Jupiter and its Galilean moons. Montage of images of Jupiter and its four largest satellites. These are known as the Galilean moons as they were discovered by the astronomer Galileo in 1610. The moons, in order of increasing orbital distance from Jupiter, are; Io (second right), Europa (centre), Ganymede (right) and Callisto (foreground). The Galileans are by far the largest moons in the Jupiter system, ranging from 3000km to 5000km diameter. The other 12 moons of Jupiter range between 12 and 180km across. The images of Jupiter and the Galilean moons used in this montage were gathered by Voyager spacecraft in 1979.