Protecting Spacefarers from Heavy Nuclei

Buzz Aldrin on the surface of the Moon (Credit: NASA)

by Marcel F. Williams

In July of 2016, researchers on the health of NASA astronauts dropped a bombshell concerning the cardiovascular mortality of the Apollo Lunar astronauts. Despite the enhanced levels of radiation exposure by astronauts at LEO and beyond LEO, there were no indications of increased levels of cancer for space faring astronauts relative to people living on Earth. The research also showed no significant increase in cardiovascular deaths (the leading cause of death for Americans) relative to people living on the Earth's surface. Surprisingly, the research did reveal that the  frequency of  cardiovascular deaths of Apollo Lunar astronauts was four to five times higher than in LEO astronauts and in NASA astronauts that have yet to have the opportunity to fly into space.

Although astronauts are exposed to enhanced levels of radiation beyond the Earth's magnetosphere, astronauts at LEO are completely shielded from the most lethal component of cosmic radiation, heavy nuclei (heavy ions), most of the time.

Cosmic radiation beyond the Earth's surface (Annual levels)

Low Earth Orbit (LEO): 20 Rem (solar maximum) to 40 Rem (solar minimum) 

Interplanetary Space: 28 Rem (solar maximum) to 73 Rem (solar minimum)

Surface of the Moon: 11 Rem (solar maximum) to 38 Rem (solar minimum) 

Surface of Mars: 8 Rem (solar maximum) to 33 Rem (solar minimum) 

Cosmic rays are relativistically accelerated particles  resulting from the  explosions of ancient super nova mostly within the galaxy. Approximately 87% of cosmic radiation particles are composed of  protons;  12% are alpha particles derived from helium atoms.  But approximately, 1% of the population of cosmic rays are composed of heavy nuclei, large ionized accelerated particles derived from heavier nuclei such as carbon, oxygen, silicon, and iron.

Most of the protons and alpha particles from cosmic radiation 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, on the other hand,  have relatively-short interaction lengths when encountering matter. Such interactions with human tissue can be significantly deleterious DNA molecules and can pose a challenge to cellular repair. 

It has been 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. With the inability of neurons to repair themselves, it has been predicted that a mere six months of heavy nuclei exposure could potentially destroy a third of the total neurons in the central nervous system.

Approximately, 15 to 20 g/cm2 of mass is required to effectively stop the penetration of heavy ions. However, its estimated that Astronauts aboard the Apollo Command Module (CM), were only provided with about 10 g/cm2 of radiation shielding. And some parts of the Command Module were more heavy shielded than other areas of the CM. The window areas were particularly thinly shielded which would have allowed heavy nuclei to easily penetrate into the CM interacting with the body tissues of the astronauts. 

Astronauts on there way to and from the Moon experience retinal flashes aboard the CM when they were outside of the Earth's magnetosphere. And NASA believes that this was the result of heavy nuclei traversing the human retina. Retinal flashes occurred at an average frequency of every 2.9 minutes which sometimes made sleeping a challenge for the astronauts.  At least 90% of the Apollo astronaut's exposure to heavy nuclei bombardment occurred during their voyage to and from the lunar surface and not on the lunar surface itself.  

But there was even less shielding against heavy nuclei for astronauts when the were aboard the Lunar Module (LM) and when they were on the surface of the Moon. However, half of the heavy nuclei would have been blocked by the mass of the Moon when astronauts were on the lunar surface.

 Since the cardiovascular health of  Apollo Lunar astronauts relative to humans on Earth has now been shown to be significantly effected after less than two weeks beyond the Earth's magnetosphere, its now clear that astronauts in the future will have to be adequately protected from heavy nuclei when traveling beyond the magnetosphere to the Earth-Moon Lagrange points, the lunar surface and on interplanetary journeys to Mars. 

