Mars and Phobos. Computer artwork of how Mars (left) and its tiny moon Phobos (upper centre) might appear from a distance of about 100 miles from the surface of Phobos. Phobos is the larger of Mars' two satellites (the other being Deimos), being about 16 miles long and about 11 miles across at its smallest dimension. It may be an asteroid long ago captured by Mars' gravity and does not have enough mass for gravity to pull it into a uniform sphere like the larger satellites and planets in the Solar System.