Editing Impact event (section)

=== Frequency and risk ===
{{Main|Asteroid impact avoidance}}
{{wide image|SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|500px|align-cap=center|Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.}}
[[File:Bolide.jpg|thumb|A bolide undergoing atmospheric entry]]

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 (algebra)|cube]] of the resulting crater's diameter, which is on average proportional to the diameter of the impactor.<ref name=ArvidsonEtAl79>{{Citation|last1=Crater Analysis Techniques Working Group|title=Standard Techniques for Presentation and Analysis of Crater Size-Frequency Data|journal=Icarus|volume=37|issue=2|pages=467–474|date=1979| postscript=.|bibcode=1979Icar...37..467C | doi = 10.1016/0019-1035(79)90009-5|last2=Arvidson| first2=R. E. |last3=Boyce|first3=J. |last4=Chapman|first4=C. |last5=Cintala|first5=M. |last6=Fulchignoni |first6=M.|last7=Moore|first7=H.|last8=Neukum|first8=G.|last9=Schultz |first9=P. |last10=Soderblom |first10=L.|last11=Strom|first11=R. |last12=Woronow |first12=A. |last13=Young|first13=R.|hdl=2060/19780014063|s2cid=118171810 |hdl-access=free}}</ref> Asteroids with a {{convert|1|km|mi|abbr=on}} diameter strike Earth every 500,000 years on average.<ref name="Paine 2002">{{Cite journal|last1=Paine|first1=Michael|last2=Peiser|first2=Benny|date=2002|title=The Frequency and Consequences of Cosmic Impacts Since the Demise of the Dinosaurs|url=https://www.researchgate.net/publication/265496068|journal=Bioastronomy 2002: Life Among the Stars}}</ref><ref name="types">{{Citation |author-link=Nick Bostrom |first=Nick |last=Bostrom |date=March 2002 |url=http://www.nickbostrom.com/existential/risks.html |title=Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards |journal=Journal of Evolution and Technology |volume=9}}</ref> Large collisions – with {{convert|5|km|mi|0|abbr=on}} objects – happen approximately once every twenty million years.<ref name="Earth-impact" /> The last known impact of an object of {{convert|10|km|mi|0|abbr=on}} or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago.<ref name="UCB2013" />

The energy released by an impactor depends on diameter, density, velocity, and angle.<ref name="Earth-impact" /> 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 escape velocity|Earth's escape velocity]], the minimum impact velocity is [[Earth escape velocity|11 km/s]] with asteroid impacts averaging around 17&nbsp;km/s on the Earth.<ref name="Earth-impact" /> The most probable impact angle is 45 degrees.<ref name="Earth-impact" />

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.<ref name="Rumpf 3433–3440"/> Large [[seiche wave]]s arising from earthquakes and large-scale deposit of debris can also occur within minutes of impact, thousands of kilometres from impact.<ref name=pnas1>[https://www.pnas.org/content/early/2019/03/27/1817407116 A seismically induced onshore surge deposit at the KPg boundary, North Dakota] {{Webarchive|url=https://web.archive.org/web/20190404144219/https://www.pnas.org/content/early/2019/03/27/1817407116 |date=2019-04-04 }} – Proceedings of the National Academy of Sciences – [[Robert DePalma]] ''et al.'', published 1 April 2019.<p>([https://www.pnas.org/content/pnas/early/2019/03/27/1817407116.full.pdf PDF direct link], [https://www.pnas.org/content/pnas/suppl/2019/03/27/1817407116.DCSupplemental/pnas.1817407116.sapp.pdf Supplementary published information])</p></ref>

