Protecting astronauts and equipment during missions outside of Earth’s protective sphere is critical to successful space exploration. The Moon is a malign environment where powerful solar radiation bombards the surface unimpeded.
The Moon’s temperature swings, the wildest in the Solar System, reach highs of 121 Celsius and lows of -146 Celsius. Some polar locations are in a permanent -240 Celsius deep freeze.
There are also micrometeorites. With no atmosphere to stop them, micrometeorites pummel the surface at extremely high velocities up to 70 km/second (157,000 mph.) Some research shows that up to 10,000 kg of micrometeorites strike the Moon every day.
Astronauts need protection from all of these hazards. Orbital images of the Moon show that there are numerous lava tubes on the Moon that could keep astronauts safe. These were formed during ancient lava eruptions. When part of a lava tube ceiling collapses it leaves an opening called a skylight that shows up in images.
This image from NASA’s Lunar Reconnaissance Orbiter shows a section of collapsed ceiling in a lava tube in Mare Tranquillitatus. Rocky debris from the collapse is seen at the bottom of the pit. The pit is about 100 meters deep, but only further exploration can determine how long or large the lava tube is. Image Credit: By NASA/GSFC/Arizona State University – http://photojournal.jpl.nasa.gov/catalog/PIA13518, Public Domain.
There are also visible sinuous rilles on the surface indicating the path and length of some tubes. While many have collapsed almost completely, others have not. Some rilles reach great lengths. Rima Sharp, a rille on the lunar near side, is up to 566 km long, though some researchers say it is actually two separate rilles.
Rima Sharp may be the longest lunar rille at 566 km in length, though some consider it to be a pair of separate rilles. Image Credit: NASA/GSFC/Arizona State University.
There’s been a lot of research into the idea of using these caves for protection. There are concepts for bases constructed inside of them where astronauts could live and work. But they need to be explored before those plans can take any concrete steps. The collapsed skylights are seen as possible entrances into some of these caves, and scientists have thought hard about how they could be used to access the caves and explore them.
A team of European researchers have designed a trio of robots with different capabilities that could work together to explore lunar caves and assess their potential. The researchers have tested them on Lanzarote, a volcanic island in Spain’s Canary Islands. The team’s work is presented in a new research letter published in Science Robotics titled “Cooperative robotic exploration of a planetary skylight surface and lava cave.” The lead author is Raul Dominguez from the Robotics Innovation Center at the German Research Center for Artificial Intelligence.
While the Moon is the next place where astronauts will get their boots on the ground, Mars is part of this work, too. Mars also has lava tubes with collapsed skylights, and astronauts visiting the planet could also benefit from their protection. Additionally, Mars has astrobiological importance that the Moon lacks.
“Exploration of lava caves on the surface of planetary bodies near Earth is of high importance for scientific research and space exploration,” the authors write. “The natural shielding that these caves offer against radiation and small meteorites makes them well suited for preserving exobiological signatures and protecting human-made facilities. The use of a robot team arises as the safest and most cost-efficient way to explore extraterrestrial lava caves because they are difficult to access.”
The team are working on a three-member team of heterogeneous robots that work together autonomously to explore lava tubes in four phases.
SherpaTT is a hybrid wheeled and legged robot that performs surface exploration and acts as a surface anchor to lower another robot, Coyote III, into the lava tube. Coyote III is a small, lightweight, highly-mobile wheeled rover. LUVMI-X is the third rover. It’s lightweight and cost effective, and performs mapping along with SherpaTT.
The three robots and their capabilities. Image Credit: Dominguez et al. 2025 SciRob
In phase 1, the entrance area around the lava tunnel is mapped by SherpaTT and LUVMI-X. In phase 2, a sensorized payload cube with high-speed cameras is ejected into the cave from LUVMI-X to collect initial data about the entrance and the cave floor. In phase 3, the Coyote III scout rover rappels into the cave while being anchored to SherpaTT. In phase 4, the scout rover explores and maps the cave in 3d.
This image shows the three members of the robotic team tested on Lanzarote in the Canary Islands. Image Credit: Dominguez et al. 2025. SciRob
The team tested the operation on Lanzarote Island in February 2023.
The researchers aimed to test each phase of the mission several times in order to gauge effectiveness and understand what improvements are needed. They were unable to test each phase as many times as they desired due to weather and other constraints, but still achieved good results.
“In MP-1, SherpaTT and LUVMI-X collaborated to generate a DEM with enough accuracy to identify the skylight and place the rovers in the correct position during the following phases,” the authors write.
“During MP-2, the skylight was explored using the payload cube, which demonstrated its capacity to recreate the skylight in 3D by emulating a free fall under lunar gravity through a pulley system,” the authors explain.
MP 3 was also successful. Coyote III was autonomously lowered into the cave without damage.
MP4, the cave-mapping portion of the test, was also validated. “Coyote III was suitable for traversing the harsh terrain and collecting point clouds through remote control,
therefore validating the locomotion and data acquisition approach,” the authors write. Coyote III successfully traversed different terrains in the cave, but was unable to execute point turns “in consolidated irregular rocky surfaces because of high friction,” the authors explain.
These panels show some of the results of the testing for mission phase 4. The testing validated the cave reconstruction and the mobility and effectiveness of Coyote III. A shows the skylight mesh image produced by Coyote III. B shows the skylight Time of Flight data visualization. C shows the rovers travelling through a narrow corridor. D shows the passage Time of Flight visualization. E shows the Coyote III cave reconstruction. F shows ground-truth rover cave reconstruction for F(i) the skylight area and F(ii) the narrow corridor area. Image Credit: Dominguez et al. 2025 SciRob
The researchers say that a “heterogeneous autonomous robot team is a promising approach” for the exploration of lunar and Martian lava caves. “The presented analog field test showed reliable and efficient performances in most of the defined MPs,” they write. ” However, the mission concept needs further research to be extrapolated to a real planetary exploration scenario.”