CCNet, 15 September 1999


    Four Milleniums Ago

    Kings and chieftains ruled, while ordinary humans
    planted their crops, raised children, paid taxes.
    Empires grew in Mexico, China, India, and Egypt,
    sun and rain blessed fertile land, while food and water
    were enough for all, and populations slowly grew.
    One day it ended with a growing dark, wind, dust,
    the sun obscured; perhaps the ground shivered.
    Then the next day, and the next were cold and dark,
    and weeks and months were dull and grey.
    Leaves turned pale or fell, grain stalks short and spindly,
    bees vanished along with flowers, ants, butterflies.
    Crops failed, starvation loomed, taxes were not paid,
    raiders came to loot for grain, rulers trembled
    and then the empires and kingdoms fell. Somewhere
    on this huge Earth a small invader, no bigger than a hill,
    had used its gigatons of energy to send a monstrous cloud
    of dust and smoke and fractured rock so high
    the Sun was blotted out for months, the Earth bled,
    and human history's brief span faltered for a moment.

    Malcolm Miller

     Roger W. Sinnott <>

     Andrew Yee <>

      Larry Klaes <>

     SCIENTIFIC AMERICAN, October 1999

     David DeGraff <>

     Patrick Michel <>

      Y.V. Skorov & N.I. Komle

      C.M. Lisse et al.

      Y.R. Fernandez et al.

       G.J. Black et al.,

         Science Daily, 14 September 1999


From Roger W. Sinnott <>


On September 11th, the LINEAR 1-meter robotic camera in Socorro, New
Mexico, operated by MIT's Lincoln Laboratory, caught the first images
of a 15th-magnitude asteroid with unusually rapid motion in the
constellation Sculptor. The Minor Planet Center at the Smithsonian
Astrophysical Observatory immediately posted this information on its
Near-Earth Object Confirmation Page, and during the next three days
11 other astrometric stations operated by amateur and professional
astronomers in Europe, Australia, South America, as well as the
United States responded with more than 50 follow-up observations.
These allowed the MPC's associate director, Gareth V. Williams, to
compute a preliminary orbit and designate the object 1999 RQ36.

Probably less than half a kilometer across, this object should
continue to brighten as it moves from Fornax into Eridanus in the
next week or so. According to Williams's orbit, it will pass only
0.015 astronomical unit from Earth -- about 6 times the Moon's
distance -- around September 23rd. At that time it will be about
magnitude 13.9, similar to Pluto's brightness, as it races
northeastward across Orion near the bright star Betelgeuse. 

While this event does not really qualify as a "near miss" (a handful
of minor planets pass this close to Earth in any given year), the
low-inclination orbit calculated by Williams suggests the object
could come considerably nearer during some future revolution. But it
is much too early to make any definite prediction, because 1999 RQ36
has not been observed long enough to calculate a definitive orbit.
Continued monitoring during the next few months will help astronomers
assess the situation further.

While asteroids of 14th or 15th magnitude are hard to spot visually
in telescopes smaller than 10- or 12 inch aperture, they are easy to
image with a CCD camera on an 8-inch instrument. The following
ephemeris gives the object's coordinates at about 0 hours Universal
Time on successive nights.

                1999 RQ36    

   Date       R. A. (2000) Decl.        Mag.
1999 09 09    01 23.4    -27 57         15.6
1999 09 10    01 28.9    -27 38         15.5
1999 09 11    01 35.2    -27 13         15.3
1999 09 12    01 42.3    -26 43         15.2
1999 09 13    01 50.5    -26 04         15.0
1999 09 14    02 00.1    -25 16         14.8
1999 09 15    02 11.5    -24 13         14.7
1999 09 16    02 25.0    -22 51         14.5
1999 09 17    02 41.5    -21 02         14.3
1999 09 18    03 01.5    -18 36         14.1
1999 09 19    03 26.3    -15 18         14.0
1999 09 20    03 56.7    -10 50         13.9
1999 09 21    04 33.7    -04 58         13.9
1999 09 22    05 17.0    +02 12         14.1
1999 09 23    06 04.7    +09 56         14.5
1999 09 24    06 53.3    +17 01         15.0
1999 09 25    07 38.6    +22 34         15.7

The announcement was made on Minor Planet Electronic Circular
1999-R44, issued today by the Minor Planet Center, Smithsonian
Astrophysical Observatory, Cambridge, Mass., U.S.A.  The above
information has been adapted from that circular, which has further
details of the discovery (

