An autonomous, robotic hydraulic excavator could build a dry stone wall that could act as a blast shield around a launch pad on the moon, a team of Swiss researchers proposes.
The excavator would use materials on site (rather than the costly practice of transporting construction materials from Soil Unpleasant the moon), collecting rocks from the lunar surface to create a ring wall with a radius of 50 to 100 meters (164 to 328 feet).
"The robot would be used both to collect the boulders and to build the wall," Jonas Walther, principal investigator on the study, told Space.com.
Walther did the research for his master's thesis at ETH Zurich and now works at the Swiss company Venturi Lab, which collaborates with other companies on the design of lunar rovers, where he specializes in the wheels.
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If humans return to the moon permanently and a base there this infrastructure will need to be protected from the exhaust fumes and debris from rockets taking off and landing. Dust, small particles and gas from rocket exhaust can all be dangerous, as was demonstrated when the Apollo12 crew brought the Surveyor 3 Oceanus Procellarum probe and was found to be damaged by dust blown away by the Intrepid lunar lander.
The rocket exhaust from the giant SpaceX is also expected to Spaceship vehicle, which will be used on NASA's Artemis 3 mission to put astronauts back on the surface of the moon will affect the lunar environment for hundreds of meters, perhaps kilometers. Hence the need for a blast shield to protect future ground infrastructure.
Walther cites a terrestrial prototype of his proposed lunar excavator. As described in a 2023 article in the journal Science A team led by Walther's co-author, Ryan Luke Johns of ETH Zurich, designed an autonomous robot to dig up material and use it to assemble a dry stone wall here on Earth. Their new research takes this technology and applies it to a lunar environment.
The challenges this technology would face on the Moon include the distance a vehicle would have to travel to collect enough raw materials for a blast shield, the energy required to do so, and the source of that energy.
The advantages, Walther and his team argue, far outweigh those of other building techniques, however. For one, a dry stone wall made from boulders plucked from the lunar surface doesn't require the energy-intensive material-handling methods that other techniques might use, such as heating to cement the material in place. Moreover, Walther points out, dry stone walls, as primitive as they may sound, can have a remarkable lifespan.
"Some dry stone walls on Earth have lasted thousands of years," Walther said. And while structures like these built on the moon wouldn't necessarily last that long, the moon's much reduced weathering (no air, no water, no wind, only space weathering from cosmic particles) means that the only forces such a wall would have to withstand would be explosions from rockets taking off or landing. Walther's team estimates that the pressure on the blast shield from gases expelled by one of SpaceX's Starship rockets would be 1,135 pascals. That's tiny compared to the standard atmospheric pressure on Earth of 101,000 pascals.
The time- and energy-consuming part, though, is gathering the materials. Walther's team estimates the excavator's loading capacity at 10 cubic meters (353 cubic feet) of boulders; to build a ring of blast shields with a radius of 50 meters (164 feet), a circumference of 314 meters (1,030 feet), and a height of 3.3 meters (10.8 feet) would require an estimated total of 1,000 cubic meters (35,314 cubic feet) of lunar boulders (defined as rocks larger than 25.6 centimeters, or about 10 inches).
The excavator must roll out and find all of this material on the moon itself. Walther's team studied images of two possible locations where a lunar base could be established, namely the Shackleton-Henson Connecting Ridge that connects the craters Shackleton and Henson in the moon's south polar region and a possible landing site for Artemis 3 and the pyroclastic deposit on the Aristarchus Plateau near the prominent crater Aristarchus in Oceanus Procellarum.
Based on images of these two areas taken by the Narrow Angle Camera on NASA's Lunar Exploration Orbiter which can detect boulders up to 2 meters (6.6 feet) wide, and using the laws of boulder frequency to estimate how many invisible smaller boulders there are (there are more smaller boulders than larger ones), Walther's team then used an algorithm to calculate the most efficient path the excavator could take to collect all the raw material and return it to the construction site, making multiple trips there and back. They arrived at a total distance traveled of between 776 and 880 kilometers (482 and 547 miles), though this figure will depend on caveats such as payload and ease of access to material (for example, much of it is expected to be collected at the base of slopes).
"I agree that the distance sounds quite large at first," Walther said, but he points out that it is not unrealistic to drive such a distance. "The Moon terrain vehicle (LTV) being developed for Artemis, or NASA's concept rover called Endurance, will be able to travel similar distances."
The excavator and dry stone wall would also use significantly less energy than alternative methods that instead cement a wall. Walther's team calculates that the excavator would use between 9 and 10 gigajoules of energy to construct just a quarter segment of the blast shield. By comparison, cast regolith, where regolith (lunar soil and rock) is heated until it melts and then poured into molds and cooled to form shaped sections, would use 1,250 gigajoules per quarter segment. Microwave heating would be even more energy-intensive, using between 6,440 and 17,500 gigajoules, depending on the density of the material. The autonomous excavator building a dry stone wall would therefore be at least two orders of magnitude less energy-intensive.
This is important on the Moon, because an isolated lunar base would have to be energy-conscious, at least to start with. The total estimated time to build the blast shield would be about 63 Earth days, but that doesn't include recharging times, and if using solar power, the excavator would have to hibernate every two weeks for the lunar night, so this would double the build time to at least 126 days. Portable recharging stations, or equipping the excavator with nuclear power in the form of a radioisotope thermoelectric generator (RTG) of the kind that the Mars rovers Curiosity And Perseverance have, but are more powerful, and could alleviate some of these delays.
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A dry stone wall may contain some holes, and Walther admits that further research is needed to determine whether these holes would pose a problem in weakening the structure or not sufficiently protecting a lunar base behind the blast shield. However, his team anticipates that the excavator could be even more useful on Mars where the density of boulders is greater over a smaller area, reducing driving time and energy consumption.
The Earth prototype shows that a lunar version can be developed in a relatively short time. Much depends on the progress of NASA's Artemis Programof Artemis2 already postponed until September 2025 at the earliest, and Artemis 3, which would have marked the first landing on the moon since Apollo17 in 1972, with no definitive timeline. Whether or not further missions will be launched after Artemis 3 is currently unknown, but if an attempt is made to develop a base on the lunar surface, the autonomous excavator and its dry stone walls could be vital in rapidly building large structures.
Walther's team's review of the excavator was published in the June 6 issue of the magazine Frontiers in space technologies.
