As I mentioned briefly earlier, there was a press briefing today that announced the official end of efforts to extricate Spirit from her sand trap at Troy. Instead, the rover drivers will now focus on improving the chances that Spirit will survive the coming winter so that she can carry on doing science once the power situation improves in the spring. There is new and interesting science they can do with a stationary lander.
But before I get to details on all of that, a few words about the politics of today's announcement, as far as I understand them. Spirit is a rover. She is the first spacecraft on Mars ever to have traveled beyond the horizon of her landing location to explore new, unseen territory. Throughout her mission, there has been continuous tension within the mission about how much to drive her. Every operational day, there'd be some people arguing to stay right where she was to study the interesting rocks at hand, and others arguing to keep pushing her toward the promise of new vistas.
Spirit's been roving on Mars for more than six years. Ten months ago, she got trapped in sand, and since then, although there has been fascinating science performed from her parking spot, the mission's main goal has been to try to get her out. And I am sure that the engineers, at least, who show up to work every day to drive Spirit, want to keep pushing and pushing, doing everything they can to salvage Spirit as a rover.
But, with little progress to show from that 10 months of work (although there has actually been surprising progress in the last few days), the order has come down from on high: stop trying to drive. Let's be clear: the ultimate decision on whether to keep Spirit driving or stop trying to extricate her did not come from within the mission, it came down from NASA Headquarters. When I got my chance to ask a question at today's briefing, I asked for clarification on whether, assuming Spirit survives the winter, they'll continue extrication efforts in the spring? Neither Ashley Stroupe (rover driver) nor John Callas (project manager) answered the question: the question was answered by Doug McCuistion. He replied:
Right now the rover is embedded in the Troy location. We do not believe that it is extractable. If it is, being a four-wheel-drive rover instead of a six-wheel-drive rover and the two wheels that John mentioned failed are both on the same side, we couldn't really make any headway....We believe that the mobility of this rover is complete and that extrication may be by accident but right now our plan is to worry about getting through the winter and then enabling this science campaign. The geophysical radio science is really a high priority of the science community. We've never done it before.
Wait. What? How can the rover not be extractable, yet still leave the possibility out there for accidental extrication? It's even more puzzling given the very definitive tone of the statements in today's press release:
After six years of unprecedented exploration of the Red Planet, NASA's Mars Exploration Rover Spirit no longer will be a fully mobile robot. NASA has designated the once-roving scientific explorer a stationary science platform after efforts during the past several months to free it from a sand trap have been unsuccessful. The venerable robot's primary task in the next few weeks will be to position itself to combat the severe Martian winter. If Spirit survives, it will continue conducting significant new science from its final location.
No mobility. Once roving. Stationary. Final location. And yet recent drives actually show the most motion from Spirit since she got stuck; and even now, there are more drives planned to attempt to increase the northward tilt of Spirit's solar panels, and future driving (assuming she survives the winter) could even accidentally extricate her.
What gives? What we're seeing is that NASA -- not the rover mission, not JPL, but NASA -- is calling the shots on this decision. They've watched the extrication efforts proceed and have now decided that enough is enough; so they have issued orders to the mission to change Spirit's goals from ones that involve roving to ones that involve being a stationary lander. So the ground rules have now changed for the Spirit side of the Mars Exploration Rover mission. Do not attempt further roving. Instead, keep the rover alive through the winter and then do lander science.
What's funny is that the next day of Spirit's operations is really not going to look any different from recent days. They are still driving the rover, after all -- in order to do science in the spring they've got to get Spirit to survive through the winter. And to do that, they've got to do their best to get the solar panels tilted northward. Spirit is located almost fifteen degrees south of Mars' equator, and Mars' rotational axis is tilted by 25 degrees. So at winter solstice, the Sun only reaches a maximum elevation of fifty degrees above the horizon. To make the most of the weak midwinter sun, Spirit would like to have solar panels tilted perpendicular to that slanting sun. That's not going to happen. But, as a rule of thumb, for every degree of northward tilt they do manage to achieve, John Callas said today, they pick up about five watt-hours of precious midwinter electrical power from the panels.
