On the night of 30 June 1908, many people in the UK took advantage of an unusual and prolonged glow in the sky to play a night game of cricket or read the newspaper outdoors, without turning on the light. They did not know that more than 5,500 kilometers away from them there had just been a gigantic and mysterious explosion in the Earth’s atmosphere, which was then called "the Tunguska event".
The big explosion
S. Semenov was outside his home in Vanavara, a small rural town in Siberian Russia, waiting for breakfast (of his name is documented only the initial). Suddenly he noticed a big flash in the sky: it was as if he had "divided into two parts", he would tell later, and half of it had ignited. As the strange strip in the sky grew larger and larger, Semenov felt an unusual and growing sensation of warmth, to the point of wanting to take off his shirt because of the heat. He was surprised and was trying to think about what to do, when he felt a strong blow; jumped a few meters and fell unconscious. It was his wife who brought him home, where he found in time to hear another series of very strong blows followed by a tremor of the ground. Frightened, lying on the ground with his face facing the floor, Semenov tried to protect his head in the fear that something might hit him. When everything seemed to be finished, he got up in the broken windows, went out and counted the damage in his garden.
Semenov’s testimony has come to this day thanks to the mineralogist Leonid Kulik, who collected it in 1930 during one of his expeditions dedicated to the study of the event in Tunguska. According to reconstructions by Kulik and other researchers, the great glow in the sky occurred around 7:17 local time on June 30, 1908. Among the first to see him were some Russian settlers, such as Semenov, and several people of the Siberian nomadic tunguse (or evenchi) people. The glow was observed from the hills north-west of Lake Baikal: it was bluish and very intense, to the point of making brighter the first hours of that summer morning.
After about ten minutes there was a strong flash in the sky, followed by an intense noise comparable to a very strong cannonade, the witnesses would later recount. As had happened in Semenov, many people were thrown meters away from where they were, pushed by a very intense wind and followed by a tremor of the ground. In an area of about 2,000 square kilometers around the point of the explosion, millions of trees were bent and cut down in a few moments by the strong shock wave.
An event like that over a city could have caused tens of thousands of deaths. It is believed that only two people died in the Tunguska event, because the area was sparsely inhabited, but no proper tests were carried out to confirm this. The ground tremor was recorded by numerous seismic stations in Europe and Asia: some detected a shock comparable to that of a magnitude 5 earthquake.
No crater
For more than ten years, no major investigations were conducted into the Tunguska event. In the 1920s, Kulik learned several details about the circumstances of the explosion, concluding that it was caused by the ground impact of a large meteorite. He persuaded the Soviet government to finance an expedition, telling that it would be useful to recover precious meteoric iron to study and use in heavy industry.
In 1927, Kulik’s expedition was able to identify where the explosion had caused the most damage. Upon their arrival, the researchers looked incredulously at the view: there was not a crater, but trees without foliage and fronds within 8 kilometers, surrounded by an expanse of trees as far as the eye could see. Many years later it would have been discovered that the area of forest felled was very wide, with a shape similar to that of a butterfly with open wings until reaching a width of 70 kilometers.
Over the next ten years, Kulik returned to the area with other expeditions to collect data and samples on the Tunguska event. He found cavities in the ground that he hypothesized could be craters caused by the meteorite, but further research led him to exclude it. The first aerial photographs were taken only in 1938 thirty years after the explosion in the sky, but they did not allow to obtain much more information.
Interest in the Tunguska event resumed after World War II, with new expeditions organized this time to analyze the soil and trees in search of some clue. The analysis led to the discovery of several metals not compatible with the surrounding environment, thus bringing new elements to the theories that the cause of the glow and the great roar on Siberia was a meteor.
Meteore
Every year about 40,000 tons of space rocks fall on Earth, but we rarely realize this because they are usually small meteors that pulverize during their high-speed impact with the atmosphere. On average, more significant events occur every ten years, such as that of Chelyabinsk in 2013, also in Russia. Since the portion of the Earth inhabited by humans is relatively limited, impacts usually occur in uninhabited areas or on the oceans and therefore do not create any particular damage. An event such as that of Tunguska is much rarer - according to some estimates one occurs every three centuries - and for this has led to dozens of research, and even today, after more than a century, it is discussed on its actual cause.
Asteroide o cometa?
The most shared hypothesis is that on the morning of June 30, 1908, an asteroid collided with Earth, and exploded in its entry into the atmosphere at an altitude between 6,000 and 10,000 meters in the sky of Siberia. The ground butterfly shape produced by the explosion and the location of the trees seems to provide further confirmation, also based on observations made after atomic tests conducted in the lower parts of the atmosphere. At the point directly below the explosion, vertical shock waves develop, which then leave the trunks of the trees standing, but destroy all horizontal elements such as branches. At a greater distance from the point of explosion, the shock wave propagates almost horizontally and then knocks down what is on its way.
