PLEASE NOTE:
*
CCNet, 32/2000 - 14 March 2000
------------------------------
QUOTE OF THE DAY
"The Rabinowitz et al. estimate for
the number of NEOs larger than
1 km is 700 +/-230, less than half the
value that has usually been
quoted in the past. Is this
important? Not very, from a strictly
scientific perspective (after all, how
important is it to produce
a new result that differs from a
previous result only a little bit
outside one-sigma error bars?).
"But from the perspective of
scientific politics and programmatics,
it could be very important.
The US Congress and NASA for example
(and other entities) have
adopted, as a definition of the goals of
the Spaceguard Survey,
finding 90% of NEAs larger than 1 km
diameter. If there are only
700 of them, then we are more than 40%
of the way towards
completing the goal, instead of just
18%. [...] If Rabinowitz et
al. are right, then modest extension of
current programs would
meet the goal.”
-- Clark Chapman
(1) HOW MANY NEAS?
David Morrison <dmorrison@arc.nasa.gov>
(2) THE POPULATION OF NEAR-EARTH ASTEROIDS
A. W. Harris, Jet Propulsion Laboratory
(3) NEA PERSPECTIVES
Clark Chapman, Southwest Research Institute
(4) NO FURTHER NEO-SEARCH PROGRAMMES NEEDED?
Benny J Peiser <b.j.peiser@livjm.ac.uk>
(5) EROS EXPLORATION AFTER ONE MONTH
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
(6) SCIENTISTS 'MUST TALK TO PUBLIC'
The Guardian, 14 March 2000
(7) GALACTIC WEATHER FORECAST?
Andrew Yee <ayee@nova.astro.utoronto.ca>
(8) RUSSIAN OBSERVATIONS OF MINOR PLANETS
Y.V. Batrakov et al., RUSSIAN ACADEMY OF
SCIENCE
(9) ORBITAL EVOLUTION AROUND IRREGULAR BODIES
A. Rossi et al., CNR,CNUCE
(10) THE VIOLENT HABITAT OF EARLY TERRESTRIAL LIFE
Eugene F. Milone <milone@ucalgary.ca>
(11) FASTEST-SPINNING ASTEROIDS
Petr Pravec <ppravec@asu.cas.cz>
(12) DRAGONS, BEOWULF, & FIRE FROM THE SKY
Mike Baillie <m.baillie@qub.ac.uk>
(13) SATURN V BLUEPRINTS
Michael Paine <mpaine@tpgi.com.au>
===========
(1) HOW MANY NEAS?
From David Morrison <dmorrison@arc.nasa.gov>
NEO News (3/13/00) How Many NEAs?
Dear Friends and Students of NEOs:
A topic of considerable discussion in the past few months
among
asteroid observers has centered on new estimates of the number of
NEAs (Near Earth Asteroids) larger than 1 km diameter. This
number
is of considerable interest, not only for its scientific value
but
also as a metric of the effectiveness of current NEA searches.
Below
are two of the first published discussions of this topic, in a
refereed paper in Nature by D. Rabinowitz and colleagues, and in
an
abstract submitted by Al Harris for the April meeting of the
Division
for Dynamical Astronomy. In addition, I have added a commentary
by
Clark Chapman.
It is worth noting that estimates of the numbers of NEAs brighter
than absolute magnitude H = 18.0 are somewhat decoupled from
estimates of the hazard (that is, the average interval between
impacts of a given energy). The hazard estimates are carried out
mainly on the basis of crater counts on the Earth and Moon. They
represent a historical, long-term average impact rate. In
contrast,
the estimates of current numbers of NEAs do not produce directly
a
flux of bodies striking the Earth. Thus if we conclude that there
are only half as many NEAs as was estimated by Shoemaker and
others
in the past, it does not follow that the hazard is only half as
great
(that is, the average time between impacts is twice as long). In
addition to uncertainties in the dynamics (that is, how likely it
is
for any given NEA to hit the Earth), there is also some
uncertainty
in converting from visible brightness, which is what observers
see,
to the impact energy, which is what matters in producing craters
and
other bad effects from impacts. To further complicate
matters, not
all authors agree on the definition of an NEA -- although these
papers quoted here do use the same definition, calling an
asteroid an
NEA if it has a perihelion less than 1.3 AU.
