## 2 Materials and Methods
### Outpost Construction Techniques
Figure 4: An inflatable module tested by ESA for extraterrestrial outpost construction [80]. Figure 5: A lunar/Martian outpost made of inflatable structures and 3D-printed protective layers (**left**) [2]. The schematic representation of the TransHab inflatable module developed by NASA in the late 1990s for astronauts’ accommodation in the International Space Station (**right**) [81]. Also, in 1992, [PERSON] and [PERSON] proposed a lunar outpost composed of a 3D flat truss that could be installed on the top of a natural valley (see Figure 1) [14]. After the installment of the truss, a layer of in situ regolith was then spread on the roof of the truss to shield the habitat from intensive temperature fluctuations, outside radiation, and possible micrometeoroid impacts. During the subsequent years, some researchers suggested similar 3D flat structures for lunar bases [23; 24; 59; 69]. The material of truss members was chiefly aluminum or light composites, such as Kevlar. In practice, it can be said that the idea of the proposed flat truss gained less attention since other forms of technology emerged in the next two decades. Apart from the 3D truss, [PERSON] also proposed several conceptual layouts of tensegrity structures for lunar outposts [15]. Tensegrity structures encompass a combination of cables and bars within an intertwined net so that the whole structure renders a stable system (see Figure 1). The idea of tensegrity-based structures did not acquire relative attention in subsequent years. From 2006 to 2010, two novel methods, including deployable structures and arch components, were proposed for outpost development on the lunar and Martian surfaces (see Figure 2). Deployable structures encompass size-variable components and, hence, can be compactly stowed during transportation from Earth to space [16]. These structures are presently used in space missions; for instance, the spaceship's antenna and solar panels are types of deployable structures. Nevertheless, only a few engineers have suggested extraterrestrial deployable outposts [6; 17]. [PERSON] et al. conducted an inclusive review on the possible deployable and inflatable structures in extraterrestrial environments [75]. Regarding arch structures, they involved a fabricated semicircular arch made of aluminum, which could provide a shelter for its inhabitants [18; 19]. A protective regolith layer was also suggested to be deposited on the arch to protect it against harsh external environmental conditions. Since a creative utilization of on-site materials significantly cuts project costs [82; 83], some researchers began to investigate the manufacture of arches using lunar/Martian regolith. [PERSON] and [PERSON] stated that sulfur-based concrete made of lunar/Martian regolith can be utilized for arch construction as it exhibits a high extent of strength and durability [68]. To fabricate such sulfur-based concrete, the regolith was heated, and then, it was allowed to be steadily cooled. Furthermore, [PERSON] et al. studied an admixture of lunar regolith simulants with aluminum powder for the fabrication of a regolith-based voussoir dome during a geothermic reaction [76]. Figure 6 shows the different stages of such an outpost. They deduced that the combination of these materials generates high-strength units that make reliable dome-shaped structures on the lunar surface. From 2010 onwards, with the emergence of 3D printing technology, architects contemplated developing 3D-printed structures on lunar and Martian surfaces [84]. The main advantage of this construction technique is the direct utilization of the in situ regolith as feedstock in the construction process [22]. In this technique, the in situ regolith is gradually sintered until its temperature reaches its melting point; then, a binding fluid is added to the mixture to compose a consolidated substance with lower density and higher strength. Since this technique is currently used by NASA and ESA to build extraterrestrial outposts, this review paper dedicates an inclusive section to 3D printing technology.