The Tunguska event

In 1908 a cosmic body exploded over Siberia and flattened some 80 million trees – the largest impact event in recorded history.

Key facts

Type

Airburst (probably a stony asteroid; a comet fragment has also been proposed)

Fell or found

Event witnessed; no meteorites recovered

Year

1908 (30 June)

Country

Russia (Siberia)

Mass

object estimated at tens of metres across; no meteorites recovered

Official name

Tunguska event (no meteorite recovered)

Shortly after seven o'clock on the morning of 30 June 1908, people living near the Podkamennaya Tunguska River in central Siberia saw a column of bluish light, almost as bright as the Sun, move across the sky. Moments later there was a blinding flash and a series of tremendous explosions. At a trading post called Vanavara, about 65 kilometres away, a man sitting on a porch was thrown from his chair and felt intense heat, as if his shirt were on fire. Windows were broken, and the ground shook.

This was the Tunguska event, the largest impact event in recorded history.

An explosion in the air

Scientists now believe that a cosmic body, probably a stony asteroid some tens of metres across, entered the atmosphere at high speed and exploded at an altitude of several kilometres. This kind of explosion is called an airburst. The energy released is estimated at several million tonnes of TNT, hundreds of times greater than the atomic bomb dropped on Hiroshima, although estimates vary.

The blast flattened an estimated 80 million trees over an area of about 2,000 square kilometres, larger than Greater London. Near the centre, trees were left standing but stripped of branches, like telegraph poles, because the blast came from directly above. Further out, trees were knocked down, pointing away from the centre in a vast radial pattern. Fires burned parts of the forest. Because the region was so remote and thinly populated, very few people were affected, and there are no confirmed records of human deaths, although some local accounts describe deaths and losses among reindeer herders and their animals.

Felt around the world

The shock wave was recorded by barometers as far away as Britain, and seismic stations detected ground vibrations. For several nights afterwards, skies over Europe and western Asia glowed unusually brightly. People reported being able to read outdoors at midnight in places where the sky should have been dark. Scientists think dust and ice high in the atmosphere scattered sunlight long after sunset.

The search for a crater

Because of the remoteness of the region and the upheavals in Russia at the time, the first scientific expedition did not reach the site until 1927. It was led by the mineralogist Leonid Kulik, who was searching for meteorites. He expected to find a crater and a huge iron meteorite. Instead, he found the vast area of flattened forest and no crater. Despite several expeditions, no large meteorite was ever found.

Later studies found tiny particles in the soil and resin of the trees that some researchers have linked to the impacting body. Most scientists think that the body was largely destroyed in the airburst, leaving no large fragments. Some have suggested that the object was a fragment of a comet, which would be made largely of ice and dust, but many researchers think a stony asteroid fits the evidence better.

Why it matters

Tunguska is a powerful reminder that impacts from space are not just events of the distant past. Objects of this size are thought to strike Earth perhaps once every few centuries on average, though estimates vary. If a similar event happened over a city, the consequences could be devastating. Tunguska has inspired research into how airbursts work and has become a key example in planetary defence planning.

In 2016 the United Nations proclaimed 30 June International Asteroid Day, chosen to mark the anniversary of the Tunguska event and to raise awareness of the importance of finding and tracking near-Earth objects.

Tunguska also shows how science works with incomplete evidence. With no meteorite to study, researchers have pieced together the story from eyewitness accounts, fallen trees, distant instrument records and microscopic particles.

Did you know?

See more

The Tunguska site