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J Borovicka - One of the best experts on this subject based on the ideXlab platform.
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satellite observation of the dust trail of a major Bolide event over the bering sea on december 18 2018
Astronomy and Astrophysics, 2020Co-Authors: J Borovicka, M Setvak, H Roesli, J K KerkmannAbstract:Context. One of the most energetic Bolide events in recent decades was detected by the US Government sensors (USGS) over remote areas of the Bering Sea on December 18, 2018, 23:48 UT. No ground-based optical observations exist.Aims. Using the satellite imagery of the dust trail left behind by the Bolide, we tried to reconstruct the Bolide trajectory. In combination with the Bolide speed reported by the USGS, we computed the pre-atmospheric orbit. Observations in various spectral bands from 0.4 to 13.3 μ m enabled us to study the dust properties.Methods. Images of the dust trail and its shadow obtained from various angles by the Multi-angle Imaging SpectroRadiometer (MISR) on board the Terra polar satellite and geostationary satellites Himawari-8 and Geostationary Operational Environmental Satellite 17 (GOES-17) were used. The initial position and orientation of the trail was varied, and its projections into the geoid coordinate grid were computed and compared with real data. Trail motion due to atmospheric wind was taken into account. Radiances and reflectances of selected parts of the dust trail were taken from the Moderate-resolution Imaging Spectroradiometer (MODIS) on board Terra. Reflectance spectra were compared with asteroid spectra.Results. The Bolide radiant was found to be 13° ± 9° from that reported by the USGS, at azimuth 130° (from south to west) and zenith distance 14°. The Bolide position was confirmed, including the height of maximum dust deposition around 25 km. The incoming asteroid had to be quite strong to maintain a high speed down to this height. The speed of 32 km s−1 , reported by the USGS, was found to be plausible. The orbit had a high inclination of about 50° and a perihelion distance between 0.95–1 AU. The semimajor axis could not be restricted well but was most probably between 1–3 AU. The dust reflectance was much lower in the blue than in the red, consistent with the material of A- or L-type asteroid. The absorption at 11 μ m confirms the presence of crystalline silicates in the dust.
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satellite observation of the dust trail of a major Bolide event over the bering sea on december 18 2018
arXiv: Earth and Planetary Astrophysics, 2020Co-Authors: J Borovicka, M Setvak, H Roesli, J K KerkmannAbstract:One of the most energetic Bolide events in recent decades was detected by the US Government sensors (USGS) over remote areas of the Bering Sea on December 18, 2018, 23:48 UT. No ground-based optical observations exist. Using the satellite imagery of the dust trail left behind by the Bolide, we tried to reconstruct the Bolide trajectory. In combination with the Bolide speed reported by the USGS, we computed the pre-atmospheric orbit. Observations in various spectral bands from 0.4 to 13.3 microns enabled us to study the dust properties. Images of the dust trail and its shadow obtained from various angles by the MISR instrument on board the Terra satellite and geostationary satellites Himawari-8 and GOES-17 were used. The initial position and orientation of the trail was varied, and its projections into the geoid coordinate grid were computed and compared with real data. Trail motion due to atmospheric wind was taken into account. Radiances and reflectances of selected parts of the dust trail were taken from the MODIS instrument. Reflectance spectra were compared with asteroid spectra. The Bolide radiant was found to be 13 +/- 9 degrees from that reported by the USGS, at azimuth 130\degr\ (from south to west) and zenith distance 14\degr. The Bolide position was confirmed, including the height of maximum dust deposition around 25 km. The incoming asteroid had to be quite strong to maintain a high speed down to this height. The speed of 32 km/s, reported by the USGS, was found to be plausible. The orbit had a high inclination of about 50\degr\ and a perihelion distance between 0.95-1 AU. The semimajor axis could not be restricted well but was most probably between 1-3 AU. The dust reflectance was much lower in the blue than in the red, consistent with the material of A- or L-type asteroid. The absorption at 11 microns confirms the presence of crystalline silicates in the dust.
