sun, starts acting on the gases and dusts that form the coma. This solar wind action blows the coma gases and dust in the opposite direction to the Sun, thus creating the tail. Radiation pressure, the outward thrust caused by sunlight, affects the development of the tail.

Comet tails are of variable density. The tail is dense when a comet produces a considerable gaseous cloud and passes close to the Sun. The tail can be up to 99

Image 13. Comet Haleâ&#x20AC;&#x201C;Bopp in April 1997. The yellowish, slightly curved, tail of dust and the blue gas tail are clearly distinguishable, both directed in the opposite direction to the Sun. (Turin Observatory)

Image 14. Comet Arendâ&#x20AC;&#x201C;Roland photographed from the Lick Observatory on 25 April 1957. The spike-like antitail seems to project from the front of the coma, due to a perspective effect. (Lick Observatory)

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100 million km long, or even more in exceptional cases (i.e. the tail of a comet in 1680 was 300 million km long and in 1843 another was 320 million km long), whereas its width can reach a maximum of 1 to 2 million km.

The density of material in a comet’s tail ranges from 10 to 100 molecules/cm3, within limits which are 1000 times lower than the ‘highest vacuum’ that can be reached in a laboratory.

Due to gravitational effects, which are greater on dust than gases, the tail’s gas and dust mixture separates to form a tail of ionised gases (plasma), which is practically straight and bluish in colour, called the plasma tail (or Type I tail), and a yellowish one made of dust which follows a curved path, called the dust tail (Type II). Solar radiation pressure pushes dust out of the comet’s trajectory, though still in its orbital plane. If dust is only occasionally freed from the nucleus, several separate dust tails can be generated. Yellow is the typical colour of a dust tail, as its particles reflect solar radiation. It is shorter than a plasma tail and its length ranges from 1 to 10 million kilometres. When the observer moves away from the comet’s orbital plane, the dust and ion tails appear clearly separated, as the ion tail is strongly affected by the solar wind.

When Earth crosses an active comet’s orbital plane, an anti-tail that seems to point towards the Sun may be visible, the result of an occasional projection effect. This is very likely caused by a part of the dust tail that is much further away from Earth than the nucleus, but still within the comet’s orbital plane, and looks as if it is pointing in the opposite direction compared to the main tail because of a perspective effect.

The dust tail is visible only because its material reflects the sunlight, while the ion tail shines due to radiation from the nucleus-ejected molecules that are excited by solar radiation. Ejection occurs at characteristic wavelengths, mainly in the blue section of the visible spectrum. In time scales of around a week, the ion tail can get separated from the comet’s head. This phenomenon is believed to be mostly due to a change in the polarity of the magnetic field that is carried by the solar wind and this interacts with the weak magnetic field that is probably a characteristic feature of comets. After separation from the old tail, a new ion tail can be generated in under an hour.

The origin of comets Interstellar origin Comets were once believed to have originated beyond the Solar System. This theory is not widely supported today, mainly because, if that was the case, most comets should move in very hyperbolic orbits, which is not confirmed by observations.

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Oort cloud and Edgeworth–Kuiper belt It is commonly thought today that numerous frozen bodies subject to the Sun’s gravitational influence are located well beyond Pluto’s orbit, as was suggested

by Jan Hendrik Oort in 1950. Oort reached this conclusion after examining the distribution of the semimajor axes of about twenty comets. He calculated that the cloud contains approximately 200 billion comets, mostly located at a distance of 50 000 to 150 000 AU. More recent investigations have brought the number of comets in the cloud to about 1000–10 000 billion. The total mass of the Oort cloud is believed to range between a tenth and a hundredth of the Sun’s mass. Most of the comet nuclei will never get near enough to the Sun to be visible from Earth. However, the occasional close transit of a star or large interstellar molecular cloud, or tidal movements generated by our Galaxy’s disc, can affect this cloud of comet nuclei, and push some of them into orbits that could take them to the inner regions of the Solar System.

This theory is also known as the Oort–Öpik theory, because in 1932 Ernst Öpik suggested the idea of a reservoir of comets in the form of a large cloud

Image 15. Sketch of the Oort cloud, a reservoir of comets on the extreme outskirts of the Solar System. (NASA)

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Table 2

Association between meteor showers and comets Comet

Period (years)

Associated Showers

Date of peak activity

ZHR

1861 I (Thatcher)

410

Lyrids

21 April

15

1 P/Halley

75.7

Eta Aquarids

5h May

35

1 P/Halley

75.7

Orionids

21 October

30

109 P/Swift–Tuttle

134

Perseids

12 August

80

21 P/Giacobini–Zinner

6.6

Draconids

9 October

20

2 P/Encke

3.3

Taurids

3 November

10

55 P/Tempel–Tuttle

33.2

Leonids

17 November

15

3200 Phaethon

1.4

Geminids

13 December

90

8 P/Tuttle

13.6

Ursids

23 December

10

(ZHR = zenithal hourly rate)

surrounding the Solar System. Without expanding too much on the problem, it is worthwhile mentioning that Oort confirmed that comets belong to the Solar System and excluded their interstellar origin. It seems then that they are formed inside the system itself and scattered outwards in all directions by gravitational perturbations caused by the planets.

Around the same time that the Oort cloud was being suggested, two other astronomers, Englishman Kenneth Edgeworth and Dutchman Gerard Kuiper, independently speculated on the existence of a disc formed of bodies created from the agglomeration of ices – the so-called Edgeworth–Kuiper belt. It was predicted to be located beyond Neptune’s orbit between 40 and 100 AU. Most of the short period comets are believed to originate in this region.

Unlike the Oort cloud, whose components are not visible because they are small in size and very far away, more than 1000 Kuiper belt objects have been discovered since 1992, with sizes around 100 km. As expected, they orbit beyond Neptune’s orbit and they represent the vanguard of the Edgeworth– Kuiper belt. Comets, then, are very likely to be primordial objects and can be rightly considered as remnants of the proto-planetary nebula from which the planets and their satellites were formed 4.5 billion years ago.

Their nuclei consist of a mixture of ice and dust with a very fragile structure that does not survive very long. In fact, whenever they pass their perihelion, comet nuclei lose some of their own material, and the nearer they get to the Sun the more they lose. A comet will thus shrink in size and scatter most of its original material into space. It has been calculated that a comet’s life can range from 1000 to 100 000 passages to its perihelion. All that remains is then debris, in the form of an agglomeration of non-volatile materials that continue to move in the same orbit as the comet, except when any planetary perturbations occur.

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Image 16. Drawing of a comet orbit, showing debris left by the comet during its revolution around the Sun. These particles enter Earth’s atmosphere and cause meteor showers when our planet periodically transits these regions of space.

The orbit itself then will be full of particles of dust and ‘pebbles’ left over from the various revolutions around the Sun. When Earth periodically crosses these regions of space during its motion around the Sun, meteor showers occur, marked by a considerable increase in the frequency of meteors. These meteors are solid particles of various sizes that overheat and vaporise leaving their typical luminous trace as they interact with Earth’s atmosphere at a speed of some tens of kilometres per second. The theory that comets generate meteors is supported by the fact that the orbits of meteor showers are similar to those of known comets.

One possible exception is the asteroid 3200 Phaethon, which does not show a coma or a tail, even though it moves in an eccentric orbit characteristic of comets. There are two explanations: either it really is an asteroid, or after losing all volatile elements it has become the dead nucleus of a comet at the end of its active life.

Comet Shoemaker–Levy 9 Previously, we mentioned Shoemaker–Levy 9 and at the end of this section it is worth mentioning the history of this famous comet. On 23 March 1993, a strange diffuse object with an elongated shape was identified near Jupiter in two photographs taken with a 46 cm diameter Schmidt telescope at Mount Palomar observatory, California. Further observations performed with the Hubble Space Telescope established that it was a comet made up of 21 separate nuclei in a line one after the other. The comet was called Shoemaker–Levy 9 (SL9), after the names of its discoverers – Eugene and Carolyn Shoemaker, with David Levy. Measurements of its position showed that the comet was in orbit around Jupiter, not the Sun, on a highly eccentric trajectory.

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Image 17. Fragments from comet Shoemaker–Levy 9 seen in an image taken by the Hubble Space Telescope in January 1994. The length of the chain of nuclei was around 600 000 km. The P and Q fragments are shown in the smaller images. (ESA/NASA/STScI)

Figure 18. Artist’s impression of the impact of comet Shoemaker–Levy 9 with planet Jupiter in July 1994. Before impacting with Jupiter, the comet was ‘stretched’ by gravitational interaction with the giant planet and then fragmented. Similar to beads on a necklace hanging on an invisible thread, the pieces continued their race towards the planet, impacting its atmosphere, and leaving some very visible traces in Jupiter’s atmosphere. (SEDS)

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By investigating its path, it was discovered that SL9 had passed just 104 000 km from the centre of Jupiter eight months before, on 8 July 1992. This explained why there was a string of nuclei. During its close transit, the comet’s original nucleus – approximately 10–15 km in diameter, but with a low cohesive force – passed within Jupiter’s Roche limit, i.e. 174 000 km from the planet’s centre. Tidal forces fragmented it into several blocks with dimensions between 2 and 4 km. It was later discovered that the fragments were going to fall on Jupiter between 16 and 22 July 1994, following their Jupiter-centred orbit. This forecast alerted the scientific world: it was the first time a collision between two celestial bodies in the Solar System had been forecast. Predictions about the effects of the impact on Jupiter’s atmosphere were not unanimous and a cautious approach was preferred. It was generally thought that any traces of the collisions would only be visible by using large instruments. However, the facts disproved this theory: the great dark spots generated by the impacts were clearly visible with small telescopes as well.

On 16 July 1994, at 20:13 UT, the first fragment, given the letter A, fell on Jupiter at a speed of about 60 km/s, followed by another twenty nuclei (at an average of 7-hourly intervals). The last nucleus, called W, fell on 22 July. The sequence of impact events left temporary traces in the shape of large dark spots up to 10 000 km in diameter, which were clearly visible before they joined together and were dispersed by Jupiter’s winds. Not all the fragments created spots, as some disappeared into Jupiter’s atmosphere without trace. At the end of July 2004, there were 11 spots in Jupiter’s southern hemisphere at –44 degrees latitude, distributed over different longitudes. The comet nuclei were identified by the letters of the alphabet (A, B, C, D, E, F, G, H, K, L, N, P2, Q2, Q1, R, S, T, U, V, W), and the same identification system was maintained for the spots.

Unfortunately, the impact events took place in Jupiter’s dark hemisphere, which is not visible from Earth. Only the Galileo probe, which at the time was travelling towards Jupiter, managed to image the flashes caused by the impacts (G, H, K, L, Q1 and W) at a distance of 1.5 AU from the planet. Luckily, the impacts occurred near the morning terminator. From Earth it was possible to see hot gas eruptions generated by the fall that quickly rose towards Jupiter’s stratosphere, where they were projected over the edge. Observations were made at all wavelengths (from UV to radio), and mainly with infrared filters centred on a methane (CH4) absorption band at 2.3 micron. Generally, Jupiter is not visible at this wavelength but the hot gas clouds billowing up beyond most of the methane layer were clearly visible. For example, the hot gas plumes generated by the impact of C, G and R fragments lasted several minutes, and were observed thanks to the NASA infrared telescope, while the Hubble Space Telescope showed A, E, G and W. 106

Image 19. The Shoemaker–Levy 9 impacts left dark stains in Jupiter’s atmosphere. (NASA/STScI)

All impact events heated Jupiter’s stratosphere by several degrees and the ammonia content in the impact sites increased 50 times. The hot gases rose about 3000 km beyond the atmospheric layer at 100 mbar, at an expansion speed of 10 km/s. Atmospheric waves expanding at about 500 m/s were also observed immediately after the impact events. From spectroscopic observations on site G, it was established that the comet nucleus penetrated the atmospheric layer with a pressure of 1000–2000 mbar, which is below the ammonia clouds.

The dark material that made up the spots came as a surprise. It was an aerosol rich in organic compounds, whose particle dimensions ranged between 0.15 and 0.3 mm, distributed among the layers with a pressure of 1 mbar to 200 mbar. The collision with SL9 injected some material into the magnetosphere and altered the intensity of Jupiter’s north aurora. On 17 July the intensity of the northern aurora was similar to that in the south, but on 27 July it was over five times stronger. It returned to normal 10 days later.

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Chapter 4

Meteors and bolides Introduction On most clear nights, in places where there is little or no light pollution, some meteors can be seen from time to time. In the Middle Ages it was believed that meteors were stars that fell towards Earth from the celestial sphere. The popular name ‘falling stars’ still testifies to this old belief, but now we know that the nature of the phenomenon is quite different.

The apparition of meteors is always ephemeral. They are bright lights that can cross several dozens of degrees in the sky and then disappear in under a second. They are caused by tiny particles of rocky material called ‘meteoroids’ entering Earth’s atmosphere. Very rarely a meteor can be brighter or visible for longer, even rivalling the brightness of a full Moon. A small percentage can leave some bright traces in the sky for a few seconds or even minutes, before dissolving and disappearing.

As with other celestial bodies, meteors can be assigned an absolute magnitude. This is the apparent magnitude (i.e. brightness) that a meteor would have if it were observed in the zenith of the observer and at a height of 100 km. As the average height at which meteors are normally seen is around this value, a meteor’s apparent magnitude seen at the zenith coincides fairly accurately with its absolute magnitude. The difference between them increases if a meteor is observed far from the zenith (since the light is increasingly absorbed by thicker layers of atmosphere) and/or the meteor is observed at a different height. This means that, in studying meteors, it is fundamental to determine their height in the atmosphere. This can be done through triangulation when there are two observers a few kilometres away from each other, observing the same event from two different directions. Measurements of this kind show that meteors normally become visible at a height between 60 and 120 km and disappear about 10 km lower, following their trajectories at speeds between 12 and 70 km/s.

Measuring meteor speeds is particularly delicate as it depends on determining the precise start and end times of a phenomenon that typically lasts, as we have already said, for a very short duration. This task is done by using a simple camera 109

equipped with a rotary shutter that stops light from entering one hundred times per second. As a result, the meteor’s trail in the photo will be broken up into tiny segments and from this we can determine its speed.

Meteor showers Imagine watching meteors for a few hours and plotting their trajectories relative to the stars onto a map of the sky. On many nights, the trajectories seem to diverge from just one or two precise points. Groups of meteors fitting this description are called ‘showers’. The fact that the trajectory radiation point (radiant) is shared proves that they all come from the same direction, so their orbits around the Sun must be extremely similar.

These showers are generally identified by the name of the constellation where the radiant is situated. We talk of ‘Perseids’ (from the Perseus constellation), ‘Leonids’ (from Leo), etc. It has sometimes been necessary to match the name of a star near the radiant in order to distinguish two showers coming from the same constellation. This was the case, for example, with the Eta–Aquarids and the Alpha–Aquarids.

The true measure of a meteor shower’s intensity – the standard to which every observer’s count is reduced – is the zenithal hourly rate, or ZHR. This is

Image 1. In the past, heavy meteor showers have aroused terror and panic, as well as understandable amazement.

This nineteenth-century painting shows the Leonid shower. The realistic scene helps us appreciate the shower’s intensity by giving the impression of filling the sky with meteor trails. (Le Ciel)

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Image 2. During the sublimation of ices, comets release particles of dust that settle around their orbit. Typical expulsion speeds, of the order of 10 m/s, are far less than the comet’s orbital speed (in this example, 42 km/s), so the dispersion around its orbit is limited. (ITASN) Image 3. Image of the spectacular bolide witnessed on 12 August 2002, taken from Tesero, Val di Fiemme, Italy, by Marco Vedovato and Ernesto Zanon (Gruppo Astofili Fiemme). The bolide’s motion is upward. The brightest star is Vega, and the bolide passed 7° south of the zenith. West is at the top and north to the left. (File ITASN).

the number of meteors that a single observer would see per hour if the shower’s radiant was at the zenith and the sky was dark enough for 6.5 magnitude stars to be visible to the naked eye.

Visibility of the showers can last from a few hours up to ten days and they recur every year around the same date. The association between showers and comets was recognised last century by Italian astronomer G. V. Schiaparelli.

He showed that all the particles making up a given meteor shower are produced by a particular comet. Today, for each of the numerous ‘young’ showers, we can identify an active comet that is responsible for their existence. On the other hand, a ‘burnt out’ comet has been identified for other showers, like the Geminids, which have been associated with the Near-Earth Asteroid 3200 Phaeton, an object for which no cometary activity is currently detectable.

Comets release dust particles along their orbits, and these are then gradually dispersed by planetary perturbations and non-gravitational effects (like radiation pressure) leading to a progressive spreading of the particles. This, in turn, affects the extension of the corresponding meteor shower. The youngest showers are more concentrated, and this has consequences for the shower’s appearance. A shower is seen when Earth moves near (or through) the comet’s orbit. You may imagine this like the crossing of a busy one-way street, with many vehicles coming toward you from the same direction. The most concentrated showers (like narrow ‘streets’) create shorter, but impressive, periods of activity. Activity from the Leonids is particularly interesting as it involves a meteor shower produced by comet Tempel–Tuttle, which was first observed in 1866. Earth intersects its orbit once a year, around 16 November, but, because of the very irregular distribution of meteoroids, very intense activity is only possible at certain epochs. Tempel– 111

Table 1

Main meteor showers Date of the maximum activity 2–3 January

No. of meteors per hour (ZHR) 145

Constellation where radiant is located (nearest star, in Greek alphabet)

Duration Possible (days) associated comet

Shower

Draco, Hercules, Bootes (b bootis)

2

Quadrantids

Blue, rapid, weak

Bootes

3

Bootids

Rapid, with persistent traces

22

Hydrids

3

Thatcher (1861 I) Lyrids I

Rapid, bright

Halley (1910 II)

Rapid, persistent

11 March

8

25 March

15

Hydra (b)

3 April

20

Virgo (a)

21 April

20

Lyra (n)

4–5 May

120

14 June

20

16 June

Virginids

Aquarius (h)

13

Scorpio–Sagittarius

50

Scorpio– Sagittarids

8

Lyra (a)

15

Lyrids II

19–20 June

6

Ophiucus (x)

30

26 June

4

Draco (g)

25 July

6

Capricornus (q)

28 July

40

30 July

8

1 August 5 August 11–12 August 16 August 19 August 12 September 9 October

Notes

8

Aquarids I

Bluish

Ophìuchids Draconids

Very slow

30

Capricornids

Bright, yellow

Aquarius (d)

17

Aquarids II

Slow, long trails

Pisces Australis (a)

25

Pisces Australids

8

Capricornus (a)

30

Capricornids (a) Bolides, yellowish

6

Aquarius (i)

40

300

Perseus (h)

16

15

Cygnus (c)

35

3

Aquarius

Pons–Winnecke (1858 II)

Aquarids Swift–Tuttle (1862 III)

7

15

Pisces

60

Var.

Draco

2

Perseids

Rapid, thin trails

Cygnids

Bright, exploding

Aquarids III Pisces Giacobini–Zinner Giacobinids or (1933 III) Draconids

Current activity reduced

20–21 October

50

Orion (n)

12

Halley (1910 II)

Orionids

Rapid, persistent

7–8 November

25

Taurus (l)

30

Encke (1819 I)

Taurids

Slow, bright

9 November

8

Cepheus (i)

4

Cepheids

15–16 November

Var.

Leo (z)

7

Tempel–Tuttle (1866 I)

Leonids

Rapid, persistent

22–23 November

Var.

Andromeda (g)

9

Biela (1852 III)

Andromedids or Bielids

Slow

12 December

50

Gemini (Castor)

6

Geminids

White, with bolides

22 December

10

Ursa Minor (b)

3

Ursids

Limited activity

29 December

12

Vela

112

15

Tuttle (1858 I)

Velids

Tuttle has a 33 year orbital period, so when the comet has just moved into the inner part of its heliocentric orbit, the most intense displays from the Leonids can be seen. They are real showers, with continuous sightings of meteors all over the sky (up to several tens of thousands of meteors per hour!). This absolutely exceptional phenomenon typically lasts a few hours.

Most meteoroids are made up of loosely consolidated, very porous rocky particles with masses less than one gram. It is estimated that the weakest meteors visible to the naked eye are objects with a mass of around a thousandth of a gram. Very bright meteors are sometimes seen during intense showers and can reach masses of several kilograms, but their fragments have never been seen to reach the ground during a shower. The brightest cometary meteors are made of material that is too fragile to survive crossing the atmosphere.

On the other hand, although the tiniest particles are around 2–50 microns in diameter, they may survive thanks to their smaller mass, because they very quickly dissipate the heat created by friction with the atmosphere. They slow down without damage and stay suspended for a long time before reaching the ground. Stratospheric flights by specially equipped aircraft have collected some significant samples of interplanetary dust particles that were found to be mostly made of fine particles, with abundances of volatile elements like carbon (C), sulphur (S), sodium (Na) and zinc (Zn). A lot of these particles have a complex and very porous structure that has not suffered significant thermal alteration.

These dust particles probably represent the only existing samples of cometary material in our laboratories, (apart from the recent samples brought to Earth by the Stardust space probe).

Image 4. A bolide’s structure during its entry into the atmosphere. The main physical characteristics of the phenomenon are indicated. (Drawing by E. Montanari)

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It is interesting that dust suspended in the upper atmosphere sometimes becomes visible as noctilucent clouds which can become visible at high latitudes as a fascinating and spectacular luminescent backdrop.

Meteor spectra When light is emitted by a meteor and dispersed by using a glass prism (or diffraction grating), a spectrum is obtained. This spectrum can supply useful indications of the mineralogical composition of the original meteoroid, although it cannot provide definitive and unequivocal proof. Needless to say, obtaining meteor spectra constitutes an observational challenge, and it is generally difficult to collect enough light to achieve a good-quality spectrum.

In general, 90% of the radiation emitted by a meteor originates from the meteoroid’s atoms. Only when a meteoroid enters the lowest layers of the atmosphere, the brightness of compressed and heated air molecules becomes predominant – but this is restricted to large meteors. The emission line spectrum of meteors is diagnostic of the body's composition and can be generally distinguished from atmospheric contamination. Unfortunately, some chemical elements (e.g. potassium), which are very important for the physics of meteoric plasma as they show intense lines in infrared images, cannot be detected in visible light spectra. Finally, various studies have shown that the general features of its spectrum tend to be independent of a meteor’s brightness.

Millman designed a classification for meteor spectra that is divided into four classes. Type X meteors show intense lines of magnesium and sodium (20% of the total), type Y meteors show lines of ionised calcium (2%), type Z meteors show lines of iron (66% ) and type W meteors are all those that do not belong to the previous classes (12%).

Table 2

Main spectral lines emitted by meteors in the optical band Element

Description

Wavelength (nm)

Colour

Ca II

Lines H and K of ionised calcium

393.4 / 396.9

Violet

Ca I

Lines of neutral calcium

422.7 / 616.2

Violet–blue–red

Fe I

Lines of neutral iron

404.6–414.4 / 426.8–442.7 488.6–498.8 / 537.1–545.6

Violet–blue Green–yellow

Mg II

Lines of ionised magnesium

516.7 / 517.3 / 518.4

Green

Na I

Neutral sodium doublet

589.0 / 589.6

Yellow

OI

Neutral atmospheric oxygen

557.7 / 615.7

Orange–red

Si II

Double ionised silicon

634.7 / 637.1

Orange–red

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Table 3

Fraction of sporadic meteors based on the apparent magnitude of meteors Magnitude limit

Fraction of sporadic meteors

–7

0.85

–3

0.54

0

0.40

+4

0.64

+7

0.71

+8

0.78

+13

0.85

Sporadic meteors Apart from the streams of meteoroids that generate showers, there is also a sporadic component of meteoroids that does not belong to any known shower.

The sporadic meteors’ ZHR is low, a few meteors an hour, but the flow is continuous and, unlike showers, not limited to certain dates. Accordingly, only 25% of all observable meteors belong to a shower, whereas all the others are sporadic. Sporadic meteoroids may either be the result of gradual diffusion of cometary streams, due to radiation pressure and mutual collisions, or they may be the debris from collisions between bodies in the main asteroid belt. It is not yet clear if a comet or asteroid origin predominates between sporadic meteors.

Sporadic meteor activity shows daytime and seasonal variations. If sporadic meteoroids were distributed evenly, we could expect most meteors at 6 o’clock local time, from the direction of Earth’s apex, when the observer is in the hemisphere that meets the meteoroids’ path. (Earth’s apex is the point in the sky determined by the intersection between the direction of Earth’s motion and the celestial sphere. The opposite point to the apex is called the anti-apex).

However, radio observations show that sporadic meteor activity peaks three times: 2 o’clock, 6 o’clock and 10 o’clock local time. These maxima are due to the presence of three separate radiants situated on the ecliptic: the apex (AP, apex source), due to geometric reasons mentioned before, solar (HE, helion source) and anti-solar (AH, anti-helion source). During the year, the HE and AH sources show some variations in intensity. The HE source reaches its maximum between April–June, while the AH source reaches its maximum twice, in April–June and again in October–December. These variations are due to the presence of a very large stream of sporadic meteors, probably associated with the old comet Encke.

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Image 5. This drawing illustrates the difference between meteoroids, meteors, bolides and meteorites. Meteoroids are bodies in interplanetary space, meteors are bright emissions caused by the meteoroidâ&#x20AC;&#x2122;s vaporisation in the atmosphere and meteorites are meteoroids that reach the ground. If a meteoroid is sufficiently big it can produce a bolide. (Drawing by E. Montanari)

Image 6. A spectacular super-bolide of magnitude â&#x20AC;&#x201C;19, taken by a station of the European Network in January 1992.

(European Fireball Network)

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Bolides The brightest meteors sometimes have peak magnitudes that rival the full moon (magnitude around –12.5). Generally, those with a magnitude brighter than –5 are called ‘bolides’, and they are produced by bodies with a mass greater than 100 grams. It is estimated that in some exceptional cases they may even reach 1 ton.

On entrance into the atmosphere, some bolides show distinct phenomena like fragmentation or emission of light bursts; this behaviour probably reflects differences in structure and resistance to the pressure by the material they are made of.

When certain bolides are associated with meteor showers, they clearly have a cometary origin. Available data, however, are limited to just a few events, including the slowest objects that produce easier traces to measure. Even with these limitations, there is a predominant number of objects whose orbits extend beyond Jupiter and which seem to be characterised by low internal strength, likely diagnostics of a cometary origin. However, there is also a population of objects which seems to have more cohesion, perhaps originating from extinct cometary nuclei or Near-Earth Asteroids. As soon as new information is collected on meteors, however, the differences between the two categories become less clear. This means that the sample we have investigated so far is not yet able to give us all the information on the real population of these objects.

Physics of bolides Let us look briefly at the main phases of the journey of a meteoroid coming into our atmosphere. According to the International Astronomical Union (IAU) conventions, the term ‘meteoroid’ indicates cosmic bodies whose mass is between 10–9 and 107 kg. The geocentric speed (that is, with respect to Earth) of a meteoroid, and generally of any body belonging to the Solar System, is between 11.2 km/s (the escape velocity from Earth) and 72.8 km/s (the sum of 42.5 km/s related to the escape velocity from the Solar System at Earth’s perihelion and 30.3 km/s of Earth’s orbital speed at the perihelion).

Image 7. A spectacular image taken by a casual witness at Lake Jackson, Wyoming, USA, on 10 August 1972. The bolide lasted almost a minute, during which its trajectory travelled 1500 km, without ever entering the thickest layers of Earth’s atmosphere. It was a rare ‘grazing’ phenomenon, during which the meteoroid only skimmed the outer layers of the atmosphere at an altitude of around 60 km before heading back towards outer space. Estimates of the meteoroid’s size range from a few metres to dozens of metres. (Sky and Telescope)

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When a meteoroid enters Earth’s atmosphere at a speed of several tens of kilometres per second (hypersonic speed), the collision with atmospheric molecules increases its surface temperature. As the body goes deeper into the atmosphere, the cosmic body moves from a discrete flow regime, when the molecules crash independently against the body, to a continuous flow regime.

The height at which the flow regime changes is called the height of transition and depends on the meteoroid’s speed and size, although it is usually between 50 and 60 km.

Below the height of transition, a motion that is originally laminar (that is, without irregularities in the flow of fluid that moves around the body) can become turbulent. Generally, when a body moves in a fluid, a pure number can be associated to quantify the turbulence, called the Reynolds number (usually NRe), depending on the body’s speed, density and fluid viscosity. This number’s value can indicate if the motion is laminar or turbulent. Motion is laminar for small NRe , below 100. For higher values, vortexes start to appear (although they are regular), while for NRe > 10 000 the vortexes disappear and an utterly turbulent trail develops.

At a height of 80–90 km, the temperature of meteoroids usually reaches 2800°C and sublimation of surface atoms begins – in other words the material turns directly from solid to gas. More mass is lost as the temperature increases to the extent that macroscopic drops of melted material can be lost. All these processes of mass loss are known as ablation. Because of the mutual collisions and atmospheric molecules, the atoms emitted by the meteoroid are ionised according to this reaction: neutral atom > positive ion + electron. The presence of alkaline metals (like potassium and sodium) can considerably increase the degree of ionisation of the emitted gases. A cloud of plasma begins to form around and behind the cosmic body. Electromagnetic radiation is emitted during the recombination process between ions and electrons and an observer on the ground will see a bright trail: a meteor. A meteor has two parts: the head

Image 8. An image of the 1999 Australian bolide trail, taken by Davison. The bolide moved from left to right. The trail’s undulations are evident. The dominant green colour is due to the camera’s night vision. The time of exposure is 1/3 of a second, comparable with the time taken by the bolide to cross the field of view. (J. Davison)

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Image 9. A residual cloud from the explosion of a daytime bolide observed in Italy on 18 June 2000. The red arrow indicates the direction of the bolide’s motion. (ITASN)

and the trail. The head contains the meteoroid that is consuming the cloud of plasma that surrounds it, while the trail is a region of plasma alone. As we said, 90% of the radiation emitted by a meteor originates from a meteoroid’s atoms.

If a meteoroid is large enough, the meteor’s head can also be very bright. As previously mentioned, when the apparent zenithal magnitude is brighter than –5, the meteor is called a bolide. The term bolide was once used for meteors whose noise was audible. The definition of bolide has not yet been established by the IAU, but the magnitude limit is normally around –6 or –8. A meteor with a brighter magnitude than –17 is called a superbolide. Meteoroids several tens of metres in diameter may produce bolides even brighter than the Sun, as seen from Earth. A classical example is the meteoroid that exploded above the region of Tunguska, central Siberia, on 30 June 1908. Superbolides are rare bodies that would need a network of global observation to be studied systematically.

Unfortunately, only a small fraction of these events is observed by networks for monitoring meteors, due to the small sky areas covered by the networks. Because of differences in atmospheric pressure between the leading and trailing parts, the meteoroid often breaks up into several parts, each of which can, in turn, become a meteor. Something similar occurred with a meteor seen from Peekskill (New York state) on the evening of 9 October 1992.

If the meteoroid’s mass is sufficiently large, then the cosmic body can survive ablation. When, its speed in the atmosphere drops to below 3 km/s, due to friction, the loss of mass and emission of radiation stops and the 119

Image 10. Image of the Peekskill bolide observed above New York on 9 October 1992. The bolide fragmented and several separate bolides were formed with similar trajectories. The meteorites found on the ground had a total mass of 12.4 kg. (Image by S. Eichmiller)

meteoroid enters the dark flight phase. From this moment a cooling of the surface begins and the body’s trajectory gets increasingly near vertical to Earth’s surface. For bodies with a geocentric speed of 15 km/s and masses between 10 g and 10 kg, the speed of impact on Earth’s surface can range from 10 to 100 m/s. When the meteoroid reaches the ground it is called a meteorite.

The probability of the meteoroid reaching the ground depends on its size and what material it is made of. A body of iron–nickel will more easily reach the ground than a rocky one. In the impact, the meteoroid slams into the ground creating a crater that is generally bigger than the body’s size. It is interesting to observe that the phases of entering the atmosphere and ablation last only a few seconds, while the possible dark flight phase that follows may last some minutes. Finally, for large meteoroids the speed may remain very high right until they impact the ground. In these cases ablation does not stop, there is no dark flight phase and an impact crater is created.

Bolide observation networks There are not many programmes dedicated to observing bolides. The Florida Fireball Patrol is active in the USA, while the Smithsonian Astrophysical Observatory’s Prairie Bright-Meteor Network (PN) was closed in 1973. This had 16 stations situated at least 250 km from each other. Every station was equipped with 4 cameras with a field of view of 90°. Another network for studying bolides was the Canadian MORP, open from 1971 to 1985.

In Europe, the European Fireball Network (EN) has been active since 1959.

It currently operates 34 stations in Germany, the Czech and Slovak Republics, 120

Image 11. Distribution of the European Network stations. (European Fireball Network)

Belgium, Austria and Switzerland. The stations are situated at least 100 km apart.

Coverage also includes Holland, thanks to an association with Dutch Meteor Society stations. The area covered is 10 million square kilometres. Every EN station has a camera with fish-eye lenses. Since the American and Canadian observation networks closed, the EN is one of the few left open in the world. The EN is the successor to the former Czechoslovak Network, founded by Ceplecha and Rajchl in 1965. Thanks to EN observations since 1959, eight meteorites associated with recorded bolides have actually been found on the ground.

The drawback of ground-based networks is their limited sky coverage that does not allow monitoring of large parts of Earth’s atmosphere.

The anomalous sound of bolides The study of sounds produced by large meteoroids of cometary or asteroid origin when crossing Earth’s atmosphere may seem a marginal field for understanding the physics of bolides. Nevertheless, as we will see, the subject is really interesting: it is a currently active field of research where even reports from visual observers can be fascinating.

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Table 4

List of meteorites found in the EN area since 1959 Date

Time (UT)

Place

Long.

Lat.

Mass (kg)

07.04.1959

19:30

Pribram

14° 02’ E

49° 40’ N

5.600

26.04.1962

11:45

Kiel

10° 09’ E

54° 24’ N

0.738

12.06.1963

12:58

Usti nad Orlici

16° 23’ E

49° 59’ N

1.260

16.09.1969

07:15

Police nad Metuji

16° 01’ E

50° 31’ N

0.840

14.11.1985

18:17

Salzwedel

11° 12’ E

52° 48’ N

0.043

01.03.1988

12:30

Trebbin

13° 10’ E

53° 13’ N

1.250

04.07.1990

18:33

Glanerbrug

06° 57’ E

52° 13’ N

0.855

06.04.2002

20:20

Neuschwanstein

10° 48’ E

47° 31’ N

1.750

Generally speaking, bolides of magnitudes –8 or less can descend into the thickest atmospheric layers (under 50 km) and generate a shock wave that, some minutes after the meteoroid has passed by, makes a sound like a ‘roar of thunder’ that is audible on the ground. This is the normal sound of a bolide. The transmission of the sound wave towards the ground depends on the temperature, density and direction of atmospheric winds. If the angle with respect to the zenith of the bolide trajectory into the atmosphere is low (an almost vertically falling bolide), it is more difficult to hear the sound. During propagation in the atmosphere, higher frequency sound waves are damped more quickly than others, so observers hear the ‘roar’ at low frequencies. The delay between observing the bolide and perceiving the sound (a phenomenon similar to the time lapse between lightning and thunder) is due to the fact that sound waves are propagated in the atmosphere at a speed of around 300 m/s, compared with around 300 000 km/s for electromagnetic radiation. For example, if we suppose a bolide is at a height of 50 km, the sound wave will reach the ground around 3 minutes after it passes by, so even a distracted observer would perceive the time lapse.

For a fraction of the bolides observed, things are not as simple as described above. At times the sound is heard contemporarily to the bolide passing overhead and not some minutes later. The sound heard simultaneously, or even before seeing the bolide, is called anomalous sound. While the normal sound of bolides has been widely studied for decades, anomalous sounds have received less attention because of their non-intuitive nature. Observers’ testimonies regarding anomalous sound are divided into two main categories: • Type I – Continuous anomalous sound: lasting some seconds, associated with very bright and slow bolides.

• Type II – Impulsive anomalous sound (or anomalous burst): lasting up to one second, associated with fast, not necessarily very bright, bolides.

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Statistics are scarce and rather uncertain because reports on anomalous sound are mostly based on visual observations. Anomalous sound type I represents 90% of cases and is a hiss or succession of small crackles. Anomalous sound type II represents the remaining 10% of cases and the observer hears a ‘pop’ or ‘click.’

According to current understanding, a bolide’s anomalous sound should be due to the phenomenon of electrophony, i.e. sound generated by electromagnetic waves. The same phenomenology is sometimes also recorded for the northern auroras. The term electrophony was coined by S.S. Stevens in 1937, while in 1940 P. Dravert introduced the name electrophonic bolides to describe a bolide with anomalous sound. Bolides’ electrophonic sound is caused by Extremely/Very Low Frequency (ELF/VLF) radio waves with a frequency between 1 Hz and 100 kHz and wavelength between 300 000 and 3 km, which through opportune ‘transducers’ generate sound waves of comparable frequency audible by the human ear (sensitivity from 20 Hz to 20 kHz). As we will see below, the theory that explains ELF/VLF generation is different for types I and II. Background on the anomalous sound The first reports on strange sounds heard contemporarily with the sighting of a bright bolide go back a long way in history in Arabic and Chinese chronicles, but these early observers could obviously not realise their importance. The problem posed by the anomalous sound is over three centuries old. In 1676, G. Montanari was the first to recognise the incongruity between the bolide’s real distance and the maximum possible period during which it was possible to hear the sound and see the bolide at the same time. The first to formally raise the matter was Edmond Halley, thanks to his work on a bolide that was seen in England in 1719. Halley lived in a time when, due to the nature of meteors, Aristotle’s vision was still accepted. But he retraced the bolide’s trajectory in the atmosphere and, as it was impossible to trace the sound heard as a normal acoustic phenomenon, since it was too high, he suggested that the anomalous sound was the fruit of people’s imagination. Sir Charles Blagdon, secretary of the Royal Society of London was more prudent. He studied the 18 August 1783 bolide and advanced a theory on meteors’ possible electrical origin, but on that occasion left the problem open concerning the audible anomalous sound.

The thing that has misled researchers is that anomalous sound is not perceived by all observers. This fact has allowed the psychological theory to dominate and therefore hinder developments in this field for a long time. A turning point came in 1980 with a theory by Australian researcher C.S.L. Keay.

In the literature, there are very interesting published descriptions concerning 123

anomalous sound events that are in sharp contrast with the psychological thesis, as this recollection from Keay shows: ‘‘At 18:44 UT on 6 April 1978 a bolide of magnitude –16 ± 2 passed by, with a southwest to northeast trajectory, above the metropolitan areas of Sydney and Newcastle (Australia) and was seen by hundreds of people. From 33 chosen testimonies, 15 talked of hisses or noises contemporary with the bolide. The physical reality of the sound was confirmed by three witnesses who perceived the sound before seeing the bolide. Two were even at home: they heard the sound, went outside and saw the bolide. In order to penetrate walls and be contemporary with the bolide, the sound’s generating physical agent had to be a radio wave. In fact, ultraviolet radiation is absorbed by the atmosphere, while optic and infrared radiation cannot penetrate buildings.’’

Another example was the anomalous sound generated by the 12 August 1998 bolide, observed from northeast Italy at 21:49 UT. In this case, some witnesses heard the anomalous sound separately (described as a continuous crackle lasting about ten seconds), before observing the bolide.

The electrophonic theory of continuous anomalous sound (or type I) As was mentioned earlier, the electrophonic theory first recorded progress during the 1970s thanks to work by Keay, who started to consider radio waves as the cause of the anomalous sound by analysing the Australian bolide of 6 April 1978. As described above, the agent responsible for the anomalous sound had to be sought in the radio dominion. Radio waves emitted by bolides could not have wavelengths in the centimetre domain, because it was known that bolides do not disturb radars, and the same goes for HF (high frequency) and VHF (very high frequency) waves. Going by exclusion, Keay realised that VLF (very low frequency) waves were the only region of the radio spectrum for which there was no negative evidence.

According to Keay’s theory, which was extended by Bronshten in 1983, VLF radiation generates in the bolide’s turbulent trail when the Reynolds number is over 106. The process is the following. We know that the bolide trail is made from plasma, a medium with very high conductivity, due to the presence of many free, charged particles. It can be shown that, when Earth’s magnetic field penetrates the plasma, its strength lines are ‘frozen’ in the turbulent trail, so that they move together with the plasma particles. Because of this freezing, the strength lines are twisted by the plasma’s turbulent vortexes, which increase the density locally, as well as increasing the magnetic field intensity and, consequently, the energy stored, the latter being proportional to the square of the field’s intensity. Basically, therefore, there is an energy transfer process from the trail of turbulent plasma to Earth’s magnetic field.

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When the ions and electrons recombine, the plasma disappears, Earth’s magnetic field uncouples from the trail, delivers excess energy that accumulated in the form of VLF radio waves and returns to the normal intensity of around 0.7 Gauss. The density of average energy in Earth’s magnetic field is 10–3 J/m3, a million times lower than the density of kinetic energy in a bolide’s trail.

Common bolides convert kinetic energy to radio energy very inefficiently, although the power of bolides is enough to create a non-negligible VLF emission.

It is important to note that this process only works if the meteoroid goes below the height of transition described in previous paragraphs and produces a turbulent trail of plasma. This can only happen for the brightest meteors, with an average magnitude of around –10 to –13. A bolide that lasts for less than 3 seconds is unlikely to move in a continuous flow regime and create electrophonic sounds. Based on Keay’s theory, we can understand why bolides that produce intense electrophonic sounds are those that are extinguished at lower heights, moving on trajectories with a small inclination on Earth’s surface.

When Keay formulated his theory, it was not known whether bolides emitted VLF radio waves, but the affirmative answer came within a relatively short time. On 13 August 1981 T. Watanabe and his group at the University of Nagoya (Japan) first recorded a brief (<0.2 s) radio emission from a Perseid bolide (magnitude –6) in the VLF range. The results were confirmed by a group of Canadian researchers, with observations of another Perseid bolide (magnitude –11) on 11 August 1993. Experiments carried out on volunteers in Keay’s laboratory established the lowest limit for the electric field of the VLF radio waves able to produce electrophonic sounds that were audible to observers. The result was a peak-to-peak amplitude of 160 V/m (with frequencies from 4 to 8 kHz), although different volunteers’ electrophonic sensitivity varied by a factor of 1000. It can, for example, depend on the length of someone’s hair and humidity content or whether they wear glasses. This explains why only some succeed in hearing electrophonic sounds.

In 1991, Keay experimentally showed that the witnesses’ sensitivity to VLF waves increased when transducers like wires, aluminium sheets, trees or bushes were in the environment. All these objects are much smaller than the wavelength typical of the VLF and when they meet an electromagnetic wave of this type, the electronic positions of the body move coherently in just one direction, creating an electric tide and determining a mechanical vibration of the object that then produces the sonorous wave audible to the human ear. A similar process acts on the observer. The importance of the environment as transducer can be another reason why the anomalous sound is perceived by some witnesses, but not by everyone. Higher frequency radio waves like those used in TV transmissions have too short a wavelength to produce mechanical 125

oscillations in macroscopic objects and, even if they were able to, the frequency of the sound produced would be too high to be audible to the human ear.

In Keay’s theory, the VLF emission and perception of the electrophonic sound is continuous and persists until the turbulent plasma trail is fed by the bolide’s head. This explains very well the anomalous sound that lasts a second or more, a compatible interval of time with the meteoroid’s ablation phase.

Clearly, if the meteoroid enters the dark flight phase, VLF emission stops together with the electrophonic sound.

It is worth adding that, in principle, electrophonic sounds can be generated by any body that enters the atmosphere, even if it is artificial, like a satellite or space shuttle.

The electrophonic theory of impulsive anomalous sound (or type II) The electrophonic theory for the impulsive anomalous sound is much more recent. It was developed in 1999 by M. Beech and L. Foschini, and calibrated for the Leonids in 2001. If a process of electrophonic bursts involved Earth’s magnetic field, similar in a way to continuous electrophonic sound, then the energy necessary to produce VLF radio waves would be very high, around 1033 J. To have an idea of what is meant by this number, suffice it to say that, in the specific case of the Leonids, only a meteoroid with a minimum mass of 800 kg could supply it. Clearly, a meteoroid of this mass is very rare and cannot justify the frequency of the reports, so it is necessary to find an alternative physical process.

In the paragraph on bolide physics we mentioned the fact that some meteoroids can disintegrate in flight due to the difference in pressure on different parts of their body. The explosion of the meteoroid produces a crashing wave in the plasma that surrounds it. The crashing wave manifests itself as a high gradient of temperature and density that is propagated by the central regions towards the outside of the bolide’s head. Due to the large differences of mass between ions and electrons that make up the plasma (even a simple hydrogen nucleus, a proton, has a mass 1840 times greater than an electron), electrons scatter more quickly because of the increase in temperature as the crashing wave moves past, and there is a spatial separation of electric charges: positive ions on one side and negative electrons on the other.

Once separation of charges has been produced like this, an electric field, which tries to remix electrons and ions and return to a condition of neutrality, is immediately established between the two regions of opposite charges. This electric field is variable over time and so, for Maxwell’s equations, generates a variable magnetic field that in turn creates an electric field and so on: the emission of a brief electromagnetic impulse results. The calculations show that 126

an electric field of amplitude 160 V/m (the lowest threshold for electrophonic sound) can be obtained by a crashing wave in motion in the plasma when electron density n and temperature T are, respectively, n ≈ 4·1018 m–3 and T = 104°C. Using the formula that ties a bolide’s absolute magnitude with the plasma’s electronic density, we can estimate that a bolide can create an electrophonic burst from around magnitude –7 downwards.

Applying this theory to the Leonids meteor shower we find that the minimum mass needed to produce an electrophonic burst turns out to be reasonable, between about 5 and 400 g, depending on the zenith angle, chemical composition and coefficient of ionisation. Keeping these factors in mind, the corresponding absolute magnitude of a bolide can oscillate between –7 and –11.

The anomalous sound of the Leonids Reports on electrophonic sounds generated by the Leonids are very rare. The first reports concerning anomalous sound go back to the 1833 storm and were reported by D. Olmsted. Anomalous sounds were also heard during the 1966 meteor storm. In general, statistical analysis of the reports of electrophonic sounds shows that there are no significant increases in testimonies during November, whereas August records a maximum due to the Perseids.

In 1998, during a peak of a high percentage of bolides on the night of 16 to 17 November, an expedition in Mongolia, led by S. Garaj, recorded by microphone for the first time some anomalous sonorous waves, heard contemporarily with the apparition of two bolides of magnitudes –6.5 and –12.

No VLF radio waves were recorded, but the receiver used was completely insensitive below 500 Hz and above 10 kHz. There were numerous testimonies of electrophonic sound from all over the world during the 2001 peak of the Leonids. Most descriptions talk of hisses and crackles lasting some seconds that were associated with the brightest bolides, and a few others talked of impulsive electrophonic sounds.

Observing Bolides Even though it is rare, the bolide phenomenon is quite fascinating. In the following paragraphs we will give some practical suggestions for systematically observing them, while in a later chapter we will see how to calculate their trajectories in the atmosphere.

The oldest method used is based on naked eye observations, still very popular with amateurs for observing common meteors. Relatively little basic knowledge is necessary for visually observing bolides. First of all, we should 127

keep in mind that accuracy in observations is directly proportional to experience in the field. For this reason it is necessary to learn to recognise the brightest stars in the sky (Sirius, Vega, Altair, Antares, Deneb etc.), the planets and the most important constellations, such as Ursa Major, Orion, Taurus, Gemini, Cygnus and Scorpio. To achieve this aim, refer to a small atlas of the night sky that shows all the stars visible to the naked eye. Useful star maps for this goal are found as appendixes in almost all introductory astronomy books.

After having learned to recognise the brightest objects in the sky, observations can begin. There are no particular preferences for the period of observation or area of sky to monitor. Meteoroids from the asteroid belt, those mostly responsible for large bolides, can fall on Earth at any moment. This point notably differentiates observations of bolide activity from those related to common meteor showers that have very precise dates for observation (see Table 1).

Observations should be made from a relatively dark place, from which it is at least possible to distinguish the brightest stars. (It is recommended to use a deckchair in order to avoid getting cramps that can interfere with concentration).

Obviously the star atlas should be close at hand, together with a small red light torch that does not dazzle your eyes, a pen and a notebook to take notes on the bolide’s trajectory over the celestial sphere. Optical aids like binoculars or telescopes are not necessary. Given the limited visual field of these instruments, the observer would risk missing the few bolide events that may occur, so all observations must be carried out with the naked eye. Small binoculars might be useful to better monitor the phenomenon of the trail, but they are not essential.

It could be useful to learn how to estimate the angular distances between two points in the celestial sphere. This estimate can be done very quickly, even if approximately, by using your hands – with your arm completely extended, your thumb subtends an angle of around 2°, your closed fist 10° and your open hand (with fingers separated) around 20°. These values can change slightly from person to person and should be individually calibrated by measuring these angles. By counting how many thumbs, fists and hands can be placed between two points of the celestial sphere and multiplying them by the previous value, the angular distance between them can be calculated. A technique of this kind can be useful to estimate the angle subtended by the trajectory crossed by the bolide or to initially establish the azimuthal coordinates of the start and end points of the trajectory, although they should be calculated more accurately after the observations.

Everyone can make their own contribution, even without making systematic observations. All it takes is a knowledge of the night sky to turn one’s testimony from a personal story into an investigation tool.

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Image 12. A painting by Russian artist P. I. Medvedev showing the Sikhoteâ&#x20AC;&#x201C;Alin bolide of 12 February 1947.

The artist was an eyewitness at the event.

Night meteors We have said that a large meteoroid can fall on Earth at any moment, so a bolide can appear either in the night or daytime, at any time or month of the year. Basically any time spent in the open is useful for observing bolides.

Naturally, night meteors will be more easily visible than those during the day.

