News Feature: Voyager still breaking barriers decades after launch
Ken Croswell
See all authors and affiliationsPNAS April 27, 2021 118 (17) e2106371118; https://doi.org/10.1073/pnas.2106371118
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.
Launched more than four decades ago, the two Voyager spacecraft keep expanding our horizons. Having flown past the giant planets in the late 1970s and 1980s, Voyagers 1 and 2 are now well beyond all their planetary targets, with Voyager 1 more than five times farther out than Neptune and Voyager 2 not far behind. “Every day is a new record for Voyager,” says the spacecraft’s project manager, Suzanne Dodd at the Jet Propulsion Laboratory near Los Angeles, CA.
New data from the fabled Voyager spacecraft have fed a controversy over the geometry and activity of the heliosphere. Image credit: NASA/JPL-Caltech.
“I never in my wildest dreams thought that I would still be working on Voyager fifty years after we wrote the proposal,” says Voyager researcher Stamatios “Tom” Krimigis of Johns Hopkins University in Laurel, MD.
During the past decade, both spacecraft reached a new realm, entering the interstellar medium: the tenuous material that fills the vast space between the stars. There, the spacecraft continue to make new discoveries.
The interstellar magnetic field has surprised researchers with both its strength and its direction, and the new data have even fed a controversy over the geometry and activity of the heliosphere—the Sun’s magnetic domain. Is the heliosphere the shape of a comet, as has long been assumed, or is it instead more spherical? And does it expand and contract when sunspots wax and wane, or is it more stable? The spacecraft have offered up some tantalizing clues.
Long Distance Voyager
Voyager 2 left Earth in 1977, followed by Voyager 1. They weren’t the first spacecraft to reach the nearest of the giant planets—that honor went to Pioneers 10 and 11. But the Voyagers were more sophisticated than the Pioneers and made many startling discoveries.
Voyager 1 took a shorter route than Voyager 2 and arrived at Jupiter first, in 1979, finding that the planet’s colorful moon Io sported erupting volcanoes. In 1980, Voyager 1 sped past Saturn, spying intricate details in the planet’s rings and discovering the first nitrogen atmosphere beyond Earth, around the moon Titan. Voyager 2 took the more scenic route, visiting Jupiter in 1979 and Saturn in 1981, then ventured past Uranus in 1986 and Neptune in 1989. Voyager 2 provided outstanding views of the green and blue planets and spotted geysers on Neptune’s large moon Triton.
The spacecraft then headed for interstellar space. As astronomers have defined it, the interstellar medium begins where the solar wind—the outflow of charged particles from the Sun—ends. This ionized gas, or plasma, presses against the cooler, denser interstellar plasma flowing around it like a pebble obstructing water in a stream. The Sun-carved cavity is called the heliosphere and its edge the heliopause, just as the top of Earth’s troposphere is called the tropopause.
When Voyager was launched, “we really didn’t know how far out the heliopause was,” says Voyager researcher Don Gurnett at the University of Iowa in Iowa City. Some thought the heliopause might be as close as Jupiter, only five times farther from the Sun than Earth is. As the spacecraft sped ever outward, estimates of the distance to the heliopause kept going up. It certainly wasn’t at Jupiter—or Saturn or Uranus or Neptune. As a result, no one knew when or where Voyager would enter interstellar space.
Soon after the Neptune encounter, Gurnett claimed that Voyager had glimpsed signs of the heliopause in the far distance. In July 1992, both Voyagers began detecting strong radio waves at frequencies between 2 and 3 kilohertz. Gurnett and his colleagues attributed these radio waves to six big flares that had erupted on the Sun more than a year earlier. The researchers said that plasma ejected during the flares had eventually hit the heliopause, causing electrons there to oscillate and emit the radio waves. The 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 vacuum 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 astronomical 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 measurement, 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 explanation for the signals. For another, his measurement 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 astronomical units from the Sun.
“Nobody wanted to hear that we would have another twenty-plus years to go before we got to the heliopause,” Gurnett says. The prediction even endangered the spacecraft themselves, because if the next big objective was really so far away, they might get turned off in order to save money.
An engineer works on the construction of the dish-shaped antenna of Voyager in July 1976. Image credit: NASA/JPL-Caltech.
