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Principal Investigator on Dawn's Asteroid Mission
From the Kennedy Space Center in Florida, I'm Elaine Marconi.
The Dawn spacecraft is about to embark on a journey to study a pair of asteroids. Why study asteroids and why is what will this mission accomplish?
Principal Investigator for the Dawn mission, Chris Russell is here with us now to give us some insight...welcome!
Thank you, I’m pleased to be here.
Tell us about the asteroid belt and specifically these two planetoids.
There’s many ways to describe the asteroid belt. One of the important ways is it’s a repository of the historical record of the solar system. In that region are the earliest bodies that were formed when the solar system formed. Some of these bodies have been relatively unscathed. And some of them grew to relatively significant sizes. We have a chance now to go out with the Dawn mission and look at these variety of bodies and to learn more of the earliest epoch of the solar system.
What are you going to learn about Ceres and Vesta that you don't already know?
Well we know very little about these two bodies at the present time. All we have are telescopic observations and even with our best telescope, with the Hubble Space Telescope for example, we cannot resolve very well the features on the surface, we can tell that there’s features on the surface, they may be craters, they may be mountains, they may just be coloration changes across the surface. But they’re teasing us, they are teasing us to go out here and explore them with our high resolution cameras.
How are the two asteroids like other planets in the solar system?
The two asteroids are very different from each other amazingly, even though
they are fairly close, but one of them is very similar to our moon and the other is very similar to the moons of Jupiter.
The first body, Vesta closest to
the sun of the two, has basaltic flows on the surface, that’s lava flows that
came out very early in its history and we can tell that from the reflected light
that we see back on Earth. If you shine a light or say sunlight on a lava flow on Earth and you shine sunlight on Vesta you get a very similar spectrum or color of the light.
The Ceres is a much darker object and when we use the few bits of information we have like how massive it is, the size it is and its shape we infer that it has lots of water in it. And the moons of Jupiter and also many of the moons of Saturn are water-ice rich and so we have it seems a representative of the inner solar system in the asteroid belt and a representative of the outer solar system and they are now close enough in the sky that one spacecraft can explore the first and leave after exploring it and go and explore the second one. Which is just a marvelous opportunity for us.
Will these studies help us understand near earth objects...is that the goal of the flight?
The goal of the flight is not to learn more about the near-Earth asteroids themselves, we get new information about the formation of the
asteroid belt as a whole and that should help us understand the near-Earth
objects which are, in general remnants of collisions and scattering of particlesfrom further on out so we’re operating in the heart of the asteroid belt, the near-Earth objects are at the edge, but we believe that the near-Earth objects are In fact representative of just scattered out of the heart of the asteroid belt.
How are Ceres and Vesta different from each other?
The first difference of Vesta and Ceres is their size, Vesta is a little more than
one half the size of Ceres in diameter it’s about 330 miles, where as Ceres is almost twice that, a little over 600 miles across. They’re also different in their density that Vesta is about the density of Mars somewhere around 4 grams in every cubic centimeter and Ceres is much less dense, about half that value.
In order to be so much less dense if it has as much rock as it should have, it should have as much rock in it roughly as Vesta then it must have had, must have some lighter constituents to it and the most obvious constituent that it would have is water and so if we calculate how much water it would take to make Ceres the observed density we get an outer layer of water or possibly water-ice of about 100 kilometers or about 60 miles thick crust of ice. So one we think is an icy body perhaps covered with dust or clay and one is a rocky similar to our moon but smaller.
What's been the hardest part about getting the mission started?
Well I think it’s the aging process, we started back in 1992 at a discovery
workshop where a engineer from the NASA Lewis Center, now NASA Glenn
told us about ion engines and what ion engines could do. It did not take me
long to realize that the ion engine was a way of doing something I had always wanted to do and that is, to reuse instruments that we might use to explore the surface of one body on another body, instead of building two missions we would save money by building one mission and exploring 2 objects.
My first idea was to explore the moon and then to go explore a dormant comet.
And I proposed that in 1994, it was pretty ambitious. And at that time, NASA was a little shy of starting off this program with a very ambitious program, a very ambitious mission. So we didn’t get selected, what did get selected was a lunar mission and a comet basically a cometary mission which basically went out to collect planetary dust and bring it back so we decided next that we needed, we loved the idea, but we needed to change the objects that we were studying so the first proposal had the moon in it and the most moon-like asteroid is Vesta and so we decided to go to Vesta.
