Voyager program
Exploring Pluto was contemplated since its discovery by Clyde Tombaugh in 1930, but Pluto presents significant challenges for exploration because of its small mass and great distance from Earth. The two probes of the Voyager program, launched in 1977 to explore Jupiter and Saturn, also had the ability for an extended mission to other targets. Pluto was rejected in favor of Titan, Uranus and Neptune.