The Tunguska meteorite impact
30th June, 1908
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| Felled trees photographed in 1938 |
At 7.14 a.m. on 30 June 1908 a massive pale blue fireball swept out of the sky and exploded high above the Tunguska River valley in Siberia.
70 kilometres away, in the town of Vanavara, people were knocked to the ground by the force of the blast.
One of these, Farmer Sergei Semenov, recalls:
I was sitting in the porch of the house at the trading station of Vanovara at breakfast time and looking towards the north . . . suddenly the sky was split in two, and high above the forest the whole northern part of the sky appeared to be covered with fire. At that moment I felt a great heat as if my shirt had caught fire . . . there was a bang in the sky, and a mighty crash was heard. I was thrown to the ground about 5-6 meters away from the porch and for a moment I lost consciousness.
At 7.18 a.m. seismic tremors caused by the blast were picked up 893 kilometres away at the Irkutsk Magnetic and Meteorological Station. About 45 minutes later the same meteorological station recorded the passage of the blast wave.
Spreading outwards across Europe the blast wave reached Potsdam, near Berlin, at 5.54 a.m., local time. At short while later it was picked up at the South Kensington Meteorological Office and a number of other weather stations across England.
The following night was exceptionally bright throughout Europe and western Siberia, and continued to be so for several weeks.
A letter to the London Times describes this atmospheric phenomenon:
Sir, Struck with the unusual brightness in the heavens, the band of golfers staying here strolled towards the links at 11 o’clock last evening in order that they might obtain an uninterrupted view of the phenomenon. Looking northwards across the sea they found that the sky had the appearance of a dying sunset of exquisite beauty. This not only lasted but actually grew both in extent and intensity till 2.30 this morning, when driving clouds from the East obliterated the gorgeous colouring. I myself was aroused from sleep at 1.15, and so strong was the light at this hour that I could read a book by it in my chamber quite comfortably. At 1.45 the whole sky, N and N.E., was a delicate salmon pink, and the birds began their matutinal song. No doubt others will have noticed this phenomenon, but as Brancaster holds an almost unique position in facing north to the sea, we who are staying here had the best possible view of it.
The Tunguska explosion levelled over 2,000 square kilometres of Siberian forest, an area equivalent to Greater London.
An expedition led by Professor Leonid Kulik of the Soviet Academy of Science travelled to the impact site in 1930. Even 22 years after the event the devastation was immense.
The professor recorded:
From our observation point no sign of forest can be seen, for everything has been devastated and burned, and around the edge of the dead area the young twenty-year-old forest growth has moved forward furiously, seeking sunshine and life. One has an uncanny feeling when one sees giant trees snapped across like twigs, and their tops hurled many metres to the south.
Kulik and his team surveyed the area chronicling the devastation. Within 30-40 kilometres of ground zero almost all of the trees had been uprooted and blown flat. Some of those sheltered by hills remained standing. Trees within 15-18 kilometres had been scorched black on the side facing the explosion. The outer limit of the tree damage was 52 kilometres. Subsequent surveys showed the area of destruction to be butterfly-shaped.
Kulik surmised that a meteorite had been the cause of the destruction but could find no trace of a crater. Thinking that he would find fragments of the object buried deep in the soil, he began extensive excavations of a marshy area that he believed to be ‘ground-zero’. No trace was found however.
Scientists now believe that the object was reduced to dust by its terminal explosion. This dust was ejected high into the atmosphere and spread around the globe by stratospheric winds.
The bright nights, documented at the time across Europe, were caused by the dust reflecting Sunlight from below the horizon.
Debate continues as to whether the object was a comet or an asteroid. Comet theorists explain away the presence of large amounts of atmospheric dust as coming from the object’s tail.
More recent expeditions to the impact area have recovered minute particles of dust from the meteor. This dust has been compared with similar material found in Antarctica (deposited there as the material settled to earth) and was found to be a close match. Both samples contain relatively large amounts of iridium.
The blast is now thought to have been caused by a stony body some 50 meters in diameter, that exploded with the force of 15-30 megatons of TNT, 6 km above the ground, at a spot 60o 55’ North, 101o 57’ East.
Futher information:
University of Bologne (Italy). Extensive research web site.