NASA’s Voyager Probes Find Puzzles beyond the Solar System
NEWS | 01 April 2025
For two decades now, the iconic twin Voyager spacecraft have been quietly overturning everything we thought we knew about the boundary between our solar system and interstellar space In humanity’s millennia of staring at the stars and decades of launching probes to explore our universe, only two spacecraft carrying working instruments have ever managed to escape the bubble of space governed by our sun. The twin Voyager spacecraft were launched in 1977 on an epic tour of the outer planets; both swung past Jupiter and Saturn, and Voyager 2’s itinerary later included Uranus and Neptune. The two probes have trekked ever outward since, and several of their instruments have continued observations despite the challenges of aging technology and waning power supplies. In 2004 Voyager 1 reached the termination shock, the beginning of its yearslong transition to interstellar space. Voyager 2 crossed the same threshold in 2007. In the years since, the twin spacecraft have been providing us with our only direct view of what lies on the outskirts of and beyond the region of the sun’s influence on space, an area scientists call the heliosphere. The Voyagers’ findings have revealed countless new puzzles about the outer heliosphere and interstellar space. These iconic spacecraft are now running out of time, but scientists are busy finding new ways to study the territory’s mysteries. On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. “We know now how little we know about the heliosphere,” says Merav Opher, a space physicist at Boston University. “It’s way more complex, way more dynamic than we thought.” Here’s what scientists do know: We Earthlings may simplistically think of the sun as a compact, distant ball of light, in part because our plush atmosphere protects us from our star’s worst hazards. But in reality, the sun is a roiling mass of plasma and magnetism that radiates particles billions of miles out into space in the form of the solar wind. The sun’s magnetic field, which travels with the solar wind, also influences the space between planets. The heliosphere grows and shrinks in response to changes in the sun’s activity levels over the course of an 11-year cycle. “You see these dramatic 11-year bumps, mins and maxes, dips and peaks throughout the whole entire heliosphere,” says Jamie Rankin, a space physicist at Princeton University and deputy project scientist of the Voyager mission. And, she notes, astronomers of all stripes are trapped in that chaotic background in ways that may affect their data and interpretations. “Every one of our measurements to date, until the Voyagers crossed the heliopause, has been filtered through all the different layers of the sun,” Rankin says. A diagram of the heliosphere shows the solar system with the solar wind and sun’s magnetic field flowing out to meet the interstellar medium. Scientists don’t yet know the shape of the heliosphere but are familiar with its three-part boundary, consisting of the inner termination shock and outer heliopause, in part because of observations gathered by the twin Voyager probes. Matthew Twombly The Voyagers earned their fame when they, too, were subjected to the sun’s whims. When the twin spacecraft originally launched, they were designed to take advantage of a rare alignment—happening just once every 176 years—that made it surprisingly economical to slip past all four outer planets: Jupiter, Saturn, Uranus and Neptune. The Voyagers discovered that Jupiter’s moon Io was a nightmarish, volcanic world nothing like Earth’s inert, gray moon. Voyager 1 surveyed Saturn’s large moon Titan and its thick, hazy atmosphere, composed predominantly of nitrogen, just like our own. Voyager 2 was the first and remains the only spacecraft to fly past Uranus and Neptune, worlds where it discovered superfast winds, more than a dozen moons and six new rings, as well as Uranus’s strange, tilted magnetic field and Neptune’s so-called Great Dark Spot storm system. The solar system would never look the same again. Even before launch, the poetry of the mission was clear. Astronomer Carl Sagan spearheaded an effort to equip each spacecraft with a Golden Record, a symbolic gesture to any other intelligent life in the universe. Each record, made to withstand the hazards of interstellar space, encoded a map of our location in the universe, photographs of daily life on Earth, and greetings and songs in countless languages. They offer a love-laden portrait of humanity to anyone who happens to find them in the centuries the spacecraft will spend floating through the cosmos. “The Voyagers are very much like biopsies of the heliosphere. We know nothing about the global