The cause of the huge explosion that took place in Siberia in 1908 remains a mystery, but a new study now provides a plausible explanation.
15 megatons
On the morning of June 30, 1908, an explosion of about 15 megatons of TNT took place near the Tunguska River, this is about 1000 times more powerful than the Hiroshima atomic bomb. As it was a very remote and unpopulated area, the victims could be counted on the fingers of one hand, but there were people who were hit and knocked down by the shock wave several kilometers away from the central location of the explosion. It is estimated that more than 80 million trees spread over an area exceeding 2,000 square kilometers were destroyed .
The explosion was recorded on seismographs across Europe and Asia; in some places, the impact wave was equivalent to that created by a magnitude 5 earthquake on the Richter scale. For several months, the US Smithsonian Astrophysical Observatory observed a decrease in the transparency of the sky due to the dust that was suspended in the atmosphere after the explosion.
Seriously, a black hole?
The cause of the Tunguska event has always been a mystery. The remote location and investigative means of the time did not allow for a rapid assessment of the damage and causes, and all subsequent studies have relied on detailed inspection of the site long after the explosion occurred. Naturally, the first explanation that was advanced was the impact of a meteorite. Estimates showed that the meteoroid (the body that entered the atmosphere) must have been about 100 meters in size (similar to the length of a soccer field).
However, repeated expeditions to the area beginning in the 1950s failed to find the meteorite or identify the crater it must have left on impact . Hence the imagination was soon triggered and more exotic (albeit less justified) theories were resorted to, such as the landing of a large spacecraft, or the fall of a mass of antimatter that was annihilated with another equivalent mass of terrestrial matter. It was even seriously discussed whether the fall of a black hole could have been the cause of the explosion. In fact, a scientific article describing this possibility was published in Nature in 1973, although it was denied a few months later by another article also published in Nature.
The most widely accepted hypothesis so far remained that of the meteoroid. To explain the absence of the crater and the remains of the meteorite, it was proposed that the body that entered the atmosphere, a fragment of asteroid or comet, disintegrated in the air, about 5 kilometers above the ground, thus creating the great explosion and the shock wave. However, this explosion must have thrown some remnant of calcined meteorite on the ground and the fact of not finding any piece continued to raise doubts about this explanation.
A colossal piece of grazing iron
A team of Russian researchers led by Daniil E Khrennikov (Federal University of Siberia) has just published a new study in which they propose that the meteoroid that flew over Tunguska only touched the planet and that it was made of iron, that is, It was similar to the Hoba we presented in this section a few weeks ago. The researchers have carried out computer simulations of the trajectories that could be followed by bodies of sizes between 50 and 200 meters, that were made up of either frozen rock or pure iron, and that approached distances from the Earth's surface of between 10 and 15 kilometres.
Calculations show that a body made of rock and ice would have completely disintegrated and could not have caused an explosion like the one in Tunguska. After multiple simulations, the researchers conclude that a piece of pure iron asteroid, between 100 and 200 meters in size , that would have 'brushed' the Earth at a speed of about 72,000 kilometers per hour, could well explain what happened. This colossal chunk of iron would have entered the atmosphere at an angle
of inclination on the ground less than 11.5 degrees, that is, following an almost horizontal trajectory, and about 10-15 kilometers of minimum altitude. The shock wave created from that altitude would have been able to knock down the trees over hundreds of kilometers and burn the most exposed surface.
During the friction with the atmosphere, due to its great mass and momentum, the meteoroid did not break and would only have lost some of the matter, but this would have been volatilized in the form of plasma and gas, and would have later dispersed on the ground without leave a clear trail. After its grazing flight, which crossed a length in the atmosphere of about 3,000 kilometers, the meteoroid was able to get out of the air again to calmly continue its swift flight through outer space.
Although the explanation seems convincing at first glance, some other researchers who have not participated in this study have already expressed doubts about whether the created blast wave would have the characteristics of what was observed in Tunguska. And it is that Khrennikov's team has not modeled the details of the propagation of the shock wave in the atmosphere. These simulations are clearly essential before definitively concluding on the validity of this hypothesis, but it must be recognized that the explanation described by the Russian team is very plausible .
The article by Khrennikov et al., Entitled "On the possibility of through passage of asteroid bodies across the Earth's atmosphere", has recently been published in the British magazine Monthly Notices of the Royal Astronomical Society and can be found at this link.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
Photo1: Image of Tunguska taken by a Russian expedition in 1920
Photo 2: Artistic recreation of a meteoroid
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