**2.1 Engineering problems: the main aspects to consider**

Four main groups of engineering problems have to be considered in habitat construction on the Moon: robustness, feasibility, sustainability and human factors.

Robustness is concerned with the habitat's resistance to structural, thermal and vibro-acoustic loads, meteoroid shocks, and radiation protection. As any house, a habitat has to bear all the loads, some of them present continuously such as the static structural loads, and others appearing occasionally as for example the vibrations from a nearby launch. Meteoroid population around the Moon follows a power law size distribution with small impactors dominating the representation. Traveling at speeds of 3–70 km/s [29], most micrometeoroids are 30–150 μm in size [30]. It has been found that micrometeoroids generally leave impacts of the same order of magnitude as their own sizes [31]. The accumulation of impact craters over time will result in a local density change of the outer shell of the habitat which may affect the mechanical resistance, thermal insulation and radiation protection effectiveness of the structure. Areal density is the most important feature in radioprotective effectiveness of a chosen material. Since all of the main structural stressors will affect the different protective properties of the structure to a greater or lesser extent, they should be considered in parallel when sizing the habitat.

Feasibility considers the technological readiness of the techniques implied in construction as well as cost and power effectiveness of the proposed methods. The mean Technological Readiness Level (TRL) of ISRU technologies reported in the 2021 *In-Situ Resource Utilization Gap Assessment Report* [9] is 3 and the highest TRL is 6 (out of 9). However, these are reported for various uses of regolith to support the human and robotic exploration of the Moon. The TRL of regolith utilization for habitat construction is hard to estimate as only small-scale prototypes of building blocks and techniques have been demonstrated with technologies plausible for lunar utilization [9, 32]. To choose among available ISRU technologies, cost effectiveness and power budget will have to be considered.

Sustainability guides the choice of materials, technologies and techniques in order to ensure a power budget-effective and scalable development and operations of the systems. Maximizing the utilization of local resources is key in achieving sustainable development on the Moon. Regolith will be the main material not only to build but also to operate and maintain facilities. For radiation protection, the degradation of the protective shell over time has to be considered and supported with timely counter-measures. The most important aspect to consider is maintaining the areal density in habitat walls over the years, possibly decades, of exploration.

Human factors regroup such aspects as the crew's mobility, well-being and safety. Surface exploration and accessibility as well as emergency shelters and escape routes have to be considered. Mundane questions such as storage become strategic engineering decisions as storing certain products can locally enhance radiation protection. The choice of the main carrying materials will be mainly guided by their mechanical properties; however the esthetic appreciation is an important factor in habitat design and should not be neglected as supplementary materials will also affect the radioprotective properties of the habitat. An important element among human factors is the visual reference system. Windows are essential in ordinary life, and observations demonstrate how the presence of windows improves human well-being [33, 34]. The fact that astronauts spend a lot of their free time in the Cupola of the International Space Station (ISS) is a clear manifest to that [35].

From a structural point of view, windows are essentially holes that, strictly engineeringly speaking, the structure would be better off without. A window stimulates local concentration of stresses which typically lead to the need of reinforcement. Radiation on the Moon adds another layer to the question of windows: what materials should be used, and how they will affect the radioprotective effectiveness of the habitat.

When the case of lunar lava tubes is considered against the main engineering problems, they evidently score high on feasibility since little preparation is required to use them. However, feasibility is complicated by the need to provide all life support and infrastructure under the ground, possibly extending many meters for ensuring safety. The main consideration regarding robustness is the potential danger of a tube falling in on itself—either upon a meteoroid impact or vibrational excitation (e.g. from a nearby landing/launch). The main show-stopper for lava tubes utilization is surface access and human factors. Humanity seeks to explore the Moon; therefore long surface expeditions are desired. With lava tubes as habitats, astronauts will have to spend a significant amount of time and energy climbing out of their homes onto the surface. For longer expeditions, a surface solar storm shelter must be envisioned to provide immediate protection. In this case, double infrastructure is required, both underground and on the surface, which will largely increase mission's costs and complexity. Most importantly, the psycho-physical effects of living underground on the Moon with no visual reference system, access to natural light or a view of the Earth must be considered. A French "Deep Time" 2021 study [36] has investigated the effects of living in similar conditions on Earth for 40 days; however the lunar case is distinct and more complex due to high levels of stress and alienation which are a part of astronaut life in space.

