What isn't expected is that this is still the general consensus today, even though a more recent computational study has provided some interesting numbers for various shielding materials.
|Shield Material (5g/cm^2)||Annual radiation dose (mSv*)|
This looks pretty good when the astronaut lifetime radiation limits are considered.
|Astronaut age||Career effective dose limits|
(mSv, average life loss)
|25||520, 15.7||370, 15.9|
|30||620, 15.4||470, 15.7|
|35||720, 15.0||550, 15.3|
|40||800, 14.2||620, 14.7|
Polyethylene shielding works better than aluminum which works better than iron because it has more carbon atoms. Adding shielding to the spacecraft and only sending crew of the appropriate age can drastically reduce the amount of life you take from astronauts on one-year long missions. Of course, most Mars missions designs are much longer duration than this and so more heavy radiation mitigation is needed.
Before discussing those options, let's think about how heavy this "light shielding" actually is. A Saturn S-IVB provides a heck of a lot of space compared to an ISS module, and has proven sufficient for a one year journey by SkyLab. It was 17.8m long by 6.6m diameter. With flat ends, this is a surface area of 4,374,982 cm^2, and at 5g/cm^2 the polyethylene shielding would weigh 21,874 kg.
On a trip to Mars one does not only have to consider the fuel required to burn in LEO to get to escape velocity, one must also consider the fuel required to do course corrections to get to and maintain Mars transit and perform Mars orbit injection. One way to reduce this fuel is to carry a large heat shield and do aerobraking. All this fuel is mass, typically with a lot of carbon in it, and so can also be used to shield the crew.
It seems reasonable at this point to wonder exactly how much radiation protection you and I get here on the surface of the Earth. The answer "enough" is sufficient so perhaps a better question is, how? Go outside and look up, what do you see? Air. How much? The answer is 1,030g/cm^2. As such, the 22t of shielding we added is providing just 0.49% as much protection.
If we want to provide sufficient radiation protection for long duration spaceflight, it seems obvious that we need to make the spacecraft smaller. Shielding just a smaller part of the spacecraft would be pointless as the crew is required to stay in there for the majority of the trip anyway.
Let's consider a 8m long cylinder of 4m diameter. Internally, it would be about 80 m^3 of pressurized volume. The surface area is 1,256,637.06 cm^2. Here's the masses required for various levels of radiation protection.
|Radiation protection (vs sea level)||Mass (kg)|
As you can see, the life expediency of the crew can be improved by almost 3.5 times by reducing the surface area of the living volume by just under half. An interesting rule of thumb: 7% of Earth normal radiation shielding takes one year of lifespan off men for every year spent on the mission.
Making the spacecraft even more cramped and arranging the storable propellant and supplies as additional shielding would permit the creation of 5% of Earth normal radiation shielding, meaning the crew could go on the long multiyear excursions required to explore Mars, but no conceivable technology today can practically provide passive shielding for 100% radiation protection.