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    THE TUNGUSKA METEORITE PROBLEM TODAY

Comission On Meteorites and Cosmic Dust, Siberian Branch of the Russian
Academy of Sciences, Tomsk University, Tomsk, Russia, Kharkov Mechnikov
Institute for Microbiology and Immunology, Kharkov, Ukraine

by Academician N.V. Vasilyev

I. INTRODUCTION

Today the Tunguska Meteorite problem can be considered as an important
part of the larger problem of the possible collision of Earth with those
cosmic bodies called Near Earth Objects (NEO). To estimate the scale of
collision danger threatening our planet, one should base research not
only on the calculations of collisions probability, but also on the
investigative results of such events in the history of the Earth.

Information about the Tunguska Event obtained during approximately 90
years of investigations is enormous. However, most of it derives from
work done after 1945 and is published in Russian science literature in
the Russian language and, therefore, has not not been available to
scientists of the West. We consider it important to present a brief
overview of the exsisting Tunguska information and to discuss its more
important aspects, which may play a key role in a final solution to the
problem.

The term "Tunguska Event" refers to the cosmic phenomenon that was
observed on June 30, 1908 in Central Siberia over the Krasnoyarsk
Territory, Irkutsk Region, and Yakutiya (Kulik, L.A., 1933, 1939;
Krinov, E.L., 1979). The most remarkable feature of the event was the
explosion of a space object of unknown origin. The event had been
observed in many settlements of the region (Astapovich, I.S., 1951;
Vasilyev N.V. et al., 1981; Vasilyev, N.V., 1984, 1986, 1988). The
flight of the object was accompanied by sound, seismic, and
electrophonic effects, which covered a vast territory. The scale of
these effects indicates the size of a bolide from - 22 to -17 magnitude.
Its brighthness was comparable to that of the Sun. Many eyewitnesses
observed a trail of iridescent bands resembling a rainbow.

When the body reached an altitude of 2.5 to 9.0 km over the area (60ø
53' N, 101ø 54' E), there occurred an explosion-like energy release. The
TNT equivalent of the effect is estimated at 10 to 20 (possibly up to
40) megatons, the estimated energy being 4.2x1023 to 1.7x1024 ergs
(Stanyukovich, K.P., Bronshten V.A., 1961; Maslov, E.V., 1963;
Boyarkina, A.P., Bronshten, V.A., 1975; Bronshten, V.A., Boyarkina,
A.P., 1975; Ben-Menahem A. 1975, Pasechnik, I.P., 1976, Zolotov, A.V.,
1969; Zotkin, I.T., Tsikulin, M.A., 1968; Korobeynikov, V.P., 1974; 1974
a; 1976; 1980; 1984, Goldin, V.D., 1986). There is some evidence
suggesting that after the explosion-like energy release at least a part
of the Tunguska Cosmic Body (TCB) continued to move in the
"pre-explosion" direction upwards (Fast, V.G. et al., 1976, Plekhanov,
G.F., in press).

The TCB "explosion" initiated a seismic wave that was recorded in the
cities of Irkutsk, Tashkent, Tbilisi and Jena. There also were pressure
disturbances (Astapovich, I.S., 1933; Whipple, F.W.,1934) 5.9 ??( 0.9
minutes (or, according to another estimate, 6.6 ??(0.2 min.) after the
explosion. Additionally, a local magnetic storm was registered and
persisted for more than four hours and caused geomagnetic disturbances
in the atmosphere similar to those that follow nuclear explosions
(Pasechnik, I.P., 1976; Ivanov, K.G., 1961). In Antarctica near the
volcano Erebus on June 30, 1908 they observed abnormal aurorae which may
have been induced by the Tunguska Event (Steel D., Ferguson, R., 1993).

The shock wave of the Tunguska explosion devastated 2,150 ??(25 km2 of
the taiga forest (Fast, V.G. et al., 1967; 1967 a; 1976; 1983), and the
flash burned vegetation over an area of about 200 km2 (Zenkin, G.M.,
Ilyin, A.G., 1964; Ilyin, A.G., et al., 1963; 1966; Vorobyev, V.A.,
1967; L'vov, Yu.A., Vasilyev, N.V., 1976) . The Tunguska explosion
resulted in a major forest fire covering an area comparable to that of
the devestated forest (Kulik, L.A., 1976; Kurbatsky, N.P., 1964; 1975).
The abnormalities of paleomagnetic properties of soils in the region,
likely to be related to this event, are described (Sydoras, S.D.,
Boyarkina, A.P., 1976).

The explosion on the Podkamennaya Tunguska River was the most striking
event in the several anomalies that occurred during that summer of 1908.
Starting on June 23, 1908 bright twilights were observed in some
locations of Western Europe, the European part of Russia, and in Western
Siberia. They increased in intensity until June 29, reaching their peak
on the evening of June 30. These anomalies included unprecedental
formation of mesospheric clouds, bright "volcanic" twilights,
disturbances of atmospheric polarization, and intense solar halos. After
July 1 these effects weakened exponentially; some after-effects were
observed until late July (Zotkin, I.T., 1961; 1966; Vasilyev, N.V.,
Zhuravlev, V.K., Zhuravleva, R.K. et al., 1965; Vasilyev, N.V., Fast,
N.P., 1972; 1976).

