Recent spacecraft observations near Venus and Mars have shown that the position and shape of the planetary bow shock do not agree well with predictions of gas dynamic models in which the obstacle boundary is defined by ionopause/magnetopause pressure balance estimations. In order to simulate some aspects of this problem, we have considered plasma flow into a ''heavy'' ion cloud, whose maximum density is comparable to that of the inflowing protons. A simple bi-ion fluid model is used which takes into account the electrostatic coupling between protons and (heavy) ions of exospheric origin; magnetic field effects are ignored. One-dimensional studies show that for subsonic flows, representing the post-shock solar wind, a critical cloud density exist at which the flow is accelerated up to the bi-ion sound speed. Above this critical value no continuous solutions exist. The flow behavior in three dimensions was studied in an axially-symmetric model. For overcritical clouds the ''critical density layer'' appears as an ''obstacle'' forming an extended proton cavity. The planetopause at Mars may find an explanation within the present model. ¿ American Geophysical Union 1992