As many as 900 large asteroids (a kilometer or greater) in the inner solar system may pose a possible threat to the Earth, according to a new study published in the June edition of the journal Science. "Some time in the future, one of these objects could conceivably run into the Earth, "warns Cornell astronomy researcher and UA alumnus William Bottke. "One kilometer (more than half a mile) in size is thought to be a magic number," says Bottke, "because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth."

Bottke, and astronomers from the UA's Spacewatch Project and Observatoire de la Côte d'Azur in Nice, France, say they now understand the mechanisms by which most asteroids, located in a belt between Mars and Jupiter, become NEAs (near-Earth asteroids) and a possible threat to Earth.

By uncovering the spatial and size distribution of large NEAs, the astronomers believe the results of their observational and computer-based study will better quantify the likelihood of future catastrophic collisions with Earth. The survey also is expected to help observational astronomers in improving their search for hard-to-find asteroids that might pose a threat to the planet.

Using a decade of asteroid search data that has painstakingly identified about 100 larger NEAs, UA Spacewatch astronomers were able to calculate the much greater number of unobserved NEAs, but not their approximate location. To obtain the orbits of the undetected NEAs, they combined their NEA population estimates with theoretical models, produced by the Cornell and Nice researchers, that show how asteroids in the main belt are transported to the near-Earth environment.

Calculating which, if any, of the 900 asteroids identified in the study could hit the Earth is tricky, says Bottke. "The problem is that fewer than half of these Earth-threatening asteroids have been discovered so far. Of those we have found, we can accurately predict whether they will strike the Earth over the next hundred years or so, but we can't project out several thousands of years. So it's possible some of these asteroids eventually will move onto an Earth-collision trajectory. It's a dangerous place out there."

Making it more dangerous, according to astronomers, is that searching for the undetected asteroids is difficult work, hampered by the limited sensitivity of astronomical instruments to the apparent slow movement and the faintness of the objects. Complicating the picture still further is that possible new threats to the Earth are being created continually by the collisions of large asteroids with each other. Most of these new asteroids behave themselves and remain in the main belt between Mars and Jupiter, orbiting the sun just like their parent asteroids. But sometimes, these newly formed asteroids can be launched into unstable regions of the asteroid belt, where their orbits can be altered by planetary orbits and moved into a possible future collision course with Earth.

Other authors of the study were Alessandro Morbidelli, Jean-Marc Petit and Brett Gladman of the Observatoire de la Côte d'Azur.

Funds for the study were provided to Spacewatch by NASA, AFOSR, the David and Lucile Packard Foundation and the Steven and Michele Kirsch Foundation. Funds to the other institutions were provided by NASA and the European Space Agency.

UA News Services has produced broadcast quality Beta SP videotape showing the new Spacewatch telescope on Kitt Peak in Arizona, including shots of a computer displayed observational run. The video is free to broadcast news organizations. Production companies and other documentary users can obtain the video for dubbing and tape fees.
Contact: Vern Lamplot, UA News Services, vlamplot@u.arizona.edu , (520) 621-1879.