The Tunguska explosion: an unexpected loud bang and explosion

Few events were as catastrophic, and mysterious, as the Tunguska explosion. The explosion occurred near the Tunguska River – hence the name – at around 7.17 am on June 30, 1908. But that is about all that is known about it!

Philip Coppens

The Tunguska explosion is notorious as being the largest impact event in recent history. It felled an estimated 80 million trees as if they were matchsticks, this over an area of 2,150 square kilometres (830 square miles). Fortunately, the area was largely uninhabited, and the explosion is believed to have only killed two people, though later reports argued several people in the nearby villages suffered fatal burns. Despite the minimal human loss it was responsible for, the explosion, which is believed to have happened at an altitude between 5 and 10 kilometres in the atmosphere, was apparently heard as far away as London, underlining the magnitude of the event.

Despite the loud bang and explosion, what caused the explosion has been the subject of decades of intense speculation. A century later, no consensus has been reached. Tunguska has even been held responsible for global warming, when Vladimir Shaidurov, of the Russian Academy of Sciences, noted how it could have been responsible for changes in the amount of ice crystals at high altitude, thus influencing the amount of solar radiation reaching the earth's surface – resulting in global warming.
The most favoured theory is that the explosion was caused by a meteoroid or comet. Estimates for the size of this meteor have ranged from 30 to a massive 1200 metres, though there is more unanimity on the energy of the blast, estimated at 10-15 megatons, or 1000 times more powerful than the atomic bomb that was dropped on Hiroshima at the end of World War II.
As the object exploded in mid air, however, no fragments of a meteor have so far been recovered, nor an impact crater, despite several candidates having been put forward – each time, without success. And without an impact crater – which you would expect with meteors – some have speculated that what exploded above Tunguska was something for more exotic, like an extra-terrestrial craft, while still others wondered whether it might not even have been a black hole, or an experiment by the famous inventor Nikola Tesla that had gone wrong.

What is known, is that at around 7.17 am, people in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the sun, moving across the sky. About ten minutes later, there was a flash and a loud knocking sound “similar to artillery fire” that went in short bursts spaced increasingly wider apart. Closer to the site of the explosion, a shock wave knocked people off their feet, while windows were broken as far as hundreds of miles away.
Once back on their feet, some of these witnesses were so flabbergasted that they believed the end of the world had begun, underlining the extra-ordinary nature of the event; some reportedly even went to their local authorities, asking what they were going to do about it! The mere passage of time – fortunately – proved that the end of the world would be for another day. However, as these things go, some have argued that “if” the explosion was due to a meteorite, and “if” it had occurred 4 hours 47 minutes later, “then” it would have completely destroyed the Russian capital of Saint Petersburg, then still ruled by the Russian Emperor. “If” this had indeed happened, “then” the history of the 20th century might have been totally different. Perhaps there would not have been a Russian Revolution, nor a Cold War, perhaps not even World War I and II – instead, perhaps, a massive rescue operation that would unite the nations of the world and a realisation that this could have happened to any city anywhere on the world.
But back to what did happen. The explosion registered on seismic stations across Eurasia, registering – it is estimated, as the Richter scale did not yet exist – 5.0 on the Richter scale. Of course, “if” the explosion had occurred on the crust of the earth, rather than in the sky, it would have registered much stronger than 5.0. Speaking of the sky: during the weeks that followed the explosion, the night skies were aglow so that people could read at night – helped, of course, by the fact that nights on the northern hemisphere were already short, so close to the summer solstice.

Decades on, some of the damage remained visible

Though several books would be devoted to the event decades later, at the time, little scientific interest was awoken by the loud bang and explosion. Systematically writing down eyewitness reports only began in 1959, five decades after the incident, when interviews were conducted with indigenous people who had been within 100 kilometres of the explosion.
The first recorded expedition arrived at the scene as late as 1921, when the Russian mineralogist Leonid Kulik, visiting the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences, deduced from local accounts that the explosion had been caused by a giant meteorite impact. He persuaded the Soviet government to fund further expeditions to the Tunguska region, based on the prospect of meteoric iron that could be salvaged to aid the Soviet industrial machine. That second expedition would occur in 1927, but to their surprise, no crater was found. “Only” a region of scorched trees about 50 kilometres across was still in evidence – this twenty years after the explosion. Indeed, a century onwards, some of the fallen trees can still be seen.
Kulik would organise three further expeditions, but each time, failed to find an impact crater. Expeditions sent to the area in the 1950s and 1960s did find microscopic glass spheres in siftings of the soil. Chemical analysis showed that the spheres contained high proportions of nickel and iridium, which are found in high concentrations in meteorites – so far the best evidence that it was indeed a meteorite that exploded over Tunguska. In June 2007, scientists from the University of Bologna argued that Lake Cheko was the potential site of the meteor’s impact. However, as early as 1961, an investigation had dismissed a modern origin of Lake Cheko. The Italian team nevertheless claims that the "expeditions in the 1960s concluded the lake was not an impact crater, but their technologies were limited."

