Phobos. Computer composite image showing the moon Phobos above the planet Mars. Phobos is the innermost and larger of the two Martian satellites, the other is called Deimos. Phobos is ellipsoid in shape, measuring 19x21x27 kilometres. It orbits Mars every 7.65 hours at a mean distance of 9380 kilometres above the Martian surface. Both Phobos and Deimos are thought to be asteroids captured by the gravity of Mars. The images of Phobos and Mars are derived from data gathered by NASA's Viking Orbiter spacecraft.