The emerging interest in lunar mining poses a threat of contamination to pristine craters at the lunar poles, which act as cold traps for water, and may harbor other valuable minerals. Lunar Prospector type missions will be looking for volatile (molecular) compounds that may be masked by the exhaust gases from landing vehicle engines. The possible self-contamination of the landing site could negate the scientific value of the soil samples taken in the vicinity of the landing site. Self-contamination may also lead to false-positive readings of resources available on the lunar surface. This innovation addresses the software and methods needed to assess the magnitude and distribution of lunar surface contamination caused by the engine exhaust of landing vehicles on known or planned descent trajectories.

Tools have been developed to model and simulate the effects of lunar landing vehicles on the lunar environment. They include simulations of 3D flux of rocket plume ejecta. The trajectories of such ejecta can be mapped onto cold trap craters to predict deposition for expected lunar landings, assuming that the collection efficiency of the 40 K cold trap surfaces is 100%.

This innovation addresses the time for the plume-induced local atmosphere above cold traps to decay to normal levels, the efficiency of gas migration into a permanently shadowed crater when the landing is outside it but nearby, and reduction of contamination afforded by moving the landing site further from the crater or by topographically shielding the crater from the direct flux of a lander’s ground jet. This work also addresses plume volatiles adsorbed onto and driven inside lunar soil and ejecta particles from their residence in the high-pressure stagnation region of the plume, and how mechanical soil dispersal across the lunar surface contributes to the induced atmosphere.

Induced atmosphere effects (magnitude and dispersal rates) are calculated for plume gas and regolith deposition inside permanently shadowed craters, locally/regionally, and globally. The redistribution of lunar volatiles is addressed by surface hopping after spontaneous desorption or plasma-induced sputtering from lunar soils, and is estimated for hypergolic combustion products of Aerozene 50 and dinitrogen tetroxide. This is the classic random walk problem for molecular diffusion. The Single Hop results of Shipley, Metzger, and Lane are iterated to develop a time-history of the migration of volatiles into lunar cold traps. The available volatiles are partitioned by the fraction that escapes versus the fraction deposited into cold traps.

The fate of small particles and volatiles (e.g., molecules) emitted from the lunar surface are estimated assuming ballistic trajectories following Keplerian orbits. Under ideal conditions, such particles either escape from the lunar exosphere or collide with the lunar surface within one orbit about the lunar center of mass. These calculations are made for ideal conditions assuming a spherical Moon with no surface terrain, a spherical gravity field with all mass concentrated at the lunar center, and no aspherical/asymmetric gravitational effects. The Maxwell-Boltzmann distribution is used to model the velocity distribution of volatiles leaving the lunar surface. An exact formulation for time-of-flight is derived for particles leaving the lunar surface. The deposition of engine exhaust products is estimated for a lunar lander descending to the surface along a ground track.

This work was done by Scott Shipley, Phillip Metzger, and John Lane of Kennedy Space Center. KSC-13914

NASA Tech Briefs Magazine

This article first appeared in the April, 2015 issue of NASA Tech Briefs Magazine.

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