Io (Jupiter I) is the first of Jupiter's first four discovered Moons, of them and the fourth outward from the planet according to current knowledge. Its radius is 1821km (a bit over a quarter or 2/7 of Earth's), its mass is 8.9x1022kg (about 1/66 of Earth's), with a mean density about three and a half times that of water (a bit over half or 7/11 Earth's). Io's orbital period is 1.75 days and it is tidally locked. Its orbit is in sync with Ganymede (1/4) and Europa (1/2). Of interest are the "real" volcanos on the surface, i.e., with temperatures above 1000k rather than Cryovolcanos like those found on other solar system moons. Flyby observations of Io have been made by the Pioneer, Voyager, and Galileo missions.