View source for Astronomy:Asteroid impact avoidance

{{Short description|Methods to prevent destructive asteroid hits}}
{{More|Asteroid impact prediction}}
[[File:DART-impact-SAAO-Lesedi-Mookodi.gif|thumb|Kinetic impactors such as the one used by the Double Asteroid Redirection Test – its impact with the asteroid moon Dimorphos photographed above – are one of many methods designed to alter the trajectory of an asteroid to prevent its potential collision with Earth.]]
'''Asteroid impact avoidance''' comprises the methods by which [[Astronomy:Near-Earth object|near-Earth object]]s (NEO) on a potential collision course with Earth could be diverted away, preventing destructive [[Astronomy:Impact event|impact event]]s. An impact by a sufficiently large [[Astronomy:Asteroid|asteroid]] or other NEOs would cause, depending on its impact location, massive tsunamis or multiple [[Earth:Firestorm|firestorm]]s, and an [[Astronomy:Impact winter|impact winter]] caused by the sunlight-blocking effect of large quantities of pulverized rock dust and other debris placed into the [[Earth:Stratosphere|stratosphere]]. A collision 66 million years ago between the Earth and an object approximately {{convert|10|km|mi|0|abbr=off}} wide is thought to have produced the [[Earth:Chicxulub crater|Chicxulub crater]] and triggered the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] that is understood by the [[Philosophy:Scientific community|scientific community]] to have caused the extinction of all non-avian dinosaurs.

While the chances of a major collision are low in the near term, it is a near-[[Law of large numbers|certainty]] that one will happen eventually unless defensive measures are taken. Astronomical events—such as the Shoemaker-Levy 9 impacts on Jupiter and the [[Physics:Chelyabinsk meteor|2013 Chelyabinsk meteor]], along with the growing number of near-Earth objects discovered and catalogued on the Sentry Risk Table—have drawn renewed attention to such threats.<ref>{{Cite web|last=Rahman|first=Shoaib|title=How Much Our Earth Is Ready To Prevent Future Asteroid Collisions?|url=https://vocal.media/futurism/how-much-our-earth-is-ready-to-prevent-future-asteroid-collisions|url-status=live|access-date=2021-11-17|website=Futurism |date=2021-04-10 |language=en|archive-url=https://web.archive.org/web/20210912125655/https://vocal.media/futurism/how-much-our-earth-is-ready-to-prevent-future-asteroid-collisions |archive-date=2021-09-12 }}</ref> The popularity of the 2021 movie ''Don't Look Up'' helped to raise awareness of the possibility of avoiding NEOs.<ref>{{Cite web|last=Powell|first=Corey S.|title=How prepared is Earth for an asteroid collision?|url=https://astronomy.com/news/2021/12/how-prepared-is-earth-for-an-asteroid-collision|url-status=live|access-date=2022-01-12|website=Astronomy |date=December 20, 2021 |language=en|archive-url=https://web.archive.org/web/20211220180825/https://astronomy.com/news/2021/12/how-prepared-is-earth-for-an-asteroid-collision |archive-date=2021-12-20 }}</ref>

In 2016, a [[Organization:NASA|NASA]] scientist warned that the Earth is unprepared for such an event.<ref>{{cite news |url=https://www.theguardian.com/science/2016/dec/13/space-asteroid-comet-nasa-rocket |title=Earth woefully unprepared for surprise comet or asteroid, Nasa scientist warns |work={{wipe|The Guardian}} |first=Alan |last=Yuhas |date=13 December 2016}}</ref> In April 2018, the [[Organization:B612 Foundation|B612 Foundation]] reported "It's 100 percent certain we'll be hit by a devastating asteroid, but we're not 100 percent sure when."<ref name="INQ-20180428">{{cite news |last=Homer |first=Aaron |title=Earth Will Be Hit By An Asteroid With 100 Percent Certainty, Says Space-Watching Group B612 |url=https://www.inquisitr.com/4881237/earth-will-be-hit-by-an-asteroid-with-100-percent-certainty-says-space-watching-group-b612/ |date=28 April 2018 |work=Inquisitr |access-date=28 April 2018 |archive-date=24 January 2020 |archive-url=https://web.archive.org/web/20200124092654/https://www.inquisitr.com/4881237/earth-will-be-hit-by-an-asteroid-with-100-percent-certainty-says-space-watching-group-b612/ |url-status=dead }}</ref> Also in 2018, [[Physicist|physicist]] [[Biography:Stephen Hawking|Stephen Hawking]], in his final book, ''Brief Answers to the Big Questions'', considered an asteroid collision to be the biggest threat to the planet.<ref name="WP-20181015">{{cite news |last=Stanley-Becker |first=Isaac |title=Stephen Hawking feared race of 'superhumans' able to manipulate their own DNA |url=https://www.washingtonpost.com/news/morning-mix/wp/2018/10/15/stephen-hawking-feared-race-of-superhumans-able-to-manipulate-their-own-dna/ |date=15 October 2018 |newspaper={{wipe|The Washington Post}} |access-date=15 October 2018 }}</ref><ref name="QZ-20181014">{{cite web |last=Haldevang |first=Max de |title=Stephen Hawking left us bold predictions on AI, superhumans, and aliens |url=https://qz.com/1423685/stephen-hawking-says-superhumans-will-take-over-ai-is-a-threat-and-humans-will-conquer-space/ |date=14 October 2018 |work=Quartz |access-date=15 October 2018 }}</ref><ref name="NYT-20180618">{{cite news |last=Bogdan |first=Dennis |title=Better Way To Avoid Devastating Asteroids Needed? |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html |date=18 June 2018 |work={{wipe|The New York Times}} |access-date=19 November 2018 }}</ref> Several ways of avoiding an asteroid impact have been described.<ref name="SPC-20190502">{{cite news |last=Wall |first=Mike |title=A Killer Asteroid Is Coming — We Don't Know When (So Let's Be Ready), Bill Nye Says  |url=https://www.space.com/killer-asteroids-warning-bill-nye.html |date=2 May 2019|work=Space.com |access-date=2 May 2019 }}</ref> Nonetheless, in March 2019, scientists reported that [[Astronomy:Asteroid|asteroid]]s may be [[Astronomy:Asteroid impact prediction|much more difficult to destroy]] than thought earlier.<ref name="PHYS-20190304">{{cite news |author=Johns Hopkins University
 |title=Asteroids are stronger, harder to destroy than previously thought |url=https://phys.org/news/2019-03-asteroids-stronger-harder-previously-thought.html

In 2022, NASA spacecraft [[Astronomy:Double Asteroid Redirection Test|DART]] impacted [[Astronomy:Dimorphos|Dimorphos]], reducing the minor-planet moon's orbital period by 32 minutes. This mission constitutes the [[Astronomy:Timeline of space exploration|first successful attempt]] at asteroid deflection.<ref name=":2">{{Cite web |last=Bardan |first=Roxana |date=2022-10-11 |title=NASA Confirms DART Mission Impact Changed Asteroid's Motion in Space |url=http://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space |access-date=2022-12-08 |website=NASA}}</ref> In 2025, CNSA plans to launch another deflection mission to near-Earth object '''2019 VL5''', a 30-meter wide asteroid which will include both an impactor and observer spacecraft.<ref name=":3">{{Cite web |last=Jones |first=Andrew |date=2023-04-11 |title=China to target asteroid 2019 VL5 for 2025 planetary defense test |url=https://spacenews.com/china-to-target-asteroid-2019-vl5-for-2025-planetary-defense-test/ |access-date=2023-04-17 |website=SpaceNews |language=en-US}}</ref><ref name=":4">{{Cite web |last=Andrew Jones published |date=2022-12-08 |title=China will launch 2-in-1 asteroid deflection mission in 2025 |url=https://www.space.com/china-asteroid-impact-mission-two-spacecraft |access-date=2023-04-17 |website=Space.com |language=en}}</ref>

== Deflection efforts ==
[[File:Asteroids-KnownNearEarthObjects-Animation-UpTo20180101.gif|thumb|upright=1.5|Known Near-Earth objects{{Snd}} as of January 2018<br />[https://www.youtube.com/watch?v=vfvo-Ujb_qk Video (0:55; July 23, 2018)]<br />(Earth's orbit in white)]]
[[File:SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|thumb|upright=1.5|Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.]]

According to expert testimony in the United States Congress in 2013, [[Organization:NASA|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 web |author=U.S.Congress |title=Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II){{Snd}} Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session |url=http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |date=19 March 2013 |page=147 |work=United States Congress |access-date=3 May 2014 }}</ref> In June 2018, the US National Science and Technology Council warned that the United States was unprepared for an asteroid impact event, and developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.<ref name="WH-20180621">{{cite web|url=https://trumpwhitehouse.archives.gov/wp-content/uploads/2018/06/National-Near-Earth-Object-Preparedness-Strategy-and-Action-Plan-23-pages-1MB.pdf|title=National Near-Earth Object Preparedness Strategy Action Plan|author=Staff|date=21 June 2018|via=National Archives|work=whitehouse.gov|access-date=22 June 2018}}</ref><ref name="GIZ-20180621">{{cite news|url=https://gizmodo.com/america-isnt-ready-to-handle-a-catastrophic-asteroid-im-1827014709|title=America Isn't Ready to Handle a Catastrophic Asteroid Impact, New Report Warns|last=Mandelbaum|first=Ryan F.|date=21 June 2018|work=Gizmodo|access-date=22 June 2018}}</ref><ref name="ICARUS-220180522">{{cite journal|last=Myhrvold|first=Nathan|date=22 May 2018|title=An empirical examination of WISE/NEOWISE asteroid analysis and results|journal=Icarus|volume=314|pages=64–97|bibcode=2018Icar..314...64M|doi=10.1016/j.icarus.2018.05.004|doi-access=free}}</ref><ref name="NYT-20180614">{{cite news|url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html|title=Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks|last=Chang|first=Kenneth|date=14 June 2018|work={{wipe|The New York Times}}|access-date=22 June 2018}}</ref>

Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project, as no known planetary defense hardware has yet been developed. It has been estimated that a velocity change of just {{nowrap|3.5/t × 10<sup>−2</sup> m·s<sup>−1</sup>}} (where t is the number of years until potential impact) is needed to successfully deflect a body on a direct collision trajectory. In addition, under certain circumstances, much smaller velocity changes are needed.<ref name="ParkRoss">S.-Y. Park and [[Biography:I. Michael Ross|I. M. Ross]], "Two-Body Optimization for Deflecting Earth-Crossing Asteroids",  ''Journal of Guidance, Control and Dynamics'', Vol. 22, No.3, 1999, pp.415–420.</ref> For example, it was estimated there was a high chance of [[Astronomy:99942 Apophis|99942 Apophis]] swinging by Earth in 2029 with a 10<sup>−4</sup> probability of returning on an impact trajectory in 2035 or 2036. It was then determined that a deflection from this potential return trajectory, several years before the swing-by, could be achieved with a velocity change on the order of 10<sup>−6</sup>&nbsp;m·s<sup>−1</sup>.<ref>Lu, Edward T. and Stanley G. Love. [https://web.archive.org/web/20160603070302/http://arxiv.org/abs/astro-ph/0509595 A Gravitational Tractor for Towing Asteroids], [[Organization:NASA|NASA]], Johnson Space Center, submitted to arxiv.org September 20, 2005. ([https://arxiv.org/abs/astro-ph/0509595 PDF document] {{webarchive |url=https://web.archive.org/web/20161005024351/http://arxiv.org/abs/astro-ph/0509595 |date=October 5, 2016 }}).</ref>

[[Astronomy:Double Asteroid Redirection Test|NASA's Double Asteroid Redirection Test]] (DART), the world's first full-scale mission to test technology for defending Earth against potential asteroid or comet hazards, launched on a SpaceX Falcon 9 rocket from Space Launch Complex 4 East at Vandenberg Space Force Base in California.<ref>{{Cite web |title=NASA, SpaceX Launch DART: First Planetary Defense Test Mission – Double Asteroid Redirection Test (DART) Mission |url=https://blogs.nasa.gov/dart/2021/11/24/nasa-spacex-launch-dart-first-planetary-defense-test-mission/ |access-date=2022-08-24 |website=blogs.nasa.gov |language=en-US}}</ref>

An impact by a {{convert|10|km}} asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the [[Biology:Biosphere|biosphere]]. There is also the threat from [[Astronomy:Comet|comet]]s entering the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a [[Astronomy:Near-Earth asteroid|near-Earth asteroid]], making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months.<ref>{{cite web | url=http://www.nss.org/resources/library/planetarydefense/2000-ReportOfTheTaskForceOnPotentiallyHazardousNearEarthObjects-UK.pdf | title=Report of the Task Force on potentially hazardous Near Earth Objects | publisher=British National Space Center | access-date=2008-10-21 | archive-url=https://web.archive.org/web/20161210142717/http://nss.org/resources/library/planetarydefense/2000-ReportOfTheTaskForceOnPotentiallyHazardousNearEarthObjects-UK.pdf | archive-date=2016-12-10 | url-status=dead }}, p. 12.</ref> Impacts from objects as small as {{convert|50|m}} in diameter, which are far more common, are historically extremely destructive regionally (see Barringer crater).

Finding out the material composition of the object is also helpful before deciding which strategy is appropriate. Missions like the 2005 ''[[Engineering:Deep Impact (spacecraft)|Deep Impact]]'' probe and the [[Engineering:Rosetta (spacecraft)|Rosetta]] spacecraft, have provided valuable information on what to expect. In October 2022, a method of mapping the insides of a potentially problematic asteroid in order to determine the best area for impact was proposed.<ref name="WP-20221021">{{cite news |last=Verma |first=Pranshu |title=There's a new tool to help blow up asteroids - Researchers from MIT and Stanford have created a tool that could improve the aim of future planetary defense missions |url=https://www.washingtonpost.com/technology/2022/10/21/asteroid-destruction-tool/ |date=21 October 2022 |newspaper=The Washington Post |accessdate=22 October 2022 }}</ref>

=== History of US government mandates ===

Efforts in [[Astronomy:Asteroid impact prediction|asteroid impact prediction]] have concentrated on the survey method. The 1992 NASA-sponsored Near-Earth-Object Interception Workshop hosted by [[Organization:Los Alamos National Laboratory|Los Alamos National Laboratory]] evaluated issues involved in intercepting celestial objects that could hit Earth.<ref>{{cite journal|last1=Canavan|first1=G. H.|last2=Solem|first2=J. C.|last3=Rather|first3=D. G.|year=1993|title=Proceedings of the Near-Earth-Object Interception Workshop, January 14–16, 1992, Los Alamos, NM|journal= Los Alamos National Laboratory LA—12476-C|url= https://ntrs.nasa.gov/search.jsp?R=19930019383}}</ref> In a 1992 report to [[Organization:NASA|NASA]],<ref name="morrison_1992">Morrison, D., 25 January 1992, [https://archive.org/details/nasa_techdoc_19920025001 ''The Spaceguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop''] {{webarchive |url=https://web.archive.org/web/20161013041451/https://archive.org/details/nasa_techdoc_19920025001 |date=October 13, 2016 }}, [[Organization:NASA|NASA]], Washington, D.C.</ref> a coordinated [[Astronomy:Spaceguard|Spaceguard]] Survey was recommended to discover, verify and provide follow-up observations for Earth-crossing asteroids. This survey was expected to discover 90% of these objects larger than one kilometer within 25 years. Three years later, another NASA report<ref>Shoemaker, E.M., 1995, ''Report of the Near-Earth Objects Survey Working Group'', NASA Office of Space Science, Solar System Exploration Office</ref> recommended search surveys that would discover 60–70% of short-period, near-Earth objects larger than one kilometer within ten years and obtain 90% completeness within five more years.

