|Mars and its inner moon, Phobos|
by Marcel F. WilliamsThe average woman on Earth is born with an approximately 38% chance of developing cancer sometime in her lifetime. And the average man is born on our planet with about a 44% chance of developing cancer sometime in his lifetime.
Oxidative stress from the production of oxygen free radicals created during the metabolism of proteins, carbohydrates, and fats (food) appears to be the primary cause of cancer and aging amongst humans and other animals on Earth. But ionizing radiation from space and from the natural geology of the Earth and in the food we eat and the water we drink can also contribute to cancer and aging.
Ionizing radiation interacts with the tissue of humans and other animals by stripping away electrons from molecules, leaving behind chemically active radicals that can be harmful to the cells of the human body. As our civilization begins to expand off the Earth in the 21st century, the human species will encounter substantially higher levels of ionizing radiation from the cosmos. Enhanced exposure to Galactic Cosmic Rays (GCR) could significantly increase the rate of cancer and aging and even brain damage amongst explorers and settlers in the New Frontier-- unless appropriate means are utilized to mitigate the potentially deleterious effects of cosmic radiation and major solar events.
Humanity and all other creatures on Earth live under a sea of air whose mass substantially reduces our exposure to cosmic radiation and the ionizing effects of solar storms. The average amount of cosmic radiation exposures experienced on the surface of the Earth is approximately 0.039 Rem. Within inhabited US territorial areas, annual cosmic radiation exposure may be as high as 0.13 Rem (Wyoming) or as low as 0.03 Rem (Puerto Rico).
The Earth's crust is also naturally radioactive thanks to uranium and thorium and a radioactive component of potassium (potassium-40) that is naturally found in our soil. Terrestrial soil contains about 6 parts per million of thorium on average and 0.7 to 11 parts per million of uranium. The Earth's oceans contain more than 4 billion tonnes of uranium 238 which has a radioactive half-life of 4.47 billion years. The world's rivers dump about 32,000 tonnes of uranium annually into the world's oceans.
The human body, of course, is naturally radioactive thanks mostly to the naturally radioactive potassium in our bodies. Human body mass typically contains about 40 grams of potassium of which 1/1000 of this element is radioactive potassium-40. Humans also ingest food and water that is naturally radioactive thanks to the natural potassium, uranium and thorium contained in these foods. So internal radiation in the human body contributes about 0.04 Rem of annual radiation exposure.
However, the inhalation of radon 222 and 220 is the predominant contributor of ionizing radiation in humans on Earth. Radon gas is a radioactive by product from the decay of uranium or thorium and has a half-life of approximately 3.8 days. Radon exposes humans and other animals to approximately 0.23 Rem annually.
Smoking, however, can add even more radiation exposure to the human lungs than radon. The inhalation of tobacco contains radionuclides polonium 210 and lead 210. While a typical non-smoking American is exposed to about 0.36 Rem of radiation annually on Earth, a smoker can add an additional 0.28 Rem of radiation exposure to the human lungs.
Typical medical diagnostic procedures that use nuclear material can add 0.06 Rem of annual radiation exposure.
Ionizing Radiation on Earth
0.039 Rem - Average annual amount of natural radiation in the human body
0.2 Rem - Average annual internal radiation exposure due to the inhalation of radon
0.28 Rem - Annual radiation exposure for individuals who smoke cigarettes
0.029 Rem - Average annual exposure to terrestrial radioactive decay
0.026 Rem - Average annual exposure to cosmic radiation in the US
0.36 Rem - Total average amount of natural and man-made ionizing radiation exposure for a person living in America
|Boeing 747 (Credit: Boeing)|
The US legal limit for radiation exposure for workers is 5 Rem per year. Personal working at a nuclear facility are normally exposed to 0.115 Rems annually. However, personal aboard an airliner are typically exposed to 0.22 Rem per year.
