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Chapter 18

The Tunguska Event

G. Longo

18.1
Introduction

In the early morning of 30th June 1908, a powerful explosion over the
basin of the Podkamennaya Tunguska River (Central Siberia), devastated 2
150 ± 50 km2 of Sibe rian taiga. Eighty millions trees were flattened, a
great number of trees and bushes were burnt in a large part of the
explosion area. Eyewitnesses described the flight of a "fire ball, bright
as the sun". Seismic and pressure waves were recorded in many obser
vatories throughout the world. Bright nights were observed over much of
Eurasia. These different phenomena, initially considered non-correlated,
were subsequently linked together as different aspects of the "Tunguska
event" (TE). Almost one century has elapsed and scientists are still
searching for a commonly accepted explanation of this event. Several
reviews and books summarize the results acquired by the intensive
investigations of the last century, e.g. Kulik (1922, 1939, 1940),
Landsberg (1924), Krinov (1949, 1966), Gallant (1995), Trayner (1997),
Riccobono (2000), Bronshten (2000), Vasilyev (1998, 2004) and Verma
(2005). Despite great efforts, the TE remains a conundrum.

18.2 The Hypotheses

The most plausible explanation of the event considers the explosion in the
atmosphere of a "Tunguska Cosmic Body" (TCB), probably a comet or an
asteroid-like meteorite.

18.2.1 Comet or Asteroid?

From his first determination of the basin of the Podkamennaya Tunguska
River as the explosion site, Kulik (1922 and 1923) used the term "Tunguska
meteorite", for the TCB, and continued searching for an iron body, similar
to one found in Arizona (Kulik 1939, 1940; Krinov 1949, 1966).
Voznesenskij (1925) hypothesized an equal probability for a stony or an
iron body composition. Shapley (1930) was the first to suggest that the
Tunguska event was caused by the impact of a comet and Kresák (1978)
indicated the comet Encke as the origin of the TCB. Fesenkov (1949), for
many years, supported the stony object hypothesis. Later, Fesenkov (1961)
worked out a definite model of an impact between a comet and the Earth's
atmosphere. From that time onward, the majority of

304 G. Longo

Russian scientists followed the cometary hypothesis (see for example
Grigorian 1998), whereas many western scientists preferred an asteroidal
model (e.g. Sekanina 1983, 1998; Chyba et al. 1993). For many reasons,
these two "schools" practically ignored each other until the inter
national workshop Tunguska96, held in Bologna (Italy) from 15th to 17th
July 1996 (Di Martino et al. 1998). In the recent past the cometary
hypothesis has been favored on the basis that a low-density object was
needed to explain the Tunguska catastrophe (Petrov and Stulov 1975; Turco
et al. 1982). Subsequently, to account for the concentra tion of energy
release of the explosion, two sub-versions of this hypothesis have been
developed, one introducing chemical reactions (Tsymbal and Shnitke 1986),
the other nuclear-fusion reactions (D'Alessio and Harms 1989). On the
other hand, it has been shown (Grigorian 1976; Grigorian 1979; Passey and
Melosh 1980; Levin and Bronshten 1986) that the fragmentation of a normal
density object can greatly increase the rate of energy deposition in a
small region near the end of the trajectory, thus appearing as an
atmospheric explosion. Detailed calculations which include the effect of
aerody namic forces that can fracture the object, and the heating of the
bolide due to friction with the atmosphere, have recently been performed,
showing that the TE is fully com patible with the catastrophic disruption
of a 60­100 m diameter asteroid of the com mon stony class (Chyba et al.
1993; Hills and Goda 1993). However, due to the uncer tainty of such input
parameters as the energy and height of the explosion or the incli nation
angle and the encounter velocity of the impactor, the same calculations do
not exclude the possibility that the TCB was a high velocity iron object,
nor rule out a carbo naceous asteroid as an explanation of the event.
Considering a "plume-forming"  atmo spheric explosion, Boslough and
Crawford (1997) have suggested that the commonly accepted energy-yield is
an overestimate and that a 3 megaton event could generate the observed
devastation. Many of the phenomena associated with the TE can be related
to the formation and collapse of an atmosphere plume, caused either by a
comet or by an asteroid. For example, the predicted ejection at altitudes
of some hundreds of kilo meters of the impactor mass can explain the
"bright nights" associated with the TE. It is difficult to definitely
support one or the other hypothesis. Therefore, one way to achieve
certainty about the nature and composition of the TCB remains the search
for some of its remnants. Numerous radiocarbon analyses of Tunguska wood
samples (Nesvetajlo and Kovaliukh 1983), chemical analyses of soil and
plants (Kovalevskij et al. 1963; Emeljanov et al. 1963; Kirichenko and
Grechushkina 1963; Iljina et al. 1971), bed-by-bed chemical analyses of
the peat formed by Sphagnum fuscum in 1850­1950 (Vasilyev et al. 1973;
Golenetskij et al. 1977a; Golenetskij et al. 1977b; Kolesnikov et al.
1977), isotopic analyses of many different soil, peat and wood samples
(Kolesnikov et al. 1979), as well as analyses of the spherules from
Tunguska soil samples collected in a radius of several tens of kilometers
from the epicenter (Florenskij et al. 1968; Jéhanno et al. 1989; Nazarov
et al. 1990) have been completed.  Nevertheless, many conclusions of this
intensive work are still uncertain, so that further investiga tions are
needed. Although almost every year there is an expedition to Tunguska, so
far no typical material has permitted a certain discrimination to be made
between an asteroidal or cometary nature of the TCB. Some papers report
that hydrogen, carbon and nitrogen isotopic compositions with signatures
similar to those of CI and CM carbonaceous chondrites were found in
Tunguska peat layers dating from the TE

