Source: https://www.mystika.info/news/mezinarodni-konference-tunguska-1996/
Timestamp: 2019-04-21 00:45:07+00:00

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G. V. Andreev, P. U. Calzolari, M. Di Martino, P. Farinella, A. Forino, V. Ye Fortov, M. Galli, S. S. Grigorian, A. W. Harris, J. G. Hills, E. M. Kolesnikov, K. Muinonen, G. Longo, A. Montanari, Z. Sekanina, E. M. Shoemaker, N. V. Vasilyev (Chairman).
P. Farinella, G. Longo (Chairman), R. Serra.
V. Svettsov (Moscow Institute for Dynamics of Geospheres) Could ponderable debris of the Tunguska bolide be found?
What danger from Tunguska-like objects?
The main available data concerning the Tunguska Meteorite event are summarized. The most remarkable feature is an explosion of a space object of unknown origin moving generally from SE to NW. The characteristics of the object suggest the flight of a bolide of -22 to -17 stellar magnitude. The explosion occurred over the area of coordinates ' N, ' E. The TNT equivalent effect is estimated to be 10-40 megatons, corresponding to an energy in the range to erg. There is some evidence suggesting that following the explosive energy release, at least a part of the Tunguska Space Body (TSB) continued to move in the pre-explosion direction upwards. The TSB exsplosion gave rise to a seismic wave recorded in Irkutsk, Tashkent, Tbilisi and Jena, and to pressure disturbances which travelled around the globe. In addition, min (or, according to another estimate, min) after the explosion, a local magnetic storm similar to geomagnetic disturbances following atmospheric nuclear explosions was registered. The shock and ballistic waves destroyed km of taiga and burnt vegetation was spread over an area of about 200 km .
The explosion was just the most striking event in the set of natural anomalies which occurred in the summer of 1908. Beginning on June 23, 1908, atmospheric optical amonalies were observed in many places of Western Europe, the European part of Russia and Western Siberia. They gradually increased in intensity until June 29 and then reached a peak in the early morning of July 1st. These anomalies included an unprecedentedly active formation of mesospheric (noctilucent) clouds, bright "volcanic" twilights, disturbances in the normal motion of the Arago and Babinet neutral points, a possible increase in the emission of the night sky, and unprecedentedly intense and long solar halos. Later on, after July 1st, these effects decreased exponentially. The area involved in these phenomena included a considerable part of the Northern Hemisphere and was limited by the Yenisey river in the East, by the Tashkent - Stavropol - Sevastopol - Bordeaux line in the South, and by the Atlantic coast in the West.
There are no explosion or impact-induced astroblemes or large fragments of the TSB. The search for finely-dispersed space material in the area of the catastrophe, over 10,000 km , did not result in the discovery of materials to be reliably identified with that of the Tunguska meteorite. Only recently in the area of catastrophe some biogeochemical elemental and isotopic anomalies have been found, which may be related to the event. The post-war expeditions revealed a complex set of ecological consequences of the Tunguska catastrophe, namely: (1) accelerated growth of young (postcatastrophic) trees and those which survived the event; (2) population-genetic effects, mainly at the epicenter and along the TSB trajectory.
In the 60's, it was suggested that, unlike the asteroidal version, a cometary hypotesis could explain the main peculiarities of the Tunguska event, namely: (1) the height of the explosion above the ground; (2) the lack of an astrobleme, as well as of any trace of a large-scale fallout of meteoritic material; (3) the set of atmospheric optical anomalies accompanying the Tunguska explosion. Recently, the very fundamentals of this hypothesis were rediscussed: Sekanina and later Chyba came to the conclusion that a comet nucleus would have had to disintegrate at a much higher altitude than it actually happened and that the Tunguska meteorite was a stony asteroid. But in this case the question of the TSB substance reappears and a number of problems remain unsolved -- such as, for example, the isotopic anomalies in the "catastrophic" layers of sphagnum bogs at the epicenter, as well as the increased concentration of volatile and chalcophile elements therein. Moreover, a return to the stony asteroid model would require a re-explanation of the 1908 atmospheric optical anomalies which were for a long time assumed to be due to penetration of the "Tunguska comet" tail into the atmosphere.
At present, the most important questions concerning the Tunguska problem may be formulated as follows: (1) Can the Tunguska explosion be explained as a result of the destruction of comet ice lumps or a meteoroid similar to carbonaceous chondrites at an altitude of 5-8 km? (2) What is the quantitative estimate of the silicate aerosol which could fall at the epicenter of the Tunguska explosion and on its dispersion train, assuming that the TSB was really a stony asteroid? (3) What is the nature of the isotopic and elemental anomalies in peat layers and wood resin dated 1908? (4) Can the atmospheric optical anomalies of 1908 be due to transport by stratospheric winds of TSB matter (the material of a stony asteroid in particular) from the site of the explosion? (5) What is the nature of the WNW segment of the "corridor" of axially symmetric deviations of forest fall vectors from the dominant radial pattern, and can it be due to anything else than the trace of the ricochet of the part of the TSB that survived the explosion, in terms of the traditional models? An unambiguous comprehensive solution of the above problems will favour a choice between the two different hypotheses.
The most informative and scientifically documented trace of the Tunguska Meteorite (TM) is the fallen and damaged forest over an area of km . The analysis of the data made it possible to get a number of very important conclusions about the nature of TM. The fall of trees is nearly radial on most of the territory, but anisotropic. It has some peculiarities in the intensity and distance, and in the direction and variance of directions. These parameters serve as a source of information about the characteristics of the air shock wave caused by TM. In the central part of the damaged region, high stumps and bare trunks of trees (the so called "telegraph poles") stand still. Most of them have fallen down later because of strong winds in chaotic directions. The diameter of the region of standing dead forest and of the chaotically fallen trees is about 3-5 km. In 1.5-2.0 km from the center of radiality (CR), the tendency to radiality in the fall directions of the trees begins to appear. The singularity point of the field of mean directions of fall (MDF) is E. Long. , and N. Lat. The variance of the fall directions decreases abruptly at a distance of 3.5-5.0 km and then a little farther it begins to rise again. The fall region itself has the shape of a butterfly; the boundary reaches 18-19 km from the CR to the N, NW and W, 26-27 km to the ESE, 37-38 km to the NE, and 40-41 km to the SSE. In the NW quadrant, the density of fallen trees is 3 times less than in the rest of the fall zone, and the mean square deviation of the fall from the mean (MSD) is 2 times as great. At long distances, the MDF shows axisymmetrical deviations from radiality. The symmetry axis passes through the CR in the direction at from the N. Along it, as well as along the orthogonal direction passing through the CR, the deviations of the MDF from the radial direction are close to zero. In the NE quadrant they are positive, and in the SE quadrant they are negative. There is some limited analogy between the SW (negative deviations) and NW (positive deviations) quadrants. The shape of the shock wave front has been reconstructed from the field of the MDF In order to get a closed front, we had to use a field of directions forming an angle of about with the MDF, but not orthogonal to it. Because the orthogonal field is a vortex one, so is the field of MDF. Nearer to the boundaries of the region of the fallen trees, the dispersion increases again. In general the MSD is inversely proportional to the aerodynamic pressure behind the front of the shock wave due to TM.
The data base of testimonies of the eyewitnesses of the Tunguska meteorite fall at the moment totals about 900 records, collected by different research groups during the years 1908-1970 using various methods. The 708 most authentic testimonies are published in the catalogue of Vasilyev et al. , 1981 . More than half were collected by the Complex Tunguska Expedition, starting in 1959. The testimonies of eyewitnesses refer to two basic sources of information: light and sound. The visible phenomena contain data about the trajectory of the object, its optical characteristics and its destruction in the atmosphere. The sound and seismic phenomena provide information on the peculiarities of the interaction of the body with the atmosphere and the ground. In this work all the aspects of the observed characteristics of the Tunguska phenomenon have been analyzed in detail. The analysis of the indications of eyewitnesses has shown the heterogeneity of the phenomenon on the entire set of observed attributes described by eyewitnesses, as well as an areal differentiation of its basic characteristics. It leads unequivocally to a very complex picture of the phenomenon. The numerous attempts to determine a trajectory of the Tunguska body on the base of visible phenomena has given different results. The difficulty of choosing between the Eastern and Southern variants of the trajectory has even led to ideas about a manoeuvering object in the atmosphere.
A more detailed study of the sound phenomena in the territory of Central Siberia has allowed us to conclude that a certain similarity exists between the sound field and the distribution of fallen trees in the region of the epicenter of the catastrophe and about a similar axis of symmetry for these two types of phenomena. This study also leads to determine a more reliable projection of the trajectory on the ground surface not only close to the epicenter, but also at significant distances from it. This candidate trajectory matches fairly well a trajectory determined recently from the whole complex of eyewitness indications. The differences of the sound field from the field of fallen trees consists in a more evident influence of a ballistic wave moving in the South-Western ( ) and North-North-Eastern ( ) directions. The indications of eyewitnesses allow also to determine an angular distance between the fronts of the ballistic wave, which were approximately situated at azimuths (from the epicenter) of about and . At a great distance from the epicenter (about 1500 km) the sound field does not degenerate in a circular pattern, testifying on the large extent and capacity of the ballistic wave. This assumption allows to explain a number of peculiarities in the distribution of maxima and minima of the field of sound phenomena. The field of visible (optical) characteristics also has a non-uniform distribution in the whole area of the observations. There is a number of populated districts, in which darkness was precisely observed in the moment of the Tunguska phenomenon. It is possible that this may be explained by the destruction of the Tunguska body in the atmosphere and by the presence of a powerful dust tail. Another optical phenomenon connected with the Tunguska event is the abnormal development of silvery clouds to the West from the territory of the catastrophe: but so far this phenomenon has received no physical explanation. The analysis of the eyewitness testimonies speaks in favour of a natural origin of the Tunguska space body, as well as about an important role of the ballistic wave in the description of the Tunguska phenomenon.
 Vasilyev N. V. et al. , 1981, Pokazaniya Ochevidtsev Tungusskogo Padeniya. Tomsk, 305 pp.
We have described in previous papers [1-2] our search for microsized particles trapped in the resin of trees that survived the Tunguska catastrophe. The tree growth rings provided information on the age of the resin and therefore on the time when the particulate was trapped. The time distribution of the particles showed clear abundance peaks centered on 1908 for some elements. This made it possible to identify Fe, Ca, Al, Si, Cu, S, Zn, Ti, Ni and other elements as possible constituents of the Tunguska Cosmic Body.
The living trees told us not only about the composition of the exploded body, but also provided information about the shock wave and about the heat caused by the explosion . No doubt some phenomena observed in the wood of surviving conifers are direct consequences of mechanical and thermal effects of the 1908 explosion. Many 1908 growth rings of trunks and branches showed traumas, while in the rings grown before 1908 there were visible traces of a kind of resin "internal haemorrhage", i. e. of a possible rupture of preexisting resin ducts when the stress reached locally the breaking value for the cells. On the other hand, also in some growth rings of 1910 and following years, we observed the formation of an anomalous number of resin ducts probably due to a damaged cambium. The 1908 ring itself generally has a normal width, showing that its growth was practically complete on 30 June, 1908. This ring, however, has an anomalously clear late wood, characterized by narrower cells with thinner walls, indicating a reduced lignification in the months following the catastrophe. Defoliation, as a consequence of the explosion damage and heat, is also responsible for the minimal width (often less than 0.1-0.2 mm) of the 1909 growth ring. In 1910-1913, some rings have a very irregular shape, due to a possible compression by the cambium damaged in 1908. Finally, an observation of the tree section as a whole indicates that trees not overthrown by the explosion were left leaning in the leeward side of the shock wave, thus causing an eccentricity in the tree section corresponding to the direction of the shock wave.
Another phenomenon observed in all the Tunguska trees examined is their accelerated growth, usually starting from 1910 but sometimes from some years later. Up to today the cause of the anomalous growth is controversial. The fact that the markedly accelerated growth was observed not only in surviving trees, but also in younger trees germinated after the catastrophe has been interpreted by some authors as a proof of genetic mutations ascribed to a nuclear explosion. However, we have found no trace of a nuclear process by examining the radiocarbon abundance in the 1903-1916 tree rings of one of our samples .
Some researchers have found correlations between the anomalous tree growth and the position of the trees. They have explained their findings by hypothesizing a scattered fertilization by a "meteoric dust" that encouraged growth in some places and not in others. We collected tree ring data for 9 spruces, 1 larch and 1 Siberian pine. A comparison of the average tree ring width over about 30 years before 1907 and exactly the same period after 1909 has confirmed the width increase for all the 11 trees examined. From these data no correlation with the tree position has been found. The trees were divided into two groups: 5 trees with an average ring width, before 1907, of about 0.4 mm and a second group having in the same period a ring width of about 1 mm. After 1909 both groups reach approximately the same ring width of about 1.2-1.5 mm with an increase for the first group by a factor 3-4, as against a factor 1.2-1.5 for the second group. Thus the trees that grew more slowly before 1908 have been more advantaged by the explosion, with respect to the others. The reason for accelerated tree growth seems to derive from the improved environmental conditions after the explosion: ash fertilization by charred trees, decreased competition for light, greater availability of minerals due to the increased distance between trees, etc. The more favourable conditions were relatively more fruitful for trees that had been more oppressed before the catastrophe and also favoured younger trees born after the explosion, so that the event had an averaging influence on the final tree dimensions.
