Impact from Outer Space

The Earth resides in a swarm of comets and asteroids that can, and do, impacts on its surface from time to time. The solar system contains a long-lived population of asteroids and comets, some fraction of which are perturbed into orbits that cross the orbits of the Earth and other planets. Spacecraft exploration of the terrestrial planets and the satellites of the outer planets has revealed crater-scarred surfaces that testify to a continuing rain of impacting projectiles. Additional evidence concerning cosmic projectiles in near-Earth space has accumulated since the discovery of the first Earth-crossing asteroid nearly sixty years ago, and improvements in telescopic search techniques have resulted in the discovery of dozens of near-Earth asteroids and short period comets each year. The role of impacts in affecting the Earth's geological history, its ecosphere, and the evolution of life itself has become a major topic of current interdisciplinary interest.

Significant attention by the scientific community to the hazard began in 1980 when Luis Alvarez and others proposed that such an impact, and the resulting global pall of dust, resulted in the mass extinctions of life forms on Earth, ending the age of dinosaurs (Alvarez and others, 1980). Additional papers and discussion in the scientific literature followed, and widespread public interest was aroused. In 1981, NASA organized a workshop "Collision of Asteroids and Comets with the Earth: Physical and Human Consequences" at Snowmass, Colorado (July 13-16, 1981). A summary of the principal conclusions of the workshop report appeared in the book Cosmic Catastrophes (Chapman and Morrison, 1989a) and in a presentation by Chapman and Morrison(1989b) at an American Geophysical Union Natural Hazards Symposium. In response to the close passage of asteroid 1989FC, the American Institute of Aeronautics and Astronautics (AIAA, 1990) recommended studies to increase the detection rate of near-Earth asteroids, and how to prevent such objects striking the Earth. The AIAA brought these recommendations to the attention of the House Committee of Science, Space, and Technology, leading to the Congressional mandate for this workshop included in the NASA 1990 Authorization Bill. In parallel with these political developments, a small group of dedicated observers significantly increased the discovery rate of Near-Earth asteroids and comets, and several of these discoveries were highlighted in the international press. Other recent activity has included the 1991 International Conference on Near-Earth Asteroids (San Juan Capistrano, California, June 30 - July 3), a meeting on the "Asteroid Hazard" held in St. Petersburg, Russia (October 9-10, 1991), and a resolution endorsing international searches for NEO's adopted by the International Astronomical Union (August 1991).

Despite a widespread perception that asteroid impact is a newly recognized hazard, the basic nature of the hazard was roughly understood half a century ago. In 1941, Fletcher Watson published an estimate of the rate of impacts on the Earth, based on the discovery of the first three Earth-approaching asteroids (Apollo, Adonis, and Hermes). A few years later, Ralph Baldwin (1949), in his seminal book The Face of the Moon, wrote

...since the Moon has always been the companion of the Earth, the history of the former is only a paraphrase of the history of the latter... [Its mirror on Earth] contains a disturbing factor. There is no assurance that these meteoritic impacts have all been restricted to the past. Indeed we have positive evidence that [sizeable] meteorites and asteroids still abound in space and occasionally come close to the Earth. The explosion that formed the [lunar] crater Tycho...would, anywhere on Earth, be a horrifying thing, almost inconceivable in its monstrosity.

Watson and Baldwin (both of whom are still alive) were prescient, but in their time few other scientists gave much thought to impacts on the Earth. Recently, however, there has been a gestalt shift that recognizes extraterrestrial impact as a major geological process and, probably, an important influence on the evolution of life on our planet. Also new is our capability to detect such objects and to develop a space technology that could deflect a potential projectile before it struck the Earth.

The Hazard of Cosmic Impacts

Throughout Its history, the Earth has been impacted by countless asteroids and comets. Smaller debris continually strike Earth's upper atmosphere where they burn due to friction with the air; meteors (which are typically no larger than a pea and have a mass of about a gram) can be seen every night from a dark location if the sky is clear. Thousands of meteorites (typically a few kilograms in mass) penetrate the atmosphere and fall harmlessly to the ground each year. On rare occasions, a meteorite penetrates the roof of a building, although to date there are no fully documented human fatalities. A much larger event, however, occurred in 1908 when a cosmic fragment disintegrated in the atmosphere over Tunguska, Siberia, with an explosive energy of more than 10 megatons TNT. But even the Tunguska impactor was merely one of the smallest of Earth's neighbours in space. Of primary concern are the larger objects, at least one kilometres in diameter. Although very rare, the impacts of these larger objects are capable of severely damaging the Earth's eco-system with a resultant massive loss of life.

