PLEASE NOTE:
*
Date sent: Fri, 20 Mar 1998 13:12:05 -0500 (EST)
From: Benny J Peiser B.J.PEISER@livjm.ac.uk
Subject: CCD II/20 March 1998
To: cambridge-conference@livjm.ac.uk
Priority: NORMAL
CC DIGEST, 20 March 1998 - late edition before the weekend
----------------------------------------------------------
(1) NINE REASONS WHY ASTEROID 1997 XF11 CHANGED ITS MIND
Darryl Pitt dpitt@interport.net
(2) GENE SHOEMAKER MIS-QUOTED?
Duncan Steel dis@a011.aone.net.au
(3) APOCALYPSE POSTPONED
NEW SCIENTIST, 20 March 1998
==================================
(1) NINE REASONS WHY ASTEROID 1997 XF11 CHANGED ITS MIND
From: Darryl Pitt dpitt@interport.net
in the op-ed section of this past wednesday's new york times,
jesse
gordon enumerated--with illustrations of a comet zig-zagging
every
different way--the "9 reasons that asteroid 1997 xf11
changed its
mind" as follows:
--human race will destroy the planet anyway
--dinosaurs were more fun to make extinct
--healthier to collide with a nonsmoking planet
--six points on asteroid driver's license
--worried about being subpoenaed by the independent counsel
--doesn't know...can't commit to anything lately...is considering
therapy
--felt humans had a negative attitude toward visit
--jury duty
--no desire to spend the rest of its days in the lobby of the
museum of
natural history
======================
(2) GENE SHOEMAKER MIS-QUOTED?
From: Duncan Steel dis@a011.aone.net.au
Dear Benny,
In that interesting article from the NY Times, Gene Shoemaker is
quoted
as saying:
>If you do the maneuver 10 to 20 years before the predicted
collision,
>then the impulse that must be delivered to the asteroid --
the change
>in velocity -- is only of the order of one or two centimeters
a year,
>to deviate a center hit on Earth to a clean miss.
In fact the required speed change required to get a potential
impactor to
miss, given a lead time of decades, is one or two centimeters PER
SECOND.
This is easy to see. Let the lead time be 20 years and the
impulse be
such that the change in speed is 1 cm/sec. Then the difference in
distance moved between the unperturbed (by our intervention) and
the
perturbed (we kicked it) orbit is 20 x 365.25 x 24 x 60 x 60 x
0.01
metres which equals 6312 kilometres, near as dammit. The Earth's
radius
is 6371 km. Thus an induced speed change of just over 1 cm/sec
would be
sufficient to get an asteroid whose path w a s due to pass
through
the centre of the Earth to miss our planet: just.
I don't know, of course, whether Gene was mis-quoted, or perhaps
he
mis-spoke himself. Easily done. Gene undoubtedly knew the impulse
required (which is still small!) but may have slipped (although
I'd
need to hear the tape to be convinced of that!).
Another part of the article states:
>As a result of the Shoemakers' search and a handful of
others', about
>1,500 Earth-crossing asteroids five-eighths of a mile or more
in
>diameter are known and their orbits are plotted. Dr.
Shoemaker believed
>there could be three times as many. Each is capable of taking
out a city,
>a country, or much more.
That is nonsense. The known population of Earth-crossers larger
than 1
km (which is what that 'five-eighths of a mile' derives from) is
ten
times less, only about 150, especially at the time of the
interview in
1996. This is undoubtedly an error on the part of the journalist,
not
Gene. And Gene knew that we had so far found only a b o u t 5 to
10%
of the Earth-crossers above that canonical 1 km size. On the
other
hand, there c o u l d be three times 1,500 at sizes >1 km
needing
discovery, although most estimates are lower (1500-3500, say;
depends
on what sorts of albedos you think they've got). And the damage
potential of 1 km asteroid is rather understated in the final
sentence
quoted above, and again I must ascribe the error to the
journalist.
Gene knew better.
Regarding:
>He discussed other ways of accomplishing
>this, such as using conventional explosives.
