On the possibility of through passage of asteroid bodies across the Earth’s atmosphere

Abstract

We have studied the conditions of through passage of asteroids with diameters 200, 100, and 50 m, consisting of three types of materials-iron, stone, and water ice, across the Earth's atmosphere with a minimum trajectory altitude in the range 10-15 km. The conditions of this passage with a subsequent exit into outer space with the preservation of a substantial fraction of the initial mass have been found. The results obtained support our idea explaining one of the long-standing problems of astronomy-the Tunguska phenomenon, which has not received reasonable and comprehensive interpretations to date. We argue that the Tunguska event was caused by an iron asteroid body, which passed through the Earth's atmosphere and continued to the near-solar orbit.

Full text

MNRAS 493, 1344–1351 (2020) doi:10.1093/mnras/staa329
Advance Access publication 2020 February 4
On the possibility of through passage of asteroid bodies across the Earth’s
atmosphere
Daniil E. Khrennikov,1Andrei K. Titov,2Alexander E. Ershov,1,3Vladimir I. Pariev4‹
and Sergei V. Karpov1,5,6‹
1Siberian Federal University, Svobodny Av. 79/10, Krasnoyarsk 660041, Russia
2Moscow Institute of Physics and Technology, Institusky Per. 9, Dolgoprudny 141700, Russia
3Institute of Computational Modeling SB RAS, Akademgorodok 50/44, Krasnoyarsk 660036, Russia
4P. N. Lebedev Physical Institute, Leninsky Prosp. 53, Moscow 119991, Russia
5L. V. Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Akademgorodok 50/38, Krasnoyarsk 660036, Russia
6Siberian State University of Science and Technology, Krasnoyarsky Rabochy Av. 31, Krasnoyarsk 660014, Russia
Accepted 2020 January 15. Received 2020 January 12; in original form 2019 June 28
ABSTRACT
We have studied the conditions of through passage of asteroids with diameters 200, 100, and
50 m, consisting of three types of materials – iron, stone, and water ice, across the Earth’s
atmosphere with a minimum trajectory altitude in the range 10–15 km. The conditions of this
passage with a subsequent exit into outer space with the preservation of a substantial fraction
of the initial mass have been found. The results obtained support our idea explaining one of the
long-standing problems of astronomy – the Tunguska phenomenon, which has not received
reasonable and comprehensive interpretations to date. We argue that the Tunguska event was
caused by an iron asteroid body, which passed through the Earth’s atmosphere and continued
to the near-solar orbit.
Key words: meteorites, meteors, meteoroids – minor planets, asteroids: general.
1 INTRODUCTION
The problem of the motion in the Earth’s atmosphere of a large
space body (SB), capable of falling on to the surface of the planet in
the form of meteorites, is now of great interest. An equally urgent
concern is the study of the conditions for the passage of such bodies
through the upper atmosphere, even without collision with the
Earth’s surface, since the shock waves produced by this passage have
a colossal destructive effect (Loh 1963; Hawkins1964; Martin 1966;
Bronshten 1983; TomGehrels 1994; Stulov, Mirskiy & Vyslyi 1995;
Nemchinov, Popova & Teterev 1999; Andruschenko, Syzranova &
Shevelev 2013; Morrison & Robertson 2019; Robertson & Mathias
2019).
Large SBs (1–10 km in size and larger) that carry the poten-
tial danger of collision with the Earth are detected by ordinary
astronomical observations. The bodies of intermediate dimensions
began to be registered relatively recently. Observations of such
bodies and the interpretation of observational data make it possible
to determine the probability of their collision with the Earth, their
properties, and the characteristic features of passage through the
atmosphere, as well as the consequences of fall. The clarification of
these questions will enable us to assess more accurately the degree
of asteroid hazard.
E-mail: vpariev@td.lpi.ru (VIP); karpov@iph.krasn.ru (SVK)
One of the fundamental problems of meteor physics is the
determination of the pre-atmospheric mass of SBs, since the
intensity of the meteor phenomenon is determined by the kinetic
energy of the body when entering the atmosphere of the planet.
