by Jim Bell
Ed Stone’s CRS instrument can tell the difference between high-energy particles (nuclei) that have come from the sun (solar energetic particles) and those that have come from outside the solar system—from elsewhere in the galaxy (cosmic rays). Some high-energy cosmic rays do make it across the heliopause boundary, and Voyager has been characterizing them for decades. However, many of the lower-energy protons and electrons that make up cosmic ray particles from elsewhere in the galaxy can’t actually pierce the bubble of the sun’s heliosphere, and instead they are diverted around the solar system, like water diverts around an island in a river. Indeed, trying to measure the full range of energetic particles that are characteristic of the interstellar wind, not just the solar wind, was a goal for Ed’s investigation way back when the instrument was being designed and built in the early 1970s.
I asked Ed whether deep in the recesses of his wildest dreams he actually harbored any hope back then, back when he was dreaming up his cosmic ray experiment in the ’60s or when he became project scientist in 1972, that at least one of the Voyagers would survive to make it to interstellar space, to measure that interstellar, galactic wind. “Well, we hoped. When we started this mission, we had, as one of the objectives, to get to 20 AU,” he told me. (“AU” is the astronomical abbreviation for “astronomical unit,” where 1 AU equals the average distance between the Earth and the sun, or about 93 million miles.) “No one knew where the boundary would be. We had to propose for a new mission extension once Voyager 2 completed Saturn, and we called it the Voyager Uranus-Interstellar mission. It was one leg at a time. Then the next leg after Uranus was the ‘Voyager Neptune-Interstellar’ mission. So ‘Interstellar’ has always been there, it’s just that none of us knew how big the bubble really is! And none of us knew how long the spacecraft could last.”
The “Interstellar” focus of Voyager’s postplanetary mission was not an accident. Ed believes that his own cosmic ray instrument, for example, was put on specifically for this possible, hoped-for extended phase of the mission, where interstellar particles could be measured in their native habitat. “Which is really kind of remarkable, when you think about it,” he confesses. There is a modern penchant for cut-rate mission budgets and strict adherence to carefully crafted specific mission goals. Ed Stone adds, almost to himself, “I’m not sure that today that would happen.”
Both Voyagers had already been accelerated to high-enough speeds, by launch and by their subsequent giant-planet gravity assists, to be on escape trajectories from the sun’s gravity. Voyager 1, which was diverted northward during its flyby of Saturn in 1980, is traveling fastest, at about 10 miles per second (more than 38,000 miles per hour). Voyager 2, which arced over the north pole of Neptune in 1989 and was then diverted southward, is not too far behind, traveling at about 9.5 miles per second (more than 34,000 miles per hour). Three other spacecraft—Pioneer 10 and Pioneer 11 launched in 1972 and 1973, respectively, and New Horizons, launched in 2006—are also on escape trajectories out of the solar system, but they are all traveling slower than the Voyagers, which are leading the race to find the edge of interstellar space.
While we’re no longer in contact with the Pioneers, Pioneer 11 could leave the heliosphere within the next decade or so, but Pioneer 10 will take much longer (perhaps thirty to fifty years or more) because it is traveling “downstream,” along the extended “tail” of the sun’s magnetic field. How far does that solar magnetotail extend? Ed says, “I’m not sure. Hundreds, hundreds of AU?” At first that may seem surprisingly long, but consider that the sun’s magnetic bubble extends more than 100 AU in the “upstream” direction, where it’s flowing against the current of the interstellar wind. Maybe it’s not so surprising, then, that it could extend two or three times as far in the other direction, where it’s flowing downstream in the same direction as the flow outside the bubble. “The edge of the sun’s magnetotail is undoubtedly a ragged or filamentary kind of thing based on the shapes of the magnetotails of the giant planets, and it probably just eventually merges with the interstellar background,” Ed adds. New Horizons is traveling upstream into the interstellar wind, like the Voyagers, but it, too, is probably twenty to thirty years away from leaving the heliosphere, based on its later start and slower speed compared to the Voyagers.
