...My question was based on the idea that it is so hard to stop cosmic rays. If WE can't stop cosmic rays from sailing right on through us, then we aren't actually absorbing much. Damage comes from what we absorb, making energy from different particles dissimilar in effect, right?
Why put a space? The particle will just cross that space.
Well, that rather depends upon the particle. Some secondary particles will decay after travelling a certain distance (or conversely expressed, a certain duration). Others will radiate in a non-linear direction from the creation point. I wasn't familiar with the identity of the secondary particles produced when cosmic rays are absorbed into shielding.
If we had wanted to, we could have launched the Shuttle with the main tank attached and leave it up there, so that now we'd have a bunch of big empty cylinders in orbit.
I had heard that suggestion a few years ago...but apparently they decided not to for one reason or another.
You are right, cosmics do not deposit much energy in us. But consider the difference- a cosmic ray hitting a carbon atom in graphite, or hitting a carbon atom in your mitochondrial RNA.
Our astronauts get about three times the dose of the average nuclear worker on Earth, for a shorter time. I put up a link somewhere above to a NASA site that goes into some detail about astronaut radiation and mitigation techniques.
I also did not explain too well about the secondaries from the shielding- I provided a link to a transport calculator that showed how little the high energy particles are attenuated- but it did not show how much of that energy is converted into other radiation. If you use a high-Z shield (look at teh Bethe-Bloch stopping power equation- Hi-Z materials stop charged particles more efficiently than low Z ones) the stuff spalls off lots of neutrons that interact further, as well as showers of energetic electrons and photons. Your shield becomes a radiation source requiring more shielding.
I think the answer is to build a lunar base and build your Mars or asteriod ship there. Heck, maybe with a beanstalk launcher! A manned ship that can explore the solar system is going to be big anyway, the shielding bulks it up a bit that's all.