The Tunguska fireball was not a meteorite but a small gaseous comet which had left no trace of itself after the impact. That's what two astronomers concluded independently in 1934. F.J.W. Whipple, head of the Kew Observatory in London, said that in view of the fact that the recorded observations of the phenomenon such as bright nights and airwaves were confined to the north of Europe, it was suggested that 'the meteor was essentially a small comet and that the tail of the comet was caught by the atmosphere'. However, he was quick to admit: 'I do not feel much confidence in this hypothesis.'
I.S. Astapowitsch of Leningrad State University expanded the idea suggested by Academician Vladimir
Vernadsky, Kulik's mentor, that the Tunguska meteorite could have been a rather dense cloud of cosmic dust, possibly associated with a comet.
Astapowitsch asserted that the bright atmospheric phenomena following the blast could have been caused by the dust tail of the nucleus of a small comet rushing towards Earth and exploding with a tremendous energy. 'The explosion gave rise to seismic and airwaves', he said, 'while the high-temperature explosive waves caused the uniform scorching over the first few kilometres'. He also estimated the blast's energy to be between 1 and 2 megatons, about 100 times higher than Whipple's calculation (current estimates vary from 10 to 20 megatons).
In 1942 William H. Christie of the Mount Wilson Observatory in California analysed all available data on the Tunguska explosion and concluded that the data agreed with the action of a comet. 'The night glows, I think, remove all the doubts as to the nature of the object which struck Earth', he said. 'It was, apparently, a small comet ... and the tail, or part of it, which was captured by Earth formed the glows. The Greenwich time of the collision was just past midnight, hence no glows were observed much west of Greenwich because this hemisphere would be on the side away from the comet.' The Siberian blast appeared to be the only recorded instance of the collision of Earth with a comet, he said. 'How long we will have to wait for another such visitor we do not know, but let us hope it again chooses a sparsely inhabited region of this globe of ours for its final resting place?'
At that time scientists knew very little about the struc ture of nuclei of comets. They believed them to consist of one or a few large stony rocks or even a 'sandbag' of small particles. The size of nuclei was also over-estimated to be several hundred kilometres. Astapowitsch and Whipple's comet hypothesis also turned out to be as questionable as Kulik's meteorite hypothesis, and it was soon dumped by scientists. It was revived in the late 1950s, when the American astronomer Fred Whipple proposed his 'dirty snowball' model for comets.
A comet is a unique cosmic phenomenon: it suddenly appears in the sky, it blazes for a few days, it wows earthlings, it disappears. A comet's awe-inspiring spectacle has always intrigued people. Its irregular appearance in the sky, its varying size, form and brightness, its exotic tail, its abrupt disappearance - these were the mysteries which ancient people were unable to solve. To them comets were omens of disaster. According to the Greek poet Homer, who lived in the 8th century bc, a comet (from the Greek word kome, meaning 'hair') was 'a red star that from his flaming hair shakes down disease, pestilence and war'. In Shakespeare's Julius Caesar, after seeing a comet, Caesar's wife Calpurnia warns him: 'When beggars die, there are no comets seen; the heavens themselves blaze forth the death of princes.' Comets are no longer considered harbingers of doom, but they still intrigue astronomers.
Even now, astronomers know very little about comets, and they can't predict when the next one will come. Comets are chunks of matter left over from the birth of the solar system 4,600 million years ago. There are somewhere between 2 trillion and 5 trillion comets that circle the solar system in a halo-like cloud - the Oort cloud - between 20,000 and 100,000 astronomical units from the Sun (one astronomical unit is the distance between the Earth and Sun, about 150 million kilometres). In the Oort cloud, comets are not packed like sardines - the neighbouring comets are typically tens of millions of kilometres apart. The Oort cloud, sometimes called the Siberia of comets because of its freezing temperatures as low as -270 degrees Celsius, is named after Dutch astronomer Jan H. Oort, who in 1950 suggested the existence of a spherical reservoir of comets swirling around the solar system.
At about the same time, Gerard P. Kuiper of the University of Chicago suggested that there is another reservoir of comets, now known as the Kuiper belt. The Kuiper belt extends between 35 and a few hundred astronomical units from the Sun, beyond the orbit of Neptune, and contains some 200 million comets waiting in dusty cold storage for a chance to whoosh across our skies and strike terror into the heart of earthlings.
About a dozen stars pass within about 200,000 astronomical units of the Sun every one million years. Occasionally these passing stars push a comet out of its orbit, sending it towards the inner solar system. It may pass the solar system once and never return again, or it may settle into an orbit to visit us regularly, like Halley's comet. Comets that take twenty or more years to orbit the Sun, including Halley's, come from the Oort cloud; comets with shorter orbital periods start their journey towards the Sun from the Kuiper belt. Orbits of some comets have periods exceeding 1 million years.
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