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Development of a compact heat flow probe for the Lunar Geophysical Network mission

Seiichi Nagihara
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Abstract Text: 

The latest Decadal Survey by the National Research Council recommends a New Frontiers-class, Lunar Geophysical Network (LGN) mission, which would deploy a ‘global, long-lived network of geophysical instruments on the surface of the Moon’. It has been envisioned that the geophysical instruments will be deployed on small landers. A heat flow probe, as well as a broadband seismometer, is considered highest-priority payload for this mission. A panel of scientists have recently recommended that a heat flow probe on such a mission should penetrated at least 3-m into the lunar regolith in obtaining measurements of thermal gradient and thermal conductivity. In addition, the instrumentation needs to be a compact system in order to be accommodated on the small lander. We have been developing a heat flow probe that meets all of these requirements. The probe utilizes a pneumatic excavation system in deploying thermal sensors into the subsurface. The deployment mechanism (~28-cm tall) of the probe spools out a glass fiber composite stem downward. The stem then forms a hollow cylinder of ~1.5-cm diameter. It pushes the penetrating cone into the regolith, while gas jets, emitted from the cone tip, blow away loosened material. Removing material from the bottom of the excavated hole allows the stem to advance deeper with minimal thrust. A short (~1.5 cm), thin (~2-mm diameter) thermal probe attached to the cone tip measures temperatures and thermal conductivities of the regolith by stopping for 30 minutes at different depths on the way down. During each stop, to measure the thermal conductivity of undisturbed regolith, the probe shuts off the gas jet and pushes the needle probe into the bottom-hole regolith. After the stem reaches the targeted, 3-m depth, the temperature sensors embedded on the fully extended stem monitor long-term stability of the thermal gradient. Our latest prototype penetrated 1.9 m into lunar regolith simulant in a vacuum chamber. Design improvement is being made on the deployment mechanism and the stem, which should enhance its penetration capability.

"B. Houdou...
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Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."