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TungusExplosionTheTunguskaevent,orTunguskaexplosion,wasanenormouslypowerfulexplosionthatoccurrednearthePodkamennaya(LowerStony)TunguskaRiverinwhatisnowKrasnoyarskKraiinRussiaabout7:14A.M.(0:14UniversalTime)onJune30,1908(June17intheJuliancalendar,inuselocallyatthetime).Theexplosionisbelievedtohavebeencausedbytheairburstofalargemeteoroidorcometfragmentatanaltitudeof5–10kilometres(3.1–6.2mi)abovetheEarth'ssurface.Differentstudieshaveyieldedvaryingestimatesoftheobject'ssize,withgeneralagreementthatitwasafewtensofmetresacrossThenumberofscholarlypublicationsontheproblemoftheTunguskaexplosionsince1908maybeestimatedatabout1,000(mainlyinRussian).ManyscientistshaveparticipatedinTunguskastudiesAlthoughthemeteoroidorcometburstintheairratherthanhittingthesurface,thiseventisstillreferredtoasanimpact.Estimatesoftheenergyoftheblastrangefrom5toashighas30megatonsofTNT(21–130PJ,with10–15megatonsofTNT(42–63PJ)themostlikely—roughlyequaltotheUnitedStates'CastleBravothermonuclearbombtestedonMarch1,1954,about1,000timesaspowerfulastheatomicbombdroppedonHiroshima,Japan,andaboutone-thirdthepoweroftheTsarBomba,thelargestnuclearweaponeverdetonated.Theexplosionknockedoveranestimated80milliontreescovering2,150squarekilometres(830sqmi).Itisestimatedthattheshockwavefromtheblastwouldhavemeasured5.0ontheRichterscale.Anexplosionofthismagnitudeiscapableofdestroyingalargemetropolitanarea.Thispossibilityhashelpedtosparkdiscussionofasteroiddeflectionstrategies.TheTunguskaeventisthelargestimpacteventoverlandinEarth'srecenthistory.Impactsofsimilarsizeoverremoteoceanareaswouldhavegoneunnoticedbeforetheadventofglobalsatellitemonitoringinthe1960sand1970s.DescriptionApproximatelocationoftheTunguskaeventinSiberiaAtaround7:17a.m.localtime,TungusnativesandRussiansettlersinthehillsnorthwestofLakeBaikalobservedacolumnofbluishlight,nearlyasbrightastheSun,movingacrossthesky.About10minuteslater,therewasaflashandasoundsimilartoartilleryfire.Eyewitnessesclosertotheexplosionreportedthesoundsourcemovingeasttonorth.Thesoundswereaccompaniedbyashockwavethatknockedpeopleofftheirfeetandbrokewindowshundredsofkilometresaway.Themajorityofwitnessesreportedonlythesoundsandthetremors,andnotthesightingoftheexplosion.Eyewitnessaccountsdifferastothesequenceofeventsandtheiroverallduration.TheexplosionregisteredonseismicstationsacrossEurasia.Insomeplacestheshockwavewouldhavebeenequivalenttoanearthquakeof5.0ontheRichterscale.[12]ItalsoproducedfluctuationsinatmosphericpressurestrongenoughtobedetectedinGreatBritain.Overthenextfewdays,nightskiesinAsiaandEuropewereaglow;[13]ithasbeentheorizedthatthiswasduetolightpassingthroughhigh-altitudeiceparticlesformedatextremelycoldtemperatures,aphenomenonthatoccurswhentheSpaceShuttlere-enterstheEarth'satmosphere.[14][15]IntheUnitedStates,theSmithsonianAstrophysicalObservatoryandtheMountWilsonObservatoryobservedadecreaseinatmospherictransparencythatlastedforseveralmonths,fromsuspendeddust.Earthimpactormodel[edit]MeteoroidairburstTheleadingscientificexplanationfortheexplosionistheairburstofameteoroid6–10kilometres(4–6miles)aboveEarth'ssurface.[citationneeded]MeteoroidsenterEarth'satmospherefromouterspaceeveryday,usuallytravellingataspeedofmorethan10kilometrespersecond(6miles/secor21,600mph).Theheatgeneratedbycompressionofairinfrontofthebody(rampressure)asittravelsthroughtheatmosphereisimmenseandmostmeteoroidsburnuporexplodebeforetheyreachtheground.Sincethesecondhalfofthe20thcentury,closemonitoringofEarth'satmospherehasledtothediscoverythatsuchmeteoroidairburstsoccurratherfrequently.Astonymeteoroidofabout10metres(30ft)indiametercanproduceanexplosionofaround20kilotons,similartothatoftheFatManbombdroppedonNagasaki,anddatareleasedbytheU.S.AirForce'sDefenseSupportProgramindicatethatsuchexplosionsoccurhighintheupperatmospheremorethanonceayear.Tunguska-likemegaton-rangeeventsaremuchrarer.EugeneShoemakerestimatedthatsucheventsoccuraboutonceevery300years.[30][31]BlastpatternsTheexplosion'seffectonthetreesneargroundzerowasreplicatedduringatmosphericnucleartestsinthe1950sand1960s.Theseeffectsarecausedbytheshockwaveproducedbylargeexplosions.Thetreesdirectlybelowtheexplosionarestrippedastheblastwavemovesverticallydownward,whiletreesfurtherawayareknockedoverbecausetheblastwaveistravellingclosertothehorizontalwhenitreachesthem.Sovietexperimentsperformedinthemid-1960s,withmodelforests(madeofmatchesonwirestakes)andsmallexplosivechargessliddownwardonwires,producedbutterfly-shapedblastpatternsstrikinglysimilartothepatternfoundattheTunguskasite.Theexperimentssuggestedthattheobjecthadapproachedatanangleofroughly30degreesfromthegroundand115degreesfromnorthandhadexplodedinmid-air.[citationneeded]SpeculativehypothesesThebehaviourofmeteoritesintheEarth'satmospherewaslesswellunderstoodduringtheearlydecadesofthe20thcentury.Duetothis,aswellasthepaucityofrelevantdataresultingfromSovietsecrecyduringtheColdWar,agreatmanyotherhypothesesfortheTunguskaeventhavesprungup,noneofwhichareacceptedbythescientificcommunity.•Comet2005NB56•Endoftheworld?•NaturalH-bomb•Blackhole•Antimatter•