The fact that a crater or a piece of the meteorite has never been found (so we call what remains and reaches the ground of a celestial body after its passage through the atmosphere) makes, according to some, less convincing the hypothesis of the asteroid. Already in the thirties it was imagined that the event of Tunguska was caused not by a large rock like an asteroid, but by a comet consisting mostly of ice and dust. This would be completely pulverized during the passage into the atmosphere, leaving a huge trail of shiny fragments, which would then reflect the sun’s rays leading to the phenomenon of the "bright nights" described in the United Kingdom and other Western European countries. In that summer of 1908 several astronomical observatories also reported a higher presence of dust in the sky, such as to complicate their observations.
The comet hypothesis found other supporters in the following decades, with speculation as to which celestial body had broken into the atmosphere in one of its periodic close-ups with the Earth, following its own orbit around the Sun. In 2001 Paolo Farinella of the University of Trieste published together with other colleagues a research in which he expounded the results of a mathematical model, performed by evaluating the probable orbit followed by the spatial object and the inclination at the entry into the atmosphere. The study concluded that it was much more likely (83 percent) that it was an asteroid from the main belt, the large region of the solar system between Mars and Jupiter dense with asteroids, and not a comet (17 percent).
Supporters of the comet hypothesis and those of the asteroid continue to confront each other today, in a very interesting scientific debate not only to reconstruct what happened in Tunguska, but also to better understand causes, dynamics and effects of the entry into the atmosphere of medium-sized celestial bodies.
What if the crater is now a lake?
It is the absence of a crater that leaves researchers puzzled: an asteroid should have produced one at the time of its impact on the ground, although others theorize that an asteroid around 60 meters in diameter could have been completely pulverized before reaching the ground.
A decade ago, a research group at the University of Bologna led by Luca Gasperini proposed some new ideas, saying they had identified a lake with characteristics compatible with those of a crater caused by the event of Tunguska. According to them, most of the celestial body that caused the explosion was pulverized in the sky, but a fragment with a maximum width of 10 meters still reached the ground where now is the Čeko Lake, which was formed just after the impact. It would be filled with water in the following years due to the proximity of a river. The hypothesis is fascinating, but has not yet found completely convincing evidence and evidence. Last year, moreover, a group of Russian researchers analyzed the soil and other samples taken in the area, arriving at the conclusion that the lake has existed for at least three centuries and not for a hundred years as hypothesized by Gasperini and colleagues.
Explosion of natural gas
However, some researchers are not convinced of an extraterrestrial origin of the Tugunska event. Against the approval of most of the scientists involved in the case, there are geologists who hypothesize that the great explosion of 1908 was caused by about 10 million tons of natural gas, leaked from the Earth’s crust: the gas would have gone up to the lower layers of the atmosphere where it would have been lit by lightning. The flames would follow the gas column to the crack in the ground and lead to the great explosion. As original as this theory is, it is considered to be implausible.
Books and screens
Tunguska’s event has inspired dozens of authors over the years, who have recounted the mysterious explosion in the sky in their novels or adapted it to their narrative needs. Isaac Asimov in The Mad Scientist, Arthur C. Clarke in the introduction of Encounter with Rama and Thomas Pynchon in his Against the Day. The story is covered in many superhero comics, is mentioned in video games such as those of the series Assassin’s Creed and in Call of Duty: World at War, and it is mentioned in the movies Indiana Jones and the kingdom of the crystal skull and in Ghostbusters, even if with a reference to 1909 and not to the correct date of 1908.
Atomic bombs
It is not easy to assess precisely the amount of energy released by the Tunguska event. The most cautious simulations speak of 3 megatons, equal to 200 times the atomic bomb dropped by the United States on the Japanese city of Hiroshima in 1945. The estimates vary a lot: there are those who calculated 10 megatons and those who went so far as to assume 30. The most reliable values, also based on more accurate computer simulations, place the event between 3 and 5 megatons.
As we have seen, the event at Tunguska could have caused enormous damage if it had happened over a densely populated area, but still contained and not such as to pose a long-term threat to our civilization. Researchers speculate that an asteroid with a diameter between 1 and 2 kilometers could instead cause the death of more than a billion people. The dust produced in the atmosphere would cloud the sky and could take up to 10 years to dissolve, preventing the proper cultivation of the fields and changing the seasons.
A century of research
Over the years more than a thousand scientific and academic researches have been produced on the Tunguska event, many of them have helped us to better understand what happens when there is a great impact in the atmosphere. However, no one has yet explained with certainty what interrupted S. Semenov’s breakfast on that morning of 30 June 1908.
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