Pluto was discovered by American astronomer Clyde Tombaugh in 1930, when it gained the title of the most distant planet in the Solar System. In his honor, NASA added a small container containing Tombaugh's ashes to the roof of the New Horizons probe, which in 2015 approached Pluto.

With the ashes was the inscription: "Here within are the remains of the American Clyde W. Tombaugh, discoverer of Pluto and the third zone of the solar system, son of Adelle and Muron, husband of Patricia, father of Annette and Alden, astronomer, teacher, jokester and friend: Clyde W. Tombaugh (1906-1997)."