Since there is virtually no atmosphere on the surface of the Moon, lunar astronauts venturing outside of their regolith shielded habitats would be exposed to the relentless penetration of heavy nuclei. Lunar astronauts would, therefore, have to wear pressure suits with enhanced shielding (~ 20 g/cm3). Helmets could easily be shielded with iron 2.6 cm2  thick.  But the helmet visor for lunar excursions would probably have to be composed of lead iron glass nearly 5 centimeters thick. Thickening the rest of the lunar pressure suit with 2.6 centimeters of iron shouldn't be too difficult. The increased weight of the iron shielded pressure suit should be easily mitigated by the Moon's 1/6 gravity. 

On the surface of Mars, the thin carbon dioxide atmosphere is still thick enough (~ 15 g/cm2) to shield astronauts on the surface from any direct interaction from most heavy nuclei. So enhanced shielding of pressure suits on the surface of Mars will probably not be required. 

Notional reusable EUS with an internally water shielded  Cygnus habitat module rendezvousing with an EUS derived  propellant producing water depot at LEO

Since traveling from LEO to other important regions within cis-lunar space can take several days, its pretty obvious that crewed spacecraft traveling within cis-lunar space will have to be  appropriately shielded against heavy nuclei. However, substantially increasing the shielding requirements of the Orion spacecraft to protect against heavy ions  could make it to heavy for the   ATV derived  Service Module (SM) to push the Orion capsule on a trajectory return to Earth. 


Delta-v to important destinations within cis-lunar space

Earth surface to LEO - 9.3 km/s to 10 km/s

LEO to EML1 - 3.77 km/s (~3 days)

LEO to EML1 - 4.5 km/s (~2 days)

LEO to EML2 - 3.43 km/s (~8 days)

LEO to EML2 - 3.95 km/s (~4 days)

Lunar surface to EML1 - 2.52 km/s (~3 days)

Lunar surface to EML2 - 2.53 km/s (~3 days)  

Notional ETLV-2 crew landing vehicle with internal radiation water shielding compartment.

However, if the ATV derived SM is replaced with a reusable EUS that uses IVF technology and orbiting water/propellant depots, then the Orion capsule  could be coupled with an appropriately water shielded Cygnus habitat.  The water shielded (20 cm thick) area within the Cygnus habitat could be dumped just before the final trajectory burns into the desired cis-lunar destinations. Water could be replenished for the Cygnus habitat for return trips at the water/propellant depots. 

Notional reusable ETLV rendezvousing with an EUS derived  propellant producing water depot at LEO

In the long run, however, it would be even more fuel efficient if an Extraterrestrial Landing Vehicle (ETLV) were also used as an orbital transfer vehicle between LEO and the Lagrange points. Again, 20 centimeters of water shielding could be provided with an designated area of the ETLV and then dumped before the final trajectory burns to the Lagrange points, Low Lunar Orbit, or to LEO. While water shielding  the ETLV crew transport area before its departure from the lunar surface would require more propellant at take off, there should be no shortages of lunar derived oxygen and hydrogen propellant on the lunar surface. 

Notional reusable ETLV rendezvousing with an EUS derived  propellant producing water depot at EML-1.

And ETLV using a ADEPT or HIAD deceleration shield could dump its water shielding just before entering Mars orbit or descending from from Mars orbit to the surface of Mars. Returning to Mars orbit, an ETLV could dock with a Mars orbiting water/propellant depot at Low Mars Orbit to add water shielding and propellant to the vehicle in order for it to return its astronauts to an interplanetary spacecraft parked in High Mars orbit.

Notional reusable ETLV returning from the martian surface rendezvousing with an EUS derived  propellant producing water depot at Low Mars Orbit before returning to High Mars Orbit. 

Protecting astronauts from the deleterious effects of heavy ion bombardment beyond the Earth's magnetosphere will increase mass shielding and propellant requirements for crewed spacecraft. But  the utilization of extraterrestrial water and regolith resources should make it easy and affordable to protect the health of astronauts from the dangers of heavy nuclie in the New Frontier.  

Links and References

Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality: Possible Deep Space Radiation Effects on the Vascular Endothelium


Substantially Enhancing the Capability of the SLS Architecture by Utilizing EUS Derived Propellant Depots and Reusable Orbital Transfer Vehicles 

Cosmic Radiation and the New Frontier 

Lunar Source Book  

THE MARTIAN RADIATION ENVIRONMENT FROM ORBIT AND ON THE SURFACE. 
  
