Download Suxun Li THE ANALYSIS OF THE SURFACE CHARACTERISTICS

THE ANALYSIS OF THE SURFACE CHARACTERISTICS AND THE TRAJECTORY
OF JILIN METEORITES SHOWER
Suxun Li
China Academy of Aerospace Aerodynamics,
Beijing, P.R. China 100074
Email: [email protected]
1. Introduction
The Jilin Meteorites shower hit the earth around three o’clock in the afternoon on March 8,
1976. It is the most massive fall of stony meteorites on record. The study of meteorites is valuable
to many research fields, such as astronomy, astrophysics, high-energy physics, cosmic chemistry
and aerodynamics. A meteorite is a mass of stone or metal that has reached the earth from outer
space. Meteorites enter the earth’s atmosphere at very rapid speeds, generating shockwaves on
its windward side. The high temperature and high pressure environment created as an aftereffect of the shockwaves causes the meteorites to shatter into smaller pieces. The shattered
pieces of meteorite fall to the ground, similar to rain, hence the phenomena is named as meteorite shower. Often, the meteorite is named after the location in which it landed. The Jilin Meteorite landed in the suburb of Jilin City in Jilin Province of China. The meteorites are usually categorized as iron meteorites, stony-iron meteorites and stony meteorites. Jilin Meteorites are stony
meteorites.
The Jilin Meteorites contain olivine, rhombic travertine, bronze pyroxene, enstatite and 43
other kinds of minerals, nine of which are not found on the earth. It also contains a variety of amino
acids, nucleic acids, fatty acids and other substances related to life. Among them, there are 11 different kinds of amino acids.
Previous reports and papers regarding the Jilin Meteorites shower are hard to find because
a long time has passed since its occurrence several decades ago. Since the Jilin Meteorite Museum once caught fire, some of the Jilin Meteorite samples were also lost. The author of the present paper was a member of the scientific research team on the site of Jilin Meteorites in 1976 and
gained first-hand data and knowledge about the Jilin Meteorite shower. The author organized pictures and records of the Jilin Meteorites shower taken in1976 and conducted some new analysis
regarding the Jilin Meteorites. This paper is separated into two sections:
(1). Study of the surface characteristics and distribution of Jilin Meteorite stones.
(2). Based on the field data gathered at the meteorite’s landing site, calculations of the trajectory of the meteorite utilizing different theories.
2. The Distribution and Surface Characteristics of Jilin Meteorite Stones
A rare meteorite shower fell on the suburb of Jilin City in Jilin Province of China around
three o’clock on March 8, 1976. A team of scientists and researchers was quickly dispatched by
Chinese Academy of Sciences and local government to the scene to interview local observers and
to collect samples of the Jilin Meteorite shower. In the end, a total number of 138 meteorite samples were collected, which total weight is 2616 kilograms. The largest stone (weighted 1770 kilogram) recovered was named as Meteorite Stone No. 1. The Jilin Meteorite Stones were discovered in a triangular area about 500 square kilometers. From the falling point of Stone No. 1, the
area spreads eastward for approximately 70 kilometers. At the east most end of the triangular area, it spreads southward for about 7-8 kilometers. The distribution of the Jilin Meteorite Stones is
shown is Figure 1.
 S. Li, 2015
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Fig. 1. The distribution of the Jilin Meteorite Stones
The meteorite stones collected were named according to their weight as Stone No. 2, No. 3 and
so on. The weight of Stone No. 1 to No. 10 and the distance between its landing point to the landing
point of Stone No. 1 are listed in Table 1.
Ta ble 1 . Jilin Meteorite Stone Weight and Distribution
Stone #
Weight W
(kg)
Distance L
(km)
No. 1
No. 2
No. 3
No. 4
No. 5
No. 6
No. 7
No. 8
No. 9
No. 10
1770
250
123
70
30~25
20~15
1~5
<1
0
12.8
16.5
19
20~22
26~28
35~45
50~60
According to local eye witnesses, strange light and sound were observed as the Stone No. 1 hit
the ground. As Stone No. 1 flew from the east direction, its bright red color gradually faded, leaving
a comet-like tail. When the Stone No. 1 hit the ground, a big boom sound was heard. Stone No. 1
produced an impact pit which was about 2 meters in diameter and 6.5 meters deep. Figure 2 shows
the actual impact pit produced by the Meteorite Stone No. 1. Figure 3 is an illustration of the Stone
No. 1 impact pit with all the reference parameters.
Figures 4-8 are the photos of the external surfaces of Stone No.1, No.2, No.3, No. 4 and No.9,
respectively. They share one common characteristic, which is that they are all covered by dark
brown fusion crust.
The size of Stone No. 1 can be determined by the 30 cm long scale shown in Figure 4a. The
thumbprint-like indentations on the surface of Stone No.1 are called regmaglypts. Regmaglypts are
most likely caused by the severe melting and abrasion of the meteorite as it enters the Earth's atmosphere. Along the flow direction, the vertical and horizontal distribution patterns of regmaglypts
changed, indicating that the meteorites tumbled and spun as they plunged through our atmosphere.
Figure 4b shows a deep groove on the surface of Stone No. 1. There was severe melting inside the
groove. The crack of the big meteorite is random, which is determined by its surface morphology.
The picture of Stone No. 2 (Figure 5) indicates the flat surface is facing the incoming flow and
the fully developed regmaglypts on the side surface are the results of high speed surrounding flows.
Judging by the smaller and not fully developed regmaglypts on the surface of Stone No. 3 and
No. 4, (as shown in Figures 6 and 7) one can conclude that the traveling speed of Stone No. 3 and
No. 4 are slower than that of Stone No. 1.
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Fig. 2. The impact pit produced by the Meteorite Stone No. 1
Fig. 3. An illustration of the Stone No. 1 impact pit
No regmaglypts were found on Stone No. 9. Instead, there are flow lines (Figure 8), which are
cooled streaks of once-molten fusion crust. According to the aerodynamic theory, such flow lines
exist in laminar flow condition. The presence of flow lines on Stone No. 9 indicates the Reynolds
number is low when fusion crust was formed on the surface, the surrounding flow was not at turbulent flow state and the ablation was not as severe as those on the bigger meteorites.
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a
b
Fig. 4. Stone No. 1 appearance (a) and Deep groove on the surface of Stone No. 1 (b)
Fig. 5. Stone No. 2
Fig. 6. Stone No. 3
Fig. 7. Stone No. 4
Fig. 8. Stone No. 9
Residents in different districts had different accounts of the meteorite’s explosions. Those east
of the landing points of Stone No. 1 and 2 heard one or two explosions, while those near the landing
points of the smaller meteorites heard smaller repeated explosions, similar to the sound of a running
airplane engine. The duration of the smaller explosions was around two minutes. Accounts for the
cause of these explosions have been provided by analysis from aerodynamicists.
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3. The Meteorite Trajectory Analysis
According to astronomers’ analysis, meteorites originated from the Asteroid Belt. When disturbed by unknown reasons, they deviated from their normal orbit. They caught up with Earth along
Earth’s revolution direction. It is believed that the travel speed of meteorites is greater than 10km/s.
When their traveling height in earth’s atmosphere is lower than 70 km., the surface of the meteorites
began to melt and to lose weight. Considering the impact of the wind at high attitude, meteorites
may begin to crack at a height between 30-15 km.
With a set of wide range conditions, we need to go through several iterations of trials to reach
conclusions based on different assumptions. For example, based on the assumed values of the
height the meteorite cracks Hcrack the meteorite traveling speed when it enters atmosphere V0, the
inital deviation angle from its orbit α0, the meteorites distribution on the ground, and the shape of
the cavity and the earth quake information collected when the meteorites hit the ground, we reached
a set of conclusions. Then, we compared these conclusions to the field data that we collected. The
set of assumed conditions which lead to calculated results that most closely matched the field data
were used in the analysis.
The following field data was used by our analysis:
(1) The ground distribution of Jilin Meteorites reflecting their paths in the air; the meteorites
traveled in the air from southeast to northwest direction at an angle of 4°~6°.
(2) The measurement data of Stone No. 1 cavity was used as the basis of Meteorite Stone No.
1 analysis.
(3) Heavy meteorites (those weights more than 20 kg) fell from the east to the west. Most of
the light meteorites (those weights less than 5 kg) fell from the west to the east.
(4) The earthquake wave and the in the air shock wave recorded by Jilin and Fengman earthquake observatories.
The time that Stone No. 1 hit the ground is denoted as T0=0, earthquake wave (Rayleigh wave)
travel speed V is 3.05 km/s，the earthquake wave arrived the observatory at time T1，in the air, the
shock wave arrived the same observatory at time T2，the distance between Stone No. 1 cavity and
the observatory L are listed in Table 2.
Ta ble 2 . Data recorded by two earthquake observatories
Station
Jilin
Fengman
T0
0
0
T2–T1
80''
58.3''
T1
15.44''
16.2''
T2–T0
95.44''
74.5''
L (km)
47
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3.1. Basic assumptions used by numerical calculations
1. Irregular shape of meteorite is simplified as sphere with equal weight. The sphere diameter

