Explorers through history have often been forced to live off the land, hunting game and scavenging roots, fruits and berries once they exhausted their own provisions. The robotic explorers that NASA plans to send to Mars and other planets similarly will need to find ways to re-energize themselves if their missions are to last more than a few weeks or months. Rather than food and water, however, these mechanized Lewis and Clarks often will look to the sun to replenish depleted batteries.