It is known that the velocity of the bodies belonging to the Solar
system at the entrance to the Earth’s atmosphere should be inside
a relatively narrow range 11.2 <Vsn <72.8 km s−1(Bronshten
1983), so that the variance of the contribution of the velocity-
squared factor to the kinetic energy does not exceed 50 times. At
the same time, the mass of a meteor body can vary in a much wider
range: from fractions of a gram (micrometeor) to tens of millions of
tons or more (the Tunguska space body), that is, by 13–15 orders of
magnitude.
The goal of this paper is to evaluate the effect on the trajectory
of the SB of its passage through dense layers of the atmosphere,
taking into account the acting forces, the initial velocity, and the
mass and its variation during the flight, to determine the conditions
for possible passage of a large SB through the atmosphere with a
minimum loss of mass without collision with the Earth’s surface.
The obtained results are compared with observational data on the
Tunguska space body with an estimated altitude of maximum energy
release of about 10–15 km to receive evidence in favour of a
new explanation of the Tunguska phenomenon, which attributes
the absence of meteoritic material on the Earth’s surface near the
epicentre to the through passage of the SB across the atmosphere
with a small loss of velocity.
C
2020 The Author(s)
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Passage of asteroid bodies 1345
Figure 1. Schematic diagram of the motion of a space body in the Earth’s
atmosphere and the angle of entry into the atmosphere (β) at a given point
relative to the X–Y coordinate system. Reis the radius of the Earth. Thickness
of the atmosphere is exaggerated. The trajectory of SB and its length within
the atmosphere are indicated by the line with the arrow.
2 PHYSICAL MODEL
First of all, let us imagine a model explaining the entry of the SB into
the Earth’s atmosphere with respect to the chosen X–Y coordinate
system coinciding with the centre of the Earth and corotating with
the rotation of the Earth (Fig. 1). The altitude of the entry of the SB
into the atmosphere is measured from the starting value h=160 km,
at which the temperature of the SB begins to increase (Bronshten
1983). This layer of the atmosphere is indicated in Fig. 1by the
value of h(the same parameter denotes the current altitude of SB
over the Earth’s surface). The angle of entry into the atmosphere
relative to the local horizontal line at the altitude his one of the
most important parameters of the problem and is denoted by β.
We denote by Lthe length along the curved trajectory of SB in
the atmosphere, dL=√dX2+dY2,andL=0 corresponds to the
entry point of the SB into the atmosphere at h=160 km.
2.1 The equation of motion with a variable mass
Ballistics of the SB are described by a system of equations, including
the equation of motion under the action of applied forces: the
force of the aerodynamic drag Ffand the gravitational force Fg=
Mg(Stulov et al. 1995):
MdV
dt=−1
2cdρhV2SV
V+Mg,
g=−GMe
r3r,dr
dt=V.(1)
Here Mis the mass, Vis the velocity of the body relative to the
Earth, tis time, Gis the gravitational constant, gis the acceleration
of gravity, Meis the mass of the Earth, cdis the drag coefficient,
and ρhis the density of the atmosphere at altitude h(NASA 1976).
Sis the area of the body’s middle cross-section, ris the current
radius vector, and ris the absolute value of the current radius vector
(the distance from the SB to the centre of the Earth). The time
dependence of Mis implied in equations (1)–(3).
We note that the contribution of the lifting force to the ballistic
of the SB is also neglected in equation (1) because we assume
that the shape of the SB is close to spherical. The Coriolis and
centrifugal forces in the rotating reference frame are negligible
for fast-moving SBs compared to the aerodynamical forces from
stratospheric winds, which we also neglect here because SBs move
much faster than the wind speed.
In accordance with the existing ideas, e.g. Stulov et al. (1995),
the main contribution to the force of aerodynamic drag is made by
the difference in pressure between the frontal and rear parts of the
SB surface (low-pressure cavity forms near the rear surface). The
expression for the force of aerodynamic drag corresponds to the
first term in equation (1). The area Sdepends on the current mass
and size of SB (equation 2). In equation (2), the initial values of the
cross-sectional area and the mass of the SB are indicated as S0and
M0:
S(t,M)=S0M(t)
M0μ
.(2)
We apply the simplified approach of isotropic loss of material
from the SB surface, which corresponds to the case μ=2/3.