In the following discussion, we examine the risks posed by impacting objects of various sizes. These projectiles could be either cometary or asteroidal. In terms of the damage they do, it matters little whether they would be called comets or asteroids by astronomical observers. We term these objects collectively NEOs (Near Earth Objects).

Every few centuries the Earth is struck by an NEO large enough to cause thousands of deaths, or hundreds of thousands of deaths if it were to strike in an urban area. On time scales of millennia, impacts large enough to cause damage comparable to the greatest known natural disasters may be expected to occur (Pike 1991). Indeed, during our lifetime, there is a small but non-zero chance (very roughly 1 in 10,000) that the Earth will be struck by an object large enough to destroy food crops on a global scale and possibly end civilization as we know it (Shoemaker and others 1990).

As described in Chapter 3, estimates of the population of NEOs large enough to pose a global hazard are reliable to within a factor of two, although estimates of the numbers of smaller objects are more uncertain. Particularly uncertain is the significance of hard-to-detect long-period or new comets, which would generally strike at higher velocities than other NEO's (Olsson-Steel 1987), although asteroids (including dead comets) are believed to dominate the flux. However, the resulting environmental consequences of the impacts of these objects are much less well understood. The greatest uncertainty in comparing the impact hazard with other natural hazards relates to the economic and social consequences of impacts.

The Relationship of Risk to Size of Impactor

Small impacting objects that produce ordinary meteors or fireballs dissipate their energy in the upper atmosphere and have no direct effect on the ground below. Only when the incoming projectile is larger than about 10 m diameter does it begin to pose some hazard to humans. The hazard can be conveniently divided into three broad categories that depend on the size or kinetic energy of the impactor:

Impacting body generally is disrupted before it reaches the surface; most of its kinetic energy is dissipated in the atmosphere, resulting in chiefly local effects.
Impacting body reaches ground sufficiently intact to make a crater; effects are still chiefly local, although nitric oxide and dust can be carried large distances, and there will be a tsunami if the impact is in the ocean.
Large crater-forming impact generates sufficient globally dispersed dust to produce a significant, short-term change in climate, in addition to devastating blast effects in the region of impact.

The threshold size of an impacting body for each category depends on its density, strength, and velocity as well as on the nature of the target. The threshold for global effects, in particular, is not well determined.

10-m to 100-m diameter impactors

Bodies near the small end of this size range intercept Earth every decade. Bodies about 100 m diameter and larger strike, on average, several times per millennium. The kinetic energy of a 10-m projectile traveling at a typical atmospheric entry velocity of 20 km/s is about 100 kilotons TNT equivalent, equal to several Hiroshima-size bombs. The kinetic energy of a 100-m diameter body is equivalent to the explosive energy of about 100 megatons, comparable to the yield of the very largest thermonuclear devices.

For the 10-m projectiles, only rare iron or stony-iron projectiles reach the ground with a sufficient fraction of their entry velocity to produce craters, as happened in the Sikhote-Alin region of Siberia in 1947. Stony bodies are crushed and fragmented during atmospheric deceleration, and the resulting fragments are quickly slowed to free-fall velocity, while the kinetic energy is transferred to an atmospheric shock wave. Part of the shock wave energy is released in a burst of light and heat (called a meteoritic fireball) and part is transported in a mechanical wave. Generally, these 100-kiloton disruptions occur high enough in the atmosphere so that no damage occurs on the ground, although the fireball can attract attention from distances of 600 km or more and the shock wave can be heard and even felt on the ground.

