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Astronomers have stumbled on an unusual asteroid hunting ground: the thousands of Hubble Space Telescope images stored in its archive.

The hunt, by Robin Evans and Karl Stapelfeldt of NASA's Jet Propulsion Laboratory in Pasadena, CA, has yielded a sizable catch of small asteroids - about 100. Their preliminary analysis suggests that a total population of 300,000 small asteroids - essentially rocks just over 1 kilometer to 3 kilometers wide (half a mile to two miles) - are orbiting between Mars and Jupiter in a band of space debris known as the main belt. Currently, there are 8,319 confirmed main belt asteroids whose orbits have been measured, and about the same number have been sighted but not confirmed.

The asteroid hunters also were intrigued that they didn't find evidence of small comets passing near Earth, a finding announced last year.

Most astronomers stalk the Hubble archive for bigger game, such as quasars, distant galaxies, and supernovae, but Evans and Stapelfeldt have discovered that the pursuit of smaller prey such as asteroids can be equally successful.

Over a three-year period, the two astronomers and their collaborators have searched through more than 28,000 Wide Field and Planetary Camera 2 (WFPC2) images, looking for wide, looping streaks of light, the telescope's tell-tale signatures of asteroids. Most of the ones they found are too faint to be observed by current ground-based search programs. Hubble captures their images purely by accident: nearby asteroids inevitably wander across the telescope's field of view while other, higher priority targets are being observed.

"The archive images are distributed fairly evenly across the sky, so we find asteroids according to both their position in the sky and their number," Evans said. "As expected, we see the asteroids concentrated towards the ecliptic plane and we see small asteroids because they are the most numerous. Small main- belt asteroids such as these are the ones most likely to evolve into Earth-crossing asteroids due to encounters with their larger neighbors. Some of the asteroids in our survey could eventually migrate toward Earth."

An accurate asteroid census is an important part of assessing how many of these small bodies there are which could potentially pose a hazard to Earth. The Hubble archives represent a newly-tapped information resource which could help scientists more precisely estimate the risks they pose to Earth.

The Hubble archival data also strongly limit the number of small comets that could be passing very near Earth, according to Evans and Stapelfeldt. Last year, Dr. Louis A. Frank of the University of Iowa in Iowa City, using data from NASA's Polar spacecraft, reported he found evidence that about a dozen small comets strike Earth's upper atmosphere each minute. Evans and Stapelfeldt estimate the such small comets should be bright enough to produce thousands of detectable trails in the Hubble archival images, but these were not seen.

The Hubble images capture an asteroid as a long trail produced by its motion across the camera's field of view. The trails appear like the streaks of light found on photos taken at night of speeding cars with their headlights on. In Hubble's case, asteroid trails show a unique curvature due to the continuously shifting position of the telescope as it orbits the Earth. This effect, known as parallax, allowed Evans and Stapelfeldt to determine distances and sizes for the asteroids spied by Hubble. A similar parallax effect is the key to depth perception in human vision: our eyes are set apart so that we can see three dimensionally.

"Asteroid trails observed by the Hubble telescope are usually curved because the telescope travels in a curved low- Earth orbit," Stapelfeldt says. "By precisely measuring the shape of the trails, we can solve for the distance to each asteroid at the time it was observed. It isn't possible to do this using a stationary telescope on the ground."

Finding asteroids isn't what the two astronomers originally had in mind. As members of the WFPC2 science team, Evans and Stapelfeldt were examining test images of distant stars and galaxies to ensure that the new camera was functioning properly. These were among the first images taken with WFPC2, which had restored sharp focus to Hubble's images when it was installed in late 1993. Stapelfeldt's wife, Deborah Padgett (also an astronomer), pinpointed the first asteroid in 1994 while looking at images on the couple's home computer. Intrigued, Evans and Stapelfeldt began combing through more than 1,600 of the science team's survey photos, finding 12 more asteroids. This discovery prompted their large-scale search, by eye, of two years worth of Hubble archival images.

Evans' and Stapelfeldt's initial results are reported in the February 1998 issue of the research journal Icarus.