Tombaugh knew even then that Pluto was not large enough to be the predicted Planet X. Eventually the [[Voyager 2]] flyby returned data that allowed [[Jet Propulsion Laboratory]] mission controllers to determine that Neptune had more mass than astronomers had supposed. (Today the New Horizons flyby established the mass of Pluto, among other physical and orbital characteristics.) | Tombaugh knew even then that Pluto was not large enough to be the predicted Planet X. Eventually the [[Voyager 2]] flyby returned data that allowed [[Jet Propulsion Laboratory]] mission controllers to determine that Neptune had more mass than astronomers had supposed. (Today the New Horizons flyby established the mass of Pluto, among other physical and orbital characteristics.) |