Voyager Still Breaking Barriers Decades After Launch
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NEWS FEATURE 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 heliosphere. Ken Croswell, Science Writer Launched more than four decades ago, the two The interstellar magnetic field has surprised re- Voyager spacecraft keep expanding our horizons. searchers with both its strength and its direction, and Having flown past the giant planets 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 Sun’s their planetary targets, with Voyager 1 more than five magnetic domain. Is the heliosphere the shape of a times farther out than Neptune 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 Earth 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 interstellar medium: 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 stars. There, the spacecraft continue to make new Voyager 1 took a shorter route than Voyager 2 discoveries. and arrived at Jupiter 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 plasma’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 solar 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 astronomical unit 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 Pluto is about 40 ’ planet s colorful moon Io sported erupting volcanoes. astronomical units from the Sun. In 1980, Voyager 1 sped past Saturn, 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- Titan. 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 Uranus 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 geysers on Neptune’s large moon Triton. 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 August 25, 2012 at 121.6 — dium begins where the solar wind 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 solar system, 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. The farthest 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 miles 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.