Ed Stone is the kind of guy who can glimpse the outer limits of human exploration just by looking at a water faucet.

We are huddled in his minimalist Caltech office, discussing his four decades of work on NASA’s twin Voyager space probes. Suddenly he decides that our conversation is getting too abstract. In the summer of 2012, Voyager 1 became the first probe to enter interstellar space, and Stone wants me (and you) to really feel what that means. So off we go to a small kitchenette down the hall for a hands-on science explainer.

Everything you need to know about the journey to interstellar space is right there in the sink, Stone explains. He pushes the handle, and a fat stream of water hits the sink’s bowl, creating a circular splash. The area within is like the space around Earth and the other planets: a region flooded by the solar wind, a fast-moving outward stream of particles from the sun. The area at the periphery of the sink is like interstellar space, where relatively cold gas between the stars sloshes weakly inward toward us. And in between?

Ed Stone teaches a class at Caltech in 1972, the same year he became project scientist for the planet-hopping Voyager mission. | Courtesy of the Archives/California Institute of Technology

“Notice how a thick ring forms here? That’s the termination shock,” Stone says excitedly. His finger slowly traces the circle in the sink. The edge between two competing flows of water — one pushed outward by the faucet’s stream and one flowing around the ring toward the drain — mirrors the sharp boundary between the solar wind and the interstellar stream. As in the sink, the boundary takes the form of a shock wave, a dramatic disruption that divides inside from outside (though it spreads in three dimensions, making it into a bubble rather than a ring). That is how Stone knew when Voyager 1 crossed into interstellar space: It passed through a barrier of turbulent particles, like a tiny boat navigating the sink bowl, traversing from inside the circle to outside.

Everywhere Stone looks, he sees frontiers. Over a career that spans the entire Space Age, he has designed some of the first scientific instruments for satellites, directed searches for life-supporting conditions on Mars and above all managed the science team of the Voyager missions that explored Jupiter, Saturn, Uranus and Neptune along with their moons — the single greatest expedition ever undertaken. Now he has transformed Homo sapiens into an interstellar species, and at age 80 he has earned a moment to reflect on what he has done.

“There’s a historic aspect of these journeys, very much like the classic journeys of exploration around the first circumnavigation of the Earth,” Stone says. “It has that same sort of character to it: going the farthest of anything that’s ever gone, reaching a region where we’ve never been before.” But his work is far from over. Really, it never will be.

Timing is Everything

Stone’s hands-on, kitchen-sink approach to exploration is deeply ingrained. As a kid in WWII-era Burlington, Iowa, he played with a crystal radio set and tinkered with hi-fi amps. “In those days, that’s what you did. Radios were the modern technology,” he chuckles. Later he attended the University of Chicago, where he was drawn to nuclear physics — the cutting-edge science of the day — until even newer possibilities intervened.

“I was on my way back to the fall term when I saw the headline that the Soviet Union had launched Sputnik,” he says. That was October 1957. By the next summer he was developing ways to study cosmic rays, enigmatic particles that originate from outer space. Space physics was a wide-open field back then, and Stone’s deep familiarity with electronic devices proved a valuable skill. By 1961, he was designing his own cosmic ray detector for the U.S. government. Three years later, he moved to Caltech and co-founded the Space Radiation Lab, where he began some of the earliest studies of solar energetic particles.

[This article originally appeared in print as "Interstellar Man."]

Stone had come of age just as NASA was growing rapidly. It had come of age at just the right time, too. In the late 1960s, a group of scientists at the Jet Propulsion Laboratory (a NASA center operated by Caltech) realized that Jupiter, Saturn, Uranus and Neptune were about to fall into a special kind of staggered lineup. A spacecraft could take advantage of the alignment to travel efficiently to all four planets, with each providing a gravitational boost in speed. Such a planetary configuration happens just once every 176 years. In 1972, President Richard Nixon approved sending a pair of identical, redundant spacecraft, Voyagers 1 and 2, to seize the opportunity. Stone became the mission’s project scientist, leader of 11 science teams. He was just 36, but he had already built and launched instruments on five different satellites and had no qualms about this huge new responsibility.

