The detection of asteroids and comets potentially dangerous to humankind
Doc. 7480
9 February 1996REPORT[1]
Rapporteur: Mr LORENZI, Italy, Liberal, Democratic and Reformers Group
Summary
There are currently estimated to be over 2 000 comets or asteroids more than a kilometre in diameter which cross the Earth's orbit. As a measure of the danger, it is sufficient to recall that the explosion of an object sixty metres in diameter near the river Tunguska (Siberia) in 1908 destroyed 2 000 km� of forest. Although the probability of an impact at the highest level of risk is low, the probable consequences of such an impact would be catastrophic and endanger the very survival of humanity.
The Assembly accordingly invites governments of member countries and the European Space Agency to support the setting-up of an international foundation and programme to prepare an inventory of potentially dangerous objects. The programme, whose purpose would be to monitor the orbit of such objects and record their configuration, density, cohesion and composition, could be implemented by co-ordinating the data collected by national observatories and by designing low-cost satellites for observations which cannot be carried out from the ground.
I. Draft resolution
1. There are two broad categories of extraterrestrial objects which have the potential to impact our planet: comets and asteroids. They are generally known among planetary scientists as Near-Earth Objects (NEOs). Their total number is unknown, but the number of Earth-crossing asteroids with sizes larger than 1 km is estimated to be about 2 000. These objects are the most dangerous and only a tiny fraction of them have been detected to date.
2. Considering that the explosion close to the Earth's surface of even an object with a diameter of 50 metres can have the effect of a 10 megaton nuclear weapon, the consequences of larger impacts would be disastrous on a global scale. The best known recent examples are the Tunguska explosion of an NEO about 60 metres in size (over Siberia) in 1908, resulting in the destruction of over 2 000 km2 of largely-unpopulated forest, and the violent impacts into Jupiter of the fragments of comet Shoemaker-Levy 9 (in July 1994); those fragments were only about 0,5 km in size, but caused devastation over a larger area than that of the Earth. Traces of other smaller impacts on our planet are frequently being discovered, as well as fossil records of cataclysmic impact events in the past.
3. The significant amount of information gathered over the last few years on asteroid and comet collisions indicates how they can trigger large-scale and large-standing ecological catastrophes, sometimes leading to mass extinctions of species. Such impacts therefore represent a significant threat to human civilisations.
4. Although, statistically speaking, the risk of major impact in the near future is low, the possible consequences are so vast that every reasonable effort should be encouraged in order to minimise them.
5. The Assembly therefore welcomes the various initiatives, such as the "Spaceguard Survey" report published by the NASA, the creation of the Working Group on Near-Earth Objects by the International Astronomical Union, and the recent decision of the NEO community to set up a spaceguard foundation to co-ordinate efforts at international level, as important steps in paving the way towards the development of a world-wide surveillance programme aimed at discovering all potentially-hazardous NEOs and tracking their orbits forward by computer so that any impact could be foreseen some years in advance, allowing preventive action to be taken as necessary.
6. The Assembly invites governments of member states and the European Space Agency (ESA) to urge the setting up and development of the above-mentioned spaceguard foundation and to give the necessary support to an international programme which would:
i. establish an inventory of NEOs as complete as possible with an emphasis on objects larger than 0,5 km in size;
ii. further our understanding of the physical nature of NEOs, as well as the assessment of the phenomena associated with a possible impact, at various levels of impactor kinetic energy and composition;
iii. monitor detected objects regularly over a period of time long enough to enable a sufficiently-accurate computation of their orbits, so that any collision could be predicted well in advance;
iv. assure the co-ordination of national initiatives, data collection and dissemination, and the equitable geographical distribution of observatories between northern and southern hemispheres;
v. participate in designing small, low-cost satellites for observing NEOs which cannot be detected from the ground, and for investigations which can most effectively be conducted from space;
vi. contribute to a long-term global strategy for remedies against possible impacts.
II. Explanatory memorandum[2]
by Mr LORENZI
Contents
2.1. The Near-Earth Object population
2.2. Impact probabilities and consequences
3.1. Discovery and follow-up
3.1.1. Astrometric observations
3.1.2. Physical observations
3.1.3. Observatories and observers
3.2. Space missions
The formation of craters on the solid bodies in the solar system is an ubiquitous and perennial process. After thirty years of space explorations, it is clear that this process has affected in the past � and will be affecting in the future � all objects: from small, rocky asteroids to planets and satellites, including the Earth.
