PLEASE NOTE:
*
CCNet, 7 December 1999
--------------------------------
QUOTES OF THE DAY
"The objects that caused the
brightest flashes that
we observed were probably a few
hundred kilograms"
--
David Dunham, 7 December 1999
"Of all the natural hazards
facing Earth, impacts are
the most dangerous. Unlike native
hazards of the
Earth's surface, impacts know no
size limit. Their
effects can be devastating over
the entire surface of
the planet. They are the only
credible natural threat
to human civilisation. But
impacts, especially those
of large bodies, are both
predictable and avoidable.
The Near Earth Object (NEO)
population constitutes
both an unprecedented hazard and
an unparalleled
opportunity. It is sometimes said
that there is a
fine line that separates a threat
from an
opportunity. The near-Earth
asteroids present us with
just this dilemma. They present us
with an
intelligence test of the highest
order, with the
highest possible stakes for the
human race."
--
John Lewis: Comet and Asteroid
Impact
Hazard on a populated Earth, 1999
(1) BRIGHTEST LUNAR IMPACTORS WERE PROBABLY LARGE OBJECTS
Joan and David Dunham <dunham@erols.com>
(2) A LITTLE LESS TO WORRY ABOUT FROM ON HIGH: THERE MAY
BE FEWER KILLER ASTEROIDS
USNews & World Report, 6 November 1999
(3) METEOR SHAKES THINGS UP
MSNBC, 6 December 1999
(4) ON THE ORBIT OF THE TUNGUSKA COSMIC BODY ACCORDING
TO BRONSHTEN
Luigi Foschini <foschini@tesre.bo.cnr.it>
(5) COMET AND ASTEROID IMPACT HAZARD ON A POPULATED EARTH
Michael Paine <mpaine@tpgi.com.au>
(6) PHAETHON AND THE GEMINID METEORS
NASA Science News, 7 November 1999
(7) RAPID SEA LEVEL DROP DURING THE MID/LATE HOLOCENE
ABC News, 3 December 1999
(8) AND FINALLY: WAS JUPITER THE STAR OF BETHLEHEM?
Press News Wire, 2 December 1999
=============
(1) BRIGHTEST LUNAR IMPACTORS WERE PROBABLY LARGE OBJECTS
From Joan and David Dunham <dunham@erols.com>
The objects that caused the brightest flashes that we observed
were
probably a few hundred kilograms, according to the messages
below,
since very little of the impact energy is converted into
light. I've
added a few comments about the observations in the messages,
using
" - " to preface my remarks.
There was a suggestion that sunglints from artificial satellites
might
be involved, but this is unlikely since the observations were
made late
at night local time when most of these would be deep in the
Earth's
shadow. Also, with six events simultaneously recorded at two or
more
separated locations, the chances are much greater that they are
lunar
phenomena than something closer. In the cases where lunar
location
information is available in the separate video records, there is
also
good agreement.
Brian Cudnik reports that he observed from the Houston
Astronomical
Society's site near Columbus, TX, at long. 96 deg. 39' 50"
W., lat. 29
deg. 37' 07" N., h 98m, about 100 km west of downtown
Houston.
There are many previous observations of probable lunar impacts,
although none of them apparently were confirmed. Many of these
were
published in a NASA Technical Report on transient lunar phenomena
that
is on the Web at http://www.mufor.org/tlp/lunar.html
One can also find there a link to a modern (about one year old)
effort
to videorecord TLP's simultaneously from different locations, a
"lunascan" project, which has other useful links, but
so far they don't
have news of the Nov. 18th lunar impacts. Apparently their
project has
concentrated more on the terminator and sunlit side of the
Moon. Also,
published in the Proceedings of the 48th convention of the
Association
of Lunar and Planetary Observers (Las Cruces, NM, June 25-29,
1997) is
a good paper by John Westfall on "Worthy of Resurrection:
Two Past ALPO
Lunar Projects", including one on "Lunar Meteor
Search" that includes a
table of meteor size, frequency, flash magnitude, and crater
diameter
that is in rather good agreement with the messages below, as well
as a
good history of efforts up to 1997.
Does anyone know of a Web (or other) reference to an account of
the
large impact observed by Canterbury monks in 1178 that apparently
caused the near-far-side crater Bruno? That's probably the first
observation of a lunar impact, although not confirmed from
observations elsewhere.