The area where these phenomena were observed is bound by the Yenisey
River in the East, by the latitude of
Tashkent-Stavropol-Sevastopol-Bordeaux in the South, and by the Atlantic
coast in the West. In August in the Western Hemisphere, the Mount Wilson
Observatory reported a decrease in the air transparency, which could be
explained by the circulation of Tunguska explosion products in the
atmosphere (Fesenkov, V.G., 1978). Concurrent with the Tunguska aerosol
cloud, there were products of another large bolide which entered the
Earth's atmosphere in May 1908 (Kondratyev, K.Ya. et. al., 1988). It
also has been suggested that the Tunguska Event influenced the ozone
layer (Turco, et al., 1982; Kondratyev, K. Ya. et al., 1988). Soon after
the Tunguska expolosion an effect similar to the well-known Bowen effect
that follows meteor showers was registered (Fast, N.P., Zalevskaya,
V.V., 1970).

It should also be noted that the summer of 1908 was quite rich in bright
bolides (Vasilyev, N.V., Zhuravlev V.K. et al.,1965; Anfinogenov, D.F.,
Budaeva L.I., 1984).

Neither expeditions to the Tunguska site prior to World War II that were
headed by L.A. Kulik, nor the post-war field work under supervision of
Florensky, Plekhanov, Zolotov and Vassilyev, found any explosion or
impact astroblems (??) or large fragments of the TCB (Florenskiy, K.P.et
al.,1960; Plekhanov, G.F., 1963; L'vov, Yu.A. et al.,1963; 1963 a ).
Search for finely-dispersed cosmic matter in the soils and peat of the
catastrophe area, over 10, 000 km2 (Kirova, O.A., 1961; Florensky, K.P.,
1963; Florensky, K.P., Ivanov, A.V. et al., 1968; Glass, B.P., 1969;
1976; Vasilyev, N.V. et al., 1971; 1973; 1974; 1974 a; 1983; Vypadenie
... 1975; Boyarkina, A.P. et al., 1976; 1976 a , Alexeeva, K.N. et al.,
1977; Kvasnitsa V.N. et al., 1979; Sobotovich, E.V. et al., 1980; 1983;
Zbic, M., 1984; Doroshin I.K., 1988) did not result in discovery of
material that could be reliably differentiated from fluctuations of the
bacrground fall of extraterrestrial matter. However, biogeochemical
element and isotopic anomalies that may be related to the event have
been discovered in the area of the catastrophe (Kovalevsky, A.F. et al.,
1963; Alexeeva K.N. et al., 1976; Zhuravlev, V.K et al., 1976;
Golenetsky, S.P. et al., 1977; 1977 a; 1980; 1981; 1983; Kolesnikov,
Kolesnikov, E.M., 1984; 1989; Kolesnikov, E.M. et al., 1979; Zhuravlev,
V.K. et al., 1976).

Increased quantities of microparticles enriched by Cu, Au, Zn and
certain other elements in the resin of the trees in the epicenter area
that survived the 1908 catastrophe are very likely related to the
Tunguska event. (Longo, G. et al., 1994).

The post-war expeditions revealed a complex range of ecological
consequences of the Tunguska explosion, namely: 1) accelerated growth of
new (post-catastrophe) trees and trees that survived the event
(Nekrasov, V.I.; Emelyanov, Yu. M., 1964; Beregnoi, V.G., Drapkina,
G.I., 1964; Shapovalova, R.D. et al., 1967; Emelyanov, Yu.M., Lukyanov,
V.B. et al., 1967; Vasilyev, N.V. et al., 1976; 1980); 2)
population-genetic effects, mainly at the epicenter area and along the
TCB trajectory (Dragavtsev, V.A. et al.,1975).

This is a general outline of the Tunguska phenomenon which, proves to be
different in principal from other impact phenomena. The many hypotheses
that have been put forward in an attempt to explain the Tunguska event
can be arranged in two groups. One includes those hypotheses that are
based on the concept of conversion of the kinetic energy of the TCB into
shock wave energy. The other group consists of hypotheses that emphasize
a release of the internal energy of the body, whether chemical or nuclear.

The first group of hypotheses involves the concept of an asteroidal
nature of the TCB (Kulik, L.A., 1933; 1939; 1939; Krinov E.L.,1949;
Astapovich, I.S., 1933; Anfinogenov, D.F., 1966; Sekanina, Z; 1983;
Chyba, C.F., 1993, Longo G. et al., 1994;), or a cometary nature
(Astapovich, I.S., 1933; Whipple, F.I.W., 1934; Fessenkov, V.G., 1961;
1978; Grigoryan, S.S., 1976; 1979; Korobeynikov, V.P. et al., 1976;
1980) . These can be classified as hypotheses based on the classical
concepts of the minor bodies of the Solar System.

