1908 7.14 a.m., 30 June. A fireball explodes in mid-air near the Stony Tunguska River in Siberia and flattens a vast forest. Nearest eyewitnesses are 70 kilometres from the explosion site, but villagers as far as 700 kilometres away see bright lights in the sky and villagers as far as 1,200 kilometres away hear loud explosions. Siberian newspapers report the explosion, but they are not sure of its nature; some suggest it to be a meteorite. After the explosion, bright 'night glows' are observed in parts of Europe and Asia. This unusual phenomenon is widely reported in newspapers in Britain, Europe and the United States, but no one knows its cause. An observatory 970 kilometres from the explosion site records a magnetic storm that began a few minutes after the blast and lasted for about four hours. Seismic waves are recorded around the world. Six microbarographs in England record air waves created by the blast.
1910 The first expedition to the Tunguska site by a non-Evenki person. A wealthy Russian merchant and goldsmith named Suzdalev is rumoured to have discovered diamonds at the site.
1921 September. Nothing is heard of the explosion until Leonid Kulik, a Russian scientist, is assigned the task of locating and examining meteorites fallen in inhabited regions of Russia before and after the First World War. During his expedition to Siberia, Kulik learns of a meteorite that had fallen near the Stony Tunguska River. The expedition ended without him visiting the explosion site.
1924 S.V. Obruchev, a Soviet geologist, conducts geological studies in the Tunguska region (but not the explosion site).
1925 A.V. Voznesensky, Director of the Irkutsk Magnetic and Meteorological Observatory in 1908, claims that the seismic and air waves recorded by his observatory on 30 June 1908 were both caused by the fall of a giant meteorite.
1926 I.M. Suslov, a Soviet ethnographer, visits Tunguska region. (The famous Suslov crater is named after him.)
1927 21 May. The first Tunguska expedition, led by Kulik, reaches the epicentre of the explosion. Kulik becomes the first scientist to visit the site.
1928 June. Kulik's second expedition reaches the explosion site. Kulik's expeditions are widely reported in British and American newspapers.
1929 Kulik's third expedition. A British scientist notices the coincidence of the date of the Tunguska explosion and the airwaves recorded in England on 30 June 1908.
1930 Other British scientists suggest that the airwaves recorded in England and the remarkable night glows of 1908 were caused by the Tunguska meteorite.
1934 The British scientist F.J.W. Whipple and the Russian scientist I.S. Astapowitsch independently propose that the Tunguska object was a comet. (Astapowitsch, in fact, expanded the idea suggested by Academician Vladimir Vernadsky.)
1938 The first aerial photographic survey of the Tun-guska region is undertaken.
1939 Kulik's fourth expedition to Tunguska. The last expedition before the start of the Second World War.
1941 The American meteorite expert Lincoln La Paz suggests that the Tunguska object was a contra-terrene (anti-matter) meteorite.
1942 14 April. Kulik dies in a German prisoner-of-war hospital.
1946 The Russian science-fiction writer Alexander Kazantsev publishes a story suggesting that the Tunguska object was an alien spaceship.
1957 The Russian mineralogist A.A. Yavnel microscopically analyses soil samples brought back by Kulik in 1929 and 1930. These samples were later proved to be of terrestrial origin.
1958 30 June. The Soviet Union releases a commemorative 40-kopeck stamp featuring a portrait of Kulik, to celebrate the 50th anniversary of the Tunguska event. The fifth expedition, the first after the Second World War, led by Kirill P. Florenskiy. The Interdisciplinary Independent Tunguska Expedition (IITE, known as KSE in Russian) is formed in the Siberian city of Tomsk to discount spaceship theories.
1960 Florenskiy supports the comet theory in an article in the US magazine Sky & Telescope. Academician Vassilii Fesenkov also presents arguments in favour of the comet theory to The New York Times. The Soviet Academy of Sciences builds a simple memorial on Kulik's grave in the town of Spas-Demensk, about 300 kilometres southwest of Moscow.
