A second wheel may now be broken on NASA鈥檚 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鈥檚 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 鈥� it seized up permanently in 2006.
Now, Spirit鈥檚 right-rear wheel is also having problems and may be permanently disabled.
Advertisement
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 鈥� 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.
鈥淭hat鈥檚 troubling,鈥� says rover project manager John Callas of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淚t鈥檚 conceivable we鈥檙e witnessing a deterioration on the wheel.鈥�
If the wheel cannot be coaxed into working again, then Spirit will likely be trapped forever, Callas says.
Low light
鈥淚t was questionable whether we could get a five-wheel-driving rover out,鈥� he says. 鈥淚f we have a four-wheel-driving rover [with] only one driving wheel on the right-hand side 鈥� then extracting the rover from its current embedded location is unlikely.鈥�
Spirit could die if it remains stuck when winter arrives six months from now at the rover鈥檚 location in Mars鈥檚 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.
No power
But Spirit鈥檚 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.
鈥淚f we extrapolate current dust accumulation rates out to the next winter six months away, it does look troubling,鈥� Callas says. 鈥淭here鈥檚 a real possibility Spirit would not have power to survive the winter at its current attitude.鈥�
Further tests were to be performed on Tuesday to try to determine the source of the wheel鈥檚 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.
Stock still
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鈥檚 radio signals as a way of measuring the motion of Mars itself. Measuring slight wobbles of the planet鈥檚 spin axis could reveal details of the world鈥檚 internal structure, but the rover needs to be stationary to get precise measurements.
The team would also monitor the rover鈥檚 accelerometers, searching for vibrations due to small meteorite impacts on the Martian surface.
Another of NASA鈥檚 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.
