Ivan Sergeyevich Petrov frowned as looked up from his breakfast of strong tea and millet broth. Something definitely was wrong here. Surely that hum outside couldn't be due to the mosquitoes? As Ivan stepped outside his log cabin and looked around warily, he was greeted by the most shocking sight of his life.
The place: The remote Siberian Podkamennaya Tunguska region in Siberia, north of the Vanavara trade post. The time: Tuesday, June 30, 1908, a quarter past 7 a.m., exactly 101 years ago. A beautiful Siberian early summer day.
But Ivan did not notice the warmth of the summer Sun. The swelling sound that had started him had reached thunder pitch and was still swelling. A huge, incandescent object was tracing a thick pall of smoke into the sky as it streaked overhead.
This sight would have sufficed to frighten Ivan and all others who watched on out of their wits. Here be dragons - was there truth in the old fairy tales after all? Or even worse: was this the day of reckoning?
Frightening though the sight was, the worst was still to come.
The object that had traced its ominous path suddenly expired in a massive explosion at an altitude of about 8 kilometers. To the horrified bystanders, it was as if the skies had caught fire. The fireball created by the explosion may have reached an initial temperature of 300000 K, it would have appeared many times brighter than the sun. A wave of intense heat set everything facing the explosion on fire - luckily almost exclusively trees in the densely forested Taiga.
As the incandescent cloud extended, its hellish temperatures and its intense brightness abated slightly, setting forth a monstrous pillar of smoke. Then the shock wave reached the ground like an enormous thunderclap, snuffing out the raging fires and snapping 60 million trees in an area of 2000 square kilometers like matchsticks. At the epicentre directly below the explosion, the charred tree trunks remained standing but all their branches were ripped off. In Vanavara, 50 kilometers distant, windows were broken and houses damaged.
Though neither Ivan nor the other stunned eye-witnesses had noticed such details - and probably, right at that moment, they wouldn't have cared about such details - we now know from the analysis of the damage wrought that it cannot have been a single explosion, but a cluster of up to four almost simultaneous explosions. Thus, the resulting cloud of smoke will have been elongated rather than spherical, before extending outwards and upwards.
In the wake of the shock wave, a shrapnel volley of mostly minute debris particles peppered the ground, but there were no large objects and therefore no craters. At least, this is what was thought until recently, when new research gave rise to some alternative theories that should be taken seriously.
A roiling pillar of plasma, dust and smoke was now pushing upwards like a giant fist, creating a chimney effect that sucked up further dust and smoke from the ground. The cloud soon reached the stratosphere and took on the typical mushroom shape that nobody was yet familiar with in 1908. Less than 40 years later, it would come to symbolize man-made nemesis. The noise of the explosion could be heard 800 km away. the shock wave ran around the Earth twice and was registered by meteorological stations everywhere. The quantities of dust carried to the stratosphere led to uncommonly bright nights in many parts of the northern hemisphere.
When the stunned onlookers had picked themselves up and dusted themselves off, they assessed the damage and found that they had been lucky. The zone of devastation was limited to densely forested areas. Many people had been badly jolted, but miraculously, nobody had been killed (Some sources mention two human casualties as well as an entire reindeer herd). In view of the fact that the energy released by the explosion was comparable to large thermonuclear devices - estimates range from 5 to 15 megatons of TNT, 300 to 1000 Hiroshima bombs! - we had gotten off miraculously lightly. The world had not ended that day, as some had feared.
In the years to come, the mysterious Tunguska event was all but forgotten. Little did the Russian elite in St. Petersburg and Moscow care what curious things occurred in remote Siberia. And then of course, the first World War broke out, and Russia entered the turmoil of the revolution and another bloody war, at the end of which the Bolsheviki had established their power. People had more pressing things to worry about, and finding out what had caused the explosion over Siberia in 1908 just did not have a high priority.
Finally, in 1927 an expedition led by the geologist Leonid V. Kulik was sent to investigate. Kulik ran into tremendous difficulties in the rugged terrain and almost drowned when fording a river. He had made up his mind that the Tunguska event had been caused by an iron meteorite like the Barringer meteor crater in Arizona. However, neither on his first nor on any of the following three expeditions could such a crater be found. The increasingly desperate Kulik interpreted a wide range of geological formations as the much-sought-after crater, which was viewed with worry and scepticism by the other expedition members. But Kulik refused to listen. World War 2 and the Stalin purges put an end to further investigations.
After the war, improved technology and infrastructure rendered the area more accessible. However, with the passing of time, some scientific evidence had been obscured. There was a seemingly remarkable finding: Those trees that had survived the the event or thrown roots afterwards appeared to be growing faster than those elsewhere. However, this could simply be due to the fact that in this devastated area the few remaining trees had more space to spread out - quite a common observation in the aftermath of extensive forest fires.
