A study of the solar wind within comet Halley's ionosheath is presented. It is shown that the plasma changes seen across the intermediate (''mystery'') transition, located approximately half way between the bow shock and the cometopause along the Giotto trajectory, are similar to those occurring across an equivalent transition present within the Venus ionosheath. As in Venus, the observed plasma changes are consistent with those expected from the onset of friction phenomena between the shocked solar wind and the main body of ionospheric plasma. It is suggested that the intermediate transition in comet Halley's ionosheath represents the outer boundary of a thick viscous boundary layer that develops from the nose of the cometopause and extends along the flanks of the ionosheath. On the basis of this interpretation it is concluded that the subsolar position of the cometopause may have reached ~3.4 105 km upstream from the nucleus at the time of the Giotto measurements. ¿ American Geophysical Union 1989