The only natural satellite of the Earth. Its existence has affected life on Earth with its bright light and tidal effects. The closest relative mass for any satellite of a planet. One side faces the Earth, while the other always faces away. Has extensive volcanic past, negligible atmosphere. Presents an old cratered surface. Thought to have been formed when a planet-sized object struck the Earth. The only other body in the solar system that mankind has landed on. Currently orbited by the GRAIL satellites.