NASA celebrated a major milestone in space exploration on Tuesday. July 4th, 1997 marked 20 years since NASA's Pathfinder mission reached Mars successfully. "Pathfinder initiated two decades of continuous Mars exploration, bringing us to the threshold of sample return and the possibility of humans on the first planet beyond Earth," Michael Meyer, lead scientist for NASA's Mars Exploration Program. Pathfinder was launched on Dec. 4, 1996, only to land on the Red Planet eight months later for the first time history.