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(meteorobs) Excerpts from "CCNet DIGEST 09/07/98"
[Several items from this issue seemed right on-topic, so
I've excerpted them below... Happy reading! -Lew Gramer]
>From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
>To: cambridge-conference@livjm.acdot uk
>Subject: CCNet DIGEST 09/07/98
>Date: Thu, 9 Jul 1998 15:25:13 -0400 (EDT)
CCNet DIGEST 9 July 1998
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(1) GREENLAND IMPACT ZONE OF GIANT METEORITE CONFIRMED
CNN Interactive
http://cnn.com/TECH/space/9807/08/metorite.dust/index.html
[snip...]
(4) METEOROID IMPACT ARTICLE
Jiri Borovicka <borovic@sunkl.asudot cas.cz>
(5) IMPACT FRAGMENTATION: FROM LABORATORY TO ASTEROID
E.V. Ryan*) & H.J.. Melosh, PLANETARY SCIENCE INSTITUTE
(6) DECODING SATELLITE IMPACT DATA
J.A.M. McDonnell & D.J. Gardner, UNIVERSITY OF KENT
(7) FORMATION & EVOLUTION OF THE PERSEID METEOROID STREAM
P. Brown & J.Jones, UNIVERSITY OF WESTERN ONTARIO
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(1) GREENLAND IMPACT ZONE OF GIANT METEORITE CONFIRMED
>From CNN Interactive
http://cnn.com/TECH/space/9807/08/metorite.dust/index.html
Impact zone of giant meteorite confirmed
(DR Online) -- Following microscopic analysis of snow-samples taken
last week, the impact zone of the giant meteorite that hit southern
Greenland last December has been located.
Last week, astronomer Holger Pedersen and geophysicist Torben Risbo of
the University of Copenhagen conducted a preliminary field
investigation on the southwestern Greenland ice cap.
Collecting snow samples by helicopter, they hoped to find traces of
meteorite dust left in the snow covering the glaciers. Some 40 samples
were taken along 3 different lines giving a very preliminary profile of
the snow-masses covering the glacier, where they scientists hope to
find the meteorite -- If it did not evaporate during entry into the.
The samples were taken to the Laboratory of the Arctic Station at
Qeqertarsuaq, Greenland, for microscopic analysis by Risbo and revealed
definite signs of meteoritic substance. Sub-millimeter size particles
that look like round brown glass, with little tails of glass trailing
behind them, were found. Other particles seem to give clues as to the
crystal-structure of the meteorite, but this can't be confirmed until
analysis has been conducted with an electron microscope.
A major field expedition on foot and by helicopter into the impact zone
planned for the end of this month may have to be pushed forward. It now
seems important to collect a much bigger amount of snow samples in
order to narrow down the area to be investigated. Also it can't be
ruled out that the meteorite, big as it was, completely evaporated
during entry, and therefore the only traces will be just dust.
Copyright 1998 CNN
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(4) METEOROID IMPACT ARTICLE
>From Jiri Borovicka <borovic@sunkl.asudot cas.cz>
Dear Dr. Peiser,
Although I am not a member of the CC DIGEST conference, I have seen
several numbers. I guess that the article we published recently in the
Astronomy & Astrophysics together with four co-authors could be
interesting for the subscribers of the CC DIGEST. I have attached the
abstract below for distribution on the network.
Sincerely
Jiri Borovicka
Ondrejov Observatory
---------------
Astron. Astrophys. 334, 713-728 (1998)
Bolides produced by impacts of large meteoroids into the Earth's
atmosphere: comparison of theory with observations I. Benesov bolide
dynamics and fragmentation
J. Borovicka(1), O.P. Popova(2), I.V. Nemtchinov(2), P. Spurny(1) and
Z. Ceplecha(1)
(1) Ondejov Observatory, Astronomical Institute of the Academy of
Sciences, CZ-251 65 Ondejov, Czech Republic
(2) Institute for Dynamics of Geospheres, Russian Academy of Sciences,
Leninsky pr. 38, build. 6, 117979 Moscow, Russia
Received 23 September 1997 / Accepted 12 January 1998
Abstract
Detailed analysis of one of the largest and well documented bolides -
the Benesov bolide (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 fragmentation at
altitudes of 38-31 km and of catastrophic disruption 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 2 g cm-3.
The comparison with the bolide PN 39434 suggests that the behavior of
Benesov is 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.
