Regolith and Radiation: The Cosmic Battle

*Yulia Akisheva, Yves Gourinat, Nicolas Foray and Aidan Cowley*

## **Abstract**

This chapter discusses regolith utilization in habitat construction mainly from the point of view of radiation protection of humans on missions of long duration. It also considers other key properties such as structural robustness, thermal insulation, and micrometeoroid protection that all have to be considered in parallel when proposing regolith-based solutions. The biological hazards of radiation exposure on the Moon are presented and put in the context of lunar exploration-type missions and current astronaut career dose limits. These factors guide the research in radiation protection done with lunar regolith simulants, which are used in research and development activities on Earth due to the reduced accessibility of returned lunar samples. The ways in which regolith can be used in construction influence its protective properties. Areal density, which plays a key role in the radiation shielding capacity of a given material, can be optimized through different regolith processing techniques. At the same time, density will also affect other important properties of the construction, e.g. thermal insulation. A comprehensive picture of regolith utilization in habitat walls is drawn for the reader to understand the main aspects that are considered in habitat design and construction while maintaining the main focus on radiation protection.

**Keywords:** habitat construction, lunar regolith, regolith simulants, ionizing radiation, space radiobiology, radiation protection

## **1. Introduction**

Living on the lunar surface will undoubtedly be a psycho-physical, technological and economical challenge. The main source of protection and support for astronauts will be their habitat. Its construction and design has to offer a counter measure against every stressor exposed onto the crew. While a habitat may be perceived as something static and frozen in the cold of lunar vacuum, it will in fact, in itself become the place of an active battlefield—the battle between radiation and matter, where the health and well-being of the people inside is at stake.

This chapter discusses the utilization of regolith in habitats. Regolith is a local source available in abundance on the lunar surface, which can be relatively easily accessed and collected. Its utilization enables a more sustainable exploration and future settlement. It also reduces the cost of a mission dramatically. However, regolith is a complex material with unique properties that result from space weathering (temperature extremes under vacuum, radiation exposure, micrometeoroid

impacts), and the techniques of its utilization and associated technologies are under development and improvement across the global space community. To complicate things further, there is a limited amount of returned lunar samples. In order to satisfy the needs in experimentation, testing and prototyping with regolith, diverse simulants are used. Simulants are specifically designed to resemble the lunar soil in its chemical, mechanical, and thermal properties. Depending on the application, some simulants are perfect replacements of regolith for research and development activities.

The main case under consideration here is regolith for radiation protection of humans. When radiation interacts with matter, it deposits a part of its energy in the target material, produces fragments of nuclei and other secondary emissions. It is important to know how effective regolith is in terms of radiation absorption or attenuation on the one hand, and what kind of secondary particles it will produce on the other. The fact that the radiation environment on the Moon is a diverse mix of particles with different energies and charges makes it complicated to optimize the utilization of regolith for dose reduction. The notion of doses is used to estimate exposure and associated risks. It is always advised to keep the risks and doses to the absolute minimum that is technologically achievable and ethically acceptable. When seeking to reduce doses in space radiation protection, we consider both the doses from primary particles and secondary emissions. In both cases regolith will act as a passive shield, and its constituent molecules will interact with radiations in their unique ways which depend on the mutual chemistry of the projectile-target pair, charge and energy of the incident particle.

As regolith will be the main construction material, it will largely define the thermo-mechanical properties of the habitat wall. It is important to look at the different protective properties in parallel and not dissociate their studies too much. For example, density is crucial for both radiation protection and thermal insulation. A holistic approach to habitat building is discussed here, while keeping the main focus on radioprotection.

