PLEASE NOTE:
*
CCNet, 8 October 1999
------------------------
QUOTE OF THE DAY
"If either object is confirmed to exist,
researchers will have
a whole new batch of theories to create about
how comets are
formed, how they evolve, and how some are set
on devastating
paths toward Earth.”
-- Roy Britt, Explorezone, 7 October 1999
(1) NEW OBSERVATIONS ELIMINATE ASTEROID IMPACT THREAT
SpaceViews, 7 October 1999
(2) ANOTHER ASTEROID THREAT DOWNGRADED
MSNBC, 7 October 1999
(3) IS PLUTO A KUIPER BELT OBJECT AFTER ALL?
SpaceDaily, 6 October 1999
(4) POSSIBLE 10th and 11th PLANET-LIKE OBJECTS ORBITING THE SUN
EXPLOREZONE, 7 October 1999
(5) ASTEROID BRAILLE
Michael Paine <mpaine@tpgi.com.au>
(6) COMET AND ASTEROID HUNTERS' DISCOVERY STREAK CONTINUES
Andrew Yee <ayee@nova.astro.utoronto.ca>
UP & DOWN & DOWN & UP: CLIMATE
RESEARCH GOES BANANAS
(7) GLOBAL WARMING CAN MAKE SEA LEVEL PLUNGE
BBC Online Network, 7 October 1999
(8) NATURE BLAMED FOR MELTING ICE
BBC Online Network, 7 October 1999
(9) COMETS & COMMUNICATION - AMATEURS vs PROFESSIONALS
W. Orchiston, NATIONAL OBSERVATORY OF NEW
ZEALAND
(10) TV OBSERVATION OF THE 1098 LEONID METEOR SHOWER
J. Watanabe et al., NATIONAL
ASTRONONOMICAL OBSERVATORY
(11) THE LEONID METEOR STORMS OF 1833 and 1966
D.J. Asher, ARMAGH OBSERVATORY
(13) METEOR STORM FORECASTING
I. Ferrin, UNIVERSITY OF LOS ANDES
(14) TRAJECTORY & ORBIT OF THE TUNGUSKA METEORITE REVISITED
V.A. Bronshten, VARSHAVSKOYE RD
16-130,MOSCOW 113105,RUSSIA
(15) SIMULATION OF PROCESSES ON COMET NUCLEI
D.W.G. Sears et al, UNIVERSITY OF
ARKANSAS
(16) WEAK RESONANCES DRIVE ASTEROIDS TOWARD TERRESTRIAL
PLANETS ORBITS
A. Morbidelli, D. Nesvorny, COTE AZUR
OBSERVATORY
(17) ORBITAL EVOLUTION OF PLANETS
J.M. Hahn & R. Malhotra, LUNAR &
PLANETARY INST
(18) INTERPLANETARY DUST AS FLUFFY AGGREGATES
R. Nakamura & H. Okamoto, KOBE
UNIVERSITY
=============
(1) NEW OBSERVATIONS ELIMINATE ASTEROID IMPACT THREAT
SpaceViews, 7 October 1999
http://www.spaceviews.com/1999/10/07d.html
New observations of a near-Earth asteroid have eliminated the
extremely small risk reported earlier this week that the object
could
collide with the Earth next century.
Astronomers had reported earlier this week that asteroid 1999
RM45
had a less than 1-in-100 million chance of striking the Earth in
either 2042 or 2050. That prediction was based on about a week's
worth of observations after its mid-September discovery by the
LINEAR
telescope in New Mexico.
However, Rob McNaught, an astronomer with the Australian National
University, was able to recover the asteroid after the
announcement
of its potential impact risk. The positions from those
observations
have refined the orbit of the asteroid and eliminated any
probability
of an impact in either 2042 or 2050.
FULL STORY at http://www.spaceviews.com/1999/10/07d.html
===================
(2) ANOTHER ASTEROID THREAT DOWNGRADED
From MSNBC, 7 October 1999
http://www.msnbc.com/news/319598.asp
Another asteroid threat downgraded
Further observations eliminate risk of catastrophic collision
By Alan Boyle
MSNBC
Oct. 7 — Further observations have led asteroid-watchers to
cross
another potential threat off their list. Earlier this week,
astronomers said there was a very slight chance that Asteroid
1999
RM45 could hit Earth in the year 2042 or 2050 — but even
that risk
has now been eliminated.
FULL STORY at http://www.msnbc.com/news/319598.asp
==================
(3) IS PLUTO A KUIPER BELT OBJECT AFTER ALL?
