On the Cover: The Mars Exploration Rovers, Spirit and Opportunity, were designed to last three months on the surface of the Red Planet. Three years and more than 180,000 images later, these hardworking robotic explorers are still making thrilling discoveries. Opportunity's Panoramic Camera (Pancam) imaged these sand dunes at the bottom of Endurance crater on sol (Martian day) 211. Some of these dunes are more than a meter tall from crest to trough. The image is processed in false color for mineral analysis. NASA/JPL/Cornell University
4 The Pioneer Anomaly—A Mystery of Cosmic Proportions: Bruce Bettslooks at the clues in this landstanding cosmic mystery.