Europa (Jupiter II) is one of Jupiter's first four discovered Moons, and of them, the smallest and the second outward from the planet. It is the seventh outward according to current knowledge. Its radius is 1564km (a quarter of Earth's), its mass is 4.5x1022kg (about a hundredth of Earth's), with a mean density about three times that of water (about 1/2 Earth's) and subsurface oceans are suspected. Europa's orbital period is 3.5 days and it is tidally locked. Its orbit is in sync with Ganymede (1/2) and Io (2x). Flyby observations of Europa have been made by the Pioneer, Voyager, and Galileo missions.