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(meteorobs) Excerpts from "CCNet 70/2001 - 22 May 2001"




This issue is chockablock full of results that are of interest (and in some
cases, hard to interpret) for meteoreaders. The discussion of evidence for
"snowy dirtballs" in addition to the well-known "dirty snowballs" was quite
startling. And indications of rapid rotation and its effects on comet heating
were also fascinating. Could we be closer to understanding the nature of
"defunct comets" or "cometary asteroids", not to mention meteoroid streams
like the Geminids that appear to be linked with "asteroidal" parent bodies?

Clear skies!
Lew Gramer


------- Forwarded Message

From: Benny Peiser <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 70/2001 -  22 May 2001
Date: Tue, 22 May 2001 11:24:33 +0100

CCNet 70/2001 -  22 May 2001
----------------------------


[...]
 
(2) MISSING CARBON-2 MOLECULE HOLDS CLUES TO COMET'S ORIGIN
    Andrew Yee <ayee@nova.astro.utorontodot ca>

(3) COPYCAT COMET CRACKS INTO THREE PIECES, MAYBE MORE
    Space.com, 21 May 2001

(4) COMET'S DEMISE REVERSES 'DIRTY SNOWBALL' THEORY
    Los Angeles Times, 21 May 2001

(5) WHAT WE LEARNED FROM COMET LINEAR
    John Wagoner <stargate@astromax.com>

[...]

(7) COMET C/1999 S4 (LINEAR) 
    Comets & Meteor Showers
    [This is a direct, unattributed excerpt from Gary Kronk!]

[...]

(11) MORE OBSERVATIONS OF SHOEMAKER-LEVY 9 IMPACT
     de Pater I, Brecht SH: SL9 impacts: VLA high-resolution
     observations at lambda=20 cm

[...]
 
(13) THE SHOEMAKER-LEVY 9 IMPACT & ENHANCED RADIAL DIFFUSION
     de Pater I, Brecht SH: SL9 impacts and simulations of enhanced
     radial diffusion

(14) FORCED PRECESSION MODELS FOR SIX ERRATIC COMETS
     Krolikowska M, Sitarski G, Szutowicz S: Forced precession models for
six erratic comets

[...]

(16) THE CHEMISTRY OF INTERSTELLAR SPACE
     Herbst E: The chemistry of interstellar space


========================================================================

(2) MISSING CARBON-2 MOLECULE HOLDS CLUES TO COMET'S ORIGIN

>From Andrew Yee <ayee@nova.astro.utorontodot ca>

Office of Public Affairs
University of Texas-Austin
P O Box Z
Austin, Texas 78713-7509
(512) 471-3151
FAX (512) 471-5812

Contact:
Rebecca Johnson or Mary Lenz
Office of Public Affairs
(512) 475-6763

Missing carbon-2 molecule holds clues to comet's origin

AUSTIN, Texas -- Astronomers seeking to chart the solar system's evolution
reaped a windfall of information when a comet disintegrated just as it made
its closest approach to the sun and they were able to observe the comet's
contents and, possibly, its origin. 

Their work will be published Friday (May 18) in the journal Science -- a
special issue devoted to studies of the comet, called C/1999 S4 (LINEAR),
that disintegrated last July.

Dr. Tony L. Farnham, a planetary scientist at The University of Texas at
Austin, and his colleagues discovered a deficiency in the molecule carbon-2
in Comet C/1999 S4 (LINEAR). Farnham, lead author of the paper, is the
Harlan J. Smith Planetary Post-doctoral Researcher in the UT Austin
department of astronomy. (LINEAR refers to the Lincoln Near Earth Asteroid
Research project funded by the U.S. Air Force to study near earth objects.)

Co-authors include Dr. David G. Schleicher and Dr. Laura M. Woodney of
Lowell Observatory in Flagstaff, Ariz.; Peter V. Birch of Perth Observatory
in Western Australia; Clara A. Eberhardy of the University of Washington in
Seattle; and Lorenza Levy of Northern Arizona University in Flagstaff. 

