Walter L. Preuninger II wrote:
one thing that stands out, in particular with the night shots of
Discovery
on the launch pad, is how dirty or marked Discovery appears.

I thought dirt on airplanes caused aerodynamic issues, such as a loss of
performance. Is this not true?


It's not dirt on the orbiter. It is thin sheen
of surface contamination deposited by the effects
of reentry heating. The deposits include calcium
and zinc oxide from gap filler materials and
RTV, bits of melted metal from surface-mounted
sensors, aluminum oxide deposited by the SRBs,
etc. Some of this stuff becomes embedded in
the outer layers of the TPS so that it can't
easily be cleaned.

- Ed Kyle