three-dimensional structure of the outer heliosphere from just these two sets of points.” —David McComas, Princeton University On their trek to interstellar space, the Voyagers had to cross a set of boundaries: first a termination shock some seven billion or eight billion miles away from the sun, where the solar wind abruptly begins to slow, then the heliopause, where the outward pressure from the solar wind is equaled by the inward pressure of the interstellar medium. Between these two stark borders lies the heliosheath, a region where solar material continues to slow and even reverses direction. The trek through these boundaries took Voyager 1, the faster of the twin probes, nearly eight years; such is the vastness of the scale at play. Beyond the heliopause is interstellar space, which Voyager 1 entered in 2012 and Voyager 2 reached in 2018. It’s a very different environment from the one inside our heliosphere—quieter but hardly quiescent. “It’s a relic of the environment the solar system was born out of,” Rankin says of the interstellar medium. Within it are energetic atomic fragments called galactic cosmic rays, as well as dust expelled by dying stars across the universe’s eons, among other ingredients. The interstellar medium varies across the galaxy, with denser and more tenuous areas alternating across the Milky Way’s spiral arms. Our sun and the bubble it creates plow through this interstellar medium, and the interaction between the sun’s dynamics and the interstellar medium influences the shape of the heliosphere. The picture of Saturn's C-ring (blue-white) and B-ring (yellow) was produced by Voyager 2 in 1981. NASA/JPL What that shape is, however, scientists don’t yet know. The heliosphere’s shape may resemble that of a comet, with a long tail trailing a compact nose where the sun pushes into interstellar space. Or perhaps the interplay between the sun’s magnetic field and the interstellar medium molds the bubble into a croissantlike shape, with two lobes stretching away from our star. The heliosphere could also take some other form that scientists haven’t even considered yet; certainty about it is difficult from our limited view on Earth. “It’s like we’re goldfish trying to measure our goldfish bowl from the inside, and we can’t even get to the edges,” says Sarah Spitzer, a space physicist at the Weizmann Institute of Science in Rehovot, Israel. The Voyager probes are the accidental exceptions to this challenge. By 1989 they had completed their planetary observations and primary mission yet were still in good health. NASA kept them going, albeit after turning off instruments that wouldn’t produce interesting data without planets to observe. Years passed, and the Voyagers trekked ever outward, swimming toward the walls of our cosmic goldfish bowl. But the goldfish weren’t floating idly by. In 2008 NASA launched the Interstellar Boundary Explorer (IBEX), which orbits Earth and samples particles called energetic neutral atoms that stream in from the edge of the heliosphere. Scientists can use IBEX measurements of these particles’ characteristics to reconstruct some of what’s happening far out there, billions of miles away. Jupiter's Great Red Spot was seen during Voyager's 1979 flyby. NASA/JPL Among IBEX’s key contributions is the discovery of a ribbon of energetic neutral atoms draped across the heliosheath. Scientists think the ribbon may be caused by particles that bounce in and out of the heliosphere. But in an example of cosmic bad luck, the Voyager spacecraft weren’t able to directly study IBEX’s ribbon: they zipped past on either side of it. “Right between them is the biggest, most glaring thing in the outer heliosphere,” says David McComas, a space physicist at Princeton and principal investigator of IBEX. It’s exactly the kind of situation that shows the limitations of relying on local observations of something as all-encompassing as the vast province of our star’s influence. “The Voyagers are very much like biopsies of the heliosphere,” McComas says. “We know nothing about the global three-dimensional structure of the outer heliosphere from just these two sets of points.” Engineers work on a high-gain antenna, designed to communicate with Earth, for one of the Voyager spacecraft in 1975. NASA/JPL IBEX is still observing, having lasted much longer than originally planned, and the spacecraft has managed to gather data throughout a complete 11-year solar cycle to watch the heliosphere’s response to the sun’s activity. But McComas is also hard at work getting another mission he leads ready for launch this autumn. He describes the Interstellar Mapping and Acceleration Probe (IMAP) mission as “IBEX on steroids,” with the same basic capabilities but at higher resolutions