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Dust particles are not really a problem in space travel, as they are few and far between and are microscopic. Cosmic rays can begin to affect anyone who has left the magnetosphere of the Earth, and can cause changes to DNA. Really harmful affects are not predicted until after about two years in space, but in traveling to Mars, the atmosphere there is not as strong, so cosmic rays could be a problem for any colonist. Scientist still are not sure of the true extent of the problems, as no one has stayed in space for much longer than a year and a half at once.