Spirit Sol 304

The trivial IDD sequence went well, so thisol we'll stow and go.

Stowing is easy. The hard part is going. It's hard because Spirit is so power-limited, and the solar insolation maps we have don't show much hope for us on the way to our next target, the imaginatively named Dark Rock. These maps show us a false-color overlay on navigation images, with the different colors reflecting how much solar energy we'll be able to collect at any given location. (This varies with the magnitude and direction of our tilt: tilting toward the sun is good, away is bad.)

In these images, red is bad, blue is good, green is in between. (Where "bad" means the rover doesn't get enough energy and could, in the worst case, die.) General policy is to lay out paths that are blue all along the way, just in case the drive faults out. Failing that, we choose paths that let us "hop" from one blue region (which we call "lily pads" even though they're not green) to another.

But in this case, we're climbing a generally south-facing slope, and there's not much blue to be seen. We're OK for the first few meters of the drive, but after that the landscape is soaked in red.

Well ... when all else fails, cheat. The different colors represent numbers on a scale from 0 to 1, where red is used for anything below 0.85. (The real floor on the insolation number is supposed to be around 0.83, but due to uncertainties in the way the map is computed, we add a small safety margin and use 0.85.) But Jeff and I find that we don't have to lower the low end very far -- only to about 0.78 or so -- before we start to see small patches of blue. It's not much, but it might be enough for us to hopscotch uphill.

If we're allowed to lower the threshold, that is. So I ask Kirk Fleming and Jake Matijevic about this, stressing that I'm OK with telling science we can't get there from here. Kirk and Jake hem and haw a little while, and come up with an absolute bare minimum insolation number of 0.75.

It's just barely enough, but it's enough. So Jeff and I get working on the first part of the overall drive to Dark Rock. Thisol we'll back uphill 4m, then -- using the same drive-extension trick I recently invented for Opportunity -- if we're on track, we'll turn and drive another 2m downhill, setting ourselves up nicely for the next sol's ~5m drive.

Dark Rock is only 20m away, as the Martian crow flies. With all the zig-zagging, and the short drive times imposed by the tiny amount of available solar power, it will take about a week to get there.

Back in my day, we'd laugh at 20 lousy meters.[1]

[Next post: sol 313 (Opportunity sol 292), November 19.]

[1] And now, of course -- Spirit being deeply embedded in Troy, and possibly unable to get out -- we'd be happy with 20 centimeters per sol. Oh, my poor little rover.


Hank Roberts said...

naive question--I understand they don't want to try to push off with the arm -- to try to lift the rover slightly while trying to drive it -- because they'd be pushing the arm into the same soft stuff the wheels are in. I gather that's a last-ditch (sorry!) choice.

What about the reach of the arm--

Can the arm drag or push anything next to/under a wheel so the wheel would ride up onto that? Or even reach the end of the arm to where the wheel would contact that as it turns for traction? maybe lift the body of the rover slightly (assuming the arm could flex without breaking the arm off)

Just thinking about the various ways I know I've gotten unstuck, push-poling from a boat (mine) on a mudbank; taking loose stuff off a 4wd truck (someone else's) sunk to the axles in lakeshore mud to stuff under the wheels for traction.

Scott Maxwell said...

That's probably the #1 question we get, and it makes me wish JPL had a FAQ to help answer it. I'm sure a lot of other people are wondering the same thing.

You're right about one of the problems -- there's not a hard surface to push against. Unfortunately, there are other problems as well. Spirit's arm can exert only about 70N of force -- the absolute max is maybe 100N -- only about 10% of the 650N she'd need to lift herself. Also, because the motor control board is shared between the arm and the wheels, we can't actually push while driving. (We could load the arm up, drive a little, load the arm up, and so on -- that is, the system wouldn't prevent us from doing that -- but we can't actually push while driving.)

In addition, this would be an incredibly risky maneuver, because it risks damaging the arm and/or its instruments, which represent 2/3 of Spirit's scientific payload.

Manipulating the surface, for example by pushing rocks in front of the wheels, is also risky but probably wouldn't help much. There aren't many helpful rocks in reach, and we could make a difference for only one wheel (the left front wheel -- the right front wheel is dead). Trying would probably consume quite a few sols, and it might not make much of a difference anyway, as the rock we put in front of the wheel might just slip right under it, through the soft soil, leaving us right back where we started.

Despite all that, strategies for using the arm are indeed on our "back pocket" list, to be reconsidered if and when the situation becomes sufficiently dire.

Right now, Spirit's big problem is that one more wheel has started giving her trouble. The right front wheel died years ago, on sol 779, and we've been dragging that anchor ever since. Just as we started the Troy extrication, the right rear wheel started stalling. While we're not yet sure whether this is because of a terrain problem or something in that wheel's drive motor, it's potentially an extremely serious problem -- and one the arm probably couldn't help us overcome even if it were five times as strong as it is.