With increasing size, asteroidal projectiles reach progressively lower levels in the atmosphere before disruption, and the energy transferred to the shock wave is correspondingly greater. There is a threshold where both the radiated energy from the shock and the pressure in the shock wave can produce damage. A historical example is the Tunguska event of 1908, when a body perhaps 60 m in diameter was disrupted in the atmosphere at an altitude of about 8 km. The energy released was about 12 megatons, as estimated from airwaves recorded on meteorological barographs in England, or perhaps 20 megatons as estimated from the radius of destruction. Siberian forest trees were mostly knocked to the ground out to distances of about 20 km from the end point of the fireball trajectory, and some were snapped off or knocked over at distances as great as 40 km. Circumstantial evidence suggests that fires were ignited up to 15 km from the endpoint by the intense burst of radiant energy. The combined effects were similar to those expected from a nuclear detonation at a similar altitude, except, of course, that there were no accompanying bursts of neutrons or gamma rays nor any lingering radioactivity. Should a Tunguska-like event happen over a densely populated area today, the resulting airburst would be like that of a 10-20 megaton bomb: buildings would be flattened over an area 20 km in radius, and exposed flammable materials would be ignited near the center of the devastated region.

An associated hazard from such a Tunguska-like phenomenon is the possibility that it might be misinterpreted as the explosion of an actual nuclear weapon, particularly if it were to occur in a region of the world where tensions were already high. Although it is expected that sophisticated nuclear powers would not respond automatically to such an event, the possible misinterpretation of such a natural event dramatizes the need for heightening public consciousness around the world about the nature of unusually bright fireballs.

On June 30th a cosmic projectile exploded in the sky over Siberia. It flattened over 2,000 square kilometers of forest in the Tunguska region. If an event like this were to occur today it could result in thousands of deaths and billions of dollars of damage.

100-m to 1-km diameter impactors

Incoming asteroids of stony or metallic composition that are larger than 100 m in diameter may reach the ground intact and produce a crater. The threshold size depends on the density of the impactor and its speed and angle of entry into the atmosphere. Evidence from the geologic record of impact craters as well as theory suggests that, in the average case, stony objects greater than 150 m in diameter form craters. They strike the Earth about once per 5000 years and -- if impacting on land -- produce craters about 3 km in diameter. A continuous blanket of material ejected from such craters covers an area about 10 km in diameter. The zone of destruction extends well beyond this area, where buildings would be damaged or flattened by the atmospheric shock, and along particular directions (rays) by flying debris. The total area of destruction is not, however, necessarily greater than in the case of atmospheric disruption of somewhat smaller objects, because much of the energy of the impactor is absorbed by the ground during crater formation. Thus the effects of small crater-forming events are still chiefly local.

Toward the upper end of this size range, the megaton equivalent energy would so vastly exceed what has been studied in nuclear war scenarios that it is difficult to be certain of the effects. Extrapolation from smaller yields suggests that the "local" zones of damage from the impact of a 1-km object could envelop whole states or countries, with fatalities of tens of millions in a densely populated region. There would also begin to be noticeable global consequences, including alterations in atmospheric chemistry and cooling due to atmospheric dust -- perhaps analogous to the "year without a summer" in 1817, following the explosion of the volcano Tambora.

Comets are composed in large part of water ice and other volatiles and therefore are more easily fragmented than rocky or metallic asteroids. In the size range from 100 m to 1 km, a comet probably cannot survive passage through the atmosphere, although it may generate atmospheric bursts sufficient to produce local destruction. This is a subject that needs additional study, requiring a better knowledge of the physical nature of comets.

1 km to 5 km diameter impactors

At these larger sizes, a threshold is finally reached at which the impact has serious global consequences, although much work remains to be done to fully 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; i.e., 10-15 km diameter 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 primary hazard derives from the global veil of dust injected into the stratosphere. 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 might threaten the survival of civilization. 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 vaporize both the projectile and the target area. Meteoric phenomena associated with high speed ejecta could subject plants and animals to scorching heat for about half an hour, and a global firestorm might them ensue. Dust thrown up from a very large crater would lead to total darkness over the whole Earth, which might persist for several months. Temperatures could drop as much as tens of degrees C. 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 ten degrees C above the pre-existing ambient temperatures. This global warming might last for decades, as there are several positive feedbacks; warming of the surface increases the humidity of the troposphere thereby increasing the greenhouse effect, and warming of the ocean surface releases carbon dioxide which also increases the greenhouse effect. 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.

Threshold Size for Global Catastrophe

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 (or perhaps several closely spaced impacts) 65 million years ago of a 10-km NEO resulted in total extinction of about half the living species of animals and plant. This so-called K-T impact may have exceeded 100 megatons in explosive energy. Such mass extinctions of species have recurred several times in the past few hundred million years; it has been suggested, although not yet 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 km or greater occur about once every 10 to 30 million years.

