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New test to check if Voyager has reached interstellar space

Jenny Winder, News Writer
Jul 24, 2014, 16:04 UTC

Sen—Two Voyager team scientists have developed a test that they say could prove once and for all whether Voyager 1 has crossed the boundary into interstellar space.

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.

In 2012, the mission team announced that Voyager 1 had effectively left the Solar System and passed into interstellar space. But, despite subsequent observations backing it up, some scientists say that the spacecraft is still within the heliosphere and has not yet reached the space between the stars.

The new study predicts 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 does not happen in the next year or two as expected, that will be confirmation that Voyager 1 has already passed into interstellar space.

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The heliosphere is a large bubble inflated by the solar wind from the Sun. The supersonic flow of solar wind abruptly slows at the termination shock. The edge of the Solar System is the heliopause. The bow shock pushes through the interstellar medium as the heliosphere plows through the galaxy. Image credit: Southwest Research Institute

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

The new model assumes that the solar wind is slowing down and, as a result, that the solar wind can be compressed. Based on this assumption, the study says Voyager 1 is moving faster than the outward flow of the solar wind and will encounter current sheets where the polarity of the magnetic field will reverse, proving that the spacecraft has not yet left the heliosphere. The scientists predict this reversal will most likely happen during 2015, based on observations made by Voyager 1.

“If that happens, I think if anyone still believes Voyager 1 is in the interstellar medium, they will really have something to explain,” Gloeckler said. “It is a signature that can’t be missed.”

Alan Cummings, a senior research scientist at California Institute of Technology and a co-investigator on the Voyager mission, believes Voyager 1 has most likely crossed into interstellar space. He said that if Voyager 1 experiences a current sheet crossing like the one being proposed in the new study, it could also mean that the heliosphere is expanding and crossed the spacecraft again.

Cummings said. “This is a puzzle. It is very reasonable to explore alternate explanations. We don’t understand everything that happened out there.”

Stephen Fuselier, director of the space science department at the Southwest Research Institute in Texas, said there is a sizeable fraction of the space community that is skeptical that Voyager 1 has entered interstellar space, but the new proposed test could help end that debate. Another good test will come when Voyager 2 crosses into interstellar space in the coming years, Fuselier and Cummings said.