The Voyager Grand Tour

Four complex spacecraft became two Voyagers

The original Grand Tour concept used four highly complex spacecraft for five outer planets. NASA cancelled it in January 1972 largely because of expected cost and approved two spacecraft for Jupiter and Saturn. Their design and trajectories nevertheless preserved a possible extension.

Evidence: NASA — Voyager 2 ↗ · NASA — Voyager Fact Sheet ↗

Every planet had to be in the correct place

A gravity-assist chain only works when arrival at each planet leads toward the next target. The 1977 launch opportunity connected Jupiter, Saturn, Uranus and Neptune. The mission therefore depended on timing and geometry, not simply on a more powerful launch vehicle.

Evidence: NASA — Voyager Planetary Voyage ↗ · NASA — Trajectories and Gravity Assists ↗

Titan for Voyager 1, the outer planets for Voyager 2

Voyager 1 took the faster Jupiter–Saturn route and a high-priority Titan flyby that sent it above the planetary plane. Voyager 2 passed Saturn at a point leading to Uranus; after success, NASA authorised Uranus and then Neptune.

Evidence: NASA — Voyager Fact Sheet ↗ · NASA — Voyager 1 ↗ · NASA — Voyager 2 ↗

Jupiter, Saturn and Uranus prepared the next encounter

Each flyby exchanged a minute amount of momentum with the orbiting planet and changed the spacecraft’s heliocentric route. Jupiter sent both Voyagers toward Saturn; Saturn and Uranus continued Voyager 2 to Neptune. NASA says the assists reduced travel time to Neptune by nearly twenty years compared with an unassisted route.

Evidence: NASA — Gravity Assist Primer ↗ · NASA — Voyager technical facts ↗ · NASA — Voyager 2 ↗