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(meteorobs) How hot are meteorites on reaching the ground?



This is an excerpt from the AMS fireball FAQ:

8.  Are meteorites "glowing" hot when they reach the ground? 

Probably not.  The ablation process, which occurs over the majority of the
meteorite's path, is a very efficient heat removal method, and was
effectively copied for use during the early manned space flights for
re-entry into the atmosphere.  During the final free-fall portion of their
flight, meteorites udergo very little frictional heating, and probably reach
the ground at only slightly above ambient temperature.   

For the obvious reason, however, exact data on meteorite impact tempertures
is rather scarce and prone to hearsay.  Therefore, we are only able to give
you an educated guess based upon our current knowledge of these events.

End of excerpt.

What the temperature of a meteoroid would be at the top of the atmosphere is
subject to many conditions.

"Space is Ice Cold. (-180 deg C)"

Space (a vacuum) doesn't have a temperature.  An object in space, distant
from any energy source, will soon radiate most of its heat away, and reach
thermal equilibrium near absolute zero, -274 deg. C. An object in space, at
1 au from the Sun, is subjected to the radiant energy of the Sun, slightly
variable around 1370 watts/m^2.  The object will reflect some, absorb some,
and reradiate some, and reach thermal equilibrium at some point, a little
warmer than before.

Now chuck this thing into the atmosphere at 40 km/sec, and heat up the
exterior to 3500-4500K.  Lots of ablation takes place, but some heat is
conducted into the interior.  Of course, a lot depends on the size and
composition of the meteoroid.  An iron would conduct heat to its interior
more quickly than a stone.  If it was a really big iron, it would zip right
thru the atmosphere and reach the ground still moving at a pretty good clip.
A 9000 kg iron would be (for simplity, assume 50% iron and 50% nickel by
weight, and a spherical shape) about 1.27 meters in diameter.  If it entered
the atmosphere perpendicular to the surface, it would arrive at the surface
retaining 6% of its original velocity, or 2.4 km/sec, and about 50% of its
original mass.  Ouch!  I guess it doesn't matter what the internal
temperature is then!

Let's take something a little lighter, say a 1000 kg iron entering on the
same 90 deg. angle.  It becomes visible at 100 km and reaches the
retardation point at 20 km.  So for about 3-4 seconds it is heated on the
exterior to 4000K.  If it were a sphere, it would be about 62 cm in diameter
(pure iron this time, nickel-iron alloy would be denser, therefore smaller).
By the end, there's "only" a couple of hundred kg left, and it's not very
big.  I would think significant heat would be conducted to the interior.
Now it falls, reaching a terminal velocity, of, say, 200 m/s.  For the next
couple of minutes it drops, subjected to a cold, but gradually warming 200
m/s air blast.  And it would arrive at the surface with a great thump,
probably a warm to the touch, but certainly neither glowing nor frozen
through.  

The whole thing would be quite different for a stone.  A stone that size and
speed would certainly undergo a catastrophic fragmentation at 30-40 km, and
nothing but a lot of little pieces would drift to the surface.     

If anyone wants a full copy of the AMS fireball FAQ, let me know.

Jim Bedient
AMS Electronic Information Coordinator