In January 2024, a Japanese lunar lander successfully touched down on the moon as part of the Smart Lander for Investigating Moon (SLIM) mission. Accompanying the SLIM on its journey were two small rovers named lunar excursion vehicles (LEV-1 and 2). LEV-2, a transformable nano rover, was particularly designed to capture images of the SLIM on the lunar surface. During the mission, LEV-2 successfully maneuvered and captured images of the lander. Understanding the mobility characteristics of LEV-2 was crucial for the success of this task. This study analyzed the mobility characteristics of LEV-2 through numerical simulation and traveling tests. In the numerical simulation, a dynamics model of LEV-2 was first developed. Subsequently, the motion behaviors of LEV2 were analyzed by utilizing the discrete element method. Traveling tests were then conducted using the LEV-2 engineering model on various terrains covered with a lunar regolith simulant. The results of both the numerical simulations and experiments revealed consistent trends across different moving modes of LEV-2. Furthermore, the simulation results indicated no significant deterioration in the mobility performance of LEV-2 on the moon when compared to that on the Earth. Based on the characteristics predicted from the simulation and the quantitative data obtained from the experiments, it was concluded that LEV-2 is supposed to travel at least 15◦slopes on the moon.

This study analyzes the surface sliding behavior of space probes in simulated extraterrestrial environments. When a space probe lands on an extraterrestrial body, its landing gear (footpad, landing legs) contacts and slides along the ground surface. The influence of various parameters (i.e., footpad size, velocity, ground condition, atmospheric pressure, and gravity) on the friction behavior of footpads was experimentally evaluated herein. First, we developed an experimental system that can perform footpad sliding tests repeatedly. Subsequently, the system was used to perform tests under various conditions: 1) on ground under normal atmospheric conditions, 2) in a vacuum, and 3) in reduced gravity. The tests performed in a vacuum and in reduced gravity indicated that the friction behavior of the footpad is largely unaffected by atmospheric pressure and gravity. The findings obtained herein offer useful design and control guidelines for space probes landing on extraterrestrial bodies.

Rovers that are used to explore craters on the Moon or Mars require the mobility to negotiate sandy slopes, on which slippage can easily occur. Such slippage can be reduced by actively readjusting the attitude of the rovers. By changing attitude, rovers can modify the position of their center of gravity and the wheel-soil contact angle. In this study, we discuss the effects of attitude changes on downhill sideslip based on the slope failure mechanism and experiments on reconfiguring the rover attitude and wheel angles. We conducted slope-traversing experiments using a wheeled rover under various roll angles and wheel angles. The experimental results show that the contact angle between wheels and slopes has a dominant influence on sideslip when compared with that of readjusting the rover's center of gravity.

The surfaces of both the Moon and Mars are covered with loose soil, with numerous steep slopes along their crater rims. Therefore, one of the most important requirements imposed on planetary rovers is their ability to minimize slippage while climbing steep slopes, i.e., the ability to generate a drawbar pull with only a small amount of slippage. To this end, the wheels/tracks of planetary rovers typically have parallel fins called lugs (i.e., grousers) on their surface. Recent studies have reported that these lugs can substantially improve the traveling performances of planetary rovers. Therefore, in this study, we conducted experiments using lightweight two-wheeled and mono-tracked rovers to provide a quantitative confirmation regarding the influence of lugs on the traveling performances of planetary rovers. Based on our experimental results, we confirmed that, although an increase in the number of lugs contributes to the high traveling performance of wheeled rovers, it does not contribute much to that of tracked rovers. Furthermore, an increase in lug height improves the traveling performances of both types of rovers.

Future planetary rovers are expected to probe over steep sandy slopes, such as crater rims, where wheel slippage can be a critical issue. One solution to this issue is to mount redundant actuators on the locomotion mechanisms of the rovers such that they can actively reconfigurate themselves to adapt to the driven terrain. In this study, we propose a mechanical model of a rover based on a wheel-soil contact model combined with the classical terramechanic theory. The effects of the rover reconfiguration on its slippage tendencies are analyzed based on slope traversing experiments and numerical simulations. The validation of the proposed contact model is also discussed based on experimental and numerical simulation results. According to the experimental results, both longitudinal and lateral slippages are greatly reduced by tilting the rover in an uphill direction. The results of the numerical simulation match the experimental results quantitatively, and indicate the possible need to include a slope failure model.