> Establishment of a lunar base will degrade the lunar vacuum. The time scale for distribution of exhaust gas across the surface of the moon is much less than the excape lifetime of the gas in the lunar atmosphere, and thus exhaust gas can be approximated as uniformly spread across the surface. A 20 person exploration base will contribute an amount of waste gas on the same order of magnitude as the daytime "natural" atmosphere. A 250 person "industrial" facility would be likely to contribute considerably more due to waste gas from various production processes such as lunar oxygen production and mining of helium 3 from the lunar regolith. This could degrade the lunar ambient to levels on the order of 3 nanotorr, replacing the mostly non-reactive gasses hydrogen, helium, and neon with more reactive gasses containing carbon and oxygen. This vacuum is still good enough to perform many important vacuum processes, such as plasma-deposition of amorphous silicon for solar cells, but processes such as molecular beam epitaxy or locating a intersecting beam accelerator on the moon will require additional vacuum pumping. In any case, though, pumping to ultrahigh vacuum will be much easier on the moon than on Earth.