In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1&nbsp;km or larger that could represent a collision risk to Earth. The 1&nbsp;km diameter metric was chosen after considerable study indicated that an impact of an object smaller than 1&nbsp;km could cause significant local or regional damage but is unlikely to cause a worldwide catastrophe.<ref name="morrison_1992"/> The impact of an object much larger than 1&nbsp;km diameter could well result in worldwide damage up to, and potentially including, [[Unsolved:Human extinction|extinction of the human species]]. The NASA commitment has resulted in the funding of a number of NEO search efforts, which made considerable progress toward the 90% goal by 2008. However the 2009 discovery of several NEOs approximately 2 to 3 kilometers in diameter (e.g. {{mp|2009 CR|2}}, {{mp|2009 HC|82}}, {{mp|2009 KJ}}, {{mp|2009 MS}} and {{mp|2009 OG}}) demonstrated there were still large objects to be detected.

United States Representative George E. Brown Jr. (D-CA) was quoted as voicing his support for planetary defense projects in ''Air & Space Power Chronicles'', saying "If some day in the future we discover well in advance that an asteroid that is big enough to cause a mass extinction is going to hit the Earth, and then we alter the course of that asteroid so that it does not hit us, it will be one of the most important accomplishments in all of human history."<ref>{{Cite journal|last=France|first=Martin|date=7 August 2000|title=Planetary Defense: Eliminating the Giggle Factor|url=https://www.airuniversity.af.edu/Portals/10/ASPJ/journals/Chronicles/france2.pdf|journal=Air & Space Power Chronicles|volume=14|pages=12|via=Air University}}</ref>

Because of Congressman Brown's long-standing commitment to planetary defense, a U.S. House of Representatives' bill, H.R. 1022, was named in his honor: The George E. Brown, Jr. Near-Earth Object Survey Act. This bill "to provide for a Near-Earth Object Survey program to detect, track, catalogue, and characterize certain near-Earth asteroids and comets" was introduced in March 2005 by Rep. Dana Rohrabacher (R-CA).<ref>National Academy of Sciences. 2010. Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies: Final Report. Washington, DC: The National Academies Press. Available at: {{cite web|url=https://books.nap.edu/catalog.php?record_id%3D12842 |title=Browse All Topics &#124; the National Academies Press |access-date=2016-10-02 |url-status=live |archive-url=https://web.archive.org/web/20140806153938/http://books.nap.edu/catalog.php?record_id=12842 |archive-date=2014-08-06 }}.</ref> It was eventually rolled into S.1281, the NASA Authorization Act of 2005, passed by Congress on December 22, 2005, subsequently signed by the President, and stating in part:

{{blockquote|The U.S. Congress has declared that the general welfare and security of the United States require that the unique competence of NASA be directed to detecting, tracking, cataloguing, and characterizing near-Earth asteroids and comets in order to provide warning and mitigation of the potential hazard of such near-Earth objects to the Earth. The NASA Administrator shall plan, develop, and implement a Near-Earth Object Survey program to detect, track, catalogue, and characterize the physical characteristics of near- Earth objects equal to or greater than 140 meters in diameter in order to assess the threat of such near-Earth objects to the Earth. It shall be the goal of the Survey program to achieve 90% completion of its near-Earth object catalogue (based on statistically predicted populations of near-Earth objects) within 15 years after the date of enactment of this Act. The NASA Administrator shall transmit to Congress not later than 1 year after the date of enactment of this Act an initial report that provides the following: (A) An analysis of possible alternatives that NASA may employ to carry out the Survey program, including ground-based and space-based alternatives with technical descriptions. (B) A recommended option and proposed budget to carry out the Survey program pursuant to the recommended option. (C) Analysis of possible alternatives that NASA could employ to divert an object on a likely collision course with Earth.}}

The result of this directive was a report presented to Congress in early March 2007. This was an Analysis of Alternatives (AoA) study led by NASA's Program Analysis and Evaluation (PA&E) office with support from outside consultants, the Aerospace Corporation, NASA Langley Research Center (LaRC), and SAIC (amongst others).

See also [[Astronomy:Asteroid impact prediction#Improving impact prediction|Improving impact prediction]].

=== Ongoing projects ===
[[File:NEA by survey.png|thumb|Number of NEOs detected by various projects.]]
[[File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif|thumb|[[Astronomy:NEOWISE|NEOWISE]]{{Snd}} first four years of data starting in December 2013 (animated; April 20, 2018)]]
The [[Organization:Minor Planet Center|Minor Planet Center]] in Cambridge, Massachusetts has been cataloging the orbits of asteroids and comets since 1947. It has recently been joined by surveys that specialize in locating the [[Astronomy:Near-Earth object|near-Earth object]]s (NEO), many (as of early 2007) funded by NASA's Near Earth Object program office as part of their Spaceguard program. One of the best-known is LINEAR that began in 1996. By 2004 LINEAR was discovering tens of thousands of objects each year and accounting for 65% of all new asteroid detections.<ref>{{cite conference|first = Stokes, G.|last = Stokes|author2 = J. Evans|date = 18–25 July 2004|title = Detection and discovery of near-Earth asteroids by the linear program|conference = 35th COSPAR Scientific Assembly|location = Paris, France|pages = 4338|bibcode =2004cosp...35.4338S}}</ref> LINEAR uses two one-meter telescopes and one half-meter telescope based in New Mexico.<ref>{{cite web|url = http://neo.jpl.nasa.gov/programs/linear.html|archive-url = https://web.archive.org/web/20040114170809/http://neo.jpl.nasa.gov/programs/linear.html|url-status = dead|archive-date = 14 January 2004|title = Lincoln Near-Earth Asteroid Research (LINEAR)|publisher = National Aeronautics and Space Administration|date = 23 October 2007}}</ref>

The [[Astronomy:Catalina Sky Survey|Catalina Sky Survey]] (CSS) is conducted at the [[Astronomy:Steward Observatory|Steward Observatory]]'s [[Astronomy:Catalina Station|Catalina Station]], located near Tucson, Arizona, in the United States. It uses two telescopes, a 1.5-meter (60-inch) f/2 telescope on the peak of Mount Lemmon, and a 68-cm (27-inch) f/1.7 [[Astronomy:Schmidt camera|Schmidt]] telescope near Mount Bigelow (both in the Tucson, Arizona area). In 2005, CSS became the most prolific NEO survey surpassing [[Astronomy:Lincoln Near-Earth Asteroid Research|Lincoln Near-Earth Asteroid Research]] (LINEAR) in total number of NEOs and potentially hazardous asteroids discovered each year since. CSS discovered 310 NEOs in 2005, 396 in 2006, 466 in 2007, and in 2008 564 NEOs were found.<ref>[https://web.archive.org/web/20040513230213/http://neo.jpl.nasa.gov/stats/ NEO discovery statistics] from JPL. Shows the number of asteroids of various types (potentially hazardous, size > 1&nbsp;km, etc.) that different programs have discovered, by year.</ref>

[[Astronomy:Spacewatch|Spacewatch]], which uses a 90 centimeter telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis equipment to search the skies for intruders, was set up in 1980 by [[Biography:Tom Gehrels|Tom Gehrels]] and [[Biography:Robert S. McMillan (astronomer)|Robert S. McMillan]] of the Lunar and Planetary Laboratory of the [[Organization:University of Arizona|University of Arizona]] in Tucson, and is now being operated by McMillan. The Spacewatch project has acquired a 1.8 meter telescope, also at Kitt Peak, to hunt for NEOs, and has provided the old 90-centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability.<ref>{{cite web|url = http://spacewatch.lpl.arizona.edu/index.html|title = The Spacewatch Project|access-date = 2007-10-23|url-status = dead|archive-url = https://web.archive.org/web/20110211071733/http://spacewatch.lpl.arizona.edu/index.html|archive-date = 2011-02-11}}</ref>

Other near-Earth object tracking programs include [[Organization:Near-Earth Asteroid Tracking|Near-Earth Asteroid Tracking]] (NEAT), [[Astronomy:Lowell Observatory Near-Earth-Object Search|Lowell Observatory Near-Earth-Object Search]] (LONEOS), [[Astronomy:Campo Imperatore Near-Earth Object Survey|Campo Imperatore Near-Earth Object Survey]] (CINEOS), Japanese Spaceguard Association, and [[Astronomy:Asiago-DLR Asteroid Survey|Asiago-DLR Asteroid Survey]].<ref>{{cite web|url = http://neo.jpl.nasa.gov/programs/|archive-url = https://web.archive.org/web/20040114063231/http://neo.jpl.nasa.gov/programs/|url-status = dead|archive-date = 14 January 2004|title = Near-Earth Objects Search Program|publisher = National Aeronautics and Space Administration|date = 23 October 2007}}</ref> [[Astronomy:Pan-STARRS|Pan-STARRS]] completed telescope construction in 2010, and it is now actively observing.

The [[Astronomy:Asteroid Terrestrial-impact Last Alert System|Asteroid Terrestrial-impact Last Alert System]], now in operation, conducts frequent scans of the sky with a view to later-stage detection on the collision stretch of the asteroid orbit. Those would be much too late for deflection, but still in time for evacuation and preparation of the affected Earth region.

Another project, supported by the [[Social:European Union|European Union]], is '''NEOShield''', which analyses realistic options for preventing the collision of a NEO with Earth. Their aim is to provide test mission designs for feasible NEO mitigation concepts. The project particularly emphasises on two aspects.<ref name=neoshield>{{cite journal |title=Science and Technology for Near-Earth Object Impact Prevention |doi=10.3030/640351 |url=https://cordis.europa.eu/project/id/640351 |access-date=21 May 2023}}</ref>

# The first one is the focus on technological development on essential techniques and instruments needed for guidance, navigation and control (GNC) in close vicinity of asteroids and comets. This will, for example, allow hitting such bodies with a high-velocity kinetic impactor spacecraft and observing them before, during and after a mitigation attempt, e.g., for orbit determination and monitoring.
# The second one focuses on refining Near Earth Object (NEO) characterisation. Moreover, NEOShield-2 will carry out astronomical observations of NEOs, to improve the understanding of their physical properties, concentrating on the smaller sizes of most concern for mitigation purposes, and to identify further objects suitable for missions for physical characterisation and NEO deflection demonstration.<ref>{{cite web|url = http://www.neoshield.net/science-technology-asteroid-impact/|title = NEOShield Project|publisher = European Union Consortium|date = 17 November 2016|access-date = 17 November 2016|archive-date = 4 March 2017|archive-url = https://web.archive.org/web/20170304190851/http://www.neoshield.net/science-technology-asteroid-impact/|url-status = dead}}</ref>

"[[Astronomy:Spaceguard|Spaceguard]]" is the name for these loosely affiliated programs, some of which receive NASA funding to meet a U.S. Congressional requirement to detect 90% of near-Earth asteroids over 1&nbsp;km diameter by 2008.<ref>{{cite web|url = http://neo.jpl.nasa.gov/neo/report.html|archive-url = https://web.archive.org/web/20031001091527/http://neo.jpl.nasa.gov/neo/report.html|url-status = dead|archive-date = 2003-10-01|title = NASA Releases Near-Earth Object Search Report|publisher = National Aeronautics and Space Administration|access-date = 2007-10-23}}</ref> A 2003 NASA study of a follow-on program suggests spending US$250–450 million to detect 90% of all near-Earth asteroids 140 meters and larger by 2028.<ref>{{cite web|url = http://impact.arc.nasa.gov/news_detail.cfm?ID=168|title = NASA NEO Workshop|author = David Morrison|publisher = National Aeronautics and Space Administration|url-status = dead|archive-url = https://web.archive.org/web/20080122123731/http://impact.arc.nasa.gov/news_detail.cfm?ID=168|archive-date = 2008-01-22}}</ref>

[[Astronomy:NEODyS|NEODyS]] is an online database of known NEOs.

==== Sentinel mission ====
{{Main|Organization:B612 Foundation|Astronomy:Sentinel Space Telescope}}

The [[Organization:B612 Foundation|B612 Foundation]] is a private nonprofit foundation with headquarters in the United States, dedicated to protecting the Earth from [[Astronomy:Impact event|asteroid strikes]]. It is led mainly by scientists, former astronauts and engineers from the Institute for Advanced Study, [[Organization:Southwest Research Institute|Southwest Research Institute]], [[Organization:Stanford University|Stanford University]], [[Organization:NASA|NASA]] and the space industry.

