Controllers have begun calling the rovers' current status "bonus time," because Spirit and Opportunity have surpassed their expected mission lengths by more than a month. Now, however, the question is whether to gamble on that success and send the rovers into more challenging -- and perhaps more hazardous - situations.

Though Spirit and Opportunity are identical in appearance, size and function, they thus far have had quite different experiences since they bounced down onto the Martian surface last January - and they now face quite opposite landscapes.

Spirit landed in a wide, flat impact basin called Gusev Crater, near the Martian equator. NASA's preliminary studies gave strong indications that Gusev was an ancient lakebed. Indeed, the spectacular images relayed by Spirit's cameras of its surroundings look exactly like dry lake beds on Earth. But the robot's instruments so far have not found any evidence of water.

Opportunity, on the other hand, has enjoyed incredible luck, discovering abundant evidence that water not only existed on Mars, but also flowed, surged, and lasted long enough to carve its legacy into the planet's surface.

For the past several weeks, after crawling along Gusev's floor and covering nearly two miles, Spirit has sighted possibly layered rock in hills just ahead. Mission controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said the rover has only about 440 yards (400 meters) and several more days of traveling to reach the base of hills the mission team has named Columbia Hills, in honor of the space shuttle astronauts.

The feat is impressive, because engineers previously had concluded the hills were unreachable within Spirit's estimated mission lifetime.

"This is the first time we've ever had a close look at hills on Mars," said James Rice of Arizona State University in Tempe, a member of the rovers' science team.

Martian hills have intrigued planetary geologists since 1997, when the Mars Pathfinder landed only about a half-mile away from a formation called Twin Peaks.

"We could only observe Twin Peaks from a distance and wonder about them," Rice said at a mission news briefing earlier this week. "But now, with a more capable rover, we can get to Columbia Hills.

Rocks in Columbia Hills could help scientists understand both how hills form on Mars and whether the ancient environment at this part of Mars was wet. As Spirit has approached the hills, its camera has taken images that show boulders and potential rock outcrops.

"These rocks are much older than what we've been driving across," Rice said. "We could find a lot of geological history locked in them. They may be some of the oldest material ever seen on Mars."

Halfway around the red planet, Opportunity is poised - stuck, even - on the rim of a crater mission controllers have named Endurance, after the ship of Antarctic explorer Ernest Shackleton.

Where Opportunity had landed in a relatively small crater, named Eagle, that proved a perfect site to begin examining rocks for water, the robot craft now sits at the edge of stadium-sized Endurance. Though controllers are confident the rover can roll down into the crater safely, the question is whether it can roll itself back out.

Even though NASA could argue it has more than gotten its money's worth from Opportunity, controllers would hate to lose the craft if they can avoid it. So they are conducting tests with a spare copy of the Mars rovers on landscape and soils that mimic the environs of Endurance.

The potential payoff could be large.

"We're looking at rocks that have very interesting surface textures," said team member Wendy Calvin of the University of Nevada in Reno. "These rocks appear to be from the same geological layer as the outcrop at Eagle crater, but they have some differences from what we saw there."

One rock, called Pyrrho, on Endurance's rim, features a braided ripple pattern. Another, called Diogenes, has disc-shaped cavities that scientists interpret as sites where water created crystals in the rocks.

While Opportunity waits for a "go" or "no go" command from NASA, it is using its panoramic camera and other instruments to study the crater's southeastern rim, supplementing a similar survey it had made earlier from the western edge.

NASA also has begun using what controllers call a "deep sleep mode" for Opportunity during the Martian night. They describe it as a more complete overnight shutdown that conserves energy, but they acknowledge it involves a calculated risk of damaging the rover's thermal emission spectrometer.

So far, the strategy has worked, because it has tripled the amount of time the solar-powered rover can work during the day, and the spectrometer has survived. Perhaps Opportunity's phenomenal good fortune will continue.

Related Links
Mars Rovers at JPL
Mars Rovers at Cornell
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