 G. Longo, R. Serra, S. Cecchini and M. Galli, "Search for microremnants of the Tunguska Cosmic Body", Planetary and Space Science, 42, n. 2, pp. 163-177 , 1994.
 R. Serra, S. Cecchini, M. Galli and G. Longo, "Experimental hints on the fragmentation of the Tunguska Cosmic Body", Planetary and Space Science, 42, n. 9, pp. 777-783 , 1994.
 G. Longo, "Zhivyie svideteli Tungusskoj katastrofy", Priroda, 1, pp. 40-47, 1996.
Trees in the area of fall of the Tunguska meteorite both survived in 1908 and perished but keeping on standing like good witnesses of the catastrophe. The possibility to get information on this phenomenon is not limited to researches on the fallen trees and on the thermal damages undergone by the survived trees. The use of dendrochronoindication (DCI) is the likely future trend in the exploration of the Tunguska phenomenon.
DCI is a complex method of study of natural processes and phenomena by the analysis of specific recording structures of trees, i. e. annual rings. Morphomerical DCI was used to define the precise date of the trees' death in the zone of the "telegraphic forest" and to assess the anomalous growth of trees in the area of the catastrophe. DCI for the date of the trees' death and the withering of their branches was performed also to understand the cause of one of the types of thermal damages in the area. Structural DCI helped to find out the so-called "friable ring", namely the annual ring of 1908, made up only of tracheids of early wood. Biogeochemical DCI due to atomic-abundance analysis of annual ring wood in the period from 1893 to 1923 demonstrated that the 1908 ring of the trees injured in the catastrohpe contains a surplus quantity of some chemical elements. A very exact dating of the observed events is an advantage of the DCI method. For example, the definition of the quantity of radiocarbon in annual rings of Pinus silvestris L. from the central part of the area of the catastrophe in the period from 1898 to 1930 showed an intimate correlation between the 11-yr cycle of solar activity and the concentration of radiocarbon.
The next stage of isotopic DCI of the Tunguska phenomenon will be the definition of the correlation of hydrogen isotopes in the exchange fraction of hydrogen in the cellulose of annual rings of wood belonging to trees growing where E. M. Kolesnikov found a reduction of hydrogen isotopes in layers of mossy turf formed after 1908.
We give a review of the results of our investigations about the nature of the Tunguska cosmic body (TCB), which nave been carried out during more than 25 years. In the first work the hypotheses of an annihilation event and of a thermonuclear character of the Tunguska explosion have been tested, and after measuring the neutron-inducting 39Ar radioactivity from K and Ca in rocks and soil under the explosion epicentre, they have been rejected . This method is 100 times more sensitive than 14C analysis of tree rings .
However, not even a gram of TCB matter has been discovered yet, although the TCB mass was of some millions of tons. At the same time, the cosmic magnetic spherules which have been found in soils in the explosion area in 1961 seem to be ordinary micrometeorites [3-5]. To search for TCB remnants, peat Sphagnum fuscum sampled in the same area has been investigated. In the "catastrophic" peat layers, containing the 1908 growth-up, silicate microspherules have been found for the first time . We have discovered that they were enriched in Na and Zn and were probably a product of cosmic matter differentiation . Then, layer-by-layer analyses of peat from the explosion epicentre have revealed positive anomalies in the contents of Fe, Co, Al, Si and some volatile elements (Zn, Br, Pb, Au) in the "catastrophic" layers, which are probably due to the conservation in peat of TCB matter . Small particles found in the tree resin formed in 1908 have a similar composition .
We have shown that Pb in the "catastrophic" peat layers has a different isotopic composition compared to those of the other peat layers and to typical Pb in this area . In the "catastrophic" peat layer of another column from the explosion epicentre only an increase of Ir content has been shown . Moreover, we also suggested to analyse the isotopic composition of light elements, which are the most abundant in comets . In the "catastrophic" peat layer of a column sampled at the Bublik swamp, a small increase of the carbon isotopic composition value ( 13C = parts in ) and, on the contrary, a decrease of the hydrogen isotopic composition value ( D = parts in ) have been found .
Recently, in the "nearcatastrophic" layers of three other peat columns from the explosion epicentre, anomalies in the isotopic composition of C and H have been also revealed. The shifts for carbon ( 13C reaches +4.3 parts in ) and hydrogen ( D reaches -22 parts in ) cannot be explained by ordinary terrestrial reasons (fall-out of terrestrial dust and fire soot; emission of oil-gas streams; climate changes; and so on). Moreover, the isotopic effects are closely connected with the area and the time of the TCB event, but are absent in the upper and the lowest peat layers and also in the control peat columns sampled in other places. These effects cannot be explained by the contamination of peat by matter similar to the ordinary meteorites. However, they may be explained by the conservation in peat of cometary matter [14,15].
 Kolesnikov E. M. et al. 1973, Geokhimiya N 8, p. 1115-1121 (Russian).  Cowan C. et al. 1965, Nature, v. 206, N 4987.  Ganapathy R. 1983, Science, v. 220, N 4602, p. 1158-1161.  Nazarov M. A. et al. 1983, Proc. Lunar Planet. Sci. Conf. 14th, p. 548-549.  Jehanno C. et al. 1989, C. R. Acad. Sci. Paris, v. 308, Serie II, p. 1589-1595.  Vasilyev N. V. et al. 1973, Meteoritika, N 32, p. 141-146 (Russian).  Kolesnikov E. M. , Lyul A. Yu. , Ivanova G. M. 1977, Astron. Vestn. v. 11, N 4, p. 209-218 (Russian).  Golenetskiy S. P. , Stepanok V. V. , Kolesnikov E. M. 1977, Geokhimiya N 11, p.\ 1635-1645 (Russian).  Longo G. , Serra R. , Cecchini S. , Galli M. 1994, Planet.\ Space Sci. , v. 42, N 2, p. 163-177.  Kolesnikov E. M. , Shestakov G. I. 1979, Geokhimiya, N 8, p. 1635-1645.  Korina M. I. et al. 1987, Proc. Lunar Planet. Sci. Conf. 18th, p. 501-502.  Kolesnikov E. M. 1988, Proc. Global Catastr. in Earth History Conf. , Snowbird, USA, p. 97-98.  Kolesnikov E. M. 1982, Doklady Akad. Nauk SSSR, v. 266, N 4, p. 993-995 (Russian).  Kolesnikov E. M. , Bottger T. , Kolesnikova N. V. 1995, Doklady Akad. Nauk, v. 343, N 5, p. 669-672 (Russian).  Kolesnikov E. M. et al. 1996, Doklady Akad. Nauk, v. 347, N 3, p. 378- 382 (Russian).
Assuming that the 1908 Tunguska event was a series of several explosions, the local centres can be determined in the following way on the base of the data from fallen trees. Given two fallen trees in two different points on the ground, it is possible to draw straight lines in directions opposite to the trees' orientation, and calculate the intersection point of these lines; this point is to be considered as "the source of the wave" which caused the trees to fall. The density in the distribution of such points, for all pairs of trees located in the region, is assumed to be proportional to the density of the probability distribution for the locations of the "centres of "explosion". Already L. A. Kulik applied this method, using tight threads to determine the epicentre of the Tunguska explosion. In the present work, we adopt this approach in a computer program, using all the data obtained by numerous expeditions and collected in V. G.\ Fast's catalogue [1,2]. Some results of the calculations are presented in this report. In particular, when using all the data from the catalogue, the calculations show that the density maximum of in the distribution of intersection points is close to the critical point, determined by V. G. Fast as the epicentre of the Tunguska explosion; other local maxima do not appear. To determine other local features, it is necessary to incorporate into the calculation the trees located in small sites of the region of forest destruction. These calculations show another local centre placed 4 km to the West of V. G. Fast's epicentre.
1. Fast V. G. , Boyarkina A. P. , Baklanov M. V. The destructions, caused by shock waves of Tunguska meteorite. Problem of the Tunguska meteorite. Iss. 2 -Tomsk: Tomsk University, 1967. pp. 62-104.
2. Fast V. G. , Fast N. P. , Golenberg N. A. Catalogue of fallen trees, caused by Tunguska meteorite. Meteoritic and meteoric research. Novosibirsk: "Nauka", Siberian Branch, 1983, pp. 24-74.
During the high-speed motion of a meteorite in the atmosphere, NO originates and subsequently changes into HNO and HNO , which falls out as acid rains (Prinn & Fegley 1987). In deposits at the K/T boundary such rains have been recorded (Gardner et al. 1992) by a sharp increase (8-20 times) of the N concentration and a positive nitrogen isotopic anomaly (from 3 to 18 parts in ). These effects were in agreement with the increase of the Ir content that is a marker of the presence of meteorite matter.
The same effects have been revealed for the first time in peat sampled at the epicentre of the Tunguska cosmic body (TCB) explosion area (Nearkuhushma peatbog) and near the settlement of Vanavara, 65 km to the South of the epicentre.
In peat from the epicentre the smooth increase of N starting from the "catastrophic" layer has a peak at the depth corresponding to the boundary of thawing of the permafrost in the summer of 1908. Acids soluble in water fallen out at the peatbog surface have probably dipped out at this level. The isotopic effect is about +3.5 parts in and, similar to the K/T boundary, is in good agreement with the threefold increase of the N concentration. Similar effects have been observed in the peat column from a region near Vanavara. This column was a control in the case of C analyses, which is a marker of the TCB matter presence in peat from the epicenter (Kolesniskov, this conference), but in the case of N analyses it has shown the traces of acid rains. This should have been expected, since the Tunguska bolide passed near Vanavara. Fortunately, the TCB explosion area is far away from industrial centres, so that these effects cannot be explained by acid fall-out.
The agreement with data on the K/T boundary, the clear connection of the N isotopic composition shifts in peat from the epicenter to the 1908 permafrost boundary, together with the synchronism of the changes of the isotopic composition and of the N concentration, allow us to connect the observed effects to acid rain fall-out after the TCB pass and explosion.
Experiments performed on the generation of tritium in condensed matter, due to interaction of dense deuterium plasma flows with solid metal surface, give an insight on the scales of the tritium generation processes both in space and inside the Earth. This process has a particular significance for the interaction of cosmic bodies with the atmospheres of both the Earth and the other planets. In particular, if these interactions took place for the Tunguska meteorite, then the consequences might be the following: (1) deuterium impoverishment of the environment; (2) enrichment of the heavier isotope of carbon; (3) influence of tritium on biological processes, also at the genetic level. A modelling of a metal atomization and formation of fine metal fractions was performed. The presence of fine metallic particles is believed to indicate the existence of a reducing medium at the moment of metal atomization. The analysis of He and He could provide important information about the directivity of nuclear fusion reactions in condensed matter.
The progress in the understanding of the Tunguska object is reviewed in the light of evidence presented in numerous recent investigations, which appeared following the publication of my 1983 paper on the object's proposed asteroidal nature. The issues addressed extensively in the present review involve: (1) the results yielded by seismic studies of the event for the object's energy, altitude, and velocity at the time of its terminal explosion; (2) the problem of atmospheric fragmentation and its implications for the ablation processes and deceleration of the impactor; and (3) estimates for the object's elemental composition, for its preatmospheric mass and velocity, and for the orientation of its heliocentric orbit. Employed in the arguments are the results now available on the impacts of the fragments of Comet Shoemaker-Levy 9 into Jupiter and the results of a recent comparative study of two huge fireballs (one cometary, one stony, both brought about by projectiles a few meters across) observed with the cameras of the European Network of fireball monitoring. It is concluded that hypotheses based on the presumed cometary origin of the object encounter unsurmountable difficulties in each of the above categories of physical characteristics and that the event's interpretation in terms of a stony projectile is not only plausible, but virtually certain.
Modern mathematical models for the quantitative simulation of the process of penetration of a celestial body in a planetary atmosphere are discussed in this presentation, including definite criticisms referred to some recent publications. The acceptable variants of the mathematical models show that the Tunguska meteorite was a small comet.
