by Jim Bell
Despite the incredible speeds of both spacecraft, the missions appear to be going in relative slow motion because of the enormous distance scale of the outer solar system. The spacecraft are traveling 10 miles every second (imagine Voyager passing through your neighborhood at that speed) but still took nearly a decade and a half beyond their last planetary encounters to pass the termination shock and to enter the turbulent heliosheath. During that long cruise outbound, Ed Stone and other Voyager scientists moved on to work on other projects, to other missions, or even to retirement. “I feel extremely fortunate to have become the project manager in 2010,” recalls Suzy Dodd. “During the twenty-year period that I was off the Project, we were just sort of sailing out towards interstellar space, not really having a good idea of how far away that was. I owe my job to the previous project managers who constantly had to fight against the mission being canceled during that time. It wasn’t easy to keep Voyager going this long, not just from a technical standpoint, but also from a financial standpoint. There were doubters out there in the early 2000s who wanted to cancel Voyager. Once we got through the termination shock, though, people thought—OK, we should be getting closer.” Indeed, once they crossed that boundary, many people began paying closer attention to the mission again, watching for changes in the densities, energies, and directions of the fields and particles measured by the Voyagers, searching for the telltale clue that the next—the ultimate—boundary had been passed.
HELIOPAUSE
After crossing the termination shock and entering the heliosheath, Voyager 1 continued speeding outward at more than 37,000 miles per hour for nearly seven more years. Slowly, over that time, the background intensity of “outside” cosmic rays (hydrogen, helium, and free electrons) slowly kept creeping upward. Ed and his team interpreted that as meaning that there was a higher intensity of cosmic rays outside the heliopause that they would eventually (soon?) encounter, but that some of these “outside” cosmic rays were still slowly leaking, or diffusing, into the heliosphere. When Voyager was deeper inside the heliosphere, these outside particles couldn’t get that far in. But now, getting closer to the edge, they could start to sense more strongly the storm waiting for them on the other side. The magnetic field lines kept slowly turning as well, until by 2010 they weren’t pointing radially outward from the sun at all—in fact, in places the field had been turned around completely and was now stagnant or even in places pointing back in toward the sun. Voyager 1 had moved into a sort of magnetic doldrum. The environment was changing over the years, but so far only gradually.
Things started getting weird, though, rather abruptly, on July 28, 2012. On that day, Ed Stone’s cosmic ray counter instrument on Voyager 1, at a distance of about 120 AU from the sun, measured a sudden and dramatic 50 percent drop in the kinds of solar energetic particles that had been seen for about a decade inside the heliosphere. At the same time, another counter measured a big increase in the cosmic ray particles formed outside the heliosphere, in the nearby galaxy. But then everything switched back a few days later, and the environment went back to more normal levels of “inside” and “outside” particles. What the heck was going on? Again, a few weeks later, in mid-August, the inside particles dropped off and the outside particles jumped up—but then again went back to normal a few days later. Things seemed to be bouncing around, and it was hard for the team to make sense of what they were seeing on their squiggly line plots. Ed Stone would later refer to this phase of the mission as the time when Voyager 1 was “dipping in” and “dipping out” of the heliosphere, along a somewhat jagged edge. “I can still remember taking the data home every night, and putting the plots on the refrigerator,” recalls Ed Stone. “I couldn’t stop thinking about them, wondering what would happen next.” Suzy Dodd remembers seeing Ed give a talk in summer 2012 where he showed that plot, telling people in the audience, “This is the first thing I look at every day when I get up in the morning. And you should do that too!” And then, on August 25, 2012, Voyager 1 saw the inside particles typical of the heliosphere drop off steeply to zero—and stay there. The outside particles jumped up dramatically at the same time—and stayed there too. The solar energetic particles were gone, replaced by nearly 100 percent interstellar cosmic rays. Was that it? Had the spacecraft just suddenly fallen off the edge of a proverbial cliff on August 25 and tumbled out into interstellar space? “It felt like I was standing on the shore of a particle beach, and the water comes up,” recalled Ed Stone. “You’re standing there and a wave comes in and gets your feet wet, and then the water recedes, and then there’s another wave that comes in, and it recedes, and then finally the next one comes in and that’s it. The tide has changed, and your feet are in the water all the time.”
