People become astronauts knowing it's a dangerous job, and measures can be taken to minimise their radiation exposure
|Local magnetic fields may provide some radiation protection in particular regions on Mars, but astronauts' living modules will need to be buried. Image: Nasa|
On Earth, we're all exposed to natural radiation from a variety of sources, including brazil nuts, bananas, and invisible radon gas. We'd be exposed to much higher doses from extraterrestrial sources if the Earth wasn't so well protected. The planet's magnetic field is carved into a teardrop-shaped bubble – the magnetosphere — by the solar wind that blows from the Sun. Although it contains some regions of higher radiation, the magnetosphere's overall effect is to shield life on Earth, and astronauts in low Earth orbit, from the harshest effects of cosmic radiation. Earth's atmosphere provides an additional buffer against all but the most energetic of particles. Astronauts travelling outside the magnetosphere, however, have to rely on the shell of their spacecraft to protect them from danger.
Nasa's rover, Curiosity, which made its spectacular landing on Mars last August, is carrying an experiment to measure radiation levels. The newly published results from the cruise to Mars highlight the fact that had astronauts taken the same journey, their radiation exposure would have reached two-thirds of the maximum, career-long dose set by Nasa in less than a year. Protection from radiation is clearly a serious issue that needs to be addressed for prolonged visits to the moon, the journey to Mars, and for other deep-space missions such as Nasa's plans to visit a near-Earth asteroid.
There are two main sources of cosmic radiation: cosmic rays, a near-constant rain of high-energy particles arriving from elsewhere in our galaxy, and more sporadic bursts of particles from the sun, called solar energetic particles. Major risks can be minimised by travelling during the quietest phase of the sun's 11-year activity cycle, but that's not a completely reliable approach either because dangerous levels of solar energetic particles can be created by solar flares even during minimum activity.
Based on current technology, the most sensible approach to protecting astronauts is to build a highly shielded module into the design of the spacecraft, ie a storm cellar. The radiation measurements from Curiosity show that a large proportion of the total exposure occurred during periods following solar flares, with quiet periods in between. If astronauts are able to shelter for a few days when necessary in a well-shielded module, they're likely to avoid the worst effects of solar energetic particles.
Once at Mars, the danger doesn't go away completely. There are several regions on the planet where local magnetic fields provide some protection, but astronauts will ideally need to bury their living modules for additional protection during extended visits.
People become astronauts in the full knowledge that going into space is a high-risk endeavour. Although highlighting the risks posed by radiation, Nasa will almost certainly deem the Curiosity measurements to be within manageable levels. The will to explore will prevail.