In the last few decades scientific research has reached a satisfactory understanding of how craters are formed, whereas the frequency of these events according to the type and size of the impacting objects is still not completely understood. Furthermore, the most recent studies on the occurrences of impacts on the Earth's surface are continuously unveiling the close connection between cosmic impacts and the development of life on our home planet, indicating how impacts have contributed, during the geologic eras, to the global evolution of the biosphere through dramatic and repeated destructive episodes, sometimes leading to mass extinctions of living organisms and species.
It is a widespread opinion, among planetary scientists, that impact events may represent a serious and continuous threat to human civilisation, having as their likely consequences the potential to trigger large‑scale and long‑standing ecological catastrophes which may cause the collapse of social infrastructures, especially in less developed countries.
Modern scientific and technological research is able to confront the threat posed by cosmic impacts to that of other major natural disasters such as volcanic eruptions, floods and earthquakes. However, such research � whose importance for the protection of the environment has not yet been fully recognised � is still poorly funded and not well co-ordinated at an international level; catastrophic impacts do not make distinctions of political boundaries and the global character of the hazard needs a corresponding international response.
A short scientific assessment of the problem is contained in this report, together with suggestions on the actions that the international community could undertake to reduce the risk to tolerable levels.
It is the opinion of the author of this report that the risk of major impacts is very low on a short temporal scale; however, their possible consequences are so devastating that every reasonable effort should be encouraged in order to minimise them.
2.1. The Near‑Earth Object population
There are two broad categories of Near‑Earth Objects (NEOs): comets and asteroids. Asteroids and comets are distinguished by astronomers on the basis of their telescopic appearance. If the object is star‑like in appearance, it is called an asteroid. If it has a visible atmosphere or tail, it is a comet. This distinction reflects in part a difference in composition; asteroids are generally rocky or metallic objects without atmospheres, whereas comets are composed in part of volatiles (like water ice) that evaporate when heated to produce a tenuous and transient atmosphere. However, a volatile‑rich object will develop an atmosphere only if it is heated by the Sun, and an old comet that has lost much of its volatile inventory, or a comet that is far from the Sun, can look like an asteroid.
For our purposes, the distinction between a comet and an asteroid is not very significant; more important is the classification of NEOs in terms of their orbits. The near‑Earth asteroids are categorised as Amors, Apollos, and Atens, according to whether their orbits lie outside that of the Earth, cross that of the Earth with period greater than one year, or cross that of the Earth with period less than one year. Cometary objects are classed as short‑period if their periods are less than 200 years, and as long‑period (or "new") if their periods are greater than 200 years.
Even more relevant to this report is the definition of an Earth‑crossing asteroid (ECA). These are the asteroids that have the potential to impact our planet. An ECA is defined as an object moving on a trajectory that is capable of intersecting the capture cross‑section of the Earth (in practice, to hit the planet) as a result of on‑going long‑range gravitational perturbations due to the Earth and other planets. In this case "long‑range" refers to periods of tens of thousands of years. For any particular NEO, it will not be clear whether it is in fact an ECA until an accurate orbit is calculated. Ultimately, however, it is only ECAs that concern us in a programme aimed at discovering potential Earth impactors.
In 1989 there were ninety known ECAs, while 128 ECAs were known at the time the "Spaceguard Survey Report" was written in 1992.[3]. The population of Earth‑crossing asteroids can be approximated by several power laws, which reflect a general exponential increase in the numbers of ECAs as we go to smaller and smaller sizes; from such distributions one can estimate the total number of asteroids having diameters larger than values of particular interest. Using the curves given in the Spaceguard Survey Report, we estimate there are 2 100 larger than 1 km in diameter (the size for which this survey seeks completeness), 9 200 larger than 500 metres (a size representative of great potential damage from tsunamis[4]), and 1,4 million larger than 50 metres (the threshold for penetration of the atmosphere). Active comets can also cross the Earth's orbit with the potential for collision. At any given size, short‑period comets contribute only an additional 1% or so to the total population, a value that is small compared to the estimated uncertainty in the ECA population.
