Editing Tunguska event (section)

==Scientific investigation==
Since the 1908 event, an estimated 1,000 scholarly papers (most in Russian) have been published about the Tunguska explosion. Owing to the site's remoteness and the limited instrumentation available at the time of the event, modern scientific interpretations of its cause and magnitude have relied chiefly on damage assessments and geological studies conducted many years after the event. Estimates of its energy have ranged from {{convert|3|–|30|MtonTNT|abbr=off}}.

Only more than a decade after the event did any scientific analysis of the region take place, in part due to the area's isolation and significant political upheaval affecting Russia in the 1910s. In 1921, the Russian [[mineralogy|mineralogist]] [[Leonid Kulik]] led a team to the Podkamennaya Tunguska River basin to conduct a survey for the [[Russian Academy of Sciences|Soviet Academy of Sciences]].<ref>{{cite web|title=The Tunguska Impact – 100 Years Later|url=https://science.nasa.gov/science-news/science-at-nasa/2008/30jun_tunguska|website=NASA Science|access-date=13 January 2019|archive-date=16 May 2021|archive-url=https://web.archive.org/web/20210516020717/https://science.nasa.gov/science-news/science-at-nasa/2008/30jun_tunguska/|url-status=live}}</ref> Although they never visited the central blast area, the many local accounts of the event led Kulik to believe that a giant [[impact event|meteorite impact]] had caused the event. Upon returning, he persuaded the Soviet government to fund an expedition to the suspected impact zone, based on the prospect of salvaging [[meteoric iron]].<ref name=":1">{{Cite web|url=https://www.aps.org/publications/apsnews/201806/history.cfm|title=This Month in Physics History|date=June 2018|publisher=American Physical Society|access-date=22 December 2018|archive-date=8 March 2021|archive-url=https://web.archive.org/web/20210308025217/https://www.aps.org/publications/apsnews/201806/history.cfm|url-status=live}}</ref>

Kulik led a scientific expedition to the Tunguska blast site in 1927. He hired local [[Evenks|Evenki]] hunters to guide his team to the centre of the blast area, where they expected to find an [[impact crater]]. To their surprise, there was no crater at [[ground zero]]. Instead, they found a zone, roughly {{convert|8|km|mi}} across, where the trees were scorched and devoid of branches, but still standing upright.<ref name=":1" /> Trees farther from the centre had been partly scorched and knocked down away from the centre, creating a large radial pattern of downed trees.
[[File:Moscow State University Tunguska event 2014-01 1389528710.JPG|thumb|Sample of burn damaged tree, museum of Moscow State University]]
In the 1960s, it was established that the zone of levelled forest occupied an area of {{convert|2150|km2|abbr=on}}, its shape resembling a gigantic spread-eagled butterfly with a "wingspan" of {{convert|70|km||abbr=on}} and a "body length" of {{convert|55|km||abbr=on}}.<ref>Boyarkina, A. P., Demin, D. V., Zotkin, I. T., Fast, W. G. "Estimation of the blast wave of the Tunguska meteorite from the forest destruction". ''Meteoritika'', Vol. 24, 1964, pp.&nbsp;112–128 (in Russian).</ref><ref>{{Cite web|url=https://i.imgur.com/n5q5EaD.jpg|title=external image from ''Meteoritika'' article|access-date=6 September 2022|archive-date=4 March 2023|archive-url=https://web.archive.org/web/20230304211831/https://i.imgur.com/n5q5EaD.jpg|url-status=live}}</ref> Upon closer examination, Kulik found holes that he erroneously concluded were meteorite holes; he did not have the means at that time to excavate the holes.

During the next 10 years, there were three more expeditions to the area. Kulik found several dozen little "pothole" bogs, each {{convert|10|to|50|m|abbr=off}} in diameter, that he thought might be meteoric craters. After a laborious exercise in draining one of these bogs (the so-called "Suslov's crater", {{convert|32|m|abbr=on|disp=sqbr}} in diameter), he found an old tree stump on the bottom, ruling out the possibility that it was a meteoric crater. In 1938, Kulik arranged for an aerial photographic survey of the area<ref>{{cite web|url=http://www-th.bo.infn.it/tunguska/APS-photolist.htm|author=Longo G.|title=The 1938 aerophotosurvey|access-date=8 October 2017|archive-date=26 February 2021|archive-url=https://web.archive.org/web/20210226195352/https://www-th.bo.infn.it/tunguska/APS-photolist.htm|url-status=live}}</ref> covering the central part of the leveled forest ({{convert|250|km2|disp=sqbr}}).<ref name=Bronshten2000>See: Bronshten (2000), p.&nbsp;56.</ref> The original negatives of these aerial photographs (1,500 negatives, each {{convert|18|by|18|cm|abbr=off|disp=sqbr}}) were burned in 1975 by order of [[Yevgeny Krinov]], then Chairman of the Committee on Meteorites of the USSR Academy of Sciences, as part of an initiative to dispose of increasingly flammable [[Nitrocellulose#Film|nitrate film]].<ref name=Bronshten2000/> Positive prints were preserved for further study in [[Tomsk]].<ref>Rubtsov (2009), p.&nbsp;59.</ref>

Expeditions sent to the area in the 1950s and 1960s found microscopic [[silicate]] and [[magnetite]] spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees. [[Chemical analysis]] showed that the spheres contained high proportions of [[nickel]] relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a [[meteor air burst]].<ref name=Florenskiy/> Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.<ref name=anomalies>{{cite journal|last1=Kolesnikov|first1=E.m.|last2=Boettger|first2=T.|last3=Kolesnikova|first3=N.V.|title=Finding of probable Tunguska Cosmic Body material|journal=Planetary and Space Science|date=June 1999|volume=47|issue=6–7|pages=905–916|doi=10.1016/S0032-0633(99)00006-9}}</ref>

