Ninety years ago today, the morning of June 30th, 1908, was calm and clear on the banks of the Tunguska river near Lake Baikal in eastern Siberia. Suddenly, and literally out of the blue, a gigantic ball of fire was seen to blaze a trail across the northern sky.
This spectacular display was followed by a loud explosion and a huge pillar of fire that was visible from a distance, rising to a great height.
The effects were evident both near and far. Trees were levelled radially in a zone extending 30 miles from the occurrence; the driver of a train 400 miles away was obliged to bring his locomotive to a standstill, as the Trans-Siberian railway heaved and quaked in front of him; earth tremors and shock-waves were recorded on seismographs and barographs as far away as London; and for many nights the skies of Europe and of Asia displayed magnificent sunsets, similar to those produced by the dust of Krakatoa when it had erupted a quarter of a century before.
The inaccessibility of the Tunguska region, and the confusion produced by war and revolution shortly afterwards, made it impossible for many years to investigate this strange affair. As a result, many wild theories gained in currency, ranging from a strike by "anti-matter" to extra-terrestrial nuclear bombardment of origin unknown.
It is now generally assumed, however, that the "Tunguska Event" was caused by the arrival of a large meteorite.
Meteorites are the flotsam and jetsam of the solar system, cosmic litter whizzing through space at unimaginable speeds. Most of them are only tiny specks of matter that glow and become visible as meteors when they encounter the friction of the atmosphere around the Earth.
Many thousands of millions of these tiny particles enter our atmosphere unnoticed every day, and slowly settle down to earth as an extremely fine dust. Now and then, however, one a little larger than its peers survives the burning passage through the atmosphere, and approaches ground intact, becoming a meteorite.
This was what happened at Tunguska; it is believed that the spectacular display may have been the remains of a comet which, as it hurtled earthwards, exploded some 20,000ft above the ground.
Fortunately, although the mighty explosion destroyed a herd of deer, it did not kill a single human being. It is a sobering thought, however, to realise that if it had fallen from the same part of the sky a mere five hours later, the rotating Earth would have presented St Petersburg beneath it, and that city and its inhabitants would have been destroyed.