Leaving the Solar System is like leaving any familiar territory without maps — you have no idea what’s coming next, or even what you’ve just journeyed through. Such is the fate of NASA’s Voyager 1 spacecraft, which, at 18.7 billion kilometres from the Sun, has been flirting with the edge of interstellar space for the past year. Conflicting data from its various experiments suggest that it both has and hasn’t left the Solar System.
Now, a team led by researchers at the University of Maryland has a new explanation for what the probe has been seeing. And they claim that the signal NASA has been waiting for to say Voyager 1 is in interstellar space — a change in magnetic field direction — isn’t that relevant after all.
It all comes down to how to interpret magnetism at the Solar System’s edge. The Sun’s magnetic field dominates throughout a bubble of space enclosing the Solar System. Beyond that bubble, the magnetic field of interstellar space takes over. The question is what happens at the boundary between these two regions.
Believing that magnetic fields are oriented in different directions inside and outside the bubble, Voyager scientists are looking for a sharp change in direction. But in the 1 September Astrophysical Journal Letters, Marc Swisdak and his colleagues put forward a different scenario: that magnetic field lines may be parallel close to the boundary. If that’s the case, then Voyager 1 could have crossed into interstellar space already.
These parallel lines could exist thanks to a phenomenon called magnetic reconnection, in which magnetic field lines break and then recombine in a sort of violently popping action. Magnetic reconnection is thought to power solar flares, and Swisdak’s team suspects something similar may be going on along the edge of the Solar System. They envision a pair of magnetic ‘islands’ that appear spontaneously near three reconnection sites. Together these phenomena combine to create a set of parallel magnetic field lines that are just outside the Solar System proper.
The work relies on a powerful magnetohydrodynamics simulation done at NASA’s Ames Research Center in Moffett Field, California. At an American Geophysical Union meeting in Mexico earlier this year, co-author Merav Opher said that the magnetic reconnection idea faces an uphill battle. “People are having a really hard time swallowing this scenario,” said Opher, of Boston University in Massachusetts.
Voyager scientists lost no time in responding to the new report. In a statement, physicist Ed Stone of the California Institute of Technology in Pasadena was noncommittal about what it might mean. “The fine-scale magnetic connection model,” he explained, “will become part of the discussion among scientists as they try to reconcile what may be happening on a fine scale with what happens on a larger scale.” Stone has been project scientist for Voyager its entire 35-year lifetime, and as a Nature feature noted earlier this year, Voyager 1 will leave the Solar System when he says it does.
There may not be long to wait. The probe has seen undisputed signs that interstellar space is nigh; its particle counters already detect far more interstellar particles than ones from the sun. In fact, Swisdak’s group says it can explain some of the particle fluctuations by Voyager 1 crossing specific parts of the magnetic reconnection maelstrom.
In the end, humanity may know its voyager has left the Solar System only when it is well and truly in the rear-view mirror.