The fragmentation of a small asteroid in the atmosphere greatly increases its aerodynamic drag and rate of energy dissipation. The differential atmospheric pressure across it disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with asteroid density. Extending our previous work, we have used a spherical atmosphere and a fitted curve to its density profile to find the damage done by an asteroid entering the atmosphere at various angles to the zenith. At zenith angle 45 degrees and a typical impact velocity of V = 17 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters D 220 meters and iron ones with D 80 meters. Most of the energy dissipation occurs in a fraction of a scale height, so large meteors appear to "explode" or "flare" at the end of their visible paths. This atmospheric dissipation of energy protects Earth from direct impact damage (e. g. , craters), but the blast wave from it can cause considerable damage. In previous work, we estimated the blast damage by scaling from data on nuclear explosions in the atmosphere that were done during the 1940s, 1950s, and 1960s. This work underestimated the blast from asteroid impacts because nuclear fireballs radiate away a larger fraction of their energy than do meteors, so less of their energy goes into the blast wave. We have redone the calculations to allow for this effect. We have found the area of destruction around the impact point in which the over pressure in the blast wave exceeds 4 pounds/inch = dynes/cm , which is enough to knock over trees and destroy buildings. We find that for chondritic asteroids entering at zenith angle 45 degrees and an impact velocity of 17 km/s, it increases rapidly from zero for those less than 50 meters in diameter (13.5 megatons) to about 2000 km for those 76 meters in diameter (31 megatons). (This is the maximum likely size of Tunguska.) The area of blast damage by stony asteroids between 70 and 200 meters in diameter is up to twice as great as would be had they dissipated their energy at sea level rather than higher in the atmosphere. If we assume that a stony asteroid 100 meters in diameter hits on land about every 1000 years, we find that a 50 meter diameter one (causing some blast damage) hits land every 125 years, while a Tunguska size impactor hits about every 400 years. If iron asteroids are about 7% of the frequency of stones of the same size, they constitute most of the impactors for which the blasted area is less than 500 km , about 20% that of Tunguska. About every 100 years an iron impactor should blast an area of 300 km or more somewere on the land area of Earth. The optical flux from asteroids 60 meters or more in diameter is enough to ignite pine forests. However, the blast from an impacting asteroid goes beyond the radius in which the fire starts. The blast wave tends to blow out the fire, so it is likely that the impact will char the forest (as at Tunguska), but the impact will not produce a sustained fire. Crater formation and earthquakes are not significant in land impacts by stony asteroids less than about 200 m in diameter because of the air protection. The situation is similar for the production of water waves and tsunami for ocean impacts. Tsunami is probably the most devastating type of damage for asteroids that are 200 meters to 1 km in diameter. An impact by an asteroid this size anywhere in the Atlantic would devastate coastal areas on both sides of the ocean.
The main results of this study are related to the creation of a complex model for the flight, the stress-strain state, the fracture of a meteoroid, and the determination of the action of a shock wave system and of radiation on the Earth's surface.
The motion of the Tunguska cosmic (celestial) body (TCB) is accompanied by its strong deformation, ablation and fracturing. The TCB's entering into the Earth's atmosphere had the following main stages: the flight of the ablating body before its destruction under the actions of gasdynamical forces, inertial forces and heat fluxes; the flight of a conglomerate of crushed body's substances, consisting of different pieces, particles and vapors; an explosion-like expansion of this conglomerate within which some additional energy may have been released; the interaction of shock waves, radiation and solid fragments with the Earth's surface.
Because of the great difficulties of an experimental study about a moving body, the mathematical modelling methods have the greatest potential for solving the problem.
The whole complex model for the investigation of the flight and blast of the TCB now includes the following units: (1) the calculation of the trajectory, the velocity of the centre of mass, the flow around the body, the ablation and change of the body shape; (2) the computation of the stress-strain state inside the body, using the deformable-body mechanics equations, and the determination of the time and place of the fracturing process and the parameters of the crushed conglomerate; (3) the computation of the trajectory and motion of the volume containing small pieces of the crushed body; (4) the calculation of the explosion-like expansion during the final stage of flight; (5) the determination of the action of the shock wave system and the radiation on the Earth's surface; (6) the determination of the cosmic body's orbit.
We consider units 2, 4 and 5 in detail and the other units only briefly. A comparison with the observed data is presented, and conjectures concerning the plausible nature of TCB are made. Several papers of the author and his colleagues have been devoted to the development of different parts of the complex model; our results are also described in the references listed below.
References:  V. P. Korobeinikov, P. I. Chushkin, L. V. Shurshalov, Astronomicheskii Vestnik 25, No. 3, p. 327 (1991);  P. I. Chushkin, V. P. Korobeinikov, L. V. Shurshalov, in the book: Modern Problems in Computational Aerodynamics, CRC Press and Mir Publishers, p. 272 (1991);  V. P. Korobeinikov, S. B. Gusev, P. I. Chushkin, L. V. Shurshalov, Computers Fluids, 21, No. 3, p. 323 (1992);  V. P. Korobeinikov, V. I. Vlasov, D. B. Volkov, CFD Journal, 4, No. 4, p. 463 (1996).
Mathematical models of the entry trajectory for various types of meteors have frequently been applied in an effort to determine the nature of the Tunguska object. This approach has been used to support both a stony asteroid and a cometary object as the most probable cause of the event. An accurate trajectory model must include an evaluation of both the mechanical fragmentation and the aerothermal ablation and must couple these two processes. Inaccuracies in the calculated ablation rate can lead to substantial errors in the predicted terminal altitude for a given entry body; this is particularly true for relatively weak, icy objects such as comets. The present study uses an analytical approximation of the mechanical fragmentation and radial spreading of the bolide and examines aerothermal ablation in some detail, including an evaluation of radiative cooling of the shock layer gases and the effect of radiation blockage by ablation products coming off the meteor's surface. Such calculation can be performed only in an approximate manner since the properties of high temperature gases are not well established at the extreme pressures and temperatures involved. It is found that the sudden release of energy approximately 8 km above the surface could have been produced by the disruption of either an asteroid or a comet. Therefore, a trajectory analysis of this type cannot be used at the present time to exclude either type of object definitively.
The following are considered as reliably established consequences of the Tunguska fall: forest fall in an area of about 2000 km surrounding the place of impact of a body with the Earth's surface, and absence of a meteoric crater. We have not found so far any substance we could consider as belonging to the Tunguska space body.
In this paper, it is shown that the question concerning the source of the explosion can be removed neglecting the hypothesis of explosion and replacing it by a model of fast evaporation with subsequent movement, deformation and braking of a gas volume, consisting of products of evaporation mixed with air.
Using the parameters of the Tunguska space body taken from the work of Korobeinikov et al. (Complex modelling of flight and explosion in atmosphere of a meteoric body, Astronomical Bulletin, 1991, v. 25, N3, pp. 327-343), we obtain an ablation parameter of 25.5. Using the asymptotical form of solution for a trajectory with large ablation parameters we see that the evaporation of a snow-ice sphere occurs in the following range of altitudes: 20;SPMlt;H;SPMlt;37 km.
Gaseous products keep on moving, being mixed with air and experiencing strong braking. The last phase of the Tunguska phenomenon consists in the fall on the Earth's surface of head, ballistic shock waves and gas driven after them.
This is a different explanation of the fall and burning of trees and of the absence of a crater at Tunguska.
Almost two decades ago L. Kresak (Bull. Astron. Inst. Czechoslov.\ 29, 129, 1978) suggested that the Tunguska bolide might be a fragment of Comet Encke, a hypothesis that Z. Sekanina critiqued in a publication a few years later (Astron. J. 88, 1382, 1983). In this paper we investigate one aspect of this putative genetic relationship, namely the required differential rotation in the lines of apsides of the two objects so as to make an impact upon the Earth possible for the Tunguska projectile, even though the comet's orbit in the current epoch is far from the condition of Earth intersection. This work was foreshadowed in a previous paper by us in which we showed how theoretical meteor radiants may be calculated for objects with orbits similar to Comet Encke (Earth, Moon & Planets 68, 155, 1995). Here we show, by applying appropriate secular perturbation theory and numerical integration techniques, that the required dispersion in the orientations of the lines of apsides can be attained within 10 kyr provided that the semimajor axes of the orbits differ by at least 0.05 AU, this being an amount easily achieved under the presently observed non-gravitational forces of Comet Encke.
Since 1985 the authors, in a series of publications, put forward and analyzed a hypothesis about the nature of the Tunguska comet body (TCB), without contradiction both from an internal and an external point of view. The hypothesis treats the TCB as a fragment of a comet, whose penetration into the low atmosphere, subsequent explosions and accompanying events were determined chiefly by its chemical composition, mass and velocity. As it is claimed by the explosion experts from the Institute of Technology, practically all components of a comet core after being evaporated, dissociated and mixed with the air, form an explosive substance resembling those used in the ammunition for volume explosion ("vacuum bombs"). Clearly, in the final part of the trajectory, the evaporation and dissociation of CH- compounds would result in the formation of an extensive volume of a detonating mixture, more than 2 km in diameter, 20-25 km in length, with a mass up to 35 Mt. The initiation of the explosion of the cloud contained in this volume was triggered by powerful electric discharges accompanying the braking and decay of this voluminous body (at the aftershocks). As a starting point for the identification of the cometary origin of the TCB, one may consider the results of a careful analysis of the data of spectral transparency of the atmosphere for the period of 1905-1914, recorded by scientists of the Smithsonian Astrophysical Observatory at the mountain station of Mt.\ Wilson (California). The subsequent analysis of the data by the group of the St. Petersburg SU showed that the TCB's entry had been preceded by the entry of a large asteroid fragment; as a result of its explosion at the height of 25 km, a dust cloud with a mass about 0.1 Mt was formed. This cloud, transforming in time, had passed over Mt.\ Wilson two times more with an interval of 60 days. During the second passage (from 16.07 to 19.08, 1908 ) the cloud had been superimposed (in the optical sense) on the air mass transferred from the TCB blast site into the mid-latitude circulation Ferrel cell (at 15-17 km heights). The air mass containing TCB explosion products was practically free of dust but rich in moisture (0.5 cm of precipitable water) and had an increased content of N-oxides and of some other compounds. These data with certainty support the comet origin of the TCB. The next stage in the development of the hypothesis was a scrupulous analysis and generalization of the information given by eyewitnesses of the TCB on the flight and explosions, as well as the conclusions made up on the basis of observations performed at the TCB fall site and of the tree fall; these formed the foundation for a working model of the event. The validity and the consistency of the model are evidently defined by objective estimates of the input parameters of the TCB (arrival angle, mass, velocity and composition), a realistic description of physico-chemical processes accompanying the TCB movement and explosion. This model makes it possible to explain practically every feature of the phenomenon: (1) Persons who witnessed the event held that the TCB flight duration was long; this is explained not only by a low TCB velocity but also by a sequential movement of 4 or 5 fragments forming a very long train in space and time. (2) The explosions following the principal one at intervals of about one minute were responsible for the spotlike pattern of the treefall, the duration (dose value) of the thermal irradiance according to eyewitnesses in Vanavara etc. (3) Fragments with their voluminous trains following the principal one penetrated into the body of the leading explosion; this was perceived as if the explosion were spreading in time and had vanished into the stratosphere ("the sky had broken"). (4) Fused fragment debris fell out in the Southern bog, their signs and even their presence in craters were noticed by evenkies soon after the fall; as a consequence, the bog turned from a dry to a wet one, and the water was bitter, quite insuitable for drinking; these circumstances suggest that the ice debris consisted of gas-hydrates with alkaline and silicate mineral inclusions. (5) An explanation is obtained for the inconsistency between the direction of the axis of symmetry of the tree fall pattern and the trajectory derived by the sound intensity isolines, the ballistic wave and seismic phenomena. (6) The presence of broken and burnt tree branches show that the shock wave arrival preceded the high-temperature explosion products, and this is characteristic of chemical explosions only. (7) The spot-like pattern of the tree fall and the fires show in addition that multiple chemical explosions took place, with a total energy estimated as high as J. These peculiarities completely rule out any hypotesis about the nature of the explosion other than chemical. (8) A well-grounded justification is obtained for the great mass of TCB (70-100 Mt) and its low entry speed into the Earth's atmosphere (7-11 km/s ); explanations are obtained for the optical anomalies.
COULD PONDERABLE DEBRIS OF THE TUNGUSKA BOLIDE BE FOUND?
The lack of recovered meteorites, despite the repeated attempts of expeditions to the fall region, has been used as an argument against an asteroidal origin of the Tunguska body . Here I argue that the lack of residual meteorites is typical for a fall of a stony or carbonaceous bolide tens of meters in size. Small meteoroids decelerate high in the atmosphere and thereby can escape complete ablation and pulverization. In contrast, a Tunguska-sized body penetrates deeper and, being subjected to greater aerodynamic load, is broken into a great deal of dispersing fragments not larger than 10 cm . Hydrodynamic simulations based on the assumption that a heavily fragmented body behaves in a way similar to a fluid show that the fragments are dispersed when the bolide is appreciably decelerated . Repulsive forces acting between the fragments  produce a swarm of segregated debris. The radiation flux at the fragment surface inside the swarm is about the black-body radiation flux with a temperature equal to that behind the shock wave of the swarm, when the velocity falls below 10 km/s. The radiation flux outside the fireball has been computed using the assumptions of model  for a stony body and treating the energy release as a line source of variable specific energy. The computations show that 3-10 cm stony fragments fully ablate either inside or outside the fireball. Only if the fragments gain significant lateral velocity (due to accidental collisions) and escape the fireball at altitudes above 15 km, could their ponderable remnants reach the ground at 5-10 km from the explosion epicenter. It is possible that microremnants found recently  represent recondensed material of the bolide.