Although about 700 very‑long‑period or new comets are known to have passed through the inner solar system during recorded history, their total population is difficult to characterise since the majority remain unobservable in the outermost regions of the solar system. From their orbital and size distributions, we estimate that Earth‑crossing new comets are about five times more abundant than Earth‑crossing short‑period comets. Thus the combined total number of Earth‑crossing comets is only about 5% to 10% that of the Earth‑crossing asteroid population. However, as noted in the Spaceguard Survey Report, the comets contribute disproportionately to the impact flux because of their higher impact speeds, relative to those of the asteroids, and they may account for as much as 25% of the total impact hazard.
The well‑observed ECAs exhibit a diversity in inferred mineralogy approaching that in the rest of the asteroid population. The majority are expected to be similar to the dark C‑type asteroids in general properties (presumably moderately low‑density, volatile‑rich bodies, coloured black due to at least several percent of opaques). There are also a large number of S types. (S's are thought to be either stony, chondrite‑like objects, stony‑iron objects, or a combination of both.) In addition, there are known examples of metallic bodies (probably like nickel‑iron alloy meteorites) and rocky, monomineralic bodies. These asteroids are often quite irregular in shape; they also tend to have rather rapid spins, but there is a great diversity in such properties. Radar images of several such objects (Castalia, Toutatis, Geographos) show complex, highly elongated forms that suggest a history of catastrophic impacts and, in the case of Castalia, of merging of two fragments to form a dumbell‑shaped double‑lobed object.
It is particularly uncertain what the physical properties of comets (dead or alive) might be like. Only one comet has been studied in detail: Comet Halley, which was the target of several flyby spacecraft missions at the time of its last apparition in 1986. The nucleus of Halley is irregular and dark, with an average diameter of about 10 km. Like other comets, it is made of a combination of ice(s), rocks, and dust, with much of the atmospheric outgassing near the Sun confined to discrete plumes or jets. The non‑volatiles include both silicates and organic materials, while the primary ices (with percentages derived for Halley) are water (80%) and carbon monoxide (15%), plus lesser quantities of formaldehyde, carbon dioxide, methane, ammonia, and hydrocyanic acid. The physical configuration of comets is even more poorly understood than that of the small asteroids, and many comets have been observed to split under rather modest tidal and thermal forces. The only direct estimate of density is derived from the tidal disruption of Comet Shoemaker‑Levy 9, yielding a value near 1 g/cc.
For a study of impacts, it is not essential to know a great deal about the physical nature of comets and asteroids. The most important properties are simply their mass and impact velocity, although it would make a difference if the projectile were double or multiple and easily came apart as it entered the atmosphere. However, any future programme for intercepting and diverting an incoming comet or asteroid will require detailed knowledge of the configuration, density, cohesion, and composition of these objects. For these reasons, in addition to their significance for basic science, spacecraft missions to comets and near‑Earth asteroids are essential.
2.2. Impact probabilities and consequences
There are only a few impact craters still recognisable as such on the surface of the Earth. This is due to tectonic processes that recycle most of the surface on geologic time‑scales, the continuous erosion of the surface by water, and the reworking of the soil by living organisms. These processes obliterate the smallest craters very quickly.
Owing to the poor number of identified craters, the evaluations of the impact probabilities, as a function of size of the impacting body, are mostly based on the observation of craters on the Moon. Our satellite still retains a very good memory of the impacts occurred in the last four thousand million years: therefore, the study of the distribution and age of lunar craters provides us with an efficient method to evaluate how the corresponding distribution on Earth has evolved with time. From these distributions it is then possible to evaluate the probability that an object of a given size may strike the Earth; figure 1 shows the results of this evaluation, in the form of a frequency curve (Chapman and Morrison,[5]).
Figure 1 � Typical intervals between impacts equal to or larger than the specific yields. Equivalent asteroid diameters are shown in the upper scale, assuming 20 km/s impact velocity and 3 g/cm^3 density (Chapman and Morrison, 1994).
The effects of an impact on the Earth vary greatly, depending on the size of the object and its velocity with respect to the planet. Small bodies, with diameters below 10 metres, cause no damage, because they are destroyed during their descent through the upper atmosphere. Larger objects (up to 100 metres) may explode in the upper atmosphere, the lower atmosphere, or hit the ground. In the last two instances the area surrounding the fall site may be severely affected. As an example, the explosion of a 50 metre object close to the Tunguska river (Siberia) in 1908, at an altitude of about 8 km, completely destroyed about 2 000 km2 of forest: had the event happened over a city, the consequences of the explosion (equivalent to about 10 megatons) would have been disastrous. Cases of this type take place every 100 to 300 years somewhere on the Earth.