Determining the exponent μis the subject of a separate study
(Bronshten 1983) taking into account the complexity of the problem
in a general case. The dependence S(t,M) is used in the numerical
solution of the differential equation (1) at each time iteration step
together with the change in the SB mass (see equation 3).
The value of the SB drag coefficient (Kutateladze 1990), in a
general case, depends on the Reynolds number Re =VR
ν,whereR
is the radius of SB and νis the kinematic viscosity of the air. At a
velocity in the range 10 <V<40 km s−1, the radius of SB is of
the order of several dozens of metres, kinematic viscosity of air at
a trajectory altitude is of the order of 5 ×10−5m2s-1 and greater.
Therefore, the Reynolds number exceeds 1010 (Bronshten 1983).
According to Kutateladze (1990), Spearman & Braswell (1993),
and Zhdan et al. (2007), if the Reynolds number exceeds the value
1.5 ×105,thenasharpdropofcdto 0.1 with a subsequent raise
on average of up to 0.9–1 takes place. In our case, the value of
cd=0.9 corresponds to the most extreme conditions for the motion
of spherical SB in the atmosphere.
2.2 The loss of the space body mass and the heat transfer with
the shock wave boundary layer
The following equation describes the loss of SB mass when moving
in the atmosphere:
dM
dt=−chρhV3S
2H.(3)
Here His the specific heat of sublimation of the SB material and ch
is the coefficient of radiation heat transfer, defined as the fraction of
the kinetic energy of the oncoming stream of molecules that goes
into the sublimation of the SB material (Bronshten 1983).
Mass-loss of SB occurs due to heating to a temperature much
higher than the melting point (Stulov et al. 1995). In our case,
the main contributor to this heating is the radiant heat transfer
between the SB and the boundary layer of the shock wave, whose
temperature reaches several thousand degrees close to the surface
of the SB (Bronshten 1983). One of the most difficult problems
in calculating the radiant heat transfer is the determination of the
radiant heat transfer coefficient (ch). Its magnitude is affected by
the velocity of motion in the atmosphere, flight altitude, air density,
temperature of the boundary layer and the nature of the processes in
the boundary layer (dissociation and ionization of air molecules),
the degree of blackness of the radiating and absorbing surfaces, etc.
According to the available data, depending on the altitude and the
SB velocity, the values of the radiant heat transfer coefficient lie in
the range 0.01 ≤ch≤0.1 (Svetsov, Nemtchinov & Teterev 1995;
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1346 D. E. Khrennikov et al.
Andruschenko et al. 2013; Johnson, Stern & Wheeller 2018) (the
maximum value is reached at an altitude of about 10 km). Taking
into account that the mass-loss rate reaches a maximum at ch=0.1,
this value will be used in further calculations. According to Svetsov
et al. (1995), this value gives the best fit with observational data.
The degree of blackness for the radiating and absorbing surfaces
was taken to be equal to 1 in the calculations because of the
high temperature of the surfaces and the formation of a dense
high-temperature plasma (Bronshten 1983). The solution of the
system of differential equations (1) and (3) together with algebraic
equation (2) was carried out by the explicit Runge–Kutta method
of the fourth order.
Note that the employed model does not involve the process of SB
fragmentation (Stulov & Titova 2001; Barri 2010; Dudorov et al.
2015), since the initial dimensions of the SB are taken to be quite
significant (from 50 to 200 m) as well as moderate velocities, when
most of the SB remains intact, despite extreme external influences.
First of all, maximum resistance to fragmentation is characteristic
of iron SBs, which is associated with the high homogeneity of
their internal structure. In contrast to the iron SBs, the internal
structure of stone and ice SBs is heterogeneous with an abundance
of numerous microcracks. The results of the study of the conditions
for the fragmentation of iron SBs will be presented in our next
paper.
We denote the mass-loss by the term ‘ablation’, which includes
two processes: The first process is the low-temperature blowing off
a liquid film from the SB surface (at a temperature about 1000◦C)
with the formation of small droplets. These droplets are typical
for a slow fall of small SBs or their fragments at the final stage
of the flight in the atmosphere. The second process is the high-
temperature sublimation of material occurring when the surface
temperature exceeds several thousand degrees. In this case, a mass-
loss occurs in the form of vapours of single atoms and their ions.
