And then we'll have other stretches where everything seems to work exactly the way we want it to. This week has been one of the latter ones.

Our drive last weekend with Opportunity was perfect, and it put the part of Purgatory Dune that we wanted to study right into the arm's "work volume" - the territory that the arm can reach.

We then developed a very aggressive three-sol plan to learn everything we can about the dune: Microscopic Imager, APXS and Moessbauer in the old wheel tracks, MI and APXS out on the surface next to the tracks so we have something to compare it to, and Mini-TES to tell us about the thermal inertia of the soil.

We're two sols into our three-sol campaign now, and so far it's gone perfectly. I wish I could really describe here the great job the Opportunity engineering team has done this week, managing power down to a tenth of an amp-hour each sol so we can squeeze every available drop of science out of the vehicle.

The sol we planned today will wrap it up, and the sol we're going to plan tomorrow will be the one we've waited two months for... driving away from Purgatory Dune.

So, if you're watching our images, in another few days you'll see Opportunity on the move again, at long last. And if you watch closely, you'll see something that might freak you out a little... we'll be driving north.

No, we haven't decided to turn tail and run away from Erebus Crater. But after a careful study of all the images we have, we've concluded that the best way to map a path to the south is to start by going north a little bit, and taking some pictures off toward both the east and the west.

Those images will complement ones we have already, and should allow us to plan the best possible southward path. So there'll be two or three drives to the north first, a lot of imaging, and then the long hard push south toward Erebus.

Over at Gusev, Spirit continues to shine. We've been really nailing our drives there lately. It hasn't hurt that the recent terrain has been just about the firmest ground we've seen anywhere at Gusev crater.

We don't really understand this, frankly, but if you look at recent Hazcam and Navcam images you'll see that we're barely even leaving tracks, the ground is so hard.

It makes for great traction and great climbing. I'm still not confident that we'll make it to the summit, since the images seem to show a transition to softer ground with more loose rocks ahead. But we're enjoying the conditions while we've got them.

And to make things even better, we seem to have found a nice piece of layered bedrock. This turned up right underneath the rover early this week after one of our long drives, and the timing was perfect. We've named it "Independence Rock", and we've maneuvered into position on it to do some arm work over the coming holiday weekend.

We like to try to give the operations team a few days off on holidays, but of course we need to keep the rovers busy every sol. If we're trying to drive every sol we need to come to work and plan every day, since the terrain around the rover is always new and different.

But when we're going to be parked in one place for several sols, we can plan multiple sols in advance and give people a little time off. So the Spirit team planned three sols yesterday and will be planning three more tomorrow, giving everybody on Earth the holiday off but keeping Spirit busy looking at Independence.

Of course we're dying to know what Independence really is. We haven't seen any MI images of it yet, but in the Pancam images it's finely layered, with a strange kind of porous texture. It looks a bit like Peace and Alligator, and a bit like Methuselah.

Methuselah was one of the many high-titanium, high-phosphorous, low-chromium outcrops we saw all over the place on Cumberland Ridge, and Peace and Alligator were a couple of very weird sulfate-rich outcrops that we found our climb up to Larry's Lookout.

I've felt all along that Peace and Alligator were probably the most important rocks we've found at Gusev, and it would be very cool to find another one like them. If I had to bet money I'd bet that Independence is similar to Methuselah, but I've been fooled by Mars so many times now that I think it's better not to make any bets. We'll find out very soon. And then it'll be time to resume the climb.

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