Again, I'd like to hear the tape. Gene knew damn well that
there's no
point in using 'conventional explosives'. You would do as well
using
just lead in a hypervelocity impact, with less chance of an
accident
along the way! At a speed of 3 km/sec a kg of lead (or
a n y t h i n g) has the same kinetic energy as the chemical
energy of
a kg of TNT. So there's no point in using conventional
explosives. One
needs things with high specific energies, and that implies
nuclear.
Duncan Steel
=========================================
(3) APOCALYPSE POSTPONED
From: NEW SCIENTIST, 20 March 1998
After a day-long drama in which it seemed there was an outside
chance that civilisation might end 30 years from now with a
catastrophic asteroid impact, astronomers declared the all clear
last Thursday. Revised calculations based on data from 1990 show
that
on 26 October 2028 asteroid 1997 XF11 should miss the Earth by
960 000
kilometres--2·5 times farther away than the Moon.
1997 XF11 was discovered by Jim Scotti of the University of
Arizona in
Tucson on 6 December last year. Gareth Williams and Brian Marsden
of
the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts, soon added it to their list of "potentially
hazardous
objects" which might hit the Earth.
Initial calculations indicated that 1997 XF11 would miss the
Earth by
about 800 000 kilometres in 2028. But the inclusion of
observations
made on 3 and 4 March showed 1997 XF11 skimming just 42 000
kilometres
above the Earth's surface--with a small chance that it would hit
us.
Marsden announced the bad news in an electronic circular sent out
on 11
March, trying to encourage more observations. He also asked
astronomers
to check their archives for any sightings from the previous
occasions
1997 XF11 came within viewing distance: in 1990, 1983, 1976, 1971
and
1957. "But we didn't think the chances were that large of
finding
something," says Dan Green, who works with Williams and
Marsden.
So the team was surprised when Eleanor Helin of the Jet
Propulsion
Laboratory (JPL) in Pasadena promptly unearthed images of the
1990
encounter on photographic plates. While Marsden talked with
reporters
who had arrived to cover the story on 12 March, Williams
recalculated
the orbit, and found the asteroid would miss the Earth by 960 000
kilometres. Don Yeomans of JPL has confirmed that result.
The impact of an asteroid the size of 1997 XF11, which is 2
kilometres
across, would release about half a million megatons of explosive
energy. That could devastate global agriculture, warns Scotti.
With sufficient warning, it could be possible to nudge an
asteroid
away by exploding nuclear weapons a kilometre or so above its
surface.
Identifying all the potential threats from outer space will
require a
ten-year, $50-million programme, says David Morrison of the NASA
Ames
Research Center in California. That's less than the budgets of
the two
asteroid disaster movies scheduled to reach the screen this
summer--but
the money has not yet been forthcoming.
Jeff Hecht, Boston
(c) 1998 New Scientist
--------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK
--------------------------------
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*
Date sent: Fri, 20 Mar 1998 09:08:13 -0500 (EST)
From: Benny J Peiser B.J.PEISER@livjm.ac.uk
Subject: CC DIGEST 20/03/98
To: cambridge-conference@livjm.ac.uk
Priority: NORMAL
CC DIGEST 20 March 1998
-----------------------
A man said to the universe:
"Sir, I exist!"
"However," replied the universe,
"The fact has not created in me
A sense of obligation."
(Stephen Crane, Collected Works VI)
(1) GENE SHOEMAKER'S VOICE IS MISSING, BUT HIS LEGACY IS FELT
Henry S.F. Cooper, The New York Times
(2) NASA'S TRANSITION REGION AND CORONAL EXPLORER MISSION SET TO
STUDY
THE SUN'S TURBULENT UPPER ATMOSPHERE
NASANews@hq.nasa.gov
(3) NATIONAL SPACE SOCIETY TEAMS WITH TIME MAGAZINE AND PARAMOUNT
PICTURES FOR IN-DEPTH EXPLORATION OF COMET THREAT
meteorobs@latrade.com
(4) WORKNOTES ON THE JOSHUA IMPACT EVENT
Ed Grondine epgrondine@hotmail.com
(5) MAKING A COMET NUCLEUS
J.M. Greenberg, LEIDEN UNIVERSITY
(6) THE RELEVANCE OF LABORATORY IMPACTS FOR THE STUDY OF COMETS
D. Koschny, MAX PLANCK INSTITUTE
===============================
(1) GENE SHOEMAKER'S VOICE IS MISSING, BUT HIS LEGACY IS FELT
From: Clark Whelton cwhelton@mindspring.com
Excerpted from the New York Times Science Section, Tuesday, March
17,
1998 page F4
Asteroid Expert's Voice is Missing, but His Legacy is Felt
by Henry S.F. Cooper
In all the discussion about Asteroid 1997 XF11 and its chances of
hitting Earth, and what to do about it, one voice badly missing
was Dr.
Eugene M. Shoemaker's. He was the geologist who first brought the
study
of rocks into space and whose work, first with impact craters and
then
with the asteroids and comets that blasted them, has made most
people
accept the notion -- revolutionary in the 1950s and 1960s -- that
big
rocks from space can make big holes in planets, with catastrophic
results.
The Palomar Asteroid Search, which he initiated in 1973, was the
first
systematic search for asteroids (and later, comets) whose
trajectories
crossed Earth's and might hit the planet.
Dr. Shoemaker was killed in an automobile accident in July near
Alice
Springs, Australia, where he and his wife, Carolyn Shoemaker, his
partner in the asteroid and comet search, were studying impact
craters.
In the spring of 1996, I spent a couple of weeks with the
Shoemakers
at their house in Flagstaff, Arizona, interviewing them for a
book I am
writing...
By good fortune, I happened to ask Dr. Shoemaker to unfold for me
the scenario of how we could deal with an asteroid one-kilometer
wide --
about five-eighths of a mile -- if we learned early enough that
it was going
to hit Earth. I taped his answer, so that in fact we are not
without
Dr. Shoemaker's voice on this subject after all. It is a soothing
and
gently persuasive voice, making whatever he said sound
reasonable,
however outlandish or devastating.
"If you could manage to get the nations of the world
organized, or
at least the lead nations, then you could think in terms of
sending up a
nuclear device," he said. "In fact you would send
several devices on
several separate spacecraft, with which you have stand-off
explosions that
would result in gentle pushes on the asteroid.
"We want to do the maneuver when it is near the Sun, near
the
asteroid's perihelion, to get the maximum effect. If you can do
it, you
can change the the semi-major axis of the orbit, you can change
its
period, so that the Earth is somewhere else when the object goes
by.
It turns out you don't have to push very hard. If you do the
maneuver
10 to 20 years before the predicted collision, then the impulse
that
must be delivered to the asteroid -- the change in velocity -- is
only
of the order of one or two centimeters a year, to deviate a
center hit
on Earth to a clean miss. Clearly you don't do just one big shot;
you
give it a series of smaller shots; you very carefully herd it
into the
new orbit that you want." He discussed other ways of
accomplishing
this, such as using conventional explosives.
"Whatever you do, you don't want to break it," he said.
"You push on it
gently -- that is why you do a stand-off explosion. If you break
it
up, then it becomes an uncontrolled problem. You wouldn't be able
to
herd it if you suddenly got pieces going off in various
directions --
it's out of control. You have multiple objects hitting the Earth,
and
they could actually cause more damage rather than less."
Although Dr. Gene Shoemaker is no longer on this planet, he,
along with
Carolyn Shoemaker and another observer, Dr. Eleanor Helin, may
have had
the last word on Asteroid 1997 XF11. Photographic plates that the
Shoemakers took in April 1990 as well as plates that Dr. Helin
took in
March 1990, which were re-examined last week, revealed extremely
faint
images of the asteroid on an earlier orbit. The pictures provided
a
longer arc of information on the asteroid's trajectory that had
been
available before, enabling astronomers at the Jet Propulsion
Laboratory
in Pasadena to determine with authority that in 2028 the comet
would
miss Earth by a distance very likely greater than that of Earth
from
the Moon.
