An artist's rendering of one of the twin Voyager spacecraft. Voyager… (NASA )
Voyager 1, the spacecraft famous for beaming back striking photos of Jupiter, Saturn and their moons more than 30 years ago, has made still another surprising discovery: the existence of an unexpected zone at the very edge of the solar system.
It had been thought that the NASA probe was already passing through the outermost section of the solar system on its way toward the heliopause — the boundary where the solar wind ends and interstellar space begins. For that reason, the existence of yet another district at our cosmic neighborhood's edge was completely unexpected, said Stamatios Krimigis, a solar physicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., and leader of the team that operates Voyager's low-energy charged particle instrument.
"Nature is imaginative," he said Monday.
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Speaking to reporters from the annual meeting of the American Geophysical Union in San Francisco, Krimigis and former Jet Propulsion Laboratory Director Edward Stone described the newly discovered region as a "magnetic highway" that connects the heliosphere, the bubble surrounding the solar system, to the vast expanse of space beyond.
NASA researchers said in September that they thought that Voyager 1 might pass out of the solar system by the end of the year. As the craft neared the heliopause, scientists expected to detect fewer particles of solar wind and more cosmic rays pouring in from interstellar space. They also expected the magnetic field to change direction.
Since late July, Krimigis said, the intensity of the solar wind particles had decreased a thousand-fold, while cosmic ray intensities rose.
"If we had looked at particle data alone, we would have said, 'We're out! Goodbye, solar system!'" he said.
Although they could tell the strength of the magnetic field had increased, Voyager's instruments never detected the anticipated change in the field's direction, said Leonard Burlaga, a member of the team that operates Voyager's magnetometer from NASA's Goddard Space Flight Center in Greenbelt, Md.
For this reason, he said, "there's no evidence we've entered interstellar space."
Rather, the highway region, which is created by a magnetic field originating from the southern hemisphere of the sun, appears to allow particles from within the heliosphere to escape into interstellar space while permitting particles from the outside to pour in.
Gary Zank, a space physicist at the University of Alabama-Huntsville, said he wasn't convinced Voyager 1 was still contained within the solar system and that it would take several months to figure out whether the probe had crossed the heliopause after all.
Different theoretical models predict a different looking boundary, he said. Either way, he said, it was "a major discovery."
"As ever, Voyager seems to have a remarkable capacity for providing observations that suggest ... we're almost right," he said. "It would be nice for the theory and the observations to agree all at once. But it may not ever happen that way."
Voyager 1 is about 11 billion miles from the sun, making it the most distant man-made object in space, according to NASA. Its twin probe, Voyager 2, is about 9 billion miles from the sun.
Launched in 1977, they are the elder statesmen of the working NASA fleet. Their scientific instruments, which are fueled by radioactive plutonium-238, will begin powering down in 2020 and are expected to stop operating in 2025.
The Voyagers should pass through the heliopause before then.
Stone, who continues to work as a Voyager project scientist at Caltech, said that researchers at NASA think the magnetic highway might be about 5 to 10 astronomical units thick — that is, 5 to 10 times the distance from the Earth to the sun. If they are right, it would take two to three years for Voyager 1 to cross the region, he said.
Then again, Stone cautioned, NASA hadn't predicted the existence of the highway in the first place, so making precise guesses about when the spacecraft will leave the solar system isn't really possible — nor is foreseeing exactly what the spacecraft might see on the other side of the bubble.
The magnetic field should still shift direction, Stone said. But he said he thought other details would remain a surprise.
"We won't know until we finally actually leave the bubble," he said.