Spirit Sol 58

Today Ray Arvidson gives the science lecture. He paints it as a justification -- an apologetic one -- for staying at Humphrey for so long. The MTES shows a nice clean basaltic signature on this rock (and lots of others around here). In particular, the MTES shows olivine in Humphrey, but for some reason it's not showing any other basaltic minerals. Maybe they're just being blocked by the dust somehow, but we needed to confirm that by brushing off the dust and looking again. When they did that, the MTES showed exactly the same results. So the next hypothesis was that the layer beneath the dust may be a "rind" -- an indurated surface formed by water vapor, which will block the layers beneath. The next step, then, is the one we're doing now -- drill deeply enough that we'll get through the rind (if there is one) and look again. "This process is not being done just to cause Mickey Mouse signatures," he concludes, "though that was kind of cool ...."

We get the MI images back, and they're not showing what we expected. We had targeted two locations, one a little below and to the right of the Mickey head, and one at the bottom of Mickey's right (stage-right) ear. The images we've gotten back so far show part of a circle in the lower left of the image, as if we were looking at the right-ear image but the arm had drooped more than expected, moving down the rock face to show the right-hand side of Mickey's face instead. This is more droop than we should have seen, so either something is wrong with the arm or something was wrong with the sequencing (which would make it my fault, just to put a point on it).

It's not the worst thing in the world from a science perspective, since the point of the right-ear observation was to see some unbrushed surface, and we do. But it's not nearly as much as we expected: we were expecting more than half of the image to be brushed surface, and this is maybe a tenth. And worse than that, it means that we can't target the instruments correctly because the arm is behaving in ways we don't understand (or it means I really screwed something up, which is bad in a different way).

The scientists are planning a big day for tomorrow. Energy permitting, they'll first RAT the reachable spot, then MB, APXS, and MI that spot -- the entire suite. They start discussing what they'll cut first if energy proves to be a limitation (which won't be surprising: when grinding, the RAT runs for a long time and draws a lot of energy). While they're discussing it, Art Thompson (the TUL for today) comes in to say that we're precluded from using the IDD at all until we fully understand what happened. He promises we'll analyze it and have an answer -- one way or the other -- by the SOWG. I know what this means for me, and hustle to the Mobility/IDD station. As I'm leaving, I hear Art telling them some good news to go along with the bad news: we've changed the way we model power, so we should be able to model more accurately. This will help enormously, because we've had to be more conservative than necessary in our plans as the available daily power budget has decreased.

Jeng is the Mobility/IDD guy for the day, and he fills me in on his concerns. First, there's the MI image that doesn't look like we expected. Second, the joint angles the IDD is reporting don't match up very well with the predictions. It's normal for there to be a difference of less than one degree, due to minor uncertainties in the position of the vehicle, inaccuracies in the terrain mesh, and so on. But the differences he's seeing exceed two degrees.

The first problem already has an answer, which one of the scientists (Doug Ming) and I were working out when Art came in with the bad news. It turns out that it isn't a right-ear image, it's an image from the other placement. The reason we're seeing Mickey's face in it is that the MI field of view is a little wider than we thought: we had thought that it would show unbrushed area only, but when we model it in RSVP and look through the MI, we see almost exactly what's showing up in the real image. So at least as far as that's concerned, we've gone from thinking that the IDD was way off to thinking that it's spot on, and it was only our expectations that were wrong.

The second problem is more troublesome, but yields to analysis in the end. Jeng and Chris and I stare at it for a little while, then we realize that the predicted angles themselves might be wrong. The predictions come out of simulating the sequence in RSVP and recording the angles the simulated rover reaches. But RSVP doesn't model the interaction of the rover and the rock surface -- it cheerfully lets the instruments pass through solid rock. And because of the inherent measurement uncertainties, we always command a slight overdrive, telling the rover to push the instrument to a point that lies inside the rock. (It actually stops moving earlier than that, when switches mounted on the instrument report contact with a solid surface.) Normally, the difference doesn't amount to much, but the IDD happens to be in an unusual pose, in which it takes a lot of shoulder movement to make a small change in the actual position of the instrument tip. When we redo the simulation without an overdrive, the differences subside into the expected range. Again, the IDD was performing splendidly, and only our expectations were wrong.

