Voyager 1 spacecraft might not have reached interstellar space

The heliosphere, in which the Sun and planets reside, is a large bubble inflated from the inside by the high-speed solar wind blowing out from the Sun. 

Pressure from the solar wind, along with pressure from the surrounding interstellar medium, determines the size and shape of the heliosphere. 

The supersonic flow of solar wind abruptly slows at the termination shock, the innermost boundary of the solar system. 

The edge of the solar system is the heliopause. 

The bow shock pushes ahead through the interstellar medium as the heliosphere plows through the galaxy. 

Credit: Southwest Research Institute

In 2012, the Voyager mission team announced that the Voyager 1 spacecraft had passed into interstellar space, traveling further from Earth than any other manmade object.

But, in the nearly two years since that historic announcement, and despite subsequent observations backing it up, uncertainty about whether Voyager 1 really crossed the threshold continues.

There are some scientists who say that the spacecraft is still within the heliosphere, the region of space dominated by the Sun and its wind of energetic particles, and has not yet reached the space between the stars.

Now, two Voyager team scientists have developed a test that they say could prove once and for all if Voyager 1 has crossed the boundary.

The new test is outlined in a study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

The scientists predict that, in the next two years, Voyager 1 will cross the current sheet, the sprawling surface within the heliosphere where the polarity of the sun's magnetic field changes from plus to minus.

The spacecraft will detect a reversal in the magnetic field, proving that it is still within the heliosphere but, if the magnetic field reversal doesn't happen in the next year or two as expected, that is confirmation that Voyager 1 has already passed into interstellar space.

"The proof is in the pudding," said George Gloeckler, a professor in atmospheric, oceanic and space sciences at the University of Michigan in Ann Arbor and lead author of the new study.

Gloeckler has worked on the Voyager mission since 1972 and has been a vocal opponent of the view that Voyager 1 has entered interstellar space.

He said that, although the spacecraft has observed many of the signs indicating it may have reached interstellar space, like cosmic rays, Voyager 1 did not see a change in magnetic field that many were expecting.

"This controversy will continue until it is resolved by measurements," Gloeckler said.

This artist’s concept shows the Voyager 1 spacecraft entering the space between stars. 

The Voyager mission team announced in 2012 that the Voyager 1 spacecraft had passed into interstellar space, but some scientists say it is still within the heliosphere, the region of space domininated by the Sun and its wind of energetic particles. 

In a new study, two Voyager team scientists are proposing a test that they say could prove once and for all of Voyager 1 has crossed the boundary. 

Credit: NASA/JPL-Caltech

If the new prediction is right, "this will be the highlight of my life," he said. "There is nothing more gratifying than when you have a vision or an idea and you make a prediction and it comes true."

The Voyager 1 and 2 spacecraft were launched in 1977 to study Jupiter and Saturn. The mission has since been extended to explore the outermost limits of the Sun's influence and beyond.

Voyager 2, which also flew by Uranus and Neptune, is on its way to interstellar space.

More information: Geophysical Research Letters DOI: 10.1002/2014GL060781