Editing Asteroid (section)

== Threats to Earth ==
{{See also|List of Earth-crossing minor planets}}

[[File:SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|thumb|Frequency of [[bolide]]s, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere]]

There is increasing interest in identifying asteroids whose orbits cross [[Earth]]'s, and that could, given enough time, collide with Earth. The three most important groups of [[near-Earth asteroid]]s are the [[Apollo asteroid|Apollos]], [[Amor asteroid|Amors]], and [[Aten asteroid|Atens]].

The [[near-Earth object|near-Earth]] asteroid [[433 Eros]] had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: [[1221 Amor]], [[1862 Apollo]], [[2101 Adonis]], and finally [[69230 Hermes]], which approached within 0.005&nbsp;[[Astronomical unit|AU]] of [[Earth]] in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of the [[Alvarez hypothesis]] that an [[impact event]] resulted in the [[Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction]], and the 1994 observation of [[Comet Shoemaker-Levy 9]] crashing into [[impact events on Jupiter|Jupiter]]. The U.S. military also declassified the information that its [[military satellite]]s, built to [[detect nuclear explosions]], had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.

All of these considerations helped spur the launch of highly efficient surveys, consisting of charge-coupled device ([[Charge-coupled device|CCD]]) cameras and computers directly connected to telescopes. {{As of|2011}}, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.<ref name=nasa_neo/> {{as of|2018|10|29}}, the LINEAR system alone had discovered 147,132 asteroids.<ref>{{cite web |title=Minor Planet Discover Sites |publisher=International Astronomical Union |department=Minor Planet Center |url=https://minorplanetcenter.net//iau/lists/MPDiscSites.html |access-date=27 December 2018}}</ref> Among the surveys, 19,266&nbsp;near-Earth asteroids have been discovered<ref>{{cite web |title=Unusual Minor Planets |publisher=International Astronomical Union |department=Minor Planet Center |url=https://minorplanetcenter.net//iau/lists/Unusual.html |access-date=27 December 2018}}<!--- using the "close approach" quote ---></ref> including almost 900&nbsp;more than {{cvt|1|km|1}} in diameter.<ref>{{cite web |series=Discovery Statistics |title=Cumulative Totals |date=20 December 2018 |publisher=NASA |department=Jet Propulsion Laboratory |url=https://cneos.jpl.nasa.gov/stats/totals.html |access-date=27 December 2018}}</ref>

In June 2018, the [[National Science and Technology Council]] warned that the United States is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.<ref name="GIZ-20180621" /><ref name="ICARUS-220180522" /><ref name="NYT-20180614">{{cite news
 |last=Chang |first=Kenneth
 |date=14 June 2018
 |title=Asteroids and adversaries: Challenging what NASA knows about space rocks
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html
 |access-date=22 June 2018
}}</ref> 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.<ref name="US-Congress-20130410">{{cite report |collaboration=House Committee on Science, Space, and Technology, One Hundred Thirteenth Congress, First Session |date=19 March 2013 |title=Threats from Space: A review of U.S. Government efforts to track and mitigate asteroids and meteors |volume=Part&nbsp;I and Part&nbsp;II |page=147 |series=Hearing before the Committee on Science, Space, and Technology |publisher=House of Representatives |url=http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |archive-date=2022-10-09 |url-status=live |access-date=26 November 2018}}</ref>

