* Voyager 2: Voyager 2 is a space probe launched by NASA in 1977. It flew by Neptune in 1989, becoming the first and only spacecraft to visit the planet. Voyager 2 provided the first close-up images of Neptune and its moons, and made several important discoveries, including the presence of a magnetic field, a thin atmosphere, and a banded cloud structure.

* Cassini-Huygens: Cassini-Huygens was a joint mission of NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI). The Cassini spacecraft orbited Saturn from 2004 to 2017, and the Huygens probe landed on Saturn's moon Titan in 2005. Although Cassini-Huygens did not visit Neptune, it did make some observations of the planet and its moons from a distance.

Robots:

* New Horizons: New Horizons is a space probe launched by NASA in 2006. It flew by Pluto in 2015, becoming the first and only spacecraft to visit the dwarf planet. New Horizons continued on to the Kuiper Belt, and is expected to make a close approach to a Kuiper Belt object called MU69 in January 2019. Although New Horizons did not visit Neptune, it did make some observations of the planet and its moons from a distance.