NASA's Voyager 1 spacecraft has entered the last region it has to cross before reaching interstellar space.
The team describes this new region as a 'magnetic highway' for charged particles. Our sun's magnetic field lines are connected to interstellar magnetic field lines, allowing lower-energy charged particles originating inside our heliosphere to zoom out and allows higher-energy particles from outside to stream in.
Until they enter this region, the charged particles simply bounce around in all directions.
The Voyager team reckons that this region must still be inside our solar bubble, as the direction of the magnetic field lines hasn't changed - which it will do, they believe, when Voyager breaks through to interstellar space.
"Although Voyager 1 still is inside the sun's environment, we now can taste what it's like on the outside because the particles are zipping in and out on this magnetic highway," says Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena.
"We believe this is the last leg of our journey to interstellar space. Our best guess is it's likely just a few months to a couple years away. The new region isn't what we expected, but we've come to expect the unexpected from Voyager."
Since December 2004 when Voyager 1 crossed a point in space called the termination shock, it's been exploring the heliosphere's outer layer, or heliosheath. Here, the solar wind abruptly slowed down from supersonic speeds and became turbulent.
Voyager 1 spent about five and a half years in this environment before the outward speed of the solar wind slowed to zero, at which point the intensity of the magnetic field also began to increase.
Voyager data from two onboard instruments that measure charged particles showed the spacecraft first entered this magnetic highway region on July 28, 2012. The region ebbed away and flowed toward Voyager 1 several times. The spacecraft entered the region again August 25, and the environment has been stable since.
"We are in a magnetic region unlike any we've been in before - about 10 times more intense than before the termination shock - but the magnetic field data show no indication we're in interstellar space," says Voyager magnetometer team member Leonard Burlaga.
"The magnetic field data turned out to be the key to pinpointing when we crossed the termination shock. And we expect these data will tell us when we first reach interstellar space."