On June 26-28, Krasnoyarsk, a city in Central Siberia, will host an all-Russian scientific and practical conference "The Centenary of the Tunguska Event". In the 20th century, Russia was the scene of two major impact events involving large meteorites. On February 12, 1947, a meteorite fell in the Sikhote-Alin mountain range, approximately 440 km northeast of Vladivostok, Russia, gouging over 30 craters with a diameter of seven to 28 meters and up to six meters deep.
The impact area for this meteorite in the form of "iron rain" covered about 10 square kilometers, from whiche about 27 metric tons of meteorite substances were recovered. About 1% of the meteorite's initial energy was released during impact, mostly dispersing into the atmosphere.
Scientists have been debating the nature of the Tunguska event, which happened 39 years earlier, one of the most mysterious and well-studied 20th century phenomena, for the last 100 years. Unlike the Sikhote-Alin meteorite, the Tunguska meteorite exploded in mid-air, leaving no fragments on the ground. Only traces of its explosion were found.
As the Sun was rising over Central Siberia on June 30, 1908, local people spotted a glowing object with a long fiery tail moving across the sky. The object subsequently exploded near the Podkamennaya (Lower Stony) Tunguska River in what is now Russia's Krasnoyarsk Territory.
Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons of TNT, with 10-15 megatons the most likely - about 1,000 times as powerful as the bomb dropped on Hiroshima, Japan.
The shock wave circled the globe twice and was registered by all existing geophysical observatories. In Irkutsk, a seismograph scale went wild. The ground trembled as far as Tashkent in Uzbekistan (Central Asia), Tbilisi in Georgia (South Caucasus) and even Jena, Germany.
Barometers at meteorological stations in Cambridge and Petersfield in the U.K. registered atmospheric pressure fluctuations. And a four-hour magnetic storm began several minutes after the blast. The storm closely resembled the geomagnetic fluctuations registered after high-altitude nuclear blasts.
Over the next few days, "white nights" and unusual silvery clouds were seen over tha vast territory from Siberia to Europe's western borders.
Starting in 1921, numerous Russian and foreign expeditions have visited the site of the Tunguska event. Most scientists now agree that the "celestial body" was travelling at several dozen km per second from east to west prior to the blast, and that it weighed from 100,000 to a million metric tons.
Although a meteorite of such proportions should have carved out a 500-meter deep crater, not a single expedition managed to locate it. Even if the meteorite had disintegrated, its fragments would also have left at least several craters. However, none were found.
Experts have offered dozens of theories to explain the Tunguska event. After the development of nuclear energy, scientists started talking about a possible UFO crash that resulted in a nuclear explosion over Central Siberia.
Postwar expeditions found some substance fused into tiny balls after the blast. The same balls were found after the United States dropped atomic bombs on Hiroshima and Nagasaki. Generations of animals endemic to the region have been afflicted by numerous mutations that may have been caused by nuclear radiation.
In 1960, a special expedition was sent to the Podkamennaya Tunguska River region on the initiative of famous Soviet rocket and spacecraft designer Sergei Korolev. Incidentally, one of its members was future Soviet cosmonaut Georgy Grechko. The expedition was ordered to find UFO fragments at the blast site, which it was thought contained thousands of metric tons of substances that would have been released by such tremendous energy. However, negligibly small amounts of such substances and no "extraterrestrial" fragments were found.
The huge iron or stone meteorite, which could not have disintegrated completely, was probably an amorphous body. Many scientists believe that the Earth encountered a comet 100 years ago. This explains why the "celestial body" was flying along an unusual trajectory.
The comet theory was first suggested by Leonid Kulik and further developed by Vasily Fesenkov, Member of the Soviet Academy of Sciences.
In the 1970s, Georgy Petrov, also a Member of the Soviet Academy of Sciences and the first director of its Space Research Institute, conducted major theoretical research of the Tunguska event. According to Petrov's findings, the unknown body had a density of about 0.01 gram per cubic centimeter - about 5-10 times less than the density of freshly-fallen snow.
The body must have resembled a 300-meter gaseous cloud at an altitude of 50 to 60 km. Although the huge clot's braking speed must have increased greatly during descent, the shock wave was travelling much faster.
According to Petrov, the "celestial body' was not originally a gas cloud because it would not have lasted a minute in outer space. A gas-dust formation or a solid-particle conglomerate could not have generated a shock wave during its entry into the atmosphere because every particle would have been moving on its own. Moreover, such a huge conglomerate would have been short-lived.
It turns out that the "meteorite" was an amorphous snowball-like body containing ice crystals and iron-and-silicate dust particles. Research conducted by the unmanned space probes Vega-1 and Vega-2, launched by the Soviet Union in December 1984 to study Venus and Halley's Comet, revealed that the comet's nucleus had a similar structure.
The comet theory also explains many after-effects of the Tunguska event, including the increased opacity of the terrestrial atmosphere after the dispersal of the comet's nucleus, glowing comet-tail particles in the night sky, as well as the lack of a crater and meteorite fragments in the disaster area.
Although scientists are gradually accepting the comet theory, many aspects of the event are still a mystery. The causes of powerful magnetic aberrations in the disaster area, as well as biological anomalies, have not yet been explained. It is unclear why pine-tree mutations have increased along the Tunguska meteorite's flight path. Experts are also unable to explain rapid tree growth in areas damaged by the explosion.
The 1908 Tunguska event probably offers unique and invaluable data for predicting the consequences of apparently inevitable asteroid impacts and preventing such disasters in the future.
Yury Zaitsev is an expert at the Russian Academy of Sciences' Space Research Institute.
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