Impacts by objects of several hundred metres in diameter could devastate a much larger region. If they would fall on the oceans, the ensuing tsunami could cause very great damage in coastal areas, where a large fraction of the human population resides.
At larger sizes, 1 km to 5 km in diameter, a threshold is finally reached at which the impact has catastrophic global consequences, although much work remains to be done fully to understand the physical and chemical effects of material injected into the atmosphere. In general, the crater produced by these impacts has 10 to 15 times the diameter of the projectile, that is 10 km to 15 km diameter crater for a 1-km asteroid. Such craters are formed on the continents about once per 300 000 years. At impactor sizes greater than 1 km, the greatest hazard derives from the global veil of dust injected into the atmosphere. The severity of the global effects of large impacts increases with the size of the impactor and the resulting quantity of injected dust. At some size, an impact would lead to massive world‑wide crop failures and consequent mass mortality, and would threaten the survival of civilisation. At still larger sizes, even the survival of the human species would be put at risk.
What happens when an object several kilometres in diameter strikes the Earth at a speed of tens of kilometres per second? Primarily there is a massive explosion, sufficient to fragment and partially to vaporise both the projectile and the target. Meteoric phenomena associated with high‑speed ejecta could subject plants and animals to scorching heat for about half an hour, and a continent‑wide firestorm might then ensue. Dust thrown up from a very large crater would lead to daytime darkness over the whole Earth, which might persist for several months. Temperatures could drop as much as tens of degrees Celsius. Nitric acid, produced from the burning of atmospheric nitrogen in the impact fireball, would acidify lakes, soils, streams, and perhaps the surface layer of the oceans. Months later, after the atmosphere had cleared, water vapour and carbon dioxide released to the stratosphere would produce an enhanced greenhouse effect, possibly raising global temperatures by as much as 10 degrees above the pre‑existing ambient temperatures. This global warming might last for decades, as there are several positive feedbacks. Both the initial months of darkness and cold, and then the following years of enhanced temperatures, would severely stress the environment and would lead to drastic population reductions of both terrestrial and marine life.
The threshold size of impactor that would produce one or all of the effects discussed above is not accurately known. The geochemical and paleontological record has demonstrated that one impact of a 10‑km object, occurred 65 million years ago on the Yucatan peninsula, resulted in total extinction of about half the living species of animals and plants. This so‑called Cretaceous‑Tertiary (K‑T) impact may have exceeded 100 million megatons in explosive energy. Such mass extinctions have occurred several times in the past few hundred million years; it has been suggested, although not yet fully proven, that impacts are responsible for most such extinction events. We know from astronomical and geological evidence that impacts of objects with diameters of 5 kilometres or greater occur about once every 10 to 30 million years.
3.1. Discovery and follow‑up
Although the probability of an impact at the highest levels of risk is extremely small (of the order of once every 300 000 to 30 million years, depending on the size), its likely consequences are so high that it seems at least prudent to reach the certainty that no asteroid or comet in that diameter range (over 1 km) is currently on an orbit which could lead it to a collision with the Earth.
Unfortunately this certainty is impossible to obtain, at present. As already said, we have discovered only a minority of the potentially hazardous objects, and for many of them our current knowledge of their motion is very poor. In recent years several studies have been conducted aimed at designing an observational strategy capable of dramatically increasing the rate of discovery.[6],[7] It is generally believed that a network of dedicated telescopes, equipped with modern CCD sensors, in operation for several decades, would discover some 95% of the objects larger than 1 km, that is those which could pose a global threat (the so‑called "Spaceguard Survey"). However, there are two orders of problems related to this plan. The first one is scientific in character and related to the discovery‑confirmation process; the second one is mainly of political nature, and involves the current situation of observers and observatories.
As a matter of fact, discovery is not sufficient. In order to be able to compute the orbit of an NEO with sufficient accuracy to guarantee that it will not collide with our planet during, say, the next couple of centuries, we need a great number of observations, spread over a rather long period of time. This activity is called "follow‑up", and is of crucial importance. It is not at all sure that the discovery network could, at the same time, conduct sufficiently reliable follow‑up; on the contrary, it is probably more useful that a number of smaller telescopes be dedicated to follow‑up observations, once the objects have been identified. Follow‑up observations can be divided in two broad classes: astrometric observations and physical observations.
