Carl Sagan in 1986: ‘Voyager has become a new kind of intelligent being—part robot, part human’

The renowned scientist reflected on the lesser-known triumphs and lofty ambitions of Voyager in Popular Science's October 1986 issue.
Bettmann/Getty / Popular Science

One of the worries that kept legendary astronomer Carl Sagan up at night was whether aliens would understand us. In the mid-1970s, Sagan led a committee formed by NASA to assemble a collection of images, recorded greetings, and music to represent Earth. The montage was pressed onto golden albums and dispatched across the cosmos on the backs of Voyagers 1 and 2.

In a 1986 story Sagan wrote for Popular Science, he noted that “hypothetical aliens are bound to be very different from us—independently evolved on another world,” which meant they likely wouldn’t be able to decipher the golden discs. But he took assurance from an underappreciated dimension of Voyagers’ message: the designs of the vessels themselves.

“We are tool makers,” Sagan wrote. “This is a fundamental aspect, and perhaps the essence, of being human.” What better way to tell alien civilizations that Earthlings are toolmakers than by sending a living room-sized, aluminum-framed probe clear across the Milky Way. 

Although both spacecraft were only designed to swing by Jupiter and Saturn, Voyager 2’s trajectory also hurled it past Uranus and Neptune. Despite numerous mishaps along the way—and because of the elite toolmaker skills of NASA engineers—the probe was in good enough shape to send back close-ups of those distant worlds. In 2012, Voyager 1 became the first interstellar spacecraft, followed soon thereafter by Voyager 2. “Once out of the solar system,” Sagan wrote, “the surfaces of the spacecraft will remain intact for a billion years or more,” so resilient is their design.

Today, the probes are 12–15 billion miles from Earth, still operable (despite experiencing recent communication difficulties), and sailing through the relative calm of interstellar space. They are expected to continue to transmit data back to Earth for another year or so, or until their plutonium batteries quit. 

It was early 20th century wireless inventor Guglielmo Marconi who suggested that radio signals never die, they only diminish as they travel across space and time. Even after communications from the Voyager spacecraft cease, perhaps the tiny voices of Earth’s first emissaries, animated by NASA’s master toolmakers nearly half a century ago, will continue to drift through the cosmos for all time, accessible to far-flung civilizations equipped with sensitive enough receivers to listen.

“Voyager’s Triumph” (Carl Sagan, October 1986)

A noted scientist tells the little-known story of the remarkable feats of the Voyager engineers, a dedicated band who repeatedly overcame technical adversity to ensure the success of these historic expeditions to the outer solar system.

Carl Sagan is Director, Laboratory for Planetary Studies, Cornell University, and, since 1970, a member of the Voy­ager Imaging Science Team. His Cosmos: A Special Edition is televised this fall. 

On Jan. 25, 1986, the Voyager 2 robot probe entered the Uranus system and reported a procession of wonders. The encounter lasted only a few hours, but the data faithfully relayed back to Earth have revolu­tionized our knowledge of the aquamarine planet, its more than 15 moons, its pitch black rings, and its belt of trapped high-energy charged particles. Voyager 2 and its compan­ion, Voyager 1, have done this before. At Jupiter, in 1979, they braved a dose of trapped charged particles 1,000 times what it takes to kill a human being [PS, July ’79); and in all that radiation they discovered the rings of the largest planet, the first active volcanoes outside Earth, and a pos­sible underground ocean on an airless world—among a few hundred other major findings. At Saturn, in 1980 and 1981, the two spacecraft survived a pummeling by tiny icy particles as they plummeted through previously un­ known rings; and there they discovered not a few, but thou­ sands of Saturnian rings, icy moons recently melted through unknown causes, and a large world with an ocean of liquid hydrocarbons surmounted by clouds of organic matter IPS, March ’81 l. These spacecraft have returned to Earth four trillion bits of information, the equivalent of about 100,000 encyclopedia volumes. 

Because we are stuck on Earth, we are forced to peer at distant worlds through an ocean of distorting air. It is easy to see why our spacecraft have revolutionized the study of the solar system: We ascend to the stark clarity of the vacuum of space, and there approach our objectives, flying past them or orbiting them or landing on their surfaces. These nearby worlds have much to teach us about our own, and they will be—unless we are so foolish as to destroy ourselves—as familiar to our descendents as the neighboring states are to those who live in America today. 

