Spirit Sol 207

The news starts off good, at least. No errors on the drive, and we turned as expected for comm.

But where the hell are we? It looks like we have a little bit of outcrop under our wheels, and we'll just need another drive to back up a little. But first we need to know where we are.

What happens next is something we call "localization." You look at pictures from the previous sol -- or earlier, if you have them -- and try to figure out where you are relative to that picture, by comparing it to the new picture. Generally, this is a game of hunting rocks, trying to find the same set of rocks in each image to use as landmarks. "Let's see, if this guy here in this image is this guy over here in that image, then there would have to be a pointy rock over there ... but there isn't ... so maybe that guy there is the same as that guy over there ...." It helps if you know roughly where you should be looking, which we do. This is laborious, but can be fun in its way.

This time it's not working out. There's a clump of rocks right in front of us that should be obvious from the previous images, but hell if we can find them. We stare at the images for at least fifteen minutes, with plenty of false starts but no luck.

More images come in, and one of the scientists takes a look at one and says, "You know, I don't think we moved."

Ah-hah. The reason we weren't able to localize the rover was that we were trying to find a solution near where we were supposed to end up. Instead we look at the area near where we started -- and sure enough, there's the rock clump. We might have gained a whole meter.

As more images and engineering data arrive, they dispel the mystery. Looks like the rover experienced about 66% slip through the first part of the drive (already more than we'd planned), then the slope became so steep that it just couldn't climb any further. The more it tried to climb, the more it slipped. In one case, visodom showed the rover experiencing 125% slip -- the rover tried to step 60cm upslope, and ended up 15cm downslope.

In the process -- and perhaps exacerbating the problem -- the left rear (goalward) wheel lost contact with the surface. When this happens, the middle wheel continues to drive goalward, causing the bogie pair to tilt the rear wheel further from the surface. The slope was already too steep, but having only five wheels to climb with sure didn't help.

"Ray, I'm sorry," says Squyres, "but we're going to have to take the rover away from you."

Not that it's Ray's fault, of course. I had the foresight to warn them yestersol that something like this might happen, but I don't feel better to have been right.

Squyres and Ray don't want us to drive nextersol. Instead they ask us to take some time to come up with a plan. John and I talk about it a while, and work something out. We clearly can't fight our way directly uphill. Instead, we can drive switchback -- find a series of gentler slopes leading uphill, mostly transverse to the goal. After a few minutes of searching, we're able to find a path.

We'd have time to implement and deliver that drive thisol, but by then the planning has already proceeded with the assumption that we won't be driving, and it's too late to change it. And anyway, there's a possible problem with the PMA, which means we can't use it thisol, which means we can't do visodom thisol, and we definitely don't want to do this drive without visodom. So instead we just get a jump on the next sol, building the sequences now; we'll execute them a sol later.

We show the switchback plan to Ray, and he nods approval. He doesn't mind that it might take us two or three sols to get back uphill to Clovis. "It's going to be worth it," he says. "This outcrop is gonna tell all." Squyres emphatically agrees: "We drove three kilometers to get to this outcrop."

[Next post: sol 212, August 7.]


Andrew Sackville-West said...

just found this blog today. Awesome. Keep it up!

Anonymous said...

Took a few days but I read it all...good stuff, very interesting.