Understanding the Fukushima Accident

SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots

In 2031, a reusable OTV-400  places a boom contracted artificial gravity habitat (AGH) and two Extraterrestrial Landing Vehicles (ETLV-2) into high Martian orbit  for a  60 day mission of human exploration of the two Martian moons, Deimos and Phobos, before refueling at a pre-deployed orbiting  fuel depot for the return trip to cis-lunar space.

The relatively weightless conditions of orbital and interplanetary space are inherently deleterious to human health. Humans and other terrestrial animals have evolved their physical and physiological attributes under our planet's heavy gravitational environment.

On the surface of the  Earth, the human heart has to counter the downward pull of gravity  in order to pump adequate amounts of blood to the head and torso when people are standing erect, while blood flowing to the lower limbs is aided by the pull of gravity. But under the microgravity conditions of space,  human blood disproportionately flows to the head and torso while blood flow to the lower limbs is reduced. This makes the human face look puffy while their legs become thinner.

Such blood flow redistribution can  sometimes cause nausea and headaches when astronauts first arrive in orbit but usually disappears after a few days in space. But other minor but annoying problems can be experienced under weightless conditions, including:

1.  Weight loss: the less strenuous conditions diminish appetite, resulting in weigh loss which could become excessive if astronauts don't exercise and eat regularly. 

2. A degraded sense of smell and taste: your favorite foods could taste a little different under microgravity

3. Clumping of tears and perspiration: there's no gravity to force trickles of water to run off the human body 

4. Facial and speech distortions: the face becomes  puffy and the voice tone and pitch becomes more nasal

5. Increased flatulence: since digestive gasses no longer rise towards the mouth, their is an increase in gas being expelled through the posterior orifice

The problems listed above could be viewed as only a minor inconvenience on short missions into space. But during long interplanetary journeys  lasting months or years, such problems could be annoying enough to enhance stress and increase tension aboard ship.

But there are other, more serious health problems that the human body may be subject to during months or years in a microgravity environment:

1. Astronauts can lose between 1 to 1.5% of their bone mass in a single month

2. Without regular exercise, astronauts can lose up to 20% of their muscle mass in just 5 to 11 days.

3. A microgravity environment can reduce  cardiovascular fitness

4. Vision problems of varying degrees of severity can occur especially in older men

5. Blood flow redistribution in a microgravity environment can effect medicines ingested or injected into the human body. 

6. Fluid loss and bone demineralization in a microgravity environment can increase the blood's calcium concentration, increasing the risk of an astronaut developing kidney stones.

7. The infected spray from the cough or the sneeze an ill person on board floats in the air instead of falling to the floor, enhancing the spread of infection aboard ship-- especially in a confined environment.

Returning to Earth after  a few months aboard the ISS, the blood pressure of some astronauts drops abnormally low when they move from a lying position to a sitting or standing position. Some astronauts even have problems standing up, walking, and turning and stabilizing their gaze. 

Since conjunction class missions to Mars may take more than seven months, there could be some question as to whether or not astronauts would  have the physical ability to perform a mission to the Martian surface.  Astronauts would have to endure weightless for several more months during their  return trip from Mars to Earth.

One possible solution to the deleterious effects of weightlessness would be temporary periods of high simulated gravity (hypergravity).   On Earth, sustained bed rest on a flat surface that is tilted  a few degrees backwards can simulate the deleterious effects of a microgravity environment, causing more blood to rush towards the head and less blood to flow into the hind limbs. But if such beds are attached to a short radius centrifuge, high levels of G forces can be experienced at the human foot level.

Short radius hypergravity centrifuge could help to mitigate the deleterious effects of a microgravity environment on the human body (Credit NASA).

Hypergravity studies have shown that protein synthesis in the leg muscles can be sustained if 2.5 G is experienced for just one hour a day at the foot level during a 21 day period. This suggest that daily exposure to a brief period of  hypergravity in space may be an effective counter measure, mitigating the loss of muscle mass in a microgravity environment. 

Production of the 8.4 meter section of the  SLS hydrogen fuel tank. Such components could be used for reusable common bulkhead interplanetary vehicles or for pressurized human space habitats (Credit Boeing Aerospace).

Short radius hypergravity centrifuges require a radius of more than 3 meters to accommodate the human body. So space habitats utilizing such  centrifuges would have to have unobstructed  internal diameters of more than six meters. Habitats derived from the SLS hydrogen fuel tanks could have an internal diameter as large as 8.4 meters. Even surrounded by 50 centimeters of water for radiation mass shielding would still give a habitat 7.4 meters in diameter of space to accommodate such machines. 

