A spacecraft that moves across the surface of a planet or moon. Its tasks can include investigating and collecting soil samples, and transmitting pictures of the landscape. Rovers are programmed to move and survive on an alien planet, but they can be remotely steered from Earth stations. NASA's Sojourner was a successful ‘microrover’ carried to the planet Mars in 1997 by the Mars Pathfinder mission. More dramatic Mars data was collected in 2004 by the twin rovers Spirit and Opportunity, launched separately in June and July 2003 on the Mars Exploration Rover mission.