Last spring Matt Heverly waved farewell to identical robotic arms he helped design for Spirit and Opportunity, two exploration rovers that NASA was sending to Mars. Then he prayed that they would make it to the red planet without being damaged. They did, surviving a rough ride into Mars’ atmosphere at 12,000 miles per hour, and successfully landing on January 3 and January 24, respectively. Never before have two mobile robots explored the surface of another planet simultaneously.