As a non-governmental organization it has conducted two lines of related research to help detect NEOs that could one day strike the Earth, and find the technological means to divert their path to avoid such collisions. The foundation's goal had been to design and build a privately financed asteroid-finding space telescope, Sentinel, which was to be launched in 2017–2018. However the project was cancelled in 2015. Had the Sentinel's infrared telescope been parked in an orbit similar to that of [[Astronomy:Venus|Venus]], it would have helped identify threatening NEOs by cataloging 90% of those with diameters larger than {{convert|140|m}}, as well as surveying smaller Solar System objects.<ref name="Discover-2013.09">Powell, Corey S. [http://discovermagazine.com/2013/september/17-hunting-season-for-asteroids "Developing Early Warning Systems for Killer Asteroids"] {{webarchive |url=https://web.archive.org/web/20161028014849/http://discovermagazine.com/2013/september/17-hunting-season-for-asteroids |date=October 28, 2016 }}, ''Discover'', August 14, 2013, pp. 60–61 (subscription required).</ref><ref name="B612-Sentinel Mission" /><ref name="New York Times-2013.02.16" />

Data gathered by Sentinel would have helped identify [[Astronomy:Asteroid|asteroid]]s and other NEOs that pose a risk of collision with Earth, by being forwarded to scientific data-sharing networks, including [[Organization:NASA|NASA]] and academic institutions such as the Minor Planet Center.<ref name="B612-Sentinel Mission">{{cite web|title=The Sentinel Mission |publisher=B612 Foundation |url=http://b612foundation.org/media/sentinelmission/ |access-date=September 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120910234035/http://b612foundation.org/media/sentinelmission/ |archive-date=September 10, 2012 }}</ref><ref name="New York Times-2013.02.16">Broad, William J. [https://www.nytimes.com/2013/02/17/science/space/dismissed-as-doomsayers-advocates-for-meteor-detection-feel-vindicated.html Vindication for Entrepreneurs Watching Sky: Yes, It Can Fall] {{webarchive |url=https://web.archive.org/web/20141104050747/http://www.nytimes.com/2013/02/17/science/space/dismissed-as-doomsayers-advocates-for-meteor-detection-feel-vindicated.html |date=November 4, 2014 }}, ''{{wipe|The New York Times}}'' website, February 16, 2013 and in print on February 17, 2013, p. A1 of the New York edition. Retrieved June 27, 2014.</ref><ref name="Space.com-2012.07.10">{{cite web| first=Mike | last=Wall| title=Private Space Telescope Project Could Boost Asteroid Mining| work=Space.com| date=July 10, 2012  | access-date=September 14, 2012| url=http://www.space.com/16501-private-space-telescope-asteroid-mining.html}}</ref> The foundation also proposes asteroid deflection of potentially dangerous NEOs by the use of [[Engineering:Gravity tractor|gravity tractor]]s to divert their trajectories away from Earth,<ref name="Discover-2013.10">Powell, Corey S. [http://discovermagazine.com/2013/oct/16-how-to-dodge-a-cosmic-bullet How to Deflect a Killer Asteroid: Researchers Come Up With Contingency Plans That Could Help Our Planet Dodge A Cosmic Bullet] {{webarchive |url=https://web.archive.org/web/20160828054622/http://discovermagazine.com/2013/oct/16-how-to-dodge-a-cosmic-bullet |date=August 28, 2016 }},  ''Discover'' website, September 18, 2013  (subscription required), and in print as "How to Dodge a Cosmic Bullet", October 2013. Retrieved July 15, 2014.</ref><ref name="B612-2002">{{cite web|title=PROJECT B612: Deflecting an Asteroid using Nuclear-Powered Plasma Drive Propulsion (home page) |publisher=Project B612 (now B612 Foundation) |date=November 26, 2002 |url=http://b612.boulder.swri.edu/ |access-date=April 15, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20110712085255/http://b612.boulder.swri.edu/ |archive-date=July 12, 2011 }}</ref> a concept co-invented by the organization's CEO, physicist and former NASA astronaut [[Biography:Ed Lu|Ed Lu]].<ref name="Nature-Vol. 438 No. 7065-Lu, Love">{{cite journal | last1 = Lu | first1 = Edward T. | last2 = Love | first2 = Stanley G. | year = 2005 | title = Gravitational Tractor For Towing Asteroids | journal = Nature | volume = 438 | issue = 7065| pages = 177–178 | doi = 10.1038/438177a | pmid = 16281025 | arxiv = astro-ph/0509595 |bibcode = 2005Natur.438..177L | s2cid = 4414357 }}</ref>

=== Prospective projects ===
[[Astronomy:Orbit@home|Orbit@home]] intends to provide distributed computing resources to optimize search strategy. On February 16, 2013, the project was halted due to lack of grant funding.<ref>{{cite web|url=http://orbit.psi.edu/?q=node/32|title=Project Stopped|website=Orbit.psi.edu|access-date=2013-10-29|url-status=dead|archive-url=https://web.archive.org/web/20130802060805/http://orbit.psi.edu/?q=node%2F32|archive-date=2013-08-02}}</ref> However, on July 23, 2013, the orbit@home project was selected for funding by NASA's Near Earth Object Observation program and was to resume operations sometime in early 2014.<ref>{{cite web|url=http://orbit.psi.edu/?q=node/32|title=orbit@home is upgrading!|website=Orbit.psi.edu|access-date=2013-10-29|url-status=dead|archive-url=https://web.archive.org/web/20140227092309/http://orbit.psi.edu/?q=node%2F32|archive-date=2014-02-27}}</ref> As of July 13, 2018, the project is offline according to its website.<ref>{{cite web|url=http://orbit.psi.edu/?q=node/33|title=The orbit@home project is currently offline|website=Orbit.psi.edu|access-date=2018-07-13|url-status=live|archive-url=https://web.archive.org/web/20180713232632/http://orbit.psi.edu/?q=node%2F33|archive-date=2018-07-13}}</ref>

The [[Astronomy:Large Synoptic Survey Telescope|Large Synoptic Survey Telescope]], currently under construction, is expected to perform a comprehensive, high-resolution survey starting in the early 2020s.

===Detection from space===
On November 8, 2007, the House Committee on Science and Technology's Subcommittee on Space and Aeronautics held a hearing to examine the status of NASA's Near-Earth Object survey program. The prospect of using the [[Astronomy:Wide-field Infrared Survey Explorer|Wide-field Infrared Survey Explorer]] was proposed by NASA officials.<ref>{{Cite web|url=http://www.spaceref.ca/news/viewsr.html?pid=25960|title=Hearing Charter: Near-Earth Objects: Status of the Survey Program and Review of NASA's 2007 Report to Congress &#124; SpaceRef Canada – Your Daily Source of Canadian Space News<!-- Bot generated title -->|access-date=2021-02-27|archive-date=2012-12-05|archive-url=https://archive.today/20121205230352/http://www.spaceref.ca/news/viewsr.html?pid=25960|url-status=bot: unknown}}</ref>

WISE surveyed the sky in the infrared band at a very high sensitivity. Asteroids that absorb solar radiation can be observed through the infrared band. It was used to detect NEOs, in addition to performing its science goals. It is projected that WISE could detect 400 NEOs (roughly two percent of the estimated NEO population of interest) within the one-year mission.

[[Astronomy:Near Earth Object Surveillance Satellite|NEOSSat]], the Near Earth Object Surveillance Satellite, is a [[Biology:Microsatellite|microsatellite]] launched in February 2013 by the [[Organization:Canadian Space Agency|Canadian Space Agency]] (CSA) that will hunt for NEOs in space.<ref name="Hildebrand2008">{{cite conference |url=http://www.lpi.usra.edu/meetings/acm2008/pdf/8293.pdf |title=The Near Earth Object Surveillance Satellite (NEOSSat) Mission Will Conduct an Efficient Space-Based Asteroid Survey at Low Solar Elongations |conference=Asteroids, Comets, Meteors |first1=A. R. |last1=Hildebrand |first2=E. F. |last2=Tedesco |first3=K. A. |last3=Carroll |year=2008 |id=Paper id 8293 |bibcode=2008LPICo1405.8293H|display-authors=0}}</ref><ref name="CalHerald20080502">{{cite news|url=http://www.canada.com/topics/news/world/story.html?id=278ed690-ccf5-4bdd-88ee-ce83eecb2db4&k=41066 |title=Canada space mission targets asteroids |work=Calgary Herald via Canada.com |first=Tom |last=Spears |date=May 2, 2008 |access-date=June 27, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20121106103756/http://www.canada.com/topics/news/world/story.html?id=278ed690-ccf5-4bdd-88ee-ce83eecb2db4&k=41066 |archive-date=November 6, 2012 }}</ref> Furthermore [[Astronomy:NEOWISE|Near-Earth Object WISE (NEOWISE)]], an extension of the [[Astronomy:Wide-field Infrared Survey Explorer|WISE]] mission, started in September 2013 (in its second mission extension) to hunt [[Astronomy:Asteroid|asteroid]]s and [[Astronomy:Comet|comet]]s [[Astronomy:Near-Earth object|close to the orbit of Earth]].<ref name="nasa20130821">{{cite web |url=http://www.jpl.nasa.gov/wise/newsfeatures.cfm?release=2013-257 |title=NASA Spacecraft Reactivated to Hunt for Asteroids |publisher=[[Organization:NASA|NASA]] |first1=D. C. |last1=Agle |first2=Dwayne |last2=Brown |date=August 21, 2013 |access-date=April 24, 2018 |archive-date=August 30, 2013 |archive-url=https://web.archive.org/web/20130830000959/http://www.jpl.nasa.gov/wise/newsfeatures.cfm?release=2013-257 |url-status=dead }}</ref><ref>{{cite web |url=https://hackaday.com/2020/07/22/the-wise-in-neowise-how-a-hibernating-satellite-awoke-to-discover-the-comet/ |title=The WISE In NEOWISE: How A Hibernating Satellite Awoke To Discover The Comet |date=July 22, 2020 |first=Tom |last=Nardi}}</ref>

=== ''Deep Impact'' ===

Research published in the March 26, 2009 issue of the journal ''Nature'', describes how scientists were able to identify an asteroid in space before it entered Earth's atmosphere, enabling computers to determine its area of origin in the Solar System as well as predict the arrival time and location on Earth of its shattered surviving parts. The four-meter-diameter asteroid, called [[Astronomy:2008 TC3|2008 TC<sub>3</sub>]], was initially sighted by the automated [[Astronomy:Catalina Sky Survey|Catalina Sky Survey]] telescope, on October 6, 2008. Computations correctly predicted that it would impact 19 hours after discovery and in the Nubian Desert of northern Sudan.<ref>{{cite web |url=https://www.newswise.com/articles/we-saw-it-coming-asteroid-monitored-from-outer-space-to-ground-impact |title=We Saw It Coming: Asteroid Monitored from Outer Space to Ground Impact |archive-url=https://web.archive.org/web/20160303201008/http://newswise.com/articles/view/550468/ |archive-date=March 3, 2016 |url-status=live |work=Newswise |access-date=March 26, 2009}}</ref>

A number of potential threats have been identified, such as [[Astronomy:99942 Apophis|99942 Apophis]] (previously known by its [[Astronomy:Provisional designation|provisional designation]] {{mp|2004&nbsp;MN|4}}), which in 2004 temporarily had an impact probability of about 3% for the year 2029. Additional observations revised this probability down to zero.<ref>{{cite web |url=http://neo.jpl.nasa.gov/apophis/ |archive-url=https://web.archive.org/web/20071027032753/http://neo.jpl.nasa.gov/apophis/ |url-status=dead |archive-date=2007-10-27 |title=99942 Apophis (2004 MN4): Predicting Apophis' Earth Encounters in 2029 and 2036}}</ref>

=== ''Double Asteroid Redirection Test'' ===

On September 26, 2022 [[Astronomy:Double Asteroid Redirection Test|DART]] impacted [[Astronomy:Dimorphos|Dimorphos]], reducing the minor-planet moon's orbital period by 32 minutes. This mission was the [[Astronomy:Timeline of space exploration|first successful attempt]] at asteroid deflection.<ref name=":2" />

=== 2019 VL5 Asteroid Deflection Mission ===

In 2025, China's CNSA intends to launch a deflection mission to near-Earth object 2019 VL5, a 30-meter wide asteroid. The mission will launch on a [[Engineering:Long March 3B|Long March 3B]] rocket and carry both an impactor and observer spacecraft.<ref name=":3" /><ref name=":4" /><ref>{{Cite web |last=Young |first=Chris |date=2023-04-12 |title=China will launch an impactor spacecraft to alter asteroid trajectory |url=https://interestingengineering.com/science/china-to-alter-asteroid-trajectory |access-date=2023-04-17 |website=interestingengineering.com |language=en-US}}</ref>

== Impact probability calculation pattern ==
[[Image:AsteroidImpactProb.png|thumb|Why asteroid impact probability often goes up, then down.]]{{One source section|date=December 2018}}
The ellipses in the diagram on the right show the predicted position of an example asteroid at closest Earth approach. At first, with only a few asteroid observations, the error ellipse is very large and includes the Earth. Further observations shrink the error ellipse, but it still includes the Earth. This raises the predicted impact probability, since the Earth now covers a larger fraction of the error region. Finally, yet more observations (often radar observations, or discovery of a previous sighting of the same asteroid on archival images) shrink the ellipse revealing that the Earth is outside the error region, and the impact probability is near zero.<ref>{{cite web|url=http://www.spaceguarduk.com/scares.htm |title=Why we have Asteroid "Scares" |publisher=Spaceguard UK |archive-url=https://web.archive.org/web/20071222114100/http://www.spaceguarduk.com/scares.htm |archive-date=December 22, 2007}}</ref>

For asteroids that are actually on track to hit Earth the predicted probability of impact continues to increase as more observations are made. This similar pattern makes it difficult to differentiate between asteroids that will only come close to Earth and those that will actually hit it. This in turn makes it difficult to decide when to raise an alarm as gaining more certainty takes time, which reduces time available to react to a predicted impact. However, raising the alarm too soon has the danger of causing a [[False alarm|false alarm]] and creating a Boy Who Cried Wolf effect if the asteroid in fact misses Earth.

== Collision avoidance strategies ==
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 [[Biography: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. {{hdl|10945/40399}}</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&q=Terminal%20intercept%20for%20less%20than%20one%20orbital%20snell&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#v=onepage&q=Terminal%20intercept%20for%20less%20than%20one%20orbital%20snell |date=May 6, 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 {{Anchor|delay2016-01-26}}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 |url=https://ui.adsabs.harvard.edu/abs/2000JBIS...53..180S/abstract |journal=Journal of the British Interplanetary Society |volume=53 |pages=180–196 |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|access-date=2019-08-30|citeseerx=10.1.1.462.7487|doi=10.2514/2.3743|s2cid=123431410 }}</ref>

Collision avoidance strategies can also be seen as either direct, or indirect and in how rapidly they transfer energy to the object. The direct methods, such as nuclear explosives, or kinetic impactors, rapidly intercept the bolide's path. Direct methods are preferred because they are generally less costly in time and money.{{citation needed|date=January 2022}} Their effects may be immediate, thus saving precious time. These methods would work for short-notice and long-notice threats, and are most effective against solid objects that can be directly pushed, but in the case of kinetic impactors, they are not very effective against large loosely aggregated rubble piles. Indirect methods, such as [[Engineering:Gravity tractor|gravity tractor]]s, attaching rockets or mass drivers, are much slower. They require traveling to the object, changing course up to 180 degrees for [[Astronomy:Space rendezvous|space rendezvous]], and then taking much more time to change the asteroid's path just enough so it will miss Earth.{{citation needed|date=May 2020}}

Many NEOs are thought to be "flying [[Astronomy:Rubble pile|rubble pile]]s" only loosely held together by gravity, and a typical spacecraft sized kinetic-impactor deflection attempt might just break up the object or fragment it without sufficiently adjusting its course.<ref name="spacesailing.net">{{Cite web|url=http://www.spacesailing.net/paper/200703_Washington_DachwaldKahleWie.pdf|archiveurl=https://web.archive.org/web/20160304091722/http://www.spacesailing.net/paper/200703_Washington_DachwaldKahleWie.pdf|url-status=dead|title=Planetary Defense Conference 2007, Washington D.C. Head-On Impact Deflection of NEAs: A Case Study for 99942 Apophis. Bernd Dachwald, Ralph Kahle, Bong Wie, Published in 2007.pg 3|archivedate=March 4, 2016}}</ref> If an asteroid breaks into fragments, any fragment larger than 35 meters across would not burn up in the atmosphere and itself could impact Earth. Tracking the thousands of buckshot-like fragments that could result from such an explosion would be a very daunting task, although fragmentation would be preferable to doing nothing and allowing the originally larger rubble body, which is analogous to a shot and wax slug, to impact the Earth.{{citation needed|date=January 2022}}

