The Mars rover Opportunity has driven about 15 feet down the rocky slope of a football-stadium-size crater with little sign that it will have any trouble climbing out once it has completed its explorations, NASA engineers said yesterday.

But with Opportunity and Spirit, its partner on the other side of Mars, operating about two months beyond their design lives, NASA planners have begun assessing the possibility that the rovers could wear out.

Mission manager Mark Adler said Spirit, examining the base of a rock outcrop dubbed "Columbia Hills," has developed a balky right front wheel and had lost a Martian day's work when its chilled electronic components could not load transmitted instructions from NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"Spirit has gotten a little hard of hearing, due to temperature, and a little bit of arthritis," Adler told reporters in a telephone briefing from the laboratory. "We're starting to see some warranty damage. They [the rovers] could continue to operate for several more months, or they could stop and fail tomorrow."

Despite these warnings, Adler described a "great week" of exploration for the rovers, which have both completed long traverses across the Martian landscape to reach secondary research targets chosen after their original missions were completed in April. The two rovers landed on opposite sides of Mars in January.

Opportunity, which had spent about a month scouting the rim of an impact crater nicknamed "Endurance," picked an entry spot last week, and has traveled downhill about 15 feet en route to a rocky outcrop called "Tennessee."

Having discovered evidence of liquid water during Mars's distant past, scientists are hoping that the lower rock layers exposed in the crater will provide new information about the planet's geological history and the possibility that it may once have harbored life.

Planners judged this scientific potential to be significant enough to undertake the descent into Endurance knowing that the six-wheeled Opportunity, much nimbler traveling downhill than uphill, may not be able to climb out.

But, Adler said, Opportunity has performed flawlessly on the rock-strewn, dusty slope. "We have had exactly the slippage we predicted," he said. "We have gone in and back out and back in again, and we have no reason to believe that we are not going to be able to get back out."

Spirit, meanwhile, completed a 0.9-mile traverse to the base of Columbia Hills. Last week it found concentrated salt in the soil below Mars's Gusev Crater region, further evidence that the planet once had liquid groundwater.

Science team member Laurence Soderblom, a U.S. Geological Survey astrogeologist, said planners intend to have Spirit climb about 60 yards to the top of Columbia Hills's "West Spur," examining rocks that have rolled down from the summit. The rovers have robotic cameras, coring tools and spectrographic equipment to sample and test the rocks and soils they encounter.

Adler said engineers noticed three weeks ago that Spirit's right front wheel was using more electric current than the others, a situation that has worsened gradually. He said planners were trying to decide whether to use the wheel until it quits, or shut it down and operate on five wheels.

"The wheels have a tremendous amount of torque," Adler said. "Even just on three or four wheels, we could easily do a traverse on flat terrain." But he also said engineers were conducting five-wheel tests to assess the rover's future.

Adler also warned that "we are headed for a confluence of bad things for the rovers," with the depths of the August-September Martian winter approaching. As Mars draws away from the sun, the rovers' solar arrays are losing efficiency in charging the batteries, he explained.

At the same time, he added, Earth and Mars are drawing farther apart, causing communications problems. "We may have to cut back, or perhaps suspend," contact for a while, or put the rovers into "deep sleep" until spring brings stronger sunshine, he said.

This image from the Mars rover Spirit shows the rock outcrop "Columbia Hills." The rover, operating about two months beyond its design life, has a problem with a front wheel.Discussing the rovers, from left, are Mark Adler, Laurence Soderblom and Scott McLennan. Adler says they "could continue to operate for several more months" or "fail tomorrow."