NASA's well-traveled Voyager 1 is headed out of the heliosphere, the fluctuating bubble in space inflated by plasma streaming outward from the sun. For years Voyager 1 has been closing in on the heliopause—the outer edge of the heliosphere—where the solar wind meets the interstellar medium. Yet despite intriguing hints, the probe remains within the heliosphere, mission scientists announced last December. It appears that Voyager 1 has discovered another wrinkle in the structure of our local space environment, a kind of magnetic highway linking the heliosphere to what lies beyond.
“We're still inside, apparently,” said Voyager project scientist Edward Stone of the California Institute of Technology in a teleconference with reporters on December 3. “But the magnetic field now is connected to the outside. So it's like a highway letting particles in and out.”
Voyager 1 crossed into the new region in August, when it registered a huge drop in the number of low-speed solar particles in its environment and a corresponding jump in the number of higher-energy cosmic-ray particles arriving from outside the solar system.
Despite the influx of cosmic-ray particles, the team concluded that Voyager 1 is still inside the heliosphere because the probe's magnetometer has not yet registered a change in magnetic field direction, as would be expected when crossing the boundary from the sun's plasma to the interstellar medium. “If we had only looked at particle data alone, we would have said, ‘Well, we are out. Good-bye to the solar system,’” says Stamatios M. “Tom” Krimigis of the Johns Hopkins University Applied Physics Laboratory. “But nature is very imaginative, and Lucy pulled up the football again.”
Launched in 1977, Voyager 1 is now more than 18 billion kilometers from the sun, farther out than any man-made object has ever traveled. It takes radio signals 34 hours to make the round-trip from Earth to Voyager and back again. Still, no one knows how much farther Voyager 1 may travel before it breaches the heliopause. “It may take several more months—it may take several more years,” Stone says.
Adapted from Observations at blogs.ScientificAmerican.com/observations