IN 1989, rock-and-roll legend Chuck Berry attended one of the biggest parties of the summer. The bash wasn’t a concert, but a celebration of two space probes about to breach the edge of our solar system: NASA’s Voyager mission

Launched from Cape Canaveral, Florida, in 1977, identical twins Voyager 1 and 2 embarked on a five-year expedition to observe the moons and rings of Jupiter and Saturn, carrying with them Golden Records preserving messages from Earth, including Berry’s smash single “Johnny B. Goode.” But 12 years later, out on the grassy “Mall” of NASA’s Jet Propulsion Laboratory, scientists celebrated as Voyager 2 made a previously unscheduled flyby of Neptune. Planetary scientist Linda Spilker remembers the bittersweet moment: the sight of the eighth planet’s azure-colored atmosphere signaled the end of the mission’s solar system grand tour.

“We kind of thought of it as a farewell party, because we’d flown by all the planets,” says Spilker. “Both of them were well past their initial lifetimes.”

Many in the scientific community expected the spacecrafts to go dark soon after. But surprisingly, the pair continued whizzing beyond the heliopause into interstellar space, where they’ve been wandering ever since, for more than three decades. Spilker, now the Voyager mission project scientist, says the probes’ journeys have shed light on the universe we live in—and ourselves. “It’s really helped shape and change the way we think about our solar system,” she says. 

Currently traveling at a distance between 12 and 14 billion miles from Earth, Voyager 1 and 2 are the oldest, farthest-flung objects ever forged by humanity. Nearly five decades on, the secret to Voyager’s apparent immortality is most likely the spacecrafts’ robust design—and their straightforward, redundant technology. By today’s standards, each machine’s three separate computer systems are primitive, but that simplicity, as well as their construction from the best available materials at the time, has played a large part in allowing the twins to survive. 

For example, the spacecrafts’ short list of commands proved versatile as they hopped from one planet to the next, says Candice Hansen-Koharcheck, a planetary scientist who worked with the mission’s camera team. This flexibility of its operations allowed engineers to turn the Voyagers into scientific chameleons, adapting to one new objective after another.

As the machines puttered far from home, new discoveries, like active volcanoes on Jupiter’s moon Io and a possible subsurface ocean on neighboring Europa, helped us realize that “we weren’t in Kansas anymore,” says Hansen-Koharcheck. Since then, many of the tools that have contributed to Voyagers’ success, such as optics and multiple fail-safes, have been translated to other long-term space missions, like the Saturn Cassini space probe and the Mars Reconnaissance Orbiter. 

Both Voyagers are expected to transmit data back to Earth until about 2025—or until the spacecrafts’ plutonium “batteries” are unable to power critical functions. But even if they do cease contact, it’s unlikely they will crash into anything or ever be destroyed in the cosmic void. 

Instead, the Voyagers may travel the Milky Way eternally, both alone and together in humanity’s most spectacular odyssey. 

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