Probably the best-determined numbers are the long-term average
flux
rates of objects with a given impact energy, since these can be
derived directly from lunar crater statistics (referring to all
NEOs,
not just the asteroidal component). Both the current number
of NEAs
and the present impact rates are less clear, with uncertainties
of at
least a factor of two in each case.
If the new estimates are correct and the number of NEAs brighter
than
H=18 is only about 1000 rather than 2000, then we are of course
doing
better than we thought in discovering these objects -- the
primary
objective of the Spaceguard Survey.
David Morrison
--------------------------------------------------------------
A reduced estimate of the number of kilometre-sized near-Earth
asteroids. D. Rabinowitz*, E. Helin, K. Lawrence, S.
Pravdo: NATURE,
2000, Vol.403, No.6766, pp.165-166
*Yale University, Department of Physics
Near-Earth asteroids are small (diameters < 10 km), rocky
bodies with
orbits that approach that of the Earth (they come within 1.3 AU
Of
the Sun). Most have a chance of approximately 0.5% of colliding
with
the Earth in the next million years. The total number of such
bodies
with diameters > 1 km has been estimated to be in the range
1,000-2,000, which translates to an approximately 1% chance of a
catastrophic collision with the Earth in the next
millennium(1,2).
These numbers are, however, poorly constrained because of the
limitations of previous searches using photographic plates. (One
kilometre is below the size of a body whose impact on the Earth
would
produce global effects(3).) Here we report an analysis of our
survey
for near-Earth asteroids that uses improved detection
technologies.
We find that the total number of asteroids with diameters > 1
km is
about half the earlier estimates. At the current rate of
discovery of
near-Earth asteroids, 90% will probably have been detected within
the
next 20 years.
Copyright 2000, Institute for Scientific Information Inc.
--------------------------------------------------------------
(2) THE POPULATION OF NEAR-EARTH ASTEROIDS
The Population of Near-Earth Asteroids
A. W. Harris, Jet Propulsion Laboratory
For purposes of defining a population vs. size, we define a
Near-Earth asteroid (NEO) as one with a perihelion less than 1.3
AU,
and we measure size in terms of absolute magnitude H, rather than
actual size. Earlier estimates of the population with H
< 18.0
(generally considered to correspond to diameter > 1 km) range
from as
many as 2000 to as few as 750. I have estimated the
population in
two ways: (1) by extrapolating a model fit over the range
where
surveys are complete already (H < 14.5) down to H = 18.0, and
(2) by
dividing the numbers of already discovered asteroids by the ratio
of
the number of new to total detections (new plus re-detections) in
the
last year. Method (1) yields a population estimate of about
1500
brighter (larger) than H = 18.0), and method (2) yields about
1000.
The uncertainty in either method is about equal to the separation
between the values.
This research was supported by NASA under contract to JPL.
Abstract
submitted to the Division for Dynamical Astronomy, American
Astronomical Society, to be published in Bul. Amer. Astron. Soc.
32
(2000).
----------------------------------------------------------------
(3) NEA PERSPECTIVES
Perspective from Clark Chapman, Southwest Research Institute
The Rabinowitz et al. estimate for the number of NEOs larger than
1
km is 700 +/-230, less than half the value that has usually been
quoted in the past. Is this important? Not very, from a
strictly
scientific perspective (after all, how important is it to produce
a
new result that differs from a previous result only a little bit
outside one-sigma error bars?) But from the perspective of
scientific politics and programmatics, it could be very
important.
The US Congress and NASA for example (and other entities) have
adopted, as a definition of the goals of the Spaceguard Survey,
finding 90% of NEAs larger than 1 km diameter. If there are
only 700
of them, then we are more than 40% of the way towards completing
the
goal, instead of just 18%.
Operationally, there is a major difference: Most people in this
field
believe that, in order to multiply the detection rate by a factor
of
8 (the shortfall as previously estimated), new and larger
telescopes
would have to be constructed more-or-less immediately. On
the other
hand, if Rabinowitz et al. are right, then modest extension of
current programs would meet the goal.
