A lunar lander and base at the south pole

A vehicle you live inside

Apollo’s lunar module gave two crew roughly 6.65 cubic meters of habitable volume for stays measured in hours, with no seats. The landers now being built carry more people, for longer, and the human factors question changes with the duration: a cabin that is tolerable for a day becomes the limiting factor over a week.

In one sixth gravity the body is light but the suit is not. Mass still has inertia, so a suited crew member is hard to start and hard to stop in a small volume, and every handhold, corner and hatch edge is something to collide with.

The egress sequence is the mission

Each surface excursion runs the same sequence: prebreathe, don the suit, depressurize, clear the hatch with a backpack on, descend the ladder, and return covered in dust. Twice a day, for weeks. Every step is crew time, and crew time is the scarce resource on the surface.

The design question is which steps the vehicle can absorb and which the crew must carry. A suit port removes donning assistance and most of the depressurization. A wider hatch removes a collision risk. A dust mitigation strategy decides whether the cabin stays habitable in week three.

What we would measure

The site: a crater rim at the south pole

When the lander is no longer home, the crew moves into the base. We would put it on the rim of a crater at the lunar south pole and nowhere else on the surface. The rim gets near-continuous sunlight, a line of sight to Earth for communications, and a much smaller temperature swing than the equator, so the habitat is not cycling between freezing and overheating.

At the pole the sun sits close to the horizon and circles it once a lunar day. Everything arriving from the sun comes in close to horizontal, so the heaviest shielding only has to face one direction at a time, and it can turn with the sun.

One large torus

The habitat is a single large inflatable torus, divided into compartments inside. For the same internal volume, several small habitats need more structure and more shielding mass than one large one. Equipment and stores inside the torus add to the shielding around the crew.

Because the shell is inflatable, parts of it can be clear. With radiation handled by the systems below, the crew can look outside.

Stacked tori would combine their fields into a solenoid, a field shaped like a tower. For a first base, one large torus is the better trade.

A magnetic radiation shield

A single loop of current produces a field that wraps around the conductor, in the same torus shape as the habitat. Run the loop around the habitat and the field surrounds it. Charged particles crossing the field are deflected. How strong and how large the field must be to turn the most energetic cosmic rays is the first question for simulation.

The loop is a superconductor. Once charged, it carries current with no resistance and needs no continuous power. Moving metal near the field induces small losses, so the loop is topped up with current every so often, at low cost.

On Earth, superconducting magnets are expensive because of the cooling. On the Moon there is no air to carry heat, so a conductor shaded from the sun can reach very low temperatures passively. Five layers of aluminized Kapton, the approach used on the James Webb Space Telescope sunshield, block nearly all of the incoming sunlight. We would use high temperature ceramic tape over low temperature wire. The tape is more fragile, but it is installed once and does not move.

A loop carrying a large current is pushed outward by its own field and has to be mechanically restrained. The restraint does not need much mass, but it cannot be left out.

Neutrons and the water wall

The field does not stop neutrons, which carry no charge. Cosmic rays striking the surface produce secondary neutrons, and they are among the most damaging radiation to the body. Hydrogen-rich materials stop them best, because a hydrogen nucleus has nearly the same mass as a neutron and takes most of its energy in each collision. Water and ultra-high-molecular-weight polyethylene both work. Lead, iron and packed regolith can produce secondary radiation of their own when struck.

Water is already on the manifest, so the question is where to keep it. We would store it in cells in the outer wall and pump it slowly around the torus so it always faces the sun. It moves sideways, not up, and slowly, so it takes little power. The pole has near-continuous sunlight at about 1.36 kW per square meter, compared with roughly 1 kW at the surface of the Earth.

A free neutron decays into a proton, an electron and an antineutrino, with a mean lifetime of about 15 minutes. Some materials, such as boron-10 and lithium-6, absorb slow neutrons and release charged particles that the field could then deflect. Whether a thin layer can do this for the faster neutrons produced at the surface is an open question.

Micrometeoroids and leaks

Micrometeoroids are stopped by two separated layers of thin aluminum foil, the bumper principle Fred Whipple proposed in the 1940s. The foil does not need a complex shape. It can be a set of triangular awnings held above the torus on a tensioning system, and one layer is replaced every so often.

Larger impacts are rare. When something does get through, a robot travelling across the membrane finds the hole and seals it. The innermost layer of the membrane is a heat-activated laminate, like the film used to laminate paper: the robot heats the area and the layer closes the hole. The same system handles punctures from inside. Whether the robot works inside or outside the membrane is still open.

Shielding the crew while they sleep

A person moving around the habitat is hard to shield. A sleeping one is easy: you know where they are and they stay there for hours. The sleeping enclosures get the most shielding in the habitat, and at night the water wall moves to cover them.

A habitat that knows where its crew is could go further and move water to shield whichever rooms are occupied.

What happens next

The base concept is at TRL 0 to 1. TRL 2 is the math and the simulations: the field strength needed, the current and length of superconductor that produce it, the restraint structure, and the mass that has to be brought from Earth. In parallel, a materials search for a layer that converts fast neutrons into charged particles.

How it connects

This study sits between the training work and the habitation work. MEK is where a crew learns to move in a suit before they fly; the lander is where that movement meets a real volume on a real schedule; the base is where they go when the lander is no longer home.

Lander figures come from public Apollo documentation and should be checked against current vehicle data before use in a proposal. The base concept has not yet been simulated. If you are designing a lander, a suit, a habitat or the operations between them, we would like to compare notes.