Since Spirit has been travelling backwards for a long time, dragging its right front wheel behind it, it's easy to lose track of orientations and what's going on where. The image below is taken from the front of the rover, but looking backwards, and the disabled wheel, the right front, is at the right. When I was reading about the failed attempts to move the rover, as I reported in the previous post, I hadn't realised that the stalled wheel that ended the test was the right rear wheel that had, up until now been functioning reasonably well. It now seems very likely that the right rear wheel has ceased to function and this leaves Spirit with only four working wheels, three of which are more or less buried.
It's therefore, as the New Scientist reported a couple of days ago, looking even more grim:
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.
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."