and taking additional measurements. IMAP will travel to what scientists have dubbed Lagrange Point 1, a stable orbit path about one million miles toward the sun from Earth. From this vantage point, the spacecraft will catch a host of particles: the same energetic neutral atoms that revealed the IBEX ribbon; so-called pick-up ions that begin as atoms in the interstellar medium, pick up a charge near the sun and reverse course to head back out toward the heliopause; and grains of interstellar dust—debris from dead stars—that sneak into the solar system. Meanwhile the probe will also observe the sun’s magnetic field and the structure of the solar wind to reveal why particles travel the way they do. With these tiny messengers, IMAP scientists hope to build a clearer map of our heliosphere and a sharper picture of what lies beyond it. The image of Neptune was based on data gathered by Voyager 2 in 1989. NASA/JPL Other scientists are scheming to collect more observations from the region directly. One more spacecraft is already on track to follow the Voyagers out of the heliosphere: NASA’s New Horizons mission, which whizzed past Pluto in 2015. The spacecraft finished studying the dwarf planet (and, in 2019, an even more distant rocky object called Arrokoth) and is on course to cross the heliopause in perhaps another decade or so. Scientists hope its instruments will still be working then, ready for humanity’s third expedition beyond the sun’s influence. Scientists have also designed a would-be mission, dubbed Interstellar Probe, that, unlike the Voyagers and New Horizons, is tailored to illuminate the outer reaches of the heliosphere and beyond. It would use a massive rocket to take a fast track out of the solar system, carrying instruments to study plasma and magnetic fields instead of planets. Ideally, it would travel far enough to look back and discern our heliosphere’s elusive shape from a distance. But that mission was not recommended as a priority in a recently released Decadal Survey that charted U.S. heliophysics for the coming decade, and this rejection has hurt the chances of the nation’s scientists sampling the interstellar medium anytime soon. (Researchers in China may be more fortunate because that country is pursuing an interstellar mission of its own.) For now scientists are stuck poring over the signals dribbling back from the Voyagers. In some ways, they have a wealth of information: about two decades’ worth of data on the boundary to interstellar space and what lies beyond from two craft at two different locations. And the returns are rich in oddities, with one spacecraft apparently crossing the termination shock five different times, perhaps as the heliosphere billowed in and out in sync with the solar wind’s fluctuating strength. An updated version of Voyager’s iconic “Pale Blue Dot” image, originally released in 1990, shows Earth as a faint speck in a streak of sunlight. The image was taken by Voyager 1 when it had completed its planetary flybys and was 3.7 billion miles away from the sun. NASA/JPL-Caltech But the Voyagers’ distant observations remain mere breadcrumbs, tantalizing glimpses of a region that lies nearly out of our reach—exactly the type of data that raises more questions than answers. For example, scientists expected that the magnetic fields of the heliosphere and interstellar space would be dramatically different, but the probes have found otherwise. In 2020 Voyager 1 entered a strange “pressure front”—a sudden increase in the magnetic field that scientists can’t explain. And even though both spacecraft are years beyond the heliopause, they continue to see small traces of the sun’s activity in the material they sail through, expanding scientists’ understanding of how far our star’s influence reaches. And, of course, the Voyagers are aging. They are by far NASA’s longest-observing spacecraft, dreamed up by scientists who never imagined the mission might go on to outlive them. At each probe’s heart is a nuclear core to power instruments and communications, but they are running on about half the power they launched with and are weakening every year. Voyager 1 has had serious episodes affecting its communications in the past couple of years, and Voyager 2 briefly lost its orientation to Earth. At this point any glitch could easily be the end. One thing is certain: no matter when their mission concludes, the Voyager spacecraft will leave scientists wanting more data from interstellar space. “The instruments are going to be shut off before we get the full picture,” Opher says. “But having the Voyagers extended as much as we can, it’s priceless.”
Author: Clara Moskowitz. Meghan Bartels. Lee Billings.
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