Months of planning are finally coming to fruition: NASA engineers are ready to begin trying to maneuver the plucky rover Spirit out of its sandy trap on Mars.
Mission managers are sober about the prospects for freeing Spirit. They will send the first commands to the rover to try to move on Monday, "but this process could take quite a while if it's possible at all," said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington.
The new plan will command Spirit to try to backtrack and make its escape using the tracks that were left in the dirt before the rover got stuck.
Spirit has been stuck in a spot of soft, sandy dirt (nicknamed "Troy") on the Martian surface since April, when it broke through what mission scientists call a "dirt crust" — a hard top layer of dirt disguising a layer of soft, talcum powder-like material below.
"Spirit did the equivalent of falling through the ice over a frozen pond," McCuistion said.
An escape plan on Mars
The rover's engineering team has spent months devising a strategy to extricate the spacecraft, working with replicas of Spirit (and its twin spacecraft, Opportunity). That effort is complicated by the fact that Spirit has a bum right front wheel, forcing it to drive backwards.
The team has tried driving the replica rovers in a variety of ways — forward, backward and sideways — in a mixture of dirt similar to that in which Spirit is stuck, to see which movements had the best chance of getting the rover out and which might make the situation worse.
"If there is a way to get the rover out, we'll find it," said John Callas, the rover project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
The plan they'll try first is to get the rover moving "forward" through its own tracks. The rover drivers suspect that moving through the soft material already churned up will be easier than breaking through more of the dirt crust. Retracing its steps downslope also means the rover won't have to try to move uphill, said Ashley Stroupe, a rover driver from JPL.
When moving the rover, the team also has to be careful not to scrap against a rock under the rover's belly. Moving forward should minimize the interaction between the rover and this rock, Stroupe said.
"This is clearly going to be a long process to either get to extrication or determine if extrication is going to work," Stroupe said.
The command to move will be sent up on Monday night, and mission controllers expect to hear back from Spirit about its progress on Tuesday. The command calls for 5 meters (16 feet) of wheel spin, though mission managers don't expect the rover to actually move that far, Callas said. The team will take a day to analyze any movement Spirit makes before sending more commands, repeating this process as long as needed.
"The reality is that we're going to see very little motion each day, at least initially," Callas said. "It's kind of like watching grass grow."
Callas said he has confidence in the rover team and the plan they have as being the best chance to extricate Spirit.
"We're in good shape; we're ready to roll on Monday," he said.
The 'spirit' of Spirit
Mars enthusiasts have been worried that Spirit's predicament could spell the end for the rover's mission, which so far has lasted 24 times longer than its original planned 90 days.
Also of concern are Spirit's periodic memory lapses, the most recent of which occurred on Oct. 24, preventing Spirit from saving science observations in its flash memory.
Spirit's sticky situation hasn't been all bad though: Mission scientists have used the forced downtime to conduct science observations of the site, which has turned out to be one of the most interesting that the rovers have encountered on Mars.
"Of course no place is a nice place to be embedded, but this turns out to be a geological treasure trove," said Ray Arvidson, deputy principal investigator for the rovers from Washington University in St. Louis.
While the rover has detected sulfate sands in a spot (called Cyclops_eye) where it punched through with its robotic arm, it didn't detect these at another nearby spot (Polyphemous_eye).
Spirit and Opportunity have been roaming the Martian surface for nearly six years now, after landing on different sides of the planet in January 2004.
McCuistion asked fans of the rover to be "hopeful but realistic" about Spirit's prospects.
If Spirit can't get out, "it's likely that this lonely spot straddling the edge of this crater might be where Spirit ends its adventure on Mars," he said.
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