Chemical analysis of [[peat bog]]s from the area also revealed numerous anomalies considered consistent with an impact event. The [[isotopic signature]]s of carbon, hydrogen, and nitrogen at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of [[iridium]], similar to the iridium layer found in the [[Cretaceous–Paleogene boundary]]. These unusual proportions are believed to result from debris from the falling body that deposited in the bogs. The nitrogen is believed to have been deposited as [[acid rain]], a suspected fallout from the explosion.<ref name=anomalies/><ref>{{cite journal|last1=Hou|first1=Q.L.|last2=Ma|first2=P.X.|last3=Kolesnikov|first3=E.M.|title=Discovery of iridium and other element anomalies near the 1908 Tunguska explosion site|journal=Planetary and Space Science|date=February 1998|volume=46|issue=2–3|pages=179–188|doi=10.1016/S0032-0633(97)00174-8|bibcode=1998P&SS...46..179H}}</ref><ref>{{cite journal|last1=Kolesnikov|first1=E.M.|last2=Kolesnikova|first2=N.V.|last3=Boettger|first3=T.|title=Isotopic anomaly in peat nitrogen is a probable trace of acid rains caused by 1908 Tunguska bolide|journal=Planetary and Space Science|date=February 1998|volume=46|issue=2–3|pages=163–167|doi=10.1016/S0032-0633(97)00190-6|bibcode=1998P&SS...46..163K}}</ref>

Other scientists disagree: "Some papers report that hydrogen, carbon and nitrogen isotopic compositions with signatures similar to those of CI and CM carbonaceous chondrites were found in Tunguska peat layers dating from the TE (Kolesnikov et al. 1999, 2003) and that iridium anomalies were also observed (Hou et al. 1998, 2004). Measurements performed in other laboratories have not confirmed these results (Rocchia et al. 1990; Tositti et al. 2006)."<ref name="Longo 2007 303–330"/>

Researcher John Anfinogenov has suggested that a boulder found at the event site, known as John's stone, is a remnant of the meteorite,<ref>{{cite journal|last1=Anfinogenov|first1=John|last2=Budaeva|first2=Larisa|last3=Kuznetsov|first3=Dmitry|last4=Anfinogenova|first4=Yana|title=John's Stone: A possible fragment of the 1908 Tunguska meteorite|journal=Icarus|date=November 2014|volume=243|pages=139–147|doi=10.1016/j.icarus.2014.09.006|bibcode=2014Icar..243..139A|s2cid=118541956}}</ref> but oxygen isotope analysis of the [[quartzite]] suggests that it is of [[Hot spring|hydrothermal]] origin, and probably related to Permian-Triassic [[Siberian Traps]] magmatism.<ref>{{cite journal|last1=Bonatti|first1=Enrico|last2=Breger|first2=Dee|last3=Di Rocco|first3=Tommaso|last4=Franchi|first4=Fulvio|last5=Gasperini|first5=Luca|last6=Polonia|first6=Alina|last7=Anfinogenov|first7=John|last8=Anfinogenova|first8=Yana|title=Origin of John's Stone: A quartzitic boulder from the site of the 1908 Tunguska (Siberia) explosion|journal=Icarus|date=September 2015|volume=258|pages=297–308|doi=10.1016/j.icarus.2015.06.018|bibcode=2015Icar..258..297B}}</ref>

In 2013, a team of researchers published the results of an analysis of micro-samples from a peat bog near the centre of the affected area, which show fragments that may be of extraterrestrial origin.<ref>{{cite journal|last1=Peplow|first1=Mark|title=Rock samples suggest meteor caused Tunguska blast|journal=Nature|date=10 June 2013|doi=10.1038/nature.2013.13163|s2cid=131239755}}</ref><ref>{{cite journal|last1=Kvasnytsya|first1=Victor|last2=Wirth|first2=Richard|last3=Dobrzhinetskaya|first3=Larissa|last4=Matzel|first4=Jennifer|last5=Jacobsen|first5=Benjamin|last6=Hutcheon|first6=Ian|last7=Tappero|first7=Ryan|last8=Kovalyukh|first8=Mykola|title=New evidence of meteoritic origin of the Tunguska cosmic body|journal=Planetary and Space Science|date=August 2013|volume=84|pages=131–140|doi=10.1016/j.pss.2013.05.003|bibcode=2013P&SS...84..131K|url=https://gfzpublic.gfz-potsdam.de/pubman/item/item_247242}}</ref>

===Earth impactor model===
[[File:Tunguska_and_Chelyabinsk_meteoroid_size.png|thumb|Comparison of possible sizes of Tunguska (TM mark) and [[Chelyabinsk meteor|Chelyabinsk]] (CM) meteoroids to the [[Eiffel Tower]] and [[Empire State Building]]]]
The leading scientific explanation for the explosion is a [[meteor air burst]] by an [[asteroid]] {{convert|6|–|10|km|mi|sigfig=1|abbr=on}} above the Earth's surface.

