A Conceptual Open Pit Mine Architecture for the Moon Environment

On an international level, the International Space Exploration Coordination Group (ISECG) is developing the global exploration roadmap (GER), which outlines a shared international vision for human and robotic space exploration (ISECG, 2018). The GER reflects an exploration strategy that begins with the International Space Station (ISS) and extends to the Moon, asteroids, Mars, and other destinations. The supplement to the GER, published in 2020 (ISECG, 2020), updates the lunar surface exploration scenario (LSES) with further architectural elements and details of exploration campaigns.

The LSES is divided into the following three phases:

1 – Boots on the Moon;

The scenario outlines lunar orbit and lunar surface activities and discusses the creation of a Gateway in a lunar orbit in the context of NASA’s Artemis missions and the timing of upcoming robotic missions developed by space agencies around the world.

The LSES also defines the key elements that are needed in the phases defined above. In the context of this paper, the following three are most important:

In Phase 1, development of a technology demonstration for small landers and robotic precursors, which can deliver cargo to the Moon.

With respect to plans to deliver humans to the Moon’s surface, SpaceX plans to use one of its starships as the first permanent infrastructure on the Moon – after its placement in the horizontal direction (Abdin et al., 2021).

The European Space Agency (ESA) has developed the Terrae Novae 2030+ Strategy Roadmap (ESA, 2021), which defines the main long-term goals for Europe – namely, the capability to launch and deliver payloads to ISS as a laboratory for technology demonstration, and then to deliver these technologies to the Moon and Mars. In the context of the Moon, Europe aims to achieve strategic autonomy in its lunar exploration activities, including long-range surface mobility, possibly culminating in international research infrastructure on the Moon. The strategy is divided into the following three main steps:

First, access to cis-lunar space is achieved through the extension of the ISS partnership beyond Low Earth Orbit (LEO); Europe is playing a critical role in the development of the transportation and staging architecture (via the Gateway partnership, an element of the Artemis program).

In Poland, the Polish Space Agency (POLSA) has announced a call for the first mission to the Moon and plans for a feasibility study in 2023. Polish institutions and companies are developing several projects such as:

Moon harvesting – A project funded by the Polish National Science Center, related to civil engineering and space mining on the Moon;

Identifying a space mine architecture that is best suited to selected mining tasks means first considering the architectures that have been developed and optimized for mines that have been dug on Earth over the past decades. The different ISRU architectures are presented in more detail in the following sections.

During long-term space missions, including a human return to the Moon, it is imperative to create a system to ensure the survival of astronauts (Chen et al., 2021). Bioregenerative life support systems (BLSS) are artificially created ecosystems that contain appropriately selected organisms such as bacteria, fungi, microalgae, and higher plants (Guo et al., 2017). These organisms convert waste, through recycling processes, into valuable resources such as H₂O and O₂, as well as food. The system of growing higher plants in space has been studied for almost 50 years. In 1978, the Controlled Life Support System program was initiated by NASA (Averner 1989). In Europe, the ESA is conducting research into the effectiveness of life support systems in the context of the MicroEcological Life Support System Alternative (MELiSSA) project (Lasseur et al., 2010). In Rokkasho (northern Japan), experiments have been conducted in Japan’s Closed Ecology Experiment Facilities (Nitta et al., 2000). China has a facility called the Lunar Palace (Hu et al., 2023), where tests are being conducted with human participants, with a view to using BLSS in future extraterrestrial habitats. Finally, Russia is conducting research at facilities called BIOS (BIOlogical closed life support System), which are located in Siberia (Gitelson & Lisovsky, 2002).

In general, these systems are designed based on phenomena occurring on Earth. BLSS connect producers (plants), consumers (humans), and reducers (microorganisms) (Guo et al., 2017). The system can use limited resources to sustainably provide humans with the elements necessary for survival beyond Earth, namely, oxygen, water, and food (Fig. 1).

Figure 1.

As indicated above, Linne et al. (2017) provide a general overview of all activities needed to develop a full ecosystem on the Moon, and new ideas are discussed during annual Space Resources Roundtable meetings. These discussions have highlighted an important paradigm change: ISRU should be based on a “service,” and not a “product.”

