Space is getting more dangerous. Just as missions will ramp up, it seems that exploring the solar system will become more deadly.

The sun is going through a quiet period. Simulations suggest that, by the 2020s, this means astronauts spending a year in space will exceed NASA's safety limits for radiation exposure, potentially thwarting missions to Mars or to asteroids.

High-energy particles from deep space called cosmic rays bombard the solar system and can damage spacecraft and human DNA. The sun's magnetic field shields us from much of this radiation, but the field's strength waxes and wanes on an 11-year cycle. The most recent peak in activity – a solar maximum – has been abnormally weak, reducing the shield's effectiveness.

Earth's own magnetic field protects people on the ground and even on the International Space Station, but a sleepy sun could be bad news for those going further afield. Nathan Schwadron at the University of New Hampshire in Durham and his colleagues used radiation readings from NASA's Lunar Reconnaissance Orbiter (LRO) to predict what will happen in the current solar cycle, which will reach a minimum in 2020.

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