Death by starvation of much of the world's population could result from a global catastrophe far less horrendous than those cataclysmic impacts that would suddenly render a significant fraction of species actually extinct, but we know only very poorly what size impact would cause such mortality. In addition to all of the known variables (site of impact, time of year) and the uncertainties in physical and ecological consequences, there is the question of how resilient our agriculture, commerce, economy, and societal organization might prove to be in the face of such an unprecedented catastrophe.

These uncertainties could be expressed either as a wide range of possible consequences for a particular size (or energy) of impactor or as a range of impactor sizes that might produce a certain scale of global catastrophe. We take the second approach and express the uncertainty as a range of threshold impactor sizes that would yield a global catastrophe of the following proportions:

It would destroy most of the world's food crops for a year, and /or
It would result in the deaths of more than a quarter of the world's population, and/or
It would have effects on the global climate similar to those calculated for "nuclear winter", and/or
It would threaten the stability and future of modern civilization.

A catastrophe having one, or all, of these traits would be a horrifying thing, unprecedented in history, with potential implications for generations to come.

To appreciate the scale of global catastrophe that we have defined, it is important to be clear what is not. We are talking about a catastrophe far larger than the effects of the great World Wars; it would result from an impact explosion certainly larger than if 100 of the very biggest Hydrogen bombs ever tested were detonated at once. On the other hand, we are talking about an explosion far smaller (less than 1 percent of the energy) the the K-T impact 65 million years ago. We mean a catastrophe that would threaten modern civilization, not an apocalypse that would threaten the survival of the human species.

What is the range of impactor sizes that might lead to this magnitude of global catastrophe? At the July 1991 Near-Earth Asteroid Conference in San Jaun Capistrano, California, the most frequently discussed estimate of the threshold impactor diameter for globally catastrophic effects was about 2 km. An estimate of the threshold size was derived for this Workshop in September 1991 by Brian Toon, of NASA Ames Research Centre. Of the various environmental effects of a large impact, Toon believes that the greatest harm would be done by the sub-micrometer dust launched into the stratosphere. The very fine dust has a long residence time, and global climate modelling studies by Covey and others (1990) imply significant drops in global temperature that would threaten agriculture worldwide. The quantity of sub-micrometer dust required for climate effects equivalent to those calculated for nuclear winter is estimated at about 10,000 Teragrams (Tg) (1 Tg = 10¹²g). For a 30 km/s impact, this translate to a threshold impacting body diameter of between 1 and 1.5 km diameter.

The threshold for an impact that causes widespread global mortality and threatens civilization almost certainly lies between about 0.5 and 5 km diameter, perhaps near 2 km. Impacts of objects this large occur from one to several times per million years.

Risk Analysis

If this estimate of the frequency of threshold impact is correct, then the chances of an asteroid catastrophe happening in the near future -- while very low -- is greater than the probablility of other threats to life that our society takes very seriously. For purposes of discussion, we adopt the once-in-500,000 year estimate for the globally catastrophic impact. It is important to keep in mind that the frequency could be greater than this, although probably not by more than a factor of two. The frequency could equally well be a factor of ten smaller.

Because the risk of such an impact happening in the near future is very low, the nature of the impact hazard is unique in our experience. Nearly all hazards we face in life actually happen to someone we know, or we learn about them from the media, whereas no large impact has taken place within the total span of human history. (If such an event took place before the dawn of history roughly 10,000 years ago there would be no record of the event, since we are not postulating an impact large enough to produce a mass extinction that would be readily visible in the fossil record). But also in contrast to more familiar disasters, the postulated impact would produce devastation on a global scale. Natural disasters, including tornadoes and cyclones, earthquakes, tsunamis, volcanic eruptions, firestorms, and floods often kill thousands of people, and occasionally several million. But the civilization-destroying impact exceeds all of these other disasters in that it could kill a billion or more people, leading to as large a percentage loss of life worldwide as that experienced by Europe from the Black Death in the 14th century. It is this juxtaposition of the small probability of occurrence balanced against the enormous consequences if it does happen that makes the impact hazard such a difficult and controversial topic.

