• 2410km radius satellite of Jupiter discovered by Galileo in 1610. A favourite target for observation by amateur astronomers, it has been viewed spectacularly by space probes in its 1.9 million km radius orbit. Callisto has a low average density of 1.8 gm/cc and is thought to consist of ice (and possibly water below the surface) surrounding a rocky core. The surface of Callisto, the largest of the Galilean satellites, is marked with bright impact craters, of which Valhalla is the most conspicuous.