NASA's Mars Exploration Rover (MER) Mission is an ongoing robotic mission of exploring Mars, that began in 2003 with the sending of two rovers — MER-A Spirit and MER-B Opportunity — to explore the Martian surface and geology.
Primary among the mission's scientific objectives is 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 landers in 1976 and Pathfinder in 1997.
The total cost of building, launching, landing and operating the rovers on the surface for the initial 90 Martian-day primary mission was US$820 million. Since the rovers have continued to function for over five years after landing they have received five mission extensions with the fifth mission extension, which was granted in October 2007, being until 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. In July 2007, Martian dust storms blocked sunlight to the rovers and threatened the ability of the craft to gather energy through their solar panels, causing engineers to fear that one or both of them might be permanently disabled. However, the dust storms lifted, allowing them to resume operations.
In recognition of the vast amount of scientific information amassed by both rovers, two asteroids have been named in their honor: 37452 Spirit and 39382 Opportunity.
The mission is managed for NASA by the Jet Propulsion Laboratory, which designed, built and is operating the rovers.