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How are astronauts protected from radiation (alpha, beta, gamma rays, uv rays) by their spacecraft? Is the spacecraft built in a special material?

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    $\begingroup$ This is a pretty broad question, do you have any specific spacecraft in mind, such as onboard the ISS? In general, there's a few methods to lower exposure (passive shielding with matter, active shielding with induced magnetic fields, reducing exposure time, selection of naturally protected regions, risk management,...), prevent onset of problems due to exposure (e.g. boost immune system's ability to cope with increased radiation, radiation treatments,...), and so on. All of these are pretty broad, unless you have some more specific regions of outer space, vehicles and missions in mind. $\endgroup$ – TildalWave Jun 5 '14 at 12:20
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This may sound strange, but part of the protection is a lack of shielding. The high-energy particles that make up a large portion of the radiation environment mostly just pass through the human body. They do, however, interact with traditional shielding materials (eg. lead) to produce secondary radiation that does affect humans. Because of this, light shielding is worse than no shielding: it's thick enough to turn high-energy, low-danger radiation into medium-energy, high-danger radiation, but not thick enough to absorb that radiation.

Hence the use of hydrogen-rich shielding (water, plastics): it's not as effective as traditional materials, but it doesn't produce secondary radiation.

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In current spacecraft, the materials for e.g. the hull are chosen primarily because they are light and strong. Aluminium is common. A few mm of aluminium blocks most of the radiation you would encounter in low Earth orbit. In the ISS, 95% of the radiation is blocked.
This is enough for low Earth orbit: these orbits are inside the Van Allen Belts, so they are protected from the worst radiation. If we want to go beyond LEO for longer periods, more protection is needed. You could make the hull thicker, but this makes the launch more expensive. One way around this would be to use a small asteroid and build the spaceship inside it, but that leaves the problem of capturing an asteroid and bringing it to Earth. We've never done that.
Another way is a magnetic shield. This requires a lot of power. I seem to remember a figure of 500 kW for a reasonable shield for a small spacecraft (I can't find the reference at the moment though). The ISS (with its massive solar arrays) can produce about 200 kW.

Some interesting related questions and answers:
Efficiency of radiation shielding
Materials

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  • $\begingroup$ Not only does using a thicker hull make the launch more expensive (because you need more fuel to lift an identical useful payload mass), it lowers your maximum useful payload mass too. The Ariane V can lift 15 tons to LEO (table 3). If the hull weighs 1 ton (figure out of thin air) then you have 14 tons you can use for other things; if the hull weighs 5 tons, that leaves you with only 10 tons for anything actually useful. I'm not sure whether this includes take-off fuel but that doesn't alter the big picture. $\endgroup$ – a CVn Jun 6 '14 at 19:05
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    $\begingroup$ You can find some info on power requirements of a dipole magnetic field in my answer to How much power would a spacecraft's magnetic shield require?. Numbers are much smaller and to the surprise of the researchers too, this actually works, which is of course great news. Refer to cited documentation there for more info. ISS solar panel arrays generate up to 120 kW (average is lower, IIRC 84 kW). $\endgroup$ – TildalWave Jun 6 '14 at 23:46
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Most space-based radiation, primarily charged particles, is very effectively blocked with hydrogen-rich materials. Scientists for the Lunar Reconnaissance Orbiter discovered that, pound for pound, polyethylene was a more effective radiation shield than aluminum.

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    $\begingroup$ See Mark's answer. Hydrogen is prefered not because it blocks radiation better, but because it minimizes secondary radiation. If a lead atom is hit by cosmic radiation, it could split into a spray of several high energy nucleaus. But hydrogen is only a single proton so it cannot multiply the radiation rays upon impact. $\endgroup$ – LocalFluff Jun 8 '14 at 7:04
  • $\begingroup$ The electronics (or the human flesh) inside a spacecraft do not care whether a radiation particle is primary or secondary. The change in energy, however, from something that passes completely through to something that lodges and displaces, is a real problem. We are seeing it now on solar cells in orbit. $\endgroup$ – Tristan Jun 9 '14 at 14:16
  • $\begingroup$ It is my humble understanding that no matter how powerful, one ray can only hit one atomic nucleus, for example in a DNA molecule or a transistor in an IC. But if it hits a heavy nucleus in a metal screen a bit away from a human, that heavier nucleus splits into a spray of high energy rays which risk hitting several DNA nucleus behind it. A hydron nucleus, on the other hand, is just one proton so it cannot split, no matter by what force it is hit. The secondary rays are numerous, and each one of them still harmful. Rather 1 hit by one single overkill than 10 hits by sufficient kills. $\endgroup$ – LocalFluff Jun 9 '14 at 14:28
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    $\begingroup$ I can't speak to the biological effects so much, but for electronics, there is a sweet spot in radiation energy that does the most damage. A high energy proton will pass through a silicon PN junction with relatively little damage. If the energy is just right, however, the proton will lodge directly in the junction, introducing a permanent flaw. Enough of these, and the junction will fail. $\endgroup$ – Tristan Jun 9 '14 at 14:31
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    $\begingroup$ @LocalFluff. Cosmic rays aren't going to split lead atoms into a "spray" of lighter nuclides. The secondary radiation is mostly in the form of X rays: en.wikipedia.org/wiki/Bremsstrahlung $\endgroup$ – Solomon Slow Feb 15 '18 at 19:54

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