Originally called the Mariner Jupiter/Saturn '77 (MJS '77) Project, Voyager's 1 and 2 (launched September 5, 1977 and August 20, 1977 respectively) took advantage of an alignment of the four Gas Giants, Jupiter, Saturn, Neptune and Uranus, which occurs every 176 years. Using gravity assist flybys and well-planned trajectories, the spacecraft imaging and data gathering instruments, they explored the four planets as well as their moons and the rings during the longest and most complex of any unmanned mission.
From the description of Voyager Guidance and Control Collection, 1966 - 1981. (Jet Propulsion Laboratory Library and Archives). WorldCat record id: 733099991