Comparison of Bulk Radiation Shielding Materials
Providing adequate radiation shielding for space habitats via bulk material is arguably the most difficult logistical challenge facing designers. With limited lifting capacity, the shear mass and volume required to protect the inhabitants for an extended period is rather unpalatable. It is for this reason that many long-term habitat concepts rely on material acquired from extraterrestrial sources such as the moon and asteroids. For anything short of a mining mission however, the shielding material will have to be integrated from the start using terrestrial materials. Additionally, the way this bulk material is fabricated and integrated into the exterior may affect the material’s degree of appropriateness. Below I address the question of what kinds of shielding may be appropriate for near-term space habitats.
Depending on the bulk material used on the exterior of a given habitat, the safety of the interior environment may be increased or decreased when compared to having no shielding at all. An exterior aluminum shield 1 centimeter thick may create a more hazardous interior environment than being directly exposed to GCR (Galactic Cosmic Rays) due to harmful secondary radiation produced from the reaction between the high-velocity protons and the aluminum. Some of the popularly cited materials for use as radiation shielding are polyethylene, liquid hydrogen, water, and epoxy*. Using these materials as examples, polyethylene stands out as more useful and practical than the rest for a bulk material.
Polyethylene
Polyethylene is a simple, inexpensive, and mass-producible material. Its constituents are found everywhere, and its chemistry is well-tested (the TRL of its application as a shielding level can be considered a TRL 6). Comprised of carbon and hydrogen, it does not tend to produce secondary neutron radiation from GCR†. In addition to radiation shielding, polyethylene is impact-resistant against debris and meteorites and is durable at the lower temperatures encountered in space.
Liquid Hydrogen
Liquid hydrogen is often cited as the most mass-efficient shielding material as it has the lowest required mass (g/cm^2) for any given radiation reduction level that has been tested and is the best sort of shielding from a theoretical perspective due to its closely packed nuclei, however this efficiency in mass comes at a cost of volume. Comparing liquid hydrogen to polyethylene, for a given protection level polyethylene requires 2.7x the mass of liquid hydrogen, however at these same levels of radiation protection liquid hydrogen requires 5x the thickness of polyethylene. This of course is without considering the added mass and applied technology required to maintain and insulate a voluminous shield of liquid hydrogen at sub 33°K while it dissipates at a rate of 1% per day. It is for these reasons that liquid hydrogen should not be considered as a primary shielding material.
Water
Water has a relatively high protection factor when compared to conventional materials, however it is still less mass efficient than polyethylene while being comparable from a density standpoint. In addition, its liquid state requires it to be stored in multiple vessels and precludes its use as a structural element. However, water (like polyethylene) does not tend to emit secondary neutron radiation (as it is composed of oxygen and hydrogen) and it does serve an extremely important function from a human perspective. For this reason, it may be useful to supplement the exterior shielding material with water storage. In this way, the water can serve human functions as well as shielding, slightly offsetting the amount of shielding needed.
Epoxy
While epoxy is a solid like polyethylene and may have benefits with respect to structure, it remains less mass efficient than both polyethylene and water. One virtue of epoxy is that it can be made to be transparent. This can be used to create somewhat radiation-resistant windows. In combination with water, even lensing effects are possible.
Based on the established radiation protection, material properties, and economics, the use of polyethylene as the basis for a composite material would be a more effective, flexible, and economical solution to the GCR and SPE (Solar Particle Event) problem when compared to other materials.
