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(meteorobs) Excerpts from "CCNet 64/2001 - 4 May 2001"
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 4 May 2001
Date: Fri, 4 May 2001 10:21:03 +0100
CCNet 64/2001 - 4 May 2001
---------------------------
"Because they are surrounded by a cloud of their own gas and dust, the
nuclei of comets never [sic] reveal their exact size. Nuclei are fre-
quently estimated to be a few miles (or kilometers) in diameter, but
only after lengthy observations. So how big is C/2001 A2? "We have no
clue," Carl Hergenrother said. "It might be small. It might be big."
Rob Britt, Space.com, 3 May 2001
(1) COMET CRACKS UNDER PRESSURE AFTER RAPID BRIGHTENING
Space.com, 3 May 2001
(2) AS COOL AS THE COSMOS
Andrew Yee <ayee@nova.astro.utorontodot ca>
[...]
(4) KNOW YOUR ENEMY COURSE/WORKSHOP
J. Mayo Greenberg <greenber@strw.leidenunivdot nl>
[...]
(8) THE FRENCH COMET SCARE OF 1773
Alain Maury <alain.maury@obs-azur.fr>
[...]
========================================================================
(1) COMET CRACKS UNDER PRESSURE AFTER RAPID BRIGHTENING
>From Space.com, 3 May 2001
http://www.space.com/scienceastronomy/solarsystem/comet_breakup_010503.html
By Robert Roy Britt
Senior Science Writer
What was at first a mundane comet zooming into the inner solar system
suddenly brightened unexpectedly this spring and was on the verge of putting
on a minor show by becoming visible to the naked eye.
Then it cracked under the pressure.
Carl Hergenrother, an astronomer at the University of Arizona's Lunar and
Planetary Laboratory, was conducting a routine sky survey on Monday with his
colleagues when he spotted a change in the comet, named C/2001 A2.
Like most comets, C/2001 A2 showed up in a telescope as just a bright spot,
which represents the core, or nucleus, of the object. When comets near the
Sun, a fuzzy halo grows around them as charged particles streaming out from
the solar disk burn off the frozen gases and dust in the core. Nearer the
Sun, tails form.
"Usually in the middle you just see one little fuzzy point source,"
Hergenrother said in a telephone interview on Wednesday. "And that's how it
looked when I shot it last week. But then on April 30th..dot it looked a little
funny."
At first, the fuzzy core of the comet looked less like a point and more like
a bar.
"And then, upon getting some better images, I could break up that bar into
two separate little bright spots."
Hergenrother and his colleagues, graduate student Matt Chamberlain and his
wife Yen Chamberlain, reported their finding May 1 in a circular of the
International Astronomical Union.
Cracking under pressure
Because they are surrounded by a cloud of their own gas and dust, the nuclei
of comets never reveal their exact size. Nuclei are frequently estimated to
be a few miles (or kilometers) in diameter, but only after lengthy
observations.
So how big is C/2001 A2?
"We have no clue," Hergenrother said. "It might be small. It might be big."
And there's also no way to know how big each of the two new pieces are.
Sometimes the smaller parts produce more light depending on the composition
of each piece, Hergenrother said. The bit that broke off could be a big
boulder, or a small chunk of crust.
And though researchers aren't sure why a comet breaks up, it has to do with
the way they blow off steam. As a comet heats up, its ice "sublimates"
directly into gas, bypassing the liquid stage. This generates outward
pressure.
"Kind of like a geyser like Old Faithful," Hergenrother explained, "the
pressure builds up and finally [a piece] pops off."
Naked-eye visible by June?
It's not unusual for comets to break apart. A spectacular example occurred
last summer when another comet, popularly called LINEAR, broke into several
pieces.
(MIT's Lincoln Near Earth Asteroid Research telescope, or LINEAR, discovers
dozens of comets each year, generating some confusion in press reports over
which comet is which. C/2001 A2, the one that just broke apart, was also
discovered by LINEAR, in January of this year.)
