I'm not sure that in the remainder of my life that I will see any manned spaceflights leave Earth orbit, but more and more discussion is occurring over this, whether it is Obama wanting NASA to land on an asteroid or the 1-way suicide mission to Mars that 500,000 people have already volunteered for. While there are many concerns with such missions, one of the biggest challenges is radiation protection. Not solar radiation, but galactic radiation. The latter is made up of high energy particles that pose a great risk to carbon-based life forms.
I've always figured that metal would be used for protection against this. Metals have a high density, not just at the bulk level, but at the atomic level. We all know that atoms have as much empty space in them as, well, outer space, so the closer together you can pack the atoms, the tighter the mesh you have for absorbing/scattering/reflecting the radiation photons. Silly me. A new report in Space Weather  ( ($) or here is a free review article) shows that plastic might be the preferred option. The plastic has to be pretty thick, but it's lightness seems to be a strong consideration for its use.
Let me explain the research in some more detail. The researchers put a set of detectors on the Lunar Reconnaissance Orbiter (LRO) and shielded some of them with "A-150 Tissue-Equivalent Plastic", something that I've never heard of. It was apparently developed to mimic human tissue and is this odd mixture of "45.14% polyethylene, 35.22% polyamide (du Pont nylon Zytel 69), 16.06% carbon black and 3.58% calcium fluoride by weight..." The shielding was 81 mm, just over 3 inches thick. The amount of shielding varied with the energy of the radiation levels, but overall reductions (from Table 1 in the report) were about between 84 and 97%! Not bad for some cheap, light-weight plastic.
These measurements were made over a 2-year period, and that raises my one concern that was not addressed in this report - how the shielding changed over time. The radiation would alter the makeup of the plastic, crosslinking and scissioning the backbones with wild abandon . This would undoubtedly alter the shielding performance over time. Since this experiment was on the LRO, the plastic is not going to be recovered for examination. Too bad. I would love to see what type of material was left. Probably lots of powder, but also some Swiss-Cheesed matrix that is a tough as an aerogel - meaning that you handle it much more kindly than you handle a newborn baby's fingers.
This doesn't mean that we are going to start building plastic spaceships anytime soon , as the metal is still needed for impact strength with space rocks and other debris, but it does mean that there should be a thick layer of plastic between the hull and the living space. And since polyethylene, nylon and carbon black can all be biosourced (for a premium price) we can have a sustainable shielding layer as we go rocketing around the solar system (propelled by non-sustainable fuels in a non-sustainable hull).
 "Space Weather" How cool is that, that you get to read about polymers in that journal? Know any chemists reading "Space Weather"?
 ALL radiation creates both crosslinks and scissions. For any given plastic, one mode dominates (for HDPE, it is crosslinking, while for PP it is scissioning) but both reactions are occurring regardless.
 Too bad, as the weight reduction would allow for some really big payloads