PLEASE NOTE:
*
CCNet DIGEST 9 July 1998
---------------------------------------
(1) GREENLAND IMPACT ZONE OF GIANT METEORITE CONFIRMED
CNN Interactive
http://cnn.com/TECH/space/9807/08/metorite.dust/index.html
(2) WHAT'S IN A NAME: APOHELE = APOAPSIS & HELIOS
Dave Tholen <tholen@galileo.IfA.Hawaii.Edu>
(3) APOLLO ASTEROID 1991 & THE ONDREJOV NEO PROGRAM
Petr Pravec <ppravec@sunkl.asu.cas.cz>
(4) METEOROID IMPACT ARTICLE
Jiri Borovicka <borovic@sunkl.asu.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
====================
(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
===========================
(2) WHAT'S IN A NAME: APOHELE = APOAPSIS & HELIOS
From Dave Tholen <tholen@galileo.IfA.Hawaii.Edu>
Benny,
Duncan Steel has already brought up the subject of a class name
for
objects with orbits interior to the Earth's. To be sure,
we've already
given that subject some thought. I also wanted a word that
begins with
the letter "A", but there was some desire to work
Hawaiian culture into
it. I consulted with a friend of mine that has a master's
degree in
the Hawaiian language, and she recommended "Apohele",
the Hawaiian word
for "orbit". I found that an interesting
suggestion, because of the
similarity to fragments of "apoapsis" and
"helios", and these objects
would have their apoapsis closer to the Sun than the Earth's
orbit. By
the way, the pronunciation would be like
"ah-poe-hey-lay". Rob
Whiteley has suggested "Ali`i", which refers to the
Hawaiian elite,
which provides a rich bank of names for discoveries in this
class, such
as Kuhio, Kalakaua, Kamehameha, Liliuokalani, and so on.
Unfortunately, I think the okina (the reverse apostrophe) would
be
badly treated by most people.
I wasn't planning to bring it up at this stage, but because
Duncan has
already done so, here's what we've got on the table so far. I'd
appreciate some feedback on the suggestions.
--Dave
==================
(3) APOLLO ASTEROID 1991 & THE ONDREJOV NEO PROGRAM
From Petr Pravec <ppravec@sunkl.asu.cas.cz>
Dear Dr Peiser,
First of all, I congratulate you and your wife on the birth of
your
child. I can imagine well how you feel as I enjoyed the birth of
my
daughter a half year ago.
I would like to draw your attention to our paper
"Occultation/eclipse
events in binary asteroid 1991 VH" that recently appeared in
Icarus [1].
You perhaps passed it over as unrelated to NEOs, although the
opposite
is true. If you need me to provide the abstract, please, let me
know.
I also want to add a basic information about the Ondrejov NEO
Program.
As Gerhard Hahn (CCNet Digest 03/07/98) and Jana Ticha (CCNet
Digest
06/07/98) have already mentioned, there are important European
efforts
in the fields of NEO search and astrometric follow-up at the
Observatoire
de la Cote d'Azur (France) and the Klet Observatory (Czech
Republic),
respectively. The Ondrejov Observatory (also in the Czech
Republic)
is another professional station in Europe contributing to NEO
observations. The primary aim of the Ondrejov NEO Program is to
do a
photometric (mainly lightcurve) follow-up for a large number of
NEOs.
Results of our photometric observations made during 1994-97 for
as much
as 51 NEAs have appeared or will appear soon in Icarus and other
journals,
for 38 of them deriving their rotational periods for the first
time,
in several cases combining our data with those by others.
(See [2], [3], [4], [5] and [6] for some of the relevant papers
where
I am the first author.) Adding unpublished periods for some 15
NEAs recently
observed from our station, we are approaching a goal of doubling
the sample
of NEAs rotational periods known so far. The probable detection
of the
binary nature of the Apollo asteroid 1991 VH (see the previous
paragraph)
is one of the most important recent results of our program. I can
say that
currently we are probably the leading station in the lightcurve
follow-up
work on NEAs.
Although the lightcurve observations of NEAs are the primary aim
of
the Ondrejov NEO Program, we also do an astrometric follow-up of
NEOs
whenever our photometric schedule allows. I do not count exact
numbers of
our positions that were published on the MPCs, but others do and
appreciate
our astrometric effort. For example, in 1997 we measured 3870
positions
of asteroids (not only NEAs), that placed us at the 9th position
among
all stations over the world (S. Nakano, report on recent
activities).
The Ondrejov NEO astrometric follow-up effort has been
appreciated
by Brian Marsden of the MPC several times since its start in 1992
(last time in CCNet DIGEST 11/05/98) and Eleanor Helin (by naming
the minor planet 4790 Petrpravec; see the citation on MPC
30095-30096).
Not bad results for it being our secondary program, is it?
The success of the Ondrejov NEO Program was achieved through a
team work
of our group consisting of Marek Wolf (of the Charles University
Prague),
Lenka Sarounova and myself. We use the 0.65-m f/3.6 CCD
telescope,
that is relatively small but it is dedicated to the NEO program
and therefore we can use it for as much observing time as weather
conditions allow. Additional information can be found on the URL
http://sunkl.asu.cas.cz/~ppravec/neo.html
(although I should update the
page soon). We believe to keep up the good work of the
photometric
and astrometric follow-up of NEOs also in the future. We also
have a
plan to build a dedicated 1.5-m NEO follow-up telescope; if we
get
a support for it, both our photometric and astrometric efforts
will be even strengthened.
Best wishes,
Petr Pravec
Ondrejov Observatory
References:
[1] Pravec, P., M. Wolf, L. Sarounova 1998. Occultation/eclipse
events
in binary
asteroid 1991 VH. Icarus 133, 79-88.
[2] Pravec, P., M. Wolf, M. Varady, and P. Barta 1995. CCD
photometry
of 6 near-Earth
asteroids. Earth Moon Planets 71, 177-187.
[3] Pravec, P., L. Sarounova, and M. Wolf 1996. Lightcurves of 7
near-Earth
asteroids. Icarus 124,
471-482.
[4] Pravec, P. M. Wolf, L. Sarounova, A. W. Harris, and J. K.
Davies 1997.
Spin vector, shape and
size of the Amor asteroid (6053) 1993
BW3. Icarus 127,
441-451.
[5] Pravec, P., M. Wolf, L. Sarounova, S. Mottola, A. Erikson, G.
Hahn,
A. W. Harris, A. W. Harris, and J.
W. Young 1997. The near-Earth
objects follow-up program II. Results
for 8 asteroids from 1982 to 1995.
Icarus 130, 275-286.
[6] Pravec, P., M. Wolf, and L. Sarounova 1998. Lightcurves of 26
near-Earth
asteroids. Icarus, in
press.
=========================
(4) METEOROID IMPACT ARTICLE
From Jiri Borovicka <borovic@sunkl.asu.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@asu.cas.cz)
© European Southern Observatory (ESO) 1998
=======================
(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.
=====================
(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.
==============
(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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