Calisto is a satellite of Jupiter discovered by Galileo Galilei in 1610. It is the third largest satellite in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto is approximately 99% of the diameter of the planet Mercury, but only a third of its mass. It is the fourth Galilean satellite in terms of distance to Jupiter. It is not influenced by the orbital resonance that affects the three interior Galilean satellites - Io, Europe and Ganymede - so it does not suffer appreciable warming from tidal forces, such as it does occur in the other three. Callisto has a synchronous rotation, that is, its period of rotation agrees with its orbital period, so that, like the Moon with Earth, it always «shows» the same face to Jupiter. Callisto's surface is not as influenced by Jupiter's magnetosphere as that of the other inland satellites since its orbit is further away.