The editors of Sky & Telescope magazine are interested in receiving
reports and images of this object for possible publication, either in
the magazine (P.O. Box 9111, Belmont, Mass. 02478, U.S.A.) or on our
Web site at

Roger W. Sinnott
Associate Editor
Sky & Telescope


From Andrew Yee <>

Extracted from INSCiGHT, Academic Press

Tuesday, 14 September 1999, 5 pm PST

Killer Impact's Wet Wallop
By Robert Evans

When a massive asteroid plunged into the Gulf of Mexico 65 million
years ago, triggering dust clouds and fires blamed for the demise of
the dinosaurs and countless other species, needless to say, it caused
quite a splash. Tsunamis raced outward from the impact at Chicxulub off
the northern coast of the Yucatán Peninsula. Now a scientist says the
cataclysmic waves may have reached as far inland as South Dakota, more
than 2500 kilometers from ground zero -- and twice as far as previously

Over the last decade, geologists have found traces of the waves in the
Gulf of Mexico and the Caribbean: 65-million-year-old rocks containing
rumpled beds of sand and signs of erosion on the ancient sea floor.
Since 1997, geologist Phil Stoffer of the U.S. Geological Survey (USGS)
in Menlo Park, California, has studied similarly contorted sand layers
from that time in Badlands National Park, South Dakota. The misshapen
beds, covering more than 300 square kilometers, probably weren't caused
by a submarine landslide, because the sea floor then was flat. Stoffer
therefore concluded that something big had disturbed the sediments.
Stoffer presented his findings on 26 August at a public lecture at the
USGS in Menlo Park.

At first a skeptic, Dennis Terry of Temple University in Philadelphia
changed his mind and joined Stoffer's team after visiting the South
Dakota site. "I stood there this summer wondering what it must have
been like to see a wall of water rushing through," he says.

But Rachel Benton, the staff paleontologist at Badlands National Park,
says she's reserving judgment until more evidence comes in. She says
the dating of the sediments -- based on fossils below the sandstone
that are slightly older than 65 million years, and remnants of tiny
glass spheres that could have been forged in the heat of a meteorite
strike -- is far from conclusive. More reliable indicators would be
elevated levels of iridium, an element well known to be concentrated in
rock layers the same age as meteorite craters, or mineral grains
deformed by a powerful impact.

Copyright 1999, The American Association for the Advancement of Science


From Larry Klaes <>

ScienceWeek BULLETIN September 13, 1999

Two of the central questions in planetary and Earth science concern
the origin of the Earth and Moon. How did these two bodies form and
what forces defined their basic physical structures?
A.N. Halliday and M.J. Drake (2 installations, CH US) present a short
review of current research in this area, the authors making the
following points:

     1) Advances in this field have come mainly with progress in
simulating the dynamics of planetary accretion, in measuring isotopes
that act as chronometers for early Solar System processes, in
analysis of noble gas isotopes that yield clues about the early
atmosphere, and in melting experiments at previously unattainable
pressures and temperatures. Although a general picture may be
emerging, many issues remain hotly debated.

     2) Planet formation is believed to begin with sticking and
frictional coagulation of dust particles in a gaseous nebula that
persists in the *circumstellar disk. The particles grow in size until
there is substantial gravitational attraction between kilometer-sized
bodies, and these coalesce further. Major collisions between small
proto-planets eventually result in objects the size of Earth.

     3) The energy of late-stage planet-building impacts would be
colossal, sufficient to melt the entire planet. *Magma oceans would
be formed, and some volatile elements would escape into space. The
most widely accepted theory for the origin of the Moon is that it
coalesced from a ring of debris produced by such a late-stage
collision between two Earth-forming proto-planets. This "Giant Impact
Theory", established over a decade ago, explains the rotational speed
of the Earth-Moon system, a critical feature that must be reproduced
by any satisfactory model. But in spite of a growing consensus, some
researchers are still opposed to the Giant Impact Theory on both
dynamical and geochemical grounds.

     4) All isotopic data are consistent with Earth being fully
formed within 50 to 100 million years after the start of the Solar
System. The isotopic record from Moon rocks is consistent with the
formation of the Moon at about the same time.

     5) The authors conclude: "We have recently come a long way in
obtaining hard constraints on the origin of Earth and the Moon. The
issues have changed from discussion of whether or not there was a
giant Moon-forming impact to debate about the accretion rates of the
Earth and the chemical, isotopic, and physical effects of such
catastrophic accretionary scenarios."

