The most recent Mars rover, called Curiosity, landed on Aug. 6, 2012, on the Aeolis Palus plain near the Gale Crater. Three previous rovers successfully landed on Mars, including the Sojourner on July 4, 1997; Spirit on Jan. 3, 2004; and Opportunity on Jan. 24, 2004.
The Sojourner was part of the Pathfinder Mission to Mars, which was designed to show that landing on another planet and gathering information was possible. The Mars Exploration Rover Mission followed suit, sending the twin rovers Spirit and Opportunity to study Martian geology. Curiosity, a much larger and more advanced rover, assesses the geology, climate, role of water in biological history on Mars and the probability of ancient microbial life. It also gathers information on habitability to aid in future manned missions to Mars.