Comparative study of models of meteoroid disruption into a cloud of fragments
Authors: Brykina Irina , Bragin Michael 
Published: October 2020

AbstractModels of meteoroid disruption into a cloud of fragments are considered: the two-parameter model, which takes into account changes in the cloud shape and density, and simple models without accounting these effects. Models are used to reproduce the energy deposition of the Chelyabinsk superbolide. Influence of ablation model is studied; for simple fragmentation models, optimal coefficient is proposed depending on heat transfer coefficient.When meteoroid breaks up into a large number of fragments, at the first stage they move with a common shock wave, before dispersing enough distance to move independently. To simulate &#160;&#160;meteoroid disruption at this stage, models of a cloud of fragments moving as a single body were proposed and used [1&#8211;7, and others]. Such a cloud is flattened by pressure forces: it is compressed in a flight direction, and spreads out in a lateral direction. Fragment cloud models differ in equations for the rate of cloud lateral expansion. Comparison of models [3, 4] was made [7] for energy deposition modeling for the Chelyabinsk and Tunguska events. Here we consider two-parameter model [6], which takes into account changes in the fragment cloud shape and density, and simple models, for example [1, 4], without accounting these effects. We also consider the above models with limited midsection radius growth. We compare models as applied to reproducing the energy deposition of the Chelyabinsk superbolide. For correct comparison, we use them with the same ablation and drag coefficient models.2. Fragmentation modelsSpherical shape of meteoroid is assumed before breakup starts, then it continues its flight as cloud of fragments and vapor, moving as a single body, and sphere is transformed into spheroid with ratio of axes k. In model [6], in addition to flattening parameter k, parameter &#947; is introduced, characterizing decrease of fragmented meteoroid density &#948; due to increase of spacing between fragments: &#948; = &#948;e/&#947;3 (&#947; &#8804; 3), where &#948;e is initial meteoroid density. Equation for the rate of increase of the fragment cloud midsection radius RS in two-parameter model [6] has a form:&#160;dRs/dt = (&#947;3/k)1/2(&#961;/&#948;e)1/2V,&#160; &#160;k = 4&#960;&#948;eRS3/(3M&#947;3),&#160; &#947; = (&#947;m - 1)( &#961;1/2 - &#961;f1/2)/(&#961;m1/2 - &#961;f1/2)&#160; &#160; &#160; (1)Here t is time, &#961; is atmospheric density, V and M are meteoroid velocity and mass; subscripts f and m correspond to values at heights of fragmentation start hf and maximum bolide brightness hm. Parameter &#947;m is adjusted to match the observed height hm.In simple models equation for the midsection radius RS isdRs/dt = c(&#961;/&#948;e)1/2V,&#160; &#160;c = const&#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160;(2)Here c = 1 and c = (7/2)1/2 correspond to models [1, 5] and [4]. Simple models, in contrast to the two-parameter model, do not take into account decrease of density of disrupted meteoroid and change of its shape. Solution of equation (2) shows that midsection radius is determined only by initial parameters, ablation does not affect it, so fragmentation problem is separated from ablation and motion problem. For two-parameter model these problems are coupled.3. Calculation resultsWe used various fragment cloud models to simulate interaction of the Chelyabinsk meteoroid with the atmosphere, solving ablation and motion equations together with equations (1), or (2), by Runge-Kutta method. Data [9] were used as initial parameters. Unknown entry mass was determined to match the observed maximum energy deposition [10]. For the drag coefficient we used: CD = 1.78 &#8211; 0.85/k. For the radiative heat transfer coefficient CH we used approximate formula depending on V, &#961;, RS and k [6, 8]. Uncertainty parameter &#968; is introduced in &#1057;&#1053; formula to account for effects of precursor, absorption by vapor layer, and other uncertain factors. Effect of CH coefficient on meteoroid mass loss, midsection radius, energy deposition, and entry mass estimate is studied by varying parameter &#968;. Constant CH values are also used.Figure 1: Energy deposition per unit height for two-parameter model (a) and models [1] (b) and [4] (c); black curve is observational data [10]; hf &#160;= &#160;45 km.Figure 1 shows that two-parameter model and simple model [1] enable satisfactory simulation of the observational energy deposition curve down to 27 km, respectively, &#160;at &#968; = 1 (and &#1057;&#1053; = 0.1), and at &#968; = 0.3 (and &#1057;&#1053; = 0.03). They give estimates of the Chelyabinsk meteoroid entry mass, close to estimates [9, 11]. Model [4] does not reproduce observational data. We found coefficient c in equation (2) for simple fragmentation models, which provides satisfactory agreement with observational data, depending on heat transfer coefficient. Two-parameter model gives much smaller increase of midsection radius than simple models. Limiting the midsection radius growth does not improve any models.AcknowledgementsThis work was performed according to the plan of Institute of Mechanics of Lomonosov Moscow State University and was partially funded by Russian Foundation for Basic Research, grant 18-01-00740.References[1] Grigoryan S.S. Motion and destruction of meteors in planetary atmospheres. Cosmic Research 17, 724&#8211;740. 1979.[2] Melosh H.J. Atmospheric breakup of terrestrial impactors. Proc. Lunar Planet. Sci. 12A, 29&#8211;35. 1981.[3] Chyba C.F., Thomas P.J., Zahnle K.J. The 1908 Tunguska explosion &#8211; Atmospheric disruption of a stony asteroid. Nature 361, 40&#8211;44. 1993.[4] Hills J.G., Goda. M.P. The fragmentation of small asteroids in the atmosphere. Astronomical J. 105 (3), 1114&#8211;1144. 1993.[5] Grigoryan S.S., Ibodov F.S., Ibadov S.I. Physical mechanism of Chelyabinsk superbolide explotion. Solar Syst. Res. 47, 268&#8211;274. 2013.[6] Brykina I.G. 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