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CCNet DIGEST, 8 January 1999
----------------------------

(1) FIRE, FLOOD AND COMET
    NEW SCIENTIST, 9 January 1999

(2) COULD SMALL NUMBERS OF HUMAN MIGRANTS HAVE WIPED OUT 85% OF
    AUSTRALIA'S LARGE VERTEBRATES 50,000 YEARS AGO?
    The BBC
    http://news.bbc.co.uk/hi/english/sci/tech/newsid_250000/250469.stm

(3) 'GHOSTLY' GALAXIES RAISE NEW QUESTIONS
    ABCNews Online
   http://abcnews.go.com/sections/science/DailyNews/darkhalo990106.html

(4) STARDUST MISSION PRELAUNCH SCIENCE BRIEFING SCHEDULED
    FOR JANUARY 13
    Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>

=========================
(1) FIRE, FLOOD AND COMET

From NEW SCIENTIST, 9 January 1999, p. 42

Review: Exodus to Arthur, by Mike Baillie
Batsford, 15.99, ISBN 0713484520

Fire, brimstone and pestilence have ravaged the Earth since the dawn of
time, wiping out cities, even whole peoples. Or so the ancient prophets
and storytellers would have us believe (there’s a fair bit in the
Bible). But could they possibly be describing real events?

As a renowned authority on tree rings and their use in dating ancient
artefacts and events, Mike Baillie might seem an unlikely author for a
book which tackles a lot of mythology. But he’s a perfect candidate,
since he recently helped complete a 5000-year continuous and global
record of annual growth patterns. It revealed five major worldwide
environmental shocks. To find out what they meant he turned to the
early texts.

His conclusion comes as a shock. Not only did the five episodes
coincide with the onset of “dark ages” for society, but they were
triggered by cometary impacts. If Baillie is right, history has
overlooked probably the single most important explanation for the
intermittent progress of civilisation. Worse, our modern confidence in
benign skies is foolhardy, and our failure to appreciate the constant
danger of comet “swarms” is the result of a myopic trust in a mere 200
years of “scientific” records. Our excuse is that Christianity probably
suppressed the dire warnings of earlier sages in an effort to downplay
their influence, as Baillie points out.

The tree-ring record points to global environmental traumas between
2354 and 2345 BC, 1628 and 1623 BC, 1159 and 1141 BC, 208 and 204 BC
and AD 536 and 545. Baillie argues that the tree rings are recording
first the biblical flood, then the disasters that befell Egypt at the
Exodus, famines at the end of King David’s reign, a famine in China
that ended the Ch’in (sic) dynasty, and finally, the death of King
Arthur and Merlin and the onset of the Dark Ages across the whole of
what is now Britain.

The biblical account of the Exodus and contemporary annals from China
speak of cometary activity preceding calamity. Previous writers have
wondered if the hail or red-hot stones that befell the Egyptians was
due to the eruption of Santorini, the Aegean volcano that destroyed
Minoan civilisation. The pillar of smoke that guided the Israelites may
have been the plume. But a single volcano is an unlikely cause of a
global downturn.

So Baillie goes a step further, arguing that a series of cometary
impacts around the size of the 20-megaton explosion at Tunguska in
Siberia might be enough to trigger earthquakes, tidal waves, volcanic
eruptions and ocean floor outgassing. This would explain why comets are
seen as portent, along with the occurrence of flooding and poisonous
fogs – all reported at the time of Exodus and during others of
Baillie’s five catastrophes.

As he admits, he has ventured beyond his professional expertise in
exploring mythology. Is it correct to assume that references to angles
and dragons are descriptions of comets? Science would be much poorer
if acknowledged experts felt unable to speculate as Baillie has done
here. But science also tends to seek solutions, and if the author is
not carried shoulder-high for broaching this important subject, it
will be because his doomsday scenario offers little in the way of an
immediate technical fix. If a couple of lumps of interplanetary rock
the size of several football pitches hit Europe tomorrow, tens of
millions would die (sic). And some astrophysicists reckon that an
event of this order is likely at least every 5000 years – killing at
least a quarter of the human population.

Baillie proposes seeding space with our DNA in an effort to survive,
but that doesn’t sound quite as much fun as carrying on as we are.
Let’s hope both he and the astrophysicists are wrong.

