Stardust Set to Deliver Comet Samples

Stardust's principal investigator, Donald Brownlee, talks about what he hopes to learn from the comet dust samples collected by the probe.

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IRA FLATOW, host:

This is TALK OF THE NATION/SCIENCE FRIDAY. I am Ira Flatow.

A little bit later in the program, Ben Franklin's 300th birthday.

But first, a look at two space projects. One, completing a round trip, and the other, it's off on a one-way ticket. In 1999 NASA launched a mission to collect and bring back samples from a comet. In 2004, the probe called Stardust swung by a comet called Wild 2, stuck out sort of a big catcher's mitt--I think it looks more like a tennis racket--and hopefully it picked up some of the particles thrown off by the comet. Well, this weekend that mission comes home. A capsule that contains, we're hoping, these particles, is scheduled to land via parachute in Utah early in the wee hours of Sunday morning. Joining me now to talk about the project is Don Brownlee. He's a professor in the Department of Astronomy, University of Washington in Seattle, and project principal investigator for the Stardust mission. He joins us by phone.

Welcome back to the program.

Professor DON BROWNLEE (University of Washington-Seattle): Good afternoon.

FLATOW: You must be a little bit excited by now.

Prof. BROWNLEE: Oh, we're very excited. We're just a little over a day away.

FLATOW: Can you actually be in touch with the probe and know it's healthy and working?

Prof. BROWNLEE: Oh, yeah. We have a bevy of people here in Utah and the Jet Propulsion Lab in Pasadena and at Lockheed Martin in Denver they're flying the spacecraft. And we have control over it until four hours before touchdown. That's when we actually release the capsule that has the samples, and then it's on its own.

FLATOW: Well, you must be holding your breath a little--just a little bit, considering what happened to the Genesis capsule that crashed to the Earth a couple of years ago.

Prof. BROWNLEE: Well, not really. We--because of the Genesis event, with a parachute not coming out, we have undergone a series of quite intensive reviews, and the source of that problem with the parachute is well-understood, and we don't have the problem. It basically uses g-switches, which is like you use to blow out...

FLATOW: Right.

Prof. BROWNLEE: ...the air bags in your car, and they were put in backwards. We know ours were put in correctly because we fully tested them on accelerometers and centrifuges before launching.

FLATOW: Do you know that the comet actually has captured particles or do you open it up and take a look and see what's in there?

Prof. BROWNLEE: No. We--two years ago we flew past this comet and we were pounded by particles. We actually had impact censors on board, and the spacecraft was being hit by rocks up to almost half a centimeter, almost the size of--between a raisin and a grape in size. They were...

FLATOW: Wow.

Prof. BROWNLEE: ...pounding on the spacecraft. I don't know--if you look at the pictures from the Deep Impact spacecraft hit its comet on the Fourth of July, you can see the pictures wobbling from picture to picture because of impact of, you know, raisin and larger size rocks on--at extremely high speed. So...

FLATOW: So the...

Prof. BROWNLEE: ...comet is a dangerous thing to do.

FLATOW: Yeah. And the thing caught in a tennis racket sort of made out of aerogel, this really wispy, smoky sort of solid, right?

Prof. BROWNLEE: Yeah, it's an amazing material. The collector basically looks like a tennis racket filled with ice cubes, so this is kind of like a big ice-cube tray, and each ice cube is not made out of ice, but it's made out of this aerogel. It's a silica aerogel; it's a glass, but it has about the same density as air. We actually got a Guinness Book of World Records for developing this lightest solid material in the world. And the particles hit this at high speed and embed in it. It's actually very similar to, like, the police when they want to recover a bullet from a rifle...

FLATOW: Right.

Prof. BROWNLEE: ...and they take it and they shoot it in to a box full of cotton. That's basically what we're doing except we're using aerogel.

FLATOW: Why was Wild 2 chosen out--from all the comets that are out there?