"We usually get a look at the surface of a comet, but this time we got to
look inside," Farnham said. The group observed the comet both before and
after it broke up, using telescopes at UT Austin's McDonald Observatory,
Lowell Observatory and Perth Observatory.

Comets are sometimes referred to as 'dirty snowballs,' because they are made
up of dust and rock held together by ice. When they venture close to the
sun, the ice vaporizes and elements inside are released.

The carbon-2 deficiency discovered by Farnham and his colleagues indicates
that the comet formed near Neptune, probably billions of years ago. Most
comets were formed during the solar system's earliest years in two regions:
near Jupiter and Saturn, and farther out, near Neptune. 

They didn't stay in those regions, however, because the force of gravity of
those giant planets catapulted the comets away, and created two comet
habitats: the Oort Cloud (a halo of Jupiter-origin comets enveloping the
solar system) and the Kuiper Belt (a belt of Neptune-origin comets orbiting
in the plane of the solar system along with the planets, beyond Neptune's
orbit).

Different lines of evidence may indicate another history for C/1999 S4
(LINEAR). Other researchers found the comet to be lacking in other
carbon-chain molecules, pointing to a Jupiter-region origin, Farnham said.
He said the discrepancy "may be telling us that it has a surface  material
different from what's inside. It's possible that the comet formed near the
Jupiter region, and other materials formed on the surface as it migrated
out," into the outer solar system. But Farnham cautioned that there is no
proof of this.

Understanding the origin of comets is important because they carry key
information about the history of the solar system that cannot be derived
from studying planets. Comets have changed very little since the solar
system formed. They are relics of an earlier time. Geological processes
(like volcanic eruptions and the movements of glaciers) have rewritten the
surfaces of most of the planets since their formation.

"Comets have been in storage, especially dynamically new comets, like this
one," Farnham explained. He said comets remain in the cold realms of the
Oort Cloud and Kuiper Belt until some large body like a planet or a passing
star gives them a gravitational kick into the inner solar system. A
'dynamically new' comet is one that has never been in the inner regions of
the solar system before.

Farnham also calculated a lower limit for the radius of the comet's nucleus
before break-up: about 0.4 kilometers. In addition, he observed an
'outburst' from the comet, at the same time other researchers using the
Hubble Space Telescope photographed a piece of the comet breaking away.
Comet C/1999 S4 (LINEAR) was not visible to the naked eye.

For more information, contact Dr. Tony L. Farhnam at (512) 471-1483, or via
e-mail at farnham@astro.as.utexasdot edu or see:
     http://www.ll.mitdot edu/LINEAR
 	
========================================================================

(3) COPYCAT COMET CRACKS INTO THREE PIECES, MAYBE MORE

>From Space.com, 21 May 2001
http://www.space.com/scienceastronomy/solarsystem/comet_breakup_010521.html

By Robert Roy Britt
Senior Science Writer

If copyright laws applied to cosmic acts, then Comet 2001 A2 would be in
court right now.

First it broke into two pieces earlier this spring just after a rapid
brightening session. Now one of those two chunks has split apart and
researchers say there may be many smaller pieces flying through space. 

If the scenario sounds familiar, it's because this isn't the first comet to
break apart in recent months. The disintegration act comes less than a year
after another comet, called 1999 S4, broke into several large pieces and
countless smaller ones, all in plain view of numerous telescopes. That event
led to a bevy of revealing scientific papers on comet formation that were
released just last Thursday.

Both of these comets have been popularly called "LINEAR," named after the
Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research
telescope, which was used to discover them.

The most recent LINEAR comet, 2001 A2, was discovered just this year on Jan.
3 and presumed initially to be intact. It brightened suddenly and
unexpectedly back in March and April. As comets approach the Sun, their
nucleus of gas and dust burns off and forms a halo, or coma, that glows with
reflected sunlight. 