The latter events can only be observed if they are very bright. a) Visual observations In general, to measure the bolideâ&#x20AC;&#x2122;s trajectory in the atmosphere and the point where it impacts the ground, it is necessary to have data from at least two observatories placed about 10Â km from each other (see chapter 10 for details).

In practice, with visual observations, the more observers there are, the better 129

it is, because observation errors can be averaged out. Naturally, the data collected must be of sufficient quality: comments like “…I saw it go from north to southeast…” are not very useful when researching meteorites or calculating their trajectories or orbits.

The naked eye is enough to see the trajectory that a bolide’s head follows between the visible stars. It is not necessary to remember the whole trajectory: it is only necessary to record the point where the bolide’s head appeared (starting point) and where it disappeared (end point). Observing the bright trail is not important; the observer must focus on its head. After having carefully observed the bolide’s trajectory and memorised the position of the start and end points, it is necessary to make a report. The observer must not be in a hurry, but focus on the trajectory, ignoring other details that could distract. The drawing on the map will lead to the equatorial coordinates of the start and end of the trajectory.

Besides this, it is essential to write down the time of the observation (specifying if it is daylight saving time, solar time or UT), and how long the phenomenon lasted. The duration is the interval of time taken by the bolide to move from the start to the end point. The estimate of time can be done by starting to mentally count the seconds from the trajectory’s starting point observed. Alternatively, you can mentally see the sequence of events again and count the seconds. The important thing is to note down the interval of time immediately, so that it is not forgotten. The duration value allows the observer to estimate the bolide’s average speed.

Sometimes, a bolide can be observed only after it has already appeared, thus losing its starting point. Luckily this is not a problem for calculating the trajectory. Just record the position of the point on the sky where the bolide was first seen. The same applies for the end point. If, during the journey from the start to the end point, the bolide hides bright and easily recognisable stars, it is worth recording it. In this way, the intermediate points of the observed trajectory can be given; these additional data are very useful for reconstructing the trajectory in the atmosphere.

The magnitude of the bolide’s head can be estimated by comparing its brightness with other celestial bodies. Remember that Venus at its greatest brightness has a magnitude of around –4.4, the Moon in the first quarter is magnitude –10, the full Moon is –12.5, while the Sun is –26.8. Unfortunately, there are no other stars of intermediate magnitude to make the comparison with. This is why estimating a bolide’s magnitude is always rather uncertain, but it is better than nothing.

The parameters illustrated above are fundamental for useful observations of bolides. Another physical phenomenon that should be monitored is possible sound perceived. If the sound is heard contemporarily with the bolide’s apparition, then it is an electrophonic sound of electromagnetic origin, not due 130

Table 5

Data that can obtained from visually observing bolides Fundamental data:

Notes:

Observation site

Latitude, longitude, metres above sea level

Equatorial coordinates of starting point

Right ascension and declination, specify the equinox

Equatorial coordinates of end point

Right ascension and declination, specify the equinox

Duration

Measured in seconds

Magnitude of head

Venus, Moon and Sun to use as reference

Sounds heard

Specify if before or after sighting

Complementary data: Colour/s

Specify sequence assumed by the ‘head’

Shape and diameter of head

Compare with the apparent diameter of the Moon

Persistence of trail

Note time of persistence in seconds

to the propagation of sound waves. As we said before, this kind of sound resembles a hiss. If the observer hears a deep sound after the bolide has disappeared, this is due to the propagation of crashing waves in the atmosphere and is communicated by normal compression waves. When observing a bolide, noting the type of sound is interesting, but not essential. Other details like colour, persistence of the trail and shape of the head are not necessary and can be ignored. Remembering too many details can create confusion, so it is better to limit oneself to essential data: the position of the start and end point, duration and magnitude of the head. Naturally, to complete the report, the geographical position where the bolide was observed must be noted: latitude, longitude and metres above sea level. b) Photographic observations Photography has been used for studying meteors since November 1885, when L. Weinek took the first photo of a meteor in Prague.

Photographic observations of bolides are very useful, especially if taken together with visual ones. An observer can start with a simple reflex camera (ideally manual), fixed on a tripod, with an average sensitivity film equipped with a 28 mm focus wide angle lens or even a normal 50 mm lens. The camera must be aimed towards the same region of sky that is being visually observed and exposure time can last between 5 and 30 minutes, based on the light pollution present. Should a bolide cross the area under observation, exposure time must be interrupted. Unless the bolide appears immediately after the beginning of the exposure, images of the stars will appear as arcs in the photo.

An image of this type is not the best aesthetically, but it has an enormous value, 131

Image 13. Photograph of the 6 February 2005 bolide, taken with an all-sky ˇ camera from the Crni Vrh Astronomical Observatory in Slovenia. (Herman Mikuz).

because it is then possible to precisely determine the coordinates of the start and end of the trajectory. The value of photography is improved when a telescope equipped with an equatorial mount is used together with a camera.

In this case, Earth’s rotation can be counterbalanced, and images of the stars will be dot-like, facilitating calculation of the trajectory coordinates. c) Observations with a video camera From a simple photo of a bolide’s trail we can find the coordinates of the trajectory points, but not the event’s temporal duration, which still has to be visually estimated. There are two ways to overcome this problem: equip the camera with a rotating sector (but this is rather complicated), or use a home video camera that is very sensitive to light. Once assembled on a tripod and pointing in the desired direction, the video camera must be able to film stars of magnitude 3 or 4, and show the date, hour, minutes and seconds on every image recorded. In this way, besides the images of the bolide and information on the trajectory, the temporal data will also be available. The only limitation could be the amplitude of the video camera’s field of view, which is around 15° compared to 74° with a 28 mm lens and 46° with a reflex’s 50 mm lens.

Historically, the bolide that has been filmed the most is the one associated with the fall of the Peekskill meteorite: well over 15 videotapes were made in various eastern locations in the USA. If a video camera is not sufficiently sensitive to record the brightest stars, it does not mean it cannot be used for researching bolides. In fact, in order to be useful, all that is required is to make sure that some elements of landscape are clearly visible, e.g. trees or roofs at a certain distance from the observer. This means that a part of the field of view is sacrificed, but if a bolide comes into the 132

area of sky being observed, the observer can easily trace the trajectory’s azimuthal coordinates by using the landscape elements as a reference. To obtain this information, it is not necessary to use a theodolite, but a simple night photograph of about 10 seconds exposure time, taken from the same point and with the same camera shot: from the azimuthal coordinates of the stars recorded on the video it is easy to trace back to the landscape elements and, from there, calculate the bolide’s coordinates.

Daytime bolides With bright, daytime bolides that are much rarer than night ones, there are no stars that can serve as reference points for the trajectory, so it will be necessary to use landscape features, like houses and trees. A bolide that was clearly visible in daylight was observed in the United States and western Canada on 10 August 1972. The meteoroid responsible for this bolide reached at least 58 km high and then it returned into space. The estimated mass of this celestial body was at least 100 tons (see box).

With visual observations of daytime bolides, observational precision is generally lower compared to night bolides, so it is best just to gather all possible information. Immediately after observing a bolide, it will be necessary to make a quick sketch of what was observed, being careful to note down the landscape’s main elements and bolide’s trajectory in relation to them. The drawing should show the azimuth and height above the horizon (in degrees) of the start and end points. The azimuth of any point in the celestial sphere is the angle, counted from north to east, between the north cardinal point and projection of the point on the horizon. The height is the vertical angle between the point and the horizon. These values can be estimated by using your hands, as we mentioned before. It is also important to write down the point from which the observation was made, the geographical location, exact time and duration of the apparition.

Photographic observations of daytime bolides are not practically feasible, while observations by video cameras should be carried out in the same way as those at night.

Meteors with anomalous trails Usually, the trail produced by meteoroids on entering the atmosphere is similar to a straight line. In a fraction of cases however, observers report anomalouslooking trails that are very curved or have numerous undulations.

The first report of an anomalous trail was in 1742. Strangely enough, the phenomenon was not noted in older Chinese, Japanese or Korean chronicles, 133

Table 6

Classification of meteors’ anomalous trails (Beech, 1988) Main classification

Description

C

Continuous curved trail

S

Continuous sinusoidal trail

Subgroups CR

Right angle curved trail

CS

Sinusoidal and curved trail

SF

Fragmented meteor with a sinusoidal component

CF

Fragmented meteor with a component curve

maybe because it was not considered to be worthy of note. For many years it was believed that these trails were due to perception errors by observers. In general, a meteor is not observed at the centre of the visual field and so, on moving his/her head, the observer might see curved trails. Recently, the continuously growing number of reports and photographic documentation of anomalous trails led to re-evaluation of the phenomenon from a physical point of view.

C and S type trails A simple phenomenological classification has been introduced for anomalous trails. Meteors that show curved traces are classified as C type, while those with a sinusoidal trend belong to the S type. Besides these two main groups, there are some subgroups with mixed cases and fragmentation.

In the scientific literature, anomalous trails represent 0.54% of all observed trails. Around 60% of these trails belong to C type, while the remaining 40% are S type. From research carried out on Italian meteor records, among 1066 recorded events there were 14 (that is 1.3%) with anomalous trails. The distribution between C and S types was well balanced, with 50% for each of them.

It is still not clear what causes anomalous trails. C types are very probably caused by the meteoroid’s rotation around its axis. This deviation is induced by spinning and is known as the Magnus effect, after German physicist H. G. Magnus who, in the middle of the 19th century, described curved trajectories of spinning cannon balls. This deviation occurs in an orthogonal sense to the axis of rotation: if the latter is parallel to the direction of the motion, the trajectory stays rectilinear. It is more complicated to interpret the S type trails and it is possible that they are due to the spinning of non-symmetrically shaped meteoroids.

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THE DAYTIME SUPER-BOLIDE OF 10 AUGUST 1972 A unique event took place on 10 August 1972 at 20h 29m UT, a daytime super-bolide of magnitude –18 crossed the skies of Utah, Wyoming, Idaho, Montana (United States) and Alberta (Canada), moving from south to north. Thanks to the length of its atmospheric trajectory, many witnesses succeeded in taking pictures. Two videos were also recorded. The best-known tape was filmed by Linda Baker of Lake Jackson (Rocky Mountains, Wyoming) and lasted 26 s.

The first article on this super-bolide appeared in Sky & Telescope magazine in October 1972, based on number of witnesses, while the first complete study was done in 1974 by Luigi Jacchia, who was then at the Center for Astrophysics of Massachusetts. The most recent, comprehensive study was carried out by Zdenek Ceplecha (1994), who corrected some aspects of Jacchia’s work. The overall description that comes out from the analysis is the following.

The meteoroid responsible for the super-bolide skimmed through Earth’s atmosphere with a geocentric speed of around 15 km/s. After having reached a minimum height of 58 km, the atmosphere did not succeed in slowing it down because of its large mass, and it started to bounce like a cobblestone thrown into water, before finally returning to space. Its estimated mass was around 105 kg, while its size ranges from 3 to 14 m in diameter, depending on the preferred density value that can be reasonably adopted.

The derived heliocentric orbit of the object before hitting Earth’s atmosphere indicates that it originated in the main asteroid belt, with an aphelion at 2.3 AU. After the encounter, the orbit had a reduced semimajor axis and lower eccentricity, so that the aphelion now should be of the order of 2.0 AU. It is possible that this body will have other close encounters with Earth in the future.

Unfortunately, the orbit was not recorded with the necessary accuracy to be able to make quantitative predictions. However, taking planetary perturbations on meteoroid motion into account, there might be a fairly close passage on 23 August 2015, at 0.115 AU from Earth.

A spectacular example of an anomalous S type trail was that shown by an Australian bolide seen at 09h 05m UT on 18 September 1999 from Ulladulla.

J. Davison was focusing a digital video camera with a night vision device on the area of the Southern Cross when, suddenly, a bolide transited. This meteor’s trail, however, did not simply undulate but it looked like the overlap of a normal rectilinear trail with a helical type. This could throw new light on the real physical nature of the S type trails.

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Chapter 5

Meteorites Historical background Meteoroids that survive entry into the atmosphere and reach Earth’s surface are called meteorites. This process of ‘stones falling from the sky’ has taken place since the birth of the Solar System. Unfortunately, only a small fraction of the available meteorites has been found and analysed: on average only some tens of meteorites are recovered a year, excluding those found in Antarctica.

The process of meteoroids coming through the atmosphere is spectacular and characterised by numerous unusual phenomena (like electrophonic sound). In fact, meteorites that were collected after these events have been revered in temples as messages from the gods since human history began.

The ancient Egyptians knew about meteorites. Hieroglyphic writings inside pyramids speak of ‘celestial iron’, a clear reference to metallic meteorites. In fact, Howard Carter found a meteoritic iron dagger in Tutankhamun’s tomb in 1922. Diligent Chinese observers have left lists of falling meteorites since 2000 BC. Utensils and ornaments made from meteoritic iron have been found in the tombs of North American Indians. The first person to discover that native Americans knew about meteorites was John Ross, in 1818. During his search for the northwest passage, Ross discovered an Eskimo tribe on Greenland’s northwest coast who used knives with meteoritic iron blades.

Meteorites are also mentioned in the Bible. In the Old Testament, Joshua’s troops were pursuing armies of the five Amorite kings when sudden rains of large stones pelted down from the sky and sealed the fate of everyone in the battle (Joshua X: 11). The New Testament also cites a temple dedicated to Artemis where there is ‘a sacred stone that fell from the sky’ (Acts of the Apostles, 19:35).

In 467 BC, a meteorite fell on Egospotami in Thrace. Pliny and Plutarch wrote about the event. According to Anaxagoras (500–428 BC), the stone arrived on Earth from another celestial body where there had been a landslide or earthquake. Anaxagoras’ theory was very interesting for the time, because of the connection with extraterrestrial bodies, but Aristotle (384–322 BC) thought differently. The great Greek philosopher believed that meteorites were stones 137

lifted by the wind and subsequently brought back to Earth. His position on this should not surprise us, because he believed that the skies did not change, reasoning that meteors and comets were only atmospheric phenomena.

Aristotle’s opinion was affirmed and approved throughout the Middle Ages.

The ascent of Christianity contributed to ending any interest in meteorites.

The Catholic Church denied that stones could fall from the sky and – in the case of falls being directly witnessed – the Devil was blamed. For centuries the small hollows that were often found in the meteorites’ black crust (known as ‘regmaglypts’ today), were seen as nothing less than the Devil’s imprints. The inquisition actively discouraged the population from being interested in meteorites, and whoever disobeyed risked being accused of heresy and practising black magic, with the likelihood of receiving the death penalty.

Things improved during the Renaissance. After the trial against Galileo, the Catholic Church gradually lost its dominance in scientific fields, but meteorites did not attract astronomers’ attention until the 18th century, when the problem of their origin started being discussed again. Aristotle’s ideas on meteorites lost credibility after 1700. It was previously thought that meteorites were formed where they were found and that they were stones struck by lightning. Since it was impossible that meteorites fell from the sky, witnesses must have been mistaken or hallucinating. When a meteorite fell in front a group of farmers in Lucé (France) on 13 September 1768, scientists in Paris – including Antoine-Laurent de Lavoisier – did not believe them.

Around 1780, American astronomer David Rittenhouse put forward the idea that meteors were caused by extraneous bodies to Earth entering the atmosphere, but he was unheeded.

Towards 7 pm on 16 June 1794, a dark and tempestuous cloud drew near Siena, Italy, from the north, then a large explosion was heard and numerous stones fell at the feet of many witnesses. From then on it was impossible to deny these falls or maintain that they had only been witnessed by ignorant people.

Father Ambrogio Soldani dared to suggest their origin was not terrestrial, saying that the stones had been condensed inside a stormy cloud.

Later that year, the ‘father of acoustics’, the German Ernst Florenz Friedrich Chladni (1756–1827), published a 63-page book on meteorites: On the Origin of the Pallas Iron and Others Similar to it, and on Some Associated Natural Phenomena. The title referred to a meteorite found in Siberia in 1749

by Russian academic Peter Simon Pallas (1741–1811), after whom the Pallasite class of meteorites was named. In his work, Chladni reasoned that there was no point in sustaining the idea that the space between planets could not hold small celestial bodies – as Newton had done previously. He supported the theory of meteorites having an extraterrestrial origin. Not only did stone or iron masses fall from the sky, but they originated from interstellar space and produced 138

Image 1. Portrait of E. F. Chladni (1756–1827), who was the first to suggest that meteorites were stones that had fallen out of the sky and favoured the existence of a link between bolides and meteorites. (Russian Academy of Science)

Image 2. Micrometeorites are collected through special devices carried by aeroplanes at high altitudes.

The example in (a) is an irregular aggregate a few hundredths of a millimetre across. The spherical particle in (b) was found in oceanic sediments and seems to have partially fused during its passage through the atmosphere. This irregular grain of dust resulted in a drop of material that then solidified. (D. Brownlee, University of Washington)

bolides as they crossed Earth’s atmosphere. Needless to say, the arguments in the book – even though they were near reality – were not approved by researchers of the time because they did not think that other bodies besides planets and comets could exist in the Solar System. Theories opposing Chladni included that of Olbers, who, proposed in 1795 that meteorites were stones launched toward Earth by the Moon’s volcanoes.

Another significant fall occurred on 13 December 1795 at Wold Cottage, England. The Royal Society asked English chemist Edward C. Howard to carry out chemical analysis on this meteorite, together with a fragment from the fall in Siena and other pieces from the Pallas meteorite. With help from French mineralogist Jacques Louis de Bournon, Howard completed the analyses and found numerous differences between the meteorites and Earth’s rocks – in particular a significant quantity of nickel associated with iron.

Howard was also the first to describe the ‘chondrule’ (which we will speak 139

about later) that he defined as ‘curious globules’. The results of Howard’s analyses were published in 1802.

With these data, it became clear that meteorites could not be stones from Earth. Many important scientists of the time, including Laplace, Poisson and Biot, were convinced of the extraterrestrial origin of the ‘flying stones’ and supported Olbers’ theory. They finally turned their backs on the old theories when a shower of around 2000–3000 meteorites hit Aigle, in Normandy, on 26 April 1803. French scientist Jean-Baptiste Biot (1774–1862) was sent to investigate. His name is important in physics for the Biot–Savart law that concerns the magnetic field produced by a wire crossed by an electric current.

Biot wrote a report for the Institut de France in July 1803, after which nobody (at least in the scientific world) had any more doubts about the fact that extraterrestrial stones could fall on Earth.

In 1805, Chladni was converted to Olbers’ theory. In fact, the Moon’s presumed volcanic activity was probably what is known today as Transient Lunar Phenomena (TLP) – temporary variations of albedo in some areas of the lunar surface, mostly situated on the edges of lava seas and near certain craters, that astronomers of the time interpreted as eruptions from volcanoes.

Astronomers who observed presumed lunar eruptions include illustrious names such as William Herschel (1738–1822), Gian Domenico Cassini (1748– 1845), Johann Schröter (1745–1816) and Jêrome de Lalande (1732–1807).

Despite progress on the origin and composition of meteorites, another ten years went by before recognising the cause–effect relationship with bolides.

Only in 1848 did British physicist James Prescott Joule (1818–1889) propose a physical process able to explain the bolide phenomenon. The theory of the meteorites’ lunar origin collapsed in 1859 thanks to American astronomer Benjamin A. Gould (1824– 1896). Gould calculated that only an infinitesimal fraction of fragments expelled by presumed lunar volcanoes could reach Earth. This low rate of falls could not explain the number of meteorites found, unless lunar volcanic activity was higher, which Image 3. Willamette’s meteorite in New York. Its mass is 14.1 tonnes, did not seem likely. but it is clearly only a part of a larger body. The less resistant material The problem concerning was destroyed during its fall through the atmosphere. (Smithsonian Institution) the origin of meteorites was 140

still unsolved. By 1854, 20 asteroids were already known and English astronomer Robert P. Greg (1826–1906) suggested that they were the remains of an exploded planet and that meteorites were small fragments from this object. His theory was well received among astronomers, but was hindered by Chladni who, after the failure of the lunar origin theory, had gone back to considering an interstellar origin. Chladni had been led to this revised theory by the fact that many bolides seemed to have a heliocentric speed over 42 km/s.

Between 1866 and 1867, research by Italian astronomer Giovanni V. Schiaparelli (1835–1910) clarified the link between meteor showers and comets. Schiaparelli found that the Perseid meteors followed an identical orbit to the Swift–Tuttle comet. However, these discoveries did not resolve the problem of the origin of large sporadic bolides, which precede finding meteorites, because any known comet could be associated with them. Chladni’s interstellar theory had not been forgotten – in fact, it was alive and flourishing.

But why did Chladni and many others after him make this mistake?

During the late 19th and early 20th centuries, only visual observations were used to calculate the bolides’ heliocentric speed and this led to overestimation of the values. In 1925, Hoffmeister published a report on bolides which stated that 79% had hyperbolic speed. In 1935, Estonian astronomer Ernst J. Öpik (1893–1985) declared that the meteorites’ interstellar origin was certain. Öpik had led an observatory campaign on sporadic bolides (using one of his methods to improve visual measures of the trajectory) and found that about 60% of bolides had hyperbolic orbits. In 1936, Fred L. Whipple founded a programme to photograph meteors with two cameras located 38 km apart and equipped with a rotating shutter. Whipple was interested in accurately determining the orbits of the Taurid and Geminid showers, but also achieved results concerning sporadic bolides – although there was no evidence for hyperbolic speed. In 1952, the first results were obtained from two wide-angle Super Schmidt cameras that Whipple had installed in New Mexico as a follow-on to the 1936 experiment. In four years, 10 000 photos were taken of meteors and bolides, but none had hyperbolic speed. In 1959, Öpik abandoned the idea of hyperbolic orbits and admitted his mistake. When photographic data started to be used to calculate bolides’ orbits, it was understood that their origin was not interstellar, but probably from the asteroid belt – so Greg’s theory was right!

In fact, it should be added that a small percentage of bolides has hyperbolic speed. Using radar, it has been found that around 1% of bolides have hyperbolic orbits. Very probably, the few cases of hyperbolic speed that exist were caused by planetary perturbations. It is possible that, after passing near a planet, some small bodies in the Solar System acquire a speed larger than the escape velocity of the Sun. 141

The structure of meteorites Let us start by becoming familiar with the meteorite classification and its three main categories: 1. Siderite – made from metal alloys (primarily iron–nickel) 2. Siderolites – made from metals and rock 3. Aerolites – made only from rock These three categories are then sub-divided into numerous groups that we will examine later. For now, it is enough to focus on the classification above. Of all the meteorites seen to fall on our planet, 94.2% are aerolites, 4.6% are siderites and 1.2% are siderolites. If we suppose that meteorites fall on Earth by chance, this composition should reflect the real abundance of meteorite populations in space. The percentages change if we consider meteorites found at random on the ground.

Meteorites have many chemical–physical characteristics that differentiate them from Earth’s rocks, but this does not mean differentiating a meteorite from a common rock is easy. Let’s look at a meteorite’s most typical characteristics, some of which are due to interaction with Earth’s atmosphere and others which are caused by the dynamic process that caused it to exist.

Meteorites – we should really call them meteoroids before they touch the ground, but for simplicity, we shall refer to meteorites – move at a hypersonic speed, typically around 10–20 km/s during flight through the atmosphere. This high speed means that collisions with molecules in the air generate a considerable quantity of heat and the meteorite’s surface reaches temperatures of several thousand degrees Celsius. Under these conditions, the material on the surface melts and is blown away by the flow of air (ablation). The ablation process stops when the meteorite reaches the ‘point of arrest’ and begins its ‘dark flight’ phase.

At this altitude, the meteorite has lost all its initial speed and so falls to the ground under the influence of Earth’s gravity alone. During this phase the melted surface cools and crystallises. Despite the high temperature of the surface, the inside of the meteorite remains at the same temperature as in outer space, because the heating process only lasts about 10 seconds. During the ablation process, the meteorite’s surface material can be removed at random. This is due to the melting at different temperatures of the minerals that make up the celestial body, as well as air turbulence effects. This leads to the creation of small ‘dimples’, called regmaglypts, that resemble at first glance a handprint left in fresh mud and typically range in size from a few millimetres to a few centimetres.

The melted mineral crust is what gives the meteorite its characteristic dark colour once it has reached the ground. Just under the melted crust is an area around 1 mm deep (it depends on the conductivity of the material that makes 142

Image 4. Example of a regmaglypt on the ferrous Sikhote–Alin meteorite. Note also the dark colour of the fusion crust. (Collection Jim Hurley)

Image 5. Comparison of the abundance of solar chemical elements and those present in chondrites. The 45° straight line shows that they are identical. (LPI)

up the meteorite) which shows signs of thermal alteration, but the rest is intact.

The melted crust can be thicker: its thickness depends on the speed the meteorite entered the atmosphere and its dynamic behaviour. The melted crust has a glassed look in aerolites and its thickness is between 1 and 10 mm, while a siderite’s crust is made of magnetised iron oxide (Fe3O4) that quickly deteriorates when it is exposed to atmospheric agents and turns into rust. A metallic meteorite crust keeps better if it lands in a dry region like a desert. The dark colour also facilitates its identification, due to its contrast with the surrounding environment.

Most meteorites are roughly spherical (like a football) or oval shaped, but some are also oriented, that is they have an ‘edge’ like a wedge. The rounded shape indicates the meteorite rotated on itself during its fall through the atmosphere and ablation was more or less symmetrical. In the second case, the shape can have two different causes: either the meteorite did not rotate at all or the rotation axis was parallel to the direction of motion. It was these pointed shapes that deceived researchers during the last few centuries and made them believe that some meteorites were stone objects from pre-history. Meteorites also have some internal characteristics that are not found in terrestrial rocks.

Chondrules As we mentioned before, it was noted in a publication of 1802 that some meteorites showed ‘curious globules’. Today, these small spheres are known 143

Image 6. Sections of different metallic meteorites can show very different structures. On the left, is the section of an octahedrite, showing inclusions of a mineral called troilite that are visible as egg-shaped areas in sections. Another octahedrite (right) shows bands of the mineral kamacite. (Smithsonian Institution)

as chondrules (from the Greek chondros meaning seed). Chondrules are immersed in the matrix of 92% of aerolites that, for this reason, are called chondrites. An aerolite that does not contain chondrules (8% of all aerolites) is called an achondrite. Chemically, the composition of chondrules is rather varied. In general, their composition is similar to the minerals from the matrix.

Chondrules are made up of silicates (olivine and pyroxene), glass and lesser quantities of iron–nickel and iron sulphide. There are two types of chondrules, monosomatic (made of a single crystal) and polysomatic (made of more than one crystal, including different minerals). Chondrules can make up over 70% of a meteorite’s mass. It is believed that chondrules are grains of material from which all other rocky bodies in the Solar System were formed by aggregation.

The formation of chondrules in the absence of gravity explains their spherical shape. Supporting this theory is the fact that the chemical composition of most primitive types of chondrites is identical to that of the Sun. Achondrites are very different. As chondrules are not present in these meteorites, they probably originated from chemically differentiated bodies.

Evidently, heat sources in chondrites’ parent asteroids were not sufficient to provoke the meltdown and differentiation of the celestial body – otherwise chondrules would have been eliminated and mixed in with the rest of the matrix. However, some metamorphic transformations due to weak heat sources can be seen in a number of chondrites.

Chondrites that did not undergo a significant process of metamorphism are called carbonaceous chondrites, named after their content of complex organic molecules.

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Widmanstätten patterns Siderites consist mainly of two different iron–nickel alloys: kamacite and taenite. Kamacite contains up to 7.5% nickel, while taenite has a percentage of over 25%. The overall percentage of nickel in siderites ranges from 5 to 25%, with an average of 8%. The percentage of nickel in a siderite can be estimated by the naked eye. In fact, alloys with different percentages of nickel crystallise differently. Siderites with an overall percentage of nickel between 6 and 14% show a distinctive intersection between the kamacite and taenite crystals.

Kamacite is in the form of thin lamellae, while taenite is located on the edge of the thin lamellae. The remaining spaces are filled with a granular mixture of kamacite and taenite called plessite. The kamacite thin lamellae are parallel to the facets of an octahedron. If, after cutting the siderite, the exposed surface is polished and wiped with a very dilute nitric acid solution, the kamacite crystals are mostly corroded. In this way, a kamacite crystal pattern can be seen inside the meteorite. The angle of intersection between the thin kamacite plates depends on the angle of the cut. For example, if the cut is parallel to one of the octahedron facets, the angle of intersection is 60°, while if it is orthogonal to the face of a cube, the angle is 90°.

The geometrical complexes of thin intersecting plates are called Widmanstätten patterns, after Count Alois de Widmanstätten, who was the first person to observe them in 1808. The patterns are the result of slow cooling of the original body containing the meteorite, occurring at a rate between 1° and 100°C per million years and at pressures under 10 000 bars. With these restrictions, meteorites showing Widmanstätten patterns must have been formed in the metallic core of a differentiated body with a diameter less than 250 km. If a siderite containing these patterns is heated for a long time, the geometry of the kamacite crystals is lost. Siderites that contain Widmanstätten patterns are called octahedrals for obvious reasons.

Neumann bands

Image 7. The Gibeon meteorite showing its Widmanstätten patterns. (Collection Jim Hurley)

Neumann bands (or lines) are found in certain siderites where Widmanstätten patterns are not present and nickel content is less than 6%. These meteorites are generally fragments of a single kamacite crystal with flaking in three orthogonal 145

Image 8. Canyon Diablo, an excellent example of a ferrous meteorite, believed to be associated with the remains of the object that impacted the ground and formed Meteor Crater in Arizona. It clearly shows a dark, external fusion crust that is bright inside. (Collection Jim Hurley)

directions. The intersection of the cut in the meteorite with the thin kamacite plates shows thin and parallel streaks (Neumann bands) that only become visible after the meteorite has been cut, smoothed and wiped with acid solution. Neumann bands are sometimes also visible in the octahedrites’ thin kamacite plates and are formed following shock-induced deformation caused by collisions, when pressures reach 10 000 bars and temperatures remain under 303°C. Neumann bands were first observed by researcher Johann G. Neumann in 1848. Siderites that have Neumann bands are called hexahedrites.

The ages of meteorites The interval of time from the formation of the parent body to the fall or discovery of a meteorite on Earth’s surface is the meteorite’s age. Is it possible to estimate a meteorite’s age? The answer is ‘yes’. Radiometric dating methods can be used. This technique is based on the fact that some unstable atomic nuclei decay in other atomic species at a certain rate, according to the law of radioactive decay. It follows that if a type 1 ‘parent’ atom decays in a type 2 stable ‘child’ nucleus, then at time t their concentration in atoms per volume unit will be given by: and

 t N1 = N 0 ⋅ exp −   τ

  t  N 2 = N 0 ⋅ 1 − exp −    τ   146

Here N0 is the number of atoms present at the moment of forming the parent body. Quantity τ is the average life of the radioactive atom. If, in a sample of a given material, N1 and N2 are measured, t can be derived. In fact, the two preceding equations lead to:

 N + N2  t = τ ⋅ ln  1   N1  In general, the reactions of radioactive decay are more complicated than the simple case of two nuclei seen here. There can be several stages of intermediate decay present. Nevertheless, if the average life of the parent (e.g. uranium) is far greater than the average lives of the intermediary radioactive nuclei, the preceding equations can still be used.

Since the number of radioactive atoms and the number of atoms generated by the decay can be measured with a mass spectrometer, the problem is, theoretically, solved. In fact, problems can occur if the number of atom children has altered over time because of the meteorite’s thermal history. For example, if the products of decay are gas atoms (as with the decay of potassium in argon) and the meteorite has been through heating phases, then the gas atoms can easily escape from the sample and the determination of its age will be distorted. Another problem occurs when a certain quantity of atom children are already present at the moment of forming the celestial body. To overcome this type of problem, it is best to measure the age against different types of radioactive atoms.

The results of these measures tell us that meteorites, in general, are about 4.5 billion years old. These results agree with current theories on the formation of the Solar System. Only some of the meteorites identified by the initials SNC, which we will speak about later, are much younger, with ages of about Table 1

Isotopes used in radiometric dating Nucleus parent

Stable final nucleus

Decay time τ (millions of years)

N

5.73∙10–3

Mg

7.40∙10–1

C

14 26

14

Al

26

I

129

129

K

40

40

Rb

Sr

49 000 13 900

87

Th

208

U

207

U

206

235 238

17 1250

Pb+6∙4He

87

232

Xe

Ar+40Ca

Pb+7∙4He

704

Pb+8∙4He

4.47·109

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1.3 billion years. An accurate chemical analysis indicates that Mars – and not an asteroid – is the parent of these meteorites.

Analysing the radioactivity of meteorites not only allows us to estimate when they were formed, but also how long they were in space. The interplanetary environment is crossed by high-energy protons from the Sun and the galactic environment. Energy from cosmic rays can be far greater than the amount supplied to a proton in a modern particle accelerator. These energetic particles, known as cosmic rays, were discovered in 1910 during balloon flights by physicist Victor Hess, winner of the 1936 Nobel Prize. If a cosmic ray proton cuts into a meteorite’s surface, it is able to trigger nuclear reactions with atomic nuclei on the surface and create new isotopes. Iron nuclei can, for example, be turned into an unstable manganese isotope.

By measuring the quantity of extraneous isotopes present in meteorites, it is possible to understand how long they were affected by cosmic radiation and the time of exposure in space can be estimated. Obviously, the exposure time indicates when the impact that expelled the meteorite from its parent body took place. Typical values of time spent in interplanetary space are 300–600 million years for siderites and about 10 million years for aerolites.

Obviously, only some hundreds of millions of years ago, there were sufficiently violent impacts to expel siderites from the asteroids’ iron nuclei.

Table 2

Origin of the classification names of meteorites Name

Origin

Achondrite

Without chondrules

Angrite

Fell at Angra dos Reis, Brazil, January 1869

Ataxite

From Greek ataxis, meaning no structure

Aubrite

Fell at Aubres, France, 14 September 1836

Chassignite

Fell at Chassigny, France, 3 October 1815

Chondrite

With chondrules

Diogenite

From Diogenes of Apollonia

Eucrite

From Greek eukritos, meaning easily distinguished

Hexahedrite

Crystals have hexahedrite shape

Lodranite

Fell at Lodran, Pakistan, 1 October 1868

Mesosiderite

From Greek mesos, meaning half, and sideros, meaning iron

Nakhlite

Fell at Nakhla, Egypt, 28 June 1911

Octahedrite

Crystals have an octahedral shape

Pallasite

From the explorer P. S. Pallas

Shergottite

Fell at Shergotty, India, 25 August 1865

Ureilite

Fell at Novo Urei, Russia, 4 September 1886

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Classification of meteorites Scientists and collectors alike classify meteorites according to certain parameters.

Previously, we mentioned the three categories of meteorites (aerolites, siderites and siderolites) and we saw their main characteristics (chondrules, Widmanstätten patterns and Neumann bands). Let us now look more closely at how meteorites are classified.

Officially, meteorites take their name from the geographical location nearest to where they fall. This rule, of course, does not apply to meteorites that Table 3

Classification of chondrites Group

Type of meteorite

Chondrites (and enstatites)

High iron content (12%–21%)

Ordinary chondrites

Low iron content.

(5%–10%)

Chondrites: rocky meteorites

Very low iron content (<2%).

Bronzite and olivine present.

Carbonaceous chondrites

Type of chondrule

Subgroup

Very distinguished

E4

Less distinguished

E5

Indistinct

E6

Melted

E7

Abundant

H3

Distinguished

H4

Less distinguished

H5

Indistinct

H6

Melted

H7

Abundant

L3

Distinguished

L4

Less distinguished

L5

Indistinct

L6

Melted

L7

Abundant

LL3

Distinguished

LL4

Less distinguished

LL5

Indistinct

LL6

Melted

LL7

Friable, a lot of H2O

Absent

CI

Friable, little H2O

Scattered

CM2

Scattered

CV2

Inclusions of calcium and aluminium.

Olivine rich in iron.

Abundant

CV3

Distinguished

CV4

Less distinguished

CV5

Abundant

CO3

Distinguished

CO4

Small chondrules

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fall in Antarctica, which have their own rules, as we will see later. Table 2 shows the origins of the classification names. As we will see, this rule is not always respected, because of the historical origin of certain names that have been maintained till now.

Let us start by classifying aerolites. As we said, aerolites are divided into chondrites and achondrites, depending on whether or not they contain chondrules. This classification dates back to the late 19th century and is still valid for an initial approximation. In more modern classification, a meteorite is identified as a chondrite when it has not been modified due to melting or differentiation of the parent body.

Chondrites Chondrites are generally like rather porous stones, with a percentage of empty spaces that can range from 7 to 18% (compared with 14% for sand). This suggests that they have never been subjected to much pressure and so are probably surface rocks from the parent asteroid. The chondrites’ melted crust is rather fragile and can be destroyed by atmospheric agents if it spends too much time between falling and being collected.

Chondrites are meteorites with low albedo: only 4.5% of incidental radiation is reflected. They are divided into ordinary and carbonaceous.

Ordinary chondrites are more numerous (hence the term ‘ordinary’), while carbonaceous chondrites are more rare and fragile. Carbonaceous chondrites are so-called because they contain organic material in the form of complex molecules of carbon, hydrogen, oxygen and nitrogen. These organic molecules do not have a biological origin.

Ordinary chondrites are divided into subgroups – H (high), L (low) and LL (very low), depending on whether the iron content is high, average or low. The number between 3 and 7 inclusive indicates the degree of thermal metamorphism undergone by chondrules. 3 is used for an unchanged sample, while 7 indicates that chondrules are fused within the matrix. So, for example, when we hear of an LL3 type chondrite, it refers to an ordinary chondrite with low iron content and abundant, clearly distinct chondrules. The subgroup initials are a very convenient way to express a lot of information easily. An example of an H-ordinary chondrite (breccia type) is one that fell in Fermo, in the province of Ascoli Piceno, Italy, on the afternoon of 25 September 1996. The meteorite’s mass was 10.2 kg and its dimensions were 19 x 16 x 24 cm. The melted crust and regmaglypts were quite visible on the surface.

First of all, carbonaceous chondrite subgroups are identified by two letters.

The letter C stands for carbon, while the other letter is the initial of the name of the meteorite representing the subgroup: I (Ivuna), M (Mighei), V (Vigarano) 150

and Or (Ornans). The degree of alteration of the chondrule is indicated by a number between 1 and 5. Numbers 1 and 2 indicate that the chondrules are altered by water, type 3 that the chondrules are unchanged, while type 4 and 5 chondrules have undergone thermal metamorphism.

A rather famous carbonaceous chondrite is Allende (CV3). On 8 February 1969, a bright bolide exploded above the village of Pueblito de Allende in Mexico. Numerous meteorite fragments fell over an area of about 150 km2.

Following organised search campaigns, around two tonnes of chondrites were recovered, with a mass varying from a few grams to about 10 kg. From traces left by cosmic radiation it was possible to estimate the time spent in space at around 5 million years. This is the interval of time that elapsed between separation from the asteroid parent and the meteorite falling to Earth. In 1973, some refractory inclusions, poor in volatile elements, were identified in the Allende meteorite. They were rich in unusual isotopes of calcium and titanium, elements that form during explosions of supernovae, due to fast capture of neutrons from iron atoms. This was the first evidence to support the theory that the formation of Solar System was initiated by a supernova explosion.

Another famous carbonaceous chondrite is the Murchison meteorite (CM2), which also fell in 1969. Some amino acids that are quite unknown on Earth were found in this meteorite.

The last type of chondrites is rich in enstatite (up to 65%). For this reason it is called an E-chondrite. Enstatite (Mg2Si2O6), a mineral from the pyroxenite group (silica) that is rich in magnesium, is also abundant on Earth. E-chondrites are rather rare and represent only about 2% of aerolites.

Before leaving chondrites, let us look briefly at the meteorite that fell in Monahans in Texas on 22 March 1998. The fall was witnessed by some

Image 9. A fragment of the Allende meteorite. Note the chondrules. (Collection Jim Hurley) Image 10. The main fragment, weighing 11.8 kg, from the fall at Peekskill, an example of a typical chondrite. The red colour above the black fusion crust comes from the car on which the meteorite fell. (Montana Meteorite Laboratory)

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youngsters and less than 48 hours later one of the two fragments from the meteorite was already being analysed at the NASA Johnson Space Center in Houston, Texas, thus excluding contamination from terrestrial material. The meteorite was type H5, which indicates moderate thermal metamorphism.

Inside it were some sodium chloride (NaCl) crystals, similar to common kitchen salt, which measured up to 3 mm in diameter. It was the first time that such large salt crystals had been found inside a meteorite. The age of the crystals, determined with the Rb/Sr method (rubidium/strontium), was 4.7±0.2 billion years. Inclusions of fluid have been found inside the salt crystals, probably representing secondary inclusions that formed after the crystals. Analyses indicate that the fluid is essentially composed of water molecules. This is the first time that water responsible for the chondrites’ morphological alterations has been identified with certainty. Previous discoveries were of doubtful origin or never confirmed. The origin of this primitive water still has to be understood – whether it originates from the meteorite’s asteroid parent or if it was deposited following impact with frozen objects like comets. Only direct exploration of asteroids and cometary nuclei can solve this problem.

Achondrites Achondrites are relatively rare meteorites, representing just 7.6% of all those that fall on Earth. Achondrites belong to a heterogeneous group: some are of igneous (volcanic) origin, while others are of the breccia type. All originate from bodies that have undergone a process of chemical differentiation.

HED type achondrites take their name from the initials of the meteorites that form the family. Infrared spectroscopic analysis in the early 1990s by Richard Table 4

Classification of achondrites Group

Achondrites: rocky meteorites.

Type of meteorite

Principal minerals

Subgroup

Aubrite

Enstatite

AUB

Angrite

Augite

ACANOM

Ureilite

Olivine, pigeonite

AURE

Howardite

Eucrite

AHOW

Eucrite

Anortite, pigeonite

AEUC

Diogenite

Enstatite

ADIO

Shergottite

Basalt

AEUC

Nakhlite

Diopside, olivine

ACANOM

Chassignite

Olivine

ACANOM

HED (Vesta)

SNC (Mars)

152

Image 11. A lunar achondrite found during a search for meteorites in the Antarctic in 1982. The cube beside it is 1 cm long. Breccias similar to this were brought back to Earth during the Apollo lunar missions. This sample reached Earth following an impact that expelled fragments of lunar soil. The recognition of the similarity between achondrite breccias and the rocks brought back to Earth by the Apollo crews was one of the most important discoveries of the 1980s. (Collection Jim Hurley) Image 12. The Murchison meteorite. (Collection Jim Hurley)

Binzel from MIT made it possible to establish that the spectrum of HEDs and asteroid 4 Vesta are very similar, indicating that it is probably the HEDs’ parent body. In May 1996, taking advantage of Vesta’s minimal distance from Earth (1.17 AU), observations with the Hubble Space Telescope established the existence of a crater, about 460 km in diameter and 13 km deep, near Vesta’s south pole. Considering that the whole asteroid measures 580 x 560 x 460 km, the formation of this crater must have been a near-catastrophic event for Vesta.

The crater was obviously formed as the result of an impact by an asteroid with a diameter of about 10 km. Fragments ejected during the collision, however, could not have been travelling fast enough to enter radically different orbits from that of Vesta. This would need variations in speed of 0.6 km/s. How did the HEDs reach Earth?

A reasonable explanation is that their escape could have been induced by the 3:1 resonance with Jupiter. Fragments produced by the impact with Vesta could have been expelled from the asteroid’s gravitational field, where the escape velocity is only 0.3 km/s, and then pulled into an orbit close to the 3:1 resonance.

Some million years later they could have reached Earth. Binzel’s later research Image 13. A fragment of the Millbillillie meteorite, established the existence of a family of a fragment from Vesta. (Collection Jim Hurley) 153

Table 5

Classification of siderites Group

Type of meteorite

Kamacite lamellae dimensions

Hexahedrites

Siderites: iron meteorites

Octahedrites

Ataxite

Subgroup

> 50 mm

H

Very large

3.3–50 mm

Ogg

Large

1.3–3.3 mm

Og

Average

0.5–1.3 mm

Om

Fine

0.2–0.5 mm

Of

Very fine

< 0.2 mm

Off

No structure

D

about 10 asteroids, with diameters of 5–10 km and a similar spectrum to Vesta’s. Eight of these asteroids also have an orbit close to the 1/3 resonance.

An example of a eucrite is the meteorite that fell in Millbillillie in Western Australia, in October 1960. This meteorite is basically a breccia with structures from impact metamorphism. Analysis of cosmic ray traces indicates exposure in space of around 20 million years.

Siderites Siderites have a double system of classification. The traditional system classifies these meteorites according to their type of internal structure: Widmanstätten patterns, Neumann bands or no structure. The other classification system in use is Wasson’s, which is based on the siderite’s chemical composition, namely concentrations of gallium, germanium, iridium and nickel. Table 5 shows the traditional classification.

Hexahedrites are composed entirely of large, cubic kamacite crystals (called ‘hexahedrons’, hence their name) and they have Neumann bands.

Kamacite crystals are smaller in octahedrites and classification is based on their thickness. Plessites have a micro-crystal structure of kamacite and taenite.

Ataxites have a high content of nickel (25%) and are made up entirely of microscopic taenite crystals. They do not have structures at macroscopic level.

The largest known meteorite, which is at Hoba West, near Grootfontein in Namibia, is an ataxite that weighs 60 tons. According to current theories, siderites originate from iron nuclei of the largest asteroids from the main belt.

Siderolites Siderolites are a small fraction of the meteorites collected. Pallasites consist of olivine inclusions embedded in a nickel–iron matrix. The olivine inclusions 154

Table 6

Classification of siderolites Group Siderolites: meteorites with iron and rock

Meteorite type

Principal minerals

Subgroup

Pallasite

Fe–Ni alloy, olivine

PAL

Mesosiderite

Fe–Ni alloy, plagioclase

MES

Lodranite

Fe–Ni alloy, pyroxene, olivine

LOD

are often single crystals with diameters of about 1 cm. The metallic part has an octahedron structure and Widmanstätten patterns. These meteorites could have originated in the area of transition between the iron nucleus and the rocky mantle of the asteroid parent. Mesosiderites are breccias of rock and metal in more or less equal parts. These meteorites probably originate from the numerous collisions between the bodies’ parents that led to a mix of silicates and metals.

Meteorites from the Moon and Mars In January 1992, ALHA 81005, the first meteorite of lunar origin, was discovered amid Antarctic ice. Comparison of the chemical composition of this meteorite and samples brought back by the Apollo missions left no doubts. Others have since been discovered and today several tens of lunar meteorites are known with a total mass of over 30 kg. (About 50 have been found in Antarctica, two in Libya and one in Australia.) Lunar meteorites are achondrites. Most are breccias and originate from the ejecta of asteroid impacts, while the others are basaltic and probably originate from lunar maria. Lunar meteorites are around 4 billion years old, about 500 million years younger than meteorites coming from asteroids. We can certainly say that lunar and martian achondrites are the only meteorites whose parent body is definitely known.

Out of all the planets in the Solar System, Mars has been observed with most interest over the last 200 years – first with telescopes on Earth and then, from the mid-1960s, with space probes, both in orbit and on the red planet’s surface. A lot of things are now known about Mars, but there is still a lot to understand. In particular, only detailed analysis of Mars rocks, done in labs here on Earth, will allow us to completely understand the planet’s geology and the geological processes that modelled its surface.

