The two robotic Voyager spacecraft (1977 - 1989) opened up the study of the giant planets (atmospheres, interiors, rings, satellites).

Voyager 1 flew past Jupiter and Saturn and then left the Solar System. Voyager 2 continued on to fly by both Uranus and Neptune.

The Galileo spacecraft (1989 - present) became an orbiting satellite of Jupiter (and dropped a probe into the planet's upper cloud levels).