IRF Kiruna


Meteoroids 2001

Conference at the

Swedish Institute of Space Physics,

Kiruna, Sweden
6-10 August 2001


[Programme]

Session 8: "Fireballs, Bolides and Meteorites"

Date: Thursday 8.30-10.05

 

The Moravka Meteorite Fall: Fireball Trajectory, Orbit and Fragmentation

J. Borovicka, P. Spurny and Z. Ceplecha (Astronomical Institute, Academy of Sciences, 251 65 Ondrejov, Czech Republic)

The Moravka meteorite fall of May 6, 2000, is only the sixth case in history, when the pre-fall trajectory could be determined from instrumental records. A very bright fireball appeared during broad daylight at 11:51:52 UT and was seen by thousands of people. Fortunately, three casual witnesses captured the fireball on video. The fireball was also detected by satellite-based infrared and visible sensors. Sonic booms were recorded by a local seismic network and an infrasound array located in Germany recorded signals from this event. Three ordinary chondrites of type H5-6 and total mass of 634 g were recovered in the vicinity of village Moravka, Czech Republic (18.53E, 49.60N) over a span of 11 km. Much more fragments certainly fell, since one video record shows multiple hierarchical fragmentation to more than hundred pieces. After a careful calibration of the videos, we were able to determine the fireball trajectory with a good precision. The initial velocity was 22.5 km/s and the trajectory slope 20 degrees to horizontal. The most resistant fragment disappeared at an altitude of 21 km when decelerated to 4 km/s. The heliocentric orbit is notable by a high inclination of 32 degrees. Details of fireball dynamics and fragmentation will also be given. 8.1

 

On the Relationship between Asteroids, Fireballs and Meteorites

A.E. Rosaev (FGUP NPC NEDRA, Yaroslavl, Russia)

Best of interest searching for parental bodies for meteors, fireballs and meteorites with well determined orbits. We use two days to study this problem in this work. First way - in-vestigation close orbits by one of number of empirical criterion's like criterion Southworth -Hawkins. The second way is to study the orbits intersections statistics. The fireballs of Prai-rie network and meteorites with well determined orbits was took into account. Obviously, that fireballs and meteorites - an essence of bodies, having potentially unstable orbits and, as an effect, small time life's. If take sufficiently natural suggestion on that, that similar objects are form in collisions NEA with each other and with comets, possible expect that crossing the orbits of asteroids and fireballs, or meteorites will indicate us on the most close on a time events of mutual collisions NEA. Really, the distribution nearly intersected orbits in the system NEA-fireball and fireball-fireball show significant non-homogenous. Possible select about 10 areas to concentrations the cross points of orbits of fireballs and NEA. Probably, they correspond to the most recent disastrous events. Results of calculations for meteorites Lost City, Prhibram, Innisfree Peekskill and Tun-guska's is given. In case of a general conclusion of this work, the hypothesis of an very close relation stud-ied meteorites with NEA put forward. 8.2

 

Relation of Meteoroid Ablation-Classification to Light Curves

D.O. ReVelle (1) and Z. Ceplecha (2)

1) Los Alamos National Laboratory, P.O. Box 1663, MS J577, Earth and Environmental Sciences Division, EES-8, Atmospheric and Climate Sciences Group, Los Alamos, NM 87545, USA; 2) Emeritus: Academy of Sciences, Astronomical Institute, Observatory, 25165 Ondrejov, Czech Republic

Classification of bolides according to their ablation coefficients allows the recognition of 4 different groups: type I, type II, type IIIA, and type IIIB. (In addition, iron bolides have also been identified by the authors, but from our limited data set, we can not evaluate them using the current effort). The possibility of determining to what type a bolide belongs from just light curve data was examined, and a criterion IT = (1/I)(dI/dt) was earlier proposed. The statistical significance of sorting bolides according to IT was also established. Recently, we have revised our original light-curve classification using ground-based radiometer data taken at the Ondrejov Observatory provided by Sandia National Laboratory (courtesy of Mr. Richard Spalding, operated by Mr. Pavel Spurny) and have determined: a) the originally proposed classification tables published in the Annals of the New York Academy of Sciences (1997) are valid and form the best possibility available at the moment, but they should be utilized only if light curves are the only form of observational data available, b) relevant results can be obtained for values of brightness separated by a time interval, dt, of 0.05 s (if a shorter interval is available, one should adjust the interval by summing all data inside each 0.05-s interval), c) associating a bolide with a single type by using only light-curve data is rarely possible, since only the probability of belonging to one of the 4 types can be determined, d) the ablation coefficient, sigma and the shape-density coefficient, K, cannot be construed as a weighted average and must be taken for each bolide type separately, e) if other precise observational data can be derived (e.g. data on heights and velocities at several points), they are certainly preferable for determining the sigma and K values. The characteristic light curve determined for each of these4 meteoroid types will be presented. PSB-10

