Europa is one of Jupiter's Galilean moons, discovered by Galileo Galilei using a primitive telescope in 1610. These four moons, Io, Europa, Ganymede, and Callisto, are by far Jupiter's largest natural satellites. Europa is the smallest of the Galilean moons. At 3000 km in diameter, it is a bit smaller than the Earth's Moon. Europa is named for a mistress of the god Zeus in Greek mythology. Jupiter, the planet Europa orbits, was named after the Roman name for Zeus.
When Europa was discovered, it helped verify that planetary bodies aside from the Sun exert their own gravity. This helped replace the geocentric view of cosmology with the heliocentric Copernican version. Europa orbits Jupiter every three days or so and orbits at a distance from Jupiter a little less than twice the distance from the Earth to the Moon. Like the other Galilean satellites and our Moon, Europa is tidally locked to Jupiter, meaning the same side always faces it while the other side faces away.
Europa's appearance is that of a white sphere with brown cracks and splotches all over it. Primarily composed of silicate rock, Europa has a thin layer of ice, which some planetary scientists believe might hide a liquid water ocean. Europa has one of the smoothest surfaces in the solar system. Because of the possibility of water oceans, Europa has been a proposed target of a lander probe for several decades, but funds have not yet materialized for a mission. Other moons which may have subsurface oceans include Ganymede and Callisto.
The Jupiter Icy Moons Orbiter (JIMO) project, which would have used ice-penetrating radar to search for such oceans, was canceled in 2005 due to a change of priorities at NASA in favor of manned missions. The spacecraft was to feature innovative features such as a nuclear fission engine and power source, which would have given it a thousand times the electrical power of conventional space probes powered by solar panels and radioelectric thermal generators.
