A moon is defined as a natural satellite that orbits a larger body--such as a planet--that, in turn, orbits a star. The moon is kept in its position both by the gravity of the object that it circles, as well as by its own gravity. Some planets are orbited by moons; some are not. Some dwarf planets--such as Pluto--possess moons. In fact, one of Pluto's moons, named Charon, is almost half the size of Pluto itself, and some planetary scientists think that Charon is really a chunk of Pluto that was torn off in a disastrous collision with another object very long ago. In addition, some asteroids are also known to be orbited by very small moons.
Discovering the little moon also reinforces the theory that most dwarf planets have moons.
The twin spacecraft flew in an almost-circular orbit until the mission ended on December 17, 2012, when the probes were intentionally sent down to the lunar surface. NASA ultimately named the impact site in honor of the late astronaut Sally K. Ride, who was America's first woman in space and a member of the GRAIL mission team.