Just a few weeks ago, Spirit's panels were actually tilted southward. But the recent drives, in which Spirit had reversed direction and tried driving backwards (in a rover sense, that is, meaning southwards), have been popping especially the left-rear wheel out of its sand trap, dramatically improving the tilt situation, to the point that the rover's pitch (the front-to-back angle of her body) has flattened to zero. She is still significantly rolled to the left, and for reasons she didn't explain, rover driver Ashley Stroupe said it'd be better if she was rolled right; but that pitch improvement is really really good news. There's no reason to believe that they won't be able to continue making some improvements during the remaining few drives that they will be able to pull off before they run too low on power, which Callas said would likely happen in about three weeks.
After that, Spirit will be able to continue doing low levels of science for a while, likely taking pictures and returning them to Earth into March or possibly April, Callas said. But the solstice doesn't come until May, and, just as on Earth, the lowest temperatures happen after the solstice. Callas said that Spirit needs to be able to generate about 160 watt-hours of power with her solar panels in order to maintain Earth control.
What will happen in the depths of winter, if the solar panels can't make 160 watt-hours per day? Every day that Spirit is active, before she goes to "sleep" for the night, Spirit sets an internal timer. When the timer expires, the rover rouses just enough to check the status of her batteries. If they are charged above a minimum level, she'll wake up further, enough to attempt contact with Earth. If, however, the batteries lack enough juice, she'll hit the snooze button and go back to sleep for another day. Keep in mind that the transmitter is a pretty power-hungry device. Callas said that if the tilt doesn't improve with the next few driving efforts, Spirit could spend six months in this sort of hibernation.
If Spirit goes in to this hibernation, we won't hear anything at all from her for months. That's going to be worrying. But Callas cautioned that this is not at all like Phoenix's end-of-mission situation; despite Spirit's silence, she's actually still electrically active and taking active steps to conserve power for a sunnier day.
Still, during that time, things will get awfully cold. Spirit's electronics are designed to be used at temperatures above minus 40 Celsius, and to survive temperatures down to minus 55. Callas said they don't expect temperatures to drop that low; they think temperatures will bottom out around minus 45. But although the low temperatures are within Spirit's design limits, he was careful to emphasize that those design limits were for a brand new rover; one that's been through the temperature extremes of three Martian years already may not be robust to these low temperatures.
But, if Spirit survives the winter lows, and spring comes and the Sun returns and the rover powers up again, what happens next? Principal investigator Steve Squyres, ever upbeat, explained that a surviving but sedentary rover presents an opportunity for a brand-new kind of science. He said he was most excited about what we would learn by performing radio tracking of Spirit for six solid months. If Spirit did not move, its position as determined through radio tracking would reflect the motion of Mars through its orbit -- something that is exceedingly precisely known -- plus the spin of Mars on its axis. Mars' spin axis wobbles very, very slightly, with those wobbles being excited by other planets and its own moons. This, too, can be modeled, but the way in which Mars wobbles depends in part on whether it has a liquid outer core, like Earth does, or is solid all the way down. Six months of tracking requires the rover be sitting still, with power enough for its power-hungry transmitter, for six months, so this experiment will not begin until well after the winter solstice, again assuming Spirit survives another winter.
But if Spirit survives another winter, and is commandable, then...will they really not try to drive her again? NASA's pronouncements today indicate that Headquarters, at least, does not want to see further driving efforts; they want to have new science to show for their continued operation of Spirit, and they believe the best way to get that is to tell the mission not to drive her anymore.
What do people within the mission think? I'm sure there are conflicting opinions; some scientists who are relieved to be back to work, others mourning the loss of mobility, maybe some doing a bit of both. I expect the rover drivers are probably the saddest, maybe even angry. I don't know, I can't speak for them.
What do I think? It's a sad day, but one I expected. As a scientist, I'm really excited about the opportunity to take advantage of the amazing longevity of Spirit to find out if Mars' core is liquid or not. But when spring comes, and the rover is still with us (because she will be!), I don't think I'll be able to keep myself from hoping that I'll yet see those wheels pop out of the soil and the rover skidding and sliding ever further down the west valley. She's a rover, dang it. She should rove.
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