Asaph Hall III (October 15, 1829 – November 22, 1907) was an American astronomer who is most famous for having discovered the moons of Mars, Deimos and Phobos, in 1877. He determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.
Deimos is the smaller and outermore of the two natural satellites of the planet Mars with a mean radius of 6.2 km, the other being Phobos. Deimos takes 30.3 hours to orbit Mars. Its systematic designation is Mars II. In Greek mythology, Deimos was the twin brother of Phobos and personified Terror.
Deimos was discovered at the United States Naval Observatory in Washington, D.C on August 12, using an astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time). Hall also discovered Phobos on August 18, 1877, at about 09:14 GMT, after deliberately searching for Martian moons.