One of the Voyager probes drifts through interstellar space in this illustration. Credit: NASA/JPL

In December 2012, a group of space scientists gathered in San Francisco to hold a vote. The lone issue on the ballot: Is Voyager 1 in, or is it out?

Throughout that year, the hardy spacecraft had beamed back tantalizing hints that it had left the heliosphere, the magnetic bubble inflated by the solar wind, and become the first human-made object to enter interstellar space. Yet mission scientists weren’t seeing what they had thought would be the unambiguous signature of Voyager’s departure: a sudden shift in direction of the magnetic field. It seemed inconceivable that the probe could have left a river of charged particles flowing from the Sun for an ocean of interstellar plasma without drastic magnetic deviations. “That was truly a shock to all of us,” says Len Burlaga, coinvestigator of the magnetometer instrument. “We figured that in a completely different region [the field direction] wouldn’t be the same.”

Subsequent measurements from the probe’s other instruments ultimately provided enough evidence to convince most of the voters in San Francisco that Voyager 1 had crossed into interstellar space in August 2012, some 122 AU from the Sun (see Physics Today, November 2013, page 18). Nonetheless, more than six years later, the spacecraft’s magnetic field readings are still conspicuously similar to those taken in the outer reaches of the heliosphere. The puzzling measurements have sent space scientists struggling to explain what Voyager is seeing just beyond the solar bubble.

“Boundaries are simple when you learn about them in books,” says Merav Opher, a space scientist at Boston University. “In nature, it’s never like in the book.” Opher and others will soon get to put their revised theoretical models to the test, as mission scientists recently reported that, judging from particle and plasma data, Voyager 2 joined its sibling in interstellar space in November. The readings of its magnetometer are expected to be announced at the International Astrophysics Conference in Pasadena, California, next week.

The first hint of Voyager 1’s heliopause crossing came on 28 July 2012, when galactic cosmic-ray intensity abruptly jumped, the concentration of solar particles plummeted, and the magnetic field intensity doubled, to about 0.4 nT. Four more inflection points occurred over the next month, marked by a fall in field intensity, then a jump, then a fall, and one last jump on 25 August, the currently accepted interstellar crossing date. Yet during those turbulent four weeks, the direction of the magnetic field barely budged. The big Science paper describing Voyager 1’s exit concluded with the observation that because of the lack of shift in field direction, “the very definition of the heliopause comes into question.”

Voyager 1 detected changes in magnetic field intensity (top graph) five separate times (B1–B5), presumably as it crossed the heliopause, in mid 2012. But the magnetic field direction, measured here in terms of azimuthal and elevation angle, remained steady. Credit: L. F. Burlaga, N. F. Ness, and E. C. Stone, Science 2013

With the benefit of hindsight, Opher and several of her colleagues now say they shouldn’t have been surprised at the magnetic complexity relayed by the interstellar probe. After all, the heliopause is not a static boundary. Interstellar magnetic fields likely pile up and drape about the solar bubble, which could result in elevated field intensity and twists in field direction. Some of those field lines could break and reconnect in new orientations (see Physics Today, February 2019, page 20). Flux tubes may tunnel through the boundary and allow solar particles to leak into interstellar space.

Another complication is that although the interstellar magnetic field direction almost certainly differs from the heliosphere’s, it’s unclear what that direction should be. Voyager 1 is humanity’s first beacon in interstellar waters—all other clues have come indirectly. One important line of evidence comes from the Interstellar Boundary Explorer (IBEX) spacecraft’s detections of energetic neutral atoms, cosmic messengers that can swim upstream through the heliopause into the solar bubble. In 2009 mission scientists reported a bright ribbon of neutral-atom emission that wraps around the nose of the heliosphere, between the positions of the two Voyager probes. Based on the IBEX data and other evidence, including the polarization of starlight and cosmic-ray anisotropies, some researchers suspect that the interstellar magnetic field is oriented toward the center of that ribbon.

The Interstellar Boundary Explorer spacecraft detected a bright flux of neutral atoms in a ribbon shape near the nose of the heliosphere. The positions of the Voyager probes (V1 and V2) are shown in relation to that of the ribbon. Credit: D. J. McComas et al., Science

Theorists are analyzing Voyager 1’s measurements in attempts to combine all those hints and hypotheses into workable models. In late 2013, Opher and colleague James Drake ran magnetohydrodynamic simulations of the heliosphere environment and found that interstellar magnetic field lines twist as they pile up in front of the advancing solar bubble. The researchers showed that the twist should result in an orientation very close to that inside the heliosphere, regardless of the direction of the interstellar field. They concluded that Voyager 1 may have to fly through the pileup region for many years before reaching pristine interstellar space.

Nathan Schwadron, a space scientist at the University of New Hampshire, says he thought he had things figured out after analyzing Voyager interstellar data collected from May 2013 to August 2014. During that time the field direction changed slowly but steadily, covering nearly 10° of ecliptic longitude, toward the position of the IBEX ribbon. Schwadron reckoned that after traversing some sort of transition region, the spacecraft had finally started to get a taste of the ambient interstellar field.

But by the time Schwadron’s paper on the subject made it into print in late 2015, the field direction had swerved away from the ribbon. “The simplified picture started to fall apart,” he says. Schwadron’s latest paper, authored with IBEX principal investigator David McComas, pins the recent deviation on several coronal mass ejections from the Sun during its last active period, in 2012. They say that the material should propagate through the heliopause as pressure waves, creating areas of compression ahead of their arrival and rarefaction in their wake. Interstellar magnetic field lines would drape differently in each of those regions. If Schwadron and McComas are right, then the field should soon reverse direction again, back toward the ribbon.

In mid 2013 Voyager 1 detected a shift in the magnetic field toward the IBEX ribbon (blue trend line). But in 2015 the direction changed (red). Credit: N. A. Schwadron and D. J. McComas, Astrophys. J. 2017

Though most researchers are focused on adjusting their models to fit the new measurements, a small but vocal minority maintain that Voyager 1’s magnetic readings demonstrate that the probe is still in the heliosphere. Voyager scientists George Gloeckler and Lennard Fisk, who voted in 2012 against proclaiming the probe interstellar, have devised a model that predicts the probe will reach the heliopause in four or five years, when it is about 160 AU from Earth. And when it does cross, they say it will detect significant magnetic field deviations. A definitive signature could come even sooner, if the spacecraft catches up to plasma that was emitted prior to one of the Sun’s periodic magnetic field reversals. In that case, a 180° flip in the measured field direction would signal that Voyager 1 is still in the heliosphere.

After several years of scratching heads and tuning models, those who have been following the two Voyager spacecraft’s exploits are now focused solely on the first interstellar magnetic results for Voyager 2, which are expected to be released at next week’s conference in Pasadena. Opher and Schwadron expect little to no change in the field direction, just as with Voyager 1. Burlaga and McComas expect a more significant shift, based on the hypothesis that the draping of the interstellar field should be different in Voyager 2’s neck of the woods.

Regardless of who’s right (an abstract of Burlaga’s talk hints that the field intensity jumped and the direction changed at least somewhat), the space science community will have a lot more work to do in the coming years to explain the probes’ two sets of readings. What everyone once thought would be the clear-cut harbinger of interstellar space will, for the foreseeable future, continue to be one of its most puzzling attributes.