305 Chapter 18 · The Tunguska Event

(Kolesnikov et al. 1999, 2003) and that iridium anomalies were also
observed (Hou et al. 1998, 2004). Measurements performed in other
laboratories have not confirmed these results (Rocchia et al. 1990;
Tositti et al. 2006). Moreover, a concentration of microparticles of
inferred cosmic origin was found in tree resins dating from the TE (Longo
et al. 1994; Serra et al. 1994). Although these data are compatible with
the hypothesis of the impact of a cosmic body, they are by no means
conclusive and are not sufficient to prove the nature of the TCB. The same
can be said about the lacustrine sediments of Cheko Lake (Sacchetti 2001)
studied in the framework of the multidisciplinary investigation as carried
out by the Italian scientific expedi tion Tunguska99 (see
http://www-th.bo.infn.it/tunguska/) (Amaroli et al. 2000;  Pipan et al.  
2000;  Gasperini et al.  2001;  Longo et al.  2001;  Longo and Di Martino
2002 and 2003; Longo et al. 2005). This field research has been
strengthened by theoretical studies and modeling. In a recent paper
(Farinella et al. 2001), a sample of possible TCB orbits has been
constructed and a dynamic model was used to compute the most probable
source of a TCB placed on each of these orbits. The results of
calculations gave a greater probability for a TCB coming from an
asteroidal source (83%), than from a cometary source (17%).

18.2.2 "Non-traditional" Hypotheses

Vasilyev (2004) states, "We should not exclude the possibility that the
Tunguska phe nomenon is a qualitatively new phenomenon for the science,
that should be analyzed from non-traditional positions". These
"non-traditional" approaches still consider an impact with the atmosphere
of "something" coming from external space. Several of them, though
published in scientific journals, were found to be technically ground
less, e.g. the hypotheses involving near critical fissionable material
(Zigel' 1983; Hunt et al. 1960), antimatter meteors (Cowan et al. 1965),
and tiny black holes (Jackson and Ryan 1973). Others consider alien
spacecrafts (Kazantsev 1946; Baxter and Atkins 1976). Kazantsev was the
first who explained the lack of fragments or impact craters in Tunguska by
an explosion in the atmosphere. Nevertheless, I think that here we can
ignore such extremely "non-traditional" hypotheses.

18.2.3 Alternative Approaches

Recently, some "alternative" approaches were presented to explain the TE.  
Different from the above-mentioned traditional or non-traditional
explanations, these alterna tive approaches deny an impact of an external
body with Earth. They claim that the event was triggered by a terrestrial
cause. I mention here two of the more discussed alternative
interpretations. The first is a tectonic interpretation (e.g. Ol'khovatov
2002), which considers the coupling between tectonic and atmospheric
processes in a "very rare combination of favorable geophysical factors."
Another recent work that should be mentioned is the "kimberlite
interpretation" (Kundt 2001), which considers the TE as caused by the
tectonic outburst of some 10 megaton of natural gas. For the volcanic
(outflow)  inter

306 G. Longo

pretation, Kundt presents the estimates of the involved mass and kinetic
energy of the vented natural gas, of its outflow timescale, supersonic and
subsonic ranges, and buoyant escape towards the exosphere. The main idea
of this latter work is contradicted by at least two facts. The first and
more obvious point against the hypothesis of an explosion from the ground
is that the eyewitness testimonies describe the trajectory of a bolide
crossing the sky (see Sect. 18.3.2). Among these testimonies, the
earliest, given a few days after the event by educated people, have a high
trustworthiness. On that basis, the first Kulik expedition (1921­1922)
gathered sufficient information to conclude, that "the meteorite fall in
the neighbor hood of the Ogniya river, a left tributary of the Vanavara
river, which is a right tributary of the Podkamennaya Tunguska (Hatanga)
river" (Kulik 1922, 1923, 1927; Landsberg 1924). The first expedition
could not go farther than Kansk, about 600 km from the Tunguska explosion
site. Five years later, Kulik discovered the site about 50 km from the
mentioned tributary of the Vanavara River. A second objection comes from
the absence of debris clearly referred to the explo sion in the epicenter
area. If we assume that the anomalous optical phenomena ob served after
the TE, were due to particles released in the atmosphere by the explosion,
we should find an increasing concentration of those particles (with
grain-size pro gressively decreasing) toward the explosion epicenter. As
also pointed out by Kundt, we do not observe a carpet of dust in the
vicinity of the epicenter as we should ob serve for the explosion of a
meteorite, but also (and more markedly) for the explosion of a diatreme or
any volcanic emission. How could an explosion "from below" dis perse dust
in the atmosphere to an extent comparable to that of the Krakatau without
leaving significant traces close to the epicenter? It seems more probable
that an ex plosion "from above" could explain this occurrence. Moreover,
geological maps of the region (Sapronov 1986) and our own observations
during the Tunguska99 expedition do not report the presence of mantle
rocks, such as peridotites or eclogites, which are usually associated with
kimberlites. Though the area is centerd on the roots of the lower Triassic
Kulikovsky paleovolcanic complex (see Fig. 18.1), which extends over an
area 25 × 20 km wide, displaying numerous, various sized craters, it is
presently a tectonically stable cratonic region, as testified by the low
intraplate seismicity. The map from the USGS catalog, which reports
significant world wide earthquakes during historical times, confirms this
stability. Finally, the "radonic storm" registered at our base camp (see
Fig. 18.2) during the Tunguska99 expedition (Longo et al. 2000; Cecchini
et al. 2003) has nothing to do with a "kimberlite" phenomenon, as
suggested by Kundt. Indeed, we registered an intensity enhancement of
gamma radiation during a thunderstorm (see Fig. 18.3) due to radon
daughters, as observed in other parts of the world, where no "kimberlite
interpreta tion" is possible. Though we cannot accept the main ideas of
Kundt, the outflow theory can help us to understand some aspects of the
TE. It is plausible, and even probable, that gas releases took place from
the permafrost dissolution (caused by the impact of the TCB and not by a
kimberlite outflow). For example, part of the multiple explosions heard
for more than half an hour by many earlier trustworthy witnesses (Kulik
1922, 1927; Obruchev 1925; Voznesenskij 1925) might probably be due to a
rapid release of gas (methane) from the permafrost layer as a consequence
of the thermal burst related to

