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Degradation of the lunar vacuum by a moon base
Acta Astronautica Vol. 21, No. 3, pp. 183-187, 1990
0094-5765/90 $3.00+ 0.00 Copyright © 1990PergamonPress plc
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DEGRADATION OF THE LUNAR VACUUM BY A MOON BASE GEOFFREY A. LANDISt NASA Lewis Research Center, 302-1, 21000 Brookpark Road, Cleveland, OH 44135, U.S.A. (Received 15 April 1988; revised version received 31 August 1989)
Abstract--High vacuum is required for many industrial processes which might be accomplished on the moon, such as electronic component and solar cell manufacturing or a large particle accelerator. Ambient pressure on the moon is in the range of l0 -12 torr (night) to l0 -~° torr (day). The effects of a 20 person base and a 250 person industrial facility on this vacuum are discussed. Exhaust from the transport spacecraft and leakage from the habitat will be roughly comparable to the daytime gas pressure for the 20 person base, and will degrade the vacuum to the range of 2 × 10-9 torr for the 250 person facility. This is higher than the desired pressures for some semiconductor manufacture processes or for a lunar-based particle accelerator.
i. THE LUNAR AMBIENT
The existing lunar atmosphere is tenuous and not well characterized. The ambient pressure is in the range of 2 x 105 molecules/cm3 during the lunar night, and 6 x 105 to < 107 molecules/cm3 during the lunar day[l-3]. This corresponds to pressures from 5 x 10 -t3 torr [0.0005 nanotorr (ntorr)] up to 0.4 ntorr, primarily consisting of hydrogen, helium, argon, and neon at night, with the probable addition of CO and CO2 in the daytime[4]. The mean free path for these pressures are in the range of hundreds to thousands of kilometers; thus, the movement of gas in the atmosphere is primarily via ballistic transport. The atmospheric escape lifetime from the sunlit side of the moon is approx. 10,000 s (15 min) for the lightest molecules (hydrogen and helium), and up to 107 s, approx. 100 days, for heavier molecules[5]. 107 s is roughly the maximum lifetime of atmosphere constituents; this is approximately the time it takes for the molecules to become ionized by the solar ultraviolet, at which time they are swept away by electric fields associated with the solar wind in times which are typically no more than a few hundred seconds[5,6]. As noted by Vondrak[6], this mechanism becomes ineffective if the atmosphere is thick, however, the gas input rate (on the order of 250,000 tons/month) required to reach such a level is considerably higher than what is likely to be produced in any near-term industrial facility.
2. REQUIRED VACUUM It seems absurd to expect that the lunar vacuum could be lost by small-scale operations on the moon. ?National Research Council Resident Research Associate, NASA Lewis Research Center.
However, high vacuum and ultra-high vacuum is needed for many industrial processes, some of which may be accomplished on the moon. Some processes which require vacuum and thus would be simpler to manufacture or use on the moon include vacuum tubes, semiconductor manufacture, solar cell manufacture, and particle accelerators. Silicon is a major component of the lunar crust. One likely low-cost process sequence for producing solar cells on the moon[7] is plasma-deposition of amorphous silicon. Such deposition processes typically have base pressures in the very high vacuum range, mid-10 -6 torr, to below 10-7 torr for some experimental set-ups. It is believed that impurities in the deposited films of concentration > 1018/cm3 cause (or exacerbate) the deleterious light-induced degradation effect; this corresponds to a base pressure of 2000 ntorr at deposition pressure 1 torr; 100 ntorr at deposition pressure 0.05 torr. Many processes for manufacturing semiconductor products require vacuum. One process for depositing high-purity layered compound semiconductors is molecular beam epitaxy (MBE). This process requires ultra-high vacuum. Base pressure for MBE is in the range of 0.1 ntorr[8,9] and can be as low as 0.03 ntorr for GaAIAs[10], where C and O2 contamination are particularly harmful. "Vacuum" tubes have a different values for the required operating vacuum, depending on the type of tube and the lifetime, noise level, etc. required. This ranges from 10-5 torr for the magnetron tubes used in fimicrowave ovens, to ultra-high vacuum of 10-~0 torr for travelling-wave tubes. The moon would be a good location for a large, high-energy particle accelerator for several reasons, one of which is the vacuum ambient. Intersecting storage ring (ISR) accelerators require very good vacuum, since any residual gas tends to scatter and 183
184
GEOFFREYA. LANDIS
defocus the beam. At a pressure of 10 ntorr the beam lifetime is typically around l h; and operating pressures of under 0.01 ntorr are required for long lifetime storage and operation[l l]. An additional problem is that whenever the beam tube is vented to atmosphere, gas is adsorbed onto the surfaces which is later desorbed by the beam current. This vacuum will be degraded by human habitation and industrial processing of materials. It is unlikely that maintaining the lunar vacuum will be an important priority of the occupants of a moonbase. The amount of degradation can be calculated by multiplying the mass of gas exhausted times the gravitational acceleration of the moon and dividing by the lunar surface area. This factor is equal to 3.2 x 10 13torr per (metric) ton of gas exhausted. Since the exhausted gas has an average lifetime in the lunar atmosphere of 100 days, the equilibrium contribution to the atmosphere is 1.3 x 1 0 t2 torr per ton of exhaust gas per month. 3. 20 PERSON BASE