[[Meteoroids]] enter [[Earth's atmosphere]] from [[outer space]] every day, travelling at a speed of at least {{convert|11|km/s|sigfig=1|abbr=on}}, the [[Escape velocity|escape velocity]] of the Earth. The heat generated by compression of air in front of the body ([[ram pressure]]) as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Early estimates of the energy of the Tunguska air burst ranged from {{convert|10|–|15|MtonTNT|lk=on|abbr=off}} to 30 megatons of TNT (130 PJ),<ref name=shoe/> depending on the exact height of the burst as estimated when the scaling laws from the [[effects of nuclear weapons]] are employed.<ref name=shoe>{{cite journal|last=Shoemaker|first=Eugene|author-link=Eugene Merle Shoemaker|title=Asteroid and Comet Bombardment of the Earth|year=1983|volume=11|issue=1|doi=10.1146/annurev.ea.11.050183.002333|journal=Annual Review of Earth and Planetary Sciences|pages=461–494|bibcode=1983AREPS..11..461S}}</ref><ref name="Sandia National Laboratories">{{cite news|url=https://share.sandia.gov/news/resources/releases/2007/asteroid.html|title=Sandia supercomputers offer new explanation of Tunguska disaster|date=17 December 2007|publisher=[[Sandia National Laboratories]]|access-date=22 December 2007|archive-date=19 February 2013|archive-url=https://web.archive.org/web/20130219203913/https://share.sandia.gov/news/resources/releases/2007/asteroid.html|url-status=dead}}</ref> More recent calculations that include the effect of the object's [[momentum]] find that more of the energy was focused downward than would be the case from a nuclear explosion and estimate that the air burst had an energy range from 3 to 5 megatons of TNT (13 to 21&nbsp;PJ).<ref name="Sandia National Laboratories"/> The 15-megaton ([[TNT equivalent|Mt]]) estimate represents an energy about 1,000 times greater than that of the [[Trinity (nuclear test)|Trinity]] nuclear test, and roughly equal to that of the United States' [[Castle Bravo]] nuclear test in 1954 (which measured 15.2 Mt) and one third that of the [[Soviet Union]]'s [[Tsar Bomba]] test in 1961.<ref>Verma (2005), p&nbsp;1.</ref> A 2019 paper suggests the explosive power of the Tunguska event may have been around 20–30 megatons.<ref>{{cite journal|title=Probabilistic assessment of Tunguska-scale asteroid impacts|journal=Icarus|volume=327|pages=83–96|doi=10.1016/j.icarus.2018.12.017|year=2019|last1=Wheeler|first1=Lorien F.|last2=Mathias|first2=Donovan L.|bibcode=2019Icar..327...83W|doi-access=free}}</ref>

Since the second half of the 20th century, close monitoring of Earth's atmosphere through infrasound and satellite observation has shown that asteroid air bursts with energies comparable to those of nuclear weapons routinely occur, although Tunguska-sized events, on the order of 5–15 [[TNT equivalent|megatons]],<ref name="Chelyabinsk">{{cite journal|last1=Borovička|first1=Jiří|last2=Spurný|first2=Pavel|last3=Brown|first3=Peter|last4=Wiegert|first4=Paul|last5=Kalenda|first5=Pavel|last6=Clark|first6=David|last7=Shrbený|first7=Lukáš|title=The trajectory, structure and origin of the Chelyabinsk asteroidal impactor|journal=Nature|date=14 November 2013|volume=503|issue=7475|pages=235–237|doi=10.1038/nature12671|pmid=24196708|bibcode=2013Natur.503..235B|s2cid=4399008}}</ref> are much rarer. [[Eugene Merle Shoemaker|Eugene Shoemaker]] estimated that 20-kiloton events occur annually and that Tunguska-sized events occur about once every 300 years.<ref name=shoe/><ref>{{cite web|url=https://www.smithsonianmag.com/science-nature/phenomena-comment-notes-86860922/|archive-url=https://archive.today/20120910221113/http://www.smithsonianmag.com/science-nature/phenom_jan95.html?c=y&page=2|url-status=live|archive-date=10 September 2012|title=Phenomena, Comment & Notes|first=John P. Jr.|last=Wiley|date=January 1995|work=Smithsonian}}</ref> More recent estimates place Tunguska-sized events at about once every thousand years, with 5-kiloton air bursts averaging about once per year.<ref name="Flux">{{cite journal|last1=Brown|first1=P.|last2=Spalding|first2=R. E.|last3=ReVelle|first3=D. O.|last4=Tagliaferri|first4=E.|last5=Worden|first5=S. P.|title=The flux of small near-Earth objects colliding with the Earth|journal=Nature|date=November 2002|volume=420|issue=6913|pages=294–296|doi=10.1038/nature01238|pmid=12447433|bibcode=2002Natur.420..294B|s2cid=4380864}}</ref> Most of these are thought to be caused by asteroid impactors, as opposed to mechanically weaker [[comet]]ary materials, based on their typical penetration depths into the Earth's atmosphere.<ref name="Flux"/> The largest asteroid air burst observed with modern instrumentation was the 500-kiloton [[Chelyabinsk meteor]] in 2013, which shattered windows and produced meteorites.<ref name="Chelyabinsk"/>

==== Glancing impact hypothesis ====
In 2020, a group of Russian scientists used a range of computer models to calculate the passage of asteroids with diameters of 200, 100, and 50 metres at oblique angles across Earth's atmosphere. They used a range of assumptions about the object's composition as if it was made of iron, rock, or ice. The model that most closely matched the observed event was an iron asteroid up to 200 metres in diameter, travelling at 11.2&nbsp;km per second, that [[Earth-grazing fireball|glanced off]] the Earth's atmosphere and returned into solar orbit.<ref>{{cite journal|last1=Khrennikov|first1=Daniil E|last2=Titov|first2=Andrei K|last3=Ershov|first3=Alexander E|last4=Pariev|first4=Vladimir I|last5=Karpov|first5=Sergei V|title=On the possibility of through passage of asteroid bodies across the Earth's atmosphere|journal=Monthly Notices of the Royal Astronomical Society|date=21 March 2020|volume=493|issue=1|pages=1344–1351|doi=10.1093/mnras/staa329|doi-access=free|arxiv=2009.14234}}</ref><ref>{{Cite web|title=Most Explosive Meteor Impact: 1908 Tunguska Explosion Caused by Iron Asteroid That Entered Earth Then Bounced Back to Space|url=https://www.sciencetimes.com/articles/25599/20200506/explosive-meteor-impact-1908-tunguska-explosion-caused-iron-asteroid-entered.htm|date=6 May 2020|website=Science Times|access-date=7 May 2020|archive-date=7 May 2020|archive-url=https://web.archive.org/web/20200507225419/http://www.sciencetimes.com/articles/25599/20200506/explosive-meteor-impact-1908-tunguska-explosion-caused-iron-asteroid-entered.htm|url-status=live}}</ref><ref>{{Cite web|title=World's largest 'explosion' could have been caused by iron asteroid entering and leaving atmosphere|url=https://siberiantimes.com/science/others/news/worlds-largest-explosion-could-have-been-caused-by-iron-asteroid-entering-and-leaving-atmosphere/|website=siberiantimes.com|access-date=7 May 2020|archive-date=7 May 2020|archive-url=https://web.archive.org/web/20200507000035/http://siberiantimes.com/science/others/news/worlds-largest-explosion-could-have-been-caused-by-iron-asteroid-entering-and-leaving-atmosphere/|url-status=live}}</ref>

====Blast pattern====
The explosion's effect on the trees near the explosion's [[hypocentre]] was similar to the effects of the conventional [[Operation Blowdown]]. These effects are caused by the [[blast wave]] produced by large air-burst explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, but remain standing upright, while trees farther away are knocked over because the blast wave is travelling closer to horizontal when it reaches them.