David Dunham, 1999 Dec. 7
_________________________________________________________
Date: Mon, 6 Dec 1999 11:30:36 -0700
To: Joan and David Dunham <dunham@erols.com>
From: Jay Melosh <jmelosh@LPL.Arizona.EDU>
Subject: References & calcs. of lunar meteor impacts
Cc: pweissman@issac.jpl.nasa.gov
Dear Joan and David:
I just heard from Paul Weissman that he estimates that your m = 3
flashes
must have been made by an object "about half a meter"
in diameter. I
have to agree with this estimate--which implies masses *much*
larger
than you have mentioned! (a half meter diameter projectile would
mass
about 500 kg). The problem is that the luminous efficiency of an
impact
onto a solid surface is *much* lower than the ca. 10% Mike Mazur
estimates for a bolide. This is discussed in detail in the
Nemtchinov
paper I mentioned in my last email, but let me work out the
consequences using Mazur's estimates for the energy released by
the
various flashes on the moon (i.e. L_obj=10^[(m-26.98)/-2.5] J/s,
and an
estimated duration of 33 milliseconds). I use Nemtchinov et al.'s
luminous efficiency estimate of
10^-4:
impactor mass,kg crater
diameter,m luminous energy,J
Magnitude,m
100
9.8
2.5e7
4.8
300
13.
7.6e7
3.6
500
15.
1.3e8
3.0
I used my web program for computing crater sizes at
www.lpl.arizona.edu/tekton/crater.html
for the crater size
computations, assuming a projectile density of 1000 kg/m^3,
impact
angle of 45 degrees, impact velocity of 71 km/sec and a target of
loose
sand (lunar regolith) with a mean density of 2500 kg/m^3.
A potentially serious problem in these estimates is the duration
of the
flash. Nemtchinov and I computed that most of the light is
emitted in a
single millisecond for a 1 m radius impactor--much shorter than
the 1/30
sec Mazur estimates! Smaller objects will produce
correspondingly shorter
flashes. However, I presume that your video camera (is it a
CCD?)
- Yes
integrates the light emitted over the duration of one frame (1/30
second?)
- Actually, with interlaced video, the even lines
are scanned in
1/60th of a second, then the odd lines
are scanned in the next
1/60th of a second to form a
1/30th-second frame. But some VCR's,
including the ones we used, can work
with the half-frames to
achieve 1/60th second time resolution.
so Mazur's estimates for total energy emitted may be correct--but
this
has to be verified before these estimates can be accepted.
If the
actual integration time was much smaller than assumed, that will
reduce
the mass of the projectile fragment accordingly.
- No, the integration time per half-frame is close
to 1/60th second;
as I understand, there is very little
"dead time" between scans
but there is some. The E flash is
curious in that I think it
peaked between two scans in my tape,
where it is almost equally
bright, but rather faint, around 7th
mag., on two successive
half-frames. But it must have been
brighter, around 5th mag.,
on a single half-frame for it to show up
so well in Sada's tape.
The other uncertainty is that Nemtchinov et al. assumed an impact
velocity of 30 km/sec. It is possible that the luminous
efficiency is
higher at 70 km/sec, and this is something that should be looked
into,
but it seems unlikely it will be off by as much as a factor of 4.
I hope this is of help to you.
- Yes, certainly, many thanks.
Sincerely, Jay Melosh
###########################################################################
Jay
Melosh
Tel: (520) 621-2806
Professor of Planetary
Science
Fax: (520) 621-4933
Lunar and Planetary
Lab
email: jmelosh@lpl.arizona.edu
University of Arizona
Tucson AZ 85721-0092
_________________________________________________________
Date: Mon, 06 Dec 1999 20:00:35 -0800
From: R Clark <rclark99a@earthlink.net>
To: dunham@erols.com
Subject: size of lunar leonid impacts
Hello Dr. Dunham,
I was very excited to hear that several impacts on the lunar
surface
had been detected during the Leonids. The possibility of such
observations has been examined several times over the years, and
generally ruled out as a difficult project with little likelihood
of a
quick success. However the high efficiency and capabilities of
modern
sensors and their widespread use by amateurs has now made the
observation a reality.
