Last year, scientists monitoring Voyager 1 noticed strange happenings that suggested the spacecraft had broken through: Charged particles streaming from the sun suddenly vanished. At the same time, there was a spike in galactic cosmic rays bursting in from the outside.
Since there was no detectable change in the direction of the magnetic field lines, the team assumed the far-flung craft was still in the heliosphere, or the vast bubble of charged particles around the sun.
The Voyager team patiently waited for a change in magnetic field direction — thought to be the telltale sign of a cosmic border crossing. But in the meantime, a chance solar eruption caused the space around Voyager 1 to echo like a bell last spring and provided the scientists with the information they needed, convincing them the boundary had been crossed in August of last year.
"It took us 10 seconds to realize we were in interstellar space," said Don Gurnett, a Voyager scientist at the University of Iowa who led the new research, published online in the journal Science.
The new observations are fascinating, but "it's premature to judge," said Lennard Fisk, a space science professor at the University of Michigan and former NASA associate administrator who was not part of the team. "Can we wait a little while longer? Maybe this picture will clear up the farther we go."
What bothers Fisk is the absence of a change in magnetic field direction.
Harvard astrophysicist Jonathan McDowell was more blunt: "I'm actually not going to believe it for another year or two until it's been solidly outside for a while."
Voyager 2 trails behind at 9 1/2 billion miles from the sun. It may take another three years before Voyager 2 joins its twin on the other side. Eventually, the Voyagers will run out of nuclear fuel and will have to power down their instruments, perhaps by 2025.
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