The international space community is continuously working on new ideas to sustain human exploration goals on the surface of the Moon and beyond. In one recent study on the future of space habitats, the European Space Agency (ESA) partnered with design, engineering, and urban planning firm Skidmore, Owings & Merrill (SOM) to bring multiple sectors together to investigate the possibilities of space architecture.
Cover image: Visualization of SOM and ESA Moon Village Concept Image Credit: © SOM | Slashcube GmbH
The research offered insights into the technical and architectural constraints of building on the Moon, and to realizing humanity’s long-term exploration goals in space. In the partnership between SOM and ESA, the Moon Village concept served as the foundation to take a deeper look into the design and engineering limits of deployable habitats.
The primary habitat module is designed to be manufactured, tested, and launched from Earth with a significant amount of its internal equipment pre-integrated for use. It utilizes a hybrid structural system composed of rigid and flexible elements, and includes several deployable systems, such as an exterior shell, a floor system, and multi-purpose racks that could be configured and secured in a stowed condition during launch. Once on the surface, the habitat would be transferred to the building site already prepared for deployment. When it’s pressurized, a team on Earth could conduct remote testing of the internal equipment for nominal performance. At this point, the crew arrives, performs any pending deployment activities, and occupies the habitat. The interior is designed to remain flexible so that a wide range of mission scenarios can be accommodated.
The habitat addresses functional, environmental, and performance constraints, while also emphasizing human-centered design principles – all of which are characterized in numerous architectural features. A single unit offers a net habitable volume of up to 390 m3 and a net habitable area of up to 104 m2. To maximize the functionality of central spaces, the vertical structure is placed at the perimeter and integrated with windows and secondary mechanical distribution systems. The primary mechanical systems are located within the composite floor assembly, with payload rack units mounted near the center in a stowed configuration; during occupancy, it is displaced to the perimeter walls. The environmental protection system includes a multi-layer assembly with structural mesh directly woven into the mega-columns to increase resistance under tension. This structural mesh supports the internal pressure loads, and the windows along the rigid elements provide visual connectivity and situational awareness for occupants. Although it is an engineering challenge to integrate windows into the envelope, they are essential to the experience of living on the Moon, and to the psychological wellbeing of the crew.
The only space habitat that is currently occupied is the International Space Station (ISS). One of the most iconic elements of the ISS is the largest glass window ever flown in space. The Cupola, which is attached to the Tranquility module, technically known as Node-3, was added to the ISS in 2010. The European-built window assembly is now an integral part of the ISS and provides astronauts with an incredible view of Earth in what would otherwise be a small space filled with equipment and tools. The window on the ISS was originally added to serve as an observation and working node operating the station’s robotic arms, and to take high resolution photographs of the planet below. Today, it maintains its view of Earth while the ISS orbits the planet at 27,580 kilometers per hour, 400 km away.
The windows on the ISS have to withstand the intense constraints of the space environment and the mass limitations of transportation into orbit. They have to simultaneously endure exposure to extreme temperatures, micro meteoroids and orbital debris (MMOD) traveling at high velocities, all while containing the internal atmospheric pressure.
The pressure differentials between the vacuum of space and the human-sustaining habitation likewise had to be considered across all structural elements. Integrating openings into the structure is a complex endeavor, and requires innovative engineering solutions. When introducing windows and glass, it is essential to include structural redundancies, but this also increases the mass. The Cupola was pre-assembled and integrated into Node-3 before being launched with the space shuttle and attached to the ISS.
This real-world precedent for a window in space demonstrates the challenges of integrating glass into orbiting habitats. Designing for surface structures on other planetary surfaces, however, requires an entirely new way of designing and engineering. Together with ESA, SOM investigated how windows could play an important role in the design of an entirely new type of habitat.
When discussing the design of a human habitat for such an extreme environment, it is important to describe the challenges that must be confronted.
The first challenge on the Moon that comes to mind for most people is the decrease in gravity relative to that of Earth’s surface. The Moon is smaller and has a lower overall density than Earth – resulting in a surface gravity that is only 16.5% the strength of Earth’s. Ironically, this does not make construction easier. The loads on any human habitat generated from the difference in pressure between the interior and exterior are about an order of magnitude greater than the weight of the materials necessary to enclose the space. Thus, the major structural problem on the Moon is holding the building together given the pressure loads, and by not holding them up against Earth’s gravity.
The second primary challenge is the lack of atmosphere. With no air in space, the mechanical systems must supply oxygen and recycle carbon dioxide in a closed loop. Additionally, without a thick atmosphere to insulate the moon’s surface, the diurnal temperature variation is also much larger – making temperature-induced stresses a significant problem for the structure and the insulation.
The Moon’s weak magnetic field, combined with the absence of atmosphere, means that the lunar surface has virtually no protection from cosmic radiation. There are three different types of radiation to worry about. First, there is the solar wind, a stream of charged particles that is constantly emitted from the sun. The second is cosmic rays, which are produced by exploding stars in the galaxy. One way settlers on the Moon can be shielded from these first two types of radiation is by covering all habitable spaces with lunar regolith, a powdery material that covers the entire surface of the Moon, or by using advanced materials.
