Edward Stone in front of a model of Voyager. He directed the Jet Propulsion Laboratory from 1991 to 2001. Stone launched his physics career at the dawn of the Space Age, turning his attention to the cosmos after the Soviet Union launched Sputnik. NASA/Jet Propulsion Laboratory

Edward C. Stone, who opened a window on the farthest reaches of the solar system while serving as the chief scientist of NASA’s Voyager mission, supervising a pair of spindly, plutonium-powered spacecraft that continue to operate billions of miles from Earth, died June 9 at his home in Pasadena, Calif. He was 88.

His death was announced by the California Institute of Technology, where he was a professor emeritus of physics, and by NASA’s Jet Propulsion Laboratory, which he directed for 10 years beginning in 1991. His daughter Susan Stone said he had been in declining health but that the cause of death was not yet known.

Stone launched his physics career at the dawn of the Space Age, turning his attention to the cosmos after the Soviet Union launched Sputnik – a shiny metal ball that became the world’s first artificial satellite – when he was a graduate student at the University of Chicago in 1957.

Over the next six decades, he designed some of the first scientific instruments for U.S. satellites; oversaw the construction of the W.M. Keck Observatory, which boasted the world’s two largest optical telescopes when it was completed in Hawaii in the mid-1990s; and spearheaded the establishment of LIGO, a billion-dollar physics experiment that in 2015 made the first direct observations of gravitational waves, ripples in space time that had eluded scientists for years.

He remained best known for serving as project scientist – and, less officially, chief spokesman – for Voyager 1 and 2. Launched two weeks apart in 1977, five years after Stone was hired for the mission, the twin probes have brought back mesmerizing photos of the giant outer planets and their moons, as well as a wealth of data about the solar system.

“We were on a mission of discovery,” he told the New York Times in 2002, looking back on the project’s origins. “But we didn’t appreciate how much discovery there would be.”

Both spacecraft visited Jupiter and Saturn, with Voyager 2 continuing to Uranus and Neptune, aided by a rare alignment of the outer planets that happens once every 176 years. The one-ton probes are now traveling through interstellar space, farther than any other human-made objects in the universe. Along with cameras and scientific instruments, they each carry a celestial message-in-a-bottle: a gold-plated record, devised with help from astronomer Carl Sagan, bearing sounds and images that would introduce potential extraterrestrials to the diversity of life on Earth.

“It was a wonderful notion,” Stone told the Los Angeles Times in 2011, reflecting on the record’s inclusion as Voyager 1 prepared to enter interstellar space. “At the time, though, just making it to Saturn was what I focused on.”

Beginning in 1979, the probes took the first close-up images of Europa, one of Jupiter’s moons, revealing the cracked, fractured surface of a frozen world that “looked like an ice pack,” as Stone put it. They studied Saturn’s vast ring system; found evidence of a thick atmosphere rich in organic compounds on Saturn’s moon Titan; tracked 1,000 mph winds gusting on the surface of Neptune; and discovered five-mile-tall geysers erupting from the icy surface of Neptune’s largest moon, Triton.

One of the mission’s most striking early findings was the revelation of volcanic activity on Jupiter’s moon Io. It was the first time active, ash-spewing volcanoes had been discovered outside Earth, and it surprised scientists who assumed that the moon would be much like the Earth’s – inert, cratered, cold and dead.

“Time after time, we just found that nature was much more inventive than our models,” Stone told a Caltech interviewer.

Edward Stone with a student in 1996. California Institute of Technology Archives

As Voyager passed the outer planets, Stone appeared on the nightly news and gave frequent interviews. While overseeing 11 investigative teams and some 200 researchers, he was credited with speeding up the pace by which the team’s scientists announced their findings, leading daily meetings in which he sought to identify the group’s most fascinating findings, then working with researchers to help make the material accessible for a general audience.

