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Voyager still breaking barriers decades after launch NEWS FEATURE As Voyagers 1 and 2 continue their epic journeys through interstellar space, they’re resolving past controversies and even helping to spark a new one: the true shape of the .

Ken Croswell, Science Writer

Launched more than four decades ago, the two The interstellar magnetic field has surprised re- Voyager keep expanding our horizons. searchers with both its strength and its direction, and Having flown past the giant in the late 1970s the new data have even fed a controversy over the and 1980s, Voyagers 1 and 2 are now well beyond all geometry and activity of the heliosphere—the ’s their planetary targets, with more than five magnetic domain. Is the heliosphere the shape of a times farther out than and Voyager 2 not far comet, as has long been assumed, or is it instead more behind. “Every day is a new record for Voyager,” says spherical? And does it expand and contract when sun- the spacecraft’s project manager, Suzanne Dodd at spots wax and wane, or is it more stable? The space- the Jet Propulsion Laboratory near Los Angeles, CA. craft have offered up some tantalizing clues. “I never in my wildest dreams thought that I would still be working on Voyager fifty years after we wrote the Long Distance Voyager proposal,” says Voyager researcher Stamatios “Tom” Voyager 2 left in 1977, followed by Voyager 1. Krimigis of Johns Hopkins University in Laurel, MD. They weren’t the first spacecraft to reach the nearest During the past decade, both spacecraft reached a of the giant planets—that honor went to Pioneers 10 new realm, entering the : the and 11. But the Voyagers were more sophisticated than tenuous material that fills the vast space between the Pioneers and made many startling discoveries. the . There, the spacecraft continue to make new Voyager 1 took a shorter route than Voyager 2 discoveries. and arrived at first, in 1979, finding that the

New data from the fabled Voyager spacecraft have fed a controversy over the geometry and activity of the heliosphere. Image credit: NASA/JPL-Caltech.

Published under the PNAS license. Published April 21, 2021.

