Spirit Sol 228

I'm the only rover driver who hasn't worked on both rovers. Brian and Jeff did most of Spirit's first drive before moving to Opportunity. Eric did the initial IDD checkout sequences, which Frank edited slightly, before moving to Opportunity; and Eric came back for a sol or two. Chris has spent several sols on Opportunity, and John is racking up a few as well. Ashitey, too. Bob never worked on Opportunity as far as I can recall, but he's off the project now.

Which leaves me. But not for long: I have a shift scheduled on Opportunity tomorrow. Technically, I'm a floater, which means in practice that I'm working on both, but it counts.

The only bad news is, I don't know anything about what they're up to or when they expect me to come in. So I go upstairs to the Opportunity room to find out.

"When do you need me tomorrow?" I ask Emily.

"Tomorrow's a restricted sol[1]," she replies. "We don't need a rover planner."

Well, that's just ducky. I guess I won't be working on Opportunity tomorrow, after all. And on the current schedule, I don't get another chance at it for a while.

And Opportunity has cool stuff going on right now, too, as Chris Salvo shows me. They found this weird mudlike stuff -- it looked like rock, but driving over it nearly destroyed much of it -- which they're going to drive back to and examine in detail with the IDD. Nobody knows what this is. One thing is certain, though: I won't be helping them find out.

Ah, well, Spirit's my first love, anyway. While I was away, they finished up at the previous location (where the rover slipped almost double the predicted amount during my last drive -- turns out it's a good thing I shortened it) and then performed a short drive to a new rock called "Ebenezer." The targets on Ebenezer are named for characters from "A Christmas Carol" -- TinyTim, Cratchit, Scrooge, Marley.

We expect to stay at Ebenezer for a while. The scientists like this rock and our power situation is excellent, so we might stay here through solar conjunction[2]. In the meantime, we'll be performing the usual extended exploration campaign -- APXS, MI, MB, RAT-brush, APXS, MI, MB, RAT-grind, APXS, MI, MB, and then maybe do it all over with different targets.

But not all in one sol, of course. Today we're just getting started, unstowing and beginning to poke at the surface. John has the work largely done by the time I get in, so it should be a cakewalk.

But, reminiscent of Ebenezer Scrooge himself, we're haunted by the Ghost of Sols Past. Yestersol they did a touch-and-go, and we discovered in thisol's downlink that the instrument placements very nearly didn't work. The APXS placement, in fact, wouldn't have opened the APXS doors (a requirement for good data) had John not happened to sequence an unusual preload move at the end of the instrument placement.

This looks like the same problem we've seen several times before, where the surface isn't where the images and terrain mesh seem to say it is. John thinks it's nothing to worry about, a combination of the fact that yestersol's images were taken in shadow (meaning they had low contrast), were heavily compressed, and were of a uniform surface. This combination of factors makes it hard for the stereo correlation to find the same features in the left-eye and right-eye images, and therefore it has a harder time figuring out how far away the terrain is. Thisol we're working with less heavily compressed images of a less uniform surface, so we shouldn't have the problem again.

But Julie's worried about it. She wants the rover to trust its sense of touch more than its sense of sight, by telling it to reach out and touch the surface so it knows where it is. "What would it take to do a MB touch before each instrument placement?" she asks.

John shrugs. "We can do that," he says. "We'll have to redo all the sequencing to use relative moves instead."

"How long will that take?"

He shrugs again. "A couple of hours," he says.

This is designed to make Julie give up on the idea. We're already an hour behind schedule, and nobody likes to slip even later. But she doesn't take the bait. "Okay," she says, "go ahead."

John looks annoyed, but it's not his problem anyway -- it's mine. I pick up the sequence and get to work.

As predicted, the changes take a couple of hours to implement. Then it gets worse. At the walkthrough, they ask for even more changes. By the time all is said and done, we're almost four hours late.

And I'm not really sure we gained anything. I've made a lot of changes under time pressure, which is always risky. If we'd been smart, we'd have weighed that risk against the risk of just leaving the original version of the sequence alone. Instead, we weighed risk against time, which was the wrong tradeoff. By now, we ought to be smarter about this stuff.

As I'm leaving, I can't help but think of that bit from The Princess Bride:

Valerie Max: Think it'll work?
Miracle Max: It would take a miracle!


[1] Because of the difference in the two planets' day lengths, the Martian day appears to rotate through our planning day -- Martian noon is 40 minutes later each Earth day, more or less. When the phasing reaches a point where the sol's downlink (whose timing depends on the Martian-day time seen by a particular rover) will arrive so late in the Earth day that we'll no longer have time to plan the next sol's activities for the rovers, we plan differently: we plan two sols' worth of activities for that rover, every other day.

Planning ahead this way restricts what we can safely do with the rover, though -- we can't plan anything for the second sol that might be unsafe if the first sol goes awry, for instance, since we have to plan them both at the same time -- so these are known as "restricted sols."

Incidentally, because the two rovers are at almost exactly opposite longitudes on Mars, their Mars times are about 12 hours apart, so at most one rover is ever in restricted sols at any particular time.

[2] When the sun will be between us and Mars, making it impossible for us to reliably communicate with the rovers for about two weeks.

No comments: