PLEASE NOTE:
*
CCNet DIGEST, 14 May 1999
-------------------------
QUOTE OF THE DAY
"Is this a case of science gone
wrong? Not at all, according to
most astronomers. If Marsden hadn't
shared his initial prediction
with the rest of the scientific
community, the JPL folks would
have had no reason to look for the
asteroid in their records.
Perhaps the only mistake was in
immediately announcing the finding
to the public. But astronomers are, for
the most part, open with
their data. The most glaring error of
the movies Deep Impact and
Armageddon is the ease with which the
government and the
scientific community keep the
probability of a global catastrophe
secret."
(1) EXPLORATORIUM MAGAZINE
Michael Paine <mpaine@tpgi.com.au>
(2) THE SKY IS FALLING
EXPLORATORIUM MAGAZINE ONLINE, Volume 22,
Number 3, Spring 1999
(3) NEAs: REMNANTS OF LARGE ASTEROID COLLISIONS
Andrew Yee <ayee@nova.astro.utoronto.ca>
(4) A THREAT FROM SPACE? A DISCUSSION OF NEAR-EARTH OBJECTS
IMPACT workshop <workshop@to.astro.it>
(5) NEW UK TELESCOPE WILL SURVEY SOUTHERN SKIES
Jacqueline Mitton <jmitton@dial.pipex.com>
(6) EJECTA PATTERN OF THE IMPACT OF COMET SHOEMAKER-LEVY 9
A.A. Pankine*) and A.P. Ingersoll, CALTECH
(7) HIGH TEMPERATURES IN RETURNING EJECTA FROM THE R IMPACT OF
COMET SL9
S.J. Kim et al., KYUNGHEE UNIVERSITY
(8) IMPACT EROSION OF PLANETARY ATMOSPHERES
W.I. Newman et al., UNIVERSITY OF CALIFORINIA
(9) COLLIDE: COLLISIONS INTO DUST EXPERIMENTS
J.E. Colwell*) and M. Taylor, UNIVERSITY OF
COLORADO
(10) A STUDY OF HILDA ASTEROIDS
M. Dahlgren, J.F. Lahulla, C.I.
Lagerkvist, ASTRONOMICAL OBSERVATORY UPPSALA
(11) HISTORICAL OBSERVATIONS OF THE LEONID METEOR SHOWER
P. Brown, UNIVERSITY OF WESTERN ONTARIO
============
(1) EXPLORATORIUM MAGAZINE
From Michael Paine <mpaine@tpgi.com.au>
Dear Benny,
The popular science website Exploratorium has a new article on
the NEO
hazard at http://www.exploratorium.edu/exploring/index.html.
It
includes audio quotes from Dr. Eleanor Helin.
Michael Paine
The Planetary Society Australian Volunteers
-----------------
(2) THE SKY IS FALLING: SOMETIMES WE CAN LEARN ABOUT OUTER SPACE
WHEN SPACE
OBJECTS COME TO US.
From EXPLORATORIUM MAGAZINE ONLINE, Volume 22, Number 3, Spring
1999
http://www.exploratorium.edu/exploring/index.html
By Mary K. Miller
By early October, backyard astronomers are picking up the
approaching
asteroid with their telescopes. The mile-wide chunk of space rock
is
on a collision course with the earth. On October 26, it's the
brightest
object in the sky, save for the moon. Traveling at nine miles per
second, the asteroid enters the earth's atmosphere on October 28.
Friction from the atmosphere heats the surface of the object to
more
than 3,000 degrees Fahrenheit. As the object sails through the
atmosphere, it produces a streak of light that we call a meteor
or
"shooting star." About ninety miles above the earth,
the extreme heat
melts the meteor's hard shell and it bursts into a crackling,
hissing
fireball.
By chance, ground zero is the Canadian prairie, and the meteorite
(as
it's now called) smacks into the earth with the force of nearly
one
million megatons of TNT, carving out a crater ten miles across
and
several thousand feet deep. The enormous force of the impact
momentarily compresses the ground, which springs back like a
rubber
handball, releasing a catastrophic explosion of dust, melted
soil, and
gases. The energy released from the impact heats the atmosphere,
and
grasslands and forests as far south as the equator burn to the
ground
within hours. Most of the meteorite is vaporized, and dust from
the
impact is thrown into the stratosphere, blocking the sun and
dropping
global temperatures about 18 degrees Fahrenheit. Photosynthesis
is
virtually halted; plants wither, and the animals that depend on
them
perish. For up to a year, farming is futile and mass
starvation sets
in. Cockroaches, rats, and other detritus-feeders survive and
prosper
on the decaying plants and animals.
Sound like the doomsday script of the newest Hollywood
blockbuster?
It's actually a probable scenario built upon the prediction that
a
cosmic collision may be in our near future. On March 11, 1998,
Brian G.
Marsden of the Minor Planet Center at the Harvard-Smithsonian
International Astronomical Union made that startling
announcement. In a
press release, he reported that in the year 2028, an asteroid
called
1997 XF11, discovered by Jim Scott of the Spacewatch program at
the
University of Arizona, would pass within 30,000 miles of the
earth, the
width of a cat's whisker in astronomical terms. The preliminary
orbital
calculation was based on observations made by three additional
astronomers, and Dr. Marsden stated that, although the chance of
a
collision with earth was small, it was not out of the question.
In his
announcement, Marsden, a respected expert in the field, appealed
to the
astronomical community to search their photographic archives for
more
observations of the asteroid, with the hope of determining the
object's
exact orbit.
In the heels of this sensational announcement, a firestorm
erupted in
the press (no doubt delighting the makers of two Hollywood movies
about
killer asteroids and comets due out later in the year). But the
announcement also prompted Eleanor Helin and her colleagues at
the Jet
Propulsion Lab to search their database of images. Within a day,
the
JPL team found photographs of 1977 XF11 dating from 1990. This
allowed
astronomers to recalculate the asteroids orbit and announce that
it
would pass at a comfortable distance from the earth -- about
500,000
miles, or twice the distance to the moon.
Is this a case of science gone wrong? Not at all, according to
most
astronomers. If Marsden hadn't shared his initial prediction with
the
rest of the scientific community, the JPL folks would have had no
reason to look for the asteroid in their records. Perhaps the
only
mistake was in immediately announcing the finding to the public.
But
astronomers are, for the most part, open with their data. The
most
glaring error of the movies Deep Impact -- about a planet-killing
comet
-- and Armageddon -- whose villain is an asteroid "the size
of Texas"
-- is the ease with which the government and the scientific
community
keep the probability of a global catastrophe secret. For one
thing,
there are all those students and backyard astronomers constantly
watching the skies. To date, about 20 percent of asteroids have
been
discovered by amateurs.
Despite what happens in the Hollywood movies, the chances of a
globe-threatening asteroid or comet hitting the earth are
increasingly
small. But not impossible. Evidence suggests that 65 million
years
ago the earth was walloped by a ten-mile-wide meteorite. A layer
of
iridium, an element that's rare on the earth's surface but common
in
some asteroids, left a fingerprint in the geologic record at
about the
same time the dinosaurs disappeared from the planet. Scientists
have
found what they think is the impact site along the Yucatán
Peninsula of
Mexico, a circular hole over a hundred miles wide that's now
buried by
sedimentary rock.
Scientists estimate that monster meteorites, those above a mile
in
width, hit only once every 300,000 years. A ten-mile-wide
impactor,
like the dino-killer, is rarer still, striking the earth on the
average
of once every 100 million to one billion years. More frequent are
smaller objects, such as the 180-foot rock that exploded over
Tunguska,
Siberia, in 1908, flattening trees for about 15 miles in all
directions. Objects of this size invade our atmosphere every few
centuries on average, causing extensive local damage. If they hit
the
ground, they create large craters a mile or so in width. The most
common meteors, however, are created by dust particles that have
an
average size of about 0.2 millimeters. Most of them never make it
through the atmosphere intact. Instead, they melt and burn high
above
the earth in spectacular displays commonly known as shooting
stars.
By chemically analyzing meteorites, scientists can determine
their
makeup, which is primarily iron, nickel, and silicate rock.
Determining
the composition of a meteorite can help us infer the composition
of the
earth's interior. Analysis also reveals a meteorite's point of
origin
and its age. The oldest space rocks are about 4.6 billion years
old,
leading scientists to conclude that that's the age of the solar
system
as well.
Scientists theorize that collisions of astral bodies created the
planets in our solar system. After the sun formed, bits of space
dust
collided to form specks, then rocks, then boulders. As the space
objects got bigger, their gravitational pull attracted smaller
ones.
Eventually these "planetesimals" of rock, metal, and
ice gobbled up
most of the small meteoroids and became the familiar nine planets
of
our solar system. A swarm of asteroids, about 18,000, were left
in a
belt between Mars and Jupiter.
After the planets were formed, a process that may have taken
almost 100
million years, asteroids continued to pelt the earth, with a
violent
collision occurring as often as once a month. Over time, the
gravitational pull of the earth and moon swept up most of the
stray
asteroids in our part of the solar system; the impact rate today
is
therefore much less than it has been in the past.
Anyone with a decent pair of binoculars can see the violent
history of
our solar system. The near side of the moon is covered with more
than
30,000 craters, yet they're conspicuously rare on the earth's
surface.
In fact, only about 150 craters have been found scattered around
the
world. Weathering and the movement of the earth's crust have
covered up
or destroyed the earth's bombardment record. One impressive
exception
is Meteor Crater in Arizona, a bowl-shaped depression nearly a
mile in
diameter.
Physicist Peter Fiske and his colleagues at Lawrence Livermore
Labs in
northern California are tracking down a meteorite impact in
Southeast
Asia, although they haven't found the crater yet. What they have
found
is a profusion of "tektites," glassy orbs created when
soil melts from
the intense pressure of an impact. From the abundance,
characteristics,
and location of these tektites, Fiske and his colleagues believe
the
impact of a mile-wide asteroid that formed them occurred
relatively
recently, about 770,000 years ago, the largest, most recent
impact
identified so far.
Because meteorite impacts have occurred so infrequently in recent
history, there aren't many clues for scientists to work with.
"Meteorite impact is the least well-understood geological
process we
know of," Fiske says. "Impacts have the potential to
cause massive
damage to the earth and life on it. But we really don't
understand the
details." He hopes to get at those details: How much hot
debris is
ejected from a large impact? How much forest is ignited? Do
medium-sized impacts (smaller than the dino-killer) cause
regional but
not global extinctions?
Sooner or later, history will repeat itself and a
globe-threatening
impactor will target our planet. There are programs in place to
monitor the orbits of known asteroids and comets, and more
near-earth
objects are being discovered by amateur astronomers and the pros
every
year. In the best of circumstances, we could have decades,
perhaps a
century, of advance warning. But what would we do if a big one
was on a
collision course with earth? Doomsday movie scripts aside,
scientists
haven't yet figured out how to prevent global catastrophe.
Launching a
nuclear weapon could just break a meteoroid into pieces and
spread out,
not prevent, the damage. If we're lucky, we'll have enough time
to
come up with a solution.
------------
About Eleanor F. Helin: Dr. Helin has been a planetary scientist
and astronomer
for more than thirty years. She is the Principal Investigator for
the Near-Earth
Asteroid Tracking (NEAT) program, an author, and discoverer of 20
comets and
asteroids. To learn more about Dr. Helin, see her biography at
the Women in
Science and Technology website. Also, visit the NEAT Program
website for
extensive information on Dr. Helin's research.
© 1999, The Exploratorium
================
(3) NEAs: REMNANTS OF LARGE ASTEROID COLLISIONS
From Andrew Yee <ayee@nova.astro.utoronto.ca>
ESA Science News
http://sci.esa.int
11 May 1999
From Near Earth Asteroids to water on Mars -- astronomers review
latest ISO results
Near-Earth Asteroids -- asteroids whose orbits bring them close
to Earth -- very likely originate from collisions between larger
asteroids that orbit the Sun between the planets Mars and
Jupiter.
This result, obtained by ESA's infrared space telescope, ISO, was
presented yesterday at the workshop on ISO results on Solar
System,
held at ESA's Villafranca Satellite Tracking Station in Spain.
Other
findings related to the atmosphere of Mars and the giant planets
--
Saturn, Jupiter, Neptune and Uranus -- were also presented during
a press conference yesterday morning.
About 250 Near-Earth Asteroids (NEA) are known so far, with sizes
ranging from one to 40 kilometres in diameter. But according to
Alan
Harris, of the German Aerospace Centre (Berlin), there are more
than 2000 NEA not yet identified. Their origin is still unclear,
but
scientists are working with two main ideas: many probably
originate
from collisions among the larger bodies in the Main Belt of
asteroids,
between Mars and Jupiter, while others might be old, 'dead'
comets
that have undergone so many fly-bys of the Sun that they have
lost
all their coma -- the beautiful comet tail of dust and gas.
The infrared observations made by Harris with ISO provide
information about the surface and mineralogy of the NEAs, and
confirm that many are fragments of larger asteroids. ISO sees a
rocky surface without much residual dust; a dusty surface is what
would be expected if the NEA were 'dead comets'. Harris, however,
is still analyzing the data and has not yet ruled out the
possibility
of finding traces of dust in one of the asteroids observed, the
orbit
of which is very similar to that of a comet.
As Harris points out, knowledge on the composition of these
objects
is necessary to predict the consequences of a future impact on
the
Earth. "We still know just a few of them, and although the
odds that
any one will hit the Earth in the near future are very low,
there's a
possibility. Therefore, the more we know about these objects, the
better", he said.
The new data about Mars refer to the water in the atmosphere: ISO
sees that it condenses and freezes close to the planet's surface.
This result will help to understand the behaviour of the Martian
atmosphere.
"We got these observations while NASA's Mars Pathfinder
mission
was also studying the planet, and the results of both missions
are
in agreement. ISO confirms the Mars Pathfinder data", said
Therese
Encrenaz, of the Observatoire de Paris-Meudon.
Encrenaz also described the detection of new molecules in the
atmospheres of the giant planets, mainly hydrocarbons like the
radical CH3, never detected before, and benzene. Benzene is a
common molecule on Earth (in petrol for instance), but is
apparently less common on other planets.ISO's spectrometers see
it now in Saturn.
The discovery of water in the atmosphere of Saturn's largest
moon,
Titan, was also reviewed by Athena Coustenis of the Observatoire
de Paris-Meudon.
USEFUL LINKS FOR THIS STORY
ISO Science Homepage
http://www.iso.vilspa.esa.es/
ISO Data Centre
http://www.iso.vilspa.esa.es/users/idc/IDC.html
[NOTE: Images supporting this article are available at
http://sci.esa.int/story.cfm?TypeID=1&ContentID=4850&Storytype=18]
==============
(4) A THREAT FROM SPACE? A DISCUSSION OF NEAR-EARTH OBJECTS
From IMPACT workshop <workshop@to.astro.it>
The Planetary Society,
The Organizers and the Scientific & Organizing Committe of
the IMPACT
Workshop (International Monitoring Programs for Asteroid and
Comet Threat),
The Italian Association for Aeronautics and Astronautics
(AIDAA), and
The "Torino Incontra" Congress Center
present
A Threat from Space?
A Discussion of Near Earth Objects
Speakers:
Andrea Carusi, President of the Spaceguard Foundation
Marcello Coradini, European Space Agency
David Morrison, Director of Space at Nasa Ames Research Center
Moderator:
Louis Friedman, Executive Director of the Planetary Society
Wednesday, June 2, 1999
8:30 PM
Centro Congressi Torino Incontra
Via Nino Costa 8
Torino, Italy
Asteroids and comets have played a very important role in the
collisional history of the solar system. Just five years ago
Comet
Shoemaker-Levy 9 collided with Jupiter, giving us a timely
reminder of
the presence of these objects in Earth's vicinity. Many times
Near-Earth objects have collided with our planet in the past with
devastating results, probably resulting in the extinction of the
dinosaurs 65 million years ago. What are the odds of
collision for Earth in the future?
"A Threat from Space? A Discussion of Near Earth
Objects" will examine
the role NEOs have played in Earth's history, the observation
programs
designed to find and track them, the potential threat NEOs could
pose
to our future, and the exploration of these objects by
spacecraft. The
present limitations in our detection capabilities, the need of
collecting
essential physical data (objects' sizes and compositions), and
the role
that space-born instruments can eventually play in the near
future will
also be discussed.
==============
(5) NEW UK TELESCOPE WILL SURVEY SOUTHERN SKIES
From Jacqueline Mitton <jmitton@dial.pipex.com>
ROYAL ASTRONOMICAL SOCIETY
PRESS NOTICE
Date: 14 May 1999
For immediate release
Ref. PN 99/13
Issued by:
Dr Jacqueline Mitton
RAS Press Officer
Office & home phone: Cambridge ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892
E-mail: jmitton@dial.pipex.com
RAS Web: http://www.ras.org.uk/ras/
* * * * * * * * * * * * * * * * * * * * * * * * *
CONTACT FOR FURTHER INFORMATION ON THIS RELEASE:
Dr Jim Emerson (Queen Mary and Westfield College, University of
London)
Phone: (+44) (0)171 975 5040
email: j.p.emerson@qmw.ac.uk
* * * * * * * * * * * * * * * * * * * * * * * * *
NEW UK TELESCOPE WILL SURVEY SOUTHERN SKIES
The quest for answers to outstanding mysteries in our universe
received a
boost this week when British astronomers learned that their
proposal for a
special new telescope to survey southern skies has been approved.