===============
(4) NO FURTHER NEO-SEARCH PROGRAMMES NEEDED?
From Benny J Peiser <b.j.peiser@livjm.ac.uk>
Long before the study by Rabinowitz, Helin, Lawrence & Pravdo
on "A
reduced estimate of the number of kilometre-sized near-Earth
asteroids”
was published in NATURE earlier this year, Clark Chapman had
publicly
argued that no additional NEO-search programmes should be funded.
The
latest research on the estimated numbers of large near-Earth
asteroids
(< 1 km) has reinforced his message that a 'modest extension
of current
programs would meet the [NASA] goal'”.
NASA’s goals are based, as we know, on Clark’s and
David Morrison’s
own definition of the ‘Impact Hazard’ as documented in
the Spaceguard
Survey. If the only authoritative goal of NEO searches continues
to be
the detection of objects larger than 1 km, for the foreseeable
future,
then, indeed, no additional search programmes are required. If
Clark is
right, the NEO search community should stop lamenting the evident
lack
of appropriate funding and simply get on with the job. If, on the
other
hand, NASA's goals as such have become antiquated - and I should
remind
readers that they are increasingly regarded as incoherent and
short-sighted by a number of researchers - then Clark’s
claims may well
turn out to be extremely counterproductive for the NEO search
community. Hence, the community would be well adviced to
ascertain what
the implications of the latest NEA estimates are - as well as the
consequences these findings have for future NEO efforts and
sesarch
programmes.
Benny J Peiser
==============
(5) EROS EXPLORATION AFTER ONE MONTH
From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
13 March 2000
Applied Physics Laboratory
FOR IMMEDIATE RELEASE
NEAR Team Reports Exciting First Month of Asteroid Eros
Exploration
After scarcely a month in orbit around asteroid Eros, NASA's Near
Earth
Asteroid Rendezvous (NEAR) spacecraft is astounding scientists
with
ever more detailed views of geologic features and with technical
scientific accomplishments.
NEAR team members have found evidence of geologic phenomena that
could
have originated on a much larger parent body from which Eros was
derived. In their search to decipher the mysteries of Eros, they
have
obtained the first ever laser range returns from an asteroid and
the
first ever x-ray detection of an asteroid. High-resolution images
are
surprising scientists by the abundance of ridges, chains of
craters,
and boulders.
"Eros in our first month of observations has proven to be a
marvelous
and fascinating object," says Dr. Andrew F. Cheng, NEAR
Project
Scientist from the Johns Hopkins University Applied Physics
Laboratory
in Laurel, Md., which manages the mission for NASA.
NEAR's first x-ray detection of Eros demonstrated the presence of
magnesium, iron, and silicon and possibly aluminum and calcium.
Their
detection was the result of a brilliant solar flare on March 2,
when
NEAR was 131 miles (212 kilometers) from Eros. That solar
explosion
made it possible for the spacecraft's x-ray spectrometer to view
the
asteroid from four times farther away than it was designed to do.
"The
solar x-ray burst caused elements on the asteroid to react and
emit
fluorescent x-rays that were measured by the spectrometer,"
says Dr.
Jacob I. Trombka of NASA's Goddard Space Flight Center, who heads
the
x-ray/gamma ray instrument team. "It was only a 600-second
window of
opportunity but it is a huge bonus for the mission. This
detection at
the higher orbit gives us confidence in our ability to develop
elemental maps when we're at our operational orbit of 50
kilometers,"
he says.
In what is the first detection of a laser return from an
asteroid, the
spacecraft's laser rangefinder, operating 180 miles (290
kilometers)
from Eros, measured topographic profiles of chains of pits or
craters.
"As we accumulate more data we will be able to determine if
the
features are from erosion, fault lines, tectonic stress lines, or
other
events," says Dr. Maria T. Zuber of the Massachusetts
Institute of
Technology and NASA Goddard Space Flight Center, who heads the
laser
rangefinder science team.
In the last two weeks, the NEAR multispectral imager has returned
more
than 2,400 images. The spacecraft has been in a nearly circular
orbit
around Eros, traveling approximately 124 miles (200 kilometers)
from
the asteroid's center, and taking images closer to an asteroid
than has
ever been done before. The unprecedented images show chains of
craters,
numerous boulders as small as 55 yards (50 meters) across, and
long
ridges that extend for several kilometers across the surface.