3.1.1. Astrometric observations
NEOs are often discovered when they pass just close enough for detection, and they may subsequently fade. After a few weeks (and sometimes only a few days), they cannot be re‑observed with the same telescope that made the discovery unless longer integration times are used. Half of the kilometre‑sized NEOs that will be detected in an all sky survey will be visible for no more than one month. Unfortunately, this observational window is too short to determine with sufficient certainty the orbit of an NEO. Many NEOs determined to be in Earth-crossing orbits could not be ruled out as threats by such limited observations. Also, the position of such an object could not be predicted with sufficient accuracy to find it again a few years later.
If the telescopes that conduct the survey are large enough (about 2 metres), they will have enough light‑gathering power to cover the whole sky, each month, with short integrations (about 10s), and still have time left over for following up discoveries from previous months. However, if metre‑class telescopes are used, they will require longer integrations to achieve the same limiting magnitude, and will not have time left over for astrometric follow‑up. Also, they may not have the pixel resolution needed to determine orbits of adequate precision for future recovery and hazard prediction. In that case, additional telescopes of 1 metre aperture or larger will be required for astrometric follow‑up.
3.1.2. Physical observations
There is both a need and an opportunity to make physical observations of a subset of NEOs discovered by a Spaceguard survey. In its barest form, the survey would identify an enormous number of objects in Earth‑approaching orbits, characterised by their orbital properties and their optical magnitudes. However, in order to identify the NEOs that would be potentially hazardous to Earth, sampling of the population's physical properties is essential. The mass‑frequency relation of the NEO population must be defined in order to reach a specified level of completeness for a threat defined by impact energy. Asteroid albedos (the fraction of sunlight reflected by the surface) are known to differ by a factor of about 30; some constituent materials of NEOs differ even more. Without information about albedo, inferred masses (and hence impact energies) could differ by more than a factor of 100. Moreover, the population of metre‑to‑km scale objects could differ greatly from that of the larger objects. Therefore, cataloguing NEO orbital properties to some limiting magnitude does not meet the minimal goals of the NEO survey. Sampling of associated masses (that is by determination of diameters and albedos) is also required.
A proper survey of NEOs, made for the purpose of raising our level of understanding about impact hazards, would have even broader goals than merely obtaining a partial or complete census of objects of particular sizes. It should seek to obtain data about physical properties that will help us understand the impact hazard more generally. For example, it is reasonable to expect that NEOs with different origins will fragment in different ways. Extinct comet nuclei with tensile strengths about 100 to 1 000 dynes/cm^2 will fragment easily, while metallic bodies, such as are represented by nickel‑iron meteorites with tensile strengths exceeding 10^9 dynes/cm^2 will not fragment easily. The fragile, cometary bodies may therefore dominate the population of NEOs smaller than some transition size (about 1 metre to 1 kilometre). Since comets originate in the outer solar system, such objects might also be expected to dominate NEOs with higher impact velocities. Thus an understanding of NEO mineralogy will be a factor in our understanding of impact hazard versus size. Ideally, the statistical completeness of the survey would be assessed for each mineralogical class, and this would require knowledge of the mineralogical type and size for as many NEOs as possible.
3.1.3. Observatories and observers
There are currently only three centres making regular observations aimed at discovering near‑Earth objects. Two of them are located in the United States, and one in Australia. All of these projects have big funding and personnel problems. A project like the proposed Spaceguard survey, on the contrary, should count on a relatively large number of observing stations (six to ten, for discovery and follow‑up) and certainly involve many scientists. A first important point, in this respect, is the geographical distribution of observatories, which should be evenly located in the northern and southern hemisphere. A second point is that the activity at all these stations should be highly co-ordinated, possibly on an international basis. A third point involves international agreements to allow observers to move from one place to another, without requiring an often impossible financial effort from the home institutions.
The organisational aspects of the problem are as important as the scientific and technical ones, because a network like the one being studied could work with good efficiency only if these problematic aspects are solved with the concurrence of many nations.
3.2. Space missions
It is not the purpose of this report to enter into the field of active protection from cosmic impacts. It is necessary to mention, however, that any attempt to catalogue unknown objects and investigate their physical nature would obtain great advantages from observations in space, both using space observatories in Earth orbit and by close recognitions of selected targets by space probes.
At present, the major space agencies are studying missions to minor bodies, such as the NEAR (NASA) or the Rosetta (ESA) missions. A great deal of new knowledge will certainly be produced by these missions, especially for what concerns the physical nature of the visited objects.
