It is well-known that branches of trees were heated and burnt at the region of the Tunguska blast (Yu.A.L’vov, N.V.Vasilijev 1976). A.E.Zlobin carried out special experimental investigation for determination of thermal properties of tree's rind and blast heat impulse. Rinds of pine and larch were investigated during heating by electrical heater. Obtained thermal properties were used for calculations of temperature distribution in cross section of branches. Heat impulse was determined during analysis with 2-D finite element method. Area of burn was closely inspected too. Clear picture of heat influence was obtained in accordance to value of heat impulse and clustering procedure (look at left Figure). Three considerable blasts here are visible (central "A", eastern "B" and western "C"). Fourth blast "D" is visible on some distance to north-west. Also three "horseshoe-like" burn structures "E" are visible on some distance to south. This picture is interpreted as blasts of four fragments and heat radiation influence by ballistic shock wave's surface (heat caustic influence).

It is necessary to mention the picture of Tunguska space body at flight, which was drawn by eyewitness T.N.Naumenko from the town Kezhma (look at left Figure). There is good correspondence between this picture and thin structure of burn area. Scale and size of comet's fragments is added by A.E.Zlobin. The picture of T.N.Naumenko demonstrate that Tunguska comet nucleus moved across the sky with low velocity. Certainly, only in case of low velocity of Tunguska comet at flight T.N.Naumenko was able to notice and draw form of the body with so many detailes.

Attentive analysis was carried out by A.E.Zlobin on the base of tree fall catalogue processing (W.H.Fast et al. 1976). There is thin structure like "horseshoe" in southern area of tree fall (look at left Figure). This structure was interpreted by A.E.Zlobin as influence of ballistic shock wave cone (gas dynamic caustic influence). The axis azimuth of "horseshoe" is in good correspondence to I.S.Astapovich's latest trajectory, which is directed from south-south-east to north-north-west (I.S.Astapovich 1965). There is good visible, that northern wing of “butterfly” forest fall was produced mainly by four final horizontal-directed blasts, but southern wing was produced mainly by ballistic and ablation shock wave. The influence of ballistic shock wave was increased due to concave form of trajectory.

Quasi 3-D modeling dealt with estimation of unsteady temperature distribution in the Tunguska space body before entering into atmosphere, estimation of mechanical properties of the body in accordance to its temperature, solution of equations of motion (three coordinates), calculations of pressure, heat and mass transfer at the region of stagnation point (for all body and then for all its fragments), strength and stress analysis for all body and then for all its fragments, deformation of all fragments, expansion of hot gas volumes in case of its known geometry, shock wave formation and heat radiation influence on forest and branches of trees, formation and raising of hot cloud, variation of local magnetic field.

It was shown in this study that Tunguska-size comets are able to penetrate considerable deep into dense atmosphere due to decrease of drag effect. A.E.Zlobin explained this decrease by forward-directed jet from cavern, which was located at the region of stagnation point of ice comet's body (or of each ice fragment). This cavern is formed due to most intensive heat and mass transfer processes at the region of stagnation point. Length of Tunguska's trajectory as function of altitude is shown at left Figure.

Expansion of hot gas volumes was calculated with the help of particle-in-cell method (PIC). A.E.Zlobin made special computer program based on PIC algorithm for calculations of gas expansion in horizontal plane. PIC-method is convenient for solution of this task because borders of hot gas volumes in the case of Tunguska blast are considerably curved. Also these calculations are more determined due to well-known geometrical form of each initial gas volume. Pressure, velocity and temperature distributions were estimated during calculations. Calculation process consisted of cells grid generation, initial positioning of particles, initial and boundary conditions set, solution of gasdynamic equations with Eulerien and Lagrangian procedures with time steps. Typical grid of cells was (50 x 50). Quantity of air particles was 4 per cell and quantity of comet's substance particles - 300 per cell. The view of initial distribution of all particles is visible in Figure. Model form of gas volume was stated approximately in accordance to form of blasts "A" and "B". The sample of pressure estimation is shown in Figure too.

Some results and conclusion
- Total energy of Tunguska impact, which determined tree fall in the form of “butterfly”, is in good correspondence to 5.6·10¹⁶ J
- Final total mass of all Tunguska fragments was 10 million tons (at the end of trajectory in atmosphere)
- The Tunguska space body was typical nucleus of comet with the same final size relations between its four fragments A,B,C,D (Table 1)
- During quasi 3-D modeling was noticed good correspondence to typical density of comet substance with average value ~0.6 g/cm³
- The most of internal substance of Tunguska comet is determined as water ice
- Substance of all Tunguska fragments was considerably uniform and all fragments were scattered on equal lateral distance ~7 km
- Approximate coordinates of the most probable regions, where small heavy fragments may be discovered, are determined. These regions are stated as initial points of blasts A,B,C,D (Table 2). Here are relative coordinates of these points, where main houses of L.A.Kulik's expedition’s base are marked too (near the Stoikovich Mountain). It was shown form of regions, where possible to discover thin sediments of Tunguska comet's substance
- Final effective velocity of fragments before blasts is determined as ~2.2 km/s (velocity of entrance into atmosphere ~11.2 km/s)
- The Tunguska trajectory in atmosphere is in good correspondence to Astapovich's latest trajectory (direction from south-south-east to north-north-west, initial trajectory inclination angle ~5°-7°)
- It is strongly confirmed that azimuth angle and inclination angle of fragments trajectories were considerably variable during motion in atmosphere. Near final point of trajectory the angle of trajectory’s inclination and vertical component of velocity were considerably increased
- Expansion of final hot gas volumes took place mainly in horizontal plane on altitude of ~4-6 kilometers
- Upper limit of compression strength of Tunguska internal comet ice is determined as 1.3·10⁶ Pa
- Sample of calculated parameters for the blast of fragment "A" is presented in Table 3
- Results of modeling are in good correspondence to theoretical model by M.N.Tsinbal, V.E.Shnitke (1986, 1988). There is the difference too. There is not necessity on considerable additional chemical energy for blasts. Also the decreasing of drag effect is taken into consideration.
- New aspects of comet danger are discovered (cavern into the comet's ice body, more deep penetration of comet nucleus into atmosphere, “horseshoe-like” burns, increased final lateral distance between fragments etc.)
- More accurate data concerning the Tunguska comet impact will be obtained on the base of full three-dimensional modeling