As a result of the Shoemakers' search and a handful of others',
about
1,500 Earth-crossing asteroids five-eighths of a mile or more in
diameter are known and their orbits are plotted. Dr. Shoemaker
believed
there could be three times as many. Each is capable of taking out
a city, a
country, or much more. Following the Shoemaker-Levy impacts, an
obviously impressed Congress asked NASA to study what would be
needed
for a full survey, and a NASA committee led by Dr. Shoemaker
recommended a program called Spaceguard, requiring an initial
five-year
effort that would cost $24 million, followed by an additional $3
million a year to continue the search. The proposal was shelved.
Mrs. Shoemaker hopes the recent scare will revive Spaceguard. At
the
very least, expanding the library of plates might prevent future
false
alarms. And as surely as the Moon follows the Sun across the sky,
the
alarm some day will be real. (c) 1998 The New York Times.
========================
(2) NASA'S TRANSITION REGION AND CORONAL EXPLORER MISSION SET TO
STUDY
THE SUN'S TURBULENT UPPER ATMOSPHERE
From: NASANews@hq.nasa.gov
Donald Savage
Headquarters, Washington, DC March 19, 1998
(Phone: 202/358-1547)
Donna Drelick/Jim Sahli
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-8955)
RELEASE: 98-48
NASA'S TRANSITION REGION AND CORONAL EXPLORER MISSION SET TO
STUDY THE SUN'S TURBULENT UPPER ATMOSPHERE
NASA's Transition Region and Coronal Explorer (TRACE) mission,
scheduled for launch at 9:40 p.m. EST (6:40 p.m. PST) March 30,
1998,
will greatly improve understanding of events in the Sun's
atmosphere,
including intense storms and flares, which can have an impact on
power
and communications systems on Earth.
The TRACE mission will join a fleet of spacecraft studying the
Sun
during a critical period when solar activity is beginning its
rise to a
peak early in the new millennium. The Sun goes through an 11-year
cycle
from a period of numerous intense storms and sunspots to a period
of
relative calm and then back again. The coming months in the Sun's
cycle
will provide solar scientists with periods of strong solar
activity
interspersed with periods when the Sun is relatively passive and
quiet.
This will give TRACE the chance to study the full range of solar
conditions, even in its relatively short planned lifetime.
TRACE will train its powerful telescope on the dynamic so-called
'transition region' of the Sun's atmosphere, between the
relatively
cool surface and lower atmosphere of the Sun where temperatures
are
about 6,000 degrees Fahrenheit, and the extremely hot upper
atmosphere
called the corona, where temperatures are up to 16 million
degrees
Fahrenheit. Using instruments sensitive to extreme-ultraviolet
and
ultraviolet wavelengths of light, TRACE will study the detailed
connections between the fine-scale surface features and the
overlying,
changing atmospheric structures of hot, ionized gas, called
plasma. The
surface features and atmospheric structures are linked by
fine-scale
solar magnetic fields.
The power of the TRACE telescope to do detailed studies of the
solar
atmosphere makes this observatory unique among the current group
of
spacecraft studying the Sun.
"The spacecraft has roughly ten times the temporal
resolution and five
times the spatial resolution of previously launched solar
spacecraft.
Its findings are eagerly awaited by the solar science
community," said
Dr. Alan Title, TRACE principal investigator from the Stanford
Lockheed
Institute for Scientific Research in Palo Alto, CA. "We can
expect to
resolve some present mysteries of the Sun's atmospheric dynamics
as
well as discover new and exciting phenomena."
TRACE will be launched into a polar orbit to enable virtually
continuous observations of the Sun, uninterrupted by the Earth's
shadow
for months at a time. This orbit will give the mission the
greatest
chance of observing the random processes which lead to flares and
massive eruptions in the Sun's atmosphere.
The TRACE telescope is really four telescopes in one. Its
30-centimeter
(12-inch) primary and six-centimeter (2-inch) secondary
super-polished
mirrors are individually coated in four distinct quadrants to
allow
light from different bandwidths (colors) to be reflected and
analyzed.
