The heliosphere is a vast region, extending more than twice as far as Pluto. It casts a magnetic "force field" around all the planets, deflecting charged particles that would otherwise muscle into the solar system and even tear through DNA. However, the heliosphere, despite its name, is not actually a sphere. Space physicists have long compared its shape to a comet, with a round "nose" on one side and a long tail extending in the opposite direction.
In 2015, using a new supercomputer model and data from the Voyager 1 spacecraft, Merav Opher, professor of astronomy and researcher at Boston University's Center for Space Physics, and her coauthor James Drake of the University of Maryland came to a different conclusion: they proposed that the heliosphere is actually shaped like a crescent--not unlike a freshly baked croissant, in fact. In this "croissant" model, two jets extend downstream from the nose rather than a single fade-away tail. "That started the conversation about the global structure of the heliosphere," says Opher.
Then, two years after the "croissant" debate began, readings from the Cassini spacecraft, which orbited Saturn from 2004 until 2017, suggested yet another vision of the heliosphere. By timing particles echoing off the boundary of the heliosphere and correlating them with ions measured by the twin Voyager spacecraft, Cassini scientists concluded that the heliosphere is actually very nearly round and symmetrical: neither a comet nor a croissant, but more like a beach ball. Their result was just as controversial as the croissant. "You don't accept that kind of change easily," says Tom Krimigis, who led experiments on both Cassini and Voyager. "The whole scientific community that works in this area had assumed for over 55 years that the heliosphere had a comet tail." 
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Now, Opher, Drake, and colleagues Avi Loeb of Harvard University and Gabor Toth of the University of Michigan have devised a new three-dimensional model of the heliosphere that could reconcile the "croissant" with the beach ball.
Unlike most previous models, which assumed that charged particles within the solar system all hover around the same average temperature, the new model breaks the particles down into two groups. First are charged particles coming directly from the solar wind. Second are what space physicists call "pickup" ions. These are particles that drifted into the solar system in an electrically neutral form--because they aren't deflected by magnetic fields, neutral particles can "just walk right in," says Opher--but then had their electrons knocked off.
The New Horizons spacecraft, which is now exploring space beyond Pluto, has revealed that these particles become hundreds or thousands of times hotter than ordinary solar wind ions as they are carried along by the solar wind and sped up by its electric field. But it was only by modeling the temperature, density, and speed of the two groups of particles separately that the researchers discovered their outsized influence on the shape of the heliosphere.
That shape, according to the new model, actually splits the difference between a croissant and a sphere. While the new model looks very different from the classic comet model, the two may actually be more similar than they appear, says Opher, depending on exactly how you define the edge of the heliosphere. Think of transforming a grayscale photo to black and white: The final image depends a lot on exactly which shade of gray you pick as the dividing line between black and white.
So why worry about the shape of the heliosphere, anyway? Researchers studying exoplanets--planets around other stars--are keenly interested in comparing our heliosphere with those around other stars. Could the solar wind and the heliosphere be key ingredients in the recipe for life? "If we want to understand our environment we'd better understand all the way through this heliosphere," says Loeb, Opher's collaborator from Harvard.
And then there's the matter of those DNA-shredding interstellar particles. Researchers are still working on what, exactly, they mean for life on Earth and on other planets. Some think that they actually could have helped drive the genetic mutations that led to life like us, says Loeb. "At the right amount, they introduce changes, mutations that allow an organism to evolve and become more complex," he says. But the dose makes the poison, as the saying goes. "There is always a delicate balance when dealing with life as we know it. Too much of a good thing is a bad thing," says Loeb.
When it comes to data, though, there's rarely too much of a good thing. And while the models seem to be converging, they are still limited by a dearth of data from the solar system's outer reaches. That is why researchers like Opher are hoping to stir NASA to launch a next-generation interstellar probe that will cut a path through the heliosphere and directly detect pickup ions near the heliosphere's periphery. So far, only the Voyager 1 spacecraft and Voyager 2 spacecraft have passed that boundary, and they launched more than 40 years ago, carrying instruments of an older era that were designed to do a different job. Mission advocates based at Johns Hopkins University Applied Physics Laboratory say that a new probe could launch sometime in the 2030s and start exploring the edge of the heliosphere 10 or 15 years after that.
"With the Interstellar Probe we hope to solve at least some of the innumerous mysteries that Voyagers started uncovering," says Opher. And that, she thinks, is worth the wait.
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