In-situ resource utilization (ISRU) is commonly cited as being a critical step towards a sustainable human presence in space, especially on the Moon. Just how crucial it is, and how much its by-products will affect other uses of the Moon, is still up for debate. A new paper from Evangelia Gkaravela and Hao Chen of the Stevens Institute of Technology dives into those questions and comes up with a promising answer - ISRU is absolutely worth it, if we can control the waste products.
The Moon is useful for more than just a place to extract resources from, or even just offering additional living space for humans. It holds a special place in the solar system by being close enough to Earth that it doesn't require too much effort to reach, yet being isolated enough that it doesn't have some of the "downsides" of Earth that affect scientists wishing to examine some part of the wider universe. In particular, it is useful as a case study for the planetary formation process, as understanding where the Moon comes from can help elucidate more complex planetary formation processes. In addition, parts of the Moon, especially the "dark side" that always faces away from the Earth, are shielded from "noise" from the Earth itself, making it highly suited for applications like radio astronomy.
However, at least in theory, ISRU could disrupt that with its waste products. Dust from its operations could obscure the "seeing" of telescopes, given the lack of rain and low gravity, both of which would remove those obstructions on Earth. Resource collection could also destroy important geological formations that could reveal the Moon's formation. Finally, the ISRU creates literal "slag," which, unless utilized for some other purpose, is just waste material and could quickly overwhelm any storage efforts for a nascent lunar colony.
To understand that impact, the authors looked at three different ISRU processes - Molten Regolith Electrolysis (MRE), Soil/Water Extraction (SWE), and Direct Water Electrolysis (DWE). For each, they analyzed the inputs, products, and "byproducts" (i.e., waste) for each system. They then used a cost function to compare the cost of utilizing in-situ resources to getting them from Earth.
MRE is one of the most commonly used ISRU methods. It melts regolith at high temperatures and releases oxygen and metals from the regolith itself. Its main byproduct is slag, though some materials would also result in silicon, which can be useful for everything from solar cell fabrication to microchip manufacturing. SWE's primary purpose is to extract water from soil, with its byproducts being dried regolith and a small amount of "volatiles" like ammonia, hydrogen sulfide, or carbon dioxide. DWE splits the water from SWE into oxygen and hydrogen, with essentially no "byproducts".
The framework Ms Gkaravela and Dr Chen use looks to minimize a cost function for a mission, which is essentially a mathematical way to add up all the costs, including the cost of waste disposal or recycling, to come up with a total mission cost, and comparing methodologies (i.e. ISRU or shipping resources from Earth) to see which has a lower cost. In all cases, ISRU, including the cost of dealing with the waste slag and other byproducts, comes in at dramatically less than the cost of shipping all the necessary materials from Earth. Quantifying the impact on some of the more esoteric scientific use cases on the Moon is more difficult. Still, even if they or their mitigation strategies multiply the mission's overall cost by three times, it would still be worthwhile to use local resources for the resources required for human settlement, such as water, oxygen, and metal.
Fraser discusses ISRU and its benefits.
Ultimately, the paper's outcome confirms what space exploration enthusiasts have thought for a long time - utilizing resources where they are already located, rather than trying to blast them out of the gravity well of our home planet, is the way to go for inexpensive missions. The costs of dealing with the waste created by that plan are negligible compared to the cost difference between ISRU and launching from Earth, even before considering the ecological damage done on Earth itself as part of creating those materials. In essence, once we can get ISRU up and running, it will undoubtedly be the best way to tackle colonizing the Moon and beyond - we just have to develop the technologies, including waste management, first.
Learn More:
E. Gkaravela & H. Chen - Logistics Analysis for Lunar Post-Mission Disposal
UT - Spacecraft, Landers and Rovers Could be Recycled for Parts on the Moon
UT - What is ISRU, and How Will it Help Human Space Exploration?
UT - A Comprehensive Blueprint for the Settlement of Mars