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A milestone on Mars

NASA / JPL / Cornell
NASA's Opportunity rover took this snapshot of the rim of Beagle Crater on July 30.

The colors have been "stretched" to emphasize subtle differences in surface

composition. At the time, the rover was about 82 feet (25 meters) from the rim.

Today marks the darkest day of the year for Mars' southern hemisphere – the winter solstice – and thus the first full winter-to-winter cycle for those hardy rovers Spirit and Opportunity. Although they're experiencing aches and pains, prospects look good that both of NASA's Red Planet robots will see another Martian spring, three Earth years after setting down for what was expected to be only a 90-day mission.

You don't hear so much about the rovers nowadays, but at NASA's Jet Propulsion Laboratory in Pasadena, Calif., project manager John Callas says, "We've been very busy."

Spirit is finishing up a huge picture-taking project even as it hunkers down for the winter on an outcrop known as "Low Ridge Haven." Meanwhile, Opportunity is closing in on Victoria Crater, the gaping hole where it may well spend the rest of its operating life.

The winter is tougher on Spirit because it's in a more southerly locale, meaning that less sunlight falls on its solar panels. Thus, the rover has to stay put for another month or so, until the days get longer and Spirit's power supply has the extra juice for locomotion.

That doesn't mean Spirit has been hibernating. Over the past few weeks, the industrious rover has been snapping and sending back more than 1,500 pictures, and the imaging team is knitting all that imagery together into a 360-degree, stereo-image panorama.

"It's effectively complete," Callas said, with just a few edges yet to be filled in.

Spirit has also been surveying the scene with its miniature thermal emission spectrometer, and spotting intriguing features such as a pair of possible meteorites nearby. Once the Spirit is moving again, those rocks could be among the first targets for closer examination, Callas said.

The science team also wants to check out some "very curious, almost fanlike features" that Spirit spotted on the way to its haven, Callas said. And the rover may well return to a rocky area known as Home Plate for further investigation. Scientists want to check out some areas they weren't able to see the first time around due to time constraints.

"There's a whole side of Home Plate that we didn't explore. ... I don't believe there's a clear hypothesis as to what Home Plate is," Callas told me today.

One of Spirit's six wheels has stopped working - and Callas said that might well slow the rover's pace even when spring arrives next February. (Mars' seasons are roughly twice as long as Earth's.)

"Clearly driving will be different," he said. "We're going to have to be a lot more careful about where we drive this vehicle."

Meanwhile, on the other side of the planet, Opportunity is trucking through the expanse of Meridiani Planum. Right now it's on the lip of a 115-foot-wide (35-meter-wide) pothole called Beagle Crater, but that's just a warmup for the main event.

Opportunity is expected to take a week or so to check out Beagle Crater, as well as some brightly banded ripples of Martian sand that have piqued scientists' interest, Callas said. Then it will move a third of a mile (500 meters) onward, to the half-mile-wide (750-meter-wide) Victoria Crater. That crater is five to eight times the size of the biggest crater Opportunity has explored to date, Endurance Crater, and should prove to be a geological gold mine (metaphorically speaking, of course).

It took six months to survey Endurance, and if NASA decides to study Victoria in the same depth, Opportunity could be occupied for years, Callas noted.

"She could spend the entirety of her remaining life inside Victoria Crater," he said.

Since Opportunity is just south of the Martian equator, winter hasn't been as much of a limiting factor as it has been for Spirit. But Oppy has its (her?) own little problems. For most of its operating life, the rover has had to contend with a heater switch that's been stuck in the "on" position. NASA works around the glitch by powering down Opportunity and putting it into a "deep sleep" during the night when necessary.

As long as the rovers can still do science, NASA doesn't intend to pull their plugs. In fact, the space agency just gave the go-ahead for a mission extension of up to a year.

To be sure, the science operations have changed quite a bit, compared with the bustling days that followed the rovers' landings in January 2004. "The science team is almost completely remote, and that's been working effectively," Callas said. For example, principal investigator Steve Squyres is usually able to monitor the mission from his home base at Cornell University in New York.

Those long-distance science relationships – in addition to other economies of scale – have brought the current operating costs down to roughly a third of what they were at the mission's peak, Callas said. Thus, a twin-rover mission that started out with a price tag of $800 million is becoming a better bargain every day.

Check out NASA's latest rover overview and our own "Return to the Red Planet" section for a recap of the past two and a half Earth years – and stay tuned for brighter days on Mars.