The Interstellar Age (33 page)

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 extending 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).

A mohawk wouldn’t seem to be Jamie’s style, though. She’s a serious young researcher focused on using
Voyager
and other measurements to make PhD-quality discoveries about the sun’s interactions with interstellar space. She works closely with Ed Stone and seemed genuinely amazed at the amount of time that he devotes to mentoring. “He is very, very patient,” she says when I ask her to characterize her famous dissertation advisor. “He doesn’t micromanage.
He doesn’t get surprised by much, because he’s seen
a lot
. He’s got to be one of the busiest people I’ve ever met, but when we meet to talk science, he’s never rushed. There is a trust there between us.” It was the kind of sentiment about Ed that I had heard from others on the
Voyager
science team as well.

Jamie’s enthusiasm for the future of the mission is exciting to soak in. “
Voyager
is like the Energizer Bunny—it keeps on running!” she says, with a touch of amazement in her voice. Aware that the team is aging and retiring, she takes her responsibility as a sort of “heir” to Ed Stone’s part of the continuing
Voyager
empire seriously. “Somebody’s going to have to run it in the future. Somebody’s going to have to learn how to operate it from the experts who are running it right now. And somebody just has to have
faith
that it has come this far for a reason. I’m sure some people thought that
Voyager
would never last this long, that it would never get to interstellar space. ‘Let’s just turn it off,’ they probably thought. It would have been so easy! But now we’re getting these interesting results because of that faith, and that’s why I’m here.”

Like Ed, Jamie believes that
Voyager 1
has left the heliosphere, telling me that “it would be quite strange to imagine some sort of connected region where all these interstellar particles somehow get in, while the magnetic fields haven’t changed. I don’t think there’s a real debate. I think part of the problem is that a lot of these competing models just can’t resolve the details of the heliosphere on the same scale that
Voyager
can see them.” Ed had told me earlier that people are starting to work on explaining smaller structures, like the ones
Voyager
is observing, but it’s still an active, ongoing area of research. Jamie gets the chance to meet with many members of the space physics community who are thinking about these problems,
either at
Voyager
team meetings or when they come to visit Ed at Caltech, and she told me that she asks them what they think. Almost without exception, she says, they tell her, “If Ed has said that it’s left, then it’s left.” She went on to praise his careful, methodical style. “Ed is very wise about how to approach these kinds of discoveries. He doesn’t jump the gun. The fact that he’s gone through the process, being skeptical, and then changing his mind because of the solar flare results—there’s a reason he’s changed his mind. He’s been analyzing data from space missions for fifty years, so I think his judgment on this is probably pretty much the best out there.”

The uncertainty over the interpretation of
Voyager 1
’s data may never be fully resolved, partly because the spacecraft has only a partially functional set of instruments. But luckily, there is another similar spacecraft, but this one with a fully functional plasma-density instrument, just a few years and a few dozen AUs behind
Voyager 1
, that could resolve any lingering controversy and in the process become the
second
human-made object to leave the solar system.
Voyager 2
, which is heading outward on a much more southerly track than
Voyager 1
, passed through the termination shock in late 2007 and is now exploring a different part of the heliosheath, but still searching for its edge.

“We measure the plasma directly with
Voyager 2,
and so soon we’ll
know
what kind of a discontinuity there is between the inside and the outside,” says Ed Stone. “In fact, we already know that the plasma flow inside the heliosphere at
Voyager 2
’s location along the flanks of the heliosphere is totally different than it was along
Voyager 1
’s path along the nose.” Specifically, the solar wind stagnated as
Voyager 1
approached the boundary, partly due to the lower solar
wind pressure as the sun was going through a minimum in its cycle of activity.