Prof. BROWNLEE: Well, we chose comet Wild 2, first of all, because it had an orbit that we could get to in our seven-year mission and collect a sample at relatively low-encounter speed and then come back to Earth. But also this comet is very unusual, and it was only captured in its present orbit in 1974. It did a very close encounter with Jupiter in 1974, similar to the comet, you know, that crashed into Jupiter that dramatically changes orbit to an orbit that now it goes through orbit of Mars, through orbit of Jupiter. Before that it was in an orbit that went from the orbit of Jupiter out past the planet Uranus. But most of the history of the solar system--this is why the comet is so important--most of the history of this comet was spent out beyond Neptune, out where Pluto is at the very edge of the solar system, what we call the Kuiper belt.

FLATOW: And so then it still has a lot of its original material on it...

Prof. BROWNLEE: It...

FLATOW: ...'cause it hasn't gone around the sun a lot and gotten blown off by the solar.

Prof. BROWNLEE: Yeah, the real importance of comets like this is that originally when the solar system--you know, the sun and planets were forming, there was a disk of dust and gas spinning, and originally this disk was filled full of objects like comets. But then most of these bodies either went into forming planets or they went into the sun, or they were thrown back out in the galaxy. And there's only a few places like out where Pluto is where these bodies have survived. And then they're relatively small bodies and many of them are extremely well-preserved. In fact, they have ice in them--it means they've never been heated to the high temperatures. So when one comes in here--I mean, the big effort on this was having the comet come in to meet us. We went halfway to Jupiter to meet it. But the samples we collected from it we believe to be exactly the same materials that went into forming the comets and the solar system four and a half billion years ago. So it's a fantastic thing. We have basically the building blocks of the solar system preserved in this sort of cosmic library, if you will, for the entire age of the solar system and preserved at very low temperatures.

FLATOW: So when it comes back, do you literally have to pick the pieces of it, like with a tweezer, out of the aerogel?

Prof. BROWNLEE: They're much smaller than you can pick up with tweezers mostly. Yeah. They're extremely small, but we have lots of very, very fancy techniques to deal with them. Actually we do this all the time. And many of these samples we study literally at the atomic, single-atom level.

FLATOW: It is true--I saw on a Web site that you're actually going to put up pictures, up to a million or a million and a half pictures of the comet so that--of the aerogel so that people can try to pick out the pieces themselves?

Prof. BROWNLEE: Yeah. We--the mission is called Stardust for actually two interesting reasons. On the main side, the front side of our ice cube tray, if you will, we exposed that when we flew past the comet, so we caught the comet dust. But one of--a major constituent of comets, we believe, are stardust, interstellar grains, particles that formed 'round other stars and were the original seed of the star formation of the solar system. So when we collect comet dust, we're actually collecting some stardust that was, you know, four and a half billion years old and was here when the solar system formed.

As we orbited the sun, we exposed the back side of the collector to stardust, interstellar grains, that are actually coming in to our solar system at the present time. Right now there's a flux of stuff from other stars from our galaxy that's flying into the solar system. We collected those, although they're smaller and they're much fewer and they're much more of a challenge to find and analyze than the much larger comet particles. Some of the comet particles would be almost millimeter size. But the current interstellar particles that are--recently fell in are small and hard to find. So people at Berkeley that did the SETI@home project came up with this very clever thing. This was run by Andrew Westphal at the University of California-Berkeley called Stardust@home where people could download pictures from these--what we call the interstellar tray, interstellar side and search for impacts of interstellar dust. And last I heard yesterday 17,000 people have signed up to do this in just about a couple days on the Web.

FLATOW: Wow. Wow. We'll be keeping our fingers crossed for your--What?--later Saturday night, early Sunday morning out there in the desert. Good luck to you.

Prof. BROWNLEE: Thank you very much.

FLATOW: Take care. We're talking with Don Brownlee, who is the Department of Astronomy, University of Washington in Seattle and the project principal investigator for NASA Stardust mission.

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