The Tunguska Explosion

On the morning of June 30, 1908, a giant bright blue fireball blazed through Earth’s atmosphere before finally exploding at a height of 5 miles (8 kilometers) in the sky above Tunguska, Siberia. The effects of the explosion were felt for 600 miles in all directions, windows in houses shattering, horses and people being knocked unconscious and trains coming to a halt for fear of derailment. Closer near the epicenter, trees were bowled over, all animal life was extinguished and a mysterious black rain fell from the skies.

Seismic waves were recorded In Europe and America, while the dust and debris injected into the atmosphere caused eerie effects across Europe, Russia and western Siberia, nights so bright it was possible to read indoors. Later it was found that atmospheric shock waves had circled the Earth twice, scientists determining that the event whatever its cause had released energy equal to 12 megatons of TNT.

Due to political instability and the isolation of the impact site an expedition to investigate the devastated area didn’t arrive until 1927 when a Soviet mineralogist named Leonid Kulik (who had an interest in fallen meteorites) arrived on the scene. Standing on a ridge he surveyed the blast area and wrote: “From our observation point no sign of forest can be seen, for everything has been devastated and burned. . . . One has an uncanny feeling when one sees 20 to 30 inch (50 to 75 centimeters) thick giant trees snapped across like twigs and their tops hurled many meters away to the south.”

Owing to problems with his superstitious guides, another month passed before Kulik finally reached the middle of the fall and found not the impact crater expected but instead a frozen swamp in the center of which was a copse of trees, unaffected, despite clearly being at the blast's focal point. Whatever had caused the blast had obviously occurred far above the ground and he found no trace of the large meteoric iron fragments for which he was searching.

So if the Tunguska explosion wasn’t caused by a meteor or small asteroid, what did cause it? There are two prevailing theories currently in vogue. One held by UFO aficionados speculates that it was an alien spacecraft burning up in the atmosphere, its engines erupting in an uncontrolled nuclear detonation. The other theory is that it was either a fragment or the head of a small comet.

Evidence for the alien spacecraft and a subsequent nuclear explosion seem sketchy at best. Claims that scabs had broken out on reindeer were cited as evidence of radiation but in view of the fact that absolutely none of the humans in the vicinity showed any signs of radiation sickness, it can only be surmised that the scabs were in fact the result of burns caused by the initial flash of heat given off by the blast.

A one percent increase in radiocarbon in the rings of trees growing in the United States during the period 1908 to 1909 seemed at first to corroborate the nuclear theory but in fact fluctuations of two percent are not uncommon and a double check on a Norwegian tree (a good deal nearer the explosion) revealed much lower radiocarbon levels during 1909.

Significant clues pertaining to the second theory were unearthed by Soviet geochemist Kirill Florensky who led expeditions to the area in 1958, 1961 and 1962. Instead of looking for large meteoric fragments Florensky’s expedition searched for microscopic particles. Their search was successful, revealing a narrow corridor of cosmic dust stretching for 150 miles (240 kilometers) northwest of the site. The dust was composed of magnetite (magnetic iron oxide) and droplets of fused rock (a low density stony composition containing flecks of iron believed to be typical of interplanetary debris found in comet’s tails). Florensky declared the mystery solved, as far as he was concerned the Tunguska blast was caused by a comet’s head vaporizing low in the Earth’s atmosphere.

Evidence confirming the cometary nature of the Tunguska object comes from further Soviet studies reported in 1977. After finding microscopic rock and iron particles in the 1908 peat layer and thousands of tonnes more scattered throughout the fall area researchers concluded that the Tunguska object was a comet of carbonaceous composition (again typical of interplanetary debris). [1]

Estimates are that events such as the one at Tunguska occur on average once every two thousand years.

[1] Many astronomers now believe that the Tunguska object was a fragment broken off Comet Encke.

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