The Interstellar Age: Inside the Forty-Year Voyager Mission

The Interstellar Age: Inside the Forty-Year Voyager Mission

by Jim Bell


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*Chosen as one of Amazon's Best Books of 2015!*
*An ALA Notable Book of 2015*

The story of the men and women who drove the Voyager spacecraft mission— told by a scientist who was there from the beginning.

The Voyager spacecraft are our farthest-flung emissaries—11.3 billion miles away from the crew who built and still operate them, decades since their launch.

Voyager 1 left the solar system in 2012; its sister craft, Voyager 2, will do so in 2015. The fantastic journey began in 1977, before the first episode of Cosmos aired. The mission was planned as a grand tour beyond the moon; beyond Mars, Jupiter, and Saturn; and maybe even into interstellar space. The fact that it actually happened makes this humanity’s greatest space mission.

In The Interstellar Age, award-winning planetary scientist Jim Bell reveals what drove and continues to drive the members of this extraordinary team, including Ed Stone, Voyager’s chief scientist and the one-time head of NASA’s Jet Propulsion Lab; Charley Kohlhase, an orbital dynamics engineer who helped to design many of the critical slingshot maneuvers around planets that enabled the Voyagers to travel so far;  and the geologist whose Earth-bound experience would prove of little help in interpreting the strange new landscapes revealed in the Voyagers’ astoundingly clear images of moons and planets.

Speeding through space at a mind-bending eleven miles a second, Voyager 1 is now beyond our solar system's planets. It carries with it artifacts of human civilization. By the time Voyager passes its first star in about 40,000 years, the gold record on the spacecraft, containing various music and images including Chuck Berry’s “Johnny B. Goode,” will still be playable.

Product Details

ISBN-13: 9780525954323
Publisher: Penguin Publishing Group
Publication date: 02/24/2015
Pages: 336
Product dimensions: 6.10(w) x 9.10(h) x 2.10(d)
Age Range: 18 Years

About the Author

Jim Bell is currently a professor in the School of Earth and Space Exploration at Arizona State University, an adjunct professor in the Department of Astronomy at Cornell University, and president of The Planetary Society. He and teammates have received more than a dozen NASA Group Achievement Awards for work on space missions, and he was the recipient of the 2011 Carl Sagan Medal from the American Astronomical Society, for excellence in public communication in planetary sciences. He is a frequent contributor to popular astronomy and science magazines like Sky & TelescopeAstronomy, and Scientific American, and to radio shows and internet blogs about astronomy and space. He has appeared on television on the NBC Today show, on CNN's This American Morning, on the PBS NewsHour, and on the Discovery, National Geographic, Wall St. Journal, and History Channels. He is the author of Postcards from Mars.

Read an Excerpt

Prelude: Outbound

I believe our future depends, powerfully, on how well we understand this Cosmos in which we float like a mote of dust in the morning sky. We’re about to begin a journey through the Cosmos . . . it’s a story about us . . . how the Cosmos has shaped our evolution and our culture, and what our fate may be.

—Carl Sagan

(Cosmos: A Personal Journey)

PHYSICS TELLS US that all things attract each other gravitationally, from pulsars to planets to petunias, even if those forces are sometimes too small to notice in everyday life. But if you look closely at the trajectories that your life has taken, you may notice the results of similar gravitational effects from the people you have known. Sometimes people around us cause massive swings in direction and speed that can propel us on toward new and undiscovered territory and experiences. That’s what happened with me and the space-exploration mission known as Voyager.

The trajectory of my life has been guided by the slow, gentle, persistent gravitational pull of two elegant robotic spacecraft and the teams of people—scientists, engineers, mentors, students—who made their missions of exploration so marvelously compelling. Taking advantage of a rare celestial alignment of the planets, those two robots, Voyager 1 and Voyager 2, gave us all our first detailed, high-resolution, glorious views of the solar system beyond Mars, revealing the giant planets Jupiter, Saturn, Uranus, and Neptune, and their panoply of rings and moons, in all their awesome wonder—not just for scientists, but also for poets, musicians, painters, novelists, moviemakers, historians, and even kids.

I happened to have been born at a time that placed me in college and graduate school right when the fruits of that fortuitous celestial alignment were ripening. By a random turn of a corner in a building, while walking back from class, I spotted a flyer from a professor who was looking for student research help. I soon found myself involved in the missions of these extraordinary projections of human technology—something I had dreamed of since I could barely read. I felt as if I had been cast out into deep space myself, seeing my life, and my world, from a completely new perspective. In one seemingly chance Forrest Gump–like encounter after another, the arc of my life has been shaped by the Voyager missions, and even to this day I find myself drawn to their power to lift the human spirit. Just think of these sophisticated creations—mere machines, yet projections of ourselves—launched into my hero Carl Sagan’s “shallow depths of the cosmic ocean,” representing the integrated abilities, hopes, dreams, and fears of the more than 100 billion people who have lived on planet Earth and who, like me, have wondered, “Are we alone?” “What else is out there?” “What is our destiny?”

These Voyagers—and by that I mean the people as well as the machines—have taken us all on a tour of the Greatest Hits of the Solar System, and we have all been privileged passengers carried along for the ride. Along the way, I went from a starry-eyed kid interested in astronomy and planetary science to a student learning the ropes from some of the greatest masters in the field, to—now—a practitioner of the art myself, with students of my own. It has been an adventure filled with astounding beauty, discovering new worlds so exotic that their alien landscapes were entirely unanticipated, facing unprecedented challenges, meeting and then saying good-bye to new friends and colleagues. . . .

