(meteorobs) Fwd Re: Halley's Comet Returns In Bits And Pieces

 Message forwarded without permission...
 Another informative reply to this thread on the 'mplist'. Again, Roy
 Tucker is not a subscriber of 'meteorobs': if you reply to this post,
 please be sure to manually place 'tucker@noao.edu' in your "Cc:" line.

   Clear skies!  Lew Gramer <owner-meteorobs@latrade.com

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Date: Fri, 23 Oct 1998 16:37:41 -0700
To: mplist@bitnik.com
From: Roy Tucker <tucker@noao.edu>
Subject: Re: Halley's Comet Returns In Bits And Pieces

At 05:37 PM 10/23/98 -0500, you wrote:
>Ron Baalke wrote:
>October 20, 1998: The last time Halley's comet visited Earth, in 1986,...
>> Comet debris particles are usually no bigger than grains of sand, and much
>> less dense. Although they
>> are very small, these tiny 'meteoroids' make brilliant shooting stars when
>> they strike Earth's atmosphere because they travel at tremendous speeds. A
>> typical debris particle is about the same size as a grain of sand, but
it is
>> much less dense, weighing only 0.01 gram.
>Maybe someone can "illuminate". The above statement regarding the size of
>meteoroids has been made also by others regarding the upcoming Leonids.
>However, acknowledging my vast ignorance on the field of astronomy and
>Astrometrics, I wonder:
>How is it possible for particles so small to burn with such brilliance
>and for so long, to give up so much visual energy that we, on the surface
>of the planet, miles upon miles under a thick coating of light-energy
>absorbing atmosphere can still see them?
>Something does not add up and it may just be my scientific training.
>Would some of the fine folks on this fine list explain this miracle?
>Thank you

Hi Miguel,

Your question is essentially, "How can we see such tiny particles as
meteors?". There are two parts to that explanation: (1) how much energy is
available, and (2) how is this energy used to produce the light that we see.

The kinetic energy of a moving object goes up as the square of the
velocity according to the relationship: K.E. = 1/2 x mass x velocity ^2.
The mass was given as 0.01 gram or 0.00001 kilogram. The velocity is
approximately 40,000 meters per second. Solving, we get 8,000 joules of
energy. A meteor may last 0.1 second so, assuming uniform deceleration and
energy conversion, it will produce 80,000 watts of power. Not all of this
energy is converted to light. The fraction that is converted to light, the
so-called "luminous efficiency", is used to generate light largely by two
mechanisms: incandescence and atomic excitation. The power radiated by a
hot object goes up as the fourth power of the temperature and directly as
the surface area of the object. The tiny size of these objects will not
limit their ability to radiate the power being generated by their passage
through the atmosphere. Although the solid object will vaporize, the vapor
produced is still travelling at high speed. Before they are decelerated to
a complete stop, they will generate whatever temperature is necessary to
radiate the power, tens of thousands of degrees. Also, excitation and
ionization of the vaporizing body and the atmosphere will produce an
emission line spectrum, adding to the light.

     I hope this helps.

					- Roy

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