What NASA's Voyager Probes Are Carrying Into Deep Space for Aliens to Find on the Golden Record

On 5 September 1977, NASA launched Voyager 1 from Cape Canaveral. Its twin, Voyager 2, had gone up sixteen days earlier. Both probes were built to survey the outer planets. But each one also carried something that had nothing to do with planetary science: a 12-inch gold-plated copper disc, a record, sealed inside an aluminium jacket, with a needle and a stylus cartridge attached so that whoever found it could actually play the thing.

The record was not a symbolic gesture. Carl Sagan chaired the committee that built it. The team had roughly six weeks to decide what humanity would say to the universe, and they took the job with the seriousness it deserved. The result is one of the strangest and most carefully reasoned objects ever made.

Both probes have now crossed into interstellar space, beyond the heliosphere, the bubble of solar wind that defines the edge of our solar system. Voyager 1 crossed that boundary in 2012. Voyager 2 followed in 2018. They are the only human-made objects in deep interstellar space. At current speed, Voyager 1 will take about 40,000 years to pass within 1.6 light-years of any other star. The record has time on its side.

The record opens, in a sense, with mathematics. The cover is engraved with a binary diagram showing how to decode the disc's rotation speed and the correct stylus angle. A pulsar map marks the Sun's position relative to fourteen known pulsars, rotating neutron stars whose frequencies are unique enough to serve as a galactic address. Any civilisation advanced enough to intercept the probes and read binary would, in theory, be able to locate Earth from that map alone.

Inside the audio tracks: 116 images encoded as analogue signals, including diagrams of human anatomy, the structure of DNA, photographs of the Indian Ocean, a nursing mother, rush-hour traffic, and the Great Wall of China. There is a mathematical definition of the number one. There is a photograph of a supermarket.

The sounds are stranger and more affecting. The record carries 55 greetings in different human languages, including a message in Sanskrit recorded by a professor at Cornell. It carries the sound of a mother kissing her child. Rain. Surf. Wind. A chimpanzee. A tractor. A train. Beethoven's Fifth Symphony. Chuck Berry's "Johnny B. Goode." A Navajo night chant. A Peruvian wedding song. Kesarba Patel's "Jaat Kahan Ho", a north Indian classical piece that made the final cut alongside Bach and Mozart.

There is also a spoken message from then-UN Secretary-General Kurt Waldheim, and a message from U.S. President Jimmy Carter, which reads in part: "We are attempting to survive our time so we may live into yours."

The pulsar map on the record's cover is the most technically precise piece of communication on the disc. Pulsars emit radiation at extraordinarily stable frequencies, they are accurate enough to be used as natural clocks. The fourteen pulsars chosen for the map were selected because their positions, relative to the Sun, triangulate Earth's location with enough precision that a civilisation with radio astronomy could identify our star.

The same map appeared earlier on the Pioneer plaques, attached to Pioneer 10 and Pioneer 11, which were launched in 1972 and 1973. Those plaques also showed a nude man and woman, which generated considerable controversy on Earth, far more, it turned out, than among any aliens.

Frank Drake, who designed the pulsar map and co-designed the record with Sagan, was also the author of the Drake Equation, the formula that estimates the number of communicating civilisations in the galaxy. Drake never claimed the probes would be found. The record was designed on the assumption that the attempt itself mattered, that building it carefully was the point, regardless of outcome.

Sagan's committee made choices that reveal as much about 1977 humanity as about any attempt at universal communication. The images were selected to show Earth without making it look like a military target, no weapons, no armies. The music was chosen to represent as many cultures as possible in the time available, though the committee later acknowledged the selection skewed toward Western classical music.

The greeting in Sanskrit was included because Sanskrit's grammatical precision made it, in the view of some linguists on the team, among the most logically structured of human languages, a quality that might help an alien civilisation parse meaning from phonetics. Whether that reasoning holds is a separate question. The committee believed it did.

The record also carries a one-hour recording of the brainwaves of Ann Druyan, who was, at the time of the recording, newly in love with Carl Sagan. She meditated on the history of Earth, on human suffering, on what it meant to be alive. That recording sits on both probes now, moving at roughly 61,000 kilometres per hour through interstellar space. It is the most private thing humanity has ever sent anywhere.

NASA's Jet Propulsion Laboratory still receives signals from both Voyager probes. The signal from Voyager 1 takes more than 22 hours to reach Earth at the speed of light. Engineers communicate with spacecraft launched before most of their team members were born, using 1970s hardware and modern software patches, keeping the mission alive year after year.

The probes will keep moving. In about 300 years, Voyager 1 will reach the inner edge of the Oort Cloud, the vast shell of icy bodies that surrounds the solar system. It will take another 30,000 years to pass through it. In approximately 40,000 years, Voyager 1 will pass within 1.7 light-years of the star Gliese 445, in the constellation Camelopardalis. Nobody will be at JPL to watch it happen.

The record itself is built to last. Gold-plated copper does not corrode in the vacuum of space. The aluminium jacket protects it from micrometeorite damage. Scientists estimate the disc could remain playable for more than a billion years. Whether anyone will play it is, of course, unknowable.

What the Voyager record actually is, not what it might become, is a document of one species at one specific moment, deciding that the attempt at contact was worth making even with no expectation of reply. The pulsar map locates Earth in the galaxy. The Sanskrit greeting and the Kesarba Patel raga and Ann Druyan's brainwaves locate something harder to map: the particular texture of being human in the late twentieth century, which is the only century we had when the probes were launched. Both probes carry that texture into deep space at the same speed, on the same disc, with no way to know if the message will ever arrive at anything that can read it, and no way to unsend it now.