Design of Lunar Soil Simulant for Lunar Landing Applications and Its Mechanical Property Evolution under Impact Loading
This study develops a Changbai Mountain volcanic ash-based lunar soil simulant tailored for the Moon's far side and South Pole, demonstrating through impact tests on various slopes that landing gradients significantly alter the spatial distribution of post-impact mechanical properties, with shear strength weakening uphill and strengthening downhill until stabilizing beyond 8°.
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Imagine standing on the surface of the Moon, looking out at a landscape that has never felt the touch of a human foot. The ground beneath you is not like soil on Earth; it is a layer of fine, jagged dust and broken rock, created over billions of years by the relentless bombardment of space rocks and the harsh vacuum of space. This material, known as lunar regolith, behaves in ways that are difficult to predict. When a spacecraft lands, its heavy feet press into this dust, and the way the ground shifts, compacts, or shifts sideways determines whether the ship stands firm or tips over. For decades, scientists have tried to recreate this alien ground in laboratories on Earth to test landing gear, but most of their models were built using soil characteristics from the side of the Moon we can see. Now, with missions targeting the dark, hidden far side and the icy South Pole, engineers need a new kind of test ground that matches the unique, rugged nature of those specific regions.
A researcher at the Beijing Institute of Space Mechanics and Electricity has taken on this challenge by developing a new, highly accurate substitute for the Moon's surface. They started with volcanic ash collected from the slopes of Changbai Mountain in China, a material that shares a similar mineral makeup with the lunar soil found on the far side and in the massive South Pole-Aitken basin. By crushing, grinding, and carefully sorting this ash into specific sizes, they created a mixture that mimics the texture and behavior of the real lunar ground. To test how this new material reacts to a landing, they built a massive, ten-meter-square sandbox filled with their simulated soil. They then dropped a model of a four-legged lander onto this surface from a height that simulated a real landing speed of 2.5 meters per second. Crucially, they did not just land on flat ground; they tested the lander on slopes of four degrees, eight degrees, and twelve degrees to see how the angle of the terrain changed the outcome.
The results revealed a striking pattern in how the ground responds to a landing on a hill. When the lander touched down on a flat surface, all four legs hit the ground at nearly the same time, and the soil under each leg was compressed evenly. However, as the slope increased, the story changed completely. The leg that touched the ground first, at the top of the hill, experienced a different kind of impact. Because the lander was still sliding slightly down the slope at the moment of contact, much of the energy went into pushing the soil sideways rather than squashing it straight down. This left the soil under the uphill leg looser and weaker, with a lower ability to hold the weight of the ship. In contrast, the leg that landed last, at the bottom of the hill, hit the ground when the lander had already stabilized. This leg drove straight down with maximum force, packing the soil tightly and creating a much stronger, more compact foundation.
This difference in compaction created a clear gradient across the landing site. The soil under the downhill leg became significantly denser and stronger, while the soil under the uphill leg remained more porous and fragile. The researcher found that this pattern became stable once the slope reached eight degrees; steeper slopes did not change the fundamental nature of this uneven distribution. The soil under the downhill leg showed a void ratio, a measure of how much empty space exists between the particles, that was lower than the uphill side, meaning the particles were packed much closer together. This uneven settling is critical for future missions because it means a lander on a slope will not settle evenly; one side will sink less than the other, potentially causing the entire spacecraft to tilt.
The researcher also confirmed that their volcanic ash mixture was an excellent stand-in for the real thing. Its particle sizes matched the range found in samples returned by the Chang'e-5 mission, and its mechanical strength fell within the same range as the soil brought back by the Apollo missions. The simulated soil behaved just like the real lunar ground, showing the same tendency to lock together when squeezed and the same way of shifting under pressure. By proving that the landing slope creates a predictable zone of weak soil uphill and strong soil downhill, this work provides a vital guide for engineers designing future landers. It suggests that for a spacecraft to land safely on the uneven terrain of the lunar South Pole or the far side, its stability calculations must account for this uneven compression, ensuring the ship can withstand the tilt that comes from landing on a hill where the ground itself is not uniform.
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