“On
Voyager 2
we haven’t seen any such slowing of the solar wind. We see it turning, as it has to, as it starts feeling the effects of the impending ‘wall’ of the interstellar wind,” explained Ed. Will
Voyager 2
’s plasma density instrument eventually reveal the large predicted jump at the putative heliopause boundary? Will the sun’s magnetic field lines smoothly merge into the interstellar field, like
Voyager 1
’s results and some new models of the heliosphere imply, or will there be a sharp change in those field directions, as had been predicted earlier based on classical models of the heliosphere? “I don’t think anybody should take it for granted,” counseled Ed Stone, “that Nature can’t throw us another curveball. I’ll be surprised if there aren’t surprises!”

In the meantime, Ed has started putting his cosmic-ray intensity plot for
Voyager 2
back on his refrigerator. He had a copy pinned to his office wall when we met recently. For now, he explained, pointing to the squiggly line, “
Voyager 2
is just up and down, small variations, no big jumps yet like we saw with
Voyager 1
in the summer of 2012.” I asked him when he thought it would cross out of the bubble. “It could be anytime. Probably a few more years. Who knows. We’re watching. We’re waiting.” Suzy Dodd told me that she “feels like the spacecraft has given me several once-in-a-lifetime events, first with the Uranus and Neptune encounters, and now their crossing of the
heliopause.”

10

Other Stars, Other Planets, Other Life

F
OR ALMOST FORTY
years our metal and silicon emissaries named
Voyager
have been speeding away from the people who launched them, first flying past the giant planets and their gaggle of icy and rocky moons, and since then sailing out into the hinterlands beyond the sun’s influence, where the once-familiar solar wind gives way to a different, unexplored interstellar wind. Once
Voyager 2
also passes the heliopause—the boundary between the solar and interstellar winds—both spacecraft will be functioning, but they won’t keep working forever. The plutonium nuclear power supplies on the
Voyagers
generate electricity for the spacecraft’s heaters, computers, and instruments at a very predictable power level. Over time, especially over the decades, that power level has slowly been dropping (from a total power level near 470 watts at launch to around 250 watts now)
as the radioactive plutonium-238 slowly decays to nonradioactive lead-206.

The pioneers in our understanding of this magical alchemy called radioactivity were the physicists Marie and Pierre Curie and their colleague Henri Becquerel in France who, when studying certain kinds of phosphorescent minerals around 1896, found out that uranium-bearing salts spontaneously emitted their own radiation. They had discovered radioactivity, and the genie was out of the bottle. The reason that radioactivity isn’t like alchemy (“turning lead into gold”), though, is that only certain starting atoms, such as uranium, have the correct unstable collection of protons, neutrons, and electrons that spontaneously (that is, with no human intervention or added energy of any kind) lose energy and turn into different, and eventually stable, elements. The predictable and steady—and for many elements, very slow—rate of decay of radioactive “parent” elements to stable “daughter” elements is what makes radioactivity such a great natural clock. Indeed, the decay of some radioactive elements takes billions of years, making these clocks inside rocks excellent natural ways to estimate the ages of samples of the Earth, the moon, and meteorites studied in the laboratory. In an interesting parallel, this phenomenon was elegantly applied when a small spot of radioactive uranium-238 was electroplated onto the cover of the
Voyager
Golden Records as a sort of timepiece, indicating to any extraterrestrial recipient who could measure the amounts of parent U-238 and its daughter radioactive-decay products the precise time that had elapsed since the spacecraft was sent out on its journey.

The element plutonium (atomic number 94, with 94 protons and 114 neutrons) has about twenty known radioactive forms, or isotopes. It and the next-lightest element, neptunium, had been
discovered in 1940 as by-products of a uranium nuclear reactor. As the next two elements were discovered beyond uranium on the periodic table, physicists decided to name them after the next planets after Uranus: Neptune and Pluto (then, and to some still, a bona fide planet). The first isotope of plutonium to be intentionally manufactured in the laboratory, plutonium-238, was created in a nuclear reactor in 1941 by UC Berkeley physicist Glenn T. Seaborg and colleagues. They recognized Pu-238 as special because it generates a lot of heat when it decays radioactively but does not generate as many harmful gamma rays and other high-energy particles as other radioactive elements. This makes Pu-238 safer and easier to work with and especially useful in RTGs like
Voyager
’s.