PLEASE NOTE:
*
CCNet DIGEST, 11 September 1998
-------------------------------
(1) MORE TROUBLE FOR FRANK'S MINI-COMETS THEORY
Andrew Yee <ayee@nova.astro.utoronto.ca>
(2) BRITISH GOVERNMENT ADVISER ATTACKS GLOBAL WARMING LOBBY:
THE TIMES, 9 September 1998
http://www.sunday-times.co.uk:80/cgi-bin/BackIssue?1617548
(3) DPS ABSTRACTS
Paolo Farinella <paolof@keplero.dm.unipi.it>
(4) THOUGHTS ON SPACE-BASED NEO DETECTION
Bob Kobres <bkobres@uga.edu>
(5) NEO SEARCH MISSIONS
Michael Paine <mpaine@tpgi.com.au>
(6) SPECTACULAR ENCOUNTER IN SPACE EXPECTED IN NOVEMBER
DESERET NEWS
http://www.desnews.com:80/tdy/y10quabf.htm>
(7) GOD'S ULTIMATE PUNISHMENT? "ASTEROID SHOWERS AFTER
FAMILY
BREAKUP EVENTS"
V. Zappala et al., ASTRONOMICAL OBSERVATORY
TORINO
(8) SYNCHRONIC BAND FORMATION IN COMETARY DUST TAILS
K. Nishioka, OLYMPUS OPT CO LTD
(9) TIDAL DISTORTION & DISRUPTION OF EARTH-CROSSING ASTEROIDS
D.C. Richardson & W.F. Bottke, UNIVERSITY
OF WASHINGTON
(10) SIMULATING ASTEROIDAL IMPACTS
I. Giblin et al., UNIVERSITY OF PISA
(11) COMPOSITION OF JETS IN ASYMETRIC IMPACTS
G.H. Miller, UNIVERSITY OF CHICAGO
===============
(1) MORE TROUBLE FOR FRANK'S MINI-COMETS THEORY
From Andrew Yee <ayee@nova.astro.utoronto.ca>
Public Information Office
American Geophysical Union
2000 Florida Avenue, N.W.
Washington, D.C. 20009
Tel (202) 462-6900 / FAX 202-328-0566
September 8,
1998
Contact: Harvey Leifert
AGU RELEASE NO.
98-31
(202) 939-3212
For Immediate
Release
hleifert@kosmos.agu.org
"Snowball comets" are just camera noise, Berkeley
researchers say
after analyzing dark pixels in Iowa data
WASHINGTON, D.C. -- Researchers at the University of California
at
Berkeley have concluded that "atmospheric holes" in
satellite
imagery are caused by instrument noise in the spacecraft's own
cameras, not by the presence of comets the size of a house
bombarding the Earth's atmosphere every few seconds. The
existence
of such comets, sometimes referred to as snowballs in space, has
been hotly debated since it was first proposed by Prof. Louis A.
Frank in 1986.
New, higher resolution images from the VIS and UVI cameras aboard
the Polar spacecraft show similar clusters of dark pixels, which
Frank and Dr. John B. Sigwarth, both of the University of Iowa,
have
recently taken as independent verification of the presence of
small
comets. Various critics of the comet theory have previously
suggested that the simple explanation for the dark pixels is
noise.
In papers scheduled for publication October 1 in the journal,
Geophysical Research Letters, Prof. Forrest S. Mozer and Dr.
James
P. McFadden of Berkeley's Space Sciences Laboratory state that
their
study "differs from all others that have objected to the
small-comet
hypothesis in that it considers events produced by the major
proponents of this hypothesis [Frank and Sigwarth] from data
provided by their own Polar instrument."
Both papers analyze raw data for one day provided by Frank and
Sigwarth and additional data in the form of 700,000 pixel
clusters,
covering 120 days, posted on the web and known as the Iowa
catalog.
McFadden, et al., investigate the characteristics of the dark
pixels
in relation to expected noise from the individual components of
the
two cameras. Using computer simulations, they show that the dark
pixels seen in the satellite data from both cameras are entirely
consistent with instrumental noise.
