A number of media events (a TV miniseries, several TV documentaries, a feature article in New Yorker, and many newspaper stories) in early 1997 have focused public interest on the hazard of asteroid and comet impacts. This short primer on the impact hazard provides answers to many of the questions raised by these films and stories.

The Earth orbits the Sun in a sort of cosmic shooting gallery, subject to impacts from comets and asteroids. It is only fairly recently that we have come to appreciate that these impacts by asteroids and comets (often called Near Earth Objects, or NEOs) pose a significant hazard to life and property. Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small, the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.

Studies have shown that the risk from cosmic impacts increases with the size of the projectile. The greatest risk is associated with objects large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Such an event could depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively different from other more common hazards that we face (excepting nuclear war), because of their potential effect on the entire planet and its population. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a groundburst explosion with energy in the vicinity of a million megatons of TNT. The corresponding threshold diameter for Earth-crossing asteroids or comets is between 1 and 2 km (approximately 1 mile). Smaller objects (down to tens of meters diameter, or the size of a large house) can cause severe local damage but pose no global threat. One historic example of such a "small" impact was the explosion of an asteroid which took place in the Tunguska wilderness of Siberia in June 1908.

At the request of the U.S. Congress, NASA has sponsored two studies of ways to discover Earth-crossing asteroids or comets before they pose any direct threat to the planet. The first study, chaired by David Morrison of Ames Research Center and completed in 1992, developed the idea of an international Spaceguard Survey of specially-designed ground-based telescopes to detect and catalogue all asteroids larger than 1 km in diameter within the next 25 years. The follow-up study, chaired by Eugene Shoemaker of Lowell Observatory and completed in 1995, concluded that advances in astronomical imaging systems could allow the Spaceguard Survey to be completed in just 10 years at a total cost of less than $50 million. Of approximately 2000 Earth-crossing asteroids inferred to exist, fewer than 200 have so far been discovered.

The proposed Spaceguard Survey would assess the population and identify any large objects that could impact the Earth through a systematic search that effectively monitors a large volume of space around our planet and detects these objects as their orbits repeatedly carry them through this volume of space. In addition, the survey would deal with the long-period comets, which are thought to constitute about 10 percent of the flux of Earth impacts. Long-period comets do not regularly enter near-Earth space; however, nearly all Earth-impacting long-period comets could be detected with advance warning on the order of a year before impact with the same telescopes used for the asteroid survey.

Although the Spaceguard Survey has not been funded in the United States or elsewhere, NASA currently supports asteroid and comet search programs at a level of about $1 million per year. These funds support the development of new detectors and computer software for automated sky searches. Currently this support goes primarily to three groups of astronomers. Tom Gehrels of the University of Arizona carries out the Spacewatch survey from Kitt Peak, Arizona. Eugene Shoemaker and Ted Bowell are completing a new search camera for the Lowell Observatory Near Earth Object Survey (LONEOS) at Flagstaff, Arizona. And Eleanor Helin and her colleagues from the NASA/Caltech Jet Propulsion Lab carry out a Near Earth Asteroid Tracking (NEAT) program in collaboration with the U.S. Air Force and Livermore National Laboratory, with instruments on Haleakala in Maui, Hawaii. All of these programs use modern electronic detectors and automated detection algorithms to identify moving NEOs against the background star fields.

At present no asteroid or comet is known to be on a collision course with the Earth. The chances of a collision within the next century with an object 1 km or more in diameter are very small (roughly 1 in ten thousand), but such a collision is possible and could happen at any time. If we did have sufficient warning, however, the incoming object could be deflected or destroyed.

Cosmic impacts are the only known natural disaster that could be avoided entirely by the appropriate application of space technology. Possible defenses against a threatening asteroid or comet have been discussed at six international meetings of astronomers and defense experts held over the past few years at the UN in New York, at Los Alamos and Livermore National Laboratories in New Mexico and California, at Erice Italy, and at Snezhinsk (Chelyabinsk-70), Russia.

In order to plan an effective defense, it is necessary to have long warning (as would be provided by the Spaceguard Survey) and to learn more about the properties of Earth-approaching objects. To learn more about the asteroids, the NASA Galileo spacecraft flew past the main-belt asteroids Gaspra and Ida en route to its 1995 rendezvous with Jupiter, and in February 1996 we launched a dedicated asteroid mission called NEAR (Near Earth Asteroid Rendezvous) to go into orbit around the asteroid Eros for an extensive year-long investigation. Imaging of near-Earth asteroids is also possible using powerful planetary radars supported by NASA at Goldstone, California, and Arecibo, Puerto Rico. Close up studies of asteroids have also been among the objectives of the Department of Defense Clementine 1 and Clementine 2 deep-space missions.

The Jan 27 New Yorker article by Tim Ferris gives an excellent picture of the current state of knowledge and concern about impacts. Also excellent are the Discovery Channel 2-hour documentory first shown on Feb 9, and the National Geographic 1-hour special first shown on Feb 26.

In contrast, the NBC miniseries "Asteroid" first shown on Feb 16-17 plays pretty loose with the facts. It is certainly true that the impact of asteroids or comets with the Earth could cause the sort of destruction depicted in the film; indeed the dangers are understated in the film. For example, there is a scene where the President reacts with disbelief that "millions of people" could die in an impact. The fact is that impact by an asteroid 4 km across, which was being discussed, would probably lead to the death of billions, perhaps as many as half the Earth's population. But it is virtually impossible that a comet could attract or dislodge several asteroids from the asteroid belt and send them on a collision course toward Earth, and this aspect of the situation depicted in the film is absurd. If an asteroid is going the hit the Earth, it is already on the way; that is why we can predict such events centuries in advance if we find the objects and track their orbits. Also, the idea of using air-borne lasers to try to defend against an incoming asteroid has no basis in fact. The only technology we have today for defense against asteroids and comets is nuclear, and we would require years of warning in order to deflect or disrupt a threatening object. Dramas that try to compress these events into a few days are fiction and should not be confused with fact. The truth is that if we found an asteroid headed our way with less than several years' warning, there is nothing we could do to protect ourselves except evacuate population from the impact site.