Catching the Planets at the Right Moment

The opportunity depended on a timetable no space agency could change. In the late 1970s, the giant planets occupied positions that made it possible for a spacecraft to travel from one to another using gravity assists. Missing that arrangement would mean losing an unusually efficient route. Voyager’s planners had to build machines that could leave on time and remain useful long after the launch crews had finished their work.

Voyager 2 launched on August 20, 1977, followed by Voyager 1 on September 5. The numbering did not describe the order in which they left Earth. Their trajectories differed, and Voyager 1 would reach Jupiter and Saturn first. Sending two spacecraft created options, but did not make the missions interchangeable. Each path determined which encounters would remain possible later.