A milestone in our knowledge of Mars was definitively recognising that the 13 achondrites known as type SNC (after the initials of the three component types: Shergottite, Nakhlite and Chassignite) are possible fragments of the martian surface. The only sample of chassignite in our possession fell to Earth near Chassigny, France, in 1815, while the shergottites’ parent fell near 155

Table 7

List of Mars meteorites* Name

Abbreviation

Year of recovery or fall

Place

Type

Mass (kg)

Allan Hills A77005

ALHA77005

1977

Antarctica

Shergottite

0.483

Allan Hills 84001

ALH 84001

1984

Antarctica

OPX

1.931

Chassigny

1815 (fell)

Champagne–Ardenne, France

Chassignite

4

Dar al Gani 476

DaG 476

1998

Al Jufrah, Libya

Shergottite

2.02

Dar al Gani 489

DaG 489

1997

Al Jufrah, Libya

Shergottite

2.15

Dar al Gani 670

DaG 670

1999

Libya

Shergottite

1.619

Dar al Gani 735

DaG 735

1997

Al Jufrah, Libya

Shergottite

0.588

Dar al Gani 876

DaG 876

1998

Al Jufrah, Libya

Shergottite

0.620

Dar al Gani 975

DaG 975

1999

Al Jufrah, Libya

Shergottite

0.028

Dar al Gani 1037

DaG 1037

1999

Al Jufrah, Libya

Shergottite

4.01

Dhofar 019

Dho 019

2000

Zufar, Oman

Shergottite

1.056

Dhofar 378

Dho 378

2000

Zufar, Oman

Shergottite

0.015

Elephant Moraine A79001 EETA79001

1979 or 1980

Antarctica

Shergottite

7.94

Governador Valadares

1958

Minas Gerais, Brazil

Nakhlite

0.158

Grove Mountains 99027

GRV 99027

2000

Antarctica

Shergottite

0.010

Grove Mountains 020090

GRV 020090

2003

Antarctica

Shergottite

0.008

Lafayette (stone)

1931

Indiana, USA

Nakhlite

0.800

Larkman Nunatak 06319

LAR 06319

2006

Antarctica

Shergottite

0.079

Lewis Cliff 88516

LEW 88516

1988

Antarctica

Shergottite

0.013

Los Angeles

1999

California, USA

Shergottite

0.698

Miller Range 03346

MIL 03346

2003

Antarctica

Nakhlite

0.715

Nakhla

1911 (fell)

Al Buhayrah, Egypt

Nakhlite

10

Northwest Africa 480

NWA 480

2000

Northwest Africa

Shergottite

0.028

Northwest Africa 817

NWA 817

2000

Morocco

Nakhlite

0.104

Northwest Africa 856

NWA 856

2001

Northwest Africa

Shergottite

0.320

Northwest Africa 998

NWA 998

2001

Northwest Africa

Nakhlite

0.456

Northwest Africa 1068

NWA 1068

2001

Northwest Africa

Shergottite

0.577

Northwest Africa 1110

NWA 1110

2001

Northwest Africa

Shergottite

0.118

Northwest Africa 1195

NWA 1195

2002

Northwest Africa

Shergottite

0.315

Northwest Africa 1460

NWA 1460

2002

Northwest Africa

Shergottite

0.070

Northwest Africa 1669

NWA 1669

2003

Northwest Africa

Shergottite

0.036

Northwest Africa 1775

NWA 1775

2002

Northwest Africa

Shergottite

0.025

Northwest Africa 1950

NWA 1950

2001

Northwest Africa

Shergottite

0.812

Northwest Africa 2046

NWA 2046

2003

Northwest Africa

Shergottite

0.063

Northwest Africa 2373

NWA 2373

2004

Northwest Africa

Shergottite

0.018

Northwest Africa 2626

NWA 2626

2004

Northwest Africa

Shergottite

0.031

156

Table 7 (cont’d)

List of Mars meteorites* Name

Abbreviation

Year of recovery or fall

Place

Type

Mass (kg)

Northwest Africa 2646

NWA 2646

2004

Northwest Africa

Shergottite

0.009

Northwest Africa 2737

NWA 2737

2000

Morocco

Chassignite

0.611

Northwest Africa 2800

NWA 2800

2007

Morocco

Shergottite

0.686

Northwest Africa 2969

NWA 2969

2005

Northwest Africa

Shergottite

0.012

Northwest Africa 2975

NWA 2975

2005

Algeria

Shergottite

0.070

Northwest Africa 2990

NWA 2990

2007

Northwest Africa

Shergottite

0.363

Northwest Africa 3171

NWA 3171

2004

Algeria

Shergottite

0.506

Northwest Africa 4222

NWA 4222

2006

Northwest Africa

Shergottite

0.017

Northwest Africa 4468

NWA 4468

2006

Northwest Africa

Shergottite

0.675

Northwest Africa 4480

NWA 4480

2006

Algeria

Shergottite

0.013

Northwest Africa 4527

NWA 4527

2006

Algeria

Shergottite

0.010

Northwest Africa 4797

NWA 4797

2001

Morocco

Shergottite

0.015

Northwest Africa 4864

NWA 4864

2007

Northwest Africa

Shergottite

0.094

Northwest Africa 4878

NWA 4878

2007

Northwest Africa

Shergottite

0.130

Northwest Africa 4880

NWA 4880

2007

Northwest Africa

Shergottite

0.082

Northwest Africa 4925

NWA 4925

2007

Northwest Africa

Shergottite

0.282

Northwest Africa 4930

NWA 4930

2007

Northwest Africa

Shergottite

0.118

Northwest Africa 5029

NWA 5029

2003

Morocco

Shergottite

0.015

Northwest Africa 5298

NWA 5298

2008

Morocco

Shergottite

0.445

Queen Alexandra Range 94201

QUE 94201

1994

Antarctica

Shergottite

0.012

Roberts Massif 04261

RBT 04261

2004

Antarctica

Shergottite

0.079

Roberts Massif 04262

RBT 04262

2004

Antarctica

Shergottite

0.205

Sayh al Uhaymir 005

SaU 005

1999

Al Wusta, Oman

Shergottite

1.344

Sayh al Uhaymir 008

SaU 008

1999

Al Wusta, Oman

Shergottite

8.58

Sayh al Uhaymir 051

SaU 051

2000

Al Wusta, Oman

Shergottite

0.436

Sayh al Uhaymir 060

SaU 060

2001

Al Wusta, Oman

Shergottite

0.042

Sayh al Uhaymir 090

SaU 090

2002

Al Wusta, Oman

Shergottite

0.095

Sayh al Uhaymir 094

SaU 094

2001 or 2001

Ad Dakhiliyah, Oman

Shergottite

0.223

Sayh al Uhaymir 120

SaU 120

2002

Al Wusta, Oman

Shergottite

0.075

Sayh al Uhaymir 125

SaU 125

2003

Al Wusta, Oman

Shergottite

0.032

Sayh al Uhaymir 130

SaU 130

2004

Al Wusta, Oman

Shergottite

0.279

Sayh al Uhaymir 150

SaU 150

2002

Al Wusta, Oman

Shergottite

0.108

Shergotty

1865 (fell)

Bihar, India

Shergottite

5

YA 1075

Antarctica

Shergottite

0.055

Yamato 793605

Y-793605

1979

Antarctica

Shergottite

0.016

Yamato 980459

Y-980459

1998

Antarctica

Shergottite

0.083

157

Table 7 (cont’d)

List of Mars meteorites* Name

Abbreviation

Year of recovery or fall

Place

Type

Mass (kg)

Yamato 984028

Y-984028

1998

Antarctica

Shergottite

0.012

Yamato 000027

Y-000027

2000

Antarctica

Shergottite

0.010

Yamato 000047

Y-000047

2000

Antarctica

Shergottite

0.005

Yamato 000097

Y-000097

2000

Antarctica

Shergottite

0.025

Yamato 000593

Y-000593

2000

Antarctica

Nakhlite

13.71

Yamato 000749

Y-000749

2000

Antarctica

Nakhlite

1.283

Yamato 000802

Y-000802

2000

Antarctica

Nakhlite

0.022

Zagami

1962 (fell)

Katsina, Nigeria

Shergottite

18

* Martian meteorites are martian rocks that were supposedly ejected from Mars by impacts and later fell to Earth as meteorites. Three well-known types are shergottites (basaltic to lherzolitic igneous rocks, named after the Shergotty, India, fall of 1865), nakhlites (clinopyroxenites or wehrlites, formed as cumulate rocks, and named after the Nakhla, Egypt, fall of 1911), and chassignites (dunitic cumulate rocks named after the Chassigny, France, fall of 1815).

Shergotty, India, in 1865. The main member of the nakhlite family fell in 1911, near Nakhla in Egypt. It is quite ironic to think that, while astronomers observed the martian surface through telescopes, fragments of the planet were being found here on Earth.

Table 7 shows the list of all known meteorites from Mars. They are all SNCs and the letter ‘A’ in the date column indicates those found in Antarctica. The naming of Antarctic meteorites follows a different rule from the other meteorites: the letters are initials that are associated with the place in Antarctica where they were found, the first two numbers indicate the year of discovery, while the other numbers indicate the progressive order in which they were analysed.

All SNC meteorites are of igneous origin, and created by the crystallisation of magma. Unlike achondrites of asteroid origin, SNCs are much younger. The age established with radioactive dating indicates values of 1.3 billion years for nakhlites and chassignites and 170 million years for shergottites, compared with about 4.5 billion years for common achondrites. The data indicate that SNCs cannot be of asteroidal origin. No asteroid is so massive as to maintain magmatic activity up to 3.2 billion years after its formation. Isotopic analysis of gases trapped in the inclusions of EETA79001 established the match with analyses of the Mars atmospheric composition made by the two Viking landers in 1976, within error bars. None of the SNCs show signs of atmospheric degradation, indicating that all of them solidified near, but not on, the martian surface. This makes trapping of the present martian atmosphere rather difficult.

Some basalt in the shergottites has a composition that is similar to Earth’s magma, while the others are dominated by olivine or pyroxene, both silicon 158

compounds. The shergottites’ composition is comparable to the martian soil that was analysed by the Viking landers and Pathfinder.

The problem with SNCs is understanding what process expelled them from the martian surface and enabled them to land on Earth. For a body to escape a planet’s gravitational field, it is necessary for departure speed to exceed escape velocity, namely 5.02 km/s on Mars. The only process able to provide sufficient kinetic energy is the formation of a crater after a small asteroid impacts on the martian surface: i.e. SNCs could be a small part of the ejecta from craters on Mars. The process of expulsion is more efficient if the asteroid impact is grazing to the planet’s surface. This mechanism is feasible, but not proven by accurate calculations taking into account processes such as oblation in the martian atmo-sphere and g effects during acceleration/ejection.

The time of exposure in space ranges from 0.7 million years for EETA79001 to 14 million years for nakhlites and chassignites. Shergottites show an intermediate period, about 3 million years. These are the intervals of time taken for SNCs to get to Earth.

ALH 84001 deserves a separate mention. It was formed about 4.5 billion years ago, so it is much older than the SNCs, and it was travelling through space for 16 million years. In 1996, D.S. McKay and colleagues published an article in the journal Science, in which the idea was advanced that ALH84001

Image 14. The presumed nanobacteria of the ALH 84001 Martian meteorite. (NASA/JPL)

159

contained proof of biological activity on Mars. ALH 84001’s fractures contained some small globules of carbonate with an average diameter of 50–100 micrometers (1 micrometer is a millionth of a metre). The composition of the globules was not homogeneous: at the centre was some orange-coloured carbonate of calcium and manganese, surrounded by alternating, clear layers of ferrous (iron) carbonate and dark-coloured magnesium carbonate. The outer regions of the globules were rich in iron and contained many granules of magnetite as well as some of iron sulphide (pyrrhotite). When the carbonate matrix was in contact with the granules of magnetite and pyrrhotite, it appeared porous, as if it had undergone a solution process. It should be noted that the matrix of carbonate can only be dissolved in an acid environment, although this would inhibit the magnetite and pyrrhotite from forming. Only bacteria could dissolve the carbonate, leaving the iron compounds intact. This was the first sign of biological activity.

Some organic molecules and polycyclic aromatic hydrocarbons (PAHs) have also been discovered in carbonate globules. The simplest example of an aromatic molecule is hexagonal benzene (C6H6). The PAHs are absent near the crust, but their concentration increases with depth inside the meteorite, meaning that they were originally part of the meteorite and not the result of later contamination. PAHs contain much more carbon-12 than carbon-13, a typical characteristic of bacterial metabolism. This was a second sign of biological activity. But are there bacteria?

In the outer parts of the carbonate globules, where magnetite and pyrrhotite granules were found, electron microscopy was able to highlight elongated, worm-shaped structures that had never been found in any meteorite before and were very similar to fossil traces of terrestrial bacteria. The length of these ‘fossils’ was around 20–100 nanometres (1 nanometre is a billionth of a metre), 10 to 100 times smaller than terrestrial fossil bacteria. This is why they were called nanobacteria. According to McKay and colleagues, these nanobacteria were responsible for the unusual properties of the carbonate globules. It was then deduced that life had appeared on Mars – at least in bacterial form.

Naturally, this theory provoked a heated scientific debate and further work on ALH 84001's characteristics. According to more recent analysis, the meteorite’s characteristics would be due entirely to inorganic processes.

The fall and identification of meteorites As mentioned above, the phenomenon most frequently associated with a body coming into the atmosphere is the formation of a luminous trail in which atmospheric gases and materials vaporised by the meteorite are suddenly heated by propagation of a shock wave. The bolide’s trail is usually white or 160

Image 15. Examples of dispersion ellipses. The sizes of the circles are proportional to the masses of the meteorites collected. (a) Fall at Camel Donga (Australia, 1984). (b) Fall at Kunashak (Siberia, 1949). (c) Fall at Homestead (Iowa, USA, 1875). (d) Fall at Johnstown (Colorado, USA, 1924). The flight directions are from right to left (drawings are not to scale).

bluish, but red and yellow are also frequently observed. The colour probably indicates the composition of the object’s external layers. For example, the rare greenish coloured wakes are probably due to the presence of copper. The incandescent material forms a bright wake, and as it subsequently cools, this can fall to the ground in the form of dust. Sometimes the bolide’s brightness can exceed that of the full moon, and the wake can last for several minutes.

Few objects remain intact on entering the atmosphere at speeds of the order of 10 km/s, because of atmospheric friction.

Around 30% of bolides survive this phase, but the most difficult part of the descent takes place during the last moments of the fall. In fact, the impacting body is very quickly slowed in the thickest layers of the atmosphere, where deceleration can reach 1000 times Earth’s gravitational acceleration. It is like hitting a wall – an object is normally completely smashed to small fragments.

Basically, all rocky meteorites retrieved from the ground came into the atmosphere relatively slowly: only a less intense deceleration avoided their disintegration. Only bodies that are particularly resistant to breaking up (like metallic meteorites) can hope to reach the ground at high speeds. In this case, impact speed can be higher than the speed of sound. There is, however, a higher limit: if impact speed with the ground is too high, the body can undergo violent fragmentation. In this case an impact crater will be evident and fragments will be scattered over a wide area.

An intermediate case involves rocky bodies with internal fracture lines, where resistance to breaking up is particularly low. In this case, they can partially break into fragments in the atmosphere and reach the ground as a shower of small 161

meteorites, over an area that sometimes reaches 10 000 square kilometres. An example was the event in Pultusk, Poland, in January 1868, which is estimated to have produced around 100 000 rocky fragments.

Since metallic meteorites have more inner cohesion, they are less subject to fragmentation, whereas it is a more frequent phenomenon for aerolites. In general, the region of the fall on the ground covers an elliptical shaped area (called a dispersion ellipse) with its semimajor axis directed along the meteorite’s line of motion. Sizes of the ellipses’ major axes vary considerably.

A typical length is around 10 km, but far higher values have been recorded.

The largest fragments are found in the front of the ellipse, while the smaller pieces tend to be at the opposite end. The reason for this asymmetry in the distribution of fragments can be explained by remembering the aerodynamic braking of the atmosphere. At the moment of fragmentation, the meteorite’s different parts are travelling at similar speeds, even though they are different in size. Atmospheric braking of a fragment is proportional to the square of its velocity, but inversely proportional to the fragment’s surface area. Bodies with larger surface areas are slowed less than smaller ones. The smaller a fragment is then, the more speed it loses and the more it tends to stay back compared to larger objects. This is why small fragments tend to fall to the back part of the ellipse of dispersion. It should be noted that not all ellipses of dispersion have this regular distribution of mass – in fact some are symmetrical. The largest masses are found in the back part of the ellipse, although it is not clear why.

Perhaps it is due to very different aerodynamic coefficients between their large and small fragments. Ellipses of dispersion, of course, do not exist on celestial bodies without an atmosphere, like the Moon or Mercury. They should be present on Venus and, to a lesser extent, on Mars. It is often noted that meteorites in the ellipse of dispersion are found in heaps; this is a sign that more than one fragmentation phenomenon affected the main body.

If the meteorite’s mass is average to small (under a tonne), the body falls on the ground (or rams deeply into it) at a speed of 350–700 km/h. Meteorites with mass greater than about one tonne retain part of their cosmic speed and reach the ground at much higher speeds. For speeds around 15 000 km/h the crash tends to shatter both the ground and the meteorite, creating a crater larger than the celestial body. In this case, there will be craters and not meteorites in the ellipse of dispersion, so that, at best, only residual fragments might be recovered. A fall of this type occurred at Sikhote–Alin in Siberia on 12 February 1947. 122 craters were found in the ellipse of dispersion, 17 with diameters between 10 and 26 metres. Meteorites with masses greater than 10 tons produce an explosive crater, like Meteor Crater in Arizona, which formed around 50 000 years ago. In another chapter, we will investigate impact craters and how meteorites that have fallen to the ground can be recognised.

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Image 16. Meteorites that fall in desert areas are generally easy to find. This chondrite weighs 1.75 kg and was found in Oman by a European expedition in 1993. (Euromet) Image 17. Image of some pisolites. Note their resemblance to the Allende chondrules. (Euromet)

Identifying meteorites is relatively easy only if someone witnesses the fall.

In all other cases the problem might not have an easy solution, at least without chemical analysis. However, without worrying about chemistry, any one of us can assess a ‘suspect’ rock. Externally, a meteorite can have all or some of the following characteristics: 1 Melted crust 2 Oriented shape 3 Bevelled edges 4 Flow lines 5 Regmaglypts 6 Fractures 7 Chondrules 8 Traces of Widmanstätten patterns or Neumann bands Characteristics (1 to 6) are quite common in all meteorites, while 7 and 8 are typical only of chondrites and siderites. The melted crust tends to disappear if the meteorite has been exposed to atmospheric agents for a long time, so its absence is not conclusive. Furthermore, the oriented shape is not apparent in all cases.

Chondrites can be confused with some sedimentary terrestrial rocks, such as sandstone, clay and limestone, which contain small spheres that are similar to chondrules, called oolites. Oolites (from the Greek word meaning ‘stone eggs’) are spherical granules with diameters of less than 2 mm. A covered nucleus can be distinguished inside an oolite. The nucleus can consist of a granule of varying nature and shape, while its covering generally has a layered structure or radial pattern. The covering’s layers can be calcareous, siliceous or ferrous.

163

Oolite is formed by the precipitation of calcium salts, silicon and iron in shallow waters and moved by the current. When these salts saturate water, the precipitation process begins on the condensation nuclei present (grains of sand) and oolites grow. The water current makes this growth both spherical and symmetrical. Once formed like this, the oolite falls to the bottom where it stays trapped in the rock that will later form. Oolites with diameters larger than 2 mm are called pisolites (from Greek, meaning ‘stone pears’). Pisolites can reach a diameter of several centimetres. Considering that sedimentary rocks are very common – they cover most of the sea bed and three quarters of the continental masses, it is not difficult to find a rock containing oolites.

Another important parameter to bear in mind is density. On average, a meteorite’s density is greater than Earth’s surface rocks, whose values are between 2.6 and 2.7 g/cm3. The density of siderites is similar to iron (8 g/cm3), while the density of aerolites is around 3.5 g/cm3. Only the density of the rare carbonaceous chondrites is comparable to common rocks: 2–2.5 g/cm3. Besides density, meteorites’ magnetic properties are different from most common rocks. A siderite can make a small magnet hanging on a thread divert from a vertical position (not to be confused with terrestrial magnetite, though!), but siderolites or aerolites also contain high quantities of iron and are also able to divert a suspended magnet.

Despite everything, if it is not possible to confidently establish whether a rock is a meteorite, the only thing to do is to ask a geochemistry lab for detailed analyses. The quantity of rock necessary for a complete analysis is just a few grams and there is no risk of being left without the meteorite.

The origin of meteorites As described in detail in chapter 6, bodies that follow orbits in the inner Solar System do not spend much time in this region, because they are expelled or fall onto a planet or the Sun. This clearing up process is relatively rapid, so it is necessary to identify another process that is still in operation and able to supply the inner Solar System with small bodies, including meteoroids. This is one of the most current problems under investigation, and only recently have possible solutions been put forward.

The main source is asteroids, both from the main belt and NEAs. The fact that they are subject to intense collisions results in them producing many fragments. It is clear that, by studying and comparing their physical properties (spectrum and composition), we can expect to find the class of asteroids that could be the origin for every class of meteorites.

The simplest association between meteorites and asteroids has been suggested for a long time: carbonaceous chondrites would originate from 164

type C asteroids; enstatite chondrites or rocky–metallic objects from S asteroids; M asteroids would essentially be made of metals and so would produce iron meteorites, and so on, for the least common types. It has been possible to establish, with a fair amount of certainty, that basaltic meteorites (about 5% of all meteorites found) originate from the family of asteroids that resulted from the partial destruction of asteroid 4 Vesta. With an average diameter of 530 km, Vesta is the third largest asteroid in the main belt. A small number of meteorites originating from the Moon and Mars should also be remembered. However, this scenario does not explain one important fact.

There is no obvious origin for ordinary chondrites, the most common type of meteorite. About 70% of meteorites are this type!

This riddle is still one of the most open and debated issues. Which objects can produce ordinary chondrites if only a very few are the same colour? One solution suggested is that ordinary chondrites originate from S type asteroids, the most common in the inner region of the asteroid belt. The fact that the meteorites are very dark and neutral, while the spectrum of S type asteroids shows a predominance of red, could be the result of so-called space weathering, probably due to the action of solar wind, cosmic rays and micrometeorite impacts, which tend to change the S asteroid surfaces and make them increasingly red as they get older.

Since small objects are probably fragments of larger bodies, we can also assume that their surfaces are younger. Recent observations show that the spectrum of small S type asteroids more closely resembles ordinary chondrites, compared with the spectrum of larger S type objects.

Other studies converge on the problem of including the processes responsible for transporting meteorites to Earth. These are substantially similar to those with NEOs (see chapter 6) but some important clarification is needed.

In particular, data exist that are difficult to explain, following measurements of chemical abundances. This allows us to go back to the duration of the object’s exposure to cosmic rays.

Imagine a meteoroid inside its parent body. If it is buried at enough depth (a few metres) it will receive a very low dose of cosmic rays. When an impact sets the meteoroid free, it starts to feel the effects of cosmic radiation. This process stops when the object reaches Earth and the bombardment of cosmic rays ceases. Like a clock whose hands stop moving, a meteorite can be read by researchers to determine how long the trip lasted from the parent body to our planet.

Once estimated, these times are generally of the order of 10 million years.

This is longer than typically needed to reach Earth from the main resonances present in the main asteroid belt. Other sources must then be called on, for example, small asteroids like Earth- or Mars-crossers. In this case, future 165

meteorites would not be deeply buried in the parent asteroid and would begin to accumulate the effects of cosmic rays well before the impact that sets them free occurs. This delicate theme still holds many mysteries and must be investigated more thoroughly.

The determination of orbits Accurately determining a meteoroid’s orbit before it impacts our planet and becomes a meteorite would be important to help understand its origin.

Unfortunately, this problem has several almost insurmountable problems.

Firstly, falls that are witnessed are rare and occur in places we cannot predict.

Additionally, the fall should be followed by suitable research that reveals the trail left by the object during its ablation phase in the atmosphere. As we mentioned earlier, cameras with rotating shutters are suitable. They must be placed in at least two different sites, in order to determine the object’s trajectory via a triangulation. Lastly, it is necessary to specify that not all meteorites that reach the ground produce a luminous trail: slow moving fragments with small mass that enter the atmosphere can do so completely invisibly.

Basically, there has to be a series of favourable conditions – summed up as being in the right place at the right time – including working equipment and being able to retrieve the fallen meteorite!

For this reason, there are very few calculated orbits. Today, 800 meteorite falls have been witnessed, but we only have accurate orbits for six of them. The first meteorite whose trail had also been photographed was found in Pribram, Czechoslovakia, in 1959. The fall occurred on 7 April 1959 at 19h 30m 20s UT, with a geocentric speed of 20.88 km/s. Four meteorites were found from this fall, with a total mass of 5.8 kg. Thanks to this success, different observation networks were activated to try to increase the finds, but results were discouraging. Only three other objects were studied – the meteorites known as Lost City (USA, 3 January 1970), Ohajala (India, 28 January 1976) and Innisfree (Canada, 5 February 1977). Resources were subsequently no longer invested in this direction and some networks were dismantled. Then, on 9 October 1992, a meteorite fell in Peekskill, USA, and it was possible to determine its orbit from the amateur films taken. The bolide associated with the fall was spectacular and disintegrated into about ten smaller bolides.

Recently the European Network, one of the few networks still active, succeeded in calculating the orbit of a meteorite that fell in Neuschwanstein (Germany) on 6 April 2002. (Its bolide was also visible from the north of Italy.)

Its orbital elements were identical to those of the Pribram meteorite. Since it is highly unlikely that two meteorites have the same orbit, this suggested a common origin. Nevertheless, the Pribram meteorite is an H5 type chondrite 166

while Neuschwanstein is an EL6 type chondrite. Their ages are also different, 12 million and 48 million years, respectively, so it is unlikely that they came from the same body., Among the NEAs, there are two objects with orbits similar to the Pribram meteorite: 4486 Mithra and 1998SJ70. Thanks to radar observations carried out in the spring of 2000 by the Arecibo and Goldstone antennas, it was possible to establish that Mithra has an elongated shape. This is typical of bodies formed by many fragments that are kept together by gravity and have a relatively high-speed rotation rate around their axis. Mithra was probably subject to fragmentation processes due to collision or tidal interaction with planets. Over time, this led to the dispersion in space of different types of fragments, resulting in a shower of bodies that occasionally fall on Earth.

Meteorites’ orbits will be discussed later in chapter 10.

Despite the lack of data, the similarity in bodies’ orbits is significant: they all have a trajectory that takes them to the asteroid belt. Another common factor is that they all underwent strong perturbation by Jupiter that significantly changed their original trajectory and eccentricity, before sending them to Earth.

Even though approximate, other orbits have been calculated by visual observers. Statistical interpretation of the data (the only option in this case) showed no correlation between the meteorites’ origin and the known meteor showers.

All of the data indicate that these objects’ orbits are prograde (they move in the same direction as the largest planets) with low inclinations and eccentricity, similar to those of many Earth-crossing asteroids.

Tektites Tektites (from Greek tektos, ‘melted’) are glassy looking objects measuring several centimetres across. A tektite’s typical mass is under 100 grams, but some can reach about 10 kg. Their density is between 2.2 and 2.8 g/cm3, while their hardness on the Mohs scale is between 6 and 7. By comparison, Earth’s quartz has a density of 2.6 g/cm3 and a hardness equal to 7 (diamonds are 10). The refraction index of tektites is between 1.48 and 1.52, while the predominant colour is black, although there are also greens, greys, browns and yellows.

Unlike meteorites, tektites are not uniformly distributed over Earth, but are found in well-defined geographical areas called ‘strewnfields’. All of the known strewnfields are between latitudes 50° North and 40° South. All tektites from the same field have several characteristics in common, including age, colour, size, etc.

Below are some types of tektites, grouped according to the four known strewnfields, and their associated craters: 167

• European

years):

strewnfield (Nördlinger Ries, Germany, age: about 14 million

• Moldavites • Australasian

(Czech Republic, green) strewnfield (no associated crater has been identified, age: about 800 000 years):

• Australites (Australia, dark, mostly black)

• Indochinites (South East Asia, dark, mostly black) • Chinites (China, black) • North American strewnfield (Chesapeake Bay impact crater, USA, age: about 35 million years): • Bediasites (USA, Texas, black) • Georgiaites (USA, Georgia, green) • Ivory Coast strewnfield (Lake Bosumtwi Crater, Ghana, age: about 1.1 million years): • Ivorites (Ivory Coast, black) The largest field covers Australasia, including most of Australia, the Philippines, the Indian Ocean, southern China and southeastern Asia.

The tektites we find in this very large field are the youngest. The ages of the tektites can be determined using isotopic analysis by measuring the K-40 and Ar-40 ratio, the potassium decaying products. Other smaller tektite fields are located in the Aouelloul impact crater in Mauritania (3 million years old) and around the Zhamanshin crater near Irgiz in Kazakistan (irgizites). In 1991, some tektites were found in Tibet, named Tibetanites, but they have not yet been completely studied.

Usually, the shape of tektites is aerodynamic, even though there are some twisted objects covered with protuberances, like the tektites from the craters of Aouelloul and Zhamanshin. The tektites of the large Australasian field are often rounded bodies shaped like spheres, discs, doughnuts, handlebars, drops, buttons and lenses, and without traces of inside deformation. The simplest way to explain these geometries is to suppose that tektites have solidified in the absence of weight. In these conditions, if the piece of melted glass does not rotate, the stable shape is the sphere, while if it is in rotation, a disc can originate. When rotation speed increases, the result is a doughnut, handlebar or drop shape (when the handlebar separates into two). The button and lens shapes can be produced by partially re-melting the spheres when they pass through the atmosphere.

Some tektites’ shapes have been reproduced in a laboratory by subjecting artificial glass spheres to jets of hot air. The first to understand the reason for the Australasian button geometry was German geologist A.W. Stelzner in 1893. Stelzner suggested that they had been made as partially melted spheres 168

and that the liquid had flowed from the front to the back of the tektite because of the airflow. The first to demonstrate this theory was Austrian Franz E. Suess, who, in 1900, built a wind tunnel in which he subjected tektite models to jets of hot air. The term ‘tektite’ was given by Suess.

Most tektites have no inside structure. In general, there are no inclusions, bubbles or crystals of any type – a characteristic that suggests 18. Some drop-shaped australites. These tektites rapid cooling. Tektites have a Image originate from Rizal in the Philippines. (Collection Jim chemical composition to some extent Hurley) intermediate between granite and terrestrial basalt. Granite is a volcanic rock that originates from the solidification of magma and is present in continental masses. Basalt is similar in origin to granite, but it has greater density and is found on ocean beds. However, in many ways, tektites are chemically similar to sedimentary rocks.

Like meteorites, tektites have been known about since prehistoric times.

But, while notable advances have been made in understanding the origin and evolution of meteorites, there is still considerable uncertainty about where tektites come from.

The first scientific study on tektites was conducted in 1844, thanks to Charles Darwin (1809–1882), the author of The Origin of Species. Darwin concluded that tektites were fragments of obsidian, a volcanic glass. However, although it is true that tektites resemble obsidian, their chemical composition is different from any lava and they contain water in far smaller quantities than common obsidian. Tektites cannot be a product of Earth’s volcanoes, because the ejection speed from a volcano (at the most around 2 km/s) would not be enough to explain the vastness of certain fields like Australasia, where speeds of around 6 km/s would be necessary. Why is the emission speed of volcanic rocks so low? Rocks cast from volcanoes pick up speed due to the expulsion of gases from the volcano. For gas coming out of a container (e.g. an active volcano) the maximum speed possible is twice the speed of sound, which is 1 km/s for a volcanic gas at a temperature of 1470 °C. The first to suggest tektites had an extraterrestrial origin was Victor Streich in 1893. Streich thought that tektites were a type of meteorite, but this is not possible because of the fields’ locations: meteorites are distributed uniformly over Earth’s surface. This characteristic of tektites’ geographical distribution poses some serious restrictions on their origin. We cannot blame some dark 169

asteroid from the main belt. The debate on tektites’ origin is still open, but a number of theories are currently being discussed.

The idea that tektites originate from the impact of small asteroids on Earth’s surface comes up against the problems of explaining the vastness of certain fields like the Australasian field, the fact that expulsion speeds of deposits from a crater are too low and the absence of any suitable impact craters. For the Australasian field to exist, a crater with a diameter of 300 km and depth of 40 km would have been necessary. It is unlikely that a structure like this would have been unnoticed for 750 000 years. With this theory, it would also be difficult to understand how tektites do not contain beads of air or water. The fact that tektites do not contain signs or traces of cosmic rays sets a limit of 900 years to the interval of time they spent in space. This means that their origin must be looked for within the Earth– Moon system. Some researchers think that tektites are ejecta coming from lunar craters, but this theory does not fit in with the composition of lunar soil. Almost all rocks and soil from the Moon are basaltic and do not have anything to do with the composition of tektites.

A theory that tektites could be derived from explosive eruptions of deep lunar volcanoes received discreet consent among researchers when it was proposed by John O’Keefe in the late 1970s. To have an ejection speed higher than the Moon’s escape velocity (2.5 km/s), O’Keefe supposed the existence of hydrogen volcanoes. In this way, the maximum possible speed of expulsion, 6 km/s, could be reached because the speed of sound for hydrogen (under hypothetical conditions) reaches 3 km/s. Most erupted material would escape the Moon’s gravitational field without falling back to the ground and this would explain how the lunar surface is not covered with tektite material.

Once expelled by the Moon, tektite clouds would not have the time to scatter and they would fall on limited areas of Earth’s surface. During the trip from the Moon to Earth, tektites would be subjected to a cooling process in the absence of gravity that would freeze them into a spherical shape, drops or handlebars (if the sphere rotates). Considering that tektites from the different fields are not homogeneous, it is necessary to accept that, in the last 35 million years, there have been six lunar volcanic eruptions of some importance.

Tektites associated with craters (Aouelloul and Zhamanshin) would be due to the impact of one large block of lunar volcanic glass, with a mass of a few million tonnes, that scattered irregular fragments over the surrounding area. In fact, these fields are limited to a size that is compatible with this theory.

Nevertheless, there is no evidence of the Moon’s geological activity in the recent past (TLPs are not enough to corroborate this theory) and nobody has yet seen a lunar hydrogen volcano. Further investigation can definitely clarify the tektites’ origin, even though the terrestrial origin of tektites is today accepted by many geochemical and isotopic studies.

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Image 19. Two fragments of Libyan desert glass. Their widths are 30 mm (left) and 35 mm (right). (Collection Jim Hurley)

Libyan desert silica glass The Egyptian–Libyan Desert is one of the most inhospitable regions on Earth.

In 1932, near the plateau of Gilf Kebir in southwest Egypt, an expedition led

by Patrick Clayton (1896–1962) recovered about 50 kg of yellowish silica glass that was quite transparent. It was the first discovery of Libyan desert silica glass (LDSG) had been discovered. Two more expeditions followed in the 1930s, but there were no others until 1971, where another 24 samples of Libyan glass were recovered. The region where the Libyan glass can be found covers an area that measures about 130 x 50 km, with the major axis approximately NNW by SSE, and the LDSG was found in sand free areas between dunes.

LDSG shows some distinctive characteristics including a low index of refraction (1.462), low density (2.2 g/cm3), high silicon content (98%) and high water content (0.064%). There is no presence of aerodynamic shapes, so they were probably formed in situ following a melting process of the sand. The meteoric elements contained in the Libyan glass have a similar relative abundance as chondrites, favouring an impact origin that must have taken place 28 million years ago. This is why the natural glass is called impactite.

Inclusion of particles of fused quartz also indicates that the Libyan glass must have been subjected to very high temperatures (>1700 °C).

Although impact by an extraterrestrial body can certainly result in these temperatures, there is still a problem in understanding how relatively homogeneous and free glass could have been created by intrusions during the brief moments immediately after the fall. The problem is still open to discussion.

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Chapter 6

Dangerous objects – Near-Earth Objects Introduction The acronym NEO stands for Near-Earth Objects, a population of bodies that orbit the Sun on trajectories that could potentially impact Earth. Some NEOs are also NEAs – Near-Earth Asteroids.

It is important to define exactly what objects we are talking about. In earlier chapters we saw how the population of small bodies is heterogeneous and that these objects are located in very different regions of the Solar System.

The locations of these objects – whether they are comets or asteroids – must not, however, be considered as being stable in time or space. These small bodies’ motion around the Sun is determined by well known and quantifiable physical laws, but, due to the role played by the perturbations of the major planets, they may move, sometimes considerably, from one region to another.

This is due to the interplay of purely physical and dynamical processes that are not easily predictable, but are highly significant.

One important physical process is that of mutual collisions. It was mentioned previously that, due to their large numbers, asteroids suffer mutual collisions which are able to cause complete shattering of the objects involved.

The destiny of the resulting fragments is quite varied. Some can fall back onto each other and roughly rebuild a sizeable fraction of the original parent body.

Others overcome their mutual gravitational attraction and escape on independent trajectories, thus starting life as single asteroids.

It is still possible to recognise the common origin of some groups of fragments, if their escape was not very violent. These are the dynamical families that have been identified in all regions of the main belt. The collisions and ensuing escape of fragments are generally not sufficient to cause objects to achieve orbits drastically different with respect to that of their parent body. In other words, the amount of kinetic energy achieved by the fragments is not sufficient to greatly alter their orbital motion around the Sun. Purely collisional processes alone, therefore, cannot normally remove an asteroid from the main belt and make it a Trojan asteroid, or put an asteroid on a cometary trajectory and make it a NEO. 173

In order to achieve more relevant orbital changes, it is necessary for dynamical processes to be added to the physical process of collision, so that the orbital parameters of freshly formed collisional fragments vary significantly. As we will see below, some other mechanisms also exist that may produce a steady orbital drift of small objects, even without the need for ancillary processes like collisions.

A key role is played by the concept of dynamical chaos, or chaotic motion.

We will see below how completely chaotic, unpredictable processes are, in fact, highly efficient, even though they operate over time scales much longer than those that affect our daily lives – millions or even hundreds of millions of years.

This fact, though, should not fool us into believing that there is only a very small chance of suffering direct or indirect consequences from events that take place over such a long time scale.

The number of bodies involved is such as to create and maintain a continuous flow of objects that move from stable to unstable regions. The population of bodies orbiting the Sun on unstable orbits includes many objects that in any given epoch may be dangerous for the terrestrial biosphere, namely bodies moving along orbits that may sooner or later cross Earth’s orbit, thus making an impact possible, though generally quite unlikely.

The origin of NEOs Fundamental processes of dynamical transfer We will describe later some non-gravitational mechanisms, like the so-called Yarkovsky effect, which is today thought to play a very important role in producing a steady orbital drift of small main belt asteroids towards the inner regions of the Solar System. Since this kind of orbital drift sooner or later causes the objects to cross some of the most important resonances with the major planets, primarily Jupiter, we will first focus on a description of the dynamical phenomena associated with these resonances.

In chapter 2 we described the mean-motion and secular resonances. We also saw how the main asteroid belt regions crossed by resonances have been nearly emptied.

It is worth outlining what would happen should an asteroid be inserted inside a fairly efficient resonance. Due to complex perturbation mechanisms that cannot be easily described without using mathematical details that are beyond the scope of this book, an object having a resonant orbit (like an asteroid in a 3:1 mean-motion resonance with Jupiter) starts immediately to have a completely chaotic orbital motion. As shown by modern numerical computations, the orbital eccentricity and inclination, which normally are fairly 174

stable and make only small periodic variations, tend to change very greatly and very quickly for such resonant orbits.

Orbital eccentricity, in particular, can reach very high values – even near the limit value of 1 – that would theoretically lead to the degeneration of the ellipse into a parabola. This would happen over periods of time that vary according to the resonance involved, although they should always be considered short (from a few to several tens of millions of years) compared with the lifetime of the Solar System.

During this ‘rapid’ variation of eccentricity, the orbital semimajor axis would not change very much. This means that the ellipse that describes the object’s motion would lengthen by moving the orbital perihelion increasingly closer to the Sun. During this transformation process, the asteroid captured by the resonance is forced to cross the orbits of planets that were previously always on the inside. It would therefore cross Mars’ orbit, then, in succession, the orbits of Earth, Venus, and even Mercury.

Image 1. This graph shows the number of NEOs over a certain diameter.

For example, there are more than 1000 objects larger than one km and around 135 000 larger than 100 m. Note that we can only be sure of having discovered all existing objects up to 5 km. Under this diameter, the graph is based on estimates and uncertainty increases as diameters get smaller.

The dotted lines on either side of the continuous line indicate the degree of uncertainty.

Image 2. A striking series of photos of Eros taken by the NEAR-Shoemaker spacecraft during its approach to the asteroid. (NASA/JPL)

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As this process goes on, the object’s fate is sealed. Upon reaching a certain limit value of eccentricity, the orbital perihelion will bring the object within the solar disc, causing the asteroid to impact our star. As described previously, it is believed that this process happens quite frequently: most resonant objects end their rapid orbital evolution by falling into the Sun. Despite its dramatic and tragic end, this process would not have much effect on the inner planets of the Solar System, though it would lead to the rapid disappearance of all objects that achieve a resonant orbit through processes such as collisions. In reality, however, things are more complex.

First, not all resonances that cross the main belt take the process of orbital transformation to the extreme limits. Although increasing eccentricity makes the perihelion approach the Sun, it also pushes the aphelion towards the most outer regions. If the acting resonance is located in the outer main belt, before the perihelion can penetrate inside the inner planets’ orbits, the aphelion will be pushed very near, or even beyond, Jupiter’s orbit.

As a consequence, sooner or later this object will go near the giant planet.

Perturbations that were already powerful in the original state can become completely unbearable during such close encounters. The asteroid will then suffer a drastic change in its orbit and its semimajor axis.

In other words, the object will be extracted from the resonance that trapped it and forced by Jupiter to follow a new, completely different trajectory.

Normally in these cases the asteroid tends to be expelled from the Solar System: another way to get rid of resonant objects.

This situation dominates with resonances whose distance from the Sun exceeds 2.8 AU. This means that it is also the most probable process for resonances like 5:2, 9:4 and 2:1, even if it does not exclude a small percentage of objects able to avoid Jupiter and survive for quite a long time in the inner planet region.

Nevertheless, even objects involved in resonances nearer the Sun (like secular resonance υ6 and mean motion resonance 3:1, see glossary for definition) that are quite protected from being captured by Jupiter, do not always follow the route that makes them fall on the Sun. Although much smaller than Jupiter, the inner planets can also produce significant perturbations on the motion of asteroids that cross inside their orbits. Obviously, given their lower mass, asteroids must go quite near the inner planets and have really close encounters in order to make drastic changes to the orbit – as we described with Jupiter.

In the case of the terrestrial planets, the most efficient at extracting asteroids from resonances are obviously Earth and Venus, because their masses are greater than those of Mars or Mercury. What happens then to an object that is made to leave a resonance following one or several close encounters? Its orbit 176

Image 3. In the diagram on the left, the thickest lines indicate the inner planets’ orbits (Mercury, Venus, Earth and Mars). The small broken lines indicate Jupiter’s orbit and its sphere of influence (these are practically invisible for the inner planets due to their relatively small sizes). The orbits shown with lighter dotted lines are those relative to an asteroid inserted in the 2:1 resonance and refer to increasing eccentricity (0 – 0.2 – 0.4 – 0.6 – 0.8). It can be noticed that for e=0.4, the asteroid is now near the region of Jupiter, while it is still quite far from Mars. This clarifies how most fragments inserted in the 2:1 resonance are not destined to become NEAs. The diagram on the right shows a similar case, but the asteroid is inserted in the 3:1 resonance. In this case, before reaching an eccentricity of 0.4, the asteroid has already become a Mars-crosser and will also cross Earth’s orbit with an eccentricity of around 0.6. It remains a long way from Jupiter. That is why the 3:1 resonance is considered to be an excellent NEA ‘reservoir’.

Image 4. Image 3 can also be explained more generally.

The vertical lines refer to the most important mean motion resonances. The almost horizontal lines show the value of the eccentricity necessary for an asteroid to bring its own aphelion inside the orbit of the various inner planets.

An asteroid that moves vertically along a resonance, increasing its eccentricity, will eventually cross the planets orbits and could increasingly become an ‘inner’ NEA. The dotted line refers to the eccentricity necessary to cross into Jupiter’s sphere of influence. Note again how the 4:1 and 3:1 resonances allow the object to cross all the inner planets’ orbits before reaching Jupiter, while the 2:1 resonance meets the giant planet first.

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drastically changes and often tends to reduce its semimajor axis. Under these conditions, the asteroid has entirely lost any memory of its initial orbit and becomes a provisional tenant of the inner region. In other words, it becomes an object representing an impact hazard for terrestrial planets – a Near-Earth Asteroid (NEA). No longer tied to a resonance, its orbit is far from being stable.

It is continually subjected to new and vigorous tugs caused by increasingly probable encounters with the inner planets that share the same space.

Evolution of the orbit continues to be highly chaotic and rather unpredictable over quite short time scales (hundreds of years). It is certainly a very complex evolution with a variety of possible end-states. Some asteroids may be inserted again into a main resonance and end up falling into the Sun, as described above. Others may survive longer by exploiting the existence of small resonances with the inner planets or they may be inserted into orbits that push them towards Jupiter, only to be expelled from the Solar System. Some may finish their days colliding with one of the terrestrial planets, especially the more massive Earth and Venus.

CLOSE ENCOUNTERS WITH PLANETS One of the most difficult epochs to calculate a NEO’s orbital evolution is when its motion is in proximity to one of the inner planets. In order to simulate what is called a close encounter, it is necessary to make some approximations that can lead to various degrees of uncertainty in the resulting calculations. This is also one of the reasons why it is practically impossible to predict for more than a hundred years the future trajectory of a NEO that undergoes more than one close encounter with a planet.

It is assumed that there is a close encounter between a NEO and a planet, when the NEO succeeds in entering the planet’s sphere of influence (or Hill sphere, 1878). This is normally defined as the planet-Sun distance multiplied by the cubic root of the ratio between the planet’s mass and three times the Sun’s mass. Obviously, the larger the planet, the larger is its sphere of influence. Once the NEO penetrates this sphere, the gravitational attraction of the Sun can no longer be considered as dominant. The system reduces itself to the NEO’s motion under the influence of the planet, or rather to a system of just two bodies. Before leaving the sphere, the NEO’s trajectory is similar to a hyperbole. During this phase, the NEO’s speed may drastically change and there are notable variations in its semimajor axis and eccentricity. The semimajor axis change can be sufficient to extract or lead the object into a certain resonance, just as it can push it more deeply into the inner planets’ region.

We should also note that a similar planetary flyby is frequently used in space missions.

To accelerate (decelerate) a probe directly into the outer (inner) regions of the Solar System and save propellant, the probe is forced to have one or more close encounters with a planet (Earth, Venus, Jupiter, etc.). By planning the trajectory and speed of arrival as accurately as possible, and applying also the necessary corrections to the motion after the flyby, the probe is directed towards its objective on the planned trajectory. This process is also called the slingshot effect or gravity assist.

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Image 5. The trajectory of asteroid A during a close encounter with planet P. Once the asteroid enters its sphere of influence S, its orbit changes dramatically. The trajectory inside the sphere of influence, shown by a dashed line, is close to a hyperbole.

Image 6. A graph showing the eccentricity/semimajor axis of the Themis dynamical family. The line on the right indicates the edge of the 2:1 resonance. As can be seen, the family has been completely cut off from it. This suggests how most fragments were inserted in the resonance and then expelled through complex dynamical processes.

Insertion into resonances Previously, we described the process of dynamical transfer that takes place when an object finds itself inside a main resonance. We also said that the time scale necessary to completely eliminate the asteroids residing in such resonances is relatively short, much less than the age of the Solar System.

It follows that all objects that might have been originally located inside the main resonances during their formation, in the early history of the Solar System, would have long ceased to exist by now, with one of the possible â&#x20AC;&#x2DC;deathsâ&#x20AC;&#x2122; as described previously. It follows that, not only should objects no longer exist inside these resonances, but, if we assume that injection into resonance is the only possible mechanism to move objects from the asteroid main belt to the inner Solar System, no asteroids (NEAs) should now exist that have orbits within the Solar Systemâ&#x20AC;&#x2122;s inner regions. The situation that is observed, however, is obviously quite different. This means that, either there is some mechanism that steadily injects main belt asteroids into resonances, or that other mechanisms of orbital evolution exist and must be taken into account to explain the existence of the NEA objects we see today. As we will see, both options are true.

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Let us focus first on the role of collisions, the first ‘classical’ mechanism historically investigated by the researchers in this field. At the beginning of this chapter, we mentioned that collisions – even highly violent ones – are not able to force fragments to achieve orbits very different to that of their parent body.

Ejection speeds only allow for moderate changes in the orbital parameters, not enough to move an object from the main belt into the region of the inner planets. Nevertheless, if the destroyed parent body was near enough to an important resonance, it is entirely reasonable that a certain number of fragments could be inserted inside this resonance. This is one possible process for restocking the NEA population. As discussed in previous chapters, collisions between asteroids are still an active phenomenon and can produce a roughly constant flow of new fragments that are pushed to some resonance, always maintaining the population of potentially hazardous asteroids. This is confirmed by analysing the structure of some of the largest asteroid families.

Some of these are perfectly bordered by one resonance or in some cases both their edges fit them perfectly. This shows that fragments expelled by families have entered the resonances and that they, in turn, have swept away the fragments over relatively short times.

Before going on, it should be remembered that, in addition to the above mentioned collisional mechanism, the Yarkovsky effect, consisting of a steady radial force acting on an object’s orbit due to the thermal emission from its surface, can also produce a drift in semimajor axis which can force an object to reach a resonant orbit, sooner or later. This mechanism, which is currently thought by most researchers to play a role even more important than that of collisions, will be described later.

Other possible sources of NEOs Asteroids are not the only candidates to be the sources of NEOs. In fact, asteroidal objects are not the only existing bodies able to closely approach the terrestrial planets. Comets must also be considered. Here, we only briefly outline the contribution that comets can make to the population of Earthapproaching objects.

A comet that moves from the Oort cloud or Edgeworth–Kuiper belt to the proximity of the Sun is generally experiencing a rather complex dynamical evolution. In this respect, it is especially important to distinguish between long period comets and medium or short period comets. The former mostly originate directly from the Oort cloud. They are perturbed in their orbital motion around the Sun by the passage of some nearby star and/or some massive molecular cloud, as well as by the tidal forces produced by the galactic gravitational field. These perturbations are able to tear a certain number of 180

ASTEROID SHOWERS The creation of an asteroid family is a rather infrequent collisional phenomenon in the history of the Solar System. The fact that fragments originating from an impact can still be easily identified as dense groups of objects, even after several hundreds of millions of years, means that the destructive event involved a ‘target’ of quite a significant size.

However, this can certainly not happen very frequently. On average, the families that can be observed today were produced by parent bodies that must have been larger than several tens of kilometres. In some cases they were well over 100 km, for example, the parents of the Themis, Eunomia and Vesta families. Under these conditions, the time that elapsed between the creation of two observable families should be around hundreds of millions of years. So they are not frequent events, although exceptionally important for the production of NEAs. Some of the largest families are located quite near the most efficient resonances.

This implies that many fragments that originated from the formation of the family were located inside a resonance. This is one of the basic processes for the possible formation and maintenance of a steady flow of small NEAs. With some families, the numbers of involved fragments may be really impressive. They may have produced huge quantities of fragments that started a chaotic trip towards the inner planet region, leading to large, temporary increases in the number of NEAs. Calculations carried out using models that simulate these processes result in impressive values. For example, the Eunomia family may have produced several thousand NEAs around one kilometre in size over a period of less than 10 million years.

This family alone may have inserted a number of NEAs into the resonances several times more than the average value that can be observed today (almost 5000 objects). Similarly, the Eos family may have injected more than 30 000 fragments of 1 km size into the 9:4 mean-motion resonance with Jupiter, and the Themis family over 100 000 objects into the 2:1 resonance.

Even if the vast majority of these objects were probably ejected from the Solar System, at least a small fraction of them may have impacted Earth in the past. These considerations result in the following overall picture: on average, the number of observable NEAs is rather constant, but sometimes, probably at intervals of several tens of millions of years, the creation of a new family suddenly lifts this number to values of ten or even one hundred times greater.

The consequence is that during these ‘hot’ periods the inner planets, including Earth, see their probability of being impacted by a NEA increase by around tens or hundreds of times. For a period of some millions of years, terrestrial planets would literally be experiencing a shower of NEAs, with an impact threat much higher than normal. Then everything will go back to normal, until a new large family is created.