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radiation of molecules in benesov Bolide spectra
Icarus, 2016Co-Authors: J Borovicka, A A BerezhnoyAbstract:Abstract We analyzed molecular radiation in the spectra of the very bright Benesov Bolide. The Benesov Bolide appeared over the Czech Republic on May 7, 1991 and reached an absolute magnitude of –19.5. It was caused by a meteoroid larger than 1 m. Small meteorites of various mineralogical types were recovered recently (Spurný et al. [2014] Astron. Astrophys., 570, A39, 14 p.). The spectrum of the Bolide, recorded on two photographic plates, is probably the richest meteor spectrum ever obtained. It contains hundreds of atomic emission lines, continuous radiation and molecular bands, and covers the whole Bolide trajectory from the altitude of 90–20 km. In this paper we focus on identification and analysis of molecular bands. The identification of FeO, CaO, AlO, and MgO, reported earlier (Borovicka and Spurný [1996] Icarus, 121, 484–510) was confirmed. In addition, radiation of N2 was probably detected. The oxides were best seen in the wake and in the radiating cloud left at the position of the Bolide flare at the altitude of 24.5 km. Trace of N2 was seen only in the meteor at lower altitudes. FeO bands are present in the spectra from the highest altitudes. We suppose that FeO was ablated directly in molecular form at high altitudes. CaO was first detected just below 50 km and its intensity, relatively to FeO, strongly increased toward lower altitudes. AlO, which is similarly refractive as CaO, behaved as FeO rather than CaO at lower altitudes. MgO was observed only in the radiating cloud. The spectrum of the cloud is unique because it contains almost no atomic lines. We compared the data with theoretical calculations of the presence of molecules in the mixture of meteoric vapors and air at various altitudes and temperatures. CN and TiO were not found. The upper limit of CN is in agreement with theory for ordinary chondrite meteoroid. Most of carbon should be in fact present in the form of CO, but CO bands are too weak to be detected. The non-detection of TiO can be explained by the fact that temperature in the wake and the cloud was lower than needed for the presence of TiO bands. However, AlO was found to be about 40 times more abundant than MgO, although comparable abundances are expected. The explanation may be that the abundances are in fact comparable but there are non-equilibrium conditions in the radiating cloud with the excitation temperature of MgO lower than that of AlO. The difference may be caused by higher ablation temperature of Al. Another non-equilibrium effect is the observed difference between the rotational (∼1000 K) and vibrational (∼3000 K) temperature of AlO molecules. This can be explained by short hydrodynamic timescale and the fact that vibrational relaxation time is significantly longer than rotational relaxation time. The vibrational temperature therefore could not decrease so quickly during the cooling and expansion of the cloud because of insufficient number of collisions. FeO and CaO could not be analyzed in detail, because their molecular constants, especially transition probabilities, are not well known. The increase of the CaO/FeO ratio with decreasing altitude could be, nevertheless, explained in scope of equilibrium chemistry.
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radiation study of two very bright terrestrial Bolides and an application to the comet s l 9 collision with jupiter
Icarus, 1996Co-Authors: J Borovicka, P SpurnýAbstract:Abstract Lightcurves and spectra of two of the brightest photographed Bolides were studied. The Sumava Bolide reached −21.5 absolute (i.e., 100 km distance) magnitude. This fragile cometary body of initial mass of about 5000 kg exhibited many flares and disappeared at an altitude of 59 km above the surface. The Benesov Bolide (−19.5 mag) was a stony body of initial mass of about 13,000 kg which radiated down to 17 km of altitude and exhibited a bright flare at 24 km accompanied with severe fragmentation. It is shown that the fragmentation was the main mode of ablation. By modeling the lightcurve of Sumava it is found that about 85% of the initial mass was lost in five subsequent breakups. Even between the breakups, the ablation coefficient is found to be much higher than the theoretical value for melting and vaporization. The spectra show that the radiation of both Bolides was produced by a gas heated to 4000–5000 K. At this temperature only the ablated material radiates; the radiation of the atmosphere was negligible. Atomic and molecular emissions are dominant for most of the trajectory, but near the maximum light thermal continuous radiation is also present. The results were generalized for the entry of the comet Shoemaker–Levy 9 into the Jupiter atmosphere. Due to the high ablation, the cometary nuclei are not expected to penetrate below the jovian clouds. As the radiating volume of the associated Bolides is optically thick, the Bolides remain relatively faint and only weak dependence of the Bolide brightness on the impactor mass is produced. The results are in accordance with the Bolide and fireball observations from the Galileo spacecraft.
P Brown - One of the best experts on this subject based on the ideXlab platform.