The behavior of cosmic bodies during their penetration through the Earth's atmosphere can be best studied on photographic records of bolides. Such records are available for bodies up to a diameter of several meters. The dynamic data on one of the brightest such event photographed in scope of the European Fireball Network (EN), the Benesov bolide, is presented here. The Benesov bolide was photographed at 23h03m53s on May 7, 1991 at three stations of the Czech part of the European Fireball Network and reached -19.5 maximum absolute magnitude at a height of 24 km. It was a stony body of initial mass of about 13,000 kg which penetrated down to 17 km of altitude. In addition to three fixed and one guided fish-eye records (f/3.5, f = 30 mm) this bolide was also photographed by two 360-mm spectral cameras at the Ondrejov Observatory. These two very precise records covering the whole trajectory and containing also the zero order of the bolide spectrum with time marks on the bolide trail enabled us to use a new fragmentation model in order to compute the velocity and other dynamical data of the bolide, including the fragments which were resolved.
We discuss the results of computer simulations of the deformation, disintegration and dynamic vaporization of low-strength (stone and ice) meteoroids during their motion in an exponential planetary atmosphere. The elastoviscoplastic model, in which the damage accumulation and the filtration of hot atmospheric gas through the system of growing cracks play an important role, is used for describing the material's behavior under intensive aerodynamic loading.
The complete system of equations for the considered material consists of the energy and momentum conservation laws plus the nontraditional conservation law expressing the compatibility of displacement gradient and mass velocity. The system is closed by the kinetic equation of the plastic flow, the equation of damage parameter evolution and a wide-range equation of state, that allow us to simulate finite strains and phase transformations from solids into fluids and gases, respectively. Unlike traditional models, a distributed source of energy is introduced in the right-hand side of the energy equation. This aims at describing the heating of the meteoroid due to heat transport from the hot atmospheric gas, which flows from the shock-compressed layer through the growing cracks in the meteoroid. The intensity of this energy source is shown to be proportional to the material damage, the pressure at the critical point of stream and the temperature difference between atmospheric gas and meteoroid material.
The use of the equations presented in divergent form gives the opportunity to exploit a conservative monotonic method of Godunov's type at the second order of approximation. The computations introduced by this method for the comet core motion in an exponential atmosphere (Shoemaker-Levy 9 in the Jovian atmosphere and Tunguska in the Earth's atmosphere) show some new features of the nuclear behavior. The hypothesis of an "explosion in flight", that is used by some investigators for simulating gasdynamic processes in a planetary atmosphere and which implies the transformation of the solid meteoroid into a gas cloud with kinetic energy consistent with its initial value should be preferred in comparison with the hypothesis of an "explosion" at the point of full stopping. This question is important for obtaining a realistic scenario of the gas flows, the shock and radiative effects in the atmosphere which are caused by a large asteroid.
It is shown that the predominant type of comet fragment fracture is an accumulation of shear cracking due to a relatively smooth rise of the aerodynamic loading on the head surface. We also show that the new mechanism of meteoroid material heating and vaporization caused by filtration of hot gas through the system of cracks in in the fractured material plays a crucial role in the process. Thus it is clear that the pure mechanical process of fracturing triggers a very strong thermal effect, leading to heating of the disintegrated solid, its melting and vaporization. This process is more intense than the conductive and radiative heat transfer.
The 5 ppb Ir anomaly discovered by the Berkeley Group (Alvarez et al.\ 1980) in an inconspicuous clay layer marking the Cretaceous/Tertiary (K/T) boundary near Gubbio, Italy, was considered a strong hint that a 10 km chondritic extraterrestrial bolide collided with the Earth at the end of the Cretaceous Period producing a crater about 200 km in diameter. The impact explosion would have released an energy equivalent to megaton of TNT, causing a global climatic catastrophe with the resulting extinction of numerous life forms, from unicellular marine plankton to the mighty dinosaurs. In the following years, the Ir fingerprint was found in numerous other K/T sites around the world associated with tangible geological evidence of an ET impact such as altered quenched melt droplets produced in an impact cloud (Smit and Klaver, 1981; Montanari et al. , 1983), spinel crystallites interpreted as products of meteoritic ablation (Robin et al. , 1993), and quartz grains with planar deformational features which can be produced in nature only by extremely energetic impacts (Bohor and Izett, 1986). By the mid 1990's, more than 3,000 interdisciplinary scientific papers had been written on the K/T boundary. While most of these papers contributed to the mounting evidence for a giant impact, the paleontological record led to a variety of questions that still have no answers: Why are turtles and crocodiles still crawling on this planet? Why were there survivors of the K/T catastrophe in practically all the ecosystems of the Earth? Debate over the nature of the K/T event took a new turn with the recognition of the K/T boundary tsunami, and proximal impact debris deposits in the Gulf of Mexico and surrounding areas (Smit and Romein, 1985; Burgeois et al.\ , 1988; Hildebrand and Boynton, 1988; Smit et al. , 1992), indicating that the suspect killer hit not far away from this region, leaving a unique set of footprints. Ultimately, these megawave deposits yielded direct geochemical and radioisotopic evidence for linking the Chicxulub impact structure hidden under the Yucatan peninsula (the largest impact crater known on Earth with a diameter of approximately 200 km; Hildebrand et al. , 1991; Pope et al. , 1991) with the K/T boundary layer: the time of mass killing was then settled at 65 Ma through 40Ar/39Ar dating of the impact melt glass from the crater itself, and the proximal tektites from Mexican and Haitian K/T sections (Swisher et al. , 1992). Despite some crucial questions about how the enormous energy delivered to the planet by the intruder effectively altered the global atmosphere and climate, and how the world biota handled the "day after", in "geo-criminalogical" terms the case of the K/T boudary mass murder can be cautiously considered closed. An intriguing question arises from this K/T verdict: is "ET impact" a serial killer of the Earth's biota? Current investigations of the record of the last, well documented 150 Ma of Earth history suggest that less energetic impact events are weekly correlated with biologic crises, and in some cases they seem to correlate with times of biologic radiation rather than mass extinction. However, impact signatures in this stratigraphic record have yet to be carefully investigated at high resolution. It is worth noting that slowly changing climates and environments controlled by global tectonics may select biologic populations which are more or less sensitive to the stress produced by small or medium size impacts. So, in the general issue of evolution vs impacts, historical contingency plays a major role in each individual event. As an example, the Tunguska event, which was so small that did not even produce a crater, may have caused a global catastrophe if it would have occurred a half a century or so later (a blink compared to the immensity of geologic time), and a few tens of degrees more to the West (an infinitesimal in the vastness of planetary distances): in this historical contingency, the Tunguska explosion would have destroyed Moscow during the Bay of Pigs crisis, and I would probably not have written this abstract, and you would not be reading it.
Refs. : Alvarez et al. , 1980, Science, 28:1095-1108; Bohor & Izett, 1986, LPSC 17:68-69; Burgeois et al. , 1988, Science 241: 567-570; Hildebrand & Boynton, 1990, Science 248:843-846; Hildebrand et al. , 1991, Geology, 19:867-871; Montanari et al. , 1983, Geology 11:668-671; Pope et al. , 1991, Nature 351:105; Robin et al. , 1993, Nature 363:611; Smit & Klaver, 1981, Nature 292:47-49; Smit and Kyte, 1984, Nature, 310:403-405; Smit & Romein, 1985, EPSL 74:155-170; Smit et al. , 1992, Geology 20:99-103; Swisher et al. , 1992, Science 257:954-958.
From independent analysis of the airwaves recorded in Britain, Shoemaker (1977) estimated the energy of the Tunguska event to be about 12 MT, an energy very similar to that obtained by Hunt et al.\ (1960) and Ben-Menahem (1975). If the Tunguska bolide was travelling at the rms velocity of Earth-crossing asteroids (17.8 km/s) and had a density like that of CI meteorites (2.3 g/cm ), appropriate for a burst height of 8-10 km, then its estimated diameter would be about 60 m. In order to estimate the frequency of impacts of this magnitude or greater, we may either survey the NEO flux astronomically or examine the geological and historical records of observed impacts.
The terrestrial record of impact of crater-forming iron meteorites indicates that an iron asteroid with an energy of 12 MT or greater strikes the Earth about once every 15,000 years. Irons constitute of meteorites recovered from observed falls. However, CI meteorites are drastically underrepresented among recovered meteorites; a more likely ratio of irons to stones is about . Hence, from the terrestrial crater record, the mean waiting interval for 12 MT impacts of stony asteroids probably is of order 450 years. Alternatively, we may use the post-mare lunar crater record to derive impact frequency. Assuming a constant impact rate for the past 3.2 Gyr, the waiting interval for 12 MT events would be about 500 years. Allowing for a possible increase in the recent impact flux (Shoemaker et al. 1990), the waiting interval might be of years. Uncertainties of at least a factor of 2 should be attached to these estimates.
Limited infrasound observations of very energetic bolides and the historical observations of large fireballs are both consistent with the impact rate derived from the terrestrial and lunar cratering records. Within statistical, modeling, and other uncertainties, the astronomical observations of NEOs greater than 60 m in diameter are also consistent with the impact rate derived from the cratering records. It is highly unlikely that the estimated lower bound derived here for the present mean waiting interval for 12 MT impact events ( years) could be as much as a factor of ten too high.
Impacts on the Earth by asteroids or comets has, until recently, been an almost completely neglected class of natural hazards. The reason is that our atmosphere shields us from any incoming body much smaller than the Tunguska event of 1908. Thus this scale of event forms the lower limit in size (equivalent to 10 MT explosion) and upper limit in frequency of occurrence (once per 300 years) of impact events. Still larger events are even less frequent, e. g. it is estimated that a 1.5 km diameter asteroid, delivering 3 x 10 megatons explosive energy, should impact the Earth only about once in 300,000 years. An event this large, however, could cause a global climatic disaster leading to mass famine and the deaths of a significant fraction of the world's population. Thus the "annual death rate" from such large events may be as great as 1,000,000,000 people/300,000 years 3,000/year, which is far greater than the death rate expected from "Tunguska" scale events, 6,000 people/300 years 20/year. A first order response to the hazard from very large impacts is to simply survey and catalog all near-Earth asteroids, D ;SPMgt; 1 km, which it is estimated can be done to 90% completeness for a cost 50 M, in about a decade. If no object were found on a collision course (by far the most likely outcome of the survey), the residual hazard from undetected NEAs plus long period comets would be perhaps only 30% of the total risk, thus such a survey can be declared cost effective, in terms of lives "saved" compared to cost. But what should be done about Tunguska-scale impacts? A death rate of 20/year worldwide is a tiny hazard, compared to many risks that are practically ignored by present policies in the world. Nevertheless, in the developed world, it is often considered to be reasonable policy to expend public funds of the order of $1M per life saved for hazard prevention of various sorts. Thus one might consider an annual budget of $20M/year to be reasonable for protection against Tunguska-sized impacts. However, since the "first world" is likely to be paying the entire cost, and the estimated loss of life from small impacts in that part of the world is nearly an order of magnitude less than for the whole world, in a more selfish sense perhaps only an expenditure of $2M/year could be justified. One can first consider addressing the small impact hazard in the same way as the hazard from large bodies: go out and find them. I have performed an analysis of the strategy of conducting discovery surveys for the "Shoemaker Report" on NEO surveys, primarily aimed at the question of discovering D ;SPMgt; 1 km bodies, but the same techniques can be scaled for discovery surveys to any size. If one considers the completeness achieved by a 10-year long survey of all the sky to a given limiting magnitude, I find that for 125 m diameter and larger bodies, a survey to 19th magnitude achieves only 2% completeness. This is about the capability of a single 0.5 m telescope fully dedicated to searching (e. g. LONEOS). A magnitude 21 survey could achieve 13% completeness. This is approximately the level of survey advocated by the "Shoemaker Report", estimated to cost $50M. In order to reach even 50% completeness requires a mag. 23 survey, which would take about eight 3m telescopes. To obtain completeness ;SPMgt;80% requires a mag. 25 survey, which would take about thirty 10m telescopes. Clearly there is no crossover in a cost-benefit curve in this scenario. At no magnitude level of survey is the cost less than the benefit in terms of lives "saved" from small impacts. An alternate scenario to consider is the possibility to maintain a defensive system ready to respond to an object on an impact trajectory, thus one might count on discovering the object days or weeks before impact, and still be able to defend against it. There are three major difficulties with this scenario. (1) The probability that such a system would ever be needed is minuscule, so one must compare the certain cost of building it against the very small probability of ever using it. The cost of building and maintaining even an extremely modest space vehicle system is bound to exceed $20M/year, thus it is hard to imagine that any system based on current technology could be cost effective. (2) A practical system capable of such short response time would likely involve nuclear explosives. The risk of accident or misuse of nuclear weapons is undoubtedly much greater than the hazard they are proposed to mitigate. (3) Finally, we have no present-day technology for short-term detection of NEOs as they approach the Earth. Optical systems can't see in the direction of the sun; thermal IR is less efficient than visual-band detection. Radar systems can't reach even as far away as the moon for 100m-sized objects. Again, any space-based or exotic-technology system is bound to cost more than the limit of 20M/year for cost-effective deterrence. But the situation may not be so pessimistic. If we ask instead, what survey instruments are required to probably detect the next Tunguska-sized impactor before it hits, the answer is that a mag. 19 survey (LONEOS) is about sufficient. The reason is that such an ongoing survey probably has 300 years, rather than just 10, to find the object. A mag. 21 survey has a ;SPMgt;90% probability of detecting the next such impactor before it hits. So my conclusion, of what to do about Tunguska-sized impacts, is NOTHING, other than what we should already be doing about larger impacts.
