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The Safety Belt of Our Solar System

Mapping the heliosphere, which shields us from harmful cosmic rays.

David McComas has a favorite “astrosphere,” the environment created by a star’s stellar wind as it buffets the surrounding interstellar…By Virat Markandeya

David McComas has a favorite “astrosphere,” the environment created by a star’s stellar wind as it buffets the surrounding interstellar medium. It belongs to a star named Mira. In an image from 2006, Mira is heading to the right, at 291,000 miles an hour, five times the speed our sun ambles through its local interstellar cloud in the Milky Way. You can make out a “bow shock” forming ahead of the star, like one would ahead of a boat sailing through water. Gas there heats and mixes with the wind of the cooler hydrogen gas blowing off Mira, and then flows to the star’s rear, forming a wake. Mira’s astrosphere, trailing behind the star to the left, looks turbulent, fragmented, and stretched. “How clearly you can see it sort of fall apart from this single structure to these turbulent smaller structures,” McComas, a professor of astrophysical sciences at Princeton, said, in a video interview recently. “I think it is very beautiful.”

When McComas isn’t admiring Mira’s astrosphere, he’s spearheading efforts to understand our own, the “heliosphere,” a bubble canonically comet-shaped. He’s eager to learn about the functions it might serve. Since 2008, McComas has been the principal investigator of the Interstellar Boundary Explorer (IBEX) mission. He oversees the data the IBEX satellite collects to disclose the nature of our solar system’s edge. He’ll also be in charge of IBEX’s successor, the Interstellar Mapping and Acceleration Probe (IMAP), which is set to launch in 2024.

What might count as our solar system’s boundary? There is no definite point at which light from our sun completely fades, or where its gravity stops being felt, so neither of those could mark it. But the heliosphere can. It “moves through the galaxy, keeping our home safe,” McComas says. The sun’s solar wind, an outflow of ionized gas, or plasma, pushes out against the galactic material between stars, also called the “interstellar medium.” The interstellar medium in our very local region is a mixture of plasma, helium, and neutral hydrogen. It is formed by warm, partially ionized clouds found in the Local Bubble, a large cavity filled with plasma that was likely produced by multiple supernova explosions, along with interstellar dust and other stellar winds. The barrier separating us from this occupies a region far beyond the orbit of Pluto, one you can define and measure.

DOES IT HAVE A TAIL?: Recent data from satellites and space probes suggest our heliosphere, butting up against the interstellar medium, might be more compact and rounded (left) than scientists thought (right). We might be missing a comet-like tail, a feature which has been spotted in the astrospheres of other stars.NASA

“In some ways,” McComas says, “it is like our ship traveling through interstellar space.” Without it—as data from the Voyager probes, launched in the 1970s, indicated—we would be bombarded on Earth by four times the amount of cosmic rays that come at us, which would be damaging to both the Earth’s ozone layer and our DNA. This year, using data from IBEX, McComas and his colleagues were able to create a 3-D map of the heliosphere—from the inside. Scientists have only had quite a limited sense of things from the outside. In the last decade, both Voyager 1 and Voyager 2 crossed the heliosphere’s threshold, a layer called the heliopause, and offered some data on the shape of our bubble ship’s front. (The Voyagers are racing ahead of the sun in the same direction.)

“These in situ measurements,” McComas and his colleagues wrote in a paper recently published in The Astrophysical Journal, “have provided the necessary ground truth as to the scale of the heliosphere, but as such, we only have direct measurements along two spacecraft trajectories at specific instances in time, providing important but very spatially and temporally limited information about the dimensions of the heliosphere.”

Unlike the Voyagers, IBEX doesn’t have a camera that collects light. Rather it has two sensors, on either side of its less-than-a-meter-wide hexagonal shape, that collect particles called energetic neutral atoms. Neutral atoms of hydrogen drift unimpeded through electromagnetic boundaries that separate interstellar space from the heliosphere. “They’re sort of meandering,” McComas says. Directly observing the interstellar neutral gas that flows into the heliosphere gives an estimate of the speed at which the heliosphere is moving relative to the interstellar medium. (At around 52,000 miles per hour, it’s a comparatively leisurely pace.)

