Astronauts bound across Phobos, the largest moon of Mars, in this 1980s artist's concept. Mars looms in the background. The oddly shaped moon is about the size of a small city, with a minuscule gravitational pull. Astronauts would have to be tied to the little moon to keep from floating off into space. A recent study suggested they might also face mild electric charges on Phobos, created by interactions with the solar wind. The charges probably wouldn't be strong enough to harm an astronaut, but could damage delicate equipment. [NASA/Pat Rawlings (Eagle Engineering)]