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As I understand it; being away from earth subjects you to significant health risks from radiation. On Earth most of this risk is deflected by Earth's magnetic field. I did not find a reference but presumably this magnetic field could be duplicated in a spaceship given sufficient energy. I also understand that some radiation shielding can be created by use of materials (lead being a well known example).

If you have a spacecraft comparable to the ISS in mass and volume (different structure of course) moving around the solar system under a constant 1g thrust (turn over and decelerate halfway), what would be the more efficient use of energy; Moving the extra mass or the energy required to generate the magnetic field?

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In space (within the Solar system), you will get mostly two types of "radiation" that have health consequences:

  • Photons of various energy, from long wave radio to gamma rays.
  • High-energy charged particles, mostly electrons and protons ejected from the Sun upper atmosphere (this is known as the solar wind).

Main source for these is, of course, the Sun. Photons, being electrically neutral, totally laugh at magnetic fields; a "magnetic barrier" will work only for charged particles. We know what UV can do to human skin despite an atmosphere, so one can imagine that some extra shielding will be needed in space.

Assuming that you have superconductors, you can maintain a powerful magnetic field for indefinite times, with energy being consumed only when a particle is indeed deflected. The shape and position of this field requires some care, though. For instance, the Earth magnetic field is not very good at protecting the Earth from solar wind; instead, it just moves around the impact point: high energy particles concentrate on polar regions, producing beautiful auroras. An awful lot of research on the subject of optimal magnetic shielding for spaceships is referenced from this page.

An aggravating circumstance of radiations in space is that it does not occur with a continuous flow; instead, it comes in bursts of considerable intensity, when solar flares occur. A good space shielding will be utter overkill most of the time, but will occasionally become an absolute necessity to avoid the crew being, well, killed over. A mitigating characteristic, though, is that the source position is well known (the Sun tends to be highly visible) and flares can be observed "visually" some hours before the onslaught of high-energy particles, giving time to raise extra shields.


Outside of the solar system, things change quite a bit. The solar wind actually creates a kind of "bubble" around the Sun, called the heliosphere, which acts a bit like a magnetic shield against the rest of the Universe. On the border of the heliosphere is a rather confused situation about which much is theorized but little is known; the Voyager 1 probe is currently moving through it. Beyond, there is not much to fear about solar wind, but a lot more about other high energy particles of many types, collectively known as cosmic rays.

We don't really know where cosmic rays come from, but the sources appear to be multiple. For our present discussion, this means that cosmic rays don't come from a unique predictable direction, and happen at seemingly random times, so shields of any kind must be up at all times. Moreover, not all of these particles are charged, so magnetic shields won't be enough.

Note that cosmic rays are also a problem within the solar system, even close to the Earth, but leaving the heliosphere increases the issue dramatically.

An extra hazard is beautifully exposed in Arthur C. Clarke's "The Songs of Distant Earth". If you are outside of the heliosphere, then you are traveling to the stars -- so you must be traveling fast, because stars are far, far away. This implies that low-energy particles or bigger fragments (e.g. stray atoms or molecules from nebulae) will have a high relative speed, and the repeated impacts will be damaging for the ship and its inhabitants. In the book, they add a big layer of ice in front of the ship, and must renew it regularly.


As for materials for more tangible shields (which will protect from neutral particles as well), a good candidate is not lead, but water. Water has a very good ratio of absorbing power per weight; also, water has other uses that lead does not offer, such as bathing, watering plants, raising fishes (tilapias offer a lot of protein while requiring only limited amounts of swimming space), and, come what may, even drinking, should the onboard stocks of decent beverages become depleted.

A popular design is a space ship as a big tumbling cylinder, creating "artificial gravity". The "ground" (the cylinder surface, from the inside) can be a big pool, and habitats would then be floating, like fish farms. The water maintains the inner ecosystem and provides excellent radiation shielding at the same time. Astronauts double as sailors.

Other possible materials include various polymers, gold (used for lunar modules on Apollo missions -- when you go to the Moon, you do it with style), and even "biological waste" from the crew. This whole radiation issue is still one of the unsolved problems for the trip to Mars, so that's an active research area.

