Phobos is the largest of the two moons of Mars, the other being Deimos. It is a small, irregularly shaped object with a mean radius of 11 km and is named after the Greek god of fear. Phobos orbits Mars at a distance of 9, 377 km, making it the closest MOON to any planet in the Solar System. It is also the fastest-orbiting natural satellite in the Solar System, taking only 7 hours and 39 minutes to complete an orbit.