When the NASA rovers Spirit and Opportunity touched down on Mars in 2004, they weren't expected to last long—perhaps 90 days, or six months at most. But 90 days stretched into five years, and a short-term science mission searching for evidence of water has turned into one of the greatest adventures of the Space Age. The rovers have trekked miles across hostile plains, climbed mountains, ventured in and out of deep craters, survived dust storms and mechanical failures, and cheated death so often that no one will venture a guess as to how much longer they might last.