The Interstellar Age

by Jim Bell

  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

  FOR 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 urani
um 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.

  In high school we learned about Voyager’s plutonium power supplies and the role that physicists had played in developing ways to power spacecraft far beyond the distances where solar panels would work. Plutonium is element 94, one of the “trans-Uranian” elements (heavier than uranium), radioactive, and one of a slew of very heavy elements on the bottom of the periodic table, only relatively recently discovered. In 1980 my best friend, Bob Thompson, asked our chemistry teacher how many elements were then known, and Dr. Manley replied that the best way to get the inside scoop would be to write a letter (remember, this is in the days before e-mail) to Dr. Glenn T. Seaborg at Berkeley and ask him about it. So he did. We all thought it was hilarious because Bobby’s “reward” for asking a question in class was more homework. A few weeks later, though, to our and Dr. Manley’s amazement, Seaborg wrote back to him. “One-hundred-six elements are now known of which the last (element number 106) has not yet been given a name,” wrote Dr. Seaborg. It was a fairly short reply, but just the fact that a Nobel Prize–winning physicist who had discovered ten elements on the periodic table took the time to write a personal letter was enough to get Bobby’s picture in the local paper—VALLEY STUDENT HAS NOBEL PEN PAL read the caption. Bob has gone on to a career in astronomy studying giant stars and designing instruments for airborne and ground-based telescopes. In 1997, two years before Seaborg died, they named “element 106” Seaborgium, in honor of Bobby’s Nobel pen pal.

  DELIVERY TIME

  Even though the plutonium on the Voyagers has decayed by only about 25 percent of its starting amount, the spacecraft are already starting to feel the pinch of looming power limitations. Even if there were something useful to photograph, for example, the cameras can no longer be turned on because they would gobble up too much power to also be able to run their heaters. The five remaining instruments use less power, but the power needs of the radio transmitters and the heaters are relentless. In addition, the spacecraft need to use tiny amounts of thruster fuel to accurately point their antennas at the Earth, and that thruster fuel is a consumable, and dwindling, resource. However, amazingly, Suzy Dodd says that only just recently did they finally switch to Voyager’s backup thrusters, because the primary thrusters were getting close to their expected lifetime limits, “after nearly 350,000 thruster cycles and thirty-four years of flight!” she said. The backups, which had never been used, are working just fine.

  Team members predict that the spacecraft will have enough power and thruster fuel to stay in communication with the Earth and operate at least one instrument until sometime around 2025, when Voyager 1 could be more than 160 AU from the sun (more than 15 billion miles away), and Voyager 2 could be out beyond 135 AU. By cycling off some of the remaining instruments and systems—those needing the highest power—when not in use after 2020 (or, at some point, off forever), mission controllers may be able to push the spacecraft’s lifetimes beyond the mid-2020s. But eventually, the power levels will drop to critically low values where some of the heaters and other engineering subsystems will have to be shut off, and then the science instruments will fail or have to be shut off, one by one, with the lowest-power instruments like the magnetometer likely staying on the longest. Even then, though, according to Suzy Dodd, it might be possible to continue to operate the Voyagers “with just an engineering signal. We’ve been talking with the DSN about that possibility.” That is, it might be possible to just stay in occasional radio contact with them well into the 2030s.

  Earth to Voyager . . . still there?

  (long pause)

  Still here . . .

  Very well. Carry on. Talk to you soon.

  There is some possible science that could come from simply monitoring the strength of that faint signal, beamed back over such vast distances. According to Ed Stone, “As long as we have a few watts left, we’ll try to measure something.” Randii Wessen says that no one really knows how long the spacecraft will keep going. “I started at JPL in 1980—at Saturn—as an intern of the Voyager science support team. I always thought that the mission would end sometime during my professional career. Now I’m not so sure.” Suzy Dodd has a specific goal: “We launched in 1977, and so if we can keep in contact, still doing science, until 2027, that would be fifty years. That’s my goal—to have Voyager operate for fifty years.” She and the Voyager team continue to have to fight to justify new NASA funding every few years. At some point, though, even the so-called engineering signal will cease from Voyagers, and they will embark on their final mission, to carry forth the Golden Record for all of humanity.

