V.V. Svetsov

Radiation emitted during the flight of asteroids and comets through the atmosphere

Abstract The radiation emitted during the flight of a cosmic body through the atmosphere is an important factor in the bodys interaction with a planet. A variety of radiation aspects are considered in this paper. Altitudes of interest here range from the dense atmosphere to above 100 km where nonequilibrium and nonstationary ionic kinetic and molecular dynamics play an important role. Structure, intensity and the spectrum of radiative shock waves generated in the atmospheres of the Earth and Mars are investigated on the basis of the detailed numerical simulations. Ignition of fires, creation of a thin thermal layer at the ground, which essentially changes flow pattern after the impact, and contamination of the atmosphere by soot, aerosols and dust may pose a threat to humanity. These problems are also considered. Registered radiation of 1–10 m bodies can supply the necessary data on their properties and refine size-frequency curves. A relatively simple method to obtain the body characteristics in flight is developed and some observational data are analyzed. In the proposed scenario the body is heavily fragmented and intensely radiates due to an increase of its effective area.

Assessment of comet Shoemaker-Levy 9 fragment sizes using light curves measured by Galileo spacecraft instruments

Abstract Two- and three-dimensional numerical radiation-hydrodynamic simulations of the bolide stage of SL-9 fragments intruding into the Jovian atmosphere have been conducted. Radiation fluxes in each wave band of the Galileo mission instruments (SSI, PPR, NIMS and UVS) in the direction of Galileo have been calculated. The simulations are based on detailed tables of spectral opacities calculated assuming thermodynamic equilibrium and the initial composition of 0.89H2+0.11He+0.00195CH4. The small amount of methane and its products strongly change the spectral absorption coefficients in the IR and visible region, compared with a pure hydrogen helium mixture. Simulations begin at the moment when the bolide head is at an altitude of 300 km. The intensity of light rapidly increases until the moment when the bolide head disappears below the clouds, but the wake still shines above the cloud tops. This changes the slope of the light intensity versus time curve and creates a plateau on the light curve. A small difference in the observed maximum intensity for K, W and N fragments registered by SSI and for L, G, H and Q1 fragments registered by PPR instruments (no more than 2.5 times) implies that the sizes of all the above-mentioned fragments vary by no more than a factor of about 1.5. A comparison of the theoretical peak intensity determined neglecting ablation with that observed for all these fragments gives an estimate of the fragment radius in the range 0.5–0.7 km. NIMS data at λ = 4.38 μm for the G fragment give the same value of radius. The radiating volume is located mainly at altitudes of about 30–100 km and slowly changes its size and shape. The brightness temperature is much smaller than the temperature of gas in the bolide head and in the hot core of the wake. This effect of screening due to absorption in cold outer layers of the wake explains a rather low brightness temperature of PPR and NIMS. A short duration of the signal in the ultraviolet is also explained by the simulations. Simulations taking into account radiation-driven ablation show that the dense vapor cloud formed at high altitudes tends to increase the shock wave radius by a factor of two or three. Taking this correction factor into account we can expect the fragment radius to be 0.3–0.4 km for K and 0.15–0.20 km for W. The theoretical profile of the light curve before and some time after the peak intensity is similar to the observed one (for K fragment until 15–20 s). The discrepancy at later time is probably due to the heating and evaporation of the clouds by the shock wave and mechanically induced ablation, which have not yet been taken into account in the physical model.

Comment on “Extraterrestrial impacts and wildfires”

Abstract Characteristic features of a fire ignited during the Tunguska event, 30 June 1908, are described on the basis of literature published in Russia. The 200-km 2 area of initial ignition is in agreement with theoretical models of thermal radiation from the fireball. However, the fire covered a larger area, about 500 km 2 . The development of fire depends on the weather, groundwater level, and forest structure, but these factors were not favorable for rapid fire propagation in the case of the Tunguska event.

COULD THE TUNGUSKA DEBRIS SURVIVE THE TERMINAL FLARE

Abstract The purpose of this paper is to show that lack of residual meteorites is typical for a fall of a stony or carbonaceous bolide tens of meters in size. A Tunguska-sized body penetrates deep into the atmosphere and is broken into a great deaf of fragments the maximum size of which is smaller than 10 cm. Numerical simulations of analogous problems show that the fragments are separated from each other at a stage of dramatic deceleration of the bolide. Computations made here give that 3–10 cm stony fragments fully ablate either inside or outside the fireball due to high radiation flux. Only if the fragments accidentally gain significant lateral velocities at altitudes above 15 km, could their ponderable remnants reach the ground at 5–10 km from the explosion epicenter. Vaporized material of the impactor does not touch the ground and moves upward along the wake.

Luminosity of the bolides created by SL-9 comet fragments in the Jovian atmosphere

Abstract 2D and 3D radiation hydrodynamic simulations of the bolide stage of the SL-9 comet fragments impact onto Jupiter have been fulfilled taking into account radiation driven ablation. Detailed tables of spectral opacities of hot Jovian atmosphere and of ablation products have been calculated and used in the simulations. It has been shown that methane and its products substantially change opacities of hot Jovian atmosphere in comparison to zero-metallicity opacities of pure helium-hidrogen mixture. Composition of the SL-9 comet in the model has been varied from pure water to pure chondritic dust, including volatile/dust ratio estimated for comet Halley. We find that a dense cloud of vapor formed at altitudes of about 150–300 km moves with fragments to an altitude of the 1 bar level and substantionally (by a factor of two-three) increases the size of the emitting volume and intensity of radiation. Radiation fluxes in each waveband of Galileo mission instruments (SSI, PPR, NIMS and UVS) have been calculated in the direction of Galileo for various sizes of the SL-9 fragments. Sizes of some of the fragments have been determined from comparision of theoretical intensities with observations, namely 0.3 – 0.8 km.