Pointing an accusing finger

After meticulously reviewing five decades of research on the Tunguska fireball, in 1983 Zdenek Sekanina put together a new analysis of the orbit, atmospheric entry and explosion of the interstellar body. He ruled out a comet as a suspect and pointed an accusing finger at an asteroid about 90 to 190 metres across. The asteroid came in from a direction close to 110 degrees east of north at an angle about 5 degrees above the horizon, and exploded at about 8 kilometres above the ground. Its speed when it entered the atmosphere was about 108,000 kilometres per hour.

In his 32-page seminal paper in The Astronomical Journal, Sekanina remarked that evidence on the forest devastation in the area of fall of the Tunguska fireball left no doubt that it exploded in mid-air and then completely disintegrated in the atmosphere. The evidence included the absence of impact craters and/or sizeable debris in the area of the fall, a near-perfect radial symmetry of the region of flattened trees to within at least 15 kilometres from its centre, and the presence of standing bare trunks (Kulik's 'telegraph poles') at the very centre of the radial of devastation.

Since the evidence pointed to only one enormous outburst, Sekanina concluded that the fireball did not fracture in flight. If such break-ups had in fact occurred, the resulting sequence of explosions would have diminished the enormous power of the final explosion.

The explosion released almost instantaneously energy in excess of 12 megatons. The eyewitnesses saw this energy, which was enough to wipe out even modern London or Tokyo, as a fireball 40 times brighter than the noonday sun. 'This conclusion is supported by accounts of a dazzling blaze of light in the sky, described in eyewitness reports from places near the centre of the explosion', Sekanina said.

The fireball dissipated a million tonnes of small particles in less than one-tenth of a second in Earth's atmosphere. Winds assisted by the expanding shock front dispersed the dust in the stratosphere, which caused the bright skies reported in the aftermath of the fall.

Sekanina rejected the notion that the Tunguska fireball was a comet. Because of the high speed at which the object was travelling when it entered the atmosphere, it resisted a very high air pressure before exploding. It is inconceivable that a comet, known for its extreme fragility, could have survived such a high pressure. The fireball, therefore, must have been a denser, stony object to survive its journey to the Siberian sky. It was probably a small member of the Apollo asteroids, believed to be nuclei of comets that have lost their volatile components.

Critics of the asteroid theory said that Sekanina's analysis was mainly based on eyewitness accounts, most of which were recorded at least two decades after the event, and seismic records. 'You can't make a sophisticated model from poor data', the American meteor specialist Richard McCrosky told Sky & Telescope. 'It seems everything he assumes must be true for his conclusion to be right.'

In the same year the American atmospheric scientist Richard Turco suggested that the bright nights were caused by noctilucent clouds, silvery clouds at high altitude that shine at night. On 30 June 1908 the wind was blowing in the right direction for the dust associated with the fireball to reach western Europe. The dust would then have settled in the atmosphere at an altitude of about 80 kilometres and remained there for several days. These dust particles and the water vapour that was also deposited by the fireball contributed to enhance cloudiness. A stony asteroid would have been too dry to provide the water needed for such clouds. Turco's analysis favoured an icy comet nucleus.

More recently, Vitalii Bronshten rejected the asteroid theory on the ground that an analogy with nuclear explosions suggested that, even upon a very strong explosion, a stony asteroid should break up into fragments of various sizes. These fragments could not completely vaporise in the intense heat of the explosion or as a result of their fall to the Earth. Some of these fragments should survive, but even after many careful searches no such fragments have ever been found. Bronshten also suggested that the noctilucent clouds, which he agreed were observed, could never gain such brightness to result in light nights.

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