On its way out of the solar system, the Voyager 1 spacecraft has encountered a “magnetic highway” of charged particles — a hint that the spacecraft may not have far to go before reaching the brink of interstellar space.

This so-called highway lies where the sun’s magnetic field and the interstellar magnetic field meet. Particles blown outward by the solar wind are speeding in one direction, while particles from cosmic rays generated outside the solar system are racing inward.

“This was a major unexpected result,” Voyager scientist Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory said in a December 3 teleconference hosted by NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Voyager has surprised us.”

Voyager 1 and 2 were launched 16 weeks apart in 1977 and are the most distant human-made objects in the universe. Voyager 1 is now more than 123 times as far from Earth as the planet is from the sun, and Voyager 2 is about 101 times as far.

The magnetic superhighway may be the outermost region the spacecraft will encounter before it leaves the bubble of charged particles that envelops the solar system, Voyager project scientist Edward Stone of Caltech said.

Three signs will indicate that Voyager 1 is leaving the bubble known as the heliosphere. The probe should encounter more high-energy particles from cosmic rays outside the solar bubble. At the same time, the gust of particles coming from the sun should die down. Lastly, the spacecraft should detect a shift in the direction of the magnetic field around it.

Voyager 1 has already spotted the first two signs, but not the third; the orientation of the magnetic field remains constant for now.

Though scientists suspect the crossing will occur soon, the exact time is unknown. “It could take several more months; it could take several years,” Stone said.

For the past seven years, the spacecraft has been in an outer region of the solar system called the heliosheath, where the solar wind particles slow down and bounce around in all directions.

Voyager 1 first encountered the highway July 28, as the sun’s particle wind dropped away abruptly and the magnetic field ramped up in strength. After drifting in and out of the region several times, the spacecraft finally entered for good August 25.

“Voyager 1 has entered a new region, never before sampled by humanity,” said space scientist David McComas of the Southwest Research Institute in San Antonio. Models of the heliosheath failed to foresee the highway region, but after the earliest clues came in July, McComas and colleagues published a study in The Astrophysical Journal explaining how it could exist.

So what will the Voyager probes encounter once they finally do leave the heliosphere? “We think we will find a magnetic field oriented more in a north-south direction, and we’ve detected radio waves of a few kilohertz,” says Stone, “but we could well be quite surprised once we get out of the bubble.”