And now the Voyagers are leaving the protective bubble of our sun and crossing over into the uncharted territory between the stars. They—and we, through them—are now interstellar travelers. Via their technology, their discoveries, and the messages that they are delivering to the galaxy on our behalf, we have all entered the Interstellar Age. This may be the ultimate legacy of the men and women and machines of Voyager. As we learn and grow as a species, as we begin to grasp the fragility of our existence and the fleeting nature of habitable environments in our solar system, we must adapt and move on. In the long run—the very long run—we will have to leave our sun’s cradle and move out into the stars. The Interstellar Age is the inevitable future of humankind, and the Voyagers are our first baby steps along that path.

I want to share that story with you here and convey, I hope, how special it has been to be witness to what historians of the future will no doubt regard as some of the most incredible voyages of exploration ever attempted.

Part One




I GAZED IN wonder at the graceful and swirling azure clouds of Neptune. I had impulsively boarded the spacecraft in 1977, at the age of twelve, seeking a place where the gravity of my world would be but a distant memory. Word of the launch came on the evening news. “A Grand Tour of the Solar System!” the announcer proclaimed. Two launches would carry our band of travelers destined for Jupiter and Saturn, and if all went well, we would forge on, perhaps past Uranus and Neptune—worlds as yet unexplored. The thought of running away from home to explore some distant land tugged at me, as it did for many preteens. In the world of my small Rhode Island town, even traveling to another area code may as well have been like traveling to Mars. Of course, it could only ever be a dream: traveling faster than any rocket had ever gone, taking two years to Jupiter, three years to Saturn, Uranus by the mid-’80s, Neptune by my twenty-fourth birthday . . .

It sounds like science fiction, but this is essentially a true story. The spaceships are called Voyager 1 and Voyager 2, and they really did launch in 1977. While the Voyagers don’t carry humans on board, they do carry our eyes and ears, our most sophisticated cognitive intelligence, our science and art and dreams. In 1977 they brought me out of that sheltered world of childhood and into a fantastic new world of learning, culture, and Big Science, first as a college student at Caltech in Pasadena, then as a graduate student in Hawaii. Voyager’s story of exploration parallels my own. Indeed, the missions have touched countless lives and careers in space science and engineering—so many of the people I know and have worked with over the decades feel as if the Voyagers propelled their lives.

The “Grand Tour” announced that day in 1977 would take advantage of a once-every-176-year planetary alignment that provided an opportunity to send a single spacecraft past all four giant outer solar system planets, using the gravity of one to slingshot the mission on a path to the next, bouncing it from one remarkable world to another, and then eventually completely out of our solar system. The last time such an alignment occurred, back in the eighteenth century, the frontier of exploration was defined by European wooden sailing ships.

Voyager 1 and 2 Trajectories. Schematic diagram of the trajectories that enabled NASA’s twin Voyager spacecraft to tour the four gas giant planets and achieve the velocity to escape our solar system. (NASA/JPL)

The twelve-year-old me had become hopelessly hooked on space exploration watching the adventures of the Apollo astronauts on the moon. My parents tell me that they woke me up on that Sunday night in July of 1969 to witness Neil Armstrong and Buzz Aldrin make history on live TV in the Sea of Tranquility. We saved the Monday MAN ON MOON! giant headlined copy of the Providence Evening Bulletin, which I later had framed. For the next three and a half years, I was glued to the television, whenever possible, watching these guys walking—and driving cars!—on the lunar surface. While I was assured by the voices of NASA engineers and space commentators that it was hard work, many of the astronauts seemed like they were having fun. I want to do that, I thought. I dressed as an astronaut for a long run of Halloweens.

I followed the exploits of the twin Viking landers sent to the surface of Mars in 1976. Even though people weren’t going, the idea of sending two car-sized robots on a 150-million-mile remote-control journey and getting them to set down, softly, onto the surface of the Red Planet was astounding. In the decades ahead I would witness firsthand even crazier Mars landing systems as the Mars Pathfinder and Spirit and Opportunity rover mission set down on Mars—successfully—using bouncing airbags, and the larger Curiosity rover did so using its Rube Goldberg–like “sky crane” landing system. Viking used old-school technology, like parachutes and retro-rockets, right out of a Bugs Bunny cartoon. While Marvin the Martian wasn’t waiting there for us, the Mars that was revealed by the Vikings turned out to be eerily like deserts on Earth, though much dustier, colder, and drier.

The early 1970s-era cameras on Viking were essentially faxing their photos back to Earth, and NASA was using what was then brand-new electronic imaging technology that needed no photographic film. Instead, it converted the sunlight reflected off of a Martian scene into radio signals, beaming them back to Earth, where the faint signals were picked up by radio telescopes the size of a baseball field. I saw the digital images these faint signals produced revealed on the nightly news. The first images came down live, and painstakingly slow—one column of picture elements or “pixels” at a time. Space photography! I want to do that, too, I thought. My parents and grandparents helped me buy a telescope and some attachments to link it up to my 35mm camera.

These days it’s hard to explain to my kids, or to my students, what it was like growing up thirsty for science in the 1970s and 1980s. Imagine a world, I implore them, where there are only three major TV networks plus another run by the government, called the Public Broadcasting Service, or PBS. Imagine further that for the most part only the government channel would have science shows on TV (not counting Star Trek—one of my favorites, to be sure, but only partly “science”). For the most part, science TV was dominated at the time by NOVA—the educational and beautifully produced show from Boston’s WGBH station that is still running strong today. But that was basically it: no Science Channel, Discovery Channel, National Geographic Channel, NASA TV, History Channel, or for that matter, no Fox, CNN, MTV, VCRs, DVRs, and no way to skip the commercials. They look at me in horror, as if I had to endure being raised by wolves in the frozen tundra. Then they shoot me a truly pitiful look when I remind them that, worse yet, we had no Internet. Gads! How did we survive?!

