Mars has two moons, Phobos and Deimos. Both are believed to be captured asteroids or other debris from early in the formation of our solar system. Phobos is the larger of Mars' two moons. It orbits Mars three times a day, and is so close to the planet's surface that in some locations on Mars it cannot always be seen. Phobos is nearing Mars at a rate of six feet (1.8 meters) every hundred years. At that rate, it will either crash into Mars in 50 million years or break up into a ring. Deimos is the smaller moon. It orbits Mars every 30 hours.