Ben Rudder is an anthropologist.

Copyright 1999, New Scientist

====================
(2) COULD SMALL NUMBERS OF HUMAN MIGRANTS HAVE WIPED OUT 85% OF
    AUSTRALIA'S LARGE VERTEBRATES 50,000 YEARS AGO?

From The BBC
http://news.bbc.co.uk/hi/english/sci/tech/newsid_250000/250469.stm

Big bird clue to mass extinction

When the Aborigines arrived in Australia about 50,000 years ago they
may have wiped out most of the large animals on the continent.
Scientists speculate that humans triggered an ecological disaster by
burning vast areas of vegetation, destroying the food that sustained
many species.

They have come to the conclusion after studying the fossilised
eggshells of an enormous flightless bird called Genyornis newtoni.

The theory, put forward in the latest edition of Science magazine, is
a controversial one.

Scientific debate

For decades, scientists have argued about the likely cause of the mass
extinctions that rocked ecosystems around the world during the
Quaternary period (1.8 million years ago to the present). These
events were mild or absent in some regions of the world, such as
Africa and Southeast Asia, moderate in others such as Europe, and
extreme in the Americas, Australia, and many oceanic islands.

The Australian mass extinction saw the loss of many large terrestrial
vertebrate species - more than 85% of the continent's large animals may
have disappeared. They included carnivorous kangaroos and a horned
tortoise nearly the size of a small car.

However, the search for an explanation to the mass extinction has
always been hindered by the difficulty in nailing down the actual
timing of the event, largely because its age pushes the limits of
radiocarbon dating.

Dating techniques

Gifford Miller of the University of Colorado and his colleagues appear
to have jumped this hurdle by using a variety of alternative dating
techniques not based on radiocarbon isotopes. They dated the extinction
of Genyornis newtoni, a ponderous flightless bird with thick, short
legs that weighed around 200 pounds - twice as much as the modern day
emu.

They analysed the birds' fossilised eggshells and set the date of
Genyornis' sudden disappearance at 50,000 years. This date loosely
matches the time of the Aborigines' arrival to the continent as
indicated by the most reliable evidence yet available.

This was also a time of moderate climate change, making it unlikely
that climate played a role in Genyornis' extinction.

Picky eater

Gifford and his team conducted further tests on the shells to reveal
clues about the bird's diet. It appears to have been a relatively picky
eater, relying on extensive shrubland for food. Many of the animal
species that disappeared around the same time as Genyornis also
fed on these shrubs and trees.

The team think that if humans disrupted the natural fire cycle by
burning the landscape periodically it could have destroyed the animals'
feeding grounds.

"I think we have compelling circumstantial evidence that the Genyornis
extinction date is applicable to the vast majority of Australian
megafauna (large animals)," said Miller. "There are certainly no secure
dates to refute this supposition."

"We suspect the systematic burning by the earliest colonisers - used to
secure food, promote new vegetation growth, to signal other groups of
people and for other purposes - differed enough from the natural fire
cycle that key ecosystems were pushed past a threshold from which they
could not recover."

Copyright 1999, BBC

===================
(3) 'GHOSTLY' GALAXIES RAISE NEW QUESTIONS

From ABCNews Online
http://abcnews.go.com/sections/science/DailyNews/darkhalo990106.html

Light’s On, But No Stars

What’s Glowing in Galaxy NGC 5907?
                                                                 
By Kenneth Chang
ABCNEWS.com

A U S T I N,   Texas,   Jan. 6 — Astronomers of the late 20th century
are perplexed by what they cannot see. Now they can’t even find what
they can see.

A ghostly halo around a nearby galaxy unexpectedly appears devoid of
bright stars, the latest piece in the puzzle of the universe’s
missing mass.

For decades, astronomers have known that the stuff they can see —
stars, galaxies, gas clouds — is only a small fraction of what’s out
there. Although unseen, the other unknown stuff, called dark matter,
must exist because its gravitational pull keeps galaxies from flying
apart.

For years, astronomers and physicists have been searching for and
guessing what dark matter might be — black holes, exotic subatomic
particles, dim stars, etc.