Hergenrother says that while comet C/2001 A2 could also further break apart
any day, he does not expect that to happen, though the two pieces will tend
to drift apart as they continue to orbit the Sun.
The comet is currently visible through binoculars, though it takes a
high-powered telescope to see the double nucleus.
And first you have to find it.
It has been visible low on the horizon near the feet of the constellation
Orion, the Hunter. But the comet is heading into the Southern Hemisphere
skies and may no longer be visible from the United States, Hergenrother
said.
It will come back into our 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 no one expects it to put on any kind of
show like Hale-Bopp or other popular comets past.
Sudden brightening
The brightness of stars and other celestial objects is measured on a scale
of apparent brightness. Smaller numbers are brighter and negative numbers
are the brightest. Magnitude 6.0 is the faintest object visible to the naked
eye under dark skies. The brightest star is minus 1.4 on the scale, and
Venus is minus 4.4 at its brightest.
Hergenrother and his colleagues have been watching this newly found comet
for months using a University of Arizona telescope. It was a typical, rather
mundane target that was not expected to brighten much.
"Then one day, all of a sudden -- whammo -- the thing got a hundred times
brighter."
The jump in visual brightness occurred March 25-26, when it rose from an
indistinct magnitude 12 to 10.7. A few days later, it was near magnitude 8.
Then it faded slightly, but began brightening again over the past two weeks
and ultimately approached magnitude 6.
Copyright 2001, Space.com
========================================================================
(2) AS COOL AS THE COSMOS
>From Andrew Yee <ayee@nova.astro.utorontodot ca>
>From The Guardian, 3 May 2001
http://www.guardianunlimited.codot uk/Print/0,3858,4179775,00.html
People expect meteorites to land in a blaze of heat, but they are in fact
icy cold, says Duncan Steel.
By Duncan Steel
Recently a woman walking her dog in York heard a whiz and a bang, and then
spotted a smoking hole in the ground. Assuming it was a meteorite impact,
the police cordoned off the area and a curator from the York Museum was
called in to dig up the supposed cosmic rock.
However, it was obvious, even from the sparse initial report, that this was
not a visitor from space. Meteorites are cold when they land, and generally
remain intact on the surface. It proved to be an exploding underground power
cable.
The chance of an extraterrestrial rock reaching the ground depends
critically on its dimensions, different things happening in distinct size
regimes. Every year the Earth accumulates about 40,000 tons of cosmic
detritus, mostly as billions of tiny flecks ranging in size from sand grains
to peas. Travelling at typically 20 miles per second, these are heated and
then evaporated by friction with the upper atmosphere, and in their death
throes they appear to us as meteors, or shooting stars.
Smaller particles -- less than a tenth of a millimetre in size -- decelerate
without being destroyed. This is possible because their relatively high
surface areas enable them to lose their energy by radiating it away, whereas
larger bodies cannot do this quickly enough, and so are melted and
vaporised. The small grains slow to a halt, and then gradually fall from
altitudes of some tens of miles until they reach the Earth's surface. Such
minute dust particles are termed micrometeorites.
This drop from altitude can take a surprisingly long time: years, in the
case of the tiniest interplanetary dust. Flying research jets in the
stratosphere, NASA scientists have collected samples by employing sticky
plates that project from the wings. Another way such cosmic dust has been
gathered for analysis is by melting and sieving large volumes of snow and
ice in the Antarctic and Greenland, where it has accumulated over millennia.
Actually, there are micrometeorites all around. In a typical house, about
one in a thousand dust particles on the mantlepiece may be of
extraterrestrial origin, depending of course on how often it is cleaned, and
how polluted your neighbourhood might be. A good repository of metallic
micrometeorites is the gutter on your roof, because they are gradually swept
down by the rain, and their high density makes them more likely to remain in
the sludge that builds up there.
