Steve Synnott never forgot the day Ed Stone let him name a moon. It was 1980 and Synnott was a member of the navigation team for the Voyager 1 and Voyager 2 spacecraft, which had just reconnoitered Jupiter. Stone was the Voyager project scientist: NASA-speak for head of the program. During the Voyagers’ close pass of the Jovian system, one of the ships captured an image—and then several images—of a small object zipping around the giant planet at a speed that saw it completing more than one revolution every Earth day. Its size and velocity and altitude could only mean it was a moon.
Even so momentous a discovery did not mean that the likes of Synnott had leave simply to present himself at Stone’s office, so the young engineer waited until the project chief made one of his frequent walking tours of the Voyager bullpens, then approached him and showed him a letter he was planning to send the International Astronomical Union (IAU)—which catalogs new space objects and approves the name that an object will bear. Synnott handed Stone the one-paragraph communication and waited while the senior scientist read it.
“Do you know its orbital period?” Stone asked when he was done, according to a conversation I had with Synnott when I was writing the book Journey Beyond Selene.
“About 18 hours,” Synnott answered, handing Stone a page of calculations.
“Its size?”
“About 60 miles.”
“Altitude?”
“One hundred thirty-eight thousand miles.”
Stone re-read the letter and then re-scanned the calculations. “Well,” he said at last, with a smile, “it looks like you’ve found yourself a moon.”
Synnott beamed back, sent his letter to the IAU, and at length got a response, which included a list of mythological names he could choose for the moon. He settled on Thebe, a nymph of the Greek god Zeus and the Roman god Jupiter, and with that, the solar system got just a tiny bit bigger.
Stone—who died on June 9, 2024, at age 88 of undisclosed causes, after half a century as head of the Voyager program—could afford to be so generous with his moons. His Voyagers would ultimately discover 48 of them, orbiting the four gas giants—Jupiter, Saturn, Uranus, and Neptune—as well as previously unknown rings or partial rings around Jupiter, Uranus, and Neptune, and volcanoes on the Jovian moon Io. The Voyagers were launched in 1977, and are currently beyond the limits of the solar system itself, traveling in interstellar space—still doing science, still beaming back data, having outlived the man who midwifed them and flew them and saw them through most of their great campaign, until his retirement in 2022.
“Ed Stone was a trailblazer who dared mighty things in space. He was a dear friend to all who knew him, and a cherished mentor to me personally,” said Nicola Fox, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington, in an official statement. “Ed took humanity on a planetary tour of our solar system and beyond, sending NASA where no spacecraft had gone before.”
It was in 1966 that NASA astronomers, studying the orbits of the four outer planets, discovered that 13 years later, in 1979, the worlds would form a tidy alignment, falling into a once-every-176-years parade that would allow a single ship—or, better still, a pair of ships—to visit them all in one go. That gave the space agency 11 years to invent and build and launch the ships—to say nothing of securing approval and funding for them in the first place. For the first six years of the project, things moved along only fitfully, and so in 1972, NASA’s Jet Propulsion Laboratory (JPL), in Pasadena Calif., which was overseeing the mission, turned the Voyager reins over to Stone, then a 36-year-old physicist. It was both a smart choice and a calculated gamble.
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Stone had joined Caltech, which co-manages JPL along with NASA, in 1964, studying space radiation. He worked on several NASA satellite missions, but had not yet held a leadership post. The NASA brass recognized his native smarts, however; even before joining Caltech, he collaborated with the Department of Defense to design a spy satellite that both photographed the Earth and, as a research bonus, measured the solar wind—the stream of charged particles flowing out from the sun—helping to determine why photographic film aboard spacecraft was forever being fogged by the energetic storms. That kind of talent was just what was needed for Voyager, but whether Stone had the leadership chops to run the program was unknown. It turned out that he did.
