Space Crew Radiation Protection

Image From the Mars Society Website

Radiation Exposure Limits

NASA currently uses a Linear No Threshold (LNT) Model for radiation exposure, with a 600 mSv space related career exposure limit, with most other space agencies using a 1 Sv limit. The LNT model is not really supported by the data, either from radiation therapies, or the global distribution of background radiation levels. Recognizing a reasonable threshold model is required to reasonably plan long term space habitation. A conservative threshold for no effect would probably be 300 mSv/year (0.82 mSv/day) for continuous exposure since there is an area in Iran with a background of 260 mSv/year that seems not to have any issues. If you’re willing to take a less conservative view based on the moderate dose subset of the radium girls (who didn’t have any detectable increase in cancer) with a safety factor of 10, you get a limit of 700 mSv/year (2 mSv/day) for continuous exposure. The NASA standard is derived from limiting excess risk of death due to radiation to a 3% increase, which you could also apply to a threshold model.

The dangers of chronic radiation exposure are also not the same for all parts of the body. Fast dividing cells like the lining of the gut or the active bone marrow are relatively vulnerable to radiation, whereas skin, muscles, and inactive bone marrow are much more resilient to radiation damage. In an adult only the bones in the skull and abdomen have active red bone marrow under normal circumstances, and the water in your cells provides some shielding for deeper tissues, so full protection isn’t even necessarily required for the whole body.

Mechanics of Radiation Shielding

Due to the contribution of secondary neutrons and the reduced relative linear energy transfer of higher energy particles, the effect of additional shielding on GCRs is very nonlinear. For metals and polymers, the first 10 g/cm^2 of shielding gets you a significant reduction in effective dose, 20 g/cm^2 gives a small reduction compared to 10 g/cm^2, and further reductions are marginal. Depending on the exact boundary conditions some simulations show an increase in effective dose due to chain target fragmentation behind metal shielding that’s 20-100 g/cm^2 that doesn’t have a polymer layer behind it.

10 g/cm^2 of PE gets you below 400 mSv/year from GCRs outside a planetary magnetosphere at solar minimum, but getting below 300 mSv/year requires more like 60g/cm^2, or being on a surface or in a sufficiently low orbit for the relevant body to take up at least 30% of the sky. Getting 60g/cm^2 of shielding over a full living volume that’s smaller or less efficient than a dumbbell habitat designed for 100s of people is impractical due to surface to volume ratios and the required shielding mass relative to a habitat without supplemental shielding.

NOAA S3 Solar Storms (frequent) are shielded completely by 10 g/cm^2 of PE shielding, with S4 storms (~3 per cycle) reduced to an approximately 100 mSv total dose, and approximately 200 mSv for an S5 storm (less than 1 per cycle). Increasing shielding to 20 g/cm^2 reduces S4 and S5 doses by about ½ for hard spectrum storms and ¾ for soft spectrum. More shielding has severe diminishing returns for additional mass, especially against hard spectrum storms.

From a practical perspective on the cost-benefit of shielding weight, an average dose of 400 mSv/year including solar storms is likely the minimum practical dose for a small expeditionary crew in interplanetary space. Under 1 Sv career radiation limits, that allows one round trip to mars. A threshold model that allows 600 mSv lifetime exposure to marginal radiation over a 0.82 mSv/day threshold would allow 4 career round trips to mars with indefinite spaceflight in LEO or stays on the lunar or Martian surface, assuming no exceptional solar storms. A 2mSv/day threshold would be limited only by solar storms and radiation belts.

Designing Efficient Shielding

Materials with a high hydrogen content provide significantly better radiation shielding than metals by weight, especially against high energy particles. Current spacecraft are almost all built out of aluminum, but we could instead switch to fiber reinforced polymer composites. While they are a little more expensive and harder to work with, and don’t end up saving much if any structural weight in most spacecraft applications, UHMWPE or Kevlar reinforced epoxy composites have equivalent radiation protection to PE, which is about 15% better than aluminum. Glass windows can also be replaced with glass fiber reinforced epoxy composites that are more durable and provide better radiation shielding.

The other way to increase shielding without adding much weight is to remember that the limbs don’t need as much radiation protection as the head and torso, so a relatively practical vest and beanie hat that provide significant radiation shielding for vulnerable organs is just as effective as adding supplemental shielding to the spacecraft hull while being much lighter. Because the apparent gravity in a habitat is a design variable, and lower spin gravity is easier, with good design and a good belt, a shielding vest that weighs as much as the person wearing it is entirely practical and can provide ~6 g/cm^2 on the torso. To keep it flexible enough it needs a high density flexible shielding material, and I think the simplest answer is a something like a quilted down vest filled with bismuth powder since it’s almost as dense as lead and non-toxic.

Getting 10 g/cm^2 of PE equivalent shielding for the astronauts vital organs doesn’t actually add much weight, and if you accept even a conservative threshold model, that’s good enough for any reasonable missions across the inner solar system without invoking new propulsion technologies to shorten the trip or active shielding to reduce exposure.

Matthew Glimcher, Aerospace Engineer