What might count as our solar system’s boundary?

If a neutral hydrogen atom passes close enough to a proton in an ionized gas, the proton will snatch the electron from the hydrogen atom, and when it snatches that electron, the proton becomes a neutral, and the hydrogen atom becomes a new proton: a process known as charge exchange. The new neutral has the properties of the ionized gas. It will have a temperature and a bulk motion, corresponding to where the neutral was created, that can be used to pick out “hot” or energetic regions. “If you have a detector that can measure these energetic neutral atoms,” says Gary Zank, a space physicist at the University of Alabama at Huntsville, who has worked on IBEX, “what you’re basically doing is learning about where those energetic neutral atoms were created.”

These can come from the heliosphere’s boundary and from beyond the boundary. Populations of neutral atoms moving at around 310 miles per second come from the region dominated by the supersonic solar wind. Atoms going around 62 miles per second come from the inner heliosheath, where the solar wind becomes subsonic as it hits against the interstellar medium. Energetic neutral particles can also come from solar wind interacting with the moon’s surface, and from processes occurring in Earth’s magnetosphere.

These particles, coming toward us from all directions, some of them bouncing off of the inside of the heliosphere, clue scientists in on how the sun’s solar wind interacts with interstellar space as our solar system drifts around the galaxy. Weighing a little more than the average American, IBEX has been orbiting Earth for more than a decade, observing the solar system’s edge with enough success to have earned McComas the chance to probe the barrier further, with IMAP.

To investigate the distant barrier of our solar system, McComas has had to grapple with an internal barrier—his dyslexia, a condition that makes it difficult to interpret letters and words. But it may have helped set him on the scientific path he’s on today, in more ways than one.


Since he had difficulty learning to read in grade school, he gravitated toward taking apart and reassembling whatever he got his hands on. In high school, he started a business, using a propane torch to solder unusual jewelry that he sold across the midwest, earning enough money over three years to actually consider foregoing college. Later, at MIT, he got involved in its center for space research, which needed a candidate to do fine assembly: a task McComas had experience in.

In a 2014 talk about his dyslexia and scientific career, titled “A Personal Journey from ‘Slow’ to the Interstellar Frontier,” McComas says that, when he discovered an interest in physics, he found it uncannily intuitive. “I felt like I could just understand the answers.” In one slide of his presentation, he shows a graphic illustrating a finding he made in the 1990s, that the sun’s solar wind is much faster at its poles. “This is sort of typical for how I’ve been able to communicate in my own area,” he said. “Putting together the different pieces of knowledge into a single graphic that, if you’re a space physicist, doesn’t need a caption.”

THE HELIOSPHERE’S SCALE: The extent of the plasma bubble protecting us extends far beyond the orbit of Pluto as it pushes against the interstellar medium, as shown in this logarithmic scale.Interstellar Probe / Jet Propulsion Laboratory / NASA

He credited dyslexia with helping him become a more collaborative scientist who can recognize people’s strengths and weaknesses, and thereby form better teams. “It took hundreds and hundreds of people to do the IBEX mission. You need people with all kinds of different skills and you need them to work together well,” he said. “So, I think there’s a diversity of thinking that’s easier for dyslexics to understand why that might be good.”

Early results from IBEX made the cover of Science in 2009. The satellite captured a mysterious ribbon structure projected over an “all-sky” map flattened to two dimensions. “The IBEX results are truly remarkable, with emissions not resembling any of the current theories or models of this never-before-seen region,” McComas said at the time. “We expected to see small, gradual spatial variations at the interstellar boundary, some 10 billion miles away. However, IBEX is showing us a very narrow ribbon that is two to three times brighter than anything else in the sky.” The neutral atoms IBEX was collecting, in other words, were not coming from all directions in more or less equal amounts. It’s a big hint to scientists that the way our sun’s magnetic fields interact with the galaxy’s magnetic field is much more complicated than they supposed. Still, scientists can use the ribbon to glean how we’re moving through the galaxy’s magnetic fields, and how those fields, in turn, influence our solar system.

