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   In 1962, a team of 10 leading astrodynamicists from Lockheed
       Sunnyvale studied the possibility of achieving a nonstop free-fall
       multiplanetary round-trip trajectory with a single spacecraft
       launched from Earth, passing Venus and Mars (or Mars and
       Venus), and returning to Earth.  They presented an astrodynamic
       argument that they believed showed that the launch energy  
       requirements were so high, the possibility was an "academic
       pastime" that might be achieved with nuclear propulsion.  See
       page 5-1 of their 180 page Final Report.
 

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