Second stalled wheel may doom Mars rover
A second wheel may now be broken on NASA's Spirit rover, dampening hopes for freeing the robot from a sand trap it has been trapped in for seven months. The injury will also increase the rover's risk of freezing to death in the coming winter. Spirit has been struggling to escape from a patch of soft, sandy soil since April. Its three left wheels are almost entirely buried and have little traction, and its right-front wheel is of no use – it seized up permanently in 2006.
Now, Spirit's right-rear wheel is also having problems and may be permanently disabled.
The right-rear wheel stalled on 28 and 21 November during attempts to start driving the rover out of the sand trap. Each wheel has its own motor, and the rover team commanded Spirit to try to spin the wheel again during a series of tests on 3 and 4 December – but it did not budge.
In the previous stalls, the wheel had at least moved a little bit before unexpectedly stopping. But no motion at all was detected in the tests.
"That's troubling," says rover project manager John Callas of NASA's Jet Propulsion Laboratory in Pasadena, California. "It's conceivable we're witnessing a deterioration on the wheel."
If the wheel cannot be coaxed into working again, then Spirit will likely be trapped forever, Callas says.
"It was questionable whether we could get a five-wheel-driving rover out," he says. "If we have a four-wheel-driving rover [with] only one driving wheel on the right-hand side ... then extracting the rover from its current embedded location is unlikely."
Spirit could die if it remains stuck when winter arrives six months from now at the rover's location in Mars's southern hemisphere, Callas says.
In previous winters, Spirit rested on slopes that angled its solar arrays in a way that captured as much sunlight as possible. That allowed the rover to power heaters designed to keep its electronic innards from freezing.
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But Spirit's solar panels are not currently at a good angle to maximise sunlight. Meanwhile, dust is slowly accumulating on the panels, reducing their efficiency. If the rover is unable to move, it could run out of power and die in the frigid Martian night.
"If we extrapolate current dust accumulation rates out to the next winter six months away, it does look troubling," Callas says. "There's a real possibility Spirit would not have power to survive the winter at its current attitude."
Further tests were to be performed on Tuesday to try to determine the source of the wheel's trouble. A rock stuck in the wheel seems unlikely at this point, since previous jams have not prevented the wheel motor from moving altogether, as was observed in this case, Callas says.
Another possibility is that brushes used to make electrical contacts inside the motor have worn out, which is the leading hypothesis for what permanently disabled the right-front wheel in 2006, he says.
If Spirit turns out to be permanently stranded, scientists have planned a set of observations to do while it remains alive. These include tracking Spirit's radio signals as a way of measuring the motion of Mars itself. Measuring slight wobbles of the planet's spin axis could reveal details of the world's internal structure, but the rover needs to be stationary to get precise measurements.
The team would also monitor the rover's accelerometers, searching for vibrations due to small meteorite impacts on the Martian surface.
Another of NASA's Mars probes, the Mars Odyssey orbiter, temporarily stopped relaying data from Spirit and its twin Opportunity back to Earth in November following a glitch, but the problem has since been resolved.