Frequency of Impacts of different sizes

We begin to address the risk of cosmic impacts by looking at the frequency of events of different magnitudes. Small impacts are much more frequent than large ones, as is shown in Figure 2.4. This figure illustrates the average interval between impacts as a function of energy, as derived from the lunar cratering record and other astronomical evidence. For purposes of discussion , we consider two cases: The threshold globally catastrophic impact discussed above, and for comparison, a Tunguska-class impact from a smaller object perhaps 100 m in diameter. In all of the examples given below, the numbers are approximate and are used only to illustrate the general magnitudes involved.

For the globally catastrophic impact:

Average interval between impacts: 500,000 years

For the Tunguska-class impacts:

Average interval between impacts for total Earth: 300 years
Average interval between impacts for populated area of Earth: 3,000 years
Average interval between impacts for world urban areas: 100,000 years
Average interval between impacts for U.S. urban areas only: 1,000,000 years

Annual risk of death from impacts

One way to address the risk is to express that risk in terms of the annual probability that an individual will be killed as a result of an impact. This annual probability of mortality is the product of (a) the probability that the impact will occur and (b) the probability that such an event will cause the death of any random individual.

For the globally catastrophic impact:

Average interval between impacts for total Earth: 500,000 years
Annual probability of impact: 1/500,000
Assumed fatalities from impact: one-quarter of world population
Probability of death for an individual: 1/4
Annual probability of an individuals death: 1/2,000,000

For the Tunguska-class impact:

Average interval between impacts for total Earth: 300 years
Assumed area of devastation and total mortality from impact: 5,000 sq km (1/10,000 of Earth's surface)
Annual probability of an individual's death: 1/30,000,000

Thus we see that the analysed risk is about 15 times greater from the large impact than from the Tunguska-class impact.

Equivalent annual deaths as a measure of risk

An alternative but equivalent way to express the risks is in terms of average annual fatalities. While such an index is convenient for comparison with other risks, we stress the artificiality of applying this approach to the very rare impact catastrophes. The concept of equivalent annual deaths strictly applies only in a static world in which the population and the mortality rate from other causes do not vary with time. This figure is obtained by multiplying the population of the Earth by the total annual probability of death calculated above. In the case of the U.S equivalent deaths, we allow for the higher than average population density in the U.S.

For the globally catastrophic impact:

Total annual probability of death: 1/2,000,000
Equivalent annual deaths for U.S. population only: 125
Equivalent annual deaths (worldwide population):2,500

For the Tunguska-class impact:

Total annual probability of death: 1/30,000,000
Equivalent annual deaths for U.S. population only: 15
Equivalent annual deaths (worldwide population): 150

These figures can be compared with the mortality rates from other natural and man-made causes to obtain a very rough index of the magnitude of the impact-catastrophe hazard. For example, the U.S. numbers can be compared with such other causes of death as food poisoning by botulism (a few per year), tornadoes (100 per year), and auto accidents (50,000 per year).

Qualitative difference for the impact catastrophe

The above analysis is presented to facilitate comparison of impact hazards with others with which we may be more familiar. However, there is a major qualitative difference between impact catastrophes and other more common natural disasters. A global impact catastrophe could lead to a billion or more fatalities and an end to the world as we know it. No other natural disasters, including the Tunguska-class impacts, have this nature. They represent just one among many causes of human death. In contrast, the potential consequences of a large impact set it apart from any other phenomenon with the exception of full-scale nuclear war.

Concern over the cosmic impact hazard motivated the U.S. Congress to request that NASA conduct a workshop to study ways to achieve a substantial acceleration in the discovery rate for near-Earth asteroids. This report outlines an international survey network of ground-based telescopes that could increase the monthly discovery rate of such asteroids from a few to as many as a thousand. Such a program would reduce the time-scale required for a nearly complete census of large Earth-crossing asteroids (ECAs) from several centuries (at the current discovery rate) to about 25 years. We call this proposed survey program the Spaceguard Survey (borrowing the name from the similar project suggested by science-fiction author Arthur C. Clarke nearly 20 years ago in his novel Rendezvous with Rama).

In addition, this workshop has considered the impact hazards associated with comets (both short-period and long-period) and with small asteroidal or cometary objects in the tens of meters to hundreds of meters size range. The object is not elimination of risk, which is impossible for natural hazards such as impacts, but reduction of risk. Emphasis, therefore, is placed upon the greater hazards, in an effort to define a cost-effective risk-reduction program. Below we summarize our conclusions with respect to these three groups of objects: ECAs, comets, and small (Tunguska-class) objects.

