Callisto (moon)
Callisto is almost the same size as the planet Mercury, being 99% the diameter of the planet, but is much less massive. Of the four Galilean moons of Jupiter, Callisto is the farthest out, with an orbital radius of about 1,880,000 kilometres. It does not form part of the orbital resonance that affects three inner Galilean satellites, Io, Europa and Ganymede. Like the Earth's Moon, Callisto rotates synchronously with its orbital period. This means that one side is always turned toward Jupiter, while the other side never sees the face of the planet. Callisto's surface is less affected by Jupiter's magnetosphere than the seven moons nearer the planet.