Voyager and its brethren are prodigies of human inven­tiveness. Just before Voyager 2 was to encounter the Uranus system, the mission design had scheduled a final course correction, a short firing of the on-board propul­sion system to position Voyager correctly as it flew among the moving moons. But the course correction proved un­necessary. The spacecraft was already within 200 kilome­ters of its designed trajectory after a voyage along an arcing path five billion kilometers in length. This is roughly the equivalent of throwing a pin through the eye of a needle 50 kilometers away, or firing your target pistol in New York and hitting the bull’s eye in Dallas.

The lodes of planetary treasure were transmitted back to Earth by the radio antenna aboard Voyager; but Earth is so far away that by the time the signal was gathered in by radiotelescopes on our planet, the received power was only 10-16 watts (fifteen zeros after the decimal point). Comparing this weak signal with the power emitted by an ordinary reading lamp is like comparing the width of an atom with the distance between Earth and the moon. (Incidentally, the first photograph ever taken of Earth and the moon together in space was acquired by one of the Voyager spacecraft.)

We tend to hear much about the splendors returned, and very little about the ships that brought them, or the shipwrights. It has always been that way. Our history books do not tell us much about the builders of the Nina, Pinta, and Santa Maria, or even the principle of the caravel. De­spite ample precedent, it is a clear injustice: The Voyager engineering team and its accomplishments deserve to be much more widely known.

The Voyager spacecraft were designed and assembled, and are operated by the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration in Pasadena, Calif. The mission was conceived during the late 1960s, first funded in 1972, but was not approved in its present form (which includes encounters at Uranus and Neptune) until after the 1979 Jupiter flyby. The two spacecraft were launched in late summer and early fall 1977 by a non-reusable Titan/Centaur booster configuration at Cape Canaveral, Fla. Weighing about a ton, a Voyager would fill a good-sized living room. Each spacecraft draws about 400 watts of power—considerably less than an average American home—from a generator that converts radioactive plutonium into electricity. The instrument that measures interplanetary magnetic fields is so sensitive that the flow of electricity through the innards of the spacecraft would generate spurious signals. As a result, this instrument is placed at the end of a long boom stretching out from the spacecraft. With other projections, it gives Voyager a slightly porcupine appearance. Two cameras, infrared and ultraviolet spectrometers, and an instrument called the photopolarimeter are on a scan platform; the platform swivels so these instruments can point toward a target world. The spacecraft antenna must know where Earth is if the transmitted data are to be received back home. The spacecraft also needs to know where the sun is and at least one bright star, so it can orient itself in three dimensions and point properly toward any passing world. It does no good to be able to return pictures over billions of miles if you can’t point the camera.

On-orbit repairs

Each spacecraft costs about as much as a single modern strategic bomber. But unlike bombers, Voyager cannot, once launched, be returned to the hangar for repairs.

As a result, the spacecraft’s computers and electronics are designed redundantly. And when Voyager finds itself in trouble, the computers use branched contingency tree logic to work out the appropriate course of action. As the spacecraft journeys increasingly far from Earth, the round-trip light (and radio) travel time also increases, approaching six hours by the time Voyager is at the distance of Uranus.

Thus, in case of an emergency, the spacecraft needs to know how to put itself in a safe standby mode while awaiting instructions from Earth. As the spacecraft ages, more and more failures are expected, both in its mechanical parts and its computer system, although there is as yet no sign of a serious memory deterioration, some robot Alzheimer’s disease. When an unexpected failure occurs, special teams of engineers—some of whom have been with the Voyager program since its inception—are assigned to “work” the problem. They will study the underlying basic science and draw upon their previous experience with the failed subsystems. They may do experiments with identical Voyager spacecraft equipment that was never launched or even manufacture a large number of components of the sort that failed in order to gain some statistical understanding of the failure mode.

In April 1978, almost eight months after launch, an omitted ground command caused Voyager 2’s on-board computer to switch from the prime radio receiver to its backup.

During the next ground transmission to the spacecraft, the receiver refused to lock onto the signal from Earth. A component called a tracking loop capacitor had failed. After seven days in which Voyager 2 was out of contact, its fault protection software commanded the backup receiver to be switched off and the prime receiver to be switched back on. But, mysteriously, the prime receiver failed moments later: It never recovered. Voyager 2 was now fundamentally imperiled. Although the primary receiver had failed, the on-board computer commanded the spacecraft to use it. There was no way for the controllers on Earth to command Voyager to revert to the backup receiver. Even worse, the backup receiver would be unable to receive the commands from Earth because of the failed capacitor. Finally, after a week of command silence, the computer was programmed to switch automatically between receivers.

