Many sci-fi conceptions of space exploration have envisioned astronauts running experiments on a planetary surface and then returning to a small base composed, principally, of the ship that took them there.
But lengthier missions, if they ever happen, will require more extensive infrastructure: habitats, launch and landing pads, blast walls, meteoroid shields, cryogenic fuel and oxidizer storage, and other facilities.
A human mission to Mars would entail astronauts living and working in structures built on the Martian surface. But what would those structures be made of? Because only so much material can be brought from Earth, Mars explorers would have to learn to build using materials found on Mars. That may seem a mundane problem, compared to getting a crew to Mars, but it presents a host of complexities—problems on which NASA scientists and engineers at the agency’s Granular Mechanics and Regolith Operations (GMRO) Lab, at the Kennedy Space Center at Cape Canaveral, Florida, are actively working.
Extraterrestrial buildings probably will be built through additive construction using a large-scale 3D printer, says NASA Research Engineer Nathan Gelino, who works at the GMRO Lab (also known as Swamp Works). Additive construction is a proven, even routine, process on Earth. But it will present significant engineering challenges off Earth, not least because of the exotic soil astronauts would need to use as building material.
Asteroids bombard rocky bodies such as moons, creating fine debris that lofts up and falls back down across the planetary surface. After millennia, this builds up a deep layer of crushed rock that scientists call regolith.
Regolith differs from sand. On Earth, sand undergoes continuous weathering from wind, rain, and tides, which erodes the grains into a much rounder shape. Those forces don’t exist on the moon, so grains of regolith are extremely sharp, like tiny shards of glass. Scientists at the GMRO Lab expect basalt regolith on Mars to have similar mineral properties to lunar basalt regolith, so they are experimenting with Earth-borne simulants—although they do have some actual moon regolith in a safe. “Those samples are incredibly rare,” Gelino says. “You don’t use those materials unless you absolutely have to.”
The material is somewhat dangerous to work with. Like any fine-grained rock dust, it is a respiratory hazard; regolith could be carcinogenic in the lungs in the same way asbestos is. Black Point 1 (BP-1), a waste product of asphalt production, is a lunar-regolith simulant that the GMRO Lab uses. It contains silica, so silicosis is a concern for investigators, requiring protective respiratory equipment.
“The more direct problem in space is abrasion,” Gelino says. “Since the grains are like glass shards, they become electrostatically charged, and they loft very easily. If you look at images of Apollo missions—where astronauts are walking on the surface of the moon—in many of them, they are covered with dirt. The charged particles stick to surfaces, including astronaut suits and gloves. You can imagine that the folds in an astronaut’s suit are filled with tiny knives. The abrasion can wear through the outer layers quickly. The Apollo astronauts found that the life of their suits, and especially their gloves, was significantly limited by this.”
And if fabrics are one problem, machines are clearly another. “Engineers will have to be careful to keep regolith out of rotary joints or sliding surfaces,” says Brad Buckles, a colleague of Gelino’s. “Abrasion has been an issue with the 3D printer we use. It’s very rough on the feed screw and the barrel and nozzle as the material is extruded through the system.”
Moreover, Buckles says, “regolith just behaves funny” compared to other additive-construction materials. It will not flow like beach sand in an hourglass; rather, it tends to bridge, pack, jam, and “rathole.” NASA engineers have developed a variety of tricks to achieve the flow necessary to 3D print with regolith. Nevertheless, those same engineers insist that regolith additive construction will work. But building infrastructure off Earth will require a rethink about infrastructure itself.
“On Earth, everything is extremely heavy,” Gelino says. “We use steel and concrete, materials that weigh tons. If you want to send 40 bags of concrete to the moon, those are going to be the most expensive 40 bags of concrete in history. And the rockets we have are limited in payload capacity.”
Thus, NASA must focus on “in-situ resource utilization,” enabling planetary explorers to live off the land. Off Earth, NASA engineers look at every pile of dirt as a valuable resource, potentially a source of hydrogen and oxygen to produce water—or the source elements to make iron-silicon-based steel. “When you get used to looking at things that way, you see the things you could do with a pile of regolith that you would never think about on Earth,” Gelino says.
To anyone who has ever made a sand castle or worked with concrete, an obvious question about 3D-printing buildings in space is: Doesn’t the process require water?
Yes and no, Gelino says. For certain concretes, water is necessary, but the good news is that water is not as scarce off Earth as popularly believed. There is harvestable water ice on Mars and in the shadows of deep craters on the moon, and water can be chemically synthesized from hydrogen and oxygen, both of which are abundant in hydrated Mars regolith minerals.
Other concretes, Gelino adds, can be made without water. “The material we’ve been using in our additive-construction experiments is regolith mixed with waste polymers,” Gelino says. “You can get polymers in the form of astronaut trash and shipping containers, or you can synthesize polymers. You can use that as a binder for regolith, with a relatively low ratio of polymer to regolith, to make a construction material pretty similar to Portland cement in compression and 20 times stronger in tensile strength.”
NASA engineers also have sintered regolith into various kinds of pavers or bricks. The process involves molding the material under high heat; the end product has a consistency like sandstone, Gelino says. The required heat (about 1,200 degrees Celsius) demands a great deal of energy—a limitation, Gelino believes, but not an insurmountable problem.
Payload capacity of delivery rockets will dictate the scale of the construction equipment that can be sent to the moon or Mars. The 3D printer that Gelino’s team is working with consists of an extruder mounted on an industrial robot arm and is capable of building a structure 6 feet tall and roughly 8 feet wide. It’s a proof-of-concept system; a flight-ready solution will need to be lighter and adapted to operate in extreme cold, radiation-exposure, and vacuum conditions.
Ideally, the additive-construction process would be entirely robotic, Gelino says. The printer might even be sent to its destination before astronauts arrive to build needed infrastructure.
NASA assesses emerging technologies on a Technology Readiness Level (TRL), measuring an innovation’s readiness for adoption, on a scale of 0 to 9. A TRL 1 innovation is a basic principle or an unrealized concept. “A TRL 9 technology is ready for flight,” Gelino says. “The GMRO Lab generally works on innovations at the TRL 1 to 5 level, really new concepts that can change the way we think about things.”
Gelino places off-Earth additive construction at a TRL 2 to 3. “We’ve proven it’s a viable concept, and now we are working to characterize and upscale it,” he says. “We need to understand more definitively the regolith and how it performs, especially its load-bearing and strength qualities as a construction material.”