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

[Jose Campos]

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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