The Spaceguard Survey is likely to discover the majority of near-Earth asteroids larger than 1 km in diameter in the foreseeable future (e. g. , Morrison et al. 1992, The Spaceguard Survey; Bowell and Muinonen 1994, Hazards due to Comets and Asteroids, p. 149). According to Rabinowitz et al. (1994, Hazards due to Comets and Asteroids, p. 285), there are about 1500 Earth-crossing asteroids larger than 1 km and about 140,000 larger than 0.1 km in diameter in the population of near-Earth objects. Moving toward Tunguska-sized bodies, the model size distribution predicts Earth-crossing asteroids larger than 50 m, with an uncertainty of a factor of three. With the help of computer simulations, we study various search strategies for these 50 m bodies. In particular, adding to the earlier computations, we simulate orbital uncertainties for the discovered objects as recently requested by several researchers. It seems plausible that, although all-sky surveys could yield better discovery statistics, the resulting orbits could make the follow-up exceedingly difficult.
Although the asteroidal vs. cometary nature of the Tunguska body is still debated, in the last decade it has become clear that it was a member of a vast and probably heterogenous population of near-Earth objects in the size range from 10 to 100 m. Fireball observations indicate that this population has a wide range of material strengths, and dynamical studies have shown a variety of evolution mechanisms and patterns. From the Spacewatch Survey some evidence has also been found that this population is overabundant with respect to a power-law extrapolation from larger sizes, and includes a component with a peculiar distribution of orbits (more Earth-like than usual) and colors. If these preliminary conclusions will be confirmed by future observations and will be proven to imply a different distribution of sources with respect to that inferred for km-sized near-Earth objects, there will be important consequences for the impact hazard issue, and also for our understanding of meteorites (which are generated from meter-sized impactors, and thus sample yet another portion of the size distribution of the near-Earth population).
Possible sources for the Tunguska-like population include: asteroids, both main-belt ones and members of the near-Earth Aten-Apollo-Amor groups; comets, coming from either the flattened Edgeworth-Kuiper belt or the isotropic Oort cloud; the Moon and Mars, which are known to deliver meteorites to the Earth. I will shortly discuss the evidence in favour and against an important contribution of each of these sources to the overall population, and comment upon the corresponding implications concerning the physical properties of the Tunguska-like bodies and the variability of their Earth impact flux.
This paper is devoted to the problem of the simulation of large space bodies crossing the atmosphere and hitting the Earth, so as to determine the amounts of energy involved, the shock waves and other disastrous effects of such an event. In particular, we have solved the problem of a large icy meteoroid moving in the Earth's atmosphere in a way similar to the supposed one for the Tunguska bolide. The solution has been obtained by a numerical method with the help of a two-dimensional axisymmetric procedure of calculation for gas-dynamical processes, taking into account the transfer of radiation in the one group parabolical diffusion approximation . Particular attention was given to the thermal explosion of the meteoroid at a height of 10 kilometers and to the parameters of the shock wave calculated in the vicinity of the fall zone near the ground surface.
To estimate the influence of the meteoroid properties on the parameters of interaction with the Earth's atmosphere, we have based our study on the model described in the reference , allowing us to get an upper estimate for the height of the body's destruction. By varying the material strength and re-entry velocity in the ranges 1.96-5 MPa and 20-35 km/s, respectively, our results show that the height of destruction was in the range between 10 and 27 km. After the event, the destructed mass keeps on braking and scattering, and the transmission of kinetic energy to the surrounding air continues.
(1) V. N. Arkhipov, V. V. Val'ko, B. V. Zamyshl'ayev, V. E. Makarov, O. N. Oushakov, Mathematical modelling of radiation-hydrodynamic processes having high energy densities, Russian J. of Computational Mechanics,1994,3, on press.
(2) V. P. Korobelnikov, V. I. Vlasov, D. B. Volkov, Modelling of space body destruction when travelling in planetary atmospheres, Mathematical modelling, 1994, Vol. 6, No. 8, p. 61.
Several aspects of Earth-impact hazard assessment can be re-evaluated in light of knowledge gained from observations and simulations of the impact of comet Shoemaker-Levy 9 with Jupiter. In particular, the threat of impact-generated plumes to satellites in low-Earth orbit should be recognized. Visible plumes from the impacts on Jupiter rose to altitudes exceeding 3000 km above the visible cloudtops before collapsing. A 2-D simulation of a 34 meter-diameter stony meteorite entering Earth's atmosphere at 20 km/s generates a plume that rises to nearly 1000 km. Such an impact event, with a kinetic energy equivalent to 3 megatons of TNT, has an expected recurrence interval of about 100 years. Possible outcomes of satellite interactions with very low density plumes would be changes in attitude and orbit, mechanical damage to protruding parts, and damage to optics and electronics by the impact of condensed particles in the plume and/or plasma within the bow shock. Higher density plumes could cause premature reentry or otherwise destroy a satellite. Detailed modeling coupled with observations of high-energy atmospheric entry events should be performed to quantify this threat to satellites in the near-Earth environment.
This work was funded by the LDRD program and was performed at Sandia National Laboratories by the U. S. Dept. of Energy under contract DE-AC04-94AL85000.
3- Los Alamos National Laboratory, EES-5, MS F665, Los Alamos, NM, 87545, U. S. A.
5- ET Space Systems, 5990 Worth Way, Camarillo, CA, 93012, U. S. A.
6- Johnson Space Center, SN-2, Houston, TX, 77058, U. S. A.
The detection capability of the three major fireball camera networks is limited by the relatively small collection area in the atmosphere sampled. Such a bias can be overcome by long-term (many decades) worth of operation, but useful statistics will still be limited to objects fainter than -20 absolute magnitude. In contrast, space based observations offer the possibility of monitoring almost all of Earth's atmosphere, with a trade-off in sensitivity. Since the mid-1970's more than 250 fireball events have been detected from orbit by infrared sensors operated by the U. S. Department of Defence (DoD). Approximately 20% of this total have also been detected by optical sensors. Here we present recently released lightcurves of many of the optically detected fireballs and discuss the implications for the mass and energy estimates of these bodies. The method of determining peak brightness from the optical sensors is discussed as is the current state of calibration of the luminous efficiencies for these large fireballs. At present only ground-based fireball spectra are available for analysis. Similar spectra obtained from space platforms are highly desirable to calibrate these satellite data at both IR and optical wavelengths.
The advantages of intercomparison between the satellite IR and optical data with seismic, infrasound and "ground truth" information are also shown. In particular, recent advances in cooperation between the NASA stratospheric dust sampling program run from the Johnson Space Centre and DoD satellite operators will be highlighted. Such collaboration may soon result in sampling of material from bolide events recorded by satellite sensors.
The spectacular breakup of comet P/Shoemaker-Levy 9 by Jupiter's tidal forces in 1992 has fueled speculation that many small (few km) bodies in our solar system may be "rubble piles": loose collections of smaller component material held together by self-gravity (Asphaug and Benz, 1996, Icarus, in press). This idea is supported by Harris (1996, LPSC 27, 493), who found that none of the 107 small asteroids he examined rotate fast enough to be in a state of tension (i.e. they would fly apart if they had tensile strength). Other evidence for rubble piles was found by Bottke and Melosh (1996, Nature, in press), whose Monte Carlo simulations showed that fast rotating rubble pile asteroids encountering the Earth could be split into multiple co-orbiting components by tidal forces. By showing that this mechanism could produce binary asteroids, they were able to reproduce the observed fraction of doublet craters on the Earth, Venus, and Mars. We now suggest that tidal disruption may also produce a significant fraction of the Tunguska-sized impactors (30-100m) found among the Earth-crossing asteroid (ECA) population. To model the tidal breakup of ECAs, we use an N-body simulation of the breakup process (Richardson, 1995, Icarus 115, 320). Several hundred self-gravitating spherical particles are arranged in an elongated pile on an orbit that closely approaches the Earth. In decreasing order of severity, the possible outcomes are: (a) "SL9-type" disruption (formation of clumps of roughly equal size along the fragment train), (b) mass shedding of clumps and/or particles (over half of the primary remains intact), (c) reshaping of the primary accompanied by spin-up or spin-down, and (d) no effect. Post-encounter statistics collected include the fraction of mass that escapes, reaccretes, or orbits the primary (or largest fragment), the number of stable clumps formed, the mass and spin of each clump, and the osculating elements of the clumps and fragments with respect to the largest clump (usually near the train mass centre). Some critical parameters that frequently determine the outcome are the asteroid's shape, rotation period, target close approach distance, encounter velocity, bulk density, and encounter orientation.
Our runs show that a km-sized ECA undergoing a type (b) disruption may produce dozens of Tunguska-sized impactors. These bodies would have nearly the same orbital elements as the primary, though their apsides and nodes would soon get scrambled by precession (;SPMlt; 0.1 Myrs). The ejecta size-frequency distribution created by tidal disruption may vary somewhat from our model results, since we assume that the rubble-pile asteroid is composed of equally sized components.
We find that mass shedding events occur more frequently at low encounter velocities with Earth than at high encounter velocities, since more time is spent within the Roche sphere. By mapping ECA encounter velocities in (a, e, i) space using the technique of Bottke et al. (1995, Hazards Due to Comets and Asteroids, U. of Arizona Press, 337), we found that most low encounter velocities occur where e and i are also low. Thus, we would expect that Earth's tidal forces would be most effective at producing Tunguska-sized impactors in these regions. We also find it interesting that this region corresponds to the same region where Rabinowitz et al. (1993, Nature, 363, 704) claim there are an excess number of small asteroids 10-50m in size. These bodies cannot easily originate as part of the main-belt "collisional cascade" (i. e.\ large bodies in the main-belt fragmenting into smaller bodies, etc.) which replenishes the Earth-crossing asteroid region (Bottke et al.\ 1996, Icarus, in press).
Using Levison's and Duncan's SWIFT integrator , we investigated the orbital evolution of transneptunian test bodies under the gravitational influence of the giant planets. Various (not only small) values of the initial eccentricity and inclination of the orbits were considered. We investigated the migration of some bodies not only to the orbit of Neptune but also further inside the Solar System. The results show that a body can decrease its perihelion from 34 to 1 AU in several tens of million years at . Some bodies were ejected into hyperbolic orbits, and the mean time up to the instant of such ejection was smaller for smaller . The amplitude and character of the variations in the orbital elements highly depend on the initial orientations of the orbits, not only for resonant orbits but also for some nonresonant transneptunian orbits. The gravitational influence of the largest objects of the Kuiper belt was investigated by using the spheres' method (two two-body problems) and some analytical estimates [1-2]. Due to this effect, bodies from the outer part of the belt can migrate to its inner part and then to the orbit of Neptune. A small number of LL-chondrites, whose ages do not exceed 8 Myr, can be explained by the long distance travelled by LL-chondrites to the Earth.
The Kuiper and main asteroid belts are considered to be the main sources of Earth-crossing objects (ECOs). Computer simulations of the evolution of disks that originally consisted of planets and hundreds of other celestial bodies located in various regions of the Solar System were carried out by the spheres' method . To get the characteristic time up to the instant of the collision of two bodies orbiting the Sun, we used other formulae than those used by Öpik and other scientists. The results showed that most Amor objects cannot come from the transjovian zone and should have come from the asteroid belt. At the late stages of disk evolution for bodies initially located near the orbit of Neptune, we obtained gaps in the distribution of perihelia of bodies near the orbits of the giant planets. Perihelia or aphelia of bodies that collided with the Earth were located mainly near the Earth's orbit. A certain number of bodies migrated from various regions of the Solar System into the family of bodies whose orbits lie entirely inside the orbit of the Earth. The number of observed bodies of this family is not large, because it is difficult to observe them.
Analytical estimates of the mean time T elapsing up to a collision of an ECO with the Earth were obtained, and resulted to be less than 75 Myr. For near-Earth objects (i. e. , objects with perihelion distance q ;SPMlt; 1.3 AU) the values of T are greater by a factor of 2 than those for ECOs. The values of T can be greater, if we take into account orbital resonances. Let us consider that bodies become new ECOs at some moment of time. After a time t there will be ECOs, where 1/k is the ratio of the number of ECOs colliding with the Earth to the number of ECOs ejected into hyperbolic orbits or colliding with other planets or the Sun. Half of all ECOs that collide with the Earth do so within Myr after these objects became ECOs. The collisional lifetime of meter-sized ECOs was obtained to be several times less than 5 Myr. This result agrees with the fact that stony meteorites are usually the result of several destructions. The number N of ECOs with diameter d;SPMgt;D does not change during the evolution, if the rate of objects becoming ECOs with d;SPMgt;D equals . For (i. e. , km), k+1=10, and Myr, we have per 100 yr. The mean time between impacts of 0.1 km bodies with the Earth cannot exceed 1000 yr.
This work was supported by the Russian Foundation for Basic Research, project no. 96-02-17892, and by ESO grant no.\ B-06-018.