Various ISRU opportunities have been discovered over the decades. For example, Warren et al. (2014) describe four groups of resources (H₂O, platinum group metals, rare Earth elements, and regolith) that could have civil engineering applications. In the case of the Moon, O₂ and H₂O are needed for life support, liquid oxygen (LOx) and liquid hydrogen (LH₂) are needed for propellants, and various metals, elements, and compounds are needed for metallurgic and chemical production processes (Anand et al., 2012). These groups have been verified using an economic analysis (Blair et al., 2002), and H₂O has been proven to be an economically valuable resource.

Vergaaij et al. (2021) offer a more detailed analysis of an economically reasonable solution for fuel production. The authors present a table showing the optimal amount of fuel and the time needed for its production for different producer and customer locations. In the context of our analysis, the following assumptions are made:

An open pit mine service is the best delivery option for a customer located on the Moon’s surface. Expected customer need is based on estimates given in Vergaaij et al. (2021) – namely, 300 t of fuel over a period of 6 months.

The main functional objective of an open pit mine built on the Moon’s surface is to excavate, process, and finally sell the lunar regolith as a function of customer needs.

According to Vergaaij et al. (2021), the minimum cost of propellant (LOx, LHx) production on the Moon’s surface, with delivery to a customer on the Moon’s surface is 177 $/kg, assuming that 300 t of fuel is produced during 0.45 of a year. The possible stochiometric ratio of LOx and LHx in the propellant is in the range of 6:1–6.5:1. Therefore, optimal production is 260 t of LOx and 40 t of LHx.

During ilmenite reduction by hydrogen, one of the outputs is H₂O. The production of 300 t of propellant requires roughly 357 t of H₂O – this assumes that H₂O electrolysis can produce hydrogen (H) and oxygen (O) in the ratio 7.94 O:H. To produce 357 t of H₂O, 2,795 t of ilmenite and 40 t of H are needed. This assumes a 31% concentration of O in ilmenite and a 30% recovery ratio. Assuming that it is possible to excavate an ilmenite-rich feedstock (which requires remote as well as in situ prospecting), which contains 9% ilmenite, there is a need to excavate 31,033 t of regolith. With an assumed optimal timeframe equal to 0.45 years, daily demand is equal to 188.8 t. This key parameter is used in the definition of the open pit mine architecture presented in Section 3.

Additional objectives are focused on two aspects: (i) construction and (ii) maintenance of the open pit mine. Only the key components are considered here.

2.2.4.

The Bioregenerative Life Support System

Human settlement in an extraterrestrial environment requires the provision of a life support mechanism. Human settlement in an extraterrestrial environment requires the provision of a life support mechanism (Verseux et al., 2022). This means implementing a system that provides humans with the basic elements necessary for survival. BLSS converts CO₂ and wastewater generated during the metabolism of the human body by microorganisms into O₂, H₂O, and food. The daily water balance of an astronaut is shown in Figure 2.

Figure 2.

2.2.4.1.

Oxygen

Researchers have compiled a mass balance for an astronaut on a long-term space mission (Ewert et al. 2019). According to this reference, an astronaut conservatively needs about 0.89 kg of O₂ per day to breathe. The same person conservatively produces about 1.08 kg of CO₂ per day, considering the same type of scheduled training. For a two-person crew, assumed to be involved in operating the projected mine on the Moon, these values would increase to 1.78 kg O₂/day and 2.16 kg CO₂/day, respectively, resulting in 670 kg O₂/year and 790 kg CO₂/year, respectively.

2.2.4.2.

Water

The drinking water requirement of the reference astronaut is 2.79 kg/day (1,020 kg/year) without taking into account the 0.50 kg/day (183 kg/year) needed for food preparation, 0.76 kg/day (278 kg/year) contained in food, and 0.48 kg needed for metabolic reactions. Human metabolic processes produce 3.04 kg/day (1,110 kg/year) of water from sweat and respiratory processes, 1.40 kg/day (510 kg/year) of water from urine, and 0.09 kg/day (32 kg/year) of water from feces (Ewert et al., 2019). The total annual H₂O demand for a two-person crew will be almost 4 t/year. At the same time, the same amount of wastewater will be produced by the crew.

2.2.4.3.

Food

Food is one of the basic elements required by future astronauts settling on the Moon. The total amount of calories a person needs per day depends on several factors. These include age, gender, height, weight, and type/level of physical activity. The recommended daily calorie requirement for an adult woman is 2,000 calories and for an adult man is 2,500 calories (Watanabe et al., 2021). In the following calculations, the number of calories required is based on two men, that is, 5,000 kcal/day. However, as half of the astronauts’ diet will be delivered from Earth, this is reduced to 2,500 kcal/day. One woman and one man are assumed to participate in the mission, with estimated requirements based on the requirements of the man.

