Editing Tunguska event (section)

===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>