Voyager 1, flying about 34 degrees north of the equator, crossed the termination shock and entered the outermost layer of the heliosphere about 9 billion miles from the sun. Meanwhile Voyager 2, about 26 degrees south of the equator, finds that the shock may be nearly a billion miles closer to the sun.

Scientists believe that the observed discrepancies may be attributed to an interstellar magnetic field pressing inward on the southern hemisphere. Voyager 2 will determine the exact location of the shock in the south when it crosses it sometime before the end of next year. Then scientists will have a better idea of how strong the magnetic field is outside of the heliospheric bubble.

Voyager 2 is also finding that the shock in the south is a source of low energy ions as was discovered by Voyager 1 in the north. Contrary to earlier predictions, however, neither Voyager 1 nor 2 have found the source of higher energy anomalous cosmic rays.

Both Voyager spacecraft were launched from the Cape Canaveral Air Force Station in Florida: Voyager 2 on Aug. 20, 1977 and Voyager 1 on Sept. 5, 1977 on a faster, shorter trajectory than its twin. The mission is managed for NASA by the Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology.

Narrator: Voyager: Living on the Edge - of the Solar System

I'm Jane Platt and you're listening to a podcast from JPL -- NASA's Jet Propulsion Laboratory in Pasadena, Calif. Some of you listening out there weren't even born when the two Voyager spacecraft launched back in 1977. Now nearly 30 years later, both spacecraft are still alive and well -- cruising beyond all the planets and about to burst through past the edge of our solar system - where no one, no spacecraft has ventured before. Joining us with the latest is Dr. Ed Stone of the California Institute of Technology, and he's been the Voyager project scientist for all these years. Ed, you have been with the Voyagers since the beginning, but it's probably a good idea to refresh everyone's memories -- what were their original goals of Voyager 1 and Voyager 2?

Stone: Well, Jane, every 176 years, the giant outer planets, Jupiter, Saturn, Uranus and Neptune are lined up on the same side of the sun, and we launched the two Voyagers in 1977 on their journey of exploration of the giant outer planets.

Narrator: And you hope to learn all kinds of things about the planets?

Stone: Yes, these are giant spheres of fluid, basically, with no solid surfaces and they each have moons which form miniature solar systems around them. So we had an opportunity to discover and explore many, many new worlds.

Narrator: And we're going to talk in just a couple of minutes about what you've learned, which is a lot, but first, some of the cool current news about the Voyagers is where they are. Can you explain for our audience where exactly the Voyagers are located now?

Stone: In mid-2006 Voyager 1 will be 100 astronomical units from the sun, and that's 100 times as far from the sun as the Earth is, 9.3 billion miles. And Voyager 2 will be 80 times as far from the sun as the Earth, they are in the very outer regions of the heliosphere, the bubble that the sun creates around itself.

Narrator: So, we talk about the Voyagers approaching the edge of the solar system, what is the edge of the solar system?

Stone: Well, the sun has a supersonic wind blowing a million miles per hour radiating outward in all directions. That wind creates a bubble around the sun. That bubble is called the heliosphere, the sphere of the sun. And, the edge of that bubble is the edge of the solar system. Beyond that is interstellar space. And the two Voyagers are headed toward the edge of interstellar space.

Narrator: So, it's not, as I understand it, a nice, clean, tidy line, where it's like, "Okay, stop sign here, you're now leaving the solar system." I know it's quite complex with the different layers, but can you take a stab at trying to explain what happens as you get out towards the edge of the solar system?

Stone: The bubble is created by the supersonic wind, which means as the wind approaches contact with interstellar wind, it must abruptly slow down and turn around and head down the tail, because the heliosphere is shaped like a comet and so where it abruptly slows down is a standing shock wave, a supersonic shock wave just like in front of a supersonic aircraft. Voyager 1 has now crossed that shock for the first time and is in the region where the wind has slowed down and is turning around to head down the tail of the heliosphere.

Narrator: Okay, so the region it's in now is called?

Stone: That region is called the heliosheath because it's the region between the shock and the heliopause, which is the surface of the bubble. That is the edge of interstellar space. Beyond that the material we will be in will be material from other stars. Inside the heliosphere, all the material is coming from the sun and that's where all the planets are, that's where all the spacecraft have been, that's where Voyager still is. But eventually Voyager will exit this bubble and be in interstellar space surrounded by material from other stars.

