(1) Obs.Cote d'Azur, France
(2) I.A.S.-Planetologia, Italy
(3) Obs. UAM, Poznan, Poland
The orbital similarity criterion by Southworth and Hawkins, customarily used to classify meteors in streams, is essentially a generalized distance in a 5-dimensional space of orbital elements: q,e,i,peri,node. Meteor orbits however, are characterized by only 4 independently measured quantities, due to the fact that a meteor can be observed only at one of the nodes, and at a heliocentric distance of 1 AU. We have therefore looked for a set of 4 variables, possibly directly deducible from observed quantities, that would allow meteor stream identification without passing through the computation of orbital elements. It turns out that some of the quantities that are used in the geometric setup of Opik's theory of close encounters are ideally suited for the task. Thus, we have been able to establish and test a stream identification criterion based on the geocentric unperturbed velocity and three angles; of these, the first two are related in a simple way to the radiant, and the third one is given by the ecliptic longitude of the Earth. Moreover, it is possible to show that 2 of the 4 quantities that enter the new criterion are quasi-invariants of the principal secular perturbation affecting meteoroid orbits, the so-called Kozai cycle. It is therefore possible to intrduce a formulation of the criterion, reduced to only these 2 variables, that allows to identify those streams, even on currently widely separated orbits, that are originated by the same parent body. A new distance function, measuring meteor orbital similarity, has been applied in the analysis of a sample of 865 precisely reduced photographic meteor orbits. The new metrics successfully detected many previously known meteor streams of 3r more members. The results have been compared with a search made using the D-criterion of Southworth and Hawkins. For 25 streams, including the Perseids, the Orionids, the Geminids, and the Quadrantids, the two searches turned out to be in satisfactory agreement. However, some discrepancies occurred that are related to the complex of near ecliptic streams like the Taurids, the Andromedids, the Virginids, and the sigma Leonids.