Stone helped to secure the funding and drive the engineering for the Voyager project, not least by repeatedly making the point to both lawmakers and engineers that if NASA did not take advantage of the planetary alignment now, it would have to wait until 2153 for its next shot. Ultimately, both spacecraft would take off on time, with Voyager 2 leaving the Florida launch pad first, on Aug. 20, 1977, and Voyager 1—which was to fly slightly faster and on a slightly shorter trajectory and thus reach Jupiter first—on Sept. 5, 1977.
Even then, there was no guarantee that NASA’s budget would support a visit to all four planets over the course of more than 10 years, and, officially, Jupiter and Saturn were the only worlds on the itinerary for both spacecraft. That being the case, Stone made the decision to, effectively, throw one of his ships away. When Voyager 1 reached Saturn, he changed its trajectory so that it would swing under the ringed planet and then fly upward, putting it on course to make a close flyby of Saturn’s giant moon Titan, a world covered in a thick haze of organic methane and ethane that had long fascinated scientists. But once committed to that route, the spacecraft would not have enough on-board fuel to reverse course, and it would thus soar up and out of the plane of the solar system.
Voyager 2, which also passed Jupiter and Saturn, would keep flying in the flat, able to make close approaches to Uranus and Neptune if the will and wallet were there to allow the missions. While Stone nursed his spacecraft along, NASA brass husbanded their budget, ultimately winning funding to keep Voyager 2 flying. On January 28, 1986—poignantly, the same day the shuttle Challenger exploded—Voyager 2 passed Uranus, studying the planet’s largest moons, while discovering 11 new ones, and mapping its tenuous rings. On August 25, 1989, the ship flew by Neptune, discovering two new moons, five fine rings, and an Earth-sized bruise in the atmosphere, known as the Great Dark Spot—a mammoth storm, where winds reach 1,000 miles per hour. It also discovered icy geysers on the Neptunian moon Triton. Voyager 2 remains the only ship to visit those two worlds.
Even then, the Voyagers weren’t finished—and neither was Stone. The spacecraft are powered by radiothermal generators, able to provide energy for 50 years or more, and, though transmitting back to Earth with a signal that has less wattage than a refrigerator bulb, they could continue their work, speeding to the edge of the solar system—and then exiting. Voyager 1 entered interstellar space on Aug. 25, 2012, and is now more than 15 billion mi. (24 billion km) from Earth. Voyager 2 left the solar system on Nov. 5, 2018, and is more than 12.5 billion mi. (20 billion km) away. Both craft continue to whisper hoarsely back to us.
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Stone would be distinguished by more than just the Voyagers. He was director of JPL from 1991 to 2001, and was at the helm when the Sojourner spacecraft—the first Mars rover—landed on the Red Planet in 1997. Overall, he was principal investigator on nine NASA missions and co-investigator on five others.
It is the Voyagers, however, for which he is best known. The ships famously carry golden records—created by another lost space legend, Carl Sagan. If an alien civilization ever found the spacecraft and played the records on a simple turntable—the state of the Earthly art at the time the ships were launched—they would see 119 pictures of our planet, as well as hearing greetings in 55 languages, and 27 selections of music, including Javanese, Japanese, Chinese, and Peruvian music; samplings of Bach, Mozart, and Beethoven; as well as “Johnny B. Goode,” by Chuck Berry and “Melancholy Blues,” by Louie Armstrong and his Hot Seven Band.
In 1978, when the Voyagers were still new and Stone was still relatively young, Saturday Night Live announced that an alien civilization had intercepted the ships, played the records, and sent back a four-word message—one that would appear on a mock-up of that week’s cover of TIME magazine, which host Steve Martin displayed. The four words were: “Send More Chuck Berry.”
History does not record if Ed Stone was watching that night, but, in the fullness of time, he likely saw the sketch and laughed. And then he returned to the job. The Voyagers were still flying, which meant he was still working. He kept at it for all but the last two years of his life. Now, his ships—interstellar emissaries of the human species—sail on without him. Godspeed, Ed Stone.
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