Tunguska: 100 years of wondering
One hundred years ago, an explosion bigger than an atomic bomb blasted Tunguska, Siberia. We still don't know what caused it, but there are plenty of theories!
Could Lake Cheko be an impact crater created by fallout from the explosion that rocked Tunguska 100 years ago? (Source: University of Bologna http://www-th.bo.infn.it/tunguska/)
Early on the morning of June 30, 1908, a massive explosion rocked the Siberian wilderness flattening more than 2,000 square kilometres of forest.
Villagers 100 kilometres away from the Podkamennaya Tunguska river basin reported seeing a fireball in the sky, feeling intense heat, hearing loud thumps and being thrown off their feet.
"The split in the sky grew larger, and the entire Northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat.
"I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few yards," reported one villager.
The explosion lit up the sky as far away as London for several days.
Nearly two decades later, the first expedition into the area led by Russian meteorite specialist Leonid Kulik found a region of scorched trees 50 km in diameter, but no crater.
It's now believed that the Tunguska event — as it's now known — was hundreds of times more powerful than the atomic bomb dropped on Hiroshima
Tunguska has long fascinated scientists, enthusiasts and sci-fi fans. Over the years, the mystery of the event has prompted a number of theories with varying degrees of scientific plausibility. These have ranged from a meteorite to the crash of an alien spacecraft; an explosion of methane or the result of Nikola Tesla's experiments with electricity near New York.
Taken during the 1927 Leonid Kulik expedition, this photograph shows trees destroyed by the blast nearly 20 years before.
The most commonly accepted scientific theory is that the 10-15 megaton blast was caused by either the impact of a cosmic object or by its explosion 5-10 kilometres above the Earth's surface.
"The theory was that it was a comet nucleus that looked like a snow drift of loosely bound dusty matter exploded in the atmosphere," says Australian astronomer, Dr Fred Watson.
However, in 2007 Italian scientists suggested that Lake Cheko, an elliptical lake eight kilometres west of the blast's epicentre, could be a crater caused by a meteorite thrown from the exploding comet or asteroid.
"Its funnel-like bottom morphology and the structure of its sedimentary deposits, revealed by acoustic imagery and direct sampling, all suggest that the lake fills an impact crater," the researchers reported in Terra Nova.
"Lake Cheko may have formed due to a secondary impact onto alluvial swampy ground; the size and shape of the crater may have been affected by the nature of the ground and by impact-related melting and degassing of a permafrost layer."
But some UK scientists say that the lake is "highly unlikely to be an impact crater."
"Several lines of observational evidence… contradicts the impact hypothesis for the lake's origin: un-crater-like aspects of the lake morphology, the lack of impactor material in and around the lake, and the presence of apparently unaffected mature trees close to the lake," they responded in Terra Nova.
Watson says the lake was not at the centre of the blast and could have been created by a fragment coming in sideways, which would explain why there were trees in the area that were older than 100 years.
Another reason Lake Cheko could be an impact crater is the fact it wasn't documented on maps until after the 1908 event, he says.
The 'alternativists'
The mystery of the Tunguska event has prompted a number of alternative theories which are not documented in scientific journals or subject to rigorous scientific investigation.
In Russia, a 2008 conference devoted to the "Tunguska Event" nearly came to blows between the "meteoreticians" and the "alternativists," says Andrei Olkhovatov, an alternatist.
"The meteorite theory is the main one. We're like the poor relatives," he says.
This year, the "alternativists" organised a separate conference at which they sketched out outlandish theories for an event they say ordinary physics cannot explain.
Physicist Boris Rodionov believes the explosion was most likely caused by US physicist Nikola Tesla (1856-1943) detonating an underground volcano in Siberia by harnessing electric charges in the air from his laboratory tower outside New York.
Other theories outlined at the conference ranged from the thesis that it was a particularly powerful bolt of lightning to the proposition that it was the result of interaction between yin and yang energy fields in the universe.
As he points to multi-coloured drawings of lightning coming from the Earth's core, Vladimir Mikhailov, a clairvoyant with an intense stare, proclaims: "My theory explains everything. I just needed a place to express myself."
Others say the enduring popularity of the meteorite theory was only due to the fact that there is more money for scientists in stressing the danger of meteorites for the Earth.
"It's all linked to financing. They are just attracting attention to the danger of meteorites, using the example of Tunguska. It worries people," says Sergei Sukhonos, who attended another conference in the village of Vanavara, the settlement nearest to the epicentre of the blast.
In 1991, Italian scientists still found evidence of fallen trees near the blast site. (Source: University of Bologna http://www-th.bo.infn.it/tunguska/)
Every day the Earth is bombarded by space dust and particles, most of it so small it vaporises in the Earth's atmosphere.
But occasionally larger objects strike Earth. Last September, a meteorite exploded and formed a 13 square metre crater filled with boiling brown water near the Peruvian town of Carancas.
Currently 5,520 thousand near Earth objects (NEOs) have been identified by NASA's Near Earth Object program, set up in 1998 to track and characterise asteroids and comets that could pose a threat to Earth.
Over 770 of these objects are asteroids of over one kilometre in length and 960 of these objects are classed as "potentially hazardous asteroids", in other words, asteroids that could come close to Earth.
And cosmic objects can certainly pack a punch.
"The rule of thumb is that a 100 metre asteroid equals a 100 mTon hydrogen bomb", says Watson.
"A hundred metre long object can do a lot of damage," he says, adding that the kinetic energy created by the entry speed determines the force of impact.
Events the size of Tunguska happen around once every 200 to 1000 years.
Understanding the Tunguska event could help us prepare for the impact of a similar sized cosmic event striking a populated place on Earth, recently wrote the Italian scientists behind the Lake Cheko theory, in American Scientist.
"The first step in preparing ourselves would be to decide whether the cosmic object that affected Siberia was an asteroid or comet.
"Although the consequences are roughly comparable in either case, the important difference is that objects in the solar system that circle far away from the sun on long-period orbits [longer than 200 years] before returning, such as comets, would hit the Earth at much greater velocities than close-orbiting bodies such as asteroids," they wrote.
Perhaps the answer to this 100-year-old mystery lies at the bottom of the lake.
The scientists plan to return to Lake Cheko this year to investigate beneath the lake bed, where they say seismic- reflection profiles show a strong acoustic reflector, "probably the echo of a dense, metre-size rocky object."