Ionizing Radiation Exposure Limits on Earth
5 Rem - annual maximum radiation exposure allowed for radiation workers in the US
0.22 Rem - The average annual cosmic radiation dose experienced by flight personnel
0.12 Rem - Annual radiation exposure experienced by workers at a nuclear power plant
0.007 Rem - Annual radiation exposure while living in a stone, brick, or concrete building
0.003 Rem - Annual radiation exposure while living near the gate of a nuclear power plant
People who live near the gate of a nuclear power facility are normally exposed to about 0.003 Rem annually. Living in a brick, stone, or concrete building would expose you to 0.007 Rem of annual radiation exposure. And each individual living inside of your home with you adds another 0.04 Rem of annual exposure.
It should be noted, however, that there are places on Earth where people are exposed to substantially higher levels of ionizing radiation. A community of over 2000 people exist in Iran, that is naturally exposed to 1 to 25 Rem of radiation annually-- with no signs of any deleterious physical or reproductive effects on that population.
In space, however, exposure to ionizing radiation would be substantially above that typically experienced on Earth.
|International Space Station (Credit: NASA)|
Cosmic radiation levels become even worse as we leave the protective proximity of the Earth's massive globe and its surrounding magnetosphere. A space habitat located near the Moon at EML4 (Earth-Moon Lagrange point Four) for instance would be exposed to as much as 73 Rem annually during the solar minimum. Being beyond the Earth's magnetosphere also exposes astronauts to the heavy nuclei components of cosmic radiation.
Cosmic rays are mostly of galactic origin, resulting from super nova explosions. Approximately 85% of cosmic radiation particles are composed of hydrogen derived protons; 13% are derived from helium atoms. Heavy nuclei are accelerated particles whose nuclei are derived from atoms heavier than hydrogen and helium. While heavy nuclei comprise only about 2% of cosmic radiation particles, they can do substantially more damage to biological tissue.
Astronauts in low Earth orbit, are only infrequently exposed to heavy nuclei bombardment thanks to the Earth's protective magnetosphere. Beyond the Earth's magnetosphere, however, astronauts frequently experience ' retinal flashes'. These visual flashes appear to be the result of heavy nuclei impacts upon the visual cortex of the human brain. Most cosmic ray ions pass harmlessly though the vacuous space between the atoms of the human body. But the relentless rain of cosmic radiation inevitably results in impacts upon our corporeal components. Heavy nuclei, especially the heaviest ions, can be particularly damaging to human tissue and especially to the human brain. Additionally, the particle impacts of cosmic radiation impacts can produce significant amounts of secondary particles such as neutrons that can enhance the deleterious effects of cosmic radiation on biological tissue.
Twenty seven Apollo astronauts returned to Earth after nearly two weeks beyond the Earth's magnetosphere with no significant deleterious effects to their body as the result of exposure to cosmic rays and its heavy nuclei component. So a few days or weeks of cosmic ray exposure beyond the magnetosphere appears to have no significant impact on human health.
However, it is estimated that during a future 6 month journey to Mars, the nucleus of one out of every three cells in the human body would receive at least one hit from a cell damaging heavy ion. While most human tissue has the ability to repair itself, this is mostly not true for the neurons of the human central nervous system. So during a mere six months of relentless cosmic ray exposure, heavy nuclei could potentially destroy a third of the neurons in the human brain-- without any repair or replacement. And rodents exposed to significant amounts of heavy nuclei bombardment have displayed some mental impairment. So protecting the human brain from significant heavy nuclei exposure during multi-month or multi-year space missions should, obviously, be a priority.
Fortunately, astronauts traveling or living beyond the Earth's magnetosphere for months or for years could easily be protected from the dangers of heavy nuclei with only about 10 centimeters of lunar regolith, or an equal mass of less than 20 centimeters of water or ice.
