Astrobiology Magazine (AM): Let's first recap where the rovers are right now.

Steve Squyres (SS): Spirit is partway up the Columbia Hills. We reached the base of the Hills about five months ago, and we've been slowly working our way up. It's been slow partly because it's a steep climb with a tired little rover.

AM: Has that wheel problem on Spirit slowed you down at all?

SS: No, it really hasn't. The difficulties with the wheel have received a lot of attention from the engineers, and quite deservedly so, but in reality it's had almost no impact on our operations.

The big reason that we've been very slow going up the Hills is we keep finding good stuff. After sprinting across the plains, which had a fairly uniform deposit of compositionally identical blocks of basalt, all of a sudden we're in this incredibly target-rich environment in the Columbia Hills with layered rocks and clear evidence of aqueous alteration - water activity.

It's fascinating stuff. It almost feels like when we were back at Eagle crater with Opportunity; it's like every rock brings some new revelation, and slowly the picture starts to fall into place.

Opportunity is perched precariously high on the southern wall of Endurance crater. We've actually managed to traverse all the way up to this spectacular outcrop we call Burns Cliff.

We're working our way eastward along the cliff, but we're not going to get all the way to the eastern end because that's too steep a terrain for us to go into safely. But we're making measurements on the cliff, and we'll probably wrap that up in another week or so. Then we're going to head out of the crater.

AM: When you head out of the crater, where will you go?

SS: We've got a lot of things ahead of us, and the team is fired up about this. The first thing we're going to do is get to the heat shield. We've been itching to go to this thing for months now.

The heat shield came off the vehicle while it was descending toward the surface, and it would've impacted the martian surface going about 200 miles per hour. It probably dug a spectacular hole; it's going to be wrecked. It's going to look like a crashed flying saucer out in the desert.

We're going to go take a look at that because we want to look down into whatever hole it dug. It's probably deeper than any hole that we can dig with our wheels. Also, no one's ever been able to examine a heat shield that's gone through the martian atmosphere.

So by examining it, particularly with our microscope, we have hopes that we might be able to help future heat shield designers come up with more efficient designs.

AM: What sort of things are you expecting to find with the microscope?

SS: The key thing is the depth to which the heat shield material has been charred by passage through the atmosphere.

When you design a heat shield you have to put a certain amount of margin into the design, because it's there to protect the stuff inside the spacecraft, and you want to be careful you don't have any kind of burn-through.

With inadequate knowledge of how a heat shield interacts with the martian atmosphere, whoever designs the heat shield has to put a lot of margin into their design to be safe, which means the heat shield is probably heavier, thicker, and more massive than it needs to be.

After we learn how this heat shield performed, someone might be able to design future heat shields where you take some of that mass and put it into other scientifically productive parts of the spacecraft.

Another possibility is that the heat shield was just barely thick enough, and we almost burned through. I doubt that, because I think it was a good design, but we'd like to see how the performance of that heat shield really was.

AM: Where the heat shield crashed, geologically speaking, is it different from other places you've examined?

SS: No, it's very average plains. There's nothing special about it geologically. But we want to see the hole that it dug.

I think after we come over the lip of Endurance crater, we're just going to sprint straight to the heat shield. It's close by, only about 150 meters, and on this kind of terrain it's very easy driving. So we should get there very fast.

After that, we have a checklist of science that we want to do on the plains. There are a few cobbles - little fist-sized rocks scattered about the plains - and we've never looked at one. So we're going to find out what those rocks are made of.

Then we're going to head south. The most appealing target within reach is this so-called "etched terrain." It lies about two kilometers to the south of us, and we believe it is similar to the rock we've seen so far, but not exposed by impact. It was exposed by the gentle erosive action of wind.

That's important, because anytime you have an impact, it's very disruptive. It's like throwing the rocks in a blender, and it really gets jumbled up. It's been hard for us to unravel the geology because the rocks that we've seen in Endurance and Eagle crater have all been busted up by impact.

The nice thing about the etched terrain is that if these same layers of sediments have been exposed by the gentle action of wind over millions of years, they will not be fragmented the way they are at the craters. I think we may be able to see more intact stratigraphy there, and a geologic record that's easier to read.

AM: And then after that, you've said that you might head for Victoria crater?

SS: Yeah. But I've got to be honest with you, Victoria crater is a long shot. It's five kilometers away, and that's a heck of a long way for this rover to go.

Not only is Victoria crater far away, but it's on the other side of the etched terrain. Now, when we look at the etched terrain from orbit, it's got some significant topography to it. We cannot tell how traversable that stuff is. It may be an absolutely impenetrable barrier - a badlands through which the rover simply cannot fight its way.

But if we can travel that distance, and if the rover can last that long, and if we can fight our way through the etched terrain - three very big ifs - on the far side is Victoria crater. It's six times bigger than Endurance crater, a great big deep thing, and I think it'll be an interesting target. Do you know the story of how we came up with the name for that crater?

AM: No, I never heard that.

SS: At the Meridiani site, we're naming all the craters after ships of exploration. So Eagle is named after the Apollo 11 lunar lander, and Endurance after Shackleton's ship.

When Ferdinand Magellan set out on his voyage around the world, he left Spain with five ships and something like 270 men. And three years later, the one surviving ship, Victoria, struggled back to Spain with 18 surviving crewmembers on board.

I figure, if we make it all the way to Victoria crater, that's going to be a good analogy. We're going to be limping up to this thing with 18 grad students running the rover.

AM: So what's the timeline for all this? For instance, how long to reach the etched terrain?

SS: We could probably make it to the etched terrain within a month, if we don't stop. There could be terrific science that could be done along the way, so if we see good stuff we'll stop.

As an example, it was about 700 meters from Eagle crater to Endurance crater. With a lot of stops, we did that in a month. So if we just hit the gas pedal and go, I think we could travel the two kilometers to the etched terrain in a month, and certainly in a couple of months.

AM: And how long to Victoria crater?

SS: I don't even want to hazard a guess. I think Victoria crater's out of reach. I don't think we're going to get there. It's a nice thing to dream about, but boy, it's far away.

AM: It's always good to have that next goal on the horizon, though.

SS: The point is that no matter how long the vehicle lasts, there's always something great out there to still go to. A rover's work is never done. It's a big planet, and a very tiny vehicle, so we're never going to run out of things to look at.

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