There are dozens of safety checks we can use when we operate these rovers. We can turn slip checking on, and we can abort a drive if we see more slip than we want. We can monitor the rover tilt, or the angles in the suspension system, or wheel currents, or you name it... and abort a drive if we don't like what we see.

The big question in a situation like this is how tightly - i.e., how conservatively - we should set our limits. Do you abort a drive if you see more than 50% slip? Or 30%? or 20%? What about tilt, what about motor current?

We've got most of our safeguards on as we approach Purgatory, and we've been setting the limits very conservatively, to make sure we keep the vehicle safe in this treacherous terrain. Of course, the unavoidable consequence of staying safe in a situation like this is that you also tend to go slowly... occasionally tripping one of the limits and aborting a drive.

That has happened to us a couple of times recently, and we're still a few tens of centimeters short now of the place where we want to do the arm work. We're commanding another drive over this weekend, and we'll see on Monday where it puts us. Once we finally are in position, we're going to bang out the arm work as quickly as we can, and then be on our way. It'll feel very good finally to be moving again.

At Gusev, we're really moving... and it feels very good! Our journey to the south side of Husband Hill is well under way now. We've been spiraling up and around the west side of the hill, heading generally south and gaining elevation as we go.

I've been surprised, as I think the whole team has, at how well this part of the climb has gone so far. Other parts of Husband Hill have been very tough going, but for some reason we're moving over this part of the hill very well.

We've knocked off several drives of 20+ meters lately, which is really good for this kind of terrain, and there was one drive where we gained about three meters in elevation. Our drive paths have been nearly straight south lately, but now we're starting to curl around toward the east, more in the direction of the summit.

So now there are two big questions: Are we really going to go for the summit? And what will we see once we get our first good view to the south?

I really don't know about the summit. We all want it, of course... who wouldn't? We're doing the first mountaineering on another planet, and it would be a little frustrating to get this close and not make it to the top. (I'm a climber when I'm not doing science, and Chris Leger, who has been the lead rover driver for Spirit much of the time lately is a really accomplished rock climber... so we both want this thing.)

However...

Mountaineering is not the point of this mission, obviously! We're here to do science, and we're only going to go to the summit if it makes sense scientifically. But there's a good chance that it will.

The summit is directly between us and the terrain to the south that we want to explore, so going over the top may be a pretty efficient route to the good stuff. Also, the summit really is going to offer a pretty good view.

Geologists in the field routinely climb to the top of the highest hill to get a good look at their surroundings and plan what they're going to do next, and we may want to do that here as well.

What it ultimately comes down to, I guess, is how hard the climb is. If we can get on top without wasting a lot of sols on our trek to the south, then we're definitely going to do it. But if we find the climbing is too hard and there's a significantly faster route to the south side, then we'll do that instead.

So I simply don't know what's going to happen. But for now the going is good, so it's east, up, and we'll just take it sol by sol.

And then what will we see? We don't really know. The views toward the south right now are enticing, but so far we haven't seen much that we hadn't already seen before, albeit from a lower elevation. Orbital images of the south side of Husband Hill show some terrace-like structures that might be exposed layers, so those may be a major target once we get over there.

There's also a big dark area that we think is probably a dune field, as many dark splotches on Mars are. And there are other features that we're simply not going to understand until we get a good look at them.

The downhill run into the Inner Basin should in principle go pretty quickly once we start on it, but it gets very steep in places, so we're going to have to be careful. Anyway, we've got interesting times ahead, and everybody on the team is pretty pumped up right now.

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