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Kosmički Hazard - Udar Asteroida Komete Ili Meteorida

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Kosmički Hazard - Udar Asteroida Komete Ili Meteorida KOSMIČKI HAZARD - UDAR ASTEROIDA KOMETE ILI METEORIDA Osnova i koncept poglavlja je urađen na osnovu članka, Drago I. Dragović [email protected] http://static.astronomija.co.rs/suncsist/planete/zemlja/sudar/, sa internet sajta [email protected] uz izmene i dodatke iz drugih izvora (wikipedia i dr.) Zemlja se mnogo puta sudarila sa nekim asteroidom ili kometom. Manji delići neprestano pogađaju gornje slojeve Zemljine atmosfere, gde se usled trenja pretvaraju u paru (najčešće su veličine graška i teže oko grama); svake noći meteori se mogu videti na vedrom nebu. Hiljade meteorita (mase od po nekoliko kilograma) svake godine ipak prodiru kroz atmosferu i bez štetnih posledica padaju na površinu naše planete. U veoma retkim slučajevima meteoriti mogu možda da probiju krov neke kuće, ali nema pouzdanih dokaza da je ikada bilo ljudskih žrtava. Mnogo ozbiljniji događaj zbio se 1908. godine, kada se jedan kosmički objekat raspao u atmosferi iznad Tunguske, u Sibiru, oslobodivši energiju veću od 10 megatona klasičnog eksploziva TNT–a. Ali čak i ovaj sibirski objekat samo je jedan od manjih članova iz Zemljinog susedstva. Pravi razlog za brigu su veliki objekti, veličine od najmanje 1 kilometra. Mada vrlo retki, sudari sa takvim objektom mogu da se ozbiljno odraze na ekosistem naše planete i izazovu masovna uništenja flore i faune. Jednim imenom nazivaju se NEO (Near Earth Objects) – objekti koji se kreću u blizini Zemlje. 3.1 Asteroidi Asteroidi (grč. ἀστήρειδής, asteroides, zvezdolik) ili planetoidi su mala čvrsta tela u planetarnim sistemima. U poređenju s planetama i patuljastim planetama, manji su i najčešće nepravilnog oblika. Nastali su od ostataka protoplanetarne materije koja se nije pripojila planetama za vreme formiranja sistema iz protoplanetarnog diska ili kasnije dinamičkom evolucijom kometa ili nekih drugih objekata. Najčešće kruže oko matične zvezde vlastitom putanjom ili kao sateliti većih planeta. Neke od njih nalazimo vezane gravitacionim silama uz planete, u grupama koje orbitiraju u putanji planeta, ispred ili iza. Iako se do nedavno mislilo drugačije, otkriveno je da asteroidi mogu imati vlastite satelite. Većina asteroida u Sunčevom sistemu nalazi se u asteroidnom pojasu između Marsa i Jupitera, te u Kajperovom pojasu. Do sada ih je otkriveno preko 170.000, a preko 11.000 ih je dobilo vlastita imena, pored kataloškog broja. Procenjuje se da bi ih u našem sistemu moglo biti nekoliko miliona. U početku su asteroidi bili podeljeni u tri grupe prema sastavu površinskog materijala, odnosno svojstvima površine: boji, albedu (koeficijentu refleksije) i spektralnom tipu. Broj grupa u ovoj podeli raste s otkrićima novih asteroida i trenutno ih ima 14. parsek.com.ua apod.nasa.gov Slika 3.1. i slika 3.2: Umetničko viđenje asteroida na osnovu postojećih podataka i fotografija 18 3.2 Komete Kometa je nebesko telo koje se nalazi u putanji oko Sunca. Komete se sastoje od prašine, smrznutih gasova, kamena i leda. Orbite kometa se stalno menjaju: originalno su bile u spoljnom Sunčevom sistemu i sklone su poremećajima prilikom