Tunguska event
The Tunguska Event, or Tunguska explosion, was a powerful explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:14 a.m.[1] (0:14 UT, 7:02 a.m. local solar time[2]) on June 30, 1908 (June 17 in the Julian calendar, in use locally at the time).[2]
Although the cause is the subject of some debate, the explosion was most likely caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3–6 miles) above Earth's surface. Different studies have yielded varying estimates for the object's size, with general agreement that it was a few tens of metres across.[3]
Although the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons[4] to as high as 30 megatons[5] of TNT, with 10–15 megatons the most likely[5] - roughly equal to the United States' Castle Bravo thermonuclear explosion set off in late February 1954, about 1,000 times as powerful as the bomb dropped on Hiroshima, Japan and about one third the power of the Tsar Bomba, the largest nuclear weapon ever detonated.[6] The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 square miles). It is estimated that the earthquake from the blast would have measured 5.0 on the Richter scale, which was not yet developed at the time. An explosion of this magnitude is capable of destroying a large metropolitan area.[7] This possibility has helped to spark discussion of asteroid deflection strategies.
Although the Tunguska event is believed to be the largest impact event on land in Earth's recent history,[8] impacts of similar size in remote ocean areas would have gone unnoticed before the advent of global satellite monitoring in the 1960s and 1970s.
Description
At around 7:17 a.m. local time, Tungus natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky. About 10 minutes later, there was a flash and a sound similar to artillery fire. Eyewitnesses closer to the explosion reported the sound source moving east to north. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of miles away. The majority of eyewitnesses reported only the sounds and the tremors, and not the sighting of the explosion. Eyewitness accounts differ as to the sequence of events and their overall duration.
The explosion registered on seismic stations across Eurasia. Although the Richter scale was not developed until 1935, in some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale.[9] It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few weeks, night skies were aglow such that one could read in their light, caused by dust suspended in the stratosphere by the explosion. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, also from the suspended dust.
The Guinness Book of World Records (1966 edition) states that the rotation of the Earth would have placed the impact such that it would have caused the complete destruction of the Imperial Russian capital, Saint Petersburg, had the impact occurred 4 hours 47 minutes later.
Selected eyewitness reports
- Testimony of S. Semenov, as recorded by Leonid Kulik's expedition in 1930.[10]
"At breakfast time I was sitting by the house at Vanavara trading post (65 kilometres/40 miles south of the explosion), facing North. [...] I suddenly saw that directly to the North, over Onkoul's Tunguska road, the sky