For many people who grew up in the 1970s and 1980s, the most emblematic space missions aren’t the pioneering flights of Yuri Gagarin, Alan Shepard or John Glenn. Or the audacious Apollo moon landings, or even the decades-spanning orbital journeys of space shuttles and Soyuz rockets to and from the International Space Station. Instead, the missions that have defined a generation’s conception of the space age are those of Voyager 1 and Voyager 2, twin spacecraft launched on a “Grand Tour” of the outer planets in the late summer of 1977. This week marks the 40th anniversary of the Voyager missions—and also the release of a new feature-length documentary, The Farthest: Voyager in Space, that celebrates their legacy.

Forty years ago, robots were already regularly reconnoitering the inner solar system, but the outlands of Jupiter, Saturn, Uranus, Neptune and beyond were largely undiscovered country. Beginning in 1979 with their flybys of Jupiter and continuing throughout the 1980s, the Voyagers sent back a steady stream of remarkable scientific results as they explored this new frontier. At Jupiter they imaged the Great Red Spot, and saw signs of a liquid-water ocean within the giant planet’s icy moon Europa. They studied Saturn’s exquisite rings, and Voyager 1 peered into the smoggy atmosphere of its largest moon, Titan, which many theorists believe resembles that of the prebiotic Earth.

After its encounter with Titan, Voyager 1 was bound for interstellar space, looking back one last time to capture a “family portrait” of Earth and the Sun’s other planets as scarcely visible dots from beyond the orbit of Pluto; in 2012 it passed the heliopause, the region of space where the solar wind dwindles and gives way to the interstellar medium. Voyager 2 plowed ahead to Uranus and then Neptune, delivering the first close-up images of these otherwise unexplored worlds. At each destination these now-vintage 20th-century missions solved old mysteries—and unearthed new ones that are still setting the agenda for outer-planets exploration in the 21st. What the Voyagers gave us, more than just pretty pictures and pristine data, was a blueprint for things to come. NASA’s Galileo and Juno missions to Jupiter, its Cassini mission to Saturn, and future missions that as yet remain but twinkles in their planners’ eyes are all direct consequences of that rich legacy.

Before the Voyagers, visiting all those planets with just two spacecraft over the course of about a decade seemed inconceivable. It was only made possible by a once-every-176-years planetary alignment that allowed the probes to hop from one world to the next via a carefully planned trajectory, siphoning off an infinitesimal fraction of each planet’s orbital momentum to pick up speed. Today each spacecraft is traveling in excess of 15 kilometers per second, and both remain operational as they continue their one-way-trip into deepest space as humanity’s farthest-flung interstellar emissaries. Even when their power sources fade and their instruments die, the missions will go on: Each vessel carries a “Golden Record”—a gold-plated 12-inch LP packed with Earthly words, music and images destined for the stars, curated by the astronomer Carl Sagan and many of his colleagues. Projected to remain intact for billions of years in the emptiness between the stars, in the fullness of time these modern-day messages in a bottle may become the most enduring and meaningful relics of our age.

The Farthest, which premiered August 23 on PBS and will be rebroadcast on September 13, covers all this and much more, offering both a retrospective on the Voyager missions and a preview of what still lies ahead for the hardy spacecraft. Directed by Emer Reynolds, the film uses rare archival footage and lush computer-generated imagery to augment its core, a series of intimate conversations with many now-retired senior scientists and engineers who worked on the missions. From the high-tension, post-Apollo politics of the Nixon administration that underpinned their probes’ approval and development, through operational glitches that nearly doomed both spacecraft, to the Voyagers’ triumphant discoveries across more than a decade of planetary encounters, the film’s interviewees recall the mission’s highs and lows with an intensity of emotion only rarely seen in the rarefied, buttoned-up, high-tech world of space science.

“Voyager 1 and Voyager 2 are robots, and they went out there alone, so it can be easy to forget that it was people who did this—people with feelings and flaws, hopes and fears, who built their lives around these two spacecraft for a period of time,” says Jim Bell, a planetary scientist at Arizona State University and author of The Interstellar Age, who is featured in the film. “One of my favorite parts of The Farthest was its use of computer animation to simulate looking over the shoulder of the spacecraft as they fly over the cloud tops of Neptune or through the rings of Saturn. All of us involved in the project have always wished we were there just like that, riding along,” Bell says. “That was the dream, and my eyes well up with tears seeing it visualized for the first time.”

Bell is quick to say he had “no meaningful role” in the Voyager flybys, but the missions were intensely meaningful for him. He was only a young college student during the missions’ final outer-planets sorties in the late 1980s, finding his way into the control room at NASA’s Jet Propulsion Laboratory to fetch coffee, copies and pizzas for the research scientists working around the clock on data from Voyager 2’s encounters with Uranus and Neptune. “The feeling of being in that room, seeing something transform from a point of light in our best telescopes to a real world, aquamarine with dark rings and crazy icy moons, really got me hooked,” Bell explains. “Voyager was my gateway drug into the field of planetary science, and there are now a bunch of us ‘Voyager babies’ working on Mars rovers, Cassini and other missions, paying it forward to give to future generations some of what that previous generation gave to us.”

NASA has estimated the total cost of the Voyager program, from its inception into today, as $865 million. This may sound like a lot but is, according to the space agency, only “roughly half the cost of one candy bar each year” per each U.S. citizen, or “a fraction of the daily interest on the national debt”—in return for new vistas on our solar system. Today NASA “flagship” missions that are remotely comparable to the Voyagers in complexity are significantly more expensive, with budgets measured in the billions, and they tend to proceed far more slowly in planning and development. Each usually requires major support from high-ranking patrons in Congress or the presidency, as well as groundbreaking technological development and the availability of fuel-saving celestial alignments that can stretch their execution out by decades. Conceived and launched in a span of only five years, the Voyagers are the agency’s fastest-moving flagships in more ways than one.

All that makes some scientists skeptical that NASA or any other space agency will achieve missions of similar audacity ever again. Bell, however, is not among them. He points to NASA’s current plans to send robotic explorers to Jupiter’s ocean-bearing moon Europa in the 2020s, as well as its ongoing New Horizons mission that flew by Pluto in 2015 and is en route to visit a smaller icy object in 2019.

“Every now and then, all the different constraints will line up to allow more flagship missions, and they will become wonderful legacy of our time,” Bell says. “Centuries from now, maybe millennia, people will look back and say, ‘Look at all the problems they had. Look at how screwed-up the world was—and yet they still managed to use a tiny fraction of their combined wealth to explore and discover and achieve greatness that wasn’t done purely for survival.’ I’m crazy optimistic about the future. It would make me too sad not to be.”