Voyager 1 Bids Farewell to the Solar System

The Voyager 1 spacecraft will soon have a new territory to explore—interstellar space. Voyager chief scientist Ed Stone discusses whether the spacecraft will have a bumpy exit from the solar system, and the chances Voyager's golden record may someday be intercepted by an advanced space-faring civilization.

IRA FLATOW, HOST:

Thirty-five years ago, NASA launched a pair of spacecraft called Voyager 1 and 2 in hopes of learning more about the outer planets of solar system, those big gas giants. The Voyagers beamed back dazzling close-ups of the big red spot on Jupiter and the rings of Saturn, but scientists wanted to see even more of what's out there, see how far the Voyagers could go before running out of fuel.

Now the moment that NASA has been waiting for is finally here because Voyager 1 is inching out of the solar system, out of the sun's magnetic bubble, and it's just a matter of time before it breaks through to interstellar space, going where no man or manmade object has gone before.

Is it going to be a smooth exit, or might there be a little bit of turbulence on the way out? And what's it going to find out there in the great beyond? Joining me now to talk more about it is Ed Stone, NASA's chief scientist on the Voyager mission, professor of physics at the California Institute of Technology in Pasadena. Welcome back to SCIENCE FRIDAY. Ed.

ED STONE: It's good to be with you.

FLATOW: It's been quite a while on this ride, has it not?

STONE: Yes, 35 years and still going at almost a million miles every day.

FLATOW: Tell us where it is. How do you know that it's at the edge of our solar system?

STONE: Well, there are several clues, but one of them is there are galactic cosmic rays, which are outside in the galaxy, created by the explosion of supernovae many tens of millions of years ago. And we know they're out there, we can see the higher energy ones, the faster ones can actually get into the bubble, but the slow ones, which are much more abundant, are out there not being able to get in.

And one of our clues is we're beginning to see an increasing rate of those as they manage to seep their way in to the very outer reaches of the solar bubble.

FLATOW: And why is this spot important for exploration?

STONE: What we're trying to do is leave the - we've been inside the solar bubble for 35 years, and 11 billion miles from the sun, and we're still inside the bubble the sun creates around itself with its supersonic expansion of its atmosphere. And so once we get outside, we will be immersed in matter that has come from other stars, these supernovae, which blew up - five, 10, 15 million years ago, create the materials out there - the magnetic field - the magnetic field of the galaxy.

Inside, it's the magnetic field of the sun. So it will be an entirely new environment for the very first time.

FLATOW: And how many more years can it operate that you can follow it?

STONE: We're very fortunate that we have plutonium-238 - natural radioactive decay of plutonium-238 provides a source of heat, which converted to electricity, every 88 years, there's half as much heat and half as much electricity. So that's very predictable. And we know we can keep all the instruments running, if nothing breaks, until 2020. And we'll have to shut off the last instrument around 2025.

FLATOW: What fascinates me most about the Voyagers - and I've been following them ever since you've been launching them - is that the transmitter on board has the power of a refrigerator light bulb. It's what...

STONE: That's right. It's a 20-watt transmitter. And we're receiving the signal from 11 billion miles away. It's really quite remarkable.

FLATOW: How can you follow with a 20-watt transmitter when your radio station's around, you know, 50,000 watts or whatever, a 20-watt transmitter?

STONE: We have very sensitive receivers on the ground, situated at three places around the Earth. It's called the Deep Space Network. And the antennas are 70 meters across. That's, you know, 200 - over 200 feet, and very, very sensitive receivers that operate just a few degrees of absolute zero to minimize the thermal noise.

FLATOW: Do the cameras - do Voyager's cameras still work?

STONE: No. We turned those off after we took the portrait of the solar system. Voyager 1 took that image back on Valentine's Day in 1990. And then we shut down all the cameras and other instruments which are there to look at planets, because we knew there would be no more planetary encounters. And we needed the space in these little tiny computers, which have only 8,000 words of memory, to better use the memory for the mission to interstellar space.

FLATOW: And what is the transmission rate of the signal?

STONE: It's very slow, only 160 bits per second...

FLATOW: Back in...

STONE: ...not megabits, bits per second.

FLATOW: One hundred sixty bits, that's even - back before those telephone modem days, in the early...

STONE: Oh, yes, much slower than the telephone modem. Yes. But it's every day, 24 hours a day, and we listen on the order of six to eight hours a day with the Deep Space Network. But it actually transmits 24 hours a day.