VISTA -
the Visible and Infrared Survey Telescope for Astronomy - is
different from
most ordinary large telescopes, which can only look at one very
small patch
of sky at a time. It will survey large areas of sky for very
faint stars and
galaxies. In a single exposure of only 10 minutes, it will be
able capture
the images of 100,000 objects.
Over time, data from the telescope will become a fundamental
source of
reference on the contents and layout of our own Galaxy and the
myriads of
galaxies that populate the universe, rather like a combination of
a detailed
map and a census. VISTA will conduct ground-breaking surveys
using infrared
detectors as well as looking at visible light. Infrared
observations are
crucial for studying newly forming stars and planetary systems,
and the most
distant galaxies.
The proposal to build the 4-metre-diameter (13-foot) telescope
came from a
consortium of 18 UK universities, led by Dr Jim Emerson of the
Physics
Department at Queen Mary and Westfield College (University of
London). They
hope to locate the telescope at one of the observatory sites in
the high
Atacama desert in the foothills of the Chilean Andes, where sky
quality for
astronomy is outstanding. It will be a significant advance over
existing
facilities for surveys of the southern sky, with its large
mirror, wide
field of view, sharp images and sensitive detectors for light and
infrared
radiation.
"We are thrilled to win funding," said Dr Emerson.
"Astronomy is one of the
UK's great scientific strengths and VISTA will help keep the UK
in the
forefront of research. It will help us to unravel the riddles of
how the
universe works by providing British astronomers with the tool
needed for
cutting-edge survey astronomy for years to come. It will truly
provide us
with a new vista on the universe surrounding us, both near and
far."
The southern skies provide a particularly fruitful area for
research as
examples of many types of objects outside our Galaxy are best
seen from the
southern hemisphere. These are some of the science survey
programmes VISTA
will undertake:
- The most distant objects in our Universe are very faint and
very red:
VISTA can observe these to understand both the origins of
galaxies and the
fate of the Universe.
- Vast clouds of dust that blot out visible light limit the
optical view of
our own galaxy. VISTA will see through most of this dust in the
infrared to
map out the content of the Galaxy.
- Stars, such as our own Sun, form surrounded by dust that blots
out their
visible light. VISTA's infrared surveys will probe through this
dust to show
where and how these stars form.
- Closer to home VISTA will find many small bodies in the outer
Solar
System, allowing us to study the pristine pieces of material out
of which it
formed.
Whenever astronomers build a new and different telescope, some of
the most
exciting discoveries are totally unexpected. VISTA will probably
be no
exception.
VISTA will be a national facility for the UK. It should be ready
for use by
the beginning of 2004. The cost is expected to be about 25
million pounds.
Funding of the project was announced on 10 May. It was among the
first
allocations from the 700 million pounds being provided by the
'Joint
Infrastructure Fund' (JIF), an initiative of the UK Government's
Department
of Trade and Industry, the Wellcome Trust, and the Higher
Education Funding
Council for England. JIF is to enable UK universities to invest
in new
facilities and equipment that will underpin basic research
projects and
ensure British Universities remain at the forefront of
international
scientific research.
Universities in the Consortium are (in alphabetical order)
Birmingham,
Cambridge, Cardiff, Durham, Edinburgh, Hertfordshire, Keele,
Central
Lancashire, Leicester, Liverpool John Moores, Nottingham, Oxford,
Queen Mary
& Westfield College, Queen's University Belfast, St Andrews,
Southampton,
Sussex, University College London.
==============
(6) EJECTA PATTERN OF THE IMPACT OF COMET SHOEMAKER-LEVY 9
A.A. Pankine*) and A.P. Ingersoll: Ejecta pattern of the impact
of
Comet Shoemaker-Levy 9. ICARUS, 1999, Vol.138, No.2, pp.157-163
*) CALTECH, DIV GEOL & PLANETARY SCI, MS 150-21, PASADENA,
CA, 91125
The collision of Comet Shoemaker-Levy 9 (SL 9) with Jupiter
created
crescent-shaped ejecta patterns around impact sites. Although the
observed impact plumes rose through a similar height of similar
to 3000
km, the radii of the created ejecta patterns differ from impact
to
impact and generally are larger for larger impacts. The azimuthal
angle
of the symmetry axis of the ejecta pattern is larger than that
predicted by the models of oblique impacts, due to the action of
the
Coriolis force that rotates ejecta patterns counterclockwise from
the
south. We study the formation of ejecta patterns using a simple
model
of ballistic plume above a rotating plane. The ejected particles
follow
ballistic trajectories and slide horizontally for about an hour
after
reentry into the jovian atmosphere. The lateral expansion of the
plume
is stopped by the friction force, which is assumed to be
proportional
to the square of the horizontal velocity. Two different
mass-velocity
distributions used in the simulations produce qualitatively
similar
results. The simulated ejecta patterns fit very well the
'crescents'
observed at the impact sites. The sizes and azimuthal angles of
symmetry axis of ejecta patterns depend on a parameter L, which
has
dimension of length and is related to the mass of the fragment.
