IAU definition of planet who had theorized on the existence of the large trans-Neptunian object Planet X, and Tombaugh been searching for Planet X when he found Pluto. Almost immediately after its discovery, however, astronomers questioned whether Pluto could be Planet X. Willy Ley wrote a column in 1956 titled "The Demotion of Pluto", stating that it "simply failed to live up to the advance publicity it received as 'Planet X' before its discovery. It has been a disappointment all along, for it did not turn out to be what one could reasonably have expected". In 1978, Pluto's moon Charon was discovered. By measuring
Exploration of Pluto launched in 2030, marking the 100th anniversary of Pluto's discovery, and spending 7-8 years traveling to the Pluto system. The Fusion-Enabled Pluto Orbiter and Lander was a 2017 phase I report funded by the NASA Innovative Advanced Concepts (NIAC) program. The report, written by principal investigator Stephanie Thomas of Princeton Satellite Systems, Inc., describes a Direct Fusion Drive (DFD) mission to Pluto. A fusion reactor would be used to send a 1000 kg orbiter and lander to the Pluto system in only four years (more than twice as fast as "New Horizons"). Global Aerospace Corporation presented a Pluto lander concept