Phobos is one of two very small satellites of the planet Mars. It was discovered along with its smaller companion, Deimos, by Asaph Hall at the Naval Observatory in Washington DC in August 1877. The moons are named after the children of the Greek god Ares – who, like the Roman Mars, is the god of war. Deimos means ‘terror’ or ‘panic’ and Phobos means ‘fear’.
Phobos has an equatorial orbit, which is almost circular. It orbits once every 7 hours 39 minutes just 5989 km above the surface of Mars. Its orbit is decaying by 1.8 cm per year, so it is expected to crash into Mars, or break up to leave a ring of fragments around the planet, within 100 million years. The orbital period of Phobos is three times faster than the rotation period of Mars, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from Mars. It orbits so close to the surface of Mars that the curvature of the planet would obscure its view from an observer standing in Mars’ polar regions.