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.

Spirit remains silent at its location, called "Troy," on the west side of Home Plate. As of 2016, there has been no communication with the rover since Sol 2210 (March 22, 2010). It is likely that Spirit experienced a low-power fault and had turned off all sub-systems, including communication, and gone into a deep sleep, trying to recharge its batteries. It is also possible that the rover had experienced a mission clock fault. If that had happened, the rover would have lost track of time and tried to remain asleep until enough sunlight struck the solar arrays to wake it. This state is called "Solar Groovy." If the rover woke up from a mission clock fault, it would only listen. Starting on Sol 2333 (July 26, 2010), a new procedure to address the possible mission clock fault was implemented. Each sol, the Deep Space Network mission controllers sent a set of X-band "Sweep & Beep" commands. If the rover had experienced a mission clock fault and then had been awoken during the day, it would have listened during brief, 20-minute intervals during each hour awake. Due to the possible clock fault, the timing of these 20-minute listening intervals was not known, so multiple "Sweep & Beep" commands were sent. If the rover heard one of these commands, it would have responded with an X-band beep signal, updating the mission controllers on its status and allowing them to investigate the state of the rover further. But even with this new strategy, there was no response from the rover. The rover had driven 7,730.50 meters (4.80 miles) until it became immobile.

What was the last thing the rover communicated?
Due to the possible clock fault, the timing of these 20-minute listening intervals was not known, so multiple "Sweep & Beep" commands were sent.