Solar Radiation Storm
Solar radiation storms are swarms of electrons, protons and heavy ions accelerated to high speed by explosions on the sun. Here on Earth we are protected from these particles by our planet's atmosphere and magnetic field.
Astronauts in Earth orbit are fairly safe, too; Earth's magnetic field extends out far enough to shield them. The danger begins when astronauts leave this protective cocoon. The Moon and Mars, for instance, have no global magnetic fields, and astronauts working on the surface of those worlds could be at risk.
Spacecraft and satellites are also affected. Subatomic particles striking CPUs and other electronics can cause onboard computers to suddenly reboot or issue nonsense commands. If, say, a satellite operator knows that a storm is coming, he can put his craft in a protective "safe mode" until the storm passes.
The type of particle most feared by astronaut safety experts is the ion, that is, an atom which has lost one or more of its charge-balancing electrons. Energetic ions can damage tissue and break strands of DNA, causing health problems ranging from nausea to cataracts to cancer.
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Cosmic rays pose a threat to astronauts bound for Mars.
Researchers discuss what a big proton storm might do to someone on the Moon.