In that bleak landscape of science communication was the TV show Cosmos, which first aired on PBS in 1980. The show’s host, the astronomer, planetary scientist, astrobiologist, Voyager imaging team member, and science popularizer Carl Sagan, was probably the first scientist I had ever encountered who spoke English. I mean common English, more like what you’d hear around the dinner table than the jargon and shorthand codes that most scientists typically use when talking about their work. But that plain talk was also laced with metaphor and analogy and evocatively grand cadences, often accompanied by the soaring and romantic electronic music of Vangelis. Sagan revealed the mysteries of the planets and moons and asteroids and comets and stars and galaxies and where we came from and where we’re going. I found myself listening to him and falling in love with the idea of doing science, of possibly even becoming a scientist. It was a captivating, mind-blowing, entertaining glimpse into the modern world of astronomy and space exploration. I would eagerly await each week’s new episode, talking about it endlessly the next day with my nerd friends at school, mimicking Sagan’s distinct, guttural staccato voice . . . “Perhaps, one day, we will sail among the stars on gossamer beams of light. . . .” My mother loved his turtleneck and tweed jacket (I would get to introduce her to him many years later, at a professional conference we both attended in Rhode Island. We were both just starstruck, but Carl was kind, warm, and thoroughly approachable).

Back in 1977, it was clear to even a teenager that Voyager would be something very different from Viking. First of all, it would embark on a long journey. It would sail on for more than a decade at least, and if nothing bad happened along the way, the plutonium-fueled nuclear power pack that generated electricity for the spacecraft could keep the systems working for perhaps fifty years. With that kind of longevity, it was possible for the spacecraft to survive long enough to cross into interstellar space—the realm outside of our sun’s protective magnetic cocoon. Voyager would then venture out into the strange and unfamiliar interstellar wind. Wild. And second, the mission had the potential to literally discover entirely new and alien worlds! Viking had made important discoveries on Mars, but the landscape and processes had generally been familiar: wind, sand, maybe a little water long ago, grinding down and carving into the rock, eroding landscapes like you might encounter on a car trip through northern Arizona, Utah, and southern Colorado. Voyager would be encountering worlds not of rock, but of ice and gas, places where the sun is only the blip of a flashlight in an otherwise black, starry sky, and where the temperature might be only a few tens of degrees above absolute zero.

Through my youthful eyes, the biggest appeal of Voyager was indeed this idea of exploring the truly unknown—throwing a bottle, of sorts, into the cosmic ocean and seeing where the eddies and currents of nature would take it. In my telescope, on a clear cold night, I could make out the reddish-brown belts and bands of Jupiter, as well as its famous Great Red Spot. It was a good-sized instrument for a young amateur astronomer, a so-called Newtonian telescope (designed by Isaac Newton, and using mirrors instead of lenses) made by a company called Meade Instruments, with a main mirror about eight inches in diameter and a tube about four feet long. With that tube held by a metal mounting post and three wide metal legs, it was a heavy, bulky, cumbersome thing to schlep outside and in from the garage and to set up every time I wanted to use it (especially in the snow), but it was so worth the effort. I could resolve the enchanting, creamy yellow rings of Saturn and learned firsthand why that planet was called “the jewel of the solar system” by the pioneers of astronomy. It always amazed me, in fact, that when I looked at Saturn I was seeing the real Saturn. Like a lot of kids at the time, I collected coins and stamps and baseball cards, and that was great, but there was always someone with an older, better, or cooler collection than mine. No one had a better Saturn in their planet collection—I was looking right at the real thing with my own eyes. I felt an unfamiliar sensation, an extraordinary lightness, as I took in the nearness of a world so distant.

In my small telescope, Uranus and Neptune, and the six or seven little moons I could sometimes find perched around Jupiter and Saturn, were just specks of light. It was hard to imagine these dots as worlds, as destinations one might visit, as lands of rock and ice, wind and volcanoes, polar caps, and panoramas so staggeringly familiar and yet patently alien. They were just dots, to me and everyone else on our little blue planet—even the largest telescopes in the world at the time couldn’t reveal their true nature. But I knew that Voyager would change that, that these dots would soon permanently become distinct places, as diverse in character as the myriad environments of our own planet, and far more exotic than our own moon (which had also, only recently, become a bona fide place, rather than a two-dimensional icon in the sky). The ability to ride along with Voyager, to be a passenger on this trailblazing journey destined to discover entirely new worlds, to see history made, was irresistible to the young me. In fact, it still is.


Famous ships of exploration are usually led by a famous captain or commander, like Christopher Columbus, Ferdinand Magellan, James Cook, Ernest Shackleton, or Neil Armstrong. The Voyagers, however, are led by a committee of captains—managers, engineers, and scientists from NASA’s Jet Propulsion Laboratory (JPL) and elsewhere who were tasked with overseeing the design, manufacture, and operation of the most ambitious robotic planetary exploration mission yet attempted—and a pair of equally powerful commanders, a project manager and a project scientist.

In NASA and JPL parlance, Voyager is a “Project” (capital P), run by a Project Office (capital O) and divided organizationally into a number of subsidiary offices. These include the Mission Planning Office, where the detailed spacecraft trajectories were designed; the Flight Science Office, which includes the science team and which is responsible for making sure the mission achieves its scientific objectives; the Flight Engineering Office, with the engineers and managers who designed and built Voyager’s power, thermal control, communications, and propulsion modules; the Flight Operations Office, which provides the procedures and software needed to plan and actually operate the spacecraft and its science instruments (and which includes two teams of JPL scientists and engineers: the spacecraft operations team, who directly communicate with the spacecraft and who monitor its status and health over time, and the science support team, who serve as the interface between the science team and spacecraft operations team); and the Ground Data Systems Office, which provides the hardware and software needed to send commands up to the spacecraft (“uplink”) as well as to receive and process data back down from the spacecraft (“downlink”).

