Editing Asteroid impact avoidance (section)

====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 [[meteorite]]s exposed to the thermal X-ray pulses of the [[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 [[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 [[1566 Icarus]], an object that makes regular close approaches to Earth, sometimes as close as 16 [[lunar distance (astronomy)|lunar distances]].<ref>{{Cite journal|last1=Goldstein |first1=R. M.|title=Radar Observations of Icarus|journal=[[Science (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 [[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#Test|''Tsar Bomba'']] would have been if a uranium tamper had been used—as each rocket vehicle's [[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 }}, ''[[Time (magazine)|Time]]'', 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 [[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 (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 [[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 [[99942 Apophis|Apophis]] (with a diameter of around {{convert|300|m|disp=or|-2|sp=us}}) 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 nuclear bomb|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 [[proximity fuze|fuzed]] to detonate over the asteroid's surface at a height of {{convert|100|m|disp=or|sp=us}} ("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|sp=us|-2}} 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>