Then on April 30th, researchers at the University of Arizona's Lunar and
Planetary Laboratory photographed the comet after its nucleus had broken
into two pieces. When a comet breaks apart, more fresh material is exposed,
which likely contributed to the sudden brightening.

The latest split

Now the comet is about 62 million miles (100 million kilometers) from Earth,
but it is no longer visible from the Northern Hemisphere. On May 14,
scientists using a European Southern Observatory telescope in Chile noticed
that one of the comet's two fragments appeared to be elongated.

On May 16, last Wednesday, it became clear that the comet had split into
three pieces. A colorful image of the latest breakup was released Friday.

Hermann Boehnhardt of the European Southern Observatory works on the team
that spotted the three large chunks. He told SPACE.com that the orbit of
2001 A2 is so different from 1999 S4 that the two comets are not likely
close relatives in terms of where they were created in the disk of gas and
dust that swirled around our Sun when it was born.

Depending on what the comet does in coming months, Boehnhardt said,
additional observations might allow researchers to figure out where it
formed and why it broke apart.

Drifting apart

When Boehnhardt and his colleagues first spotted the comet in three pieces,
roughly 310 miles (500 kilometers) separated the two freshly-split chunks.
The distance between this pair and the other piece was about 3,700 miles
(6,000 kilometers) on May 14 and increased by about 620 miles (1,000
kilometers) within two days.

No one can yet say how big the pieces are or how large the comet's nucleus
was before it broke apart.

The comet moves in an exceedingly elongated orbit around the Sun and it is
making perhaps its first return trip to the inner solar system after a long
hiatus in the Oort Cloud, a halo of comets that surrounds the solar system. 

If it is the comet's first pass, that means it has not been in the main part
of the solar system since back when it formed, likely about 4.6 billion
years ago when the rest of the solar system developed. Its contents would be
pristine, and researchers will be interested to study it and see if it adds
to the long list of discoveries made by observing the other LINEAR comet
that broke apart last summer.

Where it is, where it's going

Comet 2001 A2 can now be seen with the unaided eye by observers in the
Southern Hemisphere as a faint object in the southern constellation of Lepus
(The Hare).

It will pass through its perihelion -- the point in its orbit nearest to the
Sun -- on May 25, at a distance of about 72 million miles (116 million
kilometers). Then it will reappear in Northern Hemisphere skies at the end
of June after it swings under the Sun. By then, it might be visible to the
naked eye, though that is not certain and its recent further disintegration
clouds that possibility.

Hal Weaver, a Johns Hopkins University researcher involved in the recent
studies of Comet 1999 S4 that broke up last summer, said it's possible the
same fate might await 2001 A2.

"We'll have to keep watching over the next several months," Weaver said.
Meanwhile, he's working on a French-led team making plans to observe 2001 A2
with the Hubble Space Telescope, likely in July.

Copyright 2001, Space.com 

========================================================================

(4) COMET'S DEMISE REVERSES 'DIRTY SNOWBALL' THEORY

>From Los Angeles Times, 21 May 2001
http://www.latimes.com/news/science/science/20010521/t000042583.html

>From Times Staff

Astronomers watching the "under-performing" Comet Linear last July were
astounded when it unexpectedly burst apart, offering them a direct view of
its internal structure in its disintegrating heart. Icy comets are usually
described as "dirty snowballs." Linear was comprised of rocky matter with
only a little ice, making it more of a "snowy dirtball." 

Its fragments, scientists reported in the May 18 issue of the journal
Science, appear to be the same material that makes up planets. The comet was
relatively free of carbon-rich compounds observed in the ice of other
comets, casting some doubt on the theory that comets delivered the precursor
molecules necessary for life on Earth. 

