Astronauts aboard China's space station are set to arrange an experiment on brick samples made from simulated lunar soil and featuring mortise-tenon joint structure, which may pave the way for building houses on the moon using lunar soils.

The innovative idea was conceived by a team at Huazhong University of Science and Technology (HUST), who used simulated lunar soil to craft bricks featuring traditional Chinese masonry and woodworking techniques.

Those brick samples weighing 226 grams were transported into the orbital lab through the cargo craft Tianzhou-8 in November. The subsequent spacewalk mission will transport the samples to the exterior of the Wentian Module for endurance testing.

LUNAR AMBITIONS

So far, the major space-faring nations in the world have revealed ambitious plans to establish lunar bases. In 2017, China launched a cooperation initiative for an international lunar research station, and its first phase will see a basic facility built by 2035 in the lunar south pole region.

Engineers from across the globe have suggested multiple lunar construction methods, such as rigid, deployable, inflatable, 3D-printed and underground structures. NASA and the European Space Agency have favored the use of 3D printing. However, this 3D printing technique, which completes the structure in a single process, has a key vulnerability: a single crack could compromise the integrity of the entire structure.

Ancient Chinese craftsmen used only wood to construct magnificent buildings with the mortise-and-tenon structure, which joins pieces at right angles without the use of nails. The earliest example of this technique dates back 7,000 years to the Hemudu culture in east China's Zhejiang Province.

This has inspired modern space architects led by Ding Lieyun, a professor at HUST in the central Chinese city of Wuhan. They envisioned crafting bricks with interlocking joints on the moon for assembly.

They also developed a robotic system to handle the assembly, with the final step involving the use of 3D printing to reinforce the structure.

CHALLENGES

Constructing habitats on the moon, however, presents significant challenges. First, there are extreme temperature fluctuations, ranging from over 180 degrees Celsius during the day to minus 190 at night.

Also, the lack of an atmosphere exposes the surface to intense cosmic radiation and frequent micrometeorite impacts. The moon also experiences about 1,000 small seismic events each year.

Another uncertainty is the impact of the moon's reduced gravity, which is only one-sixth of Earth's. This will affect the molding quality, Ding explained.

The goal of sending simulated lunar brick samples to space for tests outside the Wentian is to evaluate their resilience to temperature extremes and cosmic radiation. Wentian is equipped with 22 standard payload interfaces outboard for conducting extravehicular exposure experiments.

The brick samples in the experiment come in cylindrical and slab forms: the cylinders test mechanical integrity, while the slabs assesses insulation and heat resistance.

The samples have been divided into three groups. "We're planning to retrieve the first by the close of 2025," said Zhou Cheng, a professor at the National Center of Technology Innovation for Digital Construction (NCTI-DC) under the HUST. The second retrieval is scheduled for 2026, and the final one is set for 2027, according to Zhou.

"Constructing a building is not merely for short-term use. This principle is especially critical in lunar construction. We must understand the processes of material aging and decay," said Ding.

LOCAL RESOURCES

In May 2023, the researchers from HUST obtained 500 mg of authentic lunar samples returned by China's Chang'e-5 mission. The lunar probe retrieved 1,731 grams of lunar samples, consisting primarily of rocks and soil from the lunar surface.

Using the small parcel of regolith, they initiated an investigation into in-situ construction methods.

"In-situ" is a strategy of harnessing exclusively lunar resources, eliminating the need for terrestrial material transport, Ding explained. "One brick-making process employs a caustic soda solution or sulfur to solidify lunar regolith, and it is not a purely in-situ construction."

Ding's team opted for volcanic ash from Changbai Mountain in northeast China's Jilin Province, which closely mirrors the composition of lunar regolith.

Lunar regolith, a layer of soil-like mixture distributed on the surface of the moon, is primarily composed of rock and mineral fragments, glassy particles and agglutinates.

They cleaned and dried the rocks, eliminated organic compounds, crushed them for elemental analysis, and subsequently incorporated specific additives to formulate a simulated lunar regolith. The team created five simulated lunar soil compositions to mimic the moon's diverse regolith across different regions.

The density of these bricks is comparable to that of conventional bricks, and their compressive strength exceeds that of concrete bricks by more than three times.

To prepare experimental bricks launched into space, three sintering techniques -- vacuum, inert gas and air sintering -- have been employed, according to Ding.

Among these methods, inert gas sintering yields the highest strength. In a vacuum environment, effective sintering can be achieved by heating to a temperature range of 1,000 to 1,100 degrees Celsius.

Nevertheless, the lunar environment is vastly intricate. Ding is aspiring to test bricks firing on the moon in the future. "Harnessing local lunar resources for construction can substantially cut building expenses and facilitate more profound space exploration," Ding said.

In the NCTI-DC lab's sandbox stands a building model resembling a vertically oriented eggshell. The building's interior is compartmentalized into two levels: an upper work area and a lower rest area. This is the conceptual design for a lunar dwelling Ding's team has imagined.

His blueprint also includes building a Lego-like lunar base and a lunar landing pad.

"These lunar experiments lay the groundwork for future space exploration," said Ding. "The moon can be a springboard to advance our reach into the cosmos."