Out beyond the edge of the solar system, two spacecraft are heading away from the sun at more than ten miles per second. On board, they carry sounds of Earth, engrooved on old-style long-play records made of gold-plated copper and engineered to last more than a billion years.

NASA launched the craft, Voyager 1 and Voyager 2, in 1977 to study Jupiter and Saturn. Taking advantage of a favourable alignment of the outer planets, Voyager 2 also flew close by Uranus and Neptune, the only probe ever to have done so, and beamed back additional images and data. Meanwhile, Voyager 1, which had already flown out beyond the orbit of Neptune, turned its camera round, and in 1990 took a photograph in which the earth appears as a single pixel – famously described by the astronomer Carl Sagan as a pale blue dot.

Slingshotted outwards by the gravitational fields of the planets they passed, both craft will continue towards the stars indefinitely. In about 296,000 years Voyager 2 will pass within 4.6 light years of Sirius, the brightest star in the night sky. At the time of writing both Voyagers are still sending data home, but by the mid 2020s they will finally run out of power and fall silent. After that they will only ‘speak’ again in the astronomically unlikely event that an intelligent entity finds one or both and plays the records they carry. All other things shall change, but they remain the same, till heaven’s changes have their course, and time hath lost his name.

The sounds on the records, chosen by a small group convened by Sagan, include surf on a beach, wind and thunder, the songs of birds and whales, greetings in fifty-five languages and electrical signals from the brain of a woman in love. There are also images encoded on the records: printed messages from US President Carter and UN Secretary General Waldheim, and photographs of ordinary people going about activities that were normal in the 1970s such as gurning in supermarkets. There are landscapes, plants and animals and the human body – although detailed depictions of genitals and the belly of a pregnant woman were not considered acceptable by NASA.
A diagram on the cover of the records locates the Sun in relation to the different rhythmical beat of fourteen nearby pulsars – stars that emit rotating beams of electromagnetic radiation that sweep out across space like the signals from lighthouses.

One of the greatest treasures on the discs is proclaimed in words etched into them by hand: ‘To the makers of music – all worlds, all times.’ The twenty-seven tracks range from Bach to Chuck Berry, and are so varied in origin and kind that they escape a quick summary. One of the things that stands out in many of them, however, is the human singing voice in all its brilliance and directness. This is particularly so in tracks such as ‘Tchakrulo’, a three-part harmonisation for male voices from Georgia, and the Bulgarian ‘Izlel je Delyo Hagdutin’, in which pipes accompany a female voice that is both sweet and awesomely powerful.