Tuesday, March 4, 2014

Cosmic Radiation and the New Frontier

Mars and its inner moon, Phobos
by Marcel F. Williams

The 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)
On our planet of evolutionary origin, humans are typically exposed to 0.36 Rem annually. The annual exposure to cosmic radiation aboard the ISS (International Space Station) space station, however,  can range from 20 to 40 Rem depending on whether our solar system is experiencing solar maximum or solar minimum conditions. This is one of the reasons why astronauts typically remain aboard the ISS for only a few months. The solar maximum is a period when the sun has the most sunspot activity and the solar minimum is a period when the sun has the least sunspot activity. Solar maximum conditions can help to mitigate  the rain  of galactic cosmic radiation (GCR) within the Solar System. However, large solar flares often occur during solar maximum conditions.

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

Interplanetary 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/cm2 of Martian atmosphere during the solar minimum

8 Rem - annual rate of cosmic radiation on the surface of Mars beneath 16 gm/cm2 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
Shielding astronauts below the 5 Rem per year requirement for terrestrial radiation workers in the US would require approximately 50 centimeters of iron shielding  or at least 4.5 meters of water. A rotating space station with two SLS hydrogen fuel tank derived habitat modules would require nearly 1900 tonnes of internal iron shielding. Fortunately, there's no shortage of iron ore in the lunar regolith. So exporting regolith bags heavily enriched with iron to shield rotating space stations located at EML4 or EML5 shouldn't be too difficult-- especially with reusable transport vehicles with replaceable CECE engines.

ETLV derived Reusable Lunar Regolith Shuttle

50 centimeters of water shielding would probably make it prohibitive to launch manned interplanetary vehicles from LEO.  The delta-v requirement to travel from LEO to Mars capture orbit would be around 5.2 km/s. However, the cheapest fuel source for an interplanetary vehicle would be from the Moon's low gravity well rather than from the huge gravity well of the Earth. The delta-v to LEO from Earth is over 9.3 km/s while the delta-v from the Moon to one of the Earth-Moon Lagrange points is less than 2.6 km/s. So it would  be cheaper to fuel the interplanetary vehicle at one of the Earth-Moon Lagrange points rather than at LEO.   The delta-v budget for traveling from an Earth-Moon Lagrange point to Mars Capture Orbit would be less than 2 km/s vs the 5.2 km/s delta-v of launching an interplanetary vehicle from LEO to Mars Capture Orbit.
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.
On the surface of the Moon or Mars, humans would be exposed to only half the amount of cosmic radiation thanks to the natural mass shielding of being on a planetary surface.

Interior configuration of a Regolith Habitat for the Moon or Mars
Two meters of Lunar or Martian regolith would be enough to lower annual radiation exposure below 5 Rem. The metallic shells of the pressurized habitat and the outer regolith wall would add additional radiation protection.   However, two meters of iron enriched regolith shielding could reduce radiation exposure within the Lunar or Martian habitat to terrestrial levels, if desired.

Links and References

Lifetime Risk of Developing or Dying From Cancer

Oxidative stress

What is Oxidative Stress

Biological consequences of oxidative stress-induced DNA damage in Saccharomyces cerevisiae

Oxidative DNA damage: mechanisms, mutation, and disease


Natural Radiation

Fueling our Nuclear Future

Space Faring The Radiation Challenge

Cosmic Ray Interactions in Shielding Materials

Galactic Cosmic Radiation Leads to Cognitive Impairment and Increased Aβ Plaque Accumulation in a Mouse Model of Alzheimer’s Disease

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

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