Star Light, Star Bright… Let’s Build a House on the Moon Tonight!

On July 20, 1969, astronaut Neil Armstrong became the first person to walk on the moon. Over the next 3 years, 11 others followed in his footsteps. However, astronauts have not returned to the moon in nearly 50 years. Since then, scientists have talked about sending people back to the moon or even to Mars. Some scientists want to establish permanent colonies in space in case resources ever run out on Earth. But how do you build places to live and work in an area with no construction materials, equipment, or workforce? One promising solution may be three-dimensional printing.

invented in but has really grown in the past years. D-printed construction squeezes out building materials like concrete to form walls, components, such as beams and columns, and even entire buildings. There is no limit to the types and shapes of structures that D printing can create ( Figure ).
Some scientists want to establish permanent colonies and structures in space in case resources ever run out on Earth. Structures that may be useful in space include shelters, shuttle hangars, roads, landing and launch pads, and blast protection walls [ ]. As humans spend more time in space, more complex structures may also be required, including research labs, garages, greenhouses, and other long-term buildings. D printing in space has two big advantages compared to traditional construction. First, buildings can be printed using materials found in space, like crushed rocks and dust. Second, D printing requires few or no workers. This means colonies could be ready for astronauts to move into before they even arrive.

SPACE MATERIALS: MOON ROCK AND MORE
Space shuttles and rockets have limited cargo space and do not fly too often, so sending supplies into space is expensive. In fact, it costs as much as $ million per kilogram to send supplies to Mars [ ]! For this reason, building with space materials can save billions of dollars. Three common space materials are regolith, basalt, and sulfur ( Figure ).

REGOLITH
The crushed rock and dust produced on the moon's surface after centuries of micrometeorite strikes.

BASALT
An igneous rock commonly found on the near side of the moon that resists radiation from the sun.

SULFUR
A material commonly found on Mars that is used to help the ingredients of a concrete mix stick together.
Scientists are working on technology to harvest and process these materials for use in construction.
Regolith is the layer of crushed rock and dust produced on the moon's surface after centuries of micrometeorite strikes. It is the simplest and most common material used in D-printed construction in space. Basalt is an igneous rock, meaning it is formed during lava flow. It makes up roughly % of the surface of the near side of the moon. Basalt resists radiation from the sun and can be used to build pressurized structures. Sulfur is a material common to Mars.

CONSTRUCTION BY PRINTERS, FOR HUMANS
D printing can be used to build colonies on the moon and Mars even before humans arrive. Building in advance helps keep astronauts safe when they arrive in space. For example, buildings protect astronauts from solar radiation and space debris. Pressurized buildings can even give astronauts a place to live and work without needing their spacesuits.
Unlike human workers, printers do not need air, water, or food. This saves money and makes space construction easier. Also, unlike humans, printers do not need to stop to rest. This means D printing can be completed faster than normal construction. Compared to humans, printers are also more reliable and consistent. When programmed properly, printers reduce mistakes, increase safety, and improve the quality of construction. Better construction with fewer

HOW TO PRINT
In , NASA invited teams from all over the world to a D-printing D-PRINTING A process of manufacturing or constructing items and structures by printing them in sequential layers.
competition. It was called the D-Printed Habitat Challenge, and it gave out $ . million in prizes. The competition took place in three phases. First, each team submitted their best habitat design. Next, the teams developed materials to print with based on what is available in space. Finally, the teams D-printed their designs using their materials. The goal of the competition was to improve the way NASA will build structures in space. In the end, some ideas were more successful than others. However, each idea helped NASA understand what may or may not work in space. For example, dome and bubble designs both seem promising. Teams were successful using both space materials and recycled trash in their printing mixes. However, one big challenge NASA must address in the future is scalability, or the ability to make the structures full size. Engineers know that designs that work well in models are not always as good when built full-size. NASA and companies like Made in Space are working on improving the scalability of D-printed construction.
One possible approach to in-space D printing is a method called contour crafting. Contour crafting first squeezes out material in layers.

CONTOUR CRAFTING
A method of D printing in which the material is squeezed out in layers and then smoothed out by robotic trowels. One research team wants to combine contour crafting with a robotic system called ATHLETE [ ]. ATHLETE stands for All-Terrain Hex-Limbed Extra-Terrestrial Explorer. ATHLETE is "All-Terrain" because it can travel across uneven lunar or planetary surfaces. "Hex-Limbed" means ATHLETE has six arms. These arms allow it to use several tools to navigate and build components. Finally, the term "Extra-Terrestrial" means ATHLETE is designed to be used in space. It collects energy from the sun and uses it as a source of renewable power. This means it does not have to be plugged in or supplied with fuel to run.

LOOKING FORWARD
D printing in space faces some unique challenges. It is di cult to print in environments with low gravity [ ]. Printers need built-in redundancy (extra parts) in case stu breaks. If issues do arise, printers must be easy to repair. Printers must be durable enough to work in extreme temperatures, dust storms, and moonquakes [ ]. Finally, if printers are unmanned, they need to be able to be controlled from Earth. This means they must be able to operate and communicate over long distances and without delays [ ].
Right now, D-printed construction is in a stage called proof-ofconcept and validation [ ]. This means scientists are still learning what D printing can do and if it will work in space. To guide the development of D printing, they set several goals for the future. By , scientists want to start using D-printed construction to build space colonies. By , scientists hope space D printing will be able to support colonies long-term. Finally, scientists believe that by , D-printed construction in space will be entirely independent of humans and of Earth's resources [ ]!
In the meantime, developing a reliable D printing method for space helps advance D printing on Earth. For example, building shelters in space teaches us better ways to construct low-cost, quality homes here on Earth. And by finding ways to collect space materials, we can improve the way we obtain resources on Earth. This supports faster, cheaper, and better construction on Earth and in space. Simply put, D-printed construction is out of this world!

DISCLAIMER:
The views expressed in this paper are those of the authors and do not reflect the o cial policy or position of the United States Air Force, Department of Defense, or the United States government.

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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KATELYN, AGE:
Hi, my name is Katelyn. I am years old and I like to read books. I also enjoy writing fantasy stories and creating animations. My favorite sports are curling and badminton but I like basketball as well. My favorite subjects are social studies, science, and visual arts is pretty fun too.