A Mars rover is a motor vehicle that propels itself across the surface of the planet Mars upon arrival. Rovers have several advantages over stationary landers: they examine more territory, and they can be directed to interesting features, they can place themselves in sunny positions to weather winter months, and they can advance the knowledge of how to perform very remote robotic vehicle control.
There have been four successful robotically operated Mars rovers. The Jet Propulsion Laboratory managed the Sojourner rover, the Opportunity rover, Spirit rover, and, now the Curiosity rover. On January 24, 2016 NASA reported that current studies on Mars by the Curiosity Opportunity rovers would be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective. Since June 2018, the Opportunity rover has been out of contact after going into hibernation mode in a dust storm. NASA has stated they are unsure if they will ever be able to regain contact.
Mars 2, Mars 3 were physically tethered probes; Sojourner was dependent on the Mars Pathfinder base station for communication with Earth; MER-A & B and Curiosity were on their own. Of these Curiosity is still active, and Spirit, Opportunity and Sojourner completed their missions before losing contact.
Seven rovers have been dispatched to Mars:
- Mars 2, Prop-M rover, 1971, Mars 2 landing failed taking Prop-M with it. The Mars 2 and 3 spacecraft from the USSR had identical 4.5 kg Prop-M rovers. They were to move on skis while connected to the landers with cables.
- Mars 3, Prop-M rover, 1971, lost when Mars 3 lander stopped communicating about 20 seconds after landing.
- Sojourner rover, Mars Pathfinder, landed successfully on July 4, 1997. Communications were lost on September 27, 1997.
- Beagle 2, Planetary Undersurface Tool, lost with Beagle 2 on deployment from Mars Express in 2003. A compressed spring mechanism was designed to allow movement across the surface at a rate of 1 cm per 5 seconds and to burrow into the ground and collect a subsurface sample in a cavity in its tip.
- Spirit (MER-A), Mars Exploration Rover, launched on June 10, 2003 at 13:58:47 EDT and landed successfully on January 4, 2004. Nearly 6 years after the original mission limit, Spirit had covered a total distance of 7.73 km (4.80 mi) but its wheels became trapped in sand. Around January 26, 2010, NASA conceded defeat in its efforts to free the rover and stated that it would now function as a stationary science platform. The last communication received from the rover was on March 22, 2010, and NASA ceased attempts to re-establish communication on May 25, 2011.
- Opportunity (MER-B), Mars Exploration Rover, launched on July 7, 2003 at 23:18:15 EDT and landed successfully on January 25, 2004. Opportunity surpassed the previous record for longevity of a surface mission to Mars as of May 20, 2010 and surpassed the previous record for distance traveled off-Earth as of July 28, 2014 by covering a total distance of 40.25 km (25.01 mi). NASA has been unable to establish contact with Opportunity since the 2018 Mars dust storm.
- Curiosity, Mars Science Laboratory (MSL), by NASA, was launched November 26, 2011 at 10:02 EST and landed in the Aeolis Palus plain near Aeolis Mons (informally "Mount Sharp") in Gale Crater on August 6, 2012, 05:31 UTC. The Curiosity rover is still operational as of 22:30 January 16, 2019.
- ExoMars rover, a European-Russian programme
- Mars 2020, a NASA project
- Mars Global Remote Sensing Orbiter and Small Rover, a Chinese project
Examples of instruments
Examples Landed instrumentation (Rovers)
- Alpha particle X-ray spectrometer (MPF + MER + MSL)
- CheMin (MSL)
- Chemistry and Camera complex (MSL)
- Dynamic Albedo of Neutrons (MSL)
- Hazcam (MER + MSL)
- MarsDial (MER + MSL)
- Materials Adherence Experiment (MPF)
- MIMOS II (MER)
- Mini-TES (MER)
- Mars Hand Lens Imager (MSL)
- Navcam (MER + MSL)
- Pancam (MER)
- Rock Abrasion Tool (MER)
- Radiation assessment detector (MSL)
- Rover Environmental Monitoring Station (MSL)
- Sample Analysis at Mars (MSL)
Locations of Mars rovers, in context
Mars rovers in development include:
- ExoMars, by the ESA. The rover component is planned to launch in 2020 and arrive in 2021. The rover will use stained glass to prevent UV from changing image colors, allowing for true color images of the surface of Mars.
- Chinese Mars Rover, has been planned for a pre-2020 launch, as part of a sample-return mission.
- MESR (Mars Exploration Science Rover), REX (Robot EXplorer), and MRPTA (Micro-Rover Platform with Tooling Arm) have a target destination of Mars, planned by the Canadian Space Agency (CSA).
- Mars 2020, a NASA rover based on the current rover Curiosity and planned to launch in 2020.
- 2020 Chinese Mars Mission, would include an orbiter, lander and small rover.
One experimental design, not proposed for any actual mission, is:
- Mars Tumbleweed Rover, a wind-propelled rover.
- In 2018, a kind of cushion-air rover was proposed,  which in contrast with traditional hovercrafts does not uses blowers to pressurize de gas in the chamber but rather uses stored pressurized CO2 obtained from a freezing process which does not require mechanical compression.
Cancelled or stale rover mission designs/proposals
- Astrobiology Field Laboratory, proposed in the 2000-2010 period as a follow on to MSL.
- Mars Astrobiology Explorer-Cacher (MAX-C), cancelled 2011
- Mars Surveyor 2001 rover, 
NASA Mars rover goals (circa 2010s)
NASA distinguishes between "mission" objectives and "science" objectives. Mission objectives are related to progress in space technology and development processes. Science objectives are met by the instruments during their mission in space.
The details of rover science vary according to equipment carried. The primary goal of the Spirit and Opportunity rovers is to discover "the history of water on Mars". The presence of accessible water would greatly reduce a crewed mission cost.
The four science goals of NASA's long-term Mars Exploration Program are:
- Determine whether life ever arose on Mars
- Characterize the climate of Mars
- Characterize the geology of Mars
- Prepare for human exploration of Mars
- Comparison of embedded computer systems on board the Mars rovers
- Crewed Mars rover
- Curiosity rover
- ExoMars Lander
- InSight lander
- List of artificial objects on Mars
- Mars Exploration Rover
- Mars Pathfinder
- Mars Reconnaissance Orbiter
- Mars 2020 rover mission
- Odyssey orbiter
- Radiation hardening
- Scientific information from the Mars Exploration Rover mission
- Sojourner rover
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