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using large meteoroids as global infrasound reference events
2015 AGU Fall Meeting, 2019Co-Authors: Christoph Pilger, Lars Ceranna, Alexis Le Pichon, P BrownAbstract:The explosive fragmentation of large meteoroids entering the Earth’s atmosphere is one of the strongest sources of infrasound and can be detected at distances of thousands of kilometers by arrays all over the world. Influence parameters on the detection capability are quantified for a single infrasound station and for the complete infrasound network of the International Monitoring System (IMS) operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). They are applied to a number of strong Bolides of the past 15 years including the 2013 Chelyabinsk, 2010 Sulawesi, and 2009 North Pacific events. Long-range infrasound propagation modeling and realistic atmospheric background conditions are used to identify propagation paths that connect the sources and globally distributed receivers, highlighting usual as well as unusual propagation pattern, to stations detecting and stations not detecting a meteorite event. Potential influences on infrasound detection capability are due to the directivity of the acoustic source energy emission, the long-range ducting via stratosphere and thermosphere and the diurnal change of meteorological parameters and noise conditions at the stations during the signal arrivals. Since infrasound of large Bolides has probably the most similar characteristics to an atmospheric nuclear explosion, it can be utilized as reference event for studies on the global performance of the CTBTO infrasound network. Detections and non-detections of Bolide infrasound at the more than 40 operational IMS infrasound stations are studied for the estimation of station and network performance and thus verification of nuclear test ban.
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infrasound monitoring as a tool to characterize impacting near earth objects neos
Infrasound Monitoring for Atmospheric Studies, 2019Co-Authors: E A Silber, P BrownAbstract:Infrasound, the low-frequency sound lying below the human hearing range, has the capability to propagate over very long distances in the atmosphere due to its low attenuation. Thus, infrasound can serve as a tool for monitoring explosive sources, including extraterrestrial bodies impacting the Earth’s atmosphere. This chapter describes the theoretical background on meteor physics and Bolide infrasound, as well as applications of infrasound in Near-Earth Objects (NEOs) monitoring and characterization. In addition to presenting a comprehensive list of empirical relations to estimate Bolide energy release, this chapter summarizes recent case studies where infrasound served as an instrumental tool in characterizing the source.
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refinement of Bolide characteristics from infrasound measurements
Planetary and Space Science, 2017Co-Authors: P BrownAbstract:Abstract We have detected and performed signal measurements on 78 individual Bolide events as recorded at 179 infrasound stations between 2006 and 2015. We compared period-yield relations with AFTAC nuclear period-yield data, finding these to be similar with a slight offset. Scatter in period measurements for individual Bolide is found to be caused in part by station noise levels and by attenuation effects with range. No correlation was found between the infrasound signal period and any of Bolide height at peak brightness, entry speed or impact angle. We examined in detail three well constrained Bolides having energy deposition curves, known trajectories and infrasound detections finding some evidence at shorter ranges that a component of station period scatter is due to varying source heights sampled by different stations. However, for longer-range stations in these three case studies, we were not able to assign unique source heights using raytracing due to large uncertainties in atmospheric conditions. Our results suggest that while source height contributes to the observed variance in infrasound signal periods from a given Bolide, range and station noise play a larger role.
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updated population and risk assessment for airbursts from near earth objects neos
IEEE Aerospace Conference, 2015Co-Authors: M B Boslough, P Brown, Alan W HarrisAbstract:We present a new analysis of airburst risk based on updated estimates for the population of undiscovered NEOs, taking into account the enhanced damage potential of directed airbursts. We define airbursts as events in which small (meters to tens-of-meters in diameter) asteroids deposit most of their energy in the atmosphere as large Bolides and where the total energy is comparable to or greater than small nuclear explosions (>0.1 kilotons of TNT). Our tens-of-meter population estimate from optical surveys is now much closer to Bolide frequency estimates, resolving most of an earlier discrepancy. Our Tunguska-class (∼40 meters) population estimate has doubled, and Chelyabinsk-class (∼20 meters) has increased by a factor of 2.6. Uncertainty in this population remains quite large, and can only be unambiguously reduced by expanded surveys focused on objects in the tens-of-meters size range. The assessed risk from this population is also increasing for two reasons. First, airbursts are significantly more damaging than assumed in the original risk assessments, because for typical impact geometries they more efficiently couple energy to the surface than nuclear explosions of the same energy. Second, the greater numbers mean that they are more frequent than previously thought. We review the evidence that asteroid airbursts are more damaging than nuclear explosions, and provide arguments that such events are more frequent.
