When the twin rovers Spirit and Opportunity touched down on Mars in January 2004, they were expected to drive a few hundred yards and last 90 days. But scientists were wrong; the hardy robots proved to be two of the greatest explorers of the Space Age. Now, 10 years later, one rover is down, but the other refuses to die and continues to send back stunning images and exciting new scientific information. As newer rovers continue to land on the alien planet, we look back at this remarkable journey. (Source: National Geographic Channel)