In a contiguous short review of the same research area, Frank A.
Podosek (Washington University St. Louis, US) makes the following

     1) The age of the Solar System as a whole is easier to determine
than the age of Earth. The age of the Solar System is reliably
inferred from the age of *refractory element-rich inclusions in
meteorites to be approximately 4.57 billion years, thus providing an
upper limit to the age of Earth. These inclusions are the oldest
known objects in the Solar System, and their content indicates that
the Solar System did not exist for more than approximately 1 million
years before the inclusions formed.

     2) In contrast to these ancient extraterrestrial objects, there
are no known terrestrial rocks or minerals whose formation
essentially coincides with the formation of Earth, and therefore the
age of Earth must be inferred indirectly. Several independent
approaches indicate that Earth formed approximately 100 million years
later than the Solar System as a whole.

     3) All the various isotopic chronometers are intrinsically
capable of considerably higher precision, but this precision cannot
yet be realized. It is not even clear whether the chronometers are
consistent or in conflict with each other. All methods rely on models
of varying complexity involving assumptions difficult to verify and
parameters difficult to measure.

     4) The author concludes: "For testing the giant impact scenario
in particular, it would be useful to have a quantitative theory for
whether a preexisting atmosphere is lost in the impact, whether
preexisting planetary structures (*core, mantle, and crust) are
re-equilibrated after such an impact, and how much of the Moon comes
from the impactor and how much comes from the target."

A.N. Halliday and M.J. Drake: Colliding theories.
(Science 19 Mar 99 283:1861)
QY: A.N. Halliday []

Frank A. Podosek: A couple of uncertain age.
(Science 19 Mar 99 283:1863)
QY: Frank A. Podosek []
Text Notes:

*circumstellar disk: One of the important discoveries of the 1980s
was the existence of circumstellar disks of dust around some stars,
the disks apparently replenished by unseen parent bodies such as
comets and asteroids.

*Magma: In general, any mass of molten rock.

*refractory: Refractory materials are materials resistant to
decomposition by heat, pressure, or chemical attack. The term
is most commonly applied to heat resistance.

*core, mantle, and crust: Seismic studies indicate the interior of
the Earth consists of three parts: a metallic core, a dense rocky
mantle, and a thin low-density crust. The central part of the core is
solid, but the outer part of the core is evidently liquid.

Summary & Notes by SCIENCE-WEEK [] 4Jun99

The most widely accepted theory for the origin of the Earth's moon is
that during the late stages of the Earth's accretion an impact with
another planet at least the size of Mars occurred, and the impact
generated both the hot debris that formed the moon and the angular
momentum of the Earth-moon system. In geology, the mantle of a planet
or moon is the layer that lies between the crust and the core.
Chondrites are a type of stony meteorite consisting of an
agglomeration of millimeter-sized globules (chondrules) that are
thought to be unchanged since the original condensation out of the
nebula from which the sun and solar system formed, and "chondritic"
is the term used to describe a rock composition similar to that of
chondrites, which implies an age of 4.2 to 4.5 billion years. The
term "radiogenic", on the other hand, is used to describe a rock
composition apparently resulting from varying isotope decays, and the
oldest radiogenic compositions on Earth have been dated at 3.6 to 3.8
billion years. A hafnium-tungsten chronometer is not an actual
instrument but a method of radiometric age determination using the
isotope ratios of the elements hafnium and tungsten. Hafnium is
lithophilic (silicate-loving), which means it tends to associate with
chondritic materials, while tungsten is siderophilic (metal-loving),
which means it tends to associate with metal cores, and using these
differing affinities of these elements, one can attempt a
construction of the age and origin of the moon by analysis of moon
rock samples and comparisons with Earth rocks. Lee et al (4 authors
at 2 installations, US) report a study of the age and origin of the
moon with the hafnium-tungsten chronometric method. The tungsten
isotopic compositions of 21 lunar samples were found to range from
chondritic to slightly radiogenic. The authors suggest this
heterogeneity is probably the result of late radioactive decay within
the moon itself, and that the moon formed 4.52 to 4.50 billion years
ago and its mantle has since remained poorly mixed.

