When Voyager became the first man-made object to leave the solar system in 2012, scientists hailed it as a ‘milestone’ and the beginning of an incredible new journey.
The probe, which was launched in 1977 to study the outer planets in our solar system, was thought to have drifted almost 12 billion miles (19 billion km) from Earth.
But now experts believe last September’s celebrations (it took a year to confirm the readings) may have been premature.
Two Voyager 1 scientists are unconvinced the craft is in interstellar space, believing it is more likely that it is still in the heliosphere – dominated by the Sun and its wind of energetic particles.
And they have developed a test that they say could prove once and for all if Voyager 1 has crossed the boundary.
They believe that when the spacecraft does leave the solar system it will detect the reversal of Sun’s magnetic field, which hasn’t happened yet.
However if the magnetic field reversal doesn’t happen in the next year or two as expected, it will confirm that Voyager has already passed through the threshold and 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.
Prof 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,” Prof Gloeckler said.
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 also on its way to interstellar space.
Should Voyager 1 be intercepted by any intelligent alien civilisation, they will find a gold-plated disc containing a series of multicultural greetings, songs and photographs in 55 languages.
Among them are a message in Welsh and six now extinct tongues.
One of the Voyager discs
The audio recordings include the sounds of footsteps across a polished floor, a human heartbeat and someone laughing. There is also the sound of a couple kissing and a mother with her child.
The disc even contains the brainwaves of Ann Druyan, the creative director of the Voyager Interstellar Message Project, at the moment she decided to marry her husband.
In 2012 scientists noticed that the number of charged particles interacting with the craft's sensors suddenly dropped, suggesting it had broken through into interstellar space.
There was also a corresponding rise in galactic cosmic rays hitting the craft from the other side. However there was no noticeable change in the magnetism around the craft, suggesting it was still being influenced by the Sun.
Prof Gloeckler and co-author, Len Fisk, also a professor in atmospheric, oceanic and space sciences at the University of Michigan, are basing their new test on a model they developed and published earlier this year in The Astrophysical Journal.
Ed Stone of the California Institute of Technology in Pasadena and NASA’s Voyager Project Scientist said: “It is the nature of the scientific process that alternative theories are developed in order to account for new observations.
“This paper differs from other models of the solar wind and the heliosphere and is among the new models that the Voyager team will be studying as more data are acquired by Voyager."
Dr Alan Cummings, a senior research scientist at California Institute of Technology in Pasadena and a co-investigator on the Voyager mission, believes Voyager 1 has most likely crossed into interstellar space, but he said there is a possibility that Prof Gloeckler and Prof Fisk are right and the spacecraft is still in the heliosphere.
However he said that if Voyager 1 experiences a current crossing like the one being proposed in the new study, it could also mean that the heliosphere is expanding and crossed the spacecraft again.
“If the magnetic field had cooperated, I don’t think we’d be having this discussion,” Dr Cummings said. “This is a puzzle. It is very reasonable to explore alternate explanations. We don’t understand everything that happened out there.”
The new test is outlined in a study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.