Re: Impact temp. of a metorite. (is it hot or not) can it start a fire?

Rick:
The impact temperature of a meteorite is highly dependent on many factors. 
 
As a meteorite enters the earth's atmosphere, it enounters friction against 
the air molecules.  
This has two effects: 1) It heats up the meteorite and the air, and 2) it 
ablates away much of the outer surface of the meteorite.  
This heating and ablation is responsible for the fusion crust that is 
typically found on the outer surface of meteorites.  
Air friction also has the effect of slowing the meteorite down.
Let's say that you have a meteorite that is large enough to reach the 
earth's surface without being ablated to oblivion.  
How hot is it when it hits the surface?  Well, that depends.  It depends on 
how fast it is moving when it hits.  
That depends on the geometry and mass of the meteorite.  Let's do a 
first-order calculation.  
Let's assume that we have a space rock entering the atmosphere and that 
there is no ablation.  
(In other words, if we start with a golf ball, a golf ball will impact the 
earth)  
Let's also assume that the atmosphere has the same density throughout.  
Neither of these assumptions are correct, but they allow us to make a 
back-of-the-envelope calculation which will give us a good idea of what we 
actually get.

Let's start with the high-velocity equation for drag:

F=(1/2)*C*rho*A*v^2

Here, rho is the density of air, A is the cross-sectional area of the 
meteorite, v is the velocity, and C is a factor dependent on the geometry 
of the object.  
Spherical objects have a value of C of .5, so I will use .7 due to the 
irregular shape of a typical meteorite.

I ran a few calculations for some typical sizes.  If you have a 
baseball-sized meteorite of density 3.2 g/cc, using a value of 1.2 kg/m^3 
for the density of air, 
you will find that the meteorite will slow from its approach velocity of 
roughly 11000 meters per second to its terminal velocity of 60 m/s in a 
mere 28 seconds, having traveled only 3 km.  (By comparison, the speed of 
sound is roughly 315 m/s.)
It then spends another 100 minutes or so falling before it hits the ground, 
giving it ample time to cool down below its original temperature it gained 
during entry into the atmosphere.  
(At 60 m/s, it's moving like a fastball, but not much more.  It'll still 
cause a lot of damage if your car or house is in the way, but it wouldn't 
start a fire.

Much larger meteorites will hit the ground much faster.  A 1m-radius object 
hits the ground at supersonic speeds.  Still not hot enough to spark a 
fire, I don't think, but it's pretty fast.
Something about 10 times that or greater will not even hit its terminal 
velocity before reaching the ground.  Generally, these things are still 
very hot as they impact the surface, 
and transfer a lot of heat and energy to the air and to the ground as they 
pass.
On June 30, 1908, an object roughly 60 meters across didn't even make it 
all the way to the ground.  It exploded about 5 miles above a remote site 
in Siberia, called Tunguska.  
This released enough energy to flatten thousands of acres of forest, and a 
large percentage of the flattened trees were also burned.  The explosion 
was equivalent to 
15,000,000 tons of TNT.

So, depending on the size of the meteorite, it may or may not start a fire.

---Bob Macke
MIT S.B. '96, Physics
 in St. Louis Ph.D. candidate, Physics

References:

Serway, "Physics for Scientists and Engineers," Saunders College Pub., 
Philadelphia, 1990, p. 139. (Explanation of air resistance)


The Tunguska Event

Here are some of my calculations for terminal velocity of meteorites 
of various sizes, based on the parameters described above.

r(cm)   vt (m/s)	M(kg)		Initial E (J)	Initial v (m/s)
1	31.5524		0.013404117	836765.4287	11173.71917
3.66	60.36329548	0.657175671	41024848.41	11173.71917
10	99.77744964	13.40411733	836765428.7	11173.71917
50	223.10916	1675.514667	1.04596E+11	11173.71917
100	315.524		13404.11733	8.36765E+11	11173.71917
50000	7055.331125	1.67551E+12	1.04596E+20	11173.71917
100000	9977.744964	1.34041E+13	8.36765E+20	11173.71917
1000000	31552.4		1.34041E+16	8.36765E+23	11173.71917

r=radius
vt=terminal velocity
M=mass
E=energy (kinetic)
v=velocity
Note: Where v is less than vt, impact velocity will be v