Apollo Experience Report: Protection Against Radiation  

  Natural Radiation 

 The SLS and the Case for a Reusable Lunar Lander

Congress Requires NASA to Develop a Deep Space Habitat

SLS propellant tank derived DSH @ EML1 (credit NASA)

The US Congress passed an omnibus spending bill last December requiring NASA to develop a prototype deep space habitation (DSH) module no later than 2018. It also requires NASA to provide Congress with a report on how the enactment of this bill is being complied with by the first half of 2016.

NASA has viewed a DSH  as a necessary component for safely transporting humans from cis-lunar space to Mars orbit in the 2030's and also as a gateway to the lunar surface and beyond. The Earth-Moon Lagrange points EML1 and EML2 have most often been proposed as the place where a Deep Space Habitat should be deployed.

EML2 (L2) has the advantage of requiring the lowest delta-v from LEO in order to deploy the DSH into a halo orbit around the Lagrange point. But crewed journeys from LEO to L2 also has the disadvantage  of taking as long as 8 days to reach the habitat if the low delta v of 3.43 km/s is to be taken advantage of. Such a long journey would expose astronauts to two to four times as much cosmic radiation as journeying to EML1.  A higher delta-v of 3.95 km/s could transport a crew to EML2 in just four days. But this would be higher than the 3.77 km/s delta-v requirement to transport crews from LEO to EML1. EML1 also has the advantage of  a fast 2 day journey from LEO at  4.41 km/s. Such fast journeys would reduce radiation exposure while also reducing the chance of traveling during a major solar event in half. 

The Earth-Moon Lagrange points (Credit the Artemis Project)

Another, long term, disadvantage of a DSH at EML2 is that radio transmissions between the habitat and Earth could interfere with future radio telescopes deployed on the back side of the Moon in order to avoid radio interference from the Earth's surface, Earth orbit, and space craft traveling to and from the Moon. 


Delta- V budgets between LEO and EML1 or  EML2 

LEO to EML1 (~ 2 days) - 4.41 km/s dv

LEO to EML1 (~ 4 days) - 3.77 km/s dv

EML1 to Lunar Surface (~3 days) - 2.52 km/s dv

Lunar Surface to EML1  (~3 days) - 2.52 km/s dv

LEO to EML2 (~ 8 days) - 3.43 km/s dv

LEO to EML2 (~ 4 days) - 3.95 km/s dv
 
EML2 to Lunar Surface (~3 days) - 2.52 km/s dv

Lunar Surface to EML2  (~3 days) - 2.52 km/s dv

 
Aesthetically, a Deep Space Hab positioned at EML1 would probably have the most spectacular views within cis-lunar space.  An astronaut  at  EML2 would view an Earth that is slightly smaller than viewed from the front side of the Moon while the view of Earth at EML1 would be slightly larger than is seen from the lunar surface. Both EML1 and EML2 would view a Moon that is titanic in size relative to its view from the Earth. But EML2 would only be able to view the back side of the Moon while EML1 would only be able to view the front side of the Moon.

Because of the reduced time and radiation exposure to get there, the fact that an EML1  habitat wouldn't interfere with radio telescopes on the back side of the Moon, plus the aesthetic view,  I think NASA should deploy the Deep Space Hab at EML1 rather than at EML2.

The relative visual size of the Moon and Earth: at the top, the view of the Moon from the surface of the Earth or low Earth orbit; second from the top, the view of the Earth from the surface of the Moon; third from the top, the view of the Earth from EML1; at the bottom, the view of the Moon from EML1. 

The primary purposes for an EML1 (L1) Deep Space Habitat (DSH) should be to:

1. Serve as a gateway to the lunar surface. Astronauts traveling from the Earth or from the lunar surface could dock their spacecraft at the EML1 habitat, taking advantage of the larger accommodations at the DSH while transferring from one vehicle to another.  

2. Serve as a storm shelter during the occurrence of major solar events. This will probably require at least 30 cm of water shielding for the areas within the habitat that the astronauts will be occupying. Major solar events can last for several minutes or up to several hours.

