On 35th anniversary of spacecraft’s launch, scientists ponder when it will move beyond the sun’s reach

By Nadia Drake

Web edition: September 5, 2012
Print edition: October 20, 2012; Vol.182 #8 (p. 14)

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Data from Voyager 1, launched on September 5, 1977, suggest its passage out of the solar system is not quite what scientists had expected.

Credit: NASA

NASA’s Voyager 1 spacecraft, launched 35 years ago on September 5, 1977, is bracing for a controlled plunge into interstellar space. Soon the craft will leave the solar system behind, bursting through the windy bubble blown by sun.

The question is: How soon? That boundary may be a bit farther away than expected, a team from the Johns Hopkins University Applied Physics Laboratory reports in the Sept. 6 Nature.

Now 18 billion kilometers away, Voyager 1 is the most distant spacecraft flung from Earth. Voyager 2, launched two weeks earlier, is trailing its twin by about 3.4 billion kilometers.

Earlier this year, NASA reported that Voyager 1 had detected two major signs heralding its impending exit from the solar system. But after analyzing data from an earlier series of rolling maneuvers, scientists didn’t detect the expected stream of charged particles thought to signify the solar system’s edge.

At that edge, such particles should reroute to flow in the direction of the sun’s poles, forming a stream that coats the inner surface of the bubble-shaped boundary. That Voyager 1 didn’t observe that change implies that either the spacecraft is not snuggled up against the boundary, or that the edge behaves differently than predicted, says study coauthor and space physicist Edmond Roelof.

“How do you know when you’re going to get to somewhere if you don’t know what it looks like? Everything we’re seeing is new and unpredicted,” Roelof says. “The idea that there is some sort of distinct boundary, like crossing from New Jersey to Pennsylvania — it isn’t shaping up that way.”

 For now, it’s clear that Voyager 1 is within in a region called the heliosheath, a transition area between the point where the solar wind slows down and the very edge of the bubble blown by the sun, called the heliopause.

“We’ve been seeing that the wind has more or less come to a stop. It’s in a stagnation region,” Voyager chief scientist Edward Stone of Caltech said in May. “That means we’re close to the heliopause.”

Stone suggested that Voyager 1 might have another two to three years to go before crossing into interstellar space.