Callisto was discovered by the Italian astronomer Galileo Galilei in 1610. It was named by Simon Marius after a nymph in Greek mythology who was associated with Artemis, Goddess of the Hunt. In the Grecian religion, Zeus took the form of Artemis to seduce Callisto because she didn't fancy the fellow. Then he raped her.
Its diameter is approximately 99% that of Mercury's. It orbits Jupiter. The surface is primarily dominated by impact craters which cover it almost to the point of saturation. However, underneath the surface of rocks and ice is a salty subsurface ocean 100km deep*.
Jupiter Moon, the "Jupiter Jazz" episode of Cowboy Bebop and the Sporilla from Terrahawks
Above the surface, a thin atmosphere of carbon dioxide coats the icy world. NASA's Revolutionary Concepts for Human Outer Planet Exploration has named the world as the favorite for a future Jupiter base.