The project manager leads the Project Office and is the engineering and management commander of Voyager, responsible for getting the spacecraft built and tested, keeping the mission safely operating on time and on budget, and overseeing the hundreds of contractors and several thousand engineers, technicians, and other managers on the Project. The project scientist runs the Flight Science Office and the science team—a group of scientists, engineers, technicians, managers, and students from around the world who designed, built, and operate the science instruments and who interpret the downlinked data. The project scientist is the scientific commander of Voyager, responsible for making sure the mission achieves its science goals on time and on budget and for coordinating and herding (like cats) the hundreds of scientists on the Project.

Each Voyager carries scientific instruments for eleven investigations. These include wide-angle and high-resolution cameras for imaging and spacecraft navigation; radio systems for studying gravitational fields and planetary radio emissions; infrared and ultraviolet spectrometers to measure chemical compositions; a polarization sensor for surface, atmosphere, and planetary ring composition; a magnetometer measuring magnetic fields; and four devices for studying charged particles, cosmic rays, plasma (hot ionized gases), and plasma waves. Scientists conducting each investigation are organized into instrument teams, and the leader of each instrument team is called the principal investigator (or PI). The PIs are responsible for the design, construction, and operation of each of their instruments, and together they form the Science Steering Group, which is chaired by the project scientist and which reports to the project manager.

In this kind of committee-led project, it’s critical that the two commanders at the top of the org chart, the project manager and the project scientist, are consistently on the same page and have an excellent working relationship. Each is personally responsible for the success of the mission—to NASA and, ultimately, to Congress and the taxpayers who are footing the bill. Over the course of more than four decades since the Project began, Voyager has had ten project managers. But during that entire time, the mission has had only one project scientist: Edward C. Stone.

Ed Stone is a space weatherman, a physicist who studies the ways that high-energy particles called cosmic rays travel through space and interact with the magnetic fields and atmospheres of the sun and planets. Cosmic rays are a form of high-energy radiation made of protons and the nuclei of atoms, and they travel through the universe at nearly the speed of light. Exactly where they come from is still a mystery—they could be caused by massive supernova explosions of dying stars, or by the powerful black holes in the centers of active galaxies, for example. Regardless of how they formed, scientists like Ed can use the properties of cosmic rays to understand the details of the ebb and flow of the solar wind (the stream of high-energy particles coming off the sun) and the way that wind is carried by the sun’s magnetic field and interacts with the magnetic fields of the planets. Measurements of this kind of “space weather” were some of the first scientific measurements ever made from space satellites, and Ed Stone has been a prolific scientist in this game since the beginning.

In 1972, Ed was appointed as the project scientist for Voyager. Over the course of the mission he’s had other important roles as well, including serving as the director of JPL from 1991 to 2001 and as the PI for Voyager’s Cosmic Ray Subsystem (CRS) instrument, which is making the measurements that are most closely aligned to his scientific background and interests. Project scientists have to figure out how to achieve the optimum match between the science needs of a mission and its engineering and budget constraints. They also sometimes have to make tough decisions about which experiments and which observations will or will not be done. If members of the science team can’t agree on how to carve up available resources (power, time, data volume) for competing measurements, it is the job of the project scientist to step in to arbitrate, or to just plain decide.

“It turns out,” Ed reflected, “that’s a much more critical role than I had thought ahead of time, and that’s because ultimately what science is all about is making discoveries. By deciding to make this observation rather than that one, you’re effectively deciding that that group of scientists gets to make a discovery and this group doesn’t.”

Ed Stone’s impeccable record as a careful and thoughtful scientist, his patient and friendly demeanor, and his ability to work fairly with ten project managers and hundreds of Voyager scientists and engineers have established him as an effective and respected project scientist, as well as a widely recognized spokesperson for the entire Project during press conferences and media appearances. Voyager is run by committee, and consensus is most often the ruling doctrine. But if things were different, I can easily envision Ed Stone as the king of Voyager, ruling benevolently over an empire that extends out to the farthest reaches of the known solar system, and beyond.


Astronomers traditionally study stars or galaxies; geologists typically study rock outcroppings or map oil and mineral deposits; meteorologists study the weather and climate and try to make forecasts—these are relatively traditional and established fields of scientific study. But what do you call a person who studies the science of the planets, moons, asteroids, and comets around us and has to use the theories and methods of some, many, or all those fields at once? The term “planetary scientist” is a relatively new one among academic professions, and it’s one that the Voyagers have helped establish. We’re a sort of jack-of-all-trades kind of people, thinking about science questions on scales from the planetwide (like from a mapping camera on an orbiting spacecraft), to the minuscule (like individual little rock piles or sand piles studied by a rover). Some of us are more interested in astrobiology—the study of life in space—than anything else. A common approach among planetary scientists is to use remote sensing, very remote sensing, to do our science, because except for those lucky dozen astronauts who’ve had the privilege of walking on another world, none of the rest of us can actually set foot on the places we’re studying.

We use technology to experience the place remotely. Everyone actually uses remote sensing of a sort all the time in our daily lives, using our senses to interrogate the world out of our reach to, for example, judge distances and sizes or to identify objects from their shapes or colors or smells. All animals do it one way or another; plants, too. The difference for planetary scientists is the use of robotic sensors: cameras acting as eyes to provide sight, spectrometers or sampling probes acting as organs for smell and taste, arms and scoops and drills providing a sense of touch, radio antennas for “hearing” and “talking.” And even a sixth sense comes into play sometimes, one that is familiar to hikers or geologists working out in the field: a sense of place or context enabled by mobility—the ability to roam and climb and explore a place from multiple perspectives, or to leave it entirely and head for new ground. Flyby and orbiter spacecraft, and rovers on the ground, provide these essential remote-sensing capabilities for us, sending back the pictures and sensory data from remote places.

