The Tunguska explosion took place on June 30th, 1908 in the present-day Krasnoyarsk Krai of Russia. The event is wrapped in a cloud of mystery: very little is known, for a fact, about the explosion. The first expedition to the region only occurred 20 years after the explosion. The only physical observables were the magnitude and direction of the seismic waves produced by the explosion, the magnitude and speed of the accompanying acoustic waves, and the pattern of forest damage . With so little evidence available, physicists are forced to play a role of detectives. It is not surprising, the the century of research of the event has brought some rather absurd explanations for the explosion. The hypotheses have varied from a black hole  colliding and tunneling through the Earth, to Earth being hit by a comet of antimatter , to UFO  crashes. The accepted theory, of course, is that Earth was hit by a meteor. The task of determining the physical properties of a meteor capable of producing such an explosion is a difficult one.
One of the most remarkable features of the Tunguska impact is that there is no crater. The meteor exploded and disintegrated in the atmosphere. This explosion produced shock waves that were recoded at several seismographical sites. Based on the recordings, the explosion was estimated to have released 12.5 ± 2.5 Mton of energy at the altitude of 8.5 km.
The modern analysis of the event  is rooted in the assumption that the initial fragmentation of the meteor occurs when the aerodynamic pressure is equal to the mechanical stress of the object. Based on this, one can derive the meteor speed V:
where ρsl is the atmospheric density at sea level, h is the height of the first fragmentation and H is the atmospheric scale height. Using the known height of explosion, we can relate the composition of the meteor to its velocity.
|Body type||S (Pa)||>V (km/s)|
Meteors generally lose very little mass before fragmentation. Therefore the speed during fragmentation must be greater than the escape velocity of Earth, 11.2 km/s. It then seems that an iron body is most likely. However, when one accounts for hypersonic effects and inherent uncertainty in the shape and density of the object, the stone body emerges as the only possibility. If the object is assumed to be spherical shape, the diameter of the asteroid can be calculated based on the density of stone. The object was most likely to be 60 m in diameter and weighted 4×108 kg.
One of the primary ways to quantify the released energy upon the meteor's impact with the Earth is though dendrochronology, or the study of tree rings. The tree rings provide a natural chronology of the events. They allow contemporary researchers to study environmental conditions during the entire span of tree's life. A 1990 excursion to the site of the Tunguska explosion by E. Vaganov at al  confirmed that the tree growth slowed down significantly in the five years following the explosion. The researchers analyzed the surviving trees within five kilometers of the explosion.
The researchers found several abnormalities in the tree rings in the years following the explosion. These include formation of "light" tree rings and deformation of tree cells known as tracheids. It is well established that such abnormalities can result from four physical processes: ionizing radiation, a rapid decrease in temperature, defoliation, and rapid shaking of the trees that disturbs the roots. Three of these causes can be excluded from consideration. The ionizing radiation would result in anatomical deformations of the wood, which were not observed by the researchers. The rapid decrease in temperatures is a regional effect. However no similar abnormalities were observed in trees that were farther away from the impact. Finally, the disturbance of the roots due to excessive shaking would only result in short-term effects on the tree and would not account for the entire five-year range of the abnormalities. Thus, the researchers conclude that the physical abnormalities were causedby defoliation.
The defoliation of the trees could be caused by either radiation or mechanical stress due to the explosion. However, the mechanical stress is an unlikely explanation because the stress required to remove the trees' needles would also throw down the trees. We can conclude that the radiation energy flux is the only cause of defoliation. The radiation energy flux can then be easily approximated:
where D is the radial size of the needle, P is the density of the needle, c is the heat capacity of the needle, ΔT is the difference between the temperature of protein denaturation of the needle and the temperature of air, and Δt is the time of exposure to heat radiation. Using this equation, the total heat impulse can be calculated to be 3.0±0.5 ×105 J/m2. To calculate an upper bound to this estimate, we compute the energy needed to raise the temperature of the needle contents to the boiling point of water. In order to not to cause fire, the total explosion heat impulse has to be less than 3×106 J/m2.
An interdisciplinary approach is required to effectively tackle problems of as great complexity as the Tunguska explosion. In this paper, the principles of mechanical stress and fluid dynamics were used to compute the diameter and the weight of the meteor. The object was about 60 meters in diameter, and composed of stone. Using dendrochronology, we were able to determine the heat flux of the impact to be 3.0±0.5 ×105 J/m2. The researchers in the field agree that more analysis is needed to properly quantify the event. The knowledge derived from the Tunguska explosion is useful for estimating the effects to Earth's collision with interstellar objects.
©2007 Michael Ramm. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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