From ball to rover: Transformable palm-sized rover SORA-Q for autonomous lunar exploration

Robotic technologies are expected to drive substantial advancements in planetary exploration and resource prospecting by performing a variety of tasks in extraterrestrial environments. In particular, miniature robots are ideally suited for integration into spacecraft with strict payload limitations, providing a cost-effective solution. However, the pursuit of autonomous exploration using these miniature robots presents challenges owing to constraints in computational power and battery capacity and reduced locomotion performance owing to their small size. Here, we introduce a two-wheeled centimeter-scale rover, designated Lunar Excursion Vehicle 2 (LEV-2), also known as SORA-Q (named after the Japanese words for space and sphere), which transforms into a wheeled configuration from a compact spherical form, enabling efficient traversal of soft lunar terrains. On 19 January 2024 (universal time coordinated), LEV-2 was deployed from a Japanese lunar lander, Smart Lander for Investigating Moon (SLIM), immediately before its landing on the lunar surface. After a lunar landing, the palm-sized rover accomplished autonomous lunar exploration by navigating around the SLIM lander, capturing images of both the SLIM lander and its environment and transmitting selected images through wireless communication on the lunar surface without reliance on ground-based teleoperation. This study details the system design of LEV-2 and presents the results of its in situ lunar activities, highlighting the efficacy of the proposed technologies necessary for mission implementation. Furthermore, we discuss the technical challenges encountered during the mission, including operational constraints and partial data loss, as well as the lessons learned for future exploration missions using small-scale space robots.