Callisto is the outermost of Jupiter's large Galilean moons. It orbits Jupiter at an average distance of 1,170,000 miles (1,882,700 km), and has an orbital period of 16.69 days. Callisto does not participate in the 1:2:4 orbital resonance of the other three Galilean moons, and is too far from Jupiter to be affected by the tidal forces that heat Io's and Europa's interiors.
Callisto and the third-largest satellite in the solar system. With a diameter of over 2,985 miles (4,800 km), Callisto is almost the size of Mercury. Callisto has the lowest density of the Galilean satellites (1.86 grams per cubic centimeter), and is the most heavily cratered object in the solar system. Its largest surface feature is an enormous multi-ringed impact basin named Valhalla, almost 2200 miles (3600 km) in diameter.
With a nearly-complete absence of any geological activity on its surface, Callisto is thought to be a world long dead.