Our moon may not be made of green cheese—as 16th century English writer John Heywood had speculated—but Mars's largest moon comes close. Phobos is full of holes, according to the latest data collected by the European Space Agency's Mars Express orbiter during a close (67 kilometers) flyby yesterday. Scientists think the moon formed out of a cloud of dust sometime after its parent planet was born. Its gravity is so weak that it can't pull itself completely together. In fact, orbital analysis shows that Phobos is gradually being pulled in by Mars—and millions of years from now that gravity will rip the moon apart.