Galileo, en route to Jupiter, flew within 2,400 km of the asteroid Ida to discover a natural satellite, Dactyl. Ida is a member of the Koronis family of asteroids, which are believed to have been created when an enormous body, perhaps 200-300 kin in diameter, was smashed in a collision quite some time at ter the solar system was formed about 4,500 million years ago.

"This is the first clear detection of a satellite of an asteroid," says Clark Chapman, member, Galileo imaging team, Most asteroids usually orbit the sun in the main asteroid belt between Mars and Jupiter.

Scientists at Jet Propulsion Laboratory, Pasadena, believe that this discovery will provide them with important clues for deciphering the origins and evolution of rocky asteroids, presumed to be more primitive than the planets themselves.

NASA scientists estimate that the newly discovered moon is oblong, and about 56 kin long and 1.5 kin wide, orbiting about 100 kin away from Ida. In the image captured by Galileo, it looks like a small stone by a huge heavily cratered boulder.

Says Torrence Johnson, Galileo project scientist, "It was previously thought that natural satellites of natural satellites could form, but they were not so common. Having found one so quickly, we can say that they are more common than previously thought."