bliskih prolazaka pored velikih planeta. Neke od njih prilaze blizu Suncu i isparavaju, dok neke bivaju zauvek izbačene iz njega. Karakteriše ih iznenadna pojava na nebu i rep koji višestruko nadmašuje dimenzije samog jezgra komete. Reč “kometa” je nastala od latinske reči cometes. Od grčke reči komē, koja znači “kosa na glavi”, Aristotel je prvi izveo comētēs da opiše komete kao “zvezde sa kosom.” Astronomski simbol za komete je disk sa repom. Komete su jedan od mogućih izvora vode na Zemlji. Mlazovi prašine i gasa oslobođeni na taj način formiraju veliku, ali retku atmosferu oko komete zvanu koma, a Sunčevo zračenje i Sunčev vetar prave od njega karakteristični rep koji je vidljiv na nebu i uvek je okrenut suprotno od Sunca. Gasovi i prašina takođe formiraju i svoj tzv. prašinasti rep, koji ostaje u kometinoj orbiti u obliku zakrivljenog repa. Dok je telo komete (jezgro) obično manjeg prečnika do 50 km, koma može dostići veliki prečnik, dok rep komete može biti duži i od jedne astronomske jedinice (150 miliona km). Oba repa komete bivaju osvetljena Suncem i mogu biti vidljiva sa Zemlje kada kometa prođe kroz unutrašnji Sunčev sistem. Najveći broj kometa je previše slabog sjaja da bi se video bez pomoći teleskopa, ali nekoliko njih svake decenije bude dovoljno sjajno da se vide golim okom. Pre pronalaska teleskopa izgledalo je da se komete pojavljuju niotkuda i nestaju iz vida. Obično su smatrane lošim znacima smrti kraljeva i plemića, ili naznakama skorih katastrofa. Iz antičkih izvora je poznato da su bile primećivane milenijumima. www.thunderbolts.info/.../060227comet creationscience.com Slika 3.3 i slika 3.4: Fotografije kometa 3.3 Meteori – metroidi – meteoriti Meteoroid je malo nebesko telo, tj. stenoviti ili metalni fragmenat relativno malih dimenzija. Dimenzije variraju od čestice prašine do većih komada stena. Na osnovu klasifikacije koju je usvojila Međunarodna astronomska unija 1961. godine njihova masa varira od 109 kg do 107 kg. Znači, meteoroid može biti čestica prašine, deo stene ili slični ostatak iz vremena nastajanja Sunčevog sistema koji kruži oko Sunca. Meteoridi najmanjih dimenzija se nazivaju mikrometeoridi. 19 Takve čestice nastaju i pri sudarima asteroida ili isparavanjem materijala sa kometa (zaleđeni gas se topi pod uticajem Sunčeve toplote i oslobađa zarobljene komade stena i prašine). Ako takvo telo uleti velikom brzinom u atmosferu planete, zagreva se, gori i isparava zbog ogromnog trenja, ostavljajući svetli za sobom trag. Ova pojava se naziva meteor. Dovoljno veliki meteoroidi ne uspevaju celi da sagorete u atmosferi i jedan njihov deo može pasti na zemlju. Takvi kameno-metalni ostaci zovu se meteoriti. NASA/Perseid rst.gsfc.nasa.gov Slika 3.5: Fotografija zvezde padalice Slika 3.6: Meteorit wikipedia wikipedia Slika 3.7: Šematski prikaz asteroidnog oblaka u Slika 3.8: Umetničko viđenje asteroidnog oblaka i sunčevom sistemu Sunčevom sistemu Pojas asteroida - je prostrani pojas stenovitih nebeskih tela, (asteroida (planetoida), meteoroida) različite veličine, čije se orbite oko Sunca nalaze uglavnom između orbita planeta Mars i Jupiter. Udaljenosti asteroida u pojasu se kreću između 1,7 AJ i 4 AJ. Većina asteroida u pojasu imaju ekscentricitete od 0,1 do 0,2. U samom pojasu područje sa najvećom gustinom putanja asteroida je između 2,2 AJ i 3,3 AJ. Većina meteorita koji padnu na Zemlju dolazi baš iz ovog dela Sunčevog sistema. 