“He was like this machine,” his onetime boss Norman Haynes, who served for three years as Voyager’s overall project director, told the New York Times in 1990. “You’d wind him up and, zoom! He went zipping around all day getting things done.”

Astronomer Bradford A. Smith, who led the team that interpreted Voyager’s photos, told the newspaper in 2002 that the flood of images and data sent back by the probes made Voyager “the most successful mission that NASA has carried out” – praise that was echoed by plenty of scientists over the years.

“What we know of the outer planets is a direct result of Ed Stone’s contribution,” A. Thomas Young, the former director of NASA’s Goddard Space Flight Center, once said. “He was one of two or three people that made Voyager tick.”

The success of Voyager helped launch Stone to wider prominence, leading to his appointment as head of the Jet Propulsion Laboratory, or JPL, a storied planetary science center managed for NASA by Caltech. The lab faced budget cuts in the aftermath of the Cold War, although Stone still managed to work on high-profile missions that included Mars Pathfinder, which landed the Sojourner rover on Mars in 1997; the spacecraft Galileo, which orbited Jupiter for eight years; and Cassini, which orbited Saturn for 13 years.

A tribute from the lab noted that Stone was the rare scientist involved with the mission that traveled farthest from the sun – Voyager – as well as the mission that has come closest to the sun: the Parker Solar Probe, which flew through the corona, the sun’s upper atmosphere, in 2021.

“I keep asking myself why is there so much public interest in space,” Stone told the New York Times before taking over at JPL. “It is, after all, just basic science in the end. The answer is that it provides us with a sense of the future. When we stop discovering new things out there, the concept of the future will change. Space reminds us that there is something left to be done, that life will continue to evolve. It gives us direction, an arrow in time.”

The oldest of two sons, Edward Carroll Stone Jr. was born in Knoxville, Iowa, on Jan. 23, 1936. He grew up in Burlington, Iowa, where his father ran a small construction company that his mother helped manage. His parents supported his early fascination with science, including his efforts to take apart his transistor radio and put it back together again.

“I was always interested in learning about why something is this way and not that way,” Stone recalled. “I wanted to understand and measure and observe.”

After graduating from Burlington Junior College (now Southeastern Community College) in 1956, he enrolled at the University of Chicago, receiving a master’s degree in 1959 and a doctorate in physics in 1964. By then he had married Alice Wickliffe, a fellow UChicago student. She died in December. Survivors include their two daughters, Susan and Janet Stone, and two grandsons.

With his PhD in hand, Stone joined one of his former UChicago colleagues, Rochus “Robbie” Vogt, in helping launch a space physics program at Caltech. He was named a full professor in 1976 and chaired the university’s physics, math and astronomy division in the mid-1980s, around the same time he began work on the Keck, a complex of twin 10-meter telescopes near the summit of Mauna Kea in Hawaii.

His work on the project led him to champion the proposed Thirty Meter Telescope, an even larger observatory that scientists hoped to build nearby. Construction has been halted amid protests by Native Hawaiians and other critics who oppose development on the site.

Colleagues described Stone as shy and single-minded, with few interests outside physics. “My job is my relaxation,” he liked to say. He continued to work on Voyager for decades, juggling teaching and research duties while collecting honors that included the National Medal of Science in 1991 and the Shaw Prize in Astronomy in 2019, before retiring from the mission in 2022.

By then, the probes had traveled far beyond the orbits of Neptune and Pluto. Voyager 1, the more distant of the two, is now more than 15 billion miles from Earth, still operational even as engineers have had to come up with workarounds for malfunctioning computer chips and other communication issues. The spacecraft and its twin will eventually run out of power, although Stone proudly noted that the probes will “just keep going forever,” drifting through the cosmos with their golden cargo and silent instruments.

“As far as what will happen to me, nature will have its way, I understand,” he told the Los Angeles Times in 2011. “Even if I am not there we will keep exploring, keep figuring out the science. I’m optimistic about this.”