These latter hypotheses assume a special nature of the TCB, one
different from asteroids or comets. These include the hypotheses of an
antimatter nature of the TCB (La Paz, L., 1948; Cowen, C. et al., 1965);
of the Tunguska object being a miniature black hole (Jackson, IV A.A.,
Ryan, M.P.,1973), or of its being a "solar energophore" (Dmitriev, A.N.,
Zhuravlev, V.K., 1984), or even being of technogeneous origin (Zolotov,
A.V., 1961; 1961 a; 1967; 1969; Zigel, F.Yu ., 1983).

It is both timely and appropriate that we examine the most serious
difficulties which one has to deal with in any attempt to construct an
integrated concept of the Tunguska phenomenon.

II. ON THE DIRECTION OF THE TCB FLIGHT

The first investigators of the TCB (L.A.Kulik, 1939; E.L.Krinov,1949;
and I.S.Astapovich,1951) who analyzed evidence of the object's flight in
the area of the Angara River had no doubts that it had been moving
generally from the south to the north. However, there were three options
for a more precise trajectory: a southern trajectory, proposed by
L.A.Kulik, a southeastern one proposed by E.L. Krinov, and the
southwestern path proposed by I.S.Astapovich. (See Sytinskaya, N.N.,
1955, and Levin, B.Yu., 1954 as well). By the early 1960s it was
Krinov's trajectory, namely 135ø, that was considered most realistic.

Later, however, a "corridor" of axis symmetrical deviation of the
vectors of the felled forest from the dominating radial pattern was
revealed. This deviation was interpreted as the track of the ballistic
wave ( Boyarkina, A.P.et al., 1964; Fast V.G., 1967; Zotkin, I.T., 1966;
Zolotov, A.V., 1969). The direction of the "corridor," wich was
initially estimated as 111ø,E from N (114ø, east of the true meridian)
(Fast, V.G., 1967), was later found to be 95ø, E from N (99ø, east of
the true meridian) (Fast,V.G. et al., 1976). During this period of time
a number of elderly residents of the area who had lived in the upper
reaches of the Nizhnaya Tunguska in 1908 were questioned. No
eyewitnesses from this area had been questioned in the 1920s and 1930s.
This resulted in the conclusion that the object had moved from the ESE
to the WNW (Konenkin, V.G., 1967), i.e. by the path coinciding with that
of the trajectory of the TCB. This coincidence caused revision of the
notion of the TCB trajectory, so since the year 1965 the ESE-WNW option
has cominated the literature. For several years it was assumed to be a
final result.

But later the publicaton of a catalog of eyewitness accounts made
analysis of the whole event possible. (Vasilyev, N.V. et al., 1981) Two
fundamental facts were established:

1. "Images" of the bolide observed in the area of the Angara Rriver and
observed in the area of the Nizhnyaya Tunguska River are quite
different, and the evidence seems to indicate that they belong to
different objects (Demin, D.V. et al., 1984).

2. The trajectory, calculated on the basis of Angara eyewitness
accounts, differs considerably from that determined by analyzing the
vector structure of the felled forest area and the radiant burn area.
Indeed, evidence of the Angara eyewitheness, including the report of a
district officer, suggests that the bolide flew "high in the sky," which
is hardly consistent with the path 99ø east of the true meridian. On the
contrary, the data obtained on the Nizhnaya Tunguska River, even though
agreeing with the shape the area of devestation, is in contradiction
with the Angara observations.

Another complication is that the Nizhnaya Tunguska data suggest,
virtually unambiguously, that the bolide's flight occurred in the
afternoon, whereas the data from the Angara River identify it as an
early morning flight. Attempts to resolve the conflict between both sets
of data present considerable problems.

In the search for an exit from this maze, more then one approach has
been tried. Some researchers practically ignored eyewitness accounts as
unreliable subjective material. This approach could be agreed with to a
certain extent if only it were a matter of a few inconsistent accounts,
whereas there were many hundreds of independent accounts.

Other investigators have tried their best to combine the Angara
evidence, the Nizhnyaya Tunguska data, and the geometry of the
devastated area (Zotkin I.T., a. Chigorin, A.N., 1988; Yavnel, A.A.,
1988). The results seem to be rather dubious.

Then the idea of a non-ballistic trajectory of the TCB was introduced
(Zigel F.Yu., 1983), based on the assumption that it had been moving
initially along a trajectory close to that calculated by Krinov. The
object then moved along a sloped curve trajectory and entered the space
above the joining of the Nizhnyaya and Podkamennay Tunguska rivers,
after which it continued its flight in an eastern direction and finally
to the place where it exploded.

The cause of the incompatibility of the bolide path projection with the
data of the Angara eyewitnesses remains unclear. Yet, it should be borne
in mind that the identity of the axis of symmetry of the observed forest
destruction pattern with the projection of the bolide path is only an
assumption of high probability rather than an established fact. The
axially symmetric "corridor" is the trace of the ballistic wave, where
it touched Earth's surface. It remains essentially an open
question--what its initial space position was and whether it could have
changed for some reason or other.