From SpaceDaily, 6 October 1999
http://www.spacedaily.com/spacecast/news/pluto-99c.html
Possible Pluto Debris Spotted
Boulder - October 6, 1999 - Planetary astronomers working in the
Space
Studies Department (Boulder) of San Antonio-based Southwest
Research
Institute (SwRI) suggest that some Kuiper Belt objects (KBOs) may
be
leftover shards from the giant collision that created the
Pluto-Charon
system.
Pluto-Charon is the only known double planet in the solar system,
orbiting about 40 times as far away from the Sun as Earth. It is
embedded in the Kuiper Belt of planetesimals, comets, and
miniature icy
worlds that surround our planetary system in a thick disk. The
Kuiper
Belt is a larger and more populous, icy-rich analog to its better
known
cousin, the Asteroid Belt of rocky debris orbiting between Mars
and
Jupiter.
Astronomers have suspected for more than a decade that Pluto and
its
1200 km-wide satellite, Charon, formed as a pair during a giant
collision in the ancient past between proto-Pluto and another
Kuiper
Belt object. Evidence for this collision includes the orbital
configuration, the relative masses, and the angular momentum of
the
Pluto-Charon system.
Now, SwRI astronomers Drs. Alan Stern, Robin Canup, and Daniel
Durda
have found clues that some KBOs in neighboring orbits to Pluto
may, in
fact, be debris created in the Pluto-Charon forming event. Their
results are being presented Tuesday, October 12, at the American
Astronomical Society's (AAS) Division for Planetary Sciences
meeting in
Padua, Italy.
The evidence found by the SwRI team linking some KBOs called
"Plutinos"
to Pluto-Charon comes in three forms. First, there is a close
orbital
similarity between some KBOs and Pluto that is consistent with
the
expected distribution of debris from the Pluto-Charon formation
event.
Second, the colors of Pluto and some KBOs, and Charon and other
KBOs,
suggest similar surface compositions. Third, the apparent size
distribution of the objects that suggest themselves as potential
shards
of the Pluto-Charon forming collision is similar to both
laboratory
results from studies of catastrophic collisions and asteroid belt
families known to result from collisions.
Future research will be required to prove this new hypothesis,
dubbed
"Pluto's Family." If borne out by future tests, it
would constitute the
first discovery of a genetically related, parent-daughter family
of
objects in the Kuiper Belt. Further, because the KBO region
surrounding
Pluto has been known for some time to be delivering some comets
to
Earth's vicinity, the new work suggests that a small, but
nonetheless
important, fraction of the comets observed by astronomers may
actually
consist of samples of Pluto and Charon.
SwRI is an independent, nonprofit, applied engineering and
physical
sciences research and development organization with a staff of
2,700
and an annual research volume of $304 million.
Copyright 1999, SpaceDaily
===================
(4) POSSIBLE 10th and 11th PLANET-LIKE OBJECTS ORBITING THE SUN
From EXPLOREZONE, 7 October 1999
http://explorezone.com/archives/99_10/07_new_planet.htm
By Robert Roy Britt, explorezone.com
After studying 13 comets whose orbits seem to have been affected
by a
massive object far beyond Pluto, a British scientist announced
today
that there may be a previously unknown and very large planet
orbiting
the Sun.
Separately, three astrophysicists are set to propose that a brown
dwarf lurks in the outer reaches of our solar system. Brown
dwarfs
are near-stars that never gathered enough fuel to become true
stars;
they can be many times more massive than Jupiter.
If either object is confirmed to exist, researchers will have a
whole
new batch of theories to create about how comets are formed, how
they
evolve, and how some are set on devastating paths toward Earth.
The two studies combined also open the possibility that a host of
unfound planet-like objects may one day be added to our current
nine-item list.
John J. Matese, a physics professor at the University of
Louisiana,
writes of the possible brown dwarf, based on a study of 82 comet
trajectories, in an upcoming issue of the journal Icarus. Matese
told
explorezone.com that it's "very plausible" that there
are several
unknown planets and/or brown dwarfs orbiting the Sun.
"We suggest that this (brown) dwarf may only be the most
likely to be
detected during this epoch," Matese said, adding the
cautionary note
that "the existence of the object we discuss is not
certain."
The brown dwarf would not likely have formed from the disk of gas
and
dust out of which the other planets were born, Matese and his
colleagues say.
The possible 10th planet
Writing in the Oct. 11 issue of the Monthly Notices of the Royal
Astronomical Society, Open University professor John Murray says
he
observed 13 long-period comets -- objects that orbit the Sun but
also
travel to the far reaches of the solar system -- whose
trajectories
follow a similar arc. Murray suggests the common path is the
result
of a gravitational deflection caused by a planet-like object at
least
as massive as Jupiter.
