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Voyager spacecraft on the outer edge of the s -

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[music from the audio soundtrack aboard the Voyager spacecraft]

Robyn Williams: That music is now three times as far away as Neptune, beyond the planets. It's been
travelling for 31 years, not as a sound or a radio wave but as a recording that ET could put on the
record player. CDs, you see, weren't around then. Ed Stone, once of the Jet Propulsion Lab, has
been in charge of the two Voyager spacecraft since the beginning, and he's now following them to
unexplored regions.

Ed Stone: They're both in the final frontier of the solar system, the very outer edges of the
bubble around the Sun, and they're headed toward interstellar space.

Robyn Williams: It's amazing that they've travelled for so long and they've only just reached that
kind of limit of the Earth's province, if you like.

Ed Stone: The heliosphere, as it's called, is immense. Voyager 1 today is at 105 astronomical
units, 105 times as far from the Sun as the Earth is. And to give you a sense of scale, Neptune,
our outermost planet, is only 30 times as far from the Sun as the Earth is. So we're more than
three times as far out as Neptune and we're still inside the bubble but we are in its outmost
layer.

Robyn Williams: And you're still getting signals back from both?

Ed Stone: We listen to both spacecraft every day, mainly for Voyager 2, mainly...almost exclusively
now from Australia.

Robyn Williams: From Tidbinbilla.

Ed Stone: Yes, from Tidbinbilla.

Robyn Williams: I didn't know that. Why is that the source?

Ed Stone: Because Voyager 2 is headed south and now we can see Voyager 2 only from the southern
hemisphere.

Robyn Williams: I see. And what strength are the signals you get?

Ed Stone: We have a 20-watt transmitter on the spacecraft, at 105 times as far from the Sun as the
Earth is, so the signal here is really very, very faint.

Robyn Williams: When was it expected really to snuff? When was it expected to stop sending these
signals?

Ed Stone: We didn't really know when we launched. When we launched in 1977 the space age itself was
only 20 years old, so we had no experience to tell us how long these things would last, and we
launched two because we were not sure at that time that we would get both even to Saturn, which is
a four-year mission. So we started out with just a four-year mission as mission success, and then
we added each step of the way as the spacecraft lasted longer and longer and longer.

Robyn Williams: Now you're on the edge of the heliosphere there's nonetheless a kind of borderland
to go through. What's that like?

Ed Stone: We are in this thick outer layer where there is a supersonic wind from the Sun. It's
blowing outward at 400 kilometres per second, and it creates a bubble around the Sun. But because
it's supersonic, as it approaches contact with interstellar space it goes through a shock, just
like in front of a supersonic aircraft, slows down, heats up and gets very warm, and that region,
the thick outer layer is called the heliosheath. We entered that in December 2004 with Voyager 1
which is further out, and then just last August 30th we finally entered this outer layer with
Voyager 2. So we now have both spacecraft exploring this unexplored region of space, this final
outer layer of the solar system.

Robyn Williams: Any particular discoveries yet?

Ed Stone: We discovered a number of things. For instance, we found with comparing Voyager 1 and
Voyager 2 that in fact the heliosphere is squashed. That is, it's a comet-shaped object because
there's an interstellar wind that makes it into a sort of a windsock. But the nose of the
heliosphere is not spherical, it turns out it's pushed in on the south, and Voyager 2 found this
outer layer ten astronomical units closer to the Sun than Voyager 1 did, nearly a billion miles
closer.

Robyn Williams: When does it reach really the proper interstellar space?

Ed Stone: We don't know for sure because no space craft has ever been there before. Our models
suggest that it may be another five to seven years before Voyager 1 finally exits this thin layer,
crosses the boundary layer and moves into interstellar space for the first time.

Robyn Williams: What do you expect to hear back as a result?

Ed Stone: When we enter interstellar space we will then be immersed in material that has come from
other stars, explosions of other stars nearby in the last 10 or 20 million years. So the magnetic
field out there will be different, the speed of the wind will be quite different out there than
inside the heliosphere, and the material, the galactic cosmic rays coming from the galaxy should be
more intense once we're outside the bubble because the bubble provides a certain shielding for us
in here.

Robyn Williams: I can't work out whether you get more signals or fewer.

Ed Stone: We will have a weaker radio signal of course to transmit the data back, but the signals
recorded by the spacecraft should be more intense because the magnetic field should be stronger,
the interstellar wind is denser, and the cosmic ray intensity should be higher. So we should see
more out there than we actually see now.

Robyn Williams: It gives you a kind of thrill, doesn't it, thinking that those two little beasties
for decade after decade are soldering on.

Ed Stone: It is quite remarkable. You know, when we launched 30 years...we had hoped we would get
to interstellar space, we had no idea how long it would take, but we're very fortunate; these
spacecraft survived Jupiter. Jupiter I think gave us the long life. We had to develop a spacecraft
that would survive the intense radiation of Jupiter, which is a very rapid ageing effect, and
having survived the rapid ageing effect it had a very long life to go. The other key thing was that
we were powered by the radioactive decay of plutonium-238 which has a half-life of about 88 years.
So it's just a heat source with lots of thermal couples moulded onto it to create electrical
energy, and that heat source is just gradually cooling off. That's where the energy comes from but
eventually it will cool off and we will run out of enough electrical power some time between 2020
and 2025.

Robyn Williams: So there will be an end nonetheless, but, quietly, they'll sail on for ever.

Ed Stone: They will sail on forever orbiting the centre of our galaxy, just like the Sun is, but
leaving the Sun behind.

Robyn Williams: And still carrying a picture of a man and a woman and recordings of the Earth, just
in case ET spots the little craft. Ed Stone is a professor of physics at Caltech and the Voyager
chief scientist.