Mozer, et al., investigate the distribution of the dark pixels by
altitude. They show that there is no appreciable height
dependence.
The researchers also note that the same pattern of dark pixels is
seen in images of the nighttime sky as in sunlit images, which
would
not be the case if they were caused by external objects such as
small comets. They conclude that Frank and Sigwarth's own data
processing introduces those "meaningless" dark pixel
clusters.
Outside the radiation belt, say the authors, more than 80 percent
of
the dark pixel clusters "result from the process that Frank
and
Sigwarth employ to remove bright pixels caused by energetic
particles."
GRL Space Physics and Aeronomy Editor Robert Winglee notes that
Prof. Frank has been made aware of the contents of the Mozer and
McFadden papers and has been invited to submit a response.
###
Note: Copies of the two GRL papers cited in this release are
available to media representatives upon request.
Mozer, et. al., "Small-comet 'atmospheric holes' are
instrument
noise"
McFadden, et. al., "An instrumental source for the dark
pixel
clusters in the Polar VIS and UVI experiments"
The papers are not under embargo. They include contact
information
for the authors.
============
(2) BRITISH GOVERNMENT ADVISER ATTACKS GLOBAL WARMING LOBBY
From THE TIMES, 9 September 1998
http://www.sunday-times.co.uk:80/cgi-bin/BackIssue?1617548
Geologist attacks climate claims
AN ADVISER to the Government said yesterday that some scientists
and
politicians were "talking rubbish" about global
warming.
David Bowen, a geologist at Cardiff University and vice-chairman
of
the Countryside Council for Wales, said that the computer models
on
which claims about the effects of man-made pollution were based
were
flawed. There was evidence that the Earth had gone through
periods
of warming and cooling over the past two million years, well
before
man-made pollution could have had any impact.
Yet this natural variability in the climate, recorded in Arctic
and
Antarctic ice cores, was ignored in the computer models used to
forecast the impact of man-made pollution on the Earth's weather
systems, he said. The models used "assume the climate has
been
stable for 10,000 years", he said. "Until you build in
natural
variability it is impossible to forecast the future."
Professor Bowen told the festival that the United Nations
Intergovernmental Panel on Climate Change knew of the ice-core
work,
but natural variability in the Earth's climate was not included
in
the report which paved the way for the Kyoto meeting where cuts
in
energy use were agreed.
He emphasised that he was expressing his own views: "I am
not paid
by some large American oil conglomerate."
Copyright 1998, The Times Newspapers Ltd.
================
(3) DPS ABSTRACTS
From Paolo Farinella <paolof@keplero.dm.unipi.it>
Benny,
many CCNet members may be interested in reading the abstracts of
this year's DPS conference, which are now posted at
http://www.ssec.wisc.edu/dps98/
Cheers,
Paolo
====================
(4) THOUGHTS ON SPACE-BASED NEO DETECTION
From Bob Kobres <bkobres@uga.edu>
Re: PROPOSAL FOR A NEO SEARCH MISSION UNDER NASA'S DISCOVERY
PROGRAM,
CCNet DIGEST, 9 September 1998
I'm glad to see active promotion of Space-based detectors coming
forward.
I've not seen this suggested, but it seems to me that a simple,
relatively inexpensive, 'Lookout-system' might be obtained by
orbiting a dozen solar powered detectors between the orbits of
Earth
and Venus. These instruments would track on fairly evenly spaced
(24 node crossings inside Earth's circuit), moderately inclined
orbits (~30 degrees from Earth's?) and simply relay data, in
packet
form, one to the next until receipt is acknowledged by an
Earth-based collection system. With the Sun being the back
orientation for all devices the detectors could each compass at
least 270 degrees of unique view continuously. These things
could
be pretty dumb, like Prospector, yet yield a wealth of
information
in a fairly short period of time.
Getting them into solar orbit could be a monthly affair using the
Moon's mass for velocity adjustment. Several devices could
be
placed in temporary Earth orbits by one shuttle mission and then
later be individually moved into solar orbit or there could
simply
be a series of rocket launches--whichever is more cost effective.