I asked Ed if he and the team celebrated that event in some way—maybe popping some Champagne corks or throwing a party? Don’t get me wrong, no one would ever refer to Ed Stone as a party animal, but if ever there were an occasion for a space plasma physicist, a straight-up squiggly line cosmic-ray kind of guy, to let his proverbial hair down and celebrate, surely this would be it? “Well, it was really quite remarkable,” he said. “We were having a preplanned Voyager Science Steering Group meeting at JPL, timed to coincide with the thirty-fifth anniversary of the launch of Voyager 1. So we had a dinner scheduled, and much of the team out there, and the spacecraft obliged by crossing this historic boundary just the week before its big birthday party!” I pressed him about whether he had a personal celebration of some kind, though. “No, but maybe I should have. It’s because, somehow, having waited for it for thirty-six years . . . we just weren’t sure. I really wanted some confirmation that we were out there.” Sounds like he was having fun, though I never got a straight answer about the Champagne.
Ed Stone is a careful, skeptical guy, and he wasn’t yet ready to declare victory. “We couldn’t be sure yet, because we hadn’t measured the plasma density, and we hadn’t measured the magnetic fields yet, but from a ‘particle’ point of view, we felt as if we were at least connected to the outside somehow, even if we weren’t actually outside.” He knew that crossing the edge of the solar system was a big deal, and that they’d want to make sure that it had really happened. The funky dropouts in heliospheric particles during the month before August 25 were troubling—what were they caused by? Was the edge of the heliosphere moving in and out (like the water on a beach) rather than sharp? Or had they entered some unknown, unexpected, strangely depleted region of the heliosphere that was still upstream of the edge itself? That would be an exciting discovery too. No one knew.
Because the cosmic-ray measurements could be interpreted in several ways, Ed and colleagues went looking for more clues to where they really were in other Voyager 1 data sets. Most of their conceptual cartoons and computer models predicted that when the spacecraft crossed the heliopause, there would also be a sudden change in the direction of the magnetic fields—from the bunched-up, stagnant, turned-around fields measured just inside the heliosphere boundary to the more freely streaming fields of interstellar space in this particular part of the galaxy. But disappointingly, the direction of the magnetic field didn’t change at all on August 25. Curious—the magnetic-field data said that they hadn’t crossed out of the heliosphere after all. The clincher measurement would have been the density of the ionized plasma, because everyone agreed that the density should jump somewhere between 50 to 100 times higher once Voyager 1 passed into interstellar space. But the plasma density instrument had broken back at Saturn in 1981, so there was no way to make that direct measurement. They couldn’t be sure that they crossed the threshold. Cautious Ed Stone couldn’t be absolutely sure.
Ed and the other Voyager fields and particles scientists now had a major conundrum on their hands. They couldn’t prove that Voyager 1 had crossed into interstellar space, but they had definitely crossed into some kind of new region, different from any that it had ever traveled in before. They struggled with what to do, what to report to their collea
gues in space physics and to the rest of the world that was expecting some exciting news from Voyager. In the fall of 2012, armed with the information that they had in hand and mindful of the pressure from NASA, the public, and the media to report on what Voyager 1 was experiencing, they decided to take the middle road. Voyager 1 had certainly entered a region with a dearth of the solar heliospheric particles that had been seen before, so they could report that they had, indeed, passed into some sort of depleted region of space. And the lack of any significant change in the magnetic fields between the “normal” heliosphere and this new, depleted region suggested to some researchers that there was a connection across this transition zone. Ed and colleagues coined the phrase “magnetic highway” to describe the idea of relatively seamless, high-speed magnetic-field connections across this new, mysterious boundary. Pulling all the available data together, Ed and the Voyager team went to press with a series of peer-reviewed research papers that appeared in Science magazine in June 2013, hypothesizing the discovery by Voyager 1 of a new region of perhaps interplanetary, perhaps interstellar, space—but a previously unexplored region, regardless. From a purely particle perspective, the spacecraft appeared to be outside the heliosphere. But there was no proof in the plasma, at least not yet. The Voyager team consensus in 2012—driven strongly by Ed Stone’s need for definitive proof—was that they had not yet necessarily crossed the boundary. Maybe, but maybe not.
