NASA’s Mars rover Opportunity will have to watch its step around its Endurance Crater destination to avoid a potentially mission-ending fall, while its twin Spirit continues its approach to the Columbia Hills. Mars Exploration Rover (MER) mission scientists are particularly wary about sending Opportunity into Endurance crater, which bears tempting rock outcrops for study but could be catastrophic if the rover slips. With a depth of more than 66 feet (20 meters), Endurance is 10 times deeper than Opportunity’s previous Eagle crater home.