On its journey to the stars, the Voyager 1 spacecraft has hit an unexpected, knife's-edge region at the boundary between the solar system and interstellar space.
In short, Voyager 1 is in a spot where the sun's winds have, in effect, stopped blowing.
An analysis of Voyager 1 data, released Wednesday and appearing Thursday in the journal Nature, indicates that the transition between the sun's extended atmosphere and the beginning of interstellar space is not abrupt, as theorists had expected. Instead, the solar wind gradually tapers off to zero, setting up up a narrow region of what might be described as the solar "doldrums."
Voyager 1 has been crusing through these doldrums since April 2010, putting it on the doorstep of the heliopause, the point at which the sun's atmosphere yields to interstellar space beyond. Indeed, if the new calculations are correct, humanity's first rudimentary starship – now 34 years into its journey – could break free of the solar system by the end of next year.
The evidence comes from a detector on Voyager that counts the number of protons streaming from the sun as solar wind. By counting the proton hits in all directions, the team can calculate the speed and direction of the solar wind – after adjusting for the spacecraft's speed of some 38,000 m.p.h.
During the past three years, the wind's speed has dropped from 93,000 m.p.h. to zip.
"That wasn't supposed to happen," says Stamatios Krimigis, a solar physicist and emeritus head of the Space Department at the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., who led the team that conducted the study. "We're not supposed to be in a wind that isn't going anywhere."
The data do indicate some sideways flow as seen from Voyager. The team suggests this phenomenon is likely a result of the wind's collision with the galaxy's magnetic field. But the scientists also hold out the possibility that Voyager has already passed through the doldrums and bid the heliopause adieu. In that case, the deflecting particles would be part of the interstellar plasma being deflected as the solar system is traveling through space.
The team estimated the extent of the doldrums to be roughly 2 Astronomical Units, or about twice the distance between the Earth and the sun. The distance between the Earth and sun is about 93 million miles.
The team estimates that along Voyager's trajectory, the heliopause is some 11.3 billion miles from the sun – a figure reached by combining Voyager data with measurements of the sun's magnetic field by the Cassini spacecraft now touring Saturn.
Voyager's approach to interstellar space has brought other discoveries about the sun's extended atmosphere and the solar magnetic fields that shape it.
Last week, for instance, researchers said that Voyager in 2007 entered a region where the sun's magnetic field devolves into a froth of magnetic bubbles, suggesting a solar system with magnetism's equivalent of a root-beer-like head. The bubbles are thought to be roughly 100 million miles across.
Expecting the unexpected has become something of a ritual among Voyager scientists.
Dr. Krimigis acknowledges that is group had to make some assumptions in performing its calculations that, if wrong, could mean Voyager still has a few years to reach interstellar space.
But proving the assumptions wrong would be part of Voyager's adventure, he adds.
"Nature appears to be much more original than our wildest imagination," he writes in an e-mail exchange.