Galactic Cosmic Rays
Galactic Cosmic Rays (Galactic CR) are cosmic rays that do not come from the sun, but may have been created inside our galaxy. They may have low energies but range up to the millions or billions of electron volts. Cosmic Rays come from every direction in the sky, unlike Solar CR which come from one location (the sun).
Contents
Discussion
The focus of this discussion is on the radiation concerns of cosmic rays. The lower the energy, the easier they are to shield against. For example, low energy cosmic rays can be directed by the Earth's magnetic field and to impact at the polar regions. Higher energy cosmic rays move so fast, that they have not the time to be so redirected. Low energy Cosmic Rays (CR) are completely stopped by the Earth's atmosphere. High energy CR will explode on hitting Earth's air, and cause a cascade of secondary particles which can reach ground level, and even penetrate many meters below the Earth's surface.
(Scientific experiments that wish to avoid Cosmic Rays (CR) are built in mines a couple km below the surface, or deep in Antarctic ice.)
Origin
If a Cosmic Ray hits a detector, we can not know where it came from. Did it come from the Milky Way, or some distant galaxy? There is no way to know. When we discuss Galactic CR, we are talking about Cosmic Rays that MIGHT have come from within our galaxy. From supernova shock waves, neutron star mergers, accretion disks from small black holes, massive Coronal Mass Ejections from super hot stars, and the like which exist inside our galaxy. Some Cosmic Rays are so powerful, that we think that they must be made in active galactic nucleus or quasars (see Extra Galactic Cosmic Rays) which are far from our galaxy.
Mitigation
Low energy particles can be stopped by the normal Radiation protection given to Solar Cosmic Rays. However, higher energy Galactic CR, fall into the energies which are largely stopped by the Earth's atmosphere, but are not stopped by Mars' thin atmosphere. Thus they are a real concern for Mars colonists. (For short term explorers, they will simply accept a couple years of higher radiation, similar to doses accepted by current long duration astronauts.)
These medium energy Cosmic Rays (CR) are not stopped by magnetic fields (they don't spend enough time in them to be redirected any great distance, they just punch thru). So the only way to mitigate them is to have mass. If we don't have dozens of km of thick air, then we need water or soil between us and the sky. Generally we want to have at least 4 meters of water or 2 meters of soil to bring radiation down to acceptable levels, tho more be better (because people will spend time outside with less protection and we want the average dosage to be low). Some authors on this site, who prefer almost no radiation, suggest values up to 6 times greater than this minimum. (See Radiation.)
Note that the lower energy Galactic CR are pulled towards the sun's magnetic poles by the sun's magnetic field. So when the sun is at its solar maximum, we get lower doses of the low energy Galactic Cosmic Rays than when the sun is at solar minimum. (But then again, when the sun is active, we get more Solar CR.) This has little effect on high energy Galactic CR which quickly punch thru the Sun's magnetic field. (On Earth we get Cosmic Rays both during times of Solar Maximum and Solar Minimum.)
Radiation that comes in sudden intense bursts is far more dangerous than a slow, steady trickle of radiation damage, because our bodies constantly, slowly repairs radiation damage. Galactic Cosmic Rays comes in a slow, steady trickle, so there is no need for a radiation storm shelter for sudden bursts of galactic CR.