Outside of the Voyager team, there was both concurrence and controversy. Certainly most researchers in the field wanted to see the smoking-gun evidence that could have been provided by the plasma density measurements (if the instrument was working), and agreed that a conservative, wait-and-see interpretation was warranted. But others had already been convinced by Voyager 1’s data that the spacecraft had crossed into interstellar space. For example, a group from the University of Maryland and Boston University led by space physicist Marc Swisdak used the Voyager 1 magnetic field measurements to develop a new computer model of the heliosphere that envisioned the heliopause as a “porous, multi-layered structure threaded by magnetic fields.” In their computer simulation of Voyager’s flight, published in August 2013, the spacecraft had indeed crossed the heliopause and was now in interstellar space. “We think we are outside the heliopause,” says Swisdak in an interview for Science magazine. But, he adds, in order to explain the big difference in cosmic rays but the unchanged magnetic field direction, “the boundary is very different than we thought.” He concludes, “The very nature of the heliopause may come into question.”
Meanwhile, in the months since Voyager 1 had passed whatever important boundary it passed in August 2012, Professor Don Gurnett of the University of Iowa, leader of the Voyager Plasma Wave Subsystem (PWS) investigation team, knew that there was an indirect way to measure the density of the plasma in this new region of space but that the team would have to get lucky to measure it. Gurnett’s instrument measures the size of waves that travel through the ionized atoms and molecules in the magnetic fields of the giant planets and in the solar wind, providing information on the density and temperature of those regions of space. During the Jupiter and Saturn flybys, Voyager 1’s PWS instrument could characterize the space environment well because of the waves of energy created by those planets’ powerful, rapidly rotating magnetic fields. But while quietly cruising through the outer heliosphere, there were no such powerful disturbances to create waves in the ionized gas. At least, not often. Every once in a while, though, Gurnett and others knew, an enormous burst of energy from the sun, from a solar flare or so-called coronal mass ejection event, would spew forth out into the solar system, moving outward at high speed and making waves in the plasma. So, if the sun cooperated, perhaps they would see a giant flare make some waves in the Voyager 1 PWS data, and the nature of those waves would tell them whether they were in an environment of low (solar system) plasma density or a high one (interstellar space). They would have to be patient and lucky to observe such an event from the sun. But since Voyager 1’s main plasma measurement instrument was broken, they had little choice but to wait and hope.
Gurnett’s team did see a weak solar flare event pass by Voyager 1 in real-time data radioed back in October-November 2012, but its effects on the plasma were too small to yield a good answer on the density. But then, in April-May 2013, the sun provided a remarkable and unanticipated gift to the Voyager 1 team: particles from a very large and energetic flare passed by the spacecraft and created strong, easily measurable waves in the surrounding ionized gas. Later, the earlier fall 2012 event was also detected in the higher-sensitivity recorded data that were part of the regular (every six months) transmission of data back from Voyager’s tape recorder. The electrons were moving back and forth along the magnetic field—like sound waves compressing and uncompressing in an atmosphere—resonating with a frequency that told Don Gurnett and colleagues that Voyager 1 was in a region of space with 80 times the density of ionized particles as in the solar system’s normal heliosphere. Since the very definition of the heliopause—the edge of the heliosphere—is based on such a jump in density, it was a eureka moment. “When we saw that, it took us ten seconds to say that we had gone through the heliopause,” he remarked.
Finally, Ed Stone and the rest of the Voyager team had the proof they needed, thanks to the far-flung effects of a rare, giant solar flare. There was no longer any need for waffling or conservatism, or for mysterious new depletion zones or magnetic highways—it was now official: Voyager 1 had left the solar system. Don Gurnett and his colleagues published their results in a paper in Science in September 2013 that proclaimed the historic achievement to the world. “Now that we have new, key data, we believe this is mankind’s historic leap into interstellar space,” said Ed Stone at a press conference called to announce the discovery. “The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we’ve all been asking: ‘Are we there yet?’ Yes, we are.” Humanity’s first baby steps beyond the influence of our own star had been taken—and we still had a capable, functional spacecraft out there (and another not far behind) to study interstellar space for the first time.
