Astronomers are good at forecasting. If they tell you that an eclipse will start at 10.18 p.m. on such-and-such a night, you can expect it with almost total confidence. But like meteorologists, they have their failures too.
In 1997, for example, a team of American astronomers produced a forecast that set the world agog. A huge lump of rock, they said, larger than the Empire State Building, was on a collision course with Earth. This asteroid, they reckoned, would arrive here on September 30th, 2030; needless to say, this concentrated the global mind wonderfully.
Of course they hedged a little, just as weather people sometimes do. Now and then you may hear a forecast like "occasional heavy rain, some sunny periods, and temperatures between two and nineteen Celsius".
Analogously, we were told in this case that the chances of collision were only one in several hundred, but if it happened it would virtually eliminate Earth.
A few weeks and many calculations later, the prediction was revised. The asteroid, would pass us by with several million miles to spare, so everyone relaxed and began to complain about the weather again.
But sometimes such objects do make contact with the Earth. This day 93 years ago, a gigantic ball of fire blazed across the sky in the vicinity of the Tunguska River in Eastern Siberia. This spectacular display was followed by a loud explosion and a huge pillar of fire. The asteroid, or it may have been a fragment of a comet, is believed to have been some 30 or 40 metres in diameter, and it scored a direct hit on Planet Earth.
The "Tunguska Event" was evident both near and far. Trees were levelled in a 30miles radius 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; and earth-tremors and shock-waves were recorded on seismographs and barographs as far away as London.
Fortunately, although the explosion killed an estimated 1,500 deer, not a single human being perished. It is sobering, 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 as thoroughly as Hiroshima several decades later.