NGC 5907, a spiral galaxy located 40 million light-years away in the
constellation Draco, seemed a good place to look. In 1994, astronomers
found a dim, red spherical halo enveloping the galaxy, and the
distribution of mass in the halo appeared to be what was expected for
dark matter.  Perhaps, says James Graham, an astronomy professor at
University of California, Berkeley, “The dark matter in this galaxy is
not so dark.”

Counting Stars

An international team of astronomers, including Graham, pointed the
Hubble Space Telescope at NGC 5907 to take a closer look at the red
glow. “The purpose of this experiment was to count the number of stars
and determine their properties,” Graham says.

Everywhere that astronomers have looked in the universe, the ratio of
big, bright stars to small, dim dwarf stars has been about the same,
with about half of a galaxy coming from bright stars and half coming
from the dimmer, but more numerous dwarfs. Assuming this to be true
for NGC 5907 halo as well, the astronomers expected Hubble to spot
hundreds or thousands of stars scattered through the halo. Instead,
they saw almost nothing. “We have one confirmed star,” Graham says.
“Is that pathetic? We were dismayed by that.”

Graham and Michael Liu, a graduate student at Berkeley and lead author
of the study, presented their sparse results today at a meeting of the
American Astronomical Society in Austin. “The nature of our results
isn’t what we saw,” Liu says, “but what we didn’t see.”

Dwarf-Star Halo?

The most likely explanation, according to Liu, is that the stars in the
halo are all dim, dwarf stars, much smaller than our sun. Add together
the light of enough of them and you’d see the faint halo, but you
wouldn’t be able to make out any single star, even with the
sharp-sighted Hubble. If true, the NGC 5907 halo would be the first
place that violates the big-star-to-little-star ratio that holds
elsewhere.

Similar halos of dwarfs — albeit dimmer and out of sight — may surround
other galaxies as well, accounting for a sizeable fraction of the
missing dark matter. Last summer, another group of astronomers said
half the dark matter in the Milky Way galaxy may be dark, star-sized
lumps called MACHOs, for massive compact halo objects. The two sets of
findings could be one and the same; MACHOs could just be dim dwarf
stars.

The current observations looked in the near-infrared wavelength, or
light slightly redder than red. Later this year, Hubble will take a
second look, this time in the visible spectrum. “The halo is still
there,” Graham says. “It has to come from something. We know it
doesn’t come from normal stars.”

Fuzz Balls, Up in the Sky

An analysis of 43 galaxies shows that small, ghostly galaxies that
appear to be little more than “fuzz balls” in powerful telescope images
actually contain high densities of dark matter, with only a scattering
of visible stars, says John Kormendy of the University of Hawaii.

Speaking at the national meeting of the American Astronomical Society,
Kormendy said there may be more of the small, dense galaxies than the
bright, giant galaxies. They could, thus, contain a significant portion
of the universe’s dark matter. “There may be a large population of dark
galaxies that contain too few stars to be discovered,” Kormendy said.
“They may outnumber all of the luminous galaxies combined.”

The presence of invisible matter was determined years ago by
astronomers who measured the motion of stars within galaxies. They
determined that stars and clouds that shine and can be seen from
Earth did not contain enough mass to hold the galaxies together.

Thus, there had to be other matter to provide the gravitational force
that keeps the galaxies from flying apart. Some astronomers calculated
that 90 percent of the matter in the universe cannot be seen and thus
is “missing.”

Since then, astronomers have been scrambling to try to find and
identify this missing matter because it has profound implications about
the motion and ultimate destiny of the universe. “It is as if we are in
a black room working on a black puzzle,” said Vera Rubin, a Carnegie
Institution of Washington astronomer and one of the original theorists
about cold, dark matter.

Kormendy said he and his colleagues do not claim that all of the
missing matter is in “ghostly galaxies,” but because the fuzz balls
are so numerous, “they could add up to a significant portion of the
dark matter.”

