On June 30, 1908, an explosion thundered in the air over a dense forest in Siberia, near the Podkamennaya Tunguska River. They say the fireball was 50-100 meters wide. He destroyed 2,000 square kilometers of taiga, knocking down 80 million trees. More than a hundred years have passed since then - the most powerful explosion in documented human history - but scientists are still trying to figure out exactly what happened.
Then the earth was shaking. In the nearest town 60 kilometers away, glass from windows flew out. The residents even felt the warmth of the explosion.
Fortunately, the area in which this massive explosion occurred was sparsely populated. No one died, judging by the reports, only one local reindeer herder died after being thrown into a tree by an explosion. Hundreds of deer have also turned into charred carcasses.
One of the eyewitnesses said that “the sky split in two and high above the forest the entire northern part of the sky was engulfed in fire. And then there was an explosion in the sky and a powerful crack. It was followed by a noise, as if stones were falling from the sky or guns were firing."
The Tunguska meteorite - as this event was dubbed - became the most powerful in history: it produced 185 more energy than the atomic bomb in Hiroshima (and according to some estimates, even more). Seismic waves were recorded even in Great Britain.
Nevertheless, after a hundred years, scientists are still wondering what exactly happened on that fateful day. Many are convinced that it was an asteroid or comet. But practically no traces of a large extraterrestrial object were found - only traces of an explosion - which paved the way for a variety of theories (including a conspiracy).
Tunguska is located far in Siberia, and the climate there is not the most lamp-like. Long, wicked winters and very short summers, when the soil turns into a muddy and unpleasant swamp. It is very difficult to move in such terrain.
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When the explosion rang out, no one dared to investigate the scene. Natalya Artemyeva of the Institute of Planetary Sciences in Tucson, Arizona, says the Russian authorities then had more pressing problems to indulge in scientific curiosity.
The political passions in the country were growing - the First World War and the revolution happened very soon. “Even the local newspapers didn't have that many publications, let alone St. Petersburg and Moscow,” she says.
A few decades later, in 1927, a team led by Leonid Kulik finally visited the explosion site. He came across a description of the event six years earlier and convinced the authorities that the trip would be worth the candle. Once in place, Kulik, even twenty years after the explosion, found obvious traces of the disaster.
He found a huge area of fallen trees that stretched out for 50 kilometers in a strange butterfly shape. The scientist suggested that a meteor from space exploded in the atmosphere. But he was embarrassed that the meteor did not leave any crater - and indeed, the meteor itself was gone. To explain this, Kulik suggested that the shaky ground was too soft to retain the impact marks, and hence the debris left from the impact was also buried.
Kulik did not lose hope of finding the remains of the meteorite, as he wrote about in 1938. "We could find ground masses of this nickel-iron at a depth of 25 meters, individual pieces of which could weigh one to two hundred metric tons."
Later, Russian researchers stated that it was a comet, not a meteor. Comets are large chunks of ice, not rock like meteorites, so this could explain the absence of fragments of foreign stone. Ice began to evaporate already at the entrance to the Earth's atmosphere and continued to evaporate until the very moment of the collision.
But the debate did not end there. As the exact nature of the explosion was unclear, outlandish theories continued to emerge one after another. Some have suggested that the Tunguska meteorite was the result of a collision of matter and antimatter. When this happens, the particles annihilate and release a lot of energy.
Another suggestion was that the explosion was nuclear. An even more ridiculous proposal blamed an alien ship that crashed in search of fresh water on Lake Baikal.
As you might expect, none of these theories fired. And in 1958, an expedition to the site of the explosion discovered tiny residues of silicate and magnetite in the soil.
Further analysis showed that they had a lot of nickel, which is often found in meteorite rock. Everything indicated that it was a meteorite, and K. Florensky, the author of a report on this event from 1963, really wanted to cut off other, more fantastic theories:
"While I understand the benefits of sensationalizing this issue to the public, it should be emphasized that this unhealthy interest, which has arisen as a result of distortion of fact and misinformation, should never be used as a basis for promoting scientific knowledge."
But this did not stop others from coming up with even more dubious ideas. In 1973, the authoritative journal Nature published an article in which it was suggested that this explosion was caused by the collision of a black hole with the Earth. The theory was quickly challenged.
Artemieva says ideas like this are a common by-product of human psychology. “People who love mysteries and 'theories' tend not to listen to scientists,” she says. The Big Bang, coupled with the scarcity of space remains, is fertile ground for this kind of speculation. She also says that scientists have to take responsibility for taking too long to analyze the site of the explosion. They were more concerned with larger asteroids that could cause global extinctions, like the asteroid that was left by Chicxulub crater. Thanks to him, the dinosaurs became extinct 66 million years ago.
In 2013, a group of scientists put an end to much of the speculation of the previous decades. Under the leadership of Viktor Krasnytsya from the National Academy of Sciences of Ukraine, scientists analyzed microscopic samples of stones collected from the explosion in 1978. The stones were of meteorite origin. Most importantly, the analyzed fragments were extracted from a layer of peat that was collected back in 1908.
These samples contained traces of a carbon mineral - lonsdaleite - whose crystal structure resembles diamond. This particular mineral is formed when a graphite-containing structure like a meteorite crashes into the Earth.
“Our study of samples from Tunguska, as well as studies of many other authors, have shown the meteoric origin of the Tunguska event,” says Krasnytsya. "We believe that nothing paranormal happened in Tunguska."
The main problem, he said, is that researchers have spent too much time looking for large pieces of rock. "You had to look for very small particles," like the ones his group was studying.