Send offprint requests to: J. Borovicka, (borovic@asudot cas.cz)
(c) European Southern Observatory (ESO) 1998
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(5) IMPACT FRAGMENTATION: FROM LABORATORY TO ASTEROID
E.V. Ryan*) & H.J. Melosh: Impact fragmentation: From the laboratory to
asteroids. ICARUS, 1998, Vol.133, No.1, pp.1-24
*) PLANETARY SCIENCE INSTITUTE, 620 N 6TH AVE, TUCSON, AZ, 85705
In this paper, we study the effect of target size on the fragmentation
outcome of rock targets using a 2D numerical hydrocode. After comparing
our hydrocode calculations to laboratory data (including explosive
disruption experiments) to validate the results, we use the code to
calculate how the critical specific energy (Q*) needed to
catastrophically fracture a body varies with target size in the regimes
not accessible to experiment. Impact velocity is generally kept
constant at about 2.0 km s(-1), although some higher velocity (similar
to 5 km s(-1)) simulations were run to determine a velocity dependence
for the fragmentation outcome. To reflect the asteroid population,
target diameters range from 10 cm to 1000 km, spanning the regimes
where strength and self-gravity (radially varying lithostatic stress)
each dominate resistance to fragmentation. We find that there is a
significant difference in fragmentation outcome when the lithostatic
stress is included in the computations. As expected, surface layers
fragment more easily, while the strength of the central regions
is greatly enhanced. We derive the Q* versus size relationship for
three materials, (basalt, strong-, and weak-cement mortar) each having
different static compressive strengths and representing a range of
asteroid materials. The hydrocode results showed that Q*
decreased with increasing target size in the strength regime,
with slopes of 0.43, 0.59, and 0.6 for basalt, strong and weak
mortar, respectively. This decrease is directly related to the
decrease in strain rate as target size grows. In the gravity regime, Q*
increases with increasing target size, with a slope equal to 2.6 for
all three of the materials modeled. These values are much steeper than
those previously derived from scaling theories. Ejecta velocity
distributions as a function of target size are examined as well. For
large bodies, resultant ejecta speeds tend to be well below escape
velocity, implying that these asteroids are likely to be reaccumulated
rubble piles. In simulating the creation of the asteroid family Eos, we
find that the code-calculated fragment size distribution is similar in
character to the observed data, but secondary fragment sizes are
significantly underestimated. More importantly, the determined ejecta
speeds were too low for these fragments to have achieved escape
velocity, and thus we fail to actually form the separate bodies
comprising the Eos family, and are left instead with a single rubble
pile conglomerate. (C) 1998 Academic Press.
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(6) DECODING SATELLITE IMPACT DATA
J.A.M. McDonnell & D.J. Gardner: Meteoroid morphology and densities:
Decoding satellite impact data. ICARUS, 1998, Vol.133, No.1, pp.25-35
UNIVERSITY OF KENT, PHYS LAB, UNIT SPACE SCI & ASTROPHYS, CANTERBURY
CT2 7NR, KENT, ENGLAND
The densities of interplanetary micrometeoroids have been inferred by
various techniques in the past; a valuable (albeit indirect) technique
has been the study of the deceleration profile of radar meteor trails,
for example. Impacts on the thin foils of the Micro-Abrasion Package on
NASA's LDEF satellite and the Timeband Capture Cell Experiment on ESA's
Eureca satellite now provide direct in situ measurement of the
cross-sections diameters of impacting micrometeoroids and also of space
debris particles. Combining these data with impact data from
thick-target impact craters, where the damage is mass-dependent, and
where such targets have experienced a statistically identical flux,
leads to a measure of the impactor density which is only weakly
affected by the assumed impact velocity. Comparing the space result
with those from simulations shows that the density distribution of
interplanetary particles in space has a more significant low density
component than the distributions obtained by most other recent methods
and that the mean density is in the range 2.0 to 2.4 g cm(-3) for
masses of 10(-15) to 10(-9) kg. The characteristic density - namely,
the single value which would characterize the impact behavior of the
distribution-is 1.58 cm(-3). Perforation profiles reveal that a large
fraction of the largest particles impacting the satellites are
nonspherical but that typical aspect ratios are mostly in the range
1.0-1.5. Flux distributions of the meteoroid population incident on the
Earth at satellite altitudes are derived in terms of mass and mean
diameter. (C) 1998 Academic Press.