The major contribution to the lunar atmosphere from a small exploration base is exhaust gas from the transport. Assuming a specific impulse of 400 s (90% of the theoretical specific impulse of a hydrogen/oxygen engine), landing on the moon requires 0.8 tons of propellant per ton of landed material. If we assume a 10 ton lander making one trip per month with 2 tons of supplies landed per person per month (including the personnel rotation, machinery, scientific and exploration equipment, etc.), this results in an equilibrium pressure of 0.06 ntorr for a 20 person base. This does not assume that the lander is refueled on the moon from lunar oxygen (i.e. it includes the fuel use to relaunch the lander, but does not assume that any payload is carried from the moon). In actuality, it is not correct to assume that all of the propellant expended from the ship will contribute to the lunar atmosphere. The exhaust velocity of a H2/O2 engine is 4km/s, nearly double the lunar escape velocity. Further, if the trajectory used is an insertion into low lunar orbit followed by a descent burn, for much of the engine burn the exhaust will
Source
not be directed toward the lunar surface. However, for a rough calculation here I assume that the entire engine exhaust contributes to the atmosphere. Another contribution to the generated atmosphere is air leakage from the living quarters. One estimate[12] of air leakage from an advanced longduration habitat at atmospheric pressure is 1.2 kg O: plus 4.5 kg N 2 per person per day. This would result in a pressure contribution of 0.004ntorr for a 20 person base. It has frequently been proposed that oxygen be locally generated. If this is done, it is unlikely that nitrogen dilution would be used, since nitrogen is nearly absent on the moon. Thus, the habitat pressure would be proportionately lower, and the leakage rate is expected to be reduced to 23% of that listed above. However, as discussed below, lunar generation of oxygen would itself be likely to be a source of leakage of waste gas. A human also generates about 1 kg of waste CO2/day. An efficient habitat design will recycle this gas, however, it is possible that an early moonbase design might vent some portion of the waste; likewise, some spacesuit designs may simply vent the waste CO2 rather than store it for return to the base for recycling. If so, this contribution would have to be added to the total gas loss. In addition to this leakage, air will normally be lost during ingress and egress for extra-vehicular (or extra-habitat) activities (EVA). The amount of air lost will depend on whether the airlock is simply vented during egress, or if the lock is pumped down and the exhaust air reused. In the baseline case, I will assume that the lock is simply vented. If there is one EVA per person per day, and the lock volume is 2 m 3 of air at one atmosphere pressure, this then results in a contribution of 0.0017ntorr for the 20 person base, which is somewhat less than the habitat leakage (and, like the leakage, reduced if the base is assumed to have a pure oxygen atmosphere). Table 1 summarizes the contributions of the various gas sources discussed. The daytime total atmosphere is in the range of 0.07 ntorr, comparable to the natural lunar atmosphere. During the lunar night, most of this will be adsorbed into the soil, resulting in considerably lower pressure.
Table 1. Contributionsto lunar atmosphere Contribution (ntorr) Notes
20 Person Base:
Propellant Habitat leakage Airlock losses Total
0.06 0.004 0.002 0.07
50 ton lander;all exhaust gas contributes 5.7 kg/person/day 2 m3 vented;less if pumped down for EVA
250 Person Industrial Facility:
Propellant Habitat leakage 02 production 3He mining Industrial processing Total
0.6 0.05 0.9 0.9 unknown 2.5
refueled usinglunar oxygen 5.7 kg/person/day 500tons/month;25% leakage 10tons/year; 25% leakage
Degradation of lunar vacuum
185
helium and hydrogen, and 0.87 ntorr for heavier gasses. The impact of 3He mining on the lunar atmosphere If large-scale industrialization takes place on the moon, it could be expected that the lunar habitat may has also recently been considered by Duke[14], who have hundreds of inhabitants, and considerably more concluded that stripping 100,000 tons of regolith per frequent resupply flights. In this case, the vacuum year would release an amount of trapped gas degradation will be correspondingly worse. The "roughly equivalent" to the existing lunar baseline calculated here will be for a 250 person base atmosphere. processing oxygen from lunar soil. In addition, the moon base is likely to be a place I assume here slightly less support material where various other mining, refining and manufacturrequired from Earth, 1 ton of material per person ing operations take place, producing solar cells, aluper month; however, since the lander is fueled minum and titanium structures, habitation modules from lunar-produced oxygen the fuel for the lander and probably other objects useful to further colonizamust be delivered into lunar orbit. Total gas tion. These processes will involve some amount of gas contribution to the lunar atmosphere is 460 tons/ generation and, consequently, wastage. Until the month, for a pressure contribution of 0.6 ntorr. processes are more completely specified, the contribuHabitat leakage and airlock losses will contribute tions from this processing is unknown. 