The team of scientists used data gathered by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, composed of a duo of twin spacecraft that circled Earth's Moon throughout 2012, each measuring the push and pull of the other as an indicator of lunar gravity. The team discovered that the Methone's density would be about 300 kilograms per cubic centimeter. That amounts to less than a third of the density of water, making Methone less dense than any other known moon or asteroid in our Solar System! A moon is a natural body that is in orbit around a planet, and it is kept in place by both the host planet's gravity and the gravity of the moon itself. Some planets possess orbiting moons; some do not. There are several theories explaining how Earth's Moon came to be. At this point, the favored model is termed the giant impact theory, often playfully called the Big Whack or Big Splash theory by astronomers when they are in a humorous frame of mind. These funny nicknames were derived from the central tenet of the theory, which is that a Mars-sized body, named Theia, smacked into the primordial Earth billions of years ago. The collision caused part of our planet's crust to be hurled violently into space. Some of this shattered, somersaulting debris was snared into Earth-orbit, where it formed a host of moonlets that were ultimately pulled together by gravity to evolve into our Moon.