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Astrophysicist Scott Sandford Discusses Stardust Preliminary Findings (Segment 1)
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Jesse Carpenter: NASA study finds new kind of organics in Stardust mission samples. Hi, I’m Jesse Carpenter and you’re listening to a podcast presented in three parts from the NASA Ames Research Center. Launched in 1999 the Stardust spacecraft collected dust particles from the tail of the comet Wild 2 in 2004 and returned the samples to Earth in 2006. Today, we have an interview with research astrophysicist Scott Sandford. He’s the lead author of the preliminary findings from the Stardust sample return mission. Scott, tell us why would we want to go to a comet to begin with.
Scott Sandford: Well, I think one of the principle reasons why we want to go to a comet is we think that the material that comets are made out of hasn’t been processed much since comets formed 4.5 billion years ago, when the solar system formed, and as such, they may be one of the best reservoirs in the solar system of the original, primordial stuff from which everything else in our solar system was made. So a component of this material could be organic compounds, complex organic compounds. And so if material from these comets was rained out on the Earth after its formation, and we think it was, then this material may have played a role in getting life started because these complex organics that are basically falling out of the sky could have participated in all kinds of chemical reactions that could have led on to much more interesting things like us.
Jesse Carpenter: Tell us where the samples came from and how unique that is.
Scott Sandford: Okay, well, the samples came from a comet called Wild 2, which currently is in an orbit where the closest it gets to the sun is not quite the Earth’s distance from the sun, and the furthest it gets from the sun is all the way out at Jupiter’s orbit. But we collected this material by flying a spacecraft at fairly high velocity through the coma of the comet. And the coma is the cloud of dust participles surrounding a comet. It’s the dust particles that ultimately make the tail that most people are familiar with. And so we basically swept up some of this dust as we went piling through this dust cloud.
Jesse Carpenter: Why go to this comet?
Scott Sandford: Okay, well, there are sort of several reasons why we picked Wild 2 as the comet to go to. The more mundane one is it was a comet we could get to. I mean the comet is in an orbit now where the furthest it gets from the sun is Jupiter’s orbit. The closest it gets to the sun is getting down near the Earth’s orbit. So it doesn’t take a gigantic rocket motor to get there. And so one of the reasons we went is because we could get there. However, it turns out that even if it had been harder to get to, this is a comet we would have liked to have gone to because prior to 1974, this comet was in an orbit where the closest it got to the sun was Jupiter’s orbit and spent most of its time out by Neptune. But in 1974, it almost slammed into Jupiter, just missed it, and that caused -- Jupiter bent its orbit into the current one, and so this is a comet that has not been near the sun for probably most of its lifetime and has only gotten near the sun a few times.
So it hasn’t lost all the volatiles and the ices and the things that come off a comet when it gets near the sun like most other comets. Haley’s Comet’s been near the sun probably hundreds of times and so when you go to Haley’s Comet, the surface you see is not the original surface. It’s a surface that’s been cooked many, many times. Well, in the case of Wild 2, it’s only been cooked a few times since 1974, so this meant that we had a good chance when we got to Wild 2 that we would get a sample of kind of a raw comet, a comet that’s not been, you know, slapped around much. And the images we got when we flew by Wild 2 suggests that’s the case, that, in fact, we’re looking at a very much more pristine surface. And so the hope is that means that the sample we captured is also more representative of the original comet and not of a comet that’s been processed over time.
Jesse Carpenter: Are there any preliminary observations you might be able to share with us?
Scott Sandford: We’ve noticed a number of interesting things. One is that comet dust seems to be a real zoo of things. We see all kinds of particles that clearly formed in different places, possibly at different times, and certainly under different conditions. And so there’s a very strong implication that the comet contains components from all over the solar system when it was forming, so when the solar nebula was first making the planets and the sun, there must have been a lot of mixing going on. A solar nebula may have been a big, giant, you know, washing machine at some level.
Jesse Carpenter: So what is significant about this material?
Scott Sandford: Okay, well, it’s of great interest to us because we think it is the starting material from which everything was made, and so it tells us something very important about a very important part of the history of our solar system, when it formed. It tells us what it formed from, something about the conditions under which it formed, and so on. And so it really is a kind of Rosetta Stone, in some respects, of extra terrestrial material for us to understand this key component of our history.
Jesse Carpenter: Give us an idea of the size of sample that was typically found in the aerogel.
Scott Sandford: Normally, if you tell somebody you have a mission that’s returning a sample to Earth, the first vision everyone has is the Apollo astronauts bringing big boxes of rocks back. But in the case of the Stardust mission, we brought back a modest number, thousands of sub – of micron-sized grains that hit the aerogel, so we’re talking about the particles that we collected measuring, being measured in picograms, so a billionth of a gram of material. So if you add our whole sample together, it’s much less than a milligram, which doesn’t sound like very much material for all that effort, but, in fact, it’s plenty of material for what we want to do. The size of a sample you need, to understand it, has more to do with the nature of the sample than the volume of the sample, and the nature of comet dust is to consist of conglomerates of really tiny things. And so if you get a 10-micron grain, that’s not much material, but if it contains 10 million sub micron grains that represent the composition of the comet as a whole, then that’s enough material. So fortunately for us, since we brought back these modest amounts of material, it’s sort of as we expected -- comet dust is very fine-grained, so the material we’ve got has been very complex, and there’s a rich amount of stuff to learn even in a single grain. And the fact we don’t have tons of it is not a problem.
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