In the discussion of the observations you mention questions about
the
size of the impactors that produced these flashes. You mention
size
estimates ranging from >1000 kg to ~100 grams. I am curious
about how
the latter figure reached you.
- The lower estimates were from
well-intentioned astrophysical
calculations by others who,
however, did not realize the
very low fraction of energy that
is transformed into visible
light during these impacts.
For the size of the objects that produced these flashes, I have
to
agree with the earlier figure... even though I am probably the
source
of the latter. In my thesis at the University of Arizona I
studied the
detectability of a different feature associated with lunar
impacts.
High velocity lunar impacts produce several phenomena that may be
observed. The impact produces shockwaves in the target that may
be
detected as seismic energy. The Apollo missions left a network of
seismometers which detected numerous impacts between 1969 and
1977 when
they, and the remaining active Apollo surface instruments were
foolishly shut down. (the old story about spending $40 billion to
plant
a flag but not being able to afford the $50K/yr to receive and
archive
the low but unending volume of science data still being returned)
Impacts of objects down to a few kg were detected with this
networ
- It sure would have been nice to have had ALSEP
observations
of the Nov. 18th impacts! Someone
should have thought to try
to look for flashes from separate
observatories before shutting
down the network. Of course, the
widespread availability of
inexpensive sensitive CCD video cameras
was key to this effort
(the cameras we used only cost $80), and
these didn't exist
in 1977.
Another impact phenomenon, probably the most obvious thing to
look for,
is the flash produced by the impact fireball. At velocities above
~12
km/sec (virtually all impacts of asteroidal or cometary material
at
Earth) the impacting object and some ammount of target material
(increasing with higher velocities) will be vaporized to
incandescent
temperatures. The radiation from this fireball will have its peak
intensity at visible or UV wavelengths, quickly dropping into the
IR as
the gasses disperse and cool. The fraction of the impact energy
partitioned into the initial fireball is generally at most 10%.
Only a
small fraction of this energy is released as 'visible' radiation
while
the fireball gas is still hot and dense enough to radiate
efficiently.
This has now been observed!
A very large fraction of the total impact energy (~60%) ends up
as
thermal energy in the immediate vicinity of the newly formed
impact
crater. This is what I was studying. After modeling cooling
craters to
determine their radiative characteristics, I considered how to
detect
them against the background of the cold lunar nightside with
groundbased, LEO, and lunar orbiting sensors. For space based
sensors
the optimum wavelength range is in the 1-6 micron range. In the
case of
a lunar orbiting sensor I concluded that an impact <100gm may
be
detetable. For groundbased observations most of this wavelength
range
is unavailable, although the 2 micron window might allow impacts
of a
few kg or less to be detected, depending on scattering of light
from
the sunlit portion of the disk and skyglow.
I am very pleased, and more than a little surprised, at how
quickly the
(groundbased!) detection of any lunar impact events has come
within the
grasp of modern instruments and sensors.
Richard Clark
rclark@lpl.arizona.edu
Joan and David Dunham
7006 Megan Lane
Greenbelt, MD 20770
(301) 474-4722
dunham@erols.com
=============
(2) A LITTLE LESS TO WORRY ABOUT FROM ON HIGH: THERE MAY
BE FEWER KILLER ASTEROIDS
From USNews & World Report, 6 November 1999
http://www.usnews.com/usnews/issue/991206/asteroid.htm
By Charles W. Petit
Barring some sort of human-devised defense measures, astronomers
assure
us that an asteroid will some day hit Earth hard enough to
threaten
civilization. Such blows are typically a million or so years
apart, so
the risk to mankind is low. But the lower the better, and in a
sharp
revision of this prediction, two new independent analyses
conclude that
there may not be quite as many planet-busting rocks lurking
undetected
in our neighborhood as has been feared.
The standard guesstimate has been that 2,000 or more potential
doomsday
asteroids, those more than one kilometer, or 0.6 mile wide,
near
Earth's orbit. That size stone, plummeting into the crust at
65,000
miles per hour, experts say, would disrupt life worldwide, by
both
blasting the surface and by filling the air with
sunlight-blocking
dust. An asteroid (or, possibly, a comet) around 10 kilometers
wide
probably ended the reign of dinosaurs 65 million years ago.
