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

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3. Distribution of current orbits

Before integrating the orbits listed in Table 1 over millions of years, it is worth making some comments on their distribution in the a-e plane, compared to those for both smaller and larger members of the Earth-crossing population. Fig. 1 shows this distribution for our current sample (small open circles, plus special symbols for those with uncertain orbits) and for that of Jopek et al. 1995(small full circles, plus large open circles for the four meteorite-delivering photographic fireballs). This can be compared to the distributions for the smaller chondritic meteoroids (diameter of about 1-10 cm) listed by Wetherill & ReVelle (1981) and Halliday et al. (1996), as shown in Fig. 2, and to that for the currently known Earth-crossing (Apollo and Aten) asteroids with good quality orbits, divided into three different diameter ranges (Fig. 3).

[FIGURE] Fig. 1. The position in the orbital semimajor axis vs. eccentricity plane of the sample of orbits studied in this paper (small open circles, plus special symbols for the cases when we selected multiple sets of elements: Abee, full squares; Glanerbrug, crosses; Lugo, full hexagons; Marshall Islands, open squares) and for that of Jopek et al. 1995(small full circles, plus large open circles for the four meteorite-delivering photographic fireballs Píbram, Lost City, Innisfree and Peekskill). Dashed and dotted curves correspond to orbits having perihelia and aphelia nearly tangent to the orbits of Mars, the Earth and Venus.

[FIGURE] Fig. 2. The same as Fig. 1 but for the chondritic meteoroids listed by Wetherill & ReVelle (1981, open circles) and Halliday et al. (1996, crosses). These are the orbits discussed in the recent paper on the orbital distribution of meteoroids by Morbidelli & Gladman (1998).

[FIGURE] Fig. 3. The same as Fig. 1 but for the the currently known Earth-crossing asteroids. Open circles correspond to bodies less than 1 km in diameter, full squares to the 1 to 5 km diameter range and crosses to bodies larger than 5 km. This is a subset (Apollo and Aten asteroids only) of the sample of near-Earth asteroids with good quality orbits recently studied by Gladman et al. (1999). Diameters have been estimated from reasonable guesses of albedos (as explained in Migliorini et al. (1998) when this parameter has not been directly measured.

It is interesting to note that, despite the greatly different selection effects involved in the observational methods used to collect these data, there is no striking difference in the overall appearance of the distributions. For instance, the abundance of Aten-type orbits with [FORMULA] AU is [FORMULA]-[FORMULA] for all the three samples. The fact that the bolides are somewhat more concentrated near the [FORMULA] AU line can be easily explained by the higher collision probability resulting from this orbital configuration (Wetherill 1967). Apart from this, the orbits look broadly scattered in the region of the a-e plane where collisions with the Earth are possible, with no strong clustering, e.g. near Jovian resonances; this indicates that most bolide orbits are already "dynamically evolved" when they hit the Earth, namely they have been scattered around by close planetary encounters after having been transported into near-Earth space.

Table 1 and Fig. 1 show that three of the four bolides classified in Ceplecha's IIIA and IIIB groups have orbits which are dynamically decoupled from Jupiter, i.e., they are not typically cometary orbits. In our opinion, there are two possible explanations for this, which do not necessarily exclude each other: (i) the asteroid belt may also deliver very weak, fragile and possibly porous bodies, as suggested for instance by the recently determined low density of C-type asteroid 253 Mathilde, encountered by the NEAR probe in June 1997 (Yeomans et al. 1997, Foschini 1998); (ii) the dynamical pathway between typical, Jupiter-coupled cometary orbits and "asteroidal" ones, such as those of comet P/Encke and the Taurid meteoroids (Valsecchi et al., 1995) is an important one, and quite many small bodies reach the Earth-crossing region through this route . Note, however, that neither cometary orbits nor comet-like physical properties account for a dominant fraction of our sample of bright bolides. At least in part, this is probably due to selection effects, as photographic observations have been preferentially reduced for bolides of types I and II, considered as more interesting because they are possibly associated to meteorites (Z. Ceplecha, private communication).

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999