Energy-Saving 3D Printing Techniques for Extraterrestrial Habitation Structures: History
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Contributor:
Mitra Khalilidermani
,
Dariusz Knez

Various space agencies have shown great enthusiasm for constructing habitable structures on lunar and Martian surfaces. It was found that the combination of 3D-printed components along with an internal breathable inflatable module is the most promising technique for habitat development on the Moon and Mars. Moreover, the microwave sintering method was identified as the most energy-saving and reliable approach for melting the on-site regolith for use in the 3D printing process. 

  • 3D printing
  • regolith
  • Martian outpost

1. Introduction

The Earth’s dwindling resources, together with climate change issues, have spread the notion of space civilizations. Future space habitation is envisioned to be the circumvention of mankind’s sustainability in the solar system. This is why large space agencies in the US and European Union are presently tracing the construction of permanent extraterrestrial habitats on the Moon and Mars [1]. From the safety viewpoint, a long-duration stay that provides immunity in planetary environments needs resilient structures [2], sheltering human inhabitants (or astronauts) so that they can discover the atmosphere, lithosphere, biosphere, and even hydrosphere of their surroundings. On a positive note, recent evidence of water presence on lunar and Martian surfaces has given fresh impetus for rendering off-Earth life a reality [3][4][5].
Currently, there are several space missions dedicated to advancing the concept of planetary habitation. One prominent example is the National Aeronautics and Space Administration (NASA)’s Mars Exploration Program, which includes the ongoing Perseverance rover mission. Perseverance, launched in 2020, aims to explore the geology of Mars, search for signs of ancient microbial life, and collect samples for potential return to Earth [6]. The mission also includes the Ingenuity helicopter, which successfully demonstrated powered flight on another planet. Another notable endeavor is the Artemis program, led by NASA in collaboration with international partners [7]. Artemis seeks to return humans to the Moon and establish sustainable lunar habitats, utilizing the Lunar Gateway as a staging point for crewed missions. Moreover, the Artemis program aims to accomplish the significant milestone of sending the first woman to the Moon. These missions, among others, represent significant milestones in the human journey towards planetary habitation, pushing the boundaries of human knowledge and capabilities in our quest to expand beyond Earth.
From the perspective of engineering challenges, it appears that three main categories of uncertainties have postponed the rapid development of habitable outposts on remote planets: unfavorable extra-terrestrial physics, risky characteristics of the regolith soils, and uncertain construction techniques. In the following paragraphs, these three factors are described.
The potential of habitability on a specific planet is substantially intertwined with its dominating physics rules [8]. Gravity, ambient temperature, atmospheric pressure, magnetic field, surface radiation, and seismic events are the most influential factors with respect to the suitability of human habitation on a specific planet. Such physics-based factors are very mankind-friendly on Earth; however, on distant planets, physics rules can be challenging for human life, either completely or partially. Years of space exploration missions have revealed that two promising planets, the Moon and Mars, may someday be occupied by human generations if physics requirements are addressed by technology. 
Lunar and Martian outposts can be entirely constructed regardless of whether they are on the planet’s surface or underground. Alternatively, the outpost can also be constructed using a combination of both surficial and subsurface modules. The most frequent propositions have been surficial outposts, which are less complex than underground ones. Lunar and Martian surfaces are covered by a fine-grained abrasive soil known as regolith [9]. Some characteristics of the regolith can positively contribute to the development of a prospective human outpost, while at the same time, they may impede the construction process with formidable obstacles. For instance, the regolith’s cohesionlessness facilitates the excavation operation conducted by construction robots, thereby requiring less energy for regolith removal and haulage. On the contrary, lunar and Martian dust storms perpetually disperse such cohesionless regolith particles in the atmosphere, thereby covering the robots’ cameras and lenses and, even worse, penetrating their working units [10]. The latter issue can entirely or partially hamper robots from the construction process.
To revert to underground outposts, it can be said that although the excavation of any foundation, channel, trench, and tunnel can be easily carried out in loose regolith, the instability of the roof of such structures is highly problematic. Most importantly, temperature fluctuations frequently change the physical properties of soils [11][12]. This may lead to potential failure in the underground walls of the outpost.
Extraterrestrial outpost designs were initially proposed three decades ago. Nowak et al. proposed inflatable structures made of lightweight composites to build lunar habitats [13]. Aside from lightness, these inflatable structures could easily be compacted during transportation from the Earth to the Moon. Moreover, the outpost’s erection could be carried out using low amounts of energy, and this is carried out by construction robots. Benaroya and Ettouney suggested the utilization of a 3D flat truss for the development of lunar bases [14]. The members of the truss were made of light aluminum. Furthermore, the truss was installed on a naturally available lunar valley. To protect the outpost from the outer harsh environmental conditions, the roof of the truss was covered by the local regolith. Shortly thereafter, Benaroya put forward the idea of using tensegrity assemblies for space habitats [15]. Tensegrity structures includes a series of interlocking cables and bars capable of forming an intertwined structure.
In the middle of the 2000s, the concept of deployable modules was suggested for the establishment of extraterrestrial outposts [16]. The deployable structures encompass size-variable components, which can be stowed during Earth-to-space transportation, and then, they can be expanded to a desirable size and shape. For example, two typical deployable structures include the umbrella and TV antenna. In the same years, the idea of arch structures was also offered by [17][18]. In these propositions, the main structural module was a semicircular arch made of light aluminum together with a regolith-based roof for sheltering human habitants against hostile environmental conditions (Figure 1).
Figure 1. A schematic of arch structure protected by the regolith shield layer.

