Launched in August 1977, Voyager 2 reached Jupiter on a trajectory chosen to use the planet's gravity to fling it on toward Saturn. Its closest approach came at 22:29 UT on July 9, 1979. In the months around the encounter it photographed Jupiter and its moons Amalthea, Io, Europa, Ganymede and Callisto. During a ten-hour "volcano watch" it confirmed that Io was volcanically active and showed how the moon's surface had changed since Voyager 1's visit that March.
The two Voyager spacecraft were designed to take advantage of a rare alignment of the outer planets. Voyager 1, launched 16 days after its twin on a faster path, reached Jupiter and Saturn first; Voyager 2 followed a slower route that kept open the option of continuing to the ice giants.
At Jupiter, Voyager 2's images of Europa showed a bright, smooth surface criss-crossed by dark lines, hints of an icy crust that scientists now think covers a global ocean. Between them the two Voyagers watched nine volcanoes erupting on Io, the first active volcanism seen anywhere beyond Earth.
The encounter was only the start. Voyager 2 went on to Saturn in 1981, Uranus in 1986 and Neptune in 1989, and it remains the only spacecraft to have visited the two ice giants. On November 5, 2018, still transmitting, it crossed into interstellar space.