 Ipatov, S. I. : 1988, Kinematics Phys.\ Celest. Bodies, 4, N 6, 76-82;  Ipatov, S. I. : 1995, Solar System Research, 29, N 1, 9-20;  Ipatov, S. I. : 1995, Solar System Research, 29, N 4, 261-286;  Levison, H. F., and Duncan, M. J. : 1994, Icarus, 108, 18-36.
An experimental research about the influence of a powerful gas injection in the stagnation point of a cylindrically symmetric body on the heat transfer was carried out. The experiments were performed in supersonic air flow with temperatures of 8000-20000 K, that is, corresponding to the conditions of motion of space bodies in the Earth's atmosphere with velocities of about 8-20 km/s. Our results show that the influence of gas injection on heat transfer considerably differs from that predicted by the theory.
1. Department of Physics, University of Western Ontario, London, Ontario, N6A 3K7, Canada.
2. Geological Survey of Canada, Natural Resources Canada, Continental Geosciences Division, 1 Observatory Crescent, Ottawa, Ontario, K1A 0Y3, Canada.
3. Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA.
4. Federation des astronomes amateurs du Quebec, 7642 Boul.\ Shaughnessy, Montreal, Quebec, H2A 1K4, Canada.
5. Sandia National Laboratories, Org. 5909, MS 0978, P. O. Box 5800, Albuquerque, NM, 87185, USA.
6. Los Alamos National Laboratories, P. O. Box 1663, Los Alamos, NM, 87545, USA.
7. ET Space Systems, 5990 Worth Way, Camarillo, CA, 93012, USA.
8. Battelle, Pacific NW Laboratories, Richland, WA, 99352, USA.
9. Geological Survey of Canada, Natural Resources Canada, Pacific Division, 1 Observatory Crescent, Ottawa, Ontario, K1A 0Y3, Canada.
The St. Robert (Quebec, Canada) meteorite shower (H5 chondrite) occurred on 1994 June 15 at 0h02m UT. The fireball was recorded by visual observers in the US and Canada as well as by optical and infrared sensors onboard satellites operated by the US Department of Defence. The fireball endpoint occurred at an altitude of 36 km northeast of Montreal, at which time the object underwent several episodes of fragmentation. In all, some 20 fragments totalling 25.4 kg were recovered in an ellipse measuring 8 by 3.5 km. Interpretation of all available data indicate that the fireball traveled from south-southwest to north-northeast, with a slope from the horizontal of 55-61 degrees. The most likely heliocentric orbit for the body prior to collision with the Earth suggests an entry velocity near 13 km/s with the meteoroid moving in a low-inclination orbit and having orbital perihelion located extremely close to the Earth's orbit. From satellite optical data the photometric mass consumed during the largest detonation is found to be approximately 1200 kg. Estimation of the source energy from acoustic considerations yields 0.5 kilotons TNT equivalent energy, corresponding to a mass of order 10 metric tons. This measure is uncertain to approximately one order of magnitude. Modelling of the entry of the object suggests a mass near 1600 kg, in good agreement with the satellite optical data. Cosmogenic radionuclide activities constrain the lower initial mass to be 700 kg while the upper limit from these same data is approximately 4000 kg. Seismic data possibly associated with the fireball suggest extremely poor coupling between the airwave and the ground. The St. Robert meteorite demonstrates that satellite observations offer the potential to derive masses and orbits of Earth-crossing meteoroids which are beyond the detection limits of current telescopic search programs. Having ground truth from the St-Robert meteoroid also allows calibration of satellite observations both phenomenologically and on a theoretical level.
SOME METALLIC SPHERULES IN CALCAREOUS-MARLY SEDIMENTS IN THE TUSCANY SEQUENCE (MODENA DISTRICT, NORTHERN APENNINES, ITALY).
1. University of Milano, Departement of Earth Sciences, Via Mangiagalli 34, 20133 Milano (Italy).
2. University of Modena, Departement of Earth Sciences, Piazza S.Eufemia 19, 41100 Modena (Italy).
- Wartime origin (Extrema Ratio).
Orbit determination of main belt asteroids with the method of Laplace has been well established by the author during the last years. Now it seems that this method is even more appropriate for the orbital calculation of fast moving objects.
The present work attempts to determine the real frequency of the meteoritical events of the megatonic class through the bibliographic research of similar events occurred in the last two centuries.
We use the numerical model described in Menichella et al. (Earth Moon& Planets 72, 133-149, 1996) to investigate the flux of 50-m sized asteroid fragments into chaotic resonant orbits leading them to reach an Earth-crossing status. The assumed main-belt size distribution is derived from that of known asteroids, extrapolated down to sizes m and modified in such a way to yield a quasi-stationary fragment production rate over times Myr. We use collisional physics consistent with the results of laboratory hypervelocity impact experiments and the evidence from asteroid families (Davis et al. , in "Asteroids II", pp. 805-826, Univ.\ of Arizona Press, 1989; Petit & Farinella, Celest. Mech. 57, 1-28, 1993), and analyse the sensitivity of the results to the most critical poorly known parameters.
The results of our simulations show that the main asteroid belt on average can inject into the resonant escape hatches about one Tunguska-sized fragment per year, with an uncertainty of about a factor 3. Due to their limited dynamical and collisional lifetimes (as inferred from the better known behaviour of km-sized near-Earth asteroids), only a fraction of the Tunguska-sized chaotic fragments are likely to hit the Earth, yielding an average flux of the order of one impact per century, consistent with observations (within the existing uncertainties). Large-scale stochastic collisions in the main belt can enhance this fragment flux by a factor up to 5 over intervals Myr, assuming that this corresponds to the typical dynamical timescale in the resonances. Such enhanced-flux episodes are expected to occur every several tens of Myr.
The period June 13 - 30 is identified with two types of meteoritic events. One is an excess of falls of a lithological sub-population of basaltic meteorites. The other is the occurrence of four very high energy impact events on the Earth and Moon. Three of these (including Tunguska) and their calendric clustering in June have previously been reported by J. B. Hartung in Meteorites (1976, 1987), and a fourth event, a possible lunar impact whose occurrence in 617 A. D. may have been June 26 - 27, has recently been discussed by I. A. Ahmad in Archaeoastronomy (1994). A full text will be published in the Journal of Geophysical Research (1994).
Of the 49 authenticated and adequately dated meteorite falls of all lithologies catalogued by Graham et al. (1985) as having fallen during the interval June 13 - 30, 7 or 8 are NUJ although <1 would be expected if NUJ falls were randomly distributed throughout the year.
In this investigation we present the effects of the impact of a large meteorite onto the Earth. This event is compared with other catastrophic phenomena that take place on our planet, such as earthquakes and volcanic eruptions. The probability of collision for a 10-km meteorite is estimated to be one meteorite every years. In the case of a 5-km meteorite we obtain the time scale to be years. These estimates are the results of the comparison with similar impacts on the near side of the Moon. The consequences on the Earth's biosphere are also studied.
I consider the interaction of an asteroid with a swarm of small particles. As a result of the interaction, mass loss from the surface of the asteroid takes place. It is supposed that the mass loss is proportional to the kinetic energy of the asteroid. The interaction between an asteroid and a swarm of small particles can be used to study the destruction of space objects dangerous for the Earth.
The 1994 multiple collisions of fragments of comet SL-9 with Jupiter revealed a strange feature: some fragments produced bright, easily observable phenomena but some others disappeared without visible traces. This fact together with reliable estimates for the initial mass and size of SL-9  gives the possibility of scaling the Jupiter 1994 event for atmospheres of other planets and the Sun.
The existence of a critical size and mass of the falling body was predicted in advance, before the collision : if the cubic root of the explosion volume exceeds the characteristic atmospheric height H, then the explosion becomes nonspherical and breaks the atmosphere with a cumulative, initially vertical plume. Otherwise (that is, for a body of smaller size), the possibilities of observation become negligible, since the main energy deposition (the body explosion) occurs at optical depths exceeding unity; the slow rise of the hot region to the visible surface following the explosion at least does not lead to bright flashes.
where R is the planet radius, H is the characteristic atmospheric height at the explosion level and k is a numerical coefficient of order unity which is universal for all planets, and does not depend on the density of the falling body (but can depend on its maximum strength). Using the estimated sizes of the fragments of SL-9 and the fact that some of them produced flashes and some did not, we get for Jupiter km, thus evaluating k=2-0.2.
Calculating critical sizes with this value of k we have: 0.12-0.2 km for Venus, 1.5-3.1 km for Saturn, 2.0-3.1 km for Uranus, 0.7-1.2 km for Neptune. Formally we have also 0.3-0.6 km for the critical size of a comet approaching the Earth, but the corresponding pressure at the explosion level exceeds 1 atm. In particular, this means that the Tunguska fireball did not produce a plume. The critical size for the sun becomes too high to be observed with reasonable probability.
As a conclusion I would like to attract the attention of astronomers: a collision of a rather small comet with Venus can produce bright phenomena similar to the Jupiter 1994 event.
1. Asphaug E. , Benz W. , Nature, v 370, N 6485, 120.
2. Byalko A. V. The comet chain: birth and death, Priroda, 1993, N12, 80 (In Russian).
According to the report by a missionary published in 1931, a fall of three large bodies ocurred in the Amazonian forest near the Curuca river in the region of the Alto Solimoes, on August 13, 1930. This event has been ignored up to now (Bailey et al. 1995, The Observatory 115, 250).
The evidence for the fall of at least one large body that could have had an initial kinetic energy of the order of one half that of the Tunguska event is presented. By means of satellite visible and radar images, we have found an astrobleme having the characteristics of a large, approximately circular crater about 1 km in diameter. This astrobleme is located in the forest at about 25 km from the Curuca river. On the same day and at an hour compatible with that of the report, a seismic event was recorded at La Paz (Bolivia), at a distance of 1304 km from the probable impact site. The seismic data appear to have characteristics of Lg surface waves and enabled us to estimate the magnitude of the event. A living eyewitness has been found, and his informations can be useful to find the entrance trajectory of the projectile(s).
We discuss the nature of the falling bodies, possibly asteroids or cometary debris belonging to the comet P/Swift-Tuttle. This possibility is supported by the fact that the date of August 13 coincides with that of the maximum of the Perseid meteor shower.
At the initial stage of the research on the Tunguska phenomenon the aim seemed very simple: to find in the swamps fragments of a giant meteorite. The second stage began when expedition explorers ascertained the fact of an air meteorite explosion. The energy, power and energy density of the explosion were commensurable with a nuclear explosion of 20-40 megatons of TNT equivalent. In this situation any search of the fragments had no sense. The main trend of the investigation since 1960 has become ecological, concerning forestry, marsh researches and exploration of traces and signs of the catastrophe in the biosphere and lithosphere. At that time Siberian and Moscow scientists began also searching in archives the geophysical and meteorological traces of this unusual cosmic event. An aerodynamical picture of the explosion was reconstructed with good accuracy by computer investigations on the dead forest thrown down by the shock wave. Spectrum analysis discovered the chemical elements, which have been claimed to be probable traces of the matter from the Tunguska cosmic object: Yb, Eu, Tm, La, Ce, Na, Zn, Pb , Ba, Sr, Ni, Co, W, Ag, Au, Ta, Ir, C (the last, probably, in graphite form). The microscopic silicaceous spherules may also be the melt dust of the matter of the cosmic object. Some of them contain a little amount of mixtures of Na, Zn, Au and so on.
The investigators ascertained with great accuracy the conditions under which the visitor from space finished its trajectory. But we have not yet the decisive answer on the question: what was it? Probably, the Tunguska meteorite was not a conventional astronomical object. The known meteorites don't cause the geomagnetic storms and don't pollute the environment with lantanoids. The Tunguska phenomenon is not a routine problem of physics or astronomy. It is an unusual and complex problem, which requires the co-ordination of many scientific branches. The heterogeneous information accumulated by three generations of 20th century scientists is complex and contradictory.
The third stage of the research on the Tunguska phenomenon is now beginning. Systems analysis is now the key direction. Quantitative computer models of the Tunguska phenomenon must reconstruct the event of 1908. The necessary information for a complex scenario of the Tunguska catastrophe, collected by many expeditions and archive researches, is awaiting for use! Under the initiative of academician Prof. Anatoly S. Alekseev, director of the Computer Centre in Novosibirsk Academgorodok, the organization of the international data bank "TUNGUSKA CATASTROPHE" in the Internet network begins to be carried out. The author invites all scientists, who are interested in the problem of the Tunguska phenomenon -- the enigma of this century -- to join this international project.
