An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, as the impacting body is usually traveling at several kilometres per second (km/s). The minimum impact speed for bodies striking Earth is , the Escape velocity of the Earth. While planetary atmospheres can mitigate some of these impacts through the effects of atmospheric entry, many large bodies retain sufficient energy to reach the surface and cause substantial damage. This results in the formation of impact craters and structures, shaping the dominant landforms found across various types of solid objects found in the Solar System. Their prevalence and ubiquity present the strongest empirical evidence of the frequency and scale of these events.
Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Major impact events have significantly shaped Earth's history, and the formation of the Earth–Moon system has been hypothesized to be the result of a giant impact. Interplanetary impacts have also been proposed to explain the retrograde rotation of Uranus and Venus. Impact events also appear to have played a significant role in the evolutionary history of life. Impacts may have helped deliver the building blocks for life (the panspermia theory relies on this premise). Impacts have been suggested as the origin of water on Earth. They have also been implicated in several mass extinctions. The prehistoric Chicxulub impact, 66 million years ago, is believed to be the cause not only 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.
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. One of the best-known events recorded in modern times was the Tunguska event, which occurred in 1908 in a very sparsely populated part of Siberia, Russia. The 2013 Chelyabinsk meteor event is the only known such incident in modern times to result in numerous injuries. Its meteor is the largest recorded object to have encountered the Earth since the Tunguska event. 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. 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. Impact events have been a plot and background element in science fiction.
In April 2018, the B612 Foundation reported: "It's 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent certain when." Also in 2018, physicist Stephen Hawking considered in his final book Brief Answers to the Big Questions that an asteroid collision was the biggest threat to the planet. 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. 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. On 26 September 2022, the Double Asteroid Redirection Test demonstrated the deflection of an asteroid. It was the first such experiment to be carried out by humankind and was considered to be highly successful. The orbital period of the target body was changed by 32 minutes. The criterion for success was a change of more than 73 seconds.
Impacts and the Earth
Several major impact events have significantly shaped Earth's history, having been implicated in the formation of the Earth–Moon system, the evolutionary history of life, the origin of water on Earth, and several mass extinctions. Impact structures are the result of impact events on solid objects and, as the dominant landforms on many of the System's solid objects, present the most solid evidence of prehistoric events. Notable impact events include the hypothesized Late Heavy Bombardment, which would have occurred early in the history of the Earth–Moon system, and the confirmed Chicxulub impact 66 million years ago, believed to be the cause of the Cretaceous–Paleogene extinction event.
Frequency and risk
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 diameter strike Earth every 500,000 years on average. Large collisions – with objects – happen approximately once every twenty million years. The last known impact of an object of or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago.
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, by basing it on the asteroid's brightness. The density is generally assumed, because the diameter and mass, from which density can be calculated, are also generally estimated. Due to Earth's escape velocity, the minimum impact velocity is 11 km/s with asteroid impacts averaging around on the Earth. The most probable impact angle is 45 degrees.
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 bodies of water are hit. Human populations are vulnerable to these effects if they live within the affected zone. Large seiche waves arising from earthquakes and large-scale deposit of debris can also occur within minutes of impact, thousands of kilometres from impact.A seismically induced onshore surge deposit at the KPg boundary, North Dakota – Proceedings of the National Academy of Sciences – Robert DePalma et al., published 1 April 2019.(PDF direct link, Supplementary published information)
Airbursts
Stony asteroids with a diameter of enter Earth's atmosphere about once a year. Asteroids with a diameter of enter the atmosphere about every 5 years with as much kinetic energy as the atomic bomb dropped on Hiroshima (approximately 16 kilotons of TNT), but the air burst is reduced to just 5 kilotons. These ordinarily explode in the upper atmosphere and most or all of the solids are vaporized. However, asteroids with a diameter of , and which strike Earth approximately twice every century, produce more powerful airbursts. The 2013 Chelyabinsk meteor was estimated to be about 20 m in diameter with an airburst of around 500 kilotons, an explosion 30 times the Hiroshima bomb impact. Much larger objects may impact the solid earth and create a crater.
+Stony asteroid impacts that generate an airburst Impactordiameter Kinetic energy at Airburstaltitude Averagefrequency(years) Recorded fireballs(CNEOS)(1988–2018) atmosphericentry airburst 3 kt 0.75 kt 1.3 54 16 kt 5 kt 4.6 15 47 kt 19 kt 10 2 159 kt 82 kt 27 1 376 kt 230 kt 60 1 1.3 Mt 930 kt 185 0 5.9 Mt 5.2 Mt 764 0 16 Mt 15.2 Mt 1,900 0 29 Mt 28 Mt 3,300 0 Based on density of 2600 kg/m3, speed of 17 km/s, and an impact angle of 45°
+Stony asteroids that impact sedimentary rock and create a crater Impactordiameter Kinetic energy at Craterdiameter Frequency(years) atmosphericentry impact 47 Mt 3.4 Mt 5,200 103 Mt 31.4 Mt 11,000 159 Mt 71.5 Mt 16,000 376 Mt 261 Mt 36,000 734 Mt 598 Mt 59,000 1270 Mt 1110 Mt 73,000 3010 Mt 2800 Mt 100,000 16100 Mt 15700 Mt 190,000 47000 Mt 46300 Mt 440,000 Based on ρ = 2600 kg/m3; v = 17 km/s; and an angle of 45°
Objects with a diameter less than are called meteoroids and seldom make it to the ground to become meteorites. An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these typically create a weather radar signature with a strewn field large enough to be recovered and be made known to scientists.
The late Eugene Shoemaker of the U.S. Geological Survey estimated the rate of Earth impacts, concluding that an event about the size of the nuclear weapon that destroyed Hiroshima occurs about once a year. Such events would seem to be spectacularly obvious, but they generally go unnoticed for a number of reasons: the majority of the Earth's surface is covered by water; a good portion of the land surface is uninhabited; and the explosions generally occur at relatively high altitude, resulting in a huge flash and thunderclap but no real damage.
Although no human is known to have been killed directly by an impact, over 1000 people were injured by the Chelyabinsk meteor airburst event over Russia in 2013.["Число пострадавших при падении метеорита приблизилось к 1500" (in Russian). РосБизнесКонсалтинг. Retrieved 25 February 2013.] In 2005 it was estimated that the chance of a single person born today dying of an impact is around 1 in 200,000 (due to the potential for a single event killing a large fraction of the world population). The two to four-meter-sized asteroids , , 2018 LA, 2019 MO, , and the suspected artificial satellite WT1190F are the only known objects to be detected before impacting the Earth.[Roylance, Frank (2008-10-07). "Predicted meteor may have been sighted". MarylandWeather. Archived from the original on 10 October 2008. Retrieved 2008-10-08.]
Air bursts have been recognized as a significant impact threat by the planetary defense community since at least 2010, when the National Academy of Sciences, citing Boslough and Crawford, recommended that "Because recent studies of meteor airbursts have suggested that near-Earth objects as small as 30 to 50 meters in diameter could be highly destructive, surveys should attempt to detect as many 30- to 50-meter-diameter objects as possible."
Geological significance Impacts have had, during the history of the Earth, a significant geological and climatic influence.French, B. M. (1998). Traces of catastrophe: A handbook of shock-metamorphic effects in terrestrial meteorite impact structures.
The Moon's existence is widely attributed to a huge impact early in Earth's history. Impact events earlier in the history of Earth have been credited with creative as well as destructive events; it has been proposed that impacting comets delivered the Earth's water, and some have suggested that the origins of life may have been influenced by impacting objects by bringing organic chemicals or lifeforms to the Earth's surface, a theory known as exogenesis.