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(meteorobs) Excerpts from "CCNet 111/2001 - 30 October 2001"
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 111/2001 - 30 October 2001
Date: Tue, 30 Oct 2001 11:38:37 -0000
CCNet 111/2001 - 30 October 2001
================================
[...]
(2) MCNAUGHT & ASHER'S LEONID PREDICTIONS
Michael Paine <mpaine@tpgi.com.au>
[...]
(4) RADIATIVE FORCES ....
Duncan Steel <D.I.Steel@salford.acdot uk>
[...]
==============================================================================
(2) MCNAUGHT & ASHER'S LEONID PREDICTIONS
>From Michael Paine <mpaine@tpgi.com.au>
Dear Benny
Space.com has just posted an update to Rob Britt's article about the
forthcoming Leonids event:
Leonid Meteor Shower:Prediction Revised for November Event
http://space.com/scienceastronomy/astronomy/leonids_2001.html
Here are McNaught and Asher's new predictions for the peak rates (note that
the
peak may last less than one hour). All time are for Nov. 18, 2001. This
chart is updated as of Oct. 30, 2001:
Where When
Meteors
North & Central America 4:55 a.m. EST 800 per hour
Australia; East Asia 17:24 UT 2,000 per hour
Western Australia; East,
Southeast & Central Asia 18:13 UT 8,000 per hour
regards
Michael Paine
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* LETTERS TO THE MODERATOR *
==============================================================================
(4) RADIATIVE FORCES ....
>From Duncan Steel <D.I.Steel@salford.acdot uk>
Dear John,
In your analysis (CCNet 29 Oct. 2001), you appear, unfortunately, not to
have taken into account the fact that objects radiate energy at a rate that
is strongly dependant upon their temperature. This is the Stefan-Boltzmann
law:
F = A epsilon sigma T^4
F = emitted flux (Watts)
A = surface area involved (m^2)
epsilon = emissivity (composition and structure dependant,
value from 0 to 1)
sigma = Stefan-Boltzmann constant = 5.67 x 10^-8 (SI units)
T = temperature in Kelvins
In your scenario you have the meteoroid being non-isothermal so as to have
heat being conducted from one side (sunlit) to the other. Thus one side is
higher T, hence that side emits more radiation, in the contrary sense to
that idea you describe.
The reality of the situation will depend upon the meteoroidal conductivity,
or more particularly what is termed its thermal inertia. For details see my
paper in MNRAS, volume 228, 1-17 (1987). We may expect small meteoroids to
be *very* poor conductors of heat.
In addition, the reality is that like everything else in the universe,
meteoroids spin. In fact the general rule is that objects spin at a rate
limited by their strength (i.e. they spin at just below the rate at which
they'd fly apart). For small objects the strength is set by cohesive forces.
For larger objects (like galaxies) its gravity. A few objects may spin
slowly, but most spin as fast as they can
Hope this all helps,
Duncan Steel
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