The diet will be based on soya (Glycine max L.) (450 kcal/100 g) and microalgae. Both Chlorella sp. (340 kcal/100 g) and cyanobacteria such as Spirulina sp. (300 kcal/100 g) can be used. In the following calculations, a value of 300 kcal/100 g is assumed. Daily intake is then approximately 0.2 kg of soya and 0.7 kg of microalgae. Soya would account for 70% and microalgae would account for 30% of the calorific requirement. The annual soya requirement is, therefore, 70 kg and that of microalgae is 260 kg.

The following elements related to construction and maintenance are assumed to be delivered from Earth: excavation devices, a transportation system, the internal part of the habitat, service elements for devices, hydrogen, bio supplements for plants and microalgae (N, K, and P), greenhouse components, tanks, supporting structures, pressurized modules, BLSS equipment, microalgae strain, and half of the food supply.

This section focuses on outlining the challenges and objectives of the first aspect of the mine – the source of regolith – which will further lead to initial processes shown in Fig. 3.

To meet the main objective described in Section 2, which is to sell 300 t of LOx/LHx produced during a 0.45-year period (Vergaaij et al., 2021) and provide the resources that are needed for the partial self-sustainability of the station, assuming a conservation recovery ratio of 30%, 85,632 t of R0 regolith must be excavated annually in the operational phase. This section proposes a general blueprint for an open pit mine site with the required output.

The location of the mine is determined mainly by the location of economically valuable deposits of needed resources. Here, we consider ilmenite, which can be found in numerous places on the surface of the Moon. The biggest deposits are found in the weathered basaltic regolith in the lunar maria (1%–10% by weight of ilmenite concentration) and basaltic lava depositions (9%–19% by weight concentration). As concentrations are higher in the latter case, this paper considers these regions as the source of feedstock, despite the need for an additional crushing stage.

The dimensions of areas with ilmenite deposits will define the boundaries of the mining site. The Clementine mission, along with subsequent lunar orbiters, has performed spectroscopy measurements of the lunar surface, resulting in maps of the mineralogical composition of the surficial regolith. The latter data have identified ilmenite-rich areas spanning hundreds of kilometers (Crawford et al., 2015). Nevertheless, it will be necessary to confirm the subsurface composition with seismology measurements and in situ core drilling.

The average depth of the regolith layer in the lunar maria is estimated to be in the range of 7–12 m, based on measurements conducted at the Apollo and Chang’e landing sites (Richardson et al., 2020). A maximum value of 10 m is assumed as the vertical boundary of the mining site.

Assuming that the mean width of the pit is 30 m, its length will be determined by R1 regolith requirements. The mass of R0 that must be excavated is described by the excavation process equation as follows: (1) mR0⋅αex1=mR1⋅αex2 {m_{R0}} \cdot {\alpha_{ex1}} = {m_{R1}} \cdot {\alpha_{ex2}}

During construction (from T0 to T1), demand for R1 regolith is for the construction of the habitat and the greenhouse shielding layers, construction of the road between the pit and the habitation zone, and preparation of BLSS. Total demand will be of the order of 26,946 t. Assuming a bulk density of R0 regolith is equal to 1.8 g/cm³, the average length of the excavated pit will be 49.9 m.

By analogy, the dimensions of the pit excavated during the exploitation stage (from T1 to T2) can be computed. It is assumed that the height and width of the pit will stay constant. Therefore, the annual demand for R1 regolith created by R2 regolith production and crew life support is of the order of 85,632 t, which corresponds to a pit of length 158.6 m.

The pit's shape, especially its slope angle, holds both economic and safety implications. Steeper walls mean less overburden removal, offering economic advantages in mining operations. However, the stability of these walls is governed by the balance between the shear stress, induced by the mass of the regolith in the wall M multiplied by lunar gravity g_l, and the regolith's shear strength. For a stable pit wall, the shear stress τ over every shear surface A must be lower than the regolith's shear strength τ_max: (2) τ=M⋅glA≤τmax \tau = {{M \cdot {g_l}} \over A} \le {\tau _{\max }}

On Earth, the balance between cost-efficiency and safety results in a recommended maximum slope angle of 60° (Pothinos, 2007). The Moon's unique conditions, with its lower gravity and the mechanical properties of lunar regolith (notably, its higher internal friction angle compared to Earth's soils), suggest the potential for steeper slopes. While these factors indicate an economically favorable approach with even steeper pit walls, safety remains paramount. Given the significant risks associated with wall failure and the need to maintain the shear force below the regolith's shear strength, a slope angle of 60° is retained as a conservative, yet economically mindful limit.