307 Chapter 18 · The Tunguska Event

Fig. 18.1. Satellite view of the Kulikovsky paleovolcanic complex (1 ­
lake Cheko, 2 ­ river Kimchu, 3 ­ Northern swamp, 4 ­ Southern swamp, 5 ­
river Khusma)

Fig. 18.2. The base camp of the Tungu ska99 expedition on the shore of the
lake Cheko (drawing by Andrey Chernikov)

308 G. Longo

Fig. 18.3. Gamma-ray (25 keV ­ 3 MeV) intensity enhancement registered at
the base camp of the lake Cheko during the thunderstorm of July 19, 1999.
Note the steep rise of counting rate, while it is raining. It corresponds
to gammas emitted by radon daughters

Fig. 18.4.  Aerial view of the camp of the Tunguska99 expedition (23 July
1999). Near the shore, a hole with a few meters diameter resulting from a
gas outflow can be seen on the lake bottom

309 Chapter 18 · The Tunguska Event

the main event. Indeed, in July 1999, we observed a small "crater"
originated "from below" on the Cheko Lake bottom (see Fig. 18.4). It could
be due to methane emission from decaying organic matter in the surface
layer of some tens of meters.  Obviously, this does not contradict the
known tectonic stability of the region.

18.3 Known Data

18.3.1 Objective Data

Three main kinds of objective data on the Tunguska explosion are
available:  seismic and barometric registrations, recorded immediately
after the event, information on the bright nights, observed in Eurasia in
July 1908, and data on forest devastation, systematically collected 50­70
years later and recently integrated with the data of the 1938 and 1999
aerial photographic surveys.

Seismic and Barometric Registrations

Seismic records from Irkutsk, Tashkent, and Tiflis were published
together, two years after the event (Levitskij 1910), those from Jena,
three years later. However, the first paper that connected to the TE the
origin of these seismic waves was published only in 1925 (Voznesenskij
1925). Similarly, the barograms recorded in 1908 in a great num ber of
observatories throughout the world, were associated with the TE some
twenty years later (Whipple 1930; Astapovich 1933). From the analysis of
the available seismo grams and barograms, the time that the seismic and
aerial waves started was calcu lated. The main results obtained are listed
in Sect. 18.4.

Bright Nights Observed

In 1908, the attention of astronomers and geophysicists in Europe and Asia
was drawn to some unusual phenomena, such as bright nights, noctilucent
clouds, brilliant colorful sunsets and other observations. It is difficult
to conclude that some of these phenomena are really "anomalous". For
example, in June-July, the appearance of noc tilucent clouds reaches its
maximum and it is difficult to distinguish between "usual" and "unusual"
noctilucent clouds. Therefore, I shall consider here only the bright
nights phenomenon. Bright nights ("at midnight, it was possible to read
the newspaper without artificial lights"; see Figs. 18.5 and 18.6) were
described in many papers (e.g., De Roy 1908; Shenrock 1908; Süring 1908;
Svyatskij 1908). At that time, many explanations for the bright-nights
phenomenon were proposed. Up to 1921, meager information about a great
1908 bolide was published only in some local Siberian newspapers. Nobody
con sidered a link between these phenomena, although on 4 July 1908, the
Danish astrono mer Torwald Kohl wrote: "It would be advisable to learn
whether in recent times some

310 G. Longo

great meteorite has been seen in Denmark or elsewhere" (Kohl 1908). It was
only in 1922, after his first recognition in Siberia that Kulik wrote
about a probable link between the bright nights in Eurasia and the
explosion in Central Siberia (Kulik 1922).  From that time onward, such
phenomena have been considered as two parts of the Tunguska event. The
phenomenon and its correlation to the TE, was thoroughly studied in the
1960s (Zotkin 1961; Vasilyev et al. 1965). The 4 March 1960 issue of
Science published a letter from the Committee on Meteorites of the Academy
of Science of the USSR addressed to foreign scientists and asking them to
send all the information available on the op tical phenomena of 1908
(Fesenkov and Krinov 1960). Zotkin (1961) studied the bright nights,
observed in 114 points of the globe.  He dis tinguished observations
following the 30 June from those preceding that date.  He con siders the
latter poorly reliable and of "local character", whereas the events
observed from the 30 June did not have a "local" character and were
observed in more than a hundred points of Europe and Asia. Vasilyev (1965)
considered a more complete data set and referred to 86 communi cations and
articles dated to 1908. He lists 14 cases of bright nights from 21 to 29
June 1908 and 159 cases from 30 June up to 3 July (in subsequent papers,
he indicates about twenty other cases from 4 to 28 July). He considers all
these cases related to the Tunguska event and this is not easy to explain.
It seems to me that Zotkin's approach is more acceptable. Only the bright
nights following the 30 June should be related to the Tunguska event. This
is confirmed by the global character of the phenomenon and by polarization
measurements. The "global" character of the phenomenon, observed in the
nights beginning on 30 June and 1 July 1908 are illustrated in Fig. 18.5
(Vasilyev and Fast 1976). As can be seen, the bright nights were observed
on an area of about 12 million km2, from the longitude 6.5° W (Armagh,
Ireland; see Fig. 18.6) up to 92.9° E (Krasnoyarsk) and from the latitude
41° N (Tashkent) up to 60° N (Petersburg). If the bright nights are due to
dust in the atmosphere, the light reflected should be polarized. Busch
(1908a,b) measured the daylight polariza tion in Arnsberg (Germany). His
results indicate an absence of the effect in the first half of 1908 up to
28 June, a strong effect the 1 July that gradually disappears up to 25
July. The conclusions of Zotkin were that it is difficult to accept that
dust particles could reach Great Britain from Tunguska in 22 hours.
Therefore, they were ice particles from the comet tail and the comet
nucleus exploded in Tunguska. Bronshten (1991) hypothesized that the
particles were transported from Tunguska by gravitational forces. In
Boslough and Crawford model (1997), the mass of the impactor, as well as
water from the humid lower atmosphere, are ejected above the top of the
atmosphere and within 15 minutes can extend more than 2000 km from the
impact site.