Just an idea.
bobk
Bob Kobres
====================
(5) NEO SEARCH MISSIONS
From Michael Paine <mpaine@tpgi.com.au>
Thanks for a prompt response to my proposal. My "large
stable
object" was the Earth! In fact I was describing the
ground-based
Spaceguard proposal put forward in 1992 (updated 1995 - see the
NASA
web site). I was sarcastically pointing out that one
"Discovery"
mission could easily cover the cost of the worldwide, 10 year
Spaceguard Program! In the longer term I expect (as several
authors
have pointed out) that space-based detection systems will be
needed
- especially for comet and small NEO detection. The Lunar
Prospector
mission has shown that "dumb" "cheap"
(!) spacecraft can be highly
successful in specialised missions. I believe we should be
planning
ahead for such missions, while getting on with the
"down-to-Earth"
Spaceguard project.
Cheers
Michael Paine
NSW Co-ordinator (maybe ex!)
The Planetary Society Australian Volunteers
===============
(6) SPECTACULAR ENCOUNTER IN SPACE EXPECTED IN NOVEMBER
From DESERET NEWS
http://www.desnews.com:80/tdy/y10quabf.htm>
Editor's note: This article is a Web Edition extra that does not
appear in the printed Deseret News.
By Tim Radford
The Guardian
Nov. 17 will be one of the more spectacular periodic encounters
with
a dust cloud from a comet.
Tiny fragments of stardust - the size of a grain of sand or rice
-
will hit the Earth's atmosphere at 41 miles a second, and burn up
in
a blaze of glory in the early morning sky, at the rate of at
least
one a second when Earth runs head-on into the Leonids.
Rocket launches will be suspended, the Hubble space telescope
will
look away and satellites' solar panels will be moved out of the
line
of fire. Meteor storms and showers are predictable - as with the
Perseids last month. But every 33 years, a group called the
Leonids
provide a series of spectacular autumn encounters.
If this year is a disappointment, then pin your hopes on Nov. 18,
1999, says Mark Littmann, professor of astronomy at the
University
of Tennessee at Knoxville.
"Back in 1966, they were estimated at as high as 40 meteors
a
second. This time around, a meteor a second would be very
impressive. People who saw it in 1833 said it was like the
heavens
were on fire. It is like nothing else that can be seen in the
night
time sky."
People in the Far East will probably get the best show when the
constellation Leo rises over the horizon after midnight.
"Don't
watch for just a minute or two, because it can come in
spurts," he
said.
The encounter is with a ribbon of dust shed by Comet
Tempel-Tuttle.
Meteors that burn up in the atmosphere and meteorites that hit
the
ground are a fact of life. Shooting stars appear every night. The
guess is that Earth collects an average of 500 tons of stones,
dust,
water and gases from space every day.
"Over the 4 billion years the Earth has been in
existence," Littmann
says, "we have added 16 million million million tons, but
even so we
have added less than 1 percent to the Earth's mass."
But the Leonids are the fastest arrivals of all, because the
Earth
runs into them almost head on. Humans are in no danger. But the
radio region of the upper sky will fizz, crackle and pop, and
instruments orbiting above the atmosphere will be at extra risk.
NASA engineers and satellite operators have been meeting to work
out
just how big that risk will be.
"Even though we are dealing with something the size of a
grain of
sand or smaller, traveling at 150,000 mph, it's like a
bullet,"
Littmann said.