New, more-detailed statistical estimates indicate there are only
around
700 such large asteroids with orbits that come within about 5
million
miles of Earth's orbit.
With about 400 such asteroids already discovered, the lower
estimate
boosts the chances that NASA can achieve a 10-year goal to spot,
in
concert with teams of skywatchers worldwide, at least 90 percent
of
asteroids near enough and big enough to bring global catastrophe.
The
idea is to use modern astronomical instruments to actually check
for
any giant asteroids that, like fateful bullets, have Earth's name
on
them.
The lower estimates come independently from groups lead by
astronomers
David Rabinowitz at Yale University, and William Bottke at
Cornell. The
two groups have revealed their conclusions to colleagues in
scientific
meetings in recent months and confirmed them to U.S. News while
preparing detailed reports for publication in scientific
journals.
Donald Yeomans, manager of NASA's Near-Earth Objects Program at
the Jet
Propulsion Laboratory in Pasadena, describes the new numbers as
very
persuasive. Rabinowitz believes most near-Earth asteroids are
debris
from the main asteroid belt between Mars and Jupiter, which has
tens of
thousands of asteroids, some nearly 600 miles across. He is
encouraged
by the lower number but says it is no reason to stop worrying.
"It
doesn't matter much whether the interval is a little longer.
The point is, this is something that could wipe us out and could
happen
any time."
The search goes on. New equipment and stepped up surveillance,
paid for
by both the Air Force and NASA, found 66 large near-Earth
asteroids
just in the 12 months ending in September. The most successful
asteroid
spotters, at a program called LINEAR, headquartered at MIT's
Lincoln
Laboratory near Boston, use experimental Air Force tracking
telescopes
at White Sands Missile Range in New Mexico.
Most asteroid hunters are in the business to glean basic
information
about the raw materials of Earth and other planets. However,
Richard
Binzel of MIT says the search for Earth-bashers is "probably
the most
practical application of astronomy to the benefit of society.
Most of
what we do is a fabulous endeavor to find out things about the
distant
universe, but this is something that could save a lot of
lives."
Levels of danger.
In addition to the LINEAR program, other asteroid-watching
efforts
include NEAT (for Near-Earth Asteroid Tracking), run by NASA
scientists
using Air Force telescopes in Hawaii; a University of Arizona
program,
Spacewatch, and several others in the United States and overseas.
The
clearinghouse for new sightings is the Minor Planet Center in
Cambridge, Mass., operated by the Smithsonian Institution, which
alerts
astronomers worldwide. Yeomans's team at JPL is among the world
leaders
at turning astronomical observations into an orbital calculation
and
estimating whether the asteroid's path will eventually bring it
nearor
intoEarth. Astronomers even have a new "Torino Scale,"
analogous to the
Richter earthquake scale, to assign levels of danger. So far,
everything is a safe zero on the scale (with 10 denoting a
certain
impact big enough to create global disaster).
While discovery rates are already high, astronomers say it will
take
newer, larger telescopes to find every large asteroid. If all
those
are spotted, then astronomers can begin cataloging smaller ones.
There
may be tens of thousands of potentially dangerous asteroids
ranging in
size from 50 meters to a kilometer, big enough to devastate a
state or
small nation. Such impacts occur every few centuries but most,
during
recorded times, have hit unpopulated areas.
And if a big stone is detected headed our way? "We don't
really have a
plan," says NASA's Yeomans. The presumption is that any such
asteroid
will be many cycles of its orbit away from smacking Earth, giving
many
years to decades of warning. Yeomans figures the potential killer
then
becomes a military problem.
Presumably, with enough time to do the job, nuclear explosives or
other
means could nudge an asteroid to a safe trajectory.
Copyright 1999, USNews & World Report
=============
(3) METEOR SHAKES THINGS UP
From MSNBC, 6 December 1999
http://www.msnbc.com/local/wvtm/139588.asp
BIRMINGHAM, December 6, 1999 – A meteor streaking through
the sky early
Sunday morning, lit up the night in three states. Police
dispatchers
were busy with hundreds of phone calls about the mysterious light
and
loud boom that followed.
A black and white surveillance camera caught Sunday’s
mysterious light
on tape. A pitch black street gets lighter, then bright as day.
As
quickly as it happens, everything returns to darkness.
