(Note: This post was first published on Dec. 14. It was reposted Monday — the 26th — because that's when it was broadcast on Morning Edition.)
The Voyager 1 spacecraft is 11 billion miles from the sun. And every minute, it gets 636 miles closer to its destination: the frontier of interstellar space.
The craft is currently in what NASA calls, not undramatically, "the boundary between the solar wind from the Sun and the interstellar wind from death-explosions of other stars," an area that astrophysicists also call, less dramatically, a stagnation layer.
When Voyager 1 crosses that threshold, it'll become the first man-made object to do so. That feat, along with the recent discovery of Kepler-22b, a potentially inhabitable planet, means that it's an exciting time to be an astrophysicist. Now, NASA and its two Voyager craft are heading into the great beyond.
"We're still bathed by the same solar wind that we have been for the last 30-some years," Voyager program chief scientist Dr. Ed Stone tells NPR's Steve Inskeep, in an interview airing on Monday's
That solar wind emanates from (you guessed it) the sun, and it travels at anywhere from around 1 million to 2 million miles an hour. The wind is extremely weak in the outer edges of the solar system, where Voyager is traveling. Stone says it creates a comet-shaped "bubble" around our solar system.
"But once we leave the bubble, we will be, for the first time, surrounded by winds that came from the explosion of other stars," says Stone, who has been Voyager's lead scientist since 1972.
As it goes where no man-made thing has gone before, Voyager 1 will not be able to let us know if it sees something truly remarkable out there — because it no longer sees anything, in the traditional sense.
"We turned off all the cameras in 1990," Stone says, "and all the other instruments that were designed just to look at bodies as we flew by. Space is really empty. And Voyager will never be very close to any other object in our solar system, or even in our galaxy, for that matter."
That paints a rather lonely picture. But at least Voyager 1 is being trailed by its sibling craft, Voyager 2, which is about 9 billion miles from the sun. In total, the two probes have traveled even farther — they've each logged more than 14 billion miles, as they diverted their courses to study planets in our solar system, NASA says. The agency has created an animated video depicting their paths.
Of course, the edge of interstellar space will not be marked with a handy "Now Leaving Your Solar System" sign. Scientists hope that Voyager will give them more clues about where interstellar space actually begins, and where the bubble ends, as it crosses over the boundary.
"We have only had estimates," Stone says. "The estimates varied by a factor of two... the size of the bubble is determined by this pressure of the wind blowing outward, pushing against the wind outside. And we don't know exactly... how much pressure there is outside, pushing back."
"My expectation is that it's probably, at most, another billion miles," Stone says, "and may in fact be only a few hundred million miles, before we reach the edge of the bubble. We're getting very close now."
So at the most, Stone says, Voyager 1 will leave our solar system within three years. And when it does, NASA wants some answers.
"We want to explore interstellar space itself," Stone says. "We have some ideas of what's out there, from data, and observations from Earth. We believe that we are in a cloud of material that was ejected by the explosion of a series of supernova, about 5 to 10, 15 million years ago, very near the sun. And that we will be embedded in the material from those giant explosions, and the magnetic field which was swept up by the shells of material ejected by those exploding stars. So, we're very interested in learning more precisely what's really outside of the bubble, pressing back inward."
Once they've reached interstellar space, the Voyager spacecraft will also have a chance to deliver their golden cargo — the data records that include 116 pictures, along with sounds from Earth. Those include songs from Louis Armstrong, Beethoven, and a Navajo tribe.
And in Amoy, a language from eastern China, the records carry this message: "Friends of space, how are you all? Have you eaten yet? Come visit us if you have time."
It may seem risky to send an open invitation to the universe that tells alien races to stop by for a bite. But it's not as if Earth can send many invitations. That's because of the unique way our solar system's planets were aligned in 1977, when the Voyager craft were launched.
"That was an opportunity that happens every 176 years," Stone says, "to send a spacecraft past all four of the giant outer planets: Jupiter, Saturn, Uranus and Neptune."
The Voyager spacecraft are expected to stay functional until at least 2020, and possibly until 2025.
Toward the end of the interview, Stone said that in the Voyager mission, every day seems to bring a new revelation about our solar system. In doing so, he voiced an opinion that wouldn't be out of place on the space probes' gold records.
"No matter what you think you know," he said, "what there is to learn is even more interesting."
STEVE INSKEEP, HOST:
Decades after it was launched, NASA's Voyager 1 probe is still making news. The Voyager 1 and 2 spacecraft were launched in the 1970s.
LINDA WERTHEIMER, HOST:
In the 1980s, they passed close by Jupiter, Saturn, Uranus and Neptune, making significant discoveries along the way. Now, Voyager 1 is sending back information suggesting that it is reaching the edge of the solar system.
INSKEEP: It's already far beyond any planet, and it's about to leave the space that is influenced by the energy from the sun. Dr. Ed Stone is chief scientist of the Voyager space program. He has been since the beginning, and he describes the solar system as a kind of a bubble whose size is defined by solar winds, propelled outward from the Sun.
This is a spacecraft that passed the last planet in the solar system more than a couple of decades ago, but is still within the solar system as you understand it.
DR. ED STONE: It's still inside the solar bubble. That's correct. We're still bathed by the same solar wind that we have been for the last 30-some years. But once we leave the bubble, we will be, for the first time, surrounded by winds that came from the explosion of other stars.
INSKEEP: When this was launched in the 1970s, did you anticipate the spacecraft lasting long enough that it would actually leave the solar system?
