The Density and Mass Distribution of Meteoritic Bodies in the Neighborhood of the Earth's Orbit

A study of the frequency of meteorite falls in areas which have been highly populated throughout the last century indicates that the average rate of fall of meteorites on the earth probably lies between 0.32 and 1.0 falls/year 10 • km •. The frequency of fall as a function of type and mass has been studied, and it is shown that the distribution curves for stones and irons have similar shapes. The curves are also remarkably similar to those observed for asteroids. The observed relationships are used to calculate the impact frequency of meteoritic particles upon the earth and moon as a function of their size. Introduction. There are many reasons for our wanting to know the density of meteoritic bodies in the neighborhood of the earth's orbit. Opik [1957] has recently computed the frequency of impact of bodies greater than 60 meters in diameter with the lunar surface. This study, which was undertaken before Professor Opik's unpublished paper ['The Lunar Surface as an Impact Counter'] was brought to the present author's attention, gives estimates of impact frequencies with the moon and the earth of particles weighing i gram and more. 2 The •requency o• meteorite •alls upon the earth. We can obtain an indication of the density of meteoritic bodies in the neighborhood of the earth's orbit by estimating the frequency of encounter of such bodies with the earth. At present approximately ten meteorites are seen to fall and are recovered each year over the earth. The number of observed falls has been x This paper was presented at the symposium conducted by the Committee on Space Research of the International Councfi of Scientific Unions at Nice, France, on January 11, 1960. This material, together with other papers presented at the Symposium, will appear in a volume to be published by the North-Holland Publishing Co., Netherlands. •-It has also been brought to the author's attention, since the delivery of this paper, that Gerald S. Hawkins has conducted a similar study, soon to be published in the Astronomical Journal. The computed mass distributions and frequencies of terrestrial fall are compatible with those found here. increasing rather steadily with time, in part as a result of the increasing numbers of human beings in the world and in part as a result of a growing interest in meteorites. It is important to ask: If all meteorites that actually reach the earth's surface were seen and recovered, how many would be counted each year? We can estimate the frequency of meteorite impact on the earth's surface by studying the frequency of observed meteoritic falls in areas of the world which have had high rural population densities throughout he last century and in which there is a reasonably high probability that a recovered meteorite will eventually be reported. Three such areas are Japan, India, and Western Europe. Table 1 shows, in the lefthand columns, the number of meteorites that fell in these areas during the half-centuries 1840-1889 and 1890-1939, and in the right-hand columns, the density of fall in each area. For comparison, the corresponding figures are given for the United States. The errors shown are standard 'counting errors,' assuming random events. It will be noted that the densities of fall in the three selected areas are in surprisingly close agreement. Although the total number of falls on a world-wide basis increased with time during this period, there is no obvious time-dependence in these particular areas. During the first half-century, 110 meteorites fell in these regions, whereas 100 were observed during the second. It is interesting that the observed density of fall