The past history of the inner planets may have been deeply affected, both geologically as well as biologically, by these episodic and violent events. These same mechanisms may have caused biological mass extinction events. Fortunately, we live in a period that is distant enough from the creation of the last big asteroid family…!

comets from their stable orbital motion and inject them into extremely long – practically parabolic – orbits, with an orbital eccentricity very near 1.

Under these conditions, and without further sources of disturbance, comets can approach very close to the Sun, thus crossing inner planets’ orbits.

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Nevertheless, after approaching our star at perihelion, they go back to the outer Solar System, with a high probability of leaving it and getting lost in interstellar space. They are, therefore, only fleeting apparitions and the risk that they could impact a planet during their travel is extremely low.

Some of them, however, could transit not too far from the giant planets, especially Jupiter. They may then be highly perturbed and can – as we saw with asteroids – undergo drastic orbital changes and move into less elongated elliptical orbits. At this point, they would become stable tenants of the inner Solar System and make frequent trips near the Sun, like, for example, the famous comet Halley. The more times a comet transits, the greater is the chance that it may have close encounters with a planet. Consequently the evolution could be very complex, to the extent of not differing from that illustrated for asteroids.

An evolution of this type is certainly more predictable for comets that come from the Edgeworth–Kuiper belt. This can basically be considered as an external asteroid belt. The bodies within it can mutually collide and be inserted into medium-motion resonances with the outer planets, especially Neptune.

Similar to what happens with main belt asteroids, comets from the Edgeworth–Kuiper belt undergo very significant orbital variations once inserted in a resonance. They cannot lead directly to becoming stable tenants of the inner Solar System, but they can provoke transfer processes towards the giant planets, and lead to them being captured by Jupiter and then inserted into orbits that can cross those of the terrestrial planets. More time is needed, but the result is still the one described previously.

Image 7. The series of images that led to the first discovery of an object in the EdgeworthKuiper belt. Note how this object, given its distance, has a very slow motion relative to the stars in the background. The trails left by main belt asteroids are quite different, as shown in the first three images. (ESO)

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Near-Earth Asteroids: a nearly constant population Restocking problems We have explained some possible mechanisms for injection of minor bodies into the inner Solar System. The major problem is to explain the maintenance of a population that appears to have been mostly constant over long time scales (based on the cratering records of the inner planets’ surfaces). As mentioned above, mutual collisions between asteroids (like the transfer of comets) are processes that are still active in the asteroid belt and may well constitute one possible mechanism to produce a continuous supply of NEAs. Everything makes sense qualitatively. But the key question is: does this restocking mechanism also have the correct quantitative characteristics? In other words: can collisional-based restocking completely replace the objects that constantly die during the processes described previously?

In order to answer adequately, it is necessary to consider: • The typical dynamical lifetime of NEAs; • The time necessary to transfer objects from the main asteroid belt to the region of the terrestrial planets; • The time that lapses between two successive collisions capable of inserting fragments into the resonances, if we assume only the collisional mechanisms described above. Alternatively, some other suitable mechanism must be found that may cause injection into resonances.

The above time scales should, in principle, be comparable in order to guarantee the balance required. Their typical values are around several million years, which may satisfy the requested characteristics, when we talk of objects smaller than one kilometre.

While the first two factors do not change (as they are independent of the sizes), the creation of large fragments through catastrophic destruction requires much longer periods of time. Clearly, in order to produce fragments of increasing diameter, it is necessary to shatter increasingly large asteroids. But the probability of collisionally disrupting a body decreases with the increasing size of the target asteroid, since increasingly larger projectiles become necessary.

Decreasing the probability of disruption obviously increases the time that lapses between two sufficiently destructive collisions. As this time is longer than necessary to move an object from the main belt into the inner Solar System, and longer than the subsequent dynamical lifetime of the NEAs, it follows that continuous restocking of NEAs larger than one kilometre could not be ensured only by purely collisional processes. Every now and then, for 183

example every ten million or hundred million years, this would produce one or more fragments injected into some important resonance, but these would be quickly turned into NEAs and destroyed in a few million years. As a consequence, for most of the time, there would be no kilometre-sized NEAs. The reality, however, is quite different. Direct observations of the cratering records on the Moon and Mercury, for instance, show that the flow of impactors larger than one kilometre has been nearly constant for a few billion years. We are, therefore, forced to conclude that the pure collision > destruction > resonance insertion process is not sufficient to explain the current and past NEA population.

Even more perplexing is the fact that large objects, over 4–5 km across, are known to exist in the NEA population in quite significant numbers. How could such large asteroids be produced by collisions? This is a real problem, in that it becomes necessary to shatter parent bodies several tens of kilometres in diameter, near a resonance. This is possible in principle, but over very long time scales (several hundreds of millions of years). Moreover, such events would also leave visible signs for even longer, in the form of asteroid families.

If large NEAs were produced in this way, we should see a much larger number of dynamical families than currently observed.

Moreover, there are at least two large objects among NEAs (around 20–30 km in size) that can hardly have been created by any known event of family formation. Neither can we think that the family that created them can no longer be seen. A long time must pass after the creation of a family before successive collisions of the fragments and erosion due to micro-impacts and other non-gravitational processes tend to make a family unable to be identified or too widely separated to be identified. But if this happens, the time scale for disappearance is around several hundred million years.

The question that we have to ask then is simple: where and how do NEAs larger than one kilometre originate? Since the purely collisional mechanism cannot reasonably explain their observed abundance and size distribution, other mechanisms must be identified.

At this point, another argument against the collisional mechanism must be briefly mentioned. Meteorites have been extensively analysed in order to extract any possible information about their composition, age, origin, and history. Among the several pieces of information that have been obtained in this way, there is the determination of the so-called cosmic ray exposure ages.

These are the intervals of time during which these bodies have been exposed to the implantation of ions and other particles present in the solar wind. In other words, the cosmic ray exposure ages tell us how long these bodies have existed as independent bodies orbiting the Sun, with their surfaces exposed to irradiation by the solar wind.

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What seems surprising is that these cosmic ray exposure ages turn out to be, on average, much longer than the predicted lifetime of objects created as fragments from some parent body in the asteroid main belt, directly injected into some resonant orbit, and quickly moved to the region of the terrestrial planets. The problem in this respect is that the resonances are so powerful and produce an orbital evolution so fast that it is hard to reconcile it with the significantly longer exposure ages of the meteorites. Therefore, it seems that we are missing some important piece in our puzzle. The orbital evolution that removes bodies from the asteroid main belt seems significantly slower than what we should expect, assuming that the objects reach the inner Solar System after having been injected into a resonance immediately after their creation. In the next sections, we will describe some mechanisms that can overcome this apparently crucial problem.

An additional resource: Mars-crossers Near the inner border of the main asteroid belt, there is a considerable number of objects whose orbits at perihelion approach the orbit of Mars.

Usually called Mars-crossers, these objects are not in a strongly chaotic dynamical state, as they are well out of the main resonances and appear to have a small chance of effectively interacting with the red planet. In fact, Mars has long been considered too small to seriously perturb the orbits of asteroids that go near it.

It has long been known that Mars-crossers cannot be entirely stable objects, but it was supposed that the time scales of their dynamical evolution were very long – not much shorter than the age of the Solar System. A few years ago, however, accurate calculations were made on the orbital evolution of a certain sample of Mars-crossers and the results were surprising. It turned out that, due to numerous close encounters with the red planet, most objects underwent small orbital changes. In particular, on much shorter time scales than previously expected, sooner or later the objects ended up by entering a resonance in the inner region of the main belt – especially the υ6 and 3:1 resonances, which are most effective in transporting objects to the Solar System’s inner regions. The average time for a Mars-crosser to end up in one of these resonances (after which it became a NEA) was just 20–25 million years. This is much shorter than previously expected.

Nevertheless, if we compare this with the times needed for direct injection of fragments into a resonance following a catastrophic collision, this process is a great deal slower. Mars-crossers, then, may represent a very populous source of potential NEAs that is emptied slowly and is able to constantly re-stock the population of NEAs with sizes over a few kilometres.

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How to re-stock Mars-crossers The dynamical evolution of Mars-crossers is not very slow, and it is estimated that the current population would begin to decline in around 200 million years from now, in the absence of a continuous supply of objects from other regions.

This leads, again, to the same problem that we had already faced previously: how can this resource be re-stocked? Again, computations indicate that collisions between main belt asteroids cannot move fragments from the main belt to the Mars-crosser region.

Some new and efficient dynamical mechanism of transport is needed here.

In particular, we need a mechanism that should be sufficiently effective to be able to steadily remove from the main belt a sufficient number of objects to justify the currently observed population of NEOs, and at the same time sufficiently slow as to reconcile the dynamical time scales of the transport with the measured cosmic ray exposure ages of the meteorites. This transport mechanism is the so-called ‘Yarkovsky effect’.

The Yarkovsky effect consists of a steady force acting on the object’s motion, due to the pressure of the thermal radiation emitted by the surface. It is a complicated phenomenon which depends on many physical parameters of the body, including the spin period, the orientation of its spin axis, the thermal inertia of the surface, the sense of the rotation, and the albedo. A complete and analytical theory of the Yarkovsky force has not yet been developed, due to the

Image 8. The eccentricity/ semimajor axis of all known asteroids within 2.5 AU. The transverse line borders the area occupied by Mars-crossers. The asteroids above the line have probably already begun their journey to becoming NEAs.

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fact that so many parameters are involved, and some of them, such as the thermal inertia, are still poorly known for asteroids. What is certain is that the Yarkovsky force produces either an acceleration or a deceleration of the object along its orbit, and this in turn produces a steady drift in the semimajor axis. The rate of orbital drift is roughly inversely proportional to the objectâ&#x20AC;&#x2122;s diameter.

This phenomenon moves asteroids as large as 10Â km in size, and may thus explain the existence of the largest NEAs. The orbital migration is fairly slow and subject to changes, and even inversion, if the object suffers a collision that alters its rotational state. However, sooner or later an object may be forced to achieve an orbital semimajor axis corresponding to some resonant state, leading to a possible increase in orbital eccentricity and subsequent injection into a Mars-crossing orbit. At that point, the object will follow the kind of orbital evolution involving the processes described earlier.

The complete picture of origin and maintenance We now have a comprehensive picture of the variety of phenomena that can remove an asteroid from the main belt and turn it into a NEA. A critical role is played by the resonances. In some cases, collisional fragments can be injected directly into some resonant state, triggering a fast dynamical evolution towards the NEO region. However, in most cases the evolution is much slower, being driven by a Yarkovsky orbital drift and subsequent trapping into a resonance that produces either a Mars-crossing orbit or faster transport to the region of the terrestrial planets.

The most powerful resonances may lead most objects to collide with the Sun over short time scales (a few millions of years) or to be ejected from the Solar System after some close encounter with Jupiter. The NEOs that do not follow these evolutionary paths may spend longer times in the region of the terrestrial planets, following a chaotic orbital evolution that can also lead them to collide with one of the inner planets, including our Earth. The corresponding times for the dynamical evolution, particularly when considering the Yarkovsky drift, are fully compatible with the cosmic ray exposure ages of the meteorites.

The current NEO population Where are they?

We have seen that all objects with orbits that potentially threaten to impact Earth are considered to be NEOs (Near-Earth Objects). Excluding comets and concentrating on asteroid objects and NEAs (Near-Earth Asteroids), some important distinctions can be made.

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THE YARKOVSKY EFFECT Since the discovery of the first small bodies between the orbits of Mars and Jupiter, it has been assumed that the force of gravity and collisions were the only phenomena responsible for the evolution of asteroids. It has, however, been recently understood that other forces can play an important role, e.g. the Yarkovsky effect. Ivan Osipovich Yarkovsky (1844-1902) was a Russian civil engineer who, around 1900, first proposed the effect that carries his name.

In 1909, Estonian astronomer E. J. Öpik read Yarkovsky’s work and redrafted it in a 1951 article discussing its effects on the motion of meteoroids in the Solar System. The Yarkovsky effect is a non-gravitational force originating from the radiation pressure produced by the thermal emission of the object’s surface, which is heated by the Sun. The force seems to be able to act on large asteroids that are several kilometres in size. Other non-gravitational effects, like radiation pressure, solar wind and the Poynting-Robertson effect, only influence particles under a centimetre in size and are therefore of little importance to the NEO problem.

The Yarkovsky force depends essentially on the fact that a planetary body is heated by solar radiation and reissues radiation in the form of photons. If asteroids did not rotate around their own axis, the hottest region would always be the one with the sun at ‘midday’.

However, the rotation of the object, coupled with the fact that the surface re-emits the heat after some delay, produces a thermal emission that has a component in the direction of the object’s orbital motion. The emitted photons produce a ‘rocket effect’ as they push the object in the opposite direction - the Yarkovsky effect is the photon equivalent of the Whipple rocket effect (1950) theorised for gases issued by cometary nuclei. A body in direct orbit around the Sun, with its rotation axis orthogonal to the orbital plane and an anticlockwise (or direct) rotation, receives a push that tends to increase the orbital radius, whereas, if the rotation is clockwise (or retrograde), the orbital radius tends to decrease.

Given this particular geometry, the continuous - albeit minimal - imparted push proceeds for a long time and tends to increase or decrease the value of the semimajor axis, depending on the object’s direction of rotation. The strength and time scale of this variation depend essentially on the size of the body. As the intensity of the Yarkovsky effect is directly proportional to the difference in temperature ΔT between day and night hemispheres and this increases as the body’s diameter increases, larger bodies undergo a greater effect.

Although planets like Earth have such large masses that the Yarkovsky acceleration is not noticeable, the perturbation is not negligible for bodies between a few centimetres and a few kilometres. There is also a ‘seasonal’ Yarkovsky effect. It is connected to the fact that the object rotates around an axis that is generally tilted in relation to the orbital plane. In this case, there will also be a hotter region corresponding to our autumn and once again a rocket effect in the opposite direction. As a consequence, the result is a decrease in the orbital semimajor axis. The seasonal Yarkovsky effect disappears if the rotation axis is orthogonal to the orbital plane. Together, the two effects can cause variations in the orbit’s semimajor axis of some hundredths of an AU on time scales that range from 10 million to 1000 million years for objects between 1 and 10 km. These are small variations, although sometimes big enough to insert an asteroid into planetary resonances and direct them towards the Mars-crosser region.

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If we look at the current population without worrying about how the single objects can evolve in the future, we may conclude that NEAs are a subset of a larger group of objects with the characteristics of crossing at least one of the terrestrial planets’ orbits (from Mars to Mercury). We may call them Inner Planet Crossers (IPCs).

Mars-crossers are part of this larger group, because, as we have seen, they can push themselves more deeply inside the red planet’s orbit. The importance of some particular classes of IPCs is usually emphasised in the literature. These are known as Aten, Apollo and Amor objects (AAAs).

As mentioned in chapter 2, the Amor class is the furthest out and is made up of objects with a perihelion (shortest distance from the Sun) between 1.017 and 1.3 AU. The value 1.017 is not introduced by chance. It represents Earth’s aphelion, the maximum distance from the Sun that Earth can reach. Amors are defined as bodies that always remain outside Earth’s orbit, so they cannot encounter it. On the other hand, the value 1.3 AU has been selected almost entirely arbitrarily. In fact, Amors could easily be considered as the innermost Mars-crossers.

Image 9. The 305 m diameter radio telescope in Arecibo, Puerto Rico, is the largest in the world.

It was created by covering a hollow in the limestone terrain with reflective panels. (NAIC)

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The Apollo group has a semimajor axis larger than 1 AU that exceeds Earth’s, but the perihelion distance is smaller than 1.017. In other words, Apollos cross Earth’s orbit, maintaining revolution periods around the Sun longer than one year.

Finally, Atens are objects whose semimajor axes are smaller than 1 AU (or their period is less than 1 year and they are generally interior to Earth’s orbit), but their aphelion is over 0.983 AU (larger than Earth’s perihelion). This means that Atens may cross Earth’s orbit, but with the difference of having smaller revolution periods than Earth.

Another class of IPC includes objects with orbits completely inside Earth’s orbit, although only a few of them have been discovered so far. They are defined as objects with an aphelion smaller than 0.983 AU. They are generally called Inner Earth Objects (IEOs). These objects cannot impact Earth at present, since their orbits are always nearer to the Sun, but they must be considered as potentially dangerous due to the fact that they can become Earth-crossers over short time scales.

As mentioned above, all IPCs’ orbits evolve in time. AAA and IEO objects move entirely on chaotic orbits and may jump from one class to another in very short times. Moreover, some objects can get into very long, comet-like orbits, while some real comets may achieve orbital characteristics not dissimilar from asteroids. With comets, some can still maintain their more evident physical characteristics (sublimation of the nucleus material with consequent formation of coma and tail), while others, particularly old ones, may become extinct and entirely indistinguishable in their trajectories and physical characteristics from objects of asteroidal origin.

This continuous interchange between objects of very different origin means that distinction between classes is purely conventional. They are valid for a given epoch, but can vary entirely over short time scales. That is why it is preferable to speak about NEAs or NEOs, independent of their current classification.

At this point a fundamental concept about NEOs is clear. This is essential in order to best understand all the issues relating to the risks of impact and their forecast.

NEO orbits are essentially chaotic and essentially unpredictable over long time scales (beyond a hundred years). So for longer periods, it is only possible to make an estimate of a single object’s overall probability of impacting a planet. Accurate and detailed predictions are not possible.

How many are there?

Today there are almost 5000 known objects that can be classified in the three groups defined earlier (Apollo–Amor–Aten), but their number is continuously 190

growing as there are many telescopes dedicated to their discovery and many others will enter operation shortly. Nevertheless, it is important to correctly interpret all these new discoveries before establishing the absolute number of NEOs, especially in their various size classes.

It seems likely that all existing objects with diameters over 6–10 km have now been discovered. Luckily, there are not many: around ten. The largest, Ganymede, is 38 km across and the most famous is 22 km Eros (see chapter 2).

Many factors limit our ability to discover new NEOs of smaller sizes. The discovery probability depends critically on the apparent brightness of the objects. In turn, the apparent brightness depends on the size, the surface reflectance (albedo) and distance of the object from both Earth and the Sun. The orbital properties are also important. An object can only be observed under favourable conditions when it is sufficiently bright and visible from Earth. This usually happens for limited periods of time, and each object has its own detection windows. A tiny object tens of metres across may be discovered because it passes between Earth and the Moon, whereas an object of few kilometres in size might still escape our notice, as its orbit has not yet brought it into favourable observing conditions.

The most challenging objects, in this respect, are the IEOs, since they never reach large solar elongations, and cannot be observed high in the night sky.

Image 10. Following extremely accurate radar observations, it has been possible to model the shape of asteroid Castalia. The image shows c o m p u t e r- g e n e r a t e d reconstructions for single (A) as well as double models (B).

(S. Ostro, JPL)

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The same is true for the Atens, apart from the epochs when they are close to aphelion, and they are external to Earth’s orbit. All these factors, together with many others on which we cannot dwell, make an accurate estimate of the existing NEO population difficult. Current estimates, however, list about twenty objects over 5 km, about 1100 larger than 1 km and 150 000 at least 100 m in diameter. These values obviously become more inaccurate as we consider increasingly smaller objects. Many selection effects can also favour or disfavour some groups compared to others.

Another problem is the inclination of the orbits. The greater the inclination, the further away the object will be from Earth’s orbital plane. As observations normally occur near this plane, it is easy to miss objects with orbits that are too tilted. Their number might then be higher than current estimates.

Plans for the near future include close coordination between observatories all over the world. Within about ten years, this should allow us to identify all existing NEOs over a kilometre in size. Obviously, the number of discoveries of smaller objects will also grow enormously. Discovery of Atens and IEOs will, however, be still difficult and there are some ideas to build an orbiting telescope in order to eliminate the problem of their detection close to the Sun. What are they made of?

Due to their origins and travels, NEOs represent a real melting pot of objects, made up of asteroids, and some comets, that originate from different parts of the Solar System. Beyond the issue of impact risk , it is easy to understand the enormous scientific importance that this population of planetary bodies represents. Studying them with the most modern observational technologies equates to reading a summary of the characteristics of most of the asteroid belt. NEAs belong to almost all the main taxonomic classes present in the main belt. There are S-type objects that are typical of the innermost regions of the main belt, but also C and D-type objects, whose main belt counterparts are preferentially located in the outer region and, in some cases, they may even be extinct cometary nuclei. This mixture of taxonomic classes provides valid confirmation of the NEO origins that we described previously.

There are some extremely interesting cases that have helped us locate the exact place of origin of certain distinctive types of NEA. The best example is certainly NEAs belonging to the rare taxonomic V class. Up until a few years ago this class had only one member – Vesta. With its 500 km diameter, Vesta is the third largest of the known asteroids.

Observations from Earth deduce that the surface composition of Vesta is very similar to basaltic lava, implying a complex thermal evolution experienced by this body. In particular, it is believed that Vesta melted and differentiated a 192

very long time ago, leading to the formation of a metallic core and a basaltic surface. These particular properties make Vesta an object of great interest and clearly differentiate it from nearby asteroids, both in size and orbital properties.

Consequently, it is easy to understand how a piece of Vesta’s surface could be recognised, even well away from its place of origin. Vesta has, however, another characteristic – it is the largest member of a populous family associated with it. The family was certainly created following an almost catastrophic impact that caused an enormous crater on the asteroid’s surface and splintered everything around into a myriad of small fragments. These fragments were less than a few kilometres in size and were the subject of an observation campaign a few years ago that was aimed at defining its taxonomic contents. If the results had been V type, it would have been almost mathematically certain that they were pieces of Vesta expelled during its violent cratering event. And in fact, that is what they were. In the meantime, a small, close group of type V objects has been discovered among NEAs, with very similar spectral properties to the fragments from Vesta. Further research made it possible to discover pieces of Vesta that were very near the edges of the 3:1 and ν6 resonances that are the best ways to become NEAs. The likely process is as follows: the crater created a family and some fragments were progressively inserted inside the 3:1 and ν6 resonances by means of Yarkovsky drift. After achieving a resonant orbit, the objects were quickly moved to the inner planet region, thus turning them into NEAs. This is a fantastic test of the NEA-main belt connection, facilitated by the peculiarity of Vesta’s composition.

Could some of these very small fragments have struck our planet? The answer is ‘yes’. In fact, a quite rare type of meteorite known as a eucrite has practically identical characteristics to Vesta, its family members and V-type NEAs. This connection gives us a unique opportunity to theoretically study a piece of an asteroid around 200 million kilometres away.

Image 11. A radar image of Geographos (left), one of the most famous NEAs. Note the very elongated shape, typical of a fragment produced during a catastrophic collision. The model of Geographos (right) is based on the radar image.

(S. Hudson, Washington State University)

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Image 12. The image shows asteroid 4 Vesta taken with the Hubble Space Telescope (top left), a computer reconstructed model (top right) and an elevation model (centre). This latter image clearly shows the gigantic crater, over 400 km in diameter, with a central peak that is around 13 km high.

(B. Zellner, University of Georgia, P. Thomas, University Cornell, NASA)

Image 13. A cylindrical projection map of asteroid 4 Vesta. (NASA/ STScI)

Image 14. A eucrite type meteorite, which shows a spectral similarity with the asteroid Vesta.

Combined with the rarity of this spectral type, this indicates that the meteorite is probably a piece of Vesta that landed on Earth via complex dynamical processes of transport. (R. Kempton, New England Meteoritical Services)

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POTENTIALLY DANGEROUS OBJECTS: POTENTIALLY HAZARDOUS ASTEROIDS (PHAs) A new class, named Potentially Hazardous Asteroids (PHAs) has recently been added to NEAs. As we already mentioned, all NEAs are potentially dangerous objects on time scales of some millions of years because of their chaotic dynamical evolution. However, these objects were selected based on their current orbital characteristics. Their definition is essentially phenomenological, and for the near future they represent the population with the highest risk of causing a sizeable impact with Earth. PHAs are defined as the NEAs whose orbits approach within 0.05 AU of Earth’s orbit, and which are more brilliant than 22nd magnitude.

This means that PHAs are able (at least potentially) to move nearer to Earth than 7 479 900 km and exceed 150 m in size. At the end of 2007 there were about 900 PHAs. The smallest distance between two orbits is known as the Minimum Orbit Intersection Distance (MOID) and today it is considered the most important parameter adopted by most researchers in this field.

All known close encounters of PHAs have been calculated until the year 2100. Approximations and dynamical problems, however, still exist. A large source of uncertainty, in particular, comes from our currently limited knowledge of the Yarkovsky orbital drift experienced by the bodies. Close encounters with PHAs expected until the year 2037 are shown in the appendix.

Vesta is the most striking example, but other possible bonds between families and NEAs have been discovered recently, for example, the links between the Eos family and type K fragments, or Nysa with type F fragments.

Near-Earth Asteroids as a potential economic resource Apart from the danger that NEAs represent to Earth, they also have different properties that would make their exploration desirable. From a scientific point of view, they are a set of bodies of varying nature and origin, representing a collection of objects that are indispensable in understanding the genesis and dynamics of the Solar System. From a practical point of view, NEAs are the most easily reachable bodies within the entire Solar System. In fact, some of these bodies occasionally move inside the Earth–Moon system, and, as they have a small mass, an exploration mission directed towards them (by landing on the surface) is easier than a lunar mission like Apollo 11.

From an economic point of view, NEAs are sources of raw material just waiting to be exploited. 30–60% of NEAs may be nuclei of old, extinguished comets with large quantities of water under their surface crust, while others include metallic asteroids with notable abundances of essential elements like iron, nickel, cobalt and platinum. These two types of NEA can supply the necessary materials for the human conquest of the Solar System on a large scale.

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Materials extracted from asteroids can be sent to Earth and put into a low orbit around our planet by exploiting the aerobraking effect in the high atmosphere. In this way, a heliocentric orbit can be made geocentric without using large quantities of propellant. Once in orbit, materials can be processed at the International Space Station. Working in orbit has the great advantage of eliminating the costs of launching from Earth, thus making a large number of space activities economically viable, which currently they are not. It will be possible to build larger space stations, organise interplanetary missions and build human colonies on the Moon and Mars.

So although the presence of NEAs is a risk, there are also some positive aspects. It all depends on exploiting them for our future aims.

Table 1

risk/dynamical classification of near-earth asteroids Group

Description

Definition

NEA

Potential impactors

q < 1.3 AU

Aten

Objects that cross Earth’s orbit with a semiminor axis less than Earth’s

a < 1.0 AU Q > 0.983 AU

Apollo

Objects that cross Earth’s orbit with a semimajor axis greater than Earth’s

a > 1.0 AU q < 1.017 AU

Amor

Objects that draw near Earth’s orbit and cross over Mars’ orbit

a > 1.0 AU q > 1.017 AU q < 1.3 AU

PHA

Potentially dangerous objects

MOID < 0.05 AU H < 22 Mag

a: semimajor axis of the orbit q: perihelion distance of the orbit Q: aphelion distance of the orbit H: absolute magnitude (which the object would have at the distance of 1 AU from Earth and the Sun) MOID: minimum distance between the object’s orbit and Earth’s orbit.

Table 2

NUMBER OF DISCOVERED NEAs Date

Aten

Apollo

Amor

PHA

NEA

End 2008

~500

~3000

~2500

~1000

~6000

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Chapter 7

The risk of impact and the consequences

The physical and dynamical processes able to transfer planetary objects from different regions within the Solar System to the zone of the terrestrial planets were explained in the previous chapter. The known population of Near-Earth Objects (NEOs) was then briefly described, together with some updated estimates of their total number.

We have, therefore, two pieces of information at our disposal. The first concerns the properties of the orbital evolution of NEOs (their motions in the region of the inner planets) and the second concerns the number of known objects of different sizes. We discuss below how a detailed analysis of these characteristics can lead to the determination of the intrinsic probability of impact with Earth of any given NEO or, more generally, of the entire NEO population.

Intrinsic probability of impact The first source of information, orbital evolution, comes from calculations of the orbital elements for a given date, based on available observations.

Unfortunately, these orbital parameters do not remain constant over time. As we saw in the previous chapter, it is nearly impossible to establish precisely how a certain orbit will evolve in the future when referring to asteroids and/or comets in chaotic dynamical regimes. In fact, defining the future trajectory of an object forced to move under the gravitational influence of a certain number of planetary bodies of non-negligible mass is extremely complex. It is the so-called ‘N-bodies’ problem.

As already mentioned, Kepler’s three laws for planetary motion state that planetary orbits do not change over time, and go on to analytically describe an elliptical trajectory around the Sun. This prediction, however, refers to the theoretical case of a system formed by only two bodies, a planet and the Sun, subject to their mutual gravitational interaction expressed by Newton’s law of gravitation. Things are more complicated in the real world.

Our Solar System includes a number of planets, particularly the gaseous giants, whose masses are more than sufficient to perturb an asteroid’s 197

movement in its Keplerian trajectory around the Sun. The perturbation effect is such that the orbit of a minor body can no longer be simply described by a constant ellipse. Indeed, the real orbital motion of an object in an N-body system cannot in general be described by any simple analytical function. The orbital elements, then, correspond to the instantaneous orbit described by the body (approximated by a Keplerian ellipse) at a given epoch, but they cannot be assumed to be constant, so the prediction of the motion of the body over long time intervals becomes a difficult problem.

In practical terms, the motion can be predicted by means of numerical integrations. This means that the power of modern computers must be exploited in order to compute the motion of the bodies, starting from some given initial conditions, i.e. the position and velocity of all the bodies in the system at a given epoch. By considering only a relatively short interval of time, the new positions and velocities of all the bodies can be computed for a successive epoch, taking into account the gravitational forces acting on each body at the starting time, due to the positions of the others. The new positions and velocities of all the bodies of the system are then computed for this new epoch, and the procedure is repeated by computing positions and velocities for a new epoch sufficiently close in time, and so on.

In principle, one could expect that, by performing numerical integrations in which the time steps of the procedure are arbitrarily short, one could compute the orbital motion of all the bodies of the system with any desired accuracy. However, this is not possible in reality, due to two main reasons.

The first is trivial: in principle, to achieve a very great accuracy, one should consider very short time steps, but this means that, if one wants to compute the position and velocity of a body in the distant future, the number of steps in the numerical integrations tends to increase enormously, beyond the capability of even the most powerful computers.

The second reason is more subtle, and is related to the concept of dynamical chaos that was introduced in the previous chapter. We shall now explain in greater detail what chaos means. Imagine that we want to compute the motion of a small body in an N-body system – typically, the Sun, the body and N–2 perturbing planets. We may be interested in determining the position and orbital velocity of our small body after a certain number of years, for example, 100 000 years. When we give the initial conditions of the system to carry out our numerical integration, we can assign the initial position of the bodies with an accuracy that is normally quite good, but not infinitely precise. Generally speaking, we know the initial position of our body, for example, with an accuracy that reaches a limit expressed by some number with a certain number of digits. We say that the motion is chaotic when a tiny difference in the initial conditions, for instance a change of one unit on the last digit of the number that 198

expresses the initial position, leads to a huge difference in the position and velocity of the same body at the end of the numerical integration.

Non-chaotic orbits are such that the uncertainty in the final position of a body after the integration time is very limited, though never exactly zero. To express the concept in very simple terms, a chaotic orbit is one for which the final uncertainty is huge. Strongly chaotic motion does not always occur. It generally takes place in particular situations, such as the resonant orbits described in the previous chapter. It also arises when a minor body experiences a close approach with a major planet during its orbital wandering, as in the case of a NEA passing close to Earth at a certain epoch. This is why we say that NEOs are intrinsically chaotic objects.

As a consequence, no computer can predict exactly a NEOâ&#x20AC;&#x2122;s future orbital evolution beyond limited intervals of time. A minor difference in a small decimal figure would be enough to drastically change the description of the future trajectory. As a result, we would have to consider the result we get from the numerical integrations in terms of probability.

This is the real problem with NEOs. Despite starting with an extremely accurate orbit, the fact that the object is in a resonance, albeit even a weak one, or having close encounters with one or more planets, will prevent our calculation from being considered as a real prediction of its future orbital motion.

This does not mean that there is no point in devoting time to studying the future (or past) motion of chaotic objects, but an intelligent use of the results is certainly necessary. Different numerical integrations could, for example, be carried out for a particular object by slightly varying the initial conditions within the interval of uncertainty of its starting position. We would then achieve different final results that would all be equally compatible with what we know of the objectâ&#x20AC;&#x2122;s current orbit. By taking them into account, we could express the final position and velocity at the end of the integration in terms of probability in order to achieve different end-states.

Let us imagine, for example, that we have done 10Â simulations of asteroid X. Out of these, six end up with X impacting planet Earth. We could say that the probability of X impacting Earth is roughly 6/10. Although this is certainly oversimplifying the problem, it gives an idea of the limited predictions that can be made for NEOs over long periods.

Another commonly used system involves considering a certain number of real or simulated NEOs that share some common orbital characteristic (for example, similar semimajor axes), and then computing their orbital evolution over time. None of the computed trajectories for each object will very accurately describe their real future motion, but all of them, taken together, will describe the general trend of that particular group of objects. If there were, 199

Image 1. The average time interval between impacts in relation to the energy released (in megatons).

Some of the best known impacts are shown. (LNLL)

for example, 100Â objects in the initial sample and it came out that 80 ended up in the Sun, we could say that the probability of a single object from that group colliding with the Sun is 80%.

There are other, more complex, ways to analytically achieve solutions that can also be used to calculate the probability of a clash between an asteroid and a certain planet. There may be also fully predictable orbital trajectories, if we limit ourselves to quite short time scales (under 100Â years). There are also more complicated formulas that are able to calculate any objectâ&#x20AC;&#x2122;s intrinsic probability of colliding with a certain planet, always in statistical terms. In some situations, it may be especially useful to define an average probabilistic value for some selected population of objects.

If a new object appears to have an intrinsic impact probability well above the average, despite all the uncertainties in this type of computations, it certainly deserves careful attention. Such an object would undoubtedly be kept under continuous observation in order to compute a more accurate orbit, and, in the unfortunate case of a persistent probability of impact, it should be considered as a target for a whole series of possible preventive actions that we will discuss later.

The considerations described above confirm a fundamental concept that must always be remembered when considering the issue of impact risks: it is generally pointless to predict a collision with a NEO, even if it is quite well known, over periods longer than one century.

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What can be done is to determine the intrinsic collision probability: the higher this is, the greater the real risk of an impact. On average, the intrinsic impact probability of known NEOs currently ranges from 0.5 to 4 per billion years. In other words, the average probability that a particular NEO will strike Earth in any year is 0.5 to 4 in 1 000 000 000.

These appear to be entirely negligible numbers. And this is, indeed, true for almost all cases, but it is always worth remembering that we speak here of average values, not the real probability for any given object. As was said before, it cannot be excluded that a certain NEO, at a certain epoch during its existence, may reach a probability that is much higher than the average of the NEO population. At that point, the alarm must automatically be raised.

Furthermore, the greater the number of NEOs, the greater the probability that at least one of them may reach decidedly anomalous risk values at any given epoch. It then becomes essential to introduce the second piece of information that we discussed at the beginning of the chapter: the real inventory and size distribution of the NEO population.

To obtain a more realistic figure for the impact threat, we must multiply the average intrinsic probability of impact of a single NEO by the number of existing objects. If, for example, the average probability per year for a single object was one in a billion and there were a billion objects, we would be in the undesirable position of expecting one impact per year!

Frequency of collision with NEOs In this section we will quantify the risk of impact our planet runs against the NEO population. We have seen that to have an average probability of impacting with a NEO it is necessary to multiply the single object’s probability by the total number of existing similar objects. In the previous chapter, we talked about the total numbers of existing and listed NEOs. It seems to be quite easy then to produce diagrams that illustrate the probability of impact, based on the impacting object’s diameter.

Diagrams of this type can be represented in various ways – some can be interpreted immediately and others are more technical. Sometimes it is preferable to use the probability of impact based on the energy delivered during the collision, often expressed in megatons for a comparison with energies delivered by nuclear devices. Alternatively, the energy may be replaced with the number of predictable deaths following impact.

The delivered energy corresponds to the kinetic energy (the energy due to the motion) of the impacting object. It can be calculated from an estimate of the impact speed (normally around 10 km/s) and the mass of the colliding body, derived from knowledge of its size plus an assumed value for its bulk density.

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Risk of impact: a rather unusual risk The risk of impact with an asteroid and/or comet larger than 1 km can be considered in many respects exceptional compared to risks from other natural disasters. This is basically due to two reasons: • the consequences of the event are far greater than any other disaster, whether natural or artificial (including a nuclear war); in particular, a 1 km impactor is thought to be able to produce a global catastrophe, able to kill a substantial fraction of the human population.

• the probability that such an event will take place over short time scales, comparable to our lifetimes, is extremely low.

It should be noted, however, that if the probability for such an event to occur is multiplied by the number of deaths that would result, the corresponding number of deaths per year is comparable to, or even greater than, that for ‘traditional’ natural disasters, namely some thousands of deaths per year. The risk of impacts of this kind should then be considered very carefully and assessed via non-conventional systems.

This risk has another interesting characteristic: predictability. Until around twenty years ago, the problem was entirely unknown; as a consequence, no predictions were made, and no possible countermeasures could be undertaken. In about 20 years’ time we will be able to accurately predict an event of this type in 75% of cases with such precision as to immediately undertake possible actions to eliminate or decrease the danger. If scientific and technological resources have the necessary boost, an impact with a celestial body will become more controllable than an earthquake today.

The consequences are entirely different, however, with regard to the risk from impacting objects much smaller than 1 km. These impact events are much more frequent (since the number of potentially hazardous objects increases for decreasing size). They are not as destructive as the impacts of kilometre-sized bodies, but they can, nevertheless, produce great disasters that kill thousands of people. The impact of a 100 m sized impactor on an ocean, for instance, is believed to be able to produce a tsunami more violent than that produced by many violent earthquakes (like the terrible event of December 2004 in southeast Asia). It is certain that much more effort and technological advancement will be needed in the future, in order to attain the goal of being able to discover well in advance a high percentage of small dangerous objects (50–100 m). For many years to come, the risk will remain very similar to that from traditional, unpredictable natural disasters.

By considering the diameter, energy and number of deaths, a global catastrophe can be defined as an event caused by the fall of a 1 km object. The probabilities of this happening are easy to work out, even without using the diagrams shown earlier. We know that the average probability of an individual NEO’s impact ranges from 0.5 to 4 per billion years. We also know that the number of objects over 1 km should be of the order of 1000. Multiplying the average probability by the number, the result is one impact on Earth in a little 202

over one million years. The most pessimistic value of this number predicts an impact every few hundreds of thousands years, compared with a more optimistic estimate of one impact every few hundred thousand years.

As we can see, the uncertainty is not negligible and it derives from incomplete knowledge of the real inventory of potentially dangerous objects, as well as the estimate of the average probability, which varies according to the group of objects under consideration. Knowing that the number of NEOs increases as their diameter decreases, it is clear that impacting objects around one hundred metres across are much more frequent, while impacts with objects around several kilometres are fortunately rare – such as the famous asteroid that probably caused the extinction of the dinosaurs.

The Torino Scale In June 1999, during an international congress on NEOs, a proposal to unequivocally define a scale of the NEO impact risk was introduced by Prof.

Richard Binzel of Massachusetts Institute of Technology.

In a similar way to the famous Mercalli and Richter Scales that are adopted for earthquakes, he proposed a subdivision into 10 classes of danger, with the aim of classifying every new object discovered over the next century by assessing its hazard level. Each class would reflect the asteroid’s probability of impact and delivered kinetic energy. The congress was held in the city of Torino and so, due to the location of the Congress and the undisputed scientific value of local scientists’ active research into small bodies in the Solar System, it was suggested calling this scale the Torino Scale.

Around one month later, the Congress’ Scientific Organising Committee accepted the proposal and it was made official through press conferences held in various parts of the world. This meant that, in the future, the Torino Scale would be used to give a tentative evaluation value of the potential impact hazard posed by every NEO. This parameter takes into account both the intrinsic impact probability and the damage that a possible impact could have on our planet. The table on page 204 shows the 10 classes of risk, with a short explanation of their characteristics.

Consequences of impacts The energy of the impact and the impacting object’s characteristics The fundamental parameter to describe the destructiveness of a certain impact is normally the kinetic energy W of the impacting body. It is given by the product of one half of the mass m of the incoming object and the square of the speed 203

of impact v: W=

1 2 mv 2

The kinetic energy of the impacting body is usually measured in joules but is often expressed in megatons (1 Mton = approx. 4.2·1015 joules). The colliding body’s mass is closely connected to its size and density. The size of an object en route to collide with Earth can be estimated from its brightness, while density can vary a lot according to the nature of the object. It can be far less than 1 g/cm3 for comets, while it may rise up to 5 g/cm3 for metallic asteroids.

In general, a comet is expected to have a smaller mass than an asteroid of the same size, due to the fact that cometary nuclei are known to include

TORINO SCALE Degree

Description

0

The probability of collision is zero or so low as to be considered zero. This class is also applied to small objects, like meteors and bolides that burn up in the atmosphere as well as rare falls of meteorites that seldom cause damage.

1

A routine discovery in which a pass near Earth is predicted that poses no unusual level of danger. The probability of collision is extremely low. It does not merit public attention. New observations will probably lead to it being re-assigned to Level 0.

2

The object has a close encounter with Earth, but it is not highly unusual. It deserves astronomers’ attention, but it is not necessary to make it publicly known because the chance of a collision is very low. New observations will probably lead to it being re-assigned to Level 0.

3

The object has a close encounter that deserves astronomers’ attention. Calculations show a probability of 1% or higher of an impact that could cause local destruction.

4

Like 3, but destruction would be on a regional scale.

5

A close encounter by an object that poses a serious, although uncertain, threat of regional devastation. This requires astronomers’ close attention to accurately establish if the collision will take place. If the encounter is expected in less than ten years, government plans to mitigate the threat must be developed.

6

Like 5, but involving a large object that poses a serious, but still uncertain, threat of a global catastrophe. If the encounter is expected in less than three decades, government plans to mitigate the threat may be developed.

7

A very close encounter with a large object that could take place within this century, posing an unprecedented, although still uncertain, threat of worldwide catastrophe. For such a threat within the current century, international level emergency plans must be put in place, especially to urgently and definitely determine if the collision will take place.

8

A collision is definite, but destruction would be on a local scale. These events happen on average between once every 50 years and once every several thousand years.

9

A collision is definite, but with destruction on a regional scale. These events happen on average between once every 10 000 years and once every 100 000 years.

10

A collision is definite, but with destruction on a global scale. These events happen on average no more than once every 100 000 years.

Notes: Level 0 does not have any consequences. Level 1 requires continuous monitoring of the object. Levels 2–4 require special attention and possible plans of action. Levels 5–7 are to be considered alarming and require preparation for action. Levels 8–10 represent definite collisions and require definite action.

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significant amounts of volatiles. However, due to comets’ extremely elongated and sometimes retrograde orbits (with motion headed in the opposite direction to Earth), they reach us at speeds around 50 km/s, while asteroids’ speed is generally around 20 km/s. It follows that, in practice, there will not be enormous differences in the consequences of an impact with an asteroid or comet of the same size if the energy involved is sufficiently high. For lower energy values, the atmosphere can act as a more effective shield against comets, as they are less dense and more porous, and should be more effectively ablated during the transit through Earth’s atmosphere.

All these are very general considerations, but we should remember that a more accurate treatment should consider the vast variability of the parameters involved – not least the angle at which the object enters the atmosphere.

Moreover, in practical terms, it is very hard to have, in advance, an accurate estimate of many important physical parameters of the impacting body except in the most dramatic cases. On the other hand, the colliding body may be sufficiently large (more than several hundreds of metres across) to have been discovered and identified as a potential danger well in advance.

In most cases there are no warning signs of an impact and it will be the consequences, unfortunately, that will give us a rough estimate of properties of the colliding body a posteriori.

The treatment that follows will, therefore, mostly use average values for the various relevant parameters, such as size, and this will be enough to describe the overall problem.

Through the atmosphere Most objects that encounter Earth are small, no larger than a few metres, so the atmosphere is able to act as a shield. It is estimated that a cosmic object that is destroyed during its passage through the atmosphere will usually not exceed 40 m across for metallic objects or 200 m for less dense objects.

The fact that an object disintegrates in the atmosphere does not automatically mean that the effect on the ground is entirely negligible. The 1908 Tunguska event in an uninhabited forest in central Siberia shows how dangerous such objects can be. In that case, the object, estimated to be smaller than 60 m and probably no larger than 30 m, exploded at a height of around 8 km above the ground. The energy released was around 1000 times greater than the atomic bomb dropped on Hiroshima, and the explosion destroyed more than 2150 km2 of the underlying forest.

If the Tunguska event had taken place above a city instead of an uninhabited area, we would certainly not be describing the phenomenon as if it was just a notable scientific curiosity. The recorded effects are still impressive, even after 205

The case of (99942) Apophis At the end of December 2004, news spread of the discovery of a Near-Earth Asteroid, preliminarily named 2004 MN, which turned out to have a relatively high probability (about 3%) of hitting Earth on 13 April 2029, travelling at a speed above 45 000 km/hour. According to preliminary estimates, 2004 MN was a rocky body with a size around 300 m, and the energy it would deliver during an impact would be about 1000 megatons, equivalent to the explosion of more than 65 000 Hiroshima bombs. (In comparison, Meteor Crater in Arizona was produced by an impact with an energy of only some tens of megatons, while the terrible Krakatoa volcano eruption in 1883 was equivalent to about 200 megatons.) If it occurred on land, the impact would produce an impact crater with a diameter of between 3 and 6 km. An impact over an ocean would produce a terrible tsunami event, well beyond the worst events of this kind ever recorded in historical archives. It is, therefore, not surprising that, when the orbit of this object was determined with sufficient accuracy, it received a permanent number (99942) and the name of Apophis, a god of destruction in ancient Egypt.

Following the preliminary alarm, an observing campaign was immediately organised, involving professional and amateur astronomers all around the world, and new detections of the object led to a more accurate estimate of its orbital elements. Based on the improved orbit, it was possible to exclude the possibility of an impact in 2029. The decisive observations were carried out by the 306 m radar antenna at Arecibo in Puerto Rico, which was used to derive a very accurate measurement of the object’s distance and radial velocity. Based on these observations, the probability of an impact was ruled out, and it was possible to make a detailed assessment of the extremely close approach that Apophis will have with Earth on 13 April 2029.

On that day, at 22:21 (Central European Time) the asteroid will pass at a minimum distance of about 30 000 km, well below the altitude of geostationary artificial satellites.

Among the ten known objects that have experienced very close approaches to Earth in recent years, Apophis is the largest. Two objects were previously discovered to have a closer approach to our planet, but they were only a few tens of metres in size, and in the case of an impact they would have probably been destroyed by frictional forces in the atmosphere.

All the other close approachers were discovered serendipitously during their flyby of Earth, whereas Apophis is the only one to be predicted well in advance. Recently, an Italian-led team succeeded in carrying out polarimetric measurements of Apophis using the Very Large Telescope (VLT) of the European Southern Observatory (ESO). The observations led to a better estimate of Apophis’ size , which turned out to be only a little smaller than preliminary estimates, with an average diameter of about 270 m.

When Apophis has its close encounter with Earth in 2029, it will have an apparent visual magnitude a little fainter than magnitude 3 – about three times fainter than the Pole Star.

It will be visible in the night sky of Europe, Africa and part of Asia as a fast-moving ‘star’ crossing the constellation of Cancer at a speed higher than 40 degrees per hour, slower than a typical artificial satellite, but quite fast in the field of view of binoculars or a small telescope.

The irregular shape will be detectable with a small telescope, provided the observer is able to keep it aligned with such a quickly moving target. According to current estimates, an asteroid of this size is expected to have such a close encounter with Earth about once every 1300 years, on average.

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What will happen after the 2029 flyby is still not completely clear. The close encounter will certainly have important consequences for the orbit of Apophis, and this will determine its subsequent orbital evolution and future encounters with Earth. It is not possible to compute with sufficient accuracy the orbital evolution of Apophis over long time scales. According to current information, Apophis will have many resonant returns to Earth, some of which will be also quite close, like a predicted encounter in 2036. However, it seems that, at least for the rest of this century, it is unlikely that Apophis will actually impact Earth. Future observations will be needed to better constrain the orbital evolution of this object.

At the same time, the case of Apophis is becoming a reference from the point of view of the actions that humankind will be forced to make sooner or later, following the discovery of an object on a collision path with Earth. Some scientific teams are actively involved in developing possible techniques of orbital deflection and, equally important, in convincing political institutions at national and international level that it is time to devote energy and manpower to deal with the impact risk, and to plan some kind of internationally agreed road map to develop the necessary tools to defend Earthâ&#x20AC;&#x2122;s biosphere and human civilisation from the objects that may hit our planet. This is a very hard task, since, in the worst conceivable cases, it might be necessary to take urgent decisions and to apply counter-measures whose effectiveness may not yet have been sufficiently assessed or quantitatively predicted. From the purely scientific point of view, what is currently missing is a detailed and extensive knowledge of the internal structure of asteroidal and cometary bodies. There will certainly be a lot of effort put into this line of research in the future, with some decisive inputs probably coming from dedicated space missions that are currently being designed and planned for the next few decades.

Image 2. The distribution of 136 explosions produced by the interaction of large meteoroids (some metres in diameter) with the atmosphere, observed by the US satellite defence system from 1975 to 1992. (Sky and Telescope)

207

all this time. People tens of kilometres away were lifted up into the air, together with their tents, and were unconscious for quite a long time. At a distance of 500 km, the noise of the explosion was clearly heard, while seismic vibrations were registered up to 1000 km away.

Many other phenomena similar to Tunguska are known to take place in the upper atmosphere, without being so evident. Recently, the United States Department of Defense made available formerly secret data on explosions observed by spy satellites in the atmosphere. In the period 1975–1992 no less than 10 per year were identified, and in one case the energy released exceeded a megaton, over 50 times the energy of the Hiroshima bomb. It seems, therefore, that objects delivering energies under one megaton mostly disintegrate in the atmosphere, without reaching the ground.

