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Cosmic rays are not really rays. They are charged particles traveling at near the speed of light. The majority consist of protons and electrons that are ejected from the sun during solar storms.
The less common but most dangerous particles are the extremely energetic nuclei of heavy elements traveling at near light speeds that come from outside the solar system. The highest-energy particles are the rarest but also the most hazardous.
The origin of high-energy cosmic rays is uncertain, but we think they have been ejected from exploding stars in supernovae from far, far away in distant stars in our own galaxy and remote galaxies. Because there is nothing in space to slow them down, they can travel for billions of years before smashing into Earth.
Fortunately for us, high-energy cosmic ray particles strike air molecules in the upper atmosphere and produce multiple cascades of lower-energy elementary particles. Unfortunately, they do the same thing when they collide with an atom in the body. There the low-energy particles are far less benign, but they can cause harmful genetic damage that can accumulate quickly.
Decades of study from astronauts in space and in animal-based laboratory studies on Earth indicate that the dangers from exposure to cosmic rays can affect many different parts of the body.
These include risks of cancers such as leukemia, damage to the brain and nervous tissue, and general radiation sickness. Even the gastrointestinal tract can be affected, with immediate damage to the digestive system and long-term increased cancer risks.
Ironically, metals are generally good shields against cosmic radiation, but aluminum, which is valued in the aerospace world for its lightness, is the worst and virtually ineffective. Plastic is actually a better shield.
Astronauts in low-Earth orbit where the shuttle and International Space Station operate still have enough atmosphere to provide some shielding by the thinnest of atmospheres above them, and also by the Earth itself, which blocks part of the sky. They are also exposed for relatively short times.
A trip to Mars will take at least two years, during which time the astronauts will be riddled with both high- and low-energy cosmic radiation.
A single coronal mass ejection from a solar storm could be lethal in the short term, and the long-term exposure to high-energy cosmic radiation could cause high rates of cancer and decreased life expectancy for those who would return to Earth.
Because cosmic rays can cause cancer in unprotected astronauts, a better understanding of where and how cosmic rays are accelerated will improve predictions of how many will be encountered as astronauts set sail on the new ocean of space. We do not understand the details of expected risk to astronauts over long time periods, but engineers are looking for creative solutions to the great frontier of space in propulsion, life support, reentry and others as well as mission goals and plans.
Cosmic radiation might pose the greatest risk to extraterrestrial voyagers and may be the most difficult life support problem to solve.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
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