"It was so bright. It was brilliant and the most beautiful
thing I’ve
ever seen. The light shone through the clouds about ten seconds.
It’s
beautiful blue and white light, and I’d say it could be seen
from
hundreds of miles around," said Bessemer police officer
Stephen
Williams.
Williams was patrolling 9th Ave. around 4 a.m. when he saw the
dim
light in the sky get brighter. He said it looked like someone was
turning up a dimmer switch. At one point, the light was as bright
as
day. He was relieved to find out about 10 other patrol officers
had
seen it too.
"Somebody keyed up the radio and said ‘Did you guys see
that?’ It was
pretty amazing, and we were all radioing back and forth as to
what we
pondered it might be. We got together a little later, and we were
all
still shaking,” said Williams.
About 10 seconds after the light disappeared, Williams said he
heard a
loud noise, similar to a sonic boom. "We did get a lot of
calls,
apparently the light shone bright enough to light up the inside
of
houses. The boom was very loud and it shook some people’s
windows,”
said Williams.
Brad Young was with friends in Albertville about 75 miles away
when he
saw the mysterious and light and heard the boom.
"It was like a white ball of fire flying through the sky and
had a big
long tail behind it, and then a couple of seconds later it like
bang,
made a loud bang, and then it was gone," said Young.
DON’T EXPECT A CRATER
Georgia astronomer Dave Dundee has been fielding questions about
the
sightings. He believes it was caused by a meteor. That’s
defined as a
solid body from outer space which glows with heat generated by
friction
as it enters the Earth’s atmosphere.
Roswitha Becker with Birmingham Southern’s planetarium says
what
happened Sunday night wasn’t too far out of the ordinary.
"The Earth is
bombarded by thousands of debris on a daily basis, of space dust
that
just falls on the Earth," said Becker.
Rocks and debris travel so fast, they leave glowing tunnels of
hot air
in the sky. "It’s friction. They’re hitting the
air, and they heat up
air molecules very hot next to them... It’s a florescent
sort of
effect," said planetarium worker Kurt Bachmann.
Scientists say, despite the loud boom which followed Sunday’s
bright
light, more than likely nothing ever hit the ground. National
Weather
Service expert Kevin Pence says shooting stars are a lot like
what we
see and hear in a thunderstorm.
"You know, the lightening first, and then the rapid
explosion or
expansion of a channel of air, creates sound waves," said
Pence.
Copyright 1999, MSNBC
=============
(4) ON THE ORBIT OF THE TUNGUSKA COSMIC BODY ACCORDING
TO BRONSHTEN
From Luigi Foschini <foschini@tesre.bo.cnr.it>
Dear Benny,
The two recently published papers by Bronshten about the Tunguska
Cosmic Body (TCB), and that you cited in recent issues of CCNet,
raised
several questions. I would like to add some notes to the debate.
I have read the recent paper by Bronshten about possible
constraints to
the possible orbit of the TCB (V.A. Bronshten: "Trajectory
and orbit of
the Tunguska meteorite revisited". Meteoritics and Planetary
Sciences
34, (1999), A137-A143). I think that Bronshten used the principle
"post
hoc, ergo propter hoc": indeed, he started considering
geocentric
speeds of the TCB from 25 to 40 km/s and he deduced that cometary
orbits are consistent with trajectory data. However, an asteroid
with
speed higher than or equal to 25 km/s is quite rare: it is known
that
the typical NEO speed is around 16 km/s. Therefore, when
Bronshten
selected the speed range, he implicitly considered typical
cometary
bodies. It is then obvious that he obtained that TCB was a comet.
Cheers,
Luigi
=================
(5) COMET AND ASTEROID IMPACT HAZARD ON A POPULATED EARTH
From Michael Paine <mpaine@tpgi.com.au>
Book Review: Comet and Asteroid Impact Hazard on a populated
Earth. by
Dr John Lewis, Lunar and Planetary Laboratory, University of
Arizona.
Academic Press, 1999
Dr Lewis has developed a computer program to run simulations of
asteroid
and comet impacts with the Earth or other selected planets. This
works
as a Monte Carlo simulation, where events occur randomly over
time and
various parameters such as the size, composition, speed and
trajectory
of the impactor and the human population density of the target
area are
also randomly generated. In this way the impact hazard can be
assessed
by looking at the odds over many runs or extended time periods
(hence
the Monte Carlo gambling reference).
