News coverage of a mission to Mars will often result in claims about radiation on the way to Mars, that it’s either a huge problem or will even cause everyone to die on board. However, evidence doesn’t appear to suggest the scale of the problem is anywhere near that big. Radiation, while not to be merely waved away, is not a major showstopper for any mission to Mars.
Radiation levels on the way to Mars
The readings performed by the Curiosity rover on the way to Mars show that the astronauts would be exposed to a total of 1.8 milllisieverts per day, with surface levels being about 0.64 mSv per day. Assuming a 500 day surface stay and 360 days in space, the total radiation dose the crew would be exposed to is roughly 1.01 Sievert over the total duration of the trip. This is associated with a total death risk by cancer of… five percentage points. It would go up from 21% to 26%. The radiation limit for ESA astronauts is 1 Sievert, which means that ESA astronauts would be only barely out of the limit, even if provided only with the thin metal shielding on Curiosity. Only a relatively small amount of radiation protection would be required to get the mission dose under the acceptable limit. According to an ESA study from 2004, only 9 grams per square centimeter of radiation protection is required to get within an acceptable limit, which actually is no additional shielding at all for their habitat design. The NASA limit of 2/3rds of a Sv are more problematic, however.
Also, this is for Galactic Cosmic Rays, or GCRs. These particles are highly energetic and require a load of shielding to get it down to terrestrial levels. Curiosity flew during a solar minimum, which means that the sun’s own radiation was at a minimum, however the sun’s magnetic field is also weaker, which actually increases the amount of GCRs a ship would be exposed to. During a solar maximum, GCRs are reduced significantly and only solar particles provide a significant danger. Solar particles are less energetic and can be shielded against far more effectively.
These are the kind of radiation events that actually form a real danger during the trip. However, solar particles are more easily stopped than GCRs, and the risk they provide can be made almost completely negligible by the addition of a storm shelter for the crew.
The shielding required can be as “low” as 25 g/cm2 to prevent the astronauts from being under serious risks. By putting this shelter in the middle of your spacecraft, like in Mars Direct, you can use your supplies (food and water) to keep the crew safe. Other sources note 300 kg/m2 (or 30 g/cm^2) of water also sufficient to keep the dose reasonable during these events.
So how do we solve this problem?
Using ESA astronauts instead of NASA ones, obviously.
In more seriousness, additional shielding (hydrogen-based shielding like water and plastic are optimal), careful mission planning and crew selection (an old male has lower risk than a young female) and good placement of equipment and supplies on board can significantly reduce the radiation risk posed to the crew.
Is it a problem? Yes. Is it unsolvable? No. Is it going to cause the astronauts to fry on their way there? Not at all.