The longevity of NASA's Mars Exploration Rovers, Spirit and Opportunity, is becoming the stuff of legend. Mission scientists had hoped that the wheeled robots would each last 90 sols (Martian days) on the surface and perhaps drive as far as 600 meters (2,000 feet). As of early May, both rovers had passed the 800-sol mark; Spirit had traveled 6.8 kilometers (4.2 miles) and Opportunity 7.5 km. Combined, the twin craft have shot more than 150,000 images and analyzed many dozens of rocks. Most importantly, the rovers, reporting from opposite sides of the planet, have confirmed suspicions that ancient Mars was indeed wet.
The rovers are showing their age, however. Spirit's right front wheel has seized up completely, and as the rover limps backward across the landscape, it carves a deep trench in its wake. Mobility is typically limited to less than 10 meters a day. Opportunity's instrument arm has a balky shoulder joint, and engineers have had to develop a new way to analyze rocks with a limited range of arm motion.
Recently Spirit, at a latitude of 15° south, spent many days hobbling to a sunny spot for the long, cold winter. Because the Martian southern hemisphere is tipped away from the Sun, when the Red Planet is near the far point of its eccentric orbit, only 70% as much sunlight falls on the rover's solar cells in winter as in summer. To conserve precious energy, the rover team has parked the craft on a 10° slope facing the Sun. "We're probably going to sit there for the Martian winter," says panoramic camera lead scientist Jim Bell (Cornell University). "We're basically acting like a lander for the next six months." While Spirit huddles for warmth, the team plans to analyze reachable rock and soil targets in great detail, monitor the atmosphere, and shoot an enormous 360° mosaic using all of the camera's filters.
Meanwhile, since Opportunity is driving in Meridiani Planum, close to the equator, it receives more sunlight and has no need to find slopes to perch up against in order to survive. With its six good wheels, the rover traversing up to 50 meters per day toward Victoria crater. As of early May the 800-meter-wide depression was only 1.3 km away. Arrival is expected sometime in June or July. Once there, the rover will spend the winter scouting around the edge and looking for rock outcrops to examine. Controllers may even plot a path inside for next summer.
But how long will the rovers survive? Right now there is no real answer. By winter's end they will have lived 10 times longer than their "expiration dates." "We are heading into our second Martian winter and we think we'll survive," says Bell. The science team is resigned that someday a critical part will break on each rover and both will fall silent. "But until that happens we are going to just keep driving until the wheels fall off," says Bell.