NASA's Voyager 1 is Riding the Highway Out of the Solar System

Voyager 1 is now 11 billion miles (18 billion kilometers) from the Sun some 36 years after its launch, placing it closer than ever before in becoming the first human-made object to reach interstellar space.

Research conducted with the help of the spacecraft and published in the journal Science provides new detail on the last region it will cross before it leaves the heliosphere, or the bubble around the Sun, and enters the great beyond.

Thus far, scientists have seen two to the three signs of interstellar arrival they had planned on, including charged particles disappearing as they zoom out along the solar magnetic field, and cosmic rays from outside zooming in.

What they have not seen, however, is an abrupt change in the direction of the magnetic field, which would indicate the presence of the interstellar magnetic field.

"This strange, last region before interstellar space is coming into focus, thanks to Voyager 1, humankind's most distant scout," Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena, said in a press release. "If you looked at the cosmic ray and energetic particle data in isolation, you might think Voyager had reached interstellar space, but the team feels Voyager 1 has not yet gotten there because we are still within the domain of the sun's magnetic field."

It's not exactly clear how far Voyager 1 has to go yet before it crosses this important threshold - current estimates vary from several months to several years.

The reason for the uncertainty is based in the simple fact that, beyond knowing that the heliosphere extends at least 8 billion miles (13 billion kilometers) beyond all the planets in the solar system, scientists aren't sure how big it is.

They do know, however, that it is dominated by the Sun's magnetic field and an ionized wind expanding outward from the Sun.

Voyager 1 and its twin, Voyager 2, both launched in 1977, subsequently taking a tour of Jupiter, Saturn, Uranus and Neptune before embarking on their interstellar mission in 1990, all the while sparking new discoveries and studies.

At this point, Voyager 2 is approximately 9 billion miles (15 billion kilometers) from the Sun and, like its sibling, remains inside the heliosphere.

This region allows charged particles to travel into and out of the heliosphere along a smooth magnetic field line, rather than bouncing around in all direction as if trapped on local roads. And, as a result of Voyager 1's arrival, for the first time scientists were able to detect low-energy cosmic rays originating from dying stars.

"We saw a dramatic and rapid disappearance of the solar-originating particles. They decreased in intensity by more than 1,000 times, as if there was a huge vacuum pump at the entrance ramp onto the magnetic highway," said Stamatios Krimigis, the low-energy charged particle instrument's principal investigator at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "We have never witnessed such a decrease before, except when Voyager 1 exited the giant magnetosphere of Jupiter, some 34 years ago."

Other charged particle behavior observed by Voyager 1 further indicates that the spacecraft has not left the region of transition to the interstellar medium.

For example, while crossing into the interstellar medium, the charged particles originating from the heliosphere that decreased most rapidly were those shooting straightest along the solar magnetic field lines. Meanwhile, particles moving perpendicular to the field decreased more slowly.

However, cosmic rays moving along the field lines in the highway region were somewhat more populous than those moving perpendicular to the field, whereas in interstellar space, the direction of the moving charged particles is not expected to matter.

In the span of just 24 hours, the magnetic field originating from the Sun also began piling up, like cars backed up on a freeway exit ram, the scientists explained. However, they were able to quantify that the magnetic field barely changed direction.

"A day made such a difference in this region with the magnetic field suddenly doubling and becoming extraordinarily smooth," said Leonard Burlaga, the lead author of one of the papers, and based at NASA's Goddard Space Flight Center in Greenbelt, Md. "But since there was no significant change in the magnetic field direction, we're still observing the field lines originating at the sun."