By Monday, Spirit's 90th full day on Mars, the unmanned robot and its twin, Opportunity, had accomplished nearly all of the tasks before NASA would consider 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.

The tasks included a requirement that one of the rovers travel at least 1,980 feet -- a mark Spirit surpassed 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 ground the rovers fully one-third of the time they operated on Mars.

Despite computer memory problems that left Spirit sidelined for 2 1/2 weeks, it's still spent more days at work than expected, Adler said.

For Opportunity, it still must function for another 20 martian days -- which are 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.

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 now several days into a trek toward a cluster of hills that may contain geologic evidence of a more substantially wet environment, including perhaps 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.

NASA has extended the joint mission through September. If the rovers continue to function, the Jet Propulsion Laboratory will apply for money to further extend the project, Adler said.