Tunguska (airburst, no crater)

Key facts

Country

Russia

Diameter

No crater

Date

30 June 1908

Location

60.89° N, 101.89° E

On the morning of 30 June 1908 an enormous explosion shook the forests near the Podkamennaya Tunguska River in central Siberia. It was the largest impact event in recorded history, yet it left no crater. Instead, an incoming asteroid or comet exploded in the air several kilometres above the ground, in what scientists call an airburst.

The blast. Witnesses in the sparsely populated region described a brilliant light moving across the sky, followed by a blast of heat and a shock wave that knocked people off their feet many tens of kilometres away. Windows broke in settlements far from the site. The explosion flattened an estimated 80 million trees across about 2,000 square kilometres of forest, an area larger than Greater London. Close to the centre, trees were left standing but stripped of branches, like telegraph poles, because the blast came from directly above.

The energy released is usually estimated at around 10 to 15 megatons of TNT, though estimates range more widely, a thousand times more than the atomic bomb dropped on Hiroshima. The incoming object is thought to have been perhaps 50 to 60 m across, though estimates vary.

Glowing skies. For several nights after the event, the skies over Europe and Asia glowed so brightly that people in Britain reported being able to read outdoors at midnight. Instruments in Britain and elsewhere recorded the pressure wave as it travelled around the world.

The search. Because of the remoteness of the area and the upheavals in Russia at the time, the first scientific expedition did not reach the site until 1927. It was led by the Russian scientist Leonid Kulik, who expected to find a giant crater and a huge meteorite. Instead, he found the vast area of flattened forest, with the trees pointing away from the centre, but no crater. Kulik led several further expeditions, and many more have followed. Tiny particles thought to come from the impactor have been reported in soils and tree resin, but no large meteorite has ever been found.

Asteroid or comet? Scientists still debate whether the Tunguska object was a stony asteroid or a fragment of a comet. Most now favour a stony asteroid that broke apart and exploded from the pressure of the atmosphere. Suggestions that a nearby lake, Lake Cheko, is a crater from a surviving fragment remain controversial.

Why it matters. Tunguska shows that an object only a few tens of metres across can cause devastation over a huge area without leaving a crater. If a similar event happened over a city, the results would be catastrophic. That is why astronomers search the sky for near-Earth objects, and why 30 June is now marked each year as Asteroid Day to raise awareness of the impact hazard.

Did you know?

The meteorite

The Tunguska event