Copyright 1999, The Associated Press

==============
(4) STARDUST MISSION PRELAUNCH SCIENCE BRIEFING SCHEDULED
    FOR JANUARY 13

From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>

Douglas Isbell/Donald Savage
Headquarters, Washington, DC                     January 7, 1999
(Phone:  202/358-1547)

Mary Beth Murrill
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-5011)

Vince Stricherz
University of Washington, Seattle
(Phone:  206/543-2580)

NOTE TO EDITORS:  N99-2

STARDUST MISSION PRELAUNCH SCIENCE BRIEFING SCHEDULED FOR JANUARY 13

Managers and scientists leading the team preparing the Stardust
spacecraft to gather samples of icy comet dust and return them to Earth
will conduct a media briefing on the mission and its science goals on
Wednesday, Jan. 13, at 2 p.m. EST.  The televised briefing will
originate from NASA Headquarters in Washington, DC.

Set for launch from Cape Canaveral Air Station, FL, on Feb. 6, 1999,
Stardust will be the first U.S. mission dedicated solely to a comet and
the first return of extraterrestrial material from outside the orbit of
the Moon.

The primary goal of this Discovery Program mission is to collect comet
dust and related measurements during a planned close encounter with
comet Wild 2 (pronounced "Vilt-2") in January 2004. Additionally, the
Stardust spacecraft will bring back samples of interstellar dust
particles, recently discovered material streaming into the Solar
System.  Ground-based analysis of these samples after their return in
January 2006 should yield important insights into the evolution of the
Sun and planets, and possibly into the origin of life itself.

Presenters at the briefing are scheduled to include:

Dr. Carl Pilcher, science director for Solar System exploration at
    NASA Headquarters
Dr. Kenneth Atkins, Stardust project manager at NASA's Jet
    Propulsion Laboratory (JPL), Pasadena, CA
Dr. Donald Brownlee, Stardust principal investigator from the
    University of Washington, Seattle
Joseph Vellinga, Stardust program manager at Lockheed Martin
    Astronautics, Denver, CO
Dr. John Rummel, Planetary Protection Officer, NASA Headquarters

Extensive information on Stardust, including mission-related art and
images, and a public signature disk attached to the spacecraft, is
available on the Internet at the following URL:

               http://stardust.jpl.nasa.gov/

NASA Television is located on GE-2, transponder 9C at 85 degrees West
longitude, vertical polarization, with a frequency of 3880 Mhz, and
audio of 6.8 Mhz.  There will be two-way question-and-answer capability
for media at participating NASA centers.

----------------------------------------
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*

IMPACT PROBABILITIES AND THEIR APPLICATION

From David Asher <dja@star.arm.ac.uk> and
     Mark Bailey <meb@star.arm.ac.uk>

Dear Benny,

Some remarks on impact probabilities. Presumably part of what is at
issue is whether they should be included in announcements (viz. 1997
XF11).

Paolo Farinella (CCNet, 8 Jan 1999) has given a nice 3-part summary of
how one can think of impact probabilities. Once people routinely
calculate short and intermediate term probabilities along the lines
mentioned, the faith one has in the probabilities may then largely
depend on the extent to which the modelled observational errors really
do match the true distribution of errors. (For further comments on
the possibility of such calculations see the later part of Brian
Marsden's 1998 July 27 CCNet contribution.)

Now that NEO astrometric positions are available soon after
observation, we can no doubt in future expect some group(s) to
announce impact probabilities in unusual cases. Members of the public
will then be free to place what faith they choose in the probabilities.
Nevertheless, this doesn't mean impact probabilities are an essential
part of the 'Spaceguard' procedure.

Most hazardous NEOs are as yet undiscovered; the justification for
Spaceguard has often been stated as being to increase the number of
objects (in the first instance, km-sized asteroids roughly within
Jupiter's orbit, say) for which we can give a clear yes/no answer as
to whether they will hit in the near future (of order a century). By
this argument, Spaceguard proceeds by discovering as many objects as
possible, and attempts to obtain further observations of each one until
the yes/no answer within the given time frame becomes clear (other
follow-up has scientific + long term impact hazard use). Usually the
'no' answer can be established quite soon. For the answer not to become
clear for so long, as in the case of 1997 XF11, is highly unusual, and
the acquisition of more observations becomes increasingly urgent.  If
astronomers are not to be accused of a 'cover up' and if the public are
interested in Spaceguard at all, the first case like 1997 XF11 (in
which the impact of a large object within the next century based on an
appreciable arc was not excluded - this is not to support any media
team who chose to sensationalise unlikely events) is worthy of
attention. The key point was urgently to obtain new observations, and
getting these - which involved putting out the circular, people with
access to plate libararies checking them, and anyone who could observe
the object measuring it - was a very cost effective way quickly to
allow a `no impact' answer to be known for sure.