0.05 ntorr. Finally, the lunar soil contains trapped gas at a Lunar oxygen production to fuel the lander will concentration on the order of 50ppm by wt, require 400tons 02 per month. A 25% loss rate, primarily hydrogen and helium from the solar wind, which is realistic for a low-cost industrial process, plus and carbon compounds and nitrogen. This is would contribute 0.13 ntorr. If the trans-lunar injec- only loosely bound to the soil, and physical tion ship is also to be fueled, this is an additional disturbance, as well as movement of soil by mining, contribution. It has often been proposed that lunar etc. will likely release some of the gas content. This oxygen production could be used as a cheap source contribution is expected to be negligible compared to of fuel for spacecraft to be used from Earth orbit. I other sources. assume a baseline facility designed to deliver oxygen The total contribution to the lunar atmosphere to Earth orbit at a production rate of 500 tons per from the assumed industrial facility producing both month. Lifting this from the moon will require 02 and 3He is 2.5 ntorr (see Table 1), a factor of 5-100 400 tons of fuel, and leakage losses will be about times higher than the "natural" daytime atmosphere. 200 tons. The contribution to the lunar atmosphere is This is low enough that manfacture of amorphous 0.78 ntorr. This will be considerably less, however, if silicon solar cells can be performed without any the oxygen is to be shipped by mass-driver rather additional vacuum pumping. For other processes than lifted off the surface by rocket. discussed, such as MBE, travelling-wave vacuum Mining of the lunar regolith for helium 3 (3He) to tube formation, or siting of a large accelerator on the fuel terrestrial deuterium-helium 3 fusion reactors moon, the vacuum is not good enough, and these will has recently become a topic of interest[13]. 3He require additional pumping. implanted into the lunar regolith by the solar wind While the lunar vacuum may not be sufficient for would be extracted by baking the soil, and then some operations, it must be kept in mind that even distilled. For every ton of 3He produced, about after degradation, the ambient remains a very high 3300 tons of 4He, 6100 tons of hydrogen, 3000 tons of vacuum. It is much easier to pump a starting ambient CO and COs, and 500 tons of nitrogen will be of 10-gtorr down to ultra-high vacuum levels of produced[14]. Ten tons of 3He would be required to 10 -~l than it is to reach ultra-high vacuum starting be mined per year if half the U.S. electrical consump- from atmospheric pressure. Leaks and virtual leaks tion of 285GWe is to be produced. Most of the will be little problem; there will be almost no problem byproduct gasses produced will be useful to the lunar with desorption of gasses from chambers walls that base. Except for refrigeration and pressurization use, have been exposed to ambient, and finally, the however, the helium produced will not be of great "vacuum chambers" will not be required to hold up use, and may eventually leak to the atmosphere. This to the large mechanical pressure of 10,000 tons/m 2 was not assumed in the following analysis. imposed by the Earth's atmosphere. It is an advantageous feature of the moon that Since the escape lifetime for hydrogen and helium is much shorter than that for other gasses, these must the vacuum is self cleansing by the solar ultraviolet be considered separately. If 25% of the gas content and solar wind. "Air pollution" is a temporary is lost as waste due to soil agitation during mining effect. If it is decided that a high vacuum is required, plus leakage and waste in the baking and condensa- a wait of a few hundred days will suffice for the gas tion process, production of 10 tons/yr of 3He would to be removed by the solar wind. However, this is produce 23,500 tons/yr of waste hydrogen and 4He, only true as long as the amount of atmosphere plus 12,000 tons/yr of heavier gas. Assuming an present is low enough that there is little shielding of escape lifetime of 10,000s for the hydrogen and the solar ultraviolet. This is likely to be true for the helium, this result in a contribution of 0.002 ntorr for amounts of gas discussed in the present paper. Some 4. 250 PERSON INDUSTRIAL FACILITY
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GEOFFREY A. LANDIS
amount of gas will be adsorbed by the lunar soil. Cleansing of this gas to restore the original ultrahigh vacuum will take longer, since the soil will take time to outgas. 5. ATMOSPHERE VARIATION WITH POSITION
The calculations have so far assumed that the atmosphere generated can be assumed to be evenly distributed around the moon. The gas input mechanisms discussed are either continuous or periodic with a characteristic time less than or equal to the resupply time, assumed to be 1 month. This is much shorter than the escape time, and so the overall variation with time is expected to be small. However, the immediate vicinity of intermittant gas sources, such as the exhaust plume of a lander or the area adjacent to an airlock during depressurization, will experience temporary large increases in the gas concentration. Such surges in pressure were seen, for example, in results from lunar atmosphere experiments on Apollo missions, where the observed pressure rose dramatically when the LM was depressurized, when an astronaut approached the apparatus[2] (due to exhaust gas from the astronaut's EVA suit), and on lift-off of the LM from the surface. These pressure surges were superimposed on a longer term transient due to gradual degassing of the LM. Sensitive processes would likely be shut down during such periods. Gas molecules escape from the atmosphere primarily from the sunlit hemisphere of the moon, where they have higher kinetic energy and also are subject to photionization by solar ultraviolet. Thus, the escape lifetime is determined by the gas distribution on the sunlit hemisphere. For pressures of nanotorr and below, the gas in the atmosphere can be well modelled by ballistic transport. Gas molecules leave the surface with random direction and thermal velocity profile, follow a ballistic trajectory until again intersecting the surface, and then may be temporarily adsorbed by the surface before being reemitted, again at a random direction and velocity. Temporary adsorption of gas by the surface is irrelevant to the calculation of equilibrium atmosphere pressure by a steady-state source, since the adsorbed gas neither contributes to the total pressure nor is subject to escape; however, a large amount of gas stored in the adsorption reservoir will proportionately increase the time needed to reach equilibrium pressure, and also increase the time needed to purge the atmosphere after the gas source is discontinued. A complete transport calculation would integrate over the thermal (Maxwell-Boltzmann) velocity distribution, averaging over the hemispherical angular distribution, and also take into account the spherical lunar geometry and gravitational potential. A more complete calculation would include gas-gas
