<Tr> <Td_colspan="3">--Rep. Chris Stewart (R, UT) and Dr. Michael F. A'Hearn, 10 April 2013, United States Congress </Td> </Tr> <P> Small objects frequently collide with Earth . There is an inverse relationship between the size of the object and the frequency of such events . 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 Earth every 500,000 years on average . Large collisions--with 5 km (3 mi) objects--happen approximately once every twenty 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 66 million years ago . </P> <P> The energy released by an impactor depends on diameter, density, velocity, and angle . The diameter of most near - Earth asteroids that have not been studied by radar or infrared can generally only be estimated within about a factor of two based on the asteroid brightness . The density is generally assumed because the diameter and mass are also generally estimates . Due to Earth's escape velocity, the minimum impact velocity is 11 km / s with asteroid impacts averaging around 17 km / s on the Earth . The most probable impact angle is 45 degrees . </P> <P> Impact conditions such as asteroid size and speed, but also density and impact angle determine the kinetic energy released in an impact event . The more energy is released, the more damage is likely to occur on the ground due to the environmental effects triggered by the impact . Such effects can be shock waves, heat radiation, the formation of craters with associated earthquakes, and tsunamis if water bodies are hit . Human populations are vulnerable to these effects if they live within the affected zone . </P>

If a comet with a diameter of 1-4 km struck the earth it would