Copyright 2001, Los Angeles Times

========================================================================

(5) WHAT WE LEARNED FROM THE LAST YEAR'S COMET LINEAR

>From John Wagoner <stargate@astromax.com>

SKY & TELESCOPE'S NEWS BULLETIN - MAY 18, 2001
For images and Web links for these items, visit http://www.skypub.com

Last June, amateur and professional astronomers alike focused their
telescopes on what was thought to be a mediocre comet as best. Discovered by
the Lincoln Near Earth Asteroid Search, Comet LINEAR (C/1999 S4) soon became
a cosmic curiosity. In Hubble Space Telescope observations taken in July,
the comet appeared to have erupted, with a large fragment having clearly
broken off the nucleus. In the month that followed, the comet crumbled apart
completely, faded from view, and in the process, exposed its interior to
science. 

In a series of papers published in today's Science, astronomers reveal what
they have learned thus far from the deceased comet. In addition its chemical
composition and probable origin, (see following story) astronomers now think
they may understand why the comet came apart in the first place. Originally
they assumed that Comet LINEAR's breakup was due to massive outgassing --
perhaps the result of an outburst of carbon monoxide (CO), the most probable
suspect. However spectroscopic analysis found the comet to be highly
depleted of CO, thus making a pressure build-up of gas within the nucleus an
unlikely cause. 

Instead, Hal Weaver (Johns Hopkins University), author on three of the
Science papers, believes the comet was most likely rotating quickly on its
approach to the Sun. If the rotation pole was pointed toward the Sun, says
Weaver, "there would be more efficient heat transfer throughout the comet."
But he adds that it isn't clear the comet was rotating fast enough to break
apart. The critical rotation speed depends largely on the initial size of
the comet's nucleus, a quantity yet to be determined. 

Another interesting facet of Comet LINEAR is what wasn't seen. The assumed
mass of the comet before it crumbled is 100 times larger than the mass found
afterward. Some of this is to be expected. Larger blocks are directly
detectable, as are the smallest of particles, which refract light much the
same way a thick fog would. But just as a thick fog easily hides all but the
largest of objects from view, so too did LINEAR's dust cloud. 

"Watching the unraveling of the comet allows us to look in reverse at its
formation," says Weaver. Thus determining the size of the intermediate
fragments directly relates to the size of cometary nuclei in general. An
abundance of intermediate-size building blocks imply that, on the whole,
cometary nuclei may be smaller than previously thought. 

Comet breakups are far from unusual. For example, another LINEAR discovery,
C/2001 A2 -- currently visible in Southern Hemisphere skies -- recently
broke in half on its approach to the Sun. On May 16th, the 8.2-meter Yepun
reflector of the Very Large Telescope took an image showing that a third
piece has broken off. LINEAR C/2001 A2 reaches its closest point to the Sun
on May 24th. The comet also continues to brighten. Observers report that it
is a naked-eye 5th magnitude, where the comet is about 20 deg. above the
west-southwest horizon after evening twilight. It will not be visible from
the Northern Hemisphere until late June. Here are coordinates for Comet
LINEAR C/2001 A2 at 0 hours Universal Time for the coming week:

R.A. Dec.

May 19 5h 35m -23.4 deg.
May 21 5 31 -24.1
May 23 5 27 -24.8
May 25 5 21 -25.5

A COMET'S LINEAR-AGE

Even before its slow-motion disintegration last year, Comet LINEAR (C/1999
S4) was the crosshairs of many telescopes. Its brightness allowed planetary
scientists to conduct spectroscopic studies that would be impossible on
dimmer passers-by. Curiously, Comet LINEAR proved to be unlike other bright
comets, in that it contained relatively little carbon monoxide (CO) relative
to its water content.