The typical height of the bench in open pit mines on Earth is 30 m (SRK Consulting, 2014); therefore, the walls of the proposed pit will be continuous (single segmented).

The elevation rate of the haul road is called the grade and is given as a percentage. On Earth, the maximum recommended grade for longer distances is 10% (Tannant, 2001), increasing to 20% for short distances. Based on these figures, and a mine depth of 10 m, a steeper road path can be envisaged. Therefore, the road elevation distance will be 50 m, divided into two sections, each measuring about 25 m, clustered on the narrower side of the pit (Fig. 5).

Figure 5.

The proposed architecture assumes that the mine operates in a periodic mode and is equipped with a semi-autonomous excavator. Such a solution is suitable for a low-output site, and the required high level of autonomy is expected to be feasible within the next few years (Zhao, 2020).

The excavated regolith must be first transported, before proceeding with beneficiation. This section is correlated to processes shown in Fig. 3: excavation (I) and magnetic separation (II).

The lunar environment, with its reduced gravity, lack of atmosphere, and extreme temperature variations, presents a unique set of challenges for transportation and excavation activities. These factors play a significant role in influencing the design, operation, and efficiency of transportation systems and excavation equipment on the Moon.

The primary task in the mining process is excavation of R1 regolith. Excavation activities will be carried out using an autonomous excavator, suitable for the low-output site. The excavator will be equipped with tools and systems tailored for the lunar regolith's consistency and the specific requirements of the mining site. The high level of autonomy is expected to be achievable in the near future, ensuring efficient and consistent excavation processes.

The excavated material then needs to be transported to the processing zone located within the habitat area. For this purpose, an autonomous haul truck, specifically designed for lunar conditions, is proposed. This truck can be remotely supervised from the habitat or even from Earth. Special attention will be given to the truck's design to minimize dust generation and dispersion, as lunar dust can be abrasive and harmful to both equipment and astronauts.

The estimated daily demand for R1 regolith during the exploitation stage is around 234 t (74 t during the construction stage). While trucks on Earth of medium size can carry a 25-t load, lunar trucks might be designed to carry heavier loads due to the Moon's lower gravity. However, for the sake of safety and reliability, especially in the initial stages of lunar mining, it is proposed to use trucks with a similar capacity to that of those on Earth. This would mean that approximately 10 trips would be required daily during the exploitation stage, which is feasible for a single truck.

To increase efficiency of the mine and ensure downtime of transport machinery, reinforced roads are to be built. Process compaction (Fig. 3) is the main focus point.

The construction of roads on the Moon is crucial for efficient transportation and to minimize the wear and tear on vehicles. In the proposed design, reinforced roads will be established between the mine pit and the habitation zone. The total distance to cover is 600 m.

The road's width, determined by the width of the haul trucks, is set at 6 m. This width ensures safe passage for the trucks and provides a margin for any unforeseen obstacles or deviations. The depth of the road is set at 0.5 m, providing a stable base for the heavy loads transported.

Given the absence of water on the Moon, traditional road construction methods are not applicable. Instead, the road will be constructed using R3 regolith mixed with a binder. This binder could be a type of polymer or adhesive manufactured on the Moon or brought from Earth. The mixture, in a 50/50 ratio, ensures the road's stability and durability. The construction process would involve compacting layers of the regolith–binder mixture, creating a solid and long-lasting surface.

The equation (3) mR3⋅αcp1+mbinder⋅αcp2=mroad⋅αcp3 {m_{R3}} \cdot {\alpha _{cp1}} + {m_{{\rm{binder}}}} \cdot {\alpha _{cp2}} = {m_{{\rm{road}}}} \cdot {\alpha _{cp3}} describes the relationship among the masses of R3 regolith, the binder, and the resulting road. This equation will be crucial in determining the exact quantities of materials required for road construction.

In conclusion, the establishment of a lunar mine requires careful planning and consideration of the unique challenges presented by the lunar environment. The transportation and road systems play a vital role in ensuring the efficient and safe operation of the mine. With advancements in technology and a better understanding of the Moon's geology, it is anticipated that lunar mining operations will become a reality in the near future.