Lewis points out that hazard estimates based on
"typical" events tend to
overlook unusual phenomena that can be quite destructive. For
example
the Monte Carlo simulation shows that, over thousands of years,
airburst
events like Tunguska turn out to be important sources of
fatalities.
These types of events are very localized and may leave little or
no
physical evidence so information about the hazard is unlikely to
be
reliably passed on from generation to generation. Another
significant
source of localized fatalities is the impact of iron
asteroids/meteoroids. Because they comprise just a few percent of
all
Near Earth Objects (NEOs) they tend to be ignored in
"typical" hazard
assessment and yet they are much more destructive than
"stony" NEOs of
the same size.
Although basically a handbook for the software, the book contains
a
wide range of physical information about NEOs, impacts with
planets,
effects on the human population, detection techniques and
mitigation.
It is an excellent scientific resource covering physics,
chemistry and
the environment perturbation and has 8 pages of bibliography. It
is
therefore an important reference for anyone studying the possible
influence of impacts on the biosphere and human civilisation.
The book includes a diskette with the Monte Carlo program. It
requires
GW-BASIC to run (if all else fails a copy of BASIC.EXE (GW-BASIC)
can be
downloaded from http://www2.tpgi.com.au/users/aoaugh/basic.exe).
To run
the program in a higher version of BASIC such as Quick Basic you
will
need to convert it from binary to ASCII format from within
GW-BASIC. To
do this load the program in GW-BASIC
(F3"path/filename.BAS") then save
it with the ASCII option set (F4
"path/new_filename.BAS" , A ). This is
all subject to the copyright conditions of course.
The book and program include estimates of fatalities from tsunami
generated by impacts. My own investigations have revealed a wide
difference in estimates of the size and range of tsunami
generated by
asteroid impacts (see
http://www1.tpgi.com.au/users/tps-seti/spacegd7.html.
Lewis uses the
most pessimistic of these estimates. Even with these high
estimates the
proportion of all fatalities caused by tsunami only begins to
approach
that of direct effects (such as firestorms and blast waves) on
timescales of millions of years so the assumptions about tsunami
should
not make a huge difference to outcomes over centuries. In any
case,
BASIC programmers could quite easily edit the couple of lines of
code
that cover tsunami in order to try out more optimistic
parameters.
The final paragraph is an excellent summary of the impact hazard
situation and (continuing the gambling theme) suggests that our
society
is taking a grave gamble if it ignores the NEO threat:
"Of all the natural hazards facing Earth, impacts are the
most
dangerous. Unlike native hazards of the Earth's surface, impacts
know no
size limit. Their effects can be devastating over the entire
surface of
the planet. They are the only credible natural threat to human
civilisation. But impacts, especially those of large bodies, are
both
predictable and avoidable. The Near Earth Object (NEO) population
constitutes both an unprecedented hazard and an unparalleled
opportunity*. It is sometimes said that there is a fine line that
separates a threat from an opportunity. The near-Earth asteroids
present
us with just this dilemma. They present us with an intelligence
test of
the highest order, with the highest possible stakes for the human
race."
* see Riches in the Sky, also by John Lewis - mining
resource-rich
asteroids that are more accessible than the Moon.
Review by Michael Paine, 7 Dec 1999,
=================
(6)PHAETHON AND THE GEMINID METEORS
From NASA Science News, 7 November 1999
http://science.nasa.gov/newhome/headlines/ast06dec99_1.htm
Great Geminids!
On December 13 and 14, 1999, fragments of the mysterious asteroid
3200
Phaethon will strike Earth's atmosphere and produce a beautiful
sky
show.
December 7, 1999: The 1999 Leonid meteor storm was a rare treat
for
many skywatchers in Europe and the Middle East, but a bit
disappointing
in other parts of the world. If you missed the Leonid display
because
of poor weather, or perhaps because you live in the wrong place,
there's still one more chance in 1999 to see a good meteor
shower: the
Geminids.
The shower officially begins on December 7th, but it doesn't peak
until
the morning of the 14th around 3 a.m. PST (1100 UT). Unlike the
Leonids, the Geminid's broad maximum lasts nearly a full day, so
observers around the globe have a good chance to see the show. At
its
peak the Geminids could produce as many as one shooting star
every 30
seconds.
