First campaign in LUNA
The week of 11th of November 2024 marked the start of external user campaigns in LUNA, with two teams and MUSC (DLR’s Microgravity User Support Center) working together on measuring seismic vibrations. One team from ISAE-SUPAERO (Institut Supérieur de l’Aéronautique et de l’Espace) in Toulouse, France, was testing geophones for a potential deployment by astronauts on the Moon. The sensors have already undergone environmental tests (e.g. at low temperatures) to qualify them for operation in space. An important aspect not tested so far is how to insert the sensors in the regolith, so that they have good coupling to the ground and are not tilted, to ensure a good performance. To address this issue, the team tested various designs of the spikes that connect the geophones to the ground in LUNA and did measurements to quantify the performance of the sensors, using both signals generated by hammering as well as the background seismic wavefield. The analysis of these measurements will also allow to determine the velocity of sound waves in the regolith simulant in LUNA, which can be compared to previous lab results and actual data from the Moon. Geophones were deployed on the Moon during Apollo 14, 16 and 17 to obtain information on the thickness and properties of the regolith. Near the lunar south pole, where the proposed Artemis landing sites are located, they also have the potential to detect small amounts of water ice in the regolith. The information from the LUNA reference seismometer operated by MUSC was important to understand noise conditions on the DLR campus and prepare for the measurements.
At the same time, a second team from GeoForschungsZentrum Potsdam, Germany, provided instrumentation and expertise to start the first measurement with LUNA’s optical fiber, buried beneath the regolith simulant by MUSC. Reflections from a pulsed laser signal traveling through the fiber are used to measure deformation along the fiber and in this way can provide information on ground motion in high spatial resolution, used e.g. to locate sources or characterize the subsurface. Applications of this method are steadily increasing on Earth and are also discussed for the Moon. The hammer signals of the French team provided a good opportunity to record distinct sources inside LUNA, and we also registered less regular sources like driving one of the MUSC rovers during four days of registration. Rover signals have been recorded seismically on the Moon during the Apollo missions, where they have been used to better understand properties of the regolith, as well as recently during the Indian Chandrayaan-3 mission. Combining the analysis results of the different teams for data simultaneously registered by the different instruments will allow for further synergies. As this was the first campaign, feedback from the external users about LUNA is of special interest. To quote: “It’s a great facility. The experiment was going very smoothly.”
PhD student from GFZ performing a weight drop as calibration signal for the fiber. Inset shows generated signals. ©MUSC-DLR/GFZ