Rites of Passage
In the end, the spacecraft survived. Voyager 1 shot through the heliopause on August 25, 2012 at 121.6 astronomical units, about four times Neptune’s distance and right in line with Gurnett’s prediction two decades earlier. But so controversial was the passage that NASA didn’t announce the accomplishment until thirteen months later.
Still, Voyager 1 did see some indications that it had crossed the heliopause. High-energy particles from the Sun vanished, a likely sign that the rest of the solar wind had been left behind as well. Also, cosmic rays from beyond the solar system, which the heliosphere partially blocks, intensified after Voyager’s passage. These signs alone, however, failed to convince many researchers.
There were two problems. First, Voyager 1’s plasma instrument had stopped working and so could not record the jump in particle density when the spacecraft broke from the heliosphere into interstellar space. Second, the magnetic field beyond the heliosphere was expected to point in a different direction and failed to do so. “It just so happens that nature hasn’t read the theorists’ papers and didn’t know that it was supposed to change the magnetic field direction,” Krimigis says. To this day, it’s still not clear why the magnetic field outside the heliosphere aligns with that inside.
The Sun helped confirm Voyager’s feat. Solar storms had erupted earlier in 2012, and the next year they shocked the plasma that Voyager 1 was speeding through, causing electrons there to oscillate and give off radio waves that the spacecraft detected. The frequency of those radio waves indicated that Voyager had indeed entered a much denser domain (2).
Voyager 1 thus became the first spacecraft ever to reach the interstellar medium. Contrary to some media reports, the craft had not left the solar system. Roughly a trillion icy bodies revolve around the Sun far beyond the orbits of Neptune and Pluto; every now and then one of them plunges toward the Sun and we see a new comet in the sky. The farthest of these distant icy objects are probably 1 to 2 light-years, or 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 Ocean, relatively speaking, than Voyager has to the solar system’s edge.
On November 5, 2018, Voyager 2 also crossed the heliopause. This time the passage was not controversial. The spacecraft’s plasma instrument was working and detected the leap in particle density as protons, electrons, and other charged particles struck the instrument (3, 4). It also recorded the temperature: between 30,000 and 50,000 Kelvin, much hotter than the local interstellar medium, probably because the plasma gets compressed as it hits the heliosphere. Like Voyager 1, the spacecraft saw the solar wind vanish (5) and cosmic rays from outside the solar system increase (6), but the magnetic field again failed to change direction, indicating that the result six years earlier was no fluke (7).
“But the thing that was most amazing to me,” says Krimigis, “was the fact that the crossing distance was 119.0 astronomical units.” That’s almost exactly the same distance where the spacecraft’s twin had crossed—a surprise because the solar cycle was then in a different state. Over an 11-year period, as sunspots wax and wane, the solar wind strengthens and weakens, pushing harder and then less so on the heliopause, albeit with a time delay of about 2.5 years. So the heliosphere should expand and contract. Because the pressure from the solar wind was less in 2012 than in 2018, the heliosphere should have been considerably smaller during Voyager 1’s passage than Voyager 2’s. Instead, Krimigis says, the nearly equal distances mean the heliopause must be sturdier than had been thought.
And that, he says, is attributable to the unexpectedly strong magnetic field in the interstellar medium. Voyager 1 put it at 5 microgauss, about twice the predicted value, and Voyager 2 found an even stronger interstellar magnetic field, around 7 microgauss. In Krimigis’s view, the pressure of the strong interstellar field acts like a straitjacket, suppressing most of the expansion and contraction of the heliosphere.
Krimigis asserts that the magnetic field is so strong it also changes the shape of the heliosphere. “For sixty years we’ve had the wrong image of the heliosphere,” he says. The standard view is that it’s the shape of a comet, with a nose and a long tail. The nose faces the direction the solar system is moving through the interstellar medium, while the tail trails in the opposite direction. But because the interstellar magnetic field is so strong, the magnetic pressure, which goes as the square of the field’s strength, squeezes the heliosphere and makes it round. “Absolutely it is,” Krimigis says.
His team had earlier used the Cassini spacecraft, then orbiting Saturn, to reach the same conclusion (8). Cassini detected energetic neutral atoms that Krimigis believes come from near the heliopause. These varied in sync with the sunspot cycle and did so in all directions at about the same time, suggesting that the heliopause is equidistant in all directions—in other words, that the heliosphere is round.
