Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It has a diameter of 5,268 km (3,273 mi), 8% larger than that of the planet Mercury. Its diameter is 2% larger than that of Titan, the second largest moon.
Ganymede’s discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite’s name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus’s lover. Beginning with Pioneer 10, spacecrafts have been able to examine Ganymede closely. The Voyager probes refined measurements of its size, while the Galileo craft discovered its underground ocean and magnetic field. A new mission to Jupiter’s icy moons, the Europa Jupiter System Mission (EJSM), is proposed for a launch in 2020. —