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Why was Voyager 2 able to visit Neptune after flying past Saturn and Uranus?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Voyager 2 was able to visit Neptune after passing Saturn and Uranus due to a rare orbital alignment of the outer planets that occurred during the mission's timeframe. This specific positioning allowed the spacecraft to utilize a gravity assist—often called a "slingshot" technique—where the probe used the gravitational field of each planet to increase its velocity and adjust its trajectory toward the next destination.1

While both Voyager 1 and Voyager 2 were originally slated to visit only Jupiter and Saturn, the specific timing of Voyager 2’s launch provided the necessary leeway to redirect its path. By leveraging these gravitational maneuvers, Voyager 2 was able to complete an extended mission that included flybys of Uranus in 1986 and Neptune in 1989, making it the only spacecraft to have visited those two planets.231