Development and Geotechnical Engineering Properties of KLS-1 Lunar Simulant

With the growing importance of space development and the reported shortage of water and He-3, advanced countries such as the United States and Russia as well as emerging economies such as those of Europe, China, India, and Japan have actively engaged in space and lunar exploration missions. The Moon is the nearest celestial body to the Earth and has the same geologic time scale (Weill et al. 1971). Lunar soil once had a similar chemical composition to that of the Earth, but the thin atmosphere and shortage of water on the Moon have led to significant changes in the last billion years (Taylor 1975).

Lunar soil, formed through repeated pulverization, condensation, and mixing with exposure to cosmic collisions and solar wind for long periods, continues to change even today due to collisions with meteorites and high-energy solar/space charged particles (Taylor 1982). Because the lunar surface is lacking in atmosphere, moisture, and organic materials, it is composed only of single-phase solids (Slyuta 2014). Lunar soil tends to be thicker in areas that have existed for longer geological periods, and these areas are called lunar highlands (Pieters 1986; Taylor et al. 2001). The darker surfaces, known as lunar maria, formed from volcanic eruptions caused by iron-rich magma and have a higher iron content and a low degree of oxidation (Tindle and Kelley 2012).

Research on lunar soil grew active in the 1960s when the Apollo program succeeded in carrying samples of lunar soil back to Earth. Recently, the demand for research on the lunar surface and lunar soil has increased with advanced image-analysis techniques revealing the mineralogy, chemical composition, and insufficient resources (water, He-3, and so on) on the Moon. However, only 380 kg of lunar soil exists on Earth (Schnetzler and Nava 1971), and accordingly most advanced countries have developed their own lunar simulants for use in Moon-related and space-related research (performance assessment of lunar exploration equipment and payloads under actual lunar conditions) (Morris 1980).

Real lunar regolith, which originates from basalt and anorthosite, is subject to continuous pulverization, dissolution, and crystallization. It consists of various minerals, vitreous crystals, and vapor-deposited reduced iron (Fe) (Sasaki et al. 2001). Table 1 shows basic geotechnical engineering properties of real lunar regolith (lunar soil 14163 sampled by the Apollo 14 mission) (Carrier 1973; Carrier et al. 1973). Lunar regolith shows relatively high specific gravity ($G_s=2.7–3.1$) and density ranges while having distinctively low particle composition conditions ($e_{\text{max}}$), resulting in a high compression index ($C_c$) on the Moon’s surface. Meanwhile, the friction angle shows extremely high values, whereas cohesion is close to zero even though it consists of fine particles.

However, the amount of real lunar soil samples is too limited to allow use on a macroscale or in destructive research. A well-defined and reliable simulant is thus required for various scientific and engineering studies related to lunar exploration programs (Sibille et al. 2006). The first stage of lunar simulant development is to acquire a terrestrial material that has a similar chemical composition and contains similar minerals to actual lunar soil. Finding a naturally occurring soil with a similar chemical composition and physical properties to those of lunar soil is extremely challenging. Advanced countries in space technology have developed lunar regolith simulants based on basalt, anorthosite, minerals, vitreous material, dust (including volcanic ash), and nano-Fe. The few countries in addition to Korea engaged in the development of lunar simulants are the United States, Japan, Canada, and China (Table 2) (Kanamori et al. 1998; McKay et al. 1994; Spray 2010; Weiblen et al. 1990; Zheng et al. 2009).

The lunar simulant that is considered to have the most similar characteristics to those of lunar soil is Johnson Space Center Number One (JSC-1), developed by the National Aeronautics and Space Administration (NASA) Johnson Space Center (McKay et al. 1994). JSC-1 is classified as silty sand (MS) by the Unified Soil Classification System (USCS) due to its particle-size distribution and consistency indexes and shows an average $G_s$ of 2.91 (Willman et al. 1995). Its interparticle friction angle ($\varphi$) and cohesion ($c$) values vary in ranges of 44.4–53.6° (45° at $1.65\mathrm{g}/{\mathrm{cm}}^3$) and 3.9–14.4 kPa, respectively, depending on the bulk density of the specimens (Klosky et al. 2000).

However, because most lunar simulants consist largely of crushed basalt and volcanic ash, and chemical synthesis has been employed to increase their iron (Fe) content [e.g., JSC-1 = US$19/kg; Japan’s Fuji Japanese Simulant 1 (FJS-1) = US$4/kg], they are expensive and thus less versatile. In Korea, some attempts were made to develop lunar simulants in 2010 (Hanyang University) and 2014 (Korea Aerospace University) (Kim et al. 2014; Yoo et al. 2014), but their geotechnical behaviors were insufficient to satisfy international standards.

