## 4 Conclusions  
In this research, an inclusive review was carried out to assess the different structures proposed for outpost construction on celestial bodies, especially on the Moon and Mars. The chronological overview of the proposed structures showcased the evolution of construction techniques and materials. In fact, from the early concepts of inflatable structures and flat trusses to more recent advancements in 3D printing, researchers have continued to explore innovative approaches to ensure the viability and sustainability of future extraterrestrial habitats.  
Based on the conducted investigation, six physics-based challenges were identified as the most critical obstacles to the development of lunar and Martian outposts. These challenges are microgravity, cryogenic temperature, vacuum condition, lack of magnetic field, intense surface radiations, and micrometeoroid impacts. While all of these challenges constrain planetary infrastructure development, the cryogenic temperature seems to be the most restricting factor. Cryogenic temperatures induce thermal stresses on the habitat'sstructural elements, thereby leading to potential fatigue and failure in the building modules. This limitation also precludes various materials from being applied in the construction process. Moreover, cryogenic temperatures lead to the freezing or evaporation of any fluid, such as the binder liquid utilized in 3D printing processes. Additionally, a vast amount of energy is required to keep the habitat warm for astronauts and habitants. All these issues require substantial energy consumption and material transportation costs from the Earth to space. Hence, for future works, further investigating the impact of cryogenic temperature on the manufacturing process of 3D printing operations is recommended.  
Overall, the utilization of regolith and the advancement of 3D printing technology have opened up new possibilities for constructing outposts on Moon, Mars, and other extraterrestrial environments. These techniques offer potential energy savings by utilizing local resources. It can be stated that current 3D printing technology has the capability to create early structures with the necessary strength and durability for long-term human habitation. However, to extend habitats for a larger number of inhabitants, fully ISRU operations with more powerful rovers and 3D printing machines are required.  
It was also concluded that a safe, reliable outpost is constructible if the 3D-printed components are utilized in conjunction with an internal breathable inflatable module. In this case, more than 60% of the total interplanetary transportation cost will be related to binder liquid transport. Hence, investigating binders that can be produced on site is recommended. If this is carried out, a dramatic saving in energy and costs can be acquired. To summarize, although current 3D printing technology seems to be robust for developing off-Earth outposts, there is still a need for more investigation with respect to its progression, leading to saving more transportation costs and energy.  
Conceptualization, data curation, writing--review and editing, writing--original draft preparation, visualization, and methodology: M.K. and D.K.; formal analysis, software, investigation, and resources: M.K.; validation, supervision, and project Administration: D.K. All authors have read and agreed to the published version of the manuscript.  
This project was supported by the AGH University of Krakow, Poland, subsidy 16.16.190.779.  
Not applicable.  
The authors would like to thank [PERSON], student at the AGH university of Krakow, for his support during the conduction of this research.  
The authors declare no conflict of interest.