Impacting the ground Craters Following ablation of large or dense objects in the atmosphere, some fragments may succeed in reaching the ground. In most cases they are tiny bodies that have undergone intense deceleration while crossing the atmosphere. A lot of these fragments go unnoticed and only few are recovered, usually by chance.

Only rarely do we succeed in detecting an impact event at the moment it happens, in the form of a very bright meteor (see chapter 4) and some residual part of the impacting body is recovered as a meteorite. It is important then to be able to recover these small fragments, given the important contribution they can give to scientific research.

More worrying (although luckily much less frequent) are those bodies that exceed a critical value of kinetic energy that makes them less subject to the destructive action of the atmosphere. These may be metallic objects over a few tens of metres across, or rocky bodies larger than 100 m. In this case, the impact leaves a clear sign on the Earth’s surface: a crater (see chapter 9).

Destructive effects Craters are certainly the most obvious signs of impact, but it must be taken into account that the destructive effect of the colliding body is not restricted to the area delimited by the crater rim. It is estimated that an object around 70–80 m across could completely destroy an average sized city. A metropolitan area would be wiped out after a collision with a body of 150–200 m. An object of 350 m would be enough to destroy a region, while one of 700 m would practically destroy a small nation.

208

The first direct cause of destruction is certainly the shock wave of the collision, followed by violent earthquakes and gigantic fires produced by the heat of the impact. The energy involved in events like these would be very high, up to 10 000 megatons – comparable or exceeding the amount released in a global nuclear conflict.

Nevertheless, the effects are still comparable to those that characterise various natural catastrophes that are more commonplace on our planet.

Impacts with these energy values occur on average every 60 000 years, so the risk connected with this type of impacting force – should it directly strike Earth – is not generally much higher than typical natural disasters like earthquakes, hurricanes or large volcanic eruptions.

The risk represented by these relatively small impacting forces, however, is intrinsically higher if we consider that events of this type have a higher probability of taking place over an ocean, as three quarters of the Earth’s surface is covered by sea. With this in mind, destruction increases notably, because of the predictable generation of tsunamis – large waves that propagate on the surface of the ocean over distances covering thousands of kilometres.

They travel at speeds of hundreds of kilometres an hour as a result of the enormous mass of water moved by the collision.

A typical feature of a tsunami is that the waves increase in height 10 or 20 times when they reach a continental shelf. The consequences are generally disastrous. In 1960, an earthquake in Chile produced waves that travelled 17 000 km and started a tsunami in Japan with waves that averaged 2 m in height and caused the deaths of 200 people. Around 11 000 km away, the Hawaiian islands were hit by 5 m waves that killed 60 people. In 1998, an earthquake caused a tsunami in New Guinea that destroyed entire coastal villages and provoked the death of thousands of people. Even more recently, in December 2004, a terrible tsunami that followed an earthquake in Indonesia hit the Indian Ocean, causing 300 000 deaths and immeasurable damage.

A small asteroid could cause tsunami waves up to 100 m high. Under these conditions the front of the wave would travel inland over 20 km, inundating coastal areas. The effects are easy to imagine in highly populated coastal areas of the United States, Holland or Denmark. The frequency of these events would be relatively low – one every 2000 years – so it is not strange that reports of similar events have not been found in the records left by ancient civilisations – unless we want to reinterpret events of religious or mythological tradition, like the Flood or disappearance of Atlantis. It is not at all unlikely, though, that smaller objects may have caused much more frequent tsunamis, similar to those of 1960, 1998 or 2004.

Things are more complicated when the size of the impact is even greater.

With impact energies between 10 000 and 100 000 megatons, the effects 209

Document concerning the resolutions adopted by the International Astronomical Union following the IMPACT Congress, held in Torino, July 1999 22 July 1999

IAU Press Release

«Dealing With the Impact Hazard: An International Project» Earth is constantly sweeping up particles of various sizes as it travels on its orbit around the Sun. We often see some of them burning in the atmosphere as meteors. From time to time, penetrating objects that are many meters across may cause major explosions in the air.

Recently, public concern has been raised over much less frequent but devastating impacts by km-sized asteroids or comets. It has now been realized that the risk of fatality as a result of such impacts is comparable to that of well perceived hazards like airplane crashes.

Possibly hazardous objects in the solar system can be discovered by astronomical observa­ tions, e.g., when they are recorded as faint streaks of light in long telescopic exposures because of their motions. Astronomers therefore have a special mission relating to the impact hazard, namely, that of discovering and characterizing the dangerous objects, and, hopefully, by verifying the expectation that no major impact is going to occur during the next centuries.

The vast majority of these ‘dangerous’ objects have as yet escaped discovery and, since we do not know their orbits, they may hit at any time. For the time being, only statistical calculations can be made. They show that the risk of Earth being hit by a km-sized object during the next couple of centuries is one-in-a-thousand. The risks are low but the consequences are large enough to cause concern. In fact, astronomers have now put in place efficient search programs that are already resulting in a fast stream of new discoveries of such objects.

When the newly discovered objects are investigated with regard to future encounters with Earth, it is sometimes found that, due to the necessarily imprecise, initial orbit deter­minations, the risk of a collision cannot be entirely ruled out. The likelihood of such a pos­sible disaster may also be estimated. If it is found to be significant when compared to the combined risk posed by all the unknown objects, then the asteroid or comet in question will become subject of careful monitoring. The expectation is that improved knowledge of its orbit will sooner or later show that the impact will not occur. This is at least the out­come of all monitoring programmes so far.

This type of observational work is now occupying a small group of astronomers worldwi­ de. It is an international effort, since the impact hazard is obviously of concern to the enti­re world. Possible impacting objects are best studied by means of international observing campaigns and there must be an efficient exchange of information among all scientists involved. This is also why the International Astronomical Union (IAU) has engaged itself as co-sponsor of a very well attended workshop in Torino, Italy, on June 1–4. Among the other main co-sponsors were NASA and ESA. Many related issues were debated during this meeting. For instance, how to secure a fast, efficient search such that nearly all the potentially hazardous asteroids get discovered and safely catalogued before too long, how to collaborate in order to measure their most

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important physical properties by means of ground-based as well as space-based observa­tions, how to speed up and widen the data channels for an optimal use of the world’s com­bined observational facilities, and, not least, how to inform both the public and political authorities – if ever needed – about calculations that point towards sinister events.

Even though US national agencies, i.e., NASA and the US Air Force, are presently carrying a major part of the burden of these observations and calculations and may possibly increa­ se their efforts further, the participants in the Torino meeting decided to make a strong recommendation to all governments to establish ‘National Spaceguard Centres’ and to support these financially. In this way, a proper sharing of responsibilities may be realised, so that this important work can be enhanced and reach maximum efficiency. Another urgent action item is the setting up of an expert committee, under the auspices of the IAU, that will check impact predictions and advise about their publication.

Perhaps the most visible and immediately practical result was the adoption of the so-called ‘Torino impact scale’. It was worked out by Prof. Richard Binzel of MIT (Cambridge MA, USA) as a tool for communicating the issues of impact prediction outside the professional circuit.

With some superficial similarities to the Richter scale for earthquake intensity, it divides the predictions into classes 0–10. All events that have no likely consequences belong to class 0 and higher numbers correspond to progressively more probable, and/or more serious impacts.

At the present time, no single asteroid is known that has been assigned an impact predic­ tion in a class higher than 0. This is of course fortunate and this is expected to be the nor­mal state of affairs. However, it is also likely that initial uncertainties in the calculation of an orbit of a newly discovered asteroid may temporarily place it in a higher category. This is not a cause for immediate concern, but merely signals the need for more accurate obser­vations, leading to a better determination of the orbit.

With the increased rate of discoveries of asteroids and the efficient schemes of orbital computations now in use, the new Torino Scale will most certainly become of great use and will be frequently cited as reference.

Hans Rickman (IAU Assistant General Secretary)

described previously intensify and the quantity of water vapour released by impacts in the ocean or dust injected into the atmosphere by impacts on land can start to affect the global climate. These events are beyond the limits of destructive energy experienced throughout human history as a result of natural phenomenon and they mark the transition between local and global destruction phenomena; the latter are events that can kill more than 25% of humankind.

Worldwide destruction would be guaranteed with energies around one million megatons. It is estimated that these events correspond to the impact of bodies with diameters between 1.5 and 3 km, which occur on average every few million years. The most tragic effect of these events is the injection of enormous quantities of dust into the atmosphere. The particles freed by the impact and vaporisation of the ground, as well as the force of the impact itself, would be 211

added to the particles produced by numerous fires worldwide. These fires would mostly be provoked by the heat delivered by the impacting body, followed by the impact of large numbers of impact ejecta – large pieces of rock created by the first impact and subsequently cast in all directions at high speed.

All this means that, besides the devastation of the area directly hit by the impact, the whole planetary climate would be profoundly disturbed. The atmosphere would become very opaque for long periods of time and there would also be massive destruction of the ozone layer.

For greater energies, corresponding to an impact with an asteroid around 5 km in diameter, the opacity of the atmosphere caused by dust particles would become so great that it would stop plant photosynthesis. Over a certain limit, it is thought that all ocean waters would be acidified because of the sulphur particles derived from the impact.

The K/T event (see chapter 9) is much talked about concerning the extinction of the dinosaurs and very probably led to the formation of a thick aerosol of sulphur compounds that enveloped the whole planet for a long time.

It is not difficult to understand that events of this magnitude have devastating consequences for Earth’s entire biosphere.

Quantifying the destructive effects When quantifying the intrinsic risks related to these events, the critical variable to consider is the number of deaths expected for a given impact energy and the frequency that events with this energy are produced. Even though this system looks extremely cynical, it is the only one that allows us to suitably quantify the risk and its consequences. It will especially help us to compare the degree of danger of different scenarios as impact energy varies.

Starting with energy impacts of one million megatons, the expected number of victims is around one and a half billion. The frequency of occurrence of these events is of the order of one impact per million years, so the number of Equivalent Deaths Per Year (EDPR) is around 2 000, which corresponds to an annual risk of death of around one in three million.

Smaller energy impacts, between 10 000 and a million megatons, provoke a great deal of small-scale devastation that corresponds to a few tens of EDPR. The resulting risk is, in practice, negligible. This assessment corresponds to impacts that occur on solid ground, not over an ocean. Considering the current level of population, the frequency with which an event of this type can involve a densely populated area is around one every 10 000 years. Things change if impacts with similar energy are considered at sea. The effects of a tsunami can result in a huge number of victims in coastal areas. For this type of population, the intrinsic risk of death due to a tsunami is ten times greater than that estimated previously.

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It is clear that assessing and quantifying the risk is not a simple matter, due to the interplay between frequency of the events and their destructive power.

Very generally, it can be said that the risk increases as the energy of impact increases, until a maximum that corresponds approximately to the limit of transition between regional destruction and global catastrophe events. Beyond this limit, the risk quickly goes down because of the increasing rarity of this type of impact.

It can, however, be established that the global probability of dying due to the fall of a stray planetary body per person is of the order of 1 in 20 000. The corresponding probability of death per person and per year due to these events is of the order of 1 in one million. In other words, each of us has a one-in-onemillion probability of dying within one year due to the consequences of an impact by a celestial body.

If we compare this risk with other natural or artificial calamities we will be surprised: it is comparable with dying in a plane crash or greater than in a

Comet showers and periods of biological extinctions The discovery of scores of impact craters on our planet (see chapter on craters) and the study of fossils in the various geological eras raised a problem that caused quite a stir during the 1980s and it is still discussed by experts from various research areas today. During the last 300 million years, a number of biological mass extinctions have taken place, in some cases linked to the disappearance of over 60% of existing species. These extinctions include the famous dinosaur extinction, mentioned above, around 65 million years ago. Another important extinction took place at the end of the Permian, 250 million years ago. In this case, 90% of sea and 70% of terrestrial species disappeared. It is interesting to note that these extinctions seem to repeat themselves at regular intervals – with a period of around 28–30 million years. At the same time, it was discovered that impact craters are concentrated around certain dates and that, again, these peaks recur every 30 million years.

What could have caused a periodic recurrence of such violent planetary catastrophes that lead to drastic biological extinctions?

The most favoured theory concerns comet showers that detach themselves periodically from the Oort cloud (see chapter 3), pour into the inner Solar System and then collide with Earth. This process could be connected with the periodic oscillation of the Sun above and below the plane of our Galaxy while it is orbiting the galactic nucleus once every 250 million years. The time taken to cross the galactic plane in one direction is about 30 million years, so it takes 60 million years to complete the oscillation.

During oscillation, the Sun would cross an area that is rich in high-density molecular clouds. They could violently perturb the outer cometary belt and cause comet showers to head towards the Sun and the terrestrial planets. Although very interesting, these theories still need comparisons to give them validity. While recurrence of the events remains very doubtful, the reality of the link between impacts and extinctions is very probable.

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tornado, for example. Yet, while the fear of flying or being caught up in a tornado is deeply rooted in our civilisation, this is certainly not the case with the idea of finishing one’s days under a stray asteroid.

This situation is quite surprising and might seem alarming. Nevertheless, it is right and important to analyse the problem better, beyond the cold numbers that characterise the EDPR or average probabilities.

In fact, we are speaking of unlikely disasters that cause enormous damage.

They happen on time scales much longer than an average lifespan. While tornados or plane crashes are continually in the news, catastrophes relating to NEOs are only theoretical and occurred far back in geological time, long before the beginning of our civilisation.

It is not difficult to understand then why it is hard to grab the attention of world organisations about the NEO issue and the consequences of one impacting Earth. Why deal with an event that will only probably affect future generations (no one knows when), while possibly neglecting problems that continually recur during our current existence? Is it worth investing money in research whose results will not be visible for who knows how many centuries?

An extremely careful and sensitive vision is fundamental to fully understand the necessities related to such a unique problem. For example, let us imagine that the dinosaurs were intelligent beings and as technically evolved as us. They were probably swept away by a huge cosmic catastrophe after scores of millions of years of evolution (compared to human beings who, in comparison, have only just been born...). How would we judge then the hypothetical dinosaur organisations that had neglected taking any form of prevention during their long history? And what did the last generation think about their predecessors?

The problem, seen like this, becomes of extreme importance and top priority, but it is difficult to become involved in an issue that looks so remote from daily life. Nevertheless, despite not having a complete understanding of such a unique and delicate situation, there have been some preliminary, but comforting, signs of responsibility from many world organisations with regard to the beginning of observation campaigns that are aimed at discovering NEOs, together with scientific and technical prevention studies and international collaboration.

The Tunguska event Just after 7 am local time on 30 June 1908, in Evenkia (Central Siberia) in the great forests of the taigà and south of the Arctic Circle, a flaming column suddenly appeared in the sky from the southeast. ‘A fireball as bright as the Sun’ silently came down until, at around 6–8 km altitude, there was a huge explosion and a thick cloud of smoke rose over a region between the rivers 214

FREQUENCY OF IMPACTS LIKE TUNGUSKA It is interesting to study the frequency of impacts like Tunguska or those that cause craters like Meteor Crater. The energy released is estimated to be around 12 megatons. Objects able to form craters larger than 1 km are completely vaporised or fused with the surrounding rocks, so it is only possible to find fragments of the impactor in smaller craters. In these cases, only one meteorite out of 17 is ferrous. Other, less resistant, forms of meteorite are certainly underrepresented, so it can be quite accurately established that we should expect a ratio between ferrous meteorites and other types of around 1 in 33, or 3%. The study of Earth’s craters with residues of ferrous material and energy release over about 12 megatons tells us that impacts of this type happen on average every 15 000 years. Supposing that for every ferrous meteorite another 32 fall that are not ferrous, we should expect an impact every 450 years (15 000 divided by 33). We know, however, that only 30% reach the ground, while the rest probably end up in the sea. This leads to one impact every 135 years that is able to release energy of more than 12 megatons.

Image 3. Photograph taken by Leonid Kulik in 1927, during the first scientific expedition to the Tunguska region devastated by the explosion of a cosmic body.

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Image 4. Artistâ&#x20AC;&#x2122;s representation of the Tunguska objectâ&#x20AC;&#x2122;s interaction with the atmosphere.

(K. Karlevic)

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Nizhnjaja Tunguska (Lower Tunguska) and Podkamennaja Tunguska (Stony Tunguska).

The fireball was seen within a radius of about 1500 km, and a seismic wave was recorded throughout Eurasia, while a wave of atmospheric pressure went all the way round the planet. An unusual glare in the sky was seen on the following nights, lasting about two months and visible from Europe to Siberia and California.

The forest was levelled to the ground over more than 2000 km2, with around 60 million trees losing their branches and being scattered in lines along the ground that indicated the direction of the shock wave. The energy of the explosion was estimated at around 10–20 megatons, over 1000 times more than the Hiroshima bomb. Fortunately, the region was practically uninhabited, though two victims were reported.

Evenkia is more than two and a half times the size of Italy, with just 21 000 inhabitants in 1995 (far less in 1908). The place where the explosion took place was so difficult to reach that it was not explored until 1927, when Russian scientist Leonid Kulik organised an expedition with the aim of trying to understand what had happened 19 years earlier.

All witnesses interviewed over the following decades indicated that the cause of the explosion had been the fall of a celestial body, with a diameter between 50 and 100 m and travelling at about 10 km/s. But there were also some strange aspects. Why did it not create a crater comparable to Meteor Crater in Arizona, which was formed around 50 000 years ago by a similar-sized celestial body?

And why was it not possible to locate pieces of this meteorite, despite careful searches? Until a few years ago, it seemed that the celestial body that created the explosion in Tunguska had completely disappeared. Then, in 1991, samples extracted from resin in trees that survived the catastrophe were collected during an expedition of physicists from the University of Bologna and analysed in a laboratory. These tests showed that a lot of microscopic particles incorporated in the resin in 1908 probably have an extraterrestrial origin.

However, the complete absence of any larger remnants is still an enigma.

In the 1950s, these strange aspects led some researchers to suggest theories that were more or less science fiction. Perhaps it was a nuclear explosion, possibly caused by an alien spaceship breaking down and crashing on Earth.

However fascinating it might be, almost all scientists exclude this theory, given the complete absence of radioactive residue in the entire area devastated by the explosion – even though some isolated opinion defended the idea during a recent conference in Krasnojarsk, which was dedicated to the event’s 90th anniversary.

Other original theories were even published in specialist scientific magazines concerning the impact of a mini black hole or piece of antimatter with Earth.

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Instead, since the 1970s, the debate has concentrated on the comet–asteroid alternative – especially among Russian researchers. A mini-comet theory was preferred, in the belief that it could be friable and rich in volatile compounds, unlike the rocky and metallic asteroids that had formed Meteor Crater and make up meteorites.

Current knowledge concerning the role that extraterrestrial impacts have had in the history of our planet is much greater compared to a few decades ago.

Exploration of the Moon and many other planets and natural satellites has shown that collisions with stray interplanetary bodies, and the consequential formation of large craters, are relatively common events in the Solar System.

Just under twenty years ago, a new discovery indicated that the largest of these impacts on Earth probably caused real climatic and ecological catastrophes, like the one 65 million years ago that provoked the mass extinction of dinosaurs and around two thirds of other living species. The clue was convincing, but not very obvious: in the thin clay layer that marks the global boundary between the age of the dinosaurs (the Cretaceous period) and the following Tertiary period, there was an anomalous quantity of iridium, a chemical element that is rare in the Earth’s crust, but relatively abundant in meteorites. In the early 1990s, the enormous Chicxulub crater, around 180 km in diameter, was discovered buried under hundreds of metres of sediments between the Yucatan and the Gulf of Mexico. Since the crater was also around 65 million years old, it supplied a decisive argument for the cause and effect relationship between large impacts and climatic and ecological catastrophes in the history of Earth.

Many researchers believed that if iridium had been the key to understanding the extinction of dinosaurs, it might also solve the Tunguska enigma. Over the last ten years, laboratory chemical analysis has been carried out on particles found at the site of the explosion and in the layers of polar ice formed around the year 1908. Once more there were strange aspects and contradictions. Some researchers found an excess of iridium and other rare elements in particles of peat collected from the swamps in Tunguska; others did not notice any significant anomalies. Data on the Antarctic ice have shown discrepancies up to a factor of 20 between the different measurements. A recent publication by a working group of Danish scientists analysed ice from Greenland: no trace of an anomalous abundance of iridium was found from the year 1908.

How can these contradictions be explained? In the summer of 1996, the problem was debated at length during a workshop organised by the University of Bologna for around 100 Russian and western academics, but opinions still disagree. One possibility is that the extraterrestrial iridium-rich material did not spread over Earth, but fell only in the area of the explosion. This explanation, however, is in contrast with the high altitude deduced for the explosion and the atmospheric effects, such as sunsets with unusual colours, 218

that were recorded in 1908 in very distant areas. More reasonable, perhaps, is the idea that the Tunguska projectile was an unusual celestial body: a fragment of a comet made up almost completely of ice or a big meteorite from an asteroid crust with a metallic nucleus, where iridium would be mostly concentrated. After all, there are quite a lot of large impact craters on our planet, but no anomalous abundance of iridium has been found.

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Chapter 8

Threats from NEOs and possible countermeasures It is now certain that Earth is struck by objects from space at irregular intervals.

More than 170 impact craters discovered on our planet, with sizes ranging from about 10 m to 300 km, are evidence of this. In fact, the existence of such a potential risk caused the American Congress to set up two commissions in 1991. The first of these was to determine the current state of knowledge concerning the issues of the so-called Near-Earth Objects (NEOs) and suggest how to quickly complete their discovery and physical study. The second was to examine the means and necessary techniques to divert or destroy NEOs threatening to collide with Earth.

The results of these commissions have been published in two reports that are now the guidelines for the dynamic and physical study of NEOs, as well as the mitigation of any potential threat that NEOs pose to our planet.

Programmes of discovery and physical studies Following the Congressional Commission’s recommendations for the discovery and study of NEOs, the Spaceguard Survey was set up in the early 1990s.

This organisation of professional astronomical observatories and institutes was given a mandate to discover by 2008 at least 90% of the asteroids with diameters of at least 1 km and orbits passing near Earth (to date this objective has been practically achieved). Subsequently, the American space agency, NASA, also incorporated the objective set by the Spaceguard Survey into its strategic plans and began financing NEO research programmes. Other nations were lukewarm in their response to the problem, because research is carried out irregularly and with entirely unspecific techniques. This resulted in the Spaceguard Survey not being developed as it had originally been planned.

More recently, the American Congress has instructed NASA to catalogue 90% of all NEOs less than 140 m wide by 2020, and to find ways of deflecting any headed for Earth.

The problem of discovering as many NEOs as possible that are potentially dangerous to Earth is also vitally important when adopting defence strategies.

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Naturally, the first research objective is to catalogue those objects larger than 1 km in diameter, which could cause worldwide devastation should they impact Earth. It is estimated that the number of objects in this category is just over a thousand, whilst those discovered to date, with orbits that have been calculated fairly accurately, represent around 80% of the whole population.

Discovering NEOs requires – when possible – powerful telescopes over long periods of time, in order to see large areas of sky – around several degrees square. For the work to be done thoroughly, most of it should be done automatically, including using powerful computing to manage the enormous amount of data that is continually collected.

Once a new object has been discovered, it is necessary to continually observe it over the following nights in order to record the greatest number of positions necessary to make an exact calculation of its orbit and accurately understand its future trajectory.

Follow-up work is very important and requires close international coordination between the various observatories working on this type of research. Sometimes an object is discovered but then lost through lack of follow-up observation. In recent years, the four major NEO research programmes in the USA have made the most discoveries, and these will be described in detail later on. In the last decade they have discovered more than ten times the number of objects that were discovered previously by all the other observatories in the world.

The NEO search programmes US Air Force (USAF) LINEAR (Lincoln Laboratory Near Earth Asteroid Research) The Massachusetts Institute of Technology (MIT) LINEAR programme, funded by the United States Air Force and NASA, was set up in 1997 to show how to apply advanced technologies that had been developed to oversee artificial satellites and tackle the problem of discovering and cataloguing NEOs that could threaten our planet. The LINEAR project initially used a 1 m aperture telescope with a wide field of view, equipped with the latest generation electronic sensors. It was located in the White Sands missile range at Socorro in the New Mexico desert (USA), where atmospheric conditions allow for prolonged observations all year round. Since January 2000, the LINEAR system has been reinforced with a second telescope of 1 m diameter.

The system is automatic and an enormous amount of data is collected and sent to a control centre in real time, where observations are analysed. Data on asteroid and cometary type objects are then sent to the Minor Planet Center (MPC) at the Smithsonian Astrophysical Observatory, Harvard, Massachusetts, 222

Image 1. The GTS-2 telescope. This telescope, 1 metre in diameter, is one of the most used instruments for the discovery of NEOs by the LINEAR programme. (MIT Lincoln Laboratory)

USA, which manages all orbital data relating to small bodies in the Solar System. Based on the data collected, this centre calculates the orbits and compares them with existing ones to establish if a particular object is new or if it is already in the catalogues. This work is carried out jointly with the Department of Mathematics of the University of Pisa, Italy (NEODyS, the Near Earth Objects Dynamical Site), and NASA’s Near Earth Object Program Office at JPL (Pasadena, California, USA).

The LINEAR system has shown itself to be the most efficient out of those currently in existence, leading to the discovery of more than 2000 new NEOs in about ten years – over 75% of all discoveries made in the same period. In comparison, before it was set up, the discovery rate of NEOs by observatories throughout the world was about ten a year. Besides new NEOs, LINEAR has discovered around 240 comets and 226 000 main belt and Trojan asteroids. The number of astrometric observations sent to the MPC in the same period is enormous – more than 22 million.

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NEAT (Near Earth Asteroid Tracking) The NEAT programme was set up in 1999 through a collaboration between the Jet Propulsion Laboratory (JPL) and USAF. It also uses a 1 m diameter automatic telescope, with similar characteristics to the LINEAR project. It is located on Mount Haleakala on the Hawaiian island of Maui. Once again, the telescope had previously been used for surveying artificial satellites, but from December 1995 it has been partly used to search for NEOs. A similar CCD camera – but with improved characteristics – to that used with the telescope in Hawaii, has recently been installed at the 1.26 m Schmidt telescope at the Palomar Observatory in California, which is now part of the NEAT programme.

More than 50 NEOs have been discovered since it joined the NEAT programme.

Spacewatch of the University of Arizona Spacewatch is the name of a group from the Lunar and Planetary Laboratory (LPL) of the University of Arizona, Tucson, founded by Tom Gehrels and Robert S. McMillan (who also leads it) in 1980. The asteroid research programme, started in 1984, was the first to be completely dedicated to discovering NEOs. The tools used are two wide field telescopes, 90 and 180 cm in diameter. These telescopes are situated at the Kitt Peak Observatory in Arizona. Spacewatch records include the first use of a CCD in discovering asteroids and comets, the first NEO discovered with a CCD (1989 UP), the first research group to use software to automatically identify asteroids and the discovery of the asteroid 1994 XM1. This asteroid passed only 105 000 km from Earth (less than a third of the Earth–Moon distance), one of the closest flybys of our planet ever made by a cosmic body.

At present, the Spacewatch programme’s rate of discovery is 20–30 NEOs a year. It is supported by the LONEOS (Lowell Observatory Near-Earth Object Search) project with its 58 cm aperture Schmidt telescope, located at Lowell Observatory, Arizona, as well as the University of Arizona’s Catalina Sky Survey (CSS), which uses a 42 cm Schmidt and is situated north of Tucson.

In October 2003, the LONEOS project rediscovered 1937 UB (Hermes), an asteroid that had been lost as soon as it was discovered.

The American projects mentioned above can take credit for almost all NEO discoveries. It should be noted, however, that the survey centres are nearly all located in the northern hemisphere, meaning that the south is uncovered. Therefore, all NEOs that are only visible from the southern hemisphere (below –40° in declination) cannot be seen. Fortunately, in March 2004, the CSS started a NEO research programme that uses a 50 cm Schmidt telescope located at Siding Spring Observatory in Australia.

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Image 2. The Kitt Peak Observatory, Arizona. Some of its telescopes are used in the Spacewatch programme. (KPNO)

Image 3. The Schmidt 44/60 cm, f/1.95 telescope at Lowell Observatory, which is used by the LONEOS programme.

(Lowell Observatory)

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Image 4. Number of NEAs discovered by different surveys over time. The major contribution from the LINEAR programme after 1998 is evident.

(NASA/JPL)

Possible defence against risk of impact The first step in organising a defence against possible collision between cosmic bodies and Earth is obviously discovering the dangerous objects, accurately calculating their orbits and carrying out physicalâ&#x20AC;&#x201C;mineralogical analyses on them. This is the Spaceguard Surveyâ&#x20AC;&#x2122;s objective. It also coordinates existing programmes and oversees the installation of dedicated observatories to keep the entire celestial sphere under continuous control. They are supervised by the Coordination Centre, which also plans observations, archives and analyses the discoveries. The new observatories should also be distributed in equal number throughout the northern and southern hemispheres and an international programme for continuously monitoring the already discovered objects must be activated.

Is it possible though to counter, or at least mitigate, this threat, once it has been ascertained that impacts by asteroids and/or comets pose a serious threat to humanity? The answer is yes â&#x20AC;&#x201C; even though it is quite theoretical for the time being.

Current scientific and technological knowledge comes mostly from military research and it gives humankind the basics to develop some means of defence against the threat of an asteroid or comet discovered to be on a collision course with our planet. Naturally, a lot still has to be done to improve existing techniques, considering the fact that research and experiments with defence systems are an extremely delicate matter. They frequently involve military personnel and technologies, and fall under the 1963 and 1967 international treaties that forbid using nuclear devices in space. It is desirable then that any decision or action in these fields is discussed and approved as widely as possible by the international community.

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Any attempt to protect our planet from a possible cosmic impact must keep track of the object threatening to collide with Earth, be it an asteroid or short period comet. Due to the characteristics of its orbit, such an asteroid or comet is very likely to be discovered many years before a possible impact. On the other hand, a long period comet has a highly elliptical orbit, which means that it may be discovered only a few months in advance. A typical example is comet Hyakutake, which appeared in 1996 and passed by Earth at the minimum distance of around 15 million kilometres just two months after being discovered.

In cases like this, time to react would be very short and it would be necessary to act very rapidly.

Besides warnings of a possible threat, another fundamental factor for deciding the type of defence is the dimensions of the object threatening to collide with Earth. The population of possible impactors can be divided into three categories: Category 1: from 10 to 100 m in diameter In this category, only metallic objects can reach the Earth’s surface with enough speed to produce impact craters. Examples include a small metallic asteroid, no larger than 100 m across, which caused the formation of Meteor Crater in Arizona around 50 000 years ago, and a shower of metallic fragments that came down in the Sikhote–Alin region, in the extreme east of Siberia in 1947.

Rocky bodies of these sizes are partially destroyed when they interact with the upper atmosphere and the resulting fragments quickly slow down before reaching the speed at which they fall under gravity. In these cases, most of the body’s kinetic energy is transferred to an atmospheric shock wave. (Remember that a typical speed for a cosmic object compared to Earth is several tens of kilometres per second.) Part of the energy released from the shock wave is in the form of an explosion that produces light and heat and results in a meteoric fireball or bolide, while another part produces a mechanical wave. Events of this type develop energies between 100 kilotons and 100 megatons of TNT. (By comparison, the nuclear device that destroyed Hiroshima created 15 kilotons of energy.) These phenomena generally occur at sufficient heights that they do not cause damage to Earth.

Naturally, as dimensions increase, the altitude at which the explosion occurs decreases, so the effects of the waves of heat and pressure can produce widespread damage on the Earth’s surface. This happened on 30 June 1908 in the region of Tunguska, central Siberia, where an explosion of a cosmic body measuring less than 100 m across took place at a height of 6–8 km and produced energy estimated at around 15 megatons, destroying over 2000 square km of forest. The combined effects of waves of heat and pressure 227

were very similar to a thermonuclear explosion at a similar height, the only difference being that there were no radioactive effects.

Category 2: from 100 m to 1 km in diameter Rocky or metallic objects over 100 m in diameter can reach the Earth’s surface practically intact and produce a crater that can vary from 10 to 20 times the diameter of the impacting body. The area of destruction naturally extends well beyond the cratered area and the effects, though extremely serious, can be felt on a regional scale, but with few, if any, consequences globally.

Category 3: from 1 to 10 km in diameter The impact of an object in this size range on our planet’s surface would have worldwide consequences, particularly influencing the equilibrium of the Earth’s atmosphere due to the enormous quantity of dust and gas expelled.

It is not possible to take countermeasures for objects from the first category.

The main reason is that this NEO population numbers several hundred thousand, and since these small bodies are not very bright, it is very difficult, if not practically impossible, to identify them in time to adopt even passive mitigating action, such as evacuating the area of the probable impact.

Given the practical impossibility of identifying the smallest objects and considering the somewhat limited damage caused by their possible impact, the NEOs that could collide with Earth and that would have to be deviated or destroyed, are those larger than a kilometre, as they could provoke worldwide catastrophe, and objects over 100 m, because an impact with them could cause regional catastrophes and the deaths of millions of people.

Strategies to mitigate the threat There are basically two strategies that have been identified to counter the impact threat of an object larger than 100 m: deviation of its trajectory or its destruction.

Different techniques have been studied in order to achieve these objectives. They can be grouped into two main categories: nuclear and non-nuclear.

Nuclear techniques Powerful thermonuclear explosions could be used to deflect, fragment or even pulverise an asteroid or comet that is potentially dangerous for our planet. The choice between deflection and total or partial destruction basically depends 228

on the object’s dimensions and the time left to react. Apart from numerous safety and international oversight issues, the use of nuclear type explosives clearly offers an enormous advantage in terms of energy output. In fact, nuclear explosives supply around a million times more energy, per unit of mass, than conventional explosives.

Diverting an object from its original trajectory at a distance equal to, or greater than, the Earth’s radius (6371 km) would be enough to provoke a variation in orbital speed of the order of centimetres/second. A thermonuclear explosion taking place near the target’s surface would radiate its crust with a very intense flow of highly energetic particles, X-rays and gamma rays. This would provoke the violent separation from the surface of a lenticular-shaped area with an average thickness of about 10 cm after it is struck by the shock front. Following this separation and the object’s recoil would be similar to when a rifle is fired, with a variation in speed in the opposite direction to the detached part sufficient to make it divert from its original trajectory. It has been calculated that, in order to detach enough surface material to change the speed of NEOs measuring 100 m, 1 km and 10 km by 1 cm/s, explosive charges with energies of 0.1–1 kilotons, 0.1–1 megatons and 0.1–1 gigatons of TNT respectively, would be necessary. The energy developed by thermonuclear devices available at present, around 100 megatons, would be enough to counter the threat of small objects, but, if necessary, this energy could be increased ten times or more without further testing.

The effect of a thermonuclear explosion would be substantially increased if it took place under the surface. In fact, in this case most of the explosive energy would be transferred to the target and provoke a higher variation in speed compared with a nearby explosion of the same energy. In this case, the explosion would create a crater and high-speed expulsion of a great deal of material (typically around 100 m/s, over the escape velocity for objects with sizes and densities typical of NEOs), and the corresponding reaction would be a variation in the target’s speed. It is estimated that over 30% of the energy freed in the explosion would be transmitted to the material ejected into space.

Image 5. Three different strategies to avoid a NEO collision with Earth. (LLNL)

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A deep thermonuclear charge would fragment or vaporise the object, but the problems would be enormous, even with current technology, because of problems in making a sufficiently deep cavity. The use of a series of small, high velocity rockets as penetrators has been suggested and was tested during the Space Shield programme. A deep hole would be made in the body of the asteroid or comet and a suitable thermonuclear charge then inserted. It has also been calculated that using this technique to pulverise rocky objects sized 100 m, 1 km and 10 km into tiny fragments, would require explosions generating 800 kilotons, 22 megatons and 600 megatons of TNT, respectively.

Naturally, in all the cases described above, the effectiveness of the various types of intervention depends on the object’s physical characteristics. It is necessary to understand precisely its mineralogical composition, structure, shape and rotation before taking any possible action. Sending the necessary thermonuclear charges, which can weigh up to about 12 tons, into interplanetary space is not a problem, since sufficiently powerful launchers are available. The disadvantage of nuclear techniques is that the results are difficult to control: there is always the risk that the asteroid will break up into different segments and head straight towards Earth. At this point the problem multiplies rather than being solved.

Image 6. A possible scheme for disruption of a potentially dangerous NEO. The object is destroyed by means of a thermonuclear charge inserted deep in the body’s structure. (LLNL)

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Non-nuclear techniques Different techniques to divert an asteroid or comet that threatens our planet have been assessed without having to resort to thermonuclear devices, even though this type of approach still needs studying and testing and is more difficult to put into practice than the already complex nuclear option. In theory, these techniques are conceptually feasible, and putting them into practice has been assessed from a quantitative point of view, but the technological problems involved are currently so substantial that they still belong to the realm of science fiction – let us hope not for too long.

Transferring kinetic energy. One conceptually simple method for diverting a celestial body, which we could define as ‘conventional’ because of its similarities to non-nuclear weaponry, is to impact it with a sizeable projectile in an appropriate direction. Even though this technique appears the most banal, by doing some simple calculations it can be seen that enough energy to divert an asteroid sized between 1 and 5 km can be provided by a projectile with a mass of around 1500 tonnes – around 100 times greater than a thermonuclear charge whose explosion would produce similar energy. This would be an excessively large mass to be launched into space, even considering the idea of assembling such a projectile in Earth orbit before directing it towards the target. It would require the launch of no less than 50 Space Shuttles, without considering the necessary fuel to send the projectile towards the object.

One way to increase the relative impact speed is to put the projectile on a retrograde orbit – opposite to the asteroid’s direction of travel. In this way, at the moment of the impact, the relative speed between the two bodies is the sum of their respective speeds. Increasing the speed of impact makes it possible to lower the projectile’s mass. For example, in order to divert a 2 km diameter asteroid following a circular orbit at 1 AU, it is necessary to deliver a projectile with a mass of 60 tonnes and a velocity of 10 km/s. By using a retrograde orbit, an impact speed of 60 km/s can be achieved but the mass of the projectile is reduced to 2.8 tonnes – 5% its previous value. Putting a projectile in a retrograde orbit reduces the mass by 95%, but requires higher launch speed to compensate for the Earth’s orbital speed; this is difficult to achieve with a chemical type rocket. A solar sail could supply enough kinetic energy to put the projectile into orbit, but at the expense of more time, since it would take a few years to reach its definitive orbit.

Given the relatively high speeds necessary to divert the asteroid, the biggest problem of this technique is directing the projectile so that all the kinetic energy lost in the impact goes to change the asteroid’s orbit and just detach fragments or alter its rotation period.

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Gravity tractor. The gravity tractor is a fully controlled asteroid deflection concept using the mutual gravity between a robotic spacecraft and an asteroid to slowly accelerate the asteroid in the direction of the ‘hovering’ spacecraft.

Based on early warning, provided by ground tracking and orbit prediction, it would be deployed a decade or more prior to a potential impact. Ion engines would be utilised for both the rendezvous with the asteroid and the towing phase. Since the gravity tractor does not dock with, or otherwise physically contact, the asteroid during the deflection process there is no requirement for knowledge of the asteroid’s shape, composition, rotation state or other conventional characteristics.

The gravity tractor would first reduce the uncertainty in the orbit of the asteroid via Earth tracking of its radio transponder while station-keeping with the asteroid. If, after analysis of the more precise asteroid orbit, a deflection is indeed indicated, the gravity tractor would hover above the surface of the asteroid in the direction of the required acceleration vector for a sufficient duration to achieve the desired velocity change. The orbit of the asteroid is continuously monitored throughout the deflection process so the end state is known in real time. The performance envelope for the gravity tractor includes most NEOs which experience close gravitational encounters prior to impact and those below 150–200 m in diameter on a direct Earth impact trajectory.

Mass drivers. Another conventional technique is the so-called ‘mass driver’.

This idea dates back to the mid-1970s when human space colonisation projects were studied. It basically involved moving large quantities of material from the Moon’s surface towards sites in space where human settlements would be built.

This technique could be used by exploiting the reaction to the acceleration of material in order to make the orbital speed of a small asteroid vary and then divert it from its collision route.

Although the mass driver project details vary, most use electromagnetic forces to accelerate containers of surface material. It would be necessary to send one of these tools and anchor it to the surface in order to deflect a NEO. The ‘reaction’ material would be excavated on the asteroid’s surface, put in containers and then driven into space in the most opportune direction.

Therefore, with an object rotating around its own axis, the acceleration of material could be done only when the push was in the right direction. Naturally, the ejection of material would take place over a long period, depending on the asteroid’s mass.

Apart from the enormous complexity of setting up a driver plant on a small object and the problems of excavating material in the presence of low gravity, the biggest limitation would be the energy necessary to make this plant work.

The most reasonable sources would be the Sun and/or nuclear sources.

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Space towing. The technique that uses space towing is similar to the mass driver. After having been anchored to the asteroid’s surface, the tow operates ion motors with low intensity force on the asteroid over a long period of time.

In this way, the asteroid’s revolution period can be varied just enough to avoid collision with Earth. The asteroid’s rotation around its own axis can be a problem because it continually changes the direction of the ion jets. Before operating the motors, it will be necessary to change the orientation of the asteroid’s rotation axis so that it is directed parallel to the orbital motion. At this point all it takes is to put the tow on the asteroid’s advancing/receding pole to obtain the variation of the orbital period.

Ion motors have already been tested and demonstrated that they can work for long periods of time. For example, the motor on board the Deep Space 1 probe (launched in October 1998) worked for 677 consecutive days; the ESA SMART-1 mission represents the largest and most successful example of solar electric propulsion. To accelerate the ions, the motor will require electric power of some hundred kilowatts. Considering that the use of photovoltaic cells would cover a large area and cause excessive weight, a small nuclear thermo-electric generator would be enough, like the models installed on the Pioneer and Voyager probes now travelling through the outer Solar System. To avoid environmental risks during the launch phase, the radioactive power source can only be activated after launching the tow, at a safe distance from Earth.

Solar sails. While mass drivers use material extracted by the asteroid, solar sails use pressure from solar radiation to supply a small but constant push to deflect a dangerous NEO. This technique was proposed in the late 1980s as a means of interplanetary navigation and research was started to send three probes towards Mars using solar sails bearing the names of Christopher Columbus’ three caravels.

As the pressure of solar radiation is very weak, it is clear that to obtain the necessary push to divert an object sized between 1 and 10 km over a period of a few years, it would be necessary to have sails with diameters of a hundred kilometres. Although solar sails of 1 km in diameter are possible with current technology, launch and assembly in space of structures with diameters 100 times greater represents a challenge that goes well beyond current possibilities. Another problem with this technique is anchoring the enormous structure to the asteroid, particularly considering the rotation around its axis.

Methods to solve these problems exist conceptually but putting them into practice is extremely complex. It is, however, the most ecologically clean system compared to all the others.

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Solar collectors. This technique can be compared to Archimedes’ ‘burning glass’. Solar sails with diameters of between 1 and 10 km and coated with reflecting material, could be used to collect solar light that would then be directed onto the surface of the asteroid or comet. The concentrated radiation would subject the surface to very high temperatures, causing it to vaporise and produce a jet of matter. The resultant push would be very similar to jets of gas that are present in cometary nuclei and perturb their Keplerian orbits (see chapter 3). Although this is a similar technique to the mass drivers, it is not necessary to transport and set up equipment on the body’s surface.

The solar collectors’ main advantages are that it is not necessary to anchor the system to the object (so the asteroid’s rotation is not important) or work on its surface. This technique also gives a continuous, but not explosive push, minimising the dangers of fragmentation. The system also has the advantage of being reusable. The main problems in using it stem from the considerable complexity of manoeuvring in space in order to maintain the enormous mirror’s correct orientation and distance with respect to the target. But the most serious problem in using solar collectors is probably the degradation that the reflecting surface undergoes due to gases and dust from the vaporising target’s surface.

Laser and microwave systems. An obvious solution to the problem of mirror surface degradation caused by the dust and gas ejected from the vaporising surface material is to use the energy collected by the mirror not directly, but to convert it into electric energy, then produce powerful laser or microwave beams and focus them onto the cosmic body. This system would move the solar collector a great distance from the target, thus keeping it well away from the vaporised material. The greatest disadvantage of a technique of this type stems from the fact that its technical complexity increases enormously and converting solar energy into laser or microwave radiation involves consistent losses.

There are different possible approaches then to divert a cosmic object threatening Earth without having to resort to using nuclear devices. They are obviously techniques that still need a great deal of research and are sometimes technological challenges beyond currently attainable limits. It is clear however that, although nuclear devices are simpler to use, they are not the only solution to the problem of deflecting a cosmic body on a collision course with Earth.

At the moment, however, it seems unthinkable to put up a defence system against the threat of impact on our planet by an asteroid or comet. The main reason for this is that, although these events will certainly take place at some time in the future, they actually happen very infrequently, when compared to the timescale of humankind’s evolution. The first organised civilisations 234

Image 7. Solar or laser deflection. A large mirror collects and focuses the Sun’s light, or a laser is aimed towards the NEO, in order to vaporise the object’s surface material. The momentum of the jet that is produced slowly changes the NEO’s orbit. (LLNL)

appeared on Earth around 5000 years ago, while the probability of an asteroid or comet impacting our planet and provoking a global catastrophe is estimated at once every several hundred thousand years, on average. Another important element is the very high cost of such a defence system and the low probability of a threat that cannot easily win public support. This also touches on other urgent issues – like health, nutrition, pollution, global warming, etc. – that affect most of humanity, as well as natural catastrophes that happen every year.

What could and should be done is – at less cost compared to resources invested in, for example, weaponry – to compile as soon as possible an international inventory that includes all of the undiscovered objects whose orbits cross Earth’s.

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Chapter 9

Impact craters Introduction In over 40 years of exploring the Solar System with space probes, one of the most important discoveries is that all bodies with solid surfaces – planets, satellites, asteroids and comet nuclei – are pitted by craters. The process of forming craters is one of the fundamental processes in the shaping and evolving of planetary surfaces. The celestial bodies mostly involved are the terrestrial planets, Mercury, Venus, Earth (with the Moon) and Mars, as well as asteroids and the satellites of the four giant planets.

Craters are formed when a meteoroid, asteroid or comet impacts the solid surface of another celestial body. A crater’s morphology depends on its diameter. The smallest craters are bowl-shaped, larger craters have a central peak and those with even greater diameters have a series of concentric rings that surround the peak. This morphological difference is not the direct result of the process that leads to the excavation of a crater, but the result of relaxation processes that intervene later.

According to current theories, the direct result of the impact process is a transient crater; it is circular and shaped like a bowl, with a depth/diameter ratio between 1:3 and 1:4. This ratio is independent from the diameter, impact speed, angle of descent (if it is not too grazing to the surface) and surface gravity of the impactor. The transient crater will quickly change because of gravitational instability and the collapse of the materials involved in the impact.

The final crater’s morphology depends on the the physical parameters of the celestial impactor, including acceleration due to gravity and the density and type of the surface material.

Studying the formation of a transient crater can be done by using Newtonian mechanics and thermodynamics. It is, however, much more difficult to understand the subsequent stages of collapse, because it is necessary to have in-depth understanding of the dynamics of rocks and granular materials.

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Types of impact structures In 1965, M.R. Dance first classified terrestrial impact craters into two groups: simple and complex. This classification is based on craters that are exposed to the strong erosive action of the Earth’s atmosphere, although it can also be applied to craters on all celestial bodies within the Solar System.

Simple craters Simple craters are circular, with an approximately parabolic section. There are no other morphological characteristics present, if we exclude the occasional landslide of material that slips along the inner walls towards the centre. The average depth/diameter ratio of a large sample of lunar craters of this type is 1:5. Simple craters are very common in the Solar System. Most craters on small bodies, like asteroids or small satellites, are of this type. The largest known simple crater has a diameter of 90 km and is on Amalthea, one of Jupiter’s innermost moons. Most lunar craters with diameters below 15 km are simple, as are the craters on Earth that measure less than 4 km. Analysis of the bases of these simple craters reveals the existence of a lenticular-shaped layer of brecciatype rocks, known as a breccia lens. The thickness of the breccia layer is around half the current depth of the crater, while the volume of the breccia lens is approximately half the volume of the crater. The distribution of blocks of fused rock within the breccia suggests that the lens is created when the crater’s walls collapse immediately after its formation.

Complex craters Complex craters have a more complicated structure than simple ones, due to their central peaks and multi-ring shapes. The formation process in this case is more complicated. Following the impact, a transient crater is first formed, as in the case of simple craters. The compression suffered by the ground, however, is enough to make the base of the crater rebound upwards at the point of impact and form a central peak. The process is similar to throwing a stone into water: after the impact, it causes a splash that travels upwards. At the same time, the gravity of the impacted body and substantial size of the crater cause its outer rim to collapse, partially filling the inside. The falling rim also tends to widen the crater. In the end, the ratio between depth and width will be decidedly less (from 1:10 to 1:20).

Measurements show that a complex crater becomes deeper as its diameter increases, although more slowly compared to simple craters. The depth of a complex lunar crater increases according to the law D0.3, where D is the diameter.

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Image 1. These diagrams show two fundamental types of impact crater: a simple one (above) and a complex one (below). The first example has a classical bowl shape. In the second, the structure has a central peak and clearly bevelled edges caused by a more energetic impact. (NASA)

It is the same for craters on Mercury and Venus. Diameters of complex craters range from around 5 km on Earth to 460 km at the south pole of the asteroid 4 Vesta, which we discussed in chapter 5. The giant planets’ frozen satellites also have some complex craters. The transition from simple to complex crater takes place over a narrow range of diameters and is different for every single body in the Solar System. The diameter where the transition takes place is inversely proportional to the acceleration due to gravity on the celestial body where the crater will be formed. On the Moon, the transition diameter is about 15 km, on Mercury and Mars it is around 7 km, while on Earth it is approximately 5 km. For Jupiter and Saturn’s frozen satellites the transition diameter is smaller than it would be if the satellites were made of rock instead of ice.

