Meteoroid streams, producing meteor showers if some part of the stream has a node near 1 AU, have complex structures which are only just beginning to be understood. The old simplistic idea of a narrow loop being formed about the orbit of a parent comet with one, or possibly two, terrestrial intersection(s) is now being replaced by the recognition that their dynamical evolution may render convoluted and distorted ribbon shapes with eight or more distinct showers being generated. As such the streams are excellent tracers of the sorts of orbital evolution which may be undergone by larger objects (asteroids and comets) in the inner solar system; indeed it is now known that objects presently observed as Apollo-type asteroids may also be the progenitors of streams.
Searches for showers associated with newly-discovered possible parent objects may be carried out either via the calculation of theoretical meteor radiants (which have hitherto been derived using an untenable method), or through searches of catalogues of individual meteor orbits. In order to accomplish the latter, about 68,000 radar, photographic and TV meteor orbits from various programmes in the U.S.A., the former Soviet Union, Canada and Australia are available from the IAU Meteor Data Center, and more than 350,000 orbits of very faint meteors have been determined over the past three years using a new facility in New Zealand.
The discovery amongst IRAS data of dust trails lagging behind comets has opened up a new way in which meteoroid streams may be investigated, although the relationship between these trails and the streams observed as meteor showers at the Earth is by no means clear at this stage. Similarly radar, radio and spacecraft impact observations of meteoroids near cometary nuclei have added to our knowledge.
In spite of the improvement in our understanding of meteoroid streams over the past few years it is clear that there is much still to be done. The words of W.F. Denning in 1923 are still pertinent: “Few astronomers occupy themselves with the observation and investigation of meteors, and yet it is an attractive field of work offering inviting prospects of new discoveries”.