In August and September 1977, two spacecraft left Earth on what was officially a four-year mission to Jupiter and Saturn. Nearly fifty years later, both are still working, both are outside the Sun's magnetic bubble, and both are still sending data across a gap so wide that their signals take more than 20 hours to arrive.
Voyager 1 is the most distant human-made object in existence. Nothing else is close.
The alignment that made it possible
The Voyager missions exist because of an accident of orbital mechanics. Once every 175 years, Jupiter, Saturn, Uranus and Neptune line up in an arrangement that lets a single spacecraft swing from one to the next, using each planet's gravity to bend its path and add speed.
A graduate student named Gary Flandro identified the opportunity in 1965. The window was in the late 1970s. Miss it and the next chance would come in the middle of the 22nd century.
The gravity assist is the key trick. A spacecraft flying past a planet steals a minuscule amount of the planet's orbital momentum, gaining speed relative to the Sun while the planet slows by an unmeasurable amount. Without it, reaching Neptune would have taken decades of extra travel and far more fuel than could be carried.
NASA originally proposed an ambitious "Grand Tour" of all four planets, and budget cuts killed it. The approved mission was a more modest pair of probes to Jupiter and Saturn — built, as it happened, robustly enough to keep going if things went well.
Things went well.
What they carry
Both spacecraft are essentially identical: a 3.7-metre dish antenna, a suite of instruments on a boom, and three radioisotope thermoelectric generators fuelled by plutonium-238.
That power source matters. Solar panels are useless in the outer Solar System — sunlight at Neptune is about a thousandth of its strength at Earth. The RTGs convert the heat of radioactive decay directly into electricity, with no moving parts. They produced about 470 watts at launch, roughly what a household toaster draws at low setting, and they lose around 4 watts every year.
The computers are the detail that reliably startles people. Each probe has about 70 kilobytes of memory in total. A single photo on a modern phone is fifty times larger. Commands are still written in assembly language by a small team, some of whom have worked on the mission for decades, using documentation that in places exists only on paper.
The tour
Voyager 2 launched first, on 20 August 1977. Voyager 1 launched on 5 September on a faster trajectory and overtook it.
At Jupiter in 1979, the probes found something nobody expected: active volcanoes on the moon Io, the first volcanic activity discovered beyond Earth. Navigation engineer Linda Morabito spotted an odd plume on the limb of Io in a calibration image. They also found hints of a smooth, cracked ice surface on Europa, the observation that started decades of speculation about an ocean beneath.
At Saturn in 1980 and 1981, Voyager 1 was sent on a close pass of Titan to study its thick atmosphere. This was a deliberate trade: the Titan flyby flung Voyager 1 up and out of the plane of the planets, ending any possibility of visiting Uranus or Neptune. Mission planners judged Titan worth it.
That left Voyager 2 to continue alone. It reached Uranus in 1986, finding a planet tipped almost entirely on its side, with a strangely offset magnetic field and ten previously unknown moons. In 1989 it passed Neptune, discovering the Great Dark Spot and, on the moon Triton, geysers of nitrogen erupting from one of the coldest surfaces ever measured — around minus 235 degrees Celsius.
Voyager 2 remains the only spacecraft ever to visit Uranus or Neptune. Everything humanity knows from close range about those two planets comes from a few days in the 1980s.
The Pale Blue Dot
On 14 February 1990, with its cameras about to be switched off to save power, Voyager 1 was turned around at Carl Sagan's urging to photograph the planets it had left behind.
In one frame, Earth appears as a single pale dot occupying less than a pixel, caught in a band of scattered sunlight. Sagan's reflection on that image — that everyone you have ever heard of, every human who ever lived, spent their lives on that speck — became one of the most quoted passages in popular science.
The Golden Record
Bolted to each spacecraft is a gold-plated copper phonograph record, chosen because a physical groove can survive for an extraordinarily long time in the vacuum of space.
A committee chaired by Sagan selected the contents: greetings in 55 languages, natural sounds from surf to thunder to a mother's kiss, 115 encoded images, and 90 minutes of music ranging from Bach and Beethoven to Chuck Berry, Javanese gamelan, Navajo night chant and a Bulgarian folk song.
The cover is etched with instructions for playing it and a map showing the Sun's position relative to fourteen pulsars, each labelled with its pulse rate in binary. Pulsars slow at predictable rates, so the map is also a clock: a recipient could work out when the record was launched.
Nobody involved seriously believed anyone would find it. The realistic audience was always us.
Where they are now
Voyager 1 crossed the heliopause — the boundary where the Sun's outflowing particles are stopped by interstellar material — in August 2012. Voyager 2 followed in November 2018 at a different location, giving scientists two independent measurements of a boundary nobody had ever crossed.
Both are now in interstellar space, though still well inside the Sun's gravitational reach. Voyager 1 is over 24 billion kilometres away, moving at about 17 kilometres per second. A radio signal takes more than 23 hours to reach it, and the same again to return.
They are heard by the Deep Space Network's giant dishes. The signal arriving at Earth is measured in attowatts — billionths of a billionth of a watt.
Running down
Power is the constraint on everything. As the plutonium decays, instruments are shut off one by one to keep the essential systems and the transmitter alive. Heaters have been switched off in parts of the spacecraft that engineers believed would freeze and fail; several kept working anyway, colder than they were ever tested for.
In late 2023 Voyager 1 began returning meaningless data. Engineers eventually traced the fault to a failed memory chip, wrote new code around the damaged section, and had science data flowing again by mid-2024 — a repair performed on a fifty-year-old computer from 24 billion kilometres away.
Current estimates suggest the last instruments will go dark sometime in the 2030s. After that, both probes will continue coasting, silent, essentially forever. Voyager 1 will pass within about 1.6 light-years of the star Gliese 445 in roughly 40,000 years. Nothing out there will erode them meaningfully. They will likely outlast the Earth itself.
Why they still matter
Voyager was designed by people who could not know whether any of it would work, using components chosen for reliability over performance, with margins built in by engineers who assumed nothing could be repaired.
That conservatism turned a four-year mission into a five-decade one, and it produced most of what we knew about the outer Solar System for a generation. Two objects built in the 1970s are still, today, sending home measurements of a region no other spacecraft has ever reached.
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