For observing tips see www.Geminids.com
Most well known meteor showers, like the Perseids and Leonids,
are old.
They've been observed for hundreds or even thousands of years.
The
earliest record of a modern-day meteor shower is probably a
notation in
Chinese annals dated 36 AD, regarding the Perseids, where it is
said
that "more than 100 meteors flew thither in the
morning." [ref.]
The Geminids are a different story. The first Geminid meteors
suddenly
appeared in the mid-1800's. Those early showers were
unimpressive,
boasting a mere 10-20 shooting stars per hour. Since then,
however, the
Geminids have grown in intensity until today it is one of the
most
spectacular annual showers. In 1998 observers counted as many as
140
per hour (zenithal hourly rate). Sky-watchers with clear skies
should
see at least that many this year if the Geminids continue to
intensify.
After the discovery of the Geminids in 1862 astronomers began
searching
for the parent comet. Most meteor showers result from debris that
that
boils off a comet's nucleus when it passes close to the sun. This
debris orbits the sun along with the comet, forming a thin,
elongated
stream of meteoroids that become shooting stars when they hit
Earth's
atmosphere.
Years of searching proved to no avail until finally, in 1983,
NASA's
Infrared Astronomical Satellite discovered a curious object
moving in
the same orbit as the Geminid meteoroid stream. The orbital match
was
so good that it had to be the source of the debris, but to the
surprise
of many it wasn't a comet. The source of the Geminids was
apparently a
rocky asteroid.
3200 Phaethon, as the asteroid is now known, is in a highly
elliptical
1.4 year orbit that brings it within 0.15 AU (astronomical units)
of
the Sun. It made its closest recent approach to Earth in December
1997
when it passed within 0.31 AU of our planet.
But how does an asteroid produce a meteoroid debris stream?
Comets do
it easily whenever they pass close enough to the sun to heat
their
frozen nucleus. Tiny bits of ice and dust naturally bubble away
into
interplanetary space. Rocky asteroids are made of tougher stuff,
however, so it is unclear how bits of 3200 Phaethon would break
or boil
off to form a meteoroid stream.
One of the earliest ideas was that Phaethon might occasionally
collide
with other asteroids. Collisions would create a stream of
pulverized
rocks that would account for the Geminids meteor shower.
Phaethon's
orbit passes through the asteroid belt just beyond Mars, so at
first
this hypothesis seemed likely, but more detailed studies
disagree. The
orbits of individual Geminid meteoroids are not consistent with
the
idea that they broke free while in the asteroid belt. Instead,
they
appear to have crumbled away when Phaethon was closer to the Sun.
In
this respect Phaethon is behaving like a comet.
So, which is it?
Is Phaethon a comet or an asteroid?
There are arguments in favor of both. Phaethon's spectra look
like
those of a rocky asteroid, but its orbit is similar to that of a
comet.
When Phaethon passes by the sun it doesn't develop a cometary
tail, but
bits and pieces do break off to form the Geminid meteoroids. By
studying photographic records of fireballs, scientists have
estimated
the density of the Geminid meteoroids to be between 1 and 2
gm/cc.
That's less dense than typical asteroid material (3 gm/cc), but
several
times denser than cometary dust flakes (0.3 gm/cc). Many
astronomers
now believe that Phaethon is an extinct or dormant comet that has
accumulated a thick crust of interplanetary dust grains.
Phaethon's
thick mantle gives it the outward appearance of an asteroid, but
underneath lies the nucleus of a comet.
The origin of the Geminids may not be fully understood until
future
space travelers pay a visit to the asteroid-comet 3200 Phaethon.
Until
then we can still enjoy the sky show and savor the mystery of the
enigmatic Geminids.
=================
(7) RAPID SEA LEVEL DROP DURING THE MID/LATE HOLOCENE
From ABC News, 3 December 1999
http://abcnews.go.com/wire/US/reuters19991203_3880.html
Scientists Challenge Conventional Sea Level Theory
SYDNEY (Reuters) - Australian scientists say they have
discovered evidence of rapid change in world sea levels and
of a dramatic fall in geologically recent times --
directly challenging current conventional wisdom.
