Before revision: Early mission proposals Exploring Plutowas 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.
After revision: Early mission proposals Pluto, discovered, is an interesting target for planetary exploration by Clyde Tombaugh in 1930 , but Pluto presents significant challenges for exploration because of its small mass and great distance from Earth.