==== Airbursts ====

{{Further|Meteor air burst}}

Stony asteroids with a diameter of {{convert|4|m|sp=us}} enter Earth's atmosphere about once a year.<ref name="Earth-impact" /> Asteroids with a diameter of 7 meters enter the atmosphere about every 5 years with as much [[kinetic energy]] as [[Atomic bombings of Hiroshima and Nagasaki|the atomic bomb dropped on Hiroshima]] (approximately 16 [[kiloton]]s of TNT), but the [[air burst]] is reduced to just 5 kilotons.<ref name="Earth-impact" /> These ordinarily explode in the [[Mesosphere|upper atmosphere]] and most or all of the solids are [[Evaporation|vaporized]].<ref>{{Citation|author = Clark R. Chapman & David Morrison|title = Impacts on the Earth by asteroids and comets: assessing the hazard|journal = Nature|volume =367|issue = 6458|pages=33–40|date=January 6, 1994|bibcode = 1994Natur.367...33C|doi = 10.1038/367033a0|last2 = Morrison|s2cid = 4305299|url = https://zenodo.org/record/1233151}}</ref> However, asteroids with a diameter of {{convert|20|m|abbr=on|lk=off}}, 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.

{| class="wikitable floatleft" style="text-align: center;"
|+Stony asteroid impacts that generate an airburst<ref name="Earth-impact" />
|-
! rowspan=2 | Impactor<br>diameter !! colspan=2 | [[Kinetic energy]] at !! rowspan=2 | Airburst<br>altitude !! rowspan=2 | Average<br>frequency<br>(years) !! rowspan=2 | Recorded fireballs<br>(CNEOS)<br>(1988–2018)
|-
! atmospheric<br>entry
! [[air burst|airburst]]
|-
| {{convert|4|m|abbr=on|lk=on}} || 3 [[TNT equivalent|kt]] || 0.75 kt || {{convert|42.5|km|ft||abbr=on|lk=on}} || 1.3 || 54
|-
| {{convert|7|m|abbr=on|lk=off}} || [[Little Boy|16 kt]] || 5 kt || {{convert|36.3|km|ft||abbr=on|lk=off}} || 4.6 || 15
|-
| {{convert|10|m|abbr=on|lk=off}} || 47 kt || [[Fat Man|19 kt]] || {{convert|31.9|km|ft||abbr=on|lk=off}} || 10 || 2
|-
| {{convert|15|m|abbr=on|lk=off}} || 159 kt || [[W76|82 kt]] || {{convert|26.4|km|ft||abbr=on|lk=off}} || 27 || [[Kamchatka superbolide|'''1''']]
|-
| {{convert|20|m|abbr=on|lk=off}} || 376 kt || [[TN 81|230 kt]] || {{convert|22.4|km|ft||abbr=on|lk=off}} || 60 || [[Chelyabinsk meteor|'''1''']]
|-
| {{convert|30|m|abbr=on|lk=off}} || [[B83 nuclear bomb|1.3]] [[TNT equivalent|Mt]] || 930 kt || {{convert|16.5|km|ft||abbr=on|lk=off}} || 185 || 0
|-
| {{convert|50|m|abbr=on|lk=off}} || 5.9 Mt || 5.2 Mt || {{convert|8.7|km|ft||abbr=on|lk=off}} || 764 || 0
|-
| {{convert|70|m|abbr=on|lk=off}} || 16 Mt || [[Castle Bravo|15.2 Mt]] || {{convert|3.6|km|ft||abbr=on|lk=off}} || 1,900 || 0
|-
| {{convert|85|m|abbr=on|lk=off}} || 29 Mt || 28 Mt || {{convert|0.58|km|ft||abbr=on|lk=off}} || 3,300 || 0
|-
! colspan=6 style="font-size: 0.9em; font-weight: normal; text-align: left; padding: 6px;"| Based on density of 2600&nbsp;kg/m<sup>3</sup>, speed of 17&nbsp;km/s, and an impact angle of 45°
|}