It’s easy to think of spacecraft like the Voyagers as being alive, imparting to them feelings and other human attributes. They are so far away, and it is so cold and dark. They must be lonely. Some of them, like the Mars rovers Spirit and Opportunity, are so cute with their long necks and bulging eyes! They must be plucky, intrepid, courageous, and a dozen other grand adjectives of exploration, in order to survive and thrive for so long. They are out there, working tirelessly, making discoveries and braving dangerous environments with no rest, no vacation, and no pay. We’ve got robots exploring the solar system for us!

Well, as fun (or creepy) as that is to imagine, it misses the point. They are machines, built and launched and operated remotely by smart and clever people. Spacecraft like the Voyagers are high-tech, to be sure, but not sentient or any more capable than their relatively primitive (by our twenty-first-century standards) software. “Don’t anthropomorphize the spacecraft,” Voyager imaging team member Torrence Johnson recalls Project Manager John Casani saying. “They don’t like it.”

“The sense of exploration we get with these missions is a very ‘human explorer’ kind of feeling, even though our senses are on the distant spacecraft,” my friend, planetary science colleague, and Voyager imaging team member Heidi Hammel says. “I feel like an old-fashioned mountain climber when I am making discoveries, seeing something for the first time, realizing that no human before me has ever seen what I am seeing. It takes your breath away—for just a moment you feel a pause in time as you know you are crossing a boundary into a new realm of knowledge. And then you plunge in, and you are filled with childlike joy and wonder and delight.” Like me, Heidi cut her teeth in our business with Voyager, and like me, she got hooked on the thrill of exploration and discovery. “And then you get serious again, and start thinking about how it fits into what you already know,” she continued, “and your grown-up scientist brain takes over. Those of us who have had that feeling want to keep coming back for more, and we want others to have that feeling too.”


The robotic exploration of space is, in fact, human exploration. It’s just that the humans doing the exploring haven’t left this planet. And that’s why the story of the Voyagers and their travels to the edge of the solar system and beyond is a story of the human drama of deep-space exploration. Voyager’s saga is one of discovery and adventure but also of risk and frustration, successes as well as sacrifices, consensus and conflict, the historic mingling with the mundane. Scientists, engineers, managers, technicians, artists, students, and countless other professionals designed the mission and built the spacecraft, guided them on their Grand Tour of the outer solar system, helped take the photos and make the discoveries that now fill our textbooks, and still help us communicate with the spacecraft today on their continuing interstellar voyages. When historians five hundred years from now look back, the accomplishments of this particular group of people will be among the most important remembrances of our time.

But many other people have had an important albeit indirect role as well. As a student, I learned about and joined a new organization that my hero Carl Sagan had helped form in 1980 called The Planetary Society. The Planetary Society is the world’s largest public-membership space-advocacy organization, and its beginnings are as tied to Voyager as my own. America in the late 1970s was in a state of national crisis: high inflation, high prices (and even rationing) of oil and gas, and federal budget deficits rising to levels not seen in decades. Ronald Reagan was elected president in 1980 partly as a result of a national backlash against President Jimmy Carter’s administration’s inability to get the economy back on track. Reagan interpreted his mandate to be to recover the economy by promoting business growth (this is when the term “Reaganomics” was coined) and cutting taxes and federal spending. Specifically, that meant cutting nondiscretionary federal spending—the programs not related to defense or Social Security or Medicare and other entitlements. NASA found itself on the chopping block for massive potential budget cuts, initiated by Reagan’s chief of the Office of Management and Budget (OMB), David Stockman.

I don’t know whether Stockman liked NASA or not, but it didn’t matter—even though its fraction of the federal budget was less than 1 percent (as it is today—now less than 0.5 percent, in fact), the space agency was an easy target for budget cutters. “Why should we spend money on searching for little green men,” some in Congress have asked (really), “when we have so many other pressing needs here at home?”

Why should American taxpayers support NASA? I believe it’s because we like satellite launches, space shuttles, moon landings, Mars landings, and cutting-edge materials and computers and communications technology and products . . . even Tang, at least for a while. Most important, however, as is obvious to anyone who spends time around scientists like Ed Stone or Heidi Hammel, or science popularizers like Bill Nye or Neil deGrasse Tyson, there are critically important intangibles in this pursuit that feed our souls. Some of the intangibles of supporting NASA—results for which we cannot predict their future influence on our society or our planet—include the inspiration and education of our young people, the gathering of pure knowledge about the worlds around us and our place in the universe, and of course national pride in American leadership in exploring the greatest frontier there is.

The national debate and budget-cutting angst of the early 1980s was happening right on the heels of the spectacular Voyager flybys of Jupiter in 1979 and Saturn in 1980. These planets had been visited before, though only briefly, during flybys by the Pioneer 10 and Pioneer 11 spacecraft a few years prior to Voyager. While spectacular achievements in many other ways, the Pioneer images of the giant planets were somewhat fuzzy (not much better than telescopic images from Earth, because of the relatively far flyby distances and crude digital imaging technology that was used), and little new information was obtained about the moons or rings around those worlds.

Voyager was different.

Van Gogh–like tapestries of crazily colored, swirly clouds with vibrant tones of orange, yellow, and red on Jupiter, including the first close-ups of the Great Red Spot, began showing up on TV, on space posters, and in textbooks. The clarity of form and color and the simple elegance of Saturn’s rings were revealed for the first time, including photos looking back from behind the rings, beyond Saturn, viewing the planet from a perspective impossible to achieve from Earth. And the large moons around Jupiter and Saturn were unveiled as alluring worlds—planets in their own right—one with active volcanoes (Io), another with plates of what appeared to be floating sea ice (Europa), and another with a thick, smoggy atmosphere that may be what the Earth’s early atmosphere was like (Titan). It was a grand spectacle.

