The D Discriminant and Near-Earth Asteroid Streams

A method of separating an association of orbits, a stream, from the local background using the traditional D discriminant is introduced. Applying this procedure to all 708 asteroids with perihelia less than 1.3 AU known on April 22, 1999 suggests that the orbits of Earth-approaching asteroids may be rather organized. By searching the volume around every orbit in the sample space, 14 associations are found with from 4 to 25 members that statistically stand out from the local background. A more traditional search, where orbits are gathered into strings with every orbit connected to at least one other orbit by D< D c , reveals eight additional three-member strings. Using Poisson statistics seven pairs are identified as having exceptionally similar orbits; three of these pairs are a part of an association or string. On the other hand, comparing the results of a volume search around randomized orbits indicates that perhaps 50 to 75% of these groupings are due to chance, indicating that observational effects may severely affect the background and therefore the reality of the associations that are separated out. The lack of compositional or taxonomic information about most of the 155 asteroids, 22% of the sample, hinders interpretation of the nature of the streams. Of the three associations found in an earlier study (Drummond 1991, Icarus 89, 14–25), only one remains intact as a result of the stricter statistical requirements adopted here. Three of four fireball streams (Halliday et al. 1990, Meteoritics 25, 93–99), previously found to be related to the old associations, can be loosely associated with three of the new associations, but none of the four recovered meteorites with known orbits can be associated with them. Nevertheless, theoretical meteor radiants are listed, all of which are so large and diffuse, and have such slow geocentric velocities, that it is doubtful if visual meteor showers can be linked to the new streams. However, these associations, strings, and pairs of asteroids are good candidates for meteorite sources, especially where organized falls are recognized, since they generally satisfy the immediate dynamical requirements for delivery, namely, gentle geocentric velocities produced by overtaking the Earth around sunset.