The interaction between the solar wind and unmagnetized planets was investigated using computer simulation and a three-dimensional hybrid code that treats kinetic ions and massless fluid electrons. In the simulation the planet was treated as an ionized gaseous body with uniform and constant supply of plasma. The results given in this paper showed that the shock size and magnetic barrier intensity are asymmetrical in the direction of the convection electric field because of oxygen ions escaping from the side of the planet to which the electric field was pointing. The calculated asymmetry in the magnetic barrier intensity was consistent with observations, although the direction of the calculated asymmetry in the shock size did not match the asymmetry observed near either Venus or Mars. When the planet size was comparable to the Larmor radius of protons, a multiple-shock structure was observed in the simulation. It is suggested that this structure is due to dispersion of the magnetosonic waves caused by the finite Larmor radius of protons. The simulation results also showed that ions escaped to the magnetotail through the tail rays and that the tail rays were connected with the plasma sheet. ¿ 2001 American Geophysical Union