NASA engineers are working on the next generation of robotic explorers for Mars, a mechanized fleet that will be able to go where no rover has gone before. Scientists would love to send the mobile machines to areas where satellite images suggest that water once flowed or might still remain underground. But a major obstacle is that the presumed wet spots are located in rugged terrain, like on steep hills and at the base of cliffs. The current breed of rovers cannot reach such places, so NASA engineers are in the early stages of developing their version of a sports utility vehicle robot, which they call the all-terrain explorer.