Cooper had a car accident. I figured it was only a matter of time before this happened to one of us, and I'm glad it wasn't I. The car is a wreck, but his daughter, who was in the car with him, was fine, and so is he. Except that he thinks he has a cracked rib. So he's leaving early.
So anyway.
Thisol is the last sol we'll spend around Purgatory Ripple. We've got two MI stacks to do, then we take off. One of the MI targets, "Cleat Tab," is pretty easy. The other, "Torment," lives up to its name. (I wanted to call it "Cleat Fanta," but I was overruled.) As we exited Purgatory Ripple, the front and rear wheels left separate but overlapping tracks, so that the tracks have a sort of inner wall and an outer wall. Torment is on the interior of the outer wall and we're trying to shoot a stack normal to it, which means at about a 45-degree angle to the surrounding terrain. This in itself is not hard; the problem is that by the time the IDD is close enough to get a focused image, the MI poker will contact the inner wall. This is something we want to avoid.
If we were closer to the target, we could solve it by flipping the wrist into the other configuration, which would put the MI poker on the high side of the terrain. But to reach the target, we have to stretch the arm out so far that that wrist configuration would cause a self-collision.
In the end, we just have to come straight down on it -- not normal to the wall -- and hope for the best.
While I'm doing that, Cooper and Paolo are solving a problem with the drive. The drive itself is quite simple: we just back up 2m across North Ripple, turning as we go to set ourselves up for a northward drive along a trough between two ripples. (Our ultimate goal is southeast of here; we're taking a few steps north so that we can get into position to evaluate candidate routes that would take us around Purgatory Ripple without crossing too many other ripples along the way.)
The problem has to do with the turn. We need to track our position with visual odometry so that we can tell whether we're slipping too much -- it's one of the Rules of the Road for this vehicle in the post-Purgatory era -- but in this sandy, rock-free terrain, the only features visodom can converge on are our own tracks. And we're turning so much during the backup that our tracks won't be in the cameras' field of view the whole time.
Unless we change the camera pointing in midstream. Visual odometry works by comparing successive pairs of images -- you take an image, step, take another image, and compare the two. Since it only needs two pictures at any given time, the current one and the one from the previous step, you can "reseed" it at any point. It's time-consuming, but it works. And we overestimated the drive time, so we'll have enough time for one such reseed. So Cooper and Paolo figure out a pair of pointings -- one for the first half of the drive, and one for the second -- and that's what we go with.
That's all. Except for one more problem that comes up at the CAM. "What if," Emily asks -- "what if the poker does contact the terrain after all?"
I have to admit it's a good question. "We don't expect any contact, but all the moves are in free-space mode. So if we contact the terrain, it shouldn't damage the arm, but it'll fault out the IDD sequence and keep us from stowing, which will also mean we don't drive."
We generally sequence this way. For a time, there was a war between what Jeff Biesiadecki termed the "Baumgartian" and "Bonitzian" views of this problem.[1] In the Baumgartian view, unexpected contact with the terrain is something that should immediately stop motion: the terrain isn't where you think it is, so you should stop and reassess everything from the ground. So you use free-space mode unless you're actually expecting contact. In the Bonitzian view, it's acceptable for a sequence to tolerate early contact -- as long as you've written it carefully, so that it will proceed safely whether you have early contact or not.
For a long time I was a Bonitzian, but somewhere in there I converted to the Baumgartian camp, as I think most of us did. Even leaving the safety arguments aside, the Baumgartian approach makes sequencing faster and simpler, since you don't have to validate that the sequence always works correctly in the face of any unexpected contact.
So this sequence was Baumgartian. But in this case, the Baumgartian downside -- faulting out the sequence, and blowing the drive with it -- would be worse than usual. Not as bad as harming the vehicle, but still pretty bad. The Mars Program office has been breathing down our necks to get us moving again (I'm told), so nobody really wants to risk the drive. Luckily, the fix -- converting to Bonitzian style -- is easy in this case: I simply change the MI stack moves so that they use guarded mode rather than free-space mode. This way, if the MI poker senses contact with the terrain, it won't approach the terrain any more closely but will generally go on with the sequence. We can't always make that change so easily, but in this case it happens to work out. Score one for the Bonitzians.[2]
[Next post: sol 536 (Opportunity sol 515), July 6.]
Footnotes:
[1] This is after Eric Baumgartner and Bob Bonitz, two of the original rover drivers -- both of them deeply involved in the development of the arm. Though long gone from the project, they remain important influences to this day.
[2] But this was one of the last scores for the Bonitzians. Today, we're all Baumgartians. Mainly, this is due to a project-wide change in attitude about sols: they're clearly more plentiful than we once thought, so we don't go as far out of our way to save them as we used to. In that world, the main advantage of Bonitzianism -- it can enable you to autonomously recover and go on to do other activities -- disappears, and the main advantage of Baumgartianism -- it's more paranoid about unexpected behavior -- wins out.
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