Recalculation of the Tunguska Cosmic Body Parameters on the Basis of the 1938 and 1999 Aerophotosurveys
Longo G.(1), Di Martino M.(2)
(1) Dipartimento di Fisica dell’Universitа di Bologna and INFN Sezione di Bologna, via Irnerio 46, I-40126 Bologna, Italy, E-mail: longo@bo.infn.it
(2) Osservatorio Astronomico di Torino, I-10025 Pino Torinese, Italy, E-mail: dimartino@to.astro.it
ABSTRACT A multidisciplinary investigation of the Tunguska site has been carried out in July 14-29, 1999 by the Tunguska99 expedition. In this framework, the remote sensing of a 300-km2 area near the explosion epicenter has been performed. The photos have been taken in the scale 1:8000 and 1:14000. In parallel, a line scanner made simultaneously a survey in 6 spectral bands, from optical to thermal infrared. All the pictures of these surveys have now been digitalized and archived. We have digitalized also the aerophotographic survey carried out in 1938. A third database used in our investigations is the forest devastation data collected on-site by Fast and coworkers during 12 different expeditions from 1958 up to 1979. Our aims are to estimate with higher precision the energy released by the explosion and its height, to recalculate the position of the epicentre or of the epicentres, and to obtain a 3-D Digital Terrain Model (DTM) on the central devastated area. 1. THE “TUNGUSKA99“ EXPEDITION
A two-week scientific expedition to the Tunguska explosion site has been carried on July 1999 by the Department of Physics of the University of Bologna, in collaboration with researchers of the Turin Astronomical Observatory and of the CNR Institute of Marine Geology. A camp was built in the taiga, at some hundred kilometres from the nearest roads. Personnel and researchers, mainly from Tomsk (Russia), provided local support. The participants and the equipment were transported from Italy to Krasnoyarsk by a Russian Iljushin IL-20M aircraft (Fig. 1) of the GosNIIAS Institute and from Krasnoyarsk to Tunguska by a Russian MI-26 helicopter. The expedition was fully financed by the Italian part. Tunguska99 was the technologically best-equipped expedition to the Tunguska area of the last 40 years.
The main goal of the expedition was to carry out a systematic exploration of the impact site, in order to assess the nature of the body that caused the devastation, felling more than 80 million trees. In spite of the vast amount of theoretical and experimental work done up to now [1], the nature and composition of the cosmic body and the dynamics of the event have not yet been clarified. Some, but no conclusive, data were acquired by the first Italian 1991 expedition [2-3]. The "Tunguska99" expedition [4-5] was organized in order to give
Fig. 1. The Russian flying laboratory IL-20M rent to perform the aerophotosurvey of the explosion site.
an answer about the origin of the Tunguska event, and a contribution to the international research programs aiming at the study of cosmic impacts with the Earth. One of the main tasks of the "Tunguska99" expedition was to carry out a multispectral (from visible to medium infrared) aerial photosurvey of the explosion site.
(See http://www-th.bo.infn.it/tunguska/).
2. THE AERIAL PHOTOSURVEYS
Many aspects of the Tunguska event are being studied by comparing the aerophotosurvey of the explosion site made in 1938 [6-8], with the new one (Fig. 2) performed during the Tunguska99 expedition. The 1999 aero-photographic survey covered a surface of 300 km2 between the latitudes 60° 50' 00" N and 60° 58' 30" N and between the longitudes 101°45' 00" E and 102° 05' 00" E. The photos have been taken in the scale 1:8000 and 1:14000 with coverage of 60% (long.) and 30% (lat.). In parallel, a line scanner made simultaneously a survey in 6 spectral bands, from optical to thermal infrared: 0.43-0.51 µm, 0.50-0.59 µm, 0.61-0.69 µm (Fig. 3), 0.76-0.90 µm, 1.5-2.5 µm and 8.0-12.5 µm.
Fig. 2. In this area, photographed in 1999, is located the point usually called the "epicentre" of the explosion.
Fig. 3. The Cheko lake and the Kimchu river (spectral bands: 0.61-0.69 µm).
