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The System of Planetary Masses

Space exploration has quickened interest in the determination of planetary masses. Why should anyone be interested? The reasons are manyfold. In combination with the radius, the mass yields the average density-perhaps the most fundamental planetary characteristic because of its many implications for the formation, chemical composition, and evolution of the planetary system. Mass values are also crucial for precise tests of gravitational theories that involve solar system dynamics. As an example on a more practical level, the mass of a planet can be essential in planning certain spacecraft missions: to play celestial billiards successfully by the use of a gravitational assist from one planet to reach another requires precise mass data for the usual fuellimited situation. Present plans call for use of this technique on "grand tour" missions to the major planets later in this decade and on the 1973 VenusMercury flyby mission. The latter may even be enhanced by the use of Mercury's gravity field to insure a reencounter with Mercury a half year later (1). For some purposes, of course, extreme precision in mass determination is not required. Yet for the outermost planet, Pluto, even the first significant figure is in doubt. The situation for