Data on Forest Devastation

The data on forest devastation are a second kind of objective information
source about the event. The main part of these data refers to the tree
fall and the direction of flat tened trees. From these data we can obtain
information on the coordinates of the wave propagation centers (often
called "epicenter(s)") and on the final TCB trajectory.

311 Chapter 18 · The Tunguska Event

Fig. 18.6. Photos taken during the bright night of 30 June 1908 in (a)
Armagh, (b) Greenwich, and (c) Tambov. d The Irkutsk observatory at the
beginning of the 20th century Fig. 18.5. Stations where anomalous bright
nights were observed the 30 June/1 July 1908

312 G. Longo

Though Kulik discovered the radial orientation of fallen trees as early as
1927, sys tematic measurements of fallen tree azimuths were started only
during the two great post-war expeditions organized by the Academy of
Sciences in 1958 and 1961 (Florenskij et al. 1960; Florenskij 1963), and
during the Tomsk 1959­1960 expeditions.  Under the direction of Fast, with
the help of Boyarkina, this work was continued for two decades during ten
different expeditions from 1961 up to 1979. A total of 122 people, mainly
from Tomsk University, participated in these on site measurements. The
data collected have been published in a catalog in two parts: the first
one contains the data obtained by six expeditions (1958­1965), which
include the whole set of single-tree azimuths and the azimuths averaged on
trial areas equal to 2500 m2 or 5000 m2, chosen throughout the whole
devastated forest (Fast et al. 1967). In the second part, the data
collected by the six subsequent expeditions (1968­1976) were given (Fast
et al. 1983). The data on forest devastation also give information on the
energy emitted and on the height of the explosion. Indeed, these data
include, not only fallen tree directions, but also the distances that
different kinds of trees were thrown, the pressure necessary to do this,
information on forest fires and charred trees, data on traumas observed in
the wood of surviving trees and so on (e.g. Florenskij 1963; Vorobjev et
al.  1967; Longo and Serra 1995; Longo 1996, 2005). In order to correct,
update and enlarge the fallen tree distribution data, we per formed a new
aero-photographic survey during the Tunguska99 expedition (Longo and Di
Martino 2002, 2003) (see map on Fig. 18.7). This survey was needed to
obtain a new unified catalog, which includes: (1) corrected Fast data
(Fast et al.  1967, 1983), (2) data from Kulik's 1938 aerial photosurvey
never previously analyzed, (3)  never published data collected in 1967 by
the Anfinogenov group in the central region of the site. These three
datasets have been checked and completed with our on-site measure ments
carried out in July 1999 and 2002 to obtain the coordinates of different
reference points in the same area. These data allowed us to recognize
ground elements on the aerial pictures and to connect them to the regional
topographic net. Unfortunately, a map containing all the data from Fast's
catalogs (Fast et al.  1967, Fast et al. 1983) has never been published.
In the last 40 years, the map of fallen tree azimuths used for comparison
with theoretical models (e.g. Korobeinikov et al.  1990; Boslough and
Crawford 1997) was the one constructed by A. Boyarkina, V. Fast and co
workers (Florenskij 1963; Boyarkina et al. 1964). This map contains only
the data on the azimuths measured in 1958­1961. The new unified catalog
and the new map (Longo et al. 2005) have been constructed using a number
of tree azimuths and trial areas several times larger than those
considered in Fast's analyses. Moreover, we have intro duced a reliability
degree for each trial area averaged azimuth. The reliability degree has
been assigned on the basis of the percentage of singletree azimuths that
lay in a sector of 15° centered on the averaged azimuth. A good agreement
between the new map and the horizontal aerodynamic pressure calculated on
the basis of Korobeinikov et al. (1990) model has been obtained.

No Impact Craters or Meteorite Fragments

Data on forest devastation and records of the atmospheric and seismic
waves have made it possible to deduce the main characteristics of the
Tunguska explosion, i.e. its

313 Chapter 18 · The Tunguska Event

Fig. 18.7. Flight routes of the 1999 aero-photosurvey

exact time, 00h 14m 28s UT (Ben-Menahem 1975), the coordinates of the
point usually called epicenter, 60° 53' 09" N, 101° 53' 40" E (Fast 1967),
the energy release, equivalent to 10­15 million tons of TNT (Megaton) that
corresponds to about one thousand times

314 G. Longo

the Hiroshima bomb energy, and height of the explosion (5­10 km), though
the values for the last two parameters are estimated with great
uncertainty. However, neither macroscopic fragments of the cosmic body,
nor a typical signature of an impact, like a crater, have ever been found
in an area of 15 000 km2, so that the nature and compo sition of the TCB
and the dynamic of the event have not yet been clarified.

18.3.2 Eyewitnesses Testimonies

There is a great number of eyewitness testimonies. The more complete
collection of these testimonies is provided by Vasilyev et al. (1981). It
contains direct observations of the Tunguska explosion from 386 different
points and a list of the geographical coor dinates of these points. To
these observations, the authors have added news published in newspapers,
reports and communications from many official employees for a total of 708
testimonies. It is easy to find contradictions in this material collected
for more than 60 years by very different people. Sometime these
contradictions are more appar ent than real. As an example I can remember
the contradiction recently removed by Fast VG1 and Fast NP (2005). As is
well known, two centuries before the TE, the czar Peter I introduced a
reform in the Orthodox Church. Entire villages of people that did not
recognize the reform were sent to Siberia. Therefore many Siberian regions
and villages in 1908 were populated by people following the "old faith".
For them, the daily timetable was regulated starting from the morning
prayers at "obied", i.e. 8 o'clock in the morning. When asked about the
explosion time, they answered that the explosion took place some time
before the obied, which really corresponds to the seismic wave
registrations after 7 o'clock local time. For the secular people that
collected the testi monies, the word obied means lunch, i.e. about 12­14
o'clock. Therefore, they completed the forms by noting that the eyewitness
stated that the explosion took place at noon, or even in the afternoon.
These testimonies were considered not trustworthy due to the clear
contradiction with instrumental registrations. A thorough statistical
analysis performed by the Fasts (2005) has shown that the distribution of
"midday eyewitnesses" correctly reproduces the distribution of the
population following the "old faith". To use them properly, it is
important to take into account the different trustworthi ness of the
testimonies. I think that we can distinguish the following groups of testi
monies in decreasing order of trustworthiness:

1. The testimonies collected in the days immediately following the
Tunguska explosion by the director of the Irkutsk magnetic and
meteorological observatory Voznesenskij (1925). Unfortunately,
Voznesenskij published them only 17 years later due to an excess of
scientific prudence. Immediately after the registration of the earthquake
N° 1 536, in the morning of 30 June 1908, Voznesenskij sent to all his
correspondents a re quest to report what they or other people had observed
on that morning. In his paper he gives a table with the results received
from 61 correspondents and a map

1 It was the last contribution to the Tunguska studies given by the great
researcher Vilgem Genrikovich Fast (1936­2005).

315 Chapter 18 · The Tunguska Event

showing their location on a very great territory (Fig. 18.8). Moreover, he
refers to many individual testimonies from "cultured" people (chief of
town post office, em ployees of meteorological observatories, agronomist
and so on). 2. The testimonies collected before, during and immediately
after (up to 1933)  the ex peditions of Kulik. They were collected mainly
by Obruchev (1925), Suslov (1927) and Kulik (1922, 1923, 1927). I have
mentioned in Sect. 18.2.3 that this primary information was sufficient to
understand in 1922 that research had to be directed to the north of the
Podkamennaya Tunguska River, in the neighborhood of the Vanavara River. 3.
In the period from about 1933 up to 1958 practically no new eyewitness was
ques tioned and, finally, in the 1960s, a massive material with hundreds
of new testimo nies from old people was collected in many regions. A
thorough examination of these records can still be useful as Fasts's work
shows. Fig. 18.8. A map with the dislocation of the correspondents that
sent in July 1908 their reports to the Irkutsk Observatory. The map was
published by Voznesenskij (1925) and reproduced by Krinov (1949, 1966)

316 G. Longo

No doubt that the more valuable testimonies are those written immediately
after the fall by the correspondents of the Irkutsk observatory. They are
not influenced by Voznesenskij who asks only about observations related to
the earthquake N° 1536, with out any reference to a flying body. Many of
these reports are written before the pub lication in local newspapers of
the first information on the event. These genuine re ports, synthesized by
Voznesenskij in 1925, are now stored in the Archives of the Me teorite
Committee of the Russian Academy of Sciences hereafter referred as
"Archive RAS". In the following paragraph, I give in brackets the page of
the document N° 57 of Archive RAS in which these testimonies are gathered.
To describe what seen in the morning of 30 June 1908, no one of these
reports testify something different from a flying object. Many reports are
written after questioning a great number of persons. For example, the
director of the meteorological station of Maritui states that his report
is written after the interrogation of about 500 persons on a great
territory around his station (19). I quote here some descriptions of the
corre spondents:

"a large group of local inhabitants noticed a ball of fire in the north
west coming down obliquely" (3); "the workmen saw a fiery block flying, it
seemed, from south east to north west" (4); "in the north west a pillar of
fire appeared about 8 meters in diameter ... it was accurately established
that a me teorite of very large dimensions had fallen" (5); "the local
peasants told me that they saw some sort of fiery ball flying in the
north" (6); "a loud noise was heard ... probably from a passing meteor
(aerolith)" (9); "some of local inhabitants had seen an elongated body
narrowing towards one end, about one meter in length, torn as it were from
the Sun...this body flew across the sky and fell in the north east" (16);
"the fall of an aerolith was observed ... a fiery streamer was seen" (26);
"a ball of fire appeared in the sky and moved from south east to north
west. As the ball approached the ground ... it had the appearance of two
pillars of fire" (36).

The testimony of page 16 was written the 30 June 1908 (the day of the
event), that of page 6 ­ the 1 July 1908 (the day after the event), the
others ­ from a few days up to six weeks after the event. Many
correspondents could not understand what they have seen or heard. For
example, in the letter referred to on page 6, the correspondent wrote to
the Irkutsk observatory: "I have the honor to ask submissively the
observatory to com municate and clarify what this means and could it be
dangerous for human life". These testimonies, and many others, contradict
the "alternative" approaches (see Sect. 18.2.3) that deny the impact of an
external body with Earth.

18.4 Parameters Deduced

18.4.1 Explosion Time

Studying the available seismic data, a first determination of the
explosion time as 0h 17m 12s UT was obtained by Voznesenskij (1925). This
value was used up to the 1960s. The explosion time deduced from the
barograms of 6 British meteorological stations, was equal to 0 h 15 m UT
(Whipple 1930). The independent analysis of the barograms from 13 Siberian
stations, gave an explosion time equal to 0h 16m 36s UT (Astapovich 1933).
These two sets of data were subsequently analyzed more carefully taking
into

317 Chapter 18 · The Tunguska Event

account the exact distances and the properties of seismic and atmospheric
waves. Pasechnik (1971) obtained a first result (0h 14m 23s UT), based
solely on Jena and Irkutsk's seismic data. Two additional and more
complete analyses were independently performed by Ben-Menahem (1975) and
Pasechnik (1976). They found practically the same value for the time the
seismic and aerial waves started (see Table 18.1, updated from Farinella
et al. 2001). Pasechnik (1976) calculated that the time of the explosion
in the atmosphere was 7­30 seconds earlier depending on the height and
energy of the explosion; this inter val was subsequently reduced to 2­20
seconds (Pasechnik 1986). In the 1986 paper, however, Pasechnik revised
his previous results obtaining a value equal to 0h 13m 35s ± 5s UT. The
commonly accepted explosion time is the time given by Ben-Menahem for the
instant the seismic waves started, i.e. 0h 14m 28s UT.