The possibility we have to use the 1938 aerophotosurvey is of fundamental importance because it was carried out only 30 years after the event, when the tree trunks overthrown by the shock wave were still easily recognizable on the ground (Fig. 4) and when later falls, not connected with the explosion, have not altered the devastation picture. This allows us to locate the fallen trees and to determine accurately their directions. Krinov, referring to the 1938 survey, stated: "It is a unique scientific document authenticating the only radial forest devastation of its kind in existence and caused by the explosion of a meteorite" [8]. The 1999 aerophotosurvey is now used to re-examine the 1938 aerophotographic material in order to check details of the 1908 explosion and to verify some recent hypothesis on the event.
3. FIRST RESULTS
In collaboration with the Russian "State Research Institute of Aviation Systems" (GosNIIAS) we have digitalized and archived the results of the 1999 aerophotosurvey. With the help of the ”Tomsk Creative Collective”, we have digitalized also the aerophotographic survey carried out in 1938 under the direction of L. Kulik and never completely analyzed.
The 1938 photographic material is releasing important information on the fallen tree distribution and on the location of possible secondary “epicentres” produced by the explosion. A more accurate determination of the forest area destroyed by the explosion will allow us to estimate with higher precision the energy released and could shed light on the impact dynamics. The comparison between the two aerophotosurveys will make it possible to map more accurately the areas with trees surviving the 1908 catastrophe and those with flora variation due to the impact. From this comparison we shall obtain new data on the effects of a cosmic body impact on the forestland coverage. 4. FAST MAP OF FALLEN TREE AZIMUTHS The map of fallen tree azimuths used in the last 20 years is that constructed by V. Fast and co-workers on the basis of the measured data published in two catalogues [9-10]. Fast's map reproduces only a part of the data obtained from 79000 singletree-measured azimuths. Indeed, the two catalogues contain data for 1165 trial areas, while the map uses only the averaged azimuths for 732 trial areas. The remaining data for 433 trial areas have never been used and part of them were rough data without any mathematical elaboration. In collaboration with Tomsk colleagues we have now completed Fast work obtaining a new unified catalogue and a map reproducing the averaged azimuths for 1165 trial areas. Moreover, it is worth noting that there are some doubts on the method Fast used for the compilation of his catalogue, that only partially covers some interesting areas, as the “telegraphic pole forest” and the so-called region of “chaotic fall”. Now, using the 1938 and 1999 aerial surveys, we are checking and completing the data on forest devastation obtained from Fast on-site direct measurements. The analysis of the 1938 images shows that a considerable part of the imaged area was at that time covered by the grown forest, thus masking the trees overthrown by the explosion. This drawback has been partially overcome with the help of the 1999 aerophotographic survey. On the nineties, in fact, extensive fires affected the Tunguska region and in the 1999 aerophotosurvey there are some places that appear cleared thus allowing to see the fallen trees more clearly than in 1938.
New data on forest devastation will allow an independent determination of the distribution and orientation of the fallen trees and of the coordinate of the explosion epicentre(s).
5. FUTURE WORK
Using the material mentioned above, the work we plan to perform in the near future can be summarized as follows:
· To attempt to find the cause of the enigmatic "clear spot" revealed in the epicentral area by the Russian satellite ERTS-I (1973).
· To confirm or to exclude the hypothesis that the explosion was caused by the detonation of a large methane cloud ignited by a meteor.
· To compare the vegetation development in the devastated area and in its neighbourhoods.
· To evaluate the vegetation biophysical variables, to determine the event ecological consequences.
· To elaborate a general model of the effects of impact processes on forest evolution.
· To verify the accuracy of the mathematical models concerning the interaction with atmosphere of cosmic bodies having different composition and dimensions.
6. REFERENCES
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Fig. 4. Photo from Kulik 1938 photosurvey. The parallel fallen trees indicate the direction of the blast wave.
2. Longo, G., R. Serra, S. Cecchini and M. Galli, 1994, Search for microremnants of the Tunguska Cosmic Body, Planetary and Space Science, Vol. 42, n. 2, pp. 163-177, 1994
3. Serra, R., S. Cecchini, M. Galli and G. Longo, 1994, Experimental hints on the fragmentation of the Tunguska Cosmic Body, Planetary and Space Science, Vol. 42, n. 9, pp. 777-783, 1994.
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