SLS  hydrogen fuel tank derived Skylab II concept (Credit Griffin).

So an SLS hydrogen fuel tank derived habitat such as the Skylab II concept could easily accommodate a short radius hypergravity centrifuge to keep astronauts and possibly even space tourist healthier while in orbit or on an interplanetary journy.

However, it has yet to be determined whether short centrifuge hypergravity machines can also alleviate some of the other serious physical and physiological problems associated with a weightless.  And temporary hypergravity would have no effect on the enhanced spread of infectious diseases and some of the minor but annoying problems associated with weightlessness.

To eliminate all of the problems associated with a microgravity environment during long periods of space travel, a continuous artificial gravity environment would be required.

In order to mitigate the physiological effects of Coriolis, a habitat capable of producing at least 0.5g of simulated gravity (higher than the gravity on the Moon and Mars) would require a rotation of  approximately 2rpm (rotations per minute) and a radius of at least 112 meters. That  would require a rotating habitat approximately  224 meters in diameter if twin counterbalancing pressurized habitats were utilized.

Artificial Gravity Habitat (AGH) located at EML4 or EML5 and radiation shielded with iron enriched lunar regolith to reduce annual radiation levels for inhabitants to below those allowed for radiation workers on Earth.

A rotating habitat derived from SLS hydrogen fuel tank technology under this scenario would  require two SLS launches to deploy and assemble the structure at one of the Earth-Moon Lagrange points. A core module would contain a pressurized docking section, solar panels,  plus extendable cables, boom, and twin  cable elevators on each side. Two external air locks derived from the SLS upper stage oxygen tank technology would also be deployed during this launch.

AGH core module featuring elevators and cable attachment rings for the habitat modules.

A second SLS launch to the Lagrange points would deploy two twin pressurized habitat modules that would connect to each side of the previously launched the core module. The total structural mass of the AGH (Artificial Gravity Habitat) is assumed to be less than 60 tonnes before the habitat modules are internally mass shielded by water for interplanetary journeys or with iron enriched regolith for permanent space stations.

Each AGH pressurized habitat module would provide shielded living area equivalent to a small two story homes, providing a spaciously comfortable environment for scientist and astronauts who may have to live in the confined simulated gravity habitats for several months or even a few years. 

AGH habitat module that can be internally shielded with 50 cm of water for interplanetary journeys or 50 cm of iron enriched regolith for permanent space stations.

A crewed or automated OTV-2 vehicle (derived from the ETLV-2) would be utilized to help assemble the AGH structure: docking the external airlocks to the sides of the core module and docking the habitat modules to both ends of the core module.

OTV-2 orbital transfer vehicle positioning the second habitat module to be docked with the core module of an Artificial Gravity Habitat (AGH) at EML4 or EML5.

The expandable boom would be rigid enough to allow thruster pods located at the ends of the habitat modules to increase or decrease its rate of the AGH rotation while the boom and internal cables are fully extended. Steel cables within the external boom would connect the AGH habitat modules to the core module. The cables would pull in the habitat modules before rocket burn maneuvers were conducted, expanding them again once the delta-v maneuvers are over. The hollow boom would also enhance the visibility of the AGH when crewed vehicles are approaching the central axis to dock.

For interplanetary journeys, the twin habitat modules would be shielded with water 50 cm thick. This would reduce astronaut's exposure to cosmic radiation to approximately 20 Rem per year during the solar minimum while also protecting astronauts from major solar events. Internally water shielding two levels of inhabited area within a pressurized habitat would add approximately 118 tonnes of weight to each habitat. Twin habitats, therefore, would add an additional 236 tonnes of mass to an interplanetary vehicle. So an AGH shielded for interplanetary travel would weigh nearly 300 tonnes, not including the additional mass for food, water, and air for the crew. Over the course of 1000 days, a crew of ten would add at least 50 tonnes of additional  mass to the vessel unless their was significant recycling of both air and water.

Reusable SLS hydrogen tank derived  OTV-400 utilizes a common bulkhead tank architecture for storing up to 400 tonnes of cryogenic fuel for interplanetary journeys. IVF technology would utilize ullage gases for tank pressurization and attitude control. Solar powered cryocoolers would eliminate fuel boil-off during long interplanetary journeys. 