Voyager imaging team member Carl Sagan knew, from his own experience as a public speaker, educator, and TV host, that there was enormous public support for NASA, but that it was scattered across the country and not organized in any particular way. Something had to be done to combat the looming budget cuts. Sagan, along with Bruce Murray, a planetary scientist and then director of JPL at Caltech in Pasadena, and JPL space mission engineer and manager Louis Friedman, decided to try to organize and focus public support. In 1980 they formed a nonprofit membership organization, a society that any like-minded people could join. Dues would be $15 per year, and they’d send out a bimonthly magazine with the latest space images and other related information. They called it The Planetary Society, and the magazine The Planetary Report.

I joined The Planetary Society as a high school student in 1980 (I think I saw the ads for membership in the materials distributed by my rural Rhode Island amateur astronomy club, SkyScrapers). Many members of the club were as excited as I was about the Cosmos TV show and thrilled about being part of a nationwide—worldwide—effort to promote space exploration. I let my membership lapse a few times in college when I was short on cash, joined again for good in grad school, and now I’m privileged to be the president of the society’s board of directors, a position once held by my mentor, Carl Sagan. Membership in the society skyrocketed to more than 100,000 people shortly after it was formed, partly fueled by Sagan’s enormous popularity and influence, but partly also because it provided a way for interested people to stay informed about and connected with the space program in the days before the Internet. Sagan, Murray, and Friedman took this public support to Congress and the presidential administrations over the years to help demonstrate the high level of enthusiasm for NASA and space exploration. Indeed, the society was instrumental in tapping into the success and legacy of Voyager to help avert the worst of the early 1980s budget slashing of NASA and to help set the stage for the phenomenal missions of exploration and discovery that have taken place since.

Today, thirty-five years after it was founded, more than a half million people have been members of The Planetary Society, and millions more enjoy the images, activities, articles, blogs, and tweets on our website for free ( And once again we find ourselves in austere times, where shortsighted members of our government are again looking to slash and burn federal budgets for nondiscretionary programs like NASA. So once again we are rallying the troops, tapping society members (and nonmembers as well—indeed, anyone else who cares) to write letters and e-mails to Congress to express their support for the space program. Astrophysicist and science popularizer Neil deGrasse Tyson, host of the recent television remake of Cosmos and another former president of the society, has called for an increase in NASA’s budget despite the nation’s difficult financial situation, because space exploration represents the very best of what our species does, and because he knows that the value of inspiration is priceless during tough times. The society’s new CEO, the Emmy-winning TV host, engineer, and science educator Bill Nye (“The Science Guy”), is reaching out to new members, taking advantage of contemporary venues like social media, to help keep the society vital and effective.

The missions that have come since Voyager, such as the Jupiter orbiter Galileo and the Saturn orbiter Cassini, have revealed those worlds and their rings and moons anew, with more powerful senses, at higher resolution, and over extended periods of time. The time spent by these spacecraft in the Jupiter and Saturn systems has allowed us, for the first time, to see those worlds in motion, active and evolving—by nature a difficult task for a flyby mission like Voyager. These worlds are dynamic, a fact easy to forget when all you get is a short movie of the approach or a few still snapshots of a place apparently frozen in time. The more extensive time-lapse movies that we now have of these worlds from orbiting spacecraft have brought them to life. We can close our eyes and see the colossal storms raging on Jupiter as they morph into reality. Closer to home, rovers like Spirit, Opportunity, and Curiosity and the orbiters high above them are helping us unlock the secrets of ancient, Earthlike Mars, while orbiters circle and map Mercury, Venus, the moon, asteroids, and comets (as well as our own planet!) to help put the story of our origins together. Space exploration used to be dominated by the United States and the USSR, but now it has expanded into a truly global enterprise with significant contributions from Europe, Canada, Japan, China, India, and others. We are in the midst of a golden age of the exploration of space by people across our planet. About thirty active robotic missions are out there plying the ocean of space on our behalf, poised to make some of the most profound discoveries of all time. These missions let us vicariously see and hear and taste and touch the dirt and wind and ice of other worlds, following in the footsteps of the most grand and far-flung of them all, the Voyagers.


We first crossed paths professionally, me and Voyager, when I was a college student in the 1980s, searching for a way to make a career out of my childhood dreams of astronomy and space. I applied for and—to my amazement—was accepted by both MIT and Caltech to study astronomy (as my friend Bill Nye would joke about his own acceptance by his beloved Cornell, “There must have been a clerical error of some kind”). Against the wishes of family and friends (most of whom had never heard of the place), I chose Caltech, partly because I needed to spread my wings and explore firsthand the things I was hearing about this strange new world called California, and partly because I knew that Caltech was intimately connected to JPL, the epicenter of planetary exploration in the United States.

The smell of the olive trees the first day I walked onto the campus of the California Institute of Technology in the fall of 1983 was the smell of newness and change. It turned out I had traded an insular, small town and small-state family life for an insular, small dorm and small-campus nerdy life. I had never been so challenged academically (the professors are notoriously merciless there, since many have either invented the field they are teaching and written the textbooks themselves, or they are busily distracted by, and actively working on, the cutting edge of whatever was being taught). I failed Math 1 and so they put me and a small group of other struggling students in a “special” math class called Math 0.9—just slightly less than Math 1.

One day, I saw a small ad posted on one of the bulletin boards that I would frequently peruse on my job search around campus (remember: no Internet!), looking for a student to help analyze some ultraviolet measurements of Jupiter.

That sounds like astronomy, I thought to myself. Why not check it out?

