Lunar surface systems
DraftA lunar base is not a scaled-up lander. The constraints that dominate are the 14-day night, the radiation environment, and dust, and each pushes the architecture in a specific direction.
Site selection
The south polar region is the consensus target, for a combination of reasons that exists nowhere else on the Moon.
| Factor | Polar advantage |
|---|---|
| Water ice | Permanently shadowed crater floors are cold traps holding volatiles |
| Illumination | Some crater rims receive near-continuous sunlight because the axial tilt is only 1.5° |
| Thermal | Polar surface temperatures are less extreme than equatorial swings |
| Proximity | Ice and near-permanent sunlight can sit within a few kilometres of each other |
The corresponding difficulties are also polar. The Sun stays near the horizon, so shadows are long and deep, solar arrays must be mounted vertically to face a low Sun, and optical hazard detection during landing is hard in terrain that is either glaringly lit or in total darkness. See The Moon.
Surviving the night
Away from the illuminated rims, a base faces 14 Earth days of darkness at temperatures near −173 °C. Options are limited and each is unattractive.
- Batteries
- Storing 14 days of power is prohibitive. At 130 Wh/kg, a 10 kW base needs over 25 tonnes of batteries. This does not close.
- Fuel cells with regenerative electrolysis
- Better energy density than batteries for long storage, but a complex system with consumable losses.
- Fission surface power
- A 40 kWe class reactor runs continuously regardless of illumination. The most credible option and the one under active development.
- Radioisotope heating
- Enough to keep hardware from freezing, not enough to operate. Used to survive, not to work.
- Site selection
- Place the base where the Sun almost never sets. This works only for a few specific locations near the poles, and even those have short outages.
The night is the single strongest argument for fission surface power. It is not that a reactor is more efficient than solar, but that solar simply does not work for half of every month unless the site is exceptional. See Nuclear propulsion for the related reactor technology.
Radiation and shielding
Without an atmosphere or magnetic field, surface radiation is the full galactic cosmic ray flux plus solar particle events. From Space radiation, GCR cannot be shielded away with practical launched mass, and adding aluminium can make the dose worse through secondary production.
The answer is regolith, because it is already there. Covering a habitat with a few metres of loose regolith provides shielding that would be impossible to launch, and the mass is free.
| Approach | Character |
|---|---|
| Bury the module | Excavate, emplace, cover. Simple, requires substantial earthmoving |
| Regolith bags or berms | Fill containers and stack them around the habitat |
| Sintered regolith | Fuse regolith into blocks or a shell using microwave or solar concentration |
| Lava tubes | Natural voids beneath the surface, offering shielding and thermal stability without construction |
Lava tubes are attractive and speculative. Gravity data and surface pits suggest large intact tubes exist, potentially hundreds of metres across, which would provide radiation shielding, micrometeoroid protection and a stable temperature near −20 °C without any construction at all. None has been entered.
A storm shelter is a separate requirement from bulk shielding. Solar particle events are survivable in a small heavily shielded volume occupied for hours to days, and that volume can be shielded with water, consumables and regolith rather than dedicated mass.
Dust
Lunar dust is the operational problem most consistently underestimated, and Apollo experience is unambiguous about it.
The particles are sharp, because no water or wind has ever eroded them. They are electrostatically charged by solar ultraviolet, so they adhere to everything. They are fine enough to penetrate seals and lodge in mechanisms.
| Effect | Consequence |
|---|---|
| Abrasion | Suit joints and seals wore visibly within three Apollo EVAs |
| Adhesion | Coats radiators and solar cells, degrading both |
| Mechanism jamming | Bearings, hinges and connectors fail |
| Respiratory | Fine sharp particles brought inside are a crew health concern over long exposure |
| Optical | Obscures cameras, windows and sensors |
Mitigation is an active area without a settled answer. Approaches include electrodynamic dust shields that use travelling electric fields to move particles off surfaces, suitport designs in which the suit stays outside the pressurised volume and the crew enters through its back, careful airlock design with dust removal stages, and simply keeping dust-generating operations away from sensitive hardware.
Mobility and EVA
The pressurised rover is the item that most changes what a base can do. Suited EVA is limited to hours by consumables and by crew fatigue, and it exposes the suit to dust every time. A pressurised rover lets crew travel for days in shirtsleeves and perform EVAs only at the destination, multiplying the scientifically accessible area.
Construction from local material
In-situ resource utilisation for construction is more tractable than for propellant, because the requirements are looser.
| Product | Method | Maturity |
|---|---|---|
| Radiation shielding | Pile regolith on the structure | Simple; needs only earthmoving |
| Landing pads | Sinter or melt regolith to prevent ejecta plumes | Demonstrated in laboratory |
| Structural blocks | Sinter, cast or bind regolith | Laboratory demonstrations |
| Roads and berms | Compaction and sintering | Straightforward |
| Oxygen | Molten regolith electrolysis or hydrogen reduction | Laboratory; energy intensive |
| Metals | Extract from regolith oxides | Early research |
Landing pads deserve emphasis because they solve an immediate problem. A rocket landing on unprepared regolith accelerates particles to high velocity with no atmosphere to slow them, sandblasting anything nearby. Apollo 12 landed near Surveyor 3 and its returned components showed measurable damage from that effect. Any base with repeated landings needs prepared surfaces or very large separation distances.
Oxygen production is the highest-value ISRU product after water, since oxygen is the majority of propellant mass by weight and is also breathable. The processes work in the laboratory and are energy intensive, which loops back to the power problem.
References
- Heiken, G., Vaniman, D. and French, B. M. (eds.) Lunar Sourcebook, Cambridge University Press, 1991.
- Gaier, J. R. “The Effects of Lunar Dust on EVA Systems During the Apollo Missions”, NASA/TM-2005-213610.
- Haruyama, J. et al. “Possible lunar lava tube skylight observed by SELENE cameras”, Geophysical Research Letters, 36:L21206, 2009.
- Gibson, M. A. et al. “NASA’s Kilopower Reactor Development and the Path to Higher Power Missions”, IEEE Aerospace Conference, 2017.
- Metzger, P. T. et al. “Rocket exhaust cratering and blowing of lunar soil”, Journal of Aerospace Engineering, 24:24, 2011.