Opportunity

The second of NASA's twin Mars Exploration Rovers followed Spirit by three weeks, aimed at Meridiani Planum, a plain whose hematite signature hinted at a watery past. Opportunity found rock layers laid down at the edge of an ancient salty sea, among the strongest evidence yet that liquid water once stood on the Martian surface, and outlasted its planned 90 days by more than a decade. The flight was also the first to use the three-stage Delta II Heavy.