NASA's Dawn Spacecraft Closes In On Its Target

The Dawn spacecraft will soon enter orbit around Vesta, an asteroid the size of Arizona, to study it. Principal Investigator Christopher Russell discusses how the craft will investigate the asteroid's elements and surface minerals, and what that might reveal about the birth of the solar system.

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You're listening to SCIENCE FRIDAY. I'm Ira Flatow. It's been nearly four years since NASA's Dawn spacecraft set out from Earth on a journey past the orbit of Mars to the asteroid belt and its target, Vesta, a chunk of rock the size of Arizona.

Earlier this month, after traveling over a billion miles through space, the spacecraft spotted the asteroid, snapped a fuzzy little picture, and sent it back to Earth. It's now cruising closer at a leisurely speed of 670 miles per hour, preparing to slip into orbit around the asteroid in July if everything goes according to plan.

But what is it going to tell us about the asteroid, and what's it going to do when it gets there? Why visit an asteroid? Why not some other target, like a planet? And with President Obama's vision to send astronauts to an asteroid, can this mission sort of be laying the groundwork for that?

That's what we'll be talking about. Our number is 1-800-989-8255, 1-800-989-TALK. You can tweet us a question @scifri and also send us your comment at

Christopher Russell is principal investigator of the Dawn Mission and a professor of geophysics and space physics at the University of California, Los Angeles. Welcome to SCIENCE FRIDAY, Dr. Russell.

Dr. CHRISTOPHER RUSSELL (UCLA): I'm pleased to be here.

FLATOW: It's been a long ways, hasn't it?

Dr. RUSSELL: Oh, has it ever. And you mentioned the four years since launch, but we really started this project back in 1992, when they announced the possibility of investigator-led small missions to the planets. And so it's been an even longer journey for us than just those four years.

FLATOW: Wow, and why this asteroid? I know it's big, the size of Arizona. Did that have something to do with it?

Dr. RUSSELL: Well, none of well, I guess there are some members of the team from Arizona, but...

(Soundbite of laughter)

Dr. RUSSELL: But that wasn't the reason. The reason is because in many senses it's a smaller body like the moon. So it's got a basaltic surface. It's got lava flows that came out early in its existence and, you know, froze solid and, you know, lifeless ever since. But it's very much a moon-like body, but it's about one-third of the radius of the moon, in fact less than one-third of the radius of the moon.

So it's a small, a small body, but it's got many of the same processes that the moon has and the Earth, for that matter.

FLATOW: And so how do you study it? What instruments? What kinds of photos, spectrographs, things like that?

Dr. RUSSELL: Okay, well, we have a camera onboard that will map the entire surface in both a clear filter and also in color filters. So we'll get some idea of the color in the visible.

We also are measuring the infrared and looking at the reflectance of the asteroid as a function of wavelength because there is some characteristic bands in the reflectant spectrum that enables us to determine what the minerals are on the surface.

And we're also measuring the gamma rays and the neutrons from the surface that will enable us to determine the elements in the surface, as well as - especially as well as hydrogen, which will give us some idea of is there any water on the asteroid.

FLATOW: And next, after this asteroid, Dawn is flying to another one, Ceres. It's even bigger, right?

Dr. RUSSELL: Yes, that's the biggest asteroid in the main belt. In fact, it's so large that it has been given the name Dwarf Planet. So it's in the same category as Pluto and a number of other bodies in the more distant solar system.

And that's mainly because it's so massive, although it's small, it's massive enough that it makes its surface spherical. So it's in what we call hydrostatic equilibrium rather than the, you know, an irregularly shaped rock. It's got enough mass to bring everything into a nice little tight sphere.

FLATOW: And what would you learn on Ceres that you would not learn on Dawn?

Dr. RUSSELL: Well, Ceres is - although it was, you know, although it exists - I don't know exactly where it was made, but it was probably made in a similar region - even though they're in similar locations, they're very, very different.

And what we are interested in is trying to find out what caused that difference. Ceres is much lighter. Okay, that's the first clue, that you can measure its size fairly accurately with the Hubble Space Telescope, although we'll be able to do much better with Dawn.

And you can measure its gravitational field, and therefore its mass, from Mars. The - we now know the position of our spacecraft on Mars very, very accurately, to about maybe five meters. And we can - when Ceres and Vesta rotate by, then they move Mars, move the spacecraft a little bit, and we can measure that, and we know therefore how massive it is.

And so we can then solve for the density. And the density is a clue. You know, the density of water is one in our usual units, and the mass or the average density of Ceres is two, and the average density of Vesta is about 3.5.

And we therefore infer that Ceres has rock plus water, and from the density of Vesta we infer that it has rock and iron in it. So we think that Vesta has an iron core like the Earth, but Ceres may have 100 kilometers' worth of water in it. And maybe that's separated and making an ice, or ice plus water, ocean, very deep underneath a frozen crust that's pretty dusty.

FLATOW: Is Ceres a good candidate to send space probes, or astronauts, as we're talking about?

Dr. RUSSELL: Well, when we made these discoveries about the possibility of water, the first thing that NASA did was declare this as a body of astrobiological interest so that we can't do silly things there like land on it without having a disinfected spacecraft. So we have - we didn't disinfect our spacecraft. So we're going to have to back off after we've made our Ceres measurements and get into an orbit that is safe so that we don't come down on the surface.

So yes, it's a very important discovery, and it's a very - I would say fairly benign place for life to exist, much better than the moons of Jupiter and Saturn.

FLATOW: Now, the asteroid belt is out past Mars and in between Jupiter, correct? Between Mars and Jupiter?

Dr. RUSSELL: Right, exactly.

FLATOW: And there's enough rock out there there's a lot of - you know, we see all these movies of flying through asteroid belts like they do in "Star Wars" and thousands and thousands of big rocks getting in your way. Is that what it looks like out there?

Dr. RUSSELL: If you look, you know, in the disk of it, yes, there's going to be a lot of bodies out there, but as you're cruising around, the rocks are far enough separated that the chance of running into a decent sized rock is fairly small.

And one of the things we'd like to do as we're moving between Vesta and Ceres is to visit some of these other bodies that are out there. However, there's not a lot of large bodies along our trajectory. So we don't know yet whether we'll be making any neighborly stops on the way through from Vesta to Ceres.

FLATOW: Well, thank you very much, Dr. Russell, for taking time to be with us. And good luck to you. We'll be watching.

Dr. RUSSELL: Okay, thank you very much.

FLATOW: You're welcome. Christopher Russell, principal investigator of the Dawn Mission and professor of geophysics and space physics at the University of California, Los Angeles.

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