NASA's Spirit rover (search) has completed its primary mission to Mars (search) yet continues to roll along, moving toward a cluster of hills that could yield more evidence that the planet had a wet past.
By Monday, Spirit's 90th full day on Mars, the unmanned robot and its twin, Opportunity, had accomplished nearly all of the assigned tasks that would make their joint mission a full success.
"Spirit has completed its part of the bargain and Opportunity doesn't have much left to do," said Mark Adler, manager of the $820 million double mission.
NASA (search) already has extended the joint mission through September.
The rovers' chores included a requirement that one of them travel at least 1,980 feet — a milestone Spirit passed on Saturday.
Between the two of them, the rovers also had to take stereo and color panoramas of their surroundings, drive to at least eight different locations and operate simultaneously for a minimum of 30 days.
NASA assumed technical and other problems would idle the rovers fully one-third of the time they operated on Mars. Despite computer memory problems that sidelined Spirit for 21/2 weeks, it's already spent more days at work than expected, Adler said.
Opportunity still must function for another 20 martian days — nearly 40 minutes longer than Earth days — before it meets all of its targets, Adler said.
"It's better than we could have possibly imagined," he said.
Beginning Thursday, NASA will update the rovers' software. The new programming should allow Spirit to travel farther each day while navigating on its own and help Opportunity conserve battery power at night.
Spirit landed Jan. 3 in Gusev Crater, a 90-mile-diameter depression scientists believed once contained a lake. Spirit has found traces of limited past water activity in rocks it has examined, but none of the lake deposits scientists hoped it would uncover.
Spirit is several days into a trek toward a cluster of hills that may contain geologic evidence of a more substantially wet environment, perhaps including layered rocks formed in standing water.
Opportunity has found such rocks at its landing site, halfway around the now frozen and dry planet, since it landed Jan. 24. Scientists believe a salty sea or swamp once covered that site, called Meridiani Planum.