The Earth's atmosphere and magnetic fields protect life from the Sun's radiation. This protection is even strong enough to cover astronauts and cosmonauts circling above the atmosphere. As long as they stay in close orbit, the Earth's magnetic field can still deflect enough radiation to shield the spacefarers from dangerous levels. Only during solar flares, when the Sun is unusually active and releases huge amounts of energy all at once, are the men and women in earth orbit in real potential danger from radiation.
But once people are in deep space, radiation from the Sun is a serious hazard, and a ship on a trajectory to Mars will have to be able to protect its passengers. In all likelihood, only one area of the ship will be fully shielded, and the crew will have to stay in this area from the moment a solar flare or radiation alert is sounded until normal conditions return, a process that could take days.
People traveling to Mars will also be exposed to another source of radiation called Galactic Cosmic Radiation. This radiation is constant, and there is no way to protect deep-space travelers from it. It comes from exploding stars or supernovas somewhere in the galaxy, and it contains high-energy particles called heavy ions. Because such radiation only occurs in space, no one knows for certain what effects it could have on the human body during a long interplanetary flight.
