Fireballs and the Physical Theory of Meteors

Details Fireballs and the Physical Theory of Meteors Fireballs and the Physical Theory of Meteors

Nov 1969

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969saosr.305.....m&link_type=abstract

SAO Special Report #305 (1969)

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Scientific paper

We recognize a number of uncertainties and inconsistencies in the classical theory of meteors. However, the general characteristics of the theory are verified by a sizable class of objects of moderate brightness and we therefore accept it as a first approximation. We believe the bulk density of the meteoroid to be the least well-determined parameter entering the theory and one of the most important. If we could demonstrate convincingly that densities for some class of objects are similar to the density of meteoritic stone, we could accept the composition and structure as (probably) known and reduce the uncertainties in the physical theory of meteors. On the other hand, low-density material is predicted by comet models, and proof of its existence is of substantial importance. We thus think it unjustified to assume a high density for meteoritic material. A straightforward interpretation of meteor data has, in the past, suggested that low density prevaled. The same result is now seen in the fireball data. We have attempted to alter the classical theory, following other authors in part, in order to explain observations of faint and bright meteors in terms of a high density. None of the explanations offered by previous authors can be successfully extrapolated to the very bright fireballs. Frothing of the meteoroid as suggested by Allen and Baldwin becomes less important as the body size increases and less possible as the object penetrates deeper into the atmosphere. Fragmentaion by thermal shock as proposed by Jones and Kaiser becomes decreasingly important as the body size increases, as can be demonstrated both by a mathematical model and by the existence of meteorites of less than the critical size. We have also treated three additional variations in the theory. These are fragmentation of small particles, gross fragmentation, and a reverse-rocket effect produced by high-velocity spall. We find all of these to be either inefficient or unrealistic models for disguising the true bulk density. We conclude either that almost all objects are low density or that the meteor theory or the constants employed contain a gross error. We consider this latter possibility to be slight. The small terminal masses of most fireballs lend support to our contention that they are unlike meteorites. The near absence of any large masses over a 5-year period in the Prairie Network casts serious doubt on our predicted rate of fall of meteorites. We have noted that three quite different physical effects-thermal shock, ablation, and pressure fragmentation-may produce substantial variations between the mass-number flux of meteorites outside the atmosphere and on the ground, and we belive that it is impossible at present to make a sensible extrapolation from the observed distribution to that in space.

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