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

6. Conclusions

The main conclusions from this work can be summarized as follows.

1. We confirm the conclusion by Jopek et al. (1995) that a great variety of orbital parameters and evolutions is found in the bolide population, quite comparable to that observed for the larger near-Earth objects. The main dynamical mechanisms affecting the orbits are mean motion and secular resonances (frequently overlapping each other) and close planetary encounters. This results in very chaotic evolutions, with dynamical lifetimes which can be as short as or as long as  yr.

2. The most frequent fates of these bodies are solar collision and ejection into a hyperbolic orbit. We have integrated 52 orbits (26 backward and 26 forward in time) and have found 22/52 solar collisions () and 9/52 ejections (). These percentages are very similar to those found for the near-Earth object population over a 60 Myr time span by Gladman et al. (1999), and the same holds for the average median lifetimes which can be inferred from our data, of the order of 10 Myr. On the other hand, some specific orbits evolve only slowly, and they probably account for a tail of long-lived bodies for which collisions are probably the dominant lifetime-limiting process.

3. We have only two clearly comet-like sets of initial orbital elements in our sample of starting orbits, although four bodies were classified in Ceplecha's physical "cometary" group. While our sample is probably biased by selection effects (which typically favour "meteoritic" bolides against "cometary" ones) and while there are dynamical pathways between cometary (Jupiter-coupled) and asteroidal orbits, our results provide some support to the idea that only a minor fraction (possibly 10-) of the near-Earth population would be of cometary origin. For km-sized near-Earth objects, convincing evidence for this conclusion comes from spectroscopic and rotational studies (McFadden et al. 1989, Lupishko & Di Martino 1998, Binzel et al. 1992). On the other hand, the predominance of the asteroidal component in the 1-10 m size range (especially near its upper end) is at odds with Ceplecha's (1994, Ceplecha et al., 1997) finding that very weak, "cometary" bodies are very frequent in the bolide population at these sizes. A possibility to solve this conundrum is that asteroids might also supply to the Earth very fragile and/or porous material (see e.g. Foschini 1998).

4. We have found that among the 8 encounter-dominated orbits, four, the two corresponding to the Lugo bolide (6, 7) plus EN220991 (5) and Honduras-1 (24), are initially or become temporarily later Aten-like or bodies with  AU. This is quite comparable to the observed abundance of such orbits in the near-Earth object population (Michel et al. 1999).

5. Our long-term integrations show that a dynamical mechanism poorly investigated so far, that is the overlapping of secular resonances (included those involving the inner planets) is quite efficient to transport bodies to Sun-grazing orbits, both for main-belt ( AU) and for smaller semimajor axes.

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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