Phobos is the larger of Mars’s two natural satellites and was discovered in 1877.
American astronomer Asaph Hall discovered Phobos on August 18, 1877, at the United States Naval Observatory in Washington, D.C. He found Deimos, Mars’s smaller moon, six days later. Hall named both moons after figures associated with the Greek god Ares, the counterpart of the Roman god Mars.
Phobos has an irregular, heavily cratered shape and measures about 27 by 22 by 18 kilometres. It orbits Mars at an average distance of only about 9,377 kilometres from the planet’s centre, completing an orbit in approximately 7 hours 39 minutes. Because it moves faster than Mars rotates, it rises in the west and sets in the east for observers on the surface.
Phobos is gradually spiralling inward because of tidal interactions with Mars. In roughly tens of millions of years, it may break apart or collide with Mars. Its origin remains debated; possibilities include a captured asteroid or material formed in a large impact.