At 5.2 km/s to 7 km/s, the delta-v requirements for transporting such a massive vehicle  from LEO to  Mars orbit could be prohibitive.  However, if the interplanetary vehicle was  fueled and  launched from the Earth-Moon Lagrange points to high Mars orbit, the delta-v requirements could be less than 2 km/s.

Delta- V Budget from Cis-Lunar Space to  Mars Orbit

EML1 or EML2 to Mars Capture Orbit -- 1.64 km/s

EML4 or EML5  to Mars Capture Orbit - 1.93 km/s

EML1 or EML2 to Low Mars Orbit ------ 3.04 km/s

EML4 or EML5 to Low  Mars Orbit ----- 3.33 km/s

LEO to Mars Capture Orbit ---------------- 5.2 km/s

LEO to Low Mars Orbit -------------------- 7 km/s

Liquid hydrogen and oxygen fueled cryogenic propulsion stages have been proposed by SpaceWorks with a fuel capacity of over 450 tonnes but with n inert weight of less than 30 tonnes. The ULA has proposed a cluster of six ACES boosters with a LOX/LH2 fuel capacity of approximately 700 tonnes.

Under this scenario, a common bulkhead LOX/LH2 fuel tank derived from the SLS hydrogen tank technology is utilized for a reusable interplanetary booster in order to minimize development cost. The OTV-400 would be capable of storing up to 400 tonnes of fuel for crewed interplanetary journeys to Mars, Venus, and the near Earth asteroids. The standard 400 tonne fuel tank is also utilized for large fuel depots under this scenario in order to minimize cost. Integrated Vehicle Fluid (IVF) technology would utilize ullage gases for tank pressurization and attitude control. Cryofuel boil-off would be eliminated during interplanetary journeys by using  solar powered cryocoolers.

A single SLS launch would be required to deploy the OTV-400 to LEO with enough fuel to travel to an Earth-Moon Lagrange point for refueling. Less than 350  tonnes of fuel would probably be required for a crewed interplanetary journey to high Mars orbit,  including two fully fueled Extraterrestrial Landing Vehicles.

Launching human interplanetary missions from the Earth-Moon Lagrange points to high Mars orbit rather than from LEO has several advantages.

1. To travel from an Earth-Moon Lagrange point to high Mars orbit requires less than 2 km/s of delta-v. But traveling from LEO to high Mars orbit would require more than 5 km/s of delta-v.

2. The delta-v requirement to transport water for shielding and fuel to an Earth-Moon Lagrange point is less than 2.6 km/s. The delta-v requirement to transport water to LEO is more than 9 km/s

3. The vehicles required to transport water to from the Moon to the Earth-Moon Lagrange points could be used for at least ten round trips before their CECE engines would have to be replaced or a new vehicle would be required. The vehicles required to transport water from Earth to LEO, however, would be expendable and could only be used once.

Reusable OTV-400 attached to a contracted AGH during propulsive delta-v maneuvers and a reusable  OTV-400 attached to an expanded rotating AGH after the completion of a propulsive delta-v maneuver.

The interplanetary vehicles return trip from Mars to cis-lunar space would require the OTV-400 to refuel at a previously deployed depot in high Mars orbit.  Fuel depots in orbit around Mars would initially use water exported from the lunar surface to manufacture fuel. But eventually, lunar derived water producing and exporting machines and vehicles would be placed on the surfaces of Deimos and Phobos for water production and export to the Mars orbiting fuel manufacturing depots. 

An OTV-400 derived fuel depot  (WFD-400) capable of producing cryogenic hydrogen and oxygen from stored water. The WFD-400 would be capable of transporting itself  anywhere within cis-lunar space or into orbit around Mars or Venus.

For permanent space stations at the Earth-Moon Lagrange points or in orbit around Mars, the twin habitat modules would require a radiation shield of  iron enriched regolith about 50 cm thick to reduce cosmic radiation exposure to less than 5 Rem annually (maximum allowed for radiation workers on Earth) during the solar minimum. That would require nearly 1865 tonnes of mass shielding (932 tonnes of iron enriched regolith for each habitat module). This would require one or two SLS launches of twin reusable regolith shuttles to the lunar surface-- depending on whether or not the CECE engines on the regolith shuttles are replaced after ten round trips.