1961 Florenskiy leads the sixth expedition which continues into 1962.
1964 The Russian science-fiction writers Genrikh Altov and Valentina Zhuravleva suggest that Tunguska was zapped by a laser beam sent by ETs.
1965 The American scientists Willard Libby, Clyde Cowan and C.R. Alturi present a detailed theory showing that the Tunguska object was made of antimatter.
1959 Feliks Zigel, so-called 'Father of Soviet UFOlogy', suggests that the Tunguska object was a UFO.
1966 The English translation of E.L. Krinov's book Giant Meteorites is published. Its 141-page section 'The Tunguska Meteorite' has an authoritative account of early research on Tunguska.
1973 The American theoretical physicists A.A. Jackson IV and Michael P. Ryan, Jr say that the Tunguska object was a mini black hole, which passed through Earth and exited through the North Atlantic ocean.
1976 The first Tunguska book in English is published: The Fire Came By: The Riddle of the Great Siberian Explosion by John Baxter and Thomas Atkins.
1975 The Israeli scientist Ari Ben-Menahem concludes that the explosion took place 8.5 kilometres above the ground and had energy of about 12.5 megatons of TNT.
1977 The British scientist Anthony Lawton suggests that the Tunguska fireball was in fact a giant lightning ball.
1978 The Slovak astronomer Lubar Krésak suggests that a piece of comet Encke had exploded at Tunguska.
1983 The American scientist Zdenek Sekanina proposes that the explosion was caused by a stony asteroid. The American scientist Richard Turco suggests that the bright nights were caused by noctilucent clouds produced by the dust that reached the stratosphere. The American chemist Ramachandran Ganapathy says that the globules collected by Florenskiy's 1961-62 expedition are enriched in iridium, a metal that is abundant in extra-terrestrial bodies, and contain other evidence of extra-terrestrial origin. He also discovers traces of the Tunguska fireball in an Antarctic ice core.
1984 The Russian scientists Victor Zhuravlev and A.N. Dmitriev present their plasmoid hypothesis.
1989 The first post-Cold War expedition open to international scientists.
1991 The first Italian expedition, led by Menotti Galli and Giuseppe Longo. The expedition collects particles from resin in Tunguska trees. The particles contain some elements which are commonly associated with stony asteroids. The Russian scientist Andrei Ol'khovatov publishes his 'geometeor'
theory. Geometeors are meteor-like luminous objects but of terrestrial origin.
1993 American scientists Christopher Chyba, Kevin Zahnle and Paul Thomas give the asteroid theory new weight and rigour. They say that the explosion released about 15 megatons of energy in the atmosphere at an altitude of about 8 kilometres.
1994 An unknown American writer suggests that the explosion was caused by a Nikola Tesla experiment on a death ray which got out of hand.
1996 The Russian scientist Vladimir Svetsov shows that the entire mass of the Tunguska object vaporised before it could reach the ground. Ablation of the Tunguska debris was total.
1998 Sekanina revisits his asteroid theory and presents new arguments in favour of it. The Russian scientist Vladimir Alekseev suggests that the flight of the object ended in multiple explosions which were responsible for gunfire-like sounds heard by eyewitnesses.
2001 Academician Nikolai Vasilyev, who coordinated the scientific research of 29 Tunguska investigations from 1963 to 2001, dies. A team of Italian scientists, based on an idea of the late Paolo Farinella (1953-2000), calculates 886 valid orbits of the object, of which 83 per cent are asteroid orbits and 17 per cent comet orbits. The German astrophysicist Wolfgang Kundt suggests that the explosion was caused by 10 million tonnes of methane gas which escaped from a volcanic vent.
2002 The Australian physicist Robert Foot suggests that timeline: one hundred years of an enigma the Tunguska blast was caused by a mirror matter asteroid.
2004 Dr Vitalii Bronshten, a well-known Tunguska researcher and the main supporter of the comet theory, dies.
2008 As we prepare to celebrate the 100th anniversary of the Tunguska event, there is still no final answer to the question: what really caused the explosion?
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