Water is the most valuable resource candidate on the Moon. It can be consumed by astronauts, split into oxygen for breathing, used as radiation shielding, and converted into liquid hydrogen and oxygen propellant, the lifeblood of any space transportation economy. Evidence for lunar water comes from orbital measurements and from NASA’s LCROSS mission, which slammed a kinetic impactor into the permanently shadowed floor of Cabeus crater near the lunar south pole in 2009 and detected water vapour in the ejecta plume. Yet significant uncertainty remains about how much ice exists, where it sits, and whether it appears as thin films, porous frost or consolidated layers, all of which complicate the design of extraction hardware before a single watt of power is budgeted.

The DLR team systematically searched Scopus, Web of Science and Google Scholar for studies that detailed experimental or simulated lunar water extraction with quantitative data. They recorded design, objectives, environmental parameters, masses and power figures for every qualifying study, with at least two reviewers independently verifying each entry. The resulting survey spans an enormous range of scales. Some laboratory experiments processed only a few grams of icy simulant, while the largest test used nearly twenty-nine kilograms. Simulations ranged from millimetre-scale setups to concepts larger than one metre, drawing from a few watts to several solar constants’ worth of concentrated energy.

From this survey, the authors built the first unified classification of extraction methods, grouping designs by their underlying physical mechanism. Thermal extraction dominates the field: heat from solar concentrators, electrical heaters, microwaves or future nuclear sources is applied to icy regolith, causing the ice to sublimate directly into water vapour in the Moon’s near-vacuum. Thermal methods split further by how the soil is handled. In situ approaches heat regolith where it lies, using capture domes, heated rods or corer drills, while excavated approaches truck the soil to a processing unit, either continuously through augers and conveyors or in discrete batches inside crucibles and rotating heated drums.

Beyond heat, three more exotic families appear in the classification. Mechanical extraction, exemplified by the Aqua Factorem concept from the University of Central Florida, treats ice as a mineral ore, grinding icy regolith and separating ice grains pneumatically, magnetically, vibrationally and electrostatically without melting anything. Convection-assisted extraction is represented by the Rocket Mining System developed by Masten, now part of Astrobotic, with Honeybee Robotics and Lunar Outpost, which fires rocket plumes into the soil to fluidise icy regolith and vacuum up the ejected particles. Ionisation extraction, in the form of ablative arc mining, uses an electric arc to ablate and ionise the regolith itself, steering ionised particles into collection reservoirs sorted by mass with electric and magnetic fields, potentially harvesting multiple elements at once.

The review’s most pointed criticism concerns reporting. Ice content was sometimes given as weight percent and sometimes as volume percent. Only twelve of fourteen experimental studies reported electrical power, only nine reported total simulant mass, and a mere five explicitly stated the mass of ice they started with. Simulations fared little better: just six of thirteen listed numerical starting pressures, and only two explicitly mentioned captured water rather than sublimated mass. The median experiment used roughly half a kilogram of simulant, and only one test exceeded five kilograms, leaving the crucial five-to-thirty-kilogram bracket, the bridge to pilot-scale operations, nearly empty. Most experiments ran for minutes to a few hours, and endurance beyond twenty-four hours of steady operation remains essentially untested, leaving questions of hardware degradation unanswered.

To fix this, the authors propose a standardised reporting framework for the field. They call for every study to report total simulant and ice mass, electrical and thermal power with duty cycles, total energy consumed, run times, and captured and recovered water masses, enabling direct calculation of extraction, capture and recovery efficiencies. They also stress that energy accounting must cover the whole chain, not just sublimation. Capturing water vapour, a step handled by cold traps or condensers, introduces additional power draw and losses, so the highest-level metric, total energy per kilogram of recovered water, must include every input. Comparisons, they caution, should only be made between studies at similar environmental fidelity, since a test in a dusty thermal vacuum chamber cannot fairly compete with one run in open laboratory air.

The review also shines a light on a quieter source of inconsistency: the simulants themselves. Icy lunar regolith is prepared in strikingly different ways, from the common gradual freezing or mud-pie method, which produces strong, cemented, permafrost-like material, to vapour deposition and dry-mix sintering, which avoid bulk liquid water and better resemble genuine lunar conditions. Microstructural studies show wet-mixed samples have smaller pores, lower porosity and higher cohesion than sintered ones, directly altering thermal conductivity and vapour transport. Simulant choice matters too: JSC-1, LHS-1, LMS-1, NU-LHT-2M and others differ in particle size distribution and thermal properties, which control permeability and diffusion under low pressure, meaning that two experiments using different simulants may not be comparable at all. And no current simulant fully reproduces real lunar regolith, so terrestrial performance will inevitably deviate from what a lunar mission encounters.

The identified research gaps are as much about integration as invention. Almost no study couples extraction hardware with vapour capture in a single end-to-end test; the few pilot-scale efforts, such as a drilling-based thermal extraction unit that achieved water collection rates up to roughly 0.75 millilitres per minute with recovery efficiencies above eighty percent, and the European LUWEX project validating extraction, capture and purification with up to fifteen kilograms of simulant, remain rare exceptions. Lunar dust, abrasive and electrostatically charged, is barely addressed in extraction designs despite its known threat to mechanisms and thermal surfaces. Pre- and post-processing steps such as prospecting, excavation, beneficiation, purification and storage must be folded into any honest comparison, and scalable surface demonstrations on the Moon itself will ultimately be required to prove that laboratory success survives radiation, vacuum and extreme temperatures.

What emerges from this review is less a ranking of winners and losers than a roadmap for a maturing field. The authors frame their classification as a starting point, to be refined with input from the wider in-situ resource utilisation community, and their standardised metrics as an invitation to collective agreement on terms, definitions and transparent reporting. If researchers adopt them, the next generation of water extraction studies could finally be compared apples to apples, accelerating the selection of architectures capable of turning permanently shadowed craters into the solar system’s first off-world wells. The alternative, the authors imply, is a patchwork of incomparable results that slows the very missions counting on lunar water to fuel humanity’s return to the Moon and, eventually, the journey beyond it.

Subject of Research: Lunar water extraction technologies for in-situ resource utilisation

Article Title: A review of lunar water extraction technologies: comparison, classification, and research gaps

Article References: Kiewiet, L., Fälker, S., & Zabel, P. (2025). A review of lunar water extraction technologies: comparison, classification, and research gaps. Space and Planetary Resources, 1(1), Article 4. https://doi.org/10.1007/s44461-025-00005-4

Image Credits: AI Generated

DOI: 10.1007/s44461-025-00005-4

Keywords: lunar water extraction, in-situ resource utilisation, icy regolith, thermal mining, permanently shadowed regions, LCROSS, water vapour capture, cold traps, lunar regolith simulants, ISRU, Moon, space resources