collisions and the variation of temperature over the lunar surface. For an order of magnitude calculation, however, it is sufficient to assume that all the molecules can be characterized by the average thermal energy of kT/2 per degree of freedom. At a temperature of 365 K, this yields root mean square (RMS) vertical and radial velocities of 300 and 430 m/s respectively for an 02 molecule. The horizontal d travelled on a parabolic trajectory is thus 160 km, and the time in flight 380s. This distance is sufficiently small compared to the circumference of the moon that the assumption of parabolic trajectories is justifiable. In a random walk process the expected distance from the origin equals the distance d per step times // x/N, the square root of the number of steps; thus, the area covered equals ~dZN. To cover the surface area of the moon thus required roughly 450 steps, a flight time of 47 h. This time is short compared to the escape lifetime of gas in the atmosphere, thus, the assumption of roughly uniform gas distribution is justified, and there will not be a significant difference in the amount of gas near the base compared to far from the base. For other molecules, the time is proportional to (kT/m) 3,'2 Water vapor, for example, with a molecular weight of 18, will spread across the surface considerably faster. Hydrogen and helium spread across the full surface area of the moon in a time of roughly an hour. Since the escape time for hydrogen and helium is considerably less than an hour, gas concentrations for hydrogen and helium can not be assumed uniform, and considerable variations in density will exist between areas close to the gas source to areas far away. On the night side of the moon, the typical temperature is only 100 K. Molecules thus take six times as long to diffuse across the same area, and since any given molecule will spend six times as long on the night hemisphere as on the day hemisphere, the gas reservoir on the night side will be proportionately greater. Again, it should be noted that these times are exclusive of any time spent adsorbed in the soil. These conclusions compare those of Burns et al.[15], who particularly discuss column density with respect to opacity of the atmosphere for u.v. and radio astronomy, and conclude that the local pressures may be many orders of magnitude larger than the equilibrium pressure at locations close to a gas source. As discussed previously, this conclusion is correct in the case of a pressure "surge" due to transient events. 6. CONCLUSIONS 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 escape lifetime of the gas in the lunar atmosphere, and thus exhaust gas can be
Degradation of lunar vacuum 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 3He from the lunar regolith. This could degrade the lunar ambient to levels on the order of 3 ntorr, 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 an intersecting beam accelerator on the moon will require additional vacuum pumping. In any case, through, pumping to ultrahigh vacuum will be much easier on the moon than on Earth.
REFERENCES
1. J. H. Hoffman et al., Lunar atmospheric composition results from Apollo 17, Proc. 4th Lunar Science Conf. (Suppl. 4, Geochim. Cosmochim. Acta). Vol. 3, pp. 2865-2875 (1973). 2. F. S. Johnson et al., Cold cathode gage (lunar atmosphere detector). Apollo 12 preliminary science report. NASA SP-235, p. 93 (1970).
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3. (3. Jeffrey Taylor, Geological considerations for lunar telescopes. Future astronomical observatories on the moon. NASA Conference Publication 2489, pp. 21-28 (1988). 4. R. R. Hodges Jr, The escape of solar-wind carbon from the moon, Proc. 7th Lunar Science Conf., pp. 493-500 (1976). 5. F. S. Johnson, Lunar atmosphere. Rev. Geophys. Space Phys. 9, 813-823 (1971). 6. R. R. Vondrak, Creation of an artificial lunar atmosphere. Nature 248, 657-659 (1974). 7. G. A. Landis, Lunar production of space photovoltaic arrays. 20th IEEE Photovoltaic Specialists Conference, Las Vegas, Nev., pp. 874-879 (1988). 8. S. M. Sze, Semiconductor Devices Physics and Technology, p. 333. Wiley, New York (1985). 9. M. Yokoyama and S-I Ohta, J. appL Phys. 59, 2929-3921 (1986). 10. F. Y. Juang et al., J. appl. Phys. 58, 1986-1989 (1985). 11. R. Calder et al., Proc. IX Conf. High Energy Accelerators, p. 70 (1974). 12. P. O. Quattrane, Extended mission life support systems. AAS 81-237. The Case for Mars (Edited by P. J. Boston), Vol. 57, Science and Technology Series, pp. 131-162. AAS (1984). 13. G. L. Kulcinski and H. H. Schmitt, The moon: an abundant source of clean and safe fusion fuel for the 21st century. Lunar Helium-3 Fusion Power Workshop, NASA Lewis Research Center (1988). 14. M. Duke, Lunar atmosphere. Lunar Helium-3 Fusion Power Workshop, Discussion Panel section of NASA Lewis Research Center, (1988). 15. J. O. Burns et al., Artificiallygenerated atmosphere near a lunar base. Lunar Bases andSpace Activities in the 21st Century Symposium, Houston, Tex., Paper LBS-88-024 (1988).
