NASA lit a birthday candle today for its twin Mars Exploration Rovers, Spirit and Opportunity. The Spirit rover begins its second year on Mars investigating puzzling rocks unlike any found earlier. The rovers successfully completed their three-month primary missions in April. They astound even their designers with how well they continue operating. The unanticipated longevity is allowing both rovers to reach additional destinations and to keep making discoveries. Spirit landed on Jan. 3 and Opportunity Jan. 24, 2004, respectively.