THE sun's ability to shield the solar system from harmful cosmic rays could falter in the early 2020s, just in time to threaten the health of NASA astronauts as they return to the moon.
As well as the 11-year cycle of sunspots and solar flares, the sun's activity experiences longer-term shifts lasting several decades. The sun is currently in a long-term high, having been relatively active for nearly a century, but it is not known when this will end.
To find out, a team led by Jose Abreu of the Swiss Federal Institute of Aquatic Science and Technology in Duebendorf analysed 66 long-term highs from the past 10,000 years, as recorded in fluctuating levels of rare isotopes such as beryllium-10 in ice cores from Greenland. These are produced when cosmic rays break down the nuclei of oxygen and nitrogen atoms in the Earth's atmosphere. Production of these isotopes peaks when the sun is inactive, as the weaker solar wind lets more cosmic rays enter the solar system, which hit the Earth.
Based on the duration of past highs, and the fact that the current one has already lasted 80 years, the team has calculated that its most likely total lifetime is between 95 and 116 years, and they suspect the high will probably end at the shorter end of this range
http://www.newscientist.com/article/mg20126903.700-danger-ahead-as-the-sun-goes-quiet.html?DCMP=OTC-rss&nsref=space
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