0094-5765/90 $3.00+ 0.00 Copyright © 1990PergamonPress plc
Printed in Great Britain.All rights reserved
DEGRADATION OF THE LUNAR VACUUM BY A MOON BASE GEOFFREY A. LANDISt NASA Lewis Research Center, 302-1, 21000 Brookpark Road, Cleveland, OH 44135, U.S.A. (Received 15 April 1988; revised version received 31 August 1989)
Abstract--High vacuum is required for many industrial processes which might be accomplished on the moon, such as electronic component and solar cell manufacturing or a large particle accelerator. Ambient pressure on the moon is in the range of l0 -12 torr (night) to l0 -~° torr (day). The effects of a 20 person base and a 250 person industrial facility on this vacuum are discussed. Exhaust from the transport spacecraft and leakage from the habitat will be roughly comparable to the daytime gas pressure for the 20 person base, and will degrade the vacuum to the range of 2 × 10-9 torr for the 250 person facility. This is higher than the desired pressures for some semiconductor manufacture processes or for a lunar-based particle accelerator.
i. THE LUNAR AMBIENT
The existing lunar atmosphere is tenuous and not well characterized. The ambient pressure is in the range of 2 x 105 molecules/cm3 during the lunar night, and 6 x 105 to < 107 molecules/cm3 during the lunar day[l-3]. This corresponds to pressures from 5 x 10 -t3 torr [0.0005 nanotorr (ntorr)] up to 0.4 ntorr, primarily consisting of hydrogen, helium, argon, and neon at night, with the probable addition of CO and CO2 in the daytime[4]. The mean free path for these pressures are in the range of hundreds to thousands of kilometers; thus, the movement of gas in the atmosphere is primarily via ballistic transport. The atmospheric escape lifetime from the sunlit side of the moon is approx. 10,000 s (15 min) for the lightest molecules (hydrogen and helium), and up to 107 s, approx. 100 days, for heavier molecules[5]. 107 s is roughly the maximum lifetime of atmosphere constituents; this is approximately the time it takes for the molecules to become ionized by the solar ultraviolet, at which time they are swept away by electric fields associated with the solar wind in times which are typically no more than a few hundred seconds[5,6]. As noted by Vondrak[6], this mechanism becomes ineffective if the atmosphere is thick, however, the gas input rate (on the order of 250,000 tons/month) required to reach such a level is considerably higher than what is likely to be produced in any near-term industrial facility.
2. REQUIRED VACUUM It seems absurd to expect that the lunar vacuum could be lost by small-scale operations on the moon. ?National Research Council Resident Research Associate, NASA Lewis Research Center.
However, high vacuum and ultra-high vacuum is needed for many industrial processes, some of which may be accomplished on the moon. Some processes which require vacuum and thus would be simpler to manufacture or use on the moon include vacuum tubes, semiconductor manufacture, solar cell manufacture, and particle accelerators. Silicon is a major component of the lunar crust. One likely low-cost process sequence for producing solar cells on the moon[7] is plasma-deposition of amorphous silicon. Such deposition processes typically have base pressures in the very high vacuum range, mid-10 -6 torr, to below 10-7 torr for some experimental set-ups. It is believed that impurities in the deposited films of concentration > 1018/cm3 cause (or exacerbate) the deleterious light-induced degradation effect; this corresponds to a base pressure of 2000 ntorr at deposition pressure 1 torr; 100 ntorr at deposition pressure 0.05 torr. Many processes for manufacturing semiconductor products require vacuum. One process for depositing high-purity layered compound semiconductors is molecular beam epitaxy (MBE). This process requires ultra-high vacuum. Base pressure for MBE is in the range of 0.1 ntorr[8,9] and can be as low as 0.03 ntorr for GaAIAs[10], where C and O2 contamination are particularly harmful. "Vacuum" tubes have a different values for the required operating vacuum, depending on the type of tube and the lifetime, noise level, etc. required. This ranges from 10-5 torr for the magnetron tubes used in fimicrowave ovens, to ultra-high vacuum of 10-~0 torr for travelling-wave tubes. The moon would be a good location for a large, high-energy particle accelerator for several reasons, one of which is the vacuum ambient. Intersecting storage ring (ISR) accelerators require very good vacuum, since any residual gas tends to scatter and 183
184
GEOFFREYA. LANDIS
defocus the beam. At a pressure of 10 ntorr the beam lifetime is typically around l h; and operating pressures of under 0.01 ntorr are required for long lifetime storage and operation[l l]. An additional problem is that whenever the beam tube is vented to atmosphere, gas is adsorbed onto the surfaces which is later desorbed by the beam current. This vacuum will be degraded by human habitation and industrial processing of materials. It is unlikely that maintaining the lunar vacuum will be an important priority of the occupants of a moonbase. The amount of degradation can be calculated by multiplying the mass of gas exhausted times the gravitational acceleration of the moon and dividing by the lunar surface area. This factor is equal to 3.2 x 10 13torr per (metric) ton of gas exhausted. Since the exhausted gas has an average lifetime in the lunar atmosphere of 100 days, the equilibrium contribution to the atmosphere is 1.3 x 1 0 t2 torr per ton of exhaust gas per month. 3. 20 PERSON BASE
