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(meteorobs) Excerpt from "CCNet 112/2001 - 31 October 2001"
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 112/2001 - 31 October 2001
Date: Wed, 31 Oct 2001 12:03:30 -0000
CCNet 112/2001 - 31 October 2001
================================
(1) MYSTERY SPACE BLAST 'SOLVED'
BBC Online News, 20 October 2001
[...]
(4) MORE ON RADIATIVE FORCES
John Michael Williams <jwill@AstraGatedot net>
[...]
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(1) MYSTERY SPACE BLAST 'SOLVED'
>From the BBC Online News, 20 October 2001
http://news.bbc.codot uk/hi/english/sci/tech/newsid_1628000/1628806.stm
By BBC News Online science editor Dr David Whitehouse
Astronomers may have solved the puzzle of what it was that brought so much
devastation to a remote region of Siberia almost a century ago.
In the early morning of 30 June, 1908, witnesses told of a gigantic
explosion and blinding flash. Thousands of square kilometres of trees were
burned and flattened.
Scientists have always suspected that an incoming comet or asteroid lay
behind the event - but no impact crater was ever discovered and no
expedition to the area has ever found any large fragments of an
extraterrestrial object.
Now, a team of Italian researchers believe they may have the definitive
answer. After combining never-before translated eyewitness accounts with
seismic data and a new survey of the impact zone, the scientists say the
evidence points strongly to the object being a low-density asteroid.
They even think they know from where in the sky the object came.
Completely disintegrated
"We now have a good picture of what happened," Dr Luigi Foschini, one of the
expedition's leaders, told BBC News Online.
The direction of the flattened trees is a vital clue
The explosion, equivalent to 10-15 million tonnes of TNT, occurred over the
Siberian forest, near a place known as Tunguska.
Only a few hunters and trappers lived in the sparsely populated region, so
it is likely that nobody was killed. Had the impact occurred over a European
capital, hundreds of thousands would have perished.
A flash fire burned thousands of trees near the impact site. An atmospheric
shock wave circled the Earth twice. And, for two days afterwards, there was
so much fine dust in the atmosphere that newspapers could be read at night
by scattered light in the streets of London, 10,000 km (6,213 miles) away.
But nobody was dispatched to see what had happened as the Czars had little
interest in what befell the backward Tungus people in remote central
Siberia.
Soil samples
The first expedition to reach the site arrived in 1930, led by Soviet
geologist L A Kulik, who was amazed at the scale of the devastation and the
absence of any impact crater. Whatever the object was that came from space,
it had blown up in the atmosphere and completely disintegrated.
Nearly a century later, scientists are still debating what happened at that
remote spot. Was it a comet or an asteroid? Some have even speculated that
it was a mini-black hole, though there is no evidence of it emerging from
the other side of the Earth, as it would have done.
What is more, none of the samples of soil, wood or water recovered from the
impact zone have been able to cast any light on what the Tunguska object
actually was.
Researchers from several Italian universities have visited Tunguska many
times in the past few years. Now, in a pulling together of their data and
information from several hitherto unused sources, the scientists offer an
explanation about what happened in 1908.
Possible orbits
They analysed seismic records from several Siberian monitoring stations,
which combined with data on the directions of flattened trees gives
information about the objects trajectory. So far, over 60,000 fallen trees
have been surveyed to determine the site of the blast wave.
Over 60,000 fallen trees have been surveyed to determine the site of the
blast wave
"We performed a detailed analysis of all the available scientific
literature, including unpublished eye-witness accounts that have never been
translated from the Russian," said Dr Foschini. "This allowed us to
calculate the orbit of the cosmic body that crashed."
The object appears to have approached Tunguska from the southeast at about
11 km per second (7 miles a second). Using this data, the researchers were
able to plot a series of possible orbits for the object.
Of the 886 valid orbits that they calculated, over 80% of them were asteroid
orbits with only a minority being orbits that are associated with comets.
But if it was an asteroid why did it break up completely?
