Impact Event - Sizes and Frequencies

Sizes and Frequencies

Small objects frequently collide with the Earth. There is an inverse relationship between the size of the object and the frequency that such objects hit the earth. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater's diameter, which is on average proportional to the diameter of the impactor. Asteroids with a 1 km (0.62 mi) diameter strike the Earth every 500,000 years on average. Large collisions – with 5 km (3 mi) objects – happen approximately once every ten million years. The last known impact of an object of 10 km (6 mi) or more in diameter was at the Cretaceous–Paleogene extinction event 65 million years ago.

Asteroids with diameters of 5 to 10 m (16 to 33 ft) enter the Earth's atmosphere approximately once per year, with as much energy as Little Boy, the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Objects with diameters over 50 m (164 ft) strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908. At least one known asteroid with a diameter of over 1 km (0.62 mi), (29075) 1950 DA, has a possibility of colliding with Earth on March 16, 2880.

Objects with diameters smaller than 10 m (33 ft) are called meteoroids (or meteorites if they strike the ground). An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these are typically recovered and made known to scientists.