Mars has two moons: Phobos and Deimos. Both moons are irregularly shaped and very small in size compared to the Earth’s moon. Phobos is the larger of the two, with an average radius of 6.9 miles; Deimos has an average radius of only 3.8 miles.
Both Phobos and Deimos were discovered by the astronomer Asaph Hall using a refracting telescope at the U.S. Naval Observatory in 1877. Since the discovery, astronomers have observed that Phobos is slowly drawing nearer to Mars. Astronomers now expected that Phobos will either crash into Mars in about 40 or 50 million years or break apart under gravitational stress to create a thin debris ring around the planet.