Voyager 2 launched on Aug. 20, 1977, and Voyager 1 followed 16 days later, streaking skyward as Stone watched from the control room at Cape Canaveral. (Voyager 1 earned its name because its shorter, faster trajectory meant it would be the first to arrive at Jupiter.)

The twin probes were unlike anything fl own before. Each carried 10 separate science instruments, including cameras and devices to measure magnetism and charged particles, along with a state-of-the-art eight-track-tape data recorder and a long-lasting radioactive generator. “These were also the first automated spacecraft that flew themselves,” Stone says with justifiable pride. They could take measurements, collect images and even make specific course corrections without instructions from the ground — all with just 1/100,000 the computing power of a modern smartphone.

After launch, Stone used the 18-month flight to Jupiter as a shakedown period, a quiet time to learn the quirks of his robotic emissaries. Then in March of 1979, Voyager 1 reached Jupiter, and the madness began.

Worlds of Fire and Ice

Jupiter is the largest planet in the solar system, with a diverse system of moons, and in 1979 it was nearly unexplored. One of the first and biggest surprises from Voyager 1 was Jupiter’s moon Io. In the early images, it resembled a giant pizza and showed not a trace of the expected craters. Meanwhile, attempts to measure Io’s temperature kept giving inconsistent, nonsensical results.

The solution came from Linda Morabito, one of Voyager’s navigation experts. During an early morning image review, she cranked up the contrast to look for a star she expected to see in the background. Instead, her gaze was drawn to a bizarre, umbrella-shaped patch of light beside Io. “What’s that?” she asked herself. A few hours later, after Morabito and her colleagues had studied the image, they called in Stone to take a look. “His eyes were literally twinkling,” she recalls.

Stone instantly grasped what the scientists were already discussing: The umbrella must be the plume of an enormous eruption, the first sign that Io is the most volcanically active world in the solar system. Conventional wisdom at the time was that moons are small, inert bodies, so the discovery of lava lakes and sulfur jets on Io came as a shock.

“It indicated no matter how much we thought we knew, our experience just was not broad enough,” Stone says. Planetary scientists figured out that gravitational interactions with Jupiter and its other large moons create powerful tides on Io, which in turn generates intense heat via friction. That process is now recognized as an important mechanism across the solar system.

When Voyager 2 reached Jupiter four months later, the giant planet was once again upstaged by one of its moons. Images of Io’s next-door neighbor, Europa, revealed another utterly unexpected landscape: smooth as a cue ball and nearly as white, except for a network of thin, faint brownish fractures. “It just looked like an ice pack,” Stone says. More than three decades later, planetary scientists are still deciphering this enigmatic world.

As on Io, tidal heating has energized Europa. In this case, though, the heat produced not volcanoes but a warm ocean sloshing beneath Europa’s icy crust. Today, many researchers consider Europa’s inner ocean one of the most likely places to find alien life.

Over the Rings

After Jupiter came an extended period of tense preparation for the Voyager team. In one sense, not much was happening; the probes were in free flight, slowly making their way across the 500 million-mile gulf to Saturn. But this was a make-or-break moment for Stone and his mission.

NASA had set two key goals for Voyager 1 at Saturn: study the planet’s elaborate system of rings and scrutinize its largest moon, Titan, whose composition resembles that of the early Earth. Conducting those observations required that Voyager 1’s trip to Saturn would fling it up and out of the plane of the solar system. If all went well, Voyager 2 could take a different path, venturing on to Uranus and Neptune. But if not, Voyager 2 would take over the primary flight plan, and there would be no extended planetary tour.

Voyager 1 reached Saturn in November 1980. To Stone’s excitement, it showed that Titan has a deep atmosphere, thicker than Earth’s, full of organic compounds. Thirty-five years later, planetary scientists still marvel over Titan’s methane rainstorms and billowing dunes of tar dust. As for the rings, Stone sums them up with two sharp words: “totally bizarre.” Voyager 1 observed braided patterns and contrasting spokes that rotated in unison with the planet. The spokes may be caused by electrostatic charges within the rings, but their nature remains mysterious.