Narrator: And, when do you expect that - Voyager 1 will be first and then Voyager 2 - when do you expect that they will leave our solar system?

Stone: We don't know exactly how thick this sheath region is, this outer layer of the sun's atmosphere, but it may be about 10 years more before Voyager reaches the edge of interstellar space, Voyager 1. And Voyager 2 may, in fact, although it's closer to the sun, may reach interstellar space at about the same time, which is one of the new discoveries.

Narrator: Now over the last year or two, you've been seeing some hints that it's getting close and a lot of it has to do with very complex solar physics and different phenomenon. Can you try to briefly sum up what are some of the signs that have told you we are getting close?

Stone: Well, Voyager 1 has crossed the shock. We know that because when you cross the shock the wind abruptly slows down and the magnetic field, which is carried by the wind from the sun, is compressed along with the wind itself. And we saw the magnetic field intensity go up a factor of three as we crossed the shock. And as we then eventually cross into interstellar space we will see the interstellar magnetic field increase again. So we will know we've crossed into interstellar space when that happens.

Narrator: Will it be a dramatic moment or just over a period of months or weeks?

Stone: I expect that the interface, that contact surface between the solar wind and interstellar wind, will be quite turbulent and we will see a period of turbulence before we finally reach steady interstellar conditions.

Narrator: It's amazing that almost 30 years later, and really high mileage, obviously, on these two spacecraft, that you're still able to talk to them, the Deep Space Network regularly communicates with the two spacecraft. Tell me a little bit about what has enabled the spacecraft to last this long?

Stone: Well, several keys, but the most important one is we have radioisotope thermoelectric generators, which means we have a plutonium 238 heat source which is converted to electricity with thermal couples, very simple robust power supply, which should last us until at last 2020, when Voyager one will be almost 150 times further from the sun than the Earth is and could well be into interstellar space.

Narrator: Did you have any idea, you've been working on the Voyager mission since 1972, they were launched in 1977, did you have any idea then that all these years later you'd be sitting here and still getting useful data from both spacecrafts?

Stone: Many of us hoped that we would reach interstellar space but we had no idea how long the journey would be, and of course in 1977 the space age itself was only 20 years old, so we had no way of knowing that a spacecraft could last so long, and could communicate and operate so far from the sun, but the Voyagers just keep going and with some luck they may well reach interstellar space while they still have electrical power.

Narrator: And I'm going to ask you more about what you think you, what you hope to learn once they cross into interstellar space, but I did want to mention the American Geophysical Union meeting, the May meeting, in Baltimore, you have some news about Voyager, can you sort of sum up what the news is that you're talking to your colleagues about?

Stone: Well the heliosphere is a comet shaped object, which means it has a blunt nose and it turns out that Voyager 1 is in the northern hemisphere of that blunt nose, and Voyager 2 is in the southern hemisphere. The big surprise is that Voyager 2 has found that the shock, where the wind slows down, is about nine astronomical units, about 900 million miles closer to the sun than it is in the north. That is, the front nose of the heliosphere is actually pushed in, in the south, by evidently an interstellar magnetic field which is pressing against it preferentially in the south. And that's the reason Voyager 2 may also reach interstellar space sooner than we expected because we now believe this blunt object is actually distorted by an interstellar magnetic field that's outside.

Narrator: Which sort of gets back to something I mentioned a few minutes ago, which is that there's not this nice, tidy boundary that tells you "You're now in our solar system" or you're not, correct?

Stone: That's right, it's not tidy at all because it's a fluid flow and there are pressures outside which deform it. Another thing which we've discovered is that the, with Voyager 1 which is now in this sheath region, we have found that the magnetic field there has potholes in it and it has bumps. That is that, unlike the supersonic wind, when the wind slows down it gets these potholes in the magnetic field where the field almost goes away and then it comes back and will be stronger than average. And so it's a very bumpy magnetic field in the heliosheath.

Narrator: Well it sounds as though the Voyagers are kind of getting buffeted around out there, is that a correct vision I have of them?

Stone: Well, they would be buffeted around, except that this is a better vacuum than any here on Earth, so the spacecraft itself is totally unaware of any of this. Only our sensitive instruments can see these bumps and these potholes in the magnetic field.