PNAS 2021 Vol. 118 No. 17 e2106371118 https://doi.org/10.1073/pnas.2106371118 | 1of4 Downloaded by guest on September 26, 2021 oscillation frequency is proportional to the square root of the ’s electron density, and the observed frequency of the radio waves implied a density matching that expected for the interstellar medium. Although so tenuous it would pass for a perfect vac- uum on Earth, the local interstellar medium is much denser than the outer heliosphere. Furthermore, knowing the approximate speed of the outbound material and the length of time it took to hit the boundary revealed the heliopause’s distance from the Sun: between 116 and 177 astro- nomical units, where 1 is the mean distance between the Sun and the Earth (1). Gurnett’s claim was controversial, however. “Frankly, people listened politely to my talks—I think I’m a fairly respected scientist—but nobody believed it,” Gurnett says. Krimigis, who was not involved with that measure- ment, was more blunt: “He was laughed at.” For one thing, no one had ever seen such radio waves before, and many researchers doubted Gurnett’s An engineer works on the construction of the dish-shaped antenna of Voyager in explanation for the signals. For another, his measure- July 1976. Image credit: NASA/JPL-Caltech. ment meant that the heliopause was depressingly distant. By comparison, Neptune is only 30 astronomical units from the Sun, and on average is about 40 ’ s colorful sported erupting volcanoes. astronomical units from the Sun. In 1980, Voyager 1 sped past , spying intricate “Nobody wanted to hear that we would have an- ’ details in the planet s rings and discovering the first other twenty-plus years to go before we got to the nitrogen atmosphere beyond Earth, around the moon heliopause,” Gurnett says. The prediction even en- . Voyager 2 took the more scenic route, visiting dangered the spacecraft themselves, because if the Jupiter in 1979 and Saturn in 1981, then ventured past next big objective was really so far away, they might in 1986 and Neptune in 1989. Voyager 2 pro- get turned off in order to save money. vided outstanding views of the green and blue planets and spotted on Neptune’s large moon . Rites of Passage The spacecraft then headed for interstellar space. In the end, the spacecraft survived. Voyager 1 shot As astronomers have defined it, the interstellar me- through the heliopause on , 2012 at 121.6 — dium begins where the solar the outflow of astronomical units, about four times Neptune’s dis- charged particles from the Sun—ends. This ionized tance and right in line with Gurnett’s prediction two gas, or plasma, presses against the cooler, denser inter- decades earlier. But so controversial was the passage stellar plasma flowing around it like a pebble obstructing that NASA didn’t announce the accomplishment until water in a stream. The Sun-carved cavity is called the thirteen months later. heliosphere and its edge the heliopause, just as the top of Still, Voyager 1 did see some indications that it had Earth’s troposphere is called the tropopause. crossed the heliopause. High-energy particles from When Voyager was launched, “we really didn’t the Sun vanished, a likely sign that the rest of the solar know how far out the heliopause was,” says Voyager wind had been left behind as well. Also, cosmic rays researcher Don Gurnett at the University of Iowa in from beyond the , which the heliosphere Iowa City. Some thought the heliopause might be as partially blocks, intensified after Voyager’s passage. close as Jupiter, only five times farther from the Sun These signs alone, however, failed to convince many than Earth is. As the spacecraft sped ever outward, researchers. estimates of the distance to the heliopause kept going There were two problems. First, Voyager 1’s up. It certainly wasn’t at Jupiter—or Saturn or Uranus plasma instrument had stopped working and so could or Neptune. As a result, no one knew when or where not record the jump in particle density when the Voyager would enter interstellar space. spacecraft broke from the heliosphere into interstellar Soon after the Neptune encounter, Gurnett space. Second, the magnetic field beyond the helio- claimed that Voyager had glimpsed signs of the heli- sphere was expected to point in a different direction opause in the far distance. In July 1992, both Voyagers and failed to do so. “It just so happens that nature began detecting strong radio waves at frequencies hasn’t read the theorists’ papers and didn’t know that between 2 and 3 kilohertz. Gurnett and his colleagues it was supposed to change the magnetic field direc- attributed these radio waves to six big flares that had tion,” Krimigis says. To this day, it’s still not clear why erupted on the Sun more than a year earlier. The re- the magnetic field outside the heliosphere aligns with searchers said that plasma ejected during the flares that inside. had eventually hit the heliopause, causing elec- The Sun helped confirm Voyager’s feat. Solar trons there to oscillate and emit the radio waves. The storms had erupted earlier in 2012, and the next year

