Ganymede (Jupiter III) is Jupiter's largest Moon and the third outward from Jupiter of the original four moons found centuries ago, but the seventh according to current knowledge. Its radius is 2634km (.4 Earth), its mass is 1.5x1023kg (0.025 Earth), with a mean density about twice that of water (about 1/3 Earth's) and subsurface oceans are suspected. Ganymede's orbital period is 7 days and it is tidally locked. Its orbit is in sync with Europa (2x) and Io (4x). Flyby observations of Ganymede have been made by the Pioneer, Voyager, and Galileo missions, the latter within 264km of its surface.