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infrasound production by Bolides a global statistical study
Journal of Atmospheric and Solar-Terrestrial Physics, 2012Co-Authors: T Ens, P Brown, Wayne N Edwards, E A SilberAbstract:Abstract We have examined a dataset consisting of 71 Bolides detected by satellite sensors, which provide energy and location estimates, with simultaneous measurements of the same events on 143 distinct waveforms. These Bolides have total source energies ranging from 0.02 kt TNT equivalent yield to ≈ 20 kt and probable diameters of order a few meters on average. We find that it is possible to detect large events with energies of ≈ 20 kt or more globally. Infrasonic detections of these events for stratospheric arrivals have ranges between 350–17,000 km and show clear wind-related amplitude modifications. We find that our period–yield relations are virtually identical to that found from AFTAC nuclear test data with the most robust period–yield correlation found for those events having multiple station averaged periods. We have also found empirical expressions relating maximum expected detection range for infrasound as a function of energy and low and high frequency cut-off as a function of energy. Our multi-variate fits suggest that 1 2 yield-scaling is most appropriate for long range Bolide infrasound measurements with a distance scaling exponent of ≈ 1.1 best representing the data. Our best-fit wind correction exponent is a factor of ≈ 3 smaller than found by previous studies which we suggest may indicate a decrease in the value of k with range. We find that the integral acoustic efficiency for Bolides is ≥ 0.01 % with a best lower limit estimate nearer 0.1%. Finally, we conclude that a range independent atmosphere implementation of the normal-mode approach to simulate Bolide amplitudes is ineffective at large ranges due to the large change in atmospheric conditions along source-receiver paths.
I V Nemtchinov - One of the best experts on this subject based on the ideXlab platform.
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Bolides produced by impacts of large meteoroids into the earth s atmosphere comparison of theory with observations i benesov Bolide dynamics and fragmentation
Astronomy and Astrophysics, 1998Co-Authors: Jiři Borovicka, P Spurný, O P Popova, I V Nemtchinov, Zdenek CeplechaAbstract:Detailed analysis of one of the largest and well doc- umented Bolides - the BeneBolide (EN 070591) - has been performed. The Bolide had an initial velocity of 21 km s 1 , reached a maximal absolute magnitude of 19:5 at the altitude of 24 km and radiated down to 17 km. Detailed photographic data for the light curve, geometry and dynamics of the main body and several fragments are available. This enabled us to test the theoretical radiative-hydrodynamic model used previously for the analysis of satellite-detected Bolides. The conventional analysis produces a huge discrepancy between the dynamic (80-300 kg) and photometric (5000- 13,000 kg) mass. The discrepancy might be removed assuming a low density of 0.5 g cm 3 but this is unrealistic. The radiative- hydrodynamic modeling yielded a mass of 2000 kg and density of 1-2 g cm 3 . However, the dynamics was not sufficiently well reproduced. There is direct observational evidence of meteoroid frag- mentation at altitudes of 38-31 km and of catastrophic disrup- tion at 24 km. These, however, do not explain the problem with the mass. The crucial point is that the Bolide was significantly decelerated already at the altitudes between 50-40 km, while enormous luminosity was produced below 40 km. We suggest that the meteoroid must have been fragmented into 10-30 pieces of a mass of 100-300 kg already at an altitude of 60-50 km. By creating a progressive fragmentation model with two types of fragmentation at three different altitude levels, we were able to reproduce the dynamics and luminosity sufficiently well. The best estimate of the initial mass is 3000-4000 kg for a density of 2gc m 3 . The comparison with the Bolide PN 39434 suggests that the behavior of Beneis typical for large stony meteoroids. Early fragmentation under dynamic pressures of the order of 1 Mdyn cm 2 is very important. The analysis of the light curve with the radiative-hydrodynamic model can give good order-of- magnitude estimates of mass, if no dynamic data are available.
