A major problém in meteor astronomy is why the orbits of meteoroids within particular streams are so dispersed. For streams with aphelia well within Jupiter (such as the Geminids) planetary perturbations cause insignificant dispersion but can accommodate the required motion of the nodal heliocentric distance to explain why the Geminids were not observed prior to the 1860's. The spread in the orbits wouid also require unreasonably large ejection velocities from the parent. Another dispersal mechanism is therefore required. By incorporating perturbations due to the Yarkovsky-Radzievskii effect into the model the Geminid
dispersal can be understood; by including also the effects of the radiation pressure and Poynting- Robertson forces the main observed characteristics of the stream (shower duration variation with magnitude; skew rate profile; changes in mass distribution and radiant diffuseness as the shower progresses) are explicable. The necessary spin rates (about 3000 rev/sec for 1 mm and 1000 rev/sec for 1 cm radius meteoroids) wouid be attained within a thousand years of release from the parent body, due to spin-up under solar radiation pressure. It therefore appears that the Yarkovsky- Radzievskii effect is an important source of stream dispersion which has been hitherto neglected, but should be included in future models.