The GRAIL mission was managed by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, for NASA's Science Mission Directorate in Washington. The mission was part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. A moon is defined as a natural satellite in orbit around another body that, in turn, is in orbit around its Star. The moon is kept in its position by both its own gravity, as well as its host's gravitational grip. Some planets have many moons, some have only a small number, and still others have none at all. Several asteroids inhabiting our Solar System are circled by very small moons, and some dwarf planets--such as Pluto--also host moons. Ganymede is larger than Mercury, which is the innermost--and smallest--major planet in our Solar System. The surface area of Ganymede is more than half that of the land area of Earth, and it provides scientists with a wealth of data concerning a great variety of surface features.