The major contribution to the lunar atmosphere from a small exploration base is exhaust gas from the transport. Assuming a specific impulse of 400 s (90% of the theoretical specific impulse of a hydrogen/oxygen engine), landing on the moon requires 0.8 tons of propellant per ton of landed material. If we assume a 10 ton lander making one trip per month with 2 tons of supplies landed per person per month (including the personnel rotation, machinery, scientific and exploration equipment, etc.), this results in an equilibrium pressure of 0.06 ntorr for a 20 person base. This does not assume that the lander is refueled on the moon from lunar oxygen (i.e. it includes the fuel use to relaunch the lander, but does not assume that any payload is carried from the moon). In actuality, it is not correct to assume that all of the propellant expended from the ship will contribute to the lunar atmosphere. The exhaust velocity of a H2/O2 engine is 4km/s, nearly double the lunar escape velocity. Further, if the trajectory used is an insertion into low lunar orbit followed by a descent burn, for much of the engine burn the exhaust will
Source
not be directed toward the lunar surface. However, for a rough calculation here I assume that the entire engine exhaust contributes to the atmosphere. Another contribution to the generated atmosphere is air leakage from the living quarters. One estimate[12] of air leakage from an advanced longduration habitat at atmospheric pressure is 1.2 kg O: plus 4.5 kg N 2 per person per day. This would result in a pressure contribution of 0.004ntorr for a 20 person base. It has frequently been proposed that oxygen be locally generated. If this is done, it is unlikely that nitrogen dilution would be used, since nitrogen is nearly absent on the moon. Thus, the habitat pressure would be proportionately lower, and the leakage rate is expected to be reduced to 23% of that listed above. However, as discussed below, lunar generation of oxygen would itself be likely to be a source of leakage of waste gas. A human also generates about 1 kg of waste CO2/day. An efficient habitat design will recycle this gas, however, it is possible that an early moonbase design might vent some portion of the waste; likewise, some spacesuit designs may simply vent the waste CO2 rather than store it for return to the base for recycling. If so, this contribution would have to be added to the total gas loss. In addition to this leakage, air will normally be lost during ingress and egress for extra-vehicular (or extra-habitat) activities (EVA). The amount of air lost will depend on whether the airlock is simply vented during egress, or if the lock is pumped down and the exhaust air reused. In the baseline case, I will assume that the lock is simply vented. If there is one EVA per person per day, and the lock volume is 2 m 3 of air at one atmosphere pressure, this then results in a contribution of 0.0017ntorr for the 20 person base, which is somewhat less than the habitat leakage (and, like the leakage, reduced if the base is assumed to have a pure oxygen atmosphere). Table 1 summarizes the contributions of the various gas sources discussed. The daytime total atmosphere is in the range of 0.07 ntorr, comparable to the natural lunar atmosphere. During the lunar night, most of this will be adsorbed into the soil, resulting in considerably lower pressure.
Table 1. Contributionsto lunar atmosphere Contribution (ntorr) Notes
20 Person Base:
Propellant Habitat leakage Airlock losses Total
0.06 0.004 0.002 0.07
50 ton lander;all exhaust gas contributes 5.7 kg/person/day 2 m3 vented;less if pumped down for EVA
250 Person Industrial Facility:
Propellant Habitat leakage 02 production 3He mining Industrial processing Total
0.6 0.05 0.9 0.9 unknown 2.5
refueled usinglunar oxygen 5.7 kg/person/day 500tons/month;25% leakage 10tons/year; 25% leakage
Degradation of lunar vacuum
185
helium and hydrogen, and 0.87 ntorr for heavier gasses. The impact of 3He mining on the lunar atmosphere If large-scale industrialization takes place on the moon, it could be expected that the lunar habitat may has also recently been considered by Duke[14], who have hundreds of inhabitants, and considerably more concluded that stripping 100,000 tons of regolith per frequent resupply flights. In this case, the vacuum year would release an amount of trapped gas degradation will be correspondingly worse. The "roughly equivalent" to the existing lunar baseline calculated here will be for a 250 person base atmosphere. processing oxygen from lunar soil. In addition, the moon base is likely to be a place I assume here slightly less support material where various other mining, refining and manufacturrequired from Earth, 1 ton of material per person ing operations take place, producing solar cells, aluper month; however, since the lander is fueled minum and titanium structures, habitation modules from lunar-produced oxygen the fuel for the lander and probably other objects useful to further colonizamust be delivered into lunar orbit. Total gas tion. These processes will involve some amount of gas contribution to the lunar atmosphere is 460 tons/ generation and, consequently, wastage. Until the month, for a pressure contribution of 0.6 ntorr. processes are more completely specified, the contribuHabitat leakage and airlock losses will contribute tions from this processing is unknown. 