QY: Der-Chuen Lee []
(Science 7 Nov 97) (Science-Week 28 Nov 97)

Related Background:
THE ORIGIN OF EARTH'S MOON The large impact hypothesis of the origin
of the Earth's moon is the current consensus view. The essential idea
is that the moon formed from debris ejected into a disk around Earth
by the impact of a large body. A version of this is that Earth and
its moon were created more or less simultaneously by the collision of
two large planetesimals, the resultant large body becoming Earth, and
the ejected debris formed the moon. What is accepted by nearly
everyone is that an accretion disk of debris was the first stage of
the moon's formation. Shigeru Ida et al (Tokyo Institute of
Technology, JP; University of Colorado Boulder, US) have evidently
now provided the most detailed simulation calculations of lunar
growth in an impact-generated accretion disk. Using direct N-body
simulations, they show that a single dominant moon can grow from such
a disk within a year, but to satisfy the present angular momentum and
mass constraints on the analysis, the impacting body must have been
at least twice as massive as Mars, and had to provide the resultant
system with a few times more angular momentum than it now possesses.
There is presently no explanation for the subsequent loss of angular
momentum, and the required massive size of the impacting object is
also puzzling. Although this is apparently the best set of simulation
calculations to date, the authors emphasize that further simulation
modeling is needed [*Note #1].

QY: S. Ida []
(Nature 25 Sep) (Science-Week 10 Oct 97)
Text Notes:

*Note #1: Accretion is considered an important factor in the
evolution of stars, planets, and comets. The essential idea is the
coalescence of small particles in space as a result of collisions,
and the gradual formation of larger bodies from smaller ones as a
result of gravitational attraction. An accretion disk is a disk of
gas or particles in orbit around an object, the disk formed by
inflowing matter. A simulation of the sort mentioned in the report
involves computational solutions of the dynamical equations for the
history of a chosen mass of particulate matter initially ejected from
a larger body. By solving the equations for the mathematical model,
one can follow the evolution of the accretion disk and the
agglomeration that forms the final orbiting satellite. The study
mentioned here was first presented at a meeting of the American
Astronomical Society in July, and here is part of the related
SCIENCE-WEEK (1 Aug 97) report: Until the 1980s, there were three
extant theories, with no data available to support or refute any of
them. The Fission Hypothesis proposed that the moon broke away from a
rapidly spinning proto-Earth after the proto-Earth's differentiation,
the moon forming from iron-poor crust. But the moon rocks in hand
have been found to differ chemically from those of Earth. Also, if
the proto-Earth had been spinning fast enough to break up, the
present Earth-moon system should contain a great deal more angular
momentum than is observed. The Fission Hypothesis therefore had to be
abandoned. The Condensation Hypothesis was based on the idea that the
Earth and the moon condensed simultaneously from the same cloud of
material in the solar nebula. This hypothesis did not survive because
analysis of moon rocks has shown the Earth and the moon have greatly
different densities and compositions. The Capture Hypothesis proposed
that the moon was formed elsewhere in the solar system and later
"captured" by Earth. This hypothesis was always the least popular
because it required too many coincidental events. Thus, after the
mid-1980s, there was no satisfactory theory of the moon's origin.
During the past decade, a new idea gradually developed, the
Large-Impact Hypothesis, the idea of which is that the moon formed
from debris ejected into a disk around the Earth after a major
collision of the Earth with another large body about 4.5 billion
years ago, the other body a planet perhaps as large as Mars. The
Large-Impact Hypothesis is at present the consensus theory in
planetary science.

Copyright (c) 1999 ScienceWeek
All Rights Reserved




Dying plants harvest harsh surprises from climate change

Plants may seem to sit passively as climate decides their fate, but
scientists are beginning to believe that vegetation can strongly
amplify the climate's most subtle whims--sometimes with abrupt and
devastating results. A new computer simulation indicates that plants
helped to turn the Sahara from a lush grassland thriving with hippos
and elephants to its current condition as the world's largest desert.

The Sahara's succulent sojourn faced an abrupt end about 5,500 years
ago. In a matter of centuries, rainfall levels plummeted, the green
grasslands paled to a sandy yellow, and civilizations were forced to
relocate. Many scientists have assumed that human beings, who arrived
there 7,000 years ago, overused the land, which led to the quick loss
in vegetation. But the new simulations show that a steady but slow
loss of grasses--stemming from a gradual trend toward less rainfall
beginning about 9,000 years ago--ran wildly out of control.

"Climate modelers tend to think that vegetation is not important,
because it's only about 20 percent of the planet's surface area,"
says Martin Claussen, leader of the team that designed the simulation
at the Potsdam Institute for Climate Impact Research in Germany.
"We're now seeing that we're not allowed to neglect land area."

John E. Kutzbach, a climatologist at the University of 
Wisconsin‚Madison, is enthusiastic about the results because of their
value in predicting future climate. "The idea that vegetation affects
climate hasn't been studied in detail," he says.

