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Dawn - mission to asteroids Vesta and Ceres -

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Robyn Williams: Meanwhile, just starting out, another spacecraft called Dawn is sailing on the way
to inspect two remote asteroids. The 'daddy' of this mission is Chris Russell.

Christopher Russell: They are the two most massive asteroids in the asteroid belt, to start with,
but they're very, very different. Vesta is a very dry body. Think of it perhaps a little bit like
our Moon with a lot of craters on the surface, basaltic flows on the surface, so very barren and
ancient-looking. Ceres, on the other hand, is much more of a mystery to us, but it's a much darker
object, it doesn't seem to have the same topography on it as Vesta does, it's much smoother,
rounder and bigger, and it's made of much different material, in our opinion.

Robyn Williams: What about the mystery?

Christopher Russell: The mystery of Ceres is that we do not have any meteorites from it. Vesta -
we've learned a lot about Vesta because one out of every 20 meteorites that fall on the surface of
the Earth come from Vesta, so we feel very much at home with it. We've got pieces of its surface
that we're analysing and understanding, and understanding its chemistry and how it was formed. But
when it comes to Ceres we have nothing. That could be for several reasons but a likely one is that
when you knock a piece of Ceres off and it starts its way towards the Earth it doesn't make it, it
just evaporates, and something that would evaporate under those conditions is water.

Robyn Williams: How on earth, literally, do you tell when a rock hits our surface that it came from
a particular asteroid?

Christopher Russell: Of course initially that's very controversial. First of all they took a look
at these meteorites that came down and saw that they reflected sunlight in the same way that the
asteroid did, so basically they have the same colour. And those colours have like fingerprints in
them, so you could say that the fingerprints were the same as well. But also when they studied the
meteorites and understood from their geochemistry how they were formed it made a lot of sense that
a body of the size of Vesta would in fact produce rocks of this nature. So it made a lot of sense.

Robyn Williams: The mission to go there, it's called Dawn, what does it look like?

Christopher Russell: Well, it's already launched. We launched on September 27th of last year
(2007). We're well on our way to our first encounter which will be with Mars in March of 2009. So
what we're doing right now is planning what we're going to do as we fly by, not so much to take
scientific data but to test out our instruments so that we have some data under our belt that we
can practise with as we head off to Vesta where we're going to arrive in 2011.

Robyn Williams: And how many years to get to the two asteroids?

Christopher Russell: We'll spend almost a year at Vesta and then it will take another three years
and a little bit to get to Ceres.

Robyn Williams: A year at Vesta? Doing what?

Christopher Russell: We start when we arrive by taking just some survey snapshots. So we map the
whole surface but at low resolution. Once we know where everything is then we pull in to what we
call the survey orbit, the high-altitude survey orbit, and then we do our detailed mapping with
both our cameras and also our spectrometers. The cameras will tell us about the mountains and the
topography and just the general lay of the land, the size and shape of the body, but the
spectrometers tell us about the minerals that the surface is composed of, and that's where we see
the fingerprints. Then we'll get some idea of the chemistry that went behind the flows and various
rocks that we find on the surface. Okay, that was the high-altitude.

Then we're going to pull down closer, so we go from about 700 kilometres altitude and go down to
about 200 kilometres above the surface, and at that point we start two other investigations; one of
the gravity, so we'll measure the pull of the body on the spacecraft so that we know the
distribution of mass and whether there are mountains or deep roots and things of that nature. And
also we use a gamma ray and neutron detector to tell us about the elements. The spectrometer that
gave us the fingerprints tells us about minerals, but if you're going to find out how much uranium
and thorium and aluminium and iron and things of that nature, and hydrogen which is very important,
then we use the gamma rays and the neutrons that come from the surface.

Robyn Williams: Why is it important to know so much about an asteroid?

Christopher Russell: I think a lot of us are very interested in where we came from. Vesta and Ceres
we think of as proto-planets; small planets, they didn't grow all the way to the large planets that
we know and love and live on one of them, but they are the building blocks. If those building
blocks were all cement and other ones were all iron and other ones were all clay and stuff like
that, then we might be able to figure out how to make an Earth...if we took seven of those and 105
of we're trying to find out what those building blocks are made of.

Robyn Williams: And then, after a year, on to Ceres, and is that more of a challenge?

Christopher Russell: Well, it's out further. Vesta is at 2.35 astronomical units on average,
whereas Ceres is out at 2.77 on average. In fact it's a little bit elliptical and when we encounter
it it's going out even further in the solar system, and it gets cooler the further you are from the
Sun, so we're going to run out of power, encounter less power on our solar cells when we go out
further and the spacecraft will be cooler. So it's a little bit of a technological challenge.
Another aspect is we travel through a more dangerous part of the asteroid belt where there are
debris trails and stuff like that that we're going to have to make sure that we avoid on the way
out. So there's a little bit more drama and excitement in that leg.

Robyn Williams: Can you actually steer the ship?

Christopher Russell: Yes, we can. We have a really capable engine. It's an ion propulsion engine.
It has so much capability for changing the velocity of the spacecraft, it's equal to the velocity
change that we got with the big rocket when we took off from Cape Kennedy in the beginning. It's 11
kilometres per second change in velocity, and we may have to use some of that to avoid these

Robyn Williams: A busy few years coming up.

Christopher Russell: Yes, although it's nice right now. The spacecraft is behaving very, very well
and all we have to do is just keep powering those engines and checking the dials, so to speak, so
we have a fairly easy time for the next couple of years. Then when we get that data coming in we're
going to be very, very busy.

Robyn Williams: Just imagine planning a project that takes decades like the last two speakers have
managed. That was Chris Russell at the University of California, Los Angeles, where he's a
professor of geophysics.