Conspicuous on many of the crater walls are bright markings that
Dr.
Peter C. Thomas of Cornell University says are part of the loose,
fragmental material on the surface, called regolith. This
material
appears to vary in properties across the asteroid, perhaps in
response
to impact cratering events. "We have found that Eros is
literally
covered with craters smaller than about 1 mile (1.6 km) in
diameter,"
says Dr. Clark R. Chapman of Southwest Research Institute. Both
Drs.
Thomas and Chapman are members of the multispectral imager and
near-infrared spectrometer science team.
On April 1, the spacecraft will begin descending toward a 62-mile
(100-kilometer) orbit, where the resolution of the imager will
more
than double. By the mission's end in February 2001, the total
surface
will have been imaged, measured and analyzed.
For the latest images and announcements of mission progress and
discoveries visit the NEAR Web site: http://near.jhuapl.edu
--------------------------------------------------------
Media contacts:
JHU Applied Physics
Laboratory:
NASA Headquarters:
Helen
Worth
Don Savage
Laurel, MD
20723
Washington, DC
Phone:
240-228-5113
Phone: 202-358-1547
E-mail: helen.worth@jhuapl.edu
E-mail: dsavage@hq.nasa.gov
===============
(6) SCIENTISTS 'MUST TALK TO PUBLIC'
From The Guardian, 14 March 2000
http://www.newsunlimited.co.uk/uk_news/story/0,3604,146499,00.html
By Tim Radford
Scientists need to open a dialogue with the public that is
"direct,
open and timely", according to a report from the Lords
select committee
on science and technology yesterday. Public trust, shaken by BSE
and GM
foods, could only be restored by a substantial change in science
and in
scientific policy-making.
Lord Jenkin of Roding, the committee's chairman, said it was a
paradox
that this crisis of trust should come at a time when the public
was
finding science, engineering and technology more interesting and
exciting than ever.
"But the evidence of mistrust is undeniable, and must be of
deep
concern."
There remained a culture of governmental and institutional
secrecy that
invited suspicion; the public tended to question all authority,
including scientific authority; and some issues treated by
decision
makers as scientific involved many other factors which provoked
negative attitudes to science, the committee argued.
FULL STORY at
http://www.newsunlimited.co.uk/uk_news/story/0,3604,146499,00.html
===========
(7) GALACTIC WEATHER FORECAST?
From Andrew Yee <ayee@nova.astro.utoronto.ca>
ESA Science News
http://sci.esa.int
13 Mar 2000
The Heliosphere is Tilted -- implications for the 'Galactic
weather
forecast'?
Supersonic shock waves detected at the edge of the Solar System
-- a
new study by European scientists clarifies conditions at our
Earth's
outermost shield against interstellar charged particles.
The local interstellar cloud
Our Solar System entered an interstellar cloud 10,000 years ago.
Today it is speeding through this nebulosity at Mach 2 behind a
supersonic shock wave -- in much the same way that a Concorde
crosses
the Atlantic at supersonic speed. Since its formation 4.6 billion
years ago our Solar System has encountered numerous interstellar
clouds, knots, filaments, shells and bubbles of different sizes
and
contents on its path through the Milky Way. For more than 80
years
astronomers have been attracted by these past and future
encounters,
have tried to understand the physics behind them in order to
decipher
the dynamic interplay between the interstellar material and the
Solar
System.
There is some chance that the Solar System will cross small dense
clouds that have diameters up to 100 times the distance from the
Earth to the Sun. These encounters may increase the number of
interstellar charged particles bombarding Earth, with the risk of
altering the climate here. Our interstellar environment may thus
be
important for the short and long-term prospects for life on
Earth.
Even though there is still some work to be done before it will be
possible to construct a 'Galactic weather forecast', it is clear
that
for the past 200,000 years we have been in a favourable
environment
that has not altered our climate significantly (sic!). Recent
studies
by a group of European scientists of the conditions at the
outermost
edge of the Solar System using the NASA/ESA Hubble Space
Telescope
and Voyager have shown some surprising results.