Thus
more massive impacts produce larger ejecta patterns that are
rotated
through a wider angle, (C) 1999 Academic Press.
======================
(7) HIGH TEMPERATURES IN RETURNING EJECTA FROM THE R IMPACT OF
COMET SL9
S.J. Kim*), M. Ruiz, G.H. Rieke, M.J. Rieke, K. Zahnle: High
temperatures in returning ejecta from the R impact of comet SL9.
ICARUS, 1999, Vol.138, No.2, pp.164-172
*) KYUNGHEE UNIVERSITY, DEPT ASTRON & SPACE SCI, KYUNGGIDO
449701, SOUTH
KOREA
Observations from ground-based observatories and with the Galileo
spacecraft suggest that the flares from the SL9 impacts resulted
from
ejecta falling back onto Jupiter in ballistic plumes. This
explanation
is supported by comparing the plume height as a function of time
in HST
images with the flare light curve. We show that the rotational
temperature of CO in the shock from the R impact rose from less
than
2000 K near the beginning of the main flare to about 5000 K at
its end.
This behavior agrees with a simple physical model of ballistic
plumes
with a mean molecular weight indicating they are 50% or more
jovian
air, Alternate models involving formation of molecules at the
original
impact site, or formation of dust grains to initiate the hare,
are
inconsistent with these measurements. The energy is emitted
primarily
as a hot continuum, supporting the possibility that finely
divided dust
grains are heated in the reentry shock and emit to create the
flare.
Scaling such models to the energy of the KIT event supports
proposals
that ballistic plumes were responsible for the global disaster
associated with it, (C) 1999 Academic Press.
==================
(8) IMPACT EROSION OF PLANETARY ATMOSPHERES
W.I. Newman*), E.M.D. Symbalisty, T.J. Ahrens, E.M. Jones: Impact
erosion of planetary atmospheres: Some surprising results.
ICARUS,
1999, Vol.138, No.2, pp.224-240
*) UNIVERSITY OF CALIFORINIA LOS ANGELES, DEPT EARTH & SPACE
SCI, LOS
ANGELES, CA, 90095
We have investigated by analytical and computational means the
effect
of Cretaceous-Tertiary (K/T) size impacts (5 x 10(30) erg,
9-km-radius
bolide of 10(19) g) on terrestrial atmospheres. We have extended
analytically the approximate solution due to A. S, Kompaneets
(1960,
Sov. Phys, Dokl, Engl. Transl, 5, 46-48) for the blast wave
obtained
for atmospheric nuclear explosions (idealized to isothermal
atmospheres) to ideal adiabatic atmospheres and to data-based
models of
the Earth's atmosphere. For the first time, we have been able to
obtain
analytically the particle trajectories in an isothermal
atmosphere. The
outcome of this nonlinear analysis is that a massive impact
(without
the subsequent ejection of substantial mass) would only influence
a
column of approximate to 30-km radius in the Earth's atmosphere
and
that the shocked gas would be propelled up and against the column
''wall,'' but would not escape from the planet. We examined the
validity of ''hemispheric blowoff,'' the hypothesis that all
material
in a hemisphere lying above a plane tangent to the point of
impact
radially accelerated outward and, if sufficiently energetic,
would also
be ejected. We adapted and used a state-of-the-art code (CAVEAT),
a
hybrid Los Alamos-Sandia Lagrangian-Eulerian finite difference
scheme
for multimaterial flow problems with large distortion and
internal
slip. In our CAVEAT calculations, the vapor cloud produced by the
impact produces a shock that is orders of magnitude stronger than
any
previous use of such codes. We developed new methods to test the
accuracy and convergence of CAVEAT for KIT size impact events,
and it
proved to be a robust tool. We explored a KIT size impact where
the 9-
km-radius bolide was vaporized and injected into the atmosphere
and
found no radial outflow in agreement with the analytic model but,
instead, a 50-km-radius vertical column formed with only a small
fraction of material reaching escape velocity-no more than about
7% of
the vaporized bolide plus atmospheric mass will escape the
gravitation
of the Earth. (C) 1999 Academic Press.
