What is s.callisto?

As an AI language model, I don't have access to current information. However, Callisto is one of the moons of Jupiter. It is the second largest of the four Galilean moons and the third largest moon in the solar system. It is named after Callisto, a nymph and lover of Zeus in Greek mythology. Callisto is mostly composed of water ice and rock. It is believed to have a subsurface ocean, making it a potential candidate for extraterrestrial life. Several spacecraft, including the Galileo orbiter, have visited and studied Callisto.