 

Bolide Fragmentation Theory with Application to PN and EN fireballs

D.O. ReVelle (1) and Z. Ceplecha (2)

1) Los Alamos National Laboratory, P.O. Box 1663, MS J577, Earth and Environmental Sciences Division, EES-8, Atmospheric and Climate Sciences Group, Los Alamos, NM 87545, USA; 2) Emeritus: Academy of Sciences, Astronomical Institute, Observatory, 25165 Ondrejov, Czech Republic

The simple physical concept of the conservation of energy and momentum (single-body theory) was applied to a majority of multi-station photographic observations of bolides assuming that the entry behavior could be successfully described using a constant value of the ablation coefficient (sigma), the shape-density coefficient (K), and allowing for a single sudden gross-fragmentation at one distinct altitude. If the precision of the observational data is better than +/-30 m in distance measured along the bolide trajectory, then about 40% of the events can be explained without any gross-fragmentation, about 40% are explainable with one gross-fragmention point and about 20% have definitely experienced more than one gross-fragmentation point. High values of the derived ablation coefficients from observations speak for nearly continuous fragmentation as being the main mass-loss process for these bodies. If the precision of the observational data is better than +/-15 m as described above, then the assumption of constant sigma and K is no more sustainable. There are not many data available with such a high precision. The complete solution of the problem with sigma and K being functions of time was subsequently derived and applied to 22 PN and EN fireballs each with the needed high precision. The luminous efficiencies, tau, can also be determined from this approach as well. We will present our results on sigma, K, and tau as a function of the many different variables and parameters of the problem for individual bolides during their luminous trajectory and for individual bolide types. PSB-11

 

Infrasonic Monitoring of the Global Influx Rate of Large Bolides

Douglas O. ReVelle (Los Alamos National Laboratory, Los Alamos, NM 87545, USA)

We have utilized recent infrasonic bolide observations to estimate the large bolide influx rate. These infrasonic signals are from the densest, most deeply penetrating objects entering the atmosphere. Undoubtedly, depending on the exact mass range under consideration, the total influx is about a factor of five-ten times greater. This work is a continuation or work shown at the Cornell ACM meeting in 1999 and initiated by Wetherill and ReVelle in the late 1970's using data from AFTAC (Air Force Technical Applications Center, Patrick AFB, Florida). There have been several additional large bolides detected infrasonically since 1999 that are included in our latest evaluation of the global influx rate of large bolides. Some of these have also been detected independently by US DoD Satellites. Thus, in some cases we also have independent estimates of the bolide source energy that can be used for an evaluation of the accuracy of the infrasonic source energy estimate. We have also used statistical counting error procedures to estimate the uncertainty in the influx rate as a function of the source energy, assuming that the source energy estimates are without error. For example, at a source energy of 0.2 kt (1 kt = 4.185X10(12) joules), we find a global influx rate and its estimated uncertainty of 28.1 (+/-8.9) bolides per year. 8.3

 

Bolide Fragmentation Processes: Comparisons of Bolide Data against Theoretical Bolide Models

Zdenek Ceplecha (Astronomical Institute, Czech Academy of Sciences, Ondrejov Observatory, Ondrejov, The Czech Republic) and Douglas O. ReVelle (Los Alamos National Laboratory, Los Alamos, NM 87545, USA)

This work is a practical extension and testing of theoretical work also submitted to the Meteoroids2001 conference. We have applied the fragmentation model of ReVelle to the most precise EN and PN fireballs in order to evaluate the shape change parameter, and the fragmentation scale height in comparison to the pressure or density scale heights. If the ratio of the fragmentation scale height to the density scale height is large, we recover the single-body model limit. In the opposite extreme, pancake type fragmentation is possible. This was done in order to determine if, for any of the available very precise bolide observations, a parameter range existed that allowed pancake type catastrophic fragmentation processes to occur. This is important since a number of workers in the early 1990's identified this behavior as being important for the larger bodies entering the atmosphere in the small ablation limit (Hills and Goda, Chyba, Zahnle and Thomas, etc.). We are currently examining a number of bolides with very precise observations to determine these fundamental properties and will report on our findings at the conference. 8.4