A 16-step algorithm is proposed for the determination of the genotypic and environmental variation of metameric characters in plant populations having no intercalary meristems. The model considers axial and metameric increments of trees in Pinus silvestris that grew in the region of the Tungus catastrophe in 1908. The analysis of genotypic variation of increments in sample plots is not based on the genotypic variance, but on its increase ( ) and on , the genotypic variation coefficient. The comparison between and characterizing populations situated along the trajectory of the flight of the meteorite and those situated far away from this trajectory is performed by means of F criterion. The 15th step of the algorithm presumes that the increment is a statistically elementary quantitative character, i. e. its genotypic variation coefficient is supposed to be stable. The algorithm proposed has the following limitation: the variance of the genotype-environment interaction either should not be statistically significant, or it should be equal for all the sample plots. The increases of the genotypic variances of the rate of linear growth were shown to be significantly higher for the populations situated along the trajectory of the meteorite flight as compared to those situated far from this trajectory, which suggests a strong mutagenic effect of explosion of the Tungus meteorite. Starting from 1990 we have developed the new ecologo-genetical model of quantitative characters organization . From the point of view of this model there is the phenomenon of redetermination of the spectrum of genes, when the limiting factor is change. There is new possibility for identification of adaptive polygenic systems for individual coniferous trees. Monopodial coniferous plants have no intercalary meristems and therefore an annual linear axis apical growth of the tree. Individuals with great linear increment in the cold years possess valuable genes for cold hardiness, and the great increment in the droughty years testifies to the presence of genes for drought resistance. If we know the concrete limiting factor in concrete year, we can very easily estimate the genetic variants (or genetic coefficient of variation) for different adaptive polygenes. Now we begin to use this new model for analysis of Pinus silvestris populations in the region of the Tungus catastrophe of 1908.
 V. A. Dragavtsev, Algorithms of an ecologo-genetical survey of the genetic diversity and methods of creating the varieties of crop plants for yeld, resistance and quality (Methodical recommendations, New approaches), VIR, St. Petersburg, 1995, p. 37.
- strong shock waves, arising during hyper-sonic motion of the body in the Earth's atmosphere, or during the explosion resulting from the heating of the body surface for a slow moving asteroid or comet core in the atmosphere, or during the impact on the Earth's surface, and causing local and even regional catastrophes.
- strong waves of thermal radiation arising in the same processes and leading to fire on a huge area.
- jets of solid and fluid particles of geomaterials, water, products of decomposition and combustion, which change optical properties and chemical contents of the atmosphere.
- thermal decomposition of the lithosphere geomaterials under the action of intense shock waves, leading to the appearance of large amount of carbon dioxide in the atmosphere.
- complex materials rheology connected with elastic, plastic and viscous properties of substances, porosity and microcracks growth, etc.
- dissociation, ionization and chemical reactions in the atmospheric gas and evaporation products.
Practically all peculiarities of the impact process can be described in detail by contemporary models. However, a unique complex solution of the problem is very difficult owing to the difference in temporal and space characteristics of different processes.
- crater formation and emission into the atmosphere of products of the shock decomposition of the geomaterials.
A group of five impact craterlike structures is located in the basin of the Macha River, the left tributary of the Lena River in Western Yakutia . The coordinates of the craters are N, E. The two biggest craters in the group, 300 and 180 m in diameter, form a double depression, that has an eight-shaped appearance. The rest of the craters are represented by separate conelike structures. The craters are partly filled with water. The level of water in each crater depends on the relief in this area.
* depth at the water level.
The craters were formed in sand strata from the Early Quaternary about 80 m thick, which are underlied by platform sediments of the Late Proterozoic. The two biggest craters were formed in sand and basement sedimentary rocks, while the three smallest craters were formed in the sand strata only. Remnants of embankments are preserved around the craters. Fragments of charring wood and charcoal occur in the sand of the embankments. The buried soil underlies ejected material of the embankments. Fragments of sedimentary rocks on the walls of the two biggest craters have signs of shock metamorphism, including the systems of planar features in quartz.
Five metallic particles of irregular form 1.2 mm long were extracted from the sand of the embankments of the craters. Their composition is characterized by about 98% iron content, but the nickel content is 0.2% only. The age of the craters determined by the carbon method is years .
E. P. Gurov, E. P. Gurova, N. N. Kovaliuch, Doklady Acad.\ Nauk SSSR, 1987, v. 269, n. 1, 185-188 (in Russian).
There are three main stages of sedimentation in the Boltysh structure: (1) deposition of clastic material in the lake basin with a heated bottom; (2) sedimentation of clays and pyroschists in closed freshwater lake; (3) accumulation of sea sediments during the Middle Eocene transgression to the North-Eastern slope of the Ukrainian Shield.
The Boltysh impact crater just after its formation, years ago, was a circular depression about 24 km in diameter and 580 m in depth in its central part. The crater was surrounded by an uplifted rim about 330 m in height. The floor of the depression was formed by a lake of impact melt (12 km in diameter) surrounding the central uplift, 4 km in diameter, whose surface was elevated to about 80 m above the melt surface .
Creeping and collapse of unconsolidated material from the crater walls and partly from the central uplift started from the very beginning, filling the crater with sediments. Atmospheric precipitations contributed to the process, but water was completely evaporated at the early stages of sedimentation. The formation of the crater lake started when the temperature of the surface reached or less. The heated water of the lake interacted with impactites changing them: the glass turned into montmorillonite and zeolite, oxidation occurred etc. High water temperatures contributed to dissolution of silicates and determined high concentrations of several components. Sediments referring to that stage are formed by sands, sandstones and aleurites with interlayers of sedimentary breccia. Organic remains are absent in the sediments. The cooling of water in the crater lake up to some tens of degrees in temperature caused a fall in the solubility of the dissoluted components and a precipitation of the chemogenic sediments interlayered with clastic material in the crater deposits. Those chemogenic sediments are formed by layers of clynoptilolite  and layers of carbonates enriched with P O . The total thickness of the deposits reaches 100-200 m in the central part of the crater.
The second stage of sedimentation was characterized by deposition of shales, clays and pyroschists in the isolated freshwater basin. Organic matter in the pyroschists was represented by sapropel. Remains of various organisms including fishes, gastropoda, ostracoda and phytogene detritus are abundant in clays and shales. The thickness varies from about 300 m in the central part of the crater to some tens of meters at its edges . The age of the sediments was determined by paleofloristic investigations obtaining the Paleocene . The Boltysh impact crater and surrounding area were flooded in the Eocene transgression from the Dnieper-Donets Depression to the Northern slope of the Ukrainian Shield. Accumulation of sands and aleurites of the Buchack series and marls and sands of the Kiev series of Eocene formed sedimentary deposites 80-100 m thick. The Boltysh crater formation produced the deep lake basin with heated bottom and subsequent fillings during Late Cretaceous, Eocene and Neogene.
 E. P. Gurov et al. , Meteoritika 47, 1988, pp. 175-178 (in Russian).  A. A. Valter et al. Naukova Dumka, Kiev, 1982, 326 pp. (in Russian).  E. P. Gurov et al. , Geologichesky Journal 1, 1985, pp.\ 125-127 (in Russian).  J. B. Bass et al. , Razvedka i ochrana nedr. 9, 1967, pp.\ 11-15 (in Russian).  F. A. Stanislavsky, Geologichesky Journal 2, 1968, pp. 109-115 (in Russian).
A lot of versions about the nature of Tunguska meteoric body appeared during the last 88 years. The author of this project agrees with the hypotesis of S. S. Grygorjan (correspondent member of the Russian Academy of Science): the explosion of a small comet ice nucleus, its destruction, quick evaporation, and water vapour expanding in the Earth's atmosphere.
(1) Investigation of radiation due to movements in the atmosphere at a speed of a dozen km/s.
(2) Studies of laboratory conditions for increasing the efficiency of explosions.
(3) The large-scale polygon destruction of a large icy body and its radiation flowing around will be investigated by means of large explosion plasma producer.
(4) Studies by means of high-altitude explosion of Russian ballistic missiles like SS-18 (with 8 tons of ice) or SS-11 (with 1.2 tons of ice) and US ones like MX (with 3.95 tons of ice) or Minuteman (with 1.15 tons of ice), which will be equipped with ice instead of warheads (the possibility of this experiment was mentioned by Edward Teller during the conference in Snezhensk city in September 1994). The ice in the missiles is crushed into fragments by dispersing explosive placed on the block's axis. These fragments quickly breakup in the atmosphere and the cloud of CO or water vapour generates a shock wave. The velocity of missiles is up to 8 km/s. The evaluation of shock wave characteristics will be done for speeds of 8, 20, 50, 70 km/s, consistent with the speed of a small comet nucleus.
(5) An ice body destruction experiment is proposed using the MIR and Shuttle space stations. The explosion will occur at a given distance from the space station. (6) From the experiments described above, these main results can be achieved: a valid proof of the nature of the Tunguska meteorite and the possibility to protect the Earth from small-sized icy comets.
Amino acids have been detected in Cretaceous-Tertiary sediments from the Raton Basin, New Mexico, Colorado USA. Alpha-aminoisobutyric acid, an amino acid common in meteorites but having a rare terrestrial occurrence, is associated with the Cretaceous-Tertiary boundary (KTB). The distribution of -aminoisobutyric acid as well as more common biogenic amino acids such as aspartic acid, glutamic acid, glycine, alanine, valine, isovaline, isoleucine, and leucine were measured using ion exchange chromatography. Results of analyzing samples from 40 centimeters above the KTB to 20 centimeters below the KTB indicate that -aminoisobutyric acid is obtained in the largest amounts from KTB samples by leaching the amino acid with water. It was not detectable after leaching with hydrochloric acid.
-aminoisobutyric acid concentration in KTB clay at Starkville (an outcrop in the Raton Basin) is 15.8 pmol/g S2OWH. The next highest concentration is 4.2 pmol/g at 16 centimeters above the KTB in siltstone. A concentration of 3.1 pmol/g at 20 centimeters above KTB is found in the same siltstone layer. Other samples showing detectable concentrations are at 45 centimeters above (0.3 pmol/g), 4 centimeters above (1.0 pmol/g) and 20 centimeters below the KTB (0.6 pmol/g).
Biogenic (mainly protein) amino acids are detected in all samples, at higher concentrations than -aminoisobutyric acid in most cases. No other exclusively meteoritic (and/or biologically rare) amino acid could be detected. Isovaline (common in meteorites, rare terrestrial occurrence), however, may be present but detection was extremely difficult as it was eluted with the same retention time as valine (a common protein amino acid).
The results confirm the earlier work by Zhao and Bader that possibly extraterrestrial amino acids delivered by meteoritic impacts may have persisted in terrestrial as well as marine sediments. They confirm earlier work suggesting that the bolide impacting the Earth 65 million years ago was similar in composition to an acqueous altered carbonaceous chondrite.
This is a new hypothesis for explaining the Tunguska explosion. For priority reasons I am specifying that the hypothesis has been developed in the years 1987-88. On February 16, 1989 the "Lumea" ("The World") journal published it as a brief report. On May 15, 1989 the whole paper was published in the "Lumea 89" ("The World 89") almanac, Bucharest, Romania.
My hypothesis is that the explosion in the Siberian taiga is the effect of an impact between a meteorite (or fragment) and a gas pouch that had been formed in the atmosphere at a certain altitude. The gas source could be the swamps from that area which were just defrosting (the summer just began), releasing in the atmosphere in a short time a large amount of "swamp gas". Another source of gas could be underground deposits, released not by means of a volcanic eruption (as proposed by Timofeev), but through ground fissures caused by an earthquake that had just started. The theory proposed by acad.\ Prof. A. Monin and Prof. Gh. Baremblatt can explain the gas-gathering process into a pouch with a concentration (5% would be enough) sufficient to cause a blast.
The chondritic meteor was pulverized by the gas pouch explosion, with effects similar to an atomic explosion. Other phenomena, as soil fluidisation (a very important component), discovered by American scientists, contribute to the large scale of the effects.
My proposed hypothesis offers pertinent explanations for all questions which remained unanswered or poorly explained. For example, the existence of a multiple explosive wave (Zolotov) has a good explanation in the explosion of a chain of gas pouches. Their shape and relative position caused the asymmetry of the devastated area. The soil fluidisation explains the position of the flattened trees, spread outward in a circle, the increase in the level of the freatic water and the soil deformation in the shape of waves. I am also offering an explanation for the extended shape and change of direction of the meteor, the accelerated growth of vegetation after the explosion, the persistent sky glowing, etc.
Other disasters produced by gases in last years in Siberia (June 1989 in Baskiria, April 1995 in Komi), South Korea (Tae Jon, April 1995), etc. , though with smaller effects, are arguments in favour of this hypothesis. In the "Lumea" journal of October 12, 1989, an article from "Svetskaia Rossia" and TASS agency reports regarding the computer modelling of the Baskiria explosion quoted: "Recently the hypothesis proposed by a Romanian professor (Ion Nistor) was confirmed by a series of studies performed in the Soviet Union." Another argument supporting the hypothesis!
Time-resolved photometric observations of Europa and Io allowed to register fireball flashes in the atmosphere of Jupiter during the falls of fragments A and Q2 of comet SL-9. The flash of the A fragment (July 16), with an amplitude of 0.12 mag and a duration of 0.7 sec, was registered during observations of Europa. The flash of the Q2 fragment (July 20), with an amplitude 0.11 mag and a duration of 1.0 sec, was registered during observations of Io . Similar parameters for the second flash were obtained at the Vatican Observatory . The data allowed to estimate the energy of the flashes and the fragment radii. Taking into account the work of Sekanina , where he assumed that only 0.01 of the kinetic energy of the comet fragments is transformed in light radiation of the fireball, we obtain the following estimate for the sizes of the secondary nuclei A and Q2 of SL-9: R(A) = 1.42 km for p = 0.3 g/cm (1.00 km for p=1.0 g/cm ); R(Q2)= 0.65 km for p=0.3 g/cm (0.43 km for p=1.0 g/cm ).
