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MADELEINE BRAND, host:

This is DAY TO DAY. I'm Madeleine Brand.

If you live in western Utah and northern Nevada and you're out late Saturday night, you might catch a glimpse of a fiery object streaking across the night sky. It's a little hundred-pound space capsule called Stardust, and it's carrying pieces of a comet back to Earth after a seven-year, three-billion-mile journey into space. Here to tell us more is Ira Flatow. He's host of "Science Friday" and a regular Thursday contributor to DAY TO DAY.

Hi, Ira.

IRA FLATOW (Host, "Science Friday"): Hi, how are you?

BRAND: So tell us how this hundred-pound spacecraft captured pieces of a comet.

FLATOW: Yeah, it's quite interesting. It's actually a long story, the ending of a long story. The spacecraft called Stardust rendezvoused with a comet called Wild 2. Now this is orbiting out there between Mars and Jupiter. It came with 150 miles of the comet and then it stuck out a big paddle, sort of like a giant tennis racket made out of very soft and porous material called an aerogel. And it goes by and it scoops up the dust from this comet, and if all goes well, the spacecraft and its cargo will gently parachute safely to the ground about 3 in the morning, local time--that's Saturday night--at an air force base southwest of Salt Lake City, and then these samples will be quickly whisked off to NASA space headquarters in Houston where they will be studied.

BRAND: And you said if all goes well. Could it not go well?

FLATOW: Well, it could, you know. The last time NASA tried to do something like this there was a disaster. You might remember the Genesis mission, which was bringing samples back from the sun and crash-landed in September of 2004 when its parachute failed to open due to human error, something as simple as the switches being installed backwards. Well, this spaceship has a very similar parachute mechanism on it, but a prelaunch test of the parachute system found this one to be A-OK, as they say in this business. So everyone involved is hoping that there is a much softer landing this time, on the order of about 10 miles per hour. And as you say, if you want to catch a glimpse of the fireball re-entry, you got to head toward Utah and hope for a clear night. But even if it's cloudy, you still might be able to hear the sonic boom.

BRAND: And what do scientists want to learn from this comet dust?

FLATOW: Well, it's interesting, because comets originate in something called the Kuiper Belt, and, in fact, this one has also been roaming in the area of icy, rocky bodies just beyond Jupiter. And scientists believe that comets are actual remnants, leftover pieces, from the formation of our solar system billions of years ago, and by studying the comets so we can literally go back in time. And Wild 2 was chosen because it's sort of a special comet. It's not a very well-traveled comet. You know how comets, whenever they come close to the sun, they develop that long tail? Well, this is the dust that is being blown off the comet by the heating of the sun. Wild 2 has only gone around the sun about five times in its long life, and that means that the dust on the comet has remained intact. It's sort of the original stuff, still there.

BRAND: And I understand, Ira, that this is actually interactive, that people can actually help scientists...

FLATOW: Yes.

BRAND: ...analyze these particles?

FLATOW: Yes. NASA is putting over a million pictures of the aerogel up for viewing on its Web site. And you can actually help to sift through the photos looking for the tracks made by the dust. And it looks like everyone, including space scientists, like to show off the slides from their trip.

BRAND: Ira Flatow, host of NPR's "Science Friday" and our regular Thursday contributor, thanks a lot.

FLATOW: You're very welcome.

BRAND: And there are links to the Stardust Web project at our Web page, npr.org.

DAY TO DAY returns in a moment. I'm Madeleine Brand.

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