NASA's Mars rovers keep getting bigger. This photo provides a comparison of the wheel sizes for three generations of them. The first rover on Mars was Sojourner, on the Mars Pathfinder mission launched in 1996. It was small and didn't go far. The Mars Exploration Rovers Spirit and Opportunity, launched in 2003, are bigger and have driven many times farther than expected. The Mars Science Laboratory, in development for a 200 launch, represents another leap in capability. It will carry its onboard chemistry laboratory long distances.

NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, built Sojourner, Spirit and Opportunity, and is building the Mars Science Laboratory. It has managed these missions for the NASA Science Mission Directorate, Washington.