Interestingly, Murray told explorezone.com that his research had
been
done in 1996, rejected by two science journals, and only recently
been accepted for publication.
"It was finally accepted by the Monthly Notices of the Royal
Astronomical Society in May this year, but was still not really
taken
very seriously until a few days ago, when (Matese's team)
announced
that they have come to a very similar conclusion."
Murray describes his planet-like object as a lone wanderer that
has
been captured into an orbit around the Sun after a journey
through
space. Its orbit, he says, is in the opposite direction as the
nine
known planets (a motion called retrograde). He says the object
orbits
the Sun 32,000 times farther away than Earth (some
3,000,000,000,000
miles from the Sun) where it would be extremely faint and slow
moving, and so could have escaped detection by present and
previous
searches for distant planets. He estimates the object takes 6
million years to orbit the Sun.
The telltale comets
Long-period comets are believed to originate in a vast
'reservoir' of
potential comets, known as the Oort cloud, surrounding the solar
system at distances between about 10,000 and 50,000 astronomical
units from the Sun. (One AU is the average distance between Earth
and
the Sun.) These comet define the edge of our solar system, nearly
200,000 times as far from the Sun than is the Earth. Estimates
for the
number of these comets range into the hundreds of billions.
These Oort cloud comets visit the inner solar system when their
usual, remote orbits are disturbed. Only when they pass close to
the
Sun do these icy objects grow the bright heads and tails that
give
them the familiar form of a comet (and make them observable).
Murray notes that the Oort cloud comets reaching the inner solar
system include a group coming from directions in space that are
strung
out along an arc across the sky. He argues that this could mark
the
wake of some large body moving through space in the outer part of
the
Oort cloud, giving gravitational kicks to comets as it goes.
"Not only were there more of them between 30,000 and 50,000
times the
distance of the Earth from the Sun, but their furthest points
were
found to lie in a band across the sky," Murray told
explorezone.com.
"This is precisely what would be expected if their orbits
had been
disturbed in the past by the gravitational attraction of a large
planet. Looking at the orbital revolution periods of the comets
concerned gave me a clue as to where the planet had been in its
orbit
at certain times in the past, from which a distance and present
position
of the planet could be derived."
The object would have to be at least as massive as Jupiter to
alter
the comets' orbits to the extent observed, Murray says. But
current
leading theories of how the solar system formed cannot easily
explain
the presence of a large planet so far from the Sun, so Murray
speculates that such an object, if it exists, will be planetary
in
nature and would have been captured into its present orbit
sometime
after the solar system formed, even though the probability of
such an
event seems low based on current knowledge.
"I won't be certain until somebody actually finds it,"
he said. "You
have to keep an open mind about other possible explanations for
this
non-random behavior, but at the moment, it seems to me the most
likely possibility."
Possible brown dwarf
If Murray's proposed object were several times more massive than
Jupiter, it could possibly be a brown dwarf, a massive star-like
object that fell just short of the required mass and energy to
stimulate stellar processes. Still, brown dwarfs are huge, and
Murray
argues that if the object that seems to be deflecting comets were
a
brown dwarf, it is more likely it would have been detected
already.
Matese, the University of Louisiana professor who proposes there
is a
brown dwarf out there, did similar research on comets from the
Oort
cloud.
Matese calls the possible brown dwarf a companion to the Sun. He
estimates its mass at three times that of Jupiter and puts it at
25,000 AU from the Sun (roughly in the middle of the range of
Murray's estimate.)
In the analysis of highly accurate orbits of 82 Oort cloud
comets,
Matese and his colleagues detected a pattern connecting the
orientation and shape of a comet's orbit path. They suggest that
this
can best be explained if there is an object approximately 3 times
as
massive as Jupiter orbiting in the vicinity of Oort cloud comets.
Matese says he can't predict the object's exact location, but
says it
is potentially observable by radio telescopes and next-generation
infrared telescopes. He told explorezone.com that it is possible
that
his brown dwarf and Murray's possible planet are in fact the same
object, but that he had not yet studied Murray's results
thoroughly.
At an Oct. 11 meeting of the American Astronomical Society in
Italy,
Matese and his colleagues will explain that if the brown dwarf is
confirmed to exist it will "play a major role in our
understanding of
how comets get into the Oort cloud and how they return. Comets
from
the Oort cloud are important contributors to the impact history
of
the Earth and are often claimed to be the cause of mass
extinctions
like the one in which the dinosaurs, and many other species,
disappeared from the fossil record 65 million years ago."
What's next?
Before either object might be added to any lists, it would have
to be
observed directly.