Moreover, it is possible to design low‑cost satellites, to be put into orbit around the Earth, which could perform the type of observations which are not possible from the ground, either because of the screening power of the atmosphere (especially in the infra-red domain of the spectrum), or because the objects to be observed could be located too close to the Sun.
Several initiatives have already been taken at a more political and organisational level, especially in the United States, to deepen our current knowledge of the NEO hazard and to study plans of intervention. The United States Congress, in 1990, directed the NASA to establish two working groups in order to assess detection problems (Detection WG, chaired by David Morrison) and interception and active defence problems (Interception WG, chaired by J. Rahe and J. Rather). The two working groups have produced two reports, published by the NASA in 1992.[8],[9] Subsequently, in 1994, a new directive of the US Congress led to the formation of the NASA NEO Survey Working Group, led by Eugene Shoemaker. The aim of this committee was to plan a US intervention with the goal to discover all potentially hazardous objects within ten years. The Shoemaker Committee produced this report in June 1995.[10]
In 1991 the International Astronomical Union (IAU) created the Working Group on Near‑Earth Objects, chaired by Andrea Carusi, with the specific task to raise the international consciousness of the problem, and to help the birth of national and international initiatives. The WGNEO was reappointed in 1994, and is now organising an international workshop with the aim of providing a detailed plan for a Spaceguard survey. This plan should form the basis for a future international co-operative effort in the field.
In early 1993, the European Science Foundation (ESF) initiated a new scientific network, entitled "Impact Cratering and Evolution of Planet Earth". This network, especially active in the geological and paleontological fields, was intended to allow exchange of scientists and expertise in different fields, ranging from the study of terrestrial cratering records to the investigation of mass extinction phenomena in the past. The network has been active for three years, and will probably be renewed as an ESF research programme in the future.
In April 1995, a Conference on Near‑Earth Objects was hosted by the United Nations. Representatives of the UN Office of Outer Space Affairs attended the conference. The UN is not extremely active in this field, but a broad convergence with studies on space debris has been recognised. This is an important point, because the techniques and observational methods used to monitor the size, number and movements of debris around the Earth (coming mainly from unused or destroyed spacecrafts) are very close to those which will be used for NEO observations. A greater involvement of the United Nations in these problems is therefore probable and auspicious.
Major international scientific meetings dealing with NEOs and their impact threat have been held in Flagstaff (USA), San Juan Capistrano (USA), St Petersburg (Russia), Li�ge (Belgium), Tucson (USA), Erice (Italy), Belgirate (Italy), Sagamihara (Japan), New York (USA). In late 1995 and 1996 the major opportunities will be Vulcano (Italy), Hawaii (USA), and Versailles (France).
In view of the awakening consciousness that the threat posed by cosmic impacts is real, and on the sizeable amount of information gathered in the last years, the international scientific community concurs in judging cosmic impacts an important topic for further investigation, not only because of its intrinsic scientific interest, but mainly because of the relevance it may have for society and for the Earth's environmental protection.
There is wide consensus on the statement that the first � and most important �
step of this enhanced investigation is represented by a complete and long‑standing sky survey, in search for all objects larger than 1 km, those which represent the major hazard. However, such a survey would also lead to the discovery of sizeable fractions of smaller objects, some of which could cause massive regional or local destruction. There is also broad agreement on the necessity of such a survey being undertaken on an international basis.
There are at least three reasons why searches for NEOs should be overseen, in the long run, by an international agency:
a. scientific investigations devoted to the discovery and follow‑up of NEOs, to the understanding of their physical nature and of the dynamical mechanisms that deliver them in the inner regions of the solar system, as well as to the possible phenomena associated with an impact, are best conducted in an international context, where a free exchange of knowledge and skills may allow a synergistic effort and therefore an optimal result;
b. research and experiments on defence systems are a very delicate issue, which involves people and assets formerly involved with the cold war. It would be at least prudent that any decision and directive on these matters be discussed and agreed upon in an international forum;
c. political and social initiatives have a higher probability of success if the enterprise is international in nature. In particular, sharing resources and costs is especially applicable to efforts such as the proposed Spaceguard survey, which requires the establishment of observational and communication networks.