Within the circle of academics interested in Tunguska, there is a largely theoretical debate as to whether Tunguska was a meteorite, a comet or an asteroid – all three different types of astronomical bodies that sometimes penetrate our atmosphere. The comet hypothesis is supported by the glowing skies that were observed across Europe for several evenings after the impact, possibly explained by dust and ice that had been dispersed from the comet's tail across the upper atmosphere. In 1978, Slovak astronomer Lubor Kresák suggested that the explosion was caused by a fragment of the short-period Comet Encke, which is responsible for the Beta Taurid meteor shower, which peaked when the Tunguska explosion occurred.
However, in 1983, astronomer Zdenek Sekanina published a paper criticizing the comet hypothesis. He argued that such a comet ought to have disintegrated during entry into the atmosphere; he instead proposed an asteroid was responsible. His hypothesis received a boost in 2001, when Farinella, Foschini, et al. stated that the object had arrived from the direction of the asteroid belt.

Aerial photograph from 1938 showing the extent of the damage

Whether meteorite, asteroid or comet, each hypothesis truly requires an impact crater, and despite eight decades of searching for one, none has been found, even though the epicentre of the explosion was identified by Kulik in the 1950s, when he flew over the area and was able to find a section in the centre of the devastation where trees had not been felled, but had remained upright. An object that caused this explosion is not necessarily gigantic, but would crash into the earth with such force that it would leave a clear imprint on the surface of the Earth – and would destroy trees, rather than keep them upright.
To work around this problem, some have suggested that the object disintegrated in mid-air, whereby the entire object was pulverised: nothing hit the ground and all the residue remained in the atmosphere – hence the glowing skies, they say.
Others have suggested the solution is not in the sky, but in the ground. Since the early 1990s, Andrei Ol'khovatov of the Soviet Radio Instrument Industry Research Institute, noted that Tunguska was a poorly understood strong coupling between subterranean and meteorological phenomena that science is not yet ready to understand – or has a name for. Wolfgang Kundt, an astrophysicist from Bonn University, Germany, somewhat agrees that part of the answer is to be found in the ground, arguing for a massive gas explosion. He notes that a large natural gas deposit lies below the site, a well-known fact unconnected to the event until he made it part of his theory as to what happened at Tunguska. Kundt has modelled a Tunguska “outgassing” and says it would fit with eyewitness accounts of the event.

Either way, Tunguska is unique within modern history, and hence some unique suggestions have been put forward to explain it. There is the usual suggestion of an exploding alien spaceship, but much more imaginatively, one theory believed it was caused by a piece of antimatter falling from space, followed by Albert A. Jackson and Michael P. Ryan, both physicists at the University of Texas, who proposed in 1973 that the culprit was a small black hole passing through the Earth. That theory is not only unique; it also comes with an innate problem, for it requires the presence of a second explosion on the other side of the planet, where the black hole would have exited our planet. Furthermore – and few seem to have underlined this – it would require at least one gaping hole, throughout the Earth, whereas we haven’t been able to even find an impact crater.
One “out there” theory is more inviting than others, and involves the greatest genius of the 19th century, Nikola Tesla. Oliver Nichelson was the first to argue that the explosion was the unfortunate result of an experiment by Nikola Tesla. He notes that the explosion occurred when Robert Peary was trying to reach the North Pole, and that Tesla was not in the best state of mind. Tesla was at a moment in his life when his ideas were far ahead of his time, and he began to realise that several of these ideas would never be realised as the world was simply not ready for them – or did not want them. Some of these ideas, such as the wireless transmission of electricity, were at least a century ahead of their time – noting that only recently, some scientists are investigating the possibility (again).

