Callisto, also known as 'Jupiter IV', is a natural satellite of Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System. It is the second largest moon of Jupiter, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1,880,000 km.
Callisto's rotation is tidally locked to its revolution around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand still in Callisto's sky. Callisto is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away.
The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, and is thought to have evolved predominantly under the influence of impacts.