However, there are still other problems associated with the TCB path
parameters. Most authors who have studied this issue conclude that the
angle of the TCB trajectory was relatively small (Zotkin, I.T.,
Tsikulin, M.A., 1966; 1966 a; Tsikulin M.A., 1969; Bronshten, V.A.,
Boyarkina, A.P., 1975). Still, modeling experiments, as well as the
results of mathematical simulation of the Tunguska explosion parameters
(Korobeynikov, V.P. et al., 1976; 1980; 1983, 1984) testify that the
final part of the trajectory was most probably 40ø. The transition of
the TCB flight path from a comparatively flat trajectory to a steep one
seems to have taken place when the bolide approached the point where it
exploded; it might be due to an avalanche fracturing of the object and
enlargement of its frontal surface.

Especially suspicious is the fact that the "corridor"--the impression of
the ballistic wave on the forest--is observed, as has been recently
shown, to extend beyond the epicenter of the explosion. It is as if the
object continued its flight path after exploding. (Fast, V.G. et al.,
1976; Plekhanov, G.F.a. Plekhanova L.G., in press). The most reasonable
explanation is that part of the TCB survived the explosion and continued
its flight, maintaining the same trajectory to some degree.

It is important to relate certain features of forest devastation at the
epicenter of the Tunguska explosion area.

Vasiliev II

III. Some specific features of the forest devastation at the epicenter
of the Tunguska explosion.

It has been stated that the main cause of the forest devastation in the
area of the Tunguska catastrophe was a powerful energy release that
occurred at an altitude of 2.5 to 9.0 km (Goldin, V.D., 1986). It must
have been a huge explosion in the air which initiated a spherical shock
wave. The front of the shock wave was parallel to the earth's surface in
general at the epicenter of the explosion, and inclined to it away from
the epicenter. So the vertical component of the shock wave was the most
active component, whereas the horizontal component away from t he
epicenter was increasingly dominant and most pronounced in the area of
interference of the incident and reflected waves (Demin, D.V., 1963;
Boyarkina, A.P. a. Demin, D.V. et al., 1964; Fast, V.G. et al., 1967;
1976; ). As a first approximation, this can be interpreted as such.
Around the epicenter there is a vast area of dead forest (about 8 km
across), scorched and devoid of branches, but with trees standing
upright. This is th e zone of the vertical component impact of the blast
wave. Outside this area the forest is felled radially at a distance from
12 to 40 km away from the epicenter. This is the area of impact of the
horizontal component of the blast wave. If the above model is valid,
then at the explosion epicenter there must be no radially felled forest.
The actual situation is, however, essentially different. First, the
forest was not destroyed completely at the epicenter. Within 5 to 7 km
away many groups of trees survived. (Zenkin, G.M. et al., 1963). The
trees have attracted much attention of investigators, and attempts to
explain their existance, based on the relief features, have not yielded
unambiguous results. With the area's highest altitude above sea level
being 593 m, and the height of the explosion at 2.5 km (more likely 5.5
km), we can approximate the whole area to a plane.

The structure of the felled forest area in the immediate vincinity from
the epicenter also proves to be strange.

First, the assumption that no trees within this zone were felled in a
radial pattern is not true. Surface observations have shown that there
are some leveled trees in this area as well, and the general radial
character of the felled forest is seen up to a "special point," viz. the
geometric center of the fallen forest area, as calculated by V.G. Fast
(1963).

Second, Kulik's interpretation of the felled forest area on the basis of
a large-scale photographic air survey of 1938 (L.A. Kulik, 1939) not
only corroborated the complex vector structure of the epicenter area,
but also suggested at least two or three subepicenters.

Third, the vector structures of the felled forest on hillsides facing
the epicenter, and on the the opposite hillsides, are essentially
different. This is in poor agreement with the assumption that the blast
wave center was generated high above earth's surface.

Thus, we may tentatively conclude that along with a great energy release
from 5 to 5.8 km above the earth, there were a number of low-altitude
(maybe even right above the surface) explosions that contributed to the
total picture of destruction (Kulik, L.A., 1939; Golenetsky, S.P. a.
Stepanok, V.V., 1984). This seems to be sustained by data concerning the
deposition of aerosols immediately after the explosion (Serra, R. et
al., 1994).

Thus, the features of destruction at the epicenter suggest inhomogeneity
of the physical parameters of the Tunguska explosion field and
complexity of the physical processes underlying it.

It should be emphasized that though the patchine ss (????) of the
effects associated with the Tunguska explosion has been noted in
literature more than once, its origin has not been discussed. This seems
to be due to serious difficulties of its interpretation in terms of the
existing TCB models.