2. Energy-Saving 3D Printing Techniques for Extraterrestrial Habitation Structures

Amongst physics-based challenges, the cryogenic temperature appears to be the most pressing problem for the development of extraterrestrial outposts. For instance, regarding the location of the outpost on the lunar surface, it may be said that the Shackleton crater, situated in the southern pole, is appropriate for outpost construction. In this region, the sun never sets, and it shines at a relative horizontal angle relative to the crater’s surface [19]. Thus, minor solar radiation reaches the outpost. Nevertheless, the main problem will be the cryogenic temperature derived from the polar nature of the region. Therefore, ultra-resistance to cryogenic temperatures must be regarded as a key criterion in the selection of materials and 3D-printed modules.
The regolith found on the Moon and Mars can be applied in both raw and synthesized forms. The raw regolith can be used as a shield layer to protect the outposts from meteoroid impacts, solar radiation [20], and temperature fluctuations. It can also be directly deposited on the outpost and covered with membranes for guaranteeing the stability of the walls. In this case, the geomechanical properties of the regolith are highly influential with respect to the wall’s stability [10][21]. On the other hand, the synthesized regolith can be combined with additives or binders to create new materials with increased compressive strength.
The physical and geotechnical properties of the raw regolith are also very seminal in the 3D printing process. During the manufacturing process, the regolith must be collected by rovers and is fed into the printing machine; hence, the subsequent factors must be considered. Firstly, collecting surficial lunar regolith needs more robotic energy than Martian regolith. In fact, from the Moon’s surface level to the depth of 30 cm, the regolith’s bulk density increases from 1.30 g/cm3 to 1.69 g/cm3 [22][23]. By contrast, on Mars, the regolith’s bulk density builds up from 1.30 g/cm3 (at surface) to 1.34 g/cm3 (at depth = 30 cm) [24]. And more than this, the geotechnical properties of the Martian regolith are much closer to the lunar mare regions rather than the lunar highlands. The reason is that in a similar way to the Martian environment, the lunar mare regions have been impacted less by the meteoroids compared to the lunar highlands. Thus, the construction of a prospective outpost in mare regions requires less robotic energy as the regolith’s bulk density is closer to the Martian regolith. The second consideration is related to the slope stability of the regolith’s slopes; as the gravity of the Moon is less than Mars, the regolith’s slopes remain stable, with sharper angles between their toe and the horizon. This matter also has a direct impact on the thickness (and volume) of the regolith required to be deposited on the outpost. The third consideration is pertinent to the effect of temperature fluctuations on the water volume contained in the regolith. The stress cycles derived from temperature fluctuations perpetually alter the physico-mechanical characteristics of soils [25][26][27]. Since hydro-mechanical coupling between the solid skeleton of the regolith and its frozen water continually shifts [28], the effect of temperature on poroelastic parameters must be also taken into account.
The inflatable modules are the most appropriate options for providing the breathable environment inside the habitat while 3D printing technology can be used for manufacturing the outer building components. The inflatable modules provide numerous benefits in the construction process. First of all, they can be easily designed and manufactured in various structural forms, including stowed, telescoping, cylindrical, hemispherical, etc. The second advantage is their high strength in tension conditions since the inner pressure inside the inflatable module induces large tensile stresses within the structure. The third benefit is their light weight and low occupied volume, which lead to lower transportation costs, lower energy consumption by construction robots/rovers, and increased execution flexibility. Last but not least, the inflatable structures prevent breathable air from leakage. This characteristic almost renders the inflatable structures indispensable in any future outpost structure combined with 3D-printed components.
