Asteroid air-burst over Siberia - The Tunguska Event

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Since the 1908 explosion, there have been an estimated 1,000 scholarly papers (mainly in Russian) published on the Tunguska explosion. Many scientists have participated in Tunguska studies; the best known are Leonid Kulik, Yevgeny Krinov, Kirill Florensky, Nikolai Vladimirovich Vasiliev, and Wilhelm Fast. In 2013, a team of researchers led by Victor Kvasnytsya of the National Academy of Sciences of Ukraine published analysis results of micro-samples from a peat bog near the blast epicenter showing fragments that may be of meteoric origin.

Estimates of the energy of the blast range from as low as three to as high as 30 megatons of TNT  Most likely it was between 10–15 megatons of TNT , and, if so, then the energy of the explosion was about 1,000 times greater than that of the atomic bomb dropped on Hiroshima, Japan; roughly equal to that of the United States’ Castle Bravo ground-based thermonuclear test detonation on March 1, 1954; and about two-fifths that of the Soviet Union’s later Tsar Bomba (the largest nuclear weapon ever detonated).

It is estimated that the Tunguska explosion knocked down some 80 million trees over an area of 2,150 square kilometres (830 sq mi), and that the shock wave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude would be capable of destroying a large metropolitan area, but due to the remoteness of the location no fatalities were documented. This event has helped to spark discussion of asteroid impact avoidance.
The leading scientific explanation for the explosion is the air burst of an asteroid 6–10 kilometres (4–6 miles) above Earth’s surface.

Meteoroids enter Earth’s atmosphere from outer space every day, travelling at a speed of at least 11 kilometres per second (6.8 mi/s). The heat generated by compression of air in front of the body (ram pressure) as it travels through the atmosphere is immense and most asteroids burn up or explode before they reach the ground. Since the second half of the 20th century, close monitoring of Earth’s atmosphere has led to the discovery that such asteroid air bursts occur rather frequently. A stony asteroid of about 10 metres (30 ft) in diameter can produce an explosion of around 20 kilotons, similar to that of the Fat Man bomb dropped on Nagasaki, and data released by the U.S. Air Force’s Defense Support Program indicate that such explosions occur high in the upper atmosphere more than once a year. Tunguska-like megaton-range events are much rarer. Eugene Shoemaker estimated that such events occur about once every 300 years
In 1930, the British astronomer F.J.W. Whipple suggested that the Tunguska body was a small comet. A cometary meteorite, being composed primarily of ice and dust, could have been completely vaporized by the impact with Earth’s atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies (or “skyglows” or “bright nights”) observed across Europe for several evenings after the impact, possibly explained by dust and ice that had been dispersed from the comet’s tail across the upper atmosphere. The cometary hypothesis gained a general acceptance amongst Soviet Tunguska investigators by the 1960s.

In 1978, Slovak astronomer Ľubor Kresák suggested that the body was a fragment of the short-period Comet Encke, which is responsible for the Beta Taurid meteor shower: the Tunguska event coincided with a peak in that shower, and the approximate trajectory of the Tunguska impactor is consistent with what would be expected from such a fragment.It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades.

In 1983, astronomer Zdeněk Sekanina published a paper criticizing the comet hypothesis. He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere. Sekanina argued that the evidence pointed to a dense, rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when Farinella, Foschini, et al. released a study suggesting that the object had arrived from the direction of the asteroid belt.

Proponents of the comet hypothesis have suggested that the object was an extinct comet with a stony mantle that allowed it to penetrate the atmosphere.
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