Scientists Make Best

Researchers have gained a new view of the journey ahead for the twin Voyager spacecraft and other probes bound on one-way trips into interstellar space

We live in a bubble—literally.

It's called the heliosphere, and it's made of tenuous plasma billowing from the sun. This ionized gas flows outward along magnetic field lines emerging from our star, spooling out in radial spirals tied to the sun's rotation. To venture beyond where this wind wanes against the greater flows of plasma coursing through our galaxy is, in a very real sense, to leave our solar system behind.

Yet despite the heliosphere being known and studied since the late 1950s, its hazy boundaries have only relatively recently come to light—with a surprising discovery. Just more than a decade ago, NASA's Voyager 1 beamed back data suggesting it had finally exited the heliosphere to enter interstellar space. But one measurement didn't fit expectations: the spiral magnetic field didn't straighten out like it was supposed to if the spacecraft had indeed crossed over.

"Retrospectively it made sense that there should be a transition region where the interstellar magnetic field bunches up and drapes against the heliosphere," says Jamie Rankin, deputy project scientist for the Voyager mission and a space physicist at Princeton University.

This "draping" effect is similar to how flowing water piles up around a ship's bow and along its flanks, tailward. And just like this rippling wake can reveal a ship's outline, the bending of interstellar magnetic fields around the heliosphere as our star moves through the Milky Way can provide important clues about the size and shape of the bubblelike boundary between our solar system and the rest of the galaxy. But exactly what this draping looks like and how it gives way to the pristine interstellar medium have remained open questions—that is, until now.

In a study recently published in the Astrophysical Journal Letters, Rankin and her team of researchers paint the first clear picture of the draped region by bringing together independent measurements from the twin Voyager probes and a model of the heliosphere-interstellar boundary sourced from NASA's Interstellar Boundary Explorer (IBEX), an Earth-orbiting satellite launched in 2008.

The strength of the Voyagers is that they directly measure magnetic fields and how the fields change over distance as the spacecraft travel farther out from the sun. But the Voyagers only sample the field along their trajectories, offering a blinkered view of the bubble's evolving boundaries. IBEX, on the other hand, provides a "big picture" perspective by detecting the energetic showers of atoms produced through collisions between solar wind particles and interstellar medium particles at the heliosphere's boundary. These data yield a remote view of the bubble's surface across the entire sky but without crucial relative distance measurements.

The trouble is that these two data sets don't agree. Along their outbound trajectories, both Voyagers now locally measure magnetic fields that are stronger than—and askew from—values extrapolated from IBEX's remote all-sky observations of the undraped magnetic field farther out. Reconciling these results from such very different missions is a bit like trying to piece together two sets of puzzle pieces. "There has been a lot of discussion about why the Voyager data doesn't match IBEX," says Katia Ferrière, an astrophysicist at the University of Toulouse in France, who was not involved with the study.

In the paper, the researchers show how the IBEX model and Voyagers’ measurements actually tell a consistent story. Along Voyager 1's trajectory, the results show that the field strength and direction—that is, the "drapery" around the heliosphere's edges—will persist over the next 60 years, which corresponds to another 20 billion miles of the spacecraft's journey, before finally reaching the "undraped" interstellar magnetic field predicted by IBEX. Applied to data from Voyager 2, the analysis shows this spacecraft will have to travel twice as far out as its twin to escape the piled-up magnetic fields of the heliosphere transition—a journey of about 120 years.

"[These results] paint a full picture," Ferrière says—albeit one that future missions could still add to.

To that end, NASA is planning for a 2025 launch of a successor of sorts to IBEX: the Interstellar Mapping and Acceleration Probe (IMAP). IMAP will produce even higher-resolution maps of the heliosphere's global structure and will overlap with ongoing Voyager measurements, which researchers hope will continue over the next decade despite both Voyagers being perilously low on power.

"The combination of these measurements will provide the best understanding of the heliosphere interaction with the local interstellar medium," says study co-author David McComas, a space physicist at Princeton University and principal investigator of the IBEX and IMAP missions.

A future interstellar mission to continue where Voyager 1 and 2 leave off could also further clarify the heliosphere's complex shape.

"Sending a spacecraft out the side could provide a good perspective of what this bubble looks like in the direction of the local wind and on the opposite side where people talk about there being a tail-like configuration," says Ralph McNutt, Jr., principal investigator for the Interstellar Probe mission concept study and a space physicist at the Johns Hopkins University Applied Physics Laboratory.

And the results of Rankin and her team's study suggest that the magnetic fields pile up less toward the heliosphere's portside flank, meaning that a probe passing through this transition region, as opposed to the thicker draping on the other side, could more rapidly access pristine interstellar space.

Getting samples of the undisturbed interstellar magnetic field could also help map the distribution and shape of the interstellar clouds of gas and dust that surround our solar system such as the Local Interstellar Cloud (LIC) that the heliosphere is currently traversing. Interstellar clouds can also stretch and twist surrounding magnetic fields as they move.

"The general shape of the heliosphere is governed by its motion through the LIC, but its exact shape also depends on the ambient magnetic fields," Ferrière says.

Although there is still work to do to map our bubble and surroundings, Rankin and her team's study shows the power of bringing together remote satellite and in situ spacecraft measurements.

"This study is all about connecting what we have measured to make sense of the bigger picture of what our place in the galaxy looks like," Rankin says.

Theo Nicitopoulos is a freelance writer who covers Earth and space science news.

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