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.
Mars 2 and Mars 3 were physically tethered probes; Sojourner was dependent on the Mars Pathfinder base station for communication with Earth; Opportunity, Spirit and Curiosity were on their own. As of May 2021, Curiosity is still active, while Spirit, Opportunity, and Sojourner completed their missions before losing contact. On February 18, 2021, Perseverance, the newest Mars rover, successfully landed.
Mars 3, Prop-M rover, landed successfully on December 2, 1971. 4.5 kilograms (9.9 lb) rover tethered to the Mars 3 lander. Lost when the Mars 3 lander stopped communicating about 104.5 seconds after landing. The loss of communication may have been due to the extremely powerful Martian dust storm taking place at the time or an issue with the Mars 3 orbiter's ability to relay communications.
Sojourner rover, Mars Pathfinder, landed successfully on July 4, 1997. Communications were lost on September 27, 1997. Sojourner had traveled a distance of just over 100 meters (330 ft).
Spirit (MER-A), Mars Exploration Rover (MER), 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 45.16 km (28.06 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.
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.
Sojourner disembarks Mars Pathfinder base station lander on the surface of planet Mars
Tianwen-1, a CNSA project, launched on July 23, 2020 and successfully reached Mars orbit on February 10, 2021. The Zhurong rover is expected to land on Mars in May 2021. 
Mars Tumbleweed Rover, a spherical wind-propelled rover.
In 2018, a kind of cushion-air rover was proposed, which in contrast with traditional hovercraft 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.
Curiosity's (MSL) rover "hand" featuring a suite of instruments on a rotating "wrist". "Mount Sharp" is in the background (September 8, 2012).
Examples of instruments
Opportunity's first self-portrait including the camera mast on Mars (February 14−20, 2018 / sols 4998−5004) It was taken with its microscopic imager instrument
Examples of instruments onboard landed rovers include:
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".
Mark Rober [@MarkRober] (May 3, 2020). "CONFESSION: when I was at @NASA I created this image from our CAD model of the rover before it launched" (Tweet) – via Twitter.
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^The Tumbleweed Rover is on a Roll. Anna Heiney, KSC NASA. 11 March 2004.
^Arias, Francisco. J (2018). "CO2-Cushion Vehicle for Mars. An Alternative Locomotion for Exploration Rovers". 2018 Joint Propulsion Conference. 54nd AIAA/SAE/ASEE Joint Propulsion Conference Cincinnati, OH, Propulsion and Energy, (AIAA 2018–4492). doi:10.2514/6.2018-4492. ISBN 978-1-62410-570-8.
^Arias, Francisco. J (2018). "A Method of Attaining High Pressurized Vessels in Space, the Moon and With Particular Reference to Mars". 2018 International Energy Conversion Engineering Conference. 54nd AIAA/SAE/ASEE Joint Propulsion Conference Cincinnati, OH, Propulsion and Energy, (AIAA 2018–4688). doi:10.2514/6.2018-4488. ISBN 978-1-62410-571-5.
^"Mars Exploration Rover Mission: Overview". marsrovers.nasa.gov. Archived from the original on 2012-08-28. Retrieved 2008-06-25.
^"Mars Exploration Rover Mission: Science – Looking for signs of past water on Mars". marsrovers.nasa.gov. Archived from the original on 2008-05-22. Retrieved 2008-06-25.