Researchers say they have solved a key mystery regarding Voyager 1 leaving the Solar System. It was believed to have passed beyond the influence of the Sun (the heliosphere) into interstellar space in 2013, the first spacecraft ever to do so, but measurements of the surrounding magnetic field since have caused some controversy.

Essentially, the magnetic field observed by Voyager 1 is tilted at 40 degrees to the expected interstellar magnetic field. This latest study, published in the Astrophysical Journal Letters, explains the anomaly by suggesting that the orientation of the magnetic field is being pushed and stretched by the solar wind. The authors predict that it will take 10 years for the spacecraft to pass into a region of space where the surrounding magnetic field is no longer influenced at all by the Sun.

"If you think of the magnetic field as a rubber band stretched around a beach ball, that band is being deflected around the heliopause [where the solar wind stops]," lead author of the study, Nathan Schwadron of the University of New Hampshire, said in a statement.

The direction of the interstellar magnetic field was estimated using data from NASA’s Interstellar Boundary Explorer (IBEX) spacecraft. It observed a “ribbon” of hydrogen atoms, dubbed the “IBEX ribbon”, just beyond the Solar System that seem to indicate its direction. Voyager 1’s different readings, therefore, were a cause for some confusion.

Now, using readings from the Voyager spacecraft – namely that it has observed a change in magnetic field orientation of a few degrees every year – the researchers have proposed the tilted magnetic field theory.

^{Artist's impression of the Voyager spacecraft. NASA.}

The lack of a significant change in the magnetic field when Voyager crossed beyond the Solar System had also been somewhat of a mystery, but this theory seems to show that it is a gradual change, and not a sudden change like other variables – such as the dramatic change in particles, 40 times greater, observed beyond the boundary.

The results suggest that Voyager 1 is not quite in “true” interstellar space yet, although Schwardon told IFLScience it was a “matter of philosophy”. Instead, Voyager 1 is in a sort of purgatory region which Schwardon called the “outer heliosheath”, where the solar wind and interstellar medium interact. It will not be until Voyager 1 reaches the so-called “pristine” region of the interstellar medium that is unaffected by solar wind in 10 years that it will truly be beyond the Solar System’s influence.

For all intents and purposes, though, NASA says that Voyager 1 has entered interstellar space because it has crossed the heliosphere, the bubble of solar wind around the Solar System. In 2025, the researchers expect Voyager 1 to record the same magnetic field direction in the pristine interstellar medium as the IBEX ribbon, an estimation supported by observations from the Ulysses and SOHO spacecraft.

Then, Voyager 1 will truly, almost certainly, be entirely in interstellar space and out of reach of the Sun.

^{Image in text: Artist's impression of the Voyager spacecraft. NASA.}