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Bolides produced by impacts of large meteoroids into the earth s atmosphere comparison of theory with observations ii benesov Bolide spectra
Astronomy and Astrophysics, 1998Co-Authors: Jiři Borovicka, O P Popova, A P Golub, I B Kosarev, I V NemtchinovAbstract:The unique observational spectrum of the very bright Benesov Bolide EN 070591 is compared to theoretical Bolide spectra. The −19.5 mag Bolide was induced by a meteoroid of an estimated initial mass of 4000 kg, a density of 2 g cm−3 and a kinetic energy of1012 J (0.2 kT TNT). The ablating piston model predicts spectra of large Bolides by radiative hydrodynamics calculations. We present examples of the calculated H-chondrite vapor spectral opacities and of the resulting spectra for various parameters. Both theoretical and observed spectra show that Bolide radiation is composed of atomic line emissions, molecular bands and continuum radiation. The role of the continuum increases with increasing meteoroid size and with decreasing altitude. The atomic lines are produced under the effective excitation temperature of 4000–6000 K. The lines of Fe i are too faint and the lines of Ca i are too bright in the model in comparison with the observations. Also the computed continuum level is too high. These differences can be explained by the fact that the vapors occupy a larger volume and have lower density than predicted. This is probably a consequence of a mutual interaction of fragments after the meteoroid fragmentation and of a not well understood ablation process. Other differences between the theory and the observation are described and possible model improvements are discussed.
J K Kerkmann - One of the best experts on this subject based on the ideXlab platform.
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satellite observation of the dust trail of a major Bolide event over the bering sea on december 18 2018
Astronomy and Astrophysics, 2020Co-Authors: J Borovicka, M Setvak, H Roesli, J K KerkmannAbstract:Context. One of the most energetic Bolide events in recent decades was detected by the US Government sensors (USGS) over remote areas of the Bering Sea on December 18, 2018, 23:48 UT. No ground-based optical observations exist.Aims. Using the satellite imagery of the dust trail left behind by the Bolide, we tried to reconstruct the Bolide trajectory. In combination with the Bolide speed reported by the USGS, we computed the pre-atmospheric orbit. Observations in various spectral bands from 0.4 to 13.3 μ m enabled us to study the dust properties.Methods. Images of the dust trail and its shadow obtained from various angles by the Multi-angle Imaging SpectroRadiometer (MISR) on board the Terra polar satellite and geostationary satellites Himawari-8 and Geostationary Operational Environmental Satellite 17 (GOES-17) were used. The initial position and orientation of the trail was varied, and its projections into the geoid coordinate grid were computed and compared with real data. Trail motion due to atmospheric wind was taken into account. Radiances and reflectances of selected parts of the dust trail were taken from the Moderate-resolution Imaging Spectroradiometer (MODIS) on board Terra. Reflectance spectra were compared with asteroid spectra.Results. The Bolide radiant was found to be 13° ± 9° from that reported by the USGS, at azimuth 130° (from south to west) and zenith distance 14°. The Bolide position was confirmed, including the height of maximum dust deposition around 25 km. The incoming asteroid had to be quite strong to maintain a high speed down to this height. The speed of 32 km s−1 , reported by the USGS, was found to be plausible. The orbit had a high inclination of about 50° and a perihelion distance between 0.95–1 AU. The semimajor axis could not be restricted well but was most probably between 1–3 AU. The dust reflectance was much lower in the blue than in the red, consistent with the material of A- or L-type asteroid. The absorption at 11 μ m confirms the presence of crystalline silicates in the dust.
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satellite observation of the dust trail of a major Bolide event over the bering sea on december 18 2018
arXiv: Earth and Planetary Astrophysics, 2020Co-Authors: J Borovicka, M Setvak, H Roesli, J K KerkmannAbstract:One of the most energetic Bolide events in recent decades was detected by the US Government sensors (USGS) over remote areas of the Bering Sea on December 18, 2018, 23:48 UT. No ground-based optical observations exist. Using the satellite imagery of the dust trail left behind by the Bolide, we tried to reconstruct the Bolide trajectory. In combination with the Bolide speed reported by the USGS, we computed the pre-atmospheric orbit. Observations in various spectral bands from 0.4 to 13.3 microns enabled us to study the dust properties. Images of the dust trail and its shadow obtained from various angles by the MISR instrument on board the Terra satellite and geostationary satellites Himawari-8 and GOES-17 were used. The initial position and orientation of the trail was varied, and its projections into the geoid coordinate grid were computed and compared with real data. Trail motion due to atmospheric wind was taken into account. Radiances and reflectances of selected parts of the dust trail were taken from the MODIS instrument. Reflectance spectra were compared with asteroid spectra. The Bolide radiant was found to be 13 +/- 9 degrees from that reported by the USGS, at azimuth 130\degr\ (from south to west) and zenith distance 14\degr. The Bolide position was confirmed, including the height of maximum dust deposition around 25 km. The incoming asteroid had to be quite strong to maintain a high speed down to this height. The speed of 32 km/s, reported by the USGS, was found to be plausible. The orbit had a high inclination of about 50\degr\ and a perihelion distance between 0.95-1 AU. The semimajor axis could not be restricted well but was most probably between 1-3 AU. The dust reflectance was much lower in the blue than in the red, consistent with the material of A- or L-type asteroid. The absorption at 11 microns confirms the presence of crystalline silicates in the dust.
