After all that ... after all that work, we barely even moved. The first step of the drive was a 15-degree turn-in-place -- from 150 degrees to 165 degrees. Applying conservative limits in the spirit of our current rules of the road, we allowed 15 seconds for this turn. This should have been plenty of time, since the rover nominally turns at a rate of 2 degrees per second.
But there was something we didn't know: since the steering actuator stuck on Opportunity's right front wheel, she's been turning more slowly. More like 1 degree per second. On top of that, there's some fixed setup time (for example, to steer the other wheels to the right position). What with one thing and another, the turn timed out. So nothing else happened -- we just changed our heading a bit.
Just to pour salt in the wound, it timed out at a heading of 164.949 -- just 0.051 of a degree from the commanded heading. Another tenth of a second would have been plenty.
We discovered all this when the data came down yesterday afternoon. Since then I've been brooding about one of the decisions we made with this drive, specifically the decision to continue driving 20m after we cross Ripple B. The trick we're relying on to stop us when we get bogged down hasn't been tested -- what if there's some flaw in it that we hadn't thought of? We'd be pretty embarrassed -- and the subject of intense media and management scrutiny -- if we found that we'd actually dug in rather than stopping. Making this mistake at Purgatory, when we couldn't have predicted it, was one thing. But repeating the mistake would be very bad.
Based on this, I sent out email explaining what happened yesterday ("Executive summary: we are dumb, but Opportunity is fine and can proceed") and also suggesting that we shorten the drive when we redo it, and it's this email Jim Erickson is referring to when he stops me in the hallway this morning. "Why did you change your mind?"
I tell him, and he grins, nods, and then looks sharply at me. "What I don't want to see is that we lose our courage," he says. "We still have to do our job, which is to explore Mars."[1]
Ashitey has his own take on my email. "You're so hard on yourself! Back it off!" he scolds me. "How many sols have you saved? We all make mistakes on a daily basis. You're one of the best! Don't worry about it!"
The funny thing is, I rewrote the email to take out the self-flagellating tone. I thought.
Others are philosophical. "I don't turn as quickly as I used to, either," Squyres deadpans.
Instead of cutting the drive short because the slip-detector isn't tested, we could ... wait for it ... test the slip detector. Paolo and Jeng go down to the testbed to do that while I update the sequence based on what we've learned. It turns out that the slip-detector works fine -- which we all expected, but it's nice to have demonstrated the fact -- and we can go the distance.
Along the way, we're going to pass some interesting-looking flat rocky patches surrounded by pebbles -- the scientists refer to these as "cobblestones" -- which might be the signs that we're on the fringes of Erebus Highway territory. We won't stop and take pictures of them in the middle of the drive, we'll just take a low-quality image from the end position that will help us aim the higher-quality images we'll take before our next drive.
There are two of these patches, and they need names. As it happens, just before the issue comes up, I notice something in the news. Sadly, James Doohan -- better known to the world as Star Trek's "Scotty" -- died. Scotty was an icon for all engineers, and especially those of us in the space program. (One engineering school asked its students what got them interested in engineering. Half -- half -- said, "Scotty." They gave him an honorary engineering degree.) I suggest naming one of the little rocks after him, and the suggestion is received enthusiastically.
The other is named "Apollo," as today (20 July) is the anniversary of the first Moon landing. So we get to honor both real and fictional space exploration in one drive.
I hope it works.
Footnotes:
[1] Now that I'm the rover driver team lead, I have stolen that sentiment -- sometimes the exact line -- more times than I care to admit.
No comments:
Post a Comment