At about a quarter past seven on the morning of 30 June 1908, a man sitting on the porch of a trading post at Vanavara, in central Siberia, watched the northern sky split open with fire. Moments later the heat reached him — he later told investigators it felt as though his shirt were on fire — and then a blast hurled him from the porch. He was roughly 65 kilometres from the centre of the largest impact event in recorded human history.
The numbers still astonish. The explosion above the Podkamennaya Tunguska river flattened forest across roughly 2,000 square kilometres — an area larger than Greater London — felling an estimated 80 million trees in a pattern later likened to a gigantic butterfly. Its energy is commonly estimated at ten to fifteen megatons of TNT, on the order of a thousand Hiroshima bombs, though estimates range from a few megatons to as much as thirty — everything about the object must be inferred from felled trees and distant instruments. Seismographs across Eurasia twitched; barographs as far away as Britain recorded an atmospheric pressure wave that travelled round the planet. For several nights afterwards the skies over Europe and western Asia glowed so brightly that Londoners reportedly read newspapers outdoors at midnight, the atmosphere luminous with high-altitude dust. And yet, in one of history's great strokes of luck, the blast struck one of the emptiest regions on Earth: no death was ever confirmed, though a handful were reported among the Evenki reindeer herders of the area.
Nineteen years to the crater that wasn't
Almost as remarkable as the event is how long nobody went to look. Tsarist Russia's decline, the First World War and the revolution intervened, and not until 1927 did the Soviet mineralogist Leonid Kulik lead the first scientific expedition to the site, hoping to recover meteoric iron for Soviet science and industry. He expected a crater and found none. At the very centre of the devastation the trees still stood upright — stripped of branches and bark, a ghost forest nicknamed the "telegraph poles" — while for kilometres in every direction beyond, the trunks lay felled radially outward, all pointing away from a centre that contained nothing. Kulik returned repeatedly through the 1930s, draining bogs and digging pits, and never found a fragment of his meteorite. He died in 1942 as a prisoner of war, his mystery unsolved.
The physics of an airburst
The solution is that nothing substantial ever reached the ground. The modern consensus holds that a stony asteroid — or possibly a fragment of comet — some tens of metres across, perhaps 50 to 60 metres, slammed into the atmosphere at tens of kilometres per second. At such speeds the air cannot flow out of the way: it piles up ahead of the falling body as a wall of pressure that squeezes the rock harder than its own strength can bear. Somewhere between about five and ten kilometres up, the object shattered catastrophically, its enormous kinetic energy converting to heat and blast in a fraction of a second. That is an airburst. Directly beneath the explosion the shockwave arrived vertically, snapping branches while leaving trunks standing — the telegraph poles — while farther out it struck at an angle and pushed whole trees over radially. No crater formed because no coherent projectile survived; a stony body of that size vaporises and disperses. The comet-versus-asteroid debate ran for a century, with the bright European nights counted as evidence for a comet's dust, but most modern studies favour a small stony asteroid.
Nature eventually staged a scaled-down demonstration. On 15 February 2013 a rock roughly 20 metres across exploded about 30 kilometres above Chelyabinsk, Russia, with an energy near 500 kilotons; around 1,500 people were injured, mostly by flying glass, and the whole physics lesson was captured on dashboard cameras. Tunguska was the same phenomenon writ far larger. The exotic alternatives proposed over the decades — antimatter, a miniature black hole, a crashing alien ship (an idea launched by a 1946 science-fiction story), Nikola Tesla's imaginary death ray — were always answers to a question the physics had already settled. A claim that nearby Lake Cheko is the crater of a surviving fragment remains disputed, with Russian studies dating the lake as older than 1908.
Events of Tunguska's scale are thought to strike Earth perhaps once every few centuries, which is why 30 June is now marked as International Asteroid Day. The forest has long since regrown. The warning stands.
Quiz nuggets
- The Tunguska event occurred on 30 June 1908 over the Podkamennaya Tunguska river in Siberia.
- The blast flattened around 80 million trees across roughly 2,000 square kilometres, yet left no impact crater.
- Leonid Kulik led the first scientific expedition to the site in 1927, nineteen years after the explosion.
- The object is thought to have been a stony body tens of metres across that exploded five to ten kilometres above the ground.
- International Asteroid Day is observed on 30 June, the anniversary of Tunguska.