The robots that do our interplanetary drudge work, cruising dispassionately through the solar system to places as yet too far and too dangerous for humans to explore, are generally not celebrities. You may remember that a spacecraft named Magellan explored Venus, but are you aware that out there today the Advanced Composition Explorer is reporting on solar storms, or that planetesimal chaser DAWN has finished its study of the asteroid Vesta and is now arriving at the dwarf planet Ceres? Outside a community of space reporters and enthusiasts, unmanned spacecraft attract little attention. The exceptions are the Voyager probes.

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Why is that?

In the early 1970s, when they were created at the Jet Propulsion Laboratory in Pasadena, California, Voyagers 1 and 2 were the most sophisticated spacecraft ever designed. It took 2,000 engineers and scientists five years to build and outfit them for their trip through the outer planets in the 1970s and ’80s and their current journey into interstellar space. For Voyager Project Manager John Casani and many who worked with him, they represented the high point of a career. And Casani made sure that each spacecraft would carry along on its historic journey a token of the drive and the care that went into its construction. Bolted to the main spacecraft bus underneath the thermal blankets lining the inside of each probe were the signatures of all the engineers and scientists who had been involved in the project and those of their family members—some 5,000 names in all.

“There were so many people who touched these spacecraft,” he says. “Not just the scientists and the engineers, but all the other people who touched it in so many different ways. And their families touched it too—wives, significant others, children—because those people had provided the emotional support and stability for the long hours the engineers and scientists had to work on it.”

As each spacecraft blasted off from Cape Canaveral Air Force Base in Florida—three weeks apart—in the summer of 1977, all the engineers and family members looking on knew that their names were about to cruise among the stars forever.

In the last 35 years, those signatures have been along for the ride as the Voyagers’ cameras glimpsed extraordinary sights: the roiling storms of Jupiter, the volcanoes of Io, the blank orange clouds of Titan, the fractured surface of Miranda. Together, the Voyagers revolutionized the way we view the solar system by showing us it is far more diverse than we had imagined.

Since those first thrilling pictures, other spacecraft have sent us even more spectacular photos. The Voyagers are exceptional today because of where they are—at the edge of the solar system, where they are outdistancing the particles flung outward by the sun and encountering the breath of other stars. If we could see the sun from their vantage points, it would be indistinguishable in the sea of stars around it. Just as John Casani and his team planned, the twin probes are exploring a new world—interstellar space—as they extend the long march of human movement outward. What we began with the first migration from Africa 100,000 years ago, today we continue with these two robotic scouts.

The longevity of the Voyagers has changed our perception of time. Spacecraft are built according to the imperatives of a human lifetime: the length of a federal budget cycle, an administration, a scientist’s career. But the Voyagers exist in a different realm—a measure of time so vast that humans can barely comprehend it. The Voyager signatures were penned little more than a single human generation ago; 10,000 generations from now, Voyager 2 will reach Sirius, the brightest star in our night sky, having travelled more than 25 trillion miles.

The Voyagers may confound our sense of time, but one measure they clearly mark is the speed of technological advance. A mere 20 years after the Soviets opened the Space Age with a 184-pound ball that sent signals from Earth orbit for 22 days, the two Voyagers began the billion-year mission that would unlock the secrets of the outer solar system. In the years since, almost everything technological has changed. As construction of the spacecraft started, a computer scientist named Vinton Cerf was inventing a way for computers to network with one another through what he called an “internet protocol.” The technological advance that the Voyagers represent and the race forward in the 35 years since make us hopeful that the pace of invention will continue. When Voyager 2 does reach Sirius, perhaps we will be there waiting.

Christopher Riley (chris-riley.com) is a British writer, broadcaster, and filmmaker specializing in science history. His documentary, Voyager: To the Final Frontier, premiered on the British Broadcasting Corporation network last October. Richard Corfield (richardcorfield.com), a British scientist, writer, and broadcaster, was the documentary’s story consultant.