2of4 | PNAS Croswell https://doi.org/10.1073/pnas.2106371118 News Feature: Voyager still breaking barriers decades after launch Downloaded by guest on September 26, 2021 they shocked the plasma that Voyager 1 was speeding interstellar medium, while the tail trails in the opposite through, causing electrons there to oscillate and give direction. But because the interstellar magnetic field is off radio waves that the spacecraft detected. The so strong, the magnetic pressure, which goes as the frequency of those radio waves indicated that Voyager square of the field’s strength, squeezes the heliosphere had indeed entered a much denser domain (2). and makes it round. “Absolutely it is,” Krimigis says. Voyager 1 thus became the first spacecraft ever to His team had earlier used the Cassini spacecraft, reach the interstellar medium. Contrary to some me- then orbiting Saturn, to reach the same conclusion (8). dia reports, the craft had not left the solar system. Cassini detected energetic neutral atoms that Krimigis Roughly a trillion icy bodies revolve around the Sun far believes come from near the heliopause. These varied beyond the orbits of Neptune and Pluto; every now in sync with the sunspot cycle and did so in all direc- and then one of them plunges toward the Sun and we tions at about the same time, suggesting that the see a new comet in the sky. of these heliopause is equidistant in all directions—in other distant icy objects are probably 1 to 2 light-years, or words, that the heliosphere is round. 63,000 to 126,000 astronomical units, away. Someone in the center of the continental United States who walks three west has gotten closer to the Pacific “Voyager is traveling uncharted waters. It’s in a location Ocean, relatively speaking, than Voyager has to the ’ where no mission has gone before and no mission will go solar system s edge. ” On November 5, 2018, Voyager 2 also crossed the probably for decades. heliopause. This time the passage was not contro- –Suzanne Dodd versial. The spacecraft’s plasma instrument was working and detected the leap in particle density as “ protons, electrons, and other charged particles struck But this claim is controversial. All of the evidence ’ the instrument (3, 4). It also recorded the temperature: that s been presented for a round heliosphere is in fact ” between 30,000 and 50,000 , much hotter than a misinterpretation of the data, says Nathan Schwa- the local interstellar medium, probably because the dron at the University of New Hampshire in Durham, plasma gets compressed as it hits the heliosphere. who favors the traditional comet shape instead (9). He Like Voyager 1, the spacecraft saw the argues that the energetic neutral atoms don’t neces- vanish (5) and cosmic rays from outside the solar sys- sarily come from near the heliopause and that changes tem increase (6), but the magnetic field again failed to in their number merely reflect changes in the plasma change direction, indicating that the result six years of the heliosphere as the waxes and wanes. earlier was no fluke (7). This means the Cassini data say nothing about the “But the thing that was amazing to me,” says heliosphere’s shape. When the Sun is active, Schwa- Krimigis, “was the fact that the crossing distance was dron says, it compresses and thereby heats the plasma 119.0 astronomical units.” That’s almost exactly the in the outer heliosphere, which leads to a greater same distance where the spacecraft’s twin had number of energetic neutral atoms from all directions; crossed—a surprise because the solar cycle was then conversely, when the Sun is quiet, the heliospheric in a different state. Over an 11-year period, as sun- plasma expands and thereby cools, leading to fewer spots wax and wane, the solar wind strengthens and of those atoms. He also says the interstellar magnetic weakens, pushing harder and then less so on the field would have to be much stronger than Voyager heliopause, albeit with a time delay of about 2.5 years. measured—around 20 microgauss—for magnetic pres- So the heliosphere should expand and contract. Be- sure to squeeze the heliosphere into a round shape. cause the pressure from the solar wind was less in 2012 than in 2018, the heliosphere should have been The Final Frontier considerably smaller during Voyager 1’s passage than Whatever the heliosphere’s exact shape, Voyagers 1 Voyager 2’s. Instead, Krimigis says, the nearly equal and 2 continue to dart away from it. By year’s end, distances mean the heliopause must be sturdier than Voyager 1 will be 155 astronomical units from the Sun. had been thought. The spacecraft’s signals, traveling at the speed of And that, he says, is attributable to the unexpect- light, will take more than 21 hours to get to us. Voy- edly strong magnetic field in the interstellar medium. ager 2 will be 129 astronomical units out and in 2023 Voyager 1 put it at 5 microgauss, about twice the will overtake the silent to become the predicted value, and Voyager 2 found an even stron- second farthest spacecraft of all. The two Voyagers are ger interstellar magnetic field, around 7 microgauss. In dashing away in different directions and are even Krimigis’s view, the pressure of the strong interstellar farther from each other than either is from Earth. field acts like a straitjacket, suppressing most of the They continue to transmit data about interstellar expansion and contraction of the heliosphere. space. Both can measure the electron density, be- Krimigis asserts that the magnetic field is so strong cause they detect the radio waves generated when it also changes the shape of the heliosphere. “For sixty solar eruptions cause electrons in the interstellar me- years we’ve had the wrong image of the heliosphere,” dium to oscillate. These eruptions should become he says. The standard view is that it’s the shape of a more frequent as the sunspot cycle peaks around comet, with a nose and a long tail. The nose faces the 2025. The current measurements indicate that the direction the solar system is moving through the interstellar density has increased further from the

