Voyager 2 launches — only spacecraft to visit Uranus and Neptune

On 20 August 1977 a Titan IIIE-Centaur rose from Cape Canaveral carrying Voyager 2, the spacecraft that would visit more worlds than any machine before or since. In one of spaceflight's small ironies it launched sixteen days before Voyager 1, which would overtake it on a faster path; the twins were numbered by their order of arrival at Jupiter, not their departure from Earth. Both were racing to exploit a piece of celestial luck: an alignment of Jupiter, Saturn, Uranus and Neptune that occurs roughly once every 176 years, letting a single spacecraft slingshot from giant to giant.

The tour unfolded over twelve years. Jupiter came on 9 July 1979, Saturn on 25 August 1981. Then came the part no other spacecraft has ever repeated: Uranus on 24 January 1986, a sideways-tilted world where Voyager 2 discovered ten new moons, and Neptune on 25 August 1989, where it skimmed over the cloud tops, found the Great Dark Spot, and caught geysers of nitrogen erupting from the frigid moon Triton. Almost four decades later, Voyager 2 remains the only spacecraft ever to have visited the two ice giants, and the only one to have flown past all four giant planets.

The mission refused to end. On 5 November 2018, more than 18 billion kilometres from home, Voyager 2 felt the solar wind die away around it and crossed the heliopause into interstellar space, six years behind its twin. Crucially, its plasma science instrument still worked, while the equivalent on Voyager 1 had failed back in 1980, so Voyager 2 returned humanity's first direct plasma measurements of the boundary as it crossed. Nearly half a century after launch, running on the dwindling warmth of its plutonium generators, it is still out there, whispering data home across more than a light-day of emptiness.