IV. On the geophysical effects of the Tunguska explosion

One of the most striking geophysical effects associated with the
Tunguska explosion is the local geomagnetic disturbance detected shortly
after the explosion in Irkutsk, although not recorded by any other
geophysical observatory in the world existing at that time (Plekhanov,
G.F. et al., 1960; 1961; Ivanov, K.G., 1961; 1961 a; 1962; 1964;
Obashev, S.O., 1961; Zhuravlev, V.K., 1963; Kovalevsky, A.F., 1963; ).
This disturbance was similar to some effects following middle- and
high-altitude nuclear explosions in the atmosphere, but unlike the
latter, it exhibited a kind of lag; i.e. it occurred some time after the
explosion. This has provided the main argument to account for the
geomagnetic effect of the Tunguska object as being due to the shock wave
and the period of time required for the wave to cross the distance from
the explosion point to the lower ionosphere boundary.

Later, however, I.P. Pasechnik, (1986) corrected the moment of the
Tungunska explosion on the basis of direct experimental measurements of
the velocity of the seismic wave between Vanavara and Irkutsk. It has
been found that the "lag" was actually longer than 5.9 min. This fact is
important inasmuch as the ensuing velocity of a shock wave in the
atmosphere is too low. Hence the mechanism accounting for this effect as
a consequencee of the arrival time of the shock wave in the ionosphe re
appears dubious (Dmitriev, A.N., Zhuravlev V. K., 1984). The question
thus remains open. The explanation of the "abnormal twilights" of late
June and early July, 1908, being due to cometary particles in the upper
atmosphere is not convincing.

Indeed, according to this assumption, particles of a comet's tail were
to be decelerated at an altitude of 200 km or more, whereas most light
anomalies formed at altitudes of 80 km (the zone of formation of
mesospheric clouds), 50 to 60 km (diffraction effects that caused dawn
and afterglow anomalies) and below that (atmospheric halos). >>I cannot
understand the previous sentence.<< Besides, according to this
assumption, the tail of the Tunguska "comet" should have been stretched
over Canada, but this was not observed. Recently, there has been an
attempt to ascribe the "abnormal twilights" of the summer of 1908 to
transport of material from the site of the explosion by stratospheric
winds. However, this assumption faces two contradictions:

1) At least 10 locations in Eurasia reported anomalous light effects on
the night of June 29-30, 1908. The effects were practically simultaneous
with, and even somewhat before, the Tunguska explosion. This makes it
impossible to explain the optical effects of June 30 being due to
mechanical transport of space aerosols from the site of the explosion.

2) There was a sharp exponential decrease in the intensity of the
atmospheric anomalies after the Ist of July. This suggests that the
major cause of the anomalies was due to photochmical reactions. If,
alternatively, the main cause of the anomalies were the refraction and
scattering of aerosol particles, it would be more reasonable to expect a
gradual decrease in the effects, as in the case of volcano-induced
optical anomalies. Neither explanation has been found adequate to
account for the changes of the polarimetric properties of the twilight
sky that appeared as deviations from the normal travel of the Arago and
Babinet neutral points (Vasilyev, N.V. et al., 1965).

Thus, explanation of the geophysical effects of the Tunguska object
faces serious problems.

V. Ecological consequences of the Tunguska catastrophe

Ecological consequences of the Tunguska event have been studied over the
past 30 years. They can be divided into two main types.

First is the remarkably quick revival of the taiga after the explosion,
as well as accelerated growth of young (Nekrasov, V.I., Emelyanov,
Yu.M., 1964; Emelyanov Yu.M. et al., 1967; Shapovalova R.D. et al.,
1967) trees and those which survived the event.

This effect covers a vast territory, it correlates wich the NSB
trajectory. >>I do not understand the meaning of the words following the
comma<< The effect is observed in all tree (??) species in the
region,>>I cannot understand the rest of this paragraph except for the
last sentence<< tending (for the 2nd post-catastrophic generation of
pine ) to concentrate toward the projection of the NSB path . There are
two viewpoints on the nature of the effect:

1) The early revival and accelerated growth of the forest are due to
some general results of the Tunguska explosion, such as better light and
thermal conditions in the area after the leveling of so many trees, as
well as enrichment of the soil with microelements as a result of the
forest fire . This point of view is not groundless, but it does not
explain these two facts: an evident correlation of the effect with the
projection of the NSB trajectory; and discrepancy between the zones of
the acceletrated growth of saplings and the areas of the forest fall and
forest fire. >>This second "fact" is not clear to me<<

Another possibility is that the stimulating effect of the Tunguska
explosion was due to enrichment of the poor soil of the region with
cosmogeneous microelements (Shapovalova, R.D., Lukyanov, V.B. et al.,
1967., Emelyanov, Yu.M., Lukyanov, V.B. et al., 1967; Golenetsky, S.P.,
Stepanok, V.V., 1983). Modelling experiments provide evidence that
extracts of the soils from the region enriched with rare-earth elements
(RE) can in fact stimulate germination of pine seeds, as well as the
seeds of certain other plants (Vasilyev, N.V., Kucharskaya, L.K. et al.,
1980). But rare earths are outside the classsical set of cosmogeneous
microelements.

The question has not been settled, and the effect probably cannot be
explained by conventional factors.

