LA-UR-02-5391 Approved for public release; distribution is unlimited. FIREBALL DYNAMICS, ENERGETICS, ABLATION, Title: LUMINOSITY AND FRAGMENTATION MODELING Author(s): D. O. ReVelle Submitted to: http://lib-www.lanl.gov/cgi-bin/getfile?00796986.pdf Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty- free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. FORM 836 (10/96) FIREBALL DYNAMICS, ENERGETICS, ABLATION, LUMINOSITY AND FRAGMENTATION MODELING D. O. ReVelle Los Alamos National Laboratory, P.O. Box 1663, MS J577, Earth and Environmental Sciences Division, Atmospheric and Climate Sciences Group, Los Alamos, New Mexico 87545 USA Email: [email protected] ABSTRACT quantities for a perfectly stratified hydrostatic, non-isothermal atmosphere as well (see below). We have modeled large bolide passage through The energetics approach discussed further in the Earth’s atmosphere including the effects of [13] is fundamentally equivalent to the basic dynamics, energetics, ablation, luminosity and expression for the bolide height as a function of fragmentation, including bolide porosity. In this the entry velocity, ablation parameter, mass to brief paper we outline the physical processes area ratio, entry angle, etc. with the additional involved in only a schematic manner. Bolide prediction of the computed velocity at which the luminosity was assumed to be proportional to the dark flight regime begins [7]. This is time rate of change of the kinetic energy. accomplished by obtaining the solution of a Recently, the proportionality factor (which varies transcendental equation relating the change of with mass, height and velocity), a semi-empirical the bolide kinetic energy versus time combined differential luminous efficiency, has been with the simple ablation theory expression for produced in [18, 20] using the Lost City the mass loss expected for a single body during meteorite fall as a calibration event. Two entry using a single mean (constant) ablation schemes of fragmentation have been devised for parameter. This is probably the weakest link of the prediction of bolide luminosity. In the first all predicted parameters in the model, yet it is scheme, fragmented particles are deposited into only used to predict the velocity at which the wake after triggering by the stagnation ablation ceases as a function of the D parameter pressure exceeding the bolide strength and these of ReVelle. Typical D values utilized were particles remain in the wake. In the second assumed to range from 2.303 to 4.605 (90 - 99 % scheme, the fragmented particles are drawn kinetic energy removed at the end height) . The forward collectively to produce an enhanced latter D value was calibrated through a detailed frontal area as time progresses. Considerable analysis of the photographic data of the Lost success was found in modeling the light curve of City, Innisfree and Pribram meteorite falls [11]. the Benesov bolide . The full paper with all The ablation model is basically the one relevant details and many additional cases will developed originally in [9] which predicts the be submitted shortly to Icarus for formal prevailing stagnation point ablation mechanism publication. somewhere between the limits of vaporization and melting as a function of the dimensionless 2 ablation interaction parameter, k (= V∞ /2Qvap), 1. INTRODUCTION AND OVERVIEW: where V∞ = pre-atmospheric, initial meteoroid velocity and Qvap = energy of ablation per unit We have developed a theoretical model of mass for ablation of one of at least five possible, bolide dynamics, energetics, ablation, luminosity distinct meteoroid groups). The model includes and fragmentation for use in comparisons almost all previously identified forms of heat between observational material from ground- transfer with the exception of so-called precursor based camera and other bolide detection ionization effects, i.e., free stream absorption techniques (satellite, infrasound, etc.). This which occurs well ahead of the bolide and model can compute the dynamic possibilities for interference heating effects, i.e., viscous velocity and deceleration for either an isothermal boundary layer-strong shock wave interactions at atmosphere with either a single mean (constant) progressively higher altitudes, but at transitional ablation parameter or a height variable ablation Kn number~ 1). Absorption of the radiative parameter. It can also compute the same shock wave flux is not included in the calculations due to a very large uncertainty in specific electromagnetic pass-band is both the precise mix of constituents in the parent proportional to the time rate of change of the meteoroid and in their radiative properties at kinetic energy of the bolide. In order to complete high temperatures (with multiply ionized states, the prediction currently, a semi-empirical etc.). Rotation and lift (non-ballistic forces) have luminous efficiency parameter has been utilized not been included in the analysis. Two groups from [18, 20] as a function of mass, air density [2, 8] have recently considered rotation effects and velocity. This function has been computed regarding significant flickering of light curves only for the panchromatic pass-band of the that has been observed on occasion. The former photographic emulsion used to detect these group [2] concluded that rotation was not a bright bolides. Since we have now computed the significant factor in altering meteoroid ablation equilibrium temperatures surrounding the bolide processes (at least for the fall of the Innisfree as well, we can in principle correct the prediction meteorites) for a rather unusual proposed for any other pass-band of interest. meteoroid shape, even though under certain Fragmentation is calculated in two limiting conditions rotation could be expected to extremes currently. Once triggered by the influence pulsations observed on fireball light stagnation pressure exceeding the strength of the curves (significant changes in stellar magnitude bolide (as modified by cracks, porosity, etc.), the which would translate to extremely significant fragmented particles are either transferred to the changes in the predicted light intensity). wake and remain there or are allowed after a Theoretical non-dimensional heat transfer specified time lag to be drawn forward toward expressions are computed for either laminar or the main leading fragment. The bolide turbulent convective/conductive heat transfer luminosity is now being calculated for either of through the gas cap as well as for radiative these two respective limiting conditions. emission from the strong shock front in the Porosity is explicitly included in the model absence of ablation products absorption effects. formulation and as shown in [17], can These calculations also neglect the so-called significantly effect the luminosity prediction. precursor ionization phenomena (free stream absorption effect). These are discussed further in [13]. Furthermore, we have continuously 2. MODERN BOLIDE computed all of the relevant parameters as a FRAGMENTATION MODELING continuous function of the Knudsen number, Kn, APPROACHES: RELAXATION OF for all of the extremes from free molecular flow THE SINGLE-BODY THEORY LIMITS (Kn >> 1) to continuum flow (Kn << 1). Shape change during entry is accomplished There are several possible types of using the shape change parameter, µ, which has fragmentation processes, but only two been measured directly in [19] for a number of fundamental triggering mechanisms have been very precisely measured bolides. Most cases had proposed in the recent literature. These 0 < µ < 1 , but about 1/3 of the cases had µ < 0 mechanisms include both a thermal and a and for all cases µ was usually a function of mechanical initiation process, but severe height (or equivalently time). The former regime temporal limitations of the thermal mechanism encompasses the classical self-similar, no shape argue against its general applicability, except change result (µ ≡ 2/3). The latter limit (µ< 0) perhaps for meteoroids with very high thermal includes the so-called catastrophic “pancake” conductivity, such as nickel-iron meteorites. This fragmentation regime that was rediscovered by is because the time needed to produce Zahnle, Chyba and Thomas, by Hills and Goda, fragmentation by thermal effects is far too long etc. during the predictions of the impact of compared to the available entry flight time. Comet Shoemaker-Levy-9 into Jupiter’s Among the mechanism proposed starting about atmosphere. The regimes of flight corresponding 1970 and beyond, we have: to different constant µ values along the trajectory were summarized in [16, 17, 19]. We have not i) An enhanced frontal area with the yet devised a theoretical procedure to deal with air drag increasing as the changes of µ during entry despite the fact that conventional