It could take week of tricky maneuvering to rescue the Mars rover Spirit from its current predicament. Spirit is stuck in an "insidious invisible rover trap," as described by Steve Squyres, lead scientist for the Mars Exploration Rover Project.
The trap is otherwise known as sandy soil, out on one of the red planet's plateaus. But project managers are prepared to keep at it for as long as needed and have no plans to abandon or retire the rover.
On May 6, Spirit was continuing its journey around a low plateau called "Home Plate," when it hit the trap.
Thick deposits of dirt lie just below the surface, covered and camouflaged by the normal layer of dust that blankets the Martian surface. When a rover's wheels hit these patches of dirt, "the wheel can't grip" and just spins, Squyres explained.
This is exactly what happened in Spirit's case, with the rover becoming mired up to its hubcaps, prompting the rover team to suspend all driving plans and work on a way to get the rover out of the mire.
"Spirit's in a tough situation," said project manager John Callas, of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "It will probably be some number of weeks of tests" before Spirit becomes unstuck, he said.
Getting Spirit unstuck is difficult for several reasons. For one, scientists don't know the exact make-up of the dirt the rover is stuck in.
The team is having Spirit sample its surroundings to give them some clues.
They're also going to see how the test rover back at JPL reacts when its wheels are put in a sample of ground basaltic rock called "baghouse dust." If it spins its wheels like Spirit, they can use the Earth twin to test out ways that Spirit might be able to dislodge itself. If it doesn't they'll have to try out other soil analogues, including the one they used when Spirit's twin, Opportunity, got stuck in a feature called Purgatory Dune in 2005.
"We're trying to look at everything," Callas told SPACE.com.
But there are a few issues that are complicating the rescue effort.
One added problem in Spirit's case is its hobbled right front wheel, which has been dead for three year. That dead weight makes it harder to use Spirit's five remaining wheels to pull the rover out.
"We're dug in pretty deep," Squyres said.
Spirit's left middle wheel has been stalling, further adding to the difficulty of the task. Callas said the team suspects there could be a rock stuck in this wheel. They're hoping the wheel will start spinning again if the rover is moved in a different direction.
A rock could also be stuck underneath the lander, touching its belly plate, which would also make moving the rover out more dangerous and difficult.
Rover managers have worked out a possible way for Spirit to look underneath itself with its robotic arm and practiced the maneuver with Opportunity.
One piece of good news is that gusts of wind have come along in recent days and cleaned off the dust on Spirit's solar arrays, giving the robot a power boost.
"So what this does is it buys us time," Squyres told SPACE.com, allowing mission managers to take their time in finding a solution to free the rover.
And they'll likely need that time. But that's expected, he noted, as it took several weeks to free Opportunity from Purgatory Dune.
Of course, if Spirit does get freed, there's always the chance it could stumble into another trap. The area is like "walking in a minefield," Callas said.
No retirement for rovers
Squyres acknowledges that there is a chance they won't be able to get Spirit unstuck ? the rovers have far outlasted their planned lifetime and could have technical failures at any point (Spirit has actually experienced mysterious bouts of amnesia and restarts in recent weeks.)
"It's Mars, these are old rover, anything could happen," Squyres said.
And there's the chance that permanent damage could befall one of the rovers other wheels, hobbling the robot even more.
But Spirit won't get permanently stuck for lack of effort on the part of mission scientists.
"If there's a solution, we will find it," Squyres said.
"We'll probably always keep trying as long as we can," Callas agreed.
And the rover team members are adamant that it's not yet time to retire the rovers.
"They're still very functioning vehicles," Callas said. "There's still so much we can do."
Squyres agrees, arguing that because the rovers have gone so far past their missions, it's relatively cheap and easy to keep them operating, and the science to be learned is of such interest that there's no reason not to keep them going.
"Why not push them?" Squyres said. "We've got nothing to lose. We can push them as hard as we want to now."