An electronic detector collects images over a
231,000-by-231,000-mile
field of view, nearly 25 percent of the Sun's disk. A powerful
data
handling computer enables very flexible use of the detector array
including adaptive target selection, data compression and image
stabilization.
"TRACE was completed on time, under budget, and met all
performance
goals," said Jim Watzin, Small Explorer project manager,
NASA Goddard
Space Flight Center, Greenbelt, MD. "I'm really proud of
this team.
They have produced a magnificent observatory in a manner that
saved
NASA nearly $9.7 million over the initial cost estimate."
TRACE, which
costs $49 million, is the third launch in the Small Explorer
series of
small, quickly developed, relatively low-cost missions.
TRACE will be launched on an Orbital Sciences Corp., Dulles, VA,
Pegasus-XL rocket released from an L-1011 jet aircraft at the
Western
Range, Vandenberg Air Force Base, CA. The launch window is open
for 10
minutes.
TRACE will be the first space science mission with an open data
policy.
All data obtained by TRACE will be available to other scientists,
students and the general public shortly after the information
becomes
available to the primary science team.
The TRACE telescope was designed and developed in cooperation
between
Lockheed Martin Corp. and Stanford University. The spacecraft was
designed and tested at Goddard, which manages the mission for the
Office of Space Science at NASA Headquarters, Washington, DC.
Further information about the TRACE mission can be found on the
Internet at:
http://sunland.gsfc.nasa.gov/smex/trace
TRACE science information can be found at:
http://www.space.lockheed.com/TRACE/welcome.html
==========================
(3) NATIONAL SPACE SOCIETY TEAMS WITH TIME MAGAZINE AND PARAMOUNT
PICTURES FOR IN-DEPTH EXPLORATION OF COMET THREAT
(Sorry, but this Internet discussion took already place last
night).
From: meteorobs@latrade.com
Washington, DC) - March 18 - The National Space Society, in
cooperation
with Paramount Pictures, announces a live Internet discussion
about the
potential threat of planetary impact by asteroids or comets. The
discussion, hosted by TIME.com, the online component to TIME
magazine,
will be held on March 19, at 7:00 pm ET/4:00 pm PT on Yahoo!
Chat.
The live forum comes only one week after the announcement and
later
retraction of news that an asteroid might impact Earth in 2028.
The
scare renewed debate over the likelihood of a collision and
whether
more steps should be taken now to plan for such a disaster. The
news
was timely for Hollywood, which on May 8th releases Paramount
Pictures/Dreamworks' "Deep Impact," the story of the
world's reaction
to an impending collision with a comet. The Internet chat brings
together the science and Hollywood views of such an event.
Joining the live forum will be:
- Richard D. Zanuck, co-producer of "Deep Impact." He,
along with
David Brown, first presented the idea for the film to Paramount
in the
late 1970s. After the discovery that Earth had indeed suffered
from
shattering comet impacts in its distant past, the Zanuck/Brown
team
decided the most powerful script was one based on science fact.
Zanuck,
in partnership with Brown, also produced "Jaws,"
Academy Award winner
"The Sting," the Academy Award nominated film "The
Verdict" and
"Cocoon."
- Dr. Pete Worden, member of the National Space Society Board of
Directors, an
astronomer, and former director of advanced programs for the
National Space
Council during the Bush Administration. He was recently involved
with a
proposed space mission to intercept an asteroid for the purpose
of
gathering data. Dr. Worden is currently active in the defense
space
program.
- Gerry Griffin, science consultant to "Deep Impact."
Griffin was former
director of NASA's Lyndon B. Johnson Space Center in Houston. He
also served
as a Flight Director during the Apollo 13 mission. Griffin
previously
consulted on the making of the films "Apollo 13" and
"Contact."
- Jon Favreau, "Gus Partenza" in "Deep
Impact," the medical officer on a space
shuttle headed for an Earth-crossing comet. Favreau previously
wrote,
produced, directed and starred in the independent feature
"Swingers."
In preparation for the live discussion the National Space Society
is hosting a
website offering background information and a directory of
related resources.
