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In 1962 JPL's technical staff made a detailed study of the energy
       requirements for interplanetary space travel throughout the entire
       solar system and concluded that the energy requirements were so 
       high they cannot be achieved even using nuclear propulsion and
       enormous launch vehicles larger than the Saturn V launch
       vehicle (i.e., it was a physical impossibility).  See page 9 of their
       1962 paper "Nuclear Electric Space-Craft For Unmanned
       Planetary and Interplanetary Missions."

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