With the first Mariner missions underway and the Voyager Mars landers apparently set to start in the late 1960s, JPL began to consider the Navigator missions that were to venture through the asteroid belt to explore the outer solar system. The design of this spacecraft was to be sufficiently flexible for planetary flybys and orbital missions (in some cases ferrying soft landers) and to perform high-resolution imaging and a multitude of other experiments. It would use the powerful (and expensive) Saturn V launch vehicle, be nuclear powered and, because it was thought that missions to the outer planets would not be feasible using conventional chemical propulsion since the time spent in the interplanetary transfer would be excessive, ion thrusters were to be used for continuous acceleration to the objective. In parallel, JPL issued contracts to both Lockheed and General Dynamics to study whether the existing or imminent technologies might facilitate a mission that would attempt to reconnoiter the asteroid belt in 1967—1975 and perform a flyby of Jupiter in 1973—1980. General Dynamics offered four RTG-powered designs, ranging from a simple ‘spinner’ capable of only limited scientific observations, up to a complex 3-axis-stabilized spacecraft capable of a comprehensive study.1,2,3 Meanwhile, a discovery was made that would not only make missions to the outer solar system more manageable and less time-consuming, but would also revolutionize planetary exploration.
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