But this conclusion was not final either. Meteor showers are frequent. Many small meteorites could have gotten to Earth unnoticed. Samples of meteoric origin could well have traveled this way. Some scholars have also questioned whether the peat was harvested in 1908.
Even Artemyeva says that she needs to revise her models to understand the complete absence of meteorites in Tunguska. And yet, according to the early observations of Leonid Kulik, today a broad consensus implies that the Podkamennaya Tunguska event was caused by a large cosmic body, asteroid or comet, which collided with the Earth's atmosphere.
Most asteroids have fairly stable orbits; many of them are in the asteroid belt between Mars and Jupiter. However, “different gravitational interactions can lead to dramatic changes in their orbits,” says Gareth Collins of Imperial College London, UK.
From time to time, these solids can intersect with the Earth's orbit, and hence collide with our planet. The moment such a body enters the atmosphere and begins to crumble, it becomes a meteor.
The Podkamennaya Tunguska event is interesting for scientists because it was an extremely rare case of a "megaton" event - the energy emitted during the explosion was equal to 10-15 megatons of TNT equivalent, and this is by the most conservative estimates.
This also explains why the event was difficult to fully grasp. This is the only event of this magnitude that has happened in recent history. So our understanding is limited, Collins says.
Artemyeva says there are clear milestones, which she outlined in a review that will be published in the Annual Review of Earth and Planetary Sciences in the second half of 2016.
First, a space body entered our atmosphere at a speed of 15-30 km / s.
Fortunately, our atmosphere protects us perfectly. "It will rip apart a rock smaller than a football field across," explains NASA researcher Bill Cook, chief of meteoroid research at NASA. “Most people think that these stones tumble into us from outer space and leave craters, and a column of smoke will hang above them. But the opposite is true."
The atmosphere tends to break rocks several kilometers above the Earth's surface, releasing a rain of small rocks that will cool off by the time they fall to the ground. In the case of Tunguska, the flying meteor had to be extremely fragile, or the explosion was so powerful that it destroyed all its remnants 8-10 kilometers above the Earth.
This process explains the second stage of the event. The atmosphere vaporized the object into tiny pieces, and at the same time, intense kinetic energy turned them into heat.
“This process is similar to a chemical explosion. In modern explosions, chemical or nuclear energy is converted into heat,”says Artemyeva.
In other words, any remnants of whatever entered the Earth's atmosphere turned into cosmic dust.
If everything was so, it becomes clear why there are no giant debris of cosmic matter at the site of the fall. “It is difficult to find even a millimeter grain throughout this large area. You have to look in the peat,”says Krasnitsya.
When the object entered the atmosphere and fell apart, intense heat generated a shock wave that spread hundreds of kilometers. When this air blast hit the ground, it knocked down all the trees in the area.
Artemyeva suggests that this was followed by a giant plume and a cloud "thousands of kilometers in diameter."
And yet, the history of the Tunguska meteorite does not end there. Even now, some scholars say that we are missing the obvious in trying to explain this event.
In 2007, a group of Italian scientists suggested that a lake 8 kilometers north-northwest of the epicenter of the explosion could be an impact crater. Lake Cheko, they say, had not been marked on any map prior to this event.
Luca Gasserini of the University of Bologna in Italy traveled to the lake in the late 1990s and says it is difficult to explain the lake's origin in any other way. "Now we are sure that it was formed after the impact, but not from the main body of the Tunguska asteroid, but from its fragment that survived the explosion."
Gasperini is firmly convinced that most of the asteroid lies 10 meters below the bottom of the lake, buried under the sediments. “The Russians could easily go there and drill the bottom,” he says. Despite serious criticism of this theory, he hopes that someone will extract traces of meteorite origin from the lake.
Lake Cheka as an impact crater is not a popular idea. It's just another "quasi-theory," says Artemieva. “Any mysterious object at the bottom of the lake could be removed with minimal effort - the lake is shallow,” she says. Collins also disagrees with Gasperini.
In 2008, he and his colleagues published a rebuttal to this theory, in which they stated that there were "intact old trees" next to the lake, which would have been destroyed if a large piece of rock fell nearby.
If not to talk about details, we still feel the consequences of the Tunguska event. Scientists continue to publish their work.
Astronomers can look into the sky with powerful telescopes and look for signs of other similar rocks, which can also cause massive damage.
In 2013, a relatively small meteor (19 meters in diameter) that exploded over Chelyabinsk in Russia left significant damage. This surprises scientists like Collins. According to his models, such a meteor should not cause any damage at all.
“The complexity of this process is that the asteroid collapses in the atmosphere, slows down, evaporates and transfers energy to the air, all this is difficult to simulate. We would like to learn more about this process in order to better predict the consequences of such events in the future."
Meteors the size of the Chelyabinsk one fall approximately every hundred years, and the size of the Tunguska one - once every thousand years. It was thought so before. Now these figures need to be revised. Perhaps the "Chelyabinsk meteors" fall ten times more often, says Collins, and the "Tunguska" ones arrive once every 100-200 years.
Unfortunately, we are defenseless in the face of such events, says Krasnitsya. If a similar Tunguska event occurs over a populated city, thousands, if not millions of people will die, depending on the epicenter.
But it's not that bad. The likelihood that this will happen is extremely low, according to Collins, given the huge surface area of the Earth that is covered with water. Most likely, the meteorite will fall far from where people live.
We may never know what the Tunguska meteorite was, a meteor or a comet, but in a sense it doesn't matter. What matters is that we talk about it a hundred years later, and we really care about it. Both can lead to disaster.
ILYA KHEL