The flux of Earth-crossing and moon-cratering interplanetary bodies

Methods of determining the present flux and total number of kilometer-sized Earth-crossing objects are discussed, including (1) probability considerations based on the frequency of chance rediscoveries of the lost objects, (2) evaluation of large-scale photographic surveys for the detection of fast moving objects, and (3) evaluation of close encounters of interplanetary bodies with the Earth. The results are interfaced with the lunar and terrestrial cratering history. It is shown that the discrepancies between these two independent lines of evidence are still within the margin of uncertainty set by observational biases, cratering efficiencies, and surface reflectivities of the objects, in particular as regards extinct cometary nuclei. Impacts of active comets can only be held responsible for a very small fraction of the craters, and impacts of high-albedo Apollo asteroids are consistent with a steady state. There is no definite enhancement of the present-day flux as compared with the average level of cratering during the last 3 Gyr which would require significant variations in the stellar environment of the solar system, affecting the rate of delivery of new comets from the Oort cloud. There is also no evidence of a recent major collisional event in the asteroid belt. Deviations from an equilibrium between source and sink only become effective in the size range of meteor particles, where no long-term cratering record is available. They are apparently due to a very limited number of parent objects, and appear on a time scale which is very short compared with the age of the solar system.