The National Space Society, founded in 1974, is an independent,
nonprofit
space advocacy organization headquartered in Washington, DC. Its
23,000
members and 90 chapters around the world actively promote a
spacefaring
civilization. Information on NSS and space exploration is
available at
http://www.nss.org. TM &
Copyright ) 1998 by Paramount Pictures and
DreamWorks L.L.C. ALL RIGHTS RESERVED.
========================
(4) WORKNOTES ON THE JOSHUA IMPACT EVENT
From: Ed Grondine epgrondine@hotmail.com
Many researchers have noted the impact event set out in the
biblical
book of Joshua. Recently I have been working on some material
which
appears to provide indepedent corraborative evidence as to the
historicity of this event and as to its date and location. From
the
evidence, it appears that there was an impact event in about 1584
BC,
just to the west of what is now Jerusalem, and that this
Nagasaki-sized
impact destroyed the military forces of a large number of Hittite
appenages, which were under the command of the Hittite king
T'Hantilish, known to the Greeks as Tantalus. So far I have three
independent sources for this event, a summary of which follows,
(including contemporary Hittite records, second summary below),
as well
as a broad archaeological sequence that supports the occurence of
this
impact event.
The first written source for the event, one with which the
conference
participants are probably familiar, is the biblical book of
Joshua,
which is admittedly quite distant from the event itself. To
summarize
once again the account there, the Israelites leave Egypt at the
time of
the eruption of Thera in 1628 BC. They are repulsed at Rephidim,
return
to the desert, eliminate the Kohath faction, and establish
themselves
on the east bank of the Jordan in lands which were long before
under
Horim (Hurri) control. This brings us to 1588 BC. The Israelites
begin
to move across the Jordan, and after a particularly savage attack
on
Ai, a "multi-national" force is assembled to stop them.
Under the
leadership of Joshua, the Israelites launch a night attack, and
as the
"multi-national" force regroups the impact event
occurs:
"And it came to pass, as they fled from before Israel, and
were going
down to Beth-horon, that the Lord cast down great stones from
heaven
upon them unto Azekah, and they died. More died with hailstones
than
they whom the children of Israel slew with the sword."
The Israelites then went on to slaughter and enslave a large
region,
dividing the land in 1583 BC.
The second group of written sources for this impact event are the
Hittite records of Hantilish's (Hantili's) defeat. To summarize,
Hantilish's predecessor Murshilish I (Musili I)is deafened by the
explosion of Thera as a child, and takes the throne in 1604 BC.
[The dating used here follows the Hittite scholar Michael
Astour's
sequence of dates (Hittite History and Absolute Chronology of the
Bronze Age, page 2), but adjusted back by 64 years using the
middle
chronology Babylonian astronomical cycle observation. The dates
thus
arrived at coincide very well with those independently arrived at
from
the ice cores for the Thera explosion and Joshua.] Murshilish I
marches
on Babylon in 1595 BC. On his way home Murshilish I is attacked
and
defeated by the Hurrians. There is a treaty (KUB XXXVI 106 + KBo
IX 73)
with hapiru ("raiders") which was concluded by either
Murshilish now,
or his successor Hantilish concluded it at a later date, some
time
after his murder of Murshilish in 1594 BC.
Hantilish also campaigns against the Hurri, but the Hurri defeat
him
and take his queen and heirs to Shugziya and kill them. (Edict of
Telepinush, 15-17) According to fragmentary record KBo III 46,
someone
dies in Shugziya, after the death of 3 Hittite commanders, and an
unnamed Hittite king assembles 3000 Hapiru men and garrisons them
in a
(name lost) city. (Astour, page 87)
The third group of written sources for this event are the Greek
mythological records. It appears that Hantilish was known to the
Achaeans as Tantalus, the god (Theos) Hantilish, or
T'e-Hantilish: the
king of the coastal region of Sipylus, which has been correctly
identified as Hittite Zippasla. The direct Mycenean sense of
humor
finds typical expression in its description of Tantalus's fate.
After
Tantalus dies he is sent to hell, where though surrounded by food
and
drink, he can not enjoy them, as he must hold up a large stone
with both
hands, in order to keep it from falling on his head.
