Voyager 1 and 2 (1977): The Grand Tour of the Outer Planets

The Voyager programme consists of two NASA space probes, Voyager 1 and Voyager 2, launched in August and September 1977 to explore the giant outer planets. Between 1979 and 1989 they flew past Jupiter, Saturn, Uranus and Neptune, transforming knowledge of these worlds and their moons. Each carries the Golden Record, a message for any civilisation that might find it, and both have since become the first human-made objects to reach interstellar space.

Background: the Grand Tour

In 1965 Gary Flandro, a graduate student working at NASA’s Jet Propulsion Laboratory (JPL), calculated that in the late 1970s Jupiter, Saturn, Uranus and Neptune would line up in a way that occurs only about once every 175 years. A spacecraft could use the gravity of each planet in turn to bend its path and speed it towards the next, a technique called gravity assist. A journey to Neptune that would otherwise take about 30 years could be done in around 12.

NASA planned an ambitious “Grand Tour” with several large spacecraft, but budget pressures led to its cancellation in the early 1970s. In its place came a cheaper mission, Mariner Jupiter-Saturn 1977, with two probes designed for Jupiter and Saturn only. Engineers quietly made the spacecraft robust enough that one might continue further if it survived. Shortly before launch the project was renamed Voyager.

The spacecraft and launch

Each Voyager weighs about 825 kilograms at launch and is dominated by a dish antenna 3.7 metres (12 feet) across, used to communicate with Earth. Because sunlight is too weak in the outer Solar System for solar panels, power comes from three radioisotope thermoelectric generators, which turn the heat of decaying plutonium-238 into electricity. Together they produced about 470 watts at launch.

The probes carry cameras, spectrometers, magnetometers and instruments for measuring charged particles, plasma waves and cosmic rays. Their computers have only a few tens of kilobytes of memory, far less than a modern phone, yet engineers have reprogrammed them repeatedly over the decades.

Voyager 2 lifted off from Cape Canaveral on a Titan IIIE-Centaur rocket on 20 August 1977. Voyager 1 followed on 5 September 1977. Despite launching second, Voyager 1 travelled on a shorter, faster trajectory and passed its twin in the asteroid belt in December 1977, which is why it carries the number 1.

The planetary tour

Over the next twelve years the two probes made a series of flybys, each one timed so that the planet’s gravity would redirect the spacecraft towards its next target. Every encounter was planned years in advance and the commands were sent up hours or days ahead, because radio signals took longer and longer to cross the growing distance.

Jupiter, 1979

Voyager 1 made its closest approach to Jupiter on 5 March 1979, and Voyager 2 on 9 July 1979. The images showed the turbulent cloud bands and the Great Red Spot in unprecedented detail and revealed a faint ring around the planet.

The greatest surprise was Io, the innermost large moon. On 9 March 1979 JPL navigation engineer Linda Morabito noticed a plume rising from its edge in a Voyager 1 image. It was a volcanic eruption, the first active volcanism ever seen beyond Earth, driven by tidal heating as Jupiter’s gravity flexes the moon. Voyager also showed that Europa is covered in cracked ice, raising the possibility of an ocean beneath.

Saturn, 1980–1981

Voyager 1 passed Saturn on 12 November 1980. Its path was chosen to fly close to Titan, Saturn’s largest moon, which was known to have an atmosphere. Voyager found that Titan’s atmosphere is mostly nitrogen and denser than Earth’s, but a thick orange haze hid the surface. The close Titan flyby bent Voyager 1’s path sharply out of the plane of the planets, ending any chance of visiting another world.

Voyager 2 passed Saturn on 25–26 August 1981. Together the probes revealed thousands of ringlets in Saturn’s rings, strange spoke-like features, the braided F ring and several new moons.

Uranus and Neptune

Because Voyager 1 had been sent to Titan, only Voyager 2 could continue the Grand Tour. With extended funding it reached Uranus on 24 January 1986, the first spacecraft to do so. It discovered 10 new moons and two new rings, measured the planet’s oddly tilted magnetic field and photographed the jumbled surface of the moon Miranda. The encounter received less public attention than it might have, because four days later the Space Shuttle Challenger was lost shortly after launch.