The heliosphere
Charged particles from the Sun spiral out into space and form the
solar wind. The solar wind particles follow the lines of the
solar
magnetic field and fill a region of space called the Heliosphere
that
encloses the Solar System. The solar particles at the edge of the
heliosphere form a barrier to deflect other incoming charged
particles and so partially protect the inner Solar System from
the
surrounding interstellar medium. The motion of the Solar System
through the dust, gas and nebulosity that make up the
interstellar
medium give the heliosphere a comet-like shape with a head and a
tail. At the leading edge of the heliosphere, atoms and ions from
the
interstellar medium slow down as they approach the head, forming
a
shock wave, known as the interstellar bow shock. As the leader of
the
group of scientists, French astrophysicist from the Institut
d'Astrophysique de Paris, Lotfi Ben Jaffel, explains: "The
bow shock
has been predicted for more than 30 years, but its existence has
so
far been questionable. Now it seems that we have proof".
The observations
Recent analysis of observations made in the far ultraviolet with
Hubble's Goddard High Resolution Spectrograph (GHRS) has been
carried
out by the international group of scientists. By combining
measurements from the Hubble Space Telescope with Voyager
measurements, the scientists have not only located the
interstellar
bow shock, but have also discovered that the nose of the
heliosphere
points 12 deg away from the direction from which the local cloud
is
approaching. In this way the group has been able to determine the
direction of the interstellar magnetic field which causes this 12
deg
tilt.
By observing regions free of bright stars and galaxies, the team
were
able to detect a feeble ultraviolet glow called the Fermi glow,
which
arises when incoming light from stars and the Sun passes through
the
violent transition region between the heliosphere and the
surrounding
interstellar medium. By studying this faint glow and combining
the
data with intensity measurements from Voyager, Lotfi Ben Jaffel
and
his team have been able to deduce the direction of the
interstellar
magnetic field based on the observed inclination of the
heliosphere.
This discovery is highly significant as Ben Jaffel argues:
"For many
years it has been thought that the charged particles from the
interstellar medium were hitting the heliosphere head-on. Now we
see
that these ions are deflected by the interstellar magnetic field.
Only by understanding the processes at the boundary of the Solar
System can we realise what influence the interstellar medium may
have
on our planet".
The next step -- an interstellar probe
It has been a long-standing dream for the scientists to make
direct
measurements of both the heliosphere and the interstellar medium
with
a probe. This dream may well come true. Scientists are currently
investigating the different particles of interstellar origin that
have reached the inner heliosphere using the ESA/NASA solar
explorers
Ulysses and SOHO. In the long-term, NASA is working on plans to
send
a probe to investigate the boundary between the Solar System and
the
interstellar medium. This so-called 'Interstellar Probe' will fly
into the region of the bow shock closest to Earth and try to
clarify
the complex interactions occurring at this boundary. The
scientists
are excited at the prospects: "Such a probe will explore the
nature
of the interstellar medium and help predict the long-term
influence
of charged particles from the Milky Way on our weather and
climate".
They add: "The new results from Hubble and Voyager will
undoubtedly
influence the design of the 'Interstellar Probe' and help
pinpoint
the regions of greatest scientific interest".
Notes for editors
The Hubble Space Telescope is a project of international
co-operation
between NASA and ESA.
These results will be published in the May 2000 issue of
Astrophysical
Journal.
Contacts:
Lars Lindberg Christensen
Space Telescope-European Coordinating
Facility, Garching, Germany
Phone: +49-89-3200-6306
Email: lars@eso.org
Lotfi Ben Jaffel
Institut d'Astrophysique de Paris
(CNRS-INSU), France
Phone: +33-1-4432-8076
Email: bjaffel@iap.fr
Ben-Jaffel's collaborators are Romana Ratkiewicz (Space Research
Center,
Warsaw, Poland), Olivia Puyoo (IAP), C. Emerich (IAS, IAP), M.L.
Loucif
(Observatoire d'Alger, IAP), and Jay Holberg (LPL, University of
Arizona,
Tucson).