Delta-V requirements to transport water for fuel, air, drinking, and mass shielding to LEO or to the Earth-Moon Lagrange Points

Lunar surface to EML1 --------------------- 2.52 km/s

Lunar surface to EML2---------------------- 2.53 km/s

Lunar surface to EML4 or EML5 ---------- 2.58 km/s

Lunar surface to LEO (with aerobraking) - 2.74 km/s

Earth's surface to LEO ----------------------- 9.3 km/s

Earth's surface to EML2 -------------------  12.73 km/s

Earth's surface to EML1 -------------------  13.07 km/s

Earth's surface to EML4 or  EML5 -------  13.27 km/s

Reusable lunar tankers capable of delivering more than 50 tonnes of water or regolith to the Earth-Moon Lagrange points. Their CECE engines should be capable of at least ten round trips from the lunar surface to the Lagrange points before the engines, or the entire vehicle, needs to be replaced. So after ten round trips, each vehicle would be capable of delivering more than 500 tonnes of fuel or regoltih  to the Earth-Moon Lagrange points.

Transporting an iron enriched regolith shielded  space station to high Mars orbit  would obviously require a much larger vehicle than the OTV-400. A light sail with a surface area of at least 100 square kilometers should be able to transport a few thousand tonnes to Mars within a years time. But if light sail technology is still not available,  vehicles capable of transporting a few thousand tonnes to Mars orbit  could easily be assembled by clustering four or more OTV-400 tanks around a core tank. A cluster of five OTV-400 vehicles would create an OTV-2000 interplanetary booster. A cluster of seven OTV-400 boosters would create an OTV-2800 interplanetary booster. Large clustered LOX/LH2 fuel tanks have also been proposed by the ULA for their human interplanetary vehicle concepts.

 The fuel requirements for such large interplanetary vehicles under this scenario would require one or two SLS launches of reusable water shuttles to the lunar surface. The lunar tankers would then transport water manufactured on the Moon to  fuel manufacturing depots at L4 or L5. Again, such lunar tankers should be capable of at least ten round trips before their CECE engines would need to be replaced. 

OTV-2000 is comprised of a cluster of five OTV-400 boosters.

The  OTV-400 would give NASA and possibly private space companies the delta-v capability to conduct  human missions from the Earth-Moon Lagrange points to the orbits of Mars and Venus, and to the NEO asteroids, and to Sun-Earth L4 and L5 as long as fuel depots for the return trip to cis-lunar space  are pre-deployed at those destinations.

 OTV-2000 and OTV-2800 class of interplanetary boosters could  enable human journeys from the Earth-Moon Lagrange points to the asteroid belt to places like Ceres and Vesta, again with WFD-OTV- 2000 or 2800 fuel depots pre-deployed in orbit around such large asteroids.

 Marcel F. Williams
© 2013 Mu Omega Enterprises


Links and References

What if you were born in space

Weightlessness and Its Effect on Astronauts

Effect of spaceflight on the human body (Wikipedia)

Artificial gravity maintains skeletal muscle protein synthesis during 21 days of simulated microgravity

 NASA Gives Artificial Gravity a Spin

 Short Radius Centrifuge (SRC)

Skylab II

Artificial Gravity (Wikipedia)


Artificial Gravity Visualization, Empathy, and Design

Space Launch System's Liquid Hydrogen Tank Under Construction

Cosmic Radiation and the New Frontier

Utilizing the SLS to Build a Cis-Lunar Highway

An SLS Launched Cargo and Crew Lunar Transportation System Utilizing an ETLV Architecture


A Study of CPS Stages for Missions beyond LEO

A Study of Cryogenic Propulsive Stages for Human Exploration Beyond Low Earth Orbit

Evolving to a Depot-Based Space Transportation Architecture

MIT Floating Nuclear Power Plant Design

Links and References

The Future of Ocean Nuclear Synfuel Production 

Naval Research Laboratory Explains their Nuclear Synfuel from Seawater Technology

What if You Were Born in Space?

Naval Research Laboratory Explains their Nuclear Synfuel from Seawater Technology

 Links and References:

Nuclear Navy's Synfuel from Seawater Program: An interview with Kathy Lewis of the U.S. Naval Research Laboratory

The Future of Ocean Nuclear Synfuel Production

Neil Armstrong on Being a Nerdy Engineer

 

Neil Armstrong 1930 - 2012

Obama's Space Plan 'Devastating,' Says Neil Armstrong and Other Moon Visitors

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

THE HIGH BACKGROUND RADIATION AREA IN RAMSAR IRAN

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