In [[Cielo (supercomputer)|Cielo]] simulations conducted in 2011–2012, in which the rate and quantity of energy delivery were sufficiently high and matched to the size of the rubble pile, such as following a tailored nuclear explosion, results indicated that any asteroid fragments, created after the pulse of energy is delivered, would not pose a threat of re-[[Physics:Coalescence|coalescing]] (including for those with the shape of asteroid [[Astronomy:25143 Itokawa|Itokawa]]) but instead would rapidly achieve [[Astronomy:Escape velocity|escape velocity]] from their parent body (which for Itokawa is about 0.2&nbsp;m/s) and therefore move out of an Earth-impact trajectory.<ref name="Dillow">{{cite news| last =Dillow| first =Clay| title =How it Would Work: Destroying an Incoming Killer Asteroid With a Nuclear Blast| newspaper =Popular Science| publisher =Bonnier| date =9 April 2012| url =http://www.popsci.com/technology/article/2012-04/how-it-would-work-destroying-incoming-killer-asteroid-nuclear-blast| access-date = 6 January 2013}}</ref><ref>{{Cite web |url=http://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-11-03124 |title=RAGE Hydrocode Modeling of Asteroid Mitigation:Surface and Subsurface Explosions in Porous PHO Objects |author=Weaver |display-authors=0 |year=2011 |access-date=2018-04-09 |archive-url=https://web.archive.org/web/20180409145428/http://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-11-03124 |archive-date=2018-04-09 |url-status=live }}</ref><ref>[https://web.archive.org/web/20170413053140/http://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-11-00015 Further RAGE modeling of Asteroid mitigation, surface and subsurface explosions in porous objects. Weaver et al. 2011]</ref>

=== Nuclear explosive device ===
[[File:Bravo secondary fireball.jpg|thumb|upright=1.5|In a similar manner to the earlier pipes filled with a [[Physics:Partial pressure|partial pressure]] of helium, as used in the Ivy Mike test of 1952, the 1954 Castle Bravo test was likewise heavily instrumented with line-of-sight (LOS) pipes, to better define and quantify the timing and energies of the x-rays and neutrons produced by these early thermonuclear devices.<ref>{{Cite web|url=http://archive.org/details/CastleCommandersReport1954|title=Operation CASTLE Commander's Report|date=May 21, 1954|via=Internet Archive}}</ref><ref>{{Cite web|url=https://www.youtube.com/watch?v=DFJ2MyWlXgs|title=Declassified U.S. Nuclear Test Film #34|via=www.youtube.com}}</ref> One of the outcomes of this diagnostic work resulted in this graphic depiction of the transport of energetic x-ray and neutrons through a vacuum line, some 2.3&nbsp;km long, whereupon it heated solid matter at the "station 1200" blockhouse and thus generated a secondary fireball.<ref>{{Cite web |url=http://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-03-5462 |title=Data Contribute to Certification Fred N. Mortensen, John M. Scott, and Stirling A. Colgate |access-date=2016-12-23 |archive-url=https://web.archive.org/web/20161223223806/http://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-03-5462 |archive-date=2016-12-23 |url-status=live }}</ref><ref>{{Cite web|url=http://la-science.lanl.gov/lascience28.shtml|title=LANL: Los Alamos Science: LA Science No. 28|date=June 12, 2007|archive-url=https://web.archive.org/web/20070612184310/http://la-science.lanl.gov/lascience28.shtml |archive-date=2007-06-12 }}</ref>]]

Initiating a [[Physics:Nuclear explosive|nuclear explosive]] device [[Engineering:Proximity fuze|above]], on, or slightly beneath, the surface of a threatening celestial body is a potential deflection option, with the optimal detonation height dependent upon the composition and size of the object.<ref>{{cite book|author=Simonenko, V.|author2=Nogin, V.|author3=Petrov, D.|author4=Shubin, O.|author5=Solem, J. C.|date=1994|chapter-url=https://books.google.com/books?id=xXWZolI9NkUC&pg=PA929|chapter=Defending the Earth against impacts from large comets and asteroids|title=Hazards Due to Comets and Asteroids|editor=Geherels, T.|editor2=Matthews, M. S.|editor3=Schumann, A. M.|publisher=University of Arizona Press|isbn=9780816515059|pages=929–954}}</ref><ref>Solem, J. C. (1995). "[https://web.archive.org/web/20150909023233/https://e-reports-ext.llnl.gov/pdf/232015.pdf Interception and disruption]", in ''Proceedings of Planetary Defense Workshop, Livermore, CA, May 22–26, 1995'', CONF-9505266 (LLNL, Livermore, CA), pp. 219–228 (236–246).</ref><ref>{{cite journal|last=Solem|first=J. C.|year=1999|title=Comet and asteroid hazards: Threat and mitigation|journal=Science of Tsunami Hazards|volume=17|issue=3|pages=141–154|url=http://www.tsunamisociety.org/TitlesAuthors14to18.html}}</ref> It does not require the entire NEO to be vaporized to mitigate an impact threat. In the case of an inbound threat from a "rubble pile," the [[Engineering:Proximity fuze|stand off]], or detonation height above the surface configuration, has been put forth as a means to prevent the potential fracturing of the rubble pile.<ref name="defending Earth">{{cite book |url=http://www.nap.edu/openbook.php?record_id=12842&page=77 |title=Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies ( 2010 ) National Academy of Sciences page 77|year=2010|doi=10.17226/12842|isbn=978-0-309-14968-6}}</ref> The energetic [[Physics:Neutron|neutron]]s and soft X-rays released by the detonation, which do not appreciably penetrate matter,<ref>{{cite web|url=http://physics.nist.gov/cgi-bin/ffast/ffast.pl?Formula=H2O&gtype=5&range=S&lower=0.300&upper=2.00&density=1.00 |title=Physics.nist.gov |publisher=Physics.nist.gov |access-date=2011-11-08}}</ref> are converted into heat upon encountering the object's surface matter, [[Physics:Radiation implosion|ablatively vaporizing]] all [[Line-of-sight propagation|line of sight]] exposed surface areas of the object to a shallow depth,<ref name="defending Earth"/> turning the surface material it heats up into [[Astronomy:Ejecta|ejecta]], and, analogous to the ejecta from a chemical [[Engineering:Rocket engine|rocket engine]] exhaust, changing the velocity, or "nudging", the object off course by the reaction, following Newton's third law, with ejecta going one way and the object being propelled in the other.<ref name="defending Earth"/><ref name="flightglobal">{{cite web|first=Rob|last=Coppinger|date=August 3, 2007|url=http://www.flightglobal.com/articles/2007/08/03/215924/nasa-plans-armageddon-spacecraft-to-blast-asteroid.html|title=NASA plans 'Armageddon' spacecraft to blast asteroid|url-status=dead|archive-url=https://web.archive.org/web/20110905041237/http://www.flightglobal.com/articles/2007/08/03/215924/nasa-plans-armageddon-spacecraft-to-blast-asteroid.html|archive-date=2011-09-05|quote=The warheads would explode at a distance of one-third of the NEO's diameter and each detonation's X and gamma rays and neutrons would turn part of the NEO's surface into an expanding plasma to generate a force to deflect the asteroid.|website=Flightglobal.com}}<br />{{cite web|url=http://www.flightglobal.com/news/articles/nasa-plans-armageddon-spacecraft-to-blast-asteroid-215924/|title=NASA plans 'Armageddon' spacecraft to blast asteroid|access-date=2014-08-03}}</ref> Depending on the energy of the explosive device, the resulting [[Engineering:Reaction engine|rocket exhaust]] effect, created by the high velocity of the asteroid's vaporized mass ejecta, coupled with the object's small reduction in mass, would produce enough of a change in the object's orbit to make it miss the Earth.<ref name="Dillow"/><ref name="flightglobal"/>

A Hypervelocity Asteroid Mitigation Mission for Emergency Response (HAMMER) has been proposed.<ref>{{Cite web|url=https://phys.org/news/2018-03-scientists-asteroid-deflector-massive-potential.html|title=Scientists design conceptual asteroid deflector and evaluate it against massive potential threat |date=March 15, 2018 |website=Phys.org |archive-url=https://archive.today/20180423052949/https://phys.org/news/2018-03-scientists-asteroid-deflector-massive-potential.html |archive-date=April 23, 2018 |url-status=live}}</ref> While there have been no updates as of 2023 regarding the HAMMER, NASA has published its regular Planetary Defense Strategy and Action Plan for 2023. In it, NASA acknowledges that it is crucial to continue studying the potential of nuclear energy in deflecting or destroying asteroids. This is because it is currently the only option for defense if scientists were not aware of the asteroid within a few months or years, depending on the asteroid's velocity. The report also notes there needs to be research done into the legal implications as well as policy implications on the topic. <ref>{{Cite web |date=April 2023 |title=NASA Planetary Defense Strategy and Action Plan |url=https://www.nasa.gov/sites/default/files/atoms/files/nasa_-_planetary_defense_strategy_-_final-508.pdf |access-date=April 24, 2023}}</ref>

====Stand-off approach====
If the object is very large but is still a loosely-held-together rubble pile, a solution is to detonate one or a series of nuclear explosive devices alongside the asteroid, at a {{convert|20|m|adj=on|sp=us||}} or greater stand-off height above its surface,{{Citation needed|date=August 2019}} so as not to fracture the potentially loosely-held-together object. Providing that this stand-off strategy was done far enough in advance, the force from a sufficient number of nuclear blasts would alter the object's trajectory enough to avoid an impact, according to computer simulations and experimental evidence from [[Astronomy:Meteorite|meteorite]]s exposed to the thermal X-ray pulses of the [[Organization:Z Pulsed Power Facility|Z-machine]].<ref>{{cite web |url=https://www.discovermagazine.com/the-sciences/how-to-stop-a-killer-asteroid |title=How to Stop a Killer Asteroid |magazine=Discover |first=Steve |last=Nadis |date=January 21, 2015}}<!-- {{webarchive |url=https://web.archive.org/web/20160827112933/http://discovermagazine.com/2015/march/15-how-to-stop-a-killer-asteroid |date=August 27, 2016 }} --></ref>

In 1967, graduate students under Professor Paul Sandorff at the [[Organization:Massachusetts Institute of Technology|Massachusetts Institute of Technology]] were tasked with designing a method to prevent a hypothetical 18-month distant impact on Earth by the {{convert|1.4|km|mi|adj=mid|-wide|sp=us}} asteroid [[Astronomy:1566 Icarus|1566 Icarus]], an object that makes regular close approaches to Earth, sometimes as close as 16 [[Astronomy:Lunar distance|lunar distances]].<ref>{{Cite journal|last1=Goldstein |first1=R. M.|title=Radar Observations of Icarus|journal=Science|year=1968|volume=162 |issue=3856 |pages=903–4|bibcode = 1968Sci...162..903G|doi=10.1126/science.162.3856.903|pmid=17769079|s2cid=129644095}}</ref> To achieve the task within the timeframe and with limited material knowledge of the asteroid's composition, a variable stand-off system was conceived. This would have used a number of modified [[Engineering:Saturn V|Saturn V]] rockets sent on interception courses and the creation of a handful of nuclear explosive devices in the 100-megaton energy range—coincidentally, the same as the maximum yield of the Soviets' ''Tsar Bomba'' would have been if a uranium tamper had been used—as each rocket vehicle's [[Engineering:Payload|payload]].<ref name="Time1967">[http://content.time.com/time/magazine/article/0,9171,843952,00.html "Systems Engineering: Avoiding an Asteroid"]  {{webarchive|url=https://web.archive.org/web/20130721110612/http://www.time.com/time/magazine/article/0%2C9171%2C843952%2C00.html |date=July 21, 2013 }}, ''{{wipe|Time (magazine)}}'', June 16, 1967.</ref><ref name="Day">Day, Dwayne A., [http://www.thespacereview.com/article/175/1 "Giant bombs on giant rockets: Project Icarus"] {{webarchive |url=https://web.archive.org/web/20160415041026/http://www.thespacereview.com/article/175/1 |date=April 15, 2016 }}, ''The Space Review'', Monday, July 5, 2004</ref> The design study was later published as [[Astronomy:1566 Icarus#Project Icarus|Project Icarus]]<ref name="Icarus">Kleiman Louis A., [http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=6840 ''Project Icarus: an MIT Student Project in Systems Engineering''] {{webarchive |url=https://web.archive.org/web/20071017105104/http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=6840 |date=October 17, 2007 }}, Cambridge, Massachusetts : MIT Press, 1968</ref> which served as the inspiration for the 1979 film ''Meteor''.<ref name="Day"/><ref>{{Cite web|url=http://www.ips.gov.au/IPSHosted/neo/info/refers/Bk_Icarus_MIT.htm|archiveurl=https://web.archive.org/web/20160602104006/http://www.ips.gov.au/IPSHosted/neo/info/refers/Bk_Icarus_MIT.htm|url-status=dead|title=''Project Icarus''|archivedate=June 2, 2016}}</ref><ref name="Tech1979">[http://tech.mit.edu/archives/VOL_099/TECH_V099_S0470_P003.pdf "MIT Course precept for movie"] {{webarchive |url=https://web.archive.org/web/20161104102228/http://tech.mit.edu/archives/VOL_099/TECH_V099_S0470_P003.pdf |date=November 4, 2016 }}, ''The Tech'', MIT, October 30, 1979</ref>

A [[Organization:NASA|NASA]] analysis of deflection alternatives, conducted in 2007, stated:

{{blockquote|Nuclear standoff explosions are assessed to be 10–100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks.<ref name="nasa">{{cite web|url=http://neo.jpl.nasa.gov/neo/report2007.html |title=NEO Survey and Deflection Analysis and Alternatives |access-date=2015-11-20 |url-status=dead |archive-url=https://web.archive.org/web/20160305101217/http://neo.jpl.nasa.gov/neo/report2007.html |archive-date=2016-03-05 }} Near-Earth Object Survey and Deflection Analysis of Alternatives Report to Congress March 2007</ref>}}

