In a new departure in experimental Space Plasma Physics, lithium and barium plasmas were injected into the solar wind approximately 110000 kms above the Earth's surface. The effects, monitored in-situ by two spacecraft of the AMPTE (Active Magnetospheric Particle Tracer Explorers) mission, included the formation of diamagnetic cavities, a slowing down locally of the solar wind, generation of plasma waves and heating of solar-wind electrons. These comet-like interactions between the solar wind and obstacles presented by the injected plasmas are discussed and compared with the effect of the natural obstacle of the Earth's magnetosphere first encountered by the solar wind at the bow shock. Particular reference is made to the 'artificial comet' created by a barium-ion release on 27 December 1984.