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MELISSA BLOCK host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

ROBERT SIEGEL, host:

And I'm Robert Siegel.

It may be springtime here but on the southern hemisphere of Mars, the dead of winter is approaching and that's gonna make life tough for NASA's two rovers that are currently puttering around on that planet. The rovers have lasted two years longer than expected, but both are showing clear signs of age.

And as NPR's Joe Palca reports, the extreme cold of a Martian winter could spell their doom.

JOE PALCA reporting:

The rovers were never supposed to last through the winter. They were built to last 90 days, but they've already made it through one Martian winter and scientists are hoping for two.

Though scientists filled mission control at NASA's jet propulsion laboratory in Pasadena when the rovers first landed, as the rovers lasted longer and longer most of the scientists had to return to their home universities. But thanks to some new tools developed by NASA mission managers, being back home does not mean being out of the action.

Mr. JOHN CALLAS (NASA): You could literally be in Starbucks with your laptop and your cell phone and be an active participant in the daily operations process for the rovers.

PALCA: John Callas is now project manager in Pasadena for the rovers. His job is to make sure these scientists keep getting data back from Mars. But Callas can't change the harsh conditions on the planet and now he says both rovers have begun to have problems.

Mr. CALLAS: Recently Spirit's right front wheel stopped working. It looks like it's a permanent failure. That means we have to drive with five wheels. Now when a wheel stops working on these rovers, it doesn't spin anymore so we have to drag the wheel and it makes for very difficult driving.

PALCA: Callas says Spirit was supposed to park for the winter on a nearby hill so its solar panels would point more directly toward the sun, but they've had to settle for a ridge closer by. He says other than the wheel, Spirit's in pretty good shape.

On the other side of Mars Opportunity is also having driving problems.

Mr. CALLAS: Each rover has four steering actuators, one for each front wheel and one for each rear wheel. The right front one has jammed on Opportunity.

PALCA: The stuck wheel makes it hard for Opportunity to change direction when it drives. Currently it's driving south to a huge crater called Victoria.

NASA sent the rovers to Mars to look for signs that water once covered the planet because where there's water, there could be life.

Albert Haldemann is deputy project scientist. He says the rovers have found signs that water once percolated through the Martian soil but they haven't found good evidence that lakes or rivers once covered the planet's surface.

Mr. ALBERT HALDEMANN (NASA): Once we get to Victoria crater with Opportunity, however, we may see some stunning views of large stacks of sedimentary rock that will probably just be at least spectacular and possibly also very informative scientifically.

PALCA: Even as the rovers slow down, NASA is well into planning for two more landing missions.

Barry Goldstein is project manager for the mission called Phoenix, scheduled to launch in the August 2007. That craft will go to the Martian north polar region where there's clear evidence for water in the form of ice.

Mr. BARRY GOLDSTEIN (NASA): What we're doing with Phoenix is we're gonna go to where we know the water is, dig into the water and investigate what's in the water on Mars.

PALCA: Phoenix has instruments that could detect if there was or is any sign of life in that water. Goldstein says unlike the rovers, Phoenix probably won't make it through a Martian winter. That's because in summer the frozen CO2 that also covers the north pole evaporates.

Mr. GOLDSTEIN: And in the wintertime what ends up happening is all the CO2, all the CO2 frost goes down onto the poles and what we expect will happen is we will be buried, encased in over a meter of solid CO2.

PALCA: Spirit and Opportunity don't have to worry about that prospect, but they will face temperatures more than a 150 degrees below zero.

Project manager John Callas knows that someday the end will come for his rovers and that will be big loss.

Mr. CALLAS: Because we very much have personified these vehicles. I mean we, you know, we think of them as our children because they have very human characteristics. You know, they see, they move, they interact, they respond to our commands and they think on their own and so they are very much like, you know, living creatures and we've become very fond of them. I know I've become very fond of the rovers.

PALCA: His colleague Albert Haldemann feels pretty much the same.

Mr. HALDEMANN: We're used to waking up every day and seeing the latest from the rovers and they're always there everyday, day in and day out, and if one day that stops being the case, it's gonna be very sad.

PALCA: Sadness punctuated by a sense of accomplishment.

Joe Palca, NPR News.

SIEGEL: And you can find out what a typical day on Mars is like for the rovers at our web site, NPR.org.

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