Much larger meteoroids -- say between the size of an orange and a basketball
- -- produce extremely bright meteors, called fireballs. These can light up
the night sky, producing daylight conditions for a second or two. Then the
object is gone, its atoms added to the atmosphere. In a small fraction of
cases, though, it is possible that some solid lump may reach the ground
intact. That is what we call a meteorite.
For it to survive in this way, the incoming lump needs to be lucky. First,
it is more likely to escape destruction if it is strong, made of metal or
solid rock as opposed to a loose, fragile structure.
Next, there is a greater chance of avoiding vaporisation if the object has
an entry speed close to the minimum possible: that is about seven miles per
second, identical with the Earth's escape velocity.
Finally, the survival probability of a meteoroid is much enhanced if it
happens to arrive at a fairly oblique angle. Under that circumstance it
decelerates slowly, over a prolonged period rather than just a couple of
seconds. The same applies to re-entering space probes such as the Shuttle,
or the Apollo lunar capsules. Come in at too steep an angle and you burn up;
but if the approach is too shallow, you bounce off the top of the atmosphere
like a flat stone skimmed across a pond.
Meteorites, then, are rare. But what is their temperature? Arriving on a
suitable trajectory, they are slowed from their phenomenal cosmic velocity
to essentially zero in 10 or 20 seconds. As they do so, their outer layers
are intensely heated, and ablate away. The melted exterior of a meteorite,
called a fusion crust, is obvious. But the interior is unchanged. During its
fiery plunge there is simply not enough time for the heat to be conducted
into its centre.
As a result, when meteorites reach the ground their interiors are still at
the temperature they had in space, around minus 30C. If the Earth had no
atmosphere, that would be the planet's temperature at our distance from the
Sun. It is only the natural greenhouse effect that gives our home its
pleasing ambient temperature. (The scientific debate over global warming
concerns whether manmade emissions are enhancing the greenhouse effect by a
small but significant proportion.)
One can think of the effect of atmospheric entry on a meteorite as being
similar to throwing a deep-frozen joint of meat into a fire. Five minutes
later the outside may be charred, but the middle is still icy.
Just-fallen meteorites, then, are cold. It is not unusual for an observed
fall, someone reaching the object within a few minutes, to produce a report
of ice on its exterior. Water vapour in the air first condenses on to the
meteorite and then freezes.
Do meteorites make craters? The multitude of scars on the Moon is obvious,
but there are also about 250 recognised impact craters on the Earth. The
best known is Meteor Crater in Arizona. That massive hole, 1,300 yards
across, was formed 49,000 years ago when a nickel-iron projectile about 40
yards in size slammed into the ground, releasing on impact energy equivalent
to about 20 megatons of TNT. The atmosphere does not brake such a body by
much.
Little boulders from space are different, though. The atmosphere slows them
down in one way or another. They are either totally destroyed, or else
brought to a halt. An orange-sized rock would be decelerated by the time it
reaches a height of 20 miles. From there it gets to the ground
under free fall: it plummets at the same rate as an object dropped from a
plane, reaching a terminal velocity of about 200mph.
Compared to the hypervelocity impact of a very large body like an asteroid
or a comet, that is a low speed. As a result, small meteorites do not form
craters. If one were to land on soft ground, it might bury itself a bit, but
that is all. In 1947 a large metallic body broke up on atmospheric entry
over the east of Siberia, and many of the fragments weighing over a ton
managed to penetrate the earth by a metre or so. The 60-ton meteorite that
landed in Namibia in prehistoric times -- the largest known intact meteorite
- -- sits on the surface, although it did cause a bit of a dent.
If you should hear a loud bang and find a smoking crater, suspect that
someone has been playing with dynamite. To excavate an impact crater a
projectile from space has got to be large, and would make a hole far too big
for the police to cordon off. Mind you, they would have other things to
worry about.
[Duncan Steel works at the University of Salford. His most recent book is
Target Earth, published by Time-Life.]