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(7) FORMATION & EVOLUTION OF THE PERSEID METEOROID STREAM
P. Brown & J.Jones: Simulation of the formation and evolution of the
Perseid meteoroid stream. ICARUS, 1998, Vol.133, No.1, pp.36-68
UNIVERSITY OF WESTERN ONTARIO, DEPT PHYS & ASTRON, LONDON, ON N6A
3K7, CANADA
Four major models of cometary meteoroid ejection are developed and used
to simulate plausible starting conditions for the formation of the
Perseid stream. Ln addition to these physical variants, three different
choices for initial meteoroid density (100, 800, and 4000 kg m(-3)) are
used to produce a total of 12 distinct initial models. The development
and evolution of the stream are simulated for each model by ejecting
10(4) test meteoroids at seven distinct mass categories over the full
are of 109P's orbit inside 4 AU at each perihelion passage from 59 to
1862 AD. All test meteoroids are followed to their descending nodes for
times closest to the recent perihelion passage of 109P (1992). In
addition to these integrations, we have also performed long term
integrations over the interval from 5000 to 10(5) years ago using two
plausible sets of starting orbits for 109P over this interval. We find
that the choice of cone angle and precise cutoff distance for ejection
make only minor modifications to the overall structure of the stream as
seen from Earth. The assumed density for the meteoroids has a major
influence on the present activity of the stream as radiation pressure
moves nodal points further outside Earth's orbit and hence decreases
the probability of delivery for lower density meteoroids. The initial
ejection velocities strongly influence the final distributions observed
from Earth for the first approximate to 5 revolutions after ejection,
at which point planetary perturbations and radiation effects become
more important to subsequent development. The minimum distance between
the osculating orbit of 109P at the epoch of ejection and the Earth's
orbit is the principal determinant of subsequent delivery of meteoroids
to the Earth. The best fit to the observed present flux location and
peak strengths are found from models using Jones (1995) ejection
velocity algorithm with an r(-05) dependence and densities between (0.1
and 0.8 g cm(-3). The recent activity outburst maxims observed for the
Perseids from 1989 to present show a systematic shift in location from
year to year, which is explained by changing ages of the primary
component of the meteoroids malting up the outbursts. Specifically, it
is found that from 1988 to 1990 ejecta from 1610 and 1737 are the
dominant population, while 1862 and 1610 are the primary material
encountered in the outbursts from 1991to 1994. From 1995 to 1997 the
most prevalent populations are ejections from 1479 and 1079. The older
populations tend to shift the locations of the maximums to higher solar
longitudes. A discrepancy which is present for both the 1993 and 1994
peak locations of 1-2 h between the observed and modeled flux profiles
is most likely the result of emissions from 1862, which were observed
to have a large component of their velocity out of the cometary orbital
plane. The cause of Perseid activity outbursts is found to be direct
planetary gravitational perturbations from Jupiter and Saturn that
shift the nodes of stream meteoroids inward and allow them to collide
with Earth. The last such perturbations was due to Jupiter in 1991, and
this effect combined with the return of 109P in 1992 produced the
strong displays from 1991 to 1994. On average, it is found that the
Perseids observed each year in the core portion of the stream left the
parent comet (25 +/- 10) x 10(3) years ago. From the modeling, the
total age of the stream is estimated to be on the order of 10(5) years.
"From the simulations over the last 2000 years, the progression rate of
the node of the stream is estimated at (2.2 +/- 0.2) x 10(-4)
degrees/annum. The effect of terrestrial perturbations has been
evaluated from the long-term integrations and found to play only a
minor role in the stream's development, producing a 5-10% increase in
the stream's nodal and radiant spread as compared to an identical
simulation without the Earth. The primary sinks for the stream are
found to be hyperbolic ejection due to Jupiter land to a smaller degree
Saturn) as well as attainment of sungrazing states. Both the relative
and absolute contributions of these two loss mechanisms to the decay of
the stream is found to be highly dependent on the assumed cometary
starting orbits, with as much as 35% of initially released stream
meteoroids removed by hyperbolic ejection after 10(5) years for the
smallest Perseids on some starting orbits to less than 1% removed after
the same time for larger meteoroids on other potential seed orbits. On
average, it requires 40-80 x 10(3) years for a noticeable fraction of
the initial population (>0.1%) to be removed by these mechanisms,
depending on the chosen starting orbits. (C) 1998 Academic Press.
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