A Mars rover is a motor vehicle that travels across the surface of the planet Mars upon arrival. Rovers have several advantages over stationary landers: they examine more territory, 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, all managed by the Jet Propulsion Laboratory: Sojourner, Opportunity, Spirit and Curiosity. On January 24, 2016, NASA reported that current studies on Mars by Curiosity and Opportunity (the latter now defunct) 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. In June 2018, Opportunity went out of contact after going into hibernation mode in a dust storm. NASA declared the end of the Opportunity mission on February 13, 2019, after numerous failures to wake up the rover.
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. As of June 2019, Curiosity is still active, while Spirit, Opportunity, and Sojourner completed their missions before losing contact.
Six 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 Soviet Union 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.
- Spirit (MER-A), Mars Exploration Rover, launched on June 10, 2003, and landed 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. 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 and landed on January 25, 2004. Opportunity surpassed the previous records for longevity at 5,352 sols (5498 Earth days from landing to mission end; 15 Earth years or 8 Martian years) and covered a total distance of 40.25 km (25.01 mi). The rover sent its last status on 10 June 2018 when a global 2018 Mars dust storm blocked the sunlight needed to recharge its batteries. After hundreds of attempts to reactivate the rover, NASA declared the mission complete on February 13, 2019.
- Curiosity of the Mars Science Laboratory (MSL) mission by NASA, was launched November 26, 2011 and landed at the Aeolis Palus plain near Aeolis Mons (informally "Mount Sharp") in Gale Crater on August 6, 2012. The Curiosity rover is still operational as of June 17, 2019.
- Rosalind Franklin, the European-Russian ExoMars rover to launch in 2020
- Mars 2020, a NASA rover to launch in 2020
- Mars Global Remote Sensing Orbiter and Small Rover, a Chinese project
- 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, 
- Zephyr rover, would use a rigid sail for wind propulsion.
- In 2018, a kind of cushion-air rover was proposed, which in contrast with traditional hovercrafts does not use blowers to pressurize the gas in the chamber but rather uses stored pressurized CO2 obtained from a freezing process which does not require mechanical compression.
Examples of instruments
Examples of instruments onboard landed rovers include:
- 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
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 science instruments are chosen and designed based on the science objectives and goals. The primary goal of the Spirit and Opportunity rovers was to investigate "the history of water on Mars".
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
Soviet Mars rover Prop-M (1972)
- 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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