Al Globus and Joe Strout have an analysis that space settlements in low (~500 km) Earth equatorial orbits may not require any radiation shielding at all. This is based on a careful analysis of requirements and extensive simulation of radiation effects. This radically reduces system mass and has profound implications for space settlement, as extraterrestrial mining and manufacturing are no longer on the critical path to the first settlements, although they will be essential in later stages. It also means the first settlements can evolve from space stations, hotels, and retirement communities in relatively small steps.
This huge reduction in total mass compensates for the greater energetic difficulty of launching materials from Earth to ELEO as opposed to launching from the Moon to L5, the design location of the Stanford Torus. In the early studies, the EarthMoon L5 point was chosen as the location of a settlement for the energetic advantage of launching materials from the Moon. Going from the Moon to L5 requires a delta-v 3 of 2.3 km/sec, and going from Earth to 500 km ELEO is 10 km/sec [Cassell 2015]. Using the velocity squared as our energy measure , Earth to ELEO requires 19 times more energy per unit mass. Analysis suggests that at least 19 times less mass is needed if no radiation shielding is required. Thus, the energetic advantage to launching the mass of a settlement with deep space radiation shielding from the Moon to L5 is balanced by launching far less mass from Earth if no radiation shielding is necessary.
A 500 km circular ELEO using polyethylene shielding was analyzed. Even at 10 kg/m2 shielding, the equivalent of which is very likely to be provided by any reasonable hull, the 20 mSv/yr and 6.6 mGy/yr are met. Indeed, with no shielding at all the general population limit is met and the pregnancy limit is very nearly met. This has an interesting consequence: spacewalks in ELEO may be safe enough from a radiation point of view to be a significant recreational activity.
The total mass of the 4 rpm unshielded (56 meter diameter Stanford Torus, 123 person) space colony could be launched from Earth with about 40 Falcon Heavy vehicles.
The space settlement rotation rate recommendations of [Globus 2015] are:
● Up to 2 rpm (rotations per minute) should be no problem for residents and require little adaptation by visitors.
● Up to 4 rpm should be no problem for residents but will require some training and/or a few hours to perhaps a day or two of adaptation by visitors.
● Up to 6 rpm is unlikely to be a problem for residents but may require extensive visitor training and/or adaptation over a few days. Some particularly susceptible individuals may have a great deal of difficulty.
● Up to 10 rpm adaptation has been achieved with specific training. However, the diameter of a settlement at these rotation rates is so small (under ~40 meter for seven rpm) it’s hard to imagine anyone wanting to live there permanently, much less raise children. Rotation at high rates, however, may be useful for a dedicated radiation study station in ELEO.
Note that there are two classes of people that must be accommodated: residents and visitors. For residents a few days of feeling ill at the beginning of a multiyear stay is of little concern. However, if a settlement expects many short term visitors it may be best to keep the rotation rate under about 4 rpm.
The Kalpana One space station design at 4 rpm requires 17 tons/person. The cheapest advertised price today for delivering mass to orbit is the Falcon Heavy, in development, at $90 million for 53 tons to LEO [SpaceX 2015], or $1.7 million per ton. For 17 tons that is about $29 million.
The cheapest advertised price to launch people to LEO is a bit over $26 million/seat on a Falcon 9/Dragon which includes a stay at a Bigelow space station [Bigelow 2015], also in development. It should be noted that this cost must be incurred for settlers going to any space location.
Combining these two costs gives us (rounding up) $60 million per person. This does not include materials, construction or resupply costs. We assume that government or space tourism businesses will conduct most of the research and development cost other than actually building a settlement.
To get the transportation costs to close to one million dollars, leaving some small number of millions for everything else, we need to reduce the cost of launch by about a factor of 50 to around $1.2Million/person. Notice that these are extremely rough calculations, but are sufficient for planning purposes.
Elon Musk is trying to make hundreds of flights per year economic by launching and maintaining a network of 4000-20,000 internet satellites.
To reach a 50 times price reduction will almost certainly require fully reusable launch vehicles, much improved technology and a very high flight rate, probably in the tens of thousands per year. The reusability and technology requirements are generally recognized but for some reason flight rate is often ignored. However, with fewer than 100 launches per year today, a single reusable vehicle capable of two flights a week could, theoretically, satisfy the entire launch market! Even 1,000 flights per year would only require 10 such vehicles. Large reductions in price will not come if vehicles are built in such small numbers. Launch vehicles only make money when they fly, so we need a very high flight rate, probably over 10,000 flights/year.
There are only two applications that, at the right price, could create a market requiring a flight rate of ten of thousands or more per year: space solar power (SSP) and tourism. SSP requires a very large investment up front before any income is generated and is vulnerable to terrestrial competition, particularly as batteries improve.
Tourism was a $2.3 trillion/year industry in 2014.