Using 3D printing technology, the creation of large human outposts on lunar and Martian surfaces seems to be much more conceivable as it does not need a substantial transportation of raw materials to space. However, in lunar and Martian settings, inorganic binders are not available and must be taken from the Earth to space. This imposes substantial monetary and energy costs on the project. Some investigations have revealed that the Martian regolith possesses clay and gypsum minerals [29][30][31], and in the case of water extraction on the Martian surface, new materials, which function in a similar way to Earth-based concrete/cement, can be produced. Hence, further exploration operations, such as exploratory boreholes and sample collection from the lunar/Martian surface, are suggested to realize a fully ISRU outpost construction. To study the deeper layers of regolith, the implementation of drilling operations by rovers are inevitable [32][33]. The surface layer of the Moon and Mars can also be further studied using indirect techniques, such as geophysical methods and distant remote sensing [34][35].
The choice of a suitable 3D printing method hinges on three pivotal factors: firstly, the environmental condition; secondly, the expenses related to providing raw materials; and thirdly, the energy requirements essential for executing the project within the extraterrestrial setting. However, it is noteworthy to mention that all these three factors are intertwined.
Based on the conducted research, the authors posit that microgravity and cryogenic temperature are the most critical environmental challenges for the 3D printing process on remote planets. Due to microgravity, hauling regolith to feed the 3D printing machine is a difficult task. Additionally, the regolith may not be effectively consolidated and prepared for the sintering process. The less regolith densification there is, the more energy and time needed for the sintering and printing process. Regarding the cryogenic temperature of lunar and Martian environments, it can be said that this issue especially affects the binders’ state. In other words, the cryogenic temperature makes the binder freeze or evaporate, thereby hindering the 3D printing process. Furthermore, the cryogenic temperature restricts the range of Earth-based materials that can be used in extraterrestrial habitat construction. Apart from microgravity and cryogenic temperature, other environmental challenges, such as low atmospheric pressure, lack of a magnetic field, surface radiation, and micrometeoroid impacts, are important. However, based on previous investigations, these issues can be solved by taking specific actions [19].
The energy efficiency of 3D printing processes principally relies on manufacturing technology, required raw materials, regolith sintering method, and dominant environmental challenges. Manufacturing technology determines the required raw materials; the complexity of the 3D printing process; and the size, weight, and number of printing machines. Moreover, the regolith sintering method is an essential factor in achieving a successful, energy-saving 3D printing process. The microwave sintering method requires 33–44 times less energy than the laser sintering method [36]. It is also more efficient than the solar sintering method, which depends on the outpost’s location, regolith optical properties, the position maintenance of the focal spots, and dust removal from the printing machines’ lenses [36]. Hence, a precise trade-off between the available manufacturing technology and regolith sintering methods must be realized to select the reliable 3D printing technique with optimal energy and time required for the fabrication process.
To test the reliability of extra-terrestrial outposts, as well as the experimental and numerical investigations on meteoroid impacts [37], evaluating the outpost’s response to the moonquakes and marsquakes is also recommended. Moonquakes and marsquakes can have significant implications for lunar and Martian outposts [38][39]. Understanding the frequency, intensity, and patterns of these seismic activities is crucial for designing any robust and resilient infrastructure. Moonquakes, although generally mild, can still pose a risk to surface structures and equipment [38]. Lunar habitats and resource extraction facilities must be engineered to withstand the occasional shaking and vibrations. Similarly, Martian outposts need to account for the potential impact of marsquakes. These seismic events can vary in magnitude and duration, potentially affecting the stability of structures, underground habitats, and life support systems [39]. Engineers and designers working on Martian outposts must develop seismic-resistant technologies and construction methods to ensure the safety and longevity of habitats and infrastructure.

This entry is adapted from the peer-reviewed paper 10.3390/app132312913

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