"Possibly because the object was like asteroid Mathilde, which was
photographed by the passing Near-Shoemaker spaceprobe in 1997. Mathilde is a
rubble pile with a density very close to that of water. This would mean it
could explode and fragment in the atmosphere with only the shock wave
reaching the ground."
The research will be published in a forthcoming edition of the journal
Astronomy and Astrophysics.
Copyright 2001, BBC
============================
* LETTERS TO THE MODERATOR *
============================
(4) MORE ON RADIATIVE FORCES
>From John Michael Williams <jwill@AstraGatedot net>
Hi Benny
It is VERY easy for any group of specialists to drift into error because of
disinterest in outside criticism.
Correct or not, when a specialty reaches a conclusion which can be stated in
simple, unspecialized terms, it becomes an unending battle of explanation to
justify such conclusions to persons not adhering to that specialty. This is a
responsibility which has to be accepted by such specialists.
Or, such statements must be avoided, possibly balancing error with
irrelevance.
My cure for this would be NEVER to allow peer review unless it include at
least one reviewer drawn at random from a pool of peer reviewers of ALL
disciplines.
I agree my previous message was somewhat raw: I was primarily interested in
finding a way for radiation to exert a nonradial force, one which might
accumulate to an orbital change.
I agree that the NET effect of Solar radiation will be to decrease the
effective value of the gravitational constant G, depending on
surface-to-volume (surface to mass) ratio of the orbitting body. However, I
think there also will be a component caused by heat flow which will
contribute in the direction of an increase in G.
Therefore, yes, I agree that for a teardrop-shaped body, the head would be
at higher temperature than the tail, but for two reasons, not one: (1) because
it receives more sunlight, and reradiates more, which is Duncan's first point
above; and, (2) because the constant loss of heat from the tail always would
exceed that from the head.
The radiative forces are because of momentum of radiated photons, and for
photons, momentum is proportional to energy:
E = h*f and p = h*f/c.
If the heat energy radiated from the tail totals to more than that from the
head, a net force will exist in the direction of tail-to-head. And, the tail
will be cooler BECAUSE it is exerting a greater force.
Briefly, I tried to suggest that there will be three distinct forces
determining orbital corrections caused by Solar radiation (ignoring Solar
wind): The Poynting-Robertson force, the Yarkovsky force (for rapidly
spinning bodies), and the thermal force I described in my recent CCNet
posting.
Now, let me answer the main thrust of the second of Duncan Steel's issues,
which, which addresses rotation of small bodies:
Solar radiation MUST dampen such rotation. Here is a proof, expanding my
previous explanation but stopping short of equations:
Assume a body significantly larger than a wavelength of the solar radiation
in question. Ignore solar wind.
Let the body be of ANY shape and be rotating on an axis with a component off
the direction of the radius between the center of gravity of the Sun and
that of the body.
Now, in this case, it will be possible to project the axis of rotation in
the direction of
the Sun so that sunlight will be irradiating, in the temporal average, half
of the projected area
of the body receding from the Sun and half approaching.
Now consider the momentum transferred by a Solar photon to the approaching
vs receding halves:
The photon will be blue-shifted in its interaction with the approaching half
vs the other. Momentum of a photon is proportional to frequency f, p =
h*f/c.
So, the momentum transferred to the approaching half will be greater than
that transferred by a photon of equal wavelength to the receding half. This
holds for any wavelength (with the IR cutoff above); so, summing over all
wavelengths, the momentum opposing rotation always will exceed that
enhancing it.
If so, then I think one must conclude that the rotation of small bodies is
damped by Solar radiation. Note that this argument does not hold for
symmetrical radiation, such as that of the CMB.
So, the Yarkovsky effect must apply only to relatively large bodies or those
relatively recently set into rotational motion. Or both.
I have suggested that there still will be a thermal effect, even after all
rotation, other than that of the orbital motion, has ceased.
John
jwill@AstraGatedot net
John Michael Williams
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