18.4.2 Coordinates of the Epicenter

The first contact point between the Earth surface and the shock wave from
the air burst is commonly called "epicenter," though this term is not
proper. From the data collected during the first three expeditions, Fast
(1963) obtained the epicenter coor dinates 60° 53' 42" N, and 101° 53' 30"
E. These values are very close to the final ones 60° 53' 09" ± 06" N, 101°
53' 40" ± 13" E, calculated by Fast (1967) analyzing the whole set of data
from the first part of the catalog (Fast et al. 1967). At about the same
time, Zolotov (1969) performed an independent mathematical analysis of the
same data and obtained the second values quoted in Table 18.1. The
coordinates of Fast's epicen ter with the uncertainties quoted,
corresponding to about 200 m on the ground, were subsequently confirmed in
all Fast's papers. Examining the direction of fallen trees seen on the
aerial photographic survey per formed in 1938, Kulik suggested (1939,
1940) the presence of 2­4 secondary centers of wave propagation. This
hypothesis was not confirmed, although neither was it defi nitely ruled
out, by Fast's analyses and by seismic data investigation (Pasechnik 1971,
1976, 1986). Some hints of its likelihood were given by Serra et al.
(1994) and Goldine (1998). This hypothesis is compatible with the recent
reanalysis of the direction of fallen trees made on the basis of Fast's
data integrated by those obtained from the 1938 and 1999 aerial
photosurveys (Longo et al. 2005). The high trustworthiness of earlier
eyewit nesses is also in favor of the multicenter hypothesis (Voznesenskij
1925, Archive RAS).

18.4.3 Trajectory Parameters, Height of the Explosion and Energy Emitted

The final TCB trajectory can be defined by its azimuth ( ), here given
from North to East starting from the meridian, the trajectory inclination
(h) over the horizon and the height (H) of the explosion. These parameters
can be estimated from the data on for est devastation, seismic records and
eyewitness' testimonies. The height of the explosion is closely related to
the value of the energy emitted, usually estimated to be equal to about
10­15 MT (Hunt 1960; Ben-Menahem 1975), although some authors consider the
energy value to be higher, up to 30­50 megaton

318 G. Longo

319 Chapter 18 · The Tunguska Event

(Pasechnik 1971, 1976, 1986). In agreement with the first energy range,
which seems to have more solid grounds, the height of the explosion was
found equal to 6­14 km. A height of 10.5 ± 3.5 km was obtained by Fast
(1963) from data on forest devastation. Using more complete data on forest
devastation, Bronshten and Boyarkina (1975)  sub sequently obtained a
height equal to 7.5 ± 2.5 km. From seismic data, Ben-Menahem deduced an
explosion height of 8.5 km. Data on the forest devastation examined, tak
ing into account the wind velocity gradient during the TCB flight
(Korotkov and Kozin 2000), gave an explosion height in the range 6­10 km.
A close inspection of seismograms of Irkutsk station, made by Ben-Menahem
(1975), showed that the ratio between East-West and North-South components
is about 8 :  1, even though the response of the two seismometers is the
same. Since the Irkutsk station is South of the epicenter, Ben-Menahem
(1975) inferred that this was due to the ballistic wave and therefore the
azimuth should be between 90° and 180°, mostly eastward.  How ever, it is
not possible to obtain more stringent constraints on the azimuth from
seismic data. It is not clear how Voznesenskij (1925) determined the
direction of the bolide's flight given in Fig. 18.8. Using only the
eyewitness data collected in 1908, Yavnel' (1988) obtained = 114°­138° and
h = 8°­32°. A critical analysis of the eyewitness reports written in 1908
together with those collected in the nineteen-twenties, made by Krinov
(1949) gave an azimuth = 137° with h = 17°. Analysing the data on
flattened tree directions from the first part of his catalog (Fast et al.
1967), Fast found a trajectory azimuth = 115° ± 2° as the symmetry axis of
the "butterfly" shaped region (Boyarkina et al. 1964; Fast 1967). The
independent mathematical analysis of the same data gave = 114° ± 1°
(Zolotov 1969).  Having made another set of measurements, Fast
subsequently suggested a value of = 99° (Fast et al. 1976). In this second
work, the differences between the mean measured azimuths of fallen trees
and a strictly radial orientation were taken into account. He gave no
error for this new value, but a close examination of Fast's writings
suggests that he considered an error of 2°. Koval' subsequently collected
complementary data on for est devastation and critically re-examined
Fast's work. He obtained a trajectory azi muth = 127° ± 3° and an
inclination angle h = 15° ± 3° (Koval' 2000). From a critical analysis of
all the eyewitness testimonies collected in the catalog of Vasilyev et al.
(1981), Andreev (1990) deduced = 123° ± 4° and an inclination angle h =
17° ± 4°. Zotkin and Chigorin (1991) using the data in the same catalog
obtained:
 = 126° ± 12° and h = 20° ± 12°, whereas from partial data, Zigel' (1983)  
deduced h = 5° ± 14°. A different analysis of the eyewitness data
(Bronshten 2000), gave
 = 122° ± 3° and h = 15°. In the same book a mean value = 103° ± 4° is
given ob tained from forest devastation data. Sekanina (1983, 1998)
studied the TE on the basis of superbolide theories and the analysis of
the data available and eyewitness testimo nies. He suggested an
inclination over the horizon h < 5° and an azimuth
    = 110°. From the data on fallen tree directions in our new unified
catalog (Longo et al. 2005), we obtain a single-body trajectory azimuth =
110° ± 5° and h = 30°. The same data are compatible with the hypothesis
that the cosmic body was composed by at least two bodies, falling
independently but very close one to the other, with a trajectory azimuth
~135° and an inclination of the total combined shock wave axis between 30°
and 50°. The first body, with a greater mass, emitted the maximal energy
at a height of about

320 G. Longo

6­8 km. The second, of minor mass, flew a little higher, on the right side
and behind the first body, following the azimuth ~135° in the direction of
the Lake Cheko.  The last azimuth is in agreement with what found by
Krinov (1949) and Yavnel' (1988)  analyz ing earlier eyewitness
testimonies.

