Discovery of Callisto

Callisto is one of the four large moons of Jupiter discovered by Galileo Galilei in January 1610. It is the second largest of them, and the third largest moon in the solar system after Ganymede and Titan. Its surface is the most heavily cratered known in the solar system, a sign of great age, since it shows little geological activity. Callisto orbits farther out than the other Galilean moons and so lies outside the worst of Jupiter's radiation. The Galileo spacecraft found hints of a salty ocean far beneath the ice, though this remains uncertain. Its relatively mild radiation conditions have led scientists to consider it as a site for future human exploration of the Jupiter system.

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