As the diameter of a complex crater increases, one or several prominent rings surround the central peak. This type of crater has been observed on all 239

the terrestrial planets, including the Moon, while they are rare on the surfaces of frozen satellites, due to the different physical properties of ice compared to rock.

Multiple ring basins The largest impact structures are shallow craters with a relatively flat base that are surrounded by concentric rings of mountain ranges or fractures in the crust. The classical example is the Mare Orientale on the Moon. It has a diameter of about 900Â km and the ejecta can easily be identified. There are five series of concentric slopes, at respective distances of 180, 240, 300 (Montes Rook), 465 (Montes Cordillera) and 730Â km. Outside the last ring are numerous secondary craters that were formed by fragments expelled while the basin was being formed.

Image 2. Map of the great Hellas basin on Mars. The basinâ&#x20AC;&#x2122;s diameter is about 2100 km and it is the lowest point on Mars. It was formed after the fall of an asteroid of around 160 km in diameter. (USGS/NASA)

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Another example of a basin with rings is on Callisto, the outermost of Jupiter’s four Galilean satellites. The basin has been called Valhalla, after the residence of the god Odin. This structure shows a clear central region, 600 km in diameter, surrounded by a system of fractures set out in concentric rings.

Similar, but smaller, structures are also present on Europa.

Both types of basins are influenced by the lithosphere in the hollow created during the impact. The formation of a multiple ring basin therefore indicates the presence of a layer of low viscosity material under the crust.

Not all planets have structures like these. They have been found on Venus and Earth (e.g. the Chicxulub crater, discovered in the early 1990s in the Gulf of Mexico), but not on Mars, where the enormous circular basins, Hellas and Argyre, do not show any signs of concentric rings.

Multiple ring basins are formed following a type of collapse of the transient crater that is qualitatively different from other types of craters. In fact, they seem to depend heavily on the existence of a substratum that is able to flow over the time scale of the stages of collapse of the transitory crater. Their origin, however, is not yet well understood and there is still a lot of work to be done in this field. In the following paragraphs, we will consider the formation process of both simple and complex craters.

Crater chains Impact craters are usually created by single bodies. A small fraction of craters on Solar System bodies is, however, found in chains. A crater chain is defined as a group of three or more similarly sized impact craters of the same age, positioned in a straight line. Classic examples are the approximately 20 crater chains discovered by Voyager 1 in 1979 on Ganymede and Callisto. Made up of between 4 and 25 craters, these chains vary from 60 km to more than 600 km long.

The genesis of the crater chains is not clear, but there are two main theories.

One process could be the effect of ejecta created by the formation of a main crater, but this supposes that a certain number of secondary fragments is expelled on trajectories that only differ in their range. A second theory, formulated in 1993 after the discovery of comet Shoemaker–Levy 9, refers to the fall of fragments of a cometary nucleus (like SL-9) or asteroid after it is disrupted by a planetary body’s tidal forces. This second theory does not require ad hoc processes and is more probable than the first.

Eight crater chains are also found on the Moon and it is probable that their genesis is similar to Jupiter’s satellites – in this case the body responsible for the tidal fragmentation of the parent body was Earth. The best known are the Davy Chain, 47 km long and formed by 23 craters with diameters between 1 and 241

3 km, and the Abulfeda Chain, 200 km long, and formed by 24 craters with diameters between 5 and 13 km. There is also a pair of structures on Earth that could be crater chains. The first potential chain is 700 km long and comprises a series of eight circular depressions (diameters 3–17 km) that cross the states of Kansas, Missouri and Illinois. The second is in Chad, where two possible impact structures were discovered by satellite radar observations next to the 13 km diameter Aorounga crater, which is at least 345 million years old. The nature of both structures is controversial and needs closer investigation.

The formation of craters Overall, an impact process is similar to an explosion, where the cosmic body’s initial kinetic energy is used to a large extent to hollow out the material from the target’s surface. The impact process can be divided into three phases: 1. Contact and formation of a shock wave.

2. Formation of a transitory hollow with the expulsion of material.

3. Collapse of the hollow and definitive formation of the crater.

When a meteoroid strikes the ground at a speed V of the order of 10 km/s, it produces a shock wave that propagates in the rocks of the target as well as in the impacting projectile. Because of the compression produced by the shock wave, the pressure in the rock reaches values equal to millions of times greater than atmospheric pressure. The volume originally occupied by the rock can be reduced to one third of its normal value. In this phase, the rocks react as if they were a viscous fluid. After the shock wave is formed, it expands radially from the site of the impact, compressing and accelerating the rocky layers it meets. After the shock wave has passed, the pressure drastically decreases until it annihilates itself due to a decompression wave, but the rock particles maintain around one fifth of the wave’s speed. This residual speed causes the material to be expelled at the point of impact. In the meantime even the meteoroid has been fused and partly vaporised, and is expelled together with the other debris (ejecta) from the hollow that is being created. That is how a transitory crater is formed.

The quantity of ejecta is approximately comparable to the material compressed by the shock wave at the base of the crater. As the shock wave travels, some rock minerals vitrify, others re-crystallise. In quartz crystals, for example, striations, known as planar deformation features or PDFs, are formed and oriented in several directions. Re-crystallisation can take place in a new mineral like coesite and stishovite, and can be revealed by X-ray diffraction.

The rocks thus transformed are called impactites.

The rocks also undergo a process of fragmentation and so-called shatter cones are formed. They are cone-shaped rocks that form following high 242

pressure and are recognisable in the impact craters together with microstructures in quartz, chemical changes in minerals and unusual rocky grains. Through this type of mineralogical analysis it is sometimes possible to locate impact structures even where a crater is no longer visible.

Target rocks increasingly resist deformation as the impacting force penetrates them, so the transient craterâ&#x20AC;&#x2122;s diameter is greater than its depth. The craterâ&#x20AC;&#x2122;s depth and diameter, taken separately, depend on surface properties such as density, resistance to deformation, gravity acceleration, etc. The depth to diameter ratio is independent of these parameters, as we said before.

After the transient crater has been formed, the collapse phase takes place, causing it to take on its definitive shape. But why does it collapse? For many years experts talked about an elastic rebound of the crater base, as mentioned before. But this process does not explain why the diameter at which the transition between simple and complex craters occurs is different for every celestial body.

This suggests that it is the gravity and not the elastic force that is responsible for the transient crater collapsing. For simple craters the force of gravity is limited to making material flow along the crater walls, creating the breccia lens and parabolic profile. Things are more complicated and not well understood for complex craters.

The block model manages to explain the structures of complex craters. In this model, the behaviour of a transient crater formed of discrete blocks of rock (granular material that is no longer continuous) is analysed in a constant gravitational field and with sizes proportional to the Image 3. Drawings of the formation sequence of a simple type of craterâ&#x20AC;&#x2122;s diameter. Results of impact crater. (NASA) 243

numerical simulations indicate that only above a certain diameter do oscillations from the blocks of rock form the final crater’s central peak. The model also reproduces the depth/diameter ratio that is obtained during observations.

Following the results of hypervelocity impact experiments on different target bodies and observations of nuclear explosions and lunar craters, some scaling laws have been drawn up. They give the crater’s diameter based on the impacting body’s kinetic energy, the angle of the fall, the density of the projectile and target, and the acceleration of gravity on the target’s surface.

One of the simplest of these laws is Gault’s law (1974). According to this scaling law, a crater’s diameter is calculated by: −1/2 D = 0.35ρ1/6 ( gt /1.67) p ρt

−0.165

W 0.28 (sin θ )

1/3

In this formula, all the quantities are expressed in SI units, where D is the crater’s visible diameter (in metres), measured from one crater edge to another, ρ and g are the density and acceleration of gravity with the index p/t referring to the projectile/target, W is the projectile’s kinetic energy, while q is the angle that the direction of the fall forms with the target’s surface. As we have mentioned before, the angle q does not influence the crater’s shape unless the fall is almost radial. Only when q < 10° does the crater’s shape tends to be elliptical rather than circular.

Let us look at a practical example of the formula, considering a typical rocky asteroid of 1 km in diameter with a density ρp = 2500 kg/m3 and a mass m = 1.3·1012 kg. If this impactor falls vertically (q = 90°) to Earth (crustal density ρt =3500 kg/m3, gt = 9.81 m/s2), moving at a speed of 15 km/s, its kinetic energy is W = 1.47·1020 joules and the crater’s diameter D = 7200 m. As we can see, the crater is much larger than the projectile and this scale relationship quickly increases as the kinetic energy increases. As a practical rule, we should remember that for a speed of 25 km/s, terrestrial craters that have been formed in rock have a diameter 20 times greater than the projectile, while this factor drops to around 10 on soft or sandy land.

Craters on Earth Looking at images of the Moon, Mercury, Mars or the frozen satellites of the giant planets makes us realise how impact craters had a predominant role in shaping the surfaces of the solid bodies in the Solar System. At first sight, the Earth’s surface does not seem to have undergone the same process. But this impression is deceptive, basically due to the fact that Earth is still a geologically active planet and the atmospheric elements (rain, wind, etc.) have a rapid, erosive effect on geological structures. The same is not true, however, for 244

smaller planetary bodies that have been geologically dead for many millions of years and deprived of a consistent atmosphere. Even very old impacts have left, and still leave, a nearly indelible sign.

Some evidence of ancient impacts on Earth is also still traceable and visible in images taken by artificial satellites. They obviously cannot be very old – no more than two billion years old, as volcanic and tectonic phenomena have repeatedly reshaped our planet’s surface. In some cases they can still be easily identified.

It should also be remembered that around 70% of the Earth’s surface is covered by sea. Impacts that took place in oceans are difficult to locate because the sea bed cannot be analysed in detail, and also because marine erosion and sedimentation act very quickly (geologically speaking) to eliminate geological structures. On the Earth’s surface, more than 170 impact craters have been identified to date, with sizes that range from tens of metres to over one hundred kilometres. The distribution of these structures on Earth is not homogeneous, as some regions have a high concentration and others hardly any. This is not due to non-random impacts with certain parts of our planet, but rather is related to the age of the continental crust. Obviously, the older cratonic regions will show more evidence of older impacts, but the age of the crust will never exceed two billion years – unlike other bodies in the Solar System, such as the Moon or Mercury.

Where are the cratonic regions? There are basically four: northeastern America, central-northern Europe, the southern part of Africa and Australia.

Image 4. Map showing the distribution of the biggest impact craters known on the Earth’s surface. (Natural Resources Canada)

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Image 5. One of the largest visible impact craters on the Earthâ&#x20AC;&#x2122;s surface is Popigai. Situated in northern Siberia, it has a diameter of around 100 km and its age is estimated at around 35â&#x20AC;&#x201C;40 million years. (LPI)

Image 6. Clearwater Lake in Quebec, Canada, is an interesting impact structure.

It is formed of two adjacent lakes with diameters of 26 and 36 km, respectively. The double crater is probably due to the fragmentation of the impacting body as it passed through the atmosphere. The structure is around 290 million years old.

(LPI)

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Table 1

List of Earth’s impact craters with diameters over 50 km Name

Country

Diameter (km)

Age (Ma)

Acraman

Australia

160 (90?)

570

Beaverhead

Montana, USA

60

600

Charlevoix

Canada

54

375 ± 15

Chesapeake Bay

Virginia, USA

85 (90?)

35.5 ± 0.6

Chicxulub

Mexico

170

64.98 ± 0.05

Kara

Russia

65

73 ± 3

Manicougan

Quebec, Canada

100

212 ± 1

Popigai

Russia

100

35 ± 5

Puchezh–Katunki

Russia

80

220 ± 10

Siljan

Sweden

55 (52?)

368 ± 1

Sudbury

Ontario, Canada

200 (250?)

1850 ± 3

Tookoonooka

Queensland, Australia

55

128 ± 5

Vredefort

South Africa

140 (300?)

1970 ± 100

Morokweng

South Africa

70

145 ± 0.8

Kara–Kul

Tajikistan

52

<5

Table 2

The first 300 seconds of an impact Time (seconds)

Event

0

The asteroid reaches the outer limit of the atmosphere.

2.00

The asteroid penetrates the Earth’s surface. Its speed has been reduced to 35000 km/h.

2.03

The asteroid stops at a depth of circa 1 km. Like an ultra-compressed gassy body, it explodes.

At the same moment, the decompression wave propagates from the base of the crater at a speed of circa 70 000 km/h and starts its expulsion of molten, pulverised and vaporised rock.

2.13

The expulsion of material reaches its maximum.

5.00

The mass of air from the low atmosphere removed during the asteroid’s passage, drops towards the crater.

20.00

The crater stops expanding. A transient crater 15 km wide and 4.5 km deep develops. Its outer edges reach several hundreds of metres. At the same moment, the crater base starts to rise violently.

20–120

The expelled rocks fall on to the ground, covering a radius of circa 50 km around the crater.

The ball of fire above the crater starts collapsing. This mix of solid, liquid and gassy material falls onto the crater and surrounding area. Enormous blocks of rock slip and move from the crater edge, widening it as much as 25 km, while the crater centre rises to 3 km.

300

All the explosive events terminate.

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Image 7. The very young New Quebec impact crater in Canada, which dates back to 1.4 million years ago. It currently looks like a circular lake with a diameter of 3.5 km. (LPI)

Estimates suggest that only 10% of all craters larger than 10 km and younger than 100 million years have been discovered. Most craters are recorded in those few parts of Earth’s continental surface that have not undergone substantial reshaping. Around 25 impact craters have been found in Canada, 20 in Australia and 7 in Ukraine. Large numbers of unknown craters probably exist in southern America, Asia and Africa, because in-depth research has never taken place there.

Due to erosion by Earth’s atmospheric elements, it is difficult to find well preserved, complex structures. The central peak tends to flatten and the outer edges become increasingly less evident. At times, the erosion also shows the innermost morphology by eliminating the less resistant surface layers.

Concentric rings, very similar to the rings that are created when a stone drops into water, can be recognised.

There are also some cases of double craters. These can be produced by a monolithic body that splits into two because of the tidal interaction with the planet towards which it falls – similar to the formation of crater chains – or by the fall of a binary asteroid. At other times, close to large craters, small ones can be found that have been formed by the impacts of large fragments ejected during the main impact. Generally, the remains of the impacting body can be found in craters that are not very large. In fact, if the projectile is larger and 248

creates a wider crater, the collision event generates extreme pressure and temperatures that vaporise the impacting body or melt and fuse it with the surrounding rocks, thereby making it indistinguishable.

Since the 1960s, research on impact craters has highlighted another unusual characteristic tied to impact events that is very different to other terrestrial phenomena. This is basically shock metamorphism, which we discussed previously, that leaves unique signs on the rocks that were affected. A complete list of Earth’s impact craters is shown in the appendix. The table on page 247 describes the first 300 seconds following the impact on the surface of a 1 km diameter object. NASA simulated the sequence of events with an experiment in the late 1980s.

Some of Earth’s craters Meteor Crater The most famous simple type of crater is certainly Meteor Crater (or Barringer Crater) in Arizona, which was the first impact crater to be recognised on Earth.

It was identified as such around 1920, thanks to metallic fragments from the impacting body that were found inside it and in the surrounding areas. It is a milestone in this type of research and studies have opened up new issues concerning impacts.

The crater is at latitude 35° 02ʹ north and longitude 111° 01ʹ west, with a diameter of around 1200 m and a depth of 200 m. The crater’s formation was due to the impact of a metallic body with a diameter of 40–50 m, around 50 000 years ago. Meteor Crater is not perfectly circular, but tends to resemble a square. A crater can differ from a circular shape if the meteoroid’s impact on the surface is radial or the mechanical properties of the ground where the impact occurs are not isotropic, but variable according to the azimuth.

Image 8. One of the largest, relatively young craters is Kara-Kul in Tajikistan. Partially filled with water, its diameter is 52 km and it is younger than 5 million years. (LPI)

Image 9. Image of the famous Meteor Crater in Arizona.

(D. Roddy, LPI).

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Image 10. One of the most famous impact craters still recognisable on our planet is Manicouagan in Quebec, Canada.

It currently looks like a ring-shaped lake with a diameter of just under 100 km. It is very old – around 215 million years – and has badly suffered from erosion. (LPI)

The debate on this crater’s origin was particularly heated. Geologist G.K. Gilbert carried out the first investigation in 1891. Despite the iron–nickel fragments found in the surrounding area and the absence of lava, Gilbert reached the conclusion that the crater had to be of volcanic origin. In 1902 engineer Daniel M. Barringer, convinced of the crater’s extraterrestrial nature, bought the land and began digging at the crater’s base in the hope of finding the metallic meteorite that had formed it when it fell (Barringer wanted to exploit the iron–nickel meteorite commercially). In 1929, research was suspended because of lack of funds and Barringer’s death, without having found the meteorite. It was Eugene Shoemaker (one of the discoverers of comet Shoemaker–Levy 9) who, 30 years later, showed that Meteor Crater was unquestionably caused by the impact of an extraterrestrial body, after finding minerals that could only form under conditions of very high pressures and temperatures.

Manicouagan Crater One region where searches for impact craters have been fruitful is the Canadian Shield, near Hudson Bay. In this area, the Earth’s crust is more than a billion years old and has been markedly cratered. The smaller impact structures have 250

been completely eroded by atmospheric and surface elements, but the largest ones have survived the abrasive action. A typical case is Manicouagan Crater (lat. 51° 23ʹ N, long. 68° 42ʹ W), which is shaped like a ring, has a diameter of about 70 km and is 214 million years old. The ring is now a lake, which makes the crater’s circular shape easily visible from above. This ring structure was formed after the crater’s edge eroded – being made of breccia type material, it was not very resistant. The crater base, however, is far more resistant and has remained at a greater altitude than the edge – thus forming the ring-shaped lake. The rocks on the site show abundant signs of impact metamorphism – alterations that took place following brief exposure to high pressures and temperatures.

The double craters of Clearwater and Arkenu There are only a few double craters on Earth. A typical structure of this type is the double crater of Clearwater in Quebec, Canada (lat. 56° 09ʹ N, long. 74° 18ʹ W). The craters are located in an east–west direction and the east crater has a diameter of 26 km, while the west crater is 36 km across, with an inner ring of mountains. The central peaks are not visible because the craters have been invaded by water and are now a pair of lakes. The estimated age is about 290 million years.

Another double crater has recently been discovered at Arkenu in the Libyan desert, but the origin and nature of these two structures are not yet clearly defined. The double crater is at latitude 22° 04ʹ N and longitude 23° 45ʹ E, and is clearly visible in satellite images. The large crater has a diameter of 10.3 km, while the other is around 6.8 km. Both craters are complex and the maximum estimated age is less than 140 million years. The discovery of the double circular structure was made following analyses of radar images taken by Japanese satellite JERS-1, which operated from 1990 to 1998.

Radar waves can penetrate some metres below a layer of desert sand and show up the structures buried there. At optical wavelengths, images like those obtained with the Landsat satellites only show the crater centres that are filled with sand that is darker than the surrounding areas. An international expedition in the area was organised in April 2003, and it highlighted the presence of shatter cones oriented towards the crater’s centre. Impact breccia were also found, showing the structures’ extraterrestrial origin. More studies are needed to confirm definitively the impact origin of these structures.

The Chicxulub crater and the extinction of the dinosaurs The cause of the more or less sudden disappearance of the dinosaurs, approximately 65 million years ago, has long been a source of tireless discussions 251

among scientists from different disciplines. Since the early 1980s, the theory that a 10 km wide asteroid or comet fell on Earth has had a considerable following, especially after the discovery of the famous Gubbio layer and subsequent geological and paleontological research. Near Gubbio (Umbria, Italy) a sedimentary layer with a thickness of a few centimetres was found between alluvial deposits that dated back around 65 million years. It contained a very high concentration of iridium, an extremely rare chemical element on the Earth’s surface that was expected to be found in asteroid type bodies. Further researches, carried out in other areas on Earth, confirmed the existence of similar, widely scattered thin layers, at the same time highlighting other geological signs that are characteristic of high energy impacts. The study of microfossils found both above and below the Gubbio layer seems to confirm the drastic disappearance of a very high percentage of living species over the following period.

One of the main questions concerning this theory was where the object had fallen. Where was the crater? The theory’s promoters answered that it was probably in the sea bed, but the lack of the most obvious sign was a big handicap until 1991. In that year, thanks to highly sophisticated magnetic and gravimetric measurement techniques, a circular structure was discovered. Located between the northern part of Yucatan peninsula and the Gulf of Mexico, it was covered by a thick layer of sediments that prevented its direct observation.

Further studies were able to accurately establish that the structure was what remained of an impact crater whose age was precisely calculated as 65 million years, exactly the same as the iridium-rich Gubbio layer. The crater’s diameter is

Image 11. The largest impact crater recognised to date on Earth is in South Africa. Called Vredefort, it has a diameter of around 300 km and is around 2 billion years old. (LPI)

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Image 12. A striking, albeit small (and young) crater in Wolf Creek, Australia. Its diameter is 900 metres and it is less than 300 000 years old. (LPI)

around 170 km and matches what is expected for the impact of an object sized around 10 km. The Chicxulub crater (the word ‘Chicxulub’ is of Mayan origin and is pronounced Sheek-suh-loob) practically removed every doubt about the origin of the layer of iridium: it originated from the worldwide repercussions that followed enormous quantities of dust rising from the crater and the vaporisation of the impacting body. This dust would have been rapidly distributed around the world by strong stratospheric winds, so that sunlight would have been obscured for several months, provoking a drastic climatic change.

Many, however, still doubt the connection between the Chicxulub event and the extinction of the dinosaurs and more than 30% of the marine animal genera living on Earth at that time. All the reasons for such a dramatic end are very probably there and the coincidence of the two phenomena is certainly amazing. If the event was not the direct cause of the dinosaurs’ extinction, it certainly gave the coup de grace to these species, particularly since they may have already been in crisis for other reasons. In that period, one of the major breaks in the history of life on Earth occurred, the so-called K/T event which corresponded to the transition from the Cretaceous to the Tertiary period.

Traces of tsunamis (gigantic marine waves) that destroyed everything within a radius of about a thousand kilometres have been found in the regions surrounding the crater. The crater itself has a multiple ring structure and is similar to the Moon’s Mare Orientale, even though it is much smaller. There are three concentric rings: the innermost has a diameter of 80 km, while the others are 100 and 180 km. To date, Chicxulub is Earth’s only crater to show this type of structure.

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Image 13. Despite not being visible on the surface, Chicxulub is certainly the most famous impact crater on Earth, situated between the Gulf of Mexico and the Yucatan peninsula.

65 million years old, with a diameter of around 170â&#x20AC;&#x201C;180 km, it is considered to be evidence for the catastrophic K/T event that separated the Cretaceous and Tertiary periods.

According to many researchers, the Chicxulub impact is also connected with the extinction of the dinosaurs and other living species. (Geomagnetics Canada)

Image 14. This three-dimensional map of the Chicxu-lub crater was made by precisely measuring the variations in the local gravitational and magnetic fields. The impact basin is currently covered by hundreds of metres of marine sediments.

(V. L. Sharpton, LPI)

Image 15. An artistâ&#x20AC;&#x2122;s impression of the impact that caused the Chicxulub crater around 65 million years ago and sparked off the equally famous extinction of the dinosaurs. The impacting body must have had a diameter around 10 km. (NASA)

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Chapter 10

The fall and retrieval of meteorites

As we have seen, finding fragments of cosmic origin is of the utmost importance in understanding the Solar System, as well as the processes that might lead to bodies impacting Earth.

Witnessing a meteorite falling to the ground is an extremely rare event, even though it is estimated that, on average, 1000 tonnes of meteorite material fall to Earth every year, including around 30 000 fragments about ten centimetres across and around 10 000 that fall on land. Only a small fraction of these fragments lands in areas where population is high enough to make witnessing an event probable. If we apply these numbers to an Italian region like Piedmont, it is surprising to discover that it collects around 50 kg of material of cosmic origin in one year, primarily in the form of small meteorites and microscopic dust.

Investigating events published in the media, we discover that a few meteorite falls have caused immediate damage to houses and cars or have taken place just a few metres from witnesses. The cases of people being hit directly or wounded are extremely rare. One of the more sensational examples happened in 1954, when a woman in Sylacauga (Alabama, USA) was struck by a 3.7 kg meteorite after it pierced the roof of her house while she was reading a book in bed. In 1992, a child in Uganda was hit on the head by a fragment weighing around 4 grams that came from a meteorite that exploded in the atmosphere. Luckily, serious wounds were avoided thanks to a banana tree that broke the fall of the ‘bullet’.

On 26 September 1999, a 136 gram meteorite destroyed the roof of a house in Kobe, Japan, ending its travels on a child’s bed (who was not hurt). On 28 November 2003, a fall of meteorites in the district of Orissa, eastern India, damaged several residences and caused panic among the inhabitants. A meteorite weighing 13 kg fell in Auckland, New Zealand, on 13 June 2004 and finished its travels in the living room of a residence, after piercing the roof.

Finally, Chinese documents dating back to 700 B.C. recount a long series of similar events up to the 20th century, including news of seven people who were killed by meteorites and a whole family that was killed by a large object that destroyed their house in 1907.

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Most of these extreme examples that happened a long time ago cannot be proven. It is clear, though, that the probability of someone being hit by a meteorite is much lower than being hurt in a plane or car accident, or a natural catastrophe. This is not in disagreement with the general impact hazard that was computed in chapter 7, since here we are speaking of the probability of being directly hit by a meteorite. In chapter 7 we dealt with the risk of being killed as a consequence of an impact with an extraterrestrial body, and this is different, since the risk associated with energetic events does not concern only the people located at the impact point.

On the other hand, there is a much greater chance of having the opportunity to witness the appearance of very bright bolides, even in daylight, followed by a fall of meteorites. In this case, there are many people who watch the events and their testimonies are essential in reconstructing the route taken by the body and possibly identifying where it fell. Random witnesses frequently have the impression that the object is nearby or that it is going to fall in the garden next door; in fact, the point where it finally falls is generally hundreds of kilometres away, provided that a meteorite actually reaches the ground, since the body producing the bright bolide may well be completely destroyed in the atmosphere. The probability of mistaken perceptions is high, which is why it is necessary to find the most accurate and reliable witnesses possible.

These events are also unusual due to the perception of various types of sounds, sometimes thunder, creaks, detonations, cracking of a whip and other numerous variations. Bearing in mind the observer’s distance from the object when it reaches the thick atmospheric layers at an altitude of 10–20 km, the sound should come at least half minute after the event. However, the sound is sometimes contemporary, which leads us to think that the electromagnetic waves issued by the red hot gas that surrounds the body as it falls are responsible for making the noises, as well as making objects near the observer shake. The sounds produced are called ‘electrophonic’ and were covered in depth in the chapter on meteors and bolides.

It is obvious that, by seeing a bolide or meteorite fall, the witness becomes an important collaborator. In both cases, his/her sightings should not be neglected or lost and must reach the experts. Another more important aspect is for witnesses to record the key elements. To help readers with this assignment, we will describe the elementary procedures that anyone can put into effect should they find themselves in the fortunate position of being able to do so.

Sighting bolides A bolide is an exceptionally bright meteor, which generally outshines Sirius, the brightest star visible to the naked eye. (Chapter 4 provides more detailed 256

information on the physics of bolides.) Systematic night observation of meteors and bolides is coordinated by both professional and amateur research groups, and comes under the International Meteor Organisation (IMO).

Here we shall consider exceptional situations, where a very bright bolide, sometimes at least as bright as the full moon, is sighted by a casual observer. This is the situation when bolides are seen by many people, not necessarily in daylight.

Their testimonies can make it possible to determine an object’s trajectory by triangulation methods, as well as the orbit and possibly the point of its fall.

So, what should you do when you see a bolide? Firstly, you must quickly recover from the surprise and memorise or write down two essential points: 1 The instant of the event: look immediately at your watch and make a note of the time as accurately as possible. Writing down the precise instant of the phenomenon is essential in determining the trajectory. Then by using an accurate clock, check that your time is correct, and if not, proceed to correct it.

2 The trajectory: try to have an idea of its duration (usually a few seconds).

Before moving from the location of the sighting, write down the trajectory’s direction and initial and final points in relation to the visible stars. If the event happens during the day, use elements of the landscape (buildings and trees, etc.) as a reference. The best way to record the data is to draw the position of the main stars or the elements of the landscape and the bolide’s trajectory.

It is worthwhile to immediately add the following: 3 Information on the circumstances. Make a note of the exact place, as accurately as possible, as well as the previously noted date and time of the event.

4 An estimate of the brightness. It can be compared with other celestial bodies, such as (in order of increasing brightness): Venus, the waxing Moon, Moon in its first quarter, full Moon, the Sun. 5 The bolide’s colours. These sometimes change along its trajectory, so it is worthwhile noting them in sequence.

6 The duration of the trail in seconds and its colour.

7 Possible acoustic phenomena (described previously), duration and the moment they are audible in relation to the bolide appearing.

8 The witness’s name, surname and telephone number, as well as other possible impressions or characteristics of the phenomenon that have not been considered here.

Finally, it would be quite useful to quickly find out if there were other possible witnesses in the neighbourhood and help them compile a similar account of the event. A copy of the notes should be sent to the collection centres mentioned below.

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BOLIDE DATA COLLECTION CENTRES In Italy, research into bolides is not systematic on a professional level, as there are no European Fireball Network stations. Collecting data on bolides is left to the initiative of nonprofessional astronomers. At the beginning of February 2000, the Italian Superbolide Network (ITA.S.N.) was founded by researchers studying the phenomenon. Its mission is to create a national network of visual observers of bolides who are able to supply useful information for calculating a meteoroid’s trajectory in the atmosphere and its orbit around the Sun. To download an electronic copy of the observation card at the end of the chapter and get information on observing bolides, please go to: www.fis.unipr.it/~albino/ITASN/itasn.html.

The Unione Astrofili Italiani’s meteor section also collects data on bolides. For information and to download the report form, go to: http://meteore.uai.it/ Forms in English may be found at: www.namnmeteors.org/reports.html

How to reconstruct a bolide’s atmospheric trajectory Triangulation is the fundamental method used to locate distant objects, by measuring their position from two or more different observation points. If the object is observed from a single point, it is possible to determine its direction accurately, but measuring its distance may not be simple. In everyday life, this is unconsciously estimated with the help of the landscape around us and prior knowledge of an objects’ physical size. When tracing a bolide in the sky there is nothing to compare it with and so we need to resort to triangulation.

In practice, witnesses usually supply approximate information, but there are generally enough of them to determine the trajectory, thus limiting the area of research into the meteorite.

Basically, if two observers from two different locations can report the exact direction of the points at which the bolide appeared and disappeared, it is possible to reconstruct its real trajectory. In this way, reliable estimates of where it fell can be made. This method of triangulation is based on the fact that the observers supply the position of the same points in the bolide’s trajectory, presuming that the observations are contemporary.

It is unrealistic, though, to think that two random observers will succeed in simultaneously observing the same trajectory points. Since this is not normally the case, it is necessary to use a generalised triangulation that can supply the trajectory even when two observations are not contemporary and report different points for the trajectory in the atmosphere. What follows in the next few paragraphs is a little technical and is for those who want to do calculations on bolides’ trajectories. This method here is inspired by J.G. Porter’s proposal in 1943.

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Let us take a system of Cartesian coordinates (x, y, z), with axis x oriented east, axis y oriented north and axis z oriented towards the zenith. Suppose we have two observers, O1 (x1, y1, z1) and O2 (x2, y2, z2), observing the bolide from two different positions on Earth. In this model we consider Earth to be flat, which is translated into the two observers being no more than 400 km from each other, otherwise the Earth’s curvature will start to be relevant. (In fact, the method here can also easily be applied to a spherical Earth.) The results are more accurate for bolides with low inclination and long trajectories: in these cases the full power of photographic recording can be exploited, since it is far more accurate than visual observations. Another basic assumption is that the bolide’s trajectory is in a straight line, and this is true with good approximation when observing the trail.

Observer O1 will report the azimuthal coordinates A and h (see panel on page 261) of two points of the bolide’s trajectory – let us call them P1 (AP1, hP1) and P2 (AP2, hP2). Observer O2 will do the same thing, reporting the points Q1 (AQ1, hQ1) and Q2 (AQ2, hQ2). Notice that the points P are physically different from the points Q because, not being contemporary observations, they are in different portions of the bolide’s trail. With the azimuthal coordinates of points P1 and P2 we can reconstruct two unitary vectors (versors) that go towards the observed points from O1. If i, j and k are the versors of axes x, y and z, we can write them like this:

r r r r  P1 = cos ( hP1 ) sin ( AP1 ) i + cos ( hP1 ) cos ( AP1 ) j + sin ( hP1 ) k r r r r  P2 = cos ( hP 2 ) sin ( AP 2 ) i + cos ( hP 2 ) cos ( AP 2 ) j + sin ( hP 2 ) k

(1)

Considering that two vectors and one point identify a plane, with these versors and the position of O1 we can write down the analytical equation of the plane that contains the bolide’s trajectory and passes through O1:

(x − x1 )α P + ( y − y1 )β P + (z − z1 )γ P = 0

(2)

With: α P = cos(hP1 )sin (hP 2 ) cos( AP1 ) − sin (hP1 ) cos(hP 2 ) cos( AP 2 )  (3)  β P = cos(hP 2 )sin (hP1 )sin ( AP 2 ) − cos(hP1 )sin (hP 2 )sin ( AP1 ) γ = cos(h ) cos(h )sin ( A ) cos( A ) − cos(h ) cos(h )sin ( A ) cos( A ) P1 P2 P1 P2 P2 P1 P2 P1  P

We call this plane p1. We can do the same thing for O2, using the azimuthal coordinates of points Q1 and Q2 to reconstruct a different plane from the first 259

one that contains the bolide’s trajectory and O2. The formulas are identical to the previous ones (1), (2) and (3) with P replaced by Q. We call this new plane p2.

By joining together P1 and P2, a straight line is produced that shows the bolide’s trajectory in the atmosphere. The possible point of intersection between this straight line and the Earth’s surface (the geometric impact point) represents the place to start searching for the possible meteorite. The meteorite can be placed more precisely between the end point of the trajectory observed and the geometric impact point.

For O1 the distance projected on the Earth’s surface and point P1’s altitude are obtained by using this formula:

   r = (x2 − x1 )α Q + ( y 2 − y1 )β Q + (z 2 − z1 )γ Q  P1 α Q sin ( AP1 ) + β Q cos( AP1 ) + γ Q tan (hP1 )     q P1 = z1 + rP1 tan (hP1 )

(4)

A similar formula also goes for P2, replacing P with Q and vice versa, so that the distance and altitude values are obtained for observer O2. Using (4), the bolide’s trajectory projected on the Earth’s surface can be traced on an appropriately scaled map to determine the area of the possible fall.

Proceeding with this method, the trajectory’s inclination and azimuth, and therefore the azimuthal coordinates of the bolide’s apparent radiants, P1 Q1

P2

Z

Q2

hp2

O1 (x1, y1, z1) y

Image 1. Geometry for reconstructing a bolide’s trajectory. (ITASN)

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O

Ap2

x

qp2 rp2

THE AZIMUTHAL COORDINATES In astronomy there are different systems of coordinates in use (azimuthal, equatorial, ecliptic, galactic). Here we are just concerned with the azimuthal coordinates, sometimes called the ‘horizontal system’. With this system, the main plane of reference is the astronomical horizon. To unequivocally identify a star M on the celestial sphere there must be two angles: the angular height and the azimuth.

The angular height (or simply height), h, of the star M, is the angle between the plane of the astronomical horizon and the direction of star M. It is measured in the plane or great circle that contains the observer, the star and the zenith.

Heights are measured from 0° to +90° towards the zenith, if the star is in the visible part of the celestial sphere, and from 0° to –90° towards the nadir, if the star is in the invisible part. The height of the zenith is +90°, the height of the nadir is –90°, but all the points on the astronomical horizon have height 0°. We must be careful not to confuse the height h, which is measured in degrees, with the altitude of a body in relation to the Earth’s surface, which is measured in metres or kilometres. The two quantities are quite distinct.

Azimuth A of star M is the angle, measured in the plane of the astronomical horizon, formed between the direction from the northern cardinal point (the plane that contains the observer, the zenith and the north pole) and the direction of the star (the plane that contains the observer, the zenith and the star). Azimuths are measured from 0° to 360° from north to east.

The geographical north point is azimuth 0°, east is 90°, south is 180° and west is 270°.

Once we know this, it is not difficult to estimate any celestial body’s azimuth – even with the naked eye.

can also be obtained. By estimating the average speed and correcting the atmospheric trajectory for the gravity and terrestrial rotation, it is possible to calculate the orbit that the meteoroid followed around the Sun and establish its origin.

Using pairs of visual observations it is often possible to obtain a trajectory that is physically impossible, with negative altitudes, for example, and this indicates that the data supplied are low quality and incompatible. The most difficult cases are when all the observers are in the same place in relation to the bolide’s trajectory: small observation errors are enough to achieve very different trajectories if different pairs of observers are used. It is much better when observers are fairly well distributed on both sides of the bolide’s trajectory: small errors in azimuthal coordinates concerning the trajectory points do not have much influence on the final result. The ideal case is when there are different photos showing the same bolide instead of visual observations: measuring trail positions with stars in the background is straightforward in achieving quite an accurate trajectory.

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The all-sky camera As we saw in the section on calculating trajectories in the atmosphere, knowledge of at least two points in the azimuthal coordinates of the bolide’s trail, as seen by two different observers, is essential. Visual observations are hardly ever precise enough to obtain an accurate trajectory. On the other hand, normal cameras that can be used for photographing large stellar fields do not have a wide enough field of view to incorporate the whole celestial sphere.

For example, a normal reflex camera with a 50 mm lens can frame a field of just 27° x 41°, only 5.4% of the 20 626 square degrees of the celestial sphere.

Under these conditions, being able to photograph a bolide is very unlikely because they are not predictable phenomena. The ideal situation would be to have a device like an all-sky camera which is able to photograph the whole celestial sphere in just one frame. An all-sky camera is an economic and easy enough device to build and we recommend it to anyone who wants to try to photograph the trails of the brightest bolides.

A typical all-sky camera is equipped with a spherical mirror with a convex, aluminised surface to reflect the light. The mirror’s diameter is around 40 cm, while a camera with a 50 mm lens faces this mirror at around 100 cm above the mirror’s centre. The area framed by the camera is enough to photograph the whole mirror that reflects the celestial sphere. For amateur purposes, instead of a convex mirror something similar to those used at road intersections could be used. These are available in hardware and road sign shops. A convex mirror replaces the super wide angle lens extremely well and considerably lowers the cost of the device.

Image 2. Diagram of an all-sky camera from the Australian Fireball Network, part of a project financed by the National Geographic Society, together with the Natural History Museum (London), Western Australian Museum (Perth) and the Perth Observatory. The plexiglass dome protects the mirror from the weather and is also a support for the detector. The camera faces the convex mirror that, by reflecting the whole celestial sphere, photographs the entire sky above the horizon.

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The whole celestial sphere can be simultaneously photographed with this apparatus. Since all the brightest bolides can be recorded, it is a very powerful tool. The all-sky camera is fixed, so the photos show the streaks left by the brightest stars. The number of stellar traces recorded is enough to calculate the azimuthal coordinates as well as the beginning and end of the trajectories of the possible bolides or super-bolides recorded by the camera.

With the images supplied by an all-sky camera, the azimuth is the easiest coordinate to determine because the correspondence with the real azimuths is linear. An image containing the Pole Star is enough to immediately set the scale and obtain the azimuth of all the points of interest.

Determining the height above the horizon in degrees is more complicated.

To do this, it is first necessary to determine the zenith distance of some stars in the photo, as long as their distance from the centre of the image varies with time. To achieve this, all it takes to calculate the azimuths is to choose some points along the trails left by the stars, and then obtain the corresponding zenith distance by using any software that simulates the celestial sphere. Once the zenith distances of the brightest stars are known, at different distances from the centre of the photo, an interpolation curve can be traced and so the height above the horizon of any point in the celestial sphere can be determined – bolide trajectories included.

Italian bolides in 2003 To make the argument on bolides more tangible, let us use as an example the results achieved by applying the previous formulas to some bolides observed in Italy in 2003. In that year, 2003 testimonies of 13 different bolides were collected by ITASN from observers all over the peninsula. The real number of bolides was clearly higher because the sightings are always a fraction of the total, so the number 13 must be considered as a lower limit. The list here is, therefore, representative, but not exhaustive. The collected observations were visual and in every case almost enough to trace the bolide’s apparent route on the celestial sphere. In three cases, the observations were made by more than one person and this makes possible a preliminary triangulation of the bolide’s atmospheric trajectory. There was no super-bolide, the kind of very bright bolide, sometimes with apparent magnitude less than –17, which may be observed from most of the country and even reported by newspapers.

Discussed below are the year’s most interesting bolides. As there were not enough observations for accurate reconstructions of the trajectories, the preliminary results are shown below.

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Table 1

List of bolides that were observed in 2003 N

Date

Place of observation

Mv

Duration, s

Time (UT)

No. Obser.

1

18 January 2003

Dolceacqua (IM)

–4

2.5

21:05

1

2

14 June 2003

Contigliano (RI)

–5

0.2

22:40

1

3

3 July 2003

Oria (BR)

–15

7

19:27

1

4

4 July 2003

Aci Trezza (CT)

–10

?

00:30

1

5

6 July 2003

Porto Cesareo (LE)

–9

?

21:30/22:00

1

6

6 July 2003

Valico dei 4 Mori (FI)

–3

2

21:30

1

7

18 July 2003

Dolceacqua (IM)

–5

4

21:56

1

8

2 August 2003

Montecatini Terme (PT)

–12

?

21:00

1

9

6 September 2003

Centro-Nord Italia

–4

14

21:30

5

10

11 October 2003

S. G. Galermo (CT)

–8

10

22:58

1

11

30 October 2003

Padova/Ravenna

–7

?

17:17

2

12

12 December 2003

Firenze/Bologna

–8

3

15:48

2

13

15 December 2003

Contigliano (RI)

–6

2

21:30

1

The bolide on 3 July. The bolide on 3 July is the brightest of those shown in the table. It was green, with an apparent diameter of 3°, and it was observed low above the horizon from the south of Italy on the Adriatic Sea. At the end of the trajectory, the bolide fragmented. Unfortunately, only one usable observation was collected for the calculation of this bolide’s trajectory. It is an important event because, while the bolide was falling, sounds of electrophonic origin were heard by four witnesses who described them as ‘similar to a burning fuse’ or a ‘fry up’. Of the 13 bolides reported, this is the only one where the electrophonic phenomenon was noted.

Image 3. The trajectory of the 3 July 2003 bolide projected on the Earth’s surface. (ITASN)

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Having just one observation, the provisional trajectory, projected on the Earth’s surface, was drawn up by assuming a priori a value of 70 km for the initial altitude. This leads to a southwest to northeast trajectory. The observed initial point’s vertical value places it over the Ionian Sea, while the end point is northeast of the island of Corfu, Greece. Preliminary calculations indicate that the azimuth of the trajectory was 214°, crossed with moderate inclination, around 6–10° – compatible with the electrophonic sounds heard. Based on this provisional data, the final altitude was around 40 km and the apparent radiant was located in the constellation Hydra. It did not coincide with any known meteor shower.

The bolide on 6 September. Despite its modest brightness, this bolide had the highest number of observations. It was observed from northeast Italy and fragmented in the atmosphere at the end of its journey. Carlo Pampaloni from the Gruppo Astrofili of Montelupo (GrAM) described it: ‘The phenomenon started in proximity of the Pole Star as an object of magnitude 1.5 with a yellow/green colour and slowly proceeded east. It reached the region between the constellations of Cassiopea and Perseus and the show began: it ‘lit up’ and the colour turned orange. It had a very visible white tail that was smoky and frayed. It was very bright and proceeded slowly towards the eastern horizon at a constant angular speed, ending up behind the clouds near the horizon. It could even be seen behind the clouds.’

These clouds prevented the fragmentation being observed, although it was reported by other observers. Its provisional trajectory, projected on the Earth’s surface, was northwest to southeast. The vertical value from the observed initial point placed it in the Emilia Romagna region, while the end point was near the island of Lissa, in the Adriatic Sea. Preliminary calculations indicated

Image 4. The trajectory of the 6 September 2003 bolide projected on the Earth’s surface. (ITASN)

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that the azimuth of the trajectory was 290°, with an average speed of around 15–20 km/s and a low inclination. The final altitude, reached before the explosion, was around 50 km. The trajectory’s low inclination could make us consider an artificial satellite’s entry into the atmosphere, but there were none expected over Italy for the date or time of the bolide.

The bolide on 30 October. The 30 October bolide was of average–low brightness: estimates of apparent magnitude go from –5 to –7. The bolide was of a white– blue colour and was observed from northeast Italy. The provisional trajectory, projected over the Earth’s surface, was northeast to southwest. The vertical value of the observed initial point placed it over the Adriatic Sea, while the end point was near Bologna. The calculations indicate that the azimuth of the trajectory was 68°, with an average speed of around 15–20 km/s and an inclination of around 50°. The final altitude reached before its extinction was around 30–40 km. The apparent radiant was located in the Andromeda constellation, so it did not coincide with any known meteor shower, and the orbit was compatible with an asteroid origin, with an inclination of 10–15° to the ecliptic.

The bolide on 12 December. This bolide was of average brightness (Mv = –8). The atmospheric trajectory was more difficult to reconstruct because the indications of the initial points were rather uncertain. Nevertheless, resorting to spherical trigonometry, it was possible to estimate the initial points by using the final one and the inclination of the apparent route on the celestial sphere compared to the vertical one.

The bolide was of a white–red colour and was observed from north-central Italy. The provisional trajectory was southwest to northeast. The vertical value of the observed initial point placed it near Monte Falterona, while the end point was near Ravenna. The bolide fragmented into 3 or 4 parts at the end of

Image 5. The trajectory of the 30 October 2003 bolide projected on the Earth’s surface. (ITASN)

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Image 6. The trajectory of the 12 December 2003 bolide projected on the Earth’s surface. (ITASN)

its trajectory. Preliminary results indicated that the azimuth of the trajectory was 230°, with an average speed of around 15–20 km/s and an inclination of 30°. The final altitude, reached before the explosion, was around 20–30 km.

Fragments probably fell into the Adriatic Sea. The apparent radiant placed it in the constellation of Aquila and it did not coincide with any known meteor shower. The meteoroid’s provisional orbit was at a low inclination to the ecliptic and indicated an origin in the main asteroid belt.

Sighting a fall A fall is rarely witnessed directly, when a person sees the celestial body hit the ground. It can, however, also be witnessed indirectly, either because of the noise it produces when it impacts the ground or because fresh signs of the impact are found. This was the case of a farmer who, knowing his land very well, noticed the formation of a hollow – a small crater – that previously was not there.

Both situations leave little doubt, although sometimes searching for a meteorite on the ground is not easy, maybe because the land does not show any craters, is uneven or already covered by rocks and stones. Some simple criteria can help identify a meteorite: 1 Is the object compact and a little porous?

2 Does it have an uneven shape?

3 Is it strangely heavy compared to its size?

4 Does it have a smooth black or brown crust over it?

5 Does it have a light colour inside?

6 Is it attracted by a magnet?

7 Is it different from the surrounding rocks?

The answer is ‘yes’ to most of these questions for most common meteorites – yet this does not necessarily ensure that it is a meteorite. Only a specialised laboratory can supply a definitive answer.

Neither must we be deceived by some common misbeliefs. The most obvious is that the body that has just fallen must be very hot. When it comes through the atmosphere, the temperature of the gas around it reaches 20 000K, but these circumstances last very briefly, just a few fractions of a second. The meteorite also conducts little heat so its interior does not get particularly hot.

As most meteorites are decelerated to speeds less than the speed of sound at an altitude over 15 km, its temperature is quite low in the last part of its travels.

In fact, it sometimes cools down, as the temperature of the atmosphere is very low at high altitude. For these reasons, a meteorite is mostly cold when it reaches the ground. In high summer it has been known for fallen meteorites to have a thin layer of ice over their fusion crust!

267

What about the most favourable situation? The fall has happened in front of our eyes and we have a rare piece of extra-terrestrial rock in front of us. It is on the ground or at the base of a small crater hollowed out by the crash, or it could even be buried in a hole in the ground. Its commercial value is generally low, but we have to take all necessary measures to assure that it gives the greatest contribution possible to researchers. First of all, we must proceed as with bolides, immediately making a note of the time, date and, if possible, the objectâ&#x20AC;&#x2122;s presumed trajectory and any other useful data.

Once the point of the fall has been identified, or the candidate rocky body found, the best thing to do is report it to the relevant centre or observatory in order to find expert advice and interest in the phenomenon. Under no conditions is it good practice to remove the rock from where it is, because current regulations do not allow it, unless it runs the risk of disappearing or being severely altered by the environment, e.g. if it falls into water or onto muddy soil.

Before proceeding, however, it is worthwhile following some simple procedures that do not detract from our stroke of luck. In general, as we already said, the meteorite must not be removed and should be left where it was found, without moving or touching it. The surrounding traces from the impact must be preserved and, if possible, left unchanged.

If the place is uninhabited and inaccessible, it can probably be left unattended without anything happening while the fall is reported. In some natural environments, especially if it is lying together with other rocks, a passer-by would probably not even notice the alien object, but there would be the risk of losing the exact point where it fell. In these situations it is essential to use the surrounding environment to memorise some points of reference that can, without fail, lead us back to the place. This can be done by noting the position of trees and recognisable rocks, etc. If possible, an easily recognisable sign could be placed nearby.

If a camera is available, it is worthwhile to immediately get some images showing both the point where the object fell and its position in relation to its surroundings.

It is also worthwhile making a rapid inspection of the surrounding area (at least a radius of several metres), in order to immediately identify easily visible fragments that might have been detached from the object during its impact with the ground.