"It needs to be confirmed by pictures taken with the largest
telescopes on Earth," Murray said of his possible planet.
"The
problem is that it will be very faint (about 10 million times
fainter
than the faintest star visible to the naked eye) and will move
very
slowly, so two photographs taken about six months apart would be
best
to show the movement, though most of the movement would be due to
the
Earth moving in its orbit. There is also a big area to search, as
there is a good bit of uncertainty in its predicted
position." ez
Copyright 1999, Explorezone
================
(5) ASTEROID BRAILLE
From Michael Paine <mpaine@tpgi.com.au>
Dear Benny,
Robert Clements brought this item to my attention: NASA's
Spacewarn
Bulletin 550 1 September 1999
http://nssdc.gsfc.nasa.gov/spacewarn/spx550.html
"The encounter phase of the Deep Space 1 (1998-061A)
spacecraft with
the kilometer-sized asteroid, Braille, was only a partial
success. The
camera was not oriented correctly to obtain images of the
asteroid
close up. However, the orbital parameters of the asteroid
determined
during the cruise phase has indicated that the asteroid will
probably
enter an Earth orbit and soon burn out in the atmosphere in a few
thousand years."
I have been unable to find another reference to this interesting
news
so treat it with caution, particularly the mention of "Earth
orbit".
Also this is a 2 by 1 km object so I don't think "burn out
in the
atmosphere" is the appropriate term to use!
Michael Paine
============
(6) COMET AND ASTEROID HUNTERS' DISCOVERY STREAK CONTINUES
From Andrew Yee <ayee@nova.astro.utoronto.ca>
From Albuquerque Journal, 6 October 1999
[http://www.abqjournal.com/scitech/1sci10-06-99.htm]
By John Fleck, Albuquerque Journal Staff Writer
A team of New Mexico astronomers has discovered a comet that
could be
bright enough to be seen with the naked eye next summer.
It's the latest in a string of significant finds for the LINEAR
telescope outside Socorro. In recent weeks, it has also:
* Found the fourth object ever discovered that has a chance --
extremely remote at this point -- of hitting Earth
* Rediscovered a "Trojan," a rare type of asteroid
shepherded in a
stable orbit by the planet Jupiter. The asteroid was first seen
in
1906, then lost to science until the LINEAR telescope picked it
up in
September.
In the two years since the LINEAR telescope began hunting for
faint moving objects in the sky, it "has moved our knowledge
of
the entire asteroid population forward by decades relative to the
pace of discovery before they came on line," said Steve
Chesley,
an astronomer at the University of Pisa in Italy.
LINEAR -- the Lincoln Near-Earth Asteroid Research -- uses an Air
Force telescope at the north end of White Sands Missile Range. It
is run by Lincoln Laboratory at the Massachusetts Institute of
Technology.
LINEAR uses instruments developed for satellite tracking to scan
the sky each night, looking for objects that move.
Using an automated system and a highly sensitive electronic
camera, the LINEAR team has revolutionized the hunt for asteroids
and comets.
And the pace of discovery is likely to increase, with a second
telescope at the site beginning operation this month.
The latest comet discovery is one of a series that has
established
LINEAR as the dominant comet-hunting team in the world.
Of the 14 comets currently visible through telescopes, seven were
discovered by LINEAR.
A comet's performance is notoriously difficult to predict, but
the
one found last month by LINEAR could be bright enough to be of
scientific importance, said Brian Marsden, who tracks comets for
the International Astronomical Union.
The newly discovered comet isn't going to be a dramatic sight
like
Comet Hyakutake in 1996 or Hale-Bopp in 1997, but it could be
bright enough to be easily seen with binoculars, or possibly with
the naked eye, Marsden said.
"We're doing pretty well on comets these days," said
Grant Stokes,
director of the LINEAR program.
Chesley is interested in asteroids that have the potential to
collide with Earth. On Tuesday he circulated electronic mail
asking telescope users around the world to begin tracking an
asteroid discovered by LINEAR last month.
Based on preliminary data, the object has a tiny chance of
hitting
Earth in 2042 or 2050, Chesley calculated. The risk is so small
that it "is not of serious concern to the public at
large,"
Chesley said.
More observations are needed to pin the orbit down more
precisely,
Chesley said in an e-mail interview Tuesday.
The oddest discovery is the Trojan asteroid spotted in September.
Studying old data, Gareth Williams of the International
Astronomical Union realized the object had been seen in 1904 by
Edward Emerson Barnard with what was at the time the largest
telescope in the world, at Yerkes Observatory in Wisconsin.
