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Abstract. Explosive evolution of nuclei of sungrazing comets near the solar surface, which occurs at conditions of intense interaction between the solar atmosphere and falling high-velocity comet nuclei as well as the relation of the phenomenon to the character of solar activity are analytically considered. It is found that, due to aerodynamic fragmentation of the falling body in the solar chromosphere and transversal expansion of the fragmented mass under the action of pressure gradient on the frontal surface, thermalization of the kinetic energy of the body occurs by sharp stopping of the disklike hypervelocity fragmented mass near the solar surface within a relatively very thin subphotospheric layer and has, therefore, an essentially impulsive and strongly explosive character. The specific energy release in the explosion region, erg/g, considerably exceeds the evaporation/sublimation heat of the body so that the process is accompanied by production of a high-temperature plasma. The energetics of such an explosive process corresponds to that of very large solar flares for falling bodies having masses equal to the mass of the nucleus of Comet Halley. Spectral observations of sungrazing comets by SOHO-like telescopes in a wide spectral range, including X rays, with a high time resolution, of the order of 0.1-10 s, are important for revealing solar activity in the form of an impact-generated photospheric flare.

Abstract. The cometary nature of the 1908 Tunguska cosmic body is compatible with the predictions of an analytical theory of the 1908 Tunguska explosion developed in [1976][1977][1978][1979]. The theory takes into account the three simultaneously occurring processes, namely aerodynamic destruction of the cosmic body in the Earth's atmosphere, transversal expansion of the crushed mass under the action of pressure gradient on the frontal surface of the body, and an aerodynamic deceleration of crushed expanding mass. The use, for the mechanical parameters of the Tunguska cosmic body, of the characteristics of a cometary nuclei such as that of comet Halley 1986 III and comet Shoemaker -Levy 9 1994, gives parameters of the Tunguska explosion derived from observations of Tunguska event in the Siberian taiga in 1908.

High-resolution spectral observations of young stars with dense protoplanetary discs like Beta Pictoris led to the discovery of variable emission lines of metal atoms, Na, Fe etc., that indicate the presence of fluxes of comet-like evaporating bodies falling onto the stars, FEBs. Assuming the presence of stellar atmospheres similar to the solar one, we show that passages of the FEBs through the stellar chromosphere and photosphere with velocities around 600 km/s will be accompanied by aerodynamic crushing of the nuclei, transverse expansion of the crushed matter, “explosion” of the flattened nuclei in a relatively very thin sub-photosphere layer due to sharp deceleration, and impulse production of a hot plasma. The impulsive rise of the layer's temperature and density lead to the generation of a strong “blast” shock wave and shock wave-induced ejection/eruption of hot plasma into space above the chromosphere. Observations of such impact-induced high-temperature phenomena are of interest for the physics/prognosis of stellar/solar flares as well as physics of comets.