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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Audie Cornish.

In 1977, NASA captured our imaginations by launching two Voyager spacecrafts. They've made history by exploring the outer planets of our solar system, sending reams of data back to Earth. Now, Voyager 1 has made another pioneering discovery. It's the first spacecraft to leave the vast bubble of space that surrounds our sun.

As NPR's Richard Harris reports, Voyager 1 is now in interstellar space.

RICHARD HARRIS, BYLINE: There was no question that the Voyager spacecraft would someday become the first objects made by human beings to head out into interstellar space. The question, though, was whether the probes would still be alive to send back the news.

ED STONE: Most of us felt that we could at least get to Neptune, but we had no idea how much further it would be.

HARRIS: Ed Stone has been the Voyager chief scientist for more than 40 years. He and the other scientists have been waiting and waiting and waiting for the moment when they could say that Voyager has finally entered interstellar space. That's defined as the region that's outside of a huge bubble of hot gas, which flows from the sun, the solar wind. Eventually that wind dies when it runs into the cooler gas that permeates the galaxy. That's where interstellar space begins.

Is this real deal?

STONE: This is the real deal. Voyager 1 has finally reached interstellar space. The first time a spacecraft has been in the space between the stars.

HARRIS: The dramatic moment actually happened a year ago. But the instrument on board Voyager 1 that could have told us that in real time had died in 1980. Happily, there is a second instrument on board that measures the environment around the spacecraft. Don Gurnett, at the University of Iowa, helped build that instrument back in the 1970s and he's in charge of it today. The trick, though, is his instrument can only measure the spacecraft's environment when the thin gas surrounding Voyager gets a jolt of energy, say, from the sun.

DON GURNETT: When you have big explosions on the sun, it sends out a huge shockwave.

HARRIS: And that shockwave shakes up the gas. Last fall, there was a storm on the sun and Gurnett's instrument picked up this signal.

(SOUNDBITE OF A VOYAGER SIGNAL)

HARRIS: And in May of this year, another solar storm came crashing through. The gas surrounding Voyager emitted a signal at a noticeably higher pitch.

(SOUNDBITE OF A VOYAGER SIGNAL)

HARRIS: Again, fall.

(SOUNDBITE OF A VOYAGER SIGNAL)

HARRIS: And spring.

(SOUNDBITE OF A VOYAGER SIGNAL)

HARRIS: That change in pitch shows that the spacecraft has gradually entered a part of space where the surrounding gas is much cooler and much more dense. It's outside of the sun's influence. He published his results in Science magazine.

Nobody knew exactly how far away this boundary would be.

GURNETT: It's really incredible how far we are. We're three times as far as Pluto - more than that, actually. But here's another way to put it. It takes 17 hours for the radio signal to get back to the Earth.

HARRIS: And that's traveling at the speed of light. By comparison, it takes about eight minutes for light to reach the Earth from the sun. He says the original Voyager team didn't want to believe it was that far away.

GURNETT: Maybe 30 years ago, people thought it was just beyond the orbit of Jupiter. And I don't think at the time anybody wanted to really believe we had to go another 20 years before we got there. But that's, in fact, how it turned out.

HARRIS: Like Gurnett, Ed Stone at Caltech has been waiting for this as yet another auspicious moment in Voyager's already spectacular career.

STONE: It's not only having achieved a really, a major exploration goal; sort of like the circumnavigation of the Earth the first time or landing footprints on the moon for the first time. This is the spacecraft to interstellar space. That means it's the beginning of a new journey of exploration for Voyager. So rather than ending, we're just starting the next new phase.

HARRIS: Stone says there's a lot to learn about this stuff from beyond our solar system. And Voyager 1 probably has another decade of power from its nuclear generator to keep on keeping on.

Richard Harris, NPR News.

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