0.05 ntorr. Finally, the lunar soil contains trapped gas at a Lunar oxygen production to fuel the lander will concentration on the order of 50ppm by wt, require 400tons 02 per month. A 25% loss rate, primarily hydrogen and helium from the solar wind, which is realistic for a low-cost industrial process, plus and carbon compounds and nitrogen. This is would contribute 0.13 ntorr. If the trans-lunar injec- only loosely bound to the soil, and physical tion ship is also to be fueled, this is an additional disturbance, as well as movement of soil by mining, contribution. It has often been proposed that lunar etc. will likely release some of the gas content. This oxygen production could be used as a cheap source contribution is expected to be negligible compared to of fuel for spacecraft to be used from Earth orbit. I other sources. assume a baseline facility designed to deliver oxygen The total contribution to the lunar atmosphere to Earth orbit at a production rate of 500 tons per from the assumed industrial facility producing both month. Lifting this from the moon will require 02 and 3He is 2.5 ntorr (see Table 1), a factor of 5-100 400 tons of fuel, and leakage losses will be about times higher than the "natural" daytime atmosphere. 200 tons. The contribution to the lunar atmosphere is This is low enough that manfacture of amorphous 0.78 ntorr. This will be considerably less, however, if silicon solar cells can be performed without any the oxygen is to be shipped by mass-driver rather additional vacuum pumping. For other processes than lifted off the surface by rocket. discussed, such as MBE, travelling-wave vacuum Mining of the lunar regolith for helium 3 (3He) to tube formation, or siting of a large accelerator on the fuel terrestrial deuterium-helium 3 fusion reactors moon, the vacuum is not good enough, and these will has recently become a topic of interest[13]. 3He require additional pumping. implanted into the lunar regolith by the solar wind While the lunar vacuum may not be sufficient for would be extracted by baking the soil, and then some operations, it must be kept in mind that even distilled. For every ton of 3He produced, about after degradation, the ambient remains a very high 3300 tons of 4He, 6100 tons of hydrogen, 3000 tons of vacuum. It is much easier to pump a starting ambient CO and COs, and 500 tons of nitrogen will be of 10-gtorr down to ultra-high vacuum levels of produced[14]. Ten tons of 3He would be required to 10 -~l than it is to reach ultra-high vacuum starting be mined per year if half the U.S. electrical consump- from atmospheric pressure. Leaks and virtual leaks tion of 285GWe is to be produced. Most of the will be little problem; there will be almost no problem byproduct gasses produced will be useful to the lunar with desorption of gasses from chambers walls that base. Except for refrigeration and pressurization use, have been exposed to ambient, and finally, the however, the helium produced will not be of great "vacuum chambers" will not be required to hold up use, and may eventually leak to the atmosphere. This to the large mechanical pressure of 10,000 tons/m 2 was not assumed in the following analysis. imposed by the Earth's atmosphere. It is an advantageous feature of the moon that Since the escape lifetime for hydrogen and helium is much shorter than that for other gasses, these must the vacuum is self cleansing by the solar ultraviolet be considered separately. If 25% of the gas content and solar wind. "Air pollution" is a temporary is lost as waste due to soil agitation during mining effect. If it is decided that a high vacuum is required, plus leakage and waste in the baking and condensa- a wait of a few hundred days will suffice for the gas tion process, production of 10 tons/yr of 3He would to be removed by the solar wind. However, this is produce 23,500 tons/yr of waste hydrogen and 4He, only true as long as the amount of atmosphere plus 12,000 tons/yr of heavier gas. Assuming an present is low enough that there is little shielding of escape lifetime of 10,000s for the hydrogen and the solar ultraviolet. This is likely to be true for the helium, this result in a contribution of 0.002 ntorr for amounts of gas discussed in the present paper. Some 4. 250 PERSON INDUSTRIAL FACILITY
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amount of gas will be adsorbed by the lunar soil. Cleansing of this gas to restore the original ultrahigh vacuum will take longer, since the soil will take time to outgas. 5. ATMOSPHERE VARIATION WITH POSITION
The calculations have so far assumed that the atmosphere generated can be assumed to be evenly distributed around the moon. The gas input mechanisms discussed are either continuous or periodic with a characteristic time less than or equal to the resupply time, assumed to be 1 month. This is much shorter than the escape time, and so the overall variation with time is expected to be small. However, the immediate vicinity of intermittant gas sources, such as the exhaust plume of a lander or the area adjacent to an airlock during depressurization, will experience temporary large increases in the gas concentration. Such surges in pressure were seen, for example, in results from lunar atmosphere experiments on Apollo missions, where the observed pressure rose dramatically when the LM was depressurized, when an astronaut approached the apparatus[2] (due to exhaust gas from the astronaut's EVA suit), and on lift-off of the LM from the surface. These pressure surges were superimposed on a longer term transient due to gradual degassing of the LM. Sensitive processes would likely be shut down during such periods. Gas molecules escape from the atmosphere primarily from the sunlit hemisphere of the moon, where they have higher kinetic energy and also are subject to photionization by solar ultraviolet. Thus, the escape lifetime is determined by the gas distribution on the sunlit hemisphere. For pressures of nanotorr and below, the gas in the atmosphere can be well modelled by ballistic transport. Gas molecules leave the surface with random direction and thermal velocity profile, follow a ballistic trajectory until again intersecting the surface, and then may be temporarily adsorbed by the surface before being reemitted, again at a random direction and velocity. Temporary adsorption of gas by the surface is irrelevant to the calculation of equilibrium atmosphere pressure by a steady-state source, since the adsorbed gas neither contributes to the total pressure nor is subject to escape; however, a large amount of gas stored in the adsorption reservoir will proportionately increase the time needed to reach equilibrium pressure, and also increase the time needed to purge the atmosphere after the gas source is discontinued. A complete transport calculation would integrate over the thermal (Maxwell-Boltzmann) velocity distribution, averaging over the hemispherical angular distribution, and also take into account the spherical lunar geometry and gravitational potential. A more complete calculation would include gas-gas