In the same year, NASA released a study where the asteroid [[Astronomy:99942 Apophis|Apophis]] (with a diameter of around {{convert|300|m|disp=or|-2||}}) was assumed to have a much lower rubble pile density ({{cvt|1500|kg/m3|lb/cuft|disp=or|round=25|}}) and therefore lower mass than it is now known to have, and in the study, it is assumed to be on an impact trajectory with Earth for the year 2029. Under these hypothetical conditions, the report determines that a "Cradle spacecraft" would be sufficient to deflect it from Earth impact. This conceptual spacecraft contains six B83 physics packages, each set for their maximum 1.2-megatonne yield,<ref name="flightglobal"/> bundled together and lofted by an Ares V vehicle sometime in the 2020s, with each B83 being [[Engineering:Proximity fuze|fuzed]] to detonate over the asteroid's surface at a height of {{convert|100|m|disp=or|||}} ("1/3 of the objects diameter" as its stand-off), one after the other, with hour-long intervals between each detonation. The results of this study indicated that a single employment of this option "can deflect NEOs of [{{convert|100-500|m|disp=or|||}} diameter] two years before impact, and larger NEOs with at least five years warning".<ref name="flightglobal"/><ref name="nss.org">{{Cite web|url=http://www.nss.org/resources/library/planetarydefense/2007-NearEarthObjectMitigationOptionsUsingExplorationTechnologies.pdf|archiveurl=https://web.archive.org/web/20150701020407/http://www.nss.org/resources/library/planetarydefense/2007-NearEarthObjectMitigationOptionsUsingExplorationTechnologies.pdf|url-status=dead|title=Near Earth Object (NEO) Mitigation Options Using Exploration Technologies|archivedate=July 1, 2015}}</ref> These effectiveness figures are considered to be "conservative" by its authors, and only the thermal X-ray output of the B83 devices was considered, while neutron heating was neglected for ease of calculation purposes.<ref name="nss.org"/><ref>[https://web.archive.org/web/20170310002322/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090025983.pdf Towards Designing an Integrated Architecture for NEO Characterization, Mitigation, Scientific Evaluation, and Resource Utilization]</ref>

Research published in 2021 pointed out the fact that for an effective deflection mission, there would need to be a significant amount of warning time, with the ideal being several years or more. The more warning time provided, the less energy will be necessary to divert the asteroid just enough to adjust the trajectory to avoid Earth. The study also emphasized that deflection, as opposed to destruction, can be a safer option, as there is a smaller likelihood of asteroid debris falling to Earth's surface. The researchers proposed the best way to divert an asteroid through deflection is adjusting the output of neutron energy in the nuclear explosion. <ref name="auto">{{Cite journal |last1=Horan |first1=Lansing S. |last2=Holland |first2=Darren E. |last3=Bruck Syal |first3=Megan |last4=Bevins |first4=James E. |last5=Wasem |first5=Joseph V. |date=2021-06-01 |title=Impact of neutron energy on asteroid deflection performance |journal=Acta Astronautica |language=en |volume=183 |pages=29–42 |doi=10.1016/j.actaastro.2021.02.028 |bibcode=2021AcAau.183...29H |s2cid=233791597 |issn=0094-5765|doi-access=free }}</ref>

====Surface and subsurface use====
[[File:Asteroid Capture.jpg|thumb|This early Asteroid Redirect Mission artist's impression is suggestive of another method of changing a large threatening celestial body's orbit by capturing relatively smaller celestial objects and using those, and not the usually proposed small bits of spacecraft, as the means of creating a powerful kinetic impact,<ref>{{cite journal|last1=Asphaug|first1=E.|last2=Ostro|first2=S. J.|last3=Hudson|first3=R. S.|last4=Scheeres|first4=D. J.|last5=Benz|first5=W.|date=1998|title=Disruption of kilometre-sized asteroids by energetic collisions|journal=Nature|volume=393|issue=6684|pages=437–440|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19541/1/98-0965.pdf|url-status=dead|archive-url=https://web.archive.org/web/20160306071546/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19541/1/98-0965.pdf|archive-date=March 6, 2016|doi=10.1038/30911|bibcode=1998Natur.393..437A|s2cid=4328861}}</ref> or alternatively, a stronger faster acting gravitational tractor, as some low-density asteroids such as 253 Mathilde can dissipate impact energy.]]

In 2011, the director of the Asteroid Deflection Research Center at [[Organization:Iowa State University|Iowa State University]], Dr. Bong Wie (who had published kinetic impactor deflection studies<ref name="spacesailing.net"/> previously), began to study strategies that could deal with {{convert|50|to(-)|500|m|ft|adj=mid|-diameter|-2}} objects when the time to Earth impact was less than one year. He concluded that to provide the required energy, a nuclear explosion or other event that could deliver the same power, are the only methods that can work against a very large asteroid within these time constraints.

This work resulted in the creation of a conceptual [[Engineering:Hypervelocity Asteroid Intercept Vehicle|Hypervelocity Asteroid Intercept Vehicle]] (HAIV), which combines a [[Engineering:Deep Impact (spacecraft)|kinetic impactor]] to create an initial [[Astronomy:Impact crater|crater]] for a follow-up subsurface nuclear detonation within that initial crater, which would generate a high degree of efficiency in the conversion of the nuclear energy that is released in the detonation into propulsion energy to the asteroid.<ref>{{cite web|url=http://www.space.com/21333-asteroid-nuke-spacecraft-mission.html |title=Nuking Dangerous Asteroids Might be the Best Protection, Expert Says |website=Space.com |date=29 May 2013 |access-date=2013-07-02 |url-status=live |archive-url=https://web.archive.org/web/20160401213420/http://www.space.com/21333-asteroid-nuke-spacecraft-mission.html |archive-date=2016-04-01 }} Nuking Dangerous Asteroids Might Be the Best Protection, Expert Says. Includes a supercomputer simulation video provided by [[Organization:Los Alamos National Laboratory|Los Alamos National Laboratory]].</ref>

A similar proposal would use a surface-detonating nuclear device in place of the kinetic impactor to create the initial crater, then using the crater as a rocket nozzle to channel succeeding nuclear detonations.

Wie claimed the computer models he worked on showed the possibility for a 300-metre-wide (1,000 ft) asteroid to be destroyed using a single HAIV with a warning time of 30 days. Additionally, the models showed that less than 0.1% of debris from the asteroid would reach Earth's surface. <ref>{{Cite web |author1=Mike Wall |date=2014-02-14 |title=How Nuclear Bombs Could Save Earth from Killer Asteroids |url=https://www.space.com/24696-asteroid-strike-nuclear-bombs.html |access-date=2023-04-25 |website=Space.com |language=en}}</ref> However, there have been few substantial updates from Wie and his team since 2014 regarding the research.

As of 2015, Wie has collaborated with the Danish Emergency Asteroid Defence Project (EADP),<ref>{{Cite web|url=https://sex-girls.victorianvillainn.com/?u=htzkte4&o=l7dphzp&cid=1amg06baffuh2|archiveurl=https://web.archive.org/web/20161025023222/http://eadproject.com/partners/bong-wie/|url-status=dead|title=Meet and fuck tonight in your area|archivedate=October 25, 2016|website=sex-girls.victorianvillainn.com}}</ref> which ultimately intends to crowdsource sufficient funds to design, build, and store a non-nuclear HAIV spacecraft as planetary insurance. For threatening asteroids too large and/or too close to Earth impact to effectively be deflected by the non-nuclear HAIV approach, nuclear explosive devices (with 5% of the explosive yield than those used for the stand-off strategy) are intended to be swapped in, under international oversight, when conditions arise that necessitate it.<ref>{{Cite web|url=http://asteroiddefence.com/|title=EADP|date=May 5, 2015|archive-url=https://web.archive.org/web/20150505181554/http://asteroiddefence.com/ |archive-date=2015-05-05 }}</ref>

A study published in 2020 pointed out that a non-nuclear kinetic impact becomes less effective the larger and closer the asteroid. However, researchers ran a model that suggested a nuclear detonation near the surface of an asteroid designed to cover one side of the asteroid with x-rays would be effective. When the x-rays cover one side of an asteroid in the program, they produce propulsion energy that would propel the asteroid in a preferred direction. <ref>{{Cite journal |last1=Dearborn |first1=David S. P. |last2=Bruck Syal |first2=Megan |last3=Barbee |first3=Brent W. |last4=Gisler |first4=Galen |last5=Greenaugh |first5=Kevin |last6=Howley |first6=Kirsten M. |last7=Leung |first7=Ronald |last8=Lyzhoft |first8=Joshua |last9=Miller |first9=Paul L. |last10=Nuth |first10=Joseph A. |last11=Plesko |first11=Catherine S. |last12=Seery |first12=Bernard D. |last13=Wasem |first13=Joseph V. |last14=Weaver |first14=Robert P. |last15=Zebenay |first15=Melak |date=2020-01-01 |title=Options and uncertainties in planetary defense: Impulse-dependent response and the physical properties of asteroids |journal=Acta Astronautica |language=en |volume=166 |pages=290–305 |doi=10.1016/j.actaastro.2019.10.026 |bibcode=2020AcAau.166..290D |s2cid=208840044 |issn=0094-5765|doi-access=free }}</ref> The lead researcher with the study, Dave Dearborn, said a nuclear impact offered more flexibility than a non-nuclear approach, as the energy output can be adjusted specifically to the asteroid's size and location. <ref>{{Cite web |title=Nuclear impulse could deflect massive asteroid {{!}} Lawrence Livermore National Laboratory |url=https://www.llnl.gov/news/nuclear-impulse-could-deflect-massive-asteroid |access-date=2023-04-25 |website=www.llnl.gov |language=en}}</ref>

====Comet deflection possibility====
[[File:Comet-Hale-Bopp-29-03-1997 hires adj.jpg|thumb|right|"Who knows whether, when a comet shall approach this globe to destroy it&nbsp;... men will not tear rocks from their foundations by means of steam, and hurl mountains, as the giants are said to have done, against the flaming mass?"<br />— Lord Byron<ref>As quoted in ''Conversations of Lord Byron with Thomas Medwin'' (1832).</ref>]]

Following the 1994 Shoemaker-Levy 9 comet impacts with Jupiter, [[Biography:Edward Teller|Edward Teller]] proposed, to a collective of U.S. and Russian ex-[[History:Cold War|Cold War]] weapons designers in a 1995 planetary defense workshop meeting at [[Organization:Lawrence Livermore National Laboratory|Lawrence Livermore National Laboratory]] (LLNL), that they collaborate to design a [[Engineering:Nuclear weapon design#Arbitrarily large multi-staged devices|one-gigaton nuclear explosive device]], which would be equivalent to the kinetic energy of a {{convert|1|km|mi|adj=mid|-diameter|spell=in|sp=us}} asteroid.<ref name="e-reports-ext.llnl.gov">[https://web.archive.org/web/20150909023233/https://e-reports-ext.llnl.gov/pdf/232015.pdf Planetary defense workshop LLNL 1995]</ref><ref name="Jason Mick">{{cite web|url=http://www.dailytech.com/Russia+US+Eye+Teamup+to+Build+Massive+Nuke+to+Save+Planet+from+an+Asteroid/article33569.htm#sthash.rQvVzS6m.dpuf|title=The mother of all bombs would sit in wait in an orbitary platform|date=October 17, 2013|author=Jason Mick|access-date=October 6, 2014|archive-url=https://web.archive.org/web/20141009190305/http://www.dailytech.com/Russia+US+Eye+Teamup+to+Build+Massive+Nuke+to+Save+Planet+from+an+Asteroid/article33569.htm#sthash.rQvVzS6m.dpuf|archive-date=October 9, 2014|url-status=dead|df=mdy-all}}</ref><ref name="publicintegrity.org">{{Cite web|url=http://publicintegrity.org/national-security/a-new-use-for-nuclear-weapons-hunting-rogue-asteroids/|archiveurl=https://web.archive.org/web/20160320055111/http://www.publicintegrity.org/2013/10/16/13547/new-use-nuclear-weapons-hunting-rogue-asteroids|url-status=dead|title=A new use for nuclear weapons: hunting rogue asteroids|first=Douglas|last=Birch|date=October 16, 2013|archivedate=March 20, 2016|website=Center for Public Integrity}}</ref> The theoretical one-gigaton device would weigh about 25–30 tons, light enough to be lifted on the Energia rocket. It could be used to instantaneously vaporize a {{convert|1|km|adj=on|spell=in||}} asteroid, divert the paths of [[Unsolved:Global catastrophic risk|ELE-class asteroids]] (greater than {{convert|10|km|disp=or|||}} in diameter) within short notice of a few months. With one year of notice, and at an interception location no closer than [[Astronomy:Jupiter|Jupiter]], it could also deal with the even rarer short period comets that can come out of the [[Astronomy:Kuiper belt|Kuiper belt]] and transit past Earth orbit within two years.{{clarify|is it 1 year or 2?|date=May 2019}} For comets of this class, with a maximum estimated diameter of {{convert|100|km|sp=us|||}}, [[Astronomy:2060 Chiron|Chiron]] served as the hypothetical threat.<ref name="e-reports-ext.llnl.gov"/><ref name="Jason Mick"/><ref name="publicintegrity.org"/>

In 2013, the related National Laboratories of the US and Russia signed a deal that includes an intent to cooperate on defense from asteroids.<ref>{{Cite web|url=https://www.energy.gov/articles/united-states-russia-sign-agreement-further-research-and-development-collaboration-nuclear|archiveurl=https://web.archive.org/web/20160304125747/http://energy.gov/articles/united-states-russia-sign-agreement-further-research-and-development-collaboration-nuclear|url-status=dead|title=United States, Russia Sign Agreement to Further Research and Development Collaboration in Nuclear Energy and Security|archivedate=March 4, 2016|website=Energy.gov}}</ref> The deal was meant to complement New START, but Russia suspended its participation in the treaty in 2023. <ref>{{Cite news |last=Chappell |first=Bill |date=February 22, 2023 |title=What happens now after Russia suspends the last nuclear arms treaty with the U.S.? |work=NPR |url=https://www.npr.org/2023/02/22/1158529106/nuclear-treaty-new-start-putin |access-date=April 24, 2023}}</ref> As of April 2023, there has not been an official update from the White House or Moscow on how Russia's suspended participation will affect adjacent treaties.

====Present capability====
As of late 2022, the most likely and most effective method for asteroid deflection does not involve nuclear technology. Instead, it involves a kinetic impactor designed to redirect the asteroid, which showed promise in the NASA DART Mission. <ref>{{Cite web |title=DART |url=https://dart.jhuapl.edu/Mission/index.php |access-date=2023-04-25 |website=dart.jhuapl.edu |language=en}}</ref> For nuclear technology, simulations have been run analyzing the possibility of using neutron energy put off by a nuclear device to redirect an asteroid. These simulations showed promise, with one study finding that increasing the neutron energy output had a notable effect on the angle of the asteroid's travel. <ref name="auto"/> However, there has not been a practical test studying the possibility as of April 2023.

