What Were the Voyager Missions? Two Probes That Never Stopped

The mission existed because of an accident of orbits. In the late 1970s the four outer planets were arranged so that a spacecraft could swing past each in turn, using the gravity of one to accelerate towards the next, a configuration that occurs roughly once every 175 years. Gravity assist, worked out by the mathematician Michael Minovitch as a graduate student in 1961 and applied to this route by Gary Flandro in 1965, turns a flyby into a slingshot: the probe steals a minute amount of the planet's orbital momentum and leaves faster than it arrived, which cut a journey to Neptune from thirty years to twelve and made the whole thing affordable. The original grand tour was cut back on cost grounds and rebuilt as two probes with a more modest official brief, though the engineers built them capable of the full route and the extensions were approved once the first results came in.

The scientific return reshaped planetary science in a decade:

• •Io, a moon of Jupiter, has active volcanoes, spotted by the navigation engineer Linda Morabito in a routine image, making it the most volcanically active body known and proving that tidal heating can melt a moon's interior
• •Europa's surface is young, cracked ice, which led directly to the case for a liquid ocean beneath it and to every mission proposed since
• •Jupiter has a faint ring system, previously unsuspected
• •Saturn's rings resolve into thousands of ringlets, with braided structures and dark spokes nobody has fully explained
• •Titan has a dense nitrogen atmosphere with organic chemistry in it, which made it a target and eventually brought the Huygens lander there in 2005
• •Uranus is tipped on its side with a magnetic field offset at a strange angle, and its moon Miranda has terrain that looks assembled from mismatched pieces
• •Neptune has supersonic winds, the fastest in the solar system, and its moon Triton has nitrogen geysers and orbits backwards, marking it as a captured object
• •The count of known moons around the four planets roughly tripled

Each probe carries a gold-plated copper phonograph record, assembled by a committee chaired by Carl Sagan, intended for any intelligence that might one day find it. The contents are a deliberate portrait: greetings in fifty-five languages, natural sounds from surf and thunder to a mother and child, 116 images encoded as analogue signals, and ninety minutes of music running from Bach and Beethoven through Javanese gamelan, Navajo chant, Peruvian panpipes, Blind Willie Johnson and Chuck Berry. The cover is etched with instructions for playing it, a map locating the sun relative to fourteen pulsars, and a diagram of the hydrogen atom to establish units. The realistic odds of interception are negligible, and the object's real function was always the one Sagan acknowledged, which is to say something to ourselves about what we would want to be judged by. The gold plating should last a billion years in interstellar space.

Both crossed the heliopause, the boundary where the outward flow of particles from the sun gives way to the interstellar medium, Voyager 1 in 2012 and Voyager 2 in 2018, becoming the only human objects to have done so and continuing to return measurements of a region no other instrument has sampled. They are powered by plutonium thermoelectric generators whose output declines by about four watts a year, and the teams have been switching off instruments one at a time to keep the remaining ones running, a process that will end around 2030 when there is no longer power for anything. Communication takes more than twenty hours each way, uses a transmitter of about twenty-two watts, and depends on the largest dishes available; a command sent to Voyager 1 in 2023 corrupted its data stream for months until engineers traced the fault to a failed memory chip and relocated the affected code, which was done by people reading documentation written half a century ago in a language nobody uses.

In February 1990, with its planetary work finished and its cameras about to be shut down, Voyager 1 was turned around at Sagan's insistence to photograph the solar system from six billion kilometres out. The Earth occupies less than a single pixel, a pale dot caught in a band of scattered sunlight, and the image had no scientific value whatever. Sagan's commentary on it, that everyone who ever lived has lived out their lives on that speck, is the most quoted passage in the literature of space exploration and is frequently the only thing a person knows about the mission. The camera was switched off shortly afterwards to save power. The probes will pass within a couple of light years of other stars in about forty thousand years, by which time nothing on Earth that built them will remain, and they will still be carrying the record.

Two probes launched in 1977 exploited a planetary alignment occurring once every 175 years to use gravity assists from Jupiter, Saturn, Uranus and Neptune, tripling the count of known outer moons and finding volcanoes on Io, a likely ocean under Europa's ice, rings around Jupiter, chemistry in Titan's atmosphere and geysers on Triton. Each carries a gold record of Earth's sounds, images and music. Both have crossed into interstellar space, their plutonium power will run out around 2030, and Voyager 1 took the photograph in which Earth is a single pale pixel.