NASA's Mars Exploration Rover (MER) mission was a robotic space mission involving two Mars rovers, Spirit and Opportunity, exploring the planet Mars. It began in 2003 with the launch of the two rovers to explore the Martian surface and geology; both landed on Mars at separate locations in January 2004. Both rovers far outlived their planned missions of 90 Martian solar days: MER-A Spirit was active until March 22, 2010, while MER-B Opportunity was active until June 10, 2018.

Spirit, also known as MER-A (Mars Exploration Rover – A) or MER-2, is a Mars robotic rover, active from 2004 to 2010. Spirit was operational on Mars for 2208 sols or 3.3 Martian years (2249 days; 6 years, 77 days). It was one of two rovers of NASA's Mars Exploration Rover Mission managed by the Jet Propulsion Laboratory (JPL). Spirit landed successfully within the impact crater Gusev on Mars at 04:35 Ground UTC on January 4, 2004, three weeks before its twin, Opportunity (MER-B), which landed on the other side of the planet. Its name was chosen through a NASA-sponsored student essay competition. The rover got stuck in a "sand trap" in late 2009 at an angle that hampered recharging of its batteries; its last communication with Earth was on March 22, 2010.

The rover completed its planned 90-sol mission (slightly less than 92.5 Earth days). Aided by cleaning events that resulted in more energy from its solar panels, Spirit went on to function effectively over twenty times longer than NASA planners expected. Spirit also logged 7.73 km (4.8 mi) of driving instead of the planned 600 m (0.4 mi), allowing more extensive geological analysis of Martian rocks and planetary surface features. Initial scientific results from the first phase of the mission (the 90-sol prime mission) were published in a special issue of the journal Science.On May 1, 2009 (5 years, 3 months, 27 Earth days after landing; 21 times the planned mission duration), Spirit became stuck in soft sand. This was not the first of the mission's "embedding events" and for the following eight months NASA carefully analyzed the situation, running Earth-based theoretical and practical simulations, and finally programming the rover to make extrication drives in an attempt to free itself. These efforts continued until January 26, 2010 when NASA officials announced that the rover was likely irrecoverably obstructed by its location in soft sand,

though it continued to perform scientific research from its current location.The rover continued in a stationary science platform role until communication with Spirit stopped on March 22, 2010 (sol 2208). JPL continued to attempt to regain contact until May 24, 2011, when NASA announced that efforts to communicate with the unresponsive rover had ended, calling the mission complete. A formal farewell took place at NASA headquarters shortly thereafter.