Soviet experiments performed in the mid-1960s, with model forests (made of matches on wire stakes) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in midair.<ref>{{IMDb title|id=1156463|title=Siberian Apocalypse}}</ref>

====Asteroid or comet====
In 1930, the British meteorologist and mathematician [[Francis John Welsh Whipple|F. J. W. Whipple]] suggested that the Tunguska body was a small [[comet]]. A comet is composed of [[cosmic dust|dust]] and [[Volatile (astrogeology)|volatiles]], such as water ice and frozen gases, and could have been completely vaporised by the impact with Earth's atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies (or "skyglows" or "bright nights") observed across Eurasia for several evenings after the impact, which are possibly explained by dust and ice that had been dispersed from the [[comet's tail]] across the upper atmosphere.<ref name=shoe/> The cometary hypothesis gained a general acceptance among Soviet Tunguska investigators by the 1960s.<ref name=shoe/>

In 1978, Slovak astronomer [[Ľubor Kresák]] suggested that the body was a fragment of [[Comet Encke]], a [[periodic comet]] with a period of just over three years that stays entirely within Jupiter's orbit. It is also responsible for the [[Beta Taurids]], an annual [[meteor shower]] with a maximum activity around 28–29 June. The Tunguska event coincided with that shower's peak activity,<ref>{{cite journal|last1=Kresak|first1=L'|title=The Tunguska Object: a Fragment of Comet Encke?|journal=Bulletin of the Astronomical Institutes of Czechoslovakia|date=1978|volume=29|pages=129|id={{INIST|PASCAL7830419797}}|bibcode=1978BAICz..29..129K}}</ref> the Tunguska object's approximate trajectory is consistent with what would be expected from a fragment of Comet Encke,<ref name=shoe/> and a hypothetical risk corridor has now been calculated demonstrating that if the impactor had arrived a few minutes earlier it would have exploded over the US or Canada.<ref>{{Cite web|title=Analysis of the Tunguska Event as a Semi-Hypothetical Impact Scenario|last1=Boslough|first1=Mark|last2=Chodas|first2=Paul|last3=Brown|first3=Peter|url=https://agu.confex.com/agu/fm23/meetingapp.cgi/Paper/1405477|access-date=2023-12-17|series=2023 AGU Fall Meeting|date=13 December 2023}}</ref> It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades.<ref>{{cite journal|last1=Nemtchinov|first1=I.V.|first2=C.|last2=Jacobs|first3=E.|last3=Tagliaferri|title=Analysis of Satellite Observations of Large Meteoroid Impacts|journal=[[Annals of the New York Academy of Sciences]]|volume=822|issue=1 Near–Earth Ob|pages=303–317|year=1997|doi=10.1111/j.1749-6632.1997.tb48348.x|bibcode=1997NYASA.822..303N|s2cid=122983849}}</ref> In 2019 astronomers searched for hypothesized asteroids ~100 metres in diameter from the Taurid swarm between 5–11 July, and 21 July – 10 August.<ref name="Plait">{{cite web|title=Could larger space rocks be hiding in the Beta Taurid Meteor stream? We may find out this summer|publisher=Bad Astronomy|author=Phil Plait|url=https://www.syfy.com/syfywire/could-larger-space-rocks-be-hiding-in-the-beta-taurid-meteor-stream-we-may-find-out-this|date=14 May 2019|author-link=Phil Plait|access-date=17 May 2019|archive-date=14 May 2019|archive-url=https://web.archive.org/web/20190514152052/https://www.syfy.com/syfywire/could-larger-space-rocks-be-hiding-in-the-beta-taurid-meteor-stream-we-may-find-out-this|url-status=live}}</ref> {{as of|2020|02}}, there have been no reports of discoveries of any such objects.

In 1983, astronomer [[Zdeněk Sekanina]] published a paper criticising the comet hypothesis.<ref>{{cite journal|last1=Sekanina|first1=Z.|title=The Tunguska event – No cometary signature in evidence|journal=The Astronomical Journal|date=September 1983|volume=88|pages=1382–1413|doi=10.1086/113429|bibcode=1983AJ.....88.1382S|doi-access=free}}</ref> He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere. Sekanina also argued that the evidence pointed to a dense rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when [[Paolo Farinella|Farinella]], Foschini, ''et al.'' released a study calculating the probabilities based on orbital modelling extracted from the atmospheric trajectories of the Tunguska object. They concluded with a probability of 83% that the object moved on an asteroidal path originating from the [[asteroid belt]], rather than on a cometary one (probability of 17%).<ref name=Farinella-2001>{{cite journal|last1=Farinella|first1=P.|last2=Foschini|first2=L.|last3=Froeschlé|first3=Ch.|last4=Gonczi|first4=R.|last5=Jopek|first5=T. J.|last6=Longo|first6=G.|last7=Michel|first7=P.|title=Probable asteroidal origin of the Tunguska Cosmic Body|journal=Astronomy & Astrophysics|date=October 2001|volume=377|issue=3|pages=1081–1097|doi=10.1051/0004-6361:20011054|bibcode=2001A&A...377.1081F|doi-access=free}}</ref> Proponents of the comet hypothesis have suggested that the object was an [[extinct comet]] with a stony mantle that allowed it to penetrate the atmosphere.

