The Tunguska Fireball, or explosion, is also know as the Tunguska Event which occurred on 30th June 1908. This event was a massive explosion that happened near Podkamennaya, Tunguska River in what is now Krasnoyarsk Krai of Russia. The energy of the blast was estimated to be between 10 and 20 megatons of TNT, 1,000 times more powerful than the bomb dropped on Hiroshima, or equivalent to Castle Bravo, the most powerful nuclear bomb ever detonated by the US. It felled an estimated 80 million trees over 830 sq miles. The blast measured 5.0 on the Richter scale.
"When the meteorite fell, strong tremors in the ground were observed, and near the Lovat village of the Kansk Uezd two strong explosions were heard, as if from large-calibre artillery."
Siberian Life newspaper, July 27, 1908
The Tunguska event is the largest impact event in recorded history. A modern event over a large metropolitan areas would be devastating.
The cause of the event is somewhat of a mystery, although it is widely believed to be caused by the airburst of a large meteoroid or comet about 20m in length 3.6 miles above the Earth's surface.
At around 7:15am, local villagers in the hills north west 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 loud "knocking" sound similar to artillery fire that went in short bursts spaced increasingly wider apart.
Eyewitnesses closer to the explosion reported the sound source moving during each barrage, 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; to different eyewitnesses the sequence of events and their overall duration is also different.
A flash fire burned thousands of trees near the impact site, and an atmospheric shock wave circled the Earth twice. For two days afterwards there was so much fine dust in the atmosphere that newspapers could be read at night by scattered light in the streets of London 6,213 miles away.
For such a large and widespread event, there was little scientific curiosity, possibly owing to the isolation of the Tunguska region. If there were any early expeditions to the site, the records were lost during the subsequent chaotic years, with World War I, the Russian Revolution of 1917, and the Russian Civil War.
There are many other theories about the cause of the blast, which include:
In 1989, astronomers D'Alessio and Harms suggested that a comet caring deuterium may have undergone a nuclear fusion reaction during entry to the Earth's atmosphere. They concluded that the release of nuclear energy may have been almost negligible. Independently, in 1990, Caesar Sirvent proposed that a deuterium comet, may have exploded as a natural hydrogen bomb, generating most of the energy released.
In 1973 physicists at the University of Texas proposed that the Tunguska event was caused by a micro black hole passing through the Earth. Unfortunately for this hypothesis, there is no evidence for a second explosion occurring as the black hole exited the Earth and so this theory has not gained wide acceptance.
In 1965 it was suggested that the Tunguska event was caused by the annihilation of a chunk of antimatter falling from space. However, there is no astronomical evidence for the existence of such chunks of antimatter in our region of the universe due to the lack of gamma ray's.
It is claimed by UFO aficionados that the area was the site of a crashed UFO, possibly shot down by soviet forces. The lack of investigation between the event and the first expedition of 1921 could be interpreted as a cover up.
The Tunguska site is downrange from the Baikonur Cosmodrome and has been contaminated repeatedly by Russian space debris, most notably by the failed launch of the fifth Vostok test flight on December 22, 1960. The payload landed close to the Tunguska impact site, and a team of engineers was dispatched there to recover the capsule and its two canine passengers (which survived).
In a recent scientific expedition, scientists from the University of Bologna have identified a possible crater left by the blast. Lake Cheko, although shallow, fits the proportions of a small, bowl-shaped impact crater the team say. Their investigation of the lake bottom's geology reveals a funnel-like shape not seen in neighbouring lakes. In addition, a geophysics survey of the lake bed has turned up an unusual feature about 10m down which could either be compacted lake sediments or a buried fragment of space rock.
"We have no positive proof this is an impact crater, but we were able to exclude some other hypotheses, and this led us to our conclusion," Professor Longo, the research team leader.
Intriguingly, Lake Cheko does not appear on any maps before 1929, though the researchers admit the region was poorly charted before this time.
The University of Bologna team plans to mount another expedition to the Tunguska region in summer 2008 when they aim to drill up to 10m below the lake bed to the anomaly picked up in the geophysics survey and determine whether it really is a piece of extraterrestrial rock.