RACHEL MARTIN, HOST:
The Voyager 1 spacecraft launched in 1977 on a mission to Jupiter and Saturn. Since then, it's just kept on going and going. Today, Voyager is billions of miles from the earth, and scientists have been predicting it will soon leave our solar system. But one research team says it already has and it left a year ago. NPR's Geoff Brumfiel asks, how'd we miss that?
GEOFF BRUMFIEL, BYLINE: For the past decade or so, we here at NPR have been waiting for Voyager 1 to leave the solar system.
RICHARD HARRIS, BYLINE: Cormegius(ph) and his colleagues believed that Voyager 1 passed through the beginning of the end of our solar system
STEVE INSKEEP, BYLINE: And it's about to leave the space that's influenced by the energy from the sun.
AUDIE CORNISH, BYLINE: Voyager 1 is almost there.
BRUMFIEL: That's NPR's Richard Harris, Steve Inskeep and Audie Cornish. But now Marc Swisdak, a researcher at the University of Maryland, says the spacecraft may have already left.
MARC SWISDAK: July, late July 2012 is when we think it is.
BRUMFIEL: July 27th, to be precise, which is almost my birthday.
SWISDAK: It's almost my wedding anniversary too, so there is that going for it.
BRUMFIEL: Seriously though, how'd we miss that? As it turned out, it wasn't entirely our fault. Researchers thought the solar system was surrounded by a clearly marked magnetic field.
SWISDAK: There's one at the Earth. There is one at, for instance, Jupiter, Saturn - many planets have them. And so, just by analogy we were expecting there to be something like that for the solar system.
BRUMFIEL: Scientists were waiting for Voyager to cross over the magnetic edge of our solar system and into the magnetic field of interstellar space. But in a paper in the September issue of Astrophysical Journal Letters, Swisdak and his colleagues say the magnetic fields may blend together. And so in July of 2012, when Voyager crossed from the solar system into deep space, all they saw was a little change in the field.
SWISDAK: Voyager just kept cruising along.
BRUMFIEL: But not everyone thinks Voyager has left. Ed Stone is NASA's chief scientist for Voyager. He thinks there is a magnetic edge, and until Voyager sees a change in the magnetic field, it's still in the solar system. He's hoping that change will come in coming years.
ED STONE: I think that there is a very good chance before we run out of electrical power that we will be demonstrably in interstellar space.
BRUMFIEL: Until Voyager's power goes out or the magnetic field flips, Swisdak says the scientific debate will continue. So will Voyager's journey.
SWISDAK: Basically it's just happily heading out towards pretty much nowhere.
BRUMFIEL: Geoff Brumfiel, NPR News.
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MARTIN: This is NPR News. Transcript provided by NPR, Copyright NPR.