Not that long ago, a popular Russian website asked its visitors to answer the question: what do you think the Tunguska object was? The results of the poll, which was restricted to one vote per e-mail address, were as follows:

The website is frequented by science-oriented Russians, and this is reflected in the low rating of the alien spaceship theory, which is believed to be very popular among Russian Tunguska fans.

It is surprising that, after eight decades, international scientists remain as divided as Russian web surfers. Chris Trayner of Leeds University seems to have solved the riddle of why scientists have failed to solve the riddle of Tunguska. He says that many of their problems stem from the old East-West divide, and poor access to the Russian literature. He suggests that to disentangle what might have happened in 1908, scientists need extensive translations and online indexes of the Russian source

A comet

A meteorite/asteroid An alien spaceship Other

material. The same must be said of English sources for Russian scientists.

In the meantime, let's try to disentangle what really happened. There are seven basic sources of information on the event: (1) the devastated forest and its pattern of damage (first observed nineteen years after the event); (2) records of atmospheric and seismic waves at the time of the event; (3) records of magnetic storms at the time of the event; (4) bright nights observed in parts of Europe and Asia after the event; (5) anomalous atmospheric phenomena observed after the event; (6) study of microscopic particles found at the explosion site and in Antarctica; and (7) eyewitness accounts (first collected thirteen years after the event).

Almost everyone agrees on the following points:

■ The accurate time of the event (0014 gmt; 7.14 a.m. local time) and the exact location of the epicentre (latitude 60 degrees 55 minutes north, longitude 101 degrees 57 minutes east).

■ A large incoming object, presumably a meteorite or a comet, was seen over an area 1,500 kilometres across. The object's brightness was comparable to that of the noonday sun.

■ It exploded in mid-air, between 5 and 10 kilometres above the ground. The energy of the blast was probably between 10 and 20 megatons.

■ The debris was blown upwards into the stratosphere. No significant fragments, except microscopic globules at the epicentre and in Antarctica, have been found.

■ The blast created a shock wave that levelled 2,150 square kilometres of forest, of which about 200 square kilometres had been burnt by a heat wave. In a forest fire, trees are usually burnt on the lower part of their trunks, but these trees had been burnt uniformly and continuously.

■ There is no impact crater at the epicentre, the point under the explosion where the shock wave first contacted the ground.

■ The devastated forest has the shape of a butterfly spread over the ground. The epicentre lies near the 'head' of the butterfly.

■ The heat and shock waves were felt by many eyewitnesses at Vanavara, about 70 kilometres from the explosion site. Bright lights in the sky were seen as far as 700 kilometres away; and loud explosions, like gunfire, were heard after the explosion up to 1,200 kilometres away.

■ The blast also created disturbances in the atmosphere which were recorded around the world. The impact of the blast on the ground generated seismic waves which were recorded well beyond Russia.

■ A local magnetic storm - similar to ones produced by nuclear explosions in the atmosphere - began a few minutes after the blast and lasted for about four hours.

■ The explosion caused very bright nights in parts of Europe and Asia which lasted for several nights. Noctilucent clouds were also observed. The dust in the stratosphere caused these optical anomalies.

■ There was accelerated growth in young trees that survived the blast.

The controversial points:

■ The object's shape as it raced across the sky before exploding: a 'pipe', 'pillar' or 'tube', as mentioned by some eyewitnesses? Did it leave a trail of smoke and dust?

■ Its nature, size and mass; for how long it was seen in the sky.

■ Its entry angle, its subsequent flight path, and its velocity immediately before explosion; the duration of the explosion.

■ The flight ending in multiple explosions.

■ Increased radioactivity at the explosion site; genetic effects on the local population; the accelerated growth of trees being caused by genetic mutation.

■ Craters such as the Suslov crater being formed by the impact of the exploding object.

■ Optical anomalies being observed before the event as early as 23 June.

Bordering on the ridiculous:

■ The object twice changed course in flight.

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