The Tunguska Event: What Flattened a Siberian Forest in 1908

On the morning of 30 June 1908 a huge explosion over a remote part of Siberia flattened the forest across about 2,150 square kilometres, an area larger than Greater London. It knocked people off their feet, shattered windows up to 200 kilometres away and was recorded by instruments around the world.

Scientists now agree that it was caused by a space rock, tens of metres across, that exploded in the air before it reached the ground. That is why there is no crater, and why the Tunguska event became the benchmark for thinking about the danger from asteroids.

What happened

Witnesses across central Siberia saw a fireball brighter than the Sun cross the clear morning sky from south-east to north-west. At about 7:14 local time there was a deafening explosion over the taiga near the Podkamennaya Tunguska River, followed by a column of fire and a cloud of smoke.

The blast flattened millions of trees and killed many reindeer belonging to the Evenk herders who lived in the area. Its heat was felt about 80 kilometres away. At least three people are thought to have died, a low number only because so few lived nearby. Had the same explosion happened over a city, it could have destroyed it.

Felt around the world

Seismic stations in Irkutsk, Tashkent, Tiflis and Jena recorded the shock, which was later estimated at close to magnitude 5. Pressure waves showed up on barographs as far away as London, Washington and the Dutch East Indies, though the link to Tunguska was only made years later.

For several nights afterwards the sky over Europe and western Asia glowed so brightly that people could read a newspaper at midnight without a lamp. Astronomers in Britain complained that they could not make their observations. The glow faded after a few days, but strange twilight colours lingered for weeks.

No crater

Because the area was so remote, no scientist reached it for years. In 1921 the mineralogist Leonid Kulik led a first survey for the Soviet Academy of Sciences, and in 1927 Evenk guides took his team to the centre of the destruction, where he expected to find a large crater and valuable meteoric iron.

There was no crater. Instead the team found a patch of forest at the centre where trees were still standing but stripped of their branches, surrounded by trees knocked flat and pointing away from the centre. Later surveys mapped the fallen forest as a vast butterfly shape about 70 kilometres across. Kulik led more expeditions over the next decade but never found a meteorite.

An explosion in the air

The pattern of fallen trees is the key clue. Trees directly below an explosion are hit from above and stay upright, while those further out are blown over. That points to a blast high in the air, not an impact on the ground.

The leading explanation is a meteor air burst. Objects from space hit the atmosphere at more than 11 kilometres per second, and the air in front of them is compressed and heated so violently that most break up or explode. Researchers think the Tunguska object was about 50 to 60 metres across and exploded between 5 and 10 kilometres up.

How big was the blast?

Estimates vary. Early figures, based on comparisons with nuclear weapons, put the energy at 10 to 15 megatons of TNT, about a thousand times the first atomic bomb test. Later calculations that take account of the object's motion suggest that more of the energy was directed downwards, so a smaller blast of 3 to 5 megatons could have done the same damage. Other studies give higher numbers.

Either way, it is the largest impact event on Earth in recorded history. The 2013 Chelyabinsk meteor, which exploded over a Russian city and shattered windows, was far smaller, but it gave scientists modern measurements of a similar air burst to test their models against.

Asteroid or comet?

What is still debated is the nature of the object. An asteroid, made of rock or metal, and a comet, made largely of ice and dust, could both explain an air burst, and no confirmed fragment has been found to decide between them.

Lake Cheko, about 8 kilometres from the centre, has been suggested as a crater made by a piece that broke off before the explosion, but studies of its sediments disagree. A few researchers have proposed explanations from the ground, such as a release of natural gas, but these remain on the fringe, well outside the scientific consensus.

Myths and fiction

A mystery with no crater was bound to attract other explanations. A 1946 Soviet science fiction story imagined the nuclear explosion of a Martian spacecraft, and later novels blamed everything from a human-made ship to a tiny black hole. A popular legend even claims the inventor Nikola Tesla caused it with a death-ray experiment, a story that traces back to a fictionalised biography published in 1994.

None of these needs to be taken seriously when the physical evidence fits a natural explanation so well. Our guide on how to evaluate a claim explains how to weigh an exciting story against the evidence.

Why Tunguska matters

Tunguska is the event planners point to when they explain why tracking near-Earth asteroids matters. An object only tens of metres across, too small to see easily in advance, released enough energy to wipe out a city.

In 2016 the United Nations made 30 June, the anniversary of the blast, International Asteroid Day, to raise awareness of the risk and of work on planetary defence. The site itself is now a nature reserve, where the forest has slowly grown back over the fallen trees.