The rest of the chapter will introduce lunar habitats, regolith and radiation as the main actors of the cosmic battle. Then, it will outline the problem statement underlining the particular challenges associated with habitat construction on the Moon, regolith utilization, and radiation protection. To fight the problems, the existing armor will be presented. In-situ resource utilization (ISRU) technologies, regolith simulants, and radioprotection techniques will be outlined and discussed. Any good soldier is always on the lookout for more troubles and better solutions. In the context of the cosmic battle it means to be on the lookout for improving ISRU technologies, bettering regolith simulants, and investigating the use and properties of new materials that can either be brought from Earth or made in-situ. A generic conclusion summarizes the main points regarding regolith utilization in habitat construction, mainly from the point of view of radiation protection of astronauts.

#### **1.1 Habitats for long-term exploration**

Continuous human presence and surface exploration of the Moon sets an overarching requirement on the lunar habitat that it must sustain human life for several long-term missions and withstand a harsh environment. In other words, the habitat becomes a fortress under a continuous and variable siege of the cosmic and solar radiation, extreme temperatures, and micrometeoroid bombings.

On top of robustness, the habitat must present a comfortable alternative to living on Earth. Working on the Moon for extended periods of time will be extremely challenging, stressful and may even become alienating and daunting. The least that can be done to counteract the psychological burden and physical exhaustion is that

### *Regolith and Radiation: The Cosmic Battle DOI: http://dx.doi.org/10.5772/intechopen.101437*

the well-being and comfort of astronauts becomes another top-level requirement in habitat construction.

Since the very first steps on the Moon, humanity has been envisioning a longterm presence or even a permanent settlement there. In the most recent years, the global space community focuses primarily on the cislunar space the access to which will enable frequent missions to the surface, ultimately making preparations for the Moon Village [1]. The global exploration roadmap suggests that such efforts should be made in a sustainable way [2]. This leads to the choice of using local materials in habitat construction, and in fact, maximizing their utilization both in hardware and life support.

The most straightforward way to use regolith is to cover a primary structure with it. The primary structure may be brought from Earth, e.g. inflatable or origami-inspired unfolding structure, a metallic cylinder, or even a repurposed part of a spacecraft. **Figure 1** illustrates an artistic view of what such regolith-covered habitats and storage facilities could look like. The authors interpret the image as a capture of the evolution in maturity of ISRU-technology on the Moon. It could be argued that the very first habitats will resemble the one encircled and marked by letter A (in red) since regolith seems to be either loosely piled on top of the structure or compressed and reinforced with dense tiles, which could be produced either through sintering or 3D-printing. Such an approach is feasible at the early stages of exploration. Increasing in complexity, the habitat/storage unit of type B seems to be entirely produced by additive manufacturing. The dark color could be an indicator of another material present in the mixture, e.g. a binder. The surface seems to be rough, possibly owning to the chosen 3D-printing technique which had not yet been thoroughly explored in lunar conditions. Habitat C seems to use more regolith in the material mixture, and it is also produced by additive manufacturing. The triangular and conical shapes observed on the outer layer (both in types B and C) can present a significant advantage in thermal properties of the wall due to partial shadowing this could help withstand the harsh temperature of the lunar day, which reaches up to 120°C at the equator where the solar heat flux reaches 1300 W/m2 . Finally, the image depicts how the multilayer technology can be utilized with regolith, as the underlying shelter is being covered with another layer of regolith-rich material, seemingly by 3D-printing.

Another straightforward way to benefit from regolith protection is to seek shelter underground. Lava tubes have long been studied as an alternative to living on the Moon, e.g. [3–5]. They extend meters underground and offer a natural

protection from radiation and micrometeoroids. Most commonly, it is considered that a habitable structure would either be inflated or mounted inside a lava tube. Although it may seem rather convenient and even poetic for the first settlements on the Moon to use the equivalent of caves on Earth, and despite the fact that lava tubes can provide substantial radiation protection (see Section 3.3), this solution has some important limitations which will be outlined in Section 2.1. A surface habitat is considered as the main option for living on the Moon in this chapter.