Frozen CO vaporizes even at very cold temperatures, so when seen abundantly
in the comas of Halley, Hale-Bopp, and Hyakutake, astronomers concluded that
these iceballs formed very cold, in the vicinity of Uranus or Neptune,
before being flung out into the distant cometary reservoir known as the Oort
Cloud. This trio of comets also exhibited a high ratio of deuterium to
hydrogen, typical of that found in interstellar clouds of gas. Because the
comets' D-to-H ratio was at least double that found here on Earth, cosmic
chemists began to think that only a fraction of our planet's water could
have been delivered by comets.

However, Comet LINEAR's low CO value implies that it formed somewhat closer
to the Sun's warmth, perhaps near Jupiter, before taking up residence in the
Oort Cloud. As Michael C. Mumma (NASA/Goddard) and his colleagues explain in
today's issue of Science, such comets should have D:H ratios much closer to
that of seawater, which could mean that incoming comets delivered the bulk
of Earth's water after all. "The idea that comets seeded life on Earth with
water and essential molecular building blocks is hotly debated," Mumma
notes, "and for the first time, we have seen a comet with the right
composition to do the job." But his team will have to wait for another
bright comet to test their hypothesis further, because Comet LINEAR's D:H
ratio could not be determined.

Of course, comets weren't the only small bodies that formed near Jupiter.
Asteroids did too, and the most distant ones contain up to 20 percent water.
In 1997, Harold Levison (Southwest Research Institute) and Martin Duncan
(Queen's University) estimated that roughly 8 percent of all the objects
initially present in the outer asteroid belt (3.3. to 5.0 astronomical units
from the Sun) were ejected by Jupiter into the Oort Cloud. Over time, some
of these must boomerang their way back to the inner solar system. Even if
LINEAR wasn't one of them, it's definitely caused a stir among planetary
scientists.

[...]

Copyright 2001 Sky Publishing Corporation. S&T's Weekly News Bulletin and
Sky at a Glance stargazing calendar are provided as a service to the
astronomical community by the editors of SKY & TELESCOPE magazine.
Widespread electronic distribution is encouraged as long as these
paragraphs are included. But the text of the bulletin and calendar may not
be published in any other form without permission from Sky Publishing
(contact permissions@skypub.com or phone 617-864-7360). Updates of
astronomical news, including active links to related Internet resources, are
available via SKY & TELESCOPE's site on the World Wide Web at
http://www.skypub.com/.

========================================================================

(7) COMET C/1999 S4 (LINEAR) 

>From Comets & Meteor Showers
http://comets.amsmeteors.org/comets/lcomets/1999s4.html
[The author was 'meteorobs' own Gary Kronk, kronk@amsmeteors.org]

Discovery

This is another comet found by the automatic minor planet search program
LINEAR in New Mexico. It was found on CCD images obtained between 1999
September 27.40 and September 27.45 and was reported as an unusual moving
object. The nuclear magnitude was given as 17.3-17.6. LINEAR obtained
additional images on September 28 and October 1. On October 1.40 and 1.43,
D. T. Durig (Cordell-Lorenz Observatory, Sewanee, Tennessee) obtained images
of the comet with a 0.3-m Schmidt-Cassegrain and a CCD. He noted a total
magnitude of 16.3 on the second image, as well as a coma 10 arcsec across
and a tail extending 20-25 arcsec toward PA 2000-2200. The cometary nature
was confirmed on October 1.89, when J. Ticha and M. Tichy (Klet Observatory)
imaged the comet with a 0.57-m reflector and a CCD. They determined the
total magnitude as 16.2 and noted a coma 8 arcsec across and a tail
extending more than 10 arcsec toward PA 2450. 

Historical Highlights

The first published orbit came from Brian G. Marsden (Harvard-Smithsonian
Center for Astrophysics) on October 1. His parabolic solution, based on
observations collected over five days, indicated a perihelion date of 2000
July 18 and a perihelion distance of 0.72 AU. He suggested "this comet might
become a naked-eye object next July." Marsden revised the comet's orbit on
October 6, using positions collected over eight days. The new perihelion
date was 2000 July 24.2 and a perihelion distance of 0.754 AU. His ephemeris
indicated the comet's maximum brightness could reach magnitude 4.

