Voyager may not have entered interstellar space, after all

Nearly a year after NASA trumpeted Voyager 1’s departure from the sun’s protective bubble, two mission scientists argue that the spacecraft never left. Many astronomers are doubtful about the assertion, but the debate illustrates that the transition from solar bubble to interstellar space is not clear-cut.

“My tendency is to think we are out in interstellar space, but I’m not completely convinced,” says Eric Christian, an astrophysicist at NASA Goddard Space Flight Center in Greenbelt, Md., who is not on the Voyager team. “I don’t blame skeptics for looking at other explanations.”

Voyager 1 and its twin Voyager 2 completed their tours of the outer planets in 1980 and 1989, respectively. Since then, scientists have eagerly anticipated the probes’ departure from the heliosphere, the bubble of particles that encircles the sun and planets, and their entry into the unexplored space between stars. (The probes won’t exit the solar system for another 30,000 years or so (SN: 10/19/13, p. 26), since the sun’s gravitational influence continues well past the boundary of the heliosphere.)

Then in September, after months of speculation, mission scientists finally announced that Voyager 1 had exited the heliosphere on August 25, 2012 (SN Online: 9/12/13). The proof, they said, came via a blast wave from the sun that jostled particles around the probe in April 2013. The vibrations of the particles suggested that the spacecraft was surrounded by a dense soup of galactic particles rather than a comparatively sparse fog of solar ones. Last week, on July 7, the researchers reaffirmed their conclusion after analyzing another solar outburst that reached Voyager in March.

However, Voyager has yet to detect what scientists long predicted would be the calling card of interstellar space: a shift in the direction of the magnetic field. Scientists had expected the probe to encounter particles under the influence of the interstellar magnetic field draped over the outer shell of the heliosphere, inducing an abrupt shift. But the direction has remained stubbornly constant, and researchers can’t explain why. “This whole region is a lot messier than anyone dreamed of,” Christian says.

It’s a bit too messy for George Gloeckler and Lennard Fisk, Voyager scientists at the University of Michigan in Ann Arbor. They wondered whether the magnetic field and particle density conditions measured by Voyager could exist within the heliosphere. In a paper accepted for publication in Geophysical Research Letters, Gloeckler and Fisk argue that the outer heliosphere could allow an influx of galactic particles from beyond the bubble that would explain the density measurements.

The researchers’ analysis includes a way to definitively test the idea: If Voyager 1 is within the heliosphere, Gloeckler and Fisk note, then it should still be at the mercy of the sun’s magnetic field. If that were the case, within a year or so, Voyager should detect a 180-degree flip in the field’s direction, a regular occurrence caused by the sun’s rotation. “If that happens,” Gloeckler says, “Len and I will have a big celebration.”

“It’s a very good paper,” says Stephen Fuselier, a space scientist at the Southwest Research Institute in San Antonio. “They came up with a bold, testable prediction.”

Still, Fuselier thinks Gloeckler and Fisk are wrong. While the proposal is consistent with Voyager’s density measurements, he says, it can’t explain the particle vibrations Voyager detected in April 2013. Scientists suspect that solar blast waves can induce these vibrations only after interacting with the heliosphere boundary.

Voyager should provide more clarity by next year with the help of more blast wave measurements and the possibility of the 180-degree magnetic field shift. In the meantime, Christian says, scientists have to remain patient. Trying to characterize a vast unexplored region of space with one probe is like trying to understand an ocean with a single buoy.

Within several years, another buoy should reach the vicinity of the heliosphere boundary: Voyager 2, which unlike its sibling, sports a functioning instrument to take continuous measurements of particle density and temperature. “Voyager observations have surprised us over the years,” Fuselier says. “We’re just waiting for more surprises.”