USEFUL LINKS FOR THIS STORY
* High-res images and captions
http://www.estec.esa.nl/spdwww/hst/helio.html
* Version francaise
http://www.estec.esa.nl/spdwww/hst/fr-helio.html
* Hubble European Coordinating Facility
http://www.stecf.org/
* Space Telescope Science Inst
http://www.stsci.edu/
===========
(8) RUSSIAN OBSERVATIONS OF MINOR PLANETS
Y.V. Batrakov*), Y.A. Chernetenko, G.K. Gorel, L.A. Gudkova:
Hipparcos catalogue orientation as obtained from observations of
minor planets. ASTRONOMY AND ASTROPHYSICS, 1999, Vol.352, No.2,
pp.703-711
*) RUSSIAN ACAD SCI,INST APPL ASTON,ZHDANOVSKAYA UL 8,ST
PETERSBURG 197110,RUSSIA
Ground-based photographic observations of 12 minor planets
obtained at
Nikolaev observatory (Ukraine) between 1961-1995 were reduced to
the
Hipparcos catalogue system and processed by the least squares
method
(LSM) separately and in combination with the Hipparcos NDAC (the
Northern Data Analysis Consortium) and FAST (the Fundamental
Astronomy
by Space Techniques) observations of 48 minor planets. The aim
was to
estimate the accuracy of the Nikolaev observations and to
determine the
orientation of the International Celestial Reference System
(ICRS) with
respect to the DE200/LE200 dynamic frame of reference. The
Nikolaev
observations proved to be sufficiently accurate, the unit weight
error
being of order of 0.15 ''. The results of separate processing of
Nikolaev and Hipparcos observations were not satisfactory for two
reasons. First, the Nikolaev observations only measured
accurately the
changes of orientation parameters, and second, the Hipparcos
observations only accurately measured the values of the
orientation
parameters themselves. However by combining the observations, the
accuracy was greatly enhanced. The weight of the Hipparcos
observations
was taken to be unity and that one of the Nikolaev observations
was
chosen as 0.01, in accordance with the corresponding unit weight
errors. The photocentre offsets were taken into account and three
models of these offsets were considered. In the final processing
the
offset model based on the Lommel-Seeliger law of light scattering
at
the asteroid surface was used. The values of the orientation
parameters
of the ICRS and their time changes were obtained for epoch TD
2448439.5
(in mas for epsilon and ma/year for omega). The combined solution
based
on NDAC and Nikolaev observations is epsilon(x)= 2.5+/- 1.3,
epsilon(y)
= -12.7 +/- 2.2, epsilon(z) = 1.4 +/- 3.3, omega(x) = 0.4 +/-
0.3,
omega(y) = -0.7 +/- 0.3, omega(z) = -0.9 +/- 0.6. The combined
solution
based on FAST and Nikolaev observations is epsilon(x) = 3.8 +/-
1.7,
epsilon(y) = -11.0 +/- 2.1, epsilon(z) = -3.6 +/- 3.2, omega(x) =
0.3
+/- 0.3, omega(y) = -0.6 +/- 0.3, omega(z) = -0.8 +/- 0.6. The
values
obtained for w are of the same order as their errors. The
obtained
estimates and those of Folkner et al. (1994) range in accordance
with 3
sigma-tolerances. Copyright 2000, Institute for Scientific
Information Inc.
=============
(9) ORBITAL EVOLUTION AROUND IRREGULAR BODIES
A. Rossi*), F. Marzari, P. Farinella: Orbital evolution around
irregular bodies. EARTH PLANETS AND SPACE, 1999, Vol.51, No.11,
pp.1173-1180
*) CNR,CNUCE,VIA S MARIA 36,I-56126 PISA,ITALY
The new profiles of the space missions aimed at asteroids and
comets,
moving from fly-bys to rendezvous and orbiting, call for new
spaceflight dynamics tools capable of propagating orbits in an
accurate
way around these small irregular objects. Moreover, interesting
celestial mechanics and planetary science problems, requiring the
same
sophisticated tools, have been raised by the first images of
asteroids
(Ida/Dactyl, Gaspra and Mathilde) taken by the Galileo and NEAR
probes,
and by the discovery that several near-Earth asteroids are
probably
binary. We have now developed two independent codes which can
integrate
numerically the orbits of test particles around irregularly
shaped
primary bodies. One is based on a representation of the central
body in
terms of ''mascons'' (discrete spherical masses), while the other
one
models the central body as a polyhedron with a variable number of
triangular faces. To check the reliability and performances of
these
two codes we have performed a series of tests and compared their
results. First we have used the two algorithms to calculate the
gravitational potential around non-spherical bodies, and have
checked
that the results are similar to each other and to those of other,
more
common, approaches; the polyhedron model appears to be somewhat
more
accurate in representing the potential very close to the body's
surface. Then we have run a series of orbit propagation tests,
integrating several different trajectories of a test particle
around a
sample ellipsoid. Again the two codes give results in fair
agreement
with each other. By comparing these numerical results to those
predicted by classical perturbation formulae, we have noted that
when
the orbit of the test particle gets close to the surface of the
primary, the analytical approximations break down and the
corresponding
predictions do not match the results of the numerical
integrations.