Under the conditions in consideration, the employed model includes
the sublimation as a dominant process responsible for the mass-loss
at high velocities – over 11.2 km s−1.
3 RESULTS AND DISCUSSION
As a typical example of our calculations, Fig. 2illustrates the results
of calculating the trajectory of the spherical iron SB with a radius
R=50 m entering into the atmosphere at 20 km s-1 when passing
through it at the entry angle β=11.
◦2 and a minimum altitude of
11 km. As can be seen from this figure, the perturbation of the
trajectory of the SB deviates it from the initial direction by an angle
α1=11.
◦25 when neglecting the aerodynamic drag effect and α1=
16.
◦9 when the aerodynamic drag effect is taken into account. These
results demonstrate the significant effect of aerodynamic drag on
the SB trajectory.
Fig. 3shows the dependence of the minimum trajectory altitude
for iron SBs with R=100 m on the angle of entry into the atmo-
sphere for three velocities. Calculations did not reveal significant
differences for the SBs with radii R=100, 50, and 25 m.
The complex pattern of aerodynamic fluxes around a spherical
SB, vortex formation, and stagnation zone in its rear part are
shown in Fig. 4. This figure demonstrates the extreme conditions
for the passage of a spherical SB through the dense layers of
the atmosphere as well as clearly shows the conditions for the
occurrence of the aerodynamic drag taking into account the structure
of fluxes in the stagnation zone. The results illustrated in Fig. 4
were obtained with the software package ANSYS FLUENT (Hutton
2017). This package is a universal software system of the finite-
Figure 2. Changes in the trajectory of SB during a through passage via the
atmosphere. The SB parameters are radius R=50 m, the velocity of entry
into the atmosphere is 20 km s−1, and the minimum altitude is 11 km. The
angle of deflection α1=11.
◦25 at cd=0andα2=16.
◦9atcd=0.9. The
trajectory lengths correspond to the time moment 1000 s after the entrance
into the atmosphere.
Figure 3. Dependence of the minimum trajectory altitude on the entry angle
for the R=100 m iron SB at different velocities.
volume method applied for solving various problems in aero- and
hydrodynamics (Hutton 2017). To calculate the pressure distribution
at the surface of an SB with the effect of air compression due to
pressure–density dependence, the FLUENT ‘ideal gas’ model was
used with the laminar flow regime. Zero static pressure was set to
absolute pressure 26 500 Pa at an altitude of 10 km (NASA 1976).
The default air temperature was set to 219 K.
Fig. 5shows the trajectories of an SB at different angles of entry
into the atmosphere corresponding to the passage of SBs through the
atmosphere at different minimum altitudes over the Earth’s surface.
In this section, we present the results of calculations for bodies of
several sizes, consisting of iron, stone, and ice. Calculations were
carried out for the values of the radii of the SBs with R=25,
50, and 100 m and for three materials: For iron, the specific heat of
sublimation of iron is H=6380 kJ kg-1 (Luchinsky 1985), for stone,
H=3965 kJ/kg for a specimen of lunar rock (Ahrens & O’Keefe
1971)andH=9300 kJ kg-1 for quartz (Chirikhin 2011), and for
water ice, H=2853 kJ kg-1 (Voitkovskiy 1999). In calculations with
astoneSB,weusedalargervalueofHfor quartz, since SiO2is
the basis of many natural minerals. The use of a smaller value of H
(3965 kJ kg-1 for lunar rock) results in an increase in the mass-loss
of stone SBs.
Figs 6and 7illustrate the mass-loss of an iron SB and its rate
when it is moving through the atmosphere at different velocities
and at a minimum trajectory altitude of 11 km. This altitude lies
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Passage of asteroid bodies 1347
Figure 4. (a): Distribution of the velocity modulus around a spherical SB
with radius R=100 m; and (b): distribution of the velocity vectors.
Figure 5. The trajectories of the passage of SB through the atmosphere at
entry angles (β) in the range 5◦–12◦. Material is iron, radius of SB is R=
100 m, and initial velocity is 25 km s−1.
in the range of generally accepted values of the minimum altitude
for the Tunguska space body: 10–15 km (e.g. Bronshten 1983).
