Dr. Tsubokura's Radiation Lecture Vol.95

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

189. The daily average radiation exposure from space accounts for half of the annual radiation exposure on Earth

The previous article explained that natural radiation comes from not only radioactive substances in rocks and soil but also from outer space. This radiation reaching Earth from the universe is called cosmic rays.

Our exposure to cosmic rays is very low and unlikely to cause any adverse health effects at the ground level. This is because the atmosphere surrounding the Earth shields it from cosmic rays and provides natural protection against high energetic radiation, meaning the dose of cosmic rays increases at greater distances from the ground. The daily average dose of radiation exposure received by astronauts at the International Space Station located approximately 400 kilometers above the surface of the Earth is about one millisievert, which accounts for half of the annual natural radiation exposure we receive on the Earth.

Since cosmic rays originate from the sun, the level of radiation exposure rapidly increases to a dangerous level when a sudden explosion on the surface of the sun occurs.

Although SF comics often describe people traveling through space for a long time, in addition to the fact that we do not have such vehicles yet, these types of trips are also impossible in terms of the health effects of radiation exposure. In fact, radiation protection is considered a major issue in the exploration of Mars.

190. Cosmic rays are also produced from supernova

Previous articles have explained that natural radiation comes from radioactive substances that exist in rocks and soil as well as outer space. This radiation reaching the Earth from the universe is called cosmic rays.

Besides the sun, cosmic rays are known to originate from outside of the solar system as well. Just like us, stars in the universe have a limited lifespan. At the end of their lives, stars shine brightly in the sky as they explode by burning all of their energy. The supernova, a luminous stellar explosion observed in the Magellan Cloud in 1987, attracted worldwide attention.

Cosmic rays are produced by these supernovas and can sometimes reach the Earth. Cosmic rays travel through space at an extremely high velocity—close to the speed of light. That said, as indicated by the unit called a light year, the distance between stars is tremendously large. Cosmic rays from the sun reach the Earth within a day or two, but some cosmic rays travel about 10 million years in space to reach the Earth.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 26th August and 2nd September 2018 was reproduced for MRIC Global under the author's permission.