Catastrophic Events Conference 3009.pdf THE PROPERTIES OF THE NUCLEUS OF SHORT-PERIOD COMETS DANGEROUS FOR THE EARTH. Kh.I. Ibadinov, Institute of Astrophysics of the Tajik Academy of Sciences, Bukhoro Str. 22, Dushanbe 734042, Tajikistan ([email protected]). A part of short-period comets is a Near Earth comet turned into asteroid-like body 16–18 magnitudes Objects (NEO) and be of great danger for the Earth. At (Ibadinov, 1999). time (once in 50–100 years) the Earth collide with big Observations support the existence of comet- fragment of cometary nucleus and as is generally asteroid (Comastr) objects. First, we have asteroid known this collision is catasatrophic for the Earth. Chiron and comet Harrington-Wilson. Serious Therefore it is very important to know of the orbit and arguments exist to suggest that the objects 1992 AD, the physical properties and chemical copmosition of 5441 Pholus, 1992 QB, and 1993 FW, recorded as nucleus of such Near Earth Comets (NEC) in detail. asteroids, as well as near Earth asteroids of Taurid The physical properties and the chemical complex, 2101 Adonis, 2202 Oljato, 2212 Hephaistos, composition of cometary nucleus were studied by 4183 Cuno, 4179 TA, 4341 Poseidon, 4486 Mithra, different methods in the Institute of Astrophysics of the 5143 Heracles, 5731 Zeus, and 6063 Jason, are extinct Tajik Academy of Sciences. This report include the short-period comets. The Encke's comet is potential new results of our investigations of the nucleus of stort-period asteroid. short-period comets. These results considerable Thus, a part of Near Earth Objects dangerous for supplemented our notion about short-period comets the Earth is a comet-asteroid objects. They are a icy and their relation with asteroids and meteoroids. nuclei of short-period comets covering with porous A dependence of the rate of the decrease of secular refractory mantle. The thickness of this mantle is up brightness on perihelion distance, established dozens centimetres. The mantle may consist of a previously from observations for short-period comets, graphite-like consist. The density of this mantle may be clearly points to the progressive growth of a refractory 0.1–1 g/cm3, and the its thermal conductivity is 0.01– mantle on the nucleus surface and evolution such 0.1 W/mK, and mechanical strength of the mantle is comets into asteroid-like body (Dobrovolsky, Ibadinov, 20–50 kPa. The dependence of mantle surface Gerasimenko, 1984; Ibadinov, 1993; 1999). temperature on heliocentrical distance, in first The results of laboratory simulation of a cometary approximation, may be represented as T = To / r1/2. The nucleus show that, if the insolation energy is constant, icy temperature under crust is low (~200 K at r = 1 the mantle thickness is proportional to the square root A.U.). of the insolation duration, the sublimation rate grows References: lineary with mantle thickness, the temperature of ice Dobrovolsky, O.V., Ibadinov, Kh.I., and under the mantle depends very weakly on the mantle Gerasimenko, S.I., Secular Decrease in Brightness and thickness, and the temperature distribution over depth Structure of Nuclei of Periodic Comets, Dokl. Akad. is nearly linear within the mantle (Ibadinov et al., Nauk Tajik SSR, 1984, vol. 27, no. 4, pp. 198–200. 1991). Ibadinov, Kh.I., Laboratoty Investigation of the Physical and mechanical properties of the mantle Sublimation of Comet Nucleus Model, Adv. Space models were studied experimentally under conditions Res., 1989, vol. 9, no. 3, pp. 97–112, London. that mimic true conditions. The porosity, the density, Ibadinov, Kh.I., Rahmonov, A.A., Bjasso, A.Sh., the surface temperature, the effective thermal Laboratory Simulation of Cometary Structures, in conductivity, the strength, and the coeffisient of gas Comets in Post-Halley Era, Newburn, R.L., et al., Eds., diffusion in mantle were studied in laboratory Dordrecht: Kluwer, 1991, vol. 1, pp. 299–311. experiments (Ibadinov, et al., 1991; Ibadinov, 1999). Ibadinov, Kh.I., The Surface Structure of Short- The results of laboratory experiments (Ibadinov, 1989; Period Comet Nuclei, in Meteoroids and Their Parent Ibadinov et al., 1991) in particular, ion bombardment Bodies, Stohl, J. and Williams, I.P., Eds., Bratislava, of ice conglomerates (Hashimov, Ibadinov, 1993, pp. 373–376. Shoyokubov, 1994) and Vega-1 and Vega-2, and Ibadinov, Kh.I., Growth of a Refractory Mantle on Giotto data on the nucleus of comet Halley show that a Cometary Nucleus and the Evolution of the Nucleus the dark (albedo 0.03–0.06) and high-temperature into an Asteroid-like Body, Solar System Research, (320–4000 K) portions of cometary nuclei may consist 1999, vol., 33, no. 4, pp. 319–323. of a graphite-like substance. Hashimov, N.M., Ibadinov, Kh. I., and The results of calculations of the evolution of Shoyokubov, Sh. Sh., Laboratory Investigation of the short-period comets into asteroid-like bodies show that Possibility of Formation of Refractoty Substunces in the complete mantling of cometary nucleus achieved Comets, Dokl. Akad. Nauk Tajikistan, 1994, vol. 37, after approximatly dozens revolutions of comet around no. 1, pp. 16–19. the Sun on the subsolab part of the nucleus surface the mantle more than 20 cm thick formed and short-period.