It is probable that a meteoroid may undergo different combinations of these fragmentation forms in the atmosphere. Photographic observations of meteors have indicated that, along the meteor trajectory, the fragmentation of the first and fourth types may occur more than once. By analysing photographic observations of meteors, the following four main types of meteoroid fragmentation have been identified (Levin 1962): 1) the decay of a meteoroid into significantly large pieces of non-fragmenting debris 2) the progressive disintegration of the original meteoroid into fragments that continue to crumble into smaller debris 3) the quasi-continuous fragmentation, which involves a gradual release of the smallest fragments from the surface of a parent meteoroid and their subsequent evaporation and 4) the simultaneous ejection of a large number of small particles that produce meteor flares. It has been established that fragmentation is a widespread type of ablation for the majority of meteoroids that produce meteors registered by photographic, TV, and radar methods. 1988 Ceplecha & McCrosky 1992 Ceplecha et al. The discrepancy between measurements of the bulk densities of meteoroids by different methods implied that the physical theory of meteors had to be improved by taking into account meteoroid fragmentation in the Earth's atmosphere (Levin 1962, 1963 Lebedinets 1980 Bronshten 1981 Novikov et al. The difference between these evaluations of meteoroid densities could be caused not only by the different methods used, but also because the method of determining bulk densities from drag and luminosity equations is applicable only to single non-fragmenting meteoroids, which cannot be considered to be accurate in our case. ( 1993) in determining the bulk density by applying the ``gross'' fragmentation model of meteoroids. These results were confirmed by Ceplecha et al. On the basis of observational data about the heights of approximately 6000 bright meteors produced by meteoroids in a wide mass range from 10 -4 g to 10 8 g, Ceplecha & McCrosky ( 1976) and Ceplecha ( 1988) concluded, that according to their compositionĪnd structure, the meteoroids form four main groups: I - ordinary chondrites, with a mean density of 3.7 g cm -3 II - carbonaceous chondrites, with a mean density 2.1 g cm -3 IIIA - dense cometary material, with a mean density 0.75 g cm -3 and IIIB - ordinary cometary material, with a mean density 0.4 g cm -3. 1967 Verniani 1967, 1969, 1973) was widely accepted until more recently. The opinion that meteoroids have low bulk densities equal on average to 0.26 g cm -3 (Jacchia et al. Therefore, the determination of orbits and physical characteristics of meteoroids is important for both meteor astronomy and understanding the physical properties of the meteoroids' parent bodies.īulk and mineralogical (grain) densities relate to important physical parameters of meteoroids. Thus, we can say definitely that meteoroids are remnants of comets and asteroids. It has been proved that the majority of meteoroids are products of disintegration of cometary nuclei and asteroids due to their mutual impacts. It is also important scientifically and because asteroids and comets represent a long-term danger to the biosphere and human kind as they repeatedly transfer the Earth's orbit. The investigation of meteoroids' physical parameters is of significant interest in understanding the physical nature of meteoroids' parent bodies i.e. Key words: meteors, meteoroids - techniques: photometric These results confirm the porous-structure nature of meteoroids' parent bodies i.e. The Geminid meteoroids are found to have the lowest porosity, while the Leonid and Draconid meteoroids have the most porous structure (83%). Using the relation between bulk density and mineralogical density the porosity of meteoroids was estimated. Which were determined according to the theory of quasi-continuous fragmentation of meteoroids in the Earth's atmosphere, vary from 0.4 g cm -3 (Leonids) to 2.9 g cm -3 (Geminids). The mean mineralogical densitiesĢ.2 g cm -3 (Perseids) to 3.4 g cm -3 (Quadrantids, -Aquarids, and -Capricornids).
Using a physical theory of meteors and on the basis of the results of double-station photographic observations of meteors in Dushanbe (Tajikistan), Kiev, and Odessa (Ukraine), the mean mineralogical and bulk densities of meteoroids belonging to nine meteoroid streams and sporadic background are determined. Received 25 June 2008 / Accepted 13 November 2008 Institute of Astrophysics, Tajik Academy of Sciences, Dushanbe 734042, Tajikistan
Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes
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