3. Serve as a maintenance and repair station for reusable lunar shuttles (ETLV) and orbital transfer vehicles. Flex Craft docked at the DSH could be utilized  for extravehicular repairs to  nearby water/propellant depots and associated solar arrays at EML1.

4. Test the effectiveness of various levels of water shielding required to mitigate cosmic radiation and potentially brain damaging heavy nuclei. In theory, 30 cm of water would be enough shielding to to stop the penetration of the heavy nuclei component of cosmic rays, reduce the annual exposure of cosmic radiation in general to less than 25 Rem per year, while also significantly mitigating the effects of major solar events. While an even thicker shielding of water could reduce cosmic radiation exposure, a minimal amount of shielding will be required to minimize the mass for crewed interplanetary vehicles.

5. Test the integrity and reliability of the pressurized habitat structure which could also be used for habitats on the surface of the Moon and Mars and for rotating interplanetary artificial gravity habitats.

Its probably the intent of Congress  for NASA to design the habitat module that will transport humans safely to Mars.  But because of the inherently deleterious physical and psychological effects of a microgravity environment on human beings, its unlikely that any microgravity habitat will ever be able to accomplish this goal.

 Under microgravity conditions, astronauts can lose between 1 to 1.5% of their bone mass in a single month and without regular exercise, astronauts can lose up to 20% of their muscle mass in just 5 to 11 days. A microgravity environment can reduce  cardiovascular fitness-- possibly increasing the chances of heart attaches. And vision problems of varying degrees of severity can occur-- especially in older men. 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. Unfortunately, blood flow redistribution in a microgravity environment can effect medicines ingested or injected into the human body to treat illnesses.

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.

Added to the serious problems above, there are other annoying problems in a microgravity environment that could enhance discomfort and psychological stress aboard ship such as:

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 sweat and 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. This could cause some to misinterpret another individuals expression, possibly causing tension between two individuals aboard a multiyear mission. 

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.

It might be possible to eliminate all of these deleterious microgravity related problems aboard an interplanetary vehicle by  simply rotating pressurized habitats in counter balancing pairs to produce a  significant level of simulated gravity. The  additional benefit of having two pressurized modules is that it also provides a back up module in case there are serious life threatening malfunctions at the other habitat module. 

Pressurized habitats capable of being used in space and on the surface of the Moon or Mars could also be used as counter balancing habitats for rotating spacecraft and space stations that produce some levels of artificial gravity. And development cost could be greatly reduced if the basic habitat pressurized tank can be used for  microgravity habitats, low gravity surface habitats, and for artificial gravity habitats.

Internal configuration of a  lunar habitat derived from  SLS propellant tank technology. A  regolith wall composed of kevlar sandwiched between eight rigid aluminum panels is deployed around the habitat cylinder and filled with regolith to protect astronauts from cosmic radiation, micrometeorites, and fluctuating temperatures on the lunar surface. The airlocks are derived from ETLV propellant tank technology.

NASA could significantly reduce development cost by utilizing SLS propellant tanks for both a DSH but also for lunar and martian habitats. The lunar and martian regolith habs that I've previously proposed would use an SLS propellant tank as a pressurized habitat. Once the habitat module is properly placed on the lunar surface, a kevlar regolith wall sandwiched between  eight three meter wide aluminum  panels would automatically deploy, allowing a lunar backhoe to deposit regolith shielding  within  the two meter cavity between the outer wall and the inner cylindrical wall.

Since crewed interplanetary voyages to Mars probably won't take place until the 2030's, serious funding by NASA for the development of artificial gravity habitats for interplanetary journeys probably won't have to start until the early 2020's. However, this doesn't mean that a  DSH habitat couldn't be designed to function as a microgravity habitat, as a  low gravity habitat, and as a simulated gravity habitat in order to reduce cost for both the cis-lunar program in the 2020's and for the Mars program in the 2030's.

