Viking Orbiter 1 photograph of Deimos, the smaller of the two moons of Mars. Deimos orbits Mars every 1.26 days at a distance of some 23,500 km. It is ellipsoid in shape & measures approximately 11x12x15 km. The larger craters visible here are up to 1.3 km in diameter. Like the larger Martian moon, Phobos, Deimos reflects only a few percent of the sunlight that strikes it. Its spectrum suggests a composition similar to that of bodies in the outer asteroid belt, & one theory is that both Phobos & Deimos were originally asteroid-like bodies near Jupiter, which perturbed their orbits & allowed their capture by Mars. The photograph was taken in 1977.