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")

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