These dependencies are affected both by the velocity of SB, which
increases the loss of material, and by the time of flight through the
atmosphere, which reduces this loss.
Besides less loss of mass at lower velocity (15 km s−1), we can
see the effect of lengthening of trajectory in the atmosphere from
3000 to 5000 km, which takes place for smaller size SBs as well
(Fig. 6f).
The main results of our calculation are presented in Table1, which
can be considered as upper estimates of the residual masses of SBs
for different sizes and materials after through passage of the Earth’s
atmosphere for a trajectory length of 3000 km. As can be seen from
Tab l e 1, the maximum fraction of the preserved mass is observed in
the iron SBs with a radius of 100 m at lower initial velocities. When
the velocities grow, the residual mass falls considerably. Similar
tendencies are observed for the stone SBs (without regard to their
fragmentation) with a significantly greater relative mass-loss. For
the case of ice SBs, at initial velocities higher than 15 km s−1,a
Figure 6. Change in the residual mass of the iron SBs [M(L)] along the
trajectory through the atmosphere at two values of initial velocities and for
three SB radii. Lis the length along the trajectory measured from the entry
point into the atmosphere at altitude h=160 km. The curves end when
the SB reaches the exit point from the atmosphere at the same altitude h=
160 km. The minimum altitude hmin =11 km and cd=0.9.
complete loss of mass may take place. A faster mass-loss of stone
SBs compared to iron SBs of the same size is associated with a
lower mass of stone SBs due to much lower material density. That
is, if the amount of absorbed energy from the boundary layer of
shock wave is the same for equal sizes, then the fraction of the lost
mass (relative to the initial one) will be greater for the stone SB
compared to the iron one. Complete loss of mass of the ice SBs at
any initial velocities and sizes is explained by their low mass due to
low ice density. Another reason is a low specific heat of sublimation
of ice. Larger minimum altitudes of trajectory in calculations (hmin)
indicated in the table caption for stone and ice SBs were used to
prevent these SBs from falling.
Fig. 8illustrates the variation of the velocity of SBs along the
trajectory with radii R=100, 50, and 25 m. The calculations were
carried out with drag coefficient cd=0.9 for a spherical body. The
obtained results show that the smaller the SB size, the higher the
deceleration. There is a more notable decrease in SB velocity with
a radius of 25 m in comparison with the radii of 100 and 50 m.
Fig. 8contains a very important result for further consideration,
reflecting our conclusion that this dependence lies in the basis of
the mechanism of the SB tail formation when the fragmentation of
the SB occurs – the smaller the fragment and the lower its kinetic
energy, the greater its deceleration and the lower its final velocity
compared to larger fragments.
Fig. 9demonstrates the changes in trajectories of the iron SB with
R=25 m passing through the atmosphere with initial velocities 20
and15kms
−1and the effect of lengthening of the trajectory from
3000 to 5000 km at slower velocity and a minimum trajectory
altitude of 11 km. Both SBs exit the atmosphere but at different
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1348 D. E. Khrennikov et al.
Figure 7. Change in the rate of the mass-loss of the iron SBs [dM(L)/dt]
along the trajectory through the atmosphere at two different initial velocities
and for three SB radii. The curves end when the SB exits from the atmosphere
at altitude h=160 km. The minimum altitude hmin =11 km and cd=0.9.
moments of time. The effect of lengthening of the SB trajectory is
also shown in Fig. 6.
4 APPLICATION OF THE THROUGH PASSAGE
MODEL TO THE TUNGUSKA PHENOMENON
At present, there are over 100 hypotheses about the nature of
the Tunguska phenomenon, among which three to four versions
are predominant theories (e.g. Fesenkov 1962; Bronshten 2000a;
Farinella et al. 2001; Kundt 2003; Gladysheva 2008). They include
the fall on to the Earth of a small asteroid measuring several dozen
metres (Kundt 2003), consisting of typical asteroid materials, either
metal or stone, as well as ice, which is characteristic of cometary
nuclei (Fesenkov 1962; Bronshten 2000b; Morrison & Robertson
2019). The most probable material of the Tunguska SB mentioned
in literature is ice. According to the available observational data,
there are several variants of the direction and the trajectory length
of the Tunguska SB – from 450 to 600 km, in particular, with a
propagation direction from ‘south–north’ to ‘east–west’. The value
Figure 8. Changes in the velocity of the iron SBs along the trajectory when
passing through the atmosphere for the radii R=100, 50, and 25 m at a
minimum altitude of 11 km (cd=0.9). The curves end when the SB exits
from the atmosphere at an altitude h=160 km.
