NASA’S robotic Mars geologist, Spirit, embodying America’s enthusiasm for exploration, must run a grueling gantlet of challenges before it can start examining the red planet. Spirit’s twin Mars Exploration Rover, Opportunity, also faces tough Martian challenges. “The risk is real, but so is the potential reward of using these advanced rovers to improve our understanding of how planets work,” said Dr. Ed Weiler, associate administrator for space science at NASA Headquarters, Washington.