Armed with knowledge about Jupiter's intense radiation environment, NASA launched a pair of follow-on missions, Voyager 1 and Voyager 2, in 1977. (Voyager 2 actually blasted off first – on Aug. 20, 1977 -- followed 16 days later by Voyager 1.) Voyager 1 flew by Jupiter in March 1979, then reached Saturn in 1980. It became the first spacecraft to reach interstellar space in 2012.
Voyager 2 sailed past Jupiter in July 1979 and Saturn in August 1981. Voyager 2's mission was extended so that it could visit Uranus in 1986 and Neptune in 1989. It is presently in the outermost layer of the heliosphere, where the solar wind is slowed by the pressure of interstellar gas.
Astronomers used the twin Voyager spacecraft to discover that Jupiter has faint rings, lightning and a Great Red Spot that changes color. They also learned that Jupiter's moon Io has active volcanoes.
Image: Plume from erupting volcano on Jupiter's moon Io imaged by Voyager 1 in 1979. Credit: NASA