Tunguska 2
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It was nothing of this earth, but a piece of the great outside; and as such dowered with outside properties and obedient to outside laws. -The Colour Out of Space by H.P. Lovecraft

At 7:15 on the morning of June 30, 1908, something happened in the sky above the Stony Tunguska (Podkamennaya Tunguska) river in Siberia. Many thousand people in a radius of 900 miles observed the Tunguska event and more than 700 accounts were collected later. The reports describe a fireball in the sky, larger or similar to the size of the sun, a series of explosions "with a frightful sound", followed by shaking of the ground as "the earth seemed to get opened wide and everything would fall in the abyss. Terrible strokes were heard from somewhere, which shook the air []." The indigenous Evenks and Yakuts believed a god or shaman had sent the fireball to destroy the world. Various meteorological stations in Europe recorded both seismic and atmospheric waves. Days later strange phenomena were observed in the sky of Russia and Europe, such as glowing clouds, colorful sunsets and a strange luminescence in the night.

Russian newspapers reported a supposed meteorite impact. International newspapers speculated about a possible volcanic explosion, as similar strange luminous effects were observed also after the 1883 eruption of Krakatoa. Unfortunately, the inaccessibility of the region and Russia's unstable political situation at the time prevented any further scientific investigations.

Thirteen years later Russian mineralogist Leonid Alexejewitsch Kulik of the Russian Meteorological Institute became interested in the story after reading some newspaper articles. He believed that a large meteorite fell from the sky and he hoped to recover some extraterrestrial metals from the impact site. Kulik traveled to the city of Kansk, where he studied reports about the event in the local archives. In March of 1927, he arrived at the remote outpost of Wanawara. Then on April 13, Kulik discovered a large area covered with rotting logs. A huge explosion apparently flattened more than 80 million trees across 820 square miles. Only at the epicenter of the explosion, in the "Forest of Tunguska", so called "telegraph pole trees" were still standing, with all branches and the bark burned off.

Despite exploring the entire area, Kulik and his team didn't locate a single large crater, but he did find some circular pits that were interpreted as being craters produced by small fragments. However, no meteoritic material was ever discovered at the site. Kulik suggested that an extraterrestrial body exploded in the atmosphere, causing the observed fireball and devastation. The fragments became buried in the swampy ground, which was too soft to preserve the typical morphology of an impact crater.

Despite its notoriety in pop-culture, scientific data covering this event is sparse. There are seismic and barometric registrations, recorded immediately after the event, and data on forest devastation, collected 30 to 50 years later. Based on the lack of hard data, like a crater or extraterrestrial material, and conflicting accounts of eyewitnesses many theories of widely varying plausibility were proposed over the years.

Other Theories

In 1934 Soviet scientists proposed a variation of Kulik's hypothesis. They suggested that it was a comet, not a meteorite, that struck the area. Since comets are composed mostly of ice, one would have been completely vaporized during the impact, leaving no traces behind.

After the first atomic bombs were detonated in 1945, engineer and sci-fi writer Aleksander Kasantsews developed an unusual explanation involving a nuclear explosion of possible extraterrestrial origin as the cause of the Tunguska event. Apart from the pattern of destruction in the aftermath of Hiroshima and Nagasaki, so Kasantsews, also geomagnetic disturbances recorded at the station of Irkutsk were similar to a nuclear blast.

In 1973, American physicists proposed in the journal Nature that a small black hole had collided with earth, causing some sort of matter-antimatter reaction in earth's atmosphere and resulting in the huge explosion observed over the Tunguska.

In recent years the German astrophysicist Wolfgang Kundt and later Jason Phipps Morgan of the Cornell University in Ithaca and Paola Vannucchi from the University of Florence have proposed a terrestrial explanation for the Tunguska explosion. Verneshots, named after French novelist Jules Verne and author of A Journey to the Center of the Earth, are a magma/gas mix that violently erupts from the underground. According to this model, a magmatic intrusion in the underground of Siberia formed a large bubble of volcanic gases, trapped below the basalt-layers of the Siberian Traps. Finally, in June 1908, the covering rocks were shattered by the compressed gases and a burst of burning methane and other flammable gases caused the explosions as described in eyewitness accounts. The chemical residuals dispersed in earth's atmosphere by this explosion caused the glowing clouds seen all over the world. Bubbles of gas were observed in the lakes of Siberia, but the methane comes from rotting organic material buried in the periodically frozen soil of the Taiga. Geologists mapping the area also found no traces of shattered rocks or craters formed by the escaping gases.

The most compelling explanation for the Tunguska event remains a cosmic body entering earth's atmosphere. This idea is supported by the reports describing a fireball descending on the tundra, the presence of impact-related minerals like nanodiamonds, metallic- and silicate spherules in sediments, and the mapped distribution and direction of the flattened trees, pointing away from the explosion site. The nature of this cosmic body remains unclear. Based on the energy of the explosion (estimated in 10-15 megatons of TNT) scientists propose either a 2,700 ft in diameter large comet or a 100 to 300 ft large meteorite exploding at an elevation of 3 to 6 miles above ground. However, there are also some problems with the idea that the Tunguska event was of extraterrestrial origin. Some accounts describing a series of explosions lasting more than ten minutes are hard to explain with an impact. The recovered geological evidence can also be explained by background sedimentation of cosmic dust, as many small meteorites are disintegrated every day in earth's atmosphere. In 2007, Luca Gasperini and his research team of the University of Bologna proposed that a small lake located nearby, Lake Cheko, may have formed by the impact of a fragment of the Tunguska meteorite. Lake Cheko is unusually deep for a region characterized otherwise by shallow ponds, formed by melting permafrost. There's also no record of the lake existing before 1908, but it's also true that the region was poorly mapped and explored at the time. Gasperini's proposed evidence remains controversial.

In 1998, very few felled trees remained and also few stumps of telegraph pole trees or burned trees survive until today. Seen from above, no evidence whatsoever remains. Tunguska represents a category of impactors for which we have no cratering record, slightly smaller than Meteor Crater but larger than recent meteoroids. It is difficult to estimate the recurrence interval for a Tunguska sized impact, as we don't really know the nature of the cosmic body causing the event, but it has been estimated to be as frequent as every 100 to 1,000 years. Regardless of what exactly happened, the Tunguska explosion shows our vulnerability towards impact events. If the Tunguska cosmic body would have reached earth just four hours later, instead of hitting the swamps of Siberia, it could have impacted the Russian capital of St. Petersburg.