  I hope that we stay in touch with them in engineering signal mode for a long time after science measurements end. Even just the simple act of pinging them by radio and waiting the hours—and eventually days—that it will take for the ping to be acknowledged and sent back, can teach us something about where they are and what it’s like there. An interesting example of that comes from the precursor deep-space missions to Voyager, the Pioneers. After their encounters with Jupiter and Saturn, both Pioneer 10 and Pioneer 11 embarked on their own interstellar missions, heading out of the solar system in different directions from the Voyagers: Pioneer 10 heading “downwind” in the heliosphere and close to the plane of the planets, and Pioneer 11 heading “upwind,” like the Voyagers, but only slightly above the plane of the planets. NASA’s Deep Space Network kept track of the Pioneers long after their planetary flybys, until their plutonium-based power supply systems ran out of enough juice to power the radio and other critical systems in 1995 (when contact was lost with Pioneer 11) and 2003 (Pioneer 10). Subsequent analysis of the Pioneer radio signals revealed something curious, however. The spacecraft were not as far away from us as they should have been—something was slowing them down, by a tiny amount, year by year. It certainly wasn’t from the gravity of any known objects, as that was being properly accounted for, or from any other obvious known forces. Perhaps it was some kind of new physics that could only be discovered by a long, lonely trip through deep, nearly empty space? No one knew, and the discrepancy became known as the Pioneer Anomaly.

  Over more than twenty years, astronomers, physicists, and spacecraft engineers tossed around hypotheses about the gravity of small bodies like KBOs, or dark matter, or some other cosmological effect, causing the Pioneers’ deceleration. Or maybe drag from particles in the heliosphere, or small helium gas leaks on the spacecraft that acted like mini thrusters, or some other spacecraft-related effect that hadn’t been properly accounted for. Eventually after much head scratching, physicists and spacecraft engineers finally solved the Pioneer Anomaly. With funding from The Planetary Society, they tirelessly sifted through nearly thirty years of Pioneer tracking data, some of it recovered from ancient magnetic tapes restored to modern digital data files with funding from Planetary Society members. They solved the mystery. The deceleration turned out to be from a tiny, almost insignificant force created by heat (“thermal photons”) leaking out of the plutonium power generation unit in a specific direction that happens to be opposite the sun based on the design of the spacecraft components. This tiny force directed away from the sun causes the spacecraft to recoil (Newton’s “equal and opposite reaction”) toward the sun, ever so slightly, slowing it down by the tiny amount observed. Pretty old physics, actually, but it took modern-day spacecraft fore
nsics work to track it down.

  Eventually, as the Voyagers continue on in pursuit of the limitless expanse of interstellar space, they will leave not only the realm of the solar wind but the realm of the sun’s gravity as well. Both are traveling faster than the escape velocity of the solar system, presently some 128 and 105 AU from the sun, respectively. The sun’s gravitational influence is predicted to extend one-third to halfway to the nearest stars, or maybe around 100,000 AU (or about 1.6 light-years away).

  Out there is a hypothesized spherical swarm of comets and asteroids that have been cast out of the inner solar system by encounters with the planets or the sun over their lifetimes. Every few years a new comet on a long, elliptical trajectory is discovered; some of them, like 1995’s Comet Hale-Bopp or 1996’s Comet Hyakutake, produce spectacular displays of gas and dust as their ices are boiled off by the sun’s heat into beautiful, gracefully arcing tails. Tracing the orbits of these and similar so-called long-period comets back to the outer solar system tells us that they come from enormous distances, and from any possible direction in the sky. This is what led Estonian astrophysicist Ernst Öpik and Dutch astronomer Jan Oort to hypothesize that the solar system is surrounded by a vast spherical shell of perhaps a trillion or more asteroids and comets, which we now call the Öpik-Oort Cloud (or usually, just the Oort Cloud), extending out to the edge of the sun’s gravitational influence.