Both Kutzbach and Claussen had independently used earlier computer
simulations to watch how local weather affected Saharan plants, but
Claussen's latest simulations allowed his team to be the first to see
whether the plants themselves might effect change. Each of the team's
10 simulations began with the grasslands of 9,000 years ago and ended
with the arid desert of the present. The only external force they
introduced to their simulated climate was a gradual evolution of the
planet's orbit. About 9,000 years ago the earth's perihelion, the
point at which the planet passes closest to the sun, occurred in
July, and the North Pole was leaning more toward the sun. These two
circumstances, which then meant stronger summer sunlight for the
Northern Hemisphere and thus stronger monsoons to water a thirsty
Saharan grassland, have changed slowly ever since. The northward tilt
has shifted away from the sun, and perihelion now occurs in January.

During the first few thousand years of Claussen's simulation, this
transition manifested as a gradual loss of vegetation, presumably
because the monsoons were weakening. But the grassland's condition
took a dramatic nosedive starting about 5,500 years ago, the same
time that lakes and large animals begin to disappear from the fossil
record. The team speculates that the grasses of the early Sahara
trapped moisture that could evaporate and become new clouds--and new
rain. As desert sands took over, less water recycled to the
atmosphere, so even less rain fell and more plants died. "We can now
explain the most important changes in Saharan climate without taking
human beings into account," says team member Claudia Kubatzki.

Kutzbach says that the findings of Claussen's group "open up a
research area rather than being the final word," but he agrees with
their theory that a vicious feedback cycle between vegetation and the
atmosphere could force dramatic changes. More specifically, the role
that plants play in their own sustenance can be key to their

Just because the Sahara apparently dried up because of natural causes
does not mean that humans are off the hook. Noting that as much as 30
percent of the rainfall in a tropical rain forest has cycled through
the leaves and roots of its Flora, Kutzbach and his colleagues
suggest, based on their own climate simulations, that cutting down
trees could produce a feedback cycle similar to what the earth's
changing orbit set off in the Sahara. "If you deforest, the rain
washes down the Amazon rather than going back into the clouds to form
rain," Kutzbach says. And pumping ever more carbon dioxide and other
greenhouse gases into the atmosphere may do more than slowly warm the
planet. "We can't specify what will happen," Claussen says, "but
we're assessing the danger of climate surprises."

--Sarah Simpson



From David DeGraff <>

The Amateur-Professional Minor Planet Workshop 2000 will be hosted by
Alfred University's Stull Observatory From Thursday June 8th to
Sunday  July 11th.

Alfred University is in Alfred NY, which is in the western part of
the state,  not to far from the wineries of the finfger lakes and not
too far from Niagara Falls.  Rochester is the nearest Airport, and
the University can provide transportation to and from there. We are 4
hours from Toronto, Albany,  Cleveland, and Pittsburg. 6 hours from
NYC and Washington, DC. 8 hours from Boston.

We don't have many of the details worked out yet, but we wanted to
let people start planning to make the trip.

Housing will be in the dorms a very short walk from the meeting
location. The total cost, including housing, registration and meals,
including a banquet will be around $200.

More information can be found at
including links to the Observatory and to the University.  There is
also an IDA map of New York showing Alfred's Location.  Folks out
west may snicker, but we have verydark skies for the East.

Firm information and registration dates will be posted as they become

David DeGraff
784 Stull Observatory
Alfred University
Alfred NY


From Patrick Michel <>

Dear Benny,

Please find below the First announcement of a School on Singularities
in Gravitational Systems and application to chaotic transport in the
solar system that we would like you to send to the CCNET list.

Sincerely Yours,

Patrick MICHEL


                            IN THE SOLAR SYSTEM

Dear Colleague,

We have the pleasure to inform you that a Winter School on the  topic:
"Singularities in gravitational systems - Application to the chaotic
transport in the Solar System", will take place at Arc 2000 (French
Alps, France) on March 12-18, 2000.

A participation form (to be sent via Email to:, and
to is included at the end of the present message,
after the general information and preliminary program. Please, note
that this School will be open to a limited number of people and we
recommand to return the form as fast as possible. A second
announcement, including a detailed meeting schedule and all the
relevant logistic information will be  sent later.

Directors of the School:


Observatoire de la Cote d'Azur
UMR 6529 Cassini
B.P. 4229
06304 Nice Cedex 4


OBJECTIVES: to provide a deeper knowledge to researchers working in 
the different fields of dynamics and even of plasma physics on topics
which have undergone parallel and certainly complementary developments,
among communities which have a few occasions to meet, essentially
mathematicians of dynamical systems and physicists in dynamical fields,
familiar with numerical tools.