“Voyager is traveling uncharted waters. It’s in a location where no mission has gone before and no mission will go probably for decades.”
–Suzanne Dodd
But this claim is controversial. “All of the evidence that’s been presented for a round heliosphere is in fact a misinterpretation of the data,” says Nathan Schwadron at the University of New Hampshire in Durham, who favors the traditional comet shape instead (9). He argues that the energetic neutral atoms don’t necessarily come from near the heliopause and that changes in their number merely reflect changes in the plasma of the heliosphere as the solar cycle waxes and wanes. This means the Cassini data say nothing about the heliosphere’s shape. When the Sun is active, Schwadron says, it compresses and thereby heats the plasma in the outer heliosphere, which leads to a greater number of energetic neutral atoms from all directions; conversely, when the Sun is quiet, the heliospheric plasma expands and thereby cools, leading to fewer of those atoms. He also says the interstellar magnetic field would have to be much stronger than Voyager measured—around 20 microgauss—for magnetic pressure to squeeze the heliosphere into a round shape.
The Final Frontier
Whatever the heliosphere’s exact shape, Voyagers 1 and 2 continue to dart away from it. By year’s end, Voyager 1 will be 155 astronomical units from the Sun. The spacecraft’s signals, traveling at the speed of light, will take more than 21 hours to get to us. Voyager 2 will be 129 astronomical units out and in 2023 will overtake the silent Pioneer 10 to become the second farthest spacecraft of all. The two Voyagers are dashing away in different directions and are even farther from each other than either is from Earth.
They continue to transmit data about interstellar space. Both can measure the electron density, because they detect the radio waves generated when solar eruptions cause electrons in the interstellar medium to oscillate. These eruptions should become more frequent as the sunspot cycle peaks around 2025. The current measurements indicate that the interstellar density has increased further from the value it had outside the heliopause, but no one knows what will happen next. “If you want me to just make my best guess, I would say it’s going to reach a peak and then come back down a little bit,” Gurnett says. That may indicate that interstellar material is piling up near the heliopause like snow in front of a snowplow.
Voyager 2, which has a working plasma instrument, will keep tabs on the interstellar temperature. This temperature will likely fall, because astronomers have measured the local interstellar medium’s temperature to be only 7,500 Kelvin; the high temperature just beyond the heliopause probably results as the plasma there gets compressed and heated. Both spacecraft will monitor the interstellar magnetic field. If the magnetic field gets compressed and strengthened near the heliopause, then the field should eventually weaken at greater distances. And Voyager could even see the magnetic field change direction, just as researchers had expected at the heliopause.
The ultimate goal is to sample the unperturbed interstellar medium, space so distant that the heliosphere barely affects it. “Voyager is traveling in uncharted waters,” Dodd says. “It’s in a location where no mission has gone before and no mission will go probably for decades.” The interstellar medium is of interest to astronomers in part because it is the spawning ground of stars as well as the place where they deposit newly minted chemical elements, which future generations of stars and planets inherit. In addition, astronomers must peer through interstellar gas and dust and correct for their effects to study stars and galaxies. Although researchers use ground-based and Earth-orbiting telescopes to observe the interstellar medium from afar, the two spacecraft yield unique data on its density, temperature, and magnetic field by actually being inside it. “In situ measurements are very important,” she says.
No one knows how much longer the two spacecraft will survive. They have to stay warm so that the fuel they need to keep their antennas pointed toward Earth doesn’t freeze. Voyager’s heat comes from plutonium, but as the radioactive element decays, it provides less and less power every year.
Surprisingly, Voyager 1 is the warmer spacecraft, even though it’s farther from the Sun. “There’s a story to that,” Krimigis says. After NASA launched Voyager 2, engineers noticed it was colder than expected, so before launching Voyager 1, they added more insulation to keep it warm. That extra warmth may well prolong the spacecraft’s life. And because Voyager 1’s plasma instrument has failed, that instrument doesn’t consume any energy, leaving more for heating the spacecraft. As a result, the Voyager that’s exploring a more distant—and hence likely more interesting—region of space may continue to return data even after its mate falls silent. “If we make it to 2027, then it would be fifty years since Voyager was launched,” Krimigis says. “So that’s the hope: 2027 or bust!”
Published under the PNAS license.
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