And during that week’s time the JPL engineers designed an innovative command frequency control procedure to make a few essential commands comprehensible to the damaged backup receiver.

This meant the engineers were able to communicate, at least a little bit, with the spacecraft. Unfortunately the backup receiver now turned giddy, becoming extremely sensitive to the stray heat dumped when various components of the spacecraft were powered up or down. Over the following months the JPL engineers designed and conducted a series of tests that let them thoroughly understand the thermal consequences of most operational modes of the spacecraft on its ability to receive commands from Earth. The backup-receiver problem was entirely circumvented. It was this backup receiver that acquired all the commands from Earth on how to gather data in the Jupiter, Saturn, and Uranus systems. The engineers had saved the mission. (But to be on the safe side, during most of Voyager’s subsequent flight there is in residence in the onboard computers a nominal data-taking sequence for the next planet to be encountered.)

Another heart-wrenching failure occurred just after Voyager 2 emerged from behind Saturn after its closest approach to the planet in August 1981. The scan platform had been moving rapidly in the azimuth direction—quickly pointing here and there among the rings, moons, and the planet itself during the time of closest approach. Suddenly, the platform jammed. A stuck scan platform obviously implies a severe reduction in future pictures and other key data. The scan platform is driven by gear trains called actuators, so first the JPL engineers ran an identical copy of the flight actuator in a simulated mission. The ground actuator failed after 348 revolutions: the actuator on the spacecraft had failed after 352 revolutions. The problem turned out to be a lubrication failure. Plainly, it would be impossible to overtake Voyager with an oil can. The engineers wondered whether it would be possible to restart the failed actuator by alternately heating and cooling it, so that the thermal stresses would cause the components of the actuator to expand and contract at different rates and un-jam the system. After gaining experience with specially manufactured actuators on the ground, the engineers jubilantly found that they were able to use this procedure to start the scan platform up again in space. More than this, they devised techniques to diagnose any imminent actuator failure early enough to work around the problem. Voyager 2’s scan platform worked perfectly in the Uranus system. The engineers had saved the day again.

Ingenious solutions

Voyager 1 and 2 were designed to explore the Jupiter and Saturn systems only. It is true that their trajectories would carry them to Uranus and Neptune, but officially these planets were never contemplated as targets for Voyager exploration: The spacecraft was not supposed to last that long. Because of trajectory requirements in the Saturn system, Voyager 1 was flung on a path that will never encounter any other known world; but Voyager 2 flew to Uranus with brilliant success, and is now on its way to an August 1989 encounter with the Neptune system.

At these immense distances, sunlight is getting progressively dimmer, and the spacecraft’s transmitted radio signals to Earth are getting progressively fainter. These were predictable but still very serious problems that the JPL engineers and scientists also had to solve before the encounter with Uranus.

Because of the low light levels at Uranus, the Voyager television cameras were obliged to take longer time exposures. But the spacecraft was hurtling through the Uranus system so fast (about 35,000 miles per hour) that the image would have been smeared or blurred—an experience shared by many amateur photographers. To overcome this, the entire spacecraft had to be moved during the time exposures to compensate for the motion, like panning in the direction opposite yours while taking a photograph of a street scene from a moving car. This may sound easier than it is: You have to compensate for the most casual of motions. At zero gravity, the mere start and stop of the on-board tape recorder that’s registering the image can jiggle the spacecraft enough to smear the picture. This problem was solved by commanding the spacecraft thrusters, instruments of exquisite sensitivity, to compensate for the tape-recorder jiggle at the start and stop of each sequence by turning the entire spacecraft just a little. To compensate for the low received radio power at Earth, a new and more efficient digital encoding algorithm was designed for the cameras, and the radiotelescopes on Earth were joined together with oth ers to increase their sensitivity. Overall, the imaging system worked, by many criteria, better at Uranus than it did at Saturn or even at Jupiter.

Voyager has become a new kind of intelligent being—part robot, part human. It extends the human senses to far-off worlds.

The ingenuity of the JPL engineers is growing faster than the spacecraft is deteriorating. And Voyager may not be done exploring after its Neptune encounter.

