A meteoroid stream generally considered to be an elliptical ring of relatively small thickness. Such shape is attributable to meteoroid streams in an early stage of their evolution. Differences in planetary perturbations influencing the meteoroid particles ejected from the parent body from various points in its orbit at different velocities and time can result in a significant thickening
of the stream. Our studies on the evolution of the short-period meteoroid streams have shown that these streams can produce several couples of showers active indifferent seasons of the year.
Since the discovery of Phaethon (the 'Geminid asteroid') in 1983 there has been renewed interest in the association of meteor showers with Earth-approaching asteroids. In the past the existence of several streams associated with different asteroids (in particular Adonis, Apollo, Hermes and Oljato) has been suggested but not proven. Here the 3759 meteor orbits determined at Adelaide in the 1960's have been compared to the orbits of all known
Aten-Apollo-Amor asteroids using a new and powerful search technique. Strong evidence is found for streams associated with Apollo-type asteroids Icarus, Hermes, Adonis, Oljato, Hephaistos, 5025 P-L, 1982 TA, and 1904 KB;, the final five in this list may be members of the Taurid-Arietid complex, as is Comet P/Encke.
No stream associated with 1862 Apollo was identified, but this may be due to the lack of observations at the appropriate solar longitude. No streams associated with any Aten or Amor asteroid
were found: this may be due to the limited detectability of their meteors, if they exist, because of their low geocentric velocities. In general streams were discovered for all asteroids coming within 0.1 AU of the Earth, having radiants observable from Adelaide and atmospheric velocities above about 22 km/sec : this suggests that meteoroid streams are a general feature of Earth-approaching
asteroids.
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.