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Diane's Sikhote-Alin Meteorite Page


Deutsche Version

by

Diane Neisius

On February 12, 1947, near the siberian pacific coast one of the most interesting meteorite falls of the 20th century occured. It was clear winter weather, when at 10:38 local time an intense fireball, brighter than Sun, crossed the sky and broke apart with roaring thunder just before hitting the ground. Roughly 100 tons of meteoric material were shattered in ten-thousands of fragments over the Sikhote-Alin mountains north of Vladivostok. The biggest fragments crashed into the ground, causing craters of up to 26 meters in diameter.
This event is one of the best-documented meteorite falls, but in the "West" for long time there was little or nothing known about it. Fortunally, this situation has changed the last years.

The Orbit

The Fall

The Fragmentation

The Specimens

References


 

The Orbit

Because the fall occured in bright daylight it was observed by many eyewitnesses from the cities of eastern Sibiria. By the evaluation of this data Fesenkov [1] could compute the orbit of the original body which caused the meteorite shower.
For the pictures in this section I put the slightly fitted parameters from [1] to Chris Laurel's solar system simulator Celestia 1.3.2.

The probable former orbit of the Sikhote-Alin meteoroide is similar to the orbits of many small bodies of the solar system. It is ellipse-shaped and has its point of biggest distance from Sun in the asteroid belt. This makes its origin of the belt reliable - possibly it was born from the collision of asteroids.
During its 1132 day orbit round the sun the meteoroid was for just two days inside the Earth's orbit. The diagram to the left shows the situation one month before the collision; the point of the spring equinoxe is to the left.

On January 6, 1944 the meteoroid reached its perihelion for the last time (at least as an independent celestial body). The distance from Sun here was 147.02 million kilometers. Note the difference of the virtual size of the sun compared to the point of aphelion (below).

The farthest point from Sun was reached by the Sikhote-Alin meteoroide the last time at August 6, 1945. The distance from Sun now was 468.68 million kilometers. At this position, an observer would have seen a panorama of the inner solar system.
For clearness the orbits of the planets have been plotted.Earth is marked blue, Mars is marked red to give a better overview.

While Earth was visible disklike already since the beginning of 1947, at February 1st the earth-and-moon system formed a celestial panorama above the meteoroid. Less than two weeks remained until crash.

This picture of Februar 11, 1947, six hours before impact, nicely shows the meteoroid's approach from northern of the ecliptic. To the right (in the picture at the lower right edge) the ice cap of the Earth and the polar ocean. For in February it's Winter on the northern hemisphere, they are in the shadow. More left is Canada in daylight.

Eastern Sibiria straight ahead: roughly two hours before the crash there were visible a number of details on Earth's surface. Between the clouds there is the sibirian peninsula of Kamtchatka.
The light blue line is earth's atmosphere, which Celestia displays in a correct way.

A few minutes before the fall on February 12, 00:38 universal time (10:38 local time) a visitor on the meteoroide had to make off speedy.
The picture shows in the right half the dark stripe of the wooded Sikhote-Alin mountains. In reality, it was covered by snow in February 1947.
In the ocean there is the island of Sachalin and, closer to the horizon, part of Japan.

Few seconds before impact the meteoroid reaches the outermost border of earth's atmosphere at a heigth of 100 km. By air friction it is immediately heated up until white-hot. For some of the inhabitants of the planet below it now becomes visible as a bright fireball in the sky.


 

The Fall

The fireball which crossed clear sky from north-northeastern direction over the Sikhote-Alin mountains was described by more than 240 eyewitnesses, of which some statements are given in [7]. They agree the bolide was a single body at first. A school girl described it "as though a piece of the Sun has split off." With loud thunder as of cannons it then broke apart, in a number of single explosions and not continiously, as the observers explicitly mark. The parts had "the form of candlelights, they were followed by little lights and sparks" but remained in the form a coherent flying swarm. About the colour of the event there are different statements (reddish, pink, blue or greenish). It is not clear if the statements describe the bright heads of the meteoroides or the tails (compare the video tape of the Peekskill meteorite of 1992!).
A sixteen-year-old boy thought the meteor swarm was an american nuclear weapon - the bombing of Hiroshima was just one and a half year before.
The flying swarm left a broad dark smoke trail in the sky which was visible for hours. From Iman, a city at the Transsibirian Railroad, the russian artist P. I. Medvedev recorded his personal impressions by a painting.

After the fall, the scientific research started immediately. Three days later airplanes from Chabarovsk found the fresh brown craters in the snow-covered Taiga. The first ground expedition reached the area a month later.
From that time, there were expeditions to the fall site nearly every year. A number of 100 impact craters were found at the site; overall, 23 tons of meteoric material were transported to the Science Academy in Moscow, the biggest recovered piece weighs 1.75 tons. The total mass of the fall was estimated to be of 70 to 150 tons.
Analysis showed up a big iron meteorite had fallen in the Sikhote-Alin mountains. The structure is of octaedric type, class IIAB. By [5], the composition of the material is iron (93.3%), nickel (5.3%), cobalt (0.5%), sulphur (0.5%), phosphorous (0.3%) and copper (0.03%). Non-metallic minerals schreibersite, troilite and chromite also were found.
Nevertheless outside Russia there became nearly nothing known about this big meteorite fall. Reports were, with few exceptions ([2,3]), published only in russian (e.g., [1,4,5]). Additionaly, the fall site was close to the fleet base Vladivostok and by this inside a restricted area. The situation changed not until 1991, when the Soviet Union ceased to exist. Up to now uncounted fragments made their way to private collections around the world. A "Sikhote-Alin" is for the amateur collector not only one of the cheapest but also one of the the nicest specimens.