Croswell PNAS | 3of4 News Feature: Voyager still breaking barriers decades after launch https://doi.org/10.1073/pnas.2106371118 Downloaded by guest on September 26, 2021 value it had outside the heliopause, but no one knows addition, astronomers must peer through interstellar what will happen next. “If you want me to just make gas and dust and correct for their effects to study stars my best guess, I would say it’s going to reach a peak and galaxies. Although researchers use ground-based and then come back down a little bit,” Gurnett says. and Earth-orbiting to observe the inter- That may indicate that interstellar material is piling up stellar medium from afar, the two spacecraft yield near the heliopause like snow in front of a snowplow. unique data on its density, temperature, and magnetic Voyager 2, which has a working plasma instrument, field by actually being inside it. “In situ measurements will keep tabs on the interstellar temperature. This are very important,” she says. temperature will likely fall, because astronomers have No one knows how much longer the two spacecraft measured the local interstellar medium’s temperature will survive. They have to stay warm so that the fuel to be only 7,500 Kelvin; the high temperature just they need to keep their antennas pointed toward beyond the heliopause probably results as the plasma Earth doesn’t freeze. Voyager’s heat comes from plu- there gets compressed and heated. Both spacecraft tonium, but as the radioactive element decays, it will monitor the interstellar magnetic field. If the provides less and less power every year. magnetic field gets compressed and strengthened Surprisingly, Voyager 1 is the warmer spacecraft, near the heliopause, then the field should eventually even though it’s farther from the Sun. “There’s a story weaken at greater distances. And Voyager could even to that,” Krimigis says. After NASA launched Voyager see the magnetic field change direction, just as re- 2, engineers noticed it was colder than expected, so searchers had expected at the heliopause. before launching Voyager 1, they added more insu- The ultimate goal is to sample the unperturbed lation to keep it warm. That extra warmth may well interstellar medium, space so distant that the helio- prolong the spacecraft’s life. And because Voyager 1’s sphere barely affects it. “Voyager is traveling in un- plasma instrument has failed, that instrument doesn’t charted waters,” Dodd says. “It’s in a location where consume any energy, leaving more for heating the no mission has gone before and no mission will go spacecraft. As a result, the Voyager that’s exploring a probably for decades.” The interstellar medium is of more distant—and hence likely more interesting— interest to astronomers in part because it is the region of space may continue to return data even after spawning ground of stars as well as the place where its mate falls silent. “If we make it to 2027, then it they deposit newly minted chemical elements, which would be fifty years since Voyager was launched,” future generations of stars and planets inherit. In Krimigis says. “So that’s the hope: 2027 or bust!”

1 D. A. Gurnett, W. S. Kurth, S. C. Allendorf, R. L. Poynter, Radio emission from the heliopause triggered by an interplanetary shock. Science 262, 199–203 (1993). 2 D. A. Gurnett, W. S. Kurth, L. F. Burlaga, N. F. Ness, In situ observations of interstellar plasma with Voyager 1. Science 341, 1489–1492 (2013). 3 J. D. Richardson, J. W. Belcher, P. Garcia-Galindo, L. F. Burlaga, Voyager 2 plasma observations of the heliopause and interstellar medium. Nat. Astron. 3, 1019–1023 (2019). 4 D. A. Gurnett, W. S. Kurth, Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments. Nat. Astron. 3, 1024–1028 (2019). 5 S. M. Krimigis et al., Energetic charged particle measurements from Voyager 2 at the heliopause and beyond. Nat. Astron. 3, 997–1006 (2019). 6 E. C. Stone, A. C. Cummings, B. C. Heikkila, N. Lal, measurements from Voyager 2 as it crossed into interstellar space. Nat. Astron. 3, 1013–1018 (2019). 7 L. F. Burlaga et al., Magnetic field and particle measurements made by Voyager 2 at and near the heliopause. Nat. Astron. 3, 1007–1012 (2019). 8 K. Dialynas, S. M. Krimigis, D. G. Mitchell, R. B. Decker, E. C. Roelof, The bubble-like shape of the heliosphere observed by Voyager and Cassini. Nat. Astron. 1, 0115 (2017). 9 N. A. Schwadron, M. Bzowski, The heliosphere is not round. Astrophys. J. 862, 11 (2018).

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