==================
(9) COLLIDE: COLLISIONS INTO DUST EXPERIMENTS
J.E. Colwell*) and M. Taylor: Low-velocity microgravity impact
experiments into simulated regolith. ICARUS, 1999, Vol.138, No.2,
pp.241-248
*) UNIVERSITY OF COLORADO, ATMOSPHER & SPACE PHYS LAB, CAMPUS
BOX
392,BOULDER,CO,80309
We describe the Collisions Into Dust Experiment (COLLIDE), a
microgravity experiment to study the outcome of very low velocity
(1-100 cm/sec) collisions into a simulated regolith, The
experiment was
flown on the Space Shuttle in April 1998 and included six
independent
impact experiments into a lunar soil simulant, Results of these
impact
experiments showed very little dust ejecta was produced in the
impacts,
which were highly inelastic. We present normal and tangential
coefficients of restitution for impacts into simulated regolith
at
speeds between 15 and 90 cm/sec. We find normal coefficients of
restitution of 2 to 3%, significantly lower than is suggested by
extrapolation from low-velocity impact experiments with ice, but
consistent with higher velocity impacts into powder. Our
microgravity
experiment confirms the ground-based result of Hartmann (1978,
Icarus
33, 50-62) that the presence of a regolith increases the
efficiency of
planetary accretion. The absence of dust ejecta suggests higher
energy
collisions are necessary to release the dust observed in
planetary
rings. (C) 1999 Academic Press.
=============
(10) A STUDY OF HILDA ASTEROIDS
M. Dahlgren, J..F. Lahulla, C.I. Lagerkvist: A study of Hilda
asteroids
- VI. Analysis of the lightcurve properties. ICARUS, 1999,
Vol.138,
No.2, pp.259-267
*) ASTRONOMICAL OBSERVATORY,BOX 515,S-75120 UPPSALA,SWEDEN
The lightcurves of 47 Hilda asteroids presented in Dahlgren et
al,
(1998, Icarus 133, 247-285) have been analyzed. The Hilda
population
has mean lightcurve amplitudes significantly larger than those of
main-belt asteroids of similar sizes, This is due to the very
large
mean amplitudes (0.31 mag) of the small (D < 60 km) D-type
Hilda
asteroids, The spin rate distribution of the Hilda asteroids is
markedly non-Maxwellian, mainly due to a large fraction of slowly
rotating objects, More than half of the observed population have
rotation periods longer than 12 h, The spin rate versus size
diagram
has a U-shape with slow spin rates at intermediate sizes. At
smaller
sizes (D < 60 km) where the Hilda population is totally
dominated by
D-type objects (Dahlgren and Lagerkvist 1995, Astron, Astrophys,
302,
907-914; Dahlgren et al, 1997, Astron. Astrophys, 323, 606-619)
there
is a large increase in the scatter of the spin rates, and an
increase
of the mean amplitudes of these (D-type) Hilda asteroids, This
strongly
suggests a connection between taxonomic type and collisional
evolution
among objects in the Hilda population, Possible interpretations
of
these results are discussed in terms of an early heating event in
the
Solar System and subsequent collisional evolution of the Hilda
population, (C) 1999 Academic Press.
=============
(11) HISTORICAL OBSERVATIONS OF THE LEONID METEOR SHOWER
P. Brown: The Leonid meteor shower: Historical visual
observations
ICARUS, 1999, Vol.138, No.2, pp.287-308
UNIVERSITY OF WESTERN ONTARIO,DEPT PHYS & ASTRON, LONDON, ON
N6A 3K7,
CANADA
The original visual accounts of the Leonids from 1799 to 1997 are
examined and the times and magnitude of peak activity are
established
for 32 Leonid returns during this two-century interval, Previous
secondary accounts of many of these returns are shown to differ
from
the information contained in the original accounts due to
misinterpretations, typographical errors, and unsupported
assumptions.