David Asher, Mark Bailey
Armagh Observatory


*

CCNet DEBATE: IMPACT PROBABILITIES AND TIME SCALES

From Paolo Farinella <paolof@keplero.dm.unipi.it>

Dear Benny,

I have been prompted by some recent exchanges to share some
considerations on the impact probability issue. This is a tricky
subject, but I don't concur with the view that little can be said about
it on a rational basis. After all, impact probabilities are the main
point when we speak about the NEO hazard and what should be done about
it.

1. Over very long time spans (say, >100,000 yr) NEA orbits are strongly
chaotic, and wander over a significant portion of the orbital element
space. In this case, impact probabilities can be computed by statistical
methods: in a very approximate way by using analytical theories such as
Wetherill's or Kessler's (see e.g. Milani et al., 1990, Icarus 88, 292)
or, better, by integrating many `dynamical clones' of a single object and
estimating their average encounter/collisions rates (e.g. Michel et al.
1998, AJ 116, 2023). The results are interesting, but not so relevant from
the hazard viewpoint.

2. Over short time spans (comparable to the Lyapounov time, that is
shorter than or of the order of 100 yr) impact probabilities are not
mainly a function of the chaoticity of the orbits, but of the quality
and time distribution of the observations. There are good theories to
carry out these impact probability calculations, as developed and
implemented by K. Muinonen and Ted Bowell, by the JPL group and by A.
Milani and coworkers (who have an article in press on Icarus describing
their methodology). I cannot understand why such calculations cannot be
done at the MPC too.

3. The problem is more difficult for intermediate time spans. In this
case the uncertainty of the observations and the chaotic effects
(mainly due to close planetary encounters) both play a role, and
disentagling them is not easy. In other words, the orbit has not yet
been `randomized', but at the same time the non-linear effects in the
propagation of the observational errors are far from negligible.
Therefore computing impact probabilities is harder than in either of
the extreme cases above. However, I do not doubt that detailed
quantitative analyses are possible in this case too, and I look forward
at seeing forthcoming publications by some of our colleagues mentioned
above, who are currently working on this problem.

Paolo Farinella, 6 January 1998
----------------------------------------------------------------

From Brian G. Marsden <bmarsden@cfa.harvard.edu>

Dear Benny,

I think that Paolo has basically stated the impact probability very
well, and I don't see any real conflict with what was in my piece on
Monday. I did not address there the situation with regard to very long
time spans (No. 1 on his list), where a statistical analysis could have
some meaning, as in his own interesting work on Eros. It seems to me
that all three of my examples, Toutatis, 1998 XB and "post-2028" 1997
XF11 involve intermediate time spans (his No. 3), and his
categorization of this type as "difficult" fully agrees with my own
assessment; all I suggested was that a consideration of geometry
(location of the orbital nodes) and chaoticity of the dynamics is
helpful in handling this type, rather than a purely statistical
argument. Such a statistical approach might indeed give useful
information in the short-time-span case (his No. 2), illustrated by
1997 XF11 in 2028, although I must remark that there was quite a
difference of opinion, between Muinonen-Milani on the one hand and
Yeomans-Chodas on the other, as to whether nongaussian statistics and
nonlinear effects are or are not important. It also seems to me that
geometry plays a role here too, and that there would have been much
less confusion and rhetoric if we had all concentrated at the outset
on that geometry (i.e., to state correctly the absolute minimum
distance between the orbits of 1997 XF11 and the earth, given
appropriate consideration of the likely observational errors and a
physical model that did not consider nongravitational forces or
perturbations by unconsidered objects), rather than perpetuate an
argument (in which I never participated) about impact probabilities
ranging all the way from 10^{-3} to 10^{-9772}. Such geometrical
considerations are indeed made at the MPC, if not always as a matter
of course. The role of the MPC is to receive, process and archive
observations and to advise observers on the desirability (or
otherwise) of follow-up data.

Brian G. Marsden



CCCMENU CCC for 1999

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