Nikola Tesla

Psychologist Marc J. Seifer believes that Tesla suffered a nervous breakdown in 1906, due to two men close to him dying, including Stanford White, the architect who designed the Wardenclyffe Tower, a telecommunications aerial tower intended for commercial wireless trans-Atlantic telephony and broadcasting. It is but one of a number of devices Tesla had been drawing, was toying with, or was trying to sell for commercial implementation.
Jerry Smith ponders the notion whether Tesla’s depression caused him to experiment with real people. For example, the sinking of the French ship Iena in 1907 was said to have been caused by an electrical spark. Tesla was implicated by American inventor Lee De Forest, stating that Tesla had experimented with “a dirigible torpedo” capable of destroying ships. Tesla sent his response to this serious accusation to The New York Times, but rather than deny responsibility or involvement, he merely stated that he had indeed built and tested such remotely controlled torpedoes – though did not claim he was responsible for the Iena explosion. In another letter to the newspaper, dated April 21, 1908, he reiterated the possibility of electrical wave destructions; we are a mere two months before the Tunguska explosion, when he writes "This is not a dream. Even now wireless power plants could be constructed by which any region of the globe might be rendered uninhabitable without subjecting the population of other parts to serious danger or inconvenience." In 1915, he stated bluntly: “It is perfectly practical to transmit electrical energy without wires and produce destructive effects at a distance. I have already constructed a wireless transmitter which makes this possible. [...] But when unavoidable [it] may be used to destroy property and life. The art is already so far developed that the great destructive effects can be produced at any point on the globe, defined beforehand with great accuracy.” In favour of the “Tesla connection” is the fact that his transmitter could generate the energy levels and frequencies capable of releasing the destructive force of 10 megatons, or more, of TNT. The nature of the explosion is consistent with what would happen during the sudden release of wireless power. An explosion caused by broadcast power would also not leave a crater. Furthermore, reports of upper atmosphere and magnetic disturbances coming from other parts of the world at the time of and just after the Tunguska event point to massive changes in earth's electrical condition. Tesla specifically claimed this phenomenon was one of the effects he could achieve with his high power transmitter.
As Oliver Nichelson notes: “When Tesla used his high power transmitter as a directed energy weapon he drastically altered the normal electrical condition of the earth. By making the electrical charge of the planet vibrate in tune with his transmitter he was able to build up electric fields that effected compasses and caused the upper atmosphere to behave like the gas filled lamps in his laboratory. He had turned the entire globe into a simple electrical component that he could control.” That in itself is scary enough. But despite the possibility that he could have been responsible for the Tunguska explosion, it nevertheless remains an unproven allegation.

All of these scenarios sit on the border between science speculation and science fiction. And Tunguska has indeed made it into fiction – and some of the fiction, it seems, has tried to make it back into science. In 1946, Soviet engineer Alexander Kazantsev published a science fiction story in which a nuclear-powered Martian spaceship blew up in mid-air, after taking on fresh water from Lake Baikal. This story then inspired prominent Soviet scientist Alexei Zolotov to suggest that an extraterrestrial spaceship might have exploded over Tunguska. Five decades later, the TV documentary “The Secret KGB UFO Files” would claim that Tunguska was “the Russian Roswell”, to be followed by the announcement in 2004 that the grandiosely named “the Tunguska Space Phenomenon Public State Fund”, which organised an expedition in August 2004, led by Yuri Labvin, had recovered the wreck of this spacecraft from the site. So far, it does not seem to have been put on public display. Further, as soon as the 1960s, geochemist Kirill Florensky of the Soviet Academy of Sciences, who led expeditions to the site in 1958, 1961, and 1962, reported that the only radioactivity in the Tunguska trees could be explained as fallout from atomic bomb tests – i.e. Tunguska was not a nuclear explosion.

A photograph, showing the damage done by the explosion

Recently, the “uniqueness” of the Tunguska explosion has been called into question. In 1995, New Scientist reported that a similar event might have transpired in the equally sparsely populated Brazilian jungle in 1930. The event was reported in some newspapers at the time, and was investigated by a Catholic missionary, Father Fidele d'Alviano, who wrote a report for L'Osservatore Romano, the Vatican newspaper. Though it seemed to take until 1995 before it was linked with Tunguska, as early as 1931, Leonid Kulik, who initially investigated the Tunguska explosion, mentioned the Brazilian incident.
The event occurred at 8am, August 13, 1930. Shortly before the explosion along its border with Peru in northwestern Brazil, says d'Alviano, the sun turned red and then the sky went totally dark, followed by a rain of white ash and an ear-piercing whistle. Then three fireballs streaked across the sky and exploded, their rumblings heard hundreds of kilometres around. Months later, some of the affected forest was still smouldering.
British astronomer Mark Bailey, of the Armagh Observatory argues that three house-sized objects were probably involved, resulting in a combined one-megaton explosion, or about a tenth of the estimated energy released in the Tunguska Event. The event occurred at the height of the annual Perseids meteor shower and were thus seen as a likely cause for the destruction.

Despite this second anomalous explosion, which is, in truth, not as mysterious as the Tunguska explosion, a century after the explosion occurred, we are none the wiser. We do not know what it caused the explosion; we only know that “something” was responsible for a tremendous amount of destruction. As we know so little about it, Tunguska proves that vast areas can be totally destroyed by something we have no idea what, and whether or not it will ever happen again. Worst of all, it left not a single piece of concrete evidence from which we could reconstruct what had happened. All we got, was a loud bang and an explosion, and lots of fallen trees. It could have been much worse… or perhaps whatever it was, was much worse than it should have been? We simply do not know. And that is what is so scary about the Tunguska explosion.