 

Bolide Fragmentation Modeling

Douglas O. ReVelle (Los Alamos National Laboratory, Los Alamos, NM 87545, USA)

In this talk we extend work begun at the Cornell ACM meeting. The shape change parameter is evaluated for conditions when it is negative. For the values of the shape change parameter between 0 and 2/3, ablation, shape change and deceleration can occur. For values < 0, however, large lateral growth of the body occurs. This negative region of the shape change parameter corresponds to the flight regime that was "rediscovered" by Hills and Goda and by Chyba, Zahnle and Thomas in the 1990's and analyzed in detail by Grigoryan in the 1970's. We have determined analytic expressions for the shape change parameter (assuming a constant ablation parameter and constant meteoroid velocity) and for the fragmentation scale height, Hf. We have evaluated Hf assuming that fragmentation was triggered if the stagnation pressure exceeded the body's compressive/tensile strength. If Hf >> H, the density scale height, the single-body approximation is applicable, whereas, if Hf << H, catastrophic, pancake break-up will occur. In the limit with the shape change parameter < 0 with very small ablation, large increases in the frontal cross-sectional area are predicted to occur, but only over a very limited range of conditions. In addition, as the shape change parameter becomes progressively more negative, end heights raise substantially. We also evaluate the effect of a negative shape change parameter on light emission so that a nearly complete, self-consistent model of the bolide phenomena can be formulated. PSB-12

 

Bolide Luminosity Modeling: Comparisons between Uniform Bulk Density and Porous Meteoroid Models

Douglas O. ReVelle (Los Alamos National Laboratory, Los Alamos, NM 87545, USA)

We compare predictions of normalized bolide luminosity for two fundamental bolide models, one assuming a uniform bulk density throughout the body and a second which assumes a uniform chondritic composition throughout, but with varying amounts of porosity (assumed to be filled with either water-ice or open space). The second model is based upon the uniformity of spectral observations taken over many years during periods of shower meteors from the extremes of the Geminids to the dustball-like Draconids. The first model utilized is due to ReVelle (1979, 1993) and the second is based upon the porous meteoroid model of ReVelle (1983, 1993). The standard, uniform bulk density, ablation model assumes that the drag and heat transfer area are equivalent in the positive, shape change factor limit. For porous meteoroids however, the heat transfer area can exceed the drag area by increasingly larger amounts as the body's porosity increases. ReVelle (1983) used this approach to show that the bulk density and ablation parameter compositional group identifications of Ceplecha and McCrosky (1976) were essentially correct. When these factors are introduced into the relevant model equations, a set of nearly self-consistent predictive relations are developed which readily allows comparisons to be made of the end-height variations and of the normalized luminous output of the two basic types of meteoroid models. PSB-13

 

On Electrophonic Phenomena

A.Yu. Ol'khovatov (Radio Instrument Industry Research Institute, Moscow, Russia)

During last years an idea that electrophonic sounds are caused by VLF electromagnetic radiation from a bolide's wake is promoted again. The source of the radiation is considered to be "magnetic spagetti relaxation" in a bolide's wake. The most serious problem with the theory is that level of detected VLF disturbances, accompanied a bolide is negligibly small, comparing with needed for hearing VLF radiation. For example, C. Keay experiments revealed lower limit of hearing in order of 160 V/m. It means that a bolide producing electrophonic sounds is to generate in its wake VLF radiation with the power at least in order of 10^12 W. Nor present theory neither experiments with turbulent ionized wakes predicts such superpowerful VLF radiation. Anyway, if it is realized somehow, it would lead to spectacular effects, for example, to enormous Joule heating of the wake (due to extremely large electric currents) transforming the wake into object as bright (seen from the ground) as at least the Sun. And, of course, this "super-radiation" would produce remarkable global effects - but none of them are known. The presence of "transducers" near an observer can't help the situation, as the needed level to hear their vibrations are even in order of magnitude larger, and anyway, it can't be lowered down many orders of magnitude. A solid confirmation that these estimations are correct is the fact that otherwise people would hear numerours VLF transmitters hundreds miles away! Also there were reports of electrophonic sounds during several Space Shuttle re-entries. The the hypothetical "super-radiation" (if exists) would produce a very remarkable (devastating) effect on the spaceplane. Also, in many electrophonic events the power of aerobraking, i.e. the power deposited by a meteoroid into the atmosphere, which is to be the energy source of the proposed VLF super-radiation was much less that the power of the latter. 8.5

