Mass distribution of massive meteoroids determined by occurrence of brilliant fireballs

Abstract

DETERMINATION of the mass distribution function of bodies is of prime importance for an understanding of the evolutionary processes that determine the population of the Solar System. We report here a method for measuring mass distribution of meteoroids, by determining the occurrence frequency of very brilliant fireballs. In a certain mass regime the number distribution can be described by a power law dn = (const.)m−s dm where dn is the number of particles having a mass between m and m+dm and s is the mass distribution index. s is a slowly varying function of mass, and values of s have been well established for meteoroid particles 10−12< m ≲ 101 g by various techniques, and reasonable estimates are available for large bodies m ≳ 105 g (meteorites and asteroids). There is, however, a lack of information in the mass range 101 ≲ m ≲ 105 g corresponding1 to meteoroids which yield meteors of visual magnitude − 2 ≳ Mv □ − 12 ranging from bright visual objects to very brilliant fireballs (the term fireball generally refers to any meteor brighter than Venus, Mv ≃ −4). The problem is one of sampling: while meteor (radar, visual, optical) counts over periods of hours at a single station are suitable for estimates of s for m ≲ 1 g (Mv ≳ 0), such a procedure yields a prohibitively low count for larger objects. This difficulty would be overcome by observing over a large area of the Earth's surface for many years. We determine here the occurrence frequency of very brilliant fireballs, as a method of s measurement. A fireball of magnitude equal to that of the full Moon is an outstanding spectacle likely to be well reported in an area with a high population density. With good communications and active research groups analysing reports and recording data, a reliable estimate of the influx of such large objects could be secured. In the British Isles, extensive records of fireball events covering more than 40 yr, are available2,3. Most observers' estimates were based on the great illumination produced on the landscape or inside buildings, and the personnel analysing the reports were experienced workers in the field. A detailed examination of the reports reveals a consistent rate of about two lunar fireballs (apparent visual magnitude equal to or brighter than that of the full Moon) seen in the British Isles per year. Specifically, in the period 1900–36 the average occurrence frequency was 2.4 yr−1. This frequency can be compared with that of visual meteors to obtain an estimate of s.