There is, of course, a chance that some vital subsystem will fail tomorrow, but in terms of the radioactive decay of the plutonium power source, the two Voyager spacecraft will be able to return data to Earth until roughly the year 2015. By then they will have traveled more than a hundred times Earth’s distance from the sun, and may have penetrated the heliopause, the place where the interplanetary magnetic field and charged particles are replaced by their interstellar counterparts; the heliopause is one definition of the frontier of the solar system.

Robot-human partnerships

These engineers are heroes of our time. And yet almost no one knows their names. I have attached a table giving the names of a few of the JPL engineers who played central roles in the success of the Voyager missions.

In a society truly concerned for its future, Don Gray, Charlie Kohlhase, or Howard Marderness, would be as well known for their extraordinary abilities and accomplishments as Dwight Gooden, Wayne Gretzky, or Kareem Abdul Jabbar are for theirs.

Voyager has become a new kind of intelligent being-part robot, part human. It extends the human senses to far-off worlds. For simple tasks and short-term problems, it relies on its own intelligence; but for more complex tasks and longer term problems, it turns to another, considerably larger brain—the collective intelligence and experience of the JPL engineers. This trend is sure to grow. The Voyagers embody the technology of the early 1970s; if such spacecraft were to be designed in the near future, they would incorporate stunning improvements in artificial intelligence, in data-processing speed, in the ability to self-diagnose and repair, and in the capacity for the spacecraft to learn from experience. In the many environments too dangerous for people, the future belongs to robot-human partnerships that will recognize Voyager as antecedent and pioneer.

Unlike what seems to be the norm in the so-called defense industry, the Voyager spacecraft came in at cost, on time, and vastly exceeding both their design specifications and the fondest dreams of their builders. These machines do not seek to control, threaten, wound, or destroy; they represent the exploratory part of our nature, set free to roam the solar system and beyond.

Once out of the solar system, the surfaces of the spacecraft will main intact for a billion years or more, as the Voyagers circumnavigate the center of the Milky Way galaxy.

This kind of technology, its findings freely revealed to all humans everywhere, is one of the few activities of the United States admired as much by those who find our policies uncongenial as by those who agree with us on every issue. Unfortunately, the tragedy of the space shuttle Challenger implies agonizing delays in the launch of Voyager’s successor missions, such as the Galileo Jupiter orbiter and entry probe. Without real support from Congress and the White House, and a clear long-term NASA goal, NASA scientists and engineers will be forced to find other work, and the historic American triumphs in solar-system exploration—symbolized by Voyager—will become a thing of the past. Missions to the planets are one of those things—and I mean this for the entire human species—that we do best. We are tool makers—this is a fundamental aspect, and perhaps the essence, of being human.

Greeting the aliens

Both Voyager spacecraft are on escape trajectories from the solar system. The gravitational fields of Jupiter, Saturn, and Uranus have flung them at such high velocities that they are destined ultimately to leave the solar system altogether and wander for ages in the calm, cold blackness of interstellar space—where, it turns out, there is essentially no erosion.

Once out of the solar system, the surfaces of the spacecraft will main intact for a billion years or more, as the Voyagers circumnavigate the center of the Milky Way galaxy. We do not know whether there are other space-faring civilizations in the Milky Way. And if they do exist, we do not know how abundant they are.

But there is at least a chance that some time in the remote future one of the Voyagers will be intercepted by an alien craft. Voyagers 1 and 2 are the fastest spacecraft ever launched by humans; but even so, they are traveling so slowly that it will be tens of thousands of years before they go the distance to the nearest star. And they are not headed toward any of the nearby stars. As a result there could be no danger of Voyager attracting “hostile” aliens to Earth, at least not any time soon.

So, it seemed appropriate to include some message of greeting from Earth At NASA’s request, a committee I chaired designed a phonograph record that was affixed to the outside of each of the Voyager spacecraft. The records contain 116 pictures in digital form, describing our science and technology, our institutions, and ourselves; what will surely be unintelligible greetings in many languages; a sound essay on the evolution of our planet; and an hour and a half of the world’s greatest music. But the hypothetical aliens are bound to be very different from us—independently evolved on another world. Are we really sure they could understand our message? Every time I feel these concerns stirring, though, I reassure myself: Whatever the incomprehensibilities of the Voyager record, any extraterrestrial that finds it will have another standard by which to judge us.

Each Voyager is itself a message. In its exploratory intent, in the lofty ambition of its objectives, and in the brilliance of its design and performance, it speaks eloquently for us.