Published online by Cambridge University Press: 27 February 2018
Despite the complicated structure of old meteoroid streams in Halley-type orbits, the positions of their ecliptic-plane crossings are insensitive to the initial conditions of ejection and have a form determined by resonances due to planetary perturbations. This allows the main observational features of meteor showers such as the η-Aquarids, Orionids, Lyrids and Perseids to be understood. A proportion of the particles ejected into such orbits librate about the centres of mean-motion resonances and produce compact streams which may lead to meteor storms.