Sun and comets as sources in an external flow

The interaction of the largely neutral interstellar gas with the heliosphere, on the one hand, and the interaction of the ionized solar wind with largely neutral cometary comas, on the other, have a number of phenomenological similarities. Both the sun and comets act as sources embedded within an external flow. This communication is an attempt to organize the similarities as well as the differences in a form amenable for further, more detailed intercomparisons. It is noted that the drag on a cometary nucleus or on the sun from the outer flow is zero and that the presence of neutral gas affects the stability of plasma flow patterns.