Circumstances would be more complex if the site of the fall was near a residential area, a road or on farmland. In general, the recommended action is still to take the necessary measures and prevent the sample from being damaged or removed. In particular, remember to respect private property: tell the landowner about the situation, explain its importance then advise him/her not to touch it and to protect it from possible removal. Once its scientific 268

importance is made clear, this person is likely to favour reporting it to a research centre.

It will then be the task of this centre to explain what has to be done. In general, Civil Defence authorities should be contacted to retrieve the sample and deliver it to the research centre. Before doing this, it is a good idea to fence off the point where the object fell with tape and stakes, in order to prevent anyone causing any damage.

In all cases, avoid informing the press or television before the authorities, because this can involve various problems, e.g. too many people, damage to the sample, traces on the ground, etc., that could make scientific use of the sample difficult.

Showers of fragments As explained previously, meteorites sometimes break up into fragments at several kilometres in altitude, well before reaching the ground. This event can be revealed beforehand by a bolide that produces a series of bright trails at the end of its trajectory or by a direct find of fragments of various sizes, but similar composition, in a small area on Earth.

There have been campaigns all over the world to find fragments in areas covering many square kilometres. In one case, the area was divided up into sectors and careful search instructions were given to pupils from certain schools. In this way, the ground can be covered metre by metre. The task is made much easier in areas where the ground has similar characteristics, such as large areas without grass. It is prohibitive almost anywhere in the Italian peninsula, although, if such an event was definitely known to have taken place, it would certainly be worth the effort to look.

The fall in Moravka A fall of meteorites near Moravka in the Czech Republic is one of the most documented in history and deserves to be discussed in detail. On 6 May 2000 at 11:51:46 UT a bright, daytime super-bolide (apparent magnitude –20) flew over Poland and the Czech Republic with a northwest to southeast trajectory.

The super-bolide was so bright that, despite the sunny day, witnesses reported that it lit up the ground as it went over them. There were around 500 visual testimonies, besides three videos made by casual witnesses, that made it possible to reconstruct the meteoroid’s trajectory in the atmosphere and calculate its orbit around the Sun. Military satellites in orbit around Earth also recorded the brightest part of the trajectory, which the super-bolide reached when it was at 33 km in altitude.

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Image 7. The trajectory of the daytime bolide responsible for a meteorite fall on Moravka. (European Fireball Network)

Image 8. The antennas at the receiving station in Budrio, Bologna, which are used for radar observation of meteors. (Courtesy of G. Cevolani)

The trajectory’s inclination with respect to the ground was rather low (20.4°), which favoured an increase in electrophonic sound within 250 km from the trajectory. 2.5% of the witnesses reported this type of sound (which we discussed in the chapter on meteors and bolides). A strong sonic boom was also perceived within 50 km of the end point of the trajectory. Sixteen seismic stations – both Polish and Czech – recorded the shock wave at the ground. The speed it entered into the atmosphere was estimated at 22.5 km/s, while, just before its fragmentation, the super-bolide reached a height of 21 km. These are rather typical values for a large super-bolide.

The orbit described by the meteoroid around the Sun had a semimajor axis of 1.85 AU, a perihelion distance of 0.982 AU (just inside the Earth’s orbit) and an aphelion distance of 2.7 AU, which indicates it originated in the main asteroid belt. Its inclination to the ecliptic was quite high (32°), similar to 10% of Apollo objects. The Moravka meteorite is the sixth meteorite whose original orbit around the Sun before its fall is now known.

Six meteorites with an overall weight of 1.4 kg were collected – thanks to the help of the local population in the days following the event. Unfortunately, the search was hindered by the mountainous region, which is covered with woods and does not lend itself to searches for meteorites. A good number of meteorites certainly landed on the ground (the meteoroid’s initial mass is estimated at 1500 ± 500 kg) and are still waiting to be found. The recovered meteorites have a density of 3.6 g/cm3 and, like ordinary chondrites, are classified as type H5–6.

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Table 2

The details of the meteorites recovered after their fall at Moravka N

Date found

Weight (g)

East longitude

North latitude

Altitude (m)

1

6 May 2000

214.2

18° 32’ 26.6’

49° 35’ 02.8’

561

2

13 May 2000

329.5

18° 32’ 00.4’

49° 36’ 40.5’

642

3

End May 2000

90.6

18° 30’ 14.9’

49° 40’ 39.8’

385

4

13 May 2000

235.1

18° 32’ 17.4’

49° 35’ 25.9’

538

5

31 July 2001

229.1

18° 32’ 16.6’

49° 35’ 00.8’

545

6

12 May 2000

300.8

18° 32’ 18.6’

49° 35’ 28.2’

544

Table 3

Data for the six falls of meteorites whose heliocentric orbit is known Pribram

Lost City

Innisfree

Peekskill

Moravka

Neuschwanstein

20.9

14.1

14.5

14.7

22.5

20.9

Estimated mass (kg)

1300

160

30

5000

1500

300

Trajectory’s inclination

43°

38°

68°

3°

20°

49°

Magnitude

–19

–12

–12

–16

–20

–17

Duration (s)

6.8

9.0

3.8

30

9

5.3

Initial speed (km/s)

Final height (km)

13

19

20

34

21

16

Mass found (kg)

5.8

17

4.6

12.4

1.40

3.38

Table 4

Orbital elements of the 6 meteorites Name

Moravka

Pribram

Lost City

Date (GMT)

6 May 2000

7 April 1959

4 January 1970 6 February 1977 9 October 1992 6 April 2002

Innisfree

Peekskill

Neuschwanstein

Type

H5–6

H5

H5

LL5–6

H6

EL6

a

1.85

2.401

1.66

1.872

1.49

2.4

e

0.47

0.6712

0.417

0.4732

0.41

0.67

q

0.9823

0.7894

0.967

0.986

0.886

0.793

Q

2.71

4.012

2.35

2.758

2.10

4.01

w

203.5

241.75

161.0

177.97

308.0

241.2

W

46.258

17.11

283.0

316.8

17.03

16.826

i

32.2

10.481

12.0

12.27

4.9

11.41

271

THE METEORITE AT FERMO On 25 September 1996, at around 17:30 local time, a meteorite fell a few kilometres north of Fermo in Ascoli Piceno, Italy. The event was not preceded by a bright trail, but rather an acoustic phenomenon heard by numerous witnesses. The noises were similar to detonations, then aeroplanes, helicopter blades, and lastly a hiss that enabled witnesses to determine the direction of the objectâ&#x20AC;&#x2122;s fall. Farmers found the meteorite in a hole dug out by the impact, on the side of a country road. Police then protected the site to allow experts to make tests before taking the sample away. The hole was around 60 cm deep and half a metre in diameter. The meteorite measured around 24 x 19 x 16 cm and weighed over 10 kg. It was a chondrite covered by a thin fusion crust that had some hollows, known as regmaglypts, that were similar to the hollows that fingers can make when pressing on clay. They are caused by interaction with the atmosphere and formation of very high temperature air vortexes near the surface (see chapter on meteorites).

Image 9. The meteorite that fell on 25 September 1996 in Fermo, Bologna. (Courtesy of G. Cevolani)

Image 10. The small crater dug out on the edge of a country road by the Fermo meteorite. (Courtesy of G. Cevolani)

THE METEORITE IN TURIN On 18 May 1988, a small shower of meteorites fell over the western outskirts of Turin (Italy) at around 2 pm local time. The largest fragment weighed almost half a kilo and fell in the parking area of the company Aeritalia, where it was immediately recovered. Three other smaller fragments fell in other residential areas (Pianezza, Collegno and Leumann) and were also subsequently recovered. The total weight of the material was almost one kilo. Given over to the CNR labs, Istituto di Cosmogeofisica and Fisica Generale in Turin, the meteorites were found to be chondrites. Analysis of exposure to cosmic rays showed that the fragments collected had originally been buried inside a large body, probably with a diameter of around 40 cm and a mass of 120 kg. The quantity of material retrieved amounts to just one hundredth of the original mass â&#x20AC;&#x201C; testimony to the efficient fragmentation and dispersion of material. It is also probable that other fragments fell in less inhabited areas and were never retrieved.

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Image 11. On 14 August 1992, the fall of a fragmented meteorite brought Mbale, a small village in Uganda, to sudden fame. Pieces of the object were scattered over an area of 3 x 7 km, and a total of 863 fragments were found, listed and individually studied. (Dutch Meteor Society)

Image 12. The biggest fragment from the fall in Mbale weighed 27.3 kg. (Dutch Meteor Society)

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Image 13. A spectacular view of fragments found in the fall in Mbale, sub-divided according to their sizes. Their overall mass was 150 kg, from a body whose total mass was probably around 1000 kg. (Dutch Meteor Society)

Image 14. Some fragments of the Mbale meteorite fell near peopleâ&#x20AC;&#x2122;s residences. This photograph shows damage to the roof of a house produced by one of the largest fragments. (Dutch Meteor Society)

274

ITA.S.N. - ITAlian Super-bolide Network

Bolide data collection form Date, location and observation conditions Date (day, month, year): UT (time, minutes, seconds): Name of the location: Province: Longitude East ( °, ‘, ‘): Latitude ( °, ‘, ‘): Height a.s.l (metres): Weather conditions during the sighting (clear, hazy, cloudy, overcast, rainy, snow): Observer’s conditions (still, moving): light pollution (absent, average, strong): The bolide’s trajectory Equatorial coordinates of the points at the beginning and end of the sighting (Equinox =…..............................)

RA of the initial point (h, m): Dec. of the initial point (°): RA of the end point (h, m): Dec. of the end point (°): Bright stars or planets hidden by the bolide’s head along the trajectory observed: Azimuthal coordinates of the points of the beginning and end of the sighting (alternative to the equatorial ones) (*) Height of the initial point (°): Azimuth of the initial point (°): Height of the end point (°): Azimuth of the end point (°): Specify if the coordinates were obtained via visual or photographic observations: Physical description of the bolide Apparent magnitude of the bolide’s head, at the height of brightness (* *): Maximum angular diameter of the bolide’s head (°) (* * *): Interval of time elapsed between the beginning and the end of the sighting (in seconds): Colour/s (sequence of colours taken on by the bolide’s head): Trail (time of persistence in seconds and colour): Fragmentation (number of fragments produced by the bolide’s head): Sound emitted by the bolide (no sound, hiss, detonation, thunder): Time lapsed between the first sighting of the bolide and perception of the sound (in seconds): Observer’s data Name and surname: e-mail: Postal address: Telephone:

(*) (* *) (* * *)

Values of the cardinal point azimuths are the following: North=0°, East=90°, South=180°, West=270° Magnitudes for comparison: Venus = –4, Moon at the first quarter = –10, Full Moon = –13, Sun = –27 Apparent angular diameter of the Moon is 0.5°

275

Chapter 11

Observing NEOs from space Introduction Research on asteroids, and NEOs in particular, is carried out today by astronomical observation systems all over the Earthâ&#x20AC;&#x2122;s surface. But we should ask ourselves whether satellites could contribute to increasing the rate of discovering dangerous asteroids and whether they can supply additional and complementary information compared to the ground-based systems. If the answer to these questions is yes, then we need to know whether the deployment of a space system is within our design, technological and economic possibilities.

Resorting to spacecraft is only done after an assessment of cost/effectiveness and can be justified if, on this basis, it promises results better than any alternative solution or if it is the only possible means to recover information that is regarded as valuable. To answer this question, several studies have been implemented and a number of space missions, aimed at the improving our knowledge of the minor bodies of the Solar System (asteroids and comets), have been undertaken. These missions are described in a table later in this section.

All of the space missions targeted at minor bodies of our Solar System have had the scientific objective of increasing our knowledge about the process of planetary formation and the origins of life. Furthermore, they are also contributing in technical and technological terms to future space missions that will be specifically devoted to evaluation and mitigation of the impact hazard.

The increasing public concern about the possible impact risk for our planet started in the mid-1990s as a consequence of the Shoemakerâ&#x20AC;&#x201C;Levy 9 comet impact with Jupiter. As a result, various national and international institutions, particularly the space agencies, initiated studies in order to assess the problem, identify the role of space technologies, and analyse the objectives and scenarios for possible space missions.

IMPACT: International Monitoring Programme for Asteroid and Comet Threat was the first assessment study at national level, granted to Alenia Spazio (now Thales Alenia Space) and the Astronomical Observatory of Turin in 1997 by the Civil Protection Department of Regione Piemonte (Italy). It resulted in 277

the definition of a monitoring network and a preliminary architecture, comprising both ground-based and space observatories, for discovery, follow-up and characterisation of NEOs. In the same year, ESA assigned the Study of a Global Network for Research on Near-Earth Objects to the Spaceguard Foundation, an international organisation set up by the IAU in 1996 for the global coordination of NEO observations. The study had the task of designing the Spaceguard Central Node, a facility connected with other important centres devoted to NEO research, e.g. the IAU Minor Planet Center (Cambridge, USA) and NEODyS (Pisa, Italy), to provide web services – targets of interest, plans of observation, software tools, etc. – to the NEO observers.

The two study teams came together to carry out another study, SISYPHOS: Spaceguard Integrated System for Potentially Hazardous Objects Survey, assigned

by ESA to the Spaceguard Foundation in 1998. Aimed at identifying the specific role of space-based facilities to complement and integrate the tasks accomplished by ground-based assets, the study showed that satellites can make fundamental contributions to the observation and characterisation of asteroids in spatial and/or spectral regions that are hardly accessible from ground.

A common conclusion of these preliminary studies was that an effective early warning network should comprise both ground- and space-based observation facilities, as well as data collection and coordination centres.

It should be emphasised that, in recognition of activities developed by the Italian community, Turin was selected in 1999 to host a prominent international workshop (IMPACT, Turin 1–4 June 1999) that welcomed some of the world’s greatest experts on this theme (see chapter 7). The recommendations made on this occasion – including recognising the potential of space platforms – became the main theme of a dedicated workshop. It was organised by the Planetary Society within the Third Conference of the United Nations on the Peaceful Use of Outer Space (UNISPACE III, Vienna, July 1999) and its recommendations were adopted in the UN’s final declaration. In the meeting of 2005, ESA gave a presentation on the subject to stimulate the establishment of a cooperation agreement.

Another indicator of increasing political attention to the problem was the workshop organised by the Organisation for Economic Co-operation and Development (OECD) and hosted by ESA’s ESRIN centre, 20–22 January 2003, when the major problem areas were debated. Recommendations were issued in five areas: acknowledging the problem, enabling a policy-level response, assessing risk at the national level, strengthening risk assessment through research and development, and supporting exploratory R&D for mitigation.

278

In 2002, ESA became more closely involved and in July of that year it drew up six research contracts to define possible space missions dedicated to assessing the risk of impact and retrieval of information aimed at its mitigation.

Of the six studies, three involved space observatories: • Earthguard-1: this is a very compact telescope to be embarked on a small, dedicated satellite or hosted by one that is already planned. The satellite should be located in an inner, heliocentric orbit. The mission is mainly dedicated to discovering Atens and IEOs. • EUNEOS (European Near-Earth Object Survey): this is a small space telescope (30 cm aperture) located in an inner, elliptical solar orbit. Mission objective is the discovery of PHOs which are at small elongation from the Sun, and therefore difficult to be observed from Earth-based telescopes.

• NERO (NEO Remote Observation): this is a space telescope working in visible and infrared spectral bands to discover asteroids of the Aten and IEO families and to allow the chemical and physical characterisation of any observed NEO. The satellite should be located at the second Earth–Sun Lagrangian point, to optimise its thermal control.

The other three studies concerned missions encountering asteroids: SIMONE (Smallsat Intercept Mission to Objects Near Earth): a fleet of small satellites for encounters with several asteroids that are selected to represent a variety of types. In situ science measurements will enable the characterisation of a wide diversity of physical and compositional properties of the NEO population.

• ISHTAR (Internal Structure High-resolution Tomography by Asteroid Rendezvous): a satellite carrying a radar tomography instrument for the in situ study of the inner structure of a target asteroid.

• Don Quijote: a mission for an asteroid investigation, geophysical characterisation and deflection technological experiment. It comprises two spacecraft launched separately. One of them (Sancho) will be put in orbit around the target asteroid to perform analysis before and after the arrival of the second (Hidalgo), which will impact the asteroid. The consequences of the impact will be evaluated by Sancho.

•

The role of spacecraft Preliminary considerations Following the comet impact on Jupiter in 1994, the popular perception of possible risks to our planet stimulated an increase in dedicated observatories (see chapter 8). This resulted in a significant increase in the number of 279

observation systems on the Earth’s surface. There are still problems, however, including the possible loss of newly discovered objects caused by insufficient follow-up activities to accurately determine the orbit, and the existence of objects that, due to their particular orbital characteristics, can barely be observed from Earth. An example of this could be asteroids belonging to the Aten family, as well as Inner Earth Objects (IEOs), whose orbits are always inside the Earth’s orbit and whose observation is very difficult (see chapter 2).

As far as NEOs’ physical characterisation is concerned, although several techniques could be used to achieve fundamental information on their size and composition, only some of them appear both effective and practicable. The most effective one appears to be radiometric thermal infrared observation, combined with visible and near-infrared spectroscopic or spectrophotometric observations. This type of observation appears particularly problematic for observers on Earth, especially with regard to radiometric infrared observations.

We should then ask the question whether space systems can solve these intrinsic problems with Earth’s observatories. It should be noted that, although the emphasis in this book is on the risk of impact, the information collected by spacecraft has clear scientific value, because it helps determine the ‘dimensional distribution’ of the NEO population and improve understanding of their origin, evolution and formation processes.

NEOs’ physical characterisation by means of spacecraft observations In the light of what has been discussed, the physical characterisation of these objects is of fundamental importance, particularly in relation to defining their size and mineralogical composition. However, the situation is mostly unsatisfactory. In fact, the diameters of only 63 out of 826 NEOs were known by late 1999, and this percentage has subsequently decreased since the increased rate of discovery has not been accompanied by programmes dedicated to NEO characterisation.

This situation is a consequence of the fact that simply determining its magnitude in the visible spectral band does not lead to a reliable estimate of a celestial body’s size. In fact, the quantity of solar light reflected by the asteroid depends both on its size and its albedo (reflectivity) and this parameter varies between 0.04 and 0.4 throughout the asteroid population. This means that the estimated size, based only on the visible magnitude (V), could be 100% wrong.

Even resorting to optical images of the object is impracticable as, except in a few favourable cases, even the angular dimension of sizeable asteroids, 1–2 km across, is beyond the limits of available telescopes.

The classical methodology adopted for achieving reliable estimates concerning the mineralogical composition of the asteroids’ surface uses 280

spectroscopic and spectrophotometric techniques. For our purposes, the excellent ability of the method is enough to subdivide the asteroids into taxonomic classes (see chapter 2). Remember that, in general, different taxonomic classes are characterised by different average albedos, and once they are known, they can supply an approximate estimate of the albedo that, combined with determination of the magnitude, helps define the size reasonably accurately.

There are two more techniques to be considered: polarimetry and thermal radiometry. The first of these is based on the empirical relationship between parameters that describe the variation in the degree of linear polarisation of the light reflected from the asteroid’s surface as a function of the phase angle (Sun– asteroid–Earth angle) and the albedo. This methodology requires observations at various phase angles, in order to have an appreciable measure of variation in the linear polarisation, with observations taking place over a number of weeks.

Furthermore, as the incident light on the telescope has to be separated into its components at different polarisation, large telescopes are necessary to ensure that the signal is stronger than the noise.

Thermal radiometry is a broadband technique that can be applied quickly and efficiently. It is based on the fact that asteroids (like all objects in the Solar System) are in radiation balance with their environment. As a result, the amount of incident sunlight reflected in the visible band is defined by their surface albedos. Sunlight also heats the object, which then re-radiates its energy into space in different spectral regions. Bodies orbiting at distances from the Sun that are typical of NEOs show their peak of radiation in the infrared (8–10 microns), the amount of energy depending on the object’s size and albedo. Therefore, simultaneous measurement of the thermal flow and visible brightness leads to a determination of the size and albedo.

Interpreting the emission in the infrared band requires resorting to thermal models of asteroids. Thermal emission depends on the distribution of the temperatures on the object, which, in turn, depends on generally unknown characteristics, such as surface roughness, degree of cratering, thermal inertia, axis direction and speed of rotation. Various models have been developed and applied successfully, e.g. the Standard Thermal Model or STM, and research to produce increasingly refined models is under way.

This highlights the advantage of the space-based observations. Radiometric techniques around 10 μm cannot easily be adopted from observatories on the Earth’s surface because of disturbances provoked by absorption and emissions from the Earth’s atmosphere in the same spectral band. The atmosphere also limits observation in other spectral regions of interest. For this reason, the best results using this technique have been achieved with satellites – especially IRAS and MSX, whose data helped to estimate the sizes of over 2000 asteroids. These 281

missions have clearly shown both the method’s efficiency in determining size and albedo and the effectiveness of satellites in its application.

Spacecraft discovery of Atens and IEOs As we said before, asteroids belonging to the Aten or IEO families are barely observable from Earth. Atens are characterised by a semimajor axis lower than 1 AU, but the distance of their aphelion (the farthest point from the Sun) is greater than this value. Therefore, they spend most of their orbits between Earth and the Sun, where they can only be observed at low solar elongations. They also intersect the Earth’s orbit close to their aphelion, staying in opposition – a condition that favours observation from Earth – for a very limited time.

This orbital characteristic, with its limited chances of being observed from Earth, results in a low number of discoveries of this family of asteroids. An even more difficult situation applies to IEOs whose orbits are completely inside the Earth’s orbit and whose existence has been deduced from studies of the orbital evolution of all NEO classes. According to these studies, their number is estimated at half that of Atens.

Two factors make observing IEOs from Earth extremely problematic. On one hand they always have low solar elongation values, on the other, the time available for possible observation is limited to brief periods, immediately after sunset or before sunrise.

Previous considerations identify satellite platforms as having an important role; they are able to observe low solar elongations and tackle a real problem for the Earth’s observatories, thus highlighting the fact that they are complementary sources of data.

Advantages of a space observation system It is worth summing up the spacecraft’s functions: • Ability to observe at low solar elongations and to overcome problems in obtaining the orbital characteristics of Atens and IEOs. • Through a suitable payload, obtain contemporary visible and infrared observations for NEO characterisation.

Other advantages include: • Accuracy in orbital measurements due to the absence of atmospheric turbulence.

• Independence from weather conditions that limit observations from Earth.

• Continuous 24-hour observation, not limited to night hours.

282

A space observatory for characterising and discovering NEOs From the previous considerations, it appears that a satellite system can contribute a significant added value to a NEO observing network, due to the complementarities of information it can produce with respect to the groundbased observatories.

A number of studies have been performed in order to better define this space-based observatory, as well as its mission. The major results of those activities are summarised here. Although it is beyond the scope of this publication to provide detailed trade-off and analysis, readers may be interested to look for detailed information on the various studies previously listed, particularly NERO, which is the one most in line with the mission requirements adopted here.

The starting point was obviously defining the main objectives which, based on what has been discussed up to this point, are the following: • Discovering Atens and IEOs. • Attaining satisfactory completion of a NEO census during a 2–5 year mission.

• Characterising NEOs via the thermal radiometry technique.

• Deciding on spectral band, orbital systems and focal plane technology that can minimise requirements for thermally controlling the satellite. This is particularly important for the infrared sensor, in order to avoid complex cryogenic systems.

• Deciding on systems that minimise telecommunication problems with the satellite and that impact on management costs during the mission.

• Minimising overall costs.

In what follows, cost has been considered a driving factor, with the purpose of identifying a minimum system concept capable of attaining the objectives.

Of course, options not currently considered, as well as solutions which can improve the performance, can be reconsidered, depending on the budget constraints of a possible programme.

Selection of the spectral bands To contain costs and maintain the mission’s objectives, it was decided not to include a spectrophotometric option in the instrument, limiting it to just one visible band.

Infrared represents the greatest added value of observation in space, and the 8–10 micron band minimises the thermal requirements on the sensor’s focal plane. The value chosen is 8.5 microns, very near the NEO’s emission 283

peak. This decision is also consistent with technological maturity. It would be desirable to use multi-band IR detectors, which are more suitable for the optimal use of the Near-Earth Asteroids Thermal Model (NEATM). Obviously this decision could be revised, depending on the time frame of a possible programme, if these detectors were available and tested.

Selection of the orbit Two types of orbits appear most interesting for the mission’s objectives, particularly for discovering Atens and IEOs. They consist of placing the space observatory in the L2V and L2T Lagrangian points of Venus and Earth respectively (see the discussion of Lagrangian points in chapter 2).

The first of these locations allows for more frequent and easier observations of NEOs, although it involves more time to transfer the observatory to its orbit, has a worse thermal environment – resulting in the use of a more complex thermal control system – and requires a more complex telecommunication system with Earth, because of its relatively variable position over time. Earth’s Lagrangian point has, obviously, reversed advantages and disadvantages.

An extensive simulation has been performed to compare the performances obtainable by the two orbital alternatives. Results show that L2V is preferable in terms of the NEO’s magnitude and phase angle because of easier observation; however L2T supplies the best performance regarding the cumulative number of discoveries during the mission. In particular, for a 5-year mission, the results of this simulation show a 90% and 50% inventory for Atens and IEOs respectively.

Another important element in deciding the orbit is the number of observations that are required for every newly discovered NEO. The higher this number is, the more accurate is the determination of its orbit and, consequently, of its distance – which favours efficient use of radiometric techniques for determining size and albedo. As the simulation results are decidedly in favour of L2T, the selection of this orbit is assumed in the following considerations.

Selection of the observation instruments The payload is basically a telescope operating in visible and infrared spectral bands. Its size should be based on the magnitude of the NEO to be observed, which in turn specifies the signal’s value (compared to the noise) to be detected by the instrument. A value of 5 for the signal-to-noise ratio corresponds to an apparent visible magnitude of 20, and an infrared magnitude of 10.

284

An important second requirement is astrometric accuracy in determining the NEO’s position and apparent motion. This requirement is more pressing if the duration of the mission has to be limited. A value of one arc second can be considered acceptable.

Based on the two requirements, the decision was made to configure the instrument by keeping in mind, on one hand, factors such as thermal stability, the ability to separate visible and infrared bands, and image quality, and, on the other hand, characteristics like size, complexity of construction and alignment.

The catadioptric configuration appears to be the best compromise between mission requirements and cost.

In the visible band, the focal plane detector is a CCD with 1024 x 1024 squared pixels with a 24 μm side and a reading frequency of 200 kpix/s.

Detectors with these characteristics are widely available and do not require long or expensive development.

As far as a device for the infrared focal plane is concerned, the conclusive element to be considered in the technological decision is thermal control requirements. The technology known as TEC PC HgCdTe is available in Europe and has already been widely used in space missions. It guarantees an operational band in the 5–11 micron range, so it is in line with the requirements, and needs a relatively simple thermal control system, which is very important from a cost standpoint. A focal plane of 256 x 256 squared pixels with a 35 micron side appears suitable for the mission requirements.

The preliminary selection of the instrument configuration and the technologies of its focal planes in the visible and infrared bands makes it possible to decide on the cooling system, while also taking into account further requirements dictated by the optics and focal planes. A solution already developed and available in Europe appears suitable for our purposes.

Performance and sizing of the optics The assumed reference scenario for sizing the optics is the following. NEO visible magnitude = 19 – 20.5; visible brightness of the sky = magnitude 21 per arc second squared; temperature of NEO black body = 300K; NEO diameter = 1 km; distance of NEOs = 1 AU; infrared radiance of the sky = 10–9 W/cm2/μm/sr.

In the worst visible case (V = 20.5), the trade off between the diameter of the optics and the observing time necessary to give a signal-to-noise ratio of 5, leads to a diameter of 80 cm and an integration time of 15 s.

With this value for the diameter, the same infrared signal-to-noise ratio is guaranteed with a time of integration of 0.3 s. So the integration time necessary for the visible band is the one which constrains the requirement for the satellite’s stability.

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Space missions to asteroids and comets ISEE-3/ICE (International Cometary Explorer) A precursor mission launched on 12 August 1978 which, after having accomplished its primary objective of studying the Sun as ISEE-3 (International Sun–Earth Explorer), was renamed ICE (International Cometary Explorer). After several manoeuvres to leave its orbital position, by the end of 1983 it attained a heliocentric orbit that would intercept comet Giacobini–Zimmer, passing through its tail on 11 September 1985 and sending back data on particles, fields and waves. In late March 1986, ICE passed between the Sun and Halley’s comet, observing its second comet.

Vega 1 and Vega 2 Two identical USSR spacecraft, launched respectively on 15 and 21 December 1984, for a joint mission to Venus, which was accomplished in June 1985. The main buses were redirected by Venus’ gravity to encounter Halley’s comet. Vega 1 made its closest approach (8890 km) to the comet nucleus on 6 March 1986, followed by Vega 2 at a distance of 8030 km on 9 March . They sent to Earth about 1500 images of comet Halley.

Sakigake and Suisei These were two spacecraft of the Japanese Space Agency ISAS (now JAXA), launched respectively on 7 January and 18 August 1985. Suisei started UV observation of Halley’s comet in November 1985 and passed 15 000 km from it on 8 March 1986. Sakigake, Japan’s first interplanetary spacecraft, after having tested escaping from Earth’s gravitation, encountered Halley’s comet at a distance of 6.99 million km on 11 March 1986.

Giotto This was an ESA mission to Halley’s comet. Launched on 2 July 1985, the spacecraft passed as close as 596 km from the comet on 13 March 1986, sending to Earth the first close-up images of a comet nucleus as well as evidence of organic material in it. The success of Giotto was completed by the extension of the mission, for a close (200 km) encounter with comet Grigg–Skjellerup on 10 July 1992.

Galileo Galileo was launched on 8th October 1989 as a direct mission towards Jupiter and its satellites. As it travelled through the main asteroid belt it explored the asteroids 951 Gaspra and 243 Ida (see chapter 2). Galileo also discovered Ida’s satellite Dactyl, the first probe to discover an asteroid’s satellite. In 1994, during its approach towards Jupiter, Galileo was also able to observe the impacts of comet Shoemaker–Levy 9. The mission finished on 21 September 2003 with the spacecraft’s re-entry into Jupiter’s atmosphere.

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NEAR (Near Earth Asteroid Rendezvous) NEAR-Shoemaker was launched on 17 February 1996. On 27 June 1997 it flew close to asteroid 253 Mathilde (see chapter 2). After a flyby with Earth on 23 January 1998, it passed 3 800 km from asteroid 433 Eros on 23 December 1998, without entering orbit, following a breakdown in the propulsion system. It eventually entered orbit around Eros on 14 February 2000. After observing the asteroid for about a year, the probe landed on its surface on 12 February 2001 and continued transmitting data until the mission ended on 28 February.

Deep Space 1 This US mission was launched on 24 October 1998. The probe reached its objective, comet Borrelly, on 22 September 2001 and transmitted images of its nucleus to Earth (see chapter 3).

Deep Space 1 used an ion motor and was the second probe (after ESA’s Giotto in 1986) to image a comet’s nucleus from close range.

Stardust We mentioned Stardust and the nucleus of comet Wild 2 in the chapter on comets. Launched on 7 February 1999, the probe flew by the comet on 2 January 2004 and transmitted images of its nucleus to Earth. The probe also collected grains of dust from the cometary coma and interstellar space, trapping it in a porous and very light material, called aerogel. On 14 January 2006, a capsule released by the spacecraft successfully landed on Earth, bringing back the collected samples for laboratory analysis. On 3 July 2007, an extended mission, named New Exploration of Tempel 1 (NExT), was approved to redirect the spacecraft to the comet, which was impacted by the Deep Impact spacecraft in 2005 (see below), in order to obtain good images of the crater caused by the impact – those taken soon after the event were of poor quality, due to the presence of ejected material.

Hayabusa (Muses-C) Launched on 9 May 2003, this Japanese probe’s target was asteroid 25143 Itokawa, which was reached in mid-September 2005. On 12 November a mini-lander (Minerva) was released, but without success. After the rendezvous, the probe examined the asteroid for some months at a very close range, accompanying Itokawa during its orbit around the Sun. When this period of exploration ended in November 2005, Hayabusa attempted several touchdowns to collect dust samples from its surface. In April 2007 the spacecraft started its return journey to Earth, where it is expected to release the capsule that may contain tiny samples of the nucleus in June 2010.

Rosetta This ESA mission, aimed at studying a comet’s nucleus and coma, had been planned for years and was launched on 2 March 2004 directed towards the comet 67P/Churyumov– Gerasimenko. During its long trip towards its destination, Rosetta will fly past two asteroids.

The flyby of 2867 Steins took place on 5 September 2008, and an encounter with 21 Lutetia will occur on 10 July 2010. Finally, it will approach the comet in the period January–May 2014 to carry out detailed mapping of the nucleus. In November 2014 it will release the Philae lander for in situ analysis of the nucleus and will continue to escort the comet around the Sun until December 2015.

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Deep Impact On 4 July 2005 a copper projectile weighing 370 kg was launched by the NASA Deep Impact probe, hitting comet Tempel 1’s nucleus at a speed of around 12 km/s. Both before and after the impact, numerous space- and ground-based telescopes turned their attention to the comet. Observations confirmed that the coma’s brightness increased by 50% in the hours following the impact. The increase in brightness involved light emitted by gas as well as dust.

After the impact, the coma became more irregular, due to the formation of intense outburst of dust.

Overall, the observations confirmed the theory that a comet’s nucleus is basically made up of dust and ice. Most of the ice, around 80%, is water ice, around 15% is carbon monoxide ice and the remainder is carbon dioxide, ammonia and methane ices.

In 1986, when the European Giotto spacecraft approached to within 600 km of comet Halley’s nucleus, a non-volatile, very dark crust was seen to cover its ices. The behaviour of Tempel 1’s nucleus after impact seemed to confirm this structure: it seemed as if the hole made in the comet’s outer crust had stimulated an eruption of gas and dust that expanded around the nucleus at a speed of 700–1 000 km/h.

Deep Impact is an interesting mission because it was the first to succeed in its objective of digging under a comet’s surface crust, trying to bring to light the nucleus material that is believed to have formed at the same time as the Solar System.

Dawn A NASA mission, launched on 27 September 2007 and aimed at an in-depth, comparative analysis of the two largest asteroids in the main belt, 4 Vesta and 1 Ceres. The spacecraft will orbit Vesta 2012–2013, arriving at Ceres in 2015. Both asteroids exhibit many differences and it is thought that they formed in two different regions of the primordial Solar System. The analysis will investigate the conditions and the processes taking place in our Solar System at the time of its formation.

Image of the comet Tempel 1 nucleus when it was impacted by the 370 kg projectile launched by the Deep Impact space probe. (NASA/JPL)

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Payload budgets The following values of the payload budgets are based on the NERO study: • Mass: 121 kg (including a margin of 20%).

• Electrical power: 110 W (50% of which for the cooling system).

• Data flow: this depends on the information that has to be transmitted to Earth and how much processing is done on board the satellite. The data flow can vary from 142.4 kbps to 5.8 kbps in the best and worst cases respectively. We should note that the data flow value impacts on the gain of the communication antenna and therefore, on the configuration of the whole satellite and its requirement for pointing stability. It is important then to limit the information that is transmitted to what is strictly necessary for the mission’s goals, especially the NEO’s position, visible and infrared measurements, and recognition of stars in the field of view for astrometric purposes. Of course, this minimum set of information can be complemented with other interesting information, e.g. the infrared mapping of the whole field of view, but this will increase the data flow.

Spacecraft stability This requirement is determined by the need to guarantee the telescope’s accurate pointing toward the target NEO for 15 seconds, the time required by the specified signal-to-noise ratio. Considering this value and the size of the sensor’s pixels, the requirement for stability is 0.1 arcseconds per second.

Different solutions can be adopted to comply with this requirement, ranging from a suitable attitude control system on the satellite to several possible techniques for drift compensation. In any case, this requirement does not seem to pose particular problems.

Observation strategy The observation of large regions of sky with a short time for revisitation (around 2 hours) is required to accurately assess a NEO’s orbital motion.

It is also necessary that the same NEO is observed several times during the satellite’s operational life to allow an accurate reconstruction of its orbit. The possibility to observe certain NEOs on demand, e.g. for their characterisation, should also be considered.

The simplest observation strategy appears to be one based on discontinuous steps. In this case, in order to observe an area of 300 square degrees twice every 5 days, with a frame size of 30 x 30 arcminutes, a total of 2400 frames must be observed.

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If 20% of the time is dedicated to observations on demand, the time for systematic observation equals 144 seconds per frame, a value which is enough to guarantee the required time on the target, reading of the focal plane, and manoeuvring and stabilisation of the satellite on the following frame.

Selection of the platform and the launchers Payload budgets, mass and power in particular, favour a satellite in the class 600 kg / 600 W. Such a space-based observatory can be developed using one of several buses of this class already available in Europe. There are also no major problems associated with the selection of the launcher In this section, reference has been mainly made to the NERO study, as the mission most in line with the mission objectives. Of course, other options may be considered, such as those proposed in the two other ESA studies, Earthguard-1 and EUNEOS. In any case, it is clear that the observation of NEOs can be performed by a small, space-based observatory, using a class of spacecraft and relevant technologies (most of which are already available) that ensure it is a relatively low cost mission.

The spacecraft as an element of an integrated network for observing NEOs As has been frequently reported and confirmed by studies and assessment by international experts, spacecraft must be seen as elements that can effectively complement the resources deployed on Earth, with the aim of collecting information that is difficult, if not impossible, to obtain in any other way. A global network for discovering and characterising NEOs should therefore include: • A coordinated network of ground-based astronomical observers for continuous monitoring aimed at the discovery, tracking and physical study of NEOs. • A space system for characterising NEOs and discovering Atens and IEOs, including a ground segment for its operational management.

• A network of centres on Earth for the collection, elaboration and dissemination of the data supplied from both the Earth observatories and the space segment.

A further element of the network could be represented by ground-based radars that are able to provide an extremely accurate assessment of a NEO’s distance and dynamic characteristics and carry out a morphological analysis of them. Radar techniques have already been used experimentally, although 290

they can only be applied under certain conditions of proximity to the asteroid, and therefore they can be effectively and fruitfully used for the analysis of pre-defined targets.

Conclusion In order to develop an effective system that prevents the risk of asteroids impacting Earth, the first step to be completed is without doubt obtaining a reliable estimate of the distribution of the sizes and different classes, characterised by their physical properties and mineralogical compositions.

A complete census of the different families (Apollo, Amor, Aten, IEO) is important, so that we have an inventory available that is not conditioned by the difficulty or impossibility of observing the elements of some of these orbital classes.

As shown by theoretical analyses and previous missions, a satellite system can provide an invaluable contribution. This is acknowledged by space agencies that have conducted detailed studies into the role of spacecraft and possible missions for in situ analysis and remote observation.

However, although they are undeniably valuable for obtaining detailed knowledge of a visited NEO, in situ missions must necessarily be limited in number and only aimed at well identified objects where they can perform clearly focused analysis.

In order to complete the NEO inventory, it seems more reasonable to resort to a space observatory that is complementary and integrated with the Earthâ&#x20AC;&#x2122;s network of observatories. The type of information obtainable via such a network, e.g. accurate orbital parameters and physical-chemical characterisation of a number of NEOs that represent most of the overall population, will help create a â&#x20AC;&#x2DC;risk indexâ&#x20AC;&#x2122; for every NEO by combining the probability of impact with its potential consequences. This risk index could be the basis on which to plan in situ missions as well as possible actions aimed at reducing the risk itself.

The space system described here does not represent the only possible option, but it can be considered as a demonstration of the possibility of quickly creating an effective system, without having to develop specific technologies, while also limiting cost.

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Chapter 12

Mass media relations

Scientific dissemination Disseminating information means making a concept or theory accessible for everyone to understand by using fairly non-technical language. Scientific dissemination by the mass media is important for various reasons. Firstly, most people do not have time to follow scientific developments and need other people, i.e. popular science writers, to explain them. Secondly, in general, scientific research is financed by public funding, so it is only right that taxpayers have the opportunity to learn about and understand the progress that has been made through payment of their taxes. Good popular science can reach younger audiences and trigger the desire for them to make their own contributions, as well as favour exchange between researchers of different generations – essential for a country’s long-term progress.

Producing popular science can seem easy, but this is not so. Good communicators must first understand what has to be explained to others so they must have a basic technical background. Good dissemination must not force the listener or reader to learn a ‘lesson’ or accept a given conclusion on a certain issue. That only risks alienating people from science. What is important is to provide all the necessary elements so that people can independently and critically reflect on the issue. It obviously has to be tailored to the means of communication being used. When writing a book there is plenty of room for detail, whereas writing a newspaper article means the author must be more concise and stimulating – so that the reader absorbs the information and could even be tempted to find out more. This obviously must be done without compromising professionalism.

There are often stories in the newspapers or on TV about celestial phenomena (eclipses, meteor showers, super-bolides, close encounters with comets and asteroids) that are misleading for the public. This can be partly attributed to science writers who are not always up to the task but also partly due to today’s widespread need for excitement. Astronomy, however, is already quite exciting by itself, so it is not necessary to resort to advertising style 293

techniques to grab readers’ attention and increase sales/audience. The important thing is to explain the facts correctly – the excitement can be left in the background.

In this book on asteroids and the risk they represent, we have tried to supply all the basic elements in order to understand more about the issue. As the book nears its end, we realised it might be useful to have a chapter guide that one could use to explain what an asteroid is and the threat that a NEO could be for Earth.

Close encounters with asteroids The environment Before beginning to discuss asteroids, it can be useful to explain the place they occupy in the Solar System, so that the reader has an idea of where they are ‘geographically’. Remember that eight planets orbit the Sun, plus a myriad of smaller bodies, comets and asteroids. There are over 100 000 asteroids whose orbits are accurately known, and most of these are rocky bodies, covered by impact craters, that are concentrated between the orbits of Mars and Jupiter, a region of space known as the main asteroid belt. The objects in the Kuiper belt, outside Neptune’s orbit, do not represent a danger to our planet so it is not essential to consider them here (see chapters 2 and 3).

It is necessary, though, to mention how small asteroids are, so that the reader can distinguish them from planets. It is enough to say that Ceres, the largest of the asteroids, has a diameter of around 1000 km, whilst most asteroids have a diameter of a few kilometres. Even though asteroids are small, an impact with one of them could still be dangerous because of the high speed of collision, typically tens of kilometres per second.

Asteroids’ orbits are not concentric to the Sun or orderly like the planets, but a small fraction of the known objects (around 6000) describe orbits that can bring them within 45 million km of Earth, and for this reason they are called Near-Earth Objects (NEOs). A NEO becomes a Potentially Hazardous Object (PHO) – that is, potentially dangerous to Earth – when its shortest distance from the Earth’s orbit drops to under 7.5 million kilometres – 20 times the distance between Earth and the Moon – and the body’s diameter is at least 150 m. There are currently more than 1000 known PHOs. The discovery of a new dangerous asteroid Usually, asteroids are mentioned by the mass media when a new PHO is discovered. However, reporting new discoveries must be done with due 294

caution, otherwise there is a risk of giving misleading information. The asteroid 1997 XF11 was a typical case.

Discovered by Spacewatch on 6 December 1997, this asteroid was the eleventh PHO known at the time. Like all asteroids, it was observed over the following months in an effort to accurately establish its orbit. In March 1998, following the previous months’ observations, it was discovered at the Minor Planet Center (MPC) that 1997 XF11 will pass around 54 000 km from the Earth’s centre on 26 October 2028. This is a very short distance, only 1/7 of the distance between Earth and the Moon. Clearly, this forecast was affected by certain measurement errors and it could not be excluded that the asteroid might collide with Earth instead of just grazing it. The MPC issued a circular to observatories to encourage further observations of 1997 XF11 in order to reduce uncertainty about the orbit and better define the geometry of the 2028 encounter. A popularised, non-technical version of the letter was published on the MPC’s web site. It was used by the media, which, perhaps distorting its content or material, started to speak of the future catastrophe in 2028 that was dubbed the ‘end of the world’.

Within a short time, photographic material from 1990 that had been taken at Mount Palomar showed the asteroid before it was discovered, so it was possible to compute an updated orbit that increased its 2028 shortest distance to a million km – i.e. almost three times the Earth–Moon distance. The MPC published the news in another letter and the ‘end of the world’ story disappeared from the media.

A greater understanding of the importance of the uncertainty of an asteroid’s orbit by scientific communicators would have allowed the public to better understand the whole story, especially how it went from catastrophe to salvation in just a few days, without there being any errors in the calculations.

In fact, it is necessary to observe the position of the celestial body among the stars for at least three years in order to accurately calculate the orbit of an asteroid moving around the Sun. That is the only way to know with good precision where it will be ten years later. When an asteroid is discovered, observations cover a time interval ranging from about ten days to a few months, so its preliminary orbit is uncertain. Everything that is predicted by using uncertain data is affected by the same uncertainty, and that increases as it is extrapolated further ahead in time. When astronomers refer to possible future collisions, this factor of uncertainty should always be kept in mind, explaining it correctly to readers and only speaking of potential danger.

When talking about a recently discovered and potentially dangerous asteroid, it is good practice to use the Torino Scale, introduced in 1999. The scale is divided into 10 levels of danger and we referred to it in chapter 7, when discussing the risk of impact.

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When a superbolide is observed The minor members of the asteroid population that move near Earth are numerous and they frequently fall on our planet. The atmosphere protects us from the smallest NEOs, whose diameters are no more than around ten metres.

When crossing the atmosphere at a speed of about ten km/s, these bodies are consumed by friction, break up into smaller pieces and produce super-bolides (very bright meteors) that are visible in daylight as bright spheres quickly crossing the sky. At night a super-bolide is able to illuminate the ground far more than the full moon. Classical examples of super-bolides were observed in the northwestern USA on 10 August 1972 (grazing the Earth’s atmosphere), in Lugo di Romagna (Italy) on 19 January 1993 (with a final explosion), in Yukon (Canada) on 18 January 2000, and in Austria on 6 April 2002.

Super-bolides occasionally make news in the mass media because they can easily be seen by thousands of people, especially during the summer. It must be said that, sometimes, reference is made to UFOs instead of informing the public correctly on the real nature of the phenomenon. In these cases it would be good practice to turn to associations that deal with collecting the data and calculating the super-bolides’ atmospheric trajectories for information on the phenomenon (see chapter 4).

If the fall of the asteroid seems imminent The probability that a kilometre-sized NEO will impact Earth exists and cannot be neglected. It is estimated that a 100 m diameter NEO falls on average every 1000 years. This rises to one million years for an asteroid of 1 km diameter and to 100 million years for one of 10 km.

In the first case, if the NEO is rocky it can explode in the atmosphere before touching the ground (as happened in Tunguska in 1908), while, if the composition is metallic, the body crashes onto the Earth’s surface and causes an impact crater, like the famous Meteor Crater of Arizona.

In the second case, with a diameter greater than 1 km, the effects are no longer on a local scale. The formation of an impact crater more than 20 km in diameter leads to the ejection of dust into the atmosphere, affecting the climate for a number of years.

In the last case, there is so much dust in the atmosphere as to cause mass extinction, putting the survival of higher forms of life at risk, similar to the well known extinction of the dinosaurs during the Cretaceous–Tertiary era, 65 million years ago.

Having clarified these points, it should be kept in mind that asteroids are a source of continuous surprises and a popular science writer must always be 296

ready to manage unexpected events. Usually, predicted dates for critical encounters are far in the future and cannot arouse immediate worry in the population. It could happen, however, that the predicted encounter is much closer – just a few days away – as was the case with asteroid 2004 AS1.

This asteroid was discovered by the LINEAR programme on 13 January 2004. Considering the object’s fast apparent motion, it had to be near Earth and was therefore classified as a NEO. Its estimated diameter was around 30 m. As for all asteroids of this type, the Minor Planet Center calculated a preliminary orbit in order to allow other observatories to follow it and contribute to improving the orbit. The preliminary calculations, however, showed that the asteroid would increase its brightness by 40 times the following day as it approached Earth and possibly collided with it. Luckily, the calculations were very uncertain, given the brief period of observation, and the asteroid passed by at a safe distance, around 2 million km from Earth. This event was classified level 3 on the Torino Scale, but it never appeared in the mass media because there was not enough time between the discovery and possible impact. There is nothing to prevent short notice impact forecasts a few weeks or months in advance. In such cases, the mass media have the task of correctly informing the public, without causing panic. This result can also be achieved by talking about techniques for deflecting asteroids (see chapter 8) or how measures can be taken to mitigate the effects, as we explain below.

Social consequences of an impact The problem of panic and of a possible social collapse is a serious one. At present there are no models of the economic and social consequences of an impact by an asteroid. In the case of the sudden fall of a NEO with a diameter greater than 1 km, we would move into a situation that our civilisation has never faced. The common natural calamities like cyclones, earthquakes, floods and volcanic eruptions are events on a local scale that, at most, have a socioeconomic effect at national level. The two World Wars also left some nations intact, so that they were able to act as centres of recovery for others.

Following huge amounts of dust being hurled into the atmosphere, the impact of a 2 km diameter body could destroy the agriculture on a global scale, cripple communications and lead to a serious energy crisis, striking all countries indiscriminately. Possible mass psychological reactions to such a catastrophe are unknown, but the social structure of any country would risk chaos and anarchy.

Assuming the existence of numerous areas of instability, the fall of a NEO could be the event that sparked off their uncontrolled spread around the world.

Things could improve if a warning was given several years before the impact, even if it was not possible to divert the asteroid. In this case, damage 297

could be reduced by careful planning and putting a disaster plan into place, as is already done on a local scale for other natural catastrophes: accumulation of food supplies for a 1–2 year period without agriculture, precautionary measures for fires and weather changes, strengthening of infrastructures and communication systems (which must be protected against electromagnetic interference from the impact), the industrial system made safe, evacuation of coastal areas to avoid tsunami risks and total evacuation of the area of the impact. This type of planning partially reminds us of what is already done to limit the damage from seismic events, with the only differences that the ‘earthquake’ here is worldwide, not regional, and predicted.

With these measures in place, probabilities of social survival considerably increase and we could hopefully overcome the event without enormous damage. It is predictable that Third and Fourth World countries will suffer the heaviest consequences because of their structural problems, as already happens today with other natural catastrophes. However, the one thing not to do is leave everything to chance and hope for good luck.

