Phobos and Deimos are two moons that orbit Mars. Phobos is the closest of the two, with a diameter of approximately 12 miles (19 kilometers). Deimos is the more distant of the two, with a diameter of approximately 9 miles (15 kilometers). Despite being named after the Greek gods of fear and terror, the moons of Phobos and Deimos are small, harmless and slow moving. In fact, they were likely formed after a large asteroid hit Mars, knocking debris into orbit and eventually into these moons.