Opportunity Sol 528 (Spirit Sol 549)

The weekend's drive went splendidly, racking up 32m. And we're poised to do it again, with a broad lane continuing before us.

Except for the speed bump. About 12m away from us is a decision point, where the trough sort of splits into two other troughs. Each of the branches (Left Fork and Right Fork) has a ripple in front of it, a ripple with geometry that's ominously similar to Purgatory's. Alternatively, we're sitting more or less at another fork in the road -- but the third choice that provides (Right-of-Right Fork) appears to end in a cul-de-sac with bad-looking stuff at the end. But we don't have to take any of those three paths. We could start ripple-hopping to the east, hoping to find a better trough.

Thus begins the Big Debate. Right-of-Right Fork gets thrown out pretty quickly, as just a way to store up trouble. Of Left Fork vs. Right Fork, Rob Sullivan's analysis shows that Left Fork's ripple is shorter but steeper, and angle is more worrisome than height, so that's a loser.

Which leaves us with Right Fork and ripple-hopping. The eventual decision is to prefer Right Fork: ripple-hopping affords no immediate progress toward our goal, is not guaranteed to provide a better alternative soon, and has dangers of its own. Besides, we're going to come across ripples like these sooner or later on the way to Erebus and, beyond it, Victoria. We might as well start learning to deal with them.

But still, that ripple ....

At least Jeng tells me something that makes me feel better about tackling it. The problem at Purgatory, he says, wasn't just the geometry of Purgatory itself. It was exacerbated by the fact that when we hit Purgatory, we were just descending another ripple (later called "North Ripple"). This meant the rover was initially angled down and thus "wanted" to bury itself in Purgatory. For this ripple, we'd be pretty much flat as we encountered it, which is a much more favorable geometry for climbing it.

So Jeng and I plan a route along the trough, while Rob Sullivan goes off and does more analysis and Beth Dewell calls Jim Erickson and John Callas. Jim and John are nervous about this, and also nervous about the fact that our planned path extends 20m beyond the ripple. This would mean that if our new slip-detection approach doesn't work, we could spin our wheels for 20m worth of driving in the ripple -- about half the damage we did at Purgatory, but much more than we'd be comfortable with. "You'd have a new Project Manager if you did that," Jim says grimly. "You say that like it's a bad thing," jokes John -- his deputy -- but Jim is serious. Though it didn't cost us a rover, Purgatory was a costly mistake, yet a forgivable one because it had never happened before and really was not foreseeable. But if we did it twice -- "Even I would wonder about my fitness for the job if it happened again," Jim says.

In the end they accept the drive as it stands, trusting our argument about the robustness of the slip check. Really, it's hard to get the slip checks to pass even when we're genuinely not slipping, depending as they do on the finicky mechanism of visodom used in a very difficult environment. If anything, the checks are conservative. At their suggestion, we mitigate the risk further by splitting the post-drive segment so that it effectively checks for stuckage an extra time.

Still, I'd feel a lot more comfortable if we were risking only the rover, and not also risking Jim's career.[1]

So we're done. Or not. While all this has been going on, Rob Sullivan's continued analysis turns up two other possible dangers past the initial ripple. A second ripple lies behind the first one and to its left, while a third lies a bit farther on and to the right. So even once we pass the first ripple, we've got some zig-zagging to do. I sigh, roll up my sleeves, and start hacking the sequence some more.

It's an awfully good thing Rob found those ripples -- it would really have sucked to sail past the first one, only to get stuck another couple of meters in. Our terrain mesh thisol is pretty poor for some reason, so the farther ripples didn't show up well; only his experienced geologist's eye gave us warning of the danger.

As if all that weren't enough, Rob saves the day once more. We decided to take "cleat movies" of our progress over the ripple -- small HAZCAM images subframed to the wheels, taken after each 50cm of commanded motion. Normally these are at a relatively low priority. But then we realize that taking these at a low priority is a mistake: if we do get bogged down, we won't get the images for another day, and that will be an awfully tense day. So we'd really like to get them sooner, but by the time we realize this, it's so late on the East Coast that the person who's normally responsible for this has had to go home. But Rob knows the job, so with his usual good humor and great attitude, he stays late and fixes them for us. He even sticks around through the CAM to make sure everything is ready for the spacecraft.

You know how there was a Fifth Beatle? Rob is, like, the (n+1)th Rover Planner. I love that guy.[2]

There were many obstacles thisol, figuratively and literally, but we got some good news, too. We had another cleaning event -- the dust devils worked their magic -- and we now have another 100 Watt-hours to play with each sol. Thanks, Mars!

[Next post: sol 551 (Opportunity sol 530), July 22.]


[1] But then, this is why Jim was such an awesome guy: even feeling that his career was on the line, he trusted us unless he had an articulable, rational reason not to.

[2] Much later, I even gave him an official Rover Planner T-shirt. Yes, there really are official Rover Planner T-shirts. And no, you can't buy one.

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