The comet slowly brightened during the months immediately following
discovery. It was slightly fainter than magnitude 14 during November and was
brighter than 14 as 2000 began. The total magnitude had reached 13.5 during
the first week of March, and the comet was lost in the sun's glare after
March 22.

The comet was recovered in the morning sky on May 4, by K. Kadota (Ageo,
Japan) at a magnitude of about 13. Observations by other observers began
around mid-May after the comet had gained more altitude in the morning sky
and had become brighter than magnitude 12. By the beginning of June most
observers were reporting a brightness slightly fainter than magnitude 10,
with a moderately condensed coma about 3 arcmin across. Visual observations
of the tail became fairly common in larger amateur telescopes after
mid-June, with the length typically estimated as 4-5 arcmin. As June came to
a close, most observers found the comet slightly brighter than magnitude
8.5. The coma was then about 5-6 arcmin across and the tail about 10 arcmin
long.

During June, revised predictions of the maximum brightness of this comet
indicated a maximum magnitude of 4 to 6 would be attained around July 22 or
23. Although an apparent change in the rate of brightening occurred on June
22, which indicated the comet was heading for the magnitude 6 prediction, an
minor outburst around July 5 to 6 was indicated by visual observers and
fathered hopes that the comet might still reach magnitude 4 or 5. This
brightening only lasted a few days, however, and by mid-July most observers
were giving in to the possibility that this comet would not become brighter
than magnitude 6. With respect to the brightening in early July, it is
interesting that on July 28, the Space Telescope Science Institute issued
press release giving details of observations made by the Hubble Space
Telescope on July 5, 6, and 7. The images indicate a brief outburst on the
6th and the appearance of a fragment moving toward the tail from the nucleus
on the 7th. Complete details and images can be found at the Hubble Space
Telescope web site. 

The comet continued a slow brightening as July progressed. By July 21, the
general consensus was that the comet's brightness was between magnitude 6.6
and 6.8, with a coma 5 to 6 arcmin across and a dust tail 30 to 60 arcmin
long. But something happened late on the 21st and by July 22, observers were
commonly reporting the magnitude as around 6 and, although the coma diameter
and dust tail had not changed, there was a bright, straight, long gas tail.
My personal observations on July 22.16 revealed a brighter comet than I had
seen a couple of days earlier, a a unmistakable, straight gas tail extending
at least 1 degree. This is exactly why people like me observe comets. Their
potential unpredictability makes them very interesting. 

So, on July 23 I asked the question "what will happen next?" and noted "The
gas tail seen on the 21st and 22nd was certainly short-lived and indicates
something sudden and violent occurred to the nucleus. It may only prove to
have been a new pocket of gas that was released, but observers should be on
the lookout for something more within the coma during the next few days and
weeks." Well, the comet has indeed undergone changes that became apparent
shortly before July ended. The comet's nuclear region became noticeably more
diffuse and elongated beginning on July 25, and the comet began fading at a
rate much greater than had been predicted. My final observation was made
with my 33.3-cm reflector on August 2 and revealed a slightly elongated
nebulosity with virtually no condensation. My estimate of the comet's
brightness was 9, which was over 2 magnitudes fainter than expected. 

The Hubble Space Telescope was used to observe the comet on August 5. What
it found was more than a dozen tiny comets enveloped by a cloud of dust in
the area where comet LINEAR was supposed to be. The image below is only a
small section of the photo. The large scale images and complete story are on
the Hubble web site. Interestingly, the images were obtained at about the
same time as another, wider field image was obtained with the 2.2-meter
telescope at the Mauna Kea observatory complex.

In a display of how good ground-based observations can be, the European
Southern Observatory released an image obtained at Paranal (Chile) with the
8.2-m Very Large Telescope ANTU unit on the evening of August 6. A portion
of the image is shown below. The complete image and press release is located
on the VLT web site.