Another major consideration that is just as important as chemical composition is the geotechnical engineering behavior of lunar simulants. Recently, advanced countries in space technology have focused on unmanned rovers to explore and acquire resources on the Moon. Because lunar missions are restricted by the size and weight of launchers and landers, it is essential for the unmanned rovers to exhibit stable driving performance on the lunar surface. Although the lunar surface is a key factor in research on the ability of rovers to pass or avoid obstacles as well as for self-driving algorithms, the geotechnical engineering parameters of lunar soil have a significant influence on driving performance from a terramechanics perspective (Oravec et al. 2010; Sreenivasulu and Jayalekshmi 2014).

In geotechnical engineering aspects, the rover weight acts as a normal stress (${\sigma }_v$) on the ground and determines the maximum shear strength (${\tau }_{\max }={\tau }_f$) between rover wheels and the lunar surface, along with the friction angle $\varphi$ and cohesion $c$ of lunar regolith. When the torque for a unit area becomes larger than the maximum shear strength moment at the interface due to rover operation, soil failure arises from the driving of the wheels, resulting in a wheel slip that is difficult to predict. Unexpected wheel slip is the main cause of poor driving stability and a loss of driving energy. As such, an accurate evaluation of ${\tau }_{\max }$ is necessary to ensure safe and efficient driving on the heterogeneous lunar surface (Wong 2010).

This study was conducted to develop an economical Korean lunar simulant with high geotechnical engineering similarity to actual lunar soil, to be adopted in concurrent lunar exploration projects in Korea and various international cooperative space-related studies. Based on an understanding of the Moon’s formation and geological characteristics, raw materials were selected with consideration of various factors, including the chemical composition of basalts in Korea, economic feasibility, and commercialization potential. Basic research for the development of a reliable Korean lunar simulant (KLS-1) was carried out through a series of experimental verifications with Apollo samples and common lunar simulants (JSC-1 and FJS-1) via chemical composition, particle-size distribution, specific gravity, and shear properties (friction angle and cohesion) analyses and comparisons.

JSC-1 was developed by NASA Johnson Space Center with the establishment of the Space Exploration Initiative in 1989. This lunar simulant, which exhibits vitreous characteristics, was developed from volcanic ash found near Flagstaff, Arizona. It consists of anorthosite, pyroxene, and olivine, and has weak metallic properties. It has a specific gravity of 2.9, a cohesion of 1.0, and a friction angle of 45° (Willman et al. 1995). However, JSC-1 or Johnson Space Center Number One A (JSC-1A) simulants are insufficient for large-scale simulation programs due to their scarce quantities. New lunar regolith simulants with large procurement, such as NASA/USGS-Lunar Highlands Type-2 Medium regolith simulant (NU-LHT-2M) (Stoeser et al. 2010; Zeng et al. 2010), are thus currently being actively used in large-scale lunar programs (e.g., ESA PROSPECT).

FJS-1 was developed using samples obtained from the basaltic lava of Mount Fuji in Japan. Although FJS-1 was designed to match the bulk mechanical properties and chemical composition of soils in the lunar maria (Kanamori et al. 1998), the abundance of ${\mathrm{SiO}}_2$ and lower MgO content show that FJS-1 is more quartz normative than are lunar soils. Moreover, even though physical properties such as $G_s=2.94$ and $\varphi =32.5–39.4°$ (Kanamori et al. 1998) are similar to those of lunar soil, the interparticle cohesion of FJS-1 is higher than that of KLS-1, JSC-1, and real lunar soils.

This study developed KLS-1 by recomposing sieved crushed basalt from Cheorwon, Korea. The developed simulant KLS-1 has a specific gravity of 2.9, which is the typical range (2.9 to 3.2) of lunar soils (Carrier 2003; Carrier et al. 1973). Moreover, the chemical composition and physical shear strength properties ($c$, $\varphi$) of KLS-1 also successfully represent characteristics of real lunar soils.

Generally, a lunar regolith simulant is recommended to be a material manufactured from natural or synthetic terrestrial or meteoritic components to simulate one or more physical (e.g., particle-size distribution, specific gravity, relative density, interparticle friction angle and cohesion, and compressibility) and/or chemical properties of a lunar soil (Sibille et al. 2006). A series of experimental verifications provided technical evidence that the newly developed KLS-1 is a promising reliable lunar regolith simulant for use in the fields of aerospace and geotechnical engineering.

