The Voyager Missions

01 The Grand Tour

Every 176 years, the outer planets align in a configuration that allows a single spacecraft to visit Jupiter, Saturn, Uranus, and Neptune using gravity assists — slingshot manoeuvres that rob a tiny fraction of each planet's orbital momentum to accelerate the probe onward. NASA's Jet Propulsion Laboratory recognised in the early 1960s that this alignment would occur in the late 1970s. The mission concept was called the Grand Tour. Budget pressures shrank the ambitious original plan — four sophisticated spacecraft visiting all four gas giants — to a pared-back pair of probes built as backups for a Jupiter–Saturn mission. They were launched sixteen days apart in August and September 1977, and designated Voyager 2 and Voyager 1. The numbering was reversed because Voyager 1 was launched on a faster trajectory that would overtake its twin.

Voyager 2 launched first, on August 20, 1977, atop a Titan III-Centaur rocket from Cape Canaveral. Voyager 1 followed on September 5 on a shorter, faster trajectory. Both were built around a common bus: a ten-sided aluminium frame carrying a 3.7-metre parabolic high-gain antenna, a radioisotope thermoelectric generator (RTG) power supply, and a suite of eleven instruments designed for planetary science. The spacecraft weighed 825 kilograms at launch, carried 105 kilograms of scientific instruments, and was designed to survive a five-year mission to Saturn. Neither probe was designed for interstellar flight — but they have now operated for forty-eight years.

02 The Spacecraft

Each Voyager is a marvel of 1970s engineering that has been asked to do a job its designers never imagined. The 3.7-metre high-gain dish antenna communicates with NASA's Deep Space Network at a transmission rate that has dwindled from 115,200 bits per second at Jupiter to approximately 160 bits per second today — slower than a 1980s dial-up modem. Power comes from three RTGs, which convert heat from the radioactive decay of plutonium-238 into electricity. At launch, the RTGs produced about 470 watts; today they generate approximately 220 watts. The decay of output has forced mission controllers to progressively shut down non-essential instruments and heaters, rationing power with a precision that extends the mission's remaining life one year at a time.

Attitude control is maintained by hydrazine thrusters, and the onboard computers — built from discrete logic chips with a total of 69 kilobytes of memory — still execute commands uploaded from Earth. It takes a radio signal roughly 22.5 hours to travel from Earth to Voyager 1 at the speed of light; the spacecraft's response takes another 22.5 hours to return. Every command sequence is pre-tested and uploaded with the understanding that real-time intervention is impossible. The Deep Space Network tracks the Voyagers using its 70-metre dish antennas at Goldstone (California), Madrid (Spain), and Canberra (Australia), allocating precious tracking time to receive the faint whispers arriving from the edge of the Solar System.

03 Planetary Encounters

Voyager 1 reached Jupiter in March 1979, performing its closest approach on March 5 at a range of about 349,000 kilometres. The images and data transformed understanding of the giant planet: Voyager discovered that Jupiter had rings (a thin, dusty system first detected when a star occulted behind them), captured the first detailed images of the Galilean moons revealing Io's volcanic plumes — the first active volcanoes found beyond Earth — and mapped the complex circulation of Jupiter's atmosphere. Voyager 2 followed four months later with complementary observations of different longitudes.

Saturn encounters came next, in November 1980 (Voyager 1) and August 1981 (Voyager 2). Voyager 1's trajectory was bent to allow a close flyby of Titan, Saturn's largest moon, which meant it could not continue to the outer planets. Instead, it climbed out of the ecliptic plane and headed toward interstellar space. Voyager 2, its Titan flyby traded for a continued Grand Tour, pressed on to Uranus in January 1986 — the first and only spacecraft to visit the ice giant — and Neptune in August 1989. At Uranus, Voyager 2 discovered ten new moons and mapped the planet's peculiar tilted magnetosphere. At Neptune, it found winds of 2,100 km/h, the fastest in the Solar System, and observed geysers on Triton, Neptune's large moon, before heading south and out of the ecliptic.

04 Crossing the Heliopause

The Sun does not end at a sharp boundary. Its influence extends outward through the heliosphere — a vast bubble of solar wind plasma that pushes against the interstellar medium, creating a boundary called the heliopause. Inside the heliopause, the solar wind dominates; outside, the interstellar medium prevails. In 2004, Voyager 1 crossed the termination shock, where the solar wind slows from supersonic to subsonic speeds. For the next eight years, the spacecraft traversed the heliosheath, a turbulent region of heated plasma. Then, on August 25, 2012, Voyager 1's particle detectors registered a dramatic change: the count of charged particles from the Sun dropped to near zero, while galactic cosmic rays spiked. Voyager 1 had crossed the heliopause and entered interstellar space at approximately 121 astronomical units from the Sun.

Voyager 2 made the same crossing on November 5, 2018, at about 119 AU. Because Voyager 2 carries a working plasma detector — Voyager 1's had failed in 1980 — it provided the first direct measurement of the interstellar medium's density and temperature, confirming that the plasma beyond the heliopause is about 40 times denser than inside the heliosheath. Both spacecraft now operate in a region no human-made object has ever entered, sending back data about galactic cosmic rays, the interstellar magnetic field, and the pressure of the medium through which they travel. Their crossing remains the only direct evidence of the heliopause's location and properties.

05 The Golden Record

Bolted to the side of each Voyager is a gold-plated copper disc, 30 centimetres in diameter, containing sounds and images chosen to represent humanity to any civilisation that might one day encounter the spacecraft. The Golden Record was conceived by astronomer Carl Sagan and the record committee, assembled in 1977. It carries 115 images encoded as analogue video signals, greetings in 55 languages, sounds of Earth from wind and thunder to the songs of humpback whales, and 27 pieces of music spanning cultures from Bach to Chuck Berry to an Aboriginal Australian courtship song. A stylus and cartridge are stored alongside, with instructions etched in symbolic binary code showing how to play the disc at 16⅔ revolutions per minute.

The record is not a time capsule in the usual sense — it is a message cast into the cosmic ocean with the explicit understanding that its chances of being found are effectively zero. The Voyagers will not pass near another star system for roughly 40,000 years, and even then the encounter distance will be measured in light-years. The Golden Record is an act of optimism rather than communication, a statement that we existed, we built things, and we wanted the universe to know it.

06 The Long Shutdown

Every watt of remaining power is contested. As the RTGs decay, mission controllers have been forced to triage: in 2024, they shut down the cosmic ray subsystem heater on Voyager 1 to keep more critical instruments running. The strategy is to preserve at least one instrument per spacecraft for as long as possible. Engineers estimate that the Voyagers may continue transmitting until approximately 2030, at which point the RTGs will no longer produce enough power for any single instrument to operate. Even then, the spacecraft will continue on their trajectories, silent, coasting at 17 kilometres per second toward the stars, carrying the Golden Record into the galaxy.

The Deep Space Network, itself ageing, must schedule tracking time years in advance. Voyager's signal at Earth is approximately 10 to the minus 16 watts — a billionth of a billionth of a watt — requiring the full sensitivity of the 70-metre dishes to detect. When the last signal arrives, it will be indistinguishable from noise; the spacecraft will simply fade into the background of the cosmos, still moving, still out there, no longer heard.