3.4 Udar asteroida ili komete – prirodni hazard Svakih nekoliko vekova Zemlju je pogađao NEO dovoljno veliki da izazove smrt hiljada ljudskih bića, odnosno stotine hiljada da je bio pogođen urbani deo. Ako gledamo milenijumsku skalu vremena, moguće je očekivati katastrofu uporedivu sa onima najvećim poznatim u Zemljinoj istoriji (Pike, 1991). Tokom našeg života postoji mala ali ne i nulta šansa (recimo, 1 prema 10.000) da Zemlja bude pogođena objektom toliko velikim da u globalnom smislu uništi izvore hrane i izazove kraj civilizacije kakvu danas poznamo (Shoemaker i drugi, 1990). 20 Mali objekti, kakvi su obični meteori, utroše svoju energiju u gornjim slojevima atmosfere i nemaju nikakvog direktnog uticaja na tlo ispod njih. Jedino u slučaju kada je projektil veći od 10 m, počinje da bude pretnja za ljude. U zavisnosti od veličine ili kinetičke energije koju poseduju, rizike možemo podeliti u tri kategorije: 1. Generalno uzevši, strano telo i nestaje pre sudara sa površinom, predavši svoju energiju atmosferi i izazivajući samo lokalne efekte. 2. Telo dospeva do tla dovoljno jako da izazove krater, takođe izazivajući samo lokalne efekte, mada azotni oksidi i prašina mogu da pokriju veliku površinu, a i da izazovu pojavu talasa cunamija ako padne u more. 3. Udar koji bi stvorio veliki krater proizveo bi količinu prašine dovoljnu da izazove kratkotrajnu globalnu promenu klime, praćeno sa razarajućim efektima u regionu samog udara. Granična veličina za svaku od navedenih kategorija projektila zavisi od njihove gustine, snage i brzine kao i od prirode mete. Prag dovoljan za globalnu katastrofu nije, međutim, precizno određen. Kategorija 1: Telo prečnika 10 – 100 metara Tela veličine probližno maksimalnoj navedenoj pogađaju Zemlju svake decenije, dok tela od 100 m i veća pogađaju Zemlju, u proseku, nekoliko puta u milenijumu. Kinetička energija jednog 10–metarskog projektila koji putuje atmosferom tipičnom brzinom od oko 20 km/sek adekvatna je količini od oko 100 kilotona TNT, što je jednako snazi nekoliko bombi bačenih na Hirošimu, dok je kinetička energija jednog 100–metarskog tela jednaka eksploziji oko 100 megatona TNT, što je jednako snazi najvećih hidrogenskih bombi. 10–metarski projektili, posle probijanja kroz atmosferu samo retki gvozdeni ili kameno–gvozdeni poseduju dovoljan deo početne energije da bi načinili krater na zemlji, kao što je npr. to bio slučaj u regionu Sikhote–Alin, u Sibiru 1947. U Tunguskoj 1908. godine, se telo od oko 60 m u prečniku rasprslo u atmosferi na oko 8 km visine. Oslobođena energija je iznosila oko 12 megatona, kako je utvrđeno prema udarnom talasu registrovanom u meteorološkim barografima čak u Engleskoj ili oko 20 megatona, ako se uzme u obzir prečnik razaranja. Stabla sibirske šume su bila potpuno oborena u prečniku od oko 20 km od zamišljene krajnje tačke putanje projektila, dok su neka bila isčupana ili oborena i na udaljenosti većoj od 40 km. Sigurni dokazi sugerišu da je požar izbio u krugu od 15 km od centra, izazvan intenzivnim jurišem energije. Uočeni efekti su jako slični onima koji bi se očekivali od
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