The third source of danger is solar flares, which form when the sun expels large clouds of charged particles. The large radiation emissions of solar flares require a much more substantial shielding. Fortunately, humanity has a fleet of spacecraft that monitor the sun and can provide up to one hour of advanced warning before the flare reaches the Moon. The habitat can therefore include a solar storm shelter accommodating all settlers for the duration of a solar flare.
The final set of challenges are presented by the Moon’s great distance from Earth, and the astronomical cost of delivering so much mass to the surface of the Moon. Every system has to be designed to an extreme level of robustness and be easy to maintain.
The windows on the habitat are integrated into the metallic structural columns. Each measure approximately 200 cm in height and 80 cm in width. Altogether, the 12 windows represent a total area of 10.5 m2 and a mass of approximately 1,640 kg. Their vertical orientation fits within the boundary of the rigid metal frame, but still provides a large viewing area. Each window is built using advanced technologies to defend the sensitive, fused silica glass panes from years of exposure to solar radiation and debris impacts. To minimize exposure, solid metal shutters can be deployed on the outside to protect the glass assembly. During the crew’s resting hours, when no transparency is desired, the shutters can cover the windows and later open during waking hours. Furthermore, light ingress into the module can be controlled with interior sliding shutters, which operate much like the shutters on commercial airplane windows. This practical interior feature is particularly important for the habitats that are closest to the lunar polar sites, where low sun angles are prevalent most of the day and can create glare on the inside.
The windows on the lunar habitat specifically provide the astronauts with a visual connection to their environment. Each window maximizes views out of the different levels of the habitat, while keeping the glass dimensions as minimal as possible. Naturally, the geometry of the windows needs to be articulated in a way that works with the form of the capsule. Unlike windows on Earth, the windows for this space habitat are not primarily designed to illuminate the interior and have no ventilation functionality at all. Whereas glass windows on Earth are typically engineered for expected wind loads, the Moon poses an incredible atmospheric pressure differential between the interior and exterior. This drives the structural glass design. Window sizes are limited by substrate thicknesses for safety and restrictions in weight. For serviceability, the outer and inner sacrificial pane of the multi-layered glass assembly can be replaced if there are any breaks or scratches.
The reference used for the window glass design on the Moon Village habitat is the 4-pane assembly of the Cupola on the ISS. Like the Cupola, the idea includes a multi-layered window system composed of the following elements: 11.4 mm debris pane, 2 x 25 mm pressure pane, and an inner 9.3 mm scratch panel, which together results in 155.7 kg/m2 for the glass assemblies.
All the glass substrates are borosilicate glass, which has a very high thermal resistance and is used extensively in environments with high temperature swings. Its thermal coefficient of expansion is about one-third of that of regular soda lime silica float glass, which is used for most construction projects on Earth. In addition, borosilicate glass has the advantages of increased hardness, strength, and durability, all while being lighter. These characteristics are especially desirable in space architecture, and can help us create a lunar habitat that is not only designed for survival, but also for wellness – a true home in an environment where we have never lived before.
This article was originally published in IGS Magazine’s Summer 2021 Issue: Read the full Magazine here for more thought-leadership from those spearheading the industry
Georgi Petrov is practicing architect and structural engineer. He is an Associate Director in the structures group at the New York office of Skidmore, Owings & Merrill, where he works on highrises, long span roofs, and specialty glass and steel structures in North America, Asia and the Middle East. He is a leader in pioneering SOM’s involvement into design for structures in outer space and in developing novel project delivery methods in close collaboration with fabricators. He is a both licensed Architect and Professional Engineer and an adjunct professor at NYU’s Tandon School of Engineering.
Christoph Timm is the Senior Leader of the Enclosure Group at SOM — a practice embedded among the design and engineering studios in the firm’s New York office. With two decades of experience in the creative field, Christoph has designed a wide variety of projects encompassing products, furniture, street lights, and architecture.
Christoph s expertise is in both high-performance building enclosures and in their aesthetically crafted appearance in varying light conditions. Efficiency and innovation are among the many considerations central to his design process. At SOM, Chr istoph started a construction site visit program that regularly takes young architects to different sites in New York to create a better understanding of the complexities of construction in relation to design. Outside of SOM, Chr istoph shares his expertise actively at conferences and industry events, and has lectured on design and building performance-related topics. He al so sits on scientific advisory committees for a variety of conferences.
As a Senior Designer based in New York, Daniel Inocente works on both U.S. and international projects small and large, each with its own unique circumstances and opportunities. He emphasizes the human experience and design innovation by leveraging building science, design technologies, and interdisciplinary expertise. He has worked on mixed-use, residential, commercial, transportation, aviation, government, cultural, science, education, and space projects throughout his career. At SOM, Daniel has worked on multiple tall buildings, including the Hangzhou Tower, Guiyang World Trade Center, Zhongtian Tower, Zhuhai S3 Tower, and Tour Charenton.
Daniel is also a member of a dedicated team based in New York that specializes in digital technology, and utilizes this expertise to collaborate on multiple projects across the world, conduct research, and develop design ideas. He has also had the opportunity to establish and lead interdisciplinary partnerships with the space sector, bringing in partners such as ESA, NASA, MIT, and private companies. These initiatives reflect his passion for space architecture and the important role that he believes we have in contributing to future thinking about humanity’s progress through social, technological, and cooperative endeavors.