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Astron. Astrophys. 353, 797-812 (2000)

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1. Introduction

In the last decade, the complex of interplanetary solid bodies which populate the near-Earth part of the Solar System and can collide with our planet has attracted considerable attention from planetary scientists, dynamicists, meteoriticists and even geologists. These bodies range in size from micrometric particles to multi-km asteroids/comets and have a variety of chemical, physical and dynamical properties. The genetic relationships among the subpopulations observed (with different techniques) at different sizes and with the presumed source populations are very complex, and so are the main dynamical and collisional evolutionary mechanisms at work. In particular, our knowledge is very limited in the 0.1-10 m size range, because the corresponding bodies are too small to be detected in space by astronomical techniques, and at the same time are so rare that they do not hit the Earth frequently enough to provide us with large data samples for statistical work. Only recently, observations from space-based optical sensors have provided relevant information about their flux into the high atmosphere (Tagliaferri et al. 1994). Yet, these bodies are very important, because they frequently deliver meteorites to the Earth's surface, and the relationship between meteorites and their parent asteroids is an outstanding scientific issue under rapid development. For instance, it has been recently pointed out (Farinella et al. 1998; Vokrouhlický & Farinella 1998) that for these small bodies a subtle non-gravitational force (the so-called Yarkovsky effect ) may provide significant semimajor axis mobility in the main belt, making more efficient their transport into the resonant "escape hatches" which eventually deliver them to near-Earth space (for a recent detailed discussion of the relevant data and their implications, see Morbidelli & Gladman 1998).

Some five years ago, we first tackled the dynamical side of this problem, by studying numerically the long-term orbital evolution of 17 very bright bolides, mostly ranging in size between 1 and 10 m and including the four ones observed photographically and associated with recovered meteorite falls (Jopek et al. 1995). The most important conclusion of that paper was that the main dynamical mechanisms and evolutionary patterns were fairly similar to those previously found for sizeable near-Earth objects, suggesting common sources for the two populations. Only 2/17 bodies had comet-like orbits undergoing close encounters with Jupiter, indicating a minor but non-negligible cometary component. This was in agreement with earlier results on the orbits of sporadic photographic meteors (e.g., Whipple 1938) and with a variety of other arguments and observations (Binzel et al. 1992). The four meteorite-delivering, photographically observed bodies (all ordinary chondrites) had all dynamical behaviours consistent with an origin in the inner part of the asteroid belt.

A critical aspect of this kind of work is the fact that only for a small fraction of the observed bolides data are available of sufficient quantity and quality to allow a reliable determination of pre-atmospheric entry orbital elements, from which initial conditions for the long-term orbital integrations can be calculated. On the other hand, it is clear that the statistical robustness of any conclusion we may draw from the integrations depends on having analysed a sufficiently numerous and representative sample of bodies. Thus, we have now carried out a thorough search in the available literature to identify all the other bright bolides for which orbital data are available or can be derived. As we shall see in Sect. 2, we have now identified 20 more such bodies, mostly appeared in the time interval between 1993 and 1996 and reaching at least visual magnitude -10, with inferred sizes ranging from about 0.1 to 10 m. After analysing the distribution of the corresponding orbital parameters (Sect. 3), we have derived initial conditions for the integrations with the same methods discussed in Jopek et al. (1995), as summarized in Sect. 4 below. Then, we have integrated these orbits over a longer time span than we had done in 1995 (at least 10 Myr), thanks to the increased computing speed which is currently available. The results of these integrations are discussed in Sect. 5, and the main conclusions and some remaining open problems are summarized in Sect. 6.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999