The Mars rover Opportunity poked its nose over the edge of an ancient impact crater yesterday to survey the steep escarpment it must travel to reach a rock outcrop perhaps 23 feet downhill -- a trip from which it may never return.

NASA scientists say the outcrop should offer insights into the history of surface water -- and perhaps life -- on Mars. Planners deemed the scientific potential of the outcrop spectacular enough to risk the possibility that the rover could become trapped forever in the crater's shifting sands.

"We are hoping to have our cake and eat it, too, but we are operating at the capability margins for the vehicle," said Opportunity Mission Manager Matt Wallace, speaking by telephone from NASA's Mars mission offices at the Jet Propulsion Laboratory in Pasadena, Calif. "We have been forced to make the decision with the idea that we're not coming out."

Planners were examining camera images late yesterday to decide whether to drive the rover into the crater on a brief foray today: Final descent "could take five days if there's no slippage," Wallace said. "If it's not a slam dunk or there's more sand, it could be incremental and tedious and take a week."

Three months ago, Opportunity drilled, photographed and analyzed rocks from an outcrop in a shallow crater at the Martian equator, producing evidence that liquid water soaked that part of Mars at some point in the ancient past.

In April, with its 90-day design life expiring, engineers sent Opportunity on an "extended mission" -- 750 yards to "Endurance Crater," a stadium-size hole in the ground with as much as 100 feet of rock exposed, including layers older than the water-bearing stratum that the rover had examined.

"So far, what we have is compelling evidence of water evaporation -- the final phase. It's like having a book and only being able to read the last chapter," said Cornell University astronomer Steven Squyres, lead scientist for the Mars expedition. "We don't know what we'll find in the earlier chapters -- deep water, volcanic ash or a dune field -- but the way to find out more is to go deeper."

Opportunity had difficulty getting out of the crater where it landed, spinning its wheels in soft sand before finding traction. Even as the trek across Meridiani Planum began, planners knew they would probably have decide whether to risk sending the rover into Endurance Crater knowing it might not be able to climb out.

Wallace explained that the choice was somewhat easier because Opportunity is living on borrowed time, with the ultimate length of the mission dependent on the durability of its electronics and machinery and, most of all, the efficiency of its solar arrays in recharging the batteries.

When Opportunity and its identical twin, Spirit, landed on Mars in January, the rovers were getting 900 watt-hours of power from the arrays each day. But with the onset of the Martian winter and the buildup of dust on the arrays, the batteries' efficiency has declined steadily. Wallace said Opportunity's available electricity had fallen to "the 500s" in watt-hours per day, with even less available for its instruments and motors because of the need to heat components that must endure nighttime temperatures that plunge well below minus-120 degrees Fahrenheit.

Spirit is completing a 0.9-mile traverse on the other side of Mars and may reach the base of the "Columbia Hills" -- a promontory rising from the Martian surface -- by the end of the week.

"If we knew the rover was going to last for a year, we could run around and do other things before we went to the crater," Squyres said in a telephone interview from the Jet Propulsion Laboratory. "But that's not the case. Every day there's a falloff in efficiency."

Opportunity has been scouting the crater for about a month, looking for the best way down and the shortest traverse to the lower layers of rock, but Wallace said the rover's limited electricity has sharply curtailed activity in recent days.

Planners eventually picked a spot with a sharp 25-degree downslope between 16 and 23 feet from the outcrop. The crater itself is about 140 yards in diameter and about 32 feet deep. Once at the outcrop, Opportunity will be able to sample the rocks using a variety of imaging tools, drills and probes.

NASA built a dummy escarpment and successfully used a rover replica to scale it, but Wallace described the actual slope as "rocky, windswept terrain" that may have deep pockets of sand that could trap the six-wheeled vehicle.

Getting to the outcrop, however, is a "sure thing," Squyres said. "But it could be that after we find what we're after, we'll have to spend the rest of the mission in the candy store."

The Mars rover Opportunity took this composite image of "Endurance Crater" released by NASA yesterday. This composite image of Endurance with a jagged white line around the lower left portion of the crater shows the path Opportunity took.