U. S. Missions to Mars with twin rovers equipped with tools to study a diverse collection of rocks and soils that may hold clues to past water activity on Mars. MER-A, named "Spirit," launched on June 10, 2003 for a seven-month journey to a landing site in Gusev Crater and MER-B, named "Opportunity," launched on July 7, 2003 toward a landing site in Terra Meridiani.