Phobos. At left is the first ever image of Phobos, taken by NASA's Mariner 9 space orbiter in 1971. At right is the first image of the moon captured by the Viking 1 spacecraft on 25th July 1976. Phobos is the larger and innermost of the two Martian moons. An irregular shape, it measures 19 by 21 by 27 kilometres, and orbits some 6000 kilometres above the Martian surface, taking 7.6 hours to complete an orbit. Stickney Crater (top) is 10 kilometres across.