[Reading Science] The Wheel Entering the Moon's Caves

A Wheel Without Hinges Breaks Through the Barriers of Lunar Pit and Lava Tube Exploration

The most fascinating area in lunar exploration is not the surface, but the subsurface.

'Lunar pits,' formed by the collapse of underground cavities, and the lava tubes that extend beneath them, are natural shelters that can protect against extreme temperature fluctuations and cosmic radiation. They are considered prime candidates for long-term lunar habitats and preserve geological records from the early solar system. However, due to steep slopes, rocky terrain, and the risk of falling, no country has yet been able to properly access these areas.

Field test scenes of driving performance conducted in various environments such as artificial lunar soil, extreme temperatures, mud, and bedrock. Provided by KAIST

The problem has always been the same: How do you descend? And how do you get back up? In response to these questions, a Korean research team has provided an answer using the most basic means of mobility-the wheel-but in the most extraordinary way.

The research team led by Professor Lee Daeyoung of the Korea Advanced Institute of Science and Technology (KAIST) Space Research Institute and the Department of Aerospace Engineering, in collaboration with the Unmanned Exploration Research Institute, the Korea Astronomy and Space Science Institute, the Korea Aerospace Research Institute, and Hanyang University, has developed the world's first "deployable airless wheel" suitable for lunar pit and lava tube exploration rovers.

The research results were published in the December issue of the world-renowned robotics journal, Science Robotics. The title of the paper is "Soft Deployable Airless Wheel for Lunar Lava Tube In-tact Exploration."

Abandoning Complex Machinery, Solving with Structure

Traditional lunar exploration strategies have mainly involved deploying small rovers from larger ones. However, small rovers are structurally vulnerable to steep slopes and obstacles, and the variable wheels proposed to address these limitations have faced their own challenges. In the Moon's extreme environment-characterized by cold welding, uneven thermal expansion, and highly abrasive lunar dust-hinges and mechanical drive components become points of failure.

The research team's approach was the exact opposite: "Eliminate moving parts."

Principle of Deployable Variable Wheel Design Compared to Da Vinci Bridge. Similar to the Da Vinci Bridge, which can be constructed without adhesive, the interwoven crossing structure allows both deployable deformation and shape retention without hinges or adhesives. Provided by KAIST

The starting point was the "Da Vinci Bridge," devised by Leonardo da Vinci in the 15th century. The Da Vinci Bridge supports loads using only the interlocking of wooden members, without nails or adhesives. The interlocking itself creates structural rigidity.

The KAIST research team applied this interlocking structure to the design of deployable wheels. The structure itself enables folding and unfolding, maintaining both deformation and shape retention without hinges or gears. In other words, they solved a mechanical problem with a structural solution.

To achieve this, they combined origami principles and soft robotics technology, and used elastic metal plates capable of repeated deformation even in space environments. When folded, the wheel can be stored in a compact volume, and when deployed, it naturally forms a large circular structure.

From 23cm to 50cm... Overcoming the Limits of Small Rovers

This deployable airless wheel has a diameter of only 23cm when folded but expands to a maximum of 50cm when deployed. As a result, even when mounted on a small rover, it provides the effect of large wheels, enabling mobility over steep slopes and large obstacles. The airless design also reduces the risk of damage.

Experimental results supported the design. Stable driving performance was confirmed in artificial lunar soil environments, and the structure and function were maintained even after free-fall drops from about 100 meters under lunar gravity, demonstrating high impact durability. The research team also verified structural stability through precise thermal analysis reflecting the extreme temperature cycles between lunar day and night.

Professor Lee Daeyoung of KAIST explained, "This deployable wheel is the world's first technology to provide a solution to the previously unsolved problem of entering lunar pits and lava tubes. It is significant in that it overcomes the greatest technological barrier-mobility."

Photo of the research team. From the top left: Seongbin Lee, PhD candidate at KAIST; Namseok Cho, Director of the Unmanned Exploration Research Institute; Geonho Lee, Researcher; Seungju Lee, Researcher; Junseo Kim, Master's candidate at KAIST; Gyu-jin Shim, Researcher at Korea Aerospace Research Institute; Jongtae Jang, Senior Researcher at Korea Aerospace Research Institute; Segwon Kim, Professor at KAIST; Taewon Seo, Professor at Hanyang University; Chaekyung Shim, Director of the Astronomy and Space Science Center; Daeyoung Lee, Professor at KAIST. Provided by KAIST

The Questions in Lunar Exploration Are Changing

The significance of this research is not just in creating a new type of wheel. Whereas the key question in lunar exploration used to be "How far can we go?", it now shifts to "Where can we enter?" Exploration scenarios based on subsurface access, rather than just surface traversal, have become realistic options.

Chaekyung Shim, Director of the Astronomy Center, stated, "Lunar pits and lava tubes are areas of extremely high scientific and exploratory value. This achievement has substantially lowered the technological barriers to accessing these locations." Jongtae Jang, Senior Researcher at the Korea Aerospace Research Institute, also emphasized the importance of future validation in actual lunar environments, noting, "This technology was precisely designed to account for the 300-degree temperature difference between lunar day and night."

Mobility technologies that can withstand steep slopes and the risk of falling have the potential to expand beyond the Moon to Mars, asteroids, and icy moons-next-generation deep space exploration environments. As a fundamental structure-based technology not attempted by other advanced space nations, it represents a point where Korea can secure technological presence in the global space exploration race.

This research was supported by the Ministry of Science and ICT's Innovation Research Center Program (IRC), the Basic Science Research Program, and the Korea Aerospace Administration's Exploratory Research Program. The first wheel to enter the lunar caves was thus created in Korea.