============
(7) GOD'S ULTIMATE PUNISHMENT? "ASTEROID SHOWERS AFTER
FAMILY
BREAKUP EVENTS"
V. Zappala, A. Cellino, B.J. Gladman, S. Manley, F. Migliorini:
Asteroid showers on Earth after family breakup events. ICARUS,
1998,
Vol.134, No.1, pp.176-179
ASTRONOMICAL OBSERVATORY TORINO, PINO TORINESE, TO, ITALY
The fate of a fraction of the near-Earth asteroid (NEA)
population
is to collide with terrestrial planets. Collisional events in the
main asteroid belt provide a source of NEAs. We provide the first
quantitative evaluation of the numbers of impactors produced in
different size ranges by some events which produced recognized
asteroid families in the main belt. Following certain
family-forming
events large enhancements in the rate of Earth impacts likely
occurred, In the form of 'asteroid showers' lasting 2-30 Myr,
capable of influencing the Earth's biosphere. Such a shower could
also be responsible of the hypothesized 'lunar cataclysm' 4 Gyr
ago.
Because the number of NEAs is strongly affected by such events,
the
NEA population cannot always remain in a steady state. (C) 1998
Academic Press
==================
(8) SYNCHRONIC BAND FORMATION IN COMETARY DUST TAILS
K. Nishioka: Finite lifetime fragment model 2 for synchronic band
formation in dust tails of comets. ICARUS, 1998, Vol.134, No.1,
pp.24-34
OLYMPUS OPT CO LTD, SHIBUYA KU, 1-43-2 HATAGAYA, TOKYO 151, JAPAN
Some big comets showed type II tails with many narrow striae
called
'synchronic bands,' the formation mechanism of which is still
unknown. a dynamic model for the formation mechanism of
synchronic
bands, which is based on the following process, is proposed. The
complex particles of the aggregates of the unit particles are
ejected from the nucleus of the comet and disintegrate repeatedly
into individual unit particles at various disintegration speeds.
Then, these unit particles break up and their fragments are
observed
as synchronic bands. These fragments continue to disintegrate or
sublimate into smaller pieces and finally they become too small
to
be seen at a certain normalized lifetime. The structures
calculated
with this theory fit well the observed shape and orientation of
the
synchronic bands of Comet West and Comet Seki-Lines. This dynamic
model suggests that the radii of the complex particles and the
radii
of the unit particles are of less than visible wavelength. (C)
1998
Academic Press
===================
(9) TIDAL DISTORTION & DISRUPTION OF EARTH-CROSSING ASTEROIDS
D.C. Richardson*) & W.F. Bottke: Tidal distortion and
disruption of
earth-crossing asteroids. ICARUS, 1998, Vol.134, No.1, pp.47-76
*) UNIVERSITY OF WASHINGTON, DEPT ASTRON, SEATTLE, WA, 98195
We present results of numerical simulations that show that
Earth's
tidal forces can both distort and disrupt Earth-crossing
asteroids
that have weak ''rubble-pile'' structures. Building on previous
studies, we consider more realistic asteroid shapes and
trajectories, test a variety of spin rates and axis orientations,
and employ a dissipation algorithm to treat more accurately
collisions between the particles that make up the model asteroid.
We
explore a large parameter space, including the asteroid's
periapse
q, encounter velocity with the Earth upsilon(infinity), spin
period
P, initial spin axis orientation, and body orientation at
periapse.
We parameterize the simulation outcomes by the amount of mass
stripped from the asteroid during a flyby. Our most severe
disruptions result in fragment trains similar in character to
the ''string of pearls'' created when Comet D/Shoemaker-Levy 9
was disrupted near Jupiter in 1992. Less catastrophic disruptions
cause material to be stripped off in more isotropic fashion,
leaving
a central remnant with a characteristic distorted shape. Some
ejecta
can enter into stable orbits around the remnant, creating a
binary
or multiple system. Even when no mass is lost tidal forces and
torques can modify the asteroid's shape and spin. Our results
show
that mass loss is enhanced for small values of q,
upsilon(infinity),
and P, and depends to a certain extent on the body's initial spin
orientation (for example, retrograde rotation reduces mass loss).