A type of precaution like the one explained here requires intense international coordination and a great social awareness of risk impacts. This objective can be achieved by developing widespread awareness that extends the knowledge of the NEO problem from astronomers and space experts to the general public. Clearly this targeted education cannot be left to Hollywoodtype films. If a widespread ‘NEO culture’ took root, it would also have the big advantage of increasing individual and collective self-defence skills and could create the conditions for communities to have direct control over political decisions in the field of NEO research.

Groups that could most benefit from greater knowledge would be those who deal with meteorology, seismology and climate models, public officials, Civil Defence, the Red Cross, the military, and all people and structures that would be the first in line when managing an emergency. It would be best to immediately start this precautionary programme rather than to waste precious time and be caught unprepared.

The Kuiper Belt and the legend of a tenth planet The media often report the discovery of a tenth planet in the Solar System.

The most recent example was the discovery of Sedna, on 14 November 2003 (see chapter 2). In reality, they are usually small bodies with diameters in the orders of a thousand kilometres, situated in the Edgeworth–Kuiper belt, beyond Neptune’s orbit, 30 AU from the Sun. These objects cannot be considered planets because of their small diameters and the fact that they are too numerous. Until recently, Pluto was regarded as the ninth planet in the 298

Solar System, but it is now considered to be a dwarf planet and one of the largest members of the Kuiper belt, like Ceres in the main asteroid belt.

The discovery of these bodies in the outer Solar System started in 1992 and today around 1000 objects are known. As they are so distant from the Sun, they are not very bright and identifying them is difficult, even though the number of discoveries is slowly rising. When new bodies in the Kuiper belt are reported, it must be explained that they are not planets, but icy objects orbiting at the edge of the Solar System.

Beyond the Kuiper belt, more than 50 AU from the Sun, there might be another planet that, for now, has not been discovered. However, over the last few years, a completely unfounded mythology of a planet X has spread worldwide. The myth of planet X peaked in intensity 2002–2003, thanks to the proliferation of dozens of web sites that predicted its approach to within 1 AU of Earth in the spring of 2003. Obviously, none of this happened and the myth died, but it is worth mentioning the story because it could crop up again in a different form in a few years.

According to supporters of this mythological theory, planet X has a mass four times that of Earth and orbits around a hypothetical companion star of the Sun called Nemesis. If Nemesis is located at 740 AU from the Sun, these supporters assume planet X has a 3657 year orbital period with a 32° inclination of its orbit to the ecliptic (the Earth’s orbital plane).

Now we should explain where the idea of Nemesis comes from. In the 1980s, astronomers hypothesised about the existence of this star, with its small mass compared to the Sun, in order to explain a certain recurrence in mass extinctions. However, despite research, no one has ever observed Nemesis and its existence has remained theoretical. Its revolution period around the Sun has been estimated from the recurrence of extinctions at intervals of around 26 million years, give or take a million. This puts Nemesis at an average distance from the Sun of 90 000 AU. Considering that the orbit must be elliptical to provoke extinctions, the Nemesis–Sun distance varies from a minimum of 30 000 AU to a maximum of 150 000 AU. Notice that these values are far higher than the value of 740 AU adopted by planet X supporters. If a far smaller value for the distance is assumed, Nemesis’ orbital period drops from 26 million to 20 000 years and no longer matches the recurrence of mass extinctions.

Why is it impossible for the mythological planet X to exist? With good approximation, we can say that all planets in the Solar System move along circular orbits that lie on the same plane. (In reality, the orbits have modest values of eccentricity and can tilt some degrees to the ecliptic plane, but these are non-relevant details.) This situation did not happen by chance, but derives directly from the genesis of the Solar System. It is a configuration of the utmost 299

STAGES IN THE SEARCH FOR PLANET X After discovering Uranus in March 1781 and Neptune in September 1846, astronomers discussed the possibility of another planet even further from the Sun which was responsible for the observed perturbations of the giant outer planets. The hunt for the imaginary planet X by astronomers goes on. Here is a summary of the main stages of this search.

1877 D.P. Todd of Washington predicts the position of a new planet by using observations of Uranus’ orbit. Research undertaken is not successful.

1900 Hans-Emil from Copenhagen predicts two new planets. Not found.

1908 W.H. Pickering from Harvard predicts a new planet at 52 AU from the Sun. 1915 P. Lowell in Boston publishes a paper mentioning two possible orbits for the new planet.

1930 C. Tombaugh discovers Pluto from the Lowell Observatory at Flagstaff in Arizona. The planet is just six degrees from the position predicted by Lowell. It is believed that finally the body perturbing Uranus’ and Neptune’s orbits has been found.

1960 It is noted that Pluto’s mass is too small to perturb significantly the orbits of Uranus and Neptune. The hunt is on again for the perturbing planet, which is now called planet X. X stands for the ‘unknown’ and number 10.

1971 Californian J.L. Brady uses the perihelion return of comet Halley to predict the existence of an enormous planet, with 100 times Earth’s mass, on a tilted orbit of 60° with respect to the ecliptic, located at 60 AU from the Sun and with a period of 500 years.

Research is invalidated because calculations did not account for the ‘rocket’ effect on the motion of the comet’s nucleus.

1973 D. Rawlins publishes a study of the possible orbits of a planet that is perturbing Uranus and Neptune.

1977 Discovery of asteroid Chiron, 200 km in diameter, with an orbit between those of Saturn and Uranus.

1978 Discovery of Charon, Pluto’s largest satellite. This enables Pluto’s mass to be confirmed as 0.002 Earth masses, too small to cause the perturbations of the outer planets.

1980 R.L. Duncombe and P.K. Sudelmanns from the Washington Observatory confirm the perturbations to Uranus and Neptune’s orbits.

1987 New analyses by the US Naval Observatory on the outer planets’ orbits confirm the perturbations observed from the 1800s up to the beginning of the current century.

There is a theory that there is a planet with an eccentric orbit, a high inclination to the ecliptic and a period between 700 and 1000 years. Robert Harrington, a researcher at the Observatory, does not succeed in identifying it however, despite dedicating years to calculating the new planet’s position.

1989 After the flyby of Voyager 2, refinements to Neptune’s orbital parameters show that the orbit has hardly any anomalies.

1990 Studies of infrared sources by the IRAS satellite (flown in 1983) were concluded and no new planet was discovered, although thousands of asteroids were found.

1992 R. Harrington continues his research without results.

1992–1994 Discovery of the first objects in the Kuiper belt, with diameters around 100 km.

2003 Discovery of Sedna (1800 km in diameter), one of the largest celestial bodies outside Neptune’s orbit.

2008 The search goes on.

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stability that has lasted for 4.5 billion years. A planet that moves on a comet-like, tilted orbit, that causes it to cross the Solar System at an Earth-like distance from the Sun, would find itself in such an unstable orbit from the gravitational point of view that it would be expelled from the Solar System in just a few thousand years. Given that its theoretical mass is at least four times that of Earth, it would heavily perturb the orbits of the other planets in the Solar System, leaving a sign in its wake that is not observed, since the planets’ orbits are regular. So, if ever a planet X existed, it was expelled very early on in the life of the Solar System.

More than 4 billion years have gone by since then.

Now, let us look at the basic contradictions of this theory. Suppose that the Sun is really a double star and its star companion, Nemesis, is indeed 740 AU from the Sun. According to the law of gravity, Nemesis would take 20 000 years to complete its orbit around the Sun. Nemesis cannot have the same mass as the Sun, otherwise it would be as visible to the naked eye as a very bright star.

In order not to be bright, a body’s mass must be smaller than 0.1 solar masses.

With this reasonable theory, and taking 3657 years as the period of planet X, this body should move along an orbit with an average radius of 110 AU, centred around Nemesis. Additionally, if it has a very eccentric orbit, planet X can move up to 220 AU away from Nemesis. But the assumed Sun–Nemesis distance is 740 AU, so the closest that planet X can come to Earth is 740–220 = 520 AU, 13 times the Pluto–Sun distance and not 1 AU as its supporters sustained. So, even if Nemesis existed at a distance of 740 AU from the Sun and had a massive planet, at least four times the Earth’s mass, with a 3657 year orbital period, it would be completely harmless to Earth and all the other planets in the Solar System.

So it seems obvious that the theory of a dangerous planet X is pure imagination, which however has been cleverly mixed with real news on common astronomic research to give a slight hint of credibility in the eyes of the public.

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Glossary Ablation

Removal of material by attrition, e.g. by passage through the atmosphere.

Accretion Process by which matter assembles to form larger bodies such as stars, planets, and satellites.

Aphelion Q, distance of greatest heliocentric separation for a body in an eccentric orbit.

Albedo In a planetary body, the ratio of reflected sunlight to incident sunlight. It measures the reflectivity and intrinsic brightness of an object, together with its distance from the Sun and the observer.

Amor asteroids Asteroids having orbits with perihelion distance between 1.017 and 1.3 Astronomical Units (AU). They come within 0.3 AU of Earth, but do not cross its orbit.

Apogee The point in its geocentric orbit where an object is furthest from Earth.

Apollo asteroids Asteroids having orbits with semimajor axis greater than 1 AU and perihelion distance less than 1.017 AU. Asteroid One of the small planetary bodies (also known as minor planets or planetoids), that mainly, but not exclusively, populate the region of the Solar System between the orbits of Mars and Jupiter.

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Asteroid main belt A region of interplanetary space lying between the orbits of Mars and Jupiter, where a large fraction of the asteroids are found.

Astronomical Unit, AU Average distance from Earth to the Sun. Its value is 149 597 870 km.

Aten asteroids Asteroids having orbits with semimajor axis less than 1 AU and aphelion distance greater than 0.983 AU.

Basalt

A dark, fine-grained, mafic igneous rock composed primarily of plagioclase and pyroxene.

Binary A system composed of two bodies that are physically separated but gravitationally bound to one another.

Bolide A very bright meteor that generally explodes at the end of its trajectory. The International Astronomical Union assumes that the bolide must have a brightness equal to at least magnitude –4.

Breccia A clastic rock composed of angular, broken rock fragments that are embedded into a finer-grained matrix.

Catastrophic disruption

Term applied to collisional breakup when the mass of the largest post-impact fragment is ≤50% of the original target mass.

Central relief / peak Relief formed by rocks in the middle of an impact crater, caused by a rebound effect in the middle of the crater.

Chromosphere Transition region extending for about 5000 km above the solar photosphere. The temperature of the chromosphere is about 10 000 Kelvin.

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Collisional evolution Process by which planetary bodies of the Solar System change some of their physical and dynamical characteristics, as a consequence of an impact. The impact, for example, of two asteroids can generate a high number of smaller objects on different orbits. Similarly, impacts within the asteroid belt change the dimensions of the existing objects over time.

Comet Literally ‘star with hair’. It is a celestial body full of volatile elements that, when intensively irradiated by the Sun, releases gases and dust, developing a coma. The action of the solar wind and radiation pressure moves gases and dust away from the nucleus, thus forming the cometary tails that extend in the opposite direction to the Sun. Conjunction See elongation Constellation A conventional group of stars recalling characters, animals or things. Stars of the same constellation are not necessarily near each other in space. The celestial sphere is divided into 88 constellations.

Corona The outer part of the solar atmosphere, formed by gas with a temperature of about 2 million Kelvin. Solar X-rays originate from the corona.

Cosmic rays High energy particles (mostly protons) that interact with atmospheres and solid bodies, causing alterations at atomic level.

Crater Depression formed by the impact of a body (asteroid or meteorite) or by the eruptive throat of a volcano.

Density

This is the ratio between the mass and the volume of a body. It is normally measured in g/cm3. Density of water is 1.

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Diamagnetic A material that, in the presence of an applied magnetic field, forms a field with opposing polarity. Without an applied magnetic field it does not show any form of magnetism.

Differentiation Process by which different chemical and mineralogical components of a celestial body occupy different regions. In a celestial body of planetary dimensions, differentiation produces concentric layers whose density increases from the surface to the centre. Generally metals sink to the centre to form a core, displacing the lighter silicates that form the crust and mantle.

Doppler effect A change in the frequency of a wave, e.g. sound or light, occurring when the source and the observer are in motion relative to each other.

Dynamical chaos A dynamic state in which the motion of a body can be accurately predictable only within limited time periods. In a typical chaos state, an infinitesimal variation of initial dynamic conditions leads to a final position and velocity determination which are totally different after a short period. The motion of the object is, therefore, intrinsically unpredictable. The chaos state is a natural intrinsic characteristic of some dynamic systems, and it is not only due to limited calculation ability.

Dynamical family Group of asteroids whose orbits, as far as semimajor axis, eccentricity and inclination are concerned, are similar. They are identified with statistical methods.

The origin of the family is collisional, i.e. caused by the impact of two objects whose catastrophic fragmentation produces the observed bodies.

Eccentricity

A parameter describing the form of an ellipse, defined as the ratio between the distance of the focus from the centre of the ellipse and its semimajor axis. Its value can vary from 0 (in which case the ellipse becomes a circle), and 1 (in which case the ellipse becomes a parabola). Since planetsâ&#x20AC;&#x2122; orbits around the Sun are elliptical, eccentricity is also a fundamental parameter of an orbit description.

Ecliptic The plane of the Earthâ&#x20AC;&#x2122;s orbital motion around the Sun. Its intersection with the celestial sphere is a circle, generally used as reference to describe the position of planetary bodies.

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ECO Acronym for Earth-Crossing Objects.

Edgeworth–Kuiper belt A ring of 108 to 1010 remnant icy planetesimals beyond the orbit of Neptune.

Ejecta Materials ejected from a crater either by the action of volcanism or a meteoroid impact.

Ellipse A plane curve defined as the locus of points for which the sum of the distances from two fixed points of the plane, named foci, is constant. On first approximation, orbits of planets are ellipses, where the Sun is at one of the two foci.

Elongation The planet–Earth–Sun angle. Eastern elongations appear east of the Sun in the evening; western elongations, west of the Sun in the morning. An elongation of 0° is called ‘conjunction’, one of 180° is called ‘opposition’, and one of 90° is called ‘quadrature’.

Ephemeris A list of computed positions occupied by a celestial body over successive intervals of time, or predictions of an astronomical phenomenon, such as an eclipse.

Escape velocity The minimum speed an object needs to escape from the gravitational force of a celestial body. It is proportional to the square root of the ratio between the celestial body’s mass and its radius. A body on Earth has an escape velocity of 11.2 km/s, the escape velocity of Jupiter is 59.5 km/s and the Sun has an escape velocity of 617.5 km/s.

Ferromagnetic

Iron-rich material that is magnetised under the influence of an external magnetic field.When the outer magnetic field is removed, the material keeps its magnetisation with the same orientation, but with lower intensity.

Flare An intense solar emission from regions close to sunspots, with an intense flow of particles and radiation over a wide energy range. These particles reach Earth within a few hours or days.

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Galilean satellites

Io, Europa, Ganymede and Callisto. Jupiter’s four largest satellites, discovered

by Galileo Galilei during his very first observations of the heavens with the newly invented telescope. Galileo dedicated these satellites to the Medici family, who ruled Florence at the time. For this reason they are also known as medicei.

General Relativity, theory of According to this theory formulated by Albert Einstein, the presence of mass modifies the geometrical structure of the surrounding space, and each object moves along lines of least distance (geodesics) in space–time.

Geocentric Literally, ‘Earth centred’. The term is used to indicate a trajectory or position of a body in relation to Earth.

Giant planets Jupiter, Saturn, Uranus and Neptune. They are also known as the Outer Planets, because they orbit in the outer regions of the Solar System. The term ‘giant’ concerns the size of these planets, as they are much bigger than the inner or terrestrial planets. They are also called ‘gas giants’, as they are made up mainly of gases, such as hydrogen and helium.

Gravitational constant, G The constant of proportionality in the attraction between two masses a unit distance apart. G = 6.668 x 10–8 dyn cm2 g–2.

Heliocentric

Literally ‘Sun-centred’. The term is used to indicate a trajectory or position of a body in relation to the Sun. The heliocentric distance is the distance of a planet, comet or asteroid from the Sun.

Hemisphere Literally ‘half a sphere’. The equator divides Earth into two halves, the northern and southern hemispheres. Similarly, the celestial equator divides the celestial sphere into two halves, the northern and the southern hemisphere.

Ice

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Solid state of substances that, on Earth and in normal conditions, are in the liquid or gaseous state. In planetary sciences, it frequently concerns water, carbon dioxide and methane ice. In the Solar System, these substances are in the solid state due to low temperatures.

Igneous Rock or mineral formed after the solidification of melted or partially melted material. An example is rocks of volcanic origin, such as basalt.

Impact melt (or melting) The process, or result of the process, in which the material involved in a catastrophic impact phenomenon, such as a meteoroid fall, melts due to the effect of the heat and pressure generated by the impact. Particles thus produced are found in the areas of impact craters and mainly come from the ground material that undergoes an impact, but also from the material that makes up the body of the impactor.

Inclination Orbital element formed by the angle made by the orbital plane of the planet and a reference plane, that conventionally is the ecliptic. The orbits of planets and most asteroids generally have very moderate inclinations. This is not true of many comets. When an orbital inclination is greater than 90°, the orbital motion is ‘retrograde’, i.e. it is in the opposite direction to most Solar System bodies (clockwise as seen from above the Sun’s north pole).

Inner-Earth Object (IEO) Asteroid whose orbit is inside that of Earth.

Inner planets Mercury, Venus, Earth and Mars are called this because they orbit in the Solar System’s inner region. Since their physical characteristics are similar (small in size, relatively high density, rocky composition, few or no natural satellites) and different from the other planets (Jupiter, Saturn, Uranus and Neptune), they are also known as ‘terrestrial planets’, after Earth, the largest of the group.

Interplanetary dust Solid grains of interplanetary material. They have irregular shapes and are usually smaller than one thousandth of a millimetre.

Ion An atom or molecular fragment that has gained a positive electric charge by the loss of one or more electrons.

Ionosphere Region of charged particles in the outer atmosphere of a planet. Earth’s ionosphere lies from an altitude of 60 km above its surface to 800 km and beyond.

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Isotope Chemical form with the same atomic number (i.e. number of protons) but a different mass number (i.e. number of neutrons). This means that two isotopes of the same chemical element only have different numbers of neutrons. The element’s chemical characteristics remain the same. In nature, only one isotope prevails. The others are generated by processes such as cosmic ray bombardment. Therefore, the number of isotopes in a body can be used to determine how long it has been exposed to the interplanetary environment.

Kelvin scale

Absolute scale of temperature. The absolute zero in the Kelvin scale is a physical limit that does not exist in nature. Zero degrees in the Celsius scale are equal to 273 degrees in the Kelvin scale.

Kiloton A measure of explosive power equal to that of 1000 tons of TNT (1 kton = 4.2∙1012 joules) The atomic bomb dropped on Hiroshima exploded with an energy of about 15 kilotons.

Kinetic energy Energy associated with a body’s motion. In mathematics it is equal to one half the mass times the velocity squared.

Lagrangian point

The five equilibrium points in the restricted three-body problem. In the Solar System evidence supporting this theory is given by the discovery of the existence of the Trojan asteroids, which appear to be located around two of the Lagrangian points of stability of the dynamic Sun–Jupiter system, namely L4 and L5. In particular, the two groups of Trojan asteroids orbit at the same heliocentric distance as Jupiter, forming an equilateral triangle with the Sun and the planet.

Lava The general term used to indicate molten rock erupted onto the surface of a planet.

Limiting magnitude In general, it is the apparent magnitude of the faintest star visible with the naked eye or detected by a given instrument. As far as asteroids are concerned, it sets visibility limits according to size and distance. As far as meteors are concerned, it sets limits to the number of objects detectable with the naked eye.

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Line of nodes The line of nodes is the intersection of the object’s orbital plane with the plane of reference (usually the ecliptic).

Magma

Magma is molten rock located beneath the surface of any terrestrial planet or satellite. It is capable of intrusion inside the planet’s crust and of extrusion onto the surface in case of volcanic eruptions.

Magnetic field An area in space affected by magnetic phenomena. A magnetic field is always produced by a magnetic body. Many celestial bodies, such as Earth and other planets, are sources of a measurable magnetic field.

Magnitude The measure of apparent or absolute brightness of any celestial body used in astronomy and astrophysics. The scale of magnitudes is logarithmic, in analogy with human eye behaviour. By convention, magnitude increases with a decrease of brightness. The brightest stars have an apparent magnitude of 0 or 1, while the faintest stars visible with the naked eye have an apparent magnitude of around 6.

Very bright planets and stars have negative magnitudes. For example, the apparent magnitude of the Sun is about –27.

Mars-crosser Object whose orbit crosses that of Mars.

Mean motion Average daily motion of an orbiting body, 360/P deg/day, where P is the orbital period in days.

Mean motion resonance Dynamic state that allows for the periodic recurrence of reciprocal geometric configurations of two orbiting bodies.

Megaton Amount of energy released by an explosion equivalent to 1000 kilotons.

(1 Mton = 4.2·1015 joules).

Metamorphosis Radical change of physical and chemical properties of a rock caused by external influences (fusion, very high pressure, shock, etc.).

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Meteor The visible event that occurs when a solid body enters the Earth’s atmosphere.

Meteor shower Intense and brief fall of meteors (occasionally more than 1000 per minute) originating from the debris left by a comet along its orbit. As they come from the same apparent direction when they enter the atmosphere, they appear to diverge from a common point called the radiant.

Meteorite A rock of extraterrestrial origin which survives the passage through the atmosphere and impacts with the ground.

Meteoroid A natural small (sub-kilometre) object moving through interplanetary space in an independent orbit.

Millibar The thousandth part of the ‘bar’, the unit of measure of pressure. The standard sea level pressure is 1013 millibars.

Minor planet Another name for an asteroid.

Near-Earth Asteroid (NEA)

Asteroid whose orbit is close to the Earth’s orbit or which crosses it.

Near-Earth Object (NEO) Asteroid or comet whose orbit is close to the Earth’s orbit or which crosses it.

Obliquity

The angle of inclination of a planet’s rotational axis in relation to its orbital plane.

The obliquity of the Solar System’s planets is quite modest (Earth for example has an obliquity of about 23 degrees). Therefore, planets revolve in an almost perpendicular direction to the orbital plane. A significant exception is Uranus, which revolves almost parallel to its orbit.

Occultation An event which occurs when a celestial body is hidden by another one which passes between it and the observer. The typical case is given by an asteroid which covers a star during its orbital motion.

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Oort cloud A cloud of frozen planetary bodies occupying a spherical halo around the Solar System at distances ranging from about 20 000 AU to 100 000 AU. The Oort cloud is commonly thought to contain a thousand billion objects.

Orbit The trajectory that a body follows while under the influence of the gravitational force of another, more massive object.

Orbital node The point where an orbit crosses a plane of reference. Usually, it refers to the point where the orbit crosses the ecliptic plane.

Palaeozoic

The geological era which spans from 570 to 245 million years ago.

Palimpsest A circular structure on the icy surface of planetary satellites, like Callisto and Ganymede, which lacks the significant relief that characterises normal impact craters.

They are believed to be impact structures partially erased by surface relaxation.

Paramagnetic Paramagnetic material has weak magnetism in the absence of an externally applied magnetic field. It tends to strengthen in the presence of an external field.

Periapsis The nearest point in the orbit of an astronomical object from the main object (for example, the nearest orbital point between a satellite and its planet).

Perigee The point where a satellite in orbit around Earth makes its closest approach to Earth. (It can be applied to the Moon or any artificial satellites).

Perihelion The point where an orbiting object in the Solar System is closest to the Sun. Perturbation A term used to describe alterations to an object’s orbit caused by gravitational interactions with other bodies different from the one around which the orbital motion occurs, e.g. the orbits of comets and asteroids are often perturbed by giant planets.

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Photosphere Sun’s layer that can be observed in visible light. It has a depth of about 500 km and a temperature of about 5780 Kelvin.

Planetesimal Small rocky or icy object formed from the primordial solar nebula, out of which all larger Solar System members are presumed to have accumulated.

Plasma In physics and chemistry, a plasma is an ionised gas. Plasma is considered to be a distinctive state of matter like other states of matter – solid, liquid and gas – because of its unique properties. ‘Ionised’ refers to the presence of one or more free electrons, which are not bound to an atom or molecule. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields.

Potentially Hazardous Asteroid (PHA) Object whose orbit brings it closer to Earth than 0.05 AU. A subcategory of NEAs. Poynting–Robertson effect A force caused by re-emission of solar radiation absorbed by dust grains or small meteoroids (less than 1 cm across). This process leads to a reduction in the aphelion distance and circularisation of the orbit. The particle then spirals in toward the Sun. Precession Steady variation of the direction of the line of nodes, or of the argument of the perihelion, of an orbiting body. This process is caused by gravitational forces that originate because planetary bodies are not perfectly homogeneous and spherical, and they do not orbit in the same plane.

Prograde motion Most bodies in the Solar System orbit in the same direction as the Sun’s rotation, i.e. anticlockwise as viewed from above the Sun’s north pole. Most bodies in the Solar System have a direct, prograde motion in both their rotation around their axes and their orbital revolution.

Radiant

The point from which meteor trajectories appear to diverge during meteor showers.

Radiation Energy in the form of electromagnetic waves and photons.

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Regolith A layer of fragmented rocks and dust covering the outermost crust of planets, satellites and minor bodies.

Resolution The smallest detail that can be distinguished in an image. A low resolution image shows details on a large scale, a higher resolution image makes it possible to discern details on small scales.

Retrograde motion Anticlockwise motion, both of rotation around the axis and orbital revolution.

This is the opposite direction to that of most bodies in the Solar System.

Revolution The motion of a body orbiting a larger one. Because of the planet’s angle of obliquity, the revolution of Earth around the Sun causes seasons.

Rotation The motion of a body around its axis. The Earth’s rotation around its axis, for example, causes the succession of day and night and the apparent motion of stars that rise, move across the sky and then set.

Satellite

A body that revolves around another, more massive body. In this sense, planets can be defined as satellites of the Sun.

Secular resonance This occurs when the precession of the lines of nodes or the argument of the perihelion of a comet’s or asteroid’s orbit is synchronised with the precession of one of the major planets. This can cause phenomena of dynamic chaos. One of the most important secular resonances is ν6.

Semimajor axis Half of the major axis of the ellipse that represents an unperturbed orbit, symbol a. The semimajor axis of planetary orbits gives the average heliocentric distance.

Shield volcano A volcano with a flat, dome shape formed by lava flows of low viscosity. Some volcanoes in the Hawaiian Islands are shield volcanoes (Mauna Loa, Mauna Kea), and Olympus Mons on Mars is also a shield volcano.

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Sidereal period The time necessary for an object to complete its rotation in relation to the celestial sphere.

Silicates Rocks or minerals characterised by a compound containing silicon and oxygen, which binds with other heavier chemical elements like iron and magnesium (e.g. olivine).

Solar nebula The cloud of gas and dust from which the Solar System formed about 4.5 billion years ago.

Spectrum Distribution of electromagnetic radiation intensity originating from a source, measured in different wavelengths.

Speed of light The speed of light in a vacuum is 299 792 458 m/s. According to Einstein’s special theory of relativity, this is an absolute limit for the speed of any physical signal.

Sporadic meteors Sporadic meteors can appear at any time and in any part of the sky. They do not belong to any known meteor showers.

Stratosphere Cold area of a planet’s atmosphere just above convective areas (the troposphere), usually characterised by the absence of vertical motion, but sometimes with strong horizontal motion.

Sunspots Colder and darker areas on the Sun’s visible surface (photosphere), which have very intense magnetic fields. They are usually less than 50 000 km in diameter and survive, on average, a few days or weeks.

Synchronous orbit An orbit in which an orbiting body has a period of revolution equal to the rotational period of the main body being orbited. Synchronous orbits are often used by artificial satellites for telecommunications.

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Synchronous rotation This occurs when a natural satellite takes as long to rotate on its axis as it does to complete one orbit of its planet. Therefore, it always keeps the same hemisphere pointed at the body it is orbiting. It occurs not only for our Moon but also for most other satellites of the Solar System.

Taxonomic classes

Asteroid taxonomy is based on physical characteristics derived from spectroscopy at wavelengths of visible light and infrared. Asteroids with similar albedos and spectra are distinguished by a single alphabetical letter. Belonging to the same class does not automatically mean similar mineralogical characteristics. Classes that mostly populate the main belt are those of asteroids S, C, P and D.

Tektites Natural, homogeneous glass objects formed by the complete fusion, solidification and dispersion (often in the shape of teardrops), of particles of soil as the result of an impact by an interplanetary body.

Tidal forces Strain forces on a planetary body caused by neighbouring objects (planets or satellites). In the case of tidal forces caused on a satellite by a major planet, intense internal heating can be observed. This is the process proposed to explain the volcanism observed on Jupiterâ&#x20AC;&#x2122;s satellite, Io. Trojan asteroid An asteroid located at the L4 and L5 Lagrangian points of the Sunâ&#x20AC;&#x201C;Jupiter system.

Troposphere The lowest part of a planetâ&#x20AC;&#x2122;s atmosphere, where convective phenomena continuously mix gases and cause a constant decrease in temperature with increasing altitude.

Most clouds are in the troposphere.

Tsunami A destructive phenomenon that occurs with the development of an ocean wave travelling at a speed of hundreds of kilometres per hour due to an earthquake or the impact of a cosmic body.

Ultramafic

An igneous rock consisting predominantly of mafic silicate minerals with high magnesium and iron content.

Universal Time (UT) The local mean time on the prime meridian. It is the same as Greenwich Mean Time, counted from 0h at Greenwich mean midnight.

Volatile

An element or compound that evaporates at a low temperature, e.g. hydrogen, helium, water, ammonia and carbon dioxide.

X-rays

Very short wavelength, high energy, electromagnetic radiation.

Yarkovsky effect

A non-gravitational perturbation caused by re-radiation of thermal energy that can cause the orbit of a small asteroid to drift. See text for a more detailed description.

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Appendix

List of the known impact structures on Earth Updated to December 2008 Crater name

Location

Latitude

Longitude

Diameter (km)

Age (Ma)

Visibility Studied

Haviland

Kansas, U.S.A.

37° 35’ N

99° 10’ W

0.015

< 0.001

Y

N

Dalgaranga

Western Australia, 27° 38’ S Australia

117° 17’ E

0.024

~ 0.27

Y

N

Sikhote Alin

Russia

46° 7’ N

134° 40’ E

0.027

0.000059

Y

N

Campo Del Cielo

Argentina

27° 38’ S

61° 42’ W

0.05

< 0.004

Y

Y

Sobolev

Russia

46° 18’ N

137° 52’ E

0.053

< 0.001

Y

Y

Veevers

Western Australia, 22° 58’ S Australia

125° 22’ E

0.08

<1

Y

N

Ilumetsä

Estonia

27° 25’ E

0.08

> 0.002

Y

Y

57° 58’ N

Morasko

Poland

52° 29’ N

16° 54’ E

0.1

< 0.01

Y

N

Kaalijärv

Estonia

58° 24’ N

22° 40’ E

0.11

0.004 ± 0.001

Y

N

Wabar

Saudi Arabia

21° 30’ N

50° 28’ E

0.116

0.00014

Y

N

Henbury

Northern Territory, Australia

24° 34’ S

133° 8’ E

0.157

0.0042 ± 0.0019

Y

N

Odessa

Texas, U.S.A.

31° 45’ N

102° 29’ W

0.168

< 0.05

Y

Y

Boxhole

Northern Territory, Australia

22° 37’ S

135° 12’ E

0.17

0.054 ± 0.0015

Y

N

Macha

Russia

60° 6’ N

117° 35’ E

0.3

< 0.007

Y

N

Aouelloul

Mauritania

20° 15’ N

12° 41’ W

0.39

3.0 ± 0.3

Y

N

Amguid

Algeria

26° 5’ N

4° 23’ E

0.45

< 0.1

Y

N

Monturaqui

Chile

23° 56’ S

68° 17’ W

0.46

<1

Y

N

32° 43’ S

Kalkkop

South Africa

24° 34’ E

0.64

0.250 ± 0.05

Y

Y

Wolfe Creek

Western Australia, 19° 10’ S Australia

127° 48’ E

0.875

< 0.3

Y

N

Tswaing (formerly Pretoria Saltpan)

South Africa

28° 5’ E

1.13

0.22 ± 0.052

Y

Y

25° 24’ S

Barringer

Arizona, U.S.A.

35° 2’ N

111° 1’ W

1.186

0.049 ± 0.003

Y

Y

Tabun–Khara– Obo

Mongolia

44° 7’ N

109° 39’ E

1.3

150 ± 20

Y

N

Saarijärvi

Finland

65° 17’ N

28° 23’ E

1.5

> 600

Karikkoselkä

Finland

62° 13’ N

25° 15’ E

1.5

~230

Y

Y

Liverpool

Northern Territory, Australia

12° 24’ S

134° 3’ E

1.6

150 ± 70

Y

N

Talemzane

Algeria

33° 19’ N

4° 2’ E

1.75

<3

N

Y

Lonar

India

19° 58’ N

76° 31’ E

1.75

0.052 ± 0.006

Y

Y

Tenoumer

Mauritania

22° 55’ N

10° 24’ W

1.9

0.0214 ± 0.0097

Y

N

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List of the known impact structures on Earth Updated to December 2008 Crater name

Location

Latitude

Longitude

Diameter (km)

Age (Ma)

Visibility Studied

B.P. Structure

Libya

25° 19’ N

24° 20’ E

2

< 120

Y

N

Tvären

Sweden

58° 46’ N

17° 25’ E

2

~ 455

N

Y

Holleford

Ontario, Canada

44° 28’ N

76° 38’ W

2.35

550 ± 100

N

Y

West Hawk

Manitoba, Canada 49° 46’ N

95° 11’ W

2.44

351± 20

N

Y

Roter Kamm

Namibia

27° 46’ S

16° 18’ E

2.5

3.7 ± 0.3

Y

N

Viewfield

Saskatchewan, Canada

49° 35’ N

103° 4’ W

2.5

190 ± 20

N

Y

Mishina Gora

Russia

58° 43’ N

28° 3’ E

2.5

300 ± 50

Y

Y

Rotmistrovka

Ukraine

49° 0’ N

32° 0’ E

2.7

120 ± 10

N

Y

Shunak

Kazakhstan

47° 12’ N

72° 42’ E

2.8

45 ± 10

Y

Y

Goyder

Northern Territory, Australia

13° 9’ S

135° 2’ E

3

< 1400

Y

N

Iso–Naakkima

Finland

62° 11’ N

27° 9’ E

3

> 1000

N

Y

Gusev

Russia

48° 26’ N

40° 32’ E

3

49.0 ± 0.2

N

Y

Granby

Sweden

58° 25’ N

14° 56’ E

3

~ 470

N

Y

Zapadnaya

Ukraine

49° 44’ N

29° 0’ E

3.2

165 ± 5

N

Y

Newporte

North Dakota, U.S.A.

48° 58’ N

101° 58’ W

3.2

< 500

N

Y

New Quebec

Quebec, Canada

61° 17’ N

73° 40’ W

3.44

1.4 ± 0.1

Y

N

Kgagodi

Botswana

22° 29’ S

27° 35’ E

3.5

< 180

Y

Y

Ouarkziz

Algeria

29° 0’ N

7° 33’ E

3.5

< 70

Y

N

Zeleny Gai

Ukraine

48° 4’ N

32° 45’ E

3.5

80 ± 20

N

Y

Flynn Creek

Tennessee, U.S.A.

36° 17’ N

85° 40’ W

3.8

360 ± 20

Y

Y

Steinheim

Germany

48° 41’ N

10° 4’ E

3.8

15 ± 1

Y

Y

Brent

Ontario, Canada

46° 5’ N

78° 29’ W

3.8

396 ± 20*

N

Y

Mount Toondina

South Australia, Australia

27° 57’ S

135° 22’ E

4

< 110

Y

N

Suvasvesi N

Finland

62° 42’ N

28° 10’ E

4

< 1000

N

Y

Glasford

Illinois, U.S.A.

40° 36’ N

89° 47’ W

4

< 430

N

Y

Ile Rouleau

Quebec, Canada

50° 41’ N

73° 53’ W

4

< 300

Y

N

Riachao Ring

Brazil

7° 43’ S

46° 39’ W

4.5

< 200

Y

N

Rio Cuarto

Argentina

32° 52’ S

64° 14’ W

4.5

< 0.1

Y

N

Dobele

Latvia

56° 35’ N

23° 15’ E

4.5

290 ± 35

N

Y

Mizarai

Lithuania

54° 1’ N

23° 54’ E

5

500 ± 20

N

Y

Gow

Saskatchewan, Canada

56° 27’ N

104° 29’ W

4

< 250

Y

N

Gardnos

Norway

60° 39’ N

9° 0’ E

5

500 ± 10

Y

N

320

List of the known impact structures on Earth Updated to December 2008 Crater name

Location

Latitude

Goat Paddock

Western Australia, 18° 20’ S Australia

Longitude

Diameter (km)

Age (Ma)

Visibility Studied

126° 40’ E

5.1

< 50

Y

Y

Chiyli

Kazakhstan

49° 10’ N

57° 51’ E

5.5

46 ± 7

Y

Y

Jabal Waqf es Swwan

Jordan

31° 3’ N

36° 48’ E

5.5

0.009 ± 0.001

Y

Y

Söderfjärden

Finland

63° 2’ N

21° 35’ E

6.6

~ 600

N

Y

Foelsche

Northern Territory, Australia

16° 40’ S

136° 47’ E

6

> 545

Y

N

Tin Bider

Algeria

27° 36’ N

5° 7’ E

6

< 70

Y

N

Middlesboro

Kentucky, U.S.A.

36° 37’ N

83° 44’ W

6

< 300

Y

Y

Maple Creek

Saskatchewan, Canada

49° 48’ N

109° 6’ W

6

< 75

N

Y

Kursk

Russia

51° 42’ N

36° 0’ E

6

250 ± 80

N

Y

Chukcha

Russia

75° 42’ N

97° 48’ E

6

< 70

Y

Y

Decaturville

Missouri, U.S.A.

37° 54’ N

92° 43’ W

6

< 300

Y

Y

Sääksjärvi

Finland

61° 24’ N

22° 24’ E

6

~ 560

Y

Y

Rock Elm

Wisconsin, U.S.A.

44° 43’ N

92° 14’ W

6

< 505

Pilot

Northwest Territories, Canada

60° 17’ N

111° 1’ W

6

445 ± 2

Y

N

Santa Fe

New Mexico, U.S.A.

35° 45’ N

105° 56’ W

6–13

<1200

N

N

Wetumpka

Alabama, U.S.A.

32° 31’ N

86° 10’ W

6.5

81.0 ± 1.5

Y

Y

Cloud Creek

Wyoming, U.S.A.

43° 7’ N

106° 45’ W

7

190 ± 30 Ma

N

Y

Kärdla

Estonia

59° 1’ N

22° 46’ E

4

~ 455

N

Y

Crooked Creek

Missouri, U.S.A.

37° 50’ N

91° 23’ W

7

320 ± 80

Y

N

Piccaninny

Western Australia, 17° 32’ S Australia

128° 25’ E

7

< 360

Y

N

Lockne

Sweden

63° 0’ N

14° 49’ E

7.5

455

Y

Y

Wanapitei

Ontario, Canada

46° 45’ N

80° 45’ W

7.5

37.2 ± 1.2

N

N

Couture

Quebec, Canada

60° 8’ N

75° 20’ W

8

430 ± 25

Y

N

Serpent Mound

Ohio, U.S.A.

39° 2’ N

83° 24’ W

8

< 320

Y

Y

Des Plaines

Illinois, U.S.A.

42° 3’ N

87° 52’ W

8

< 280

N

Y

Beyenchime– Salaatin

Russia

71° 0’ N

121° 40’ E

8

40 ± 20

Y

N

Vepriai

Lithuania

55° 5’ N

24° 35’ E

8

> 160 ± 10

N

Y

Neugrund

Estonia

59° 20’ N

23° 40’ E

8

~ 470

N

La Moinerie

Quebec, Canada

57° 26’ N

66° 37’ W

8

400 ± 50

Y

N

321

List of the known impact structures on Earth Updated to December 2008 Crater name

Location

Latitude

Longitude

Diameter (km)

Age (Ma)

Visibility Studied

Elbow

Saskatchewan,

50° 59’ N

106° 43’ W

8

395 ± 25

N

Y

Canada Bigach

Kazakhstan

48° 34’ N

82° 1’ E

8

5±3

Y

Y

Glover Bluff

Wisconsin, U.S.A.

43° 58’ N

89° 32’ W

8

< 500

Y

Y

Crawford

South Australia, Australia

34° 43’ S

139° 2’ E

8.5

> 35

Y

N

Calvin

Michigan, USA

41° 50’ N

85° 57’ W

8.5

450 ± 10

N

Y

Ilyinets

Ukraine

49° 7’ N

29° 6’ E

8.5

378 ± 5*

N

Y

Connolly Basin

Western Australia, 23° 32’ S Australia

124° 45’ W

9

< 60

Y

N

Mien

Sweden

56° 25’ N

14° 52’ E

9

121 ± 2.3

Y

Y

Red Wing

North Dakota, U.S.A.

47° 36’ N

103° 33’ W

9

200 ± 25

N

Y

Ragozinka

Russia

58° 44’ N

61° 48’ E

9

46 ± 3

N

Y

Lumparn

Finland

60° 9’ N

20° 6’ E

9

~ 1000

N

Y

Vista Alegre

Brazil

25° 57’ S

52° 41’ W

9.5

< 65

Y

N

Flaxman

South Australia, Australia

34° 37’ S

139° 4’ E

10

> 35

Y

N

Paasselkä

Finland

62° 2’ N

29° 5’ E

10

< 1800

Y

Y

Upheaval Dome

Utah, U.S.A.

38° 26’ N

109° 54’ W

10

< 170

Y

Y

Eagle Butte

Alberta, Canada

49° 42’ N

110° 30’ W

10

< 65

N

Y

Karla

Russia

54° 55’ N

48° 2’ E

10

5±1

Y

Y

Kelly West

Northern Territory, Australia

19° 56’ S

133° 57’ W

10

> 550

N

N

Bosumtwi

Ghana

6° 30’ N

1° 25’ W

10.5

1.07

Y

N

Ternovka

Ukraine

48° 08’ N

33° 31’ E

11

280 ± 10

N

Y

Wells Creek

Tennessee, U.S.A.

36° 23’ N

87° 40’ W

12

200 ± 100

Y

Y

Avak

Alaska, U.S.A.

71° 15’ N

156° 38’ W

12

3 – 95

N

Y

Serra da Cangalha Brazil

8° 5’ S

46° 52’ W

12

< 300

Y

Y

Vargeao Dome

Brazil

26° 50’ S

52° 7’ W

12

< 70

Y

N

Nicholson

Northwest Territories, Canada

62° 40’ N

102° 41’ W

12.5

< 400

N

N

Aorounga

Chad, Africa

19° 6’ N

19° 15’ E

12.6

< 345

Y

N

Marquez

Texas, U.S.A.

31° 17’ N

96° 18’ W

12.7

58 ± 2

N

Y

Kentland

Indiana, U.S.A.

40° 45’ N

87° 24’ W

13

< 97

Y

Y

Deep Bay

Saskatchewan, Canada

56° 24’ N

102° 59’ W

13

99 ± 4

N

Y

Sierra Madera

Texas, U.S.A.

30° 36’ N

102° 55’ W

13

< 100

Y

Y

322

List of the known impact structures on Earth Updated to December 2008 Crater name

Location

Latitude

Spider

Western Australia, 16° 44’ S Australia

Longitude

Diameter (km)

Age (Ma)

Visibility Studied

126° 5’ E

13

> 570

Y

N

Gweni–Fada

Chad

17° 25’ N

21° 45’ E

14

< 345

Y

N

Zhamanshin

Kazakhstan

48° 24’ N

60° 58’ E

14

0.9 ± 0.1

Y

Y

Jänisjärvi

Russia

61° 58’ N

30° 55’ E

14

700 ± 5

Y

N

Logoisk

Belarus

54° 12’ N

27° 48’ E

15

42.3 ± 1.1

N

Y

Kaluga

Russia

54° 30’ N

36° 12’ E

15

380 ± 5

N

Y

Ames

Oklahoma, U.S.A.

36° 15’ N

98° 12’ W

16

470 ± 30

N

Y

Suavjärvi

Russia

63° 7’ N

33° 23’ E

16

~ 2400

Oasis

Libya

24° 35’ N

24° 24’ E

18

< 120

Y

N

Lawn Hill

Queensland, Australia

18° 40’ S

138° 39’ E

18

> 515

Y

N

El’gygytgyn

Russia

67° 30’ N

172° 5’ E

18

3.5 ± 0.5

Y

N

Dellen

Sweden

61° 48’ N

16° 48’ E

19

89.0 ± 2.7

Y

N

Glikson

Western Australia, 23° 59’ S Australia

121° 34’ E

~19

< 508

Y

N

Amelia Creek

Northern Territory, Australia

134 ° 50’ E

~20

1640 ± 600

Y

N

20° 55’ S

Logancha

Russia

65° 31’ N

95° 56’ E

20

40 ± 20

N

N

Gosses Bluff

Northern Territory, Australia

23° 49’ S

132° 19’ E

22

142.5 ± 0.8

Y

Y

Rochechouart

France

45° 50’ N

0° 56’ E

23

214 ± 8

Y

N

Lappajärvi

Finland

63° 12’ N

23° 42’ E

23

73.3 ± 5.3

Y

Y

Ries

Germany

48° 53’ N

10° 37’ E

24

15.1 ± 0.1

Y

Y

Boltysh

Ukraine

48° 45’ N

32° 10’ E

24

65.17 ± 0.64

N

Y

Presqu’ile

Quebec, Canada

49° 43’ N

74° 48’ W

24

< 500

Y

N

Haughton

Nunavut, Canada

75° 22’ N

89° 41’ W

23

39

Y

N

Kamensk

Russia

48° 21’ N

40° 30’ E

25

49 ± 0.2

N

Y

Strangways

Northern Territory, Australia

15° 12’ S

133° 35 ‘ E

25

646 ± 42

Y

N

Steen River

Alberta, Canada

59° 30’ N

117° 38’ W

25

91± 7*

N

Y

Clearwater East

Quebec, Canada

56° 5’ N

74° 7’ W

26

290 ± 20

Y

Y

Mistastin

Newfoundland/ Labrador, Canada

55° 53’ N

63° 18’ W

28

36.4 ± 4*

Y

N

62° 8’ N

Keurusselkä

Finland

24° 36’ E

~30

<1800

Y

N

Shoemaker (formerly Teague)

Western Australia, 25° 52’ S Australia

120° 53’ E

30

1630 ± 5

Y

N

SlatIslands

Ontario, Canada

87° 0’ W

30

~ 450

Y

N

48° 40’ N

323

List of the known impact structures on Earth Updated to December 2008 Crater name

Location

Latitude

Yarrabubba

Western Australia, 27° 10’ S Australia

Longitude

Diameter (km)

Age (Ma)

Visibility Studied

118° 50’ E

30

~ 2000

Y

N

Manson

Iowa, U.S.A.

42° 35’ N

94° 33’ W

35

73.8 ± 0.3

N

Y

Clearwater West

Quebec, Canada

56° 13’ N

74° 30’ W

36

290 ± 20

Y

Y

Carswell

Saskatchewan, Canada

58° 27’ N

109° 30’ W

39

115 ± 10

Y

Y

Saint Martin

Manitoba, Canada 51° 47’ N

98° 32’ W

40

220 ± 32

N

Y

Mjølnir

Norway

29° 40’ E

40

142 ± 2.6

N

Y

Woodleigh

Western Australia, 26° 3’ S Australia

114° 39’ E

40

364 ± 8

N

Y

73° 48’ N

Araguainha

Brazil

16° 47’ S

52° 59’ W

40

244.4 ± 3.25

Y

N

Montagnais

Nova Scotia, Canada

42° 53’ N

64° 13’ W

45

50.5 ± 0.76

N

Y

Kara–Kul

Tajikistan

39° 1’ N

73° 27’ E

52

<5

Y

N

Siljan

Sweden

61° 2’ N

14° 52’ E

52

376.8 ± 1.7

Y

Y

Charlevoix

Quebec, Canada

47° 32’ N

70° 18’ W

54

342 ± 15*

Y

Y

Tookoonooka

Queensland, Australia

27° 7’ S

142° 50’ E

55

128 ± 5

N

Y

Beaverhead

Montana, U.S.A.

44° 36’ N

113° 0’ W

60

~ 600

Y

N

Kara

Russia

69° 6’ N

64° 9’ E

65

70.3 ± 2.2

N

Y

Morokweng

South Africa

26° 28’ S

23° 32’ E

70

145 ± 0.8

N

Y

Puchezh–Katunki

Russia

56° 58’ N

43° 43’ E

80

167 ± 3

N

Y

Chesapeak Bay

Virginia, U.S.A.

37° 17’ N

76° 1’ W

90

35.5 ± 0.3

N

Y

Acraman

South Australia, Australia

32° 1’ S

135° 27’ E

90

~ 590

Y

N

Manicouagan

Quebec, Canada

51° 23’ N

68° 42’ W

100

214 ± 1

Y

Y

Popigai

Russia

71° 39’ N

111° 11’ E

100

35.7 ± 0.2

Y

Y

Chicxulub

Yucatan, Mexico

21° 20’ N

89° 30’ W

170

64.98 ± 0.05

N

Y

Sudbury

Ontario, Canada

46° 36’ N

81° 11’ W

250

1850 ± 3

Y

Y

Vredefort

South Africa

27° 0’ S

27° 30’ E

300

2023 ± 4

Y

Y

324

SP-1310

Settore Protezione Civile

THE ASTEROID hazard

Protezione Civile Regione Piemonte

Osservatorio Astronomico di Torino

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An ESA Communications Production Copyright 2009 Š European Space Agency

THE ASTEROID Hazard Evaluating and Avoiding the Threat of Asteroid Impacts