Opportunity Sol 276 (Spirit Sol 297)

Ashitey's drive went just about perfectly over the weekend, leaving us just where we wanted to be: on a broad lane of exposed rocks, like a cobblestone highway leading us straight to Burns Cliff.

But our first stop is the Bermuda Triangle, or Triangle of Doom. This is a broad triangular (duh) expanse of soft sand -- seemingly the same kind of nasty stuff we were caught in for the last a couple of weeks -- whose apex crosses the highway ahead. We're not sure we can cross it, so our first step, over the next couple of days, will be to drive to it so that we can investigate it up close. Whether we can cross it or not, we'll be able to get better pictures of the cliff from there, and maybe decide whether we want to proceed or not.

The triangle is only 15m away, the kind of distance I'm used to thinking of as a single sol's driving -- and an easy one, at that. Of course, there's a catch: at the most recent traversability meeting, Jim Erickson said he didn't want us driving farther than about five meters per sol. The rationale for this is a good one. We don't have a lot of experience driving on this exact terrain, and the price for failure is very high: if we slip off of the rocks into the soft sand downhill, there's no guarantee that we can ever get uphill again, or indeed out of the crater. Even if we can, we'll squander a lot of precious time trying. So I don't disagree with the rule, but if we follow it strictly, it'll take us three sols to do a single sol's worth of driving.

I think I can get it down to two.

I propose my idea to Andy and Frank. "I think we can pick a spot five meters out where we can say, if the rover makes it here, we'd be willing to push a button to say to go another three meters on the same heading. So why not tell the rover that? Pick a point to serve as our prediction for the end of the nominal drive, and tell the rover that if it's within some tight radius of that point, go another three meters."[1]

That automatically cancels out the biggest objection to longer drives, namely, the a priori uncertainty about the rover's behavior in this terrain. If we don't make it to the predicted point, we simply don't do anything additional. But if we do, then we know our predictions are reasonably accurate, and it's reasonable to extend the drive in that case. And in the admittedly unlikely event that we can do this two sols in a row, we can make it to the triangle one sol sooner.

Frank and Andy agree readily. But that leaves us with a project manager to convince. Andy calls Jim and asks him to join us downstairs, and Frank starts pulling up the relevant imagery so we can make some good choices about drive distance and an appropriate radius for the circle. This means we're ready for Jim's questions, which is probably part of the reason we're able to sell him on the idea without a lot of trouble. (Well, "sold" is relative. His official response, delivered with his characteristic suspicious squint, is: "Okay, let's do something dangerous." But in Jim Erickson terms, that's sold.)

So that's our plan: five meters of nominal driving, with an optional three-meter extension at the rover's discretion. The radius of the circle varies through the day. When I we talk to Jim, we're thinking in terms of 25 or 30cm. But the more we look at the terrain, the wider the circle grows, reflecting our increasing confidence -- 50cm, 70cm, 75cm.

Then Squyres gets into the act. He's eager to get to the triangle and loves the idea of pushing the drive as aggressively as possible. He also recognizes that the larger the circle is, the more likely the rover will be within it and decide to drive farther. So he pushes for a bigger circle, and gets us to go back to the drawing board and bias the drive uphill so that we have a larger safe zone. The circle's eventual size: 90cm.

Part of what bounds the circle's size is whether we'll end up on rock. But there's another limiting factor. If we're expecting (say) 20% slip, then we don't want to choose a circle that would allow 50% slip, even if the rover would be safe anywhere within that larger circle. The whole point of the extension is that if the rover ends up anywhere in the chosen zone, it's okay to do another three meters like the first five. But if it slips far more than we expected, then even if it's currently safe, the added slip means we don't understand what the vehicle is doing, and so we don't want it to drive on without our stopping and taking a look at the situation. I think Steve wanted a larger safe zone (indeed, I know he did), but he understands our logic and acquiesces.

After the CAM, he apologizes -- graciously, though unnecessarily. "Hey, I'm sorry for pushing you guys on the drive stuff."

"No problem," I reassure him. "It's always good to have more people we can blame if anything goes wrong."


[1] Another example of just inventing a technique on the fly and seeing it suddenly become standard practice. Now we do this kind of thing all the time. I'd forgotten that there was a time when it was novel, not one of those background "of course we do that" things.

1 comment:

Anonymous said...

Are there patents on any of this? It seems like "a method for driving a vehicle on another planetary body where a second maneuver is initiated if the onboard navigation system indicates the first maneuver completed successfully" or something like that would be neat to have your name on.