On the morning of 30 June 1908, a massive explosion shook the sparsely-populated Eastern Siberian taiga. It was a visitor from outer space: a comet or asteroid explosively disintegrated in the atmosphere and caused a great amount of damage locally.
The Tunguska event caused dust to be thrown high into the atmosphere where it remained for some time. As far away as the UK and Ireland there were reports of bright nigh-time lights caused by sunlight reflecting off the dust. It also affected the climate around the world for several days, as our records show. Some newspaper cuttings are included in the gallery at the bottom of the page
Early in the morning – a little after 7am local time – in the plains of central Siberia populated only by a handful of natives and some Russian settlers, “the sky split in two and fire appeared high and wide over the forest. The split in the sky grew larger, and the entire northern side was covered with fire.” This is an account from an eyewitness who was at Vanavara trading post, some 65 km (40 miles) south of the epicentre. He went on to say that his shirt felt as if it were on fire, the ground shook, and shortly afterwards a great blast pushed him off his chair and threw him a few metres away.
Soon after, a hot rushing wind came, “which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn, a part of the iron lock snapped.”
A Large Explosion
Anyone familiar with the details of the events in Japan in August 1945 might recognise this sequence as being extremely similar to the events close to ground zero of the nuclear explosions above Hiroshima and Nagasaki, except of course the Tunguska event – named after the nearby river called Stony Tunguska (‘Podkamennaya Tunguska’, literally ‘Tunguska under the stones’) – took place several decades earlier. Indeed, this has led to some science-fiction theories about nuclear-powered alien spacecraft exploding over the Siberian skies. More down-to-Earth commentators note that any large explosion will exhibit similar patterns. A notable feature of nuclear explosions, but missing from the account above – because the eyewitness was too far to see it – is the razing to the ground of any buildings or trees near the epicentre, possibly combined with spreading of fires.
The flattening of more than 80 million trees over 2150 square kilometres (830 square miles) in a radial pattern around the epicentre was indeed confirmed by the expeditions, which went out to explore the origin of this extraordinary event.
The Visitor From Space
In the decades that followed, most scientists have agreed on two most likely explanations for what had occurred at Tunguska. It is quite clear that an object from outer space entered the atmosphere over central Siberia. Because no crater was ever found on the ground, and no meteorite either, it is thought that the object disintegrated (exploded) before reaching the surface. The blast area and other effects point to that conclusion, too. The two options, then, are what the object itself might have been. Some argue that it was a small asteroid, others that it must have been a small comet (or a comet fragment). The glowing skies – suggesting high levels of water vapour in the atmosphere – are more likely to occur due to a comet, but some investigations of the resin of the impacted trees noted presence of material common in asteroids and rare in comets.
There are numerous other indications, too, which go in favour of one hypothesis and against the other. 110 years later, the matter still hasn’t been resolved. If it was an asteroid, it was probably about 36 m (120 ft) in size and 100,000 metric tons (220 million pounds) in weight. A comet – being less dense – would have to be slightly larger.
Central Siberia is a very remote and sparsely populated place, even for other Russians, so the first expedition to the area to investigate what might have happened was not mounted until 1927; almost 20 years after the event. The leader of this and several subsequent expeditions was a Russian mineralogist by the name of Leonid Kulik, who was surveying the nearby regions in the 1920s and heard local accounts of this strange explosion. Apart from two unconfirmed reports there are thought to have been no human casualties caused by the Tunguska event, but hundreds of reindeer on which the local tribes depended had died in the blast.
It is chilling to think what damage such an impact might cause if it fell on a metropolitan area. Greater London (1,572 square km or 607 square miles) covers only three-quarters of the area where the trees were completely knocked down and has a population of nearly 9 million! It is lucky, then, that about 71% of the Earth is covered by water, and only about a tenth of the remaining 29% is thought to be urban land. This means we would have to be very unlucky indeed to see such an impact hit a densely populated region (this is one thing Deep Impact – the film, not the spacecraft – got right, and Armageddon did not).
To commemorate the Tunguska Event and to raise public awareness about the asteroid impact hazard, the United Nations has declared 30th June ‘Asteroid Day’.
Looking Out And Ahead
Space scientists specialising in small bodies of the Solar System are keenly aware that there are millions upon millions of rocks floating through space near the Earth. While an object would have to be very large indeed to wreak havoc on the entire planet – and those have all been discovered and accounted for, and none will impact the Earth in the coming centuries at least – an object a few tens of metres across can still cause significant devastation, as the Tunguska Event shows us.
For that reason astronomers and governments around the world have been cataloguing ‘near-Earth objects’ (NEOs) of all shapes and sizes for decades already. Some of the more notable project are LINEAR (Lincoln Near-Earth Asteroid Research), Pan-STARRS (Panoramic Survey Telescope and Rapid Response System), and NEOWISE (Near-Earth Object WISE [Wide-field Infrared Survey Explorer]).
Most of the text above was adapted from a detailed article by Rok Nežič written to commemorate 110 years since the Tunguska Event on our blog, Astronotes, in the summer of 2018. You can find it by clicking this link.
Closer to the 100th anniversary of Tunguska Event, Bill Napier and David Asher – both linked to Armagh Observatory at the time – wrote an article about the Tunguska Event for the Royal Astronomical Society’s magazine, Astronomy & Geophysics (A&G), in February 2009. It is freely available on the Oxford University Press website; follow this link..