Raymond
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Mysterious ImpactIn June 1908 a gigantic explosion ripped open the dawn sky above the swampy taiga forest of western Siberia, leaving a scientific riddle that endures to this day.
A dazzling light pierced the heavens, preceding a shock wave with the power of a thousand atomic bombs which flattened 80 million trees in a swathe of more than 800 square miles.
Evenki nomads recounted how the blast tossed homes and animals into the air. In Irkutsk, 950 miles away, seismic sensors registered what was initially deemed to be an earthquake. The fireball was so great that a day later, Londoners could read their newspapers under the night sky.
What caused the so-called Tunguska Event, named after the Podkamennaya Tunguska river near where it happened, has spawned at least a half a dozen theories.
The biggest finger of blame points at a rogue rock whose destiny, after travelling in space for millions of years, was to intersect with Earth.
Even the most ardent defenders of the sudden impact theory acknowledge there are many gaps. They strive to find answers, believing this will strengthen defences against future Tunguska-type threats, which experts say occur with an average frequency from one in 200 years to one in 1,000 years.
If a rock was the culprit, the choices lie between an asteroid -- the rubble that can be jostled out of its orbital belt between Mars and Jupiter and set on collision course with Earth -- and a comet, one of the "icy dirtballs" of frozen, primeval material that loop around the Solar System.
Comets move at far greater speeds than asteroids, which means they release more kinetic energy pound-for-pound upon impact. A small comet would deliver the same punch as a larger asteroid.
But no fragments of the Tunguska villain have ever been found, despite many searches.
Finding a piece is important, for it will boost our knowledge about the degrees of risk from dangerous Near Earth Objects (NEOs), say Italian researchers Luca Gasperini, Enrico Bonatti and Giuseppe Longo.
When a new asteroid is detected, its orbit can be plotted for scores of years in the future.
Comets are far less numerous than asteroids but are rather more worrying, as they are largely an unknown entity.
Most comets have yet to be spotted because they take decades or even hundreds of years to go around the Sun and pass our home. As a result, any comet on a collision course with Earth could quite literally come out of the dark, leaving us negligible time to respond.
Estimates, based on the scale of ground destruction, range from 10 feet to 227 feet.
All agree that the object, heated by friction with atmospheric molecules, exploded far above ground.
But there is fierce debate as to whether any debris hit the ground.
But what if neither comet nor asteroid were to blame?
Lake Cheko does not have the typical round shape of an impact crater, and no extraterrestrial material has been found, which means "there's got to be a terrestrial explanation," Wolfgang Kundt, a physicist at Germany's Bonn University told the New Scientist magazine.
He believes the Tunguska Event was caused by a massive escape of 10 million tons of methane-rich gas deep within Earth's crust. Evidence of a similar apocalyptic release can be found on the Blake Ridge on the seabed off Norway, with a "pockmark" of 280 sq. miles.
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