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DAVID GREENE, HOST:

Okay. As 2013 comes to a close, we've been taking a look back at some of this year's biggest news events. One history-making announcement came from NASA's Voyager One mission. Scientists said the space probe had become the first explorer from Earth to enter the mysterious realm of interstellar space. NPR's Nell Greenfieldboyce reports that this was a big moment for humanity and for the researchers who have worked with Voyager since the beginning.

NELL GREENFIELDBOYCE, BYLINE: Voyager 1 and its twin, Voyager 2, both blasted off in 1977, over 35 years ago. Voyager 1 flew by Jupiter, then Saturn, then on towards interstellar space, the unknown region that lies between stars. Tom Krimigis works at Johns Hopkins University's Applied Physics Lab. He was the youngest principal investigator for Voyager when it started. Now he's 75.

TOM KRIMIGIS: We knew that we had a long-lived spacecraft in our hands and the principle question was whether, for example, for me, whether I would expire before Voyager does, before we got into the galaxy, that is.

GREENFIELDBOYCE: The telltale signs they'd been waiting for came last year. Voyager's data showed that something unusual had happened on or about August 25, 2012.

KRIMIGIS: We were absolutely certain that a big boundary was crossed.

GREENFIELDBOYCE: They saw a sharp decrease in lower-energy particles that come from inside our solar system and a big increase in the higher-energy particles that come from outside our solar system. Did this mean Voyager had crossed into interstellar space?

KRIMIGIS: Most of us felt, I would say, 90 percent sure.

GREENFIELDBOYCE: This spring, they were able to get the proof they needed when Voyager registered the effects of a solar eruption that had happened months before. The chief scientist for Voyager is Ed Stone. He's now a professor at Caltech. Back in 1972, when he and his team were designing the mission, he was 36 years old.

He says even though the original goal was to visit the outer planets, his team had always hoped that Voyager would still be working when it finally left the bubble of charged particles created by our sun.

ED STONE: Like circumnavigating the globe - I don't know how many ships started out, but I think only one made it. This has the same kind of element of risk. It was an obvious thing to try to do, but it was not obvious that it could be done.

GREENFIELDBOYCE: He says for the first time we really have left home, even though the scenery may look like just more dark space.

STONE: The spacecraft itself is unaware that any of this is happening and if you were an astronaut, you'd be unaware of it as well.

GREENFIELDBOYCE: But he and other longtime Voyager scientists are very aware that Voyager is somewhere new and unexplored.

NORMAN NESS: It's quite a romantic idea, of course.

GREENFIELDBOYCE: Norman Ness is a physicist with the University of Delaware. He's now 80 years old, but still remembers how happy he was on that day back in the 1960s when he got a telegram saying he'd been picked for the Voyager program.

NESS: When I was a kid growing up, science fiction was a great fascination for me.

GREENFIELDBOYCE: He never thought interstellar exploration would be a part of his future. Voyager is now around 12 billion miles away from Earth. Its transmissions take over 17 hours to reach our planet. Voyager veteran Don Gurnett, of the University of Iowa, says the distance the spacecraft has traveled over his lifetime is mind-boggling.

DON GURNETT: But on the other hand, compared to the vast distances between stars and in the universe, it's just puny.

GREENFIELDBOYCE: The scientists should get another shot at seeing this strange boundary region when Voyager 2 gets there. And they'll keep listening to Voyager 1 until its power runs out in about a decade. After that, the spacecraft will keep moving outwards, finally getting close to another star in about 40,000 years. Nell Greenfieldboyce, NPR News.

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