Spirit Sol 206

Oh, damn it. I'm there when the downlink starts, and one of the first things we see is an indication of another goal error. So at least the rover thinks we didn't make it.

Scott Doudrick is sitting to my right, monitoring the incoming data stream. "Bus voltage is 26.9," he says. "It didn't go very far."

But then we get the rover's reported coordinates, and they're only about a meter away from the target. Maybe we're not in such bad shape after all. The rover apparently entered a limit cycle, which means it was unable to find the goal and danced around it. Eventually, it was stopped by the time-of-day limit.

What the hell happened?

The mid-drive and post-drive images clear up the matter. Just before it took the last step -- what should have been the final 2m of driving -- the rover was perfectly positioned, with the outcrop dead ahead. (Well, dead behind -- we were driving backward. But right where it should have been, anyway) The next image shows the rover two or three meters downslope of the target, with a lot of back-and-forth tracks visible in part of the image.

It's a bug. That final step of the drive told the rover to go to a Cartesian position 2m behind it, but the rover was unable to find that position because steep slopes confuse it. So it roverdanced, trying to find the right spot. This wouldn't have taken it so far from the target by itself, but the rover kept slipping downhill as it went -- most of the resulting position error is from slip. (Or at least this is my explanation. John has a different one. Pointing to a nearby copy of the _Weekly World News_ that shows a family of Martian Eskimos, he deadpans, "I think the Eskimos did it.")

In retrospect, we could have just told it to back up 2m, rather than the more complex command we used. But if the rover hadn't started from the right spot, the more complex command might have worked better. It was a gamble, and we lost.

So now we're about 3m downslope of our target, and have to spend another sol climbing uphill.

Which makes it twice as much of a shame that the drive there is turning into a huge pain, but at least Squyres -- who's as eager as anyone to get there -- has a good attitude about it. "We're all of nine meters above sea level," he notes, "but in the annals of interplanetary robotic mountaineering, this is an unprecedented achievement."

So we'll keep trying.

Courtesy NASA/JPL-Caltech. The Clovis outcrop, perfectly positioned smack behind us -- right before Spirit becomes confused and roverdances downhill.

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