If such habitats are to be used for long  interplanetary journeys  in the future, they must be as comfortably spacious as possible while also minimizing mass.  NASA is evaluating several types of potential Deep Space Habitats derived from current technology:


Habitat Modules Derived from Current Technologies

SLS full class propellant tank derived:
Dry mass: 22.4 tonnes
Habitable volume - 519 m3

SLS minimum class propellant tank derived:
Dry mass: 17.3 tonnes
Habitable volume: 353 m3 

BA-330:
Dry mass 20 to 23 tonnes
Habitable volume: 330 m3 

ISS node & MPLM:
Dry mass: 35.5 tonnes
Habitable volume: 108 m3

ISS hab & MPLM: 
Dry mass: 32 tonnes
Habitable volume: 90 m3


The ISS derived habitats only provide between 2.8 meters  to  3 meters cubed of habitable volume per tonne. The Bigelow BA-330 would provide significantly more volume, between 14 m3 and 17 meters cubed of habitable volume but within severely confined areas. The SLS propellant tank derived habitats, however,  would provide between 20 m3 and 23 m3 of habitable space per tonne (35% to 64% more habitable volume per tonne). Since SLS propellant tanks will already be in production for SLS launches, manufacturing more tanks for Deep Space Habitats and for surface habitats for the Moon and eventually for Mars should greatly reduce development cost for a Deep Space Hab. 

The  SLS Block B with its upper stage would probably only be able to deploy  about 30 to 32 tonnes of  mass  to EML1, allowing it  to  easily deploy an SLS propellant tank derived DSH to EML1. 

An SLS propellant tank technology derived DSH @ EML1. There are four docking ports for four large vehicles. There are also four docking ports for four personal Flex Craft vehicles. There is also one crew hatch for pressure suit excursions. Twin solar panels provide power for the DSH with a central heat radiator extending between them. A crewed  MPCV and a crewed ETLV-2 are docked at the DSH in preparation for an ETLV visit to an outpost on the lunar surface. A lone floating Flex Craft has been utilized to inspect the exterior of the reusable ETLV-2 before departure (MPCV: Credit: ESA).  

Internally radiation shielding two levels of the DSH habitat area within an SLS derived habitat, above and below,  with 30 centimeters of water within that same area within the 8.4 meter in diameter tank would require approximately 71 tonnes of water. If the DSH is accompanied in its halo orbit at EML1 by  nearby water/propellant depots for missions to the lunar surface then at least 30 tonnes more of water will probably be required to be sent to EML1 on an annual basis.

Internal configuration of a DSH microgravity habitat derived from SLS propellant tank technology. The pressurized interior inhabited by humans is surrounded with 30 centimeters of water to stop heavy nuclei and to mitigate the effects of major solar evens while also reducing radiation exposure for the crew to less than 25 Rem per year during solar minimum conditions. The airlocks are derived from ETLV propellant tank technology.  

Supplying  large amounts of water to EML1 could easily be accommodated by additional SLS launches.  But since NASA currently has only 16 RS-25 engines in stock from the old Space Shuttle program,  the number of SLS launch vehicles will limited to just four until until new  RS-25 engines are in production from Aerojet Rocketdyne in 2022 or 2023. This  means that an aggressive SLS program cannot really begin until 2022 or 2023.

Four of the RS-25  engines will be dedicated to an  SLS launch in 2018 to test the MPCV (Multipurpose Crew Vehicle).  Another four engines will be used by NASA for the first crewed MPCV mission beyond the Earth's magnetosphere. That only leaves enough engines available for two additional SLS launches until  new engines are in production.

One SLS launch would be enough to deploy the DSH to EML1. But a the final engines available for one more SLS launch would not be able to transport enough water to appropriately radiation shield the DSH. This could mean that DSH deployment might have to be delayed until 2022 or 2023.
 
In previous articles, I  have suggested  that NASA needs to commit itself to developing a reusable single staged Extraterrestrial Landing Vehicle (ETLV) for crewed and robotic  missions to the surface of the Moon, the moons of Mars, and to the Martian surface (with an ADEPT or HIAD deceleration shield).  An essential component of a reusable ETLV would be an ETLV derived water/propellant depot (WPD) that would be capable of using solar electricity to produce LOX and LH2 from water. Serious funding from Congress for  the development of the ETLV and the associated landing vehicles and orbiting depots derived from it should start in 2017, in my opinion, at a funding level of at least $1.5 billion per year.