========================================================================
* ABSTRACTS *
========================================================================

(11) MORE OBSERVATIONS OF SHOEMAKER-LEVY 9 IMPACT

de Pater I, Brecht SH: SL9 impacts: VLA high-resolution observations at
lambda=20 cm
ICARUS 151 (1): 1-24 MAY 2001

We present high resolution (0.3 R-J) VLA observations of Jupiter at a
wavelength of 20 cm which were taken during and after the impacts of Comet
D/Shoemaker-Levy 9. The observations are presented both as a function of
central meridian longitude lambda (cml) and, after applying tomographic
techniques, as a function of jovicentric longitude lambda (III) Following an
impact all radiation peaks (both the main and high latitude emission peaks)
usually shift inward, toward the planet, and brighten up; the first impact,
however, triggered a decrease rather than an increase in the intensity of
the radiation peaks. Although the high latitude regions always brighten,
they do not brighten up as much as the main radiation peaks during the first
few days of the impacts, whereas they brighten up significantly more than
the main radiation peaks later in the impact week. The high latitude
regions, in particular in the southern hemisphere, move slightly closer to
the magnetic equator. (C) 2001 Academic Press.

Addresses:
de Pater I, Univ Calif Berkeley, Dept Astron, 601 Campbell Hall, Berkeley,
CA 94720 USA
Univ Calif Berkeley, Dept Astron, Berkeley, CA 94720 USA
Bay Area Res Corp, Orinda, CA 94563 USA

Copyright ) 2001 Institute for Scientific Information

========================================================================

(12) MODIFICATION OF THE JOVIAN RADIATION BELT BY SHOEMAKER-LEVY 9

Brecht SH, de Pater I, Larson DJ, Pesses ME: Modification of the Jovian
radiation belts by Shoemaker-Levy 9: An explanation of the data
ICARUS 151 (1): 25-38 MAY 2001

The SL9 impacts provided a unique data set with which to examine some
surprising physical mechanisms at work. This paper examines some of the data
in light of the collisionless shock acceleration paradigm, called diffusive
shock acceleration. The research finds that both the shock acceleration
mechanism and an enhanced diffusion mechanism are required to explain the
data. Direct numerical simulations and analytic models are used to compare
these mechanisms with the data collected. The results are consistent with
the data. In addition further analysis of these SL9 VLA data strongly
suggests that for periods of days the relativistic radiation belts drift in
a direction opposite to that predicted by normal drift theory. (C) 2001
Academic Press.

Addresses:
Brecht SH, Bay Area Res Corp, POB 366, Orinda, CA 94563 USA
Bay Area Res Corp, Orinda, CA 94563 USA
Univ Calif Berkeley, Berkeley, CA 94720 USA
Sci Applicat Int Corp, Mclean, VA 22102 USA

Copyright ) 2001 Institute for Scientific Information

========================================================================

(13) THE SHOEMAKER-LEVY 9 IMPACT & ENHANCED RADIAL DIFFUSION

de Pater I, Brecht SH: SL9 impacts and simulations of enhanced radial
diffusion
ICARUS 151 (1): 39-50 MAY 2001

