The first of the two rovers, Spirit, met the success criteria set for its prime mission. Spirit gained check marks in the final two boxes on April 3 and 5, when it exceeded 600 meters (1,969 feet) of total drive distance and completed 90 martian operational days after landing.

Opportunity landed three weeks after Spirit. It will complete the two-rover checklist of required feats when it finishes a 90th martian day of operations April 26. Each martian day, or "sol," lasts about 40 minutes longer than an Earth day.

"Given the rovers' tremendous success, the project submitted a proposal for extending the mission, and we have approved it," said Orlando Figueroa, Mars Exploration Program director at NASA Headquarters, Washington, D.C.

The mission extension provides $15 million for operating the rovers through September. The extension more than doubles exploration for less than a two percent additional investment, if the rovers remain in working condition. The extended mission has seven new goals for extending the science and engineering accomplishments of the prime mission.

"Once Opportunity finishes its 91st sol, everything we get from the rovers after that is a bonus," said Dr. Firouz Naderi, manager of Mars exploration at NASA's Jet Propulsion Laboratory, Pasadena, Calif., where the rovers were built and are controlled. "Even though the extended mission is approved to September, and the rovers could last even longer, they also might stop in their tracks next week or next month. They are operating under extremely harsh conditions. However, while Spirit is past its 'warranty,' we look forward to continued discoveries by both rovers in the months ahead." JPL's Jennifer Trosper, Spirit mission manager, said even when a memory-management problem on the rover caused trouble for two weeks, she had confidence the rover and the operations team could get through the crisis and reach the 90-sol benchmark. "We never felt it was over, but certainly when we were getting absolutely no data from the spacecraft and were trying to figure out what happened, we were worried," she said.

Trosper was less confident about Spirit's prospects for reaching the criterion of 600 meters by sol 91, given the challenging terrain of the landing area within Gusev Crater. On sol 89 Spirit accomplished that goal and set a short-lived record for martian driving, with a single-sol distance of 50.2 meters (165 feet) that pushed the odometer total to 617 meters (2,024 feet). Two days later, Opportunity shattered that mark with a 100-meter (328-foot) drive.

Beyond the quantifiable criteria, such as using all research tools at both landing sites and investigating at least eight locations, the rovers have returned remarkable science results. The most dramatic have been Opportunity's findings of evidence of a shallow body of salty water in the past in the Mars Meridiani Planum region.

"We're going to continue exploring and try to understand the water story at Gusev," said JPL's Dr. Mark Adler, deputy mission manager for Spirit. Spirit is in pursuit of geological evidence for an ancient lake thought to have once filled Gusev Crater.

Reaching "Columbia Hills," which could hold geological clues to that water story, is one of seven objectives for Spirit's extended mission. Opportunity has a parallel one, to seek geologic context for the outcrop in the "Eagle" crater by reaching other outcrops in the "Endurance" crater and perhaps elsewhere. Other science objectives are to continue atmospheric studies at both sites to encompass more of Mars' seasonal cycle, and to calibrate and validate data from Mars orbiters for additional types of rocks and soils examined on the ground.

Three new engineering objectives are to traverse more than a kilometer (0.62 mile) to demonstrate mobility technologies; to characterize solar-array performance over long durations of dust deposition at both landing sites; and to demonstrate long-term operation of two mobile science robots on a distant planet. During the past two weeks, rover teams at JPL have switched from Mars-clock schedules to Earth-clock schedules designed to be less stressful and more sustainable over a longer period.