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Impact Crater: Labeled composite image of the South Pole taken by New Mexico State Univ./Marshall Space Flight Center, using the Tortugas 24-inch telescope. Image: NMSU/MSFC
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Physics software simulates parameters for extracting water from the Moon.
Preliminary data from NASA’s Lunar CRater Observing and Sensing Satellite (LCROSS) indicates that the mission successfully uncovered water during the Oct. 9, 2009, impacts into the Moon’s south pole.
Water and other compounds found on the Moon represent potential resources that could sustain future lunar exploration. According to Edwin Ethridge, a materials scientist in the Materials & Processes Laboratory at NASA Marshall Space Flight Center, Huntsville, Ala., in-situ resources are very important because they do not have to be launched out of Earth’s gravitational well. Ethridge cited estimates that one ton of water and one ton of oxygen per year would be required for the early stages of a manned outpost.
“We are looking at the process of extracting the water from the soil. Run [the water] through a purification system and you could drink it. We will extract water [and then] electrolysis can be used to split the water into hydrogen and oxygen,” says Ethridge.
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Transient solution of the penetration of microwave energy (0.5 GHz) into lunar soil simulant with heating of the soil. Colors represent constant temperature isotherms. An AVI movie (available on www.rdmag.com) shows the progressive heating of the soil over a period of 10 hours. Image: Ed Ethridge, NASA MSFC
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The extraction process
As a principal investigator examining the use of microwaves for the extraction of volatiles from lunar soil, Ethridge asserts that microwave processing to extract water has unique advantages over other processes.
"Because of the high vacuum, the thermal conductivity of lunar soil is very low,” he says. Microwave energy is advantageous because it heats from the inside out. This means that the excavation of lunar soil could be unnecessary, thereby minimizing “moondust”.
The basic components of the microwave extraction system include a microwave source, waveguides to deliver the energy to the soil, and a cold trap to capture the water vapor. First, the microwave energy penetrates and heats the soil; because ice is relatively transparent to microwave energy, heat is transferred from the soil particles to the water ice condensed onto the surface of the soil. On the Moon, water ice transforms directly to water vapor by sublimation. Once in the cold trap, the water vapor will transform back to ice. In addition to the system components, a power source and a rover to transport the extraction system will be necessary.
Simulating the Moon
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Demonstration hardware to test the beaming of microwave energy down into lunar soil simulant (in the box) with the microwave hardware mounted on a mobile platform. Initial test of the coupling of microwave energy into the simulant. Image: Ed Ethridge, NASA MSFC
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The microwave processing parameters and hardware requirements for water extraction presented a complex problem. Ethridge used
COMSOL multiphysics software to address the challenges, in part because COMSOL has a microwave physics module that works with the main COMSOL program.
COMSOL was used to calculate the microwave penetration into and heating of simulated lunar soil. The properties of the simulant are approximated by complex electric permittivity and magnetic permeability measured in the lab. “[With COMSOL], calculations can be performed on different geometries, for a range of microwave frequencies and different power levels, for the simulated lunar soil,” says Ethridge. “Since the temperature varies with time as the soil heats, temperature-dependent soil dielectric properties can be incorporated into the model along with temperature dependent thermal conductivity of the soil.”
For the simulation, Ethridge used the RF Module for COMSOL to model the microwave power penetration and attenuation into the soil. When the model was running without error, the physics of heating and heat flow were added. A transient analysis was used to determine heating as a function of time. An animation shows lines of constant temperature as the heating progresses. Ethridge explains, “COMSOL permits the calculation of microwave penetration and heating that could be expected with different experiment scenarios.”
The next step for Ethridge is to develop a model of the percolation of water vapor from the soil. Currently, Southern Research Institute, Birmingham, Ala., is measuring the Darcy constant (describing the flow of a fluid through a porous medium) for lunar soil simulant. Once the data is generated, a COMSOL model of the water vapor transport from the simulant will be developed.
Published in R & D magazine: Vol. 52, No. 1, February, 2010, p.18.