 

The Fragmentation

Following physical model equations, a meteoroid of Sikhote-Alin's size and velocity should loose nearly half of its origin mass by atmospheric friction [6]. Therefore, the body should have been of roughly 200 tons before entering Earth's atmosphere. This correspondends to a sphere of 4 meters in diameter.
The graphics displays the size by an inserted sitting picture of the author. The shape of the body is hypothetic; however, for an octaedrite, a spheric-ellipsoidic to polyedric form is plausible [4].

The following diagrams keep close to the publication of Krinov [4], where the breakup of the meteoroide in the atmosphere is derived from the positions of specimens in the strewnfield. The phases of the breakup are clearly distinct. Slow "crumbling" which surely also occured during flight is not considered.

1st fragmentation

After the parent body had entered the denser layers of the atmosphere at a height of 80 kilometers, it began to glow intense and became visible as fireball. At a height of 10 kilometers, air drag is so high that the material strength of iron cannot stand it longer. The body breaks up, and this is called first fragmentation.
Following it, the smaller fragments are decelerated more than the bigger ones, the swarm is pulled into a narrow debris cloud. Pieces of this first fragmentation are plotted in red in all diagrams.

2nd fragmentation

At roughly 3 to 4 kilometers the next phase of breakup is reached. The air density now is at a value that the bigger fragments, even if they posses a much smaller cross-section than the original body, cannot stand the pressure. The two biggest pieces of the swarm, now flying at the top, also break up in a second fragmentation.
The new fragments, much smaller than their parents, are decelereated more. So the rest of the swarm can catch up them. The pieces of the second fragmentation are plotted in blue in the diagrams.

3rd fragmentation

Closer to the ground, at a height of 1500 meters, air density has increased still more. Again some of the pieces can't stand the pressure, and for the third time there is a breakup.
Pieces of both former fragmentations are concerned by this. First, the biggest still living body from the first fragmentation, now at the top of the swarm, breaks up. Second, three bodies from the second fragmentation are literally crushed by air drag and explode in thousands of little pieces. Both is plotted in green in the diagram.
The flying swarm now is so close to ground that there is no time to "sort right" by air drag all these new pieces to the main swarm. So, they fall down to "wrong places" - at least if compared to a standard meteoric strewnfield.

4th fragmentation

The biggest still existing pieces of the swarm have lost most of their initial velocity, but they make still 500 meters per second when they crash into the ground. By the impact they are crushed to thousands of splinters which are thrown out between the craters. The biggest of the craters has a diameter of 26 meters and is 6 meters deep.
These splinters, also called "scrapnels", are created during the impact, which overall is the fourth breakup of pieces out of the swarm. Therefore it is called fourth fragmentation.
Much of the pieces from earlier fragmentation phases have been decelerated so much by air drag they now fall free down to ground. Thanks to the snow their landing is considerably soft, so additionally to the scrapnels a lot of individuals are found in the fall site.

The diagram shows a schematic map of the Sikhote-Alin strewnfield. North is up, the colours correspond to the above shown fragmentation diagrams.
Red is the main fall ellipse, overlaid are the secondary ellipses of the second and third fragmentation. Fragments from the "green" zones usually are "too small" to have a position so far at the top of the strewnfield.
Black circles mark big craters. Here are found the many scrapnels from the fourth fragmentation; all specimens from the red and blue/green ellipses are individuals.


 

The Specimens

By the clear distinct phases of the fragmentation of Sikhote-Alin, which is a lucky circumstance, in the strewnfield much different specimens are found. Two main classes are easy to detect: the silvery sharp-edged "scrapnels" and the blackish, more rounded "individuals". The latter ones were small enough to be decelerated in a sufficient rate when still in air; so they could land unbroken.
The black crust of the individuals has been created by melting of the surfaces due to air friction. It is a fact that the breakup phases of the Sikhote-Alin meteoroide delivered so much individuals that the general phenomena of ablation of meteoroides in the atmosphere were understand in a detailed way for the first time. There are specimens which have melt pockets (rhegmaglyphs) of different deep belonging to different fragmentation phases.
Rhegmaglyphs obviously form if behind the supersonic boom which a meteoroid pushes before itself air eddies form around the body. At the high velocities and temperatures the eddies literally drill holes into the surface. Specimens from first fragmentation which traveled for a long time alone often possess deep hemisperic holes. Specimens of second fragmentation, which traveled less long for their own posses less deep shell-like holes, while some pieces of the third fragmentation are still round like there were just melted off the sharp edges. And the specimens of the fourth fragmentation, which never had to suffer air friction, are still sharp-edged.
To the left there are some examples of specimens from the Sikhote-Alin meteorite strewnfield. There are two smaller scrapnels from the fourth fragmentation, on both is visible how they have been stretched during the impact.
The other specimens are individuals which still possess their black fusion crust. The bigger one has developed rhegmaglyphs and probably is from second fragmentation. The smaller individual is more round and has only traces of melt pockets; possibly it origins from third fragmentation.
The shown specimens are of masses 24, 21, 8 and 5 grams and belong to the private collection of the author. Not for sale.


 

References

[1] Fesenkov, V. G., in: Meteoritika 9 (1951), p. 27
[2] Krinov, E. L., in: Sky and Telescope, May 1956, p. 300
[3] Krinov, E. L., in: Sky and Telescope, Feb. 1969, p. 87
[4] Krinov, E. L., in: Meteoritika 34 (1975), p. 3
[5] Yavnel, A. A., in: Meteoritika 34 (1975), p. 21
[6] Hills, J. G. und Goda, M. P., in: Astronomical Journal 105 (1993), S. 1114
[7] Gallant, R. A., in: Sky and Telescope, Feb. 1997, p. 50



Created: 03-Aug-2002
Updated: 05-Okt-2004
© 2002, 2004 Diane Neisius