Nature and destruction of the Tunguska cosmical body

Abstract

The problem of the nature of the Tunguska Cosmical Body (TCB) is closely related to the mechanism of its disintegration which ended with an explosive disburse. The only hypothesis capable of explaining such a process is the idea that TCB was a fragment of a comet. Three alternative hypotheses are considered here. They assume the TCB to be: (i) a fragment of a stony asteroid; (ii) a porous snowball; and (iii) a plasmoid. It is shown that the first of them is not plausible because the fragmentation and explosion of an asteroid should result in the scattering of numerous fragments over the ground. They, however, have not been found on the terrain for many years by many tens of explorers who have carried out a careful analysis of the soil and peat. Bodies like a porous snowball or a plasmoid cannot exist in the Solar system (they should be unstable). The history of the cometary hypothesis is followed in the article, starting from its original formulation in a book by the well-known astrophysicist H. Shapley (1930. Flights from Chaos. A Survey of Material Systems from Atoms to Galaxies. N.Y.: McGraw-Hill, p. 57–58. Russian translation: From Atoms to Milky Ways. Moscow: ONTI, 1934), who has priority over F. J. W. Whipple [the latter having put this hypothesis 4 years later ( Whipple, 1934. On phenomena related to the Great Siberian meteor. Quart. Journ. of the Roy. Meteorol. Soc. 60, 505–513)]. Simultaneously with the evolution of this hypothesis our ideas of the structure of cometary nuclei have also varied. Estimates of the main parameters of the TCB — its initial mass m, initial velocity v and energy of explosion E e — are examined. The following quantities are assured to be the most probable: m=2×10 6 t, v=31 km/s, E e=5×10 23 erg. The mentioned value of v corresponds to the Zotkin–Kresak hypothesis that the TCB was a fragment of the Encke comet. Sekanina’s criticism against this hypothesis is examined. During recent years, three analytical theories of the sequential disintegration of large bodies in the atmosphere have been put forward by Grigoryan (1979: Motion and disintegration of meteorites in the planetary atmospheres. Cosmic Res. 17(6), 875–893), Hills and Goda (1993: The fragmentation of small asteroids in the atmosphere. Astron. J. 105(3) 1114–1144) (their theory is physically equivalent to Grigoryan’s theory), and by Chyba et al. (1993: The Tunguska 1908 explosion: atmospheric disruption of a stony asteroid. Nature, 36 (1) 40–44.). The comparison of all the three theories is presented. It is shown that the theory of Chyba et al. overestimates the altitudes of disruption of the meteoroid compared to other theories. The results of the numerical simulations of the process in the free-Lagrangian and the Eulerian approximation (Svettsov V.V., Nemchinov I.V., Teterev A.V., 1995. Disintegration of large meteoroids in Earth’s atmosphere: theoretical models. Icarus 116, 131–153) are also analysed. The results of numerous studies examined either do not contradict the cometary hypothesis of the TCB nature or the discrepancies (e.g. in the case of Chyba et al., 1993) can simply be explained. An interpretation is presented for the anomalous sky glow observed after the TCB fall, west of the treefall epicenter, in Russia and in West Europe. It was first put forward by the author ( Bronshten, 1991: Nature of the anomalous illumination of the sky related to the Tunguska event. Solar System Res. 25(4), 490–504) and considers the secondary scattering of solar light by the dust of the head of the comet that entered the Earth’s atmosphere simultaneously with the TCB.