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IRA FLATOW, host:

You're listening to SCIENCE FRIDAY. I'm Ira Flatow.

NASA has certainly got its money's worth out of its Mars Rovers. Designed to operate for just a few weeks, Spirit and Opportunity have been chugging along, collecting data from Mars for, oh, six years.

But this past spring, Spirit got stuck in the sand. It's caught in a patch of Martian soil that's tricky to navigate, and NASA says it may be stuck there for good.

So what do you do if you're driving the Rover, and you've lost your locomotion? Do you give up on Spirit? Our next guest says no, and he should know about it. He's Steven Squyres - is principal investigator for NASA's Mars Exploration Rover Mission and a professor of astronomy at Cornell University in Ithaca. Welcome back.

Mr.�STEVEN SQUYRES (Principal Investigator, NASA's Mars Exploration Rover Mission; Professor of Astronomy, Cornell University): Good to talk to you, Ira, how are you?

FLATOW: So how stuck are we?

Mr.�SQUYRES: It's been pretty rough lately. You know, we're continuing to turn the wheels. We're continuing to try to move the vehicle. You know, in a really good day, we'll go five centimeters, a couple inches.

FLATOW: Wow.

Mr.�SQUYRES: It's a very, very difficult predicament that we've gotten into. Not only is the sand that we're in very soft and very deep, but two of Spirit's six wheels have no failed. They don't turn anymore, and when you're dragging two dead wheels, it's hard to go anywhere.

FLATOW: I say that every morning when I get up.

(Soundbite of laughter)

FLATOW: All is not lost, though, right? You can use it as a stationary base if you have to?

Mr.�SQUYRES: Yeah, there are a couple things. I mean, first of all, we're not going to stop trying to move this thing. We're not going to stop trying to move the wheels. Even if we can only move short distances, distances of a few centimeters, there's a substantial scientific value to that. So we are going to keep turning Spirit's wheels and seeing what happens.

But even if Spirit never leaves its current location, there is a lot of science that we can do, including some new stuff we've never done before.

FLATOW: Such as?

Mr.�SQUYRES: Well, the one I'm most excited about, believe it or not, we think we can use Spirit to find out whether or not the core of Mars is made of liquid or solid iron. It turns out that if you track a spacecraft very precisely, you can measure its position in space with enormous accuracy. And if the Rover's not going anywhere, then the only motions you're seeing are the motion of Mars and its orbit, which we understand very well, and then the spin of Mars on its axis.

So by tracking the spacecraft, we can determine that spin very precisely, and what we can measure is how the spin axis of Mars is oriented in space.

Now, the spin axis of Mars doesn't stay fixed in space. It wobbles a little bit like a top or a gyroscope. And it turns out that if Mars has a solid core, it wobbles in one fashion, and if it has a liquid core, it wobbles a little differently.

And we think that by tracking Spirit's radio signal for about six months, as long as the Rover's not much, that we can tell the difference between those two wobbles and find out whether it's got a liquid core or a solid core.

FLATOW: And you want to know that why? What does that mean?

Mr.�SQUYRES: Well, that's a fundamental characteristic of the inside of a planet. Some planets have liquid in their interior, some are solid. The Earth has a solid inner core and a liquid outer core. We want to learn what the inside of Mars is like.

We've done a great job of studying the surface of Mars, but if you want to understand the planet as a whole, how it's evolved over time, knowing what's going on inside is important, too.

FLATOW: 1-800-989-8255 is our number. You can also tweet us. Our Twitter is @scifri, @-S-C-I-F-R-I, talking with Steven Squyres.

When will you know from what I understand, let me rephrase this I understand you also have a problem about the upcoming winter, whether you'll survive the winter or not, sitting there.

Mr.�SQUYRES: Winters are always tough for this Rover because we're pretty far from the equator with Spirit. When it gets to be wintertime, the sun drops low in the sky, and if you're a solar-powered Rover, that's a bad thing.

So we expect this Rover this winter, rather, to be particularly harsh. In previous winters, we've been able to kind of tilt the solar arrays towards the sun by driving on a sunward-facing slope. We can't do that right now.

So it's going to be a tough winter for this vehicle. We think the chances for survival are actually quite good, but we think it's likely that the Rover is going to have to drop into a kind of a hibernation mode for a number of months, and we may actually go two, three, four, six months, even without hearing from the vehicle.

But then when springtime comes, and the sun rises higher in the sky, good chance that Spirit will wake up and start talking to us again.

FLATOW: So it's so crippled that you can't even now tilt it to the right direction that it would need to be tilted?

Mr.�SQUYRES: We could the only way you can tilt this vehicle is by driving it onto a hillside that faces the direction you want to tilt, and we're only moving small distances right now as we turn the wheel. So that's tough to do.

FLATOW: What about all the suggestions that have been coming in, things like using the robot's arm, you know, to lift it up and all that kind of stuff?

Mr.�SQUYRES: Yeah, those are great. I have gotten so many wonderful emails from people because, you know, people have gotten wrapped up in this drama. And everybody has had an experience of getting their car stuck somewhere and trying to figure out how to get it out. So this is a familiar predicament to many people, and we've gotten so many suggestions.

You know, some of them are actually very useful, very helpful, and some of them we have been using. Others, for example using the arm wish we could do it. You know, I wish we could use the arm to help us, but that arm was designed for precision placement of delicate scientific instruments. It is not strong enough, I'm afraid, to push spirit out from its current location.

So we're stuck to what we can do with the wheels, and we're doing the best we can with that.

FLATOW: Gosh, I remember those days we used to rock the old car out of the sand back in the if you can't do that. But can you use the arm to shove sand under the wheels? Remember in the beach, if you got too deep, your wheels would start spinning because the sand is holding the car up now, and there's no traction.