Figure 9. Difference in the iron SB trajectories with radius R=25 m when
passing through the atmosphere with two different initial velocities and a
minimum altitude of 11 km (cd=0.9).
of the angle of entry into the atmosphere mentioned in literature is
30◦–40◦. The radius of the Tunguska SB was estimated based on the
amplitude of the shock wave recorded by the seismic stations and
amounted to about 25 m. The minimum trajectory altitude of the
Tunguska SB approximately corresponded to the point of maximum
energy release.
In Fig. 10, we show the results of comparative calculations of the
velocity variations of the iron, stone, and ice SBs with radii 100 and
50 m along the trajectory of through passage across the atmosphere
for an initial velocity of 20 km s−1. Stone SBs lose their velocity
faster than iron SBs and ice SBs do not survive passage through the
atmosphere.
In Fig. 11, we demonstrate an unusual trajectory of a stone SB
with R=50 m compared to an iron one with the same size. At point
1, the SB penetrates the atmosphere at altitude 160 km, at point 2,
it reaches the minimum altitude of 11 km, at point 3, it exits the
atmosphere at altitude 160 km with a subsequent re-entry due to a
significant decrease in velocity, and at point 4, it is near the point of
fall. There is a considerable lengthening of trajectory of the stone
SB compared to the iron body, which passes through the atmosphere
Tab l e 1 . The ratios of the preserved mass of SB Mout to the initial mass Min for different materials
at different initial velocities Vand initial sizes R. Calculations are performed for minimum altitude
hmin =11 km, except for the stone SB with R=25 m (hmin =18 km), and for the ice SBs with radii
R=100, 50, and 25 m with corresponding hmin =18, 23, and 28 km.
Material Iron Stone Ice
R(m) 100 50 25 100 50 25 100 50 25
Mout/Min (V=15 km s-1 ) 0.69 0.48 0.25 0.49 0.32 0.3 0.017 0.014 0.01
Mout/Min (V=20 km s-1 ) 0.52 0.27 0.08 0.298 0.11 0.1 0 0 0
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Passage of asteroid bodies 1349
Figure 10. Comparative variation of the velocity of the iron, stone, and
ice SBs along the trajectory with R=100 m (a) and R=50 m (b). The
initial velocity 20 km s−1and the minimum trajectory altitude is 11 km.
The curves for iron and stone SBs in panel (a) end when the SB exits the
atmosphere at an altitude h=160 km. The curves for ice SBs end when the
mass of the SBs vanishes completely. The long curve for the stone SB in
panel (b) corresponds to an unusually long trajectory of the stone SB shown
in Fig. 11.
Figure 11. The trajectory of a stone SB with radius R=50 m. Initial
velocity is V=20 km s−1and minimum altitude is 11 km. At point 1:
V=20 km s-1, relative mass M/Min=1, and h=160 km; at point 2: V=
14.2 km s-1,M/Min =0.3, and h=11.26 km; at point 3: V=7.6 km s-1,
M/Min =0.13, and h=159.7 km; at point 4: V=5.9 km s-1,M/Min =0.11,
and h=11 km (with a subsequent fall). The inset shows the comparison of
trajectories of the stone and iron SBs with an equal radius R=50 m.
with a minimum loss of velocity and minimum deflection due to a
high initial mass (its trajectory is shown in the inset). Although quite
improbable, such an SB could manifest itself as a pair of explosive
phenomena in the atmosphere separated by thousands of kilometres
in distance and tens of minutes in time.