MOTIVATIONS: The theory of chaos plays a major role in various domains,
not only in modern physics (e.g., celestial mechanics, fluid mechanics,
solid state physics, ...) but also in other branches of knowledge such
as biology and economics. Major results have been obtained specially in
astronomy and gravitational systems.

A first kind of chaos due to the interaction between different types of
resonances (mean motion and secular), refered as "weak chaos", has been
detected in the motion of terrestrial planets of our Solar System by
Jacques Laskar; theoricists in the 60s had already discovered this kind
of chaos and the presence of invariant tori in hamiltonian system with
small perturbations (KAM theorem).

Another source of chaos exists and does not involve the interaction
between resonances. However, it is also responsible for the fast
transport as well as for the diffusion outside the Solar System of
small bodies such as comets and potentially dangerous asteroids
(Near-Earth asteroids). The sources of this kind of chaos are planetary
close approaches, and this is the main topic of the school.

The objectives and organization of the school are thus:

- To provide a clear introduction to mathematical methods as well as a
state of the art on the theory of singularities in gravitational
systems. One third of the school will be devoted to this aspect.
Lectures will be presented by mathematicians and physicists who are
used to numerical tools.
  The second part of the school will be devoted to the modelisation and
to the construction of mappings in which delta functions are used to
simulate close approaches as "shocks".

  The third part will present the state of the art on the diffusion of
comets, asteroids, meteroids and planetary rings. The application of
the knowledge on close approaches (singularities) to the determination
of spacecraft's trajectories, like Cassini, will also be presented.
  Since, the gravitational forces are not the only forces which have a
denominator proportional to the square of the distance, the community
working on plasma physics may also be interested in this school.

- Between the proposed lectures, a certain number of short
communications on the topic will be presented mainly by young


Location and fees: the school will take place in Arc 2000, one of the
best ski resort of the French Alps, easily reachable by train from
Paris as well as from many european cities. Inscription fees and full
board for 6 nights in a good standing Hotel will not exceed a total of
2800 French Francs per person (extra charges for single rooms).   

                      PRELIMINARY PROGRAM

Lectures will be presented from 8:30 to 12:00 and from 17:00 to 20:00.
Free time can be devoted to physical and/or intellectual activities.

Alessandra CELLETTI (4h):

"Classical Results on Collisions and the Levi-Civita Transformation"
The Levi-Civita and KS transformations; Regularization by inversion in
velocity space; Birkhoff's transformation; Perturbation theories.
Joerg WALDVOGEL (4h):

"Triple Collision and Close Triple Encounters" Painleve's theorem and
Sundman's theorem; triple collision and Siegel's series; close triple
encounters; McGehee's triple-collision manifold..
Yves ELSKENS (3h):

"Dynamical and Kinetic Aspects of Collisions" Hard collisions and
hyperbolic dynamics; Statistics: few-body problem and chaos; Many-body
viewpoint: kinetic theory, H-theorem; Elastic and inelastic collisions.

In addition, two seminars by Gabriella DELLA PENNA and Corrado
FALCOLINI are already scheduled on regularizing transformations.

Giovanni VALSECCHI (4h):

Close planetary approaches; semi-analytical models; application to the
dynamics of meteoroids.

Andrea MILANI (4h):

Close planetary approaches; numerical tools; transitory proper orbital
elements for Near-Earth asteroids.

Jean-Marc PETIT (3h):

Modelisation of the dynamics of planetary rings; chaotic diffusion.

Hans RICKMAN (3h):

Dynamics of long period comets; origin and diffusion through
the solar system; The problem of short-period comets: from the
Edgeworth-Kuiper belt to the Jupiter family; Monte-Carlo models and
Markov process.

Yves LANGEVIN (2h):

Planetary transfer of space probes.

3 hours of communications will be added to these lectures.


                     Participation form

Name: .................

Full address: ........................

Professional situation: ..........................

E-mail: ............................

Tel.: ....................................

