Microwaving Moondust with COMSOL

Multiphysics software calculates the process of extracting cryogenic trapped water under the Moon’s surface using microwave energy.

In 1999, NASA’s Lunar Prospector revealed concentrated hydrogen signatures detected in permanently shadowed craters at the lunar poles. While scientists have long speculated about the source of vast quantities of hydrogen at the poles, recent discoveries made by NASA’s Lunar CRater Observing and Sensing Satellite (LCROSS) are shedding new light on the question of water on the Moon.

Labeled Composite Image of the South Pole taken by New Mexico State University/Marshall Space Flight Center, using the Tortugas 24 in. telescope. (NMSU/MSFC) " class="caption" align="left">Water and other compounds found on the Moon represent potential resources that could sustain future lunar exploration. In situ resources are very important since they do not have to be launched out of Earth’s gravitational well. It costs about $50,000 per pound to launch a payload to the Moon. And since water is one of the resources that will have to be resupplied to a manned lunar outpost, water would be part of the total cost of the payload taken to the Moon. One ton of water and one ton of oxygen per year would be required for the early stages of a manned outpost. This makes it necessary that a water extraction process will be developed for use at an outpost. And once the water is extracted, oxygen can be obtained from the water by electrolysis.

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." class="caption" align="left">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. Additionally, microwave energy is advantageous because it heats from the inside out. This means that the excavation of lunar soil could be unnecessary, thereby minimizing Moon dust and the negative aspect of perhaps having to strip-mine the Moon.

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 (Figure 2). First, the microwave energy penetrates and heats the soil and, since 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.

Since the microwave processing parameters and hardware requirements for water extraction is a complex multiphysics problem, NASA employed simulation to address the challenges. COMSOL Multiphysics is being 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. Calculations can be performed on different geometries, for a range of microwave frequencies and different power levels, for the simulated lunar soil. 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.

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