1) Large ECAs (diameter greater than 1 km, impact energy greater than a100,000 megatons). These objects constitute the greatest hazard, with their potential for global environmental damage and mass mortality. About two thousand such objects are believed to exist in near-Earth space, of which fewer than 10 percent are now known. Between a quarter and a half of them will eventually impact the Earth, but the average interval between such impacts is long -- more than 100,000 years. While some of these objects may break up during entry, most will reach the surface, forming craters if they strike on the land. On average, one ECA in this size range passes between the Earth and the Moon every few decades.

The proposed Spaceguard Survey deals effectively with this class of objects. Telescopes of 2- to 3-m aperture can detect them out to a distance of 200 million kilometers. Since their orbits bring them frequently within this distance of the Earth, a comprehensive survey will discover most of them within a decade and can achieve near completeness within 25 years. Specifically, the survey modeled here, covering 6000 square degrees of sky per month to magnitude V = 22, is calculated to achieve 91 percent completeness for potentially hazardous ECAs in 25 years. The most probable outcome of this survey will be to find that none of these objects will impact the Earth within the next century, although a few will need to be followed carefully to ensure that their orbits do not evolve into Earth-impact trajectories. In the unlikely case (chances less than 1 percent) that one of these ECAs poses a danger to the Earth over the next century or two, there will be a warning of at least several decades to take corrective action to deflect the object or otherwise mitigate the danger.

2. Comets. Comets with short periods (less than 20 years) will be discovered and dealt with in the same manner as the ECAs described above; they constitute only about 1 percent of the ECA hazard in any case. However, comets with long periods (more than 20 years), many of which are entering the inner solar system for the first time, constitute the second most important impact hazard. While their numbers amount to only 5 to 10 percent of the ECA impacts, they approach the Earth with greater speeds and hence higher energy in proportion to their mass. It is estimated that as many as 25 percent of the objects reaching the Earth with energies in excess of 100,000 megatons are long period comets. On average, one such comet passes between the Earth and Moon per century, and one strikes the Earth every few hundred thousand years.

Since long-period comets do not (by definition) pass frequently near the Earth, it is not possible to obtain a census of such objects. Each must be detected on its initial approach to the inner solar system. Fortunately, comets are much brighter than asteroids of the same size, as a consequence of outgassing stimulated by solar heating. Comets in the size range of interest will generally be visible to the Spaceguard Survey telescopes by the time they reach the asteroid belt (500 million km distant), providing several months of warning before they approach the Earth. However, the short time-span available for observation will result in less well-determined orbits, and hence greater uncertainty as to whether a hit is likely; there is a greater potential for "false alarms" with comets than asteroids. Simulations carried out for this report indicate that only 35 percent of Earth-crossing intermediate- and long-period comets (ECCs) greater than 1 km in diameter will be detected with at least three months warning in a survey of 6000 sq degrees per month. By increasing the area of the survey to include the entire dark sky, as many as 77 percent could be detected.. Increasing telescope aperture to reach fainter magnitudes (V = 24) improves the discovery rate still further. Because of the continuing hazard from comets, which may appear at any time, the cometary component of the Spaceguard Survey should be continued even when the census of large Earth-crossing asteroids is essentially complete.

3. Smaller Asteroids, Comets, and Meteoroids (diameters from about 100 m to 1 km; energies from 20 to 100,000 megatons). These impacts are below the energy threshold for global environmental damage, and they therefore constitute a smaller hazard in spite of their more frequent occurrence. Unlike the large objects, they do not pose a danger to civilization. The nature of the damage they cause depends on the size, impact speed, and physical nature of the impacting object; only a fraction of the projectiles in this size range will reach the surface to produce a crater. However, detonation either at the surface or in the lower atmosphere is capable of severe local damage, generally on a greater scale than might be associated with a large nuclear weapon. Both the Tunguska (1908) and Meteor Crater impacts are small examples of this class. The average interval between such impacts for the whole Earth is a few centuries; between impacts in the inhabited parts of the planet is a few millennia; and between impacts in densely populated or urban areas is of the order of 100,000 years. About 300,000 Earth-crossing objects probably exist in this size range, with several passing between Earth and Moon each year.

