In 1844, the protagonist of French writer Alexandre Dumas' novel 'The Three Musketeers' shouted, 'All for one! One for all!' Soon, it seems we will be able to see the Three Musketeers in action on the Moon. They are not human but robotic musketeers.
German and Spanish scientists have developed collaborative robots to explore lava tubes on the Moon, successfully demonstrating their functionality in volcanic environments on Earth. Two robots explored the entrance of the cave, moving on four legs and wheels. The underground exploration robot descended underground using a rope lowered by another robot.
◇Exploring lava tubes for a manned lunar base
Carlos Pérez-del-Pulgar, a professor at the School of Industrial Engineering at the University of Málaga in Spain, noted on the 14th in the international academic journal 'Science Robotics' that 'three types of exploration robots working together have proven their ability to perform missions in the lava tubes of Lanzarote, Spain.'
Humanity has resumed manned lunar exploration, which had been halted since the 1972 Apollo 17 mission. The United States plans to send astronauts to the Moon by 2027 with its Artemis program. Rather than just landing once, the plan includes establishing a long-term habitation base. China and Russia have also stated their intentions to build lunar bases.
The reason Spanish researchers are trying to explore lava tubes with robots is that they are the best locations for establishing a manned lunar base. The Moon has no atmosphere and is exposed to radiation that is 150 times stronger than that on Earth. Meteorites also frequently fall. Lava tubes can protect against such dangers. The surface of the Moon can reach temperatures of 127 degrees Celsius during the day and drop to minus 173 degrees Celsius at night, but inside the tubes, temperatures are stably maintained.
The research team developed three types of robots to find and explore the skylights, which are entrance holes formed when the ceilings of lava tubes collapse. The name comes from the fact that light enters the tubes. First, SherpaTT moves on four legs to explore the surface. LUVMI-X is a rover that carries cargo. Lastly, Coyote III explores the interiors of the caves.
Lava tubes could serve as shelters for humans on missions to the Moon or Mars, but exploring underground caves is very risky. Scientists plan to send robots in first to explore and create internal maps before astronauts enter. Spanish researchers have proven this possibility in an actual cave.
◇Exploring the cave interior by descending with a rope
Scientists have already found lava tubes on the Moon. Last year, a research team from the University of Trento in Italy announced in the international academic journal 'Nature Astronomy' that they found an entrance leading to underground lava tubes in the 'Sea of Tranquility' area by analyzing radar observation data sent by NASA's Lunar Reconnaissance Orbiter (LRO). Although the existence of underground caves on the Moon had been theorized for 50 years, it was the first time it had actually been confirmed.
This time, SherpaTT and the LUVMI-X rover explored the area around the entrance of the underground cave and created a map. Coyote III utilized ground-penetrating radar (GPR) to understand the internal structure of the lava tube. The next step involves underground exploration. LUVMI-X launched a small cubic probe, similar to a CubeSat, into the cave to create a 3D map of the cave entrance.
The final step involves directly exploring inside the cave. The robotic musketeers established the optimal path for descent based on the 3D map of the cave entrance. Coyote III descended by means of a rope, receiving power while fixed to SherpaTT, the largest robot. Coyote III then disconnected from the rope and moved 235 meters inside the lava tube, creating a 3D map.
The first author of this paper is Raúl Domínguez, a researcher at the German Research Center for Artificial Intelligence (DFKI). The DFKI consortium developed robots to explore the lunar lava tubes with support from the European Commission (EC). The research team mentioned that they will advance the capabilities of the robotic musketeers further. Professor Pérez-del-Pulgar explained that 'maintaining wireless communication with surface equipment is a major challenge for robots to explore large cave systems,' as radio frequency signals cannot pass through rocks.
◇Robotic dogs complete cave exploration tests on the surface
Marco Hutter, a professor at the Swiss Federal Institute of Technology in Zurich (ETH), said in Nature that 'just as human experts form teams and mobilize various technologies, robots can also complement each other to carry out missions.'
The Spanish researchers noted that they were inspired by the underground exploration robot competition held by the U.S. Defense Advanced Research Projects Agency (DARPA) in 2021. Most teams operated various types of robots, including wheeled rovers, quadrupedal robots, and drones. During the competition, the quadrupedal robot ANYmal, developed by Professor Hutter, took first place.
NASA has already been training robots in Earth's lava tubes to prepare for the construction of manned bases on the Moon and Mars. NASA's Jet Propulsion Laboratory unveiled a robotic dog designed for Mars exploration in 2021 at the annual meeting of the American Geophysical Union, which was modified from Spot, a quadrupedal robot developed by Boston Dynamics, a subsidiary of Hyundai Motor. It was named 'Au-Spot' because of its autonomous walking capability.
In the past, Mars exploration has been handled by rovers such as Opportunity and Curiosity. However, wheeled rovers could only move on flat terrain. The terrain the scientists want to explore is mostly rugged and located below the surface. NASA scientists indicated that Spot is ideal for exploring the rough terrains of Mars as it can walk down underground and get back up when it falls.
References
Science Robotics (2025), DOI: https://doi.org/10.1126/scirobotics.adj9699
Nature Astronomy (2024), DOI: https://doi.org/10.1038/s41550-024-02302-y