We breathe a collective sigh of relief, but we repeat the experiment with a couple of other positions just to make sure. In each case the non-overdrive prediction compares well with the actuals. We tell Mark Adler, who looks very, very relieved.

So it turns out to be just one of those things. There happened to be nothing wrong, but what if Jeng had been right? We might have caused severe damage to the vehicle if he hadn't raised the alarm. Being on the downlink team is a tough job: you don't want to panic unnecessarily, but you don't want to ignore real problems, either. Every time anything unusual happens, you get to roll those dice.

With the IDD concerns addressed, the SOWG is free to ratchet up the complexity. Tomorrow, we RAT the rock, then MB and APXS the RATted spot. The following sol, we'll MI it and MB it again (presumably for longer the second time). This will be straightforward, so I start sequencing in the meeting, with the goal of finishing before it ends. But it turns out to be the shortest SOWG meeting ever, at a mere 23 minutes. Oh, well.

I continue sequencing downstairs. Relatively speaking, it's an easy night. We got very amusing email from the staffing office, of all places. Some guy in New Hampshire says he can see spider-like creatures in the Spirit images, at 600x magnification (since the images are 1024x1024, this would be about 1 or 2 pixels). For some odd reason, nobody seems to have replied to his email on the subject. My favorite response to people like this is to tell them that they're absolutely right, and not to let anyone tell them otherwise. (Not that I've ever done this, but I'm always SO tempted.) There's a lot of support for this idea. Art rags on the MI PUL: "If you'd been doing your job, you'd have found this!" He adds that if he'd seen the email earlier, he'd have had the entire SOWG call this guy (whose phone number was included in the email). That would have been some SOWG meeting.

Since it's a slow night, several of us fall to chatting. Art tells a few MER and Pathfinder war stories, as well as some from his Shuttle days. One reason the engineer-scientist relationship on MER is so good, he says, is that Squyres enforced some flight discipline on the scientists from the get-go, so that they have some idea of what the engineering situation is. The scientists normally take a more academic perspective, without taking into account what needs to be done in order for things to work. (Of course, he adds in fairness, this is the engineer's perspective; the scientists would tell the story another way.) On one Shuttle mission, the science team came in and swapped out some circuit boards in one of their experiments without telling anyone. When this was discovered, it caused them to fail a review. They didn't fly that experiment. This is the sort of thing that wasn't allowed on MER, Art says, and it's one reason the scientists understand us better: they know more about what constraints we're under, and that there's a good reason when we tell them no.

Art discloses another interesting MER fact: Justin Maki wanted to put a mirror on the lander, so that the rover could take a picture of itself. They just weren't able to find the right spot, and the idea was eventually abandoned.

Art also mentions, in passing, that some people are talking about a 150-sol total mission (90 nominal, 60 extended). Holy cow! It makes me wonder again whether I'll get to participate in the extended mission.

Word comes that there are a couple of astronauts on the floor. ("Good ones?" I ask. "Or just Shuttle astronauts?" This gets a laugh, but I'm ashamed of it. Well, maybe a little ashamed.[1]) The astronauts attend our activity plan approval meeting, dressed in their blue uniforms. It turns out that Albert Haldemann, one of the scientists, knows one of them -- because he was with them at one point, trying out for the astronaut program. Albert didn't make it, obviously, but he says working on MER is the next best thing.

They discuss tomorrow's big announcement, the water news. Albert jokes: "How many times can we discover water on Mars before people get fed up with us?" I guess we'll find out tomorrow.

Courtesy NASA/JPL-Caltech.


[1] We're just jealous. You can see how much we love our jobs, but I still doubt you could find three people on MER who wouldn't have instantly dropped everything we were doing in exchange for a single Shuttle flight.

1 comment:

Kim said...

Oho, it's a good thing you didn't have this post up before that fateful lunch in January!! *wags finger* "Just shuttle astronauts", indeed... ;)