For ten years the robots have roved across Mars, making exciting discoveries about water in the planet's past. (Image: © by Karl Tate, Infographics Artist)

Sent to Mars to conduct a 90-day investigation, both of the Mars Exploration Rover vehicles far outlasted engineers' expectations. They launched in 2003 and both arrived at Mars in January 2004. The Spirit rover operated for five years, finally expiring during a cold Martian winter. The Opportunity rover is still making discoveries in 2014.