Mr.�SQUYRES: One thing that we've talked about would be using the arm to try to nudge a rock underneath the left, front wheel. It doesn't help to put anything under the right front wheel because that wheel doesn't turn. It's one of the ones that has failed.

We've talked maybe, come springtime, about moving the arm to try to nudge a rock under that left front wheel and get a little traction that way. We'll see. If the Rover survives the winter, and springtime comes, and that looks like it's still a useful thing to try, then maybe we'll try it.

FLATOW: Charles(ph) in Birmingham, Alabama. Hi, welcome to SCIENCE FRIDAY.

Mr.�SQUYRES: Hi, thank you for taking my call. I just had a quick question and, I guess, a comment. I really appreciate the Rover being up on Mars and it being our first step toward, I guess, space exploration. I guess my question is: What are we working towards to further along our exploration in space, knowing that us living on Earth, we have so much knowledge out there in space.

I guess my question is: What are we what is our next step past Mars?

FLATOW: Thanks, Charles.

Mr.�SQUYRES: Well, there are a lot of things still about to happen on Mars. Besides the continued operation of Spirit and Opportunity and a couple of orbiters, several orbiters that are orbiting Mars right now, there's a new mission called the Mars Science Laboratory Mission that's going to launch in 2011, is going to put a big, more capable Rover than Spirit and Opportunity on to the surface and operate that for probably a fairly extended period of time.

What I'd like to see happen on Mars sometime in the future is a mission that would actually bring rocks back from the Martian surface: land on the surface, collect rocks, bring them back to Earth and put them into the best laboratories on Earth and really find out in much more detail what those are like.

Look further out in the solar system. You know, my favorite thing out there has always been Europa. It's one of the moons of Jupiter. It's got a crust of ice. Beneath that crust of ice, there appears to be an ocean of liquid water, and boy, I'd love to know what's in that ocean.

FLATOW: Yeah, but there was a plan, if I remember back in my dusty memory, of something called a Mars Sample Return Mission that was planned. Did that just get scrubbed, or there was no money for it after a while, or what happened with that?

Mr.�SQUYRES: We've been talking about Mars Sample Return for a very long time, bringing samples back. It turns out that when you look hard at what it really takes to do it, it's an enormously challenging mission technically, and it's going to be very, very expensive.

And so it's not something that's in the immediate future right now. We've got to probably take the problem and kind of break it down into pieces, have one spacecraft that goes and collects the samples, another one that goes later and picks them up and takes them into Mars orbit, maybe another one that goes and finds those and brings them back to Earth.

So by breaking up the problem into several parts, you can probably make the whole thing a bit more affordable.

FLATOW: So where is Spirit's sister-ship, so to speak, sister Rover Opportunity? Where is that vehicle?

Mr.�SQUYRES: Opportunity's on the other side of the planet, has six good wheels and is doing very well. We just today pulled up to the rim of the freshest impact crater we've ever seen on Mars with either Rover.

It's a crater called Conception. It's named after one of Magellan's ships, and geologically it formed yesterday. Now, I don't know when it actually formed, but it's the youngest, freshest, most-recent crater that we've ever seen on Mars.

It's kind of a scary-looking place, actually. I just saw the first good pictures of it this morning. If you go to marsrovers.jpl.nasa.gov, you can see the pictures virtually as they come down from the spacecraft, and it's a very rocky, blocky, kind of dangerous-looking place. So we're going to have to watch our step around Conception, but it's a very interesting feature, unlike anything we've seen before.

FLATOW: Is that where you were headed all this time?

Mr.�SQUYRES: No, this is just an interesting stop along the way. Where we're headed is a crater called Endeavor, which is huge. This thing is more than 10 miles in diameter. It's an enormous feature. It's still quite far away. But this is sort of an interesting little way-station on the way to Endeavor.

FLATOW: So when getting back to Spirit, when do you decide to give up trying to pull her out of the sand there?

Mr.�SQUYRES: Well, we're never going to give up trying to move the vehicle, okay? Our intention is to continue to try to move the vehicle whenever we can because even if all we get is small amounts of motion, even if it only moves five centimeters, 10 centimeters, we have an arm on the front of the vehicle, and it's only about the size of a human arm.

It doesn't have a very long reach, and so if you can just reposition the Rover even slightly, you can bring new patches of soil within reach of that arm, and you can study new things. So we're going to keep trying to move it, but how far we're going to move is hard to say, and our recent experience suggests not very far.

One thing that we do have to realize, though, is that very soon, probably simply within the next few weeks, we're going to have to stop all attempts to move the vehicle as winter becomes more severe. We just won't have the power to do it, and that's the point at which we'll just kind of hunker down and try to survive until spring comes, and that's coming real soon.

FLATOW: So it has a power-down mode to just go to sleep for a while?

Mr.�SQUYRES: It has yeah, it has kind of a hibernation mode that it goes into, and we don't command it to do it. We don't tell the Rover it's time to hibernate. The Rover senses that it has very little power and goes into that hibernation mode all by itself and kind of stays that way until the power comes back up again. That's the way it preserves as much power on board as it can.

FLATOW: So you just have to keep your fingers crossed that, come spring, it'll power itself back up?

Mr.�SQUYRES: Keep your fingers crossed, be very patient and hope for the best. That's all we can do.

FLATOW: All right, Steve, thank you, and good luck to you.

Mr.�SQUYRES: Thanks very much, good to talk to you again.

FLATOW: Thank you, Steve. Steven Squyres is the principal investigator for NASA's Mars Exploration Rover Mission and professor of astronomy at Cornell University in Ithaca, and he was talking to us from a studio at the campus there in Cornell.

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