Spirit, also known as MER-A (Mars Exploration Rover - A) or MER-2, is a robotic rover on Mars, active from 2004 to 2010. It was one of two rovers of NASA's ongoing Mars Exploration Rover Mission. It landed successfully on Mars at 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.

On January 21, 2004 (sol 17), Spirit abruptly ceased communicating with mission control. The next day the rover radioed a 7.8 bit/s beep, confirming that it had received a transmission from Earth but indicating that the craft believed it was in a fault mode. Commands would only be responded to intermittently. This was described as a very serious anomaly, but potentially recoverable if it were a software or memory corruption issue rather than a serious hardware failure. Spirit was commanded to transmit engineering data, and on January 23 sent several short low-bitrate messages before finally transmitting 73 megabits via X band to Mars Odyssey. The readings from the engineering data suggested that the rover was not staying in sleep mode. As such, it was wasting its battery energy and overheating - risk factors that could potentially destroy the rover if not fixed soon. On sol 20, the command team sent it the command SHUTDWN_DMT_TIL ("Shutdown Dammit Until") to try to cause it to suspend itself until a given time. It seemingly ignored the command.  The leading theory at the time was that the rover was stuck in a "reboot loop". The rover was programmed to reboot if there was a fault aboard. However, if there was a fault that occurred during reboot, it would continue to reboot forever. The fact that the problem persisted through reboot suggested that the error was not in RAM, but in either the flash memory, the EEPROM, or a hardware fault. The last case would likely doom the rover. Anticipating the potential for errors in the flash memory and EEPROM, the designers had made it so that the rover could be booted without ever touching the flash memory. The radio itself could decode a limited command set - enough to tell the rover to reboot without using flash. Without access to flash memory the reboot cycle was broken.  On sol 19 (January 24, 2004) the rover repair team announced that the problem was with Spirit's flash memory and the software that wrote to it. The flash hardware was believed to be working correctly but the file management module in the software was "not robust enough" for the operations the Spirit was engaged in when the problem occurred, indicating that the problem was caused by a software bug as opposed to faulty hardware. NASA engineers finally came to the conclusion that there were too many files on the file system, which was a relatively minor problem. Most of these files contained unneeded in-flight data. After realizing what the problem was, the engineers deleted some files, and eventually reformatted the entire flash memory system. On February 6 (sol 33), the rover was restored to its original working condition, and science activities resumed.  Spirit drove from Bonneville crater in a direct line to the Columbia Hills. The route was only directly controlled by the engineers when the terrain was difficult to navigate; otherwise, the rover drove in an autonomous mode. On sol 159, Spirit reached the first of many targets at the base of the Columbia Hills called West Spur. Hank's Hollow was studied for 23 sols. Within Hank's Hollow was the strange-looking rock dubbed "Pot of Gold". Analysing this rock was difficult for Spirit, because it lay in a slippery area. After a detailed analysis with the AXPS-and the Mossbauer instrument it was detected that it contains hematite. This kind of rock can be built in connection with water.  As the produced energy from the solar panels was lowering due to the setting sun and dust the Deep Sleep Mode was introduced. In this mode the rover was shut down completely during the night in order to save energy, even if the instruments would fail. The route was selected so that the rover's panels were tilted as much as possible towards the winter sun.  From here, Spirit took a northerly path along the base of the hill towards the target Wooly Patch, which was studied from sol 192 to sol 199. By sol 203, Spirit had driven southward up the hill and arrived at the rock dubbed "Clovis". Clovis was ground and analyzed from sol 210 to sol 225. Following Clovis came the targets of Ebenezer (Sols 226-235), Tetl (sol 270), Uchben and Palinque (Sols 281-295), and Lutefisk (Sols 296-303). From Sols 239 to 262, Spirit powered down for solar conjunction, when communications with the Earth are blocked. Slowly, Spirit made its way around the summit of Husband Hill, and at sol 344 was ready to climb over the newly designated "Cumberland Ridge" and into "Larry's Lookout" and "Tennessee Valley". Spirit also did some communication tests with the ESA orbiter Mars Express though most of the communication was usually done with the NASA orbiters Mars Odyssey and Mars Global Surveyor.  During 2007 Spirit spent several months near the base of the Home Plate plateau. On sol 1306 Spirit climbed onto the eastern edge of the plateau. In September and October it examined rocks and soils at several locations on the southern half of the plateau. On November 6, Spirit had reached the western edge of Home Plate, and started taking pictures for a panoramic overview of the western valley, with Grissom Hill and Husband Hill visible. The panorama image was published on NASA's website on January 3, 2008 to little attention, until January 23, when an independent website published a magnified detail of the image that showed a rock feature a few centimeters high resembling a humanoid figure seen from the side with its right arm partially raised.  Towards the end of June 2007, a series of dust storms began clouding the Martian atmosphere with dust. The storms intensified and by July 20, both Spirit and Opportunity were facing the real possibility of system failure due to lack of energy. NASA released a statement to the press that said (in part) "We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense". The key problem caused by the dust storms was a dramatic reduction in solar energy caused by there being so much dust in the atmosphere that it was blocking 99 percent of direct sunlight to Opportunity, and slightly more to Spirit.  Normally the solar arrays on the rovers are able to generate up to 700 watt-hours (2,500 kJ) of energy per Martian day. After the storms, the amount of energy generated was greatly reduced to 128 watt-hours (460 kJ). If the rovers generate less than 150 watt-hours (540 kJ) per day they must start draining their batteries to run survival heaters. If the batteries run dry, key electrical elements are likely to fail due to the intense cold. Both rovers were put into the lowest-power setting in order to wait out the storms. In early August the storms began to clear slightly, allowing the rovers to successfully charge their batteries. They were kept in hibernation in order to wait out the remainder of the storm.

How did the dust storms affect the rover?
The storms intensified and by July 20, both Spirit and Opportunity were facing the real possibility of system failure due to lack of energy.