Most of the engineering problems can be partially answered with regolith utilization. Nevertheless, some additional materials seem inevitable and even desirable—to compensate for certain peculiar behaviors of regolith, thus optimizing material choices for habitat construction.

#### **2.2 Regolith problems: the peculiar behavior of a special material**

The lunar soil has been unprotected and constantly bombarded by meteoroids and radiation for several billion years. Such space weathering effects led the material to be crushed and mixed. Particles range in size from a few μm up to a couple of 100 μm, and differ largely in shapes. A distinct property of regolith grains is their extreme adherence and sharpness. These characteristics make regolith uniquely difficult to operate in an effective and safe way. Grains interlock among each other and stick to materials that they come in contact with. They are extremely light, as the average density of a grain is about 3.0 g/cm3 and most particles measure only a few μm.

A particularly peculiar behavior of regolith on the lunar surface is levitation. Previously considered as the result of meteoritic impacts, particle levitation has recently been tied to the charge buildup from exposure to protons [8]. The difference in charge from the side exposed directly to the solar wind and the side away from the Sun causes charged regolith particles to levitate in attempt to cross the line of difference. This line is the place of the switch between the lunar day and night.

#### **2.3 Radiation problems: the duel of ionizing radiation and radiobiology**

In the context of lunar settlement or long-duration missions, astronauts will experience continuous low dose exposure. This type of exposure is higher than

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

that on Earth, which is protected by its magnetosphere and atmosphere, yet it is significantly lower than the single doses delivered as part of radiotherapy. However, some of the radiobiological effects and mechanisms are the same in both cases. Historically, the space sector has been borrowing the findings from radio therapeutic treatments and radiobiology to calculate mission health risks. But space radiation poses important scientific questions about the effects of low doses on cellular and organ levels which can be useful in radio diagnostics and the case of repeated doses.

The so-called absorbed dose, often simply called *dose*, is a measure of energy deposition of a particle in a target material, which is the human tissue in this case. There are several methods to go from energy deposition to the notion of dose which takes into account the biological effects and the harm to organs and the body. Calculating such doses helps to quantify the harmfulness of exposure and crosscompare protective solutions. Based on doses and the associated health risks, which largely depend on the medical history of a person and can be outlined as acute (e.g. nausea) and cumulative short- (e.g. cataracts) and long-term ones (e.g. nervous system function degradation, carcinogenesis), the total mission risks are estimated for astronauts. NASA proposed a model of risk of exposure-induced death (REID) which calculates the risk of death from cancer depending on the age, sex and previous exposure of the astronaut. REID has a hard limit of 3% which means that the total career lifetime exposure should not lead to an increase in the probability of mortality from cancer higher than 3%.

To determine whether a mission is acceptable in terms of radiation exposure, national space agencies set certain exposure limits. As such, there is a short-term limit on 30 day exposure and a career limit, which varies slightly from one agency to another and is also defined by gender in some cases. The former is set by NASA to 250 mSv [37] and the latter averages at 1 Sv across agencies [38]. There is also a specific limit on the exposure to blood-forming organs (BFO). The limit for short-term non-cancer effects is 250 mGy-Eq [37]. Radiation protection solutions must respect these limits and even go above and beyond in looking for dose reduction methods. That is the existing working principle in the context of lunar exploration and settlement, and the global space community is currently putting efforts together to establish specific exploration-type mission limits for joint space activities [38].
