100 years after the Tunguska Event, questions still remain

An event of hydrogen-bomb proportions had rocked the earth exactly a century earlier, more than half a century before Edward Teller conceptualised the weapon of mass destruction, and we still do not know the exact cause of the event, known as the Tunguska Event.

The Tunguska Event, or Tunguska explosion, was a massive explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:02 a.m. local solar time on 30 June 1908.

The explosion was most likely caused by the airburst of a large meteoroid or comet fragment at an altitude of 5-10 kilometres above the Earth's surface. Different studies have yielded varying estimates for the object's size, with general agreement that it was a few tens of metres across and weighed 560,000 metric tons.

Although the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons of TNT, with 10-15 megatons the most likely - about 1000 times as powerful as the bomb dropped on Hiroshima, Japan and about one third the power of the Tsar Bomba, the largest nuclear weapon ever detonated.

The explosion knocked over an estimated 80 million trees over 2,150 square kilometres. It is estimated that the earthquake from the blast would have measured 5.0 on the Richter scale, which was not yet developed at the time. An explosion of this magnitude is capable of destroying a large metropolitan area.

However, there is still plenty of disagreement on the actual cause and size of the event. Some suggest the asteroid that caused the extensive damage was much smaller than the generally agreed view, and consequently, resulted in a smaller explosion - equivalent to 3 to 5 megatons of TNT.

To justify their stand, they say that prior estimates may have overstated the devastation the event caused. The forest back then was unhealthy, according to foresters, so it would not have taken as much energy to blow down such trees. In addition, the winds from the explosion would naturally get amplified above ridgelines, making the explosion seem more powerful than it actually was.

Moreover, opinion is strongly divided between two possible causes - comet or asteroid. While the fact that the explosion left no crater lends support to the comet theory with its icy material, proponents of the asteroid theory point out to the relative abundance of rocky asteroids vis-à-vis comets - by a few orders of magnitude.

There have been many other explanations over the last few decades, some probable, some not quite so. Here are a few of them:

1. According to German astrophysicist Wolfgang Kundt, who advanced a scenario that has been proposed in the late 1980s by the Russian scientist Andrei Olchowatow, the Tunguska explosion could be explained by a violent eruption of natural gas (mostly methane) that ignited after rising into the upper atmosphere.
2. In 1990, César Sirvent proposed that a deuterium comet, i.e., a comet with an anomalous high concentration of deuterium in its composition, could have exploded as a natural hydrogen bomb, generating most of the energy released.
3. In 1973, Albert A Jackson and Michael P Ryan, physicists at the University of Texas, proposed that the Tunguska event was caused by a "small" (around 1020 g to 1022 g) black hole passing through the Earth.
4. In 1965, Cowan, Atluri, and Libby suggested that the Tunguska event was caused by the annihilation of a chunk of antimatter falling from space.
5. Various UFO aficionados have claimed that the Tunguska event was the result of an exploding alien spaceship or even an alien weapon going off to "save the Earth from an imminent threat". These claims appear to originate from a science fiction story penned by Soviet engineer Alexander Kazantsev in 1946, in which a nuclear-powered Martian spaceship, seeking fresh water from Lake Baikal, blew up in mid-air.
6. Nikola Tesla, who pioneered radio and modern alternating current electric power (AC) systems was often seen as a mad scientist. One story alleges he test-fired a death ray on the evening of 30 June 1908, and once he found out about the Tunguska event, he dismantled the weapon, deeming it too dangerous to remain in existence.