Only once every 175 years are the outer planets aligned in their orbits
so that we can take advantage of gravity-assist trajectories to achieve
encounters with Jupiter, Saturn, Uranus, and Neptune on one mission. The
gravity-assist technique uses one planet’s gravity field and motion
through space to alter the spacecraft’s flight path and propel it
outward toward the next planet. Voyager 1’s trajectory, which was
selected to best view Titan, has now propelled the spacecraft out of the
ecliptic plane, while Voyager 2’s path will remain in this plane to
provide future encounters with Uranus and possibly with Neptune.