As much as some folks are keen on sending people to Mars as soon as possible, it’s become obvious that protecting any astronauts from an unsafe level of radiation before they even get to Mars is going to be a tricky business.
There are two main problems for astronauts leaving our home planet; one is cosmic rays, which are usually turbo-speed protons from outside of our solar system. Some cosmic rays are blocked by our Earth's magnetosphere, and the remainder are usually stopped by our atmosphere. The other problem comes direct from the Sun itself; the Sun also flings electrons and protons in our direction in the solar wind. The solar wind is mostly stopped by our magnetosphere, but if you’re going out a bit further, we won’t have that protection.
The solar wind is usually relatively easy to protect yourself from; with a slightly thicker wall than the bare minimum on your spacecraft, you can usually protect your crewmembers from a solar wind related battering. However, cosmic rays are harder to stop. The protons which make up cosmic rays typically have more energy to them, so shielding has to be more robust. The second problem with cosmic rays is that sometimes they’re more than just a proton; they can be an entire helium nucleus (two protons, and two neutrons), making them a projectile that’s both very high speed and four times the mass of a solar wind particle. These enormous cosmic rays can break apart, at an atomic level, the material they crash into, filling the interior of your spacecraft with radiation, which is not great for anyone trying to live in there.
Once a spacecraft leaves the Earth’s protective bubble, not only does the cosmic ray dose increase dramatically, but you’ve also got a much less protected place to deal with the solar wind. And if the Sun decides to unleash a solar flare in your direction, you’ve got an awful lot of protons coming your way from the Sun, in addition to the galaxy in general pelting you with helium nuclei.
Unprotected, a solar flare can rapidly give you radiation sickness, which makes you tired and also makes you vomit. Fortunately for all involved, most spacecraft have thick enough walls that the crew should be protected from solar flares, but it’s generally considered good practice to reduce all possible risks. On the other hand, cosmic rays are not so easily stopped.
Because cosmic rays are fundamentally a charged particle, using a miniature magnetosphere surrounding the spacecraft would be an effective way of keeping them away from both your crew and the walls of the spacecraft; if this could be built into a spacecraft, you wouldn’t need to bulk up the outer surfaces of the craft for radiation protection. However, actually doing so is a bit beyond us at the moment. There have been a number of proposed magnet configurations developed, and a recent simulation of three different styles indicated that the magnetic shielding could, in fact, reduce the overall radiation dose an astronaut would receive. This is not a given, because to create such a magnetic field, you need to add extra stuff to your spacecraft; the more mass you have, the more stuff Galactic cosmic rays can bash into, filling your craft with extra radiation. However, these portable magnetospheres are only just in the design phase --the next big steps will be building them, making them lighter, easier to power and making sure they work they way we hoped they would. At this point, all we can really say is that it should be possible. We'll have to wait and see if it's also practical.
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