The Saturn encounter was a success, so NASA moved ahead with Plan A: Voyager 1 shot off toward the stars while Voyager 2 set course to Uranus and beyond. But it was hardly smooth sailing. During Voyager 2’s Saturn flyby, it lost movement in its scan platform, a turntable that aims the instruments at the correct targets. For the next couple of years, the engineers developed commands that got the scan platform moving again. They also reprogrammed Voyager 2 with better ways to take images in the very low light of the outer solar system, and with a more efficient way to transmit data. By the time it reached Uranus on Jan. 24, 1986, Voyager 2 was ready to go.

The Final Planets

Stone’s tone turns almost dreamy as he recounts Voyager 2’s journey to two completely unexplored worlds. At Uranus, Voyager 2 found that the planet’s magnetic field points sideways, indicating an internal configuration utterly unlike any other planet’s. This creates a unique interaction between the planet and the solar wind, a topic of particular interest to Stone. But again, some of the biggest surprises came from the satellites, especially Miranda. “It is only 300 miles in diameter and yet has one of the most complex geological surfaces we’ve seen,” he says — a jumble of mismatched geometric shapes, perhaps from an ancient catastrophe that shattered it into pieces, which later coalesced into the messy moon we see today.

Three years later, in August 1989, Voyager 2 unleashed yet another set of mind-benders at Neptune. It found winds of over 1,000 mph, the fastest anywhere in the solar system, and dark eruptions on Neptune’s largest moon, Triton. “It’s the coldest object we visited — it’s only 38 kelvins [minus 390 degrees Fahrenheit] — and yet there are geysers,” Stone marvels. The geysers evoked Io’s volcanoes, but here the activity is powered by frozen nitrogen rather than molten sulfur. That is one of the lasting lessons from Voyager: Even the coldest worlds can be dynamic and complex.

Stone closed out Voyager’s planetary adventures as he began, in his focused yet unflappable style. Heidi Hammel, now executive vice president of AURA (the organization that helps oversee the Hubble Space Telescope), was a postdoc working with Stone during the Neptune flyby. “Ed always was a calm spot in the midst of chaos,” she says. “I’ve tried to emulate him: Listen to everyone, synthesize the ideas and present it to the public with joy and wonder.”

With Neptune receding, Stone helped guide Voyager 1 to take a look back, creating a panoramic shot of most of the planets lined up around the sun, including the Pale Blue Dot photo of a lonely Earth. Carl Sagan considered it one of the iconic images of the Space Age. After that, there were no more worlds to visit. The Voyagers raced ahead into the void, and Stone contemplated his next move.

New Frontiers

Stone was not idle for long. On Jan. 1, 1991, he took over the entire Jet Propulsion Lab, and he found himself immersed in a space science program struggling with deep budget cuts. If Stone was demoralized, he didn’t show it. He gives just a hint of a shrug: “We cannot do everything we’re smart enough to know how to do, and it’s always a challenge what choices you make.”

One tough choice hit him immediately. NASA had been planning a complex pair of probes called the Comet Rendezvous Asteroid Flyby (CRAF) and the Saturn-bound Cassini probe, but their bloated cost was out of line with the agency’s post-Cold War budget. President George H.W. Bush canceled CRAF, and Cassini seemed headed for oblivion as well. Stone’s team reprised some of the tricks developed by the Voyager team to make the probe radically simpler and cheaper, “and we didn’t lose any part of the science package,” he says. Cassini launched in 1997 and still makes stunning discoveries about Saturn and its moons.

Carolyn Porco, Cassini’s lead imaging scientist and another of Stone’s Voyager-era disciples, credits him with being a model leader. “He was respectful and fair and never, ever arrogant or condescending,” she says. “Everyone loved Ed. We should all be like him, enthusiastic and enjoying every opportunity that life has to give.”

The other signature mission from Stone’s days as JPL director is Mars Pathfinder and its little rover, Sojourner, which landed in 1997. Pathfinder served two important functions, he notes: “It was part of learning how to do things on a smaller scale, and how to rove on the surface of Mars — because you’re unlikely to land on the most interesting location.” Sojourner’s mobility became the template for the successor Mars rovers: Spirit, Opportunity, Curiosity and the upcoming Mars 2020. Stone points out that Pathfinder was also the first NASA mission that became an Internet sensation. “For the first time, it was possible for the public to engage whenever they wanted to,” he says.