Ed Stone and other space physicists worked up computer models and predictions for when these spacecraft might reach the heliopause. The models use information on changes in the strength and shape of the solar wind measured over many decades by the Voyagers and Pioneers. “During the ’90s, we would have a meeting every few years,” Ed says, “and there would be a histogram showing the range of predictions. And that histogram just kept moving out in time as Pioneer kept moving out in space, not yet crossing the boundary. It seemed like the edge of the bubble was always 20 AU beyond where we were! Things were changing slowly, but really, we didn’t know. By the early 2000s, however, all the different ways of estimating the size of the heliosphere were converging on the first major boundary—called the termination shock—occurring at about 90, plus or minus 10, AU. So if we saw that in just a few years’ more travel, we’d know that we were getting closer to exiting the bubble.”
They also used images and other information from astronomers about the nature of similar “bubbles” seen around other nearby stars. Young stars embedded in so-called stellar nurseries—clouds of gas and dust such as the nearby Orion Nebula—are particularly useful because their darker stellar wind cocoons are easily visible against the brighter nebula gas and dust in the background. In places where the boundaries between the stellar and interstellar materials around other stars can be seen—for example, in Orion Nebula images from the Hubble Space Telescope—there appears to be a strong shock wave at the upstream sides of those boundaries (where those stellar winds are running into the background interstellar winds), almost like the shock wave near the nose of a supersonic airplane. Aeronautics experts, as well as astrophysicists, commonly refer to that leading shock wave as a bow shock (or just a bow wave, if the difference in fluid speeds is not as high), an analogy to the bow of a ship plying its way through the water.
Indeed, Voyager and other spacecraft measurements (going back to Mariner 2 in 1964) have shown that the solar wind is supersonic, meaning that a shock wave should exist close to its leading, or upstream, edge. The direction of the sun’s motion relative to nearby stars and to the overall motion of the Milky Way galaxy is well known, and so which direction is upstream is also well known. It just happens to be in the direction that both Voyagers are traveling. Ed and his colleagues predicted that a classic bow wave should exist at this upstream end of the heliosphere, since the relative velocity of the sun compared to the local interstellar medium was predicted to be about 15 miles per second. The fast-moving solar wind, plowing head-on into a fast interstellar wind moving in the opposite direction (like two rivers flowing into each other from opposing valleys), was expected to produce quite a strong wave front, perhaps even a shock wave (though they couldn’t be sure). Voyager 1 was heading closest toward the actual predicted position of the bow wave itself (along the “nose” of the wave front) and thus was predicted to be able to get there first. Exactly when Voyager 1 might cross that bow wave was anyone’s guess, however. Ten years after Neptune? Twenty? Thirty? No one knew where the edge of the heliosphere would be, but everyone knew that the Voyagers’ plutonium power supplies wouldn’t last forever. If the spacecraft hadn’t crossed the line by the late 2010s or early 2020s, the power supplies might not last long enough to see it happen.
AN INTERSTELLAR MISSION
As both Voyagers sped on toward their interstellar destinies, the science and operations teams transitioned into a different kind of mission. The imaging team was essentially disbanded, now that there was nothing new left to photograph, and the cameras had been shut off. The same is true of the ultraviolet spectrometer team, although the instrument is still used to collect occasional “automated” astrophysi
cal measurements of nearby interstellar hydrogen. Shutting off or curtailing instruments helps to save power, which is slowly dwindling on both spacecraft. While Voyager’s radioactive plutonium power supplies will take eighty-eight years to drop to half their power levels, more than forty years have now passed since that plutonium was produced, meaning that power is down to around 75 percent of maximum values. The most important thing this power is used for is to heat the computer, radio transmitter and receiver electronics, and the remaining instruments. If left to soak in the cold of deep space, the temperatures of those systems would quickly drop to just a few tens of degrees above absolute zero, causing solder joints to break, resistors to crack, or any of a number of other possible fatalities.