=== Kinetic impact ===
[[File:HRIV Impact.gif|thumb|The 2005 ''Deep Impact'' collision with the {{convert|8|by|5|km|0|spell=in|adj=on|sp=us}} comet Tempel 1. The impact flash and resulting ejecta are clearly visible. The impactor delivered 19 gigajoules (the equivalent of 4.8 [[Ton|ton]]s of TNT) upon impact.<ref>{{Cite web|url=https://www.nasa.gov/mission_pages/deepimpact/spacecraft/impactor.html|archiveurl=https://web.archive.org/web/20160623220100/http://www.nasa.gov/mission_pages/deepimpact/spacecraft/impactor.html|url-status=dead|title=NASA - Deep Impact's Impactor|archivedate=June 23, 2016|website=www.nasa.gov}}</ref> Impact created a crater estimated to be about 150 meters in diameter.<ref>{{cite web |title=In Depth - Deep Impact (EPOXI) |url=https://solarsystem.nasa.gov/missions/deep-impact-epoxi/in-depth/ |website=NASA Solar System Exploration |access-date=11 October 2022}}</ref> The comet "returned to preimpact conditions only 6 days after the event".<ref>{{cite journal | doi = 10.1086/499301 | bibcode=2006AJ....131.1130S | volume=131 | issue=2 | title=Photometry and Imaging Results for Comet 9P/Tempel 1 and Deep Impact: Gas Production Rates, Postimpact Light Curves, and Ejecta Plume Morphology | year=2006 | journal=The Astronomical Journal | pages=1130–1137 | last1 = Schleicher | first1 = David G. | last2 = Barnes | first2 = Kate L. | last3 = Baugh | first3 = Nicole F.| s2cid=123344560 | doi-access= }}</ref>]]

The impact of a massive object, such as a spacecraft or even another near-Earth object, is another possible solution to a pending NEO impact. An object with a high mass close to the Earth could be sent out into a collision course with the asteroid, knocking it off course.

When the asteroid is still far from the Earth, a means of deflecting the asteroid is to directly alter its [[Physics:Momentum|momentum]] by colliding a spacecraft with the asteroid.
[[File:Dart impact replay.webm|thumb|upright=1.2|Compiled timelapse of DART's final 5.5 minutes until impact]]
A [[Organization:NASA|NASA]] analysis of deflection alternatives, conducted in 2007, stated:

{{blockquote|Non-nuclear kinetic impactors are the [[Engineering:Deep Impact (spacecraft)|most mature approach]] and could be used in some deflection/mitigation scenarios, especially for NEOs that consist of a single small, solid body.<ref name="nasa" />}}

This deviation method, which has been implemented by [[Astronomy:Double Asteroid Redirection Test|DART]] and, for a completely different purpose (analysis of the structure and composition of a comet), by NASA's [[Engineering:Deep Impact (spacecraft)|Deep Impact]] space probe, involves launching a spacecraft against the [[Astronomy:Near-Earth object|near Earth object]]. The speed of the asteroid is modified due to the law of conservation of momentum:

{{Center|1=M{{Subscript|1}} x V{{Subscript|1}} + M{{Subscript|2}} x V{{Subscript|2}} = (M{{Subscript|1}} + M{{Subscript|2}}) x V{{Subscript|3}}}}

with V{{Subscript|1}} velocity of the spacecraft, V{{Subscript|2}} velocity of the celestial body before impact, and V{{Subscript|3}} the velocity after impact. M{{Subscript|1}} and M{{Subscript|2}} respective mass of the spacecraft and of the celestial body. Velocities are [[Vector space|vectors]] here.

The European Union's NEOShield-2 Mission<ref>{{cite web|url=http://www.neoshield.net/mitigation-measures-kinetic-impactor-gravity/kinetic-impactor-asteroid-deflection-spacecraft/|title=Kinetic impactor -|date=2016-08-29|access-date=2016-11-17|archive-date=2022-03-19|archive-url=https://web.archive.org/web/20220319000029/http://www.neoshield.net/mitigation-measures-kinetic-impactor-gravity/kinetic-impactor-asteroid-deflection-spacecraft/|url-status=dead}}</ref> is also primarily studying the Kinetic Impactor mitigation method. The principle of the kinetic impactor mitigation method is that the NEO or Asteroid is deflected following an impact from an impactor spacecraft. The principle of momentum transfer is used, as the impactor crashes into the NEO at a very high velocity of {{cvt|10|km/s|km/h mph|||}} or more. The momentum of the impactor is transferred to the NEO, causing a change in velocity and therefore making it deviate from its course slightly.<ref>{{cite web|url = http://www.neoshield.net/mitigation-measures-kinetic-impactor-gravity/kinetic-impactor-asteroid-deflection-spacecraft/|title = NEOShield Project|publisher = European Union Consortium|date = 17 November 2016|access-date = 17 November 2016|archive-date = 19 March 2022|archive-url = https://web.archive.org/web/20220319000029/http://www.neoshield.net/mitigation-measures-kinetic-impactor-gravity/kinetic-impactor-asteroid-deflection-spacecraft/|url-status = dead}}</ref>

As of mid-2021, the modified [[Astronomy:AIDA (mission)|AIDA mission]] has been approved. The NASA [[Astronomy:Double Asteroid Redirection Test|Double Asteroid Redirection Test]] (''DART'') kinetic impactor spacecraft was launched in November 2021. The goal was to impact [[Astronomy:Dimorphos|Dimorphos]] (nicknamed ''Didymoon''), the {{convert|180|m|adj=on|sp=us||}} [[Astronomy:Minor-planet moon|minor-planet moon]] of near-Earth asteroid [[Astronomy:65803 Didymos|65803 Didymos]]. The impact occurred in September 2022 when Didymos is relatively close to Earth, allowing Earth-based telescopes and planetary radar to observe the event. The result of the impact will be to change the orbital velocity and hence orbital period of Dimorphos, by a large enough amount that it can be measured from Earth. This will show for the first time that it is possible to change the orbit of a small {{convert|200|m|adj=on|sp=us||}} asteroid, around the size most likely to require active mitigation in the future. The launch and use of the [[Astronomy:Double Asteroid Redirection Test|Double Asteroid Redirection Test]] system in March 2023 showed the world that asteroids could be safely redirected without the use of nuclear means. The success of this mission proved that kinetic methods of deflection are by far the best methods of asteroid deterrence.The second part of the [[Astronomy:AIDA (mission)|AIDA]] mission{{ndash}}the ESA HERA spacecraft{{ndash}}has been approved by ESA member states in October 2019. It would reach the Didymos system in 2026 and measure both the mass of Dimorphos and the precise effect of the impact on that body, allowing much better extrapolation of the [[Astronomy:AIDA (mission)|AIDA]] mission to other targets.<ref>{{Cite web |title=NASA - NSSDCA - Spacecraft - Details |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=HERA |access-date=2022-10-12 |website=nssdc.gsfc.nasa.gov}}</ref>

=== Asteroid gravity tractor ===
{{main|Engineering:Gravity tractor}}
{{wide image|NASA-Animation-ARM-opt-800-20150325.gif|800px|align-cap=center|The [[Astronomy:Asteroid Redirect Mission|Asteroid Redirect Mission]] vehicle was conceived to demonstrate the "[[Engineering:Gravity tractor|gravity tractor]]" planetary defense technique on a hazardous-size asteroid. The gravity-tractor method leverages the mass of the spacecraft to impart a force on the asteroid, slowly altering the asteroid's trajectory.}}

Another alternative to explosive deflection is to move the asteroid slowly over time. A small but constant amount of thrust accumulates to deviate an object sufficiently from its course. Edward T. Lu and Stanley G. Love have proposed using a massive uncrewed spacecraft hovering over an asteroid to gravitationally pull the asteroid into a non-threatening orbit. Though both objects are gravitationally pulled towards each other, the spacecraft can counter the force towards the asteroid by, for example, an [[Engineering:Nuclear electric rocket|ion thruster]], so the net effect would be that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. While slow, this method has the advantage of working irrespective of the asteroid's composition or spin rate; [[Astronomy:Rubble pile|rubble pile]] asteroids would be difficult to deflect by means of nuclear detonations, while a pushing device would be difficult or inefficient to mount on a fast-rotating asteroid. A gravity tractor would likely have to spend several years beside the asteroid to be effective.

A [[Organization:NASA|NASA]] analysis of deflection alternatives, conducted in 2007, stated:

{{blockquote|"Slow push" mitigation techniques are the most expensive, have the lowest level of technical readiness, and their ability to both travel to and divert a threatening NEO would be limited unless mission durations of many years to decades are possible.<ref name="nasa" />}}

=== Ion beam shepherd ===
{{main|Engineering:Ion-beam shepherd}}

Another "contactless" asteroid deflection technique has been proposed by C. Bombardelli and J. Peláez from the Technical University of Madrid. The method involves the use of a low-divergence ion thruster pointed at the asteroid from a nearby hovering spacecraft. The momentum transmitted by the ions reaching the asteroid surface produces a slow but continuous force that can deflect the asteroid in a similar way as the gravity tractor, but with a lighter spacecraft.

=== Focused solar energy ===
[[Biography:H. Jay Melosh|H. J. Melosh]] with I. V. Nemchinov proposed deflecting an asteroid or comet by focusing [[Physics:Solar energy|solar energy]] onto its surface to create thrust from the resulting vaporization of material.<ref name=":0">{{Cite journal|last1=Melosh|first1=H. J.|last2=Nemchinov|first2=I. V.|date=1993|title=Solar asteroid diversion|journal=Nature|volume=366|issue=6450|pages=21–22|doi=10.1038/366021a0|bibcode=1993Natur.366...21M|s2cid=4367291|issn=0028-0836}}</ref> This method would first require the construction of a space station with a system of large collecting, concave [[Physics:Mirror|mirror]]s similar to those used in [[Engineering:Solar furnace|solar furnace]]s.

Orbit mitigation with highly concentrated sunlight is scalable to achieving the predetermined deflection within a year even for a global-threatening body without prolonged warning time.<ref name=":0" /><ref name=":1">{{Cite journal|last=Vasylyev|first=V. P.|date=2012-12-22|title=Deflection of Hazardous Near-Earth Objects by High Concentrated Sunlight and Adequate Design of Optical Collector|journal=Earth, Moon, and Planets|volume=110|issue=1–2|pages=67–79|doi=10.1007/s11038-012-9410-2|s2cid=120563921|issn=0167-9295}}</ref>

Such a hastened strategy may become topical in the case of late detection of a potential hazard, and also, if required, in providing the possibility for some additional action. Conventional concave reflectors are practically inapplicable to the high-concentrating geometry in the case of a giant shadowing space target, which is located in front of the mirrored surface. This is primarily because of the dramatic spread of the mirrors' focal points on the target due to the [[Physics:Optical aberration|optical aberration]] when the optical axis is not aligned with the Sun. On the other hand, the positioning of any collector at a distance to the target much larger than its size does not yield the required concentration level (and therefore temperature) due to the natural divergence of the sunrays. Such principal restrictions are inevitably at any location regarding the asteroid of one or many unshaded forward-reflecting collectors. Also, in the case of secondary mirrors use, similar to the ones found in [[Astronomy:Cassegrain reflector|Cassegrain telescopes]], would be prone to heat damage by partially concentrated sunlight from primary mirror.

In order to remove the above restrictions, V.P. Vasylyev proposed to apply an alternative design of a mirrored collector – the ring-array concentrator.<ref name=":1" /> This type of collector has an underside lens-like position of its focal area that avoids shadowing of the collector by the target and minimizes the risk of its coating by ejected debris. Provided the sunlight concentration ~ 5 × 10<sup>3</sup> times, a surface [[Physics:Irradiance|irradiance]] of around 4-5 MW/m<sup>2</sup> leads to a thrusting effect ~ 10<sup>3</sup> N. Intensive [[Physics:Ablation|ablation]] of the rotating asteroid surface under the focal spot will lead to the appearance of a deep "canyon", which can contribute to the formation of the escaping gas flow into a jet-like one. This may be sufficient to deflect a 0.5-km asteroid within several months and no addition warning period, only using ring-array collector size ~ 0.5 of asteroid diameter. For such a prompt deflection of the larger NEOs, 1.3-2.2&nbsp;km, the required collector sizes are comparable to the target diameter. In the case of a longer warning time, the required size of the collector may be significantly decreased.

[[File:Ring array asteroid.gif|center|frame|Artist's impression of asteroid deflection using an innovative ring-array solar collector.]]

=== Mass driver ===
A [[Physics:Mass driver|mass driver]] is an (automated) system on the asteroid to eject material into space, thus giving the object a slow steady push and decreasing its mass. A mass driver is designed to work as a very low [[Physics:Specific impulse|specific impulse]] system, which in general uses a lot of propellant, but very little power. This essentially uses the asteroid against itself in order to divert a collision. 

Modular Asteroid Deflection Mission Ejector Node, (MADMEN), is the idea of landing small unmanned vehicles such as space rovers to break up small portions of the asteroid. Using drills to break up small rocks and boulders from the surface, debris would eject from the surface very fast. Because there are no forces acting on the asteroid these rocks will push the asteroid off course at a very slow rate. This process takes time but could be very effective if implemented correctly. <ref>Olds, John R, et al. Multiple Mass Drivers as an Option for Asteroid Deflection Missions, SpaceWorks Engineering, Inc. (SEI), Atlanta, Georgia, 30338, http://www.sei.aero/archive/AIAA-2007_S3-7.pdf. </ref> The idea is that when using local material as propellant, the amount of propellant is not as important as the amount of power, which is likely to be limited.

=== Conventional rocket engine ===
Attaching any [[Astronomy:Spacecraft propulsion|spacecraft propulsion]] device would have a similar effect of giving a push, possibly forcing the asteroid onto a trajectory that takes it away from Earth. An in-space rocket engine that is capable of imparting an impulse of 10<sup>6</sup> N·s (E.g. adding 1&nbsp;km/s to a 1000&nbsp;kg vehicle), will have a relatively small effect on a relatively small asteroid that has a mass of roughly a million times more. Chapman, Durda, and Gold's white paper<ref>Chapman, Clark R. and Daniel D. Durda. [http://www.internationalspace.com/pdf/NEOwp_Chapman-Durda-Gold.pdf The Comet/Asteroid Impact Hazard: A Systems Approach] {{webarchive |url=https://web.archive.org/web/20160304002442/http://www.internationalspace.com/pdf/NEOwp_Chapman-Durda-Gold.pdf |date=March 4, 2016 }}, Boulder, CO: Office of Space Studies, Southwest Research Institute, Space Engineering and Technology Branch, Johns Hopkins University Applied Physics Laboratory.</ref> calculates deflections using existing chemical rockets delivered to the asteroid.

Such direct force rocket engines are typically proposed to use highly-efficient [[Engineering:Electrically powered spacecraft propulsion|electrically powered spacecraft propulsion]], such as [[Engineering:Ion thruster|ion thruster]]s or VASIMR.

