The entire future of human space exploration rests on a patch of lunar ice. For the past two years NASA has focused on designing a new crew vehicle and launch system that could return astronauts to the moon by 2018. The agency’s ultimate goal is to establish a permanent lunar base and use it for a human mission to Mars. But the grand plan depends on a risky prediction that NASA will find water ice in a permanently shadowed crater basin at one of the moon’s poles. Plentiful ice deposits would be an asset for lunar colonists, who could use the water for life support or convert it to hydrogen and oxygen rocket fuel. And two orbiters sent to the moon in the 1990s, Clementine and Lunar Prospector, found evidence of ice in perpetually shadowed polar areas where consistently frigid temperatures would preserve the water carried to the moon by comet and meteorite impacts. But some scientists have disputed Clementine’s radar data, and the anomalous neutron emissions observed by Lunar Prospector could have been caused by atomic hydrogen in the lunar soil instead of ice.