Easier said than done, it turns out. What was happening on Mars may have looked fairly benign as the Hazcam images were coming down, but this IDD campaign was one of the hairier things we've ever tried with either rover.

The initial approach wasn't too bad. We were on familiar ground, and the rover planners managed to put Hillary within reach of the arm with just a couple of very sweet drives down off the summit. But our position at the end of that second drive looked pretty dodgy: A very steep slope (we were pitched up more than 27 degrees) and some very loose-looking crud under the wheels.

In a position like this, nobody was real comfortable about deploying the IDD, because we had no way of knowing how stable the vehicle was. Swinging the IDD out changes the center of gravity of the rover, and if you're on really unstable ground that could cause unanticipated motion of the vehicle, potentially whacking delicate components (like the instruments on the arm) into rocks.

So, before deploying the IDD, we did a set of "wheel wiggles"... turning the front and rear steering actuators back and forth, just to make sure the thing had settled properly. Unfortunately, with each wiggle, the rover moved! It shimmied back down the slope something like 2 cm with each wiggle. A little motion isn't unexpected when you wiggle wheels on a slope this steep, of course, but it wasn't exactly a confidence-builder. We weren't sure whether it was safe to put the instruments on Hillary or not.

So what to do? This was a crucial rock, and we really didn't want to back off from it. At the same time, we had to know whether or not it was 100% safe to put the IDD instruments onto it. The scheme we came up with, in a very long day of planning, was to deploy the IDD partway -- to a position where it was extended, but where there was zero chance of the instruments touching the rock even if the rover shifted. Then, the next morning, we'd see how much of a shift there had been, and make a go/no-go decision regarding the actual measurements.

The go/no-go came last Saturday, with members of the team calling in from airports, cars, everywhere (I was in my car on the way to Boston at the time). The news was great... the rover tilted only 0.005 degrees as a result of deploying the arm, and it did not slip by any discernable amount. So we were go for the deploy, and over the next several days we did two detailed IDD campaigns on Hillary... one at a spot called Khumjung and the other at a spot called Namche Bazaar. It all worked, and we got everything we hoped for out of it.

The science result, in a nutshell, is that the summit rocks are nearly indistinguishable from rocks we found near Jibsheet, hundreds of meters from here. That includes how they look, and also what they're made of. And the rocks here are tilted at a very different angle from the ones at Jibsheet. All in all, it was a crucial piece of the puzzle in trying to work out the geology of Husband Hill.

From where I sat, the Hillary campaign was one of the MER uplink team's finest hours. In a situation like that, perched on loose ground at a 27-degree angle, at the summit of a mountain on Mars, we're right at the ragged edge of the vehicle's performance capability. The team thought it through carefully, kept the vehicle safe, got the science, and in the process nudged outward the performance envelope of what you can achieve with a robot on another planet. It was a pretty cool thing to watch.

So now we're on our way again, starting the descent of Haskin Ridge. The first move was a very nice, pulling back from Hillary, shooting a little more data on it, and then driving 41 meters to the east. We're on new ground now, and we're going to be seeing some new sights. Spirit has really been on a roll lately.

Unfortunately, it was a week of frustration over on the Opportunity side of Mars. You get good luck and bad luck in this game, just like anything else, and we've had more than our share of bad luck at Meridiani lately.

First there was the unexpected drive partway into something we called Telluride Dune, a little drift that was nasty enough that it triggered our slip-check alarm and stopped a drive. Fair enough; that's the way things are supposed to work when the going gets dangerous in this kind of terrain. No sooner did we back out of that, though, than we got hit by another of the mystery reboots that Opportunity has encountered a few times in the past several months.

This was different from what hit us on Sol 596... that one we understand and can trace to a simple bug in the software. But this one we don't understand yet, and it cost us a couple more sols. And then, just as soon as we recovered from that, we had a minor problem at one of the Deep Space Network stations and lost two more sols. Not a good week.

Each event was benign individually, and each was unrelated to the others. So it really was nothing other than a run of bad luck, for a rover that's had more than its share of good luck. Still, the end result was a week in which we made zero progress toward the Mogollon Rim, so we're going to try to really get things moving in the week ahead.

One good piece of news is that this latest mystery reboot was almost identical to one that happened a couple of months ago... and somewhere in that fact may lie the clue that'll help us figure it out. The reboots do no harm to the vehicle, and we've gotten very good at recovering from them. But they cost us sols when they happen, and that's never a good thing. So we're working hard on this one.

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