=== Asteroid deflection strategies ===
{{Main|Asteroid deflection strategies|Asteroid impact avoidance}}
[[File:Dart-poster3.jpg|thumb|[[Double Asteroid Redirection Test]] in 2022 demonstrated that spacecraft impact is a viable option for [[Asteroid impact avoidance|planetary defense]].]]
Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness.<ref>{{cite journal|last1=Canavan|first1=G. H |last2=Solem|first2=J. C.|year=1992|title=Interception of near-Earth objects|journal=Mercury|issn=0047-6773|volume=21|issue=3|pages=107–109|url=https://www.researchgate.net/publication/253052410|bibcode=1992Mercu..21..107C}}</ref> There are various methods for changing the course of an asteroid/comet.<ref name="HallRoss">C. D. Hall and [[I. Michael Ross|I. M. Ross]], "Dynamics and Control Problems in the Deflection of Near-Earth Objects", ''Advances in the Astronautical Sciences, Astrodynamics 1997'', Vol. 97, Part I, 1997, pp. 613–631.</ref> These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy ({{Anchor|interception2016-01-26}}interception,<ref>{{cite journal|last=Solem|first=J. C.|year=1993|title=Interception of comets and asteroids on collision course with Earth|journal=Journal of Spacecraft and Rockets|volume=30|issue=2|pages=222–228|doi=10.2514/3.11531|bibcode=1993JSpRo..30..222S|url=https://digital.library.unt.edu/ark:/67531/metadc1090076/}}</ref><ref>Solem, J. C.; Snell, C. (1994). "[https://books.google.com/books?id=xXWZolI9NkUC&dq=Terminal+intercept+for+less+than+one+orbital+snell&pg=PA1013 Terminal intercept for less than one orbital period warning] {{webarchive |url=https://web.archive.org/web/20160506210107/https://books.google.com/books?id=xXWZolI9NkUC&pg=PA1013&lpg=PA1013&dq=Terminal+intercept+for+less+than+one+orbital+snell#v=onepage&q=Terminal%20intercept%20for%20less%20than%20one%20orbital%20snell&f=false |date=6 May 2016 }}", a chapter in ''Hazards Due to Comets and Asteroids'', Geherels, T., ed. (University of Arizona Press, Tucson), pp. 1013–1034.</ref> rendezvous, or remote station).

Strategies fall into two basic sets: fragmentation and delay.<ref name="HallRoss"/><ref>{{cite journal|last=Solem|first=J. C.|year=2000|title=Deflection and disruption of asteroids on collision course with Earth|journal=Journal of the British Interplanetary Society |volume=53|pages=180–196|url=http://www.jbis.org.uk/paper.php?p=2000.53.180 |bibcode=2000JBIS...53..180S}}</ref> Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or are small enough to burn up in the atmosphere. Delay exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time, or more correctly when some point on Earth's surface intersects the impactor's orbit when the impactor arrives. Since the [[Earth]] is approximately 12,750&nbsp;km in diameter and moves at approx. 30&nbsp;km per second in its orbit, it travels a distance of one planetary diameter in about 425 seconds, or slightly over seven minutes. Delaying, or advancing the impactor's arrival by times of this magnitude can, depending on the exact geometry of the impact, cause it to miss the Earth.<ref name="RossParkPorter">{{cite journal|last1=Ross|first1=I. M.|last2=Park|first2=S.-Y.|last3=Porter|first3=S. E.|title=Gravitational Effects of Earth in Optimizing Delta-V for Deflecting Earth-Crossing Asteroids|journal=Journal of Spacecraft and Rockets|volume=38|issue=5|date=2001|pages=759–764|hdl=10945/30321|url=https://calhoun.nps.edu/bitstream/handle/10945/30321/AIAA-3743-490.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://calhoun.nps.edu/bitstream/handle/10945/30321/AIAA-3743-490.pdf |archive-date=2022-10-09 |url-status=live|access-date=2019-08-30|citeseerx=10.1.1.462.7487|doi=10.2514/2.3743|s2cid=123431410 }}</ref>

"[[1566 Icarus#Project Icarus|Project Icarus]]" was one of the first projects designed in 1967 as a contingency plan in case of collision with [[1566 Icarus]]. The plan relied on the new [[Saturn V]] rocket, which did not make its first flight until after the report had been completed. Six Saturn V rockets would be used, each launched at variable intervals from months to hours away from impact. Each rocket was to be fitted with a single 100-megaton [[nuclear warhead]] as well as a modified [[Apollo Service Module]] and uncrewed [[Apollo Command Module]] for guidance to the target. The warheads would be detonated 30 meters from the surface, deflecting or partially destroying the asteroid. Depending on the subsequent impacts on the course or the destruction of the asteroid, later missions would be modified or cancelled as needed. The "last-ditch" launch of the sixth rocket would be 18 hours prior to impact.<ref name="Portree">{{cite magazine |author=David S. F. Portree |title=MIT Saves the World: Project Icarus (1967) |url=https://www.wired.com/2012/03/mit-saves-the-world-project-icarus-1967/ |magazine=Wired |access-date=21 October 2013}}</ref>