Spirit, mission designation MER-A (Mars Exploration Rover - A), is the first of the two rovers of NASA's ongoing Mars Exploration Rover Mission. It landed successfully on Mars on 04:35 Ground UTC on January 4, 2004, three weeks before its twin Opportunity (MER-B) landed on the other side of the planet. Its name was chosen through a NASA-sponsored student essay competition. The rover completed its planned 90-sol mission. Following mission completion and aided by cleaning events that resulted in higher power from its solar panels, Spirit went on to function effectively over twenty times longer than NASA planners expected, as well as logging about 10 kilometers of driving. The mission was originally planned for about 1 km. This allowed more extensive geological analysis of Martian rocks and planetary surface features. Initial scientific results from the first phase of the mission (roughly, 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; 21.6 times the planned mission duration), Spirit became stuck in soft soil. 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 (6 years and 22 days after landing; 24.6 times the planned mission duration), when NASA officials announced that the rover was likely irrecoverably obstructed by its location in soft soil, though it will continue to perform scientific research from its current location.

As of January 2010, the rover team continues to operate the lander as a static science station. However, if the rover's static position cannot be improved so that its solar panels can be tilted more towards the Sun, or if the wind does not clean the rover's solar panels (which are covered with dust), the rover may be able to sustain operations only until May 2010 due to lack of electrical power. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover project for NASA's Office of Space Science, Washington.

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