Stone stepped down from JPL in 2001, but his touch remains in the current cadence of Mars exploration, alternating between big-budget missions like Curiosity and smaller ones like the MAVEN orbiter, which studies the Martian atmosphere.

To the Edge and Beyond

Once Stone was free of his director’s duties at JPL, he could again focus on the Voyager probes, both now headed inexorably to interstellar space. Their targets were no longer planets but the particles and fields that define the sun’s outer environment. Ever since the 1990s, team scientists had been waiting to reach the termination shock — the boundary that Stone modeled for me in his sink, where the 250-mile-per-second solar wind plows into interstellar material moving less than a tenth as fast. The sudden slowdown should be accompanied by a distinctive pattern of radio signals and particle flows, but detecting it was no simple matter.

As the faster of the two probes, Voyager 1 was poised to reach interstellar space first, but its instrument for measuring the solar wind had stopped functioning in 1980. That forced Stone and his colleagues to rely on indirect evidence, leading to a difficult series of efforts to determine when the craft had truly crossed over.

According to Stone, Voyager 1 reached the termination shock in December 2004, at a distance of 9 billion miles from Earth. Voyager 2, pointing almost 90 degrees away from its twin, hit the shock in 2007 at a distance of just under 8 billion miles. Evidently the sun’s outer boundary layer is lopsided. Piling on the confusion, the termination shock still doesn’t qualify as true interstellar space because it is thoroughly blended with the solar wind. Stone watched over the readings from the Voyagers’ remaining five active instruments, waiting for the momentous entry into the undiluted interstellar flow. And waiting some more.

At long last, in 2013 all the data were in, all the disputes were settled. Voyager 1 officially entered interstellar space on Aug. 25, 2012. Let that stand as the day humans became an interstellar species.

It’s hard to imagine a more extreme territory to conquer, but Stone sounds as restless as ever. “We’re outside, but we’re still not out in pristine interstellar wind. We’re just barely into interstellar space,” he says. “And we don’t actually leave the solar system for another roughly 40,000 years.” He means that the sun’s gravitational influence extends far beyond the solar bubble, all the way to the edge of the Oort Cloud some 9 trillion miles away — and Voyager 1 won’t get there for 400 more centuries.

For a moment Stone seems frustrated, but a moment later he’s eagerly discussing his next projects: He is helping to build the world’s largest telescope, which will scrutinize distant galaxies, and serving as adviser on a space probe that will dive closer to the sun than ever before. After 56 years, Stone is still working full time, seeking out frontiers as methodically as ever.

No plans to retire, then? He offers a half-quizzical, half-smiling look. “My job is my relaxation, really. I’m very lucky.”

From Hawaii to the Edge of Heaven

What do you do after pushing the boundaries of the solar system? How about looking to the edge of the cosmos?

Stone has signed on as executive director of the Thirty Meter Telescope (TMT), an observatory to be built in Hawaii. The scope will feature a 30-meter (100-foot) light-collecting surface designed to achieve three times the resolution of the Hubble Space Telescope. The TMT was to be completed in 2024, but local groups have protested its location because they regard it as sacred ground; last December, Hawaii’s Supreme Court revoked TMT’s construction permit.

“We are assessing our next steps forward,” Stone says. He knows too well that great acts of exploration What do you do after pushing the boundaries of the solar system? How about looking to the edge of the cosmos? Stone has signed on as executive director of the Thirty Meter Telescope (TMT), an observatory to be built in Hawaii. The scope will feature a 30-meter (100-foot) light-collecting surface designed to achieve three times the resolution of the Hubble Space Telescope.

The TMT was to be completed in 2024, but local groups have protested its location because they regard it as sacred ground; last December, Hawaii’s Supreme Court revoked TMT’s construction permit.“We are assessing our next steps forward,” Stone says. He knows too well that great acts of exploration can't be rushed. — CSP