Friday's mission marks the first search for water on the Red Planet, but it's not the first time a spacecraft has probed the question of life on Mars. The Viking 1 and Viking 2 probes found volcanoes, lava plains, immense canyons and craters and wind-formed features in an unsuccessful hunt for life in 1976. Nothing landed on Mars again until 1997, when NASA's Mars Pathfinder bounced to the ground encased in air bags. The craft explored an ancient Martian riverbed while a small rover studied rocks. Some Mars missions have failed altogether.
Scientists suspect that a ruptured fuel line sent it off course, never to be heard from again.
The latest attempt will occur in the frigid dirt 500 miles north of the planet's south pole. Launched aboard a Delta rocket from Cape Canaveral on Jan. 3, the probe has traveled 470 million miles in an elliptical orbit to its planned landing on Mars Friday. The probes will strike the planet at 400 mph and bury themselves in up to 3 feet of soil. Drills will bore for soil samples, which will be put into tiny ovens and heated. If water is present, an analyzer will detect water vapors.