FLATOW: Mm-hmm. What's going to happen to - you talked about the bubble that's created by the sun. What happens to all the magnetic fields and things as it leaves the bubble?

STONE: Well, the magnetic field that's in interstellar - interplanetary space is from our sun. It's carried off by this solar wind, which is an ionized plasma, so it carries the magnetic field out with it, wrapping it into a large spiral, because the sun is rotating. And so the magnetic field inside is east-west, because it's this spiral shape. Outside, the magnetic field is generated by the motion of the plasma, which is outside, which is orbiting the center of our galaxy. And that creates a local magnetic field, which is then distorted, and by the explosions of these supernovas.

So outside, we're actually going to be seeing a different direction, more like north-northwest, we believe. But, of course, until we get there, we won't be sure.

FLATOW: 1-800-989-8255 is our number. Talking with Ed Stone, who was the head of the Voyager mission on SCIENCE FRIDAY, from NPR. How would you describe the success of both Voyagers? Beyond anybody's wildest expectations?

STONE: Oh, much beyond. What we discovered at the planets were things that we didn't know were there to be discovered. That's really the big bonus for science and for explorations, when you find things that you hadn't even thought you should be - were there to be found. And that's been the case time after time, and it's still the case. As we head out of the solar bubble, we are constantly surprised by what we observe and how it has changed our understanding of the interaction of our solar bubble with what's outside.

FLATOW: Ed, I want to play something for you and see if you can recognize it.

STONE: OK.

(SOUNDBITE OF RECORDING)

UNIDENTIFIED MAN: (Foreign language spoken)

UNIDENTIFIED WOMAN: (Foreign language spoken)

FLATOW: Any guess, eh?

STONE: Well, I'm thinking that those are the greetings from Earth that are on the record, which are on each spacecraft.

FLATOW: That's right. There's a record - I think - did Carl Sagan have something to do with putting that record together?

STONE: Yes. He put together a group of individuals who decided what to put on these 16-and-two-thirds RPM grooved records, double-sided, which had greetings from many languages of Earth, music from around the Earth, images of the various aspects of life on Earth, and so on. It's really quite a remarkable collection. It's a message from Earth, basically.

FLATOW: And the idea is that there might be intelligent life out there that might pick up the Voyager.

STONE: Well, that's certainly - one would hope so. But, in fact, the probability of that happening is very tiny. The real thing is that it's a message to us here on Earth that we had gotten to the point where we could actually send such a message into interstellar space. It will orbit the center of our galaxy for billions of years, long after the sun and the Earth are gone. This spacecraft, these two spacecraft will be orbiting silently around the black - giant black hole at the center of our galaxy, just like all the stars and the sun do.

FLATOW: What can we actually learn as it goes into deep space and, you know, people talk about outer space? And I guess this is really the first time anything that we've sent has - going into outer space.

STONE: This will be the first time we have something in interstellar space. That's - this will be our first interstellar probe, and Voyager 1 finally gets beyond the edge of the bubble. Yes.

FLATOW: And what would it - what could it send back that would be surprising to you?

STONE: Well, it could tell us that that the magnetic field is different than we think it is. We think we have some idea of what its direction is and how strong it is, but that's indirect information, if you like. And we could find that the intensity of these cosmic ray particles which are outside are quite different. We have various estimates which range more than an order of magnitude, factor of 10, between the various estimates. So there's - I suspect we'll be surprised when we find what's really out there.

There's also - right outside the heliosphere, we think, is a source of kilohertz radio emissions. That's the audio frequency, but the radio emissions generated in interstellar space just outside of the heliosphere.

FLATOW: Audio - it sounds sort of - in the frequencies we could hear them, if there were as - is the way to hear them.

STONE: Yes. We could hear them if they were sound waves. They're actually radio waves, very low-frequency radio waves, which we first discovered when Voyager 1 was beyond Saturn's orbit and the radio waves could actually get in that far into the heliosphere. But, again, we could be surprised when we get out there that our ideas maybe are not quite what the - quite what nature does.

FLATOW: Ed Stone, all the best luck to you. We'll meet back in another 35 years, OK?

STONE: OK. Thank you.

FLATOW: Ed Stone, NASA's chief scientist on the Voyager mission and professor of physics at California Institute of Technology, Caltech, in Pasadena. We're going to take a break. And after the break, the dark side of the universe, talking more about space, perhaps another mirror, parallel world out there. You thought about that. We're going to talk about that when we get back. So stay with us. Michael Turner joins us after this break. We'll be right back.

(SOUNDBITE OF MUSIC)

FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.

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