The ribbon is like an imprint. It maps onto locations in the sky where the energetic neutral atoms would be seen to propagate from a source in straight lines as they pass unimpeded. “If you’re looking with a detector,” says Zank, “what you will see is in a certain radial direction all these energetic neutral atoms coming right back at you.” It may not be unlike situations where we might catch a glint of sunlight in the ocean. On a sunny day, there is always a region where there’s a much brighter sort of pattern of light, like a slightly broad line, says Zank. “The reason you see that brighter sort of light from the reflected light from the ocean,” he says, “is that this is the light that is directed straight at your eyes.”

Data started streaming in after Christmas, and it looked like IBEX’s instruments were glitching.

In that November issue of Science, McComas ventured six theories to explain it. The list of potential explanations soon crossed over 10, though several are variations on a theme. Today, McComas is more-or-less convinced that one of them is right.

What seems to be happening, McComas explained, is that a fraction of the charged particles that were part of the solar wind, and are now neutralized, are propagating radially out, past the heliopause into the local interstellar medium. They become trapped and gyrate around the magnetic field draped across the heliosphere. Eventually, the energetic neutral atoms reneutralize and radiate back in, toward the heliosphere. This series of charge exchanges could explain the ribbon, which originates a few hundred astronomical units beyond the heliopause. (One astronomical unit, the distance between Earth and the sun, is 93 million miles.)

IBEX is able to probe plasmas at distances of a few hundreds of astronomical units. IMAP will take it up to 500 astronomical units or more. “There’s a fundamental limit on how far you can see with neutral atoms,” says McComas, “It’s still a really long ways.” The limit is related to the atoms’ energies and charge exchange. IMAP, at a cost of over $600 million, will carry a suite of 10 instruments to allow McComas to, among other things, better verify his idea of what causes the ribbon structure. Measurements with greater sensitivity can pinpoint its source.

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IMAP may help astrophysicists decide conclusively what our heliosphere looks like. The standard view seems to be that it looks like a comet, but Merav Opher, a space plasma physicist at Boston University, has been arguing, using simulations, for a croissant-shaped heliosphere, rounder and with lobes. “It has profound implications about how stars are encased in their own bubbles and how those bubbles filter galactic cosmic rays,” she says. Opher and her colleagues predicted how the interstellar magnetic field would press on the heliosphere, making it asymmetric particularly in its southern part, which the Voyagers verified. It cannot be a complete sphere because the plasma has to flow out. “You need a horn,” Opher says. “You need an exit of the plasma.”

IMAP, which will map energetic neutral atoms in a higher energy regime than IBEX, will help constrain the models, particularly whether the heliosphere has a long tail, extending thousands of astronomical units, or a relatively more compact tail a fraction of that length. Still, Opher and her colleagues believe that in-situ measurements are crucial, and propose such a probe to go out several hundred astronomical units beyond the heliosphere for the 2030s.

With the launch of IMAP, McComas is looking forward to doing “a very complete science job.” What would be ideal is discovering what sort of physics fundamentally controls our solar system’s evolving space environment, and thus, the origin of the ribbon. One might hope this would include a surprise or two. It wouldn’t be the first time. In our video call, McComas recalled an episode of sheer anxiety shortly after IBEX launched into space, in October 2008, from the Kwajalein Atoll atop a Pegasus rocket that was dropped from an airplane. Data started streaming in after Christmas, and it looked like IBEX’s instruments were glitching.

“The first swathe [of data] showed this bright thing down here, south of the equator,” McComas told me, which wasn’t predicted by the prevailing theories. With his physical copy of the Science issue featuring his work on the cover handy, he pointed to a part of the all-sky map. The second swathe of data, he said, showed the bright swathe as well. It wasn’t until the spacecraft made an adjustment, and a structure started to emerge, that the team exhaled: It was the ribbon. They were seeing something real.


Virat Markandeya is a science writer based in Delhi.

Lead image: oobqoo / Shutterstock

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