Gaspra, Deimos and Phobos

This picture assembly shows asteroid 951 Gaspra (top) compared with Deimos (lower left) and Phobos (lower right), the moons of Mars. The 3 are presented on the same scale and nearly the same lighting conditions. Gaspra is about 17 kilometers (10 miles) long. The three bodies of are irregular shape due to past catastrophic conditions. However, their surfaces appear very different, perhaps due to differences in composition, but more probably because of the effects of very different histories. Phobos and Deimos images were obtained by the Viking Orbiter spacecraft in 1977; Gaspra image is the best of a series obtained by the Galileo spacecraft on October 29, 1991.