And to continue the study in the way I had envisioned it. We also decided to go to two smaller asteroids, minor, minor-minor planets you might say, and we picked: Glassenappia and also Lutetia these are much smaller although their important size but even though that this was again daring and the next step that we should be taking in space, it did not get selected. The next time we had an opportunity to propose was 1998 and this was the time when Deep Space 1 was being launched.
I guess they had been reading my proposals for years finally test these ion engines, so they took the ion engines and were going to put them on a spacecraft that would test them out and then they could become a workhorse for the planetary program. When my proposal went in and this time Vesta and one other asteroid, I believe it was Glassenappia again, when the proposal went in and it got to the evaluation committee it was
just about the time they tried to fire the engines on Deep Space 1, and the
engines didn’t fire properly.
They stopped thrusting and it took them a few
weeks to get them thrusting again. Eventually the engines started thrusting
and everybody was happy. However, NASA was still a little edgy about
selecting, putting their money behind an ion-propulsion mission.
Now but, you know, time was on my side, I was still young and so I re-proposed again in the year 2000. This time, nature was helping out because Vesta and Ceres had got closer together and once they get close enough together in the sky, then a spacecraft can go from one to the other fairly easily.
Now Vesta and Ceres have different orbits and it takes Vesta 17 years before it can lap Ceres and catch up with it again, so we call that the synodic period, the period which it takes one object to initially be lined up, say with the sun, and then go around and get lined up again.
And so this position from going one to the other, that relative position in the sky only happens every 17 years. But, I was fortunate enough to be around when it happened and writing a proposal and so we sent in the proposal and we said we can do both Vesta and Ceres. Nobody else had thought of that. The competition said, “Oh, we’d like to do Vesta.” But nobody said we’d like to do Vesta and Ceres and made the case for it and at that point we were selected. And as they say the rest is history, we put the mission together and now it’s sitting on the pad all stacked up and ready to go it just needs to have the fuel tank, tanker come over and pour whatever it does into those rockets.
What made you want to work on this mission?
Well when I was very young, which seems only just yesterday I worked on the Apollo program and I got very interested in the moon the physics of the
moon, the interior of the moon, the surface of the moon, and how the moon was made and I initially when we started this as I mentioned, I was interested in finishing up our exploration of the moon use this mission to do things that we had not finished doing on the Apollo Program.
When that opportunity was taken away from me in the sense, in you now a figurative sense by selecting the Lunar Prospector mission, I though that was a good thing to do, but I would have preferred to do it myself but I still had the same objective in mind, to learn about other bodies that were similar, and Vesta was the most analogous body to the moon, and then what I was looking for in these series of proposals that I wrote were complimentary bodies that would contrast with Vesta and teach us more by that contrast we learn a lot in life by taking two objects and comparing them, like an apple and an orange. You know, you could study an apple forever but not really realize what makes an apple a fruit.
Now if you compare with a orange and say, “well an orange is a fruit and an
apple is a fruit.” What’s in common?” then you learn more about the generic
class and that’s what we’re trying to do now we’re not trying to tell oranges
from apples we’re tying to find out why the apples and oranges are alike.
How is the Dawn mission different from previous flights through the asteroid belt?
Well there are lots of ways, most of the missions through the asteroid belt have not spent very much time there, they have been heading someplace else, like Galileo heading off to Jupiter and Cassini heading off to Saturn. they looked at asteroids on the way by, but they just took a fleeting glance at them. We had one mission early in the Discovery Program called NEAR that was in orbit around Eros, that was a near-Earth object.
it’s an asteroid that gets very near to the Earth, it’s very small, basically a uniform block of rock and we didn’t learn very much about the formation of the solar system from it, we understood a little bit more of what near-Earth objects are like but we didn’t really get, we really weren’t able to travel backward in time very far with that particular investigation. So what we’re trying to do is to go out to the heart of the asteroid belt to orbit these bodies, not to just fly by like Galileo and Cassini, and really do detailed in-depth investigation of them. And again, using one set of instruments, one spacecraft and two extraterrestrial bodies.
Chris, thanks so much for joining us.
For more on the Dawn mission, visit nasa.gov.
I'm Elaine Marconi.
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