collisions and the variation of temperature over the lunar surface. For an order of magnitude calculation, however, it is sufficient to assume that all the molecules can be characterized by the average thermal energy of kT/2 per degree of freedom. At a temperature of 365 K, this yields root mean square (RMS) vertical and radial velocities of 300 and 430 m/s respectively for an 02 molecule. The horizontal d travelled on a parabolic trajectory is thus 160 km, and the time in flight 380s. This distance is sufficiently small compared to the circumference of the moon that the assumption of parabolic trajectories is justifiable. In a random walk process the expected distance from the origin equals the distance d per step times // x/N, the square root of the number of steps; thus, the area covered equals ~dZN. To cover the surface area of the moon thus required roughly 450 steps, a flight time of 47 h. This time is short compared to the escape lifetime of gas in the atmosphere, thus, the assumption of roughly uniform gas distribution is justified, and there will not be a significant difference in the amount of gas near the base compared to far from the base. For other molecules, the time is proportional to (kT/m) 3,'2 Water vapor, for example, with a molecular weight of 18, will spread across the surface considerably faster. Hydrogen and helium spread across the full surface area of the moon in a time of roughly an hour. Since the escape time for hydrogen and helium is considerably less than an hour, gas concentrations for hydrogen and helium can not be assumed uniform, and considerable variations in density will exist between areas close to the gas source to areas far away. On the night side of the moon, the typical temperature is only 100 K. Molecules thus take six times as long to diffuse across the same area, and since any given molecule will spend six times as long on the night hemisphere as on the day hemisphere, the gas reservoir on the night side will be proportionately greater. Again, it should be noted that these times are exclusive of any time spent adsorbed in the soil. These conclusions compare those of Burns et al.[15], who particularly discuss column density with respect to opacity of the atmosphere for u.v. and radio astronomy, and conclude that the local pressures may be many orders of magnitude larger than the equilibrium pressure at locations close to a gas source. As discussed previously, this conclusion is correct in the case of a pressure "surge" due to transient events. 6. CONCLUSIONS 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 escape lifetime of the gas in the lunar atmosphere, and thus exhaust gas can be
Degradation of lunar vacuum 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 3He from the lunar regolith. This could degrade the lunar ambient to levels on the order of 3 ntorr, 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 an intersecting beam accelerator on the moon will require additional vacuum pumping. In any case, through, pumping to ultrahigh vacuum will be much easier on the moon than on Earth.
REFERENCES
1. J. H. Hoffman et al., Lunar atmospheric composition results from Apollo 17, Proc. 4th Lunar Science Conf. (Suppl. 4, Geochim. Cosmochim. Acta). Vol. 3, pp. 2865-2875 (1973). 2. F. S. Johnson et al., Cold cathode gage (lunar atmosphere detector). Apollo 12 preliminary science report. NASA SP-235, p. 93 (1970).
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3. (3. Jeffrey Taylor, Geological considerations for lunar telescopes. Future astronomical observatories on the moon. NASA Conference Publication 2489, pp. 21-28 (1988). 4. R. R. Hodges Jr, The escape of solar-wind carbon from the moon, Proc. 7th Lunar Science Conf., pp. 493-500 (1976). 5. F. S. Johnson, Lunar atmosphere. Rev. Geophys. Space Phys. 9, 813-823 (1971). 6. R. R. Vondrak, Creation of an artificial lunar atmosphere. Nature 248, 657-659 (1974). 7. G. A. Landis, Lunar production of space photovoltaic arrays. 20th IEEE Photovoltaic Specialists Conference, Las Vegas, Nev., pp. 874-879 (1988). 8. S. M. Sze, Semiconductor Devices Physics and Technology, p. 333. Wiley, New York (1985). 9. M. Yokoyama and S-I Ohta, J. appL Phys. 59, 2929-3921 (1986). 10. F. Y. Juang et al., J. appl. Phys. 58, 1986-1989 (1985). 11. R. Calder et al., Proc. IX Conf. High Energy Accelerators, p. 70 (1974). 12. P. O. Quattrane, Extended mission life support systems. AAS 81-237. The Case for Mars (Edited by P. J. Boston), Vol. 57, Science and Technology Series, pp. 131-162. AAS (1984). 13. G. L. Kulcinski and H. H. Schmitt, The moon: an abundant source of clean and safe fusion fuel for the 21st century. Lunar Helium-3 Fusion Power Workshop, NASA Lewis Research Center (1988). 14. M. Duke, Lunar atmosphere. Lunar Helium-3 Fusion Power Workshop, Discussion Panel section of NASA Lewis Research Center, (1988). 15. J. O. Burns et al., Artificiallygenerated atmosphere near a lunar base. Lunar Bases andSpace Activities in the 21st Century Symposium, Houston, Tex., Paper LBS-88-024 (1988).