An ETLV derived and SLS deployed water/propellant depot at EML1 near solar power station. The solar powered facility would be capable of storing up to 100 tonnes of water while also producing and storing up to 60 tonnes of LOX/LH2 propellant. 

In 2020 or 2021, a WPD could be deployed to  LEO with at least 50 tonnes of water using no SLS upper stage or 80 tonnes of water with an upper stage.   At LEO, the WPD  would  electrolyze water into hydrogen and oxygen and then  liquefy and store the hydrogen and oxygen within five propellant tanks capable of storing up to 60 tonnes of rocket propellant. The WPD would then self deploy itself into a halo orbit at EML1.

Once a WPD has been deployed to EML1 then private commercial providers could deliver water to EML1. Space X will be testing its new Falcon Heavy in 2016. Such a vehicle might be able to supply 10 to 15  tonnes of water to EML1 per launch. The ULA also has plans to develop a heavy lift version of their future Vulcan rocket  (the Vulcan Heavy) which should also be capable of delivering 10 to 15 tonnes of water to EML1 per launch. So starting in 2020, commercial launches could be used to deliver 10 to 15 tonnes of water per month to EML1 (120 to 180 tonnes per year). Monthly commercial water deliveries will continue to EML1 until  lunar water manufacturing and exporting facilities on the lunar surface are complete in the middle or late 2020's.

Artist rendition of Space X Falcon Heavy (Credit: Wikipedia)

Artist rendition of ULA Vulcan Heavy

Once the water has been delivered to EML1 and fairly close (within a few hundred meters) of the water/propellant depot, the WPD will rendezvous with the water tankers, extracting and depositing the water with WPD's water tank. The WPD will dock at an solar power station where it will use that power to convert some of the water into LOX and LH2 while storing the rest.

After the DSH is deployed to EML1, the WPD will also rendezvous with the DSH,  transferring water to the habitat for radiation shielding, drinking, and air production. Once ETLV spacecraft are ready for robotic and crewed missions to the lunar surface, they will be fueled by the WPD at EML1.

 The last remaining engines from the Shuttle era can then be used to launch the MPCV to EML1, testing the ability of the SLS to deliver astronauts safely to the Earth-Moon Lagrange points while also checking out the integrity and functionality of the Deep Space Habitat.

Since long periods of time under  microgravity conditions  is inherently deleterious to human health,  time aboard the DSH at EML1  should be constrained. For astronauts over the age of 40, the most vulnerable astronauts to microgravity visual damage,  I'd limit missions confined to microgravity environments to only 16 days. For astronauts under the age of 40, I'd limit the stay at EML1 to less than 31 days. Such short stays at EML1 would ensure that the astronauts would not receive enough radiation in the DSH to prevent them from participating in future interplanetary missions where they will be exposed to months and even years of cosmic radiation bombardment.

While thicker water shielding for the DSH could further reduce radiation exposure, it would also add substantial amounts of  mass to an interplanetary vessel. One of the goals of the DSH should be to replicate conditions for astronauts aboard an interplanetary vessel. So the DSH should only provided with enough water shielding similar to that of an interplanetary vehicle. And an interplanetary habitat only has to be shielded to a  level that would enable astronauts to complete a three year round trip to and from Mars and Mars orbit without exposing them to more than 50% of the recommended lifetime radiation exposure recommended by NASA-- which would be about 100 Rem for the least vulnerable passengers (women 25 years of age).


Links and References


Spending Bill To Accelerate NASA Habitation Module Work

Deep Space Habitats

Commonality between Reduced Gravity and Microgravity Habitats for Long Duration Missions

Building an L1 depot in phases

Habitat Concepts for Deep Space Exploration

NASA Mega-Rocket Could Lead to Skylab 2 Deep Space Station

BA-330

Solar Storm and Space Weather

Cosmic Radiation and the New Frontier

 NASA Contracts Production of New RS-25 Engines for the Space Launch System

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

ULA Future Full Spectrum Lift Capability

The SLS and the Case for a Reusable Lunar Lander

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