THE PLAN FOR SUSTAINABLE
SPACE DEVELOPMENT
- establishing an initial infrastructure on the Moon -

Space radiation is a real problem but one that seems to have some solutions. There is not one solution for every radiation setting but each situation calls for a somewhat different solution. There are a number of promising interventions such as drugs that would help address the damage that radiation could cause. But simple shielding from mass is something that is fairly well understood and relatively easily applied. Ideally, we'd like to have crew wearing electronic radiation badges that would not only tell them when radiation levels are elevated but also keep track of their overall exposure.

TYPES OF RADIATION
There are about four times of radiation of concern in space:

• Trapped radiation in the Van Allen radiation belts,
  
• Solar wind,
  
• Solar particle events (e.g. flares & coronal mass ejections), and
  
• Galactic cosmic rays (GCRs).

Solar particle events (SPEs) are a significant concern. Also the energies of the individual particles are not particularly high, during an SPE, the quantity over a short period of time are so high that they can cause radiation sickness or even death. During the Apollo program, there was one such SPE event that might have cause serious problems. Fortunately that was between missions and so we simply got lucky! We can't count on luck for the long term.

MITIGATION STRATEGIES
Trapped radiation and the solar wind are not so big a problem that they needs any particular solution. For solar particle events (whether during transit or on the surface of the Moon or Mars) the crew will need to get into and temporarily remain in a "storm shelter" which could simply be their water-bearing provisions located next to the place where they spend their sedentary time. Since they would have these provisions already, there would be little if any additional mass required to provide this shielding. How thick would these water-bearing provisions need to be to protect against an SPE? This NASA website indicates that 20cm thick should be sufficient.

GCRs are another matter. Although the number of particles per minute is much less than that during an SPE, the energies of GCRs can be much higher (traveling at nearly the speed of light). As such, they can shoot through quite a bit of material before stopping. However, the graph to the left shows that the 20cm thick of water-bearing provisions would reduce the overall GCR exposure energies by about half. This significantly helps keep the crew within their career limits.

LOW EARTH ORBIT
There is an interesting proposal that orbiting settlements in low Earth orbit (LEO) could start with tourist hotels in equatorial LEO (ELEO) where the orbit doesn't pass through the radiation of the southern Atlantic anomaly and so the orbit would experience much less radiation. Learning from these hotels, eventually spinning habitats of a kilometer in diameter could be developed as the first settlements in which full gravity, 24/7 would be possible. With this and the low radiation it might be possible to raise children and so achieve a reproducing space settlement. Unlike the Moon and Mars, because there are no material resources in ELEO, everything would have to be shipped to these settlements so there is an interesting trade-off in terms of the advantages and disadvantages of such settlements. Concerns have been raised about trapped radiation reaching into equatorial orbits during a severe solar storm. This question deserves greater examination. But for radiation, ELEO may or may not need any radiation shielding.

TRANSIT TO THE MOON
The three day transit to the Moon may not require any special shielding. Observations of the Sun may all for predictions with enough advanced warning such that launches would be held off until there are breaks between the flares. Or, one would need to ship water-bearing provisions with any crew that went to the Moon in order to provide a storm shelter.

TRANSIT TO MARS
While traveling to Mars, it is inevitable that crew will be hit by one or more solar storms. However, if the crew's sedentary area were surrounded by their water-bearing provisions and water-bearing waste, this 20 cm thick shielding would both protect them during an SPE as well as reduce their GCR radiation to below career limits. We spend most of our time doing sedentary activities so the crew would be shielded for most of the time but would be able to leave their sedentary areas for 30 or 60 minutes at a time to stretch their legs, exercise, or whatnot while still remaining within their career limits.

In free space, GCR radiation is about 1.9 mSv/day. With the shielding cutting this by about half, this would come to 0.95 mSv/day. For a long-stay Mars mission of about 260 days in free space and 619 days in a shielded habitat on Mars, this would come to about 247 mSv while in transit and 294 mSv on the surface of Mars. Together this would come to 541 mSv which is well below the 1,000 mSv career limit of typical a 47 year old male. So, there are solutions.

*** Mass of such shielding ***

LUNAR AND MARS SURFACE HABITATS
The Plan for Sustainable Space Development advocates that large, flat-roofed inflatable habitats be landed prior to crew arrival. Telerobots would push regolith (dirt) on top prior to inflation. If the telerobots could push a modest 30 grams/cm (or about 19 cm thick) of regolith on top of the habitat prior to inflation then this would reduce the GCR radiation by about 1/2. This would also protect the crew from SPEs. After arriving, the crew could maintain the telerobots which could push yet more regolith on top of the habitat 24/7 keeping well ahead of the crew's need for radiation shielding. By so doing, radiation exposure would never be the reason why crew would need to return to Earth.

EXTRA VEHICULAR ACTIVITIES (EVAs)
The Plan described on these web pages calls for lunar and Martian bases to be more about "development" rather than "exploration". As such, the crew would be spending most of their time in doors developing and expanding the infrastructure and living areas. Scientific exploration would be left to international astronauts to do. For those who remain indoors, the shielding over the habitat would do much to address the radiation problem.

But for those doing EVAs, there is a nice alignment between the need to limit exposure to toxic dust and the need to limit exposure to radiation. In particular, if the cab of the crew rover was lined with either water or polyethylene shielding to about 20 cm, this would both protect them from solar storms as well as significantly reduce their GCR exposure. These crew could none-the-less collect samples by operating robotic manipulators on the front of their vehicle or they could briefly suit-up, step out of their vehicle, examine and collect samples, and then return to their shielded vehicle. The amount of time exposed to the full radiation would be pretty small.