For macroscale utilization, the economic feasibility of lunar regolith simulants is an important criterion. Because the costs of 1 kg of JSC-1 and FJS-1 lunar simulants are reported to be US$19 and US$4, respectively, the expected material costs to simulate a $1{\mathrm{m}}^2$ lunar surface area with 34 cm thickness (approximately 1 t in mass) are estimated to be US$30,000 and US$4,000, respectively. Meanwhile, the material, transportation, and production (labor for sieving and recomposing) costs are estimated to be US$2,000 for 1 t of KLS-1 production in this study. Fig. 8 represents the large-scale KLS-1 production system (Baek et al. 2015) which is currently being installed at the Korea Institute of Civil Engineering and Building Technology (KICT). With the in situ supply of crushed basalts, the system has a capacity to treat (drying, sieving, and separating) 1.5 t of raw material in a single cycle. The entire system is perfectly sealed and pneumatically controlled to minimize dust. Crushed basalts are immediately heated and dispersed to be completely dried and separated into single particles. Dried particles are then transported via air flow into layers of sieves (#4–#200, and finer). All remaining particles on each sieve are moved by air and collected in their primary containers. Separated particles can be used separately, and an automatic vacuum and mass control system can recompose KLS-1 based on the detailed mass composition presented in the Preliminary Production of KLS-1 section. The system is estimated to produce 1 t of KLS-1 per 8 h (1 day) of operation. Moreover, the cost of KLS-1 is expected to be less than US$1,000/t, representing the most economically feasible lunar regolith simulant. Therefore, due to its high similarity to real lunar soil and potential for tonnage quantities, KLS-1 is a promising root lunar regolith simulant (Sibille et al. 2006) for application in various research programs, including macroscale drilling and excavation.

Lunar regolith is an essential component for understanding and simulating the surface environment of the Moon for concurrent and future lunar missions. However, real lunar regolith samples returned to the Earth are priceless and in quantities that are far too small to be used for macroscale research programs. Thus various lunar soil simulants have been developed by different countries and organizations using terrestrial materials to represent the physical and chemical properties of real lunar soils. However, most lunar simulants have been made with exotic materials and complicated processes, and many are in short supply.

This study developed a new lunar regolith stimulant—KLS-1—using natural basalt from Korea with a simple manufacturing procedure. The chemical and physical (geotechnical engineering) properties of KLS-1 were verified to represent the real characteristics of lunar soil 14163.

To find the most appropriate mother material for KLS-1, natural basalts in Korea were examined and classified in terms of chemical composition and total iron content ($\mathrm{FeO}+{\mathrm{Fe}}_2{\mathrm{O}}_3$). Among eight candidate samples, Quaternary intrusive basalt from Cheorwon, Korea shows the highest appropriateness to become the mother material for KLS-1 production. After powderization and precision sieving, separated basalt grains were recomposed according to the particle-size distribution of lunar soil 14163. The resulting mixture is Korea Lunar Simulant—Type 1, with a particle-size distribution (SM: silty sand; $G_s=2.94$) and a chemical composition having high similarity to those of real lunar soil.

Geotechnical shear strength properties of KLS-1 were evaluated via a series of laboratory direct shear tests within the context of Mohr–Coulomb failure criteria. The peak internal friction angle of KLS-1 is high and increases with density, with negligible cohesion. The measured peak friction angle was 44.91° on average, and interparticle cohesion was $c=1.85\mathrm{kPa}$. Standard laboratory oedometer apparatus testing yielded a compression index of $C_c=0.029$ and a recompression index of $C_r=0.0043$ for KLS-1 simulant. All mechanical properties ($G_s$, $\varphi$, $c$, $C_c$, and $C_r$) of KLS-1 were consistent with real lunar soil and other major simulants, demonstrating the high reliability of using KLS-1 in the area of aerospace engineering.

This study focused on basic research (particle-size distribution and chemical composition) for the development of KLS-1. In addition to chemical and physical reliability, economic feasibility is another important advantage of KLS-1. Based on the sufficient supply of raw materials and a concurrently developed automatic producing system, KLS-1 is expected to be the most inexpensive root lunar simulant to be deployed in various macroscale lunar research programs. Meanwhile, further research is required to evaluate other geotechnical engineering components such as compressibility. Moreover, additional processes such as plasma treatment or glass fragment bonding are expected to provide a strong representation of irregular agglutinates and dust particles to further improve the similarity of KLS-1.

The research described in this paper was financially supported by grants from the Strategic Research Projects—(1) Geotechnical Engineering Preparations for Lunar Exploration: Soil Mechanics with Lunar Soil and Laboratory Demonstration of Lunar Environments; and (2) Development of environmental simulator and advanced construction technologies over TRL6 in extreme conditions—funded by the Korea Institute of Civil Engineering and Building Technology. The authors also acknowledge Dr. Moonkyung Chung, Dr. Kiseok Kwak, and Dr. Jin-Tae Han at KICT for their sincere support and advice on this research.