The size of the crater an impactor leaves has an inverse relationship with the frequency that such impacts occur. This means tht large impacts are much less common than small ones. We take a hit from a house sized object every few months and, though it releases the energy of a nuclear bomb, we only experience a flash and bang as the impactor is torn apart by friction high in the atmosphere. These airbursts can be dangerous though as the shockwave smashes glass nearby and bursts near the surface can cause severe flash burns. Objects above twenty kilometres across no longer strike the planet but an extinction level collision with an eight to twelve kilometre asteroid is almost certain within the next hundred million years or so . This would be just as bad as portrayed in movies — mega-tsunami, darened sky and quite possibly a global firestorm due to the frictional heat of reentry of the superheated rock blasted into space by the formation of the temporary crater.

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