NASA’s Mars Exploration Rover Mission (MER) is an ongoing robotic space mission involving rovers Spirit, Opportunity and now Curiosity, exploring the planet Mars. It began in 2003 with the sending of the two rovers, Spirit and Opportunity, to explore the Martian surface and geology and continues today. The mission’s scientific objective was to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars. The mission is part of NASA’s Mars Exploration Program, which includes three previous successful landers: the two Viking program landers in 1976 and Mars Pathfinder probe in 1997. The total cost of building, launching, landing and operating the rovers on the surface for the initial 90-Martian-day (sol) primary mission was $820 million. Since the rovers have continued to function beyond their initial 90 sol primary mission, they have each received five mission extensions. The fifth mission extension was granted in October 2007, and ran to the end of 2009. The total cost of the first four mission extensions was $104 million, and the fifth mission extension is expected to cost at least $20 million.