In the time since I last wrote an update, we've passed a couple of really major milestones. The first, of course, is that we've now been on Mars for one full martian year. This, to me, is maybe the most significant milestone we've hit. The others, like 90 sols, one Earth year, and so forth, seem kind of artificial, since they really have nothing to do with what's happening on the planet where the rovers live.

But this one means that we've now seen Mars in all its seasons. The seasons really do change in a very substantial way on Mars, especially if you depend on solar power for survival. In the months ahead we know that we can expect hazy skies to clear, but we also know that the temperatures will drop dramatically, especially at the Gusev site, and that the sun is going to sink lower and lower in the northern sky. It shouldn't be too big a deal at Meridiani, but for Spirit we're starting to think real hard about where we're going to spend the winter, and making plans to get there in time to stay alive.

The other big milestone is that we've finally published a big batch of very detailed papers about the rocks at Meridiani, in a special issue of the journal Earth and Planetary Science Letters. This was a long time coming. It's very hard to get science done and get papers written at the same time we're doing flight operations every day! But the papers are out now, and they provide a much more comprehensive description of what we've found at Meridiani than we were able to do in the very short papers that we published more than a year ago. Next up will be another big volume of papers about the Spirit site, to be published in the Journal of Geophysical Research in the next couple of months.

On Mars, Spirit is well down off of Haskin Ridge and heading for Home Plate. On the way, we've stopped off at three big outcrops, named Larry's Bench, Seminole, and Algonquin. Algonquin is the one that we're parked at now. They're all similar to one another, and all dramatically different from anything we've seen anywhere else. The rocks have undergone very little alteration, and contain a lot of olivine... much more olivine than we've seen anywhere else.

Next stop should be Comanche, a big and very red outcrop between us and Home Plate. It looks like Comanche may be different from Algonquin and the rest, based on Pancam and Mini-TES data... we'll see what we see when we get there.

Over at Meridiani, progress has been slow. A couple of weeks ago, we encountered a problem with the IDD, and we're still dealing with it. The IDD on Opportunity, you'll recall, is the one that has a stuck heater on the shoulder joint. What this means is that the shoulder has seen much larger temperature ranges than any other piece of the rover. Motors can encounter problems if they undergo too much thermal stress, and we found a couple of weeks ago that the "azimuth" joint on the shoulder - the one that swings the arm left and right - wasn't moving when we asked it to.

When something like this happens, you proceed carefully, since you don't want to make the problem worse simply because you don't understand it yet. So the engineers at JPL have been methodically working their way through various fault scenarios, using tests on Earth or tests on Mars to knock them off one by one until only one's left.

At first, we feared that it might be a mechanical problem, perhaps like the one that incapacitated the right front steering motor on Opportunity. But we've ruled out all the plausible mechanical issues and narrowed it down to an electrical problem, probably within the motor itself. More analysis is needed, but it seems pretty likely that the problem is a consequence of deep thermal cycling.

We're not out of the woods yet, and I'm not making any predictions. However, the good news is that late last week we got the motor to move properly. It was just a small move, but by adjusting some of the parameters that control the motor we were able to compensate for what we think may be the problem inside the motor, and get it to move just the way we asked it to. We're going to continue to take our time with this, but I have high hopes that you may be seeing the IDD deployed out in front of the rover again before too long. We may have to operate it a big differently from here on out, but I'm also optimistic that we'll be doing science with it again pretty soon.

I'll try to do better with updates during the weeks ahead...

This article was written for Cornell University's Athena Mars Exploration Rovers website.

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