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Three generations of NASA Mars rovers. At centre front is a flight spare for the first Mars rover mission, the Mars Pathfinder Project, measuring 65 centimetres long. At left is a test rover for the Mars Exploration mission, measuring 14.6 metres long, and at right is a test rover for the Mars Science Laboratory (MSL) mission, measuring 3m long. The Mars Pathfinder Project rover, Sojourner, landed on Mars on the 4th July 1997. The Mars Exploration rovers, Spirit and Opportunity, landed in January 2004. The MSL rover Curiosity is due to land on the 5th August 2012. Photographed in the Jet Propulsion Laboratory's Mars Yard testing area, Pasedena, California, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA//JPL-CALTECH/SCIENCE PHOTO LIBRARY)