There are three types of cosmic rays; Galactic, anomalous and solar. The Galactic ones come from supernova remnants such as the Crab Nebula and Loop I. The Anomalous cosmic rays come from the boundary zone between the interstellar medium and the sun's solar wind about 100 AU from the sun. The solar cosmic rays come from active flares and other regions on the solar surface. The most energetic cosmic rays are the Galactic kind, and these are the most troublesome for interplanetary travel.
Because the Earth's magnetic field shields astronauts from most of the harmful cosmic rays, astronauts near the earth do not have a major problem with them compared to the particles in the van Allen belts or solar flares. But there is no such shielding in interplanetary space for long voyages. Within a few years, astronauts can build-up nearly 200-400 rems of exposure to them which equals a lifetime dose limit. They are hard to shield because they come from all directions and when the penetrate matter they can spawn secondary bursts of particles much like 'air showers' seen here on earth. Cosmic rays are modulated by the solar activity cycle and in the inner solar system, there are somewhat fewer of the energetic kinds during sunspot maximum conditions than during sunspot minimum...that's right, they are out-of-phase with the solar cycle.
For more information, visit the Goddard Cosmic and Heliospheric Learning Center page which has a good primer on them.
Copyright 1997 Dr. Sten Odenwald
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