The planets were aligning and Ed Stone was the man hired by Nasa to make the most of a one-in-176-year opportunity.
An intern at the American space agency calculated that a rare orientation meant a spacecraft launched in about 1977 could make a “Grand Tour” of all four giant planets — Jupiter, Saturn, Uranus and Neptune — over little more than a decade by using their gravitational pulls as a slingshot.
To run the ambitious scheme, Nasa turned to Stone, a wiry, energetic and serious-minded 36-year-old expert in cosmic rays and space probe instrumentation who would prove a gifted project manager.
Appointed project scientist in 1972, Stone spent 50 years overseeing a mission that initially cost $865 million (the equivalent of about £3.5 billion today) and saw 1,500 engineers construct twin spacecraft that at launch in 1977 were the weight and size of a small car. The Voyager probes have travelled billions of miles at speeds in excess of 35,000 miles per hour yet their transmitters would be barely strong enough to power a lightbulb in a household refrigerator.
Stone had a scientist’s detached and rational eye when it came to sending machines rather than people into space. “The complexity of the system is magnified greatly when you have to support human life. There is so much science you can do at a much lower cost,” he pointed out to a reporter. But he also recognised the value of the human touch, especially for an organisation seeking to maintain public enthusiasm for space exploration while enduring budget cuts as euphoria waned following Neil Armstrong’s “giant leap”.
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The Voyager 1 and 2 probes were, as Stone put it, “Earth’s ambassadors to the stars”. Each was loaded with a time capsule: 12-inch “Golden Records” with pictures, music and sounds featuring greetings to the universe in 55 languages, including Welsh.
Stone co-ordinated 11 teams of scientists who rushed to interpret the daily data. Often working late into the night, he selected key findings and presented them in a manner digestible for a mainstream audience at press conferences, acting as the straight man while the astronomer Brad Smith (obituary, August 6, 2018) compared moons of Jupiter and Saturn to pizzas and hamburgers.
Astronomers were astounded by the lively signals and colourful images phoned home by the probes. They found active volcanoes on Io, a moon of Jupiter, and learnt new details about the turbulent atmosphere of Jupiter, the cracked icy shell of its moon, Europa, and the ring structure of Saturn. A storm as big as Earth raged on Neptune with winds of 1,500mph, while geysers shot dark plumes five miles high on Triton, its largest moon. A famous photograph of Earth taken in 1990 at a distance of 3.7 billion miles became known as the Pale Blue Dot.
“I keep asking myself, why is there so much public interest in space?” Stone mused to The New York Times in 1990. “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 older of two sons, Edward Carroll Stone was born in Knoxville, Iowa, in 1936, and grew up in Burlington on the Illinois border. His father, Edward Sr, was a construction superintendent and his mother, Ferne (née Baber), managed the family’s garage door business. Fascinated by the technology of the Atomic Age, as a boy he devoured science magazines, built radios and played the French horn in the town band.
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After an undergraduate degree in physics from a college in Burlington he earned a doctorate in physics at the University of Chicago. Eager to work in space exploration after the Soviet Union launched Sputnik in 1957, Stone designed science instruments for space missions and joined the California Institute of Technology (Caltech) in 1964. He became a professor and worked with the Caltech-run Nasa Jet Propulsion Laboratory (JPL) near Los Angeles.
Originally due to visit only two planets over five years to appease cost-conscious politicians, the Voyagers proved vastly more durable and useful. They are still operating but may finally run out of power in about 2025.
Like his probes, Stone endured. He was the Voyager project chief scientist from 1972 until his retirement in 2022. Wrapped up in his work and not one for small talk, he often went home for dinner then returned to the office.
Stone was named director of the JPL in 1991, guiding projects including Mars Pathfinder, which landed the first Mars rover. After leaving the JPL in 2001 he worked full-time at Caltech. Earlier he oversaw construction of an observatory on the summit of Mauna Kea, a Hawaiian volcano, and the establishment of the Laser Interferometer Gravitational-Wave Observatory, which in 2015 made the first direct detection of ripples in the fabric of space and time, confirming a prediction of Einstein’s general theory of relativity.
He met his wife, Alice (née Wickliffe), on a blind date while they were students in Chicago and they married in 1962. A writer, she died in December 2023. Their daughters, Susan and Janet, survive them.
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In 2019 he won the Shaw Prize in Astronomy, which carries an award of $1.2 million, for his contribution to Voyager. Another gift he greatly appreciated was that he was still alive when the probes crossed the heliopause, in effect the boundary of the solar system. In 2012 Voyager 1, which is presently more than 15 billion miles from our planet, became the first human-made object to enter interstellar space. Voyager 2 followed suit six years later, when Stone was 82.
They were climactic achievements in a career founded on precise computation and wide-eyed wonder. “You learn by observing and calculating,” Stone reflected, “and each time you do, you find that nature’s even more powerful than you could imagine.”
Ed Stone, Voyager project leader, was born on January 23, 1936. He died after a period of ill health on June 9, 2024, aged 88