The ad was posted by Mark Allen, a Caltech/JPL research faculty member. He grilled me in his strong New York accent about my background and previous experience during the interview. Despite consistently hearing myself answer “no,” “none,” or “I don’t know what that means” to his questions, I felt good about him. Up to that point, my life’s work experience had consisted of picking car parts in my father’s junkyard, plotting chemical assay results in a metal refinery lab run by the father of one of my high school friends, and taking out the trash along with other odd jobs in a mall clothing store for large women. Incredibly, Mark offered me the job anyway. Maybe it was the plotting experience (that’s mostly what the work he needed turned out to be about). Or maybe it was a clerical error. Whatever the reason, it changed my life.

Working every day in South Mudd, Caltech’s building that houses its Division of Geological and Planetary Sciences, was an absolute delight for a young space junkie. There were posters and murals and space paraphernalia all over the place, and the halls and offices were lousy with famous (to me, at least) faculty, staff, grad students, and postdocs who were working on missions like Voyager and Viking. I would faithfully print and analyze plots for Mark, but while waiting for my plots to print or for my computer job to process (I was low on the priority totem pole), I would wander the halls and daydream about the far-off places the people around me were exploring. It was much more fun than classes and homework and exams, and so my grades continued to suffer. I kept my head just above the waterline and was lucky not to “flame out” like some of my other fellow Math 0.9 friends—but it was close.

I grew up thinking that astronomers studied everything: stars, galaxies, planets, moons, whatever. It’s all in space, right? But it turns out they’re compartmentalized, balkanized, self-segregated by distance, and then by energy: solar system, galactic, extragalactic, cosmologic, and then from microwave/infrared (low energy) to UV and gamma rays (high energy) within each of those realms. At a party with these people, you would not want to confuse a high-energy extragalactic cosmologist with a near-Earth asteroid hunter, believe me! But I also learned that there are different kinds of solar system researchers out there who are not astronomers but who are geologists, or chemists, or physicists, or meteorologists. They happen to study nearby solar system objects (or maybe meteorite samples of them) rather than astronomical objects, and at Caltech at the time, most of them didn’t use telescopes but instead used images and other data taken by robotic space missions to do their science. Many of them were designing or flying their own cameras and other instruments in space. I had found my tribe! It turned out that the particular flavor of astronomy, space mission, and hands-on engineering experience that I was looking for had a name: planetary science. And at Caltech, I could actually get a degree in planetary science! I switched my major.

The halls of South Mudd are where I first met G. Edward Danielson Jr., or just “Ed,” as he liked to be called. A cheerful, big, sometimes shy gentleman, Ed was a member of the Caltech/JPL technical staff who specialized in designing, building, and operating cameras in space, for missions like Mariners 6, 7, and 10, Viking, and the Hubble Space Telescope. He was also a member of Voyager’s imaging team and spent a lot of time looking at and analyzing the incredible images sent back from Jupiter and Saturn just a few years earlier. I would run into Ed at the printer, where I would often have to sheepishly hand him back his printout—which I was holding and admiring—of some amazing Voyager image of Saturn or Viking image from Mars that had printed out before my plot. He started getting into the habit of giving them back to me, saying something like “Oh, that’s the wrong one” or “Oh, the contrast is wrong in this one” or some such. But I was onto him—he knew how much I treasured each of those printouts, even the underexposed or oversaturated ones. It was a fun little game, and soon I felt comfortable enough to talk with him about what he was doing. That’s how Ed turned me on to a new and growing field called image processing.

While I was too inexperienced and naïve to know it at the time, I later came to realize that at his core Ed was really a tinkerer, more of an engineer and a science enabler than a pure scientist. He cared about helping to make science discoveries from the Voyager images, for sure, but he cared more about how the cameras were working, so that he could help make sure that they were taking the best possible photos that could be taken. For a flyby mission, you get only one shot—so you want to make sure the cameras are pointed in the right direction, and you have to make sure that the exposures are at the right level. Point toward empty space or set the exposure time too short: all-black image. Point in the right direction but take too long an exposure: all-white image, or at least lots of uncorrectable saturation. Accidentally point at the sun: fry the camera. The stakes were high, and so guys like Ed who were responsible for getting it right had to get it right. I had never encountered that kind of pressure among scientists or engineers before.

As Mark Allen’s project was finishing up, it turned out Ed Danielson was looking for some help processing images from the Voyager 2 flyby of Saturn so that they could understand how the camera was changing over time, as it got older and farther away from the sun, to help the team prepare to get it right for the flyby of Uranus the following year. I will never know if Ed really needed a student to do that work or if he made the job up for me to keep me around. I was delighted and jumped at the chance. It was like hitching another ride on the Voyagers.

Back in the day, image processing was done on the (single) computer used by each faculty group (it was, conveniently, located near the printer), and it was a pretty competitive ordeal to get time to run programs and analyze images. As an undergraduate working among a group of active and productive faculty, postdocs, and grad students, I was the last guy in the queue, and so to get time on the image-processing computer, I often had to come in to work during the graveyard shift, catching Ed at either the end of his workday or the beginning of the next. Those nights were ghostly quiet, with just the hum of the nearby computer fans or the distant whine of the janitor’s vacuum cleaner to keep me company. As I gazed upon image after image from Voyager’s close flyby of Saturn, my thoughts would sometimes wander. I’d imagine myself as a passenger on that ship, imagine the gasps my fellow travelers and I would make as we passed through the flat disk of Saturn’s glorious, gossamer rings. And flat indeed! Although the main rings span the width of more than twenty Earths, they are only about thirty feet thick! If Saturn’s rings were a DVD, that DVD would only be about ten atoms thick, or about 100,000 times thinner than a human hair. The cool thing was that no one knew exactly why they were so thin, but I figured the answer was probably right there among the images I was working with.

