On the morning of 30 June 1908, a brilliant meteor flew over central Siberia. It exploded about 10km above the ground with the power of a hydrogen bomb, flattening some 2,000 sq km of forest. Some Russian newspapers carried eye-witness reports. Meteorologists reported blips in barograph records. An observatory in Irkutsk determined the explosion's co-ordinates from the earth tremor it made and saw a prolonged geomagnetic storm following the earthquake. That night, a bright glow illuminated the sky over Russia and Europe.
The region's isolation and the turmoil of the following two decades caused the event to be forgotten until the 1920s. In 19, the Russian meteor expert Leonid Kulik first saw the "Land of Dead Forest": an expanse of millions of scorched and flattened trees. Thereafter he organised several expeditions to the region, searching unsuccessfully for the meteor's crater. These were heroic efforts, conducted far from civilisation in swampy mosquito-infested tundra. The accounts of these expeditions, the characters involved and the politics around them are some of the most interesting parts of Vladimir Rubtsov's book.
Kulik's expeditions, and the many others that followed, mapped the devastation. Later investigators reported suspicions of weak radioactivity, traces of exotic elements, genetic mutations in surviving trees and the odd tree surviving in the middle of the explosion region. Although the evidence of these things remains vague, Rubtsov gives it much emphasis. The absence of any meteorites on the ground left the field open for wacky explanations for the event. These often reflected the fashion of the time. In the 1940s it was an exploding nuclear-powered spacecraft; in the 1960s a collision with an anti-matter blob; in the 1970s a mini black hole.
Most investigators agree that the main features of a Tunguska-like event were seen in 1994 when fragments of Comet Shoemaker-Levy 9 (SL9) exploded in Jupiter's atmosphere. The speed of the rocks was sufficient for their kinetic energy to vaporise them. The super-hot gases then shot back into space along the "hole" that the hypersonic rock had made when entering the atmosphere. The vaporised rock then cooled and condensed to a cloud of fine dust that slowly sank through the air. Applied to the Tunguska event, this sequence explains the absence of any meteoric material on the ground, the magnetic disturbance and the high sunlit dust cloud.
Tellingly, The Tunguska Mystery totally ignores the SL9 impact until the very last pages, and then dismisses it as irrelevant. Rubtsov is a member of the "alternative" Independent Tunguska Exploration Group. He also has a strong interest in extra-terrestrial civilisations. It shows. At all points he seems intent on emphasising the mystery of it all while strongly hinting that he favours exploding spaceships.
Opportunities to discuss some current mysteries are missed. Many observers heard the approaching meteor as a rumble or explosion before they saw it, a phenomenon called "electrophonic sound". Its physics and physiology are not understood but it is now well authenticated. The sound may stem from disturbances in the local geomagnetic field as it tangles with ionised tail gases in a meteor's trail. Amazingly, although Rubtsov mentions electrophonic sound, he seems unaware that meteors leave ionised trails. He suggests that the magnetic storm was caused by ionised gases from a nuclear explosion!
Despite all this, the book is a good read. It style is chatty, with entertaining anecdotes. It is nicely produced, with clear maps and diagrams, rare photos, extensive references and a helpful index. The biographical details of the scientists involved, mostly Russian, give a feel for the "First Circle" world of Soviet science. However, critical readers may be irritated by Rubtsov's rhetorical style where his scientific arguments are weak.
The Tunguska Mystery
By Vladimir Rubtsov
Published 17 September 2009