The Interstellar Age (32 page)

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 colleagues 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.