• Larger of the satellites of Mars, discovered by Hall in 1877 and having an ellipsoidal shape of some 20km average size. Phobos is an average 9380km from Mars, taking about eight hours per orbit, and has been revealed by space probes as a lumpy, cratered body. Its spectrum resembles those of some asteroids, suggesting that it may be a captured asteroid, like Mars’s smaller satellite Deimos.
  • Pair of unmanned space missions sent to Mars by the former Soviet Union. Phobos 1 was an almost complete failure, shutting down, perhaps because of a faulty command from Earth, as soon as it reached Mars in 1988, while Phobos 2 lasted 57 days in 1989 and provided infrared, gamma ray and magnetic data as well as photographs of Mars and its namesake satellite.