This section encapsulates all activities related to building and sustaining a greenhouse. The main involved processes are fertilization, cultivation, and PBR (Fig. 3).

The greenhouse must be able to provide the minimum amount of calories consumed by the two astronauts, as described in Section 2.2.4.3. In the case under consideration, we assume that half of the astronauts’ diet will be brought from Earth, while the rest will be produced in situ.

Two aspects of soya production are described: fertilization and cultivation. The details are included in Appendix. Information related to microalgae is described in the PBR section and the details are also included in Appendix.

To ensure sustainability of the mine for the crew, and production of O₂, this section describes Electrolysis, Life Support, and Waste Utilization from Fig. 3.

BLSS will be based on chemical and biological processes. Some O₂ (0.5 t) will be created during photosynthesis in PBR. The remaining amount will be created during electrolysis (0.25 t). The crew’s total H₂O requirement, 4 t, is met by H reduction. The amount of food needed to meet the nutritional needs of the astronauts will be provided by the cultivation of soybeans (70 kg year⁻¹) and microalgae (0.26 kg year⁻¹).

Between T0 and T2, the O₂ demand is 0.75 t, H₂O demand is 4 t, and food demand is 70 kg (soybeans) and 260 kg (microalgae).

In sum, the process can be described as the following equation: (7) αls1⋅mO2+αls2⋅mH2O+αls3⋅mfood+αls4⋅mmicroalgae =αls5⋅mCO2+αls6⋅mwastewater \matrix{ {{\alpha _{ls1}} \cdot {m_{{{\rm{O}}_2}}} + {\alpha _{ls2}} \cdot {m_{{{\rm{H}}_2}{\rm{O}}}} + {\alpha _{ls3}} \cdot {m_{{\rm{food}}}} + {\alpha _{ls4}} \cdot {m_{{\rm{microalgae}}}}} \hfill \cr {\;\;\;\;\;\;\;\;\;\;\;\;\; = {\alpha _{ls5}} \cdot {m_{C{O_2}}} + {\alpha _{ls6}} \cdot {m_{{\rm{wastewater}}}}} \hfill \cr }

Waste reuse is well described in the literature. For example, Ewert and Stromgren (2019) reported than an adult astronaut weighing 82 kg produces 1.08 kg of CO₂ per day. From this, it follows that a crew of two will produce 0.8 t per year. This CO₂ will be supplied to PRB and the soybean crop to support photosynthesis. Human metabolic processes produce approximately 4.5 kg d⁻¹ of wastewater (Ewert and Stromgren, 2019). Hence, over a period of 1 year, with a two-person crew, almost 4 t of wastewater will be produced. This will be used as a nutrient source for microalgae culture (Acién et al., 2016).

Between T0 and T2, the CO₂ demand is 0.73 t and wastewater demand is 4 t.

In sum, the process can be described as the following equation: (8) αwu1⋅mCO2+αwu2⋅mwastewater=αwu3⋅mO2+αwu4⋅mH2O {\alpha _{wu1}} \cdot {m_{{\rm{C}}{{\rm{O}}_2}}} + {\alpha _{wu2}} \cdot {m_{{\rm{wastewater}}}} = {\alpha _{wu3}} \cdot {m_{{{\rm{O}}_2}}} + {\alpha _{wu4}} \cdot {m_{{{\rm{H}}_2}{\rm{O}}}}

To ensure the possibility of a continuous mine operation, as well as storage of essential materials, the need of a storage system was felt. This section deals with storage of most critical substrates and products (Fig. 3).

Ore storage (regolith R1–R5) can be resolved with a basic heap architecture, as the material does not need any sophisticated methods to shield it from the environment. Creating a heap has the significant benefit of being easily accessible from all sides. Therefore, further transport to and from a reactor can be addressed by simpler solutions, namely, a bucket transporter.

However, the storage of liquid O₂ or H comes with multiple challenges. Two main methods are considered: actively controlled and passively pressurized tanks.

Quantities of substrates, products, and reactants are summarized in the four tables presented below. Table 1 shows the demand for regolith in different states for the selected processes. Table 2 shows the demand for reactants in different states to carry out the selected processes. Similarly, Tables 3 and 4 present the products (outputs) of the selected process. In all tables, empty rows are omitted.

Data given in Tables 1–4 are based on the descriptions and equations given in Section 3. The numerical values of coefficients presented in this section are given in Table 5.