As for the archaeological record of this event, there are
destruction
levels at Jericho, Hormah, Gibeon, and Arad which have been dated
to
1550 BC, though I do not know if these dates are still valid. It
also
appears that the Hittites were so weakened by this impact event
that
they were unable to prevent the migration of the Gasgas (Kaskas)
and
Achaeans, so every LM IB site could be added to the
archaeological
support for this hypothesis.
From what I have experienced so far in the e-forums, I do not
think
that any amount of argument over written records is going to
persuade
the academic community to even consider the hypothesis. Physical
evidence of the event itself must be found. So far I have
contacted a
friend who lives in Jerusalem about searching for the meteorites
in the
fall field. I have also sent a message of inquiry with a specific
description of the location of the fall field to a meteorite
collecting
group, and since these collectors pay the same price as gold for
meteorites, I hope that solid evidence should not be long in
coming.
I think that additional text materials might justify a more
professional search for the meteorites by geologists, and I am
going to
look for more text material, immediately as I can find the time.
As a
final method of attack on this problem, I plan to investigate the
work
which has been done on finds of meteoritic iron artifacts and the
records of their trade.
Altogether there is more than I can handle, so I thought I'd take
the opportunity to share my findings so far with the conference.
=========================
(5) MAKING A COMET NUCLEUS
J.M. Greenberg: Making a comet nucleus, ASTRONOMY AND
ASTROPHYSICS,
1998, Vol.330, No.1, pp.375-380
LEIDEN UNIVERSITY, ASTROPHYSICS LAB, POSTBUS 9504, NL-2300 RA
LEIDEN,
NETHERLANDS
The chemical composition of a comet nucleus can be very strictly
constrained by combining the latest results on: the core-mantle
interstellar dust model, the solar system abundances of the
elements,
the space observed composition of the dust of comet Halley, and
the
latest data on the volatile molecules of comet comae. The
distribution
of the components in the comet nucleus fall naturally into two
basic
categories - refractories and volatiles. The refractory
components are
tightly constrained to consist of about 26% of the mass of a
comet as
silicates (a generic term for combinations of the elements Si,
Mg, Fe),
23% complex organic refractory material (dominated by carbon),
and
about 9% in the form of extremely small (attogram)
carbonaceous/large
molecule (PAH) particles. The remaining atoms are in an H2O
dominated
mixture containing of the order of 2-3% each of CO, CO2, CH3OH
plus
other simple molecules. The H2O abundance itself is very strictly
limited to similar to 30% of the total mass of a comet - not much
more
nor much less. The refractory to volatile (dust to gas) ratio is
about 1:1, while the dust to H2O ratio is approximate to 2 : 1.
The
maximum mean density of a fully packed nucleus would be
approximate to
1.65g cm(-3). The morphological structure of the component
materials,
following the interstellar dust into the final stage of the
presolar
cloud contraction, is as tenth micron silicate cores with organic
refractory inner mantles and outer mantles of 'ices' with each
grain
containing many thousands of the attogram carbonaceous/large
molecule
particles embedded in the icy and outer organic fraction.
Copyright
1998, Institute for Scientific Information Inc.
===========================
(6) THE RELEVANCE OF LABORATORY IMPACTS FOR THE STUDY OF COMETS
D. Koschny, The relevance of laboratory impacts into ice and
ice-silicate targets for the study of comets, ADVANCES IN SPACE
RESEARCH, 1997, Vol.20, No.8, pp.1569-1576
MAX PLANCK INSTITUTE, PO BOX 20, D-37189 KATLENBURG DUHM, GERMANY
This paper points out the relevance of impact experiments into
ice and
ice-silicate mixtures for the study of comets. After a brief
summary of
the properties of cometary surfaces, a short overview over the
available papers dealing with experimental impacts in ice and
ice-silicates is given. Then, the crater rate onto comet Wirtanen
is
estimated. The conclusion identifies future necessary work. (C)
1997
COSPAR. Published by Elsevier Science Ltd.
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