In 1975 Fred Whipple questioned the possibility of a comet striking Tunguska. He estimated that if we take the mass of the comet as 1 million tonnes, as suggested by Fesenkov, the chance of such a comet striking Earth every 100 years would be about 1 in 20,000. 'It appears unlikely, therefore, that the Tunguska explosion was produced by a bona fide active comet a hundred or so meters in dimension ... more likely, however, the Tunguska object was an inactive, low-density, friable body ... There is no reason to suspect it was interstellar.'
Most contemporary scientists also reject the idea of a comet. 'Comets are fluffy in comparison with asteroids and burn up quickly in the atmosphere', Richard Stone writes in Discover magazine. 'For one to have produced an explosion as big as the one over Tunguska, it would have started out as a million-tonne object. The vast swath of gas and dust left by such an object on its way down might well have shut out the sun or altered the climate.' Zdenek Sekanina, an expert on comets at NASA's Jet Propulsion Laboratory, agrees: 'The effect on life on Earth would have been horrendous. It would have been a global catastrophe, comparable to nuclear winter. The effects on mankind would have been so overwhelming that we could not discuss the topic, because we would not be here.' You and I are still discussing the topic; therefore, the Tunguska fireball was not a comet. QED.
Although the probability of such low-density objects colliding with Earth is obviously quite small, there is still hope for the comet theory - and it comes from Down Under. Assuming a speed of 108,000 kilometres per hour, as the Australian scientists Duncan Steel and Richard Ferguson present their case, seven hours before the impact the Tunguska object would have been about
750,000 kilometres from Earth. Active comets produce tails that stretch millions of kilometres away from the Sun, so it is possible that there could have been an encounter between Earth and the tail of the comet. This encounter could produce an aurora in the hours before the impact. But did anyone observe an aurora seven hours before the Tunguska blast?
Steel, a well-known authority on the threat posed by asteroids, was at a conference on asteroids, comets and meteors in Sweden in 1989, when the Russian scientists Nikolai Vasiliyv and G. Andreev circulated a short report on Soviet research on Tunguska. He was intrigued by the following paragraph in the report:
A special item in this respect could be the search for the original diary entries by Mouson who observed auroras from near the Erebus volcano at Antarctica during the summer of 1908. There is information in Shackleton's accounts that on June 30, Mouson registered an aurora which he visually considered to be anomalous. Unfortunately, Shackleton's accounts do not contain further details.
Steel soon figured out that 'Mouson' was 'Mawson', after transliteration from the Latin script to Cyrillic and then back again, and 'the summer of 1908' was in fact 'the Antarctic winter of 1908'. Coincidentally, at that time Steel was at the University of Adelaide, where Mawson's notebooks from the Antarctic Expedition of 1907-09, led by Sir Ernest Shackleton, are archived in the Mawson Institute for Antarctic Research. Mawson (later Sir
Douglas Mawson) was a young geologist on the expedition, and kept a diary of his observations. Steel and Ferguson made extensive searches of Mawson's diaries and all other expedition papers, but failed to find any record of aurora australis at the time of the Tunguska blast. However, they found a record of an exceptional aurora seven hours before the blast. Was this aurora caused by the Tunguska fireball? If yes, then the fireball was, as Mawson would have said, a fair dinkum (genuine) comet.
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