Shutting off instruments and scaling back mission operations also helps save money. NASA’s entire annual budget allocation from Congress has averaged around $17 billion per year lately, or about 0.4 percent of the entire federal budget. Of that, all the science done within NASA costs about $5 billion per year, and of that, all the solar-system science—robotic planetary missions and data analysis, laboratory studies, technology development—has averaged about $1.5 billion per year. My colleague Casey Dreier at The Planetary Society has recently pointed out that that’s about the same as what Americans paid for dog toys last year. Don’t get me wrong—I love my dogs and I want them to have fun! But it’s important to put the costs to the taxpayers of this kind of grand exploration into perspective. And of that $1.5 billion per year, it costs about $5 million a year to keep the Voyager missions going. A significant amount of money, to be sure. Voyager scientists led by Ed Stone, along with JPL Project Manager Suzy Dodd and her mission operations team work hard to justify that $5 million request every year. “Voyager has been through something like eleven major reviews of its extended, extended mission,” says Suzy Dodd. Everyone involved takes it very seriously that they make sure to use these amazing far-flung laboratories as efficiently as possible, to push the frontier of human knowledge ever outward, and to learn as much as we can about the far reaches of our solar system. Spread out over the lifetime of the project, the total cost of Voyager has been about a dime per year for every American. That seems quite a bargain.
“Now that we’re in interstellar space,” Suzy Dodd says, “we’ve reached the rarefied air of being an untouchable spacecraft.” The Voyagers belong to all of us, they represent all of us, they will speak to the ages for all of us.
After Neptune, mission operations became known at JPL and NASA HQ as the Voyager Interstellar Mission. The goal of the Voyager Interstellar Mission, according to NASA, is “to extend the NASA exploration of the solar system beyond the neighborhood of the outer planets to the outer limits of the sun’s sphere of influence, and possibly beyond.” An important part of that, both officially and in the dreams of scientists like Ed Stone, has been to search for and find the heliopause, the boundary beyond the outer limits of the sun’s magnetic field and the outward flow of the solar wind, and to directly measure the interstellar fields, particles, and waves beyond the influence of the sun. In other words, to extend the reach of human senses into interstellar space. Quite a dream.
With no working cameras, the Voyagers may be blind, but they are nonetheless still capably feeling their way through the outer solar system. Five different science instruments are still being used, many almost daily since the start of the Interstellar Mission, to touch and smell and taste the distant heliosphere. These instruments measure the plasma ions in the solar wind (“plasma” is a physics term for an ionized gas consisting of positively charged ions and negatively charged electrons—a common example is the gas inside a fluorescent lamp); the compositions, directions, and energies of solar wind particles and interstellar cosmic rays; the strength and orientation of the solar or interstellar magnetic fields; and the strengths of natural radio waves that are thought to be originating from nearby interstellar space. One important part of one of Voyager 1’s instruments was not operating properly, however. The spacecraft’s instrument that was designed to measure the density of ionized hydrogen plasma in interplanetary space had stopped working shortly after the Saturn flyby in 1981. There were other ways to indirectly measure the amount of hydrogen Voyager 1 was encountering, but its inability to make a direct density measurement would lead to some controversy later.
Although Voyager 1 had a head start after completing its planetary mission at Saturn in 1981 and was already forty times farther from the sun than at launch, the Interstellar Mission didn’t officially begin until Voyager 2 passed Neptune in 1989, at a distance of about 31 AU. Even at those ranges, Voyager’s instruments were still measuring a steady stream of high-energy solar particles and magnetic fields heading out into space on somewhat “radial” trajectories—that is, radiating generally away from the sun, like air inside an expanding balloon. The fields and particles were not perfectly radial, however, but were instead deflected somewhat—as if responding to the looming edge of the heliosphere at some unknown distance ahead.