This is confirmed by the fact that the agreement gets better and
better
for orbits farther away from the primary. Finally, we have found
that
in terms of CPU time requirements, the performances of the two
codes
are quite similar, and that the optimal choice probably depends
on the
specific problem under study. Copyright 2000, Institute for
Scientific Information Inc.
=============================
* LETTERS TO THE MODERATOR *
=============================
(10) THE VIOLENT HABITAT OF EARLY TERRESTRIAL LIFE
From Eugene F. Milone <milone@ucalgary.ca>
Benny,
Andre Glikson's essay has got to be the most comprehensive
and
intelligent overview to have appeared thus far, although I am a
little
puzzled by his final comments which seem to contradict his
arguments
about apparent uniqueness. There is nothing that says that SOME
things,
after all, cannot be unique!
- gene milone
===========
(11) FASTEST-SPINNING ASTEROIDS
From Petr Pravec <ppravec@asu.cas.cz>
Dear Benny,
Daniel Fischer was right in writting that "1998 KY26 is not
the
fastest-spinning asteroid anymore" on the March 13 CCNet.
There are
at least two asteroids for that a spin period shorter than 10
minutes
has been revealed from photometric observations: 1999 SF10 with a
period of 2.47 min and 1999 TY2 with a period of 7.28 min, both
observed by Carl Hergenrother from Catalina Station, the latter
also by
my colleagues from Ondrejov Observatory. We submitted a
manuscript on
the two as well as one other superfast rotators to Icarus a few
months
ago, see a html version of the manuscript on
http://sunkl.asu.cas.cz/~asteroid/3FRAs/
Best regards,
Petr Pravec
Ondrejov Observatory
============
(12) DRAGONS, BEOWULF, & FIRE FROM THE SKY
From Mike Baillie <m.baillie@qub.ac.uk>
Benny,
you might point out to Bev Ewen-Smith and the list that I discuss
the
sixth century context of BEOWULF in my book Exodus to Arthur:
catastrophic encounters with comets (Batsford 1999).
The interesting point being that Chinese stories note dragons
wrestling
in ponds in 503 and 524 while in the West Beowulf wrestles
Grendel's
mother in a pond in a context which has independently suggested
dates
between 495 and 533. Why would two such similar concepts appear
at
almost exactly the same times so far apart? Here the Chinese
context
helps by telling us that their dragons are associated with
fireballs.
But better still they tell us that where the dragons passed
"all the
trees were broken" which could have a Tunguska type
ring to it.
Surprisingly, in Beowulf as Grendel's mother takes off across the
moors
"They followed the (her) tracks along forest paths..."
which sounds
remarkably similar to the Chinese version. In Heaney's recent
translation he says "The forest paths were marked all over
with the
monster's tracks...".
Mike Baillie
=============
(13) SATURN V BLUEPRINTS
From Michael Paine <mpaine@tpgi.com.au>
Dear Benny,
I looked further at the Saturn V story and prepared a short
article for
SPACE.com: Saturn 5 Blueprints Safely in Storage
http://www.space.com/spacehistory/saturn_five_000313.html
Paul Shawcross from NASA has since contacted me and pointed out
that
he did not make the comments attributed in the article to him. He
was
simply quoting a FAQ page in his posting on CCNet. I apologise to
Paul and the unknown author of the FAQ.
regards
Michael Paine
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