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This study utilizes gas dynamic calculations in conjunction with observational bow shock models to investigate the solar wind flow patterns about the terrestrial planets. Average dayside bow shock position could be predicted for the earth by theory with an error of only -•2%, given the observed shape and location of the magnetopause. Accordingly, our findings confirm the validity of the singlefluid gas dynamic approximation for describing this major aspect of solar wind flow past the earth. Modeled using gas dynamic theory, the solar wind interactions with Venus and Mars exhibit very significant differences. At Mars the mean inferred altitude of the solar wind-obstacle interface varies from 510 km at the stagnation point to almost 1000 km near the terminator. The effective magnetic moment required to produce a magnetosphere of this size for average solar wind dynamic pressures and terrestrial-type internal current systems is 1.4 ___ 0.6 x 1022 G cm 3. Gas dynamic modeling of the January 21, 1972, Mars 3 and July 20, 1976, Viking 1 lander particles and fields observations supports the conclusion that the Martian obstacle to the solar wind lies at altitudes too high for it to be associated with only an ionospheric or atmospheric interaction. In contrast with Mars, our modeling of the Venus observations has found that the bow wave is closer to the planet than would be expected for a purely ionospheric obstacle. The subsolar width of the Venus ionosheath in the Venera and PVO measurements is only 60% and 90%, respectively, of that predicted by the gas dynamic model. This result is attributed to the presence of solar wind-neutral atmosphere interactions in the lower ionosheath that are not included in the gas dynamic code. 19 et al., 1982], cometary processes (i.e., charge exchange and photoion pickup) in the ionosheath can exert a great influence over the flow [Wallis, 1972; 1973]. While passing through the inner solar system, most active comets are thought to deflect the solar wind at large distances through charge exchange and photoion-pickup-induced mass loading of the incident solar wind flow [Biermann et al., 1967]. Clarification of the role played by the neutral atmosphere in the Cytherean interaction is critical to the use of Venus as an analog of the solar wind flow about comets. Third, at Mars we use gas dynamic models to infer the averge height of the solar wind-obstacle interface and evaluate the controversial

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