Phobos

The Martian moon Phobos orbits only a few thousand miles above the Red Planet's surface. Its proximity to its planet is one of the reasons astronomers were unable to see the satellite until the late 19th century. In fact, the moon is getting closer to Mars over the centuries, and eventually will either break up or be pulled into the Martian surface.

Mark Watney uses it to navigate after he leaves the safety of the Hab on his long journey to the ARES IV landing sight.