A manned mission to Mars is a big dream for many, but certainly there are obstacles ahead, one of which will be radiation. The radiation of Mars has been measured by Curiosity’s Radiation Assessment Detector from inside the spacecraft, and data suggests that the levels are way higher than what could be stopped with current radiation shielding.
According to NASA:
The findings, which are published in the May 31 edition of the journal Science, indicate radiation exposure for human explorers could exceed NASA’s career limit for astronauts if current propulsion systems are used.
Two forms of radiation pose potential health risks to astronauts in deep space. One is galactic cosmic rays (GCRs), particles caused by supernova explosions and other high-energy events outside the solar system. The other is solar energetic particles (SEPs) associated with solar flares and coronal mass ejections from the sun.
Current spacecrafts are capable of protecting astronauts from SEPs, but not GCRs.
If someone is exposed to 1 Sievert (Sv) of radiation over time there is a 5% increase in risk of that person getting cancer. NASA has set the limit to a 3% increase in risk for its astronomers. Curiosity’s trip to Mars exposed it to an average of 1.8milliSv per day, and the exposure for the whole trip is said to be equivalent to “getting a whole-body CT scan once every five or six days“, which won’t be good for human explorers.
But technology to shield humans from this sort of radiation is available on Earth. Big and heavy materials can be used as shields, but the problem will be when travelling in space. The fuel costs will be very, very high if current propulsion methods are used.[Source]