Mars and Me: Opportunity Sol 264 (Spirit Sol 285)

Somehow I don't have a good feeling about yestersol's drive. This is borne out when I walk into the SOWG meeting and Cooper says, "Did you hear about what happened?"

Sure-fire bad news opener.

"The Brain Tractor Beam kicked in," he says. This is Frank's concept: the Dino Brain -- Wopmay -- has magic powers and can remotely screw up the rover's drives. "We hit a submerged rock and got 100% slip. Now we're back near the Brain somehow, with a solar panel near the rock."

Oh, shit. This is very bad.

We don't know enough about what happened yet to decide how we're going to try to fix it, so we can't tell the SOWG meeting what we'll need. Since we're going to be their highest priority, this means they can basically do nothing, so they decide to give us some time and reconvene in an hour or so.

So Brian and I go talk to Mark Maimone, who's Mobility/IDD today. He's got a graph up, showing the rover struggling to climb uphill and breaking free at last. He's also got an image showing Wopmay off our port quarter. It's about 1.6m from the center of the WEB, which would make it about 30cm -- roughly a foot -- from the edge of the solar panels, in the worst case.

This is not a lot of room to maneuver, especially because any motion on our part -- even steering the wheels -- will tend to cause us to slip further toward Wopmay. In actuality, we have a little more clearance than that. The farthest tip of the solar panels is downhill from the tallest part of the rock, giving us a little extra room. But it's still not a lot.

Clearly, we need to get away, and fast. Mark's suggestion is to go downhill, to use gravity instead of fighting it. This frustrates me, because I'd been thinking exactly the same thing the past couple of sols: gain cross-slope distance from Wopmay by going downhill, then come back uphill later. But I hadn't said anything, so shame on me. Anyway, Mark's suggestion is the right one, and we adopt it as our tentative plan.

Mark has another image, a very surprising one. It's the rear HAZCAM image, and it shows a rock about a meter behind the rover. That's not so unusual. The unusual part is that the rock wasn't there before.

The rock's in the spot where we got stuck, too, which is telling. And there's a big hole behind it. It looks like what happened was this: as we were driving uphill, the rover somehow pushed a rock up out of the ground, then got itself stuck on the rock.

Think of a pond in winter. It's got a thin crust of ice over the top, and as you walk around on it, the ice starts to crack. Something like that is happening underneath Opportunity. This is not precisely news -- for one thing, a few sols ago, a rear HAZCAM showed a curious kind of ridge along the wheel track, caused by the rover crunching through a lightly cemented top layer of the soil. We've seen similar effects on Spirit. But up to now, all that had been revealed underneath the cemented layer was soil, so nobody really cared. Yestersol's drive was the first time we managed to kick up a rock.

Even stranger, one of the scientists, Rob Sullivan, looks back at the previous sols' images of the spot where the rock -- Andy christens it "Bane" -- was dislodged. Even in hindsight, there's no sign of the thing. Which has a disturbing implication: we can't tell where these submerged rocks might be, so our drives might stall on any of them at any time.

Later, I'd come to realize that kicking up hidden rocks, probably performing the first extraterrestrial excavation in history, was seriously cool. But at that moment, my reaction was: we're sitting right next to a rock whose size makes it a threat to the vehicle, any motion we make will tend to slip us closer to the threat, and now you're telling me we're in a mine field.

For the future, Rob has some practical advice. His guess is that unearthing (unmarsing?) these rocks is more likely when we're driving over terrain we've already driven over a lot. Where the ice is already cracked, we're more likely to fall through, so to speak. That doesn't help us thisol, maybe for the next couple of sols, since we have no choice but to drive away through terrain we've already spent a lot of time in. But at least it gives us hope that this won't be a problem forever. (We also get some welcome feedback from Steve Squyres: "Don't feel any pressure from the science side to get us to the other side of Wopmay fast. Our number one goal is for you to keep the rover safe. I've grown attached to these rovers.")

But for now, we need to worry about the short term. We decide not to try anything fancy. We're going to back away, mostly downhill, in a short shallow arc that simultaneously avoids both Wopmay and Bane, then tighten the arc to take us more directly across the slope. If all goes well, we'll end up a few meters from Wopmay, plenty of room to do an uphill drive on the next sol.

Turns out it's hard to type with your fingers crossed.

Courtesy NASA/JPL-Caltech. I cannot describe the sick horror I felt when I first saw this image: Wopmay, just off Opportunity's port quarter. This is not how I wanted my first Opportunity drive to go. Even now, it dredges up shame and fear and embarrassment.