The Sun and Radiation Exposure for Astronauts During Long-Duration Space Flight
Radiation exposure during space flight can be categorized based on a property known as linear energy transfer (LET). There is low LET radiation, the main source of which is from the Sun by way of solar particle events (solar flare activity), and high LET radiation, which is present throughout space as galactic cosmic radiation (GCR).
The energy in the Sun, as in all stars, is produced by nuclear fusion. In the Sun, in its current state, this consists mostly of hydrogen fusing to create helium. As the hydrogen is exhausted, more and more helium will undergo nuclear fusion reactions to create carbon, oxygen, and some other light elements. As this continues, the Sun will become less yellow and more red, ultimately growing to become a red giant star in about five billion years. Stars bigger than the Sun produce more of still larger atoms as they get older. This can work all the way up to iron in extremely large stars. Beyond iron, it takes an explosion known as a supernova to create very heavy elements such as gold and actually much of the production of elements heavier than oxygen comes from supernova explosions. The Sun is too small to become a supernova. As it grows to become a red giant, however, its surface will reach beyond Earth’s orbit. Mercury will be swallowed in this process and vaporized. As the Sun expands, the orbits of Venus, Earth, Mars, and the outer planets will widen, so that Mars and the outer planets will have a good chance of escaping. Venus very likely will be swallowed, but it is not clear whether or not Earth will escape.
While high solar flare activity can expose a spacecraft outside of Earth’s magnetic protection to potentially lethal doses of radiation, astronauts can be shielded against most of this fairly easily. Shielding against much of the GCR, on the other hand, is problematic. While the doses of high LET radiation from GCR would not be fatal in the short term, it is thought that there is some risk that astronauts spending several months going to Mars and back would develop cancer later on. Estimating exactly what the risk is, and for which types of cancer, however, is rather complicated. Based largely on studies of the populations exposed to ionizing radiation as a result of the Hiroshima and Nagasaki atomic bombs, and to some extend on results of the Chernobyl nuclear accident, we know that a lot of radiation causes a lot of cancer. But there is no evidence that a little bit of radiation causes a little bit of cancer, and there is some evidence that exposures a little bit higher than the normal radiation background on Earth actually may reduce one’s cancer risk. Thus, really knowing what would happen to astronauts as a result of exposure to intermediate radiation doses (resulting from the GCR with shielding against solar flare activity) is not possible, based on available information. For this reason, some scientists are developing anti-radiation cocktails, mixtures of various substances which can reduce the damage that high radiation doses cause in human cells.