The present lunar atmosphere, arising from natural sources with a total rate less than 0.010 kg/s, has a mass of less than 10^4 kg and surface number densities less than 10^7 /cm^3. The primary mass loss mechanism is due to the interplanetary electric field resulting from the motion of the solar wind. This causes rapid loss of gases to the lunar exosphere within 10^6 to 10^7 s. This loss has been confirmed by observations of lunar module exhaust gases (refs. 28, 29). If the atmosphere is dense, however, the cleansing effect of the solar wind decreases and thermal escape becomes the dominant loss mechanism due to the relatively higher collision rate among particles. The comparative effectiveness of these two loss mechanisms is illustrated in figure 5-34 for an oxygen atmosphere.
The use of the present lunar "vacuum" for industrial purposes as well as for scientific purposes (e.g., astronomical observations) will most likely necessitate the maintenance of a sufficiently "lunar-like" exosphere rather than allowing a substantial atmospheric mass to build up. Figure 5-35 presents growth curves of the lunar atmosphere for various constant gas addition rates. A release rate of about 10-100 kg/s would cause a transition to a long-lived atmosphere which occurs at a total mass of 10^3 kg (ref. 30). Release rates at about 1000 kg/s will produce an atmosphere which will exert aerodynamic drag on orbiting or departing vehicles (ref. 31). At gas release levels at or below 0.1 kg/s the lunar atmosphere would increase at most to a mass of 10^6 kg. Furthermore, if the artificial source of gas is shut off, the time scale for the Moon's atmosphere to return to its natural state is on the order of weeks (10^6 to 10^7 s). Due to the modeling techniques in determining these effects, order-of-magnitude accuracy should be attributed to these estimates.
Three sources can be identified within the framework of large-scale lunar operations as potentially releasing substantial quantities of gases into the lunar exosphere: mining and processing of lunar materials, leakages from the Moon base environment, and fuel expenditures during transportation of personnel and materials to and from the lunar surface. To build the Stanford Torus, raw material is propelled from the lunar surface by a mass launcher and not by the use of chemical rockets. Without materials processing on the lunar surface a potential source of gases is eliminated, and by using nonchemical methods to lift the required lunar materials off the lunar surface, the mass released during transport through the Moon's atmosphere is minimized.
It is believed that mining operations will not release a significant amount of gases into the atmosphere. If it is assumed that 10^10 kg of lunar materials are mined during a l0 yr period and that l0 percent of the trapped gases in the lunar soil (10^-4 to 10^-5 of its mass) are released during normal mining operations, the average release rate is 3 X 10^-4 kg/s, which is substantially less than the natural source rate (personal communication from Richard R. Vondrak, Stanford Research Institute, July/August 1975).
Losses due to leakage from a Moon base have been estimated by NASA experts (ref. 18) based upon a projected loss rate per unit surface area. The yearly leakage loss is approximated to be 18,000 kg which would result in a release rate of 6 X 10^-4 kg/s. Again this is insignificant in comparison to the natural source rate. It should also be noted that the Moon base considered by Nishioka et al. (ref. 18) includes a processing plant and would most likely be larger than the lunar facility considered here. The actual release rate would then be even smaller than that given above.
By far the most significant source for release of gases into the lunar environment is the exhaust products released by chemical rockets in the initial establishment of the lunar base and its continual resupply. It has been estimated that 1 kg of propellant will be expended by the lunar landing vehicle for each kilogram of payload landed ( ref. 18). The mass of lunar base, estimated at 17 X 10^6 kg, is assumed to be delivered to the lunar surface over a 2.5 yr construction phase. An annual resupply rate of 0.4-0.5 X I0^6 kg has been calculated. These figures give release rates of 0.2 kg/s during the establishment of the base and 0.02 kg/s thereafter by averaging the expenditures of propellant over an entire year. This is believed to be valid due to the rapid diffusion of gases released on the lunar surface (Vondrak, personal communication).
Although establishing a lunar base as required for the construction of the Stanford Torus will most likely result in gas release rates at times greater than that occurring naturally, a sparse lunar exosphere will still be preserved given the magnitudes of the calculated release rates. Furthermore, a long-lived atmosphere will not result and if the critical sources of gas are halted, the lunar atmosphere will return to its natural state within weeks.
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