Narrator: Okay, let me just go back a bit, and again, you have 30 years of these two amazing spacecraft. This is a tough question for you, I'm sure, but do you have some highlights you can rattle off, some of the most important things you've learned in that time or some of the most exciting discoveries for you.

Stone: Well, generally the most important thing we learned is how diverse the bodies of the solar system are. Each one is unique and that's because they've had a different history, different evolution. Jupiter, with it's great red spot is just the largest of dozens of giant hurricane-like storm systems. And two of Jupiter's moons, Io, has a 100 times more volcanic activity than Earth. Europa has an ice crust probably on the liquid water ocean. On to Saturn, we've found Saturn's rings are riddled with wakes from moons, which are orbiting inside the rings and outside the rings. And there is a moon there called Enceladus, which is the whitest, brightest object in the solar system and has a very fresh surface. And there's the moon Titan, which has an atmosphere in which liquid natural gas rains on the surface. On to Uranus, where we found the magnetic pole down near its equator, and we found a moon, although it was only 300 miles across, it's one of the most complex surfaces we've yet seen. And on to Neptune, the furthest planet from the sun that we've visited, yet it has the fastest winds, with the least energy from the sun to dry them, and its moon Triton, 40 degrees above absolute zero, yet we found geysers erupting from its polar ice cap.

Narrator: And what's amazing, as you rattled off these discoveries thinking that really, the Voyagers paved the way for future, current exploration, actually. For example, you mentioned Enceladus, well we recently had exciting news from the Cassini spacecraft about evidence of underground water on Enceladus, and very often when there is a new discovery it's based on comparisons with what was learned by Voyager.

Stone: Voyager really revealed the complex diversity of the solar system and created the opportunity then for Galileo to return to Jupiter to explore more closely in orbit and now Cassini to explore more closely in orbit at Saturn and to drop the probe, the Huygens probe into the moon, Titan's, atmosphere. So yes, Voyager really has revealed the diversity of the solar system.

Narrator: Do you have any absolute favorite pictures from the Voyagers?

Stone: I have several; one is of Jupiter with its great red spot, with Io and Europa sort of hanging in the foreground, so that one can see some of the diversity there. Saturn's rings are another wonderful image that I remember because of all the wakes and features in the rings one can see. At Uranus, probably the most interesting image was that of Miranda, with its very complex geological surface which was surprising for such a small, cold moon. And then at Neptune, of course, Neptune's great dark spot which has since disappeared, and the polar ice cap on Triton with the evidence of the plumes, with deposits on the icy surface.

Narrator: Very cool stuff. And we do have those online, I'll be giving the URL in just a couple of minutes. But, what kinds of things do you hope to learn beyond our solar system, in interstellar space?

Stone: We'd like to find out what's outside, because what is outside has come from the explosion of supernova, and other large stars, five to ten million years ago. And they have spewed out material during those explosions, and we will be imbedded for the first time in material and winds from other stars and magnetic fields, which have come from other objects in the galaxy. So it would be our first direct measure of what's outside, pressing back on this bubble in which we all live.

Narrator: Well, sounds great. Is there anything else, anything else at all that you'd like people to know about the Voyagers that I have not mentioned?

Stone: Well, the Voyagers, whether they reach interstellar space or not under power, will be, and Newton tells us, they will be human-kind's first interstellar probes. They will be the first objects launched from Earth to reach interstellar space. And the only question is, will they still be sending back what they're learning when they reach interstellar space?

Narrator: I know from interactions with you that you're a very modest man, and that's a wonderful trait, but I want to just ask you if you are not sometimes just bursting with pride all these years later. This is, these incredible discoveries and the thing's still working.

Stone: Well it's really been a wonderful journey of a lifetime, there's no question about that, all of us on Voyager have really been very fortunate to have been on the frontier of discovery for so many years. And here it is, almost 30 years after launch, and we're still on the frontier of discovery, it's really, for a scientist, this is really a wonderful, wonderful mission.

Narrator: Well, I want to thank you very much for taking the time to talk with us, and we'll have to definitely have you back as you find more things and you get closer to the edge of the solar system.

Stone: Thank you Jane.

Narrator: We've been talking to Dr. Ed Stone of Caltech, and he's been the scientist for Voyager 1 and Voyager 2 since the very inception back in the 1970s.

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