Evidence for the existence of groups of meteorite‐producing asteroidal fragments

Abstract

Abstract-The MORP camera network in western Canada observed 56 events which we associate with meteorites larger than 0.1 kg. An additional 33 Prairie Network (central USA) fireballs with published orbits were previously identified as the sources of meteorites of at least 0.25 kg. A comparison of the MORP orbits with each other and with the PN orbits, using the D' criterion of orbital similarity, exhibits a surprising number of small values. This suggests there are groups of related objects among the 89 events. We evaluate the probability of small values of D' arising by chance from a group of random orbits that has the distribution of orbital elements expected for meteorites. There is an excess of small values of D' among the 89 meteoritic objects over the expectation for random orbits and a marked excess of very small values. Four groups comprising a total of 16 objects account for this excess. These groups exhibit a preference for the larger masses of the population and a very strong concentration of perihelia just slightly inside the Earth's orbit.Although it has been shown by others that gravitational perturbations will disperse Earth-crossing streams in times that are much less than cosmic-ray exposure ages, the properties of the four groups suggest they may be streams of fragments that crossed the Earth's orbit only recently. Such streams may include a considerable fraction of meteorites falling at a given time. Orbital evolution of these streams could alter the sample of meteorites arriving on Earth over time intervals that are less than the accumulation time of the Antarctic collections.

Decoherence time scales for “meteoroid streams”

“…Additionally, we have shown that the multiple-member streams of Halliday et al (1990) found via D' clustering also do not constitute 2.5% departures from randomness; random sets of fireballs show similar frequency of such clumps. We conclude that the current level of departure from randomness in the worldwide fireball database is statistically insignificant.…”

“…Those authors did note that the light production and fragmentation pattern of Ridgedale was quite different from that of Innisfree; they ascribed this to Innisfree having more fractures before entering the Earth's atmosphere. In a final paper, Halliday et al (1990) examined the MORP and Prairie network (McCrosky et al 1979) databases and selected several fireball groups as potentially corresponding to streams of meteoroids in Earth-crossing orbits; Innisfree/Ridgedale are part of the most significant grouping that they found, and that grouping was also proposed to be linked to a stream of asteroids by Drummond (1991).…”

“…In particular, note that D' uses differences in q rather than a, even though q changes less during multiple planetary flybys. Halliday et al (1990). Note that the collision probability heavily selects particles along the fringes of the distribution (corresponding to perihelion and aphelion near 1 AU), and thus greatly reduces the variety of Earth-impacting orbits.…”

“…Using observations from the MORP camera network operated in western Canada, Halliday et al (1990) identified 56 events which they believed to be meteorite-dropping. Combined with 33 events from the Prairie network, they had a database of 89 meteorite-dropping orbits to be probed for clustering.…”

“…Thus, because a match of D' < 0.009 is not remarkable in a world where over 400 fireballs have been compiled, we conclude from our modelling that a P¯Ìbram/ Neuschwanstein match is not a statistically significant departure from randomness. Halliday et al (1990) searched fireball databases and identified "clusters" of orbits in which each member falls within some D' cutoff of the other members. Drummond (1991) performed a search for streams of asteroids and reported that some of his asteroid streams matched the fireball streams identified by Halliday et al In this section, we examine the claims made by Halliday et al (1990).…”