D is calculated from W   D 3 , where the W is the weight of the meteorite and the density of
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meteorite   3.88 10 3 kg/m3
2. Meteorite caught up with earth along earth’ orbit. The traveling speed of meteorite is around
44 kg/s by general statistic value. The speed of the earth flying around the sun is approximately
29-30 km/s. The difference of the two speeds is deemed as the initial speed of meteorite enters atmosphere V0 =15-25 km/s.
W
3. Ballistic coefficient is calculated from  
, where W is the weight of the sphere, S is
C dS
the area of the cross-section of the sphere and the drag coefficient exerted on the meteorite Cd is
taken as a Cd=0.92~1.0.
4. After meteorite entered the atmosphere at entrance angle α0, height H0 and speed V0, its
motion is treated as particle motion without any rotations, as depicted in Figure 9.
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Y=H/km
α0
V∞y
V∞x
αi
1
V∞
25
V∞
V
Vw
Ve
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V0
γe
O
-Z/km
X/km
Fig. 9. An illustration of coordinates and velocity
5. Assume that all meteorites cracked between height Hcrack =1 8 to 20 km.Without considering
the weight change caused by ablation, the ballistic coefficient is used as a constant.
6. The effect of the curvature of the earth surface is ignored.
3.2. Equations of plane motion
Based on the assumptions listed in the previous section, the following equations of motion
were used for the numerical calculations.
Dynamic pressure q 