### **1.2 Regolith**

Committing to a sustainable long-term exploration implies one key material choice—regolith. Abundant on the surface, it will serve as the main force to fight back the cosmic oppressors on the Moon. Regolith, or the lunar soil, will make up the bulk of habitat walls and thus, will act as a shield against incoming radiation particles, heat, and meteoroid projectiles.

*Regolith* collectively refers to the megaregolith crust consisting of boulders, large particles, grains and powder, or dust. Lunar observations and sample return from the Apollo and Luna missions have resulted in an extensive knowledge of the bulk regolith properties and deciphering some of the history of lunar geology.

Regolith is a complex material. It consists of a mixture of crystalline rock fragments, minerals, breccias, agglutinates, and glasses [6]. Chemical composition of the lunar soil has been thoroughly studied. For radiation protection, it is the most important property as the mutual chemistry of the radiation-matter pair will define the nature of their interactions, and the results in secondary emissions and doses. Two types of regolith are distinguished: mare and highlands, and both are mixes of metallic oxides, dominated by silicon dioxide up to 42–45% in weight [7]. The composition then varies slightly, namely highlands regolith contains more aluminum oxide than the mare type (approximately 25% and 13% respectively [7]). Mare regions contain high levels of titanium dioxide—between 2% and 10% versus the average of 0.5% in highlands soils [7]. It is approximated that the top 30 cm consist of the lunar dust—particles smaller than 100 μm in size with the bulk density of 1.5 g/cm3 [8]. These loose grains are accessible for collection and utilization in habitat construction. On the Moon, this will make up the majority of ISRU activities.

Currently, the global space sector is investing into its capacity-building related to ISRU technologies [9]. Regolith utilization ranges in ideas from piling-up to sintering, binding with adhesives and 3D-printing. In order to investigate the properties and behavior of raw materials as well as processed products (e.g. regolith bricks), numerical simulations and experiments are carried out. Simulations mainly concern the thermo-mechanical behavior of bulky solids, e.g. how regolith flows and what thermal insulation properties it has. Experiments are usually set up to verify predictions and observe behavior. Humanity currently possesses 382 kg (Apollo program) [10] and 321 g (Luna missions) [8] of lunar regolith from sample return missions, which manifest the greatness of the pioneering efforts in space exploration beyond the Low Earth Orbit (LEO). However, these resources cannot nearly satisfy the global scientific interest and technological demonstration needs in preparation of a lunar outpost. The solution is to simulate the material using its earthly counterparts.

Regolith simulants are like siblings—arguably originating from similar material but having different characteristics. This is due to the fact that simulants are often made to serve different scientific and technical purposes. Literature classifies simulants according to their most prominent properties [11–13]. As such, some are best at simulating mechanical behavior of the lunar soil, and others are almost the exact copies in chemical composition as the returned samples. Continuing the sibling

#### *Regolith and Radiation: The Cosmic Battle DOI: http://dx.doi.org/10.5772/intechopen.101437*

analogy, the differences among regolith simulants may be compared to the different talents that siblings have, which often result from parental investment and resource allocation to activities that nourish those talents.

The first step in working with regolith consists in choosing the appropriate regolith simulant. The main objectives of a habitat are to sustain human life and well-being. Protection from radiation becomes the key player in early habitat planning and regolith simulant considerations as it is one of the main oppressors in the lunar environment. Like under any attack, the forces of resistance must be pulled together. In radioprotective terms, passive shielding is a technique of protection when a material stands in-between a radiation source and the target. The forces of resistance are then the material's nature, or its chemical composition, and areal density. The choice of a passive shield will be based on the most probable radiationmatter interactions, and material optimization will seek to reduce the negative effects of radiation exposure on human health. The interactions between radiation particles and materials produce a diverse variety of results, ranging from energy deposition to nuclear fragmentation and DNA break-down, to mention some. The uniqueness of each interaction originates from the incoming particle's energy, charge and mass. Therefore, the specific radiation environment on the Moon presents a particular challenge to be considered in habitat construction.
