We inhabit a small planet orbiting a medium-sized star about two-thirds of the way out from the center of the Milky Way galaxy—around where Track 2 on an LP record might begin. In cosmic terms, we are tiny: were the galaxy the size of a typical LP, the sun and all its planets would fit inside an atom’s width. Yet there is something in us so expansive that, four decades ago, we made a time capsule full of music and photographs from Earth and flung it out into the universe. Indeed, we made two of them.

The time capsules, really a pair of phonograph records, were launched aboard the twin Voyager space probes in August and September of 1977. The craft spent thirteen years reconnoitering the sun’s outer planets, beaming back valuable data and images of incomparable beauty. In 2012, Voyager 1 became the first human-made object to leave the solar system, sailing through the doldrums where the stream of charged particles from our sun stalls against those of interstellar space. Today, the probes are so distant that their radio signals, travelling at the speed of light, take more than fifteen hours to reach Earth. They arrive with a strength of under a millionth of a billionth of a watt, so weak that the three dish antennas of the Deep Space Network’s interplanetary tracking system (in California, Spain, and Australia) had to be enlarged to stay in touch with them.

If you perched on Voyager 1 now—which would be possible, if uncomfortable; the spidery craft is about the size and mass of a subcompact car—you’d have no sense of motion. The brightest star in sight would be our sun, a glowing point of light below Orion’s foot, with Earth a dim blue dot lost in its glare. Remain patiently onboard for millions of years, and you’d notice that the positions of a few relatively nearby stars were slowly changing, but that would be about it. You’d find, in short, that you were not so much flying to the stars as swimming among them.

The Voyagers’ scientific mission will end when their plutonium-238 thermoelectric power generators fail, around the year 2030. After that, the two craft will drift endlessly among the stars of our galaxy—unless someone or something encounters them someday. With this prospect in mind, each was fitted with a copy of what has come to be called the Golden Record. Etched in copper, plated with gold, and sealed in aluminum cases, the records are expected to remain intelligible for more than a billion years, making them the longest-lasting objects ever crafted by human hands. We don’t know enough about extraterrestrial life, if it even exists, to state with any confidence whether the records will ever be found. They were a gift, proffered without hope of return.

I became friends with Carl Sagan, the astronomer who oversaw the creation of the Golden Record, in 1972. He’d sometimes stop by my place in New York, a high-ceilinged West Side apartment perched up amid Norway maples like a tree house, and we’d listen to records. Lots of great music was being released in those days, and there was something fascinating about LP technology itself. A diamond danced along the undulations of a groove, vibrating an attached crystal, which generated a flow of electricity that was amplified and sent to the speakers. At no point in this process was it possible to say with assurance just how much information the record contained or how accurately a given stereo had translated it. The open-endedness of the medium seemed akin to the process of scientific exploration: there was always more to learn.

In the winter of 1976, Carl was visiting with me and my fiancée at the time, Ann Druyan, and asked whether we’d help him create a plaque or something of the sort for Voyager. We immediately agreed. Soon, he and one of his colleagues at Cornell, Frank Drake, had decided on a record. By the time NASA approved the idea, we had less than six months to put it together, so we had to move fast. Ann began gathering material for a sonic description of Earth’s history. Linda Salzman Sagan, Carl’s wife at the time, went to work recording samples of human voices speaking in many different languages. The space artist Jon Lomberg rounded up photographs, a method having been found to encode them into the record’s grooves. I produced the record, which meant overseeing the technical side of things. We all worked on selecting the music.

I sought to recruit John Lennon, of the Beatles, for the project, but tax considerations obliged him to leave the country. Lennon did help us, though, in two ways. First, he recommended that we use his engineer, Jimmy Iovine, who brought energy and expertise to the studio. (Jimmy later became famous as a rock and hip-hop producer and record-company executive.) Second, Lennon’s trick of etching little messages into the blank spaces between the takeout grooves at the ends of his records inspired me to do the same on Voyager. I wrote a dedication: “To the makers of music—all worlds, all times.”

To our surprise, those nine words created a problem at NASA. An agency compliance officer, charged with making sure each of the probes’ sixty-five thousand parts were up to spec, reported that while everything else checked out—the records’ size, weight, composition, and magnetic properties—there was nothing in the blueprints about an inscription. The records were rejected, and NASA prepared to substitute blank discs in their place. Only after Carl appealed to the NASAadministrator, arguing that the inscription would be the sole example of human handwriting aboard, did we get a waiver permitting the records to fly.

In those days, we had to obtain physical copies of every recording we hoped to listen to or include. This wasn’t such a challenge for, say, mainstream American music, but we aimed to cast a wide net, incorporating selections from places as disparate as Australia, Azerbaijan, Bulgaria, China, Congo, Japan, the Navajo Nation, Peru, and the Solomon Islands. Ann found an LP containing the Indian raga “Jaat Kahan Ho” in a carton under a card table in the back of an appliance store. At one point, the folklorist Alan Lomax pulled a Russian recording, said to be the sole copy of “Chakrulo” in North America, from a stack of lacquer demos and sailed it across the room to me like a Frisbee. We’d comb through all this music individually, then meet and go over our nominees in long discussions stretching into the night. It was exhausting, involving, utterly delightful work.

“Bhairavi: Jaat Kahan Ho,” by Kesarbai Kerkar

In selecting Western classical music, we sacrificed a measure of diversity to include three compositions by J. S. Bach and two by Ludwig van Beethoven. To understand why we did this, imagine that the record were being studied by extraterrestrials who lacked what we would call hearing, or whose hearing operated in a different frequency range than ours, or who hadn’t any musical tradition at all. Even they could learn from the music by applying mathematics, which really does seem to be the universal language that music is sometimes said to be. They’d look for symmetries—repetitions, inversions, mirror images, and other self-similarities—within or between compositions. We sought to facilitate the process by proffering Bach, whose works are full of symmetry, and Beethoven, who championed Bach’s music and borrowed from it.