Injection of neutral gases into space plasmas has emerged as a powerful technique in reproducing many of the naturally occurring processes in planetary physics in a controlled way. In this article, we present thorough simulation results aimed at explaining and interpreting the AMPTE experiments. The results show the formation of a diamagnetic cavity and a shocklike structure, intense wave activity upstream of the shock, the sideways deflection of the bulk of the AMPTE cloud, the Rayleigh--Taylor instability, and the ejection of a tenuous cloud of plasma. All these results are in qualitative agreement with AMPTE observations. ¿ American Geophysical Union 1993