The chief difficulty in the asteroid hypothesis is that a stony object should have produced a large crater where it struck the ground, but no such crater has been found. It has been hypothesised that the asteroid's passage through the atmosphere caused pressures and temperatures to build up to a point where the asteroid abruptly disintegrated in a huge explosion. The destruction would have to have been so complete that no remnants of substantial size survived, and the material scattered into the upper atmosphere during the explosion would have caused the skyglows. Models published in 1993 suggested that the stony body would have been about {{convert|60|m}} across, with physical properties somewhere between an ordinary [[chondrite]] and a [[carbonaceous chondrite]].{{citation needed|date=November 2014}} Typical carbonaceous chondrite substance tends to be dissolved with water rather quickly unless it is frozen.<ref>{{cite news|title=Arctic Asteroid!|url=https://science.nasa.gov/science-news/science-at-nasa/2000/ast01jun_1m|website=Science at NASA|access-date=8 October 2017|archive-date=16 May 2017|archive-url=https://web.archive.org/web/20170516210100/https://science.nasa.gov/science-news/science-at-nasa/2000/ast01jun_1m|url-status=live}}</ref>

[[Christopher Chyba]] and others have proposed a process whereby a stony asteroid could have exhibited the Tunguska impactor's behaviour. Their models show that when the forces opposing a body's descent become greater than the cohesive force holding it together, it blows apart, releasing nearly all its energy at once. The result is no crater, with damage distributed over a fairly wide radius, and all the damage resulting from the thermal energy the blast releases.<ref>{{Cite journal|last1=Chyba|first1=Christopher F.|last2=Thomas|first2=Paul J.|last3=Zahnle|first3=Kevin J.|date=January 1993|title=The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid|url=http://dx.doi.org/10.1038/361040a0|journal=Nature|volume=361|issue=6407|pages=40–44|doi=10.1038/361040a0|bibcode=1993Natur.361...40C|issn=0028-0836}}</ref>

During the 1990s, Italian researchers, coordinated by the physicist [[Giuseppe Longo]] from the [[University of Bologna]], extracted resin from the core of the trees in the area of impact to examine trapped particles that were present during the 1908 event. They found high levels of material commonly found in rocky asteroids and rarely found in comets.<ref>{{cite journal|last1=Longo|first1=G.|last2=Serra|first2=R.|last3=Cecchini|first3=S.|last4=Galli|first4=M.|title=Search for microremnants of the Tunguska Cosmic Body|journal=Planetary and Space Science|date=February 1994|volume=42|issue=2|pages=163–177|doi=10.1016/0032-0633(94)90028-0|bibcode=1994P&SS...42..163L}}</ref><ref>{{cite journal|last1=Serra|first1=R.|last2=Cecchini|first2=S.|last3=Galli|first3=M.|last4=Longo|first4=G.|title=Experimental hints on the fragmentation of the Tunguska Cosmic body|journal=Planetary and Space Science|date=September 1994|volume=42|issue=9|pages=777–783|doi=10.1016/0032-0633(94)90120-1|bibcode=1994P&SS...42..777S}}</ref>

Kelly ''et al.'' (2009) contend that the impact was caused by a comet because of the sightings of [[noctilucent cloud]]s following the impact, a phenomenon caused by massive amounts of water vapour in the upper atmosphere. They compared the noctilucent cloud phenomenon to the exhaust plume from NASA's [[Space Shuttle Endeavour|''Endeavour'' Space Shuttle]].<ref>{{cite journal|last1=Kelley|first1=M. C.|last2=Seyler|first2=C. E.|last3=Larsen|first3=M. F.|title=Two-dimensional turbulence, space shuttle plume transport in the thermosphere, and a possible relation to the Great Siberian Impact Event|journal=Geophysical Research Letters|date=22 July 2009|volume=36|issue=14|doi=10.1029/2009GL038362|bibcode=2009GeoRL..3614103K|s2cid=129245795|doi-access=free}}</ref><ref>{{cite web|url=http://news.cornell.edu/stories/2009/06/researchers-connect-shuttle-plume-1908-explosion|title=A mystery solved: Space shuttle shows 1908 Tunguska explosion was caused by comet|access-date=25 June 2009|last=Ju|first=Anne|date=24 June 2009|work=[[Cornell Chronicle]]|publisher=[[Cornell University]]|archive-date=31 August 2018|archive-url=https://web.archive.org/web/20180831223202/http://news.cornell.edu/stories/2009/06/researchers-connect-shuttle-plume-1908-explosion|url-status=live}}</ref> A team of Russian researchers led by Edward Drobyshevski in 2009 suggested that the near-Earth asteroid {{mpl|2005 NB|56}} may be a possible candidate for the Tunguska object's parent body as the asteroid made a close approach of {{cvt|0.06945|AU|LD|lk=out|sigfig=2}} from Earth on 27 June 1908, three days before the Tunguska impact. The team suspected that {{mp|2005 NB|56}}'s orbit likely fits with the Tunguska object's modelled orbit, even with the effects of weak non-gravitational forces.<ref>{{cite arXiv|title=A search for a present-day candidate for the Comet P/Tunguska-1908|first1=E. M.|last1=Drobyshevski|first2=T. Yu|last2=Galushina|first3=M. E.|last3=Drobyshevski|eprint=0903.3313|date=March 2009|class=astro-ph.EP}}</ref> In 2013, analysis of fragments from the Tunguska site by a joint US-European team was consistent with an iron meteorite.<ref>{{cite web|url=http://blogs.discovermagazine.com/d-brief/2013/07/01/meteoroid-not-comet-explains-the-1908-tunguska-fireball/|title=Meteoroid, not comet, explains the 1908 Tunguska fireball|website=DiscoverMagazine.com blog|date=1 July 2013|access-date=29 October 2013|archive-date=4 July 2013|archive-url=https://web.archive.org/web/20130704043529/http://blogs.discovermagazine.com/d-brief/2013/07/01/meteoroid-not-comet-explains-the-1908-tunguska-fireball/|url-status=dead}}</ref>

