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.
Large collisions – with 5 km (3 mi) objects – happen approximately once every twenty million years. Asteroids with a 1 km (0.62 mi) diameter strike Earth every 500,000 years on average. 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. There is an inverse relationship between the size of the object and the frequency of such events. Small objects frequently collide with Earth. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the " National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit, but we're not 100 percent certain when." Also in 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. Impact events have been a plot and background element in science fiction. An extrasolar impact was observed in 2013, when a massive terrestrial planet impact was detected around the star ID8 in the star cluster NGC 2547 by NASA's Spitzer Space Telescope and confirmed by ground observations. The Comet Shoemaker–Levy 9 impact provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. Its meteor is the largest recorded object to have encountered the Earth since the Tunguska event. The 2013 Chelyabinsk meteor event is the only known such incident in modern times to result in numerous injuries. One of the best-known recorded events in modern times was the Tunguska event, which occurred in Siberia, Russia, in 1908. Throughout recorded history, hundreds of Earth impacts (and exploding bolides) have been reported, with some occurrences causing deaths, injuries, property damage, or other significant localised consequences. The prehistoric Chicxulub impact, 66 million years ago, is believed to not only be the cause of the Cretaceous–Paleogene extinction event but acceleration of the evolution of mammals leading to their dominance and in turn, setting in place conditions for the eventual rise of humans. They have also been implicated in several mass extinctions. Impacts have been suggested as the origin of water on Earth. Impacts may have helped deliver the building blocks for life (the panspermia theory relies on this premise). Impact events also appear to have played a significant role in the evolutionary history of life. Major impact events have significantly shaped Earth's history, and have been implicated in the formation of the Earth–Moon system. Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Impact craters and structures are dominant landforms on many of the Solar System's solid objects and present the strongest empirical evidence for their frequency and scale. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, though atmospheres mitigate many surface impacts through atmospheric entry. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect.
A major impact event releases the energy of several million nuclear weapons detonating simultaneously when an asteroid of only a few kilometers in diameter collides with a larger body such as the Earth (image: artist's impression).Īn impact event is a collision between astronomical objects causing measurable effects.
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