The famed Mars rover Spirit has been stuck fast since spring in a patch of slippery Martian soil, five of its wheels partially buried in sediment. When engineers tried to coax the rover out of its sand trap, it only dug itself deeper.
Now the rover's drivers are starting a new push to free Spirit. This time the robot's moves have been carefully honed at a NASA test lab here that precisely mimics the spot where Spirit is trapped. It's too early to say whether the new attempts to extricate the rover will be successful. But if they are, the sandbox will deserve much of the credit.
"Without it, we'd be just shooting in the dark," says Bill Nelson, engineering team chief for the Mars rovers at NASA's Jet Propulsion Laboratory. "We'd have to do our experiments on Mars. ... Better to make your mistakes here."
Efforts to free the rover are on hold for the moment while the rover team tries to figure out why one of Spirit's six wheels started stalling late last month. If the wheel starts working again, NASA will once more try to heave the rover out of its sand pit. Engineers should get test results on the balky wheel in the next few days.
The spot Spirit blundered into in April is a potential deathtrap. Its treacherous sand, the texture of finely milled flour, is so slippery that the rover's wheels can't gain any traction. Worse still, Spirit is sitting atop a boulder that could jam into the robot's undercarriage. If that happened, the rover would never rove again.
To help plot the rover's escape routes, scientists and engineers concocted a mixture to mimic the Martian sediments: 3,000 pounds of diatomaceous earth, used in swimming pool filters, and 3,000 pounds of dry potter's clay. Then they combined those ingredients in a mixer that contractors use to prepare plaster for homes.
Wheelbarrows dumped the dusty stuff into an 8-foot-by-20-foot sand box, where it was groomed to the right slope. Laboratory staffers regularly fluff it with rakes and tamp it down by treading on a plywood board placed atop the pile.
NASA drivers have commanded duplicate rovers through more than 40 test runs in the sandbox, trying out every possible individual and combination of moves. They learned some valuable lessons: For instance, wiggling the rover's wheels will help it make progress; both forward and sideways motion could help it escape; and despite its advanced age, Spirit is still up to the task of rolling uphill, which will be the easiest way off the slope where it's trapped.
Even more important, rover caretakers also discovered that one of the moves they considered would actually send Spirit downhill into a crater, where all its wheels would get embedded.
That's the beauty of the sandbox.
"We can make mistakes here," Nelson says. "But Mars is not forgiving."