Impact of asteroids and comets
on Earth: the Tunguska event

    On July 1994, the comet Shoemaker-Levy 9 fell on Jupiter, exploding in the dense clouds of the higher atmosphere of the giant planet. The same year, the US Department of Defense, made of public domain the records of the period 1974-1992 of military satellites for quick monitoring of the launch of intercontinental ballistic missiles. The scientific interest for these data is due to the fact that satellites recorded meteoroid airbursts. In the considered time period, 136 airbursts with energy greater than 1 kton (kiloton: energy equivalent to one thousand of TNT tons) were recorded. However, the right number is probably 10 times greater, because of the limited space covered by satellites.

    These events woke up again, both in scientists and in people, the interest in cosmic impacts onto Earth, a type of event often considered negligible or romantic, as the August meteor shower, or considered as something that occurred in remote time, as the impact that killed dinosaurs about 65 millions of years ago. The renewed interest in the impact problem has put in foreground one of the most intriguing and fascinating enigma that science knows: the Tunguska event. On June 30, 1908, just after 7 AM, local time, something exploded at about 8 km high over the Siberian taiga', near the river Stony Tunguska. The explosion was seen brighter than the Sun till 500 km of distance and the sonic boom was heard till 1200 km. The pressure waves were recorded by meteorological stations in Copenhagen, Berlin, Zagreb, and Greenwich, and the origin of these anomalous waves was discussed during the congress of the British Meteorological Society in 1908.

    However, it was only in 1927 that it was possible to link the two events, when the Russian scientist L. Kulik went in the siberian forest in order to understand what happened 19 years ago. Kulik was the first man that reached the explosion place, because during all these years no local nomadic hunters did dare to go there, being terrified by tales on that event. After 19 years, what Kulik saw was terrible and he found difficult to maintain ordered the group of local bearers, which were terrified by what they saw. After all, 2150 square kilometres of siberian taiga' were knocked down and uprooted, and 1000 square kilometres near the epicenter the trees were burned.

    During the following years, scientists from all over the world studied the event, trying to find a theory to explain all the phenomena observed. The Cold War made it difficult to communicate and two hypotheses dominated: the Russian (comet) and the American (asteroid).

    Only in 1996, a first International Conference, organized by prof. G. Longo, was held in Bologna. There, Russian and American scientists could discuss together the event. The abstract presented were: 27 from countries of the former URSS, 15 from Europe, 11 from United States, 1 from Japan, and 1 from Brazil. Abstracts are available on line, while a selection of  papers was published in a special issue of the international journal Planetary and Space Science, n. 2/3, 1998.

Bologna 6 May 1999

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