A laser interferometer gravitational wave observatory (LIGO) is being developed with sensitivities that will have a high probability of detecting gravitational waves from astrophysical sources. Detectors are also planned by others in Europe and eventually in space. A major component of the proposed LIGO is a total of 16 km (10 miles) of 1.2 m (48 in.) dia tube at a pressure of less than 10-8torn It will be of 304 L stainless steel procured directly from the steel mills with the initial hydrogen content specially reduced. (Target is 1 ppm by weight.) Projections of the outgassing rates of hydrogen and of water vapor as a function of time will be given and the uncertainties discussed. Based on these, a preliminary analysis of the vacuum system will be presented.

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