Voyager 1 — beyond the heliosphere

In 1965, JPL’s Gary Flandro showed that the outer planets’ late-1970s alignment could let one spacecraft visit several worlds. A planetary flyby bends its path and, in the heliocentric frame, transfers some of the planet’s orbital energy to it. A comparable alignment recurs roughly once in 175 years.

A ten-sided aluminium bus houses the electronics. A 3.66-metre dish dominates the spacecraft and must remain accurately pointed towards Earth. Separate booms carry three radioisotope generators, a science platform and a magnetometer as far as 13 metres from the bus.

Ten bays around the bus protect the electronics. The CCS executes commands, the FDS prepares telemetry, and the AACS controls the pointing of the antenna and instruments. An eight-track tape recorder stored data for later transmission when a direct Earth link was unavailable.

Solar panels would not provide enough power at the outer planets. Each Voyager therefore carries three radioisotope thermoelectric generators. Decaying plutonium-238 produces heat that thermocouples convert into electricity. At launch they supplied roughly 470 W of electrical power.

Voyager 2 launched on a Titan IIIE–Centaur rocket on 20 August 1977. Voyager 1 followed on 5 September. The latter took a shorter, faster path, overtook its twin by December 1977 and reached Jupiter first.

Voyager 1 flew past Jupiter in March 1979. Its pictures revealed complex storms and a faint ring. Linda Morabito identified a volcanic plume above the moon Io: the first observed active volcanism beyond Earth. Voyager 2 arrived in July and imaged Europa’s cracked ice in greater detail.

Voyager 2 reached Uranus on 24 January 1986 and Neptune on 25 August 1989. It remains the only spacecraft to have visited both planets at close range. At Uranus it found more moons and imaged Miranda; at Neptune it observed fast winds, the Great Dark Spot and plumes on Triton.

Voyager 1 crossed the heliopause on 25 August 2012 and Voyager 2 on 5 November 2018. The heliopause marks the edge of the region dominated by the solar wind. It is neither the end of the Solar System nor a departure beyond the Oort Cloud. The probes measure particles, magnetic fields and plasma waves in local interstellar space.

In 2023, Voyager 1’s FDS computer stopped returning intelligible data. In 2024 engineers traced the fault to a memory chip and moved its code to other parts of memory. Engineering and then science data returned across tens of billions of kilometres.

Both probes carry a gold-plated copper record with sounds of Earth, music, 115 images and greetings in 55 languages. Carl Sagan chaired the selection committee. The cover bears playback instructions, a pulsar map locating the Sun and a scale based on hydrogen.