Experimental investigation of the soil interaction and wear of lunar rover wheel-based sensors

Introduction: Multiple missions are currently planned to explore the lunar south polar region and study the occurrence of water in and around permanently shadowed regions [1]. As part of the Rover Permittivity Sensor (RPS) for the European Space Agency [2,3], a contribution to the Rashid-3 mission [4], we have developed several testbeds to investigate the soil interaction of wheel-based sensors. RPS includes two sensing electrodes on the rover wheel to measure the dielectric permittivity of the lunar soil along the traverse and infer its potential ice content and density. For such contact-based sensors, the mechanical coupling to the soil is a major source of uncertainty in measurement quality. Thus, it is important to characterise this interface with respect to soil sinkage and compaction, considering reduced gravity and the wide range of the particle size distribution. Furthermore, due to the suspected abrasive nature of lunar regolith, mechanical wear is expected at the wheel-soil interface. For missions to the lunar south pole or similar destinations, the extremely wide temperature range of the environment also needs to be considered, as it might alter the mechanical properties. Currently, no standardised environmental testing procedure exists for this scenario. The two custom-designed testbeds presented here allow studying both the geotechnical interaction between the wheel-based sensors and the soil, and the wear over the sensors' lifetime.

Soil interface testbed: The setup features a wheel analogue installed on a movable gantry and can be installed inside a dust-tolerant thermal-vacuum chamber [5]. With this system, the wheel with a functional RPS sensor can be pressed against or rolled over (ice-bearing) regolith samples, while measuring load and displacement. This enables studies of sensor-soil interaction effects on the permittivity measurement, as well as the compaction of the soil. The system is planned to include a gravity-offloading system and a cooled regolith bed, with target temperatures of around 150 K. Fig. 1 shows the initial testing setup for the vertical axis, before integration into a full gantry.

Wear testbed: The setup comprises a turntable platform on which a wheel mock-up can continuously be rolled. Both the turntable and the wheel are motorised, allowing defined slip ratios to be set. The turntable includes a ring-shaped trench, 89 cm in outer radius, filled with regolith simulant, up to 10 cm deep and 13 cm wide. Bigger rocks can also be placed in the trench to simulate higher surface roughness. The wheel is free to move vertically and is offloaded with counterweights to set specific wheel loads for different gravity or rover conditions. Fig. 2 shows the wheel wear testbed after first room temperature tests. A liquid nitrogen evaporation cooling system is being implemented to cool the wheel during operation and test the robustness of wheel and its surface-mounted sensors in a lunar polar environment.

Fig. 1: First iteration of the soli interface testbed vertical axis, used for static sinkage tests

Fig. 2: Wheel wear testbed

Conclusion: The two testbeds provide a combined capability for testing RPS wheel-based sensors with glass-fibre-reinforced polymer (GFRP) caps under various relevant lunar environment aspects (temperature, vacuum, ice-regolith mixtures) and motion types (static, rolling, slip, continuous/steady-state rolling), while studying load, wheel/sensor sinkage, and wear. Furthermore, the testbeds can also be used to study other similar wheel-based systems and materials, making them a useful tool for exploration rover development programs.

Acknowledgments: Part of this work has been performed in the context of the Rover Permittivity Sensor project for the European Space Agency (Contract No. 4000148937/25/NL/PA/dp).

References: [1] Reiss, P. (2024), PNAS, 121(52). [2] Reiss, P. et al. (2025), European Lunar Symposium. [3] Gscheidle, C. et al. (2025), EPSC. [4] Almatroushi, H. et al. (2025), EPSC. [5] Witzel, T. et al. (2025), EPSC.