V 2 , V is meteorite speed
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Density    ( H ) ，values are taken from standard atmospheric table
V

  arcsin  w sin   , where α is angle between the meteorite trajectory and the horizontal
V

plane and γ is the angle between the wind speed (Vw) and the horizontal projection of the meteorite
trajectory
dV
q
  cos   g sin  , g is the acceleration of gravity, β is Ballistic coefficient
dt

d
q
q
  cos  
sin 
dt
V
V
dH
 V Y  V sin  , where H is vertical distance
dt
dL
 V X  V cos  , L is horizontal distance
dt
2
V  V 2  V w2  2VV w cos 
When the wind speed Vw equals to zero, only the impact of gravity (g) is considered in the
above plane motion equations. When the wind speed Vw value varies with height，both the effect of
wind and gravity are considered in the above plane motion equations.
When H ≥ 15000 m, we assume
When 10000 m ≤ H ≤ 15000 m
When 0 ≤ H < 10000 m
Vw = 0
Vw = 0.01(15000 – H)
Vw = 50 – 0.0045(10000 – H)
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The relationship among the coordinates, the wind speed and the meteorite traveling speed is shown
in Figure 9.
3.3. Results Analysis
1. The angle between the meteorite trajectory and the horizontal plane is the major determining factor for the landing location of the meteorites with different weight. Compared with filed data,
α0 value is close to 20°.
2. The wind effect on the heavy meteorite is small. The wind effect on smaller meteorite (<
5 kg) cannot be neglected. Figure 10 shows the meteorite trajectory with and without the presence
of wind field. Smaller meteorite tends to fall from west to east when its height is below 10 km.
The angle between the wind speed (Vw) and the horizontal projection of the meteorite trajectory can
be over 90° and close to 100°. This simulated results agree with the field data.
3. Earth's rotation effect had a slight impact on meteorites’ east-west movements.
4. The air density is low and the air resistance is small when the height is above 50 km. The
meteorite traveling speed variation is less and the meteorite traveling trajectory is almost a straight
line no matter its weight.
The high dynamic pressure occurred at the height lower 25km for bigger meteorite (weight
more than 100 kg). The maximum dynamic pressure (q) value can be more than 4×105 kg/m2. The
high dynamic pressure causes a large amount of ablation, which leads to deep pits on the meteorite
surfaces. The high dynamic pressure occurred at the height above 20 km for smaller meteorite
(weight less than 5 kg). The maximum dynamics pressure (q) value is less than 5×104 kg/m2. The
ablation is less due to the smaller localized dynamic pressure value. The meteorite surfaces has
blackened color without apparent pits. Some edges on the meteorite are still sharp. The calculated
maximum dynamic pressure values variation with the vertical height for different weight meteorites
is shown in Figure 11.
5. During the process of a meteorite entering the earth atmosphere, a shock wave is generated when a meteorite’s traveling speed is higher than the speed of sound. Figure 12 illustrates
the shockwave formation in the leading edge area of a meteorite during its entering process. An
area of high temperature and high pressure is formed between the shockwave and the leading
edge. The shock wave at the leading edge has highest intensity. Its wave front is pushed by meteorite and it is traveling at the same speed as the meteorite. The intensity of the shock wave b ecomes weaker far behind the meteorite, and it travels at the speed of sound along the Mach line.
The sound of explosion is one type of strong pressure wave, just like the shock wave is a type of
pressure wave. The higher the intensity of the shock wave the bigger the pressure difference
across the shock wave. The sound generated by the shock wave is similar to the sound of an explosion.
Fig. 10. The traveling trajectories of the Jilin Meteorite in the air with and without wind
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Fig. 12. Flow structure around a meteorite
Fig. 11. Max. Dynamic Pressure vs. vertical height
(V0=15 km/s，α = –18°, H 碎 =18 km)
4. Closing Remarks
The present study is a complementary study for a previous study on Jilin meteorites shower
performed by the author. In the present paper, the shape and characteristics of typical meteorites are
studied. The falling trajectories of meteorites are also calculated. Due to the lack of some determining conditions, the conclusion of the study is not unique. The analysis results can be used as reference to study some meteorite phenomena which have not been looked into before.
REFERENCES
1. Chinese Academy Joint Investigation group of Jilin Meteorite Shower. An investigation report on the Jilin Meteorite
Shower // Science in China. 1977. Vol. 8, No. 1. P. 38-45. (in Chinese)
2. Jilin Meteorite Museum // Jilin Meteorite Shower (in Chinese), June, 1976.
3. The Report on Number One Stone of Jilin Meteorite Shower. The Dug-out Star (in Chinese).
4. Suxun Li. Visualization of Jilin Meteorite Fusion Crust Structture // Proceedings of 12th Asian Symposium on
Visulization, 2013.
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