Copyright 1999, Albuquerque Journal
================
UP & DOWN & DOWN & UP: CLIMATE RESEARCH GOES BANANAS
(7) GLOBAL WARMING CAN MAKE SEA LEVEL PLUNGE
From the BBC Online Network, 7 October 1999
http://news.bbc.co.uk/hi/english/sci/tech/newsid_467000/467928.stm
Global warming can lead to a dramatic fall in sea level, says a
US geologist.
This suggestion is the opposite of the generally-expected effect
of
rising temperature. And while it is unlikely to happen in the
near
future, Dr John Bratton of the US Geological Survey says the
process
behind it could offset the sea level rises which are predicted to
flood low-lying areas of the world. It could also explain
mysterious
plunges in sea level in warmer periods in the Earth's geological
past.
Ice cool
The sea level drops could be caused by the melting of
"clathrates".
These are sea-floor crystals of ice which enclose gases such as
methane. When the crystals melt, the gas bubbles away and other
gases
trapped in the ocean sediments below could also be released.
In the worst circumstances, the "hole" left behind
could result in a
sea level drop of 25 metres but Dr Bratton told BBC News Online
that
his conservative estimates suggest a drop of up to 1.5m.
"Any
temperature rise will start to melt clathrate," he says.
"The
apparent massive hydrate melting about 60 million years ago was
triggered by an increase in bottom water temperatures of about
four
degrees centigrade.
"Therefore, it appears that the process could get going with
an
increase of even one or two degrees, especially in the polar
regions
where gas hydrate is abundant."
Balancing act
This is quite possible but any actual drop in sea level would be
countered by the simultaneous melting of the Earth's polar ice
caps.
The rising temperatures would also cause ocean waters to expand.
Dr Bratton says the predicted drop resulting from clathrate
melting
"is of the same order of magnitude as those associated with
thermal
expansion of the oceans, melting of non-polar ice and melting of
the
West Antarctic ice sheet."
If correct, this is good news for threatened coastal areas but Dr
Bratton warns that the release of methane, a greenhouse gas, to
the
atmosphere could itself have a significant effect in driving
further
climate warming.
Positive feedback
Asked if this is a worry, Dr Bratton says: "Yes, definitely,
although
not everyone agrees the effect would be that significant relative
to
anthropogenic forcing by carbon dioxide emissions.
"Almost everyone agrees that hydrates melt when climate
warms. The
debate is now about whether hydrates may actually drive natural
climate warming or whether they just go along for the ride."
Dr
Bratton's research is published in the journal Geology.
By BBC News Online's Damian Carrington.
Copyright 1999, BBC
===========
(8) NATURE BLAMED FOR MELTING ICE
From the BBC Online Network, 7 October 1999
http://news.bbc.co.uk/hi/english/sci/tech/newsid_467000/467847.stm
By Environment Correspondent Alex Kirby
US scientists have suggested that the gradual melting of the huge
West
Antarctic ice sheet may not be the fault of humankind - at least
not
yet.
The work warns that climate change could hasten the total
collapse of
the sheet.
The research, published in the journal Science, concludes that
the
melting, which began nearly 10,000 years ago, may well continue
until
the sheet collapses completely.
The sheet is about 360,000 square miles in area, roughly the size
of
Texas and Colorado together.
If it did collapse, global sea levels would rise by up to five or
six
metres, enough to submerge many coastal regions. But collapse is
unlikely, on present trends, for another 7,000 years. Professor
Howard
Conway of the University of Washington, one of the scientists
involved
in the research, said there might be nothing anyone could do to
slow or
reverse the melting.
Accelerated melting
But he said climate change resulting from human activities could
bring
about the collapse of the sheet sooner than natural causes would.
"Collapse appears to be part of an ongoing natural cycle,
probably
caused by rising sea levels which were inititated by the melting
of the
northern hemisphere ice sheets at the end of the last ice age.
"But the
process could easily speed up if we continue to contribute to
warming
the atmosphere and oceans."
Much of the ice sheet on the Antarctic land mass lies below sea
level,
which makes it very vulnerable to rising sea levels.
The researchers used evidence gathered from raised beaches - land
which
rose after the ice pressing it down had melted - and through
radar
imaging of sub-surface ice structures.
This enabled them to establish that the ice sheet has both
thinned and
decreased in area since the last glacial maximum, 20,000 years
ago.
The ice was once up to 800 metres thick. But melting has relieved
its
pressure on the land so much that some of the raised beaches are
now 30
metres above sea level.
The researchers also found that the boundary between floating ice
and
ice grounded on the sea floor has withdrawn by an average of
about 100
metres annually for the last 7,600 years, a total of about 12,800
kilometres (800 miles) since the last ice age. That average is
similar
to the current rate, and there is no indication that the retreat
is
slowing.
