Based on an instrument that measures charged particles, Voyager entered the magnetic highway on July 28, 2012. The region was in flux for about a month and stabilized on Aug. 25.

Each time Voyager re-entered the highway, the magnetic field strengthened, but its direction remained unchanged. Scientists believe the direction of the magnetic field lines will shift when the probe finally enters interstellar space.

Other clues that Voyager has reached interstellar space could be the detection of low-energy cosmic rays and a dramatic tapering of the number of solar particles, Stone said.

Voyager 1 and a sister spacecraft, Voyager 2, were launched 16 days apart in 1977 for the first flybys of Jupiter, Saturn, Uranus and Neptune.

Voyager 2, traveling on a different path out of the solar system, has experienced similar, though more gradual changes in its environment than Voyager 1. Scientists do not believe Voyager 2, which is about 9 billion miles (14.5 billion km) from Earth, has reached the magnetic highway.