The Spaceguard Survey will discover as many hundreds of objects in this size range every month. By the end of the initial 25-year survey, it will be possible to track the orbits of as many as 100,000, or about 10 percent of the total population. If the survey continues for a century, the total will rise to about 40 percent. Since the interval between such impacts is greater than 100 years, it is moderately likely that the survey will detect the "next Tunguska" event with ample warning for corrective action. However, in contrast to the ECAs and even the long-period comets, this survey will not achieve a near-complete survey of Earth-crossing objects in the 100-m size range in less than a several centuries with current technology. If there is a societal interest in protecting against impacts of this size, presumably advanced technologies will be developed to deal with them.

Survey Network: Cost and Schedule

The proposed Space guard Survey network consists of six telescopes of 2- to 3-meter aperture together with a central clearinghouse for coordination of the observing programs and computation of orbits. It also requires access to observing time on existing planetary radars and optical telescopes for follow-up. For purposes of this discussion, we assume that the Space guard Survey will be international in operations and funding, with the United States taking a leadership role through the Solar System Exploration Division of NASA's Office of Space Science and Applications.

The Space guard Survey Telescopes

The six survey telescopes required for the Space guard Survey are new instruments optimised for the discovery of faint asteroids and comets. While it is possible that one or more existing telescopes could be retrofit for this purpose, we expect that the most cost-effective approach is to design and construct telescopes specifically for this project. For purposes of this Report, we consider a nominal telescope design of 2.5 m aperture and 5.2 m focal length with a refractive prime-focus corrector providing a field-of-view of at least 2 degrees. The telescope will have altitude-azimuth mounting and be capable of pointing to an accuracy of a few arcsec and tracking to a precision of a fraction of an arcsec at rates up to 20 times sidereal. We assume that each telescope will be located at an existing observatory site of proven quality, so that no site surveys or new infrastructure development (roads, power, etc.) is required. The nominal aperture of 2.5 m is optimized for the ECA survey, but we note that larger telescope aperture (3 m or even more) would permit long-period comets to be detected at greater distances and thereby provide both greater completeness and months of additional warning.

An instrument of very similar design has recently been proposed by Princeton University for a wide-angle supernova survey. We believe that the SPACE GUARD Survey Telescopes could similarly be built for about $6 million each, including observatory building, but not including the focal plane of several mosaiked CCD detectors or the supporting data processing and computation capability. For each telescope, we allocate $1 million for the focal plane and $1 million for computer hardware and software, for a total cost per installation of $8 million. If these six telescopes were purchased together, the capital costs would thus be about $48 million.

For an estimate of operating costs, we assume that each telescope will require the following staffing: 2 astronomers, 2 administrative support personnel, 3 telescope operators, 1 each senior electronic and software engineers, and 2 maintenance and support technicians, for a total of 11 persons. Additional funds will be needed for transportation, power, sleeping accommodations for observers, and other routine costs associated with the operation of an observatory; the exact nature of these expenses depends on the location and management of the pre-existing site where the telescope is located. The total operations for each site should therefore run between $1.5 million and $2.0 million per year. In making this estimate we assume that each survey telescope is dedicated to the Space guard effort, and that it will be in use for about three weeks (100-150 hours) of actual observing per month. If it is intended that the telescope be used for other unrelated purposes when the Moon is bright, we assume that the other users will pay their prorata share of operation costs.

The Space guard Survey Operations Center should provide overall coordination of the international observing effort, including rapid communications among the survey telescopes and those involved in follow-up observations. The Spac eguard Survey Operations Center will also compute orbit ephemerides and provide an ongoing evaluation of the hazard posed by any object discovered by the Survey. Similar functions are performed today for the much smaller number of known asteroids by the Minor Planet Center in Cambridge, Massachusetts. Scaling from that operation, we estimate an initial cost of $2 million for computers and related equipment, and an annual operating cost of $2 million.

A third component of the Space guard Survey Program is follow-up, including radar and optical observations. As noted previously in this Report, it would be desirable to have one or more dedicated planetary radars and large-aperture optical telescopes (4-m class). However, we anticipate that a great deal of useful work could be done initially using existing planetary radars and optical facilities. Therefore, for purposes of this Report, we simply allocate a sum of $2 million per year for the support of radar and optical observing on these instruments.

