A more modern approach uses two photographs taken less than an hour apart and examined through a stereomicroscope that allows the asteroid to appear suspended above the background of stars. More than 200 asteroids have been identified that regularly intersect the orbit of the earth, and over geologic time asteroids in similar orbits have struck the earth. Many of these small asteroids have orbits that intersect the earth's. Asteroids have been implicated in several mass extinctions of large numbers of animal and plant species in the past. In 1992, scientists reported that the appearance of patterns of shattered quartz crystals imbedded in Triassic shale and other fossil evidence suggest that another major mass extinction, about 200 million years ago, was caused by three closely spaced asteroid impacts.
Some asteroids are actually nuclei of comets that are no longer active.
The pictures revealed that Ida has a natural satellite, Dactyl. See T. Gehrels and M. S. Matthews, ed. Licensed from Columbia University Press. All rights reserved.