What was the Tunguska event?
On 30 June 1908 a large space rock exploded in the air over a remote part of Siberia, flattening around 80 million trees across more than 2,000 square kilometres without leaving a crater.
Early on the morning of 30 June 1908, near the Podkamennaya Tunguska River in central Siberia, local people saw a blinding light cross the sky and felt a blast of heat. Moments later came a deafening explosion. Witnesses tens of kilometres away were knocked off their feet, and windows broke in a trading post far from the centre. Seismographs and barometers in distant countries recorded the shock, and for several nights afterwards the skies over Europe glowed so brightly that people could read outdoors at midnight.
Because the region was so remote, the first scientific expedition, led by the Russian mineralogist Leonid Kulik, did not reach it until 1927. He expected a giant crater and meteorite. Instead he found a vast area of flattened forest, roughly 2,150 km², with trees lying radially outwards from a centre where, oddly, some trees still stood upright but stripped of branches. That pattern is the signature of an airburst: an explosion in the air directly above.
Today most scientists think the culprit was a stony asteroid or possibly a comet fragment, a few tens of metres across, perhaps around 50 to 60 metres, which disintegrated about 5 to 10 km above the ground. Estimates of the energy released range widely, commonly around 10 to 15 megatonnes of TNT. With no crater and no large pieces found, the evidence comes from the felled trees, tiny melted spherules in the soil, and the eyewitness reports.
Tunguska is the largest impact event in recorded history. Had it struck a city, the damage would have been catastrophic. It remains a key benchmark for planetary defence, and the anniversary, 30 June, is now marked each year as Asteroid Day.