The Erebus Highway drive worked splendidly, putting us right where it was supposed to. Now, back on the other side of the world, I get to help pull off another miracle: an 11m drive followed by a scuff. This is just like we did on Serpent way back on sol 72 or thenabouts, only starting from much farther away. This should be a good test of our abilities: have we learned enough since then to make something like this work? Chris is more confident than I am, but I sure hope he's right.
We break down the responsibilities more or less like this. Chris is going to worry about getting us there, and Ashley and I will work out the obstacle-checker helper sequence and the scuff sequence itself. The obstacle checker is going to have an important responsibility thisol: keeping us from driving over a cliff. Really. The dune (ripple?) we want to scuff is right on the edge of a cliff, and if we go a meter or so past it, it's bye-bye, Spirit.
The way the obstacle checkers work is this. We work out a list of known obstacles, and for each one, we define a circular hazard region around it. Essentially, the obstacle is the center of a circle we want the rover to stay away from. Then we plan a path that avoids all the circles, but just in case, the obstacle checker runs in parallel with the drive, periodically checking off its list and making sure the rover is outside of each circle. If the rover strays inside a circle, the checker issues a command that stops the rover from driving any farther.
While there are several other obstacles, the most interesting, and the one I attack first, is the cliff itself. When I rotate the terrain mesh so that we're looking down on it, I notice that the cliff face is a rough semicircle. Hmmm. Working by eye, I estimate the center of that circle, pick a radius -- eight meters -- and ask RSVP to show the circle I've just defined. It matches up with the cliff face almost perfectly on the first try.
"Not gonna get much better than that," I say nonchalantly and go on.
For the scuff, we have a couple of choices. We've done two basic kinds of these. The first was the one we did on Serpent, where we started by planting a wheel where we wanted the scuff to begin. We then turned the rover about five degrees to the left, then ten to the right, then five to the left, leaving it where it started. Next, we backed up 10cm and repeated the wiggle maneuver. The result was a beautiful, broad and deep scoop in the ripple face.
The other kind of scuff we've done didn't involve a ripple; we were just scraping off a top layer of soil. That worked differently. We spun both front wheels forward, holding the other wheels steady, to scuff away any material directly under them. Then we drove the rover a few cm backward with the front wheels locked in place -- using the middle and rear wheels to drag the front two. Then we repeated the whole maneuver, spinning the front wheels to kick away any material that accumulated under them while we dragged them and then dragging them back again, and so on. The result was just what we wanted: a shallow, clean scuff that exposed the immediate subsurface layer.
In this case, we're after the deep scoop, so I pick the first one. This is quite a blast from the past: I vividly recall building this scuff sequence the first time, and though we have to modify it slightly for this new situation, I have a lot of fun revisiting it.
So the sequence is built, and we cross our fingers that this very aggressive go-and-scuff will work. As it happens, today comes with a reminder that Murphy's Law is very much in force. They missed yesterday's downlink on both vehicles due to some kind of DSN station problem, and today we find out the details. It turns out that rodents chewed through some cables at one of the antennae -- the one our downlink was coming in on -- at the DSN complex in Goldstone.
Chris has the perfect pun for the occasion. "Rats!" he exclaims.
[Next post: sol 607, September 17.]