Their 90-day warranty expired awhile ago, but NASA's twin Mars rovers Spirit and Opportunity are still trundling along the Martian surface en route to their next destinations more than five years after landing on the red planet. But just how long they can keep going is anyone's guess ? it could be three days, or it could be three years.
"We have no way of knowing what the future holds for the rovers at this point," said Mars Exploration Rover Mission principal investigator Steve Squyres of Cornell University. "The mission could easily end tomorrow, but the miracle could continue."
That miracle began with the January 2004 landings of the two rovers. Spirit got to Mars first, touching down at Gusev Crater on Jan. 3. A few weeks later, Opportunity bounced to a stop in the vast plains of Meridiani Planum on the other side of the planet.
The road has been a bumpy one ? sometimes literally ? with stuck wheels, broken wiring and Spirit's one bum wheel periodically hampering the mission.
But even with those hiccups, the past five years (that's 20 times the planned lifetimes of the rovers) have yielded many insights into the planet's past and present and taught mission controllers numerous lessons about running a mission from millions of miles away.
Among the many remarkable discoveries the rovers have made are the clues that show the planet has not always been as cold and dry as it is at present ? at one time, it was warm and wet enough to support life.
More than 1,200 sols (or Martian days) into the mission, Spirit analyzed a patch of dirt and found it was rich in silica, which provided some of the strongest evidence yet that the Martian surface was once wet.
Opportunity found salty areas in Merdiani Planum that have garnered many watery theories to explain how they go there.
These scientific findings aren't the only useful information to come out of the mission.
"Spirit and Opportunity helped invent a whole new discipline ? robotic field science," Squyres said. "They've taught us how to organize a team of scientists and engineers to operate robotic rovers on a distant planet. We all had to learn to work together effectively year after year to squeeze the most possible science from the rovers."
Team members learned many of the perils of maneuvering a wheeled robot from hundreds of millions of miles away.
"We now know how to negotiate sand dunes and piles of rocks," Squyres said, "and perhaps more importantly, how to avoid them."
That experience will help with future missions, such as the upcoming Mars Science Laboratory, currently slated to launch in 2011.
The snags hit along the sustained mission have left the rovers with some bruises. Spirit, for example, has been driving backwards since one of its wheels jammed in 2006, and a broken electrical wire has reduced the movement capabilities of Opportunity's robotic arm.
Spirit also had a glitch earlier this year that caused it to not report in to mission controllers as planned, but eventually the rover resumed normal behavior. Spirit was on its way to its next target, von Braun, a cap-rock about 800 feet (250 meters away).
Opportunity has fared a little better; it's been "the lucky vehicle since Day 1," Squyres said. However, the rover did recently adjust its course to get around obstacles while working its way toward its next destination, Endeavor Crater, some 7 miles (12 kilometers) away.
On the horizon
Endeavor will be the largest crater that Opportunity has yet investigated; it is about 12 miles (20 km) in diameter and hundreds of meters deep.
"Endeavor is an intriguing target because the rocks close to it look different from the ones surrounding the other craters Opportunity has visited," said deputy principal investigator Ray Arvidson of Washington University in St. Louis. "Part of Endeavor crater's rim is sticking up ? Mars' ancient bedrock exposed ? and rocks nearby may be suggestive of acidic lakes on Mars' surface billions of years ago."
The trip won't come without its costs though.
"We'll have to double the odometer reading on a five-year-old vehicle to get there," Squyres said. "And it will take at least two years to reach it."
Opportunity can manage about 300 feet (100 meters) of traveling on an average day.
"It'll be a long march across the plains, but it will be well worth it," Squyres said. "The deeper the crater, the older the history of Mars we can look at."
Spirit won't have quite as long a trip as its target is a bit closer, but the journey will still take a few months. Spirit's mechanical issues make its trip a little trickier.
"Spirit is the more challenging rover to operate," Squyres said. "There's not as much wind at its location to clean the solar arrays, and that affects the vehicle's power."
Luckily, Spirit recently got a power boost when some Martian winds wiped some of the dust off its solar panels.
"Also, Spirit has to travel more challenging terrain," Squyres added. "The rocks and loose sand at Spirit's location are treacherous. Of course, to top if all off, Spirit is driving backwards."
But the trip to von Braun and its subsequent target, a feature dubbed Goddard, will be worth it.
"Home Plate, where Spirit spent the winter, is a volcanic structure eroded down so we can see the layers," Arvidson said. "And we think von Braun and the neighboring Goddard structure may be made of the same stuff."
The rocks that Spirit has checked out show evidence of "water-charged explosive volcanism," Arvidson said. "Such areas could once have supported life."
Predicting the end
Even with the periodic glitches and mechanical issues encountered so far, the rovers have stood up to the test.
"It's like a good old car that keeps on running," Arvidson said.
And while mission controllers are aware of the limitations of the rovers, it's almost impossible to predict when the rovers might stop for good.
"They're so far out of warranty it's hard to predict," Arvidson said.
The most vulnerable parts of the rovers are any parts that move, Arvidson told SPACE.com. The rovers are also exposed to large swings in temperature throughout the day that stress their systems.
Though the rovers will be checking out rocks and other features on the way to their primary targets, the team will also likely minimize the movement of certain rover parts needed to investigate the Martian environment, Arvidson said.
"We want to save those capabilities for the really juicy targets," he said.
Many mechanical issues can be worked around too, as Spirit is currently doing with its broken wheel.
"We've got an enormous amount of functional redundancy built into these vehicles," Squyres told SPACE.com in an email. "So a lot can go wrong and still allow us to do good science."
Predicting how fast different mechanical systems will wear out is difficult though, and of course that's not the only limiting factor.
"If nothing mechanical stops us, then sooner or later the capacity of the batteries will degrade to the point where they no longer can hold enough charge," Squyres said. "But we don't know how long that will take either."
If the rovers get to the point where their roving days are over, they could still operate for a time as a weather station, something like Viking 1 did when its primary mission was finished, Arvidson said.
"If we lose mobility completely, then yes, we can poke around at the place where we stop," Squyres said. "But if the scenery isn't changing, sooner or later you reach a point of diminishing returns."
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