GWEN IFILL: Now, the story of an unexpected and fantastic voyage. Twenty-five years ago today, the United States launched an unmanned spacecraft, "Voyager 2." Two weeks later, its identical twin, "Voyager 1," followed.
Their original mission: A quick five-year tour of Jupiter and Saturn, taking pictures and sending radio transmissions home. But more than two decades later, the robotic probes are still going and going and going, heading for the edge of the solar system, nearly eight billion miles from Earth-- twice as far as Pluto-- traveling at speeds of over 35,000 miles per hour. Zipping along at 186,000 miles per second, the "Voyagers'" radio signals still take more than nine hours to reach earth.
"Voyager 1" got to Jupiter in March, 1979, discovering volcanoes on Jupiter's moon Io, and taking this snapshot of an eruption rising over the planet's horizon. A year later, "Voyager 1" passed within 39,000 miles of Saturn's clouds, discovering spoke-like features in the planet's rings.
In a 1986 close encounter, "Voyager 2" studied one of the moons of Uranus, Miranda. Pictures showed a mountainous landscape with deep canyons. And while passing 3,000 miles above Neptune in 1989, "Voyager 2" discovered that the blue planet has the fastest winds in the solar system, with speeds of up to 1,200 miles per hour.
And the two one-ton spacecraft are also prepared for any surprising encounters with other extraterrestrial travelers, with identical golden discs fastened to each craft, engraved with sounds from Earth: A kiss, a mother's lullaby, and 90 minutes of music, from Bach to Chuck Berry. Scientists say "Voyager" could still be traveling in interstellar space 20 years from now.
GWEN IFILL: For more about the Voyager mission I'm joined by Jay Bergstralh, associate director of the Solar System Exploration Division at NASA. He worked on the Voyager missions during its first years.
JAY BERGSTRALH: Thank you.
GWEN IFILL: So back in 1977, what exactly did we think that the Voyager mission was going to accomplish?
JAY BERGSTRALH: We knew it was going to fly by Jupiter and Saturn. There was the possibility always for Voyager 2 to go on to Uranus and Neptune. We knew Voyager 1 would end with the Saturn encounter and a fly-by of Saturn's big satellite, Titan.
GWEN IFILL: So, detail the discoveries. What do we know now that we didn't know in 1977?
JAY BERGSTRALH: Oh, my goodness.
GWEN IFILL: Where do we start?
JAY BERGSTRALH: That could take a long time. We start with the fact that wherever we went we found completely unexpected things. We discovered Jupiter has a ring. We discovered that Io, Jupiter's inner most large satellite is the most volcanically active body in the solar system.
We discovered a few more things about the other large satellites as well. Going on to Saturn we discovered that its rings were not just land and featureless but up close looked like grooves on a phonograph record.
And Titan is covered by a photochemical smog. We went on to Uranus and discovered that had its satellite Miranda has the most exaggerated geography that we have seen, a cliff six kilometers high on a body that's only a couple hundred kilometers across.
GWEN IFILL: Almost like it was smashed together.
JAY BERGSTRALH: That's one speculation - that at some point in the past it broke up and then came smashing back together again.
GWEN IFILL: So we see these things but what do they mean? What is the practical application of this kind of information?
JAY BERGSTRALH: The practical information - well, for one thing, looking at Jupiter and Saturn and especially at Neptune, we see weather, climate, going on under completely different conditions than we find on Earth.
If we are going to test our models of climate circulation or of atmospheric circulation, that is weather and climate on earth, and we can't do it under controlled conditions in a laboratory, the phenomena are too big to see .. to do an experiment, so to speak, to find out the if the models work we have to go to place where the conditions are different, and they are very different in the outer planets.
GWEN IFILL: Compare the kind of conditions that we're used to on earth. When we find that there are storms, for instance, on planets that we didn't know there were storms on, does that translate to the way we see a storm -- for instance, and I guess this the bottom line question here -- is there life from what we have seen, as we know it?
JAY BERGSTRALH: That's something else entirely. From what Voyager found, I don't think that we would say anything pro or con about life elsewhere. Mostly what we have learned about life and the possibility of it being elsewhere has come from earthbound biologists in the last ten years or so.
Wherever we go on earth - even into what's seemed like the most hostile possible environments for life -- as long as there is liquid water and some source of chemical energy we find life.
And we can speculate about other places on the solar system where these conditions might be met, one of them might be on Mars, where we've recently established there's certainly a lot of ice, water ice. Another might be on Europa, one of Jupiter's satellites, a more recent mission, Galileo, has given us pretty good evidence that there's an ocean of liquid water, a global ocean of liquid water beneath the Europus ice crust.
GWEN IFILL: But Voyager in itself may not have made those kinds of discoveries but it spawned other missions, which were much more focused going to other planets, didn't it?
JAY BERGSTRALH: Yes. Certainly we had as I just mentioned Galileo went into orbit around Jupiter in December of 1995. We have Casini on its way to Saturn; it will reach there in July of 2004.
GWEN IFILL: As Voyager traveled further away than I can imagine, farther and farther away from us, how does it keep going? It's too far away from the Sun now to be running on any kind of solar energy.
JAY BERGSTRALH: Well, it keeps going by inertia; it simply keeps going.
GWEN IFILL: But there is energy which is powering radios and computers.
JAY BERGSTRALH: Yes. Electrical energy is provided by a radioisotope generator, a nuclear source. It's powered by plutonium 238; it's an isotope that won't -- you can't make a bomb out of it. It just sit there's and gradually decays, emits alpha particles and produces heat. And we can convert the heat into electrical power.
GWEN IFILL: And how long can that electrical power keep these two spacecraft going?
JAY BERGSTRALH: We think for about another 20 years. That's a matter of some extrapolation there's some uncertainty but about 20 more years.
GWEN IFILL: And going to where?
JAY BERGSTRALH: What we're looking for is the true edge of the solar system -- the boundary between the Sun's influence and truly interstellar space. We think that Voyager II excuse me Voyager I may be getting close to that, but that's a subject of deep interest.
GWEN IFILL: How will scientists know when you reach that boundary?
JAY BERGSTRALH: We predict that there will be a shock, a boundary shock where the interstellar medium meets the solar wind extending out from the Sun, where there's equilibrium, a shock layer and that should be recognizable by the instruments on the spacecraft.
GWEN IFILL: The Christian Science Monitor writing about this mission described it as a transmission from reconnaissance to understanding, that is, instead of previous missions, which just went out and took pictures to see what was out there, this was a mission that was actually sending back information that would help us understand this solar system and beyond that we live in. Would you agree with that?
JAY BERGSTRALH: Well, I would agree with that. There was a lot of reconnaissance in it. It was our first really capable spacecraft to go to the outer planets. A few years before we had the Pioneer 10 and 11 fly by Jupiter and in 1979 I think Pioneer 10 flew by Saturn. Those were great spacecraft too, and they lasted a long time.
But their instruments were far less capable than what we have on Voyager. But, again, wherever we went with Voyager, we found things that were unexpected and sometimes kind of downright weird.
GWEN IFILL: What more can scientists ask for? Jay Bergstralh, thank you very much for joining us.
JAY BERGSTRALH: Thank you, my pleasure.