This artist rendering provided by NASA shows Voyager 1 at the edge of the solar system.
This NASA diagram shows the two Voyager spacecraft inside the magnetic bubble around the sun. But Marc Swisdak believes it's already crossed the heliopause into interstellar space.
The Voyager 1 spacecraft launched in 1977 on a mission to Jupiter and Saturn. It kept on going. Today it's billions of miles from Earth, and scientists have been predicting it will soon leave the solar system.
NPR has been on Voyager watch since at least 2003, when longtime science correspondent Richard Harris provided this warning of Voyager's impending departure.
But now Marc Swisdak, a physicist at the University of Maryland, says the spacecraft may have already left. "Late July 2012 is when we think it [left]," he says.
How did we miss that? As it turns out, it wasn't entirely our fault. Researchers thought the solar system was surrounded by a clearly marked magnetic field bubble.
"There's one at the Earth, there's one at Jupiter, Saturn — many planets have them. And so just by analogy we were expecting there to be something like that for the solar system," Swisdak says.
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 2012, when Voyager crossed from the solar system into deep space, "Voyager just kept cruising along," Swisdak says. All scientists saw was a change in the field's direction.
But not everyone thinks Voyager has left. Ed Stone is NASA's chief scientist for Voyager. He thinks there is a magnetic edge to the solar system, and until Voyager sees a change in the magnetic field, it hasn't left the solar system. He's hoping that change will come in the next few years.
"I think that there is a very good chance before we run out of electrical power that we will be demonstrably in interstellar space," he says.
Until Voyager's power goes out or the magnetic field flips, the scientific debate will continue. So will Voyager's journey, Swisdak says: "Basically it's just happily heading out toward ... pretty much nowhere."