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Crossing Over

In his 2015 book THE INTERSTELLAR AGE, author and planetary scientist Jim Bell describes the moment the Voyager team realized that Voyager 1 had crossed the heliopause (the edge of the bubble of solar wind generated by our sun), and reached interstellar space:

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.

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.

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