Like the other planets in the solar system, the Earth is constantly pelted with impacting debris. The debris ranges in size from tiny particles of interplanetary dust to larger comets and asteroids that cause catastrophic events. There is evidence that asteroids, comets and a constant rain of interstellar dust particles brought life-forming elements and compounds to the Earth. Early in Earth's history, comets may have formed the oceans.
Cosmic impacts are a natural part of geologic history.
Large impacts on Earth are extremely rare, but they can do tremendous damage, exceeding that of any other known natural disaster. The atmosphere effectively shields us from most impacts that have less energy than 10 megatons of TNT, which corresponds to the impact of a 100-foot diameter comet or asteroid. Larger objects penetrate to the lower atmosphere or surface where they explode, causing great local damage. The damage is primarily caused by vast amounts of dust lofted into the atmosphere, but damaging chemical changes in the atmosphere and oceans also occur and hot ejecta can ignite large-scale fires. Painting by Donald E. Davis .