18.5 Tunguska-like Impacts

The Tunguska event is the only phenomenon of this kind that has occurred
in historical time. The consequences of the event can be directly studied
in situ. From such a study we can obtain a great amount of information
useful to better understand and predict the characteristics of future
Tunguska-like impacts, i.e. due to bodies with diameters equal to a few
tens of meters. Many different models have been proposed to describe the
impact with our planet by bodies having these dimensions. I mention here
only some recent models, which imply a greater impact frequency and,
therefore, a greater hazard.

18.5.1 Recent Models and Impact Frequency

The frequency of Tunguska-like impacts is highly dependent on the emitted
energy, the explosion height and the entry angle. Most of the published
models for Tunguska have assumed that the explosion was essentially from a
point source. Recent models consider that such events are more analogous
to explosive line charges, with the bolide's kinetic energy deposited
along the entry column.

Plume-forming Impacts

Boslough and Crawford (1997) explain the TE as due to a "plume-forming"  
atmospheric explosion, i.e. as associated with the ejection and collapse
of a high plume. I report here a brief description of the three
overlapping phases of the plume formation, as summarized by Stokes et al.
(2003):

1. Entry phase. When a bolide penetrates a planet atmosphere, it
encounters gases at high speed that both slow it down and heat it up. A
"bow shock" develops in front of the bolide where atmospheric gases are
compressed and heated. Some of this energy is radiated to the bolide,
causing ablation (i.e., melting and vaporization that remove material of
the bolide's surface) and deformation. The rest of the energy is deposited
along the long column created by the bolide's passage; much of the
bolide's kinetic energy is lost in this manner. In some cases, aerodynamic
stresses may over come the bolide's tensile strengths and cause it to
catastrophically disrupt within seconds of entering the atmosphere.
Airblast shock waves produced by this sequence of events may reflect off
the surface causing great devastation. 2. Fireball phase. The events
taking place during the entry phase produce a hot mix ture of bolide
material and atmospheric gas called a fireball that is ballistically shot
upward by the impact. Since it is incandescent, it radiates energy away in
visible and

321 Chapter 18 · The Tunguska Event

near infrared wavelengths. Buoyant forces cause the fireball to rise
because it is less dense than the surrounding atmosphere. The fireball's
energy expands most easily along the low-density high sound speed entry
column that was created by the bolide' passage. 3. Plume phase. The
expanding fireball (and associated debris) rushes back out the entry
column, ultimately reaching altitudes of many hundreds kilometers above
the top of the atmosphere. After ~10 minutes of cooling and contracting at
these heights, however, the plume splashes back onto the upper atmosphere,
releasing additional energy as it collapses and impacts.

Boslough and Crawford (1997) re-examined the phenomena associated with the
TE in the context of their model. They found that a 3 megaton
plume-forming event could generate the seismic waves that were actually
observed, whereas Ben-Menahem (1975) considering the waves generated by a
point explosion has obtained the generally ac cepted value of about 12.5
megaton. Boslough and Crawford (1997) obtained a qualita tive agreement
between the calculated wind speed at different distances and the tree fall
shown on the map (Boyarkina et al. 1964) used for almost 40 years. This
agreement would be improved using our new unified catalog and the
corresponding map (Longo et al. 2005). The most convincing aspect of the
plume-forming model is that it not only account for forest devastation and
seismic and pressure waves but, for the first time, it gives a simple and
reasonable explanation of the magnetic field disturbance and of the
"bright nights" associated with the Tunguska event. The resulting plume,
100 seconds after the impact, is given in Fig. 18.9. As shown, a mixture
of dust, water and tropo spheric air is ejected above the top of the
atmosphere. It is this material, transported westward rapidly enough, that
caused the bright nights within 12 hours at distances up to 6 000 km.
Shuvalov (1999) developed a similar plume-forming model. Firstly, he
considered a volumetric absorption in the projectile of the radiation
emitted by shock compressed atmospheric gas. Subsequently, Shuvalov and
Artem'eva (2002) improved the model considering a surface absorption of
the radiation. They elaborated a 2D numerical model with radiation and
ablation for the impact of Tunguska-like bodies and obtained re sults
similar to those of Boslough and Crawford (1997) for the plume formation
and the ejection in the upper atmosphere of hot vapor and air. All the
authors of plume-forming simulations consider their calculations as pre
liminary and underline the necessity of developing a totally
self-consistent 3D nu merical model using realistic topography and
including simultaneously radiation and ablation, disruption of the bolide,
formation and evolution of a fireball and of a plume.

