An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream. Small objects frequently impact Earth.

There is an inverse relationship between the size of the object and the frequency that such objects hit the earth. Asteroids with a 1 km diameter impact the Earth every 500,000 years on average. Large collisions with five kilometer objects happen approximately once every ten million years.

The last known impact of an object of 10 km or more in diameter was the Cretaceous-Tertiary extinction event 65 million years ago. Asteroids with diameters of 5-10 m impact 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 of diameters of over 50 meters strike the Earth approximately once every thousand years, producing explosions comparable to the one observed at Tunguska in 1908. At least one known asteroid with a diameter of over 1 km, (29075) 1950 DA, has a calculated probability of colliding with Earth in March 2880, with a Torino scale rating of two. Throughout recorded history, hundreds of minor impact events (and exploding bolides) have been reported, with some occurrences causing deaths, injuries, property damage, or other significant localised consequences.

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