Finding rocks in the Hubble archives.

When astronomers scour the thousands of images taken by the Hubble Space Telescope, they're usually seeking distant stars and galaxies. When Robin W. Evans and his colleagues sifted through the archives, they found a bounty much closer to home: 100 members of the main belt of asteroids, which lies between the orbits of Jupiter and Mars.

Most of the asteroids, which range from 1 to 3 kilometers in diameter, are too tiny to have been detected by ground-based telescopes. Small main-belt asteroids are of keen interest because they have a relatively high probability of being kicked into an Earth-crossing orbit.

According to Evans, an astronomer at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., the Hubble images indicate that few small comets pass close to Earth. If they did, he and his collaborators would have found many more trails. The finding is at odds with the claim that thousands of small comets pelt Earth's upper atmosphere each day (SN: 12/20 & 27/97, p. 389).

Hubble's second-generation wide-field and planetary camera, installed in late 1993, recorded the asteroids by accident as they streaked across its field of view. Coauthors Karl R. Stapelfeldt of JPL and Deborah L. Padgett of the California Institute of Technology in Pasadena were examining test images of stars and galaxies on their home computer when Padgett spied a telltale streak. They, Robins, and the rest of the team have since viewed more than 28,000 Hubble pictures and picked out the asteroids.

Hubble's motion as it orbits Earth causes the asteroid streaks to appear curved, and the degree of curvature reveals the rock's distance from Earth. This study suggests that the asteroid belt contains about 300,000 small asteroids, the researchers report in the February Icarus.