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The Voyager 1 spacecraft is now more than 11 billion miles away from Earth. It blasted off 35 years ago today on a mission to Jupiter and Saturn, and it carried a record filled with music and the sounds of our planet in case it encountered intelligent life on its journey. Scientists have been eagerly waiting for Voyager 1 to become the first human-made object to leave the solar system. As NPR's Nell Greenfieldboyce reports, there are new signs that it might be getting close to that frontier.

NELL GREENFIELDBOYCE, BYLINE: The people who work on Voyager 1 and its twin, Voyager 2, have been involved in this program for a long time. Norman Ness is a professor at the University of Delaware. He recalls that he sent NASA a proposal for a Voyager instrument back in 1969.

NORMAN NESS: And in December received a telegram from NASA that I had been selected for the Voyager mission.

GREENFIELDBOYCE: He was ecstatic, because these probes were going to visit far-off giant planets. The prospect of leaving the solar system, and exploring interstellar space, was not really on his mind.

NESS: Not only didn't we think the spacecraft might have problems living that long, we didn't know if NASA was going to continue to support the mission after the primary mission had been accomplished.

GREENFIELDBOYCE: Well, NASA kept funding it. And even though Voyager's technology is primitive by today's standards - for example, each spacecraft has an on-board eight-track tape recorder - it just keeps working.

NESS: We're getting anywheres from five to eight hours of data every day from each of these spacecraft, and it's quite remarkable.

GREENFIELDBOYCE: Of the two, Voyager 1 is the farthest away from Earth. Don Gurnett of the University of Iowa has worked on the mission since the mid-1970s. He says Voyager 1's radio signals now take more than 16 hours to reach us.

DON GURNETT: And you know, astronomers like to think of distances in terms of light years. Well, we're not anything like a light year, but we're now a substantial fraction of a light day from the Earth. And I just find that extremely impressive.

GREENFIELDBOYCE: The question is when will Voyager 1 finally leave our solar system? It's already way beyond the planets - it even looked back and snapped photos of them, including a famous one of Earth, looking like a pale blue dot. For years, scientists have been waiting for Voyager 1 to exit the bubble of charged particles that stream out from our sun, then it would truly be in the space between stars.

Just in the last couple months, researchers have been getting some tantalizing hints that Voyager 1 might be almost there. Ed Stone is the Voyager project scientist at Caltech. Last night in a public lecture at NASA's Jet Propulsion Lab, he showed off some brand new data. Voyager 1 has been seeing decreases in the number of low-energy particles that come from inside our solar system. At the same time, it's been seeing increases in the high-energy particles that come from outside our solar system.

ED STONE: So there is some kind of connection between where Voyager is and the outside, which lets the particles that are inside out, and lets the particles outside in.

GREENFIELDBOYCE: This is uncharted territory. No one really knows just what the boundary with interstellar space is like. So Stone can't say how long it will be before Voyager passes through it.

STONE: I can't tell you whether it's days, months or years. I really can't tell you. That's the nice thing. From a science point of view, there is so much that we're learning that we had no way of really understanding before Voyager.

GREENFIELDBOYCE: Still, scientists are excited that they could be getting close. Voyager 1 can keep talking to Earth for about another decade. That's how long the plutonium that powers it should last. After it falls silent, it will still keep going. But it will be tens of thousands of years before it gets anywhere close to another star. Nell Greenfieldboyce, NPR News.

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