Since 1989, communications technicians at the NASA Deep Space Network facilities in California, Australia, and Spain have faithfully captured data from the spacecraft for six to eight hours almost every day, using the “smaller” 34-meter (111-foot-wide) DSN radio telescopes to capture the faint signals from the meager 23-watt transmitters on the Voyagers. By the time those radio signals travel for more than ten hours at the speed of light, across vast distances now more than 100 AU from Earth, that 23 watts has faded to only 0.0000000000000001 watts, or barely a flea’s whisper. But the Voyagers do a good job of pointing their antennas right at the Earth, and the DSN does a good job of pointing its antennas right at the Voyagers, and the very narrow transmitter frequency is pretty far from Earthbound or celestial radio noise sources, and so—somewhat incredibly it seems—it all works. About every six months or so, the DSN points its even larger, more sensitive, 70-meter (230-foot-wide) radio telescopes at Voyagers, and the spacecraft are commanded to download a bunch of higher-quality twice-a-week plasma-wave-instrument “wide band” (higher resolution, more sensitive) data, which are used to infer plasma densities that have been stored on the 8-track tape recorder instead of transmitted in real time. Once the data are confirmed to have arrived safely at Earth, the tape recorders are rewound and the process starts again . . . year after lonely year. . . .
Voyager’s fields and particles scientists predicted that the spacecraft should pass through several distinct parts of the heliosphere before finally popping out of the bubble and reaching interstellar space. The first new and different place they figured they would encounter would be a boundary known as the termination shock. Ed Stone likes to give public talks about Voyager, and he is an enthusiastic and engaging speaker. One of his favorite slides is his “heliosphere in my kitchen sink” movie, where he tries to describe, using a kitchen-sink analogy, the kinds of places that the Voyagers will visit on their way out. Here’s how it works: Empty your sink, angle the faucet so that it points toward one side or the other of the drain (not in the middle, where I’m assuming your drain is), and turn the water on full-throttle. The water crashes into the bottom of the sink and fans out into a lovely circular disk of water maybe five or six inches across that is flowing radially away from the impact point. That’s like the deep inside of the heliosphere’s bubble, except in the real solar system, the solar wind and the sun’s magnetic field are wound (by the sun’s twenty-five-day rotation period) into a huge Archimedean spiral with arms that move outward roughly radially away from the sun. But then look carefully at the water on the side of that bubble opposite the drain. That water is starting to slow down, to thin out, and to change direction because of the slight upward slope of the sink. There’s probably a turbulent little zone there where the water is bubbling and churning a bit. That place where the water stops being radial and changes speed and direction is the termination shock and it marks the transition to a new and turbulent part of the flow, where the water i
s turned around and heads toward the drain. In the actual heliosphere, the termination shock occurs where the solar wind changes speed and direction because of the pressure of the interstellar wind coming from outside the heliosphere. The region just beyond the termination shock is called the heliosheath (the skin or covering on the heliosphere), and no one knew how large that region would turn out to be, because no one knew how much pressure was coming from the outside. The next stop beyond the heliosheath, though, should be the actual edge of the heliosphere—the heliopause.
Over the first decade of their Interstellar Mission, both Voyagers measured the density of the solar wind slowly decreasing, as it was spreading out at greater and greater distances. In December 2004, twenty-four years after passing Saturn and twenty-seven years after launch, at a distance of 94 AU from the sun, Voyager 1 noticed a sudden drop in the speed of the solar wind (from million-mile-per-hour supersonic speeds to “just” a quarter-million-mile-per-hour subsonic speeds) and a jump in the density of the solar heliospheric particles, like a traffic jam on a busy freeway. At the same time, the instruments were able to sense an increase in the strength of the sun’s magnetic field. Voyager 1 had crossed the termination shock. In August 2007, far to the south and at a distance of 84 AU from the sun, Voyager 2 crossed the termination shock as well. Both spacecraft were now in the more turbulent heliosheath. Next stop: the heliopause. But when? A year? A decade? More? I figured there must have been a betting pool. But Ed Stone said, stoically, “No, no. There was no betting. Just the histograms that we kept track of with everyone’s estimates. I don’t even know if there’s a record of who voted for what.” Pity. I bet Ed would have won the kitty.