As is always the case with mysterious events, there has been no shortage of extravagant explanations. These run the gamut from a miniature black hole over an alien spaceship in distress that suffered a fatal explosion of its propulsion system when attempting an emergency landing on the Earth to a block of anti-matter and probably to even more far-fetched notions. These are just the "cosmic" theories, others speculate on terrestrial causes, such as a release of subterranean methane.
Leonid Kulik's meteorite theory likely wasn't far off mark. However, the iron meteorite he sought would not have exploded in mid-air, it would have hit the ground and caused a crater. Any crater associated with an explosion of this magnitude would certainly have been found by now. Even though the object did not leave a crater, it did leave some traces: dust deposits created in the explosion have been found in resin of felled trees; their composition is indicative of an extra-terrestrial origin.
A mid-air explosion would be consistent with eyewitness reports. A small body with a diameter of distinctly less than 100 meters, entering the atmosphere at a high velocity, slows down abruptly upon encountering the denser low atmosphere and transforms its massive kinetic energy into thermal energy - raw heat.
Possible culprits are either a comet fragment or an asteroid. A comet consists mainly of ice, so it would have disintegrated at higher altitudes than the 8 km that follow from eyewitness reports and computer simulations based on the damage pattern.
However, comets can be very heterogeneous beasts. Our knowledge of their interior structure and composition is far from complete. If there is one thing that can be said about spacecraft missions to comets, it's that they have reliably yielded very surprising findings. We can safely assume that the European comet probe Rosetta, the first spacecraft to conduct long-term observations of an active comet nucleus, will increase the scientific knowledge about comets manifold.
Asteroids also are heterogeneous objects - the Tunguska object, if it was an asteroid, could have been a carbonaceous chondrite, a carbon-rich, primordial chunk of rock, which likely would have survived atmospheric entry down to low altitudes.
There is no clear distinction between asteroids and comets. An extinct comet (or comet fragment) that has lost most or all of its volatile components would end up as a porous, carbonaceous rock. A recent paper by Cornell geologists links the event to the noctilucent clouds widely observed in the aftermath and concludes that it must have been a comet.
Was there a crater after all?
Recently researchers have focused their interest on Lake Cheko in the Tunguska region, wondering whether this could be the impact crater caused by a large explosion fragment that hit the soft Taiga ground with a relatively low velocity (compared to the speed of atmospheric entry). Final proof has not been obtained, but this theory would be consistent with the mid-air explosion scenario.
Living with the Risk
This event probably was caused by the explosion of a small celestial body that had entered the atmosphere. Food for thought: Relatively small bodies with diameters of less than 100 meters normally do not reach the surface - unless they are metallic - and therefore are less destructive than their larger brethren. A 200 meter asteroid could devastate an entire country, with objects larger than 800 meters we can expect global consequences ... I mean global environmental consequences. Even if a much smaller blast devastated "only" one country, the human disaster and the ensuing political and economical turmoil would already affect everyone, everywhere.
Even if airbursts have less dire effects, an explosive yield of several megatons of TNT is a terrible thing to happen. What if the Tunguska event had not taken place over the vast emptiness of Siberia, but over a city? The destruction and casualties would have matched those of a nuclear attack.
The 1908 Tunguska event 1908 probably was not the only such event in recent history. In 1930, there was an explosion estimated at more than a megaton of TNT over the Brazilian Mato Grosso. Only a few years later, a similar event took place over Guyana. Since then, we've had a few large atmospheric explosions of this kind in various parts of the world, but luckily not with the same force as Tunguska.
Some of the observed event correlate remarkably well with yearly meteorite showers: Tunguska and the beta-taurids, Mato-Grosso and the perseids, Guyana (Rupununi) and the geminids. It is not unreasonable to assume that among the mostly small pieces of cometary debris there also lurk a few more massive chunks. A chilling thought.
It is not easy to assess the statistical frequency of Tunguska-class events, as they do not leave permanent scars in the landscape and many of them may have gone undetected. Estimates range from some tens to a thousand years. Compared to large impacts, this is a high frequency; we are definitely not talking about millions of years between consecutive events here. As the world population continues to increase and uninhabited wilderness regions are on the retreat, human civilization is increasingly at risk.
Small asteroids are very difficult to observe with telescopes and therefore cannot be easily tracked. This makes it difficult to plan effective countermeasures based on today's technology. The main problems are detection and accurate orbit determination and prediction. In view of the risk and the expected damage in case of a direct hit on an urban area, it seems wise to invest in technology that allows improved observability of the population of small asteroids.
If we do nothing, the next "Tunguska" will again find us unprepared. Only then it might not be a "Tunguska event", but a "London event" or a "Los Angeles event" or a "Tokyo event". Asteroid and comet impacts are the one type of natural disaster that we can not only predict, but also avert - at least a major part. So let's get going, we should not waste time.
Sandia-Lab Web Site with Supercomputer Simulations showing the possible evolution of the explosion. These simulations support the theory of a complete annihilation of a small body in the atmosphere but raise the question whether the object may have been smaller than previously assumed.
"Mysterious World" by Arthur C Clarke on youtube (extremely interesting!)