List of missions to the outer planets

The nine missions include two, Ulysses and New Horizons, whose primary objectives were not outer planets, but which flew past Jupiter to gain gravity assists en route to a polar orbit around the Sun (Ulysses), and to Pluto (New Horizons).

Cassini–Huygens also flew past Jupiter for a gravity assist on its mission to explore Saturn.

Voyager 2 is the only spacecraft to have visited Uranus, making a single flyby as part of its grand tour of the outer planets.

Voyager 2 is the only spacecraft to have visited Neptune, making a single flyby as part of its grand tour of the outer planets.

New Horizons is the only spacecraft that visited dwarf planet Pluto (in 2015) and the trans-Neptunian object 486958 Arrokoth (in 2019).

Montage of planets and some moons that the two Voyager spacecraft have visited and studied. It is the only program that visited all four outer planets.
Summary of missions to the outer planets and beyond.
True color NASA image of Neptune
True color NASA image of Neptune