We present detailed calculations on enhanced radial diffusion models and
show that many, though not all, of the phenomena observed during the week
that Comet Shoemaker-Levy 9 crashed into Jupiter can be explained by a
sudden increase in the radial diffusion coefficient. Our calculations use
estimates for the enhancement in the diffusion coefficient which come from
self-consistent calculations of the electromagnetic turbulence generated by
the impacts (Brecht er al. 2001, Icarus). These calculations suggest that
the diffusion coefficient is enhanced at least a few million times above the
nominal value during a short period of time (minutes). Our model shows that
Jupiter's main radiation peaks brighten up much more than the high latitude
regions, as is indeed observed following impacts during the first few days
of the impact week. The calculations also suggest that the largest
enhancements in intensity and largest inward shift of the radiation peaks
occur at jovicentric longitudes similar to 100 degrees less than or similar
to lambda (III) less than or similar to 250 degrees, i.e., the longitude
range where the B = constant contours are furthest from the planet. This
longitude range agrees with the region where the strongest enhancements have
indeed been observed. The dramatic increase in the intensity of the high
latitude peaks following impacts which took place later in the week is
attributed to a direct acceleration of electrons by the upward propagating
shock. Finally, compared to the observations, the radial diffusion models
predict much larger enhancements in the radiation peaks than observed. We
attribute this, as well as the initial decrease in intensity on July 16-17,
to a large loss of electrons caused by pitch angle scattering. (C) 2001
Academic Press.

Addresses:
de Pater I, Univ Calif Berkeley, Dept Astron, 601 Campbell Hall, Berkeley,
CA 94720 USA
Univ Calif Berkeley, Dept Astron, Berkeley, CA 94720 USA
Bay Area Res Corp, Orinda, CA 94563 USA

Copyright ) 2001 Institute for Scientific Information

========================================================================

(14) FORCED PRECESSION MODELS FOR SIX ERRATIC COMETS

Krolikowska M, Sitarski G, Szutowicz S: Forced precession models for six
erratic comets
ASTRONOMY AND ASTROPHYSICS 368 (2): 676-688 MAR 2001

The nongravitational motion of six "erratic" short-period comets is studied
on the basis of published astrometric observations. We present the
precession models which successfully link all the observed apparitions of
the comets: 16P/Brooks 2, 21P/Giacobini-Zinner, 31P/Schwassmann-Wachmann 2,
32P/Comas Sola, 37P/Forbes and 43P/Wolf-Harrington. We used the Sekanina's
forced precession model of the rotating cometary nucleus to include the
nongravitational terms into equations of the comet's motion. Values of six
basic parameters (four connected with the rotating comet nucleus and two
describing the precession of spin-axis of the nucleus) have been determined
along the orbital elements from positional observations of the comets. The
solutions were derived with additional assumptions which introduce
instantaneous changes of modulus of reactive force, and of maximum cometary
activity with respect to perihelion time. The present precession models
impose some constraints on sizes and rotational periods of cometary nuclei.
According to our solutions the nucleus of 21P/Giacobini-Zinner with
oblateness along the spin-axis of about 0.29 (equatorial to polar radius of
1.41) is the most oblate among six investigated comets.

Addresses:
Krolikowska M, Polish Acad Sci, Space Res Ctr, Bartycka 18A, PL-00716
Warsaw, Poland
Polish Acad Sci, Space Res Ctr, PL-00716 Warsaw, Poland
Univ Bialystok, Inst Theoret Phys, PL-15424 Bialystok, Poland

Copyright ) 2001 Institute for Scientific Information

========================================================================

(16) THE CHEMISTRY OF INTERSTELLAR SPACE

Herbst E: The chemistry of interstellar space
CHEMICAL SOCIETY REVIEWS 30 (3): 168-176 MAY 2001

Interstellar space is not empty, but contains gaseous and particulate matter
that is concentrated into very large regions known as interstellar clouds.
In the denser and cooler clouds, the gas is molecular and most of the
molecules detected are organic in nature. The gas-phase molecules are
synthesised from precursor atoms by rapid exothermic reactions in the gas
and on the surfaces of tiny dust particles. Since dense interstellar clouds
collapse to form stars and planetary systems, the molecules produced in the
clouds may be eventually incorporated into solid bodies such as comets,
meteors, and even planets.

[meteoroids, meteorOIDs!]

Addresses:
Herbst E, Ohio State Univ, Dept Phys & Astron, Columbus, OH 43210 USA
Ohio State Univ, Dept Phys & Astron, Columbus, OH 43210 USA

Copyright ) 2001 Institute for Scientific Information

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