An artist's concept shows a NASA Mars exploration rover on the surface of Mars. The twin rovers Spirit and Opportunity were launched in 2003 and arrived at sites on Mars in January 2004. An artist's concept shows a NASA Mars exploration rover on the surface of Mars. The twin rovers Spirit and Opportunity were launched in 2003 and arrived at sites on Mars in January 2004. We probably should not project human traits onto machines.

The identical robotic explorers, Spirit and Opportunity, were able to trek up to 109 yards each Martian day. They found evidence for liquid water among many other things, with Opportunity traveling farther than any autonomous vehicle on any world: over 45 km (28 miles) over more than 5000 days. In 2004, NASA launched two exploration vehicles to the red planet: the Spirit and Opportunity rovers. These two Mars Exploration Rovers were originally designed for 90-day missions to image, explore, and investigate the Martian surface. Yet these twin solar-powered rovers far exceeded their design lifetimes.

Buoyed by the success of two robotic rovers exploring the surface of Mars, NASA scientists are building smarter and more- agile robots that can rappel down cliffs, slither between cracks and even have the sense to detect trouble. Scientist Silvano Colombano stood next to one of the new machines on a recent morning at NASA Ames Research Center in Mountain View, where he demonstrated an eight-legged robot called Scorpion. It can climb up steep hills and plunge into rough terrain where its wheeled counterparts can't go. Next to Colombano, computer engineer Maria Bualat showed off the K9 rover, a robot similar to the Mars rovers, but one that can perform tasks 10 times faster than its cousins, which have been exploring the Red Planet since early last year. The new robots might get their chance in a few years.

NASA's twin Mars rovers, Spirit and Opportunity, have already rewritten the book on the Red Planet's history, their amazing discoveries transmitted to an audience of millions. But is not content to let NASA rest on its laurels. She's designing future generations of robotic explorers to bring back even more science for the buck. Her goal: a robot that can be dropped off on a planet and wander around on its own, eliminating the kind of intense supervision from Earth that Spirit and Opportunity require--their every move must be meticulously choreographed in advance and on a daily basis. "I want to plop a rover on Mars and have it call back when it finds interesting science," Howard says.