There are at present about 1,000 Earth-crossing bodies of asteroidal appearance that have diameters greater than 1 km. It is calculated that on the average about 3 of these bodies impact the Earth every million years. Because there are many more small bodies than large ones, impacts of 10-km-diameter objects, as postulated by Alvarez and others to explain the Cretaceous/Tertiary extinction, occur less frequently. Nevertheless, it is expected that the frequency of impact of these larger bodies will be about once every 40 million years. The cratering record on the Earth and the Moon is in agreement with this estimate. It is likely that bodies as large as 20 km in diameter have struck the Earth during the last 3 billion years. A somewhat smaller but possibly comparable impact rate of active comet nuclei of similar size is also expected.
The bodies of asteroidal appearance represent a quasi-steady-state population. Losses by collision and perturbation out of the solar system are balanced by supply on a 107 to 108 year time scale of new objects: asteroidal fragments and extinct comet nuclei. The relative importance of these two sources is at present uncertain.