But cosmic rays would not be the only danger astronauts could experience from ionizing radiation. A major solar storm could expose an unprotected crew to up to 1000 Rem over a short time period. Just 600 Rem of acute radiation exposure can cause radiation poisoning and even death. But 20 centimeters of water or ice would appear to be enough to reduce radiation exposure during a major solar event to well below NASA's 25 Rem per month radiation exposure limit.
|Apollo 16 astronaut on the lunar surface (Credit: NASA)|
Ionizing Radiation in Space
73 Rem - annual amount of cosmic radiation in interplanetary space during the solar minimum
28 Rem -annual amount of cosmic radiation in interplanetary space during the solar maximum
Surface of the Moon:
38 Rem - annual amount of cosmic radiation on the Lunar surface during the solar minimum
11 Rem - annual amount of cosmic radiation on the Lunar surface during the solar maximum
Surface of Mars:
33 Rem - annual rate of cosmic radiation on the surface of Mars beneath 16 gm/cm3 of Martian atmosphere during the solar minimum
8 Rem - annual rate of cosmic radiation on the surface of Mars beneath 16 gm/cm3 of Martian atmosphere during the solar maximum
|Martian Surface (Credit: NASA)|
Ionizing Radiation Exposure Limits for NASA Astronauts for a maximum 3% lifetime excess risk of cancer mortality
25 Rem - maximum 30 day exposure limit to ionizing radiation
50 Rem - maximum annual exposure limit to ionizing radiation
100 Rem - maximum career exposure limit to ionizing radiation for a 25 year old woman
150 Rem - maximum career exposure limit to ionizing radiation for a 25 year old man
175 Rem - maximum career exposure limit to ionizing radiation for a 35 year old woman
250 Rem -maximum career exposure limit to ionizing radiation for a 35 year old man
250 Rem -maximum career exposure limit to ionizing radiation for a 45 year old woman
325 Rem -maximum career exposure limit to ionizing radiation for a 45 year old man
300 Rem -maximum career exposure limit to ionizing radiation for a 55 year old woman
400 Rem -maximum career exposure limit to ionizing radiation for a 55 year old man
NASA's annual limit for radiation exposure is 50 Rem. But the lifetime exposure limit for a 25 year old woman is only 100 Rem. Philosophically, I don't believe that a single space mission should ever end the extraterrestrial career of a young individual. So the radiation shielding levels proposed here are designed to limit total cosmic ray exposure during an entire mission to less than 50 Rem. A rotating interplanetary habitat module exposing astronauts to less than 25 Rem per year during an interplanetary journey would require nearly 50 centimeters of water to protect against cosmic radiation and major solar events. The internal shielding requirement for the inhabited areas for two rotating SLS fuel tank derived habitat modules would require nearly 240 tonnes of water shielding. This water shielding could be provided from lunar water resources shuttled to an interplanetary space craft located at one of the Earth-Moon Lagrange points.
|ETLV derived Reusable Water Tanker Lunar Shuttle|
|ETLV derived Reusable Lunar Regolith Shuttle|
|Lunar Regolith Habitat with automatically deployed regolith wall. A similar habitat could be used to protect astronauts from cosmic radiation on the surface of Mars.|
|Interior configuration of a Regolith Habitat for the Moon or Mars|
Links and References
Lifetime Risk of Developing or Dying From Cancer
What is Oxidative Stress
Biological consequences of oxidative stress-induced DNA damage in Saccharomyces cerevisiae
Oxidative DNA damage: mechanisms, mutation, and disease
THE HIGH BACKGROUND RADIATION AREA IN RAMSAR IRAN
Fueling our Nuclear Future
Space Faring The Radiation Challenge
Radiation Protection for Human Missions to the Moon and Mars
Radiation Hazards and the Colonization of Mars: Brain, Body, Pregnancy, In-Utero Development, Cardio, Cancer, Degeneration
Mission to Mars: Health Risk Mitigation
(Rich Williams: NASA Chief Health and Medical Officer)
Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars
Regolith Biological Shield for a Lunar Outpost from High Energy Solar Protons
Lunar Station Protection: Lunar Regolith Shielding
Radiation exposure in the moon environment
Utilizing the SLS to Build a Cis-Lunar Highway