Copyright 1999, BBC
================
(9) COMETS & COMMUNICATION - AMATEURS vs PROFESSIONALS
W. Orchiston: Comets and communication: Amateur-professional
tension
in Australian astronomy. PUBLICATIONS OF THE ASTRONOMICAL SOCIETY
OF
AUSTRALIA, 1999, Vol.16, No.2, pp.212-221
*) NATIONAL OBSERVATORY OF NEW ZEALAND,CARTER OBSERV, POB 2909,
WELLINGTON,NEW ZEALAND
Australasian amateur astronomers, Grigg and Ross, discovered four
different comets between 1902 and 1907. Controversy surrounding
these
discoveries led to a deterioration in relations between
Australia's
leading amateur astronomers and Baracchi at Melbourne
Observatory,
and to the eventual transfer of the 'Australian Central Bureau'
to
Sydney Observatory. Copyright 1999, Institute for Scientific
Information Inc.
=========
(10) TV OBSERVATION OF THE 1998 LEONID METEOR SHOWER
J. Watanabe*), S. Abe, H. Fukushima, D. Kinoshita: TV observation
of
the Leonid meteor shower in 1998: No strong activity over Japan.
PUBLICATIONS
OF THE ASTRONOMICAL SOCIETY OF JAPAN, 1999, Vol.51,
No.4, pp.L11-L14
*) NATIONAL ASTRONONOMICAL OBSERVATORY,MITAKA,TOKYO 1818588,JAPAN
We carried out a high-sensitivity TV observation of the Leonid
meteor
shower at the Nobeyama Radio Observatory, from 15h20m UT through
20h24m UT on 1998 November 17. We detected 58 Leonid meteors,
along
with 99 sporadic meteors. The peak of the activity, as expected
around 20h UT; was not clearly observed. The average influx rate
of
meteoroids was 1.4 x 10(-5) km(-2) s(-1) (mag less than or equal
to
+8) during our observation. Copyright 1999, Institute for
Scientific
Information Inc.
=========
(11) THE LEONID METEOR STORMS OF 1833 and 1966
D.J. Asher: The Leonid meteor storms of 1833 and 1966. MONTHLY
NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 1999, Vol.307, No.4,
pp.919-924
ARMAGH OBSERV,COLL HILL,ARMAGH BT61 9DG,NORTH IRELAND
The greatest Leonid meteor storms since the late eighteenth
century
are generally regarded as being those of 1833 and 1966. They were
evidently due to dense meteoroid concentrations within the Leonid
stream. At those times, the orbit of Comet 55P/Tempel-Tuttle was
significantly nearer that of the Earth than at most perihelion
returns, but still some tens of Earth radii away. Significantly
reducing this miss distance can be critical for producing a
storm.
Evaluation of differential gravitational perturbations, comparing
meteoroids with the comet, shows that, in 1833 and 1966
respectively,
the Earth passed through meteoroid trails generated at the 1800
and
1899 returns. Copyright 1999, Institute for Scientific
Information
Inc.
=========
(13) METEOR STORM FORECASTING
I. Ferrin: Meteor storm forecasting: Leonids 1999-2001. ASTRONOMY
AND
ASTROPHYSICS, 1999, Vol.348, No.1, pp.295-299
*) UNIV LOS ANDES,CTR THEORET ASTROPHYS,MERIDA 5101,VENEZUELA
We present a method for meteor storm forecasting, that we apply
to
the Leonids in 1999-2001. The method makes use of a plot where
the
particle density distribution around the comet is mapped (Fig. 1)
and
isolines of equal meteor intensity are drawn. The most
significant
result found is the existence of a ''ridge'' or region of high
particle density, that corresponds to the great Leonid storms and
that we identify with the ''dust trails'' that Sykes et al.
(1990)
and Sykes & Walker (1992) found behind all periodic comets.
We
present detailed calculations of the trajectories of meteoroids
that
will reproduce this ridge. We predict the intensity of upcoming
Leonid showers by the position of the Earth in relation to the
isolines. For 1999 we predict a zenith hourly rate (ZHR) of 3.5
K+/-1
K. For the year 2000 we can only limit the intensity to 5 K <
ZH R <
20 K. And for 2001 the ZHR will only reach to 400+/- 100.
Copyright
1999, Institute for Scientific Information Inc.