Dr Robert Baker of the University of New England, in the
New South Wales country town of Armidale, has tapped the
secrets of worm coatings on once-submerged rocks to shake
established theory that sea levels are presently as high
as they have ever been.
Based on height measurements of worm coatings on rocks now
well above sea level, and carbon dating tests which show
them to be as recent as 3,500 years old, Baker argues that
sea levels have not been steady since the last ice age, as
is commonly believed.
Instead, he told Australia's ABC television, it changed
rapidly 3,000-5,000 years ago.
"It means that the whole natural system is unstable, it's
been unstable for 130,000 years."
Baker and his colleagues at New England University say the
sea level may have fallen quickly 3,500 years ago, by as
much as a meter in just 10-50 years.
This means that the current rise in the sea level --
normally associated with environmental warming caused by
the so-called greenhouse effect -- might not be that
unusual, Baker said.
He also said that his evidence pointed to the controversial
conclusion that sea levels had once been higher than they
are now.
"The conventional wisdom has been that sea levels
haven't
been higher. (Contrary) evidence was something that they
weren't prepared to accept," he said.
Baker's theories, which he first aired 20 years ago, were
initially rejected, but are now about to receive a wider
audience with their publication in the respected journal
Marine Biology.
The implications go further than greenhouse and global
warming. Baker said big movements in sea levels could
explain the migration of Australian Aboriginies and give
clues about the fate of ancient civilizations such as in
Egypt.
Copyright 1999, ABC News
==================
(8) AND FINALLY: WAS JUPITER THE STAR OF BETHLEHEM?
From Press News Wire, 2 December 1999
http://www.prnewswire.com/cgi-bin/stories.pl?ACCT=104&STORY=/www/story/12-02-1999/0001087025&EDATE=
Astronomer Identifies Star of Bethlehem with Roman Coin He Bought
at
New York Coin Show: Coin Held Clue to Finding Wise Men's Star -
It was
Jupiter
NEW BRUNSWICK, N.J., Dec. 2 /PRNewswire/ -- With an ancient Roman
coin
he bought at a New York coin show Michael Molnar, a computer
programmer
with a doctorate in astronomy, says he has identified the Star of
Bethlehem as Jupiter.
Molnar was able to determine with the coin the date of the star's
appearance. He realized that the Romans put the astrological sign
of Aries on the coin to represent Judea which they took over in
AD
6. Molnar explains his findings in his book, The Star of
Bethlehem: The Legacy of the Magi ($25, Rutgers University Press,
800-446-9323).
"My fellow astronomers looked for spectacular planetary
alignments, blazing comets, and exploding stars," says
Molnar. "I
stumbled on the answer from a Roman coin that I bought for
$50.
Aries on the coin showed me where the Star of Bethlehem
appeared."
With that important clue about Aries he figured that a
special
celestial event in Aries showed the Wise Men that a king was born
in Judea. Molnar then researched what happened in Aries
that
attracted the Wise Men.
"Jupiter was the star of kings," says Molnar.
"And everybody
knows from the Bible that the star rose in the east, which is
exactly what Jupiter did in Aries on April 17, 6 BC. This
date
agrees very well with estimates of Christ's birth."
Molnar's findings have been hailed by other astronomers as an
important milestone in understanding what happened in the sky
over
Bethlehem two millennia ago.
"Molnar's Star of Bethlehem is a fascinating
contribution," says
Professor Owen Gingerich, a leading authority on astronomy
history
at Harvard University. He calls Molnar's book, "the
most original
and important contribution of the entire twentieth century"
to
understanding the Star of Bethlehem.
The Star of Bethlehem: The Legacy of the Magi, $25, Rutgers
University
Press, 800-446-9323, is also available from retail bookstores and
Amazon.com.
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CCNet-ESSAY: ON THE CAUSE OF ICE-AGES
By Fred Hoyle and Chandra Wickramasinghe
"The renewal of ice-age
conditions would render a
large fraction of the world's
major food-growing
areas inoperable, and so would
inevitably lead to the
extinction of most of the present
human population.
Since bolide impacts cannot be
called up to order, we
must look to a sustained
greenhouse effect to
maintain the present advantageous
world climate.
This implies the ability to inject
effective
greenhouse gases into the
atmosphere, the opposite of
what environmentalists are
erroneously advocating."