The BBC website, on 30th June 2008, had an article about the 100 year anniversary of the comet explosion that flattened some 80 million trees in over 2000 square kilometres near Tunguska. It says
The blast was 1,000 times more powerful than the atomic bombs dropped on Hiroshima and Nagasaki and generated a shock wave that knocked people to the ground 60km from the epicentre.
The cause was an asteroid or comet just a few tens of metres across which detonated 5-10km above the ground.
To many, this event - the biggest space impact of modern times - serves as a reminder of the continuing threat posed to our planet by objects from space.
The international Spaceguard survey programme has been working to identify the Near-Earth Objects larger than 1km - the class of object could cause a "nuclear winter" if one were to strike the planet, possibly threatening civilisation.
Objects the size of the one that caused the Tunguska impact are too small to be seen by present-day surveys.
But there is no guarantee the next object will explode over the sea or a sparsely populated wilderness. This begs an obvious question: how prepared are we for the next one?
Dr Richard Crowther is head of the United Nations Near Earth Object (Neo) programme. He told the BBC News website: "Tunguska reminds us that these impact events have occurred in the relatively recent past.
"The surveys suggest that objects of this size are numerous enough to anticipate similar events in the relatively near future."
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