=== Asteroid laser ablation ===
{{main|Astronomy:Asteroid laser ablation}}

Similar to the effects of a nuclear device, it is thought possible to focus sufficient laser energy on the surface of an asteroid to cause flash vaporization / ablation to create either in impulse or to ablate away the asteroid mass. This concept, called [[Astronomy:Asteroid laser ablation|asteroid laser ablation]] was articulated in the 1995 SpaceCast 2020<ref>{{cite web|url=http://csat.au.af.mil/2020/index.htm|title=Welcome to SpaceCast 2020|work=Center for Strategy and Technology|publisher=Air University|url-status=dead|archive-url=https://web.archive.org/web/20090302104514/http://csat.au.af.mil/2020/index.htm|archive-date=2009-03-02}}</ref> white paper "Preparing for Planetary Defense",<ref>{{cite web |url=http://www.nss.org:8080/resources/library/planetarydefense/1994-DetectionAndInterceptionOfAsteroidsOnCollisionCourseWithEarth.pdf |title=Preparing for Planetary Defense: Detection and Interception of Asteroids on Collision Course with Earth |access-date=2016-05-22 |archive-date=2016-06-25 |archive-url=https://web.archive.org/web/20160625011024/http://www.nss.org:8080/resources/library/planetarydefense/1994-DetectionAndInterceptionOfAsteroidsOnCollisionCourseWithEarth.pdf |url-status=dead }}<br />{{cite report|title=SpaceCast 2020|chapter=Preparing for Planetary Defense|chapter-url=http://csat.au.af.mil/2020/papers/app-r.pdf|publisher=Air University|url-status=dead|archive-url=https://web.archive.org/web/20101026185205/http://csat.au.af.mil/2020/papers/app-r.pdf|archive-date=2010-10-26}}</ref> and the 1996 Air Force 2025<ref>{{cite web|url=http://csat.au.af.mil/2025/index.htm|title=Welcome to Air Force 2025|work=Center for Strategy and Technology|publisher=Air University|url-status=dead|archive-url=https://web.archive.org/web/20081220115949/http://csat.au.af.mil/2025/index.htm|archive-date=2008-12-20}}</ref> white paper "Planetary Defense: Catastrophic Health Insurance for Planet Earth".<ref>http://www.nss.org:8080/resources/library/planetarydefense/1996-PlanetaryDefense-CatstrophicHealthInsuranceForPlanetEarth-Urias.pdf {{Webarchive|url=https://web.archive.org/web/20160624223239/http://www.nss.org:8080/resources/library/planetarydefense/1996-PlanetaryDefense-CatstrophicHealthInsuranceForPlanetEarth-Urias.pdf |date=2016-06-24 }}<br />{{cite report|title=Air Force 2025|chapter=Planetary Defense: Catastrophic Health Insurance for Planet Earth|author=John M. Urias |author2=Iole M. DeAngelis |author3=Donald A. Ahern |author4=Jack S. Caszatt |author5=George W. Fenimore III |author6=Michael J. Wadzinski |date=October 1996|chapter-url=http://csat.au.af.mil/2025/volume3/vol3ch16.pdf|publisher=Air University|url-status=dead|archive-url=https://web.archive.org/web/20070717145438/http://csat.au.af.mil/2025/volume3/vol3ch16.pdf|archive-date=2007-07-17}}</ref> Early publications include C. R. Phipps "ORION" concept from 1996, Colonel Jonathan W. Campbell's 2000 monograph "Using Lasers in Space: Laser Orbital Debris Removal and Asteroid Deflection",<ref>{{Cite web |url=http://www.nss.org:8080/resources/library/planetarydefense/2000-LaserOrbitalDebrisRemovalAndAsteroidDeflection-Campbell.pdf |title=Archived copy |access-date=2016-05-22 |archive-url=https://web.archive.org/web/20161005024315/http://www.nss.org:8080/resources/library/planetarydefense/2000-LaserOrbitalDebrisRemovalAndAsteroidDeflection-Campbell.pdf |archive-date=2016-10-05 |url-status=dead }}</ref> and NASA's 2005 concept Comet Asteroid Protection System (CAPS).<ref>{{Cite web |url=http://www.nss.org:8080/resources/library/planetarydefense/2005-CometAsteroidProtectionSystem(CAPS)-NASA.pdf |title=Archived copy |access-date=2016-05-22 |archive-url=https://web.archive.org/web/20160625020733/http://www.nss.org:8080/resources/library/planetarydefense/2005-CometAsteroidProtectionSystem(CAPS)-NASA.pdf |archive-date=2016-06-25 |url-status=dead }}</ref> Typically such systems require a significant amount of power, such as would be available from a [[Physics:Space-based solar power|Space-Based Solar Power Satellite]].

Another proposal is the Phillip Lubin's DE-STAR<ref>{{Cite web|url=https://www.deepspace.ucsb.edu/projects/directed-energy-planetary-defense|title=DE-STAR}}</ref> proposal:

* The DE-STAR project,<ref>{{Cite web|url=https://spie.org/news/lubin-video|archiveurl=https://web.archive.org/web/20150609084807/http://spie.org/x104781.xml|url-status=dead|title=Philip Lubin: A space-based array for planetary defense|archivedate=June 9, 2015|website=spie.org}}</ref> proposed by researchers at the University of California, Santa Barbara, is a concept modular solar powered 1&nbsp;µm, near infrared wavelength, laser array. The design calls for the array to eventually be approximately 1&nbsp;km squared in size, with the modular design meaning that it could be launched in increments and assembled in space. In its early stages as a small array it could deal with smaller targets, assist [[Physics:Solar sail|solar sail]] probes and would also be useful in cleaning up [[Astronomy:Space debris|space debris]].

=== Other proposals ===
[[Image:Solarsail msfc.jpg|thumb|NASA study of a solar sail. The sail would be {{convert|0.5|km||||}} wide.]]

* Wrapping the asteroid in a sheet of reflective plastic such as aluminized PET film as a [[Physics:Solar sail|solar sail]]
* "Painting" or dusting the object with [[Chemistry:Titanium dioxide|titanium dioxide]] (white) to alter its trajectory via increased reflected radiation pressure or with [[Earth:Soot|soot]] (black) to alter its trajectory via the [[Yarkovsky effect]].
* [[Astronomy:Planetary science|Planetary scientist]] Eugene Shoemaker in 1996 proposed<ref>--in a lecture to the [[Organization:Arizona Geological Society|Arizona Geological Society]] in 12–96.</ref> deflecting a potential impactor by releasing a cloud of steam in the path of the object, hopefully gently slowing it. Nick Szabo in 1990 sketched<ref>[https://www.cs.cmu.edu/afs/cs.cmu.edu/usr/mnr/st/std070 Is an asteroid capture possible/feasible?; Asteroid movement/retrieval; Asteroid relocation/mining; etceras...] {{webarchive |url=https://web.archive.org/web/20161106170026/https://www.cs.cmu.edu/afs/cs.cmu.edu/usr/mnr/st/std070 |date=November 6, 2016 }}, Space-tech Digest #70 [bulletin board], [[Organization:Carnegie Mellon University|Carnegie Mellon University]], July 19–25, 1990.</ref> a similar idea, "cometary aerobraking", the targeting of a comet or ice construct at an asteroid, then vaporizing the ice with nuclear explosives to form a temporary atmosphere in the path of the asteroid.
* Coherent digger array<ref>{{Cite arXiv |eprint = astro-ph/9803269|last1 = Lu|first1 = Edward T.|title = Breaking and Splitting asteroids by nuclear explosions to propel and deflect their trajectories|last2 = Love|first2 = Stanley G.|year = 1998}}</ref><ref>{{Cite journal |arxiv = 0705.1805|last1 = Lu|first1 = Edward T.|title = Asteroid Deflection: How, where and when?|journal = Chinese Journal of Astronomy and Astrophysics Supplement|volume = 8|pages = 399|last2 = Love|first2 = Stanley G.|year = 2007|bibcode = 2008ChJAS...8..399F}}</ref> multiple 1-ton flat tractors able to dig and expel asteroid soil mass as a coherent fountain array, coordinated fountain activity may propel and deflect over years.
* Attaching a tether and ballast mass to the asteroid to alter its trajectory by changing its center of mass.<ref>{{cite web |title=Near-Earth Object Threat Mitigation Using a Tethered Ballast Mass|author=David French|publisher=J. Aerosp. Engrg.|date=October 2009 | url=http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JAEEEZ000022000004000460000001&idtype=cvips&gifs=yes&ref=no}}</ref>
* [[Physics:Explosively pumped flux compression generator|Magnetic flux compression]] to magnetically brake and or capture objects that contain a high percentage of [[Chemistry:Meteoric iron|meteoric iron]] by deploying a wide coil of wire in its orbital path and when it passes through, [[Physics:Inductance|Inductance]] creates an [[Physics:Electromagnet|electromagnet]] solenoid to be generated.<ref>{{cite web |url=http://www.obtronics.net/htc/technogy/elec/elmag_04.htm |title=How to Colonize an Asteroid Solenoids |url-status=dead |archive-url=https://web.archive.org/web/20060103083009/http://www.obtronics.net/htc/technogy/elec/elmag_04.htm |archive-date=2006-01-03 }}</ref><ref>{{cite web |url=http://www.nss.org/adastra/volume18/durda.html |title=National Space Society, From Ad Astra, Volume 18 Number 2, Summer 2006 |access-date=2013-11-25 |archive-url=https://web.archive.org/web/20170721141759/http://www.nss.org/adastra/volume18/durda.html |archive-date=2017-07-21 |url-status=dead }}</ref>

== Deflection technology concerns ==
[[Biography:Carl Sagan|Carl Sagan]], in his book ''Pale Blue Dot'', expressed concern about deflection technology, noting that any method capable of deflecting impactors ''away'' from Earth could also be abused to divert non-threatening bodies ''toward'' the planet. Considering the history of genocidal political leaders and the possibility of the bureaucratic obscuring of any such project's true goals to most of its scientific participants, he judged the Earth at greater risk from a man-made impact than a natural one. Sagan instead suggested that deflection technology be developed only in an actual emergency situation.

All low-energy delivery deflection technologies have inherent fine control and steering capability, making it possible to add just the right amount of energy to steer [[Astronomy:List of asteroid close approaches to Earth|an asteroid originally destined for a mere close approach]] toward a specific Earth target.

According to former NASA astronaut [[Biography:Rusty Schweickart|Rusty Schweickart]], the gravitational tractor method is controversial because, during the process of changing an asteroid's trajectory, the point on the Earth where it could most likely hit would be slowly shifted across different countries. Thus, the threat for the entire planet would be minimized at the cost of some specific states' security. In Schweickart's opinion, choosing the way the asteroid should be "dragged" would be a tough diplomatic decision.<ref name="wired">{{cite news|url=https://www.wired.com/wiredscience/2009/12/saving-earth-from-an-asteroid/ | title=Saving Earth From an Asteroid Will Take Diplomats, Not Heroes | last=Madrigal | first=Alexis | date=16 December 2009 | publisher=WIRED | access-date=17 December 2009}}</ref>

Analysis of the uncertainty involved in nuclear deflection shows that the ability to protect the planet does not imply the ability to target the planet. A nuclear explosion that changes an asteroid's velocity by 10 meters/second (plus or minus 20%) would be adequate to push it out of an Earth-impacting orbit. However, if the uncertainty of the velocity change was more than a few percent, there would be no chance of directing the asteroid to a particular target.

Additionally, there are legal concerns regarding the launch of nuclear technology into space. In 1992, the {{wipe|United Nations}} adopted a resolution that provides strict rules regarding sending nuclear technology to space, including preventing the contamination of space as well as protecting all citizens on Earth from potential fallout. <ref>{{Cite web |title=NPS Principles |url=https://www.unoosa.org/oosa/en/ourwork/spacelaw/principles/nps-principles.html |access-date=2023-04-25 |website=www.unoosa.org}}</ref> As of 2022, the UN is still considering the safety and legal issues of launching nuclear powered items into outer space, particularly given the expanding field of space travel as more private organizations take part in the modern space race. The UN Committee on Peaceful Uses of Outer Space recently emphasized the point of the previous resolution, saying it is the responsibility of the member states to ensure the safety of everyone regarding nuclear power in space. <ref>{{Cite web |title=Nuclear power in space is focus of IAEA and UN events : Nuclear Policies - World Nuclear News |url=https://www.world-nuclear-news.org/Articles/Nuclear-power-in-space-is-focus-of-IAEA-and-UN-eve |access-date=2023-04-25 |website=www.world-nuclear-news.org}}</ref>

== Planetary defense timeline ==
[[File:Space Laser Satellite Defense System Concept.jpg|thumb|right|upright=1.25|The 1984 [[Engineering:Strategic Defense Initiative|Strategic Defense Initiative]] concept of a generic space based Nuclear reactor pumped laser or a hydrogen fluoride laser satellite,<ref>{{Cite web|url=https://www.fas.org/spp/starwars/program/sbl.htm|title=Space Based Laser. FAS.}}</ref> firing on a target, causing a momentum change in the target object by laser ablation. With the proposed Space Station Freedom (ISS) in the background.]]