{{meteoroid_size_comparison.svg}}
The February 2013 [[Chelyabinsk meteor|Chelyabinsk]] [[bolide]] event provided ample data for scientists to create new models for the Tunguska event. Researchers used data from both Tunguska and Chelyabinsk to perform a statistical study of over 50&nbsp;million combinations of bolide and entry properties that could produce Tunguska-scale damage when breaking apart or exploding at similar altitudes. Some models focused on combinations of properties which created scenarios with similar effects to the tree-fall pattern as well as the atmospheric and seismic pressure waves of Tunguska. Four different computer models produced similar results; they concluded that the likeliest candidate for the Tunguska impactor was a stony body between {{convert|164|and|262|ft|m|order=flip|abbr=on}} in diameter, entering the atmosphere at roughly {{convert|34000|mph|km/h|order=flip|abbr=on}}, exploding at {{convert|6|to|9|mi|km|0|order=flip|abbr=on}} altitude, and releasing explosive energy equivalent to between 10 and 30 megatons. This is similar to the blast energy equivalent of the 1980 volcanic [[eruption of Mount St. Helens]]. The researchers also concluded impactors of this size hit the Earth only at an average interval scale of millennia.<ref name="nasatusk" />

==== Lake Cheko ====
{{See also|Lake Cheko}}
In June 2007, scientists from the [[University of Bologna]] identified a lake in the Tunguska region as a possible impact crater from the event. They do not dispute that the Tunguska body exploded in midair, but believe that a {{convert|10|m|ft|adj=on}} fragment survived the explosion and struck the ground. [[Lake Cheko]] is a small bowl-shaped lake about {{convert|8|km|abbr=on}} north-northwest of the hypocentre.<ref name=italy2>{{cite journal|title=A possible impact crater for the 1908 Tunguska Event|journal=[[Terra Nova (journal)|Terra Nova]]|volume=19|issue=4|page=245|year=2007|doi=10.1111/j.1365-3121.2007.00742.x|bibcode=2007TeNov..19..245G|last1=Gasperini|first1=Luca|last2=Alvisi|first2=F|last3=Biasini|first3=G|last4=Bonatti|first4=E|last5=Longo|first5=G|last6=Pipan|first6=M|last7=Ravaioli|first7=M|last8=Serra|first8=R|doi-access=free}}</ref>

The hypothesis has been disputed by other impact crater specialists.<ref name=rincon1>{{cite news|last1=Rincon|first1=Paul|title=Team makes Tunguska crater claim|url=http://news.bbc.co.uk/2/hi/science/nature/6239334.stm|work=[[BBC News Online]]|date=26 June 2007}}</ref> A 1961 investigation had dismissed a modern origin of Lake Cheko, saying that the presence of metres-thick silt deposits on the lake bed suggests an age of at least 5,000 years,<ref name=Florenskiy>{{cite journal|last=Florenskiy|first=K P|author-link=Kirill Florensky|title=Preliminary results from the 1961 combined Tunguska meteorite expedition|journal=Meteoritica|volume=23|year=1963|url=http://abob.libs.uga.edu/bobk/tungmet.html|access-date=26 June 2007|archive-date=20 July 2008|archive-url=https://web.archive.org/web/20080720064557/http://abob.libs.uga.edu/bobk/tungmet.html|url-status=live}}</ref> but more recent research suggests that only a metre or so of the sediment layer on the lake bed is "normal [[lacustrine deposits|lacustrine sedimentation]]", a depth consistent with an age of about 100 years.<ref>{{cite journal|last1=Gasperini|first1=L.|title=Reply – Lake Cheko and the Tunguska Event: impact or non-impact?|journal=[[Terra Nova (journal)|Terra Nova]]|volume=20|issue=2|pages=169–172|date=April 2008|doi=10.1111/j.1365-3121.2008.00792.x|last2=Bonatti|first2=Enrico|last3=Longo|first3=Giuseppe|bibcode=2008TeNov..20..169G|s2cid=140554080|doi-access=free}}</ref> [[Echo sounding|Acoustic-echo sounding]]s of the lake floor support the hypothesis that the Tunguska event formed the lake. The soundings revealed a conical shape for the lake bed, which is consistent with an impact crater.<ref name=sciam>{{cite journal|last1=Gasperini|first1=Luca|last2=Bonatti|first2=Enrico|last3=Longo|first3=Giuseppe|title=The Tunguska Mystery|journal=[[Scientific American]]|date=2008|volume=298|issue=6|pages=80–86|doi=10.1038/scientificamerican0608-80|jstor=26000644|pmid=18642546|bibcode=2008SciAm.298f..80G}}</ref> Magnetic readings indicate a possible metre-sized chunk of rock below the lake's deepest point that may be a fragment of the colliding body.<ref name=sciam/> Finally, the lake's long axis points to the Tunguska explosion's [[hypocenter|hypocentre]], about {{convert|7.0|km|abbr=on}} away.<ref name=sciam/> Work is still being done at Lake Cheko to determine its origins.<ref>{{cite news|url=http://news.nationalgeographic.com/news/2007/11/071107-russia-crater.html|title=Crater From 1908 Russian Space Impact Found, Team Says|work=[[National Geographic]]|date=7 November 2007|access-date=8 October 2017|archive-date=15 May 2018|archive-url=https://web.archive.org/web/20180515183648/https://news.nationalgeographic.com/news/2007/11/071107-russia-crater.html|url-status=dead}}</ref>

The main points of the study are that:
{{Blockquote|Cheko, a small lake located in Siberia close to the epicentre of the 1908 Tunguska explosion, might fill a crater left by the impact of a fragment of a cosmic body. Sediment cores from the lake's bottom were studied to support or reject this hypothesis. A {{convert|175|cm|in|adj=mid|-long}} core, collected near the center of the lake, consists of an upper c. {{convert|1|m|in|adj=mid|-thick}} sequence of lacustrine deposits overlaying coarser chaotic material. {{chem|210|Pb}} and {{chem|137|Cs}} indicate that the transition from lower to upper sequence occurred close to the time of the Tunguska event. Pollen analysis reveals that remains of aquatic plants are abundant in the top post-1908 sequence but are absent in the lower pre-1908 portion of the core. These results, including organic C, N and δ<sup>13</sup>C data, suggest that Lake Cheko formed at the time of the Tunguska event.