 

Recent Infrasonic Observations of Large Bolides

Peter Brown (Los Alamos National Laboratory, Los Alamos, New Mexico, USA, and Department of Physics and Astronomy University of Western Ontario London, Ontario, Canada), Douglas O. ReVelle and Rod Whittaker (Los Alamos National Laboratory, Los Alamos, New Mexico, USA)

We expect a minimum of several dozen bolides to impact the Earth each year and penetrate low enough in the atmosphere to produce infrasonic waves. These objects have energies from 10-2 kT to many hundreds of kT and many of these are also simultaneously observed by US DoD satellites. Here we present a summary and discussion of more than 15 infrasonically observed bolide events detected since 1996. These data demonstrate the capability of infrasound arrays to reliably locate the source of bolide explosions in the atmosphere at heights of typically 20-30 km and at ranges exceeding 5000km. We find, for example, that a 0.2kT bolide detonation can be detected at ranges of 3300 km under good conditions. Most notable among these recent events is a multi-kt event recorded off the coast of Mexico on 25 Aug, 2000 which was recorded by six infrasound stations and a large (~20 kt) detonation in the South Pacific on 18 Feb, 2000. The modelling and interpretation of some individual events and comparison with other instrumental records of the same bolides will be highlighted. PSB-14

 

The Tagish Lake Meteorite Fall : Interpretation of Physical and Orbital Data

Peter Brown (Los Alamos National Laboratory, Los Alamos, New Mexico, USA, and Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada), Douglas O. ReVelle (Los Alamos National Laboratory, Los Alamos, New Mexico, USA) and Alan Hildebrand (Department of Geology and Geophysics, University of Calgary)

The Tagish lake meteorite fell 18 Jan, 2000 at 16:43 UT in Northern British Columbia Canada. Some 500 meteorites were later found on the frozen ice-surface of Tagish Lake. The fireball accompanying the meteorite fall was widely recorded by ground-based photographers/videographers, earth-orbiting satellites, seismic and infrasound sensors. The associated meteorites have proven unique; reflectance spectra from Tagish Lake is the first to match that of D-class asteroids (Hiroi et al., 2001) and the bulk density of TL is the lowest measured for any meteorite at 1.67 g cm-3 (Zolensky, pers comm). Here we will discuss the data relating to the fireball and associated modelling to determine the orbit of TL and probable physical structure. Most notably, the fireball data suggest that TL is intermediate between Type II and III fireballs. Our modelling indicates the initial body had a porosity near 50%. Type III objects are presumed to be related to cometary bodies and suggests that TL and by extension D-asteroids might be intermediate in physical structure between primitive chondritic asteroids and cometary nuclei. 8.6

 

Common Ground-based Optical and Radiometric Detections of Fireballs within the Czech Part of the European Fireball Network

Pavel Spurny (Astronomical Institute, Academy of Science, Ondrejov Observatory 251 65 Ondrejov, The Czech Republic) and Richard E. Spalding (Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-0978, USA)

Since August 1999 two radiometric systems equiped with optical sensors are operated at two stations of the Czech part of the European fireball network (EN). During this period we have obtained several very detailed lightcurves for bright fireballs recorded also photographically in scope of the EN. First results of this study will be presented. PSB-15

 

The EN310800 Vimperk Fireball: Probable Meteorite Fall of an Athen-type Orbit Meteoroid

Pavel Spurny and Jiri Borovicka (Astronomical Institute, Academy of Science, Ondrejov Observatory 251 65 Ondrejov, The Czech Republic)

We report a detection of an unique fireball photographed at two Czech stations of the European Fireball Network. This slow-moving fireball with initial velocity of only 15 km/s reached the maximum absolute brightness -14 and penetrated down to almost 20 km. The meteorite fall of several pieces of the total mass of several kilograms is highly probable. However, no meteorite has been recovered yet. From one very rough spectral record and also from its behavior in the atmosphere we found that it was stony meteoroid, probably ordinary chondrite. The main exceptionality of this fireball is in its heliocentric orbit, with semimajor axis only 0.8 AU, eccentricity 0.3, aphelion 1.03 AU and inclination 17 degrees. This rare Aten type orbit is only third one in the history of decades-long operation of the European Fireball Network. PSB-16


[Programme]

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