First robotic attempt to measure heat flow of the Moon: Deployment of LISTER on Blue Ghost Mission One to Mare Crisium

On March 2, 2025, Firefly Aerospace became the first United States-based company to successfully soft-land a robotic spacecraft on the Moon. The Blue Ghost lander deployed all 10 NASA-supported payloads under the Commercial Lunar Payload Services. The Lunar Instrumentation for Subsurface Thermal Exploration with Rapidity (LISTER) was one of them. LISTER measured temperature and thermal conductivity of the lunar regolith of the landing site at 8 depths down to 1 m for the purpose of quantifying the endogenic heat flow of the Moon. To penetrate to the subsurface, LISTER used the pneumatic excavation technique in which the deployment mechanism spooled out a 6.4-mm diameter stainless steel tube and blew pressurized nitrogen gas through a nozzle attached to the leading end of the tube.  The gas jet, rapidly expanding in the lunar vacuum, removed the regolith ahead of the nozzle, while the spooling motor applied weight to advance deeper into the subsurface. The thermal sensors were encased in a stainless-steel needle, 28-mm long and 2.8-mm diameter, attached to the gas nozzle. When the needle sensor reached a depth targeted for thermal measurements, LISTER stopped the gas jet and inserted the needle into the bottom-hole regolith. Each thermal measurement sequence took 2 hours. During the first hour, the needle thermally equilibrated with the regolith. Then, the needle was electrically heated with a constant power of 50 mW for 30 minutes, followed by a 30-minute cool-off period. Thermal conductivity of the regolith was determined by modeling the rise and fall of the needle temperature during the 2^nd hour using a finite-element heat transfer model.

Prior to the mission, it was hoped that LISTER would reach greater than 1-m depth into the subsurface, where temperature of the regolith is not significantly affected by the insolation cycles.  Then, the endogenic heat flow would have been obtained simply as the product of the thermal gradient and the thermal conductivity of the regolith depth interval penetrated. Because LISTER did not reach that depth, the heat flow is being determined as the lower boundary condition for a one-dimensional (vertical) finite-element heat transport model that simulates the interaction between the upward flow of the endogenic heat and the downward propagation of the insolation-induced thermal waves. The history of the insolation-induced surface temperature swings at the landing site, which is the surface boundary condition for the heat transport model, has been reconstructed from the ephemeris of the landing site and surface temperatures determined from flyovers by the Diviner radiometer onboard the Lunar Reconnaissance Orbiter. The equilibrium temperature and thermal conductivity of the regolith determined at 8 depths by LISTER provide key constraints to the model. Our early results suggest endogenic heat flow values of 13 to 14 mW/m², comparable to what was observed at the Apollo 17 site (16 mW/m²). A more thorough inversion is now being carried out to optimize the heat flow determination and estimate its uncertainty.