An ablation model is described that can be used to estimate the effect on a large meteoroid of passage through a planetary atmosphere. The effect on ablation and deceleration of breakup due to aerodynamic pressure is investigated. Results from arc‐jet wind‐tunnel tests and strength measurements of meteorite samples have been used to help determine the material properties needed in the calculations. Results are given from a series of calculations of the ablation and breakup of bronzite and carbonaceous chondrite meteoroids in the earth's atmosphere. It is shown that evidence from meteor observations and meteorite falls can be interpreted in terms of a preponderance in space of meteoroids of density comparable to that of meteorites rather than low density. Carbonaceous chondrite is a likely candidate as the most common type of meteoroid in space.

It is suggested that fragmentation may not be the general explanation of the cometary meteor density anomaly. Observations, in ground laboratory facilities, show that terrestrial and extraterrestrial rock will froth during ablation, and the froth may subsequently be sloughed as a result of aerodynamic pressure forces. A new concept, based on these observations, is introduced to explain the density variations of some cometary meteors. Cometary meteor data are analyzed using the frothing‐sloughing concept to find the variation of the luminous efficiency factor. The results show a decreasing luminous efficiency factor with increasing speed. These results appear to explain the observed discrepancies of the velocity histograms obtained from photographic and radar observations and suggest that, for very bright cometary meteors, the apparent variation of density is largely a variation of luminous efficiency.

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