Air friction at supersonic velocities I know that if an object moves in the air, it can experience two types of drag, laminar and turbulent. For instance, I have a meteor of ideal spherical shape falling from edge of space, say, 100 km up at the Earth surface with initial velocity of 1 km per second. I would consider turbulent drag, but is it still applicable at supersonic velocities? How do I estimate meteors velocity when it hits the ground?
 A: The drag force, $F$, on the meteor is given by:
$$ F = C_d \frac{1}{2}\rho v^2 A$$
where $\rho$ is the fluid mass density, $v$ is the meteor speed, $A$ is the meteor cross-sectional area, and $C_d$ is the drag coefficient (discussed below).
The fluid mechanics of meteor reentry is quite complicated.  Over the important ranges of altitude, the meteor is supersonic and a bow shock forms in front of the meteor.  Consequently, the flow adjacent to the meteor is subsonic.  The flow field is therefore non-trivial.
For a quick answer, though, meteor trajectories can be calculated by assuming $C_d=0.7$.  If you want to be more accurate, a plot of $C_d$ vs far-field mach number can be found in this paper:  "ESTIMATING THE DRAG COEFFICIENTS OF METEORITES FOR ALL MACH NUMBER REGIMES" by R. T. Carter, P. S. Jandir, and M. E. Kress:

From the plot, you can see that the rule-of-thumb value of 0.7 underestimates the drag at high speed and overestimates at low speed.
You will also need to know atmospheric density vs altitude.  A NASA report from 1976 defines the standard values for this and is available online here.  There are also online web calculators, such as this one, that are based on the same report.  (For data above 86 km, see this report.)
