Voyager 2

Voyager 2 launched on 20 August 1977, before its twin Voyager 1, but followed a longer route through the outer Solar System. The spacecraft visited Jupiter in 1979 and Saturn in 1981. Its Saturn encounter was planned to permit a gravity-assisted journey onward to Uranus and Neptune, destinations no spacecraft had yet reached. It flew past Uranus in January 1986 and Neptune in August 1989, using each planet's gravity to alter its path. These rapid flybys made the mission a rare survey of all four giant planets, rather than a series of orbital studies.

At Jupiter, Voyager 2 returned observations of the planet, its rings and moons, adding to the discoveries of Voyager 1. At Uranus it became the first spacecraft to conduct a close flyby and identified ten previously unknown moons and two rings. Its observations also showed that Uranus has a strongly tilted, off-centre magnetic field. At Neptune, the probe discovered six moons and four rings and observed the planet's dynamic atmosphere, including the feature known as the Great Dark Spot. Together, the encounters revealed that the outer planets and their satellite systems were more varied than brief ground-based observations had suggested.

After Neptune, Voyager 2 continued away from the Sun. It crossed the heliopause—the boundary where the outward-flowing solar wind gives way to the interstellar medium—in 2018. Measurements made on the far side of this boundary provide direct evidence about the environment beyond the heliosphere. Interstellar space here means the region outside the heliosphere; the spacecraft remains within the Milky Way and is not near another star.

Voyager 2 carried imaging, spectroscopic, magnetic-field, plasma and particle instruments, along with a radio-science experiment. The spacecraft sent data to Earth by radio, and planetary flybys depended on careful navigation and long-distance tracking. Gravity assists at the giant planets changed the spacecraft's speed and direction without requiring a large onboard propulsion system. Its long journey required mission controllers to manage limited electrical power and to reduce instrument use as the spacecraft aged.