The solar wind interacts directly with planets whose intrinsic magnetic fields are too small to protect their atmospheres and ionospheres. The research will focus on the interaction of the solar wind with non-magnetic solar system bodies such as Venus, Mars, Titan, and comets. Research in this area contributes to a broader understanding of solar-planetary relations in general. The proposed research activity consists of theoretical modeling of the interactions of the solar wind, and other external plasma media, with Venus, Mars, Titan, and comets. Both test particle and magnetohydrodynamical calculations will be undertaken as part of this research effort.