( ) I will certainly attend
( ) I might possibly attend; please send me the second announcement
( ) I will not attend; you can delete me from the mailing list

To be sent to:
and to


Y.V. Skorov*) & N.I. Komle: Mass and energy balance in the near-surface
layers of a cometary nucleus. ICARUS, 1999, Vol.140, No.1, pp.173-188


In the near future, several space missions are scheduled, which
will closely investigate short-period comets. Some of these (e.g.,
ROSETTA) will also deliver landing probes for in situ investigations of
the nucleus' surface. Therefore there is now renewed interest in the
structure and behavior of cometary surface layers. In this paper we
discuss in detail some basic features that may constitute the
'landscape' of a comet nucleus: gas outflow from gaps and holes is
considered, based on a fundamental kinetic approach and previous work.
Both the gas emission from ice-dominated walls and the outflow through
'dusty' channels with a sublimating icy bottom are calculated. We
attribute special emphasis to the influence of recondensation on the
energy balance. It is found that-assuming realistic temperature
profiles as a function of depth-there exist areas of net recondensation
along the walls of ice channels, which might act as interior heat
sources. Furthermore, our Monte Carlo model is applied to interpret
some results obtained in various comet simulation (KOSI) experiments,
which were performed in the years 1989-1993 with porous ice samples
irradiated in the space simulator at DLR Cologne/Germany, It was found
that when using the measured temperature profile inside the ice sample
Monte Carlo calculations predict that the recondensation region
coincides with the region of the steepest temperature gradient. Another
result relevant for the understanding of cometary surface phenomena is
that at least in first order the temperature difference between a
sublimating ice front and the surface temperature of an overlying
porous dust mantle or nonvolatile cohesive residuum is almost
independent of the thickness of the nonvolatile layer. (C) 1999
Academic Press.


C.M. Lisse*), Y.R. Fernandez, A. Kundu, M.F. AHearn, A. Dayal, L.K.
Deutsch, G.G. Fazio, J.L. Hora, W.F. Hoffmann: The nucleus of comet
Hyakutake (C/1996 B2). ICARUS, 1999, Vol.140, No.1, pp.189-204


Infrared, optical, and radio continuum observations were made of the
long-period comet C/Hyakutake 1996 B2 during its close approach to
Earth in March 1996, Using these observations to characterize the
comet's nucleus, we find an estimated nuclear radius of 2.4 +/- 0.5 km
(1 sigma) from photometric resolution of the nucleus in the thermal
infrared at 8-20 mu m on 25 March 1996, no detectable optical nuclear
emission above that of the coma on 19-23 March 1996, and a 3 sigma
upper limit to the radius of 2.7 km (assuming an emissivity of 0.9) in
the radio at 3.6 cm on 27 March 1996. The infrared color temperature of
the nucleus was consistent with a 320-K blackbody, and assuming a
2.4-km radius, the maximum effective temperature at 3.6 cm was 230 K,
We explain the optical nondetection of the nucleus as due to excess
emission from a halo of small, cold, and high optical albedo dust
particles surrounding the nucleus; such particles would have low
emissivity in the infrared and radio. A surrounding halo of icy dust
grains emitting water in addition to the nucleus accounts for the small
nuclear size but large production rate of water from C/Hyakutake;
otherwise an anomalously large fraction of the nuclear surface, nearly
100%, must be active. A fast rotation period of 6.30 +/- 0.03 h due to
coma dust cross-sectional variations was found from optical and
infrared imaging on March 20-23. The minimum bulk tensile strength
required to stabilize the comet against centrifugal breakup due to this
rotation, similar to 10(3) dynes cm(-2), is similar to that found for
other comets. (C) 1999 Academic Press.

Y.R. Fernandez*), D.D. Wellnitz, M.W. Buie, E.W. Dunham, R.L.
Millis, R.A. Nye, J.A. Stansberry, L.H. Wasserman, M.F. AHearn, C.M.
Lisse, M.E. Golden, M.J. Person, R.R. Howell, R.L. Marcialis,
J.N. Spitale: The inner coma and nucleus of Comet Hale-Bopp: Results
from a stellar occultation. ICARUS, 1999, Vol.140, No.1, pp.205-220