STONE: Well, we hoped that, in fact, it would. And part of the original design of the mission was an interstellar phase. But, of course, none of us knew exactly how large the bubble was that we were inside. So, we didn't know whether the spacecraft could last as long as it has. When the two Voyagers were launched, in 1977, the space age itself was only 20 years old. So there was no evidence or experience that would suggest that they could last as long as they have.
INSKEEP: Oh, no. Wait a minute. You're saying that up until now no one has known exactly how big the solar system is, as you define it - the area that is covered by the solar wind.
STONE: That's correct. We have only had estimates, so we didn't know whether we were inside of a smaller or larger heliosphere. And that's because the size of the bubble is determined by this pressure of the wind blowing outward, pushing against the wind outside. And we don't know exactly - although we know a lot more today than we did 20 years ago - exactly how much pressure there is outside pushing back.
INSKEEP: OK, so just in terms of miles, if you can give me a figure. If we think of the solar system as this giant ball with the Sun as a point at the center of it, how big across is it?
STONE: It's a comet-shaped object, so we're headed toward the nose of the comet. Voyager 1, now, is 11 billion miles from the Sun. And my expectation that is that it's probably, at most, another billion miles. And may, in fact, be only a few hundred million miles before we reach the edge of the bubble. We're getting very close now.
(SOUNDBITE OF LAUGHTER)
INSKEEP: I'm just imagining an interstate road sign: One Billion Miles to Go.
STONE: Right, that's three years for Voyager 1. One billion miles every three years.
INSKEEP: And when you say it's comet-shaped and you're heading toward the nose of the comet, you're saying that 11, 12 billion miles, give or take, is the closest exit from the solar system. If you'd gone the other direction it'd be much farther.
STONE: That is exactly correct. If we had gone down the tail it would be a very long journey to get outside into interstellar space.
INSKEEP: And, of course, there is just immeasurable distances beyond - beyond the solar system.
STONE: Yes. Yes, that's correct. The two Voyager spacecraft will be orbiting the center of our galaxy but very distant from any other object in the galaxy.
INSKEEP: Is this boundary line, that you seem to be approaching, the last thing of interest that you could imagine Voyager encountering before it goes out of contact or goes out of service?
STONE: We want to explore interstellar space itself. We have some ideas of what's out there, but from observations from Earth. We believe that we're in a cloud of material that was ejected by the explosion of a series of supernovae about five to 10, 15 million years ago, very near the sun, and that we will be embedded in the material from those giant explosions and the magnetic field that was swept up by the shells of material ejected by those exploding stars. So we're very interested in learning more precisely what's really outside of the bubble pressing back inward.
INSKEEP: So, you want to get out into interstellar space. Is it strange, at all, to think of the fact that you are exploring the farthest reaches of the solar system and trying to go beyond, with the very latest in 1970s technology?
STONE: Yes. Yes...
(SOUNDBITE OF LAUGHTER)
STONE: ...that's kind of peculiar that, in fact, we have instruments that were developed, and spacecraft that was developed, in the early '70s. The computers on board this spacecraft - it's a totally automated spacecraft - the computers have 8,000 words of memory.
INSKEEP: That's - well, nothing.
STONE: Yes, it's nothing.
INSKEEP: I mean I'm sure many people have a smartphone that has far more memory than that today.
STONE: Oh, yes. They're measured in, you know, billions of words, not thousands.
INSKEEP: Would you put any different equipment - obviously it'd be better equipment. But would you ask for any different capabilities for this spacecraft if you were sending it off today on the same mission?
STONE: We would certainly have more up-to-date instruments, which are more sensitive. We would want to measure more of the energetic particles than we can. We did not cover all of the interesting energies, as it turns out. We can only infer some of what's out there, even with the instruments on Voyager. And there are many other observations with modern spectrometers that we could make, of the environment out there, that we can't make with Voyager.
INSKEEP: What powers Voyager?
STONE: It's powered by the natural radioactive decay of Plutonium-238 and thermocouples bolted to this heat source; very simple, robust power supply. The radioactive decay half-life is 88 years and that's one reason why the spacecraft are still working, is because we have a very long life battery, if you like. But we do know that our power level drops by four watts every year, and so we have to systematically turn things off one at a time.
About the year 2020, we'll have to turn off one of the science instruments. And by 2025, we'll have to have all the science instruments off and that will be the end of the mission.
INSKEEP: So you have been tracking this spacecraft for coming up on 35 years now.
STONE: That's right. And we have our 35th anniversary of launch in 2012. And we still have a great journey to go.
INSKEEP: You ever get bored waiting on it to reach the next thing?
(SOUNDBITE OF LAUGHTER)
STONE: Well, because we're exploring a part of space nothing has been before, every day we look at the data and we realize that it's even more interesting than we had imagined. That's one of the lessons from the Voyager mission, is that no matter what you think you know, what there is to learn is even more interesting.
INSKEEP: Well, Dr. Stone, thanks very much.
STONE: Well, thank you.
(SOUNDBITE OF THEME MUSIC, "STAR TREK: VOYAGER")
INSKEEP: Dr. Ed Stone is the chief scientist on the Voyager space program.
There's something about that music that just make you want to talk that way.
It's MORNING EDITION from NPR News. I'm Steve Inskeep.
WERTHEIMER: And I'm Linda Wertheimer.
(SOUNDBITE OF THEME MUSIC, "STAR TREK: VOYAGER") Transcript provided by NPR, Copyright NPR.