The earth’s magnetic field and the atmosphere protect life from much of the radiation prevalent in space. That radiation consists primarily of two types: energetic charged particles from the sun and super-energetic particles, typically referred to as galactic cosmic rays (GCR), from deep space sources far beyond our solar system.
The sun’s particles, mostly protons and electrons, can be quite intense during a solar storm but are in an energy range such that a modest amount of shielding will block them. Though they are above the atmosphere, crews on the ISS are fairly well protected by the deflection properties of the earth’s magnetic fields. (Particles trapped by these fields create the Van Allen Belts and the aurora at the north and south poles.)
Cosmic rays, on the other hand, are very sparse but their extremely high energies makes them difficult to shield against. And a bit of shielding can, in fact, be a bad thing since when a GCR runs into another particle, it will create a shower of many additional particles, which if not blocked by additional shielding, will greatly multiply the radiation dosage to any living tissue they encounter.
They are so energetic, cosmic rays are hardly affected by the earth’s magnetic fields. However, the earth’s atmosphere is thick enough to absorb most of them such that the showering particles are either blocked or converted to particles like muons that interact every little and are eventually absorbed deep in the earth’s surface.
For human settlements on the Moon, Mars or other body, the local materials can provide material shielding sufficient to block both solar radiation and cosmic rays. For spacecraft traveling in space, keeping the total mass as minimal as possible is a top design requirement, at least with current propulsion systems. While solar radiation can be fairly easily blocked (an extra heavily shielded “safe room” could be available during a solar flare), cosmic rays are much more problematic. Designing spacecraft habitats inside out such that a crew on the way to, say, Mars is surrounded by all the equipment, food storage containers, fuel tanks, etc. would definitely help but could still result in substantial cumulative dosages. (Even those dosages, though, are likely to be minor risks compared to all the other risks on a Martian mission.)
An alternative, or supplement, to material shielding is to use a magnetic field that deflects particles from the spacecraft’s habitat. Such a system should be designed in such a way that no magnetic fields reside inside the internal volume of the spacecraft where people live.
There have been various designs proposed over the years for magnetic shields. During the recent NIAC Symposium (see earlier posting), plasma physicist John Slough described what looks to be a viable design that he calls the Magnetospheric Torus (MDT). It appears to be doable with current technology, e.g. high-temperature superconducting magnets are now commonplace, and is quite effective, even for the highest energy GCR.
A prototype system can be tested and optimized on earth by examining how well the system deflects all those muons flowing through us constantly. Slough describes the MDT design in this video of his NIAC talk (his presentation starts at around 27:00):
Space radiation is often portrayed in the press as some sort of deal-breaker for long distance spaceflight. That is simply not true. Whether building really big spaceships that allow for a lot of material shielding or using magnetic shielding (or employing both approaches), radiation can be dealt such that it becomes a relatively minor issue.