Tunguska event

At about a quarter past seven on the morning of 30 June 1908 (17 June in the Russian calendar of the time), a fireball streaked across the sky of central Siberia and exploded above the forest near the Podkamennaya Tunguska River. The blast flattened some 2,150 square kilometres of taiga, killed at least three people and was registered by instruments as far away as Germany and Britain. Scientists generally attribute it to a small asteroid or comet that burst in the air, making it the most powerful impact event on Earth in recorded history.

The region lay in the Yeniseysk Governorate of the Russian Empire, in what is now Krasnoyarsk Krai. It was a land of marshes, wooded hills and permafrost, frozen for eight or nine months of the year. Its few inhabitants were mostly Evenks, who lived in small family groups and herded reindeer, hunted and fished, along with Russian settlers in small river villages such as Vanavara, about 60 kilometres south of the blast.

Witnesses across central Siberia saw a body brighter than the Sun travelling from south-east to north-west under a clear sky. Over the Tunguska basin it exploded with a deafening roar and a column of fire. Several million trees were knocked down in a radial, butterfly-shaped pattern, while those directly beneath the blast were stripped of their branches but left standing. Fires broke out, reindeer and dogs were killed, and windows were shattered up to about 200 kilometres away. A farmer at Vanavara later described being hurled from his porch by a wave of heat and losing consciousness.

Seismographs in Irkutsk, Tashkent, Tiflis and Jena recorded the shock, now judged equivalent to a magnitude of close to 5, and barographs around the world registered the pressure wave. For several nights afterwards, skies across western Eurasia glowed so brightly that people could read a newspaper at midnight.

At least three Evenks died: one man thrown against a tree, whose broken arm became fatally infected, another who died of shock, and a shaman who probably perished in a forest fire. Others lost consciousness for hours or days. Many Evenks saw the event as the wrath of their fire god, and their reluctance to speak of it, together with burial customs that led them to avoid sites of death, partly explains why so little is known about the human toll.

Remoteness and the upheavals of war and revolution delayed scientific study. The mineralogist Leonid Kulik led a first survey in 1921, and in 1927 he reached the centre of the devastated zone, expecting a crater. He found none, only the standing, scorched trees at ground zero surrounded by a vast area of fallen timber. Further expeditions followed, and in 1938 he arranged an aerial survey. Later teams found tiny metallic and silicate spheres in the soil and tree resin whose chemistry suggested an origin beyond Earth, though some laboratory findings have not been confirmed.

The prevailing explanation is that a body perhaps 50 to 60 metres across exploded some 5 to 10 kilometres above the ground. Energy estimates vary widely, from about 3 to 30 megatons of TNT, with older figures commonly around 10 to 15 megatons. Whether the object was a stony asteroid or a fragment of a comet remains debated, and no piece of it has been positively identified. The airburst over Chelyabinsk in 2013, though far smaller, has reinforced the airburst model. Tunguska remains the benchmark for assessing the danger from near-Earth objects, and the site is now a nature reserve.

Our 1900 map shows the Russian Empire, including Siberia, under Nicholas II a few years before the blast; the 1914 map shows it on the eve of the Russian Revolution that followed.