Fig. 12 shows trajectories of an ice SB with R=100 m at
different entry angles and changes in mass. This figure demonstrates
Figure 12. Variation of the ice SB mass along the trajectory with an
initial velocity 20 km s−1and an entry angle in the range 10◦≤β≤
15◦for different minimum altitudes of the trajectory. The initial SB radius is
100 m.
Figure 13. Variation of the ice SB mass along the trajectory of fall with an
initial velocity 25 and 15 km s−1and entry angle 30◦.
a dramatic loss of mass at angles over 11◦. At angle 10◦, the initial
mass is preserved due to the high altitude – over 50 km (Fig. 3).
Fig. 13 shows the reduction of the masses of ice SBs with
R=100, 50, and 25 m on the trajectory of collision with the
surface of the Earth. The residual fractions of the mass at an initial
velocity of 15 km s −1are 49 per cent, 21.3 per cent, and 4.8 per cent,
respectively, for radii R=100, 50, and 25 m of the SBs. The length
of the trajectory until the moment of the collision with the surface
of the Earth is about 325 km for the initial velocity of 15 km s -1 .
At an entry velocity of 25 km s−1for radii R=100and50m,SBs
fall with a preservation of 6 per cent and 0.000 04 per cent of the
initial mass respectively. For radius R=25 m and entry velocity
of 25 km s −1, an ice SB loses all its mass completely within a
trajectory length of about 329 km.
Of course, the fall of SB with preservation of a significant part of
the initial mass results in the formation of a crater with a diameter
largerthan1km(Stulovetal.1995). But the fact is that there
are no craters near the epicentre and around. The actual length
of the trajectory based on the results of visual observations was
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1350 D. E. Khrennikov et al.
estimated to be about 450–700 km, which is over 1.5 times longer
than the calculated value for the ice SB. Therefore, the hypothesis
of the ice origin of the Tunguska SB, which enters the atmosphere
at an angle β=30◦–40◦, is hardly justified from this point of
view.
Moreover, Fig. 12 shows the decrease in the mass of the ice SB
with an initial radius of 100 m along the trajectory at small angles
of entry into the atmosphere. As can be seen from the figure, the
passage of the ice SB through the atmosphere while preserving a
significant fraction of mass is possible only at a minimum altitude
above 40 km, which contradicts with the estimated minimum
altitude of about 10–15 km in the Tunguska event.
Our calculations showed that the trajectory length of the ice SB
when it passes through the atmosphere at a minimum altitude of
15.5 km and small entry angles (less than 15◦) until the moment of its
complete loss of mass even at a radius of 100 m is two times shorter
compared to the case of the iron SB. Thus, the through passage of
the ice SB at small entry angles with a minimum trajectory altitude
10–15 km is impossible.
For the ice SB with a radius of 25 m, the length of the trajectory to
the moment of the total loss of mass is reduced by four to five times.
In addition, it was shown that a considerable part of the initial mass
is preserved by iron and stone SBs with radii of 100, 50, and 25 m
at an entry angle of 30◦. But their fall would be accompanied by the
formation of craters with a diameter larger than 1 km and a depth
over 200 m.
As the final comments, which can be considered as the plan
for future research, we can mention the following problems. In
our work, we did not deal with the problem of the formation of a
shock wave, although when comparing the Tunguska phenomenon
with the Chelyabinsk meteorite with a size of about 10 m and an
altitude of maximum energy release of about 30 km, we have no
reason to doubt that the body that is 10–20 times larger with an
altitude of maximum energy release of 10–15 km at a velocity
of 20 km s−1will create a shock wave with a huge amplitude
and destructive force, capable of causing tree-fall over an area
exceeding 1600 km2. Experimental modelling of the knock-down
effect of a shock wave from the source with cylindrical geometry
was performed by Zotkin & Tsikulin (1966). The cylindrical source
of the shock wave was modelled by a long detonating cord inclined
at a certain angle to a plane planted with small sticks, which
imitated trees in the Siberian forest. It was shown that the shape
of the area of fallen sticks was similar to the shape of real tree-
fall territory. However, Zotkin & Tsikulin (1966) did not model
the dependence of the strength of the cylindrical shock wave on
the height of its source above the ground. Instead, they added a
point explosive at the lower end of their cord to model a presumed
spherical component of the shock wave. Because rates of the
mass and energy losses of the SB that caused the Tunguska event
depend strongly on its altitude above the ground (as evident from
Fig. 7), a sharp increase in energy release close to the minimum
altitude reached by the through passing SB can be interpreted as
an explosion creating a spherical component of the shock wave.
