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

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2. Selection of data

After an extensive search in the literature, we have chosen a new sample of 20 bolides, all having magnitudes brighter than -10. The orbital parameters and estimated masses of these bodies are listed in Table 1, where the corresponding references are also indicated. In a first stage we have restricted ourselves to the time window from 1993 to 1996, but later on we decided to include also three older interesting bolides (Abee, Glanerbrug and EN220991), which had appeared in 1952, 1990, and 1991, respectively. We recall that the Abee and Glanerbrug bolides were associated with meteorite falls, in both cases fairly rare types of material. The Abee meteorite, an E4 enstatite chondrite, is of particular interest owing to the relationship between this type of meteorites and a particular source region in the main asteroid belt (Gaffey et al. 1992, Farinella et al. 1994). The Glanerbrug meteorite, which penetrated the roof of a house, was a rare inhomogenous kind of chondrite, with darker and lighter breccias, classified as LL and L chondrites respectively, within a fine-grained matrix (Jenniskens et al. 1992).


[TABLE]

Table 1. Catalogue of bolides in chronological order (orbital elements given in the 2000.0 heliocentric ecliptic reference system). The question mark in the Type column indicates large uncertainty in the classification.
Notes:
a) Calculated from data in Griffin et al. (1992). Meteorite recovered. Here the mass indicated is the mass of the meteorite.
b) Calculated from data in Jenniskens et al. (1992). Meteorite recovered.
c) Borovicka & Spurný (1998). Probable meteorite fall.
d) Calculated from data in Cevolani et al. (1993, 1994).
e) Calculated from data in Foschini (1998).
f) Spurný (1997). Probable meteorite fall. For bolides 18 (Tisza) and 20 (Hradec)
g) Tagliaferri et al. (1995).
h) Brown et al. (1996). Meteorite recovered.
i) Spurný & Borovicka (1997).
l) Calculated from data in Borovicka et al. (1999).


Note that most orbits in our list have been derived by means of data obtained from the photographic technique (Spurný 1997, Spurný & Borovicka 1997). In these cases, it is possible to calculate with good precision the orbit of the meteoroid, by means of the gross-fragmentation model described by Ceplecha et al. (1993). It is worth noting that recently Ceplecha has improved the model, which can currently reach a precision of about 1 m along the atmospheric trajectory of the fireball (Ceplecha 1999).

The gross-fragmentation model allows one to calculate the ablation coefficient of the body and therefore to establish in a fairly reliable way some physical properties of the original meteoroid. According to the value of the ablation coefficient, Ceplecha & McCrosky (1976) classified meteoroids into four groups, as follows:

  • I: stony

  • II: carbonaceous chondritic

  • IIIA: cometary

  • IIIB: soft cometary

Later on, Ceplecha (1994) used the relatively abundant meteor data for sizes smaller than 1 m to draw some inferences about the poorly known meteoroid population in the 1-10 m size range. He concluded that carbonaceous bodies are the most common at 1 m size, whereas at 10 m the very weak IIIB-group cometary bodies are the dominant component. This was also recently confirmed by satellite observations (Ceplecha et al. 1997). As for our sample of bright bolides, in most cases we know the classification of either the delivered meteorites or the photographic fireballs, according to Ceplecha's methodology (see Table 1). Only for Lugo and Honduras there is some uncertainty, due to the limited available data. We will comment later on about the implications of this physical information; note, however, that Ceplecha's groups do not really refer to the chemical-mineralogical composition of the bodies, but rather to their physical and structural properties. Whereas in the case of meteorites some comparisons are possible with laboratory measurements for different meteorite types, nobody does really know so far how a comet fragment would look like and interact with the atmosphere.

For some fireballs for which photographic data were not available, we have derived the orbital parameters from satellite observations (Brown et al. 1996, Tagliaferri et al. 1995), visual observations by occasional witnesses (Borovicka et al. 1999, Cevolani et al. 1993, Griffin et al. 1992, Jenniskens et al. 1992), seismic records (Cevolani et al. 1994, Foschini 1998), or with a combination of these data and methods. In these cases, the orbital elements are quite difficult to calculate and in order to derive them it is necessary to adopt an interdisciplinary approach, based on concepts and methods from different disciplines, including hypersonic aerodynamics, physics of shock waves, optics, seismology and acoustics. Actually, in recent years several new theories on the aerodynamics of large meteoroids (i.e. larger than some meters) in the Earth's atmosphere have been proposed (e.g. Chyba et al. 1993, Hills & Goda 1993, Lyne et al. 1996), but all rely on a number of approximations and there is plenty of open problems (Ceplecha 1995), in particular when meteoroids of size of the order of 10 m are involved. Some possible solutions are being discussed in the current literature (e.g. Borovicka et al. 1998a, 1998b, Foschini 1998, 1999), but there is still a clear need for both relevant data and improved theories and models.

Therefore, in order to take into account the large uncertainties which in some cases affect the derivation of the orbital elements, for several bolides (Abee, Glanerbrug, Lugo and Honduras) two or three alternative solutions have been recalculated from different sets of starting data, as indicated in Table 2. In another case (Marshall Islands) two solutions corresponding to different values of the bolide's velocity were already given in the original paper of Tagliaferri et al. (1995). For these five bodies, the two or three alternative sets of orbital parameters are listed separately in Table 1, and have been used in the following stage of our work to derive different sets of starting conditions for the numerical integrations. Thus, we have dealt with a total of 26 orbits, some of them corresponding to the same physical object.


[TABLE]

Table 2. The input data used for the recalculation of the orbital elements of four bolides. The azimuth and elevation are the horizontal coordinates of the radiant point. The azimuth is measured clockwise from the North point on the horizon. All information comes from the sources quoted in Table 1.


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Online publication: December 17, 1999
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