Pollen assemblages confirm the presence of two different units, above and below the ~100‐cm level (Fig. 4). The upper 100‐cm long section, in addition to pollen of taiga forest trees such as Abies, Betula, Juniperus, Larix, Pinus, Picea, and Populus, contains abundant remains of hydrophytes, ''i.e.'', aquatic plants probably deposited under lacustrine conditions similar to those prevailing today. These include both free-floating plants and rooted plants, growing usually in water up to 3–4 metres in depth (Callitriche, Hottonia, Lemna, Hydrocharis, Myriophyllum, Nuphar, Nymphaea, Potamogeton, Sagittaria). In contrast, the lower unit (below ~100 cm) contains abundant forest tree pollen, but no hydrophytes, suggesting that no lake existed then, but a taiga forest growing on marshy ground (Fig. 5). Pollen and microcharcoal show a progressive reduction in the taiga forest, from the bottom of the core upward. This reduction may have been caused by fires (two local episodes below ~100 cm), then by the TE and the formation of the lake (between 100 and 90&nbsp;cm), and again by subsequent fires (one local fire in the upper 40&nbsp;cm).<ref>{{Cite journal|first1=Luca|last1=Gasperini|first2=Enrico|last2=Bonatti|first3=Sonia|last3=Albertazzi|first4=Luisa|last4=Forlani|first5=Carla A.|last5=Accorsi|first6=Giuseppe|last6=Longo|first7=Mariangela|last7=Ravaioli|first8=Francesca|last8=Alvisi|first9=Alina|last9=Polonia |first10=Fabio |last10=Sacchetti|title=Sediments from Lake Cheko (Siberia), a possible impact crater for the 1908 Tunguska Event|journal=[[Terra Nova (journal)|Terra Nova]]|volume=21|number=6|pages=489–494|date=December 2009|doi=10.1111/j.1365-3121.2009.00906.x|bibcode=2009TeNov..21..489G|doi-access=free}}</ref>|sign=|source=|title=}}

In 2017, new research by Russian scientists pointed to a rejection of the theory that the Tunguska event created Lake Cheko. They used soil research to determine that the lake is 280 years old or even much older; in any case clearly older than the Tunguska event.<ref>{{Cite news|url=http://ec-rgo-sfo.com/novosti/1183-ozero-cheko-starshe-tungusskogo-meteorita|script-title=ru:Озеро Чеко Старше Тунгусского Метеорита|trans-title=Lake Cheko is Older than the Tunguska Meteorite|last=Lebedeva|first=Yuliya|access-date=17 January 2018|archive-date=18 January 2018|archive-url=https://web.archive.org/web/20180118064548/http://ec-rgo-sfo.com/novosti/1183-ozero-cheko-starshe-tungusskogo-meteorita|url-status=live}}</ref> In analyzing soils from the bottom of Lake Cheko, they identified a layer of radionuclide contamination from mid-20th century nuclear testing at [[Novaya Zemlya]]. The depth of this layer gave an average annual sedimentation rate of between 3.6 and 4.6&nbsp;mm a year. These sedimentation values are less than half of the 1&nbsp;cm/year calculated by Gasperini ''et al.'' in their 2009 publication on their analysis of the core they took from Lake Cheko in 1999. The Russian scientists in 2017 counted at least 280 such annual [[varves]] in the 1260&nbsp;mm long core sample pulled from the bottom of the lake, representing an age older than the Tunguska event.<ref>{{cite journal|last1=Rogozin|first1=D. Y.|last2=Darin|first2=A. V.|last3=Kalugin|first3=I. A.|last4=Melgunov|first4=M. S.|last5=Meydus|first5=A. V.|last6=Degermendzhi|first6=A. G.|title=Sedimentation rate in Cheko Lake (Evenkia, Siberia): New evidence on the problem of the 1908 Tunguska Event|journal=[[Doklady Earth Sciences]]|date=October 2017|volume=476|issue=2|pages=1226–1228|doi=10.1134/S1028334X17100269|bibcode=2017DokES.476.1226R|s2cid=134128473}}</ref>

Additionally, there are problems with impact physics: It is unlikely that a stony meteorite in the right size range would have the mechanical strength necessary to survive atmospheric passage intact while retaining a velocity high enough to excavate a crater that size on reaching the ground.<ref>{{cite journal|last1=Collins|first1=G.S.|last2=Artemieva|first2=N.|author2-link=Natalia Artemieva|title=Evidence that Lake Cheko is not an impact crater|journal=[[Terra Nova (journal)|Terra Nova]]|year=2008|volume=20|issue=2|pages=165–168|doi=10.1111/j.1365-3121.2008.00791.x|bibcode=2008TeNov..20..165C|s2cid=31459798|doi-access=free}}</ref>