Space guard Management and Cost-Sharing

The total estimated capital costs for the Space guard Survey are $50 million, with operating costs of $10-$15 million per year. We anticipate that these costs would be shared among several nations with advanced technical capability, with the maximum expenditure for the U.S. (or any other nation) of less than half the total amount. For purposes of U.S. budgeting, we assume that NASA will pay the cost of two telescopes ($16 million) and the Operations Center ($2 million), and will support operating costs of $5 million per year.

Management of the U.S. component of the Space guard Survey could be accomplished by NASA in one of two ways. (1) The telescopes could be constructed and operated by universities or other organizations with funding from NASA Headquarters through grants or contracts, as is done today with the NASA IRTF telescope on Mauna Kea (owned by NASA but managed by the University of Hawaii under a five-year contract) or the 0.9-m Space watch Telescope on Kitt Peak (owned and operated by the University of Arizona with partial grant support from NASA). (2) NASA could construct and operate the telescopes itself through one of its Center's (JPL or Ames, for example); the Centers might contract with universities or industry for operations but would retain a more direct management control. Similarly, the Space guard Survey Operations Center could be located at a NASA Center or could be supported by grants or contracts at a university or similar location, such as the present Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics. In any case, international cooperation and coordination is essential, and an international focus is required from the beginning in planning and supporting this program.

Initial Steps

The construction of the new Space guard Survey telescopes will require approximately four years from the time funding is available. In the meantime, several steps are essential to ensure a smooth transition from the present small surveys to the new program. (1) An international coordination effort should be initiated by NASA, independent of but coordinated with the International Astronomical Union Working Group on Near Earth Objects, in order to plan for the orderly development of the Space guard Survey network. (2) The small cadre of current asteroid observers should be strengthened. Additional expenditures of about $1 million per year on existing teams would allow for expansion of personnel, purchase of badly needed new equipment, and greater sky coverage. Consequently, the discovery rate of E C A's should double to quadruple, thereby also increasing our confidence in modelling the population of such objects and planning the requirements for operation of the full-up survey. (3) In order to gain additional experience with the kind of automated C C D scanning techniques proposed for the Space guard Survey, efforts should be made as soon as possible to place in operation a telescope that utilizes these techniques; one such option is the proposed 1.8-m Space watch telescope at the University of Arizona. Efforts are also required in studying the use of C C D arrays and in developing appropriate software to support C C D scanning. (4) Continuing support should be provided for research on near-Earth asteroids and comets, including their dynamics and their physical properties. For purposes of this study, we assume an increase of $2 million/year beyond current NASA expenditures for these programs, to be maintained during the transition period.

Conclusion

The Space guard Survey has been optimized for the discovery and tracking the larger ECAs, which constitute the greater part of the cosmic impact hazard. If any large ECAs threaten impact with the Earth, they could be discovered with ample lead-time to take corrective action. The Space guard system also will discover most incoming long-period comets, but the warning time may be only a few months. Finally, the great majority of the new objects discovered by the Space guard Survey will have diameters of less than 1 km; these should be picked up at a rate of about a thousand per month. It is therefore reasonably likely that even the "next Tunguska" projectile (20 megatons energy) will be found by the Space guard Survey if it is continued for several centuries

The Space guard Survey should be supported and operated on an international basis, with contributions from many nations. The total costs for this system are of the order of $50 million in capital equipment, primarily for the six survey telescopes, and $10-15 million per year in continuing operating support. However, these estimates will vary depending on the aperture and detailed design of each telescope, the nature of the international distribution of effort, and the management of the survey. In particular, larger telescopes would be appropriate if greater emphasis is to be given to the search for long period comets. Whatever the exact cost, however, the proposed system can provide, within one decade of its initial operation, a reduction in the risk of an unexpected large impact of about 50 percent at a relatively modest cost. Of course, additional and much greater expenditure would be required to deflect an incoming object if one should be discovered on an impact trajectory with the Earth, but in that unlikely event the cost and effort would surely be worth it. The first and essential step is that addressed by the Space guard Survey: to carry out a comprehensive survey of near-Earth space in order to assess the population of near-Earth asteroids and comets and to identify any potentially hazardous objects.

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