We discuss the properties of the nucleus and inner coma of Comet
Hale-Bopp (C/1995 O1) as derived from observations of its occultation
of Star PPM 200723 on 5 October 1996, while the comet was 2.83 AU from
the Sun, Compared to previous occultations by active comets, this is
possibly the closest to the nucleus one has ever observed, Three chords
(lightcurves) through the comet's inner coma were measured, though only
one chord has a strong indication of measuring the occultation, and
that was through thin cirrus. We have constrained the radius of the
nucleus and properties of the coma using a simple model; there is a
large valid section of parameter space. Our data show the optical depth
of the coma was greater than or equal to 1 within 20 to 70 km of the
center of the (assumed spherical) nucleus, depending on the coma's
structure and the nucleus' size. The dependence of the dust coma's
opacity on cometocentric distance, rho, was steeper than expected for
force-free, radial how being probably as steep as or steeper than
1/rho(1.4) within 100 km of the nucleus (though it is marginally
possible to fit one coma hemisphere with a 1/rho law), Assuming the
dust coma flowed radially from a spot at the center of the nucleus and
that the coma's profile was not any steeper than rho(-2) the upper
limit to the radius of the nucleus is about 30 km, though relaxing
these assumptions limits the radius to 48 km, The chord through the
coma does not show the same coma structure within 100 km of the nucleus
as that which is apparent in larger-scale (similar to 700 km/pixel)
imaging taken just before the event, suggesting that (a) the star's
path sampled the acceleration region of the dust, and/or (b) azimuthal
variation in the inner coma is different than that seen in the outer
coma. (C) 1999 Academic Press.


G.J. Black, P.D. Nicholson, W.F. Bottke, J.A. Burns, A.W. Harris: On a
possible rotation state of (433) Eros. ICARUS, 1999, Vol.140, No.1,


Due to its extremely prolate figure, Asteroid (433) Eros may exhibit an
unusual nonprincipal axis rotation state. As a result of a relatively
small difference between the maximum and intermediate moments of
inertia, a small perturbation such as the gravitational torques
experienced during a close planetary encounter could have induced a
rather large amplitude libration about the longest axis. Observation of
such a state will permit measurement of the moment of inertia ratios;
this task may be possible once the NEAR spacecraft enters orbit around
Eros in early 2000 to begin an extensive study of its composition and
structure. (C) 1999 Academic Press.


From Science Daily, 14 September 1999

14 September 1999

Your Online Identity: Researchers Study Human Interaction Online
Through Game Played In Virtual Community

Interacting online with people from throughout the world is a daily
occurrence for millions of Internet users, yet most do it with little
perspective on the virtual identity they are projecting. Now a
multiplayer online game created by researchers at the Georgia Institute
of Technology is offering insight to virtual community designers and

Called "The Turing Game," its object is to differentiate imposters from
players telling the truth. Games can cover aspects of gender, age,
race, religion, nationality, native region or any other cultural marker
of the users' choice. Differentiating imposters by the content and
style of their online written communication will reveal insights into
how various cultural markers affect a person's virtual identity,
researchers said.

"Rather than just studying identity online, why not create a way for
everyone -- netizens and scholars alike -- to learn more about it
through personal experience?" said Joshua Berman, a Georgia Tech
College of Computing doctoral student who developed The Turing Game
with his advisor Dr. Amy Bruckman. "And why not try to make it fun as
well as intellectually engaging?"

Bruckman compares The Turing Game to the old game show called "To Tell
the Truth." "You have a panel of people with all but one of them
pretending to be something they are not," she explained. "The audience
asks questions via the computer, trying to determine which panelist is
telling the truth."

The Turing Game is based on the "Turing Test," named after British
mathematician Alan M. Turing. Its intention is to see if a person could
distinguish the differences between men and women without being able to
see them -- basically doing it with written responses.

The game is a research tool for Berman's dissertation, which will
explore identity and culture in online communities from two
complementary perspectives. "I hope to help virtual community members
understand the actions which create their public identities, and to
help virtual community designers be aware of the cultural and social
affordances of the societies they " Berman said.

Virtual communities, which are growing in popularity, are creating new
educational and cultural opportunities that would not otherwise be
possible. For example, U.S. students can regularly meet online with
students across the world to play educational games and share project

"That's a powerful learning experience," Bruckman said. "The community
support found in a virtual community can provide students with a lot of

But while virtual community support is a powerful tool, it is not
fulfilling its potential effectiveness, Bruckman said. Community
designers and members must first have a better understanding of virtual

"Identity in online environments is still poorly understood," Berman
said. "As online culture becomes an increasing part of everyday
culture, it becomes more and more important for us to understand how it
affects who we are. Our research aims to expand the body of knowledge
about identity and culture online. We hope to expand that understanding
not just for scholars, but for everyone who plays The Turing Game."
Researchers hope to answer what they call some crucial questions for
virtual community designers. "Is it possible to create a genderless
classroom? A raceless courtroom? A rich environment where a user can be
not just a pseudonym, but a person with a full history of culturally
bound " they ask on their Web site. "The Turing Game is a participatory
collaborative learning experience to help us understand these


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