NASA's Mars Exploration Rover Mission (MER) is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars. It began in 2003 with the sending of the two rovers—MER-A Spirit and MER-B Opportunity—to explore the Martian surface and geology.
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-sol primary mission was US$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.
The exploration of Mars is the study of Mars by spacecraft. Probes sent from Earth, beginning in the late 20th century, have yielded a dramatic increase in knowledge about the Martian system, focused primarily on understanding its geology and habitability potential.
Engineering interplanetary journeys is very complicated, so the exploration of Mars has experienced a high failure rate, especially in earlier attempts. Roughly two-thirds of all spacecraft destined for Mars failed before completing their missions, and there are some that failed before their observations could begin. However, missions have also met with unexpected levels of success, such as the twin Mars Exploration Rovers operating for years beyond their original mission specifications.
As of 24 September 2014, two scientific rovers were on the surface of Mars beaming signals back to Earth (Opportunity of the Mars Exploration Rover mission, and Curiosity of the Mars Science Laboratory mission), with five orbiters currently surveying the planet: Mars Odyssey; Mars Express; Mars Reconnaissance Orbiter; Mars Orbiter Mission; and MAVEN.