The second type of ecological consequences of the Tunguska event
includes genetic impact. Linear increments of the Tunguska pine trees
were processed with an algorithm discriminating the contributions of
genotypic and phenotypic variations (Dragavtsev, V.A. et al., 1975) .
This work revealed that the frequency of mutations in these pines has
sharply increased. Again, as with many other effects of the Tunguska
event, its genetic impact is of a patchy character, concentrating toward
the epicenter area and the "corridor" of the TSB trajectory. The thermal
influence of the forest fire could hardly be of any importance in this
instance since the contours of the areas of the mutagenic effect, forest
fire, and forest fall are quite different. The nature of the mutagenic
factor remains unknown.

VI. On the substance of the Tunguska object

The diligent search for large fragments of the Tunguska cosmic object,
which had begun in the late 1920's and ended in 1962, has shown totally
negative results. There have been no traces of astroblemes. >>I don't
know the word "astroblemes"<< The geomorphological formations around the
epicenter once thought to be small meteorite craters proved to be of
purely terrestrial origin (swamps, lakes, thermokarst holes, etc.).
Attempts to find fragments of the meteorite through rough mineralogical
analyses of soil, as well as with the help of magnetometers, metal
detectors, etc., also failed (Plekhanov, G.F., 1963). As a result,
beginning from the 1960's, through the initiative of K.P. Florensky, the
search strategy for the TCB substance was radically altered. Since then
efforts have been aimed at the search for and analysis of
finely-dispersed space material.

During the subsequent 30 and more years of investigation a number of
cosmochemical, geochemical, analytical and other techniques have been
applied. The principal results of this work can be summarized as follows:

1. Small amounts of finely-dispersed silicate and magnetite space
material are present in soils and peats in and around the region of the
explosion (Florensky, K.P., 1963; Florensky, K.P., Ivanov, A.V.et al.,
1968; Vasilyev, N.V. et al., 1973). However, there is no direct evidence
that these materials have anything to do with the TCB. On the countrary,
there is good reason to believe we are dealing with fluctuations of the
background fall of space dust. 2. Information on the iridium anomaly in
the Antarctic ice (Ganapathy,R., 1983) and Tunguska peat (Korina M.I. et
al., 1987) dated back to 1908 rests on isolated findings and requires
further verification.

3. Traces of enhanced concentration of nitrates in the Greenland ice
dated back to 1908 are absent (Rasmussen, K.L. et alll., 1984).

4. Increased concentration of microspherules enriched with copper, zinc,
gold, and some other volatile and chalcophile elements is found in the
1908 layers of peat and wood resin at a number of locations in the
region (Longo, G. et al., 1994; Kolesnikov, E.M. et al., 1977) .
Cosmogeneous nature of these anomalies is probable but they should be
differentiated from aerosols produced by the burning of peat (Doroshin,
I.K., 1988) and (possibly) wood, as well as from volcanic ash .

5. There have been revealed in the 1908 layers of peat, both at the
epicenter region and in the zone of the supposed dispersion train of
explosion products, substantial shifts in isotopic compositions of
carbon (toward its heavier isotopes), hydrogen (toward the lighter ones)
and lead. According to Kolesnikov (Kolesnikov, E.M., 1988; Kolesnikov
E.M., Shestakov, G.L., 1970) , these shifts are due to the dispersed
substance of a space body of the approximate composition of carbonaceous
chondrite.

6. A number of local geochemical anomalies were discovered at the
Tunguska site, although their association with the TCB requires further
investigation. This is, first of all, the rare earth (primarily
ytterbium) anomaly. Concentration of that element in soil, as well as in
the 1908 peat layers, is abnormally high (Zhuravlev, V.K., 1976;
Levchnko M.A., Terentiiva A.S., 1976). The ytterbium content reaches its
peak in the point of intersection of the extension of the TCB trajectory
(provided that its slope was about 40 degrees) with the Earth's surface.
Increased concentration of RE occurs mainly in the upper (i.e. recent)
layers of soil, and not in deeper layers, close to the substratum .
Along with the quantitative shifts, the affected zone shows a drastic
change in the RE ratio (Dozmarov S.V., in press).

Thus, the key to the solution of the Tunguska problem--i.e.,
determination of the TCB's elemental as well as isotopic composition--is
not yet at hand, and this line of inquiry must be continued.

Vasilyev IIb

VII On radioactivity in the Tunguska catastrophe site

The hypothesis of a nuclear nature of the Tunguska sxplosion was tested
by searching for radionuclides at the epicenter of the explosion area
(Zolotov, A.V., 1961; 1967; 1969; Plekhanov, G.F., 1963; 1964;
Kirichenko, L.V. a Grechushkina, M.P., 1963; Kirichenko, L.V., 1975;
1975 a; Mechedov, V.N, 1967; Kolesnikov, E.M, et al., 1975; Vasilyev,
N.V. et al., 1976 ; Stadolko I, in press). The results of this research
may be summarized as follows:

1. Radioactivity at the epicenter of the Tunguska explosion is within
the range of fluctuations of the present background radiation. At the
same time, however, its magnitude is somewhat higher at the epicenter
than at the periphery of the region. Most radionuclides are concentrated
in the upper horizons of the soil and peat and have been accumulated
from global fallout after nuclear tests.

