On June 30, 1908, an aerial explosion of a power similar to a thermonuclear weapon knocked down trees in an area of 2,150 square kilometers. The event took place near the Podkamennaya River in Tunguska (Russia), and about thirty hypotheses have been proposed to explain it. But what really happened in Tunguska?
Humanity, or at least modern man, had had few opportunities to witness events of this kind. In fact, there were still several decades to go before nuclear bombs could produce effects as devastating as those that occurred in June 1908. Apart from some volcanic eruptions or major earthquakes, no phenomenon had been able to knock down people standing more than 400 kilometers from the site. This explains the fascination that the so-called Tunguska Event holds for most people, and why there are so many hypotheses trying to explain what happened that summer.
The Facts
On June 30, 1908, an explosion occurred that was detected by numerous seismic stations around the world. Its magnitude was such that it could even be recorded by a barometer in a meteorological office in the United Kingdom, located thousands of kilometers away, thanks to fluctuations it caused in atmospheric pressure. Much less subtle, the effects in the vicinity of "ground zero" included the felling of all trees in an area of more than 2,000 square kilometers.
In towns located 400 kilometers from the epicenter of the explosion, the shockwave was powerful enough to break windows and knock down adults. At 600 kilometers from the impact, in the Kansk district, residents say that some sailors and horses were knocked down by the shockwave, while houses shook and crockery broke. Even the driver of the famous Trans-Siberian train had to stop because he feared the vibrations would derail the convoy.
Some witnesses reported that during the weeks following the event, the nights were so bright that in several places in Russia and Europe one could read after sunset without the need for lamps or candles. You can imagine what that meant at a time when almost no one had seen a car or an airplane, where life was simple and predictable, in a world that didn't know television and radio was just being born.
Records show that even in the United States several astronomical observatories, including those at Mount Wilson and the Smithsonian, found a significant decrease in atmospheric transparency that lasted for several months. That effect, the result of the amount of dust thrown into the atmosphere, would be associated years later with nuclear bombs, and would be called nuclear winter, given the potential capacity that such an explosion has to plunge the planet into a long and dark winter due to the darkening of the atmosphere.
In fact, experts estimate that the Tunguska Event released energy equivalent to an atomic bomb of 10 to 15 megatons, where one megaton equals the explosion of one million tons of TNT. To give you an idea of what this means, let's remember that the one dropped on Hiroshima had "only" 0.015 megatons, and it devastated an entire city.
Fortunately, Siberia was a very sparsely populated area. In many regions even today you can travel hundreds of kilometers without finding any trace of humanity. If the Tunguska Event had occurred over a densely populated area, like Europe or America, the dead would have been counted in the millions. Most of the witnesses to the 1908 explosion belonged to the Tungus people, a nomadic tribe of Mongol origin that survived in the cold steppes by herding reindeer. These people said they saw an object fall that "shone like the Sun". It must not have been easy for them to sleep that night!
The Investigation
Such an amazing event deeply intrigued scientists. But despite this, no geologists or other experts were sent to the impact site until 1921. You have to remember that, in terms of distances, the world was much larger in 1908 than in the 21st century. Just crossing the Atlantic was a major event for most people, and such an expedition took years of planning. And the tsarist government of Russia seemed more interested in passing it off as a "divine warning" against ongoing revolutionary unrest than in finding out the truth behind the event.
However, 13 years later, in 1921, the Soviet Academy of Sciences sent scientist Leonid Kulik to Tunguska to study the event. Kulik took a large number of photographs but was unable to discover any crater. He described a region more than 50 kilometers in diameter in which not a single tree was left standing, and made several drawings of the shape of the area. Intrigued precisely by its appearance, which looked a lot like a butterfly, he deduced that possibly two simultaneous explosions had occurred, slightly separated from each other. He returned to the area 17 years later, but could not discover anything more.
In the 1950s and 1960s, better-equipped expeditions were sent, and scientists were able to collect samples that later turned out to be identified as crystalline microlites and other objects of extraterrestrial nature. Recently, in 1999, Italian scientists who traveled to Tunguska showed that Lake Cheko, 50 meters deep and 450 kilometers in diameter, did not exist before 1908. In the sediments of the Cheko bottom, materials of possible extraterrestrial nature were also found.
The Hypotheses
As is obvious, such an event has generated all kinds of hypotheses. First, if you use Google to find information, you'll see how thousands of sites dedicated to UFOs, conspiracy theories, or the supernatural speculate about the possible crash of an extraterrestrial mothership, the detonation of a secret nuclear device (decades before they were "officially" invented), time machines that explode when traveling to the past, antimatter, natural atomic bombs, or practically any nonsense you can think of.
However, the scientific community, based on the evidence collected, witness testimonies, and computerized models, agrees that it was a small comet that caused the Tunguska Event. Composed basically of ice and dust, a comet (or perhaps a "piece of comet") exploded without ever touching the ground, completely vaporized by the heat generated by friction with the Earth's atmosphere. Models show that all the ice contained in the comet can sublimate, directly becoming gas, which when dispersed in the atmosphere would eliminate all traces of the explosion.
The trees would have been felled mainly by the atmospheric shockwave and, to a lesser extent, by the thermal wave. Some think it is very unlikely that a comet went unnoticed in the days (and nights) before the event, but the trajectory of fall suggests that the culprit of the explosion came from a direction almost coincident with the Sun, which would have made it very difficult for astronomers to detect it. Especially if it was small or had exhausted the volatile substances responsible for generating the flashy tails characteristic of these bodies.
Regardless of the type of celestial body involved, or even if the origin of the explosion had been different, the Tunguska Event should be a reminder of how important it is for humanity to have an early warning system that alerts us to the possibility of impact of a meteorite or asteroid on the planet. An explosion 10 times smaller than this, but that took place over any major city in the world, would not leave a single building standing.