Recognising that:
our current knowledge of the quantity, distribution, and actual orbits of Near‑Earth Objects is very limited;
an inventory of NEOs as complete as possible with present techniques is only possible through a co-operative, co-ordinated programme of observation and data collection;
cosmic impact is an environmentally significant phenomenon which has played a major role in the evolution of life on Earth;
in any given year there is a very low probability that a large cosmic impact may occur, which would destroy human civilisation or even a significant fraction of life on Earth. However, the threat is real and requires further internationally co-ordinated, public educational efforts;
a significant near-term cosmic impact threat is a naturally produced, atmospheric explosion of a small NEO being mistaken for a nuclear explosion, at a time and place of international tension. These events have been observed. Such an event could be misinterpreted as a nuclear attack and trigger an unfortunate reaction;
the gathering of additional physical knowledge on Near‑Earth Objects and their effect on Earth is a scientifically and socially important endeavour.These multidisciplinary efforts should be conducted in an open, co-ordinated, international manner. Dedicated international astronomical facilities similar to the proposed {\em Spaceguard System} should be developed. The assets and technologies of many countries can contribute to the gathering of these data, through ground and robotic space observations. The skills and technologies, necessary for any large, complex investigation, should be utilised;
unless a specific and imminent threat becomes obvious, actual construction and testing of systems that may have the potential to deflect or mitigate a threat should be deferred, it is, therefore, recommended that:
a co-ordinated, international search programme be initiated to detect all Near‑Earth Objects brighter than magnitude 22;
capabilities for obtaining accurate astrometric observations, as necessary to determine accurate ephemerides, be established;
co-ordination between scientific and humanitarian organisations and governments be encouraged concerning NEOs;
local and national initiatives to establish observational tools and networks be encouraged;
the Council of Europe take the initiative to promote these studies and activities among member states.
Reporting committee: Committee on Science and Technology.
Budgetary implications for the Assembly: none.
Reference to committee: Doc. 7163 and Reference No. 1993 of 30 January 1995.
Draft resolution unanimously adopted by the committee on 8 January 1996.
Members of the committee: Mr Roseta (Chairman), Mrs Terborg, Mr Birraux (Vice-Chairpersons), MM. About, Bartodziej, Bauer, Beaufays, Benetatos, Berger, Bog�r, Mrs Brenden, MM. Caccia, Cherribi, Cunliffe, Deguara, Galley, Golu, Gregory, Gui�, Guidi, Mrs Hoffmann, MM. Hurta, Johansson, Korakas, Leers, Lekberg, Lenzer, L�pez Valdivielso (Alternate: Robles Orosco), _uczak, Maass, Metelko, Mocioi, Moln�r (Alternate: Szalay), Nania (Alternate: Lorenzi), Olrich, Petersen, Po�as Santos, Po_�la, Proke�, Ruka, Mrs Schicker, MM. Sinka, Speroni, Mrs Stiborov�, Mr Theis, Sir Donald Thompson, Mr Jack Thompson (Alternate: Lord Newall), MM. Tiuri, Toshev, Y�r�r, Weyts.
N.B. The names of those members who took part in the vote are printed in italics.
Secretaries to the committee: Mr Perin and Miss Theophilova.
[1]. By the Committee on Science and Technology.
[2]. Prepared with the assistance of Dr Andrea Carusi, Chairman of the Working Group on Near-Earth Objects of the International Astronomical Union (IAU), Rome, Italy.
[3]. Morrison, D. (editor), 1992, "The Spaceguard Survey, Report of the NASA International Near-Earth Objects Detection Workshop", NASA, Office of Space Science and Application.
[4]. Tidal waves.
[5]. Chapman, C.R., and Morrison, D., 1994, "Impacts on the Earth by Asteroids and Comets: Assessing the Hazard", Nature, 367, 33.
[6]. Morrison, D. (editor), 1992, "The Spaceguard Survey, Report of the NASA International Near-Earth Objects Detection Workshop", NASA, Office of Space Science and Application.
[7]. Gehrels, T. (editor), 1994, "Hazards due to Comets and Asteroids", University of Arizona Press, Tucson.
[8]. Morrison, D. (editor), 1992, "The Spaceguard Survey, Report of the NASA International Near-Earth Objects Detection Workshop", NASA, Office of Space Science and Application.
[9]. Rather, J.D.G., and Rahe, J.H. (editors), 1992, "Report of the NASA Near‑Earth Objects Interception Workshop", NASA, Office of Space Science and Application.
[10]. Shoemaker, E.M. (editor), 1995, "Report of the Near‑Earth Objects Survey Working Group", NASA, in press.