[meteorite-list] Stardust SRC Hot to the touch?

[Chris Peterson]

Hi Martin-

The "mantra" that meteorites _must_ be cold is obviously incorrect. The 
reality is that meteorites _usually_ arrive with their surfaces near ambient 
and their interiors cool.

A typical meteorite (to the extent that such a thing exists) is the product 
of fragmentation from a larger body. It is protected from the heat of entry 
right up to the moment of fragmentation. It then begins to heat, and a 
fusion crust forms, but this is probably only for a second or less (the low 
mass object slows very rapidly). There is no opportunity for the object to 
absorb much energy. It then falls through -40° air for several minutes, 
significantly reducing its temperature.

So how does a meteorite manage to occasionally arrive hot? Probably the same 
way as Stardust. You need a near grazing entry angle and low speed, so that 
the cosmic velocity is lost at high altitude over a long time. This means 
that the deceleration and ram pressure are low enough that you don't get 
fragmentation (or very minimal fragmentation). There is also more time for 
the body to absorb energy. The size is also important. Too big and the heat 
capacity of the volume dominates- the energy that can be absorbed by the 
available surface area isn't enough to significantly heat the interior. Too 
small, and the interior will quickly equilibrate to air temperature during 
dark flight.

Hot meteorites must occur, they just aren't very common ("hot" meaning a bit 
uncomfortable to the touch, not flaming or hot enough to start fires). There 
may be a slight bias towards hot meteorites in reports because the same 
conditions that can produce them (long, slow entries) are also responsible 
for the most impressive fireballs. Falls produced by more common, and less 
impressive entries, may be somewhat more likely to go unwitnessed.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: "Martin Horejsi" <accretiondesk at gmail.com>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Wednesday, January 18, 2006 6:39 AM
Subject: [meteorite-list] Stardust SRC Hot to the touch?


Hi Ron and All,

I am wondering why the Stardust SRC might arrive to earth too hot to
touch, yet the mantra for meteorites is they must be cold even though
history is littered with witnesses claiming the stones and irons were,
you guessed it, too hot to touch.

Here is an excerpt from the Stardust press kit, page: 14-15:

Following a normal entry, the capsule will be hand-carried to the
helicopter for trans-
port. The capsule will be warm; temperature of the heat shield could
be as high as 60 Celsius (140 Fahrenheit), so handlers will be
required to wear protective gloves.

Cheers,

Martin