{| class="wikitable" style="text-align:center; float:left; margin-top:0;"
|+Stony asteroids that impact sedimentary rock and create a crater<ref name="Earth-impact" />
|-
! rowspan=2 | Impactor<br>diameter !! colspan=2 | [[Kinetic energy]] at !! rowspan=2 | [[Impact crater|Crater]]<br>diameter !! rowspan=2 | Frequency<br>(years)
|-
! [[Atmospheric entry|atmospheric<br>entry]]
! impact
|-
| {{convert|100|m|abbr=on|lk=on}} || 47 [[TNT equivalent|Mt]] || 3.4 Mt || {{convert|1.2|km|abbr=on|lk=on}} || 5,200
|-
| {{convert|130|m|abbr=on|lk=off}} || 103 Mt || 31.4 Mt || {{convert|2|km|abbr=on|lk=off}} || 11,000
|-
| {{convert|150|m|abbr=on|lk=off}} || 159 Mt || 71.5 Mt || {{convert|2.4|km|abbr=on|lk=off}} || 16,000
|-
| {{convert|200|m|abbr=on|lk=off}} || 376 Mt || 261 Mt || {{convert|3|km|abbr=on|lk=off}} || 36,000
|-
| {{convert|250|m|abbr=on|lk=off}} || 734 Mt || 598 Mt || {{convert|3.8|km|abbr=on|lk=off}} || 59,000
|-
| {{convert|300|m|abbr=on|lk=off}} || 1270 Mt || 1110 Mt || {{convert|4.6|km|abbr=on|lk=off}} || 73,000
|-
| {{convert|400|m|abbr=on|lk=off}} || 3010 Mt || 2800 Mt || {{convert|6|km|abbr=on|lk=off}} || 100,000
|-
| {{convert|700|m|abbr=on|lk=off}} || 16100 Mt || 15700 Mt || {{convert|10|km|abbr=on|lk=off}} || 190,000
|-
| {{convert|1000|m|abbr=on|lk=off}} || 47000 Mt || 46300 Mt || {{convert|13.6|km|abbr=on|lk=off}} || 440,000
|-
! colspan=5 style="font-size: 0.9em; font-weight: normal; text-align: left; padding: 6px;" | Based on [[Density|ρ]] = 2600&nbsp;kg/m<sup>3</sup>; [[Speed|v]] = 17&nbsp;km/s; and an angle of 45°
|}
{{clear|left}}

Objects with a diameter less than {{convert|1|m|ft|abbr=on}} 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 [[United States Geological Survey|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.{{citation needed|date=December 2013}} 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.{{citation needed|date=December 2013}}

Although no human is known to have been killed directly by an impact{{Disputed inline|.22Although_no_human_is_known_to_have_been_killed_directly_by_an_impact.22_-_not_true|date=January 2017}}, over 1000 people were injured by the Chelyabinsk meteor airburst event over Russia in 2013.<ref>["Число пострадавших при падении метеорита приблизилось к 1500" (in Russian). РосБизнесКонсалтинг. Retrieved 25 February 2013.]</ref> In 2005 it was estimated that the chance of a single person born today dying of an impact is around 1 in 200,000.<ref>{{cite web |url=https://www.newscientist.com/article/mg18825221.900-the-word-torino-scale.html |title=The word: Torino scale |work=[[New Scientist]] |page=56 |date=25 October 2005}}</ref> The two to four-meter-sized asteroids {{mpl|2008 TC|3}}, {{mpl|2014 AA}}, [[2018 LA]], [[2019 MO]], [[2022 EB5]], and the suspected artificial satellite [[WT1190F]] are the only known objects to be detected before impacting the Earth.<ref>[Roylance, Frank (2008-10-07). "Predicted meteor may have been sighted". MarylandWeather. Archived from the original on 10 October 2008. Retrieved 2008-10-08.]</ref><ref>{{cite web |title=The First Discovered Asteroid of 2014 Collides With The Earth – An Update |publisher=NASA/JPL |date=3 January 2014 |url=http://neo.jpl.nasa.gov/news/news182a.html |access-date=11 January 2014 |archive-date=11 February 2017 |archive-url=https://web.archive.org/web/20170211170917/http://neo.jpl.nasa.gov/news/news182a.html |url-status=dead }}</ref><ref>{{cite web|url=https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2022%20EB5&view=VOP |title=Small-Body Database Lookup |publisher=Ssd.jpl.nasa.gov |date= |accessdate=2022-03-16}}</ref>