An
elongated asteroid was found to be far easier to disrupt than a
spherical one, though the orientation of the ellipsoid at
periapse
can noticeably change the outcome. The size and orbital
distribution
of the ejecta are discussed, along with the applications of this
technique towards an understanding of doublet craters, crater
chains, and asteroids with peculiar shapes and spins. (C) 1998
Academic Press
==================
(10) SIMULATING ASTEROIDAL IMPACTS
I. Giblin*), G. Martelli, P. Farinella, P. Paolicchi, M.
DiMartino, P.N. Smith: The properties of fragments from
catastrophic
disruption events. ICARUS, 1998, Vol.134, No.1, pp.77-112
*) UNIVERSITY OF PISA, DIPARTIMENTO MATEMAT,I-56100 PISA,ITALY
We report and discuss the results from a series of catastrophic
disruption experiments involving 21-cm spherical targets of
alumina
cement. These experiments were performed in the open air using a
contact charge technique to simulate an impact at similar to 6
km/s,
typical of collision velocities between asteroids in the main
belt.
The 1992 experiments reported here, the most recent in an
extensive
experimental program initiated by Giuseppe Martelli before his
death
in 1994, follow directly from those described in I. Giblin et al.
(1994a, Icarus 110, 203-224), with a number of improvements to
our
instrumentation and analysis. By using two high-speed cameras at
a
mutual angle of 60 degrees we have made possible a
three-dimensional
analysis of fragment velocities alongside the standard size,
shape,
ejection angle and rotation rate measurements which can easily be
made from appropriately oriented single film records. In this
paper
we report on the results of the 1992 experiments, together with
various unpublished data from 1989. We make a comparison between
these sets of data and between our data and those of other
researchers in this field. Also, we compare our results to those
of
the most recent semi-empirical model (SEM) of P, Paolicchi et al.
(1996, Icarus 121, 126-157) and to appropriate data concerning
real
asteroids, focusing on the dynamical families, which are believed
to
be remnants from the catastrophic disruption of precursor
asteroids.
A secondary purpose of this paper is to document our experiences
and
techniques in the implementation and analysis of these
experiments.
We find considerable variation in the slope of the fragment size
distribution, even between closely similar experiments. Fragments
are found to be slightly flatter and/or more elongated than those
from some previous work, but in agreement with the previous study
mentioned above. Fragment velocities are generally between 4 and
20
m/s with a few fast fragments observed up to 35 m/s. Only a weak
correlation is found in either linear or angular velocity versus
mass. We also test for the existence of a possible ''radiant
point''
from which fragment velocities approximately originate and find
that
although it serves as a useful component in a model of the
break-up,
the location of such a point is not constant in our experiments.
(C)
1998 Academic Press
============
(11) COMPOSITION OF JETS IN ASYMETRIC IMPACTS
G.H. Miller: Jetting in oblique, asymmetric impacts. ICARUS,
1998,
Vol.134, No.1, pp.163-175
UNIVERSITY OF CHICAGO, DEPT GEOPHYS SCI, SHOCK WAVE LAB, 5734
S ELLIS AVE, CHICAGO, IL, 60637
Experiments were conducted to determine the composition of jets
emitted in asymmetric oblique impacts. The jets were sampled
indirectly using witness plates, which were quantitatively
analyzed
by SEM, The results indicate that decreasing the thickness of the
inclined target plate increases the relative abundance of
projectile
in the jet. Increasing the impact angle has the same effect. No
systematic dependence of jet composition on impact velocity was
observed, Thin plate theory, variations of which have been
applied
to several problems in planetary geophysics, gives the jet
composition as a function of plate thickness and impact angle,
This
theory gives compositional dependencies on plate thickness and
impact angle that are opposite to the experimentally observed
trends
and predicts compositional abundances that differ in magnitude
from
experimental values by over 40 wt% in all cases. A critical
review
of the theory, which is extended to explicitly include shock
entropy
production, and a comparison of the theory with hydrocode
simulations, reveals several assumptions that are not valid in
the
present application. The revised thermodynamic model predicts
peak
jet temperatures that are significantly lower than calculated
using
earlier models. If the experimentally observed compositional
trends
apply to the collisions of meteoroids with planets, then the
relative abundance of meteoroid in the jet will be greater in
oblique impacts than in normal-incidence ones. (C) 1998 Academic
Press
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