* In their 1964 book, ''Islands in Space'', Dandridge M. Cole and Donald W. Cox noted the dangers of planetoid impacts, both those occurring naturally and those that might be brought about with hostile intent. They argued for cataloging the minor planets and developing the technologies to land on, deflect, or even capture planetoids.<ref>{{cite book|title=Islands in Space: The Challenge of the Planetoids|author=Dandridge M. Cole|author2=Donald W. Cox|publisher=Chilton Books|date=1964|pages=7–8}}</ref>
* In 1967, students in the Aeronautics and Astronautics department at MIT did a design study, "Project Icarus", of a mission to prevent a hypothetical impact on Earth by asteroid 1566 Icarus.<ref name="Day" /> The design project was later published in a book by the MIT Press<ref name="Icarus" /> and received considerable publicity, for the first time bringing asteroid impact into the public eye.<ref name="Time1967" />
* In the 1980s NASA studied evidence of past strikes on planet Earth, and the risk of this happening at the current level of civilization. This led to a program that maps objects in the Solar System that both cross Earth's orbit and are large enough to cause serious damage if they hit.
* In the 1990s, US Congress held hearings to consider the risks and what needed to be done about them. This led to a US$3 million annual budget for programs like [[Astronomy:Spaceguard|Spaceguard]] and the [[Astronomy:Near-Earth object|near-Earth object]] program, as managed by [[Organization:NASA|NASA]] and USAF.
* In 2005 a number of astronauts published an open letter through the [[Organization:Association of Space Explorers|Association of Space Explorers]] calling for a united push to develop strategies to protect Earth from the risk of a cosmic collision.<ref>{{cite news|title=Astronauts push for strategies, spacecraft to prevent calamitous asteroid strike | url=http://www.post-gazette.com/pg/05332/613441.stm|access-date=2008-01-18 | work=Pittsburgh Post-Gazette | date=November 28, 2005}}</ref>
*  It is currently (as of late 2007) estimated that there are approximately 20,000 objects capable of crossing Earth's orbit and large enough (140 meters or larger) to warrant concern.<ref>{{cite web |url=http://democrats.science.house.gov/press/PRArticle.aspx?NewsID=2036 |title=Subcommittee Questions NASA's Plan for Detecting Hazardous Asteroids |url-status=dead |archive-url=https://web.archive.org/web/20110506052124/http://democrats.science.house.gov/press/PRArticle.aspx?NewsID=2036 |archive-date=2011-05-06 }}</ref> On the average, one of these will collide with Earth every 5,000 years, unless preventive measures are undertaken.<ref name="Yeomans">{{cite web | url=http://democrats.science.house.gov/media/File/Commdocs/hearings/2007/space/08nov/Yeomans_testimony.pdf | title=Testimony Before The House Committee On Science And Technology Subcommittee On Space And Aeronautics: Near-Earth Objects (NEOS){{Snd}} Status Of The Survey Program And Review Of Nasa's Report To Congress | author=Donald K. Yeomans | date=2007-11-08 | url-status=dead | archive-url=https://web.archive.org/web/20080131081137/http://democrats.science.house.gov/media/File/Commdocs/hearings/2007/space/08nov/Yeomans_testimony.pdf | archive-date=2008-01-31 }}</ref> It was anticipated that by year 2008, 90% of such objects that are 1&nbsp;km or more in diameter will have been identified and will be monitored. The further task of identifying and monitoring all such objects of 140m or greater was expected to be complete around 2020.<ref name="Yeomans" /> By April 2018, astronomers have spotted more than 8,000 near-Earth asteroids that are at least 460 feet (140 meters) wide and it is estimated about 17,000 such near-Earth asteroids remain undetected.<ref>{{cite web | url=https://www.space.com/40239-near-earth-asteroid-detection-space-telescope.html | title=About 17,000 Big Near-Earth Asteroids Remain Undetected: How NASA Could Spot Them | website=Space.com | date=10 April 2018 }}</ref> By 2019, the number of discovered near-Earth asteroids of all sizes totaled more than 19,000. An average of 30 new discoveries are added each week.<ref>{{cite web| url = https://www.nasa.gov/planetarydefense/faq| title = Planetary Defense Frequently Asked Questions {{!}} NASA| date = 29 December 2015| access-date = 26 November 2021| archive-date = 10 August 2018| archive-url = https://web.archive.org/web/20180810005201/https://www.nasa.gov/planetarydefense/faq/| url-status = dead}}</ref>
*  The [[Astronomy:Catalina Sky Survey|Catalina Sky Survey]]<ref>{{Cite web|url=https://catalina.lpl.arizona.edu/|archiveurl=https://web.archive.org/web/20161019211512/http://www.lpl.arizona.edu/css/|url-status=dead|title=Home &#124; Catalina Sky Survey|archivedate=October 19, 2016|website=catalina.lpl.arizona.edu}}</ref> (CSS) is one of [[Organization:NASA|NASA]]'s four funded surveys to carry out a 1998 U.S. Congress mandate to find and catalog by the end of 2008, at least 90 percent of all near-Earth objects (NEOs) larger than 1 kilometer across. CSS discovered over 1150 NEOs in years 2005 to 2007. In doing this survey they discovered on November 20, 2007, an asteroid, designated [[Astronomy:2007 WD5|{{mp|2007 WD|5}}]], which initially was estimated to have a chance of hitting [[Astronomy:Mars|Mars]] on January 30, 2008, but further observations during the following weeks allowed NASA to rule out an impact.<ref>{{cite web |title=Catalina Sky Survey Discovers Space Rock That Could Hit Mars|date=21 December 2007 |first=Lori |last=Stiles |url=http://uanews.org/node/17415|access-date=2007-12-22 |url-status=usurped |archiveurl=https://web.archive.org/web/20080510115105/http://uanews.org/node/17415 |archivedate=2008-05-10}}</ref> NASA estimated a near miss by {{convert|26,000|km}}.<ref>{{cite web | title=Recently Discovered Asteroid Could Hit Mars in January | url=http://neo.jpl.nasa.gov/news/news151.html | archive-url=https://web.archive.org/web/20071224120701/http://neo.jpl.nasa.gov/news/news151.html | url-status=dead | archive-date=2007-12-24 | access-date=2007-12-22}}</ref>
* In January 2012, after a near pass-by of object [[Astronomy:2012 BX34|2012 BX34]], a paper entitled "A Global Approach to Near-Earth Object Impact Threat Mitigation" was released by researchers from Russia, Germany, the United States, France, Britain, and Spain, which discusses the "NEOShield" project.<ref>Leonard David. [http://www.space.com/14370-asteroid-shield-earth-threat-protection-meeting.html Asteroid Threat to Earth Sparks Global 'NEOShield' Project] {{webarchive |url=https://web.archive.org/web/20160309203347/http://www.space.com/14370-asteroid-shield-earth-threat-protection-meeting.html |date=March 9, 2016 }}, SPACE.com, 26 January 2012.</ref><!-- http://www.thestatecolumn.com/science/neoshield-project-to-combat-asteroid-threat-to-earth/#ixzz1ko16PL2  dead link -->
* In November 2021, NASA launched a program with a different goal in terms of planetary defense. Many common methods previously in place were meant to completely destroy the asteroid. However, NASA and many others believed this method was far too unreliable so they funded the [[Astronomy:Double Asteroid Redirection Test|Double Asteroid Redirection Test]] or (DART) mission. This mission launched a small unmanned spacecraft to crash into the asteroid to break it up, or to deflect the rock away from Earth.  <ref>Atkinson, Nancy. We Already Have the Technology to Save Earth from a "Don't Look up" Asteroid, SciTechDaily, 10 July 2022, https://scitechdaily.com/we-already-have-the-technology-to-save-earth-from-a-dont-look-up-asteroid/. </ref>
* In January 2022, The NASA-funded [[Astronomy:Asteroid Terrestrial-impact Last Alert System|Asteroid Terrestrial-impact Last Alert System]] (ATLAS)—a state-of-the-art asteroid detection system operated by the University of Hawaiʻi (UH) Institute for Astronomy (IfA) for the agency's Planetary Defense Coordination Office (PDCO)—has reached a new milestone by becoming the first survey capable of searching the entire dark sky every 24 hours for near-Earth objects (NEOs) that could pose a future impact hazard to Earth. Now comprising four telescopes, ATLAS has expanded its reach to the southern hemisphere from the two existing northern-hemisphere telescopes on Haleakalā and Maunaloa in Hawai’i to include two additional observatories in South Africa and Chile. <ref>{{Cite web |last=Talbert |first=Tricia |date=2022-01-31 |title=NASA Asteroid Tracking System Now Capable of Full Sky Search |url=http://www.nasa.gov/feature/nasa-asteroid-tracking-system-now-capable-of-full-sky-search |access-date=2022-08-24 |website=NASA}}</ref>
* As of March 1, 2023, we have proof from NASA that DART does indeed work. It was successful in both targeting and making contact with an asteroid moving at high speeds  and, was successful in redirecting its course. This data showed that we can successfully move an asteroid with a diameter up to half a mile. <ref>Furfaro, Emily. NASA's DART Data Validates Kinetic Impact as Planetary Defense Method, NASA, 28 Feb. 2023, https://www.nasa.gov/feature/nasa-s-dart-data-validates-kinetic-impact-as-planetary-defense-method. </ref>

== See also ==
{{colbegin|colwidth=18em}}
* [[Astronomy:Asteroid impact prediction|Asteroid impact prediction]]
* [[Astronomy:Asteroid Redirect Mission|Asteroid Redirect Mission]]
* [[Astronomy:Asteroid Day|Asteroid Day]]
* Asteroids in fiction
* [[Organization:B612 Foundation|B612 Foundation]]
* [[Astronomy:Colonization of the Moon|Colonization of the Moon]]
* Framework Programmes for Research and Technological Development
* [[Unsolved:Global catastrophic risk|Global catastrophic risk]]
* [[Engineering:Gravity tractor|Gravity tractor]]
* [[Astronomy:Lost minor planet|Lost minor planet]]
* [[Astronomy:Near-Earth Asteroid Scout|Near-Earth Asteroid Scout]]
* [[Astronomy:Near-Earth object|Near-Earth object]]
* [[Astronomy:Potentially hazardous object|Potentially hazardous object]]
* United States Space Force<ref>{{Cite news |url=https://www.defense.gov/Explore/News/Article/Article/2205526/spacecom-space-force-officials-discuss-planetary-defense-astronaut-launch/ |title=Spacecom, Space Force Officials Discuss Planetary Defense |date=June 2, 2020 |first=C. Todd |last=Lopez |work=DOD News |publisher=U.S. Dept. of Defense}}</ref>
{{colend}}

== Sources ==
{{NASA |article=NASA, SpaceX Launch DART: First Planetary Defense Test Mission |url=https://blogs.nasa.gov/dart/2021/11/24/nasa-spacex-launch-dart-first-planetary-defense-test-mission/ |author=Linda Herridge |accessdate=24 August 2022}}

== References ==
=== Citations ===
{{Reflist|30em}}

=== General bibliography ===
{{refbegin}}
* [[Biography:Luis Walter Alvarez|Luis Alvarez]] et al. 1980 paper in ''Science'' magazine on the great mass extinction 65 million years ago that led to the proliferation of mammal species such as the rise of the human race, thanks to [[Astronomy:Impact event|asteroid-impact]], a controversial theory in its day, now generally accepted.
** {{cite journal | last1=Alvarez | first1=L. W. |  last2=Alvarez | first2=W. | last3=Asaro | first3=F. | last4=Michel | first4=H. V. | year=1980 | title=Extraterrestrial Cause for the Cretaceous-Tertiary Extinction: Experiment and Theory | url=http://www.es.ucsc.edu/~pkoch/EART_206/09-0305/Alvarez%20et%2080%20Science%20208-1095.pdf | journal=Science | volume=208 | issue=4448 | pages=1095–1108 | bibcode=1980Sci...208.1095A | doi=10.1126/science.208.4448.1095 | jstor=1683699 | pmid=17783054 | s2cid=16017767 | access-date=2021-07-07 | archive-date=2015-09-24 | archive-url=https://web.archive.org/web/20150924001940/http://www.es.ucsc.edu/~pkoch/EART_206/09-0305/Alvarez%20et%2080%20Science%20208-1095.pdf | url-status=dead }}
* Clark R. Chapman, Daniel D. Durda & Robert E. Gold (February 24, 2001) ''Impact Hazard, a Systems Approach'', white paper on public policy issues associated with the impact hazard, at [http://www.boulder.swri.edu/clark/neowp.html boulder.swri.edu]
* Donald W. Cox, and James H. Chestek. 1996. ''Doomsday Asteroid: Can We Survive?'' New York: Prometheus Books. {{ISBN|1-57392-066-5}}. (Note that despite its sensationalist title, this is a good treatment of the subject and includes a nice discussion of the collateral space development possibilities.)
* Izzo, D., Bourdoux, A., Walker, R. and Ongaro, F.; "Optimal Trajectories for the Impulsive Deflection of NEOs"; Paper IAC-05-C1.5.06, 56th International Astronautical Congress, Fukuoka, Japan, (October 2005). Later published in Acta Astronautica, Vol. 59, No. 1-5, pp.&nbsp;294–300, April 2006, available in [http://www.esa.int/gsp/ACT/publications/pub-mad.htm esa.int – The first scientific paper proving that Apophis can be deflected by a small sized kinetic impactor.]
* David Morrison. [http://www.csicop.org/si/9705/asteroid.html "Is the Sky Falling?"], ''[[Unsolved:Skeptical Inquirer|Skeptical Inquirer]]'' 1997.
* [https://web.archive.org/web/20060114085628/http://impact.arc.nasa.gov/downloads/NEO_Chapter_1.pdf?ID=113 David Morrison, Alan W Harris, Geoff Summer, Clark R. Chapman, & Andrea Carusi  ''Dealing with Impact Hazard'', 2002 technical summary]
* Kunio M. Sayanagi. [https://arstechnica.com/journals/science.ars/2008/04/04/how-to-deflect-an-asteroid "How to Deflect an Asteroid"]. ''[[Ars Technica]]'' (April 2008).
* Russell L. Schweickart, Edward T. Lu, Piet Hut and Clark R. Chapman; "The Asteroid Tugboat"; ''Scientific American'' (November 2003). Vol. 289, No. 5, pp.&nbsp;54–61. {{JSTOR|26060526}}.
{{refend}}
* Furfaro, Emily. NASA's DART Data Validates Kinetic Impact as Planetary Defense Method, NASA, 28 Feb. 2023, [https://www.nasa.gov/feature/nasa-s-dart-data-validates-kinetic-impact-as-planetary-defense-method].

== Further reading ==
'''General'''
* Air Force 2025. ''[https://web.archive.org/web/20060616060433/http://www.au.af.mil/au/2025/volume3/chap16/v3c16-2.htm Planetary Defense: Social, Economic, and Political Implications]'', {{wipe|United States Air Force}}, Air Force 2025 Final Report webpage, December 11, 1996.
* Belton, M.J.S. ''[https://books.google.com/books?id=Dw0A7T0fy6AC Mitigation of Hazardous Comets and Asteroids]'', {{wipe|Cambridge University Press}}, 2004, {{ISBN|0521827647|978-0521827645}}
* Bottke, William F. ''[https://books.google.com/books?id=JwHTyO6IHh8C Asteroids III]'' (Space Science Series), University of Arizona space science series, University of Arizona Press, 2002, {{ISBN|0816522812|978-0816522811}}
* Burrows, William E. [https://www.amazon.com/dp/B00HBQIFTY/ ''The Asteroid Threat: Defending Our Planet from Deadly Near-Earth Objects''].
* Lewis, John S. ''[https://books.google.com/books?id=0xMZITJqidcC Comet and Asteroid Impact Hazards on a Populated Earth: Computer Modeling]'' (Volume 1 of Comet and Asteroid Impact Hazards on a Populated Earth: Computer Modeling), [[Company:Academic Press|Academic Press]], 2000, {{ISBN|0124467601|978-0124467606}}
* Marboe, Irmgard : ''Legal Aspects of Planetary Defence.'' Brill, Leiden 2021, ISBN 978-90-04-46759-0.
* Schmidt, Nikola et al.: ''Planetary Defense: Global Collaboration for Defending Earth from Asteroids and Comets.'' Springer, Cham 2019, {{ISBN|978-3-030-00999-1}}.
* Verschuur, Gerrit L. (1997) [https://books.google.com/books?id=4gnumbZdVqEC ''Impact!: The Threat of Comets and Asteroids''], [[Organization:Oxford University Press|Oxford University Press]], {{ISBN|0195353277|978-0195353273}}

== External links ==
* [https://spectrum.ieee.org/aerospace/space-flight/deflecting-asteroids "Deflecting Asteroids"] (with solar sails) by Gregory L. Matloff, ''IEEE Spectrum'', April 2012
* [https://newton.spacedys.com/neodys2/ Near Earth Objects Directory]
* [https://web.archive.org/web/20070429053149/http://neo.jpl.nasa.gov/neo/report2007.html Nasa's 2007 Report to Congress on NEO Survey Program Including Tracking and Diverting Methods for High Risk Asteroids]
* [https://web.archive.org/web/20100325002654/http://star.arm.ac.uk/impact-hazard/ Armagh University: Near Earth Object Impact Hazard ]
* [https://purl.fdlp.gov/GPO/gpo41041 Threats from Space: A Review of U.S. Government Efforts to Track and Mitigate Asteroids and Meteors (Part I and Part II): Hearing before the Committee on Science, Space, and Technology, House of Representatives, One Hundred Thirteenth Congress, First Session, Tuesday, March 19, 2013 and Wednesday, April 10, 2013]

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