How many moons does Jupiter have? Jupiter has at least 63 known moons. The largest four are called Io, Europa, Ganymede and Callisto. These four moons are called the Galilean satellites because they were first seen in 1610 by the astronomer Galileo Galilei. Ganymede is the largest moon in the solar system, with a diameter of 3,260 miles. Io has a lot of active volcanoes and is covered with sulfer. Callisto may have a water ocean beneath its heavily cratered icy, rocky surface. Europa, which is covered by a cracked, icy surface, may also have a liquid water ocean. The other moons are smaller and have irregular shapes. Most of these small moons are thought to be asteroids which were caught by Jupiter's strong gravity.