Jiři Borovicka - One of the best experts on this subject based on the ideXlab platform.
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Bolides produced by impacts of large meteoroids into the earth s atmosphere comparison of theory with observations i benesov Bolide dynamics and fragmentation
Astronomy and Astrophysics, 1998Co-Authors: Jiři Borovicka, P Spurný, O P Popova, I V Nemtchinov, Zdenek CeplechaAbstract:Detailed analysis of one of the largest and well doc- umented Bolides - the BeneBolide (EN 070591) - has been performed. The Bolide had an initial velocity of 21 km s 1 , reached a maximal absolute magnitude of 19:5 at the altitude of 24 km and radiated down to 17 km. Detailed photographic data for the light curve, geometry and dynamics of the main body and several fragments are available. This enabled us to test the theoretical radiative-hydrodynamic model used previously for the analysis of satellite-detected Bolides. The conventional analysis produces a huge discrepancy between the dynamic (80-300 kg) and photometric (5000- 13,000 kg) mass. The discrepancy might be removed assuming a low density of 0.5 g cm 3 but this is unrealistic. The radiative- hydrodynamic modeling yielded a mass of 2000 kg and density of 1-2 g cm 3 . However, the dynamics was not sufficiently well reproduced. There is direct observational evidence of meteoroid frag- mentation at altitudes of 38-31 km and of catastrophic disrup- tion at 24 km. These, however, do not explain the problem with the mass. The crucial point is that the Bolide was significantly decelerated already at the altitudes between 50-40 km, while enormous luminosity was produced below 40 km. We suggest that the meteoroid must have been fragmented into 10-30 pieces of a mass of 100-300 kg already at an altitude of 60-50 km. By creating a progressive fragmentation model with two types of fragmentation at three different altitude levels, we were able to reproduce the dynamics and luminosity sufficiently well. The best estimate of the initial mass is 3000-4000 kg for a density of 2gc m 3 . The comparison with the Bolide PN 39434 suggests that the behavior of Beneis typical for large stony meteoroids. Early fragmentation under dynamic pressures of the order of 1 Mdyn cm 2 is very important. The analysis of the light curve with the radiative-hydrodynamic model can give good order-of- magnitude estimates of mass, if no dynamic data are available.
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Bolides produced by impacts of large meteoroids into the earth s atmosphere comparison of theory with observations ii benesov Bolide spectra
Astronomy and Astrophysics, 1998Co-Authors: Jiři Borovicka, O P Popova, A P Golub, I B Kosarev, I V NemtchinovAbstract:The unique observational spectrum of the very bright Benesov Bolide EN 070591 is compared to theoretical Bolide spectra. The −19.5 mag Bolide was induced by a meteoroid of an estimated initial mass of 4000 kg, a density of 2 g cm−3 and a kinetic energy of1012 J (0.2 kT TNT). The ablating piston model predicts spectra of large Bolides by radiative hydrodynamics calculations. We present examples of the calculated H-chondrite vapor spectral opacities and of the resulting spectra for various parameters. Both theoretical and observed spectra show that Bolide radiation is composed of atomic line emissions, molecular bands and continuum radiation. The role of the continuum increases with increasing meteoroid size and with decreasing altitude. The atomic lines are produced under the effective excitation temperature of 4000–6000 K. The lines of Fe i are too faint and the lines of Ca i are too bright in the model in comparison with the observations. Also the computed continuum level is too high. These differences can be explained by the fact that the vapors occupy a larger volume and have lower density than predicted. This is probably a consequence of a mutual interaction of fragments after the meteoroid fragmentation and of a not well understood ablation process. Other differences between the theory and the observation are described and possible model improvements are discussed.