Foschini Hypersonic F F Flow

Let me mention two other representations of impacts that consider the
bolide energy deposited along an entry column. Foschini (1999, 2001)
developed a model studying the hypersonic flow around a small asteroid
entering the Earth's atmosphere.  This model is compatible with
fragmentation data from superbolides. Foschini considers a bow shock in
the front of the cosmic body that envelops the body. As the air flows
toward the rear of the body, it is re-attracted to the axis. Therefore,
there is a rotation of the

322 G. Longo

stream in the sense opposite to that of the motion and this creates an
oblique shock wave (wake shock). Since the pressure rise across the bow
shock is huge when com pared to the pressure behind the body, it can be
assumed that there is a vacuum be hind the cosmic body. According to the
model, the condition for fragmentation de pends on two regimes: steady
state, when the Mach number does not change, and unsteady state, when the
Mach number undergoes strong changes (Foschini et al.  2001). In the
latter case, the distortion of shock waves causes the amplification of
turbulent kinetic energy. So, a sudden outburst of pressure that can
overcome the mechanical strength of the body, starting the fragmentation
process is expected. On the other hand, in the first case ­ the steady
state ­ the effect of compressibility suppresses the turbu lence, and then
the viscous heat transfer becomes negligible. The cosmic body is sub
jected to a combined thermal and mechanical stress. The key point in
fragmentation is how the ablation changes the hypersonic flow.  The
existence of asteroids with an extremely low density, such as Mathilde
(~1300 kg m­3), suggests that such a body could have an increased
efficiency in deceleration. A possible process by means internal cavities
could increase the deceleration and airburst effi Fig. 18.9. A plume due
to a total energy deposition of 15 megaton, 100 seconds after the impact
of a stony asteroid (Boslough and Crawford 1977). Material within all but
the outermost shell has been ejected from within the troposphere, and
contains the mass of the impactor, as well as water from the humid lower
atmosphere

323 Chapter 18 · The Tunguska Event

ciency is shown on Fig. 18.10. Following these lines Farinella et al.
(2001)  concluded that an object like asteroid Mathilde could explain the
TE.

Anfinogenov Spindle

Anfinogenov (1966) and Anfinogenov and Budaeva (1998) proposed a
qualitative model of the energy emitted by a "semi-infinite" linear
source. The bolide begins disrupting and vaporizing when it enters the
stratosphere and releases an increasing energy as it moves down. The
energy emission is schematically described by the four cylinders shown in
Fig. 18.11. In region I some 20% of mass and energy is lost, about 80% is
emitted Fig. 18.11. The energy emission in the "Anfinogenov spindle". h ­
height from the Earth sur face; r ­ distance from Fast epicenter Fig.
18.10. In the Foschini model, as the cosmic body enters the Earth
atmosphere, the ablation re moves the surface, discovering the internal
cavities, which act as something similar to a parachute, thereby
increasing the deceleration

324 G. Longo

in regions II and III and less than 1% in region IV. The maximal energy
emission is reached at a height of 6­8 km. The resulting shock wave has
the form of the so-called "Anfinogenov spindle". On the basis of the tree
fall data and earlier eyewitness testimonies we consider that the TCB was
a multiple bolide formed by at least two bodies of similar mass (Longo et
al. 2005). They likely entered the atmosphere very close to each other
following parallel trajectories with azimuths ~135°. The second body flew
slightly higher, behind the first, and was decelerated by the shock wave.
The resulting summary shock wave from the different spindles had an
inclination angle of it symmetry axis ~45°.

18.5.2 Global and Local Damages

No doubt that a KT impact causes global damages, but the local character
of damages from Tunguska-like events is questionable. It depends on the
target. For the majority of the Earth's surface, which is water, there
would be no damage (the lower limit for tsunami generation is about 10
times the Tunguska energy). Also, most of the land surface is still
sparsely populated. The situation is quite different for a Megaton explo
sion in a large city or a populated region. Apart from the direct damages
and casualties, we cannot exclude that some country could interpret that
it had suffered a nuclear attack. Even at the time of the real Tunguska
explosion, its consequences would have been very different, if the cosmic
body would have reached the Earth about four hours later. Instead of
hitting a non-populated forest at about 60° N, it could have impacted the
Russian capital of St. Petersburg at the same latitude. Under these
conditions, the Russian participation to World War I and the Russian
Revolution would not have been possible. The whole history of humanity in
the 20th century would be different.  In short, the consequences, even of
a "modest" impact are highly dependent on the target.

18.6 Concluding Remark

From the models mentioned in Sect. 18.5, it was deduced that the Tunguska
explosive yield has been overestimated by a factor 3­4. This means that
the interval between Tunguska-like events can be about three times less
than usually expected. The expected frequency for such events, from the
present value of about twice in a millennium can approach the century
timescale. Therefore, the Tunguska-like impacts may present a more serious
hazard than previously estimated. The real Tunguska event is the only
phenomenon of this kind that happened during relatively recent time and
that can be studied directly. The analyses of the data and samples
collected during recent in situ expeditions have made it possible to check
some characteristics of the Tunguska event. Many of its aspects are still
unclear. Therefore, it is important to further both theoreti cal and
experimental research on this phenomenon. For example, most of the scien
tists consider that the Tunguska event was due to the impact with the
atmosphere of an asteroid or a comet. A clear choice between these two
hypotheses has important practical consequences. The knowledge of the
nature of the object, which explosion

325 Chapter 18 · The Tunguska Event

caused the devastation observed, will make it possible to verify and
develop the models of the explosion mechanisms and fragmentation of cosmic
bodies in the atmosphere. Broadening the study to the known impacts, will
allow obtaining better estimates of the impact probability for cosmic
bodies with different composition and dimensions.

Acknowledgments

Thanks are due to all the participants to the Tunguska91 and Tunguska99
expeditions (see full list in Serra et al. 1994 and Amaroli et al. 2000),
whose work made it possible to write the present review. I have the
pleasure to thank L. Gasperini and M.  Pipan for their contribution in the
analysis of the "kimberlite" hypothesis and M. Di Martino for discussions
on the tree fall. I am grateful to I. Doroshin and his collaborators for
gen erously sharing their copy of the Archives of the Meteorite Committee
of the Russian Academy of Sciences containing the original documents from
the eyewitnesses of the TE. I thank B. Bidiukov and his collaborators for
providing an electronic copy of many Russian publications on the TE. I
gratefully acknowledge the referees Eric W.  Elst, Eugeny M. Kolesnikov
and Wolfgang Kundt who provided useful comments and suggestions for
improving the present paper. One of the referees (WK) disagrees with the
infall interpretation; his objections stimulated integrations to better
elucidate the interpre tation favored by the author.

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