They say it was the mother of all cosmic explosions. The blast, which took place on June 30, 1908, in Tunguska, Siberia, also remains one of the greatest mysteries of the world. Russia is now organising an international conference in Moscow to mark the centenary of the explosion.

The Siberian riddle has fascinated the world since an object entered the atmosphere over western China and whizzed north, leaving a 5,000 degree hot trail in the sky, to hit the banks of the Tunguska river. The explosion has exposed the fragility of mankind to a blitzkrieg from outer space. It has also led to a wide range of theories.

A virtual search for an explanation in the company of scientists engaged in unraveling its mystique is revealing.

The explosion knocked over an estimated 80 million trees over 2,150 sq km and shook the seismograph at Irkutsk, 1,000 km away, and the one in Washington DC. The region also experienced black rain — condensation mixed with dirt and debris. The impact resulted in fluctuations in atmospheric pressure strong enough to be detected in London. Over the next few weeks, the night skies were so bright that people in Europe read in the light. One groups of peasants even asked the local priest if “the end of the world” was near.

The paranoia was not without cause as the explosion could have claimed millions of lives, because of the earth’s rotation, had it happened at a different time. However, the only known casualty is an old shepherd who was thrown 40 ft up into a tree. The news of the manner in which he died stayed within his family till Russian scientist Leonid Kulik reached Tunguska.

“Even a cursory examination exceeded all eyewitness accounts and what I had imagined,” says Kulik as he rewinds to April 13, 1927, when he started his study. For Kulik, it was the beginning of a decade-long affair during which he did an aerial photographic survey which showed that the blast had left a huge butterfly-shaped scar.

For many such as Russian scientist Shaidurov, the effects are worrying. “The impact on Tunguska’s climate is never considered. The explosion is responsible for environmental changes post 1908 as it caused a considerable stirring of the high layers of the atmosphere and changed its structure, triggering a rise in global temperatures,” he argues.

However, for many others pinpointing the cause is more important. There are theories aplenty. Expeditions in the 1950s and 1960s found microscopic glass spheres in soil containing high proportions of nickel and iridium, thus raising the question of the object having been a meteorite.

“It was a fragmentary asteroid that exploded in the atmosphere,” say Christopher Chyba and Kevin Zahnle of NASA and Paul Thomas of Wisconsin University. This is endorsed by Slovak astronomer L’ubor Kresak.

D’Alessio and Harms of McMaster University, Canada, feel there is a nuclear angle to the event “as some deuterium in a comet may have undergone a nuclear fusion”.

Russian scientist Aleksander Kazantsev, who was tasked to assess the Hiroshima fallout, supports this. “There is a connection between Tunguska and Hiroshima. The strange forest of trees, stripped of branches but still standing, was also found at Hiroshima. The mushroom cloud and the black rain that followed the Hiroshima blast find an echo in reports from Tunguska,” he says.

F J W Whipple lends weight to the comet theory. “It was the nucleus of a comet that exploded midair,” he says.

However, astronomer Zdenek Sekanina is critical of the comet hypothesis. “If it was a comet, it would have disintegrated. But it apparently remained intact,” he says and finds support in Italian Farinella Foschini.

“Perhaps it was a tiny black hole that entered the earth’s atmosphere and imploded,” say Albert A Jackson and Michael P Ryan, physicists at Texas University.

The craziest explanations include one that says that the explosion was “caused by a nuclear-powered UFO that crashed after losing propulsion”, as well as the one that terms it “the result of a deliberate attack by extraterrestrials”. However, the prize in the crazy category goes to the theory that it was a failed experiment by Nikola Tesla, who dreamt of wireless weapons and power transmitters.

The alien angle finds favour with Russian scientist Yuri Lavbin, who heads the Tunguska Space Phenomenon public state fund. “It was an extra-terrestrial spaceship,” he says. Lavbin points to a huge white stone on the top of a crag and says, “It’s called the ‘reindeer stone’ and is made of a crystalline matter that is not typical of this region.”

A meaningful silence fills the sky above the reindeer stone. A hesitant minute later, the rain comes drumming down, seemingly breaking the solitude of Tunguska.

UFOs in focus

There have been several incidents that raise questions about life in outer space. For two of them, the case is far from closed

The Roswell incident

On July 8, 1947, the Roswell Army Air Field issued a statement that its personnel had recovered a crashed “flying disc” from a ranch near Roswell, New Mexico, USA. Later that day, the commanding general of the Eighth Air Force stated that a weather balloon had been recovered rather than a “flying saucer”.

In 1978, Major Jesse Marcel, who was involved with the original recovery of the debris in 1947, said the military had covered up the recovery of an alien spacecraft. The US military, however, maintains that what was recovered was a top-secret research balloon.

Many UFO proponents believe that the wreckage was of a crashed alien craft and that the military covered up its recovery. This ranks as the most-famous alleged UFO incident.

Numerous books, articles, television specials and even a made-for-TV movie brought the 1947 incident fame and notoriety so that by the mid-1990s, strong majorities in polls, such as a 1997 CNN/Time poll, believed that aliens had visited earth and the government was covering it up.

The Rendlesham Forest incident

This is the name given to a series of reported sightings of unexplained lights and the alleged landing of an extraterrestrial spacecraft in Rendlesham Forest, Suffolk, England, in late December 1980. It is perhaps the most famous UFO event to have happened in Britain.

Britain’s ministry of defence denied that it posed any threat to national security and stated that it was thus never investigated as a security matter.

www.wikipedia.org

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The New Sunday Express: Saturday May 3 2008 18:22 IST