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In January 2004, NASA landed two mobile robotic spacecraft, or rovers, on opposite sides of the planet Mars. These rovers, named Spirit and Opportunity, were each sent on their own scientific mission of exploration. Their objective was to work as robotic geologists. After more than a year the vehicles are still in excellent health, and returning vast amounts of scientific information on the ancient water processes that helped form Mars. Key to the success of the rovers was the development of their advanced mobility system. In this paper the mobility assembly, the mechanical hardware that determines the vehicles mobility capability, is described. The details of the design, test, and performance of the mobility assembly are shown to exceed the mission requirements. The rovers' ability to traverse the Mars terrain with its combination of rocks, craters, soft soils, and hills was verified, and the system design validated.