NASA’s long-lived Voyager 1 spacecraft, which is heading out of the solar system, has reached a “magnetic highway” leading to interstellar space, scientists said on Monday.

The probe, launched 35 years ago to study the outer planets, is now about 11 billion miles (18 billion km) from Earth. At that distance, it takes radio signals traveling at the speed of light 17 hours to reach Earth. Light moves at 186,000 miles (300,000 km) per second).

Voyager 1 will be the first manmade object to leave the solar system.

Scientists believe Voyager 1 is in an area where the magnetic field lines from the sun are connecting with magnetic field lines from interstellar space. The phenomenon is causing highly energetic particles from distant supernova explosions and other cosmic events to zoom inside the solar system, while less-energetic solar particles exit.

“It’s like a highway, letting particles in and out,” lead Voyager scientist Ed Stone told reporters at an American Geophysical Union conference in San Francisco.

Scientists don’t know how long it will take for the probe to cross the so-called “magnetic highway,” but they believe it is the last layer of a complex boundary between the region of space under the sun’s influence and interstellar space.

“Our best guess is it’s likely just a few months to a couple years away,” Stone said.

Voyager 1 hit the outer sphere of the solar system, a region called the heliosphere, in 2004 and passed into the heliosheath, where the supersonic stream of particles from the sun - the so-called “solar wind” - slowed down and became turbulent.

That phase of the journey lasted for 5.5 years. Then the solar wind stopped moving and the magnetic field strengthened.