Clearly, making a detailed prediction for the patterns of tree-fall
in the framework of our hypothesis of a through-passed SB as a
cause for the Tunguska event will be an important subject of future
research.
In solving the main problems in this work, we confined ourselves
to the need to make an upper estimate for calculating the residual
mass of space body using the parameters maximizing the mass-
loss. Finally, in this paper, we did not consider the problem of the
mass-loss of the space body due to its fragmentation. This will
be the subject of future research and the results will be published
elsewhere (Khrennikov et al. 2020).
5 SUMMARY
Based on the obtained results, we can make the following state-
ments:
(i) The conditions for the possible through passage of a large
space body composed of various materials across the Earth’s
atmosphere with a minimal loss of mass and without collision with
the surface of the planet are established. It was shown that this
corresponds to the entry angles of space body into the atmosphere
β≤11.
◦5.
(ii) It was shown that the Tunguska space body could hardly
consist of ice, since the length of the trajectory of such a body in the
atmosphere before the complete loss of its mass would be less than
the length of its trajectory estimated on the basis of observational
data. This statement is valid for estimates performed for the value
of the radiation heat transfer coefficient ch=0.1 as well as making
allowance for uncertainties and variations of the values mentioned
in the literature.
(iii) The value of the angle of entry into the atmosphere of 30◦–
40◦mentioned in the literature for the Tunguska space body looks
unrealistic, since it corresponds to the trajectory of a fall of a
body with a large residual mass and trajectory length, which is
1.5–2 times shorter than the estimated trajectory length based on
the observational data. Such a fall would be accompanied by the
formation of a large crater, absent near the epicentre and around.
(iv) Probably, the most realistic version explaining the Tunguska
phenomenon is the through passage of the iron asteroid body as the
most resistible to fragmentation across the Earth’s atmosphere at a
minimum altitude of 10–15 km with the length of the trajectory in
the atmosphere of about 3000 km and a subsequent exit of this
asteroid body into the outer space to the near-solar orbit. This
version is supported by the fact that there are no remnants of this
body and craters on the surface of the Earth. Within this version,
we can explain optical effects associated with a strong dustiness of
high layers of the atmosphere over Europe, which caused a bright
glow of the night sky.
If we admit the version of the complete loss of mass of SB after
the passage of the epicentre or close to it, then the evidence of
its reality would be the presence of droplets of meteoric iron of
millimetre sizes on the Earth’s surface along the trajectory of SB
and around. It follows from Fig. 8that the smaller the SB size
and its mass are, the faster it loses a velocity (the amplitude of the
shock wave near the epicentre also becomes smaller). Finally, when
the velocity of a diminishing SB reduces to such an extent that
its surface temperature approaches 1000◦C, the sublimation ceases
and the dominant mechanism of mass-loss consists of blowing off
a liquid film from the surface of the body. In this case, the SB
becomes the source of a huge amount of droplets, which will
be sprayed by the SB. However, such microformations have not
been found despite intensive searches around the epicentre and
far beyond. The absence of iron droplets around the epicentre is
explained by the high velocity of the SB during through passage
across the Earth’ s atmosphere – always over 11.2 km s−1when the
surface temperature exceeds several thousands of degree Celcius.
The dominant mechanism of mass-loss at these temperatures is the
sublimation of material in the form of single atoms, which can be
found on the Earth’s surface as iron oxides, which do not differ from
the same widespread iron oxides of terrestrial origin.
MNRAS 493, 1344–1351 (2020)
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Passage of asteroid bodies 1351
ACKNOWLEDGEMENTS
Authors thank A. B. Klyuchantsev for aerodynamic calculations
with the software package ANSYS FLUENT. The manuscript bene-
fited from many suggestions and comments made in the constructive
report by the reviewer, Dr Darrel Robertson, whom we thank for
careful reading of the manuscript. We are grateful to Doug Black
of Hamilton, Canada, for correcting the English in the final version
of the manuscript.
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