Why airplanes taxi

On Feb 8, 6:31*am, Gig 601XL Builder
wrote:
terry wrote:
The resistance is a function of the density of gas molecules not
pressure
density is related to pressure by
D=PM/RT
M=molecular wt
P=pressure
R=gas constant
T=temperature ( absolute )
ie the lower the temperature the higher the density
so what is the temperature in interestella space?

estimates are from between 3 - 20 K. But with Global Warming who knows.

yes, substitute 3 for T and 1 E-11 for P in the above equation, use
2 E-3 for MW ( any gas in interstellar space is likely to be
predominantly hydrogen, and you get 8E-16 kg/m3 for density, Very
small indeed, but then the resistance this provides to a spacecraft
is proportional to density. area. v squared.
when you chuck in a v that is anywhere near the speed of light and
square it, you start to get numbers that are no longer negligible.