Dactyl could be a smaller fragment of debris from the collision that ended up in orbit around Ida, but there is a problem - computer models suggest Dactyl would almost certainly be destroyed by an impact from another asteroid. The family is thought to have formed 1 or 2 billion years ago during an asteroid collision. Ida is a major member of the Koronis family of over 300 asteroids, all of which share similar orbits. Thanks to the larger asteroid's weak gravity, Dactyl is unlikely to be an object captured into orbit, but the alternative - that Ida and Dactyl formed alongside each other - raises as many questions as it answers. (Image credit: NASA / JPL / Ted Stryk)Ģ43 Ida, an asteroid designated as a minor planet, has a moon, measuring just 0.99 miles (1.6 km) across on its longest axis. Dactylĭactyl imaged by NASA's Galileo Orbiter. Callisto's surface, however, has remained essentially unchanged for more than 4.5 billion years, developing its dense landscape of overlapping craters across aeons. Jupiter's larger moons are directly in the firing line, and end up soaking up more than their fair share of impacts, but Callisto's inner neighbors - influenced by greater tidal forces - have all experienced geological processes that wiped away most of their ancient craters. Its main claim to fame is the title of most heavily cratered object in the solar system its dark surface is covered in craters down to the limit of visibility, the deepest of which have exposed fresh ice from beneath and scattered bright 'ejecta' debris across the surface.Ĭallisto owes its cratered surface to its location in the Jupiter system - the giant planet's gravity exerts a powerful influence, disrupting the orbits of passing comets and often pulling them to their doom, most spectacularly demonstrated in the 1994 impact of Comet Shoemaker-Levy 9. The outermost of Jupiter's Galilean moons, Callisto is the third-largest moon in the solar system, and is only slightly smaller than Mercury. This image of Callisto was taken from NASA's Galileo spacecraft.
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