=============
(14) TRAJECTORY & ORBIT OF THE TUNGUSKA METEORITE REVISITED
V.A. Bronshten: Trajectory and orbit of the Tunguska meteorite
revisited. METEORITICS & PLANETARY SCIENCE, 1999, Vol.34,
No.SS,
pp.A137-A143
VARSHAVSKOYE RD 16-130,MOSCOW 113105,RUSSIA
A critical survey is presented of all determinations of the
azimuth
and inclination of the Tunguska meteorite's trajectory based
either
on eyewitness testimonies or on the mathematical treatment of the
forest-leveling field in the area of the catastrophe. The
eyewitness
testimonies collected in the neighborhood of the Nizhnyaya
Tunguska
River indicate the most probable azimuth of the trajectory
projection
to be 104 degrees from the north to the east, which is close to
the
most recent azimuth estimate from the forest-leveling field, 99
degrees. For the most part of the trajectory, its inclination
could
not exceed 15 degrees. However, it is seen from aerodynamic
calculations that the combined action of the gravity field and
a nonzero aerodynamic lift could increase the inclination to 40
degrees as the end of the trajectory was approached. Meteoroid
orbits
are calculated for a broad family of trajectories with azimuths
ranging from 99 degrees (Fast et al., 1976) to 137 degrees
(Krinov,
1949) and geocentric velocities ranging from 25 to 40 km/s.
Orbits
with large azimuth values (120 degrees and larger) are shown to
belong to the asteroidal type. They are succeeded by the orbits
of
short-period and long-period comets, whereas very small azimuth
values and large geocentric velocities correspond to the region
of
hyperbolic orbits. Certain restrictions on the possible
trajectory
azimuths and geocentric velocities of the Tunguska body are
imposed
by this study. Copyright 1999, Institute for Scientific
Information
Inc.
===========
(15) SIMULATION OF PROCESSES ON COMET NUCLEI
D.W.G. Sears*), H.W. Kochan, W.F. Huebner: Laboratory simulation
of
the physical processes occurring on and near the surfaces of
comet
nuclei. METEORITICS & PLANETARY SCIENCE, 1999, Vol.34, No.4,
pp.497-525
*) UNIVERSITY OF ARKANSAS,DEPT CHEM & BIOCHEM,COSMOCHEM
GRP,FAYETTEVILLE,AR,72701
Laboratory comet simulation experiments are discussed in the
context
of theoretical models and recent ground-based and spacecraft
observations, especially the Giotto observations of P/Halley. The
set-up of various comet simulation experiments is reviewed. A
number
of small-scale experiments have been performed in many
laboratories
since the early 1960s. However, the largest and most ambitious
series
of experiments were the comet simulation experiments known as
KOSI
(German = Kometen Simulation). These experiments were prompted by
the
appearance of Comet P/Halley in 1986 and in planning for the
European
Space Agency's Rossetta mission that was originally scheduled to
return samples. They were performed between 1987 and 1993 using
the
German Space Agency's (DLR) space hardware testing facilities in
Cologne. As with attempts to reproduce any natural phenomenon in
the
laboratory, there are deficiencies in such experiments while
there
are major new insights to be gained. Simulation experiments have
enabled the development of methods for making comet analogues and
for
exploring the properties of such materials in detail. These
experiments have provided new insights into the morphology and
physical behavior of aggregates formed from silicate grains
likely to
exist in comets. Formation of a dust mantle on the surfaces and a
system of ice layers below the mantle caused by chemical
differentiation have been identified after the insolation of
the artificial comet. The mechanisms for heat transfer between
the comet's surface and its interior, the associated gas
diffusion
from the interior of the surface, and compositional, structural,
and
isotopic changes that occur near the surface have been described
by
modeling the experimental results. The mechanisms of the ejection
of
dust and ice grains from the surface and the importance of
gas-drag
in propelling grains have also been explored. Copyright 1999,
Institute for Scientific Information Inc.
==========
(16) WEAK RESONANCES DRIVE ASTEROIDS TOWARD TERRESTRIAL
PLANETS ORBITS
A. Morbidelli, D. Nesvorny: Numerous weak resonances drive
asteroids
toward terrestrial planets orbits. ICARUS, 1999, Vol.139, No.2,
pp.295-308
*) OBSERV COTE AZUR,BP 4229,F-06304 NICE 4,FRANCE
A systematic exploration of the chaotic structure of the asteroid
belt is presented, first taking into account only the
perturbations
provided by the four giant planets and then including also the
effects of the inner planets. We find that both the inner belt (a
<
2.5 AU) and the outer part of the main belt (a > 2.8 AU) are
mostly
chaotic. In the outer part of the belt, chaos is due to the
presence
of numerous mean-motion resonances with Jupiter and three-body
resonances, Jupiter-Saturn-asteroid. In the inner belt; chaos is
generated by mean motion resonances with Mars and three-body
resonances, Mars-Jupiter-asteroid. Due to the chaoticity of the
belt,
asteroids tend to slowly migrate in eccentricity. This phenomenon
of
''chaotic diffusion'' allows many bodies in the inner belt to
become
Mars-crossers. The number of asteroids leaking out from the inner
belt is large enough to keep the population of Mars-crossing
asteroids in steady state, despite of the short dynamical
lifetime of
the latter (similar to 25 Myr). We speculate that chaotic
diffusion
could have substantially eroded the high-eccentricity part of the
asteroid belt, thus providing the impactors responsible for the
Late
Heavy Bombardment phase of the early Solar System. (C) 1999
Academic
Press.