I used to tell my friends that I was working at the edge of space. That’s because my job was to pore over the Voyager images and, literally, find the edges of space—the pixels where the planet ended and space began. Many of Voyager’s images, especially the ones taken when the spacecraft was really close to Saturn, have parts of the planet’s edge (called the limb) or the rings’ edges gracefully arcing through the photos. I would identify those parts of the image, and using some special software that Ed and others on the Voyager team had devised, I would try to fit a smooth mathematical curve to those edges. The curve was an estimate of how the edge of the planet should be curved when viewed from Voyager at the time and place that each photo was taken, if Voyager were precisely where it was predicted to be and if the camera were behaving exactly as predicted. But the spacecraft was never precisely where it was predicted to be, because of the slight push and pull of the gravity of Saturn and its moons, and the cameras never behaved exactly as predicted, because of the strange ways that the cold temperatures or the intense radiation from the magnetic fields close to Saturn itself could introduce artifacts into Voyager’s old-style vidicon camera system (which, unlike a modern digital image detector, would capture images using a cathode ray tube and electron scanning gun, like in old TV sets). So my lovely curves would never fit perfectly the first time, and I’d have to go back and fine-tune them a little to get a better curve to fit the limb, nudging the inferred position of the spacecraft a little, or tweaking some aspect of the lens distortion.

Table of Contents

Prelude: Outbound 1

Part 1 Alignment

1 Voyagers 7

2 Gravity Assist 41

3 Message in a Bottle 71

Part 2 The Grand Tour

4 New Worlds among the King's Court 103

5 Drama within the Rings 133

6 Bull's-Eye at a Tilted World 161

7 Last of the Ice Giants 191

Part 3 Looking Back, Looking Ahead

8 Five Billion People per Pixel 223

9 The Edge of Interstellar Space 241

10 Other Stars, Other Planets, Other Life 273

Postscript: NewSpace 293

Notes and Further Reading 297

Acknowledgments 309

Index 313

What People are Saying About This

From the Publisher

“In THE INTERSTELLAR AGE Professor Jim Bell celebrates Voyager -- NASA's most famous space probe. In this task, Bell not only shines as a leading planetary scientist, you get the sneaky feeling he knew the probe personally, serving in this case as Voyager's personal biographer to the benefit of us all.”
--Neil deGrasse Tyson, Astrophysicist, host of Cosmos: A Spacetime Odyssey, author of Space Chronicles: Facing the Ultimate Frontier

“Come along with Dr. Bell on an extraordinary adventure. He loves to travel. He relishes a long voyage. He doesn't mind leaving home forever and never coming back. If it involves a once in lifetime journey through the cold emptiness of space, so much the better, as long as the ports of call are planets—up close and personal. These voyages changed what we know of the cosmos and our place within it. Jim Bell's story will change you, too.”
--Bill Nye, CEO, the Planetary Society

“The Interstellar Age is dawning and this book recounts the saga of humanity's greatest solar system exploration on the way out. Part scientific autobiography, part top-notch science writing, Jim Bell's book is a welcome addition to the history of the Voyager Missions.”
--Jon Lomberg, space artist, science journalist, and co-creator of the Voyager Interstellar Record

“Everyone loves Voyager, and every few years the plucky spacecraft give us yet another reason to remind us why. Now that Voyager 1 has passed beyond the veil of the helioshere and plunged into interstellar space, Jim Bell takes up the narrative of what that mission has meant and makes his personal relationship, which dates from his days as a student, stand for the near-universal bond most of us feel. A timely, humane, and informative read.”
--Stephen Pyne, historian, and author of Voyager: Seeking New Worlds in the Third Great Age of Exploration

“Jim Bell captures the excitement, the challenges, and the joy of the historic Voyager journey of discovery.”
--Professor Ed Stone, Caltech, Voyager Project Scientist, former director of the NASA Jet Propulsion Laboratory

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The Interstellar Age: Inside the Forty-Year Voyager Mission 2 out of 5 based on 0 ratings. 1 reviews.
Anonymous More than 1 year ago
Without a doubt Jim Bell is a planetary science rockstar -- his long list of credentials include involvement with many NASA robotic space exploration missions. But alas, the Voyager mission is not one of them -- that threw me for a loop because I was expecting a true insider to relate the Voyager story -- something along the lines of Roving Mars or Carrying the Fire. To be fair, Bell did have a very brief involvement with the Voyager mission in grad school. But, ultimately, as I kept turning the pages, I still felt bummed that my I wasn't getting a true behind-the-scenes account. The Interstellar Age presents an overview of the "greatest hits" of the Voyager mission, mainly discussing the discoveries at Jupiter, Saturn, Uranus, and Neptune. However, there was so much more (see list below) that I wanted to read about that was either glossed over in favor of discussing other topics or just left out. (1) Way too many pages (30!) were devoted to the Golden Record. I just didn't feel enough space was devoted to the Voyager mission before it left Earth. I would have much rather read more about the development, design, construction, and testing of the spacecraft and scientific instruments -- 12 pages was not near enough. (2) I wanted more details on how the ground communicated with the crafts after they left Earth. (3) The six-page section devoted to The Planetary Society seemed a bit self-promotional since Bell is President of the Board of Directors. I do see it's relevance to the text, but it seemed a bit lengthy. (4) Bell's tendency to use italics for extra, and in my opinion, unnecessary emphasis is both mildly amusing and a bit annoying. (5) I did a very quick flick through Part 3 (the last 80 or so pages). These pages are not exclusively devoted to the Voyager mission. Despite setting aside The Interstellar Age on page 123, Bell does write very well -- his tone is very conversational and very accessible -- even to those who are just discovering the exciting world of spaceflight and exploration and want to learn all they can. Bell has a knack for taking complicated ideas or confusing bureaucratic stuff and presenting it in a very clear way that is easy to understand. And the few instances when he waxes poetically (for example about his experience in the visitors' gallery in the Spacecraft Assembly Facility) are awe-inspiring. But ultimately, my reading expectations were not met. Disclosure: I received a complimentary review copy from the publisher in exchange for an honest review. The opinions are my own.