SEATTLE — NASA’s venerable Voyager 1 probe could exit the solar system and enter interstellar space as early as next year, a new study suggests.

Voyager 1, which is now about 17.7 billion kilometers from Earth, has entered an unexpected “transition zone” at the edge of the solar system, according to the study. This finding, along with observations by NASA’s Cassini spacecraft, hints that Voyager may be about to go where no man-made object ever has — into the space between the stars — a few years earlier than previously thought.

“Perhaps by the end of 2012, we will be out in the galaxy,” said the study’s lead author, Stamatios Krimigis, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

The sun’s sphere of influence, composed of solar plasma and solar magnetic fields, is called the heliosphere. This gigantic structure is about three times wider than the orbit of Pluto. On the outskirts of the heliosphere lies the heliosheath, a turbulent region at the outer reaches of the solar system.

At the edge of the heliosheath is the heliopause — the demarcation line between our solar system and interstellar space.

Voyager 1, which launched in 1977, is plying the heliosheath, as is its twin, Voyager 2. Recently, Voyager 1 stumbled into a part of the heliosheath that scientists did not know existed.

In this region, the outward speed of the solar wind — the charged particles streaming from the sun — is essentially zero. Voyager 1’s measurements show it dropping from about 209,000 kilometers per hour in August 2007 down to zero by April 2010. And it has not picked up speed since.

It is not that the solar wind has ceased altogether out there; rather, it apparently is being blown sideways by a powerful interstellar wind, researchers said. This finding, first announced at a conference in December, is reported more fully in the new study and backed up with several more months’ worth of data.

The nature and extent of this “transition zone” should come as a surprise to many scientists, who had predicted a relatively sharp boundary between the heliosheath and interstellar space, researchers said.

“It’s at variance with all the theoretical models that anybody has come up with so far,” Krimigis said in an interview. “Nobody predicted that we would go through this region of zero velocity, where essentially the solar wind would be sort of sloshing around and not doing anything.”

Krimigis and his colleagues also wanted to figure out just how far Voyager 1 has to go before it reaches interstellar space. So they enlisted the aid of another NASA spacecraft, the Saturn-studying Cassini probe.

The researchers looked at Cassini’s measurements of energetic neutral atoms flowing out of the heliosheath. This information, combined with Voyager 1 observations of charged particles, gave them an idea of how wide the heliosheath is, and by extension where its edge — the heliopause — is located. The team calculated that interstellar space likely begins about 18.2 billion kilometers from Earth. So Voyager 1 appears to be almost there. Since the probe covers about 531 million kilometers every year, it could leave the solar system as early as next year — a surprise, since previous estimates had pegged the probe’s exit at 2015 or so.

That is not a sure thing, however; the calculation has some uncertainty attached to it. In fact, the heliopause could lie anywhere from about 16.1 billion to 22.5 billion kilometers from Earth, researchers said.

“It could happen any time, but it may be several more years,” said Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena, Calif., who was not involved in the new study.

So researchers will doubtless be scrutinizing Voyager 1’s data over the coming months and years, looking for any signs that the probe has officially crossed the heliopause.

“I think we’ll know it when we cross it,” Stone said. “The direction of the [magnetic] field will change when we get out there, and probably its strength as well.”

Another likely sign, Krimigis said, would be a sudden drop in the density of hot particles, which are common in the heliosheath, and higher readings of the colder particles thought to populate interstellar space.

Krimigis and his colleagues report their results in the June 16 issue of the journal Nature.



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