2. The vertical distribution of radionuclides in the soil and peat does
not indicate the presence of radionuclides from test explosions before
1945. The only exception is a result of layer-by-layer radiometry of
sphagnum peat in the region of Vanavara (1960) where a second maximum of
radionuclide concentration was discovered at the depth of 35 cm.
(Kirichenko, L.V., Grechushkina M.P., 1963). This effect was not studied
in detail and it is not known which radionuclides were responsible for it.

3. Investigation of the isotopic composition of inert gases accumulated
in rocks near the epicenter did not reveal any peculiarities that could
be explained by the action of neutron irradiation on the natural
environment at the epicenter (Kolesnikov, E.M. et al., 1975) .

4. Analysis of 514 0C (???) content in the rings of the trees that
survived the 1908 explosion (Cowen, C., Atluri, C.R., Libby, W, 1965;
Vinogradov, A.P. et al., 1966; Lehrman, I.C. et al., 1967; Nesvitaylo,
V.D. a. Kovalyuch, N.N., 1983; Firsov, L.V. et al., 1984), revealed its
reliable excess over background values for the ring of 1909, which could
be expected if 514 0CO 42 0content was abnormally high in the summer of
1908. >>(I do not understand the previous sentence.<< The effect is of a
global character and has been traced both in the region of the Tunguska
explosion (Nesvetaylo, V.D., a. Kovalynkh, N.N., 1983) and outside
(Cowen, C. et al., 1965; Vinogradov A.P. et.al.,1966). Usually it is
explained by interference in the years 1908-1909 of two solar maxima,
viz. the 11-year and 100-year cycles. However, such an interpretation
does not explain the causes of the pathy character>>what is "pathy
character"?<< of the effect at the epicenter of the Tunguska event
(Firsov, L.V.,et. al., 1984;). The "solar" nature of the effect would be
proved if it were detected for other similar periodsl. But such work, as
far as we know, has yet to be perfomed.

Thus, the results of a search for radioactivity in the region of the
Tunguska explosion negates a nuclear hypothesis. It should be noted,
however, that a search for traces of radionuclide fallout half a century
after a nuclear explosion in the atmosphere is a challenging task
(especially taking into account comtamination from recent atmospheric
nuclear tests.

Along with attempts to detect radionuclides retained from the 1908
event, some efforts were made to find their traces by indirect methods.
First and foremost, variation of thermoluminecent properties of minerals
in the explosion area were studied. It is well known that shifts in the
intensity of thermoluminesence are a reliable, even if indirect,
indication of the exposure of minerals to ionizing radiation. This
method was successfully used to determine the level of radioactivity at
the epicenter of the nuclear explosion over Hiroshima some years after
the Tunguska event. Similar work was carried out in the epicenter area
of the Tunguska explosion (Vasilenko, V.B. et al., 1967; Bidyukov, B.F.,
et al.; 1990). Results suggest that background characteristics of
thermoluminescent minerals in this region were distorted by two opposing
factors. The first reduced thermoluminescent properties of minerals in
the immediate vicinity of the epicenter. Judging from the similarity
between the contours of this zone and the area of the radiant burn of
the trees, the effect was generated by the light flash. This seems
plausible, since annealing of minerals leads to a decrease in their
thermoluminescent properties and even to the loss of properties. But it
remains unclear why this effect manifested itself only in the area of
the light flash and not in the entire zone of the forest fire.

Working in parallel with the first factor, a second factor intensified
thermoluminescence of the minerals. This factor was at its maximum in an
area tending to the projection >>"tending to the projection" meaning not
clear<

VIII. Discussion.

Beginning in the 1960's, there was a revival of the cometary hypothesis
that had been put forward in the 1930's by I.S. AStapovich and F.
Whipple, and later developed by V.G. Fesenkov. It was suggested that
unlike the asteroidal version the cometary hypothesis could explain in a
conclusive way the main peculiarities of the Tunguska event and
distinguish it from other impact phenomena, namely: 1. the above ground
character of the explosion; 2. the lack of an astrobleme, as well as any
trace of large-scale fallout of meteorite matter; 3. atmospheric optical
anomalies that accompanied the Tunguska explosion. The cometary
hypothesis has had favorable effects on the progress of the Tunguska
studies. In its framework there were attempts to calculate the main
parameters of the TCB ; for example, its ity (???), mass, enegry,
strength characteristics, the slope of the object's trajectory, as well
as a description of the mechanism of the object's destruction (Fesenkov,
V.G., 1961; Pokrobski, G.I., 1966; Tsikulin, M.A., 1969; Boyarkina,
A.P., Bronshten, V.A., 1975; 1975 a; Petrov, G.I. a. Stulov, V.P .,
1975; Grigorian, S.S., 1976; 1979; Korobeynikov , V.P. et al., 1974;
1974 a; 1976; 1980; 1983; 1984; Kresak, L., 1978; Turco, R.P. et al.,
1982; Zynball, M.N., Shnitke, V.E., 1986; 1988). Later, however, results
of further studies have complicated the situation, which today seem to
be at a critical point. Some obstacles which the cometary hypothesis ran
into have been discussed above. It should be added that the authors of
some theoretical models (Petrov, G.I., Stulov, V.P., 1975, Turco R.P. et
al., 1982) proceeded from the assumption of a low (of the order of 10
5-2 0g cm 5-3 0, or even super-low 10 5-3 0g cm 5-3 0) density for the
object. Direct investigation of Halley's comet has shown, however, that
the real density of cometary ice is about 1 g cm 5-3 0. Conseqently, the
models of the TCB as a "super-loose lump of cosmic snow" or a "gigantic
cosmic snowball" should be rejected.

