Callisto is a satellite of Jupiter discovered by Galileo Galilei in January 1610; it is the outermost of the Galilean satellites. With a radius of 2,410 km, Callisto is, after Ganymede and Titan, the third biggest satellite in the solar system. Its distance to Jupiter is 1,882,700 km or 26 Jovian radii. Its density is 1.8 g/cm3, typical of icy objects. Callisto has been investigated by the Voyager 1 and 2 spacecraft in 1979, then by the Galileo orbiter between 1995 and 2003. The surface of Callisto is heavily cratered and consists of a mixture of ice and dust. A great basin, Valhalla, over 500 km in diameter, is the signature of a large major impact.