Voyager 2 flew past Neptune on 25 August 1989, after a journey of 12 years. It found a large storm called the Great Dark Spot, the fastest winds measured on any planet, six new moons and complete but faint rings. On Neptune’s largest moon, Triton, it saw dark plumes rising from a frozen nitrogen surface, a form of activity on one of the coldest bodies in the Solar System.

The Golden Record

Each Voyager carries a 12-inch gold-plated copper phonograph record, together with a stylus and instructions engraved on its cover showing how to play it and where Earth is. The idea followed earlier plaques on the Pioneer 10 and 11 probes and the Arecibo message of 1974.

The contents were chosen in 1977 by a committee chaired by Carl Sagan, including Frank Drake, Ann Druyan, Timothy Ferris, Jon Lomberg and Linda Salzman Sagan. The record holds 116 encoded images, spoken greetings in 55 languages, sounds such as wind, birdsong and a heartbeat, and about 90 minutes of music ranging from Bach and Beethoven to Javanese gamelan, Navajo chant and Chuck Berry’s “Johnny B. Goode”. It also includes printed messages from President Jimmy Carter and UN Secretary-General Kurt Waldheim.

Sagan stressed that the record would be found only if advanced spacefaring beings encountered it, which is very unlikely. It is best seen as a time capsule and a statement about humanity.

The Pale Blue Dot and interstellar space

On 14 February 1990 Voyager 1, then about 6 billion kilometres from the Sun, turned its cameras back to take a “family portrait” of the planets. One frame showed Earth as a speck less than a pixel wide, caught in a streak of scattered sunlight. Sagan, who had argued for the picture, made it the theme of his 1994 book Pale Blue Dot. Soon afterwards the cameras were switched off permanently to save power and memory.

After the planetary encounters, the mission was renamed the Voyager Interstellar Mission. On 17 February 1998 Voyager 1 overtook Pioneer 10 to become the most distant human-made object. It crossed the termination shock, where the solar wind slows abruptly, in December 2004; Voyager 2 did so in 2007.

On 25 August 2012 Voyager 1 crossed the heliopause, the outer boundary of the Sun’s bubble of charged particles, and entered interstellar space. Voyager 2 followed on 5 November 2018, providing the first measurements of the boundary from a working plasma instrument. By the mid-2020s Voyager 1 was more than 24 billion kilometres from Earth, so far that its radio signals take over 22 hours to arrive.

Edward Stone, the project scientist from 1972 to 2022, guided the science for half a century. The ageing probes have survived several crises, including a fault in Voyager 1’s flight data system in late 2023 that engineers fixed remotely in 2024.

By the numbers

Timeline

• 1965: Gary Flandro identifies the rare planetary alignment that makes a Grand Tour possible.
• 1977, 20 August: Voyager 2 launches from Cape Canaveral.
• 1977, 5 September: Voyager 1 launches.
• 1979, 5 March: Voyager 1 passes Jupiter; volcanoes on Io are discovered days later.
• 1980, 12 November: Voyager 1 flies past Saturn and Titan.
• 1986, 24 January: Voyager 2 makes the first flyby of Uranus.
• 1989, 25 August: Voyager 2 makes the first flyby of Neptune.
• 1990, 14 February: Voyager 1 takes the Pale Blue Dot image.
• 2012, 25 August: Voyager 1 enters interstellar space.
• 2018, 5 November: Voyager 2 enters interstellar space.

Legacy

Voyager provided detailed views of all four giant planets and dozens of moons, and for Uranus and Neptune it remains the only close-up source. Its discoveries of volcanic Io, icy Europa and hazy Titan inspired later missions such as Galileo, Cassini-Huygens, Juno and Europa Clipper. NASA put the cost of the mission from 1972 through the Neptune encounter at about US$865 million.

The Golden Record and the Pale Blue Dot also gave the mission a cultural life far beyond science. Like the Hubble Space Telescope later, Voyager changed how the public imagined the universe and Earth’s place in it.

Why it matters

Voyager turned the outer planets from blurry points of light into real worlds, and it carried humanity’s first physical message into interstellar space. Decades after the end of their planetary tour, the probes still send data from beyond the Sun’s influence, making Voyager one of the most productive journeys of exploration ever made.