I don't have much to add to this "Free Spirit" update posted this evening to JPL's website, except, wow, it's really come to using the arm (though not for digging or pushing):
The list of remaining maneuvers being considered for extricating Spirit is becoming shorter. Results are being analyzed Wednesday, Jan. 13, from a drive on Sol 2143 (Jan. 12, 2010) using intentionally very slow rotation of the wheels. Earlier drives in the past two weeks using wheel wiggles and slow wheel rotation produced only negligible progress toward extricating Spirit.
The right-front wheel has not rotated usefully since Sol 2117 (Dec. 16, 2009). With the right-rear wheel also inoperable since Sol 2099 (Nov. 28, 2009), Spirit now drives with only four wheels.
Pending results of the latest drive, the rover team is developing plans for their final few attempts, such as driving backwards and using Spirit's robotic arm to sculpt the ground directly in front of the left-front wheel, the only working wheel the arm can reach. Such activities may take several sols to implement, but time is getting short as winter approaches and the team needs to focus on Spirit's winter survival.
The amount of energy that Spirit has each day is declining as autumn days shorten on southern Mars. If NASA does determine that the rover will not be able to get away from its current location, some maneuvers to improve the tilt toward the winter sun might be attempted.The "using Spirit's robotic arm to sculpt the ground" bit is the hail-Mary pass from the underdog team trying to extricate Spirit. Using Spirit's precision-tool robotic arm to do anything that is not science jeopardizes future science; this is a last-ditch effort. This really must be pretty much the end of their list.
Of course, using the arm to do anything, including science, jeopardizes future science; so late in the mission, and being mindful of Opportunity's broken shoulder motor, the arm motors have to be regarded as "consumables." Still, using it to do anything that's not science is, at least, trading not-science for science, and depending on how hard they push it, could shorten its life further. But I've got no idea what they really mean by "sculpting." In the past, they've used the arm to push gently on the surface to see how resistant the soil is to being packed. It does seem that, to be of any use at all, they have to get the arm pretty darn close to the wheel, which is something that I am guessing is giving the rover drivers jitters. Anybody out there have their fingers on a photo that shows how close either rover has allowed its arm to get to either front wheel in the past?
But, I feel I always have to issue this reminder -- this isn't like Phoenix' waning days. Spirit's still got a highly useful set of science tools (including several on the tip of that arm, so let's hope nothing happens to any of its motors in support of extrication). There's still lots of work she can do even if she's permanently stuck.
Meanwhile, in a bit of happier news from Mars, Opportunity has finally wrapped up two months of work at Marquette and has hit the road again. I am sure the science from Marquette will be fascinating, and I hope to get an update on what exactly that was at the Lunar and Planetary Science Conference in March. But Opportunity's our roving rover, and it's past time she kicked up some dust again. Let's make tracks for Endeavour!