For long-distance manned missions, such as a mission to Mars, we are inevitably going to have to shield astronauts from cosmic radiation, especially in the event of a solar flare or SEP. What materials provide the best protection from the kinds of high-energy cosmic radiation astronauts would be exposed to on these journeys?
There's a pretty good Wikipedia page about it, which lists a couple of options:
- Water makes for fairly good radiation shielding (also discussed with land based radiation here), but is relatively heavy and is consumed during flight.
- Liquid hydrogen is also good, and is used as fuel, so it will already be on board. However, this too is consumed during flight.
- We could change the materials that spacecraft are made out of. Since hydrogen rich materials work well to shield the most common types of cosmic radiation, some plastics could work. However, this would require some reengineering to be practical.
- Like JKor said, human waste works well, but has "grossness" problems. However, this is unique in that it increases instead of decreases as flight goes on, so it could supplement liquid hydrogen and water.
One of the largest problems with bringing up extra shielding is that it tends to be heavy, and more weight == more cost.
The Wikipedia page mentions active magnetic shielding, but that is at this point mostly a theoretical idea.
Shielding is important for unmanned missions as well (though not as much), as radiation can have effects on computer systems by interfering with magnetic storage - see this National Geographic article and this NASA press release for an instance of such an event happening on Voyager 2.
With respect to potential travel in our own solar system there are two general types of radiation that have our concern!
The first type of radiation is solar radiation, which mostly consists of low- to intermediate-energy protons, electrons and x-rays from our own star. We would shield against the protons with low molecular mass materials. Typically hydrogen-bearing materials such as Lithium-Hydride are used for this because of how effective they are at stopping the protons as well as neutrons that might come from future reactors and because of how light they are. The electrons and photons (x-rays) are best stopped with high-Z materials. High-Z materials are comprised of elements that have many electrons per atom. While high-Z materials are used to stop electrons and photons, they are also useful in stopping other charged particles to include assisting with protons.
The second type of radiation is Galactic Cosmic Rays (GCRs). GCRs are typically very high energy massive particles such as carbon, and iron atoms. Due to their energetic nature and how massive these particles are, they are very hard to stop. Stopping GCRs requires thick layers of high-Z materials, which are typically dense and heavy. Heavy shielding is expensive and difficult to get into space. I will not go so far as to say that we cannot shield against GCRs, but I will say that the weight of contemporary shielding materials makes it seem as if current approaches to GCR shielding are not practical.
Our star is a type-G main sequence star, which produces Helium through proton-proton fusion at its core. Because of the dynamics of fusion in our star, ionized Helium nuclides are the primary product of this fusion. However, some of the Helium produced from proton-proton fusion is itself fused, which produces carbon. As stars become more massive they start fusing heavier elements, which can be ejected into space. Iron-56 is the heaviest element that can be produced from traditional stars, with the heaviest elements being produced by much more energetic events such as a supernova.
The energy produced from the fusion of these isotopes ionizes gases near the edge of our star, producing copious quantities of protons and electrons, which are flung into space during coronal mass ejections. Numerically speaking the majority of radiation from our star as well as other stars are in the form of protons, electrons and photons, with lesser quantities of heavy nuclides. Statistically speaking, the heavier the nuclides, the rarer it is to find them streaming in space. While I am mainly speaking about our star, the same is true of other stars, regardless of their mass.
Other stars do indeed produce protons, electrons and photons that stream into our solar sphere of influence; however, these other stars eject radiation in all directions, with only a very small fraction of them being ejected in the narrow cone angle to make it to our solar system. Much of the charged radiation from other stars is also deflected by the sun's magnetic field. As a result, the vast majority of protons and electrons in our solar system were ejected from our star and not other stars and the ones that are not are mostly of the same energy as the protons and electrons ejected from our own star. Because of this we essentially neglect non-solar protons and electrons from our radiation exposure calculations because they are negligible in their effect on absorbed dose.
However, the heavy elements ejected from super-energetic events such as supernova are traveling at near light speeds and as a result, have a profound effect on biological tissue and electronics that they encounter. Even though they make up a very small fraction of the total particle count per unit volume in space, the effects that they can have on absorbed dose is not negligible. Therefore when we talk about galactic cosmic rays, we generally are talking about the energetic heavy ions from extra-solar energetic events and not the protons and electrons from normal, everyday extrasolar sources.
One possible material that was mentioned in Scientific American is fecal matter. The hydrocarbons in it can absorb the radiation safely.
However, most of the general public would reject this possibility because of the grossness factor (just like recycling water by purifying and sanitizing urine).
There is talk that it might be possible to use magnets as a shield from cosmic radiation:
Astronauts travelling to the International Space Station are protected from much of this radiation by the Earth’s atmosphere as well as by its “magnetosphere”, the magnetized bubble of plasma surrounding the Earth created by its magnetic field. However, people on longer flights will not have this natural shielding and are therefore at greater risk.
injecting a supersonic plasma into a 1.5 m long vacuum vessel lined with magnetic coils, with a target magnet placed at the far end of the vessel. Using both optical imaging and an electromagnetic probe, Bamford’s team showed that the target magnet deflected the plasma such that the volume of space surrounding the magnet was almost entirely free of plasma particles.
An image showing how Earth's magnetic field takes care of this:
I know this is not as good as an answer than most here. I would say that a blimp could be inflated around the ship and the gas electrified to create electromagnetic shielding. This method is light weight.