) Guardian Newspapers Limited 2001
=====================================================================
(4) KNOW YOUR ENEMY COURSE/WORKSHOP
>From J. Mayo Greenberg <greenber@strw.leidenunivdot nl>
Dear Benny,
It would be of interest to your readers in your newsletter the website for
the meeting on "The Physical Properties of Potential Earth Impactors: KNOW
YOUR ENEMY"
http://emcsc.ccsem.infndot it/
I also think it would be of interest to include the Purpose of the
Course/Workshop
PURPOSE OF THE COURSE
The main objectives of the course are to learn what we know now and what and
how we can learn more about the physical and chemical properties of
potential earth impactors; namely, asteroids and comets, with the ultimate
aim of providing the best possible data for mitigation procedures. A major
concern to the world is the potential devastation produced by impacts of
cosmic objects from space For this reason we consider as a subtitle for the
school: 'KNOW YOUR ENEMY'. The lectures will be devoted to consideration of
all possible means from theoretical to experimental to remote observational
to space observational, to space in situ measurements for obtaining the
internal structure and composition of asteroids and comets. The course will
include a series of workshops which will summarize and supplement the
lectures and provide suggestions for further studies in the application of
all techniques which can be used to provide a data base on cosmic object
properties. A significant consequence of the course will be its application
to knowledge of the formation of the Solar System. This course, as was the
case for all the previous ones in the Space Chemistry School, is highly
interdisciplinary, bringing together experts with a wide variety of
chemical, physical and technical backgrounds in theory, laboratory
astrophysics, ground based observations and space observations as well as
applications.
============================
* LETTERS TO THE MODERATOR *
============================
(8) THE FRENCH COMET SCARE OF 1773
>From Alain Maury <alain.maury@obs-azur.fr>
While reading a book on French astronomer Messier, I found the following,
which talks about a comet scare in Paris in 1773, unwillingly generated by
French astronomer Joseph-Gertme Lefrangois de Lalande. It was also certainly
the last time when the French Academy of Sciences worried about possible
collisions between near earth objects and the earth :-)))
In 1773, the French astronomer Lalande meant to present at one of the
meeting of the french sciences academy a memoir on " Riflexions sur les
comhtes qui peuvent approcher la Terre" (reflexions on comets which can
approach Earth). Taken by some other activities, he could not present his
memoir, and the rumor very quickly spread that he was about to announce the
collision of a comet with Earth. Michaud, in "Bibliographie universelle
ancienne et moderne ", Paris 1854 (reed. 1966.. T .28 P 106-109) writes:
"On se demanda ce que contenait le mimoire; On y apprit que l'on
devait y voir les effets que pourrait produire une comhte qui
viendrait choquer la Terre : le bruit se ripandit que la comhte allait
arriver, qu'elle itait annoncie par Lalande. L'alarme que fit nantre cette
pridiction pritendue fut si ginirale que le lieutenant giniral de police
voulut lire le mimoire ; Il ne trouvbt rien qui p{t motiver les terreurs
qui s'itaient ripandues ; il en ordonna la prompte publication ;
Quand le mimoire fut publii, personne ne voulut y croire ; on y itait
persuadi que l'auteur avait supprimi la fatale pridiction, pour ne pas
effrayer par l'annonce d'une catastrophe ` laquelle il n'y avait
aucun moyen d'ichapper."
"It was asked what the memoir contained. It was learned that the
effects which could produce a comet which would collide with earth would
be described. The rumor spread that the comet was going to come, and
that it was announced by Lalande. The alarm which this alleged prediction
caused was so general that the lieutenant general of the police asked to
read the memoir. He could not find anything which could cause the terrors
which had spread. He ordered its quick publication. When the memoir
was published, nobody wanted to believe it. Everybody was convinced
that the author had suppressed the fatal prediction, not to scare by the
prediction of a catastrophy to which there was no way to escape"
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