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.
As of May 2021[update], there have been six successful robotically operated Mars rovers; the first five, managed by the American NASA Jet Propulsion Laboratory, were (by date of Mars landing): Sojourner (1997), Spirit (2004–2010), Opportunity (2004–2018), Curiosity (2012–present), and Perseverance (2021–present).
[1][2][3][4][5] The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.
As of November 2023, Curiosity is still active, while Spirit, Opportunity, and Sojourner completed their missions before losing contact.
The primary goal of the Spirit and Opportunity rovers was to investigate "the history of water on Mars".