In a paper by Hammel and Nelson , the parameters of brightness flashes on Io of amplitude mag observed on 26 July 1983 are given. That observation was obtained with the 1.52-m telescope through a 420 nm filter at the Palomar observatory. Although the flash on Io registered by Hammel and Nelson is noticeably greater than the one we registered in 1994 on Jupiter, they did not observe any visible new spot that might be compared with the spots that formed on Jupiter after the collision of comet SL-9. This fact suggests that the 1983 Io brightness flash was not caused by the light echo from a possible fireball on Jupiter. The most probable reason for that flash could be the fall on Io itself of a 1-2 km asteroid or an icy cometary nucleus. As a result of this collision, fragments containing matter from the surface layers of Io, such as Na, S2 and other elements, could be thrown away. In our opinion, such bodies ejected from the surface of Io could form a cometary train that in 1993 was detected by Shoemaker and Levy as a new comet consisting of 21 secondary nuclei. As a possible consequence of the fall of an asteroid on Io -- the reason of its brightness increase in 1983 -- a new crater appeared with a diameter of some km. The formation of such a crater might be proven by photographs of Io taken from the Galileo spacecraft in 1996.
 K. I. Churyumov, V. V. Kleshchonok, Time-resolved photometry of Io and Europa in the course of impacts of A and Q secondary nuclei of D/Comet Shoemaker-Levy 9, Proceedings of the European SL-9/Jupiter Workshop, February 13-15, 1995, Garching, Germany, pp. 87-92.  G. J. Consolmagno, G. Menard, A search for light echoes of A, H, and Q events, European SL-9/Jupiter Workshop, February 13-15, 1995, Garching, Germany, p. 25.  Z. Sekanina, Disintegration phenomena expected during collision of comet SL-9 with Jupiter, Science 262, pp. 382-387, 1993.  H. B. Hammel, R. M. Nelson, Bright flash on Jupiter in 1983, Nature 1, N11, p. 46, 1993.
The Tunguska explosion, which occurred on June 30, 1908 over the Podkamennaya Tunguska River, has not yet been satisfactorily explained. Eye-witness reports indicate a cosmic collision with a bolide entering the atmosphere at small incidence angle. The absence of a crater suggests that the bolide exploded in the atmosphere dispersing its debris over a wide area. The discovery of Ir-enriched spherules close to the explosion area led to the conclusion that about 2 tons of extraterrestrial material had been dispersed over the explored area of 20,000 km  or, by extrapolation, 50,000 tons for the entire Earth. The finding by Ganapathy  of an Ir anomaly in snow-ice samples from Antarctica led to an estimated mass of 7 million tons, two orders of magnitude higher. This inconsistency and the uncertain chronology of Ganapathy's snow-ice core prompted us to carry out new analyses. We summarize below the results of our search for extraterrestrial material in Antarctic snow-ice samples and in peat samples from the explosion site. We also report compositional data about spherules found in the vicinity of the explosion site.
1. Antarctic samples. Samples were collected at South Pole Station, very close to the place where the core used by Ganapathy was recovered. High-sensitivity Instrumental Neutron Activation Analyses (INAA) reveal over the interval of time 1895-1940 a fluctuating iridium content, but we have not observed strong values like those reported by Ganapathy. Considering a residence time of dust in the atmosphere of 4 years, we can derive from our data an upper limit for the Tunguska event Ir infall of g/cm , 20 times lower than Ganapathy's result. Assuming a chondritic composition and a uniform dispersion around the Earth, this leads to a maximum mass of 80,000 tons for the Tunguska bolide , which is equivalent to the steady state flux of micrometeorites accreted by the Earth over the considered period of time.
2. Peat samples collected near the epicenter. Samples were collected in three different swamps. The chronology of this soft material was derived from the counting of the annual growing phases of sfagnum. These samples, containing essentially organic matter, were first calcinated at low temperature. The small amount of residue was examined under the scanning electron microscope (SEM), then irradiated for INAA. We have not found a single extraterrestrial particle in any sample, from the explosion level and from lower and higher horizons as well. We did not measure either any Ir anomaly.
-- Pop. A: 5 spherules that cannot be distinguished from Fe-Ni micrometeorites.
-- Pop. B: 3 spherules poorer in Ni (800-1800 mg/g) and Ir (90-120 ng/g).
-- Pop. C: the rest (72 to 90% of the samples), which contains no or a very small amount of Ni and Ir.
We consider that Pop. A are particles of the normal infall of micrometeorites. Pop. C might result from an anthropogenic contamination, but we have no sufficient information about the collection and storage conditions to make a firm statement about that. We have to note, however, that the gold coating could be responsible for the small amount of Ir of these spherules. The 3 spherules of Pop. B are puzzling. Their high Ir content is unusual for terrestrial (natural or industrial) products and they have no equivalent in polar snow micrometeorite collections: they might result from the 1908 event. We have to note, however, that all spherules were collected on the ground and that we have no indication about their date of deposition. In addition, the absence of samples collected at distant places makes impossible any comparison and all the conclusions hazardous.
Conclusions and prospect. Our results do not permit to claim that remains from the Tunguska event have been identified. Part of their inaccuracy is due to the limited amount of material. However, the presently available data give useful information for future explorations. Two lines of investigation are envisaged: (i) Search for remains in Greenland ice cores. The geographical position of Greenland in the Northern Hemisphere and at latitudes close to the one of the Tunguska site makes this place suitable to keep a record of the 1908 event. (ii) Search in the vicinity of the site. Collection of samples on the ground both close to and far away from the epicenter would greatly help the identification of the 1908 event contribution. The deposition timing, permitting a precise identification of the 1908 layer, is also highly desirable: such a timing is available in peat samples, but the existence of a lake close to the epicenter offers a better possibility to collect large amounts of sediments of the right age.
 Florensky K. P. , Meteoritika, 23, p. 3, 1963.  Ganapathy R. , Science, 220, p. 1158-1161, 1983.  Rocchia R. et al. , Geol. Soc. Am. Special Paper 247, p.\ 189-193, 1990.  Zbik M. , J. Geoph. Res. , 89 suppl., p.\ B605-B611.  Jehanno C. et al. , C. R. Acad. Sci. Paris, 308, II, p. 1589-1595, 1989.
The Tunguska catastrophe differs from other known falls in some peculiarities, indicating a comet nature for the Tunguska bolide (TB). Among them there is the absence of explosive craters and meteorite fragments, the absence of smoke traces while moving in the atmosphere, the multiple explosions, the appearence of a great deal of water in all the atmospheric layers after the explosion, and, what is most important, the low speed of movement in the atmospheric part of the trajectory, connected with a bending of its ground projection.
The danger of the collision of such objects with the Earth is caused not only by the release of kinetic energy but also by the nature of their constituents. The reason is that, except water and the silicate materials, most components of comet nuclei such as hydrocarbons (CH4, C2H4, C2H2 etc.) and their oxygen-, nitrogen- and sulfur-containing derivates, when in the gaseous state form explosive mixtures with oxygen in the air, similar to a "vacuum bomb". A calculation of the explosive characteristics of such gas mixtures shows that the evaporation and explosion of a comet nucleus 200-350 m in diameter and with mass tons is required to generate an energy of about J. This corresponds to the size of a small comet nucleus and to the estimated TB size. It is possible to conclude that the entry into the atmosphere, evaporation and explosion, when being mixed with air, of one ton of comet substance (without silicaceous constituents) is equvalent to an explosion of about 2.5 tons of TNT. The fact that the comet substance contains a mixture of active components, with the heat of formation of part of them being negative (clearly these materials were formed from basic parent molecules under the action of solar radiation), triggers the explosion of their mixtures with air and gives higher limits on the assumed explosiveness.
The explosive properties of the each component in the mixture with air are rather close (D=17-2.3 km/s, C, P=1.8-2.7 MPa). This allows one to neglect in the calculations the exact concentrations of the different compounds in the mixture. The calculation of the parameters specifying the distribution of the explosive shock wave for the mentioned quantity of comet substance matches well the observed wood disruption in the region of the catastrophe and the air waves of the Tunguska explosion measured by meteostations in Siberia and Europe.
After the shock wave the region of the explosion was subjected to the action of the explosive transformation products, heated up to C (in the epicentre), and of thermal radiation of wavelengths 2.4-8.2 m, since the oxidation products consisted mainly of water and carbonic acid vapours.
Thus a collision of the Earth with a comet nucleus or its fragments will result not only in the release of kinetic energy but also in an inevitable process of mixing of the comet substance (evaporated or crushed) with air, triggering a chemical explosion comparable in capacity, shock wave action and thermal effect of the explosion products with the largest thermonuclear explosions.
L. A. Kulik was the first to pay attention to the peculiarities of the thermal damage of the taiga in the region of the "fall" of the Tunguska bolide (TB): the uniformity and invariability of burn over a large area, the widespread burn of tree tops within the radius of about 15 km, and, what is most important, the burn of breaks of tree branches and tops. It is believed that the "radial" burn process was caused by a light flash at the bolide explosion. However, the analysis of the results of field research and of eyewitness testimonies cast some doubt upon the light nature of the burn source.
The investigation of processes of charring and combustion of various substrata of wooden origin allows us to determine the energy exposure at a given distance from the epicentre of the Tunguska explosion. Some cases of ignition of wood bedding (a litter) were noted at a range of 33-34 km. This requires a heat flux of 5.5-12.5 J/m . The burn of tree bark at a range of 16 km is possible for an exposure to a heat flux of 10-20 J/m . The continuous ignition at a range of 12-14 km requires a heat flux of 12-35 J/m . The intensity of thermal damage of the vegetation under the action of light radiation should essentially grow from the periphery to the centre, and the numerical values of energy exposure should follow the Lambert-Bouguer-Beer law of scattered radiation. Then the value of energy exposure in the epicentre should be about 900 J/m , whereas even at 90-100 J/m practically everything that can burn burns down (Hiroshima). Actually, it has been ascertained that some groups of trees survived near the epicentre of the explosion, and alive seeds remained in the ground. Also, there is a contradiction between the thermal damage pattern in the region and the radiation laws on one side, and the order of effects on the other -- at first the breaking of branches and then a burn at the site of a break, testifying that it was not the action of light that caused the burn and fire.
The hypothesis of the explosion of a mixture of comet substance with air allows us to understand the peculiarities of the thermal damage of the TB "fall" region. After the pass of a detonation wave through a gas-air mixture, a gas cloud with a temperature of C and pressure of about 2 MPa would form, when the expanding products of the explosion are being cooled. Nevertheless, if the height of the explosion centre is about 6 km, they should reach the epicentre with a temperature of C, whereas at a distance of 10 km from the epicentre their temperature would be C. The action of hot gases would last for a time increasing from 5 sec at the epicentre to 10 sec at a distance of 15 km from the epicentre.
Except for the immediate thermal action, the cloud of explosion products consisting mainly of CO2, H2O, NO, NO2, CO at C generates radiation mainly in the IR region of the spectrum (2-12 m). Note that the atmosphere is most transparent to radiaton just in this range.
Eyewitnesses of the explosion from Vanavara (65 km from the epicentre) felt a flux of heat corresponding to an energy exposure of 0.4 J/m . Then the energy exposure of the area should be 16 J/m at a distance of 10 km from the epicentre and 60 J/m in the epicentre. Thus, if heat radiation was felt at the TB explosion at a distance of 65 km, but at the same time it did not cause vegetation annihilation near the epicentre, the radiation maximum of the source of thermal damage fell at a wavelength of 2-12 m, that is the temperature of the radiation source did not exceed C.This is precisely the temperature induced by gas mixtures explosions.
The complexity of the pattern of thermal damage consequences observed at the site of the Tunguska catastrophe is connected with the fact that fire and burn were caused by the action of two factors -- high-temperature gaseous products of the explosion of a mixture of comet substance with air and thermal radiation of the cloud formed by the explosion products.
Two discoveries are the most important in the history of the research on the Tunguska phenomenon: the evidence of an actual meteorite explosion in air and the decoding of the geomagnetic records of 30 June 1908 taken at the Irkutsk observatory. But if the former discovery is now well known, the latter has in fact been forgotten. The records of three magnetographs in the Irkutsk magnetic observatory were found only in 1959. They had no common features with the known effects of meteors, but were very similar to the artificial regional geomagnetic storms following high-altitude thermonuclear explosions. This geophysical effect was absolutely unexpected by scientists. It indicated that the Tunguska cosmic object had a very high density of internal energy, which was comparable to that of nuclear bombs. The electrical current system in the ionosphere, generated by the Tunguska meteorite explosion, lasted over four or five hours. This fact indicates the mistake made by some authors in attempting to explain this strange effect by the influence of the explosion shock wave on the ionosphere.
The discovery of the geomagnetic effect has a very important significance for engineers developing nuclear vehicles for comet destruction. Indeed, the geomagnetic effect of 30 June 1908 indicates that either some comets contain an unknown source of high-density plasma, or the Tunguska object was not a comet, but was a dangerous cosmic object unknown to astronomers and physicists. This conclusion may influence the conceptual approach to the creation of an asteroid-comet protection system for the Earth.
Division of Geol. Planet. Sci.

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