## 3 Results and Discussion
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 [119, 120, 121], 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 [122, 123]. The surface layer of the Moon and Mars can also be further studied using indirect techniques, such as geophysical methods and distant remote sensing [124, 125]. 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 [20]. 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 [96].