===========
(17) ORBITAL EVOLUTION OF PLANETS
J.M. Hahn*), R. Malhotra: Orbital evolution of planets embedded
in a
planetesimal disk. ASTRONOMICAL JOURNAL, 1999, Vol.117, No.6,
pp.3041-3053
*) LUNAR & PLANETARY INST,3600 BAY AREA BLVD,HOUSTON,TX,77058
The existence of the Oort comet cloud, the Kuiper belt, and
plausible
inefficiencies in planetary core formation all suggest that there
was
once a residual planetesimal disk of mass similar to 10-100 M+ in
the
vicinity of the giant planets following their formation. Since
removal of this disk requires an exchange of orbital energy and
angular momentum with the planets, significant planetary
migration
can ensue. The planet migration phenomenon is examined
numerically by
evolving the orbits of the giant planets while they are embedded
in a
planetesimal disk having a mass of M-D = 10-200 M+. We find that
Saturn, Uranus, and Neptune evolve radially outward as they
scatter
the planetesimals, while Jupiter's orbit shrinks as it ejects
mass.
Higher mass disks result in more rapid and extensive planet
migration. If orbital expansion and resonance trapping by Neptune
are
invoked to explain the eccentricities of Pluto and its cohort of
Kuiper belt objects at Neptune's 3:2 mean motion resonance, then
our
simulations suggest that a disk mass of order M-D similar to 50
M+ is
required to expand Neptune's orbit by Delta a similar to 7 AU, in
order to pump up Plutino eccentricities to e similar to 0.3. Such
planet migration implies that the solar system was more compact
in
the past, with the initial Jupiter-Neptune separation having been
smaller by about 30%. We discuss the fate of the remnants of the
primordial planetesimal disk. We point out that most of the
planetesimal disk beyond Neptune's 2:1 resonance should reside in
nearly circular, low-inclination orbits, unless there are (or
were)
additional, unseen, distant perturbers. The planetesimal disk is
also
the source of the Oort cloud of comets. Using the results of our
simulations together with a simple treatment of Oort cloud
dynamics,
we estimate that similar to 12 M+ of disk material was initially
deposited in the Oort cloud, of which similar to 4 M+ will
persist
over the age of the solar system. The majority of these comets
originated from the Saturn-Neptune region of the solar nebula.
Copyright 1999, Institute for Scientific Information Inc.
===============
(18) INTERPLANETARY DUST AS FLUFFY AGGREGATES
R. Nakamura*), H. Okamoto: Optical properties of fluffy
aggregates as
analogue of interplanetary dust particles. ADVANCES IN SPACE
RESEARCH, 1999, Vol.23, No.7, pp.1209-1212
*) KOBE UNIVERSITY,INFORMAT PROC CTR,ROKKO DAI 1-1,KOBE,HYOGO
657,JAPAN
Based on the appearance of chondritic porous aggregates collected
from stratosphere, we have modeled interplanetary dust particles
as
fluffy aggregates consisting of submicron-sized spherical
monomers.
The optical properties are calculated by a modified version of
the
discrete dipole approximation where the dipole polarizability is
determined by the first term of scattering coefficient in the Mie
theory. The accuracy of our calculations is confirmed through the
comparison with the rigorous solutions for cluster of spherical
monomers. It is found that fluffy aggregates have larger cross
sections and less resonant structures in the scattering profiles
in
comparison with the volume-equivalent sphere. Our model results
in consisted behaviors with the observation of zodiacal light,
i.e., slow rise towards the backward direction and negative
polarization between the scattering angles 160 degrees less than
or
equal to theta less than or equal to 180 degrees, For two sets of
optical constants of dielectric materials, these properties are
riot
sensitive to the size of an aggregate as long as the monomer size
is
smaller than the incident wavelength. (C) 1999 COSPAR,
Published by
Elsevier Science Ltd.
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