However, these failures have not affected the "core" of the cometary
hypothesis, since most of its supporters proceeded from more realistic
estimates of the cometary ice density, assuming it to be equal to 1 g cm
5-3 0. But recently published material has cast doubt on the very
fundamentals of this hypothesis. We mean, first of all, the detailed
calculations, made by Z.Sekanina (1983) who came to the conclusion that,
due to the strength characteristic of the comet nucleus, it would have
had to disintegrate at a much higher altitude than is now accepted.
Then, mathematical models have suggested that gigantic carbonaceous
chondrites have to disintegrate at an altitude of 30 km, a
circumstantial argument in favor of this inference being the explosion
of the Revelstoke carbonaceous chondrite, as well as high-altitude
explosions of meteoroides (Bronshten, V.A., 1981, Levin, B.Yu.,
Bronshten V.A., 1985), confirmed by aerospace methods. Since the
chemical composition of the Tunguska object in its dominant conventional
models is identical to that of a comet nucleus or to a carbonaceous
chondrite, verification of this data would be of crucial importance to
the Tunguska problem. Both Z. Sekanina (1983) and C. Chyba et al. (1993)
arrived at the conclusion that the TCB should be classified as a stony
asteroid. An iron meteorite as large as this would have reached the
Earth's surface and formed an astrobleme. A comet nucleus or a
carbonaceous chondrite would have disintergrated at a much higher
altitude than the Tunguska body did. But if this it the case, then the
question of the TCB substance reappears. A number of problems remain
unsolved: isotopic anomalies in the 1908 layers of sphagnum bogs at the
epicenter, as well as increased concentration of volatile and
chalpophile elements in the sphagnum, to cite two such problems.

A return to the stony asteroid model would require an explanation of the
1908 atmospheric optical anomalies, which for a long time were assumed
to be due to penetration of the "Tunguska comet" tail into the atmosphere.

At present the study of the problem has reached a phase in which it
becomes possible to rigorously formulate the principal questions, which,
if resolved, can restrict the spectrum of acceptable TCB models.
Primarily, these are the following closely related questions:

1. Can the Tunguska explosion be explained as a result of destruction of
comet's ice lumps or a meteoroid similar to carbonaceous chondrites at
an altitude of 2,5-9 km? Are Sekanina and Chyba right in denying such a
possibility? Or are Bronshten, V.A., a. Boyarkina, A.P., ( 1975, 1976)
Korobeynikov, V.P. et al., (1983, 1984), Grigorian, S.S. (1976) and
other investigators who advocate the cometary hypothesis? If the former
are right, then it becomes imperative to revise a large number of
calculations dealing with the mechanism of destruction of the Tunguska
object that have been published since 1963. It is also necessary to
re-explain the isotopic and elemental anomalies at the epicenter of the
region, and to re-interpret the atmospheric optical anomalies of the
summer of 1908.

2. What is the origin of the isotopic and elemental anomalies in 1908
peat and wood resin layers? Are they due to precipitation of remnants of
the TCB or to some other processes? If indeed isotope shifts and
increased concentrations of volatile and chalcophile elements dated to
1908 in these natural objects are due to some material of the TCB, then
in this case the paradox of the absence of cosmic matter in the area of
the explosion comparable with the scale of the phenomenon is eliminated.
Thus, it appeares critical to make check investigations in control areas
not associated with the Tunguska event.

3. Can the atmospheric optical anomalies of 1908 be due to transport
from the explosion site by stratospheric winds of TCB matter--the
material of a stony asteroid, in particular?

4. What is the nature of the WNW segment of the "corridor" of axially
symmetric deviations of forest fall vectors from the dominating radial
pattern? And can it be due to anything else than the trace of the
ricochet of the part of the TCB that survived the explosion, in terms of
the traditional models?

5. Is it possible to resolve the contradiction between the TCB
trajectory as defined from evidence of the Angara eyewitnesses and the
direction of flight of the body as suggested by the vector pattern of
the forest fall? Unambiguous and comprehensive solutions to the above
problems will lead to a choice between the opposing two hypotheses. It
is now too early to predict the outcome. However, our present ability to
pose the question is a significant achievement in the history of the
development of the Tunguska problem.

Aknowledgement

In conclusion, we wish to thank our many collegues in Russia and abroad,
especially the scientific workers in more than 120 astronomical and
geophysical observatories who provided archival information relating to
the period 1907-1909.

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