[meteorite-list] Fw: Astronomy Picture of the Day

[Jeff Kuyken]

www.meteorites.com.au/odds&ends/myths.html

Cheers,

Jeff

----- Original Message -----
From: Jose Campos
To: David Pensenstadler ; Meteorite List
Sent: Monday, November 20, 2006 5:21 AM
Subject: Re: [meteorite-list] Fw: Astronomy Picture of the Day


Hi David,
I suppose that you mean the METEOROID (and not the meteorite), has spent
eons in space temperature.
As you know, when the  METEOR enters our atmosphere, it is quicky heated as
it passes through the denser layers of the Earth's atmosphere, reaching a
surface temperature of around 2000 K at aprox. 80 Kms height -  but only a
few thenths of a millimiter inwards from its surface, will be heated to that
temperature.

This surface temperature depends on several parameters, such as its orbital
velocity relative to Earth's, its angle of entry thru the atmosphere, its
mass  and the amount of its surface exposed to ablation.

This process takes only a few seconds,  but if it does not loose all its
mass during the flight thru the atmosphere, it slows down to a critical
velocity of around 3 km per second - that's when it stops shining  and its
surface COOLS DOWN to form the typical crust that we often see on
METEORITES.

As it reaches its terminal velocity, it starts a  free fall towards the
ground - that phase is called the dark flight (obvioulsly, seldom mentioned
in popular newspaper articles), and it LASTS FEW MINUTES - enough time to
cancel off what might still be left of its initial  low internal body
temperature, if any, at that stage. When you pick  the meteoriteup  from the
ground, it does not feel to be abnormally hot (as sometimes claimed),
neither abnormally cold.

Regards,
José Campos
Portugal


----- Original Message -----
From: David Pensenstadler
To: Jose Campos ; Meteorite List
Sent: Sunday, November 19, 2006 4:58 PM
Subject: Re: [meteorite-list] Fw: Astronomy Picture of the Day


Jose and list:

If the meteorite has spent eons at space temperature and only spends a few
seconds heating the surface as it descends through our atmosphere, why
shouldn't it still be cold from the low internal temperature of the
material?

Dave

Jose Campos <josecamposcomet at netcabo.pt> wrote:
Hi Stefan,

Thanks for the link.
However, the captation that goes with it says:
Quote " If you are lucky enough to find a meteorite just after impact, do
not pick it up -- parts of it are likely to be either very hot or very
cold". End of quote.

That is unfortunately, quite a common popular belief, that from an
educational point of view, must be fought off.

During the meteorite's free fall to the ground, it has enough time to cool
down to the point that, on being pick up from the ground, a person do not
experiences any heat or, for that matter any cold from it - and its
temperature will not be much different from any other stone near by.

José Campos
Portugal




----- Original Message -----
From: "Stefan Brandes"
To: "Meteorite-list"
Sent: Sunday, November 19, 2006 10:29 AM
Subject: [meteorite-list] Astronomy Picture of the Day


> Nice pic of the day today!
>
> http://antwrp.gsfc.nasa.gov/apod/
>
> Stefan
>
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> Meteorite-list mailing list
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