In the summer of 1977, two spacecraft left Earth within sixteen days of each other. They were built with the computing power of a pocket calculator, designed for a four-year mission to Jupiter and Saturn, and expected to fall silent long before the century ended.
They are still transmitting.
The alignment that made it possible
The Voyager missions exist because of an accident of orbital mechanics. Once every 176 years, the outer planets line up in an arrangement that allows a single spacecraft to visit several of them in sequence, using each planet’s gravity to slingshot on to the next.
That alignment was available in the late 1970s. Missing it meant waiting nearly two centuries. The gravity-assist technique — stealing a tiny amount of a planet’s orbital momentum to accelerate a spacecraft — made a tour possible that no rocket of the era could have achieved on propulsion alone.
Voyager 2 launched first, on the slower path that would take it past all four giant planets. Voyager 1 launched second on a faster trajectory, reached Jupiter earlier, and then took a route past Saturn’s moon Titan that flung it up and out of the plane of the solar system.
What they found
The scientific return reshaped planetary science, largely because nearly everything was a surprise.
At Jupiter, the moons turned out to be worlds rather than points of light. Io was found to be volcanically active — the first active volcanism discovered beyond Earth, driven by tidal heating. Europa showed a young, cracked, nearly crater-free ice surface, the first hint of the ocean now thought to lie beneath it. Jupiter turned out to have a faint ring system nobody had predicted.
At Saturn, the rings resolved into thousands of distinct ringlets with braided and kinked structures that took years to explain. Titan proved to have a thick nitrogen atmosphere, opaque at visible wavelengths, hiding a surface that would not be seen properly for another quarter century.
Voyager 2 alone continued to Uranus and Neptune, and remains the only spacecraft to have visited either. It found Uranus tipped on its side with a bizarrely offset magnetic field, and at Neptune discovered supersonic winds and a moon, Triton, with active nitrogen geysers erupting from a surface at around 38 kelvin.
Two spacecraft, one launch window, and the only close-up data humanity has ever collected on two of the eight planets.
Into interstellar space
After the planetary encounters, both spacecraft kept going. The mission became a study of the heliosphere — the vast bubble of charged particles blown outward by the Sun.
Voyager 1 crossed the heliopause, the boundary where the solar wind gives way to the interstellar medium, in 2012. Voyager 2 followed in 2018 on a different trajectory. The crossings were identified not by any dramatic marker but by a sharp change in the density of charged particles and the direction of the magnetic field.
They are the only spacecraft to have made in-situ measurements of interstellar space, and they found it different from predictions in ways that are still being worked through.
Keeping them alive
Both spacecraft are powered by radioisotope thermoelectric generators, which convert heat from decaying plutonium into electricity. The output declines by a few watts every year, and it has been declining for nearly five decades.
Mission operations has become an exercise in careful triage. Instruments have been switched off one at a time. Heaters have been sacrificed, in some cases leaving components operating well below their rated temperatures — and working anyway. Engineers have swapped to backup thruster sets and rewritten commands for computers whose original programmers have retired.
The communication challenge is equally severe. Signals take more than twenty hours each way, and arrive with a power measured in fractions of a billionth of a watt, requiring the largest dish antennas on Earth to detect at all.
Sometime in the 2030s the power budget will fall below what even a single instrument requires, and both spacecraft will go quiet. They will keep travelling regardless, on trajectories that will carry them past other stars in tens of thousands of years.
The record
Each Voyager carries a gold-plated copper phonograph record holding sounds and images selected to represent Earth: greetings in dozens of languages, music from many cultures, whale song, thunder, a heartbeat.
The odds of any of it ever being found are effectively zero, and the people who assembled it knew that. It was never really addressed outward. It is a statement about what a species thought was worth saying about itself, launched at a moment when it had just learned how to leave.
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