At least, that’s the happy-ending Hollywood version of the story. There is, in fact, still some controversy and uncertainty—primarily from outside of the Voyager team but some even from inside—about whether the spacecraft has truly left the heliosphere. “I don’t think it’s a certainty Voyager is outside now,” wrote space physicist David McComas of the Southwest Research Institute in September 2013. He and other colleagues remain puzzled by some inconsistencies in the available data. “It may well have crossed,” he concluded, “but without a magnetic field direction change, I don’t know what to make of it.”
Indeed, George Gloeckler, a space physicist from the University of Michigan and an original member of the Voyager team since the start of the project in the 1960s, has stated flatly, “We have not crossed the heliopause.” He and Michigan colleague Lennard Fisk have developed a model of what Voyager 1 has measured that they claim can be explained by the continued piling up and compression of particles and magnetic fields within the heliosphere, behind a yet-to-be-crossed heliopause that still lies ahead. Gloeckler confessed in a Science interview, “We’re way out there, by far a minority, but we can explain every Voyager result in a pretty natural way,” based on their solar wind pile-up model. “That’s quite a different story than typical models of the heliosphere,” countered Ed Stone when I asked him for a reaction to the ongoing skepticism about whether Voyager 1 has indeed left the heliosphere. But he seems open-minded. “If they’re right, that will change our understanding of the kind of physics that is involved in these interactions between stars and their surroundings.”
“We’re learning what’s out there,” he continued diplomatically, reacting to another set of competing hypotheses that invoke a rattier, more turbulent heliopause boundary consisting of tendrils of extended heliosphere extendi
ng into the interstellar medium. “If we’re really inside some strange extension of the heliosphere, what some colleagues might call a ‘flux tube,’ then it’s a really big one because we’ve now been in it for more than two years,” Ed explained. “So in some sense, that model of the edge of the heliosphere is different than the model that most people envision.” Yet he remains gracious and diplomatic to a fault. “But if they’re right, then maybe such flux tubes are important. Maybe they’re a typical feature of the interaction of a stellar magnetic field and the interstellar magnetic field. Rather than a simple cometlike bubble, maybe there are these strange regions where the magnetic fields are connected.” It’s that kind of collegial open-mindedness that has made Ed Stone the natural scientific father of the Voyagers for more than forty years.
I had a chance to meet Jamie Sue Rankin recently, a second-year Caltech graduate student who is working with Ed Stone on the analysis and computer modeling of some of the Voyager cosmic ray data. Jamie is Ed’s only current grad student, and she counts herself blessed to be working with Voyager data during such an exciting time in the mission, as compared to the last couple of decades when there really hadn’t been much going on during the outbound cruise. “I moved to Pasadena in September 2012, roughly a week after this happened,” she says, pointing to the huge drop in “inside” heliosphere particles that Ed had been tracking on his refrigerator plot. What timing!
Jamie was born in 1988, eleven years after the Voyagers launched and just a year before the Neptune encounter, and I asked her how it feels to be working on a space mission that is much older than she is. “That is a strange thing . . .” she says. “It has a technology that I haven’t even seen before. I mean—magnetic tape recorders? I’ve never even pushed Play on a magnetic tape recorder!” She made me feel old. During that semester that she arrived at Caltech, she followed Ed through the whirlwind of team meetings and debates about whether they had crossed the heliopause. “When I walked into that first team meeting, I was definitely the youngest person in the room by at least twenty years,” she recalled. But she said it was a great environment, and that they were incredibly supportive and eager to welcome a new person into the field. One of the managers even suggested that they should put Jamie into one of the press briefings with a mohawk, to try to help get younger people interested in the mission (and following in the footsteps of the Curiosity rover’s famous “Mohawk Guy,” JPL systems engineer Bobak Ferdowsi).
