Phobos is one of the two moons of Mars, the other being Deimos. It is the larger and closer of the two, measuring about 22 kilometers (14 miles) across. Phobos orbits Mars at a very low altitude, completing a full orbit in just about 7 hours and 39 minutes, which is faster than Mars rotates on its axis. The surface of Phobos is covered in craters and grooves, indicating a history of impacts and geological activity. It is thought to be a captured asteroid, composed mainly of carbon-rich rock and ice. Due to its irregular shape and small size, Phobos is often described as resembling a potato.