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  • $\begingroup$ Would a small but heavy-duty shield be sufficient? Since radiation appears in bursts, and these are detectable ahead of time, there could be, say, a small, well-shielded room on the ship where the crew could wait out through the burst. $\endgroup$ – SF. Jul 31 '13 at 13:52
  • $\begingroup$ Sci-fi comes to the rescue. In "Red Mars", the crew for the colonization mission to Mars experiences a solar flare, and they take shelter between their water tanks -- tanks that they needed to bring with them anyway. That's a free shield. Why bother with a big magnet if you already have water ? $\endgroup$ – Thomas Pornin Jul 31 '13 at 13:56
  • $\begingroup$ There is also some interesting results that are worth highlighting regarding metal foams with borated water, that seem to have good halving masses researchgate.net/publication/… $\endgroup$ – lurscher Nov 13 '13 at 15:28
  • $\begingroup$ Gold on Apollo lunar module??? I believe it was aluminized kapton (something like mylar), which only appears to have a gold color. See en.wikipedia.org/wiki/Space_blanket $\endgroup$ – Anthony X Aug 21 '16 at 23:25
  • $\begingroup$ In fact NASA produced both aluminized and goldized Kapton; and Kapton itself is only a component of the multi-layer sheets covering the lunar modules. Most of what you see on the outside is only gold-coloured, but some gold still lingers in there. Gold covering was also used inside the module, for some instruments; and the coating of the astronauts' helmet visors was pure gold. $\endgroup$ – Thomas Pornin Aug 22 '16 at 13:50
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Another item to consider with solar flares is that since there is the potential ability to "see them coming", there is a good potential for having a significantly reduced amount of shielding. Specifically, the ship could have a sort of bomb shelter, but in this case a solar flare shelter. A subsection of the ship large enough at least for the inhabitants could be shielded much more heavily than the rest of the ship, thereby reducing the costs associated with shielding (energy usage and $ in general).

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  • $\begingroup$ Very good idea. $\endgroup$ – Kilisi May 24 '16 at 9:08
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The danger of radiation are overstated.

A conjunction class mission will give an astronaut 31.8 rems in transit from cosmic rays (both ways), 10.6 rems from cosmic rays on Mars (assuming you stay about a year there), solar flares in transit will cause 5.5 rems (assuming you have a cosmic ray storm shelter), and 4.1 rem on Mars (the planet body screens out radiation from below and the atmosphere helps against solar radiation).

This is less than the lifetime radiation dose of an airline pilot and lower than some long term stays on the International Space Station. It has zero chance of doing harm in the short term and gives a 1.1% chance of a guy getting a fatal cancer over the rest of his life.

You could reduce this radiation exposure a smidge by putting sandbags on top of the habitat on Mars to screen out a little bit more, but time on Mars is valuable. There are better things to do.

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Cosmic radiation can be shielded by mass. This may require hulls several feet thick, of something like ice. A two-foot thick hull encompassing a habitat only 15 feet in diameter would comprise a very large portion of its total mass. However, the walls don’t have to be thicker when a habitat is larger. For a habitat that is 100 feet in diameter, 2 foot thick walls is a much smaller portion of its total mass. So using mass to shield looks terrible with small habitats but much more doable with large ones. These walls would also protect against meteors. So there is bound to be a push for larger habitats as we become able to build them.

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    $\begingroup$ Two feet actually would make the problem worse. When cosmic ray particles collide with the nucleus of an atom, they create a shower of new particles, which each can create another shower of particles when they collide with something. You aren't protected unless there is enough material to disperse the energy so the cascade stops. It would take roughly 5 m of water to protect you. Source $\endgroup$ – kim holder Aug 21 '16 at 23:06
  • $\begingroup$ I just picked the number 2 feet out of the air to make a point. I am aware of this idea about scatter. I know water is a better mass than many substances (it's the hydrogen) So a given thickness of water would block more than the same mass of, say stone. Plastics are being considered too, for the hydrogen content. I don't know that research has determined how thick it should be. Last I read it was still up to debate. If you are aware of any research with definitive answers on how much mass, of what substance, is needed I'd love to hear about it. $\endgroup$ – Johnny Robinson Aug 22 '16 at 2:39
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    $\begingroup$ It is true it is up in the air, however 2 feet would definitely make matters worse, not better. The paper referenced makes a good case that 5 m would be enough, but it does have to be tested in space. Stack Exchange has a voting system in order to encourage people to write answers that are as clear, informative, and well researched as possible. Comments like my previous one are to help people improve their answers. $\endgroup$ – kim holder Aug 22 '16 at 2:55

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