The strongest Leonid storms are shown to follow a Gaussian
activity
profile and to occur after the perihelion passage and nodal
longitude
of 55P/Tempel-Tuttle. The relationship between the Gaussian width
of
the strongest returns and their peak activity is established, and
the
particle density/stream width relationship is found to compare
favorably to that expected based on observations of IRAS cometary
dust
trails. Variations in the width of the 1966 storm as a function
of
meteoroid mass are shown to be consistent with that expected from
classical gas-drag meteoroid ejection treatments. The five
largest
storms from 1799 to 1966 are found to peak at solar longitudes
systematically larger than 55P/Tempel-Tuttle's nodal longitude at
the
same epochs, suggesting an asymmetry in the dust ejection
perpendicular
to the cometary orbital plane. The dust-distribution about
55P/Tempel-Tuttle is reevaluated with these new data and
predictions
are made for the 1999-2000 showers, (C) 1999 Academic Press.
----------------------------------------
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*
LETTERS TO THE MODERATOR, 14 May 1999
-------------------------------------
(1) THE MOON MAKERS, OR WAS PLANETARY DEFENSE INVENTED 90 YEARS
AGO?
Sir Arthur Clarke, Sri Lanka
(2) UK PARLIAMENTARY REPORT ON NEOs
Duncan Steel <dis@star.arm.ac.uk>
(3) SPACE POWER THEORY
James Oberg <JamesOberg@aol.com>
==========
(1) THE MOON MAKERS, OR WAS PLANETARY DEFENSE INVENTED IN 1917?
From Sir Arthur Clarke, Sri Lanka
Dear Benny and Jay,
I have to pass on this fascinating and
astonishing item from space
artist Roy Miller...
>From: "Ron Miller" <rmiller@crosslink.net>
>To: "Arthur C Clarke"
>Date: Fri, 14 May 1999 07:54:09 +0000
>I just read Hammer of God this past week (sorry I didn't get
to it
>sooner! I almost always read your books as soon as they come
>out--don't know what happened with this one . . . ). Enjoyed
it very
>much, of course. I suppose I don't have to tell you about the
>imminent-asteroid-collision novel, The Moon Makers, by Arthur
Train
>and Robert Wood? In it a nuclear-powered space craft makes a
>rendezvous with an asteroid named Medusa, which is on a
collision
>course with the earth. By setting off a small, continuous
nuclear
>reaction on its surface, the astronauts give the asteroid
just enough
>sideways thrust that, several million miles later, it misses
the
>earth. I find it fascinating that this was written in 1917.
Completely new to me - but I can add info about Wood - one of the
most
celebrated scientists of his day, famous for his work in
spectroscopy -
and for exposing the 'N-rays' nonsense, which destroyed the
reputation
of French science for a decade.
Quite an uncanny likeness to the cold-fusion affair - though in
this
case I believe/hope the outcome will be different.
I've just learned somthing that's made my day - the translator of
2001
into Hungarian (printed in 1973) is now the President of the
country!!
Will have to send him the handsome Millennium edition that's
being
planned....
All best,
Arthur 14 May 99
=================
(2) UK PARLIAMENTARY REPORT ON NEOs
From Duncan Steel <dis@star.arm.ac.uk>
Dear Benny,
Readers of this list will undoubtdely be interested to see the
16-page
report on NEOs recently published by the (U.K.) Parliamentary
Office of
Science and Technology (POST). Go
to:
http://www.parliament.uk/post/reports.htm
and click on the link for the specific report (POST Report
number 126, April 1999).
Duncan Steel
============
(3) SPACE POWER THEORY
From James Oberg <JamesOberg@aol.com>
My new book on "Space Power Theory", commissioned by
the US Space
Command (and hence not subject to copyright) is now online at
http://www.spacecom.af.mil/usspace/SPT/overview.htm
for your entertainment and provocation.
-----------------
CCNet-LETTERS is the discussion forum of the Cambridge-Conference
Network. Contributions to the on-going debate about near-Earth
objects,
the cosmic environment of our planet and how to deal with it are
welcome. To subscribe or unsubscribe from CCNet-LETTERS, please
contact
Benny J Peiser at <b.j.peiser@livjm.ac.uk>.
The fully indexed archive
of the CCNet, from February 1997 on, can be found at
http://abob.libs.uga.edu/bobk/cccmenu.html
*
POST-LATE HEAVY BOMBARDMENT
ASTEROID/COMET IMPACT RATES & OCEANIC
MEGA-IMPACTS: IMPLICATIONS FOR EPISODIC CRUSTAL EVOLUTION AND
PLATE
TECTONICS
Andrew Glikson (05/14/99) <geospectral@spirit.com.au>