|
[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
|