===Geophysical hypotheses===
Though scientific consensus is that the Tunguska explosion was caused by the impact of a small asteroid, there are some dissenters. Astrophysicist [[Wolfgang Kundt]] has proposed that the Tunguska event was caused by the release and subsequent explosion of 10&nbsp;million tons of natural gas from within the Earth's crust.<ref>{{cite journal|last1=Kundt|first1=Wolfgang|title=The 1908 Tunguska catastrophe: An alternative explanation|journal=Current Science|date=2001|volume=81|issue=4|pages=399–407|jstor=24104960}}</ref><ref>{{cite journal|first=N.|last=Jones|title=Did blast from below destroy Tunguska?|journal=[[New Scientist]]|date=7 September 2002|volume=2359|page=14|url=https://www.newscientist.com/article/mg17523591.900-did-blast-from-below-destroy-tunguska.html|access-date=17 September 2017|archive-date=31 May 2015|archive-url=https://web.archive.org/web/20150531221204/http://www.newscientist.com/article/mg17523591.900-did-blast-from-below-destroy-tunguska.html|url-status=live}}</ref><ref>{{cite book|doi=10.1007/978-3-540-32711-0_19|chapter=Tunguska (1908) and Its Relevance for Comet/Asteroid Impact Statistics|title=Comet/Asteroid Impacts and Human Society|date=2007|last1=Kundt|first1=Wolfgang|pages=331–339|isbn=978-3-540-32709-7}}</ref><ref>[http://www.spacedaily.com/2006/080629040419.kk7pcjnx.html "100 years on, mystery shrouds massive 'cosmic impact' in Russia"] {{Webarchive|url=https://web.archive.org/web/20150924104203/http://www.spacedaily.com/2006/080629040419.kk7pcjnx.html |date=24 September 2015 }}, [[Agence France-Presse]], 29 June 2008. Retrieved 8 October 2017.</ref><ref>Choi, Charles Q., [https://www.foxnews.com/story/massive-tunguska-blast-still-unsolved-100-years-later/ "Massive Tunguska Blast Still Unsolved 100 Years Later"], [[Fox News Channel]], 4 July 2008. Retrieved 8 October 2017.</ref> The basic idea is that natural gas leaked out of the crust and then rose to its equal-density height in the atmosphere; from there, it drifted downwind, in a sort of wick, which eventually found an ignition source such as lightning. Once the gas was ignited, the fire streaked along the wick, and then down to the source of the leak in the ground, whereupon there was an explosion.

The similar [[verneshot]] hypothesis has also been proposed as a possible cause of the Tunguska event.<ref>{{cite journal|last1=Phipps Morgan|first1=J|last2=Reston|first2=T.J|last3=Ranero|first3=C.R|title=Contemporaneous mass extinctions, continental flood basalts, and 'impact signals': are mantle plume-induced lithospheric gas explosions the causal link?|journal=Earth and Planetary Science Letters|date=January 2004|volume=217|issue=3–4|pages=263–284|doi=10.1016/s0012-821x(03)00602-2|bibcode=2004E&PSL.217..263P}}</ref><ref>{{cite journal|last1=Vannucchi|first1=Paola|last2=Morgan|first2=Jason P.|last3=Della Lunga|first3=Damiano|last4=Andronicos|first4=Christopher L.|last5=Morgan|first5=W. Jason|title=Direct evidence of ancient shock metamorphism at the site of the 1908 Tunguska event|journal=Earth and Planetary Science Letters|date=January 2015|volume=409|pages=168–174|doi=10.1016/j.epsl.2014.11.001|bibcode=2015E&PSL.409..168V|url=https://zenodo.org/record/894872}}</ref> Other research has proposed a geophysical mechanism for the event.<ref>{{cite journal|last1=Ol'khovatov|first1=A. Yu.|title=Geophysical Circumstances Of The 1908 Tunguska Event In Siberia, Russia|journal=Earth, Moon, and Planets|date=November 2003|volume=93|issue=3|pages=163–173|doi=10.1023/B:MOON.0000047474.85788.01|bibcode=2003EM&P...93..163O|s2cid=122496016}}</ref><ref>{{cite journal|last1=СКУБЛОВ|first1=Г.Т.|last2=МАРИН|first2=Ю.Б.|last3=СКУБЛОВ|first3=С.Г.|last4=БИДЮКОВ|first4=Б.Ф.|last5=ЛОГУНОВА|first5=Л.Н.|last6=ГЕМБИЦКИЙ|first6=В.В.|last7=НЕЧАЕВА|first7=Е.С.|title=ГЕОЛОГИЧЕСКИЕ И МИНЕРАЛОГО-ГЕОХИМИЧЕСКИЕ ОСОБЕННОСТИ РЫХЛЫХ И КОРЕННЫХ ПОРОД ИЗ ЭПИЦЕНТРА ТУНГУССКОЙ КАТАСТРОФЫ 1908 Г|journal=ЗАПИСКИ РОССИЙСКОГО МИНЕРАЛОГИЧЕСКОГО ОБЩЕСТВА|trans-journal=Proceedings of the Russian Mineralogical Society|trans-title=GEOLOGICAL AND MINERALOGICAL-GEOCHEMICAL PECULIARITIES OF LOOSE SEDIMENTS AND PRIMARY ROCKS IN EPICENTER OF TUNGUSSKAYA CATASTROPHE IN 1908|language=ru|date=2010|volume=139|issue=1|pages=111–135|url=http://olkhov.narod.ru/skublov_2010.pdf}}</ref><ref>{{cite journal|last1=СКУБЛОВ|first1=Г.Г.|last2=МАРИН|first2=Ю.Б.|last3=СКУБЛОВ|first3=С.Г.|last4=ЛОГУНОВА|first4=Л.Н.|last5=НЕЧАЕВА|first5=E.С.|last6=САВИЧЕВ|first6=A.A.|title=МИНЕРА-ЛОГО-ГЕОХИМИЧЕСКИЕ ОСОБЕННОСТИ КОРЕННЫХ ПОРОД, РЫХЛЫХ ОТЛОЖЕНИЙ И КАТАСТРОФНЫХ МХОВ УЧАСТКА СЕВЕРНОЕ БОЛОТО (РАЙОН ТУНГУССКОЙ КАТАСТРОФЫ 1908 Г.)|journal=ЗАПИСКИ РОССИЙСКОГО МИНЕРАЛОГИЧЕСКОГО ОБЩЕСТВА|volume=140|issue=3|date=2011|pages=120–138|trans-title=MINERALOGICAL-GEOCHEMICAL FEATURES OF PRIMARY ROCKS, LOOSE SEDIMENTS AND CATASTROPHIC MOSSES IN THE NORTHERN SWAMP AREA (REGION OF THE TUNGUSKA CATASTROPHE IN 1908)|trans-journal=Proceedings of the Russian Mineralogical Society|language=ru|url=http://olkhov.narod.ru/tunguska_skublov_2011.pdf}}</ref>