Tuesday, September 30, 2014

Living and Reproducing on Low Gravity Worlds

1972 photograph of Apollo astronaut, Eugene Cernan, walking towards the LRV on the lunar surface (Credit: NASA)

by Marcel F. Williams

Establishing a permanent human presence beyond our planet of evolutionary origin is one of the long term goals of human space travel. The expansion of human settlements throughout the solar system has the potential to dramatically increase the economic wealth of human civilization while also greatly enhancing the survival of our species.

Some space advocates believe that the long term colonization of the solar system will require the manufacture of titanic artificial worlds that rotate to  produce simulated Earth-like gravities within their interior surfaces.  But there are still others  who believe that low gravity worlds such as  the Moon and Mars could be utilized as near term destinations for human colonization.

But can Homo sapiens really live and reproduce on hypogravity worlds?

In his classic 1972 song 'Rocket Man', Elton John says: "Mars ain't the kind of place to raise the kids..."

Well, maybe!

Planets and Moons within the solar system that are potentially suitable for human colonization:


surface gravity relative to the Earth: 0.17g 

diameter relative to the Earth: 27.3%

surface area relative to the Earth: 7.4%


surface gravity relative to the Earth: 0.38g 

diameter relative to the Earth: 53.1%

surface area relative to the Earth: 28.4%


surface gravity relative to the Earth: 0.38g 

diameter relative to the Earth:  38.3%

surface area relative to the Earth:  14.7%


surface gravity relative to the Earth: 0.13g 

diameter relative to the Earth:  37.8%

surface area relative to the Earth:  14.3%

Note: Land area comprises ~ 29% of the Earth's surface with ~71% covered by water

Continuous exposure to microgravity conditions over weeks and months is inherently deleterious to human health. And there is growing evidence that long term microgravity exposure can also significantly  lower  fertility in humans and other mammals, possibly leading  to sterility. This suggest that crewed interplanetary missions requiring several months of space travel may require interplanetary vehicles capable of producing artificial gravity during the journey. Obviously, humans can't colonize Mars by sterilizing their passengers before they get there!

But it is currently unknown how much gravity is required to mitigate or eliminate significant infertility in humans. However,  if the lower gravity of the Moon or Mars turns out to seriously effect the long term fertility of humans then daily exposure to-- hypergravity-- through short armed centrifuges may be a possible solution.

Short radius hypergravity centrifuge could help to mitigate the possibility of infertility on lower gravity worlds such as the Moon and Mars.   (Credit NASA)
But the lower gravity on extraterrestrial worlds could have another deleterious effect that may effect human reproduction and even the ability of people to return to the normal gravity of the Earth's surface. Bone mineral loss under microgravity conditions is already known to occur in astronauts living in space for several weeks. And significant bone loss could distort the shape of the female pelvis to a degree that endangers her and a potential infant during attempted childbirth. Unfortunately, while hypergravity centrifuges may mitigate muscle loss in low gravity environments, they appear to have no effect on bone mineral loss.  Rigorous exercise in microgravity, however,  does seem to lower the rate of bone mineral loss-- but does not stop it.

Predicted time limits beyond the Earth  for significant  bone loss in humans that could  risk  skeletal fractures once astronauts return to Earth

Space (microgravity) - 36 weeks (60 weeks with exercise)

Moon (1/6 gravity) - 96 weeks

Mars (2/5 gravity) - 159 weeks

The predicted level of tolerable bone loss for humans in space is about 36 weeks. However, if astronauts exercise rigorously for a few hours every day then their stay in space can be extended to 60 weeks (more than a year). So it seems logical that rigorous exercise should enable humans to mitigate or even eliminate significant bone mineral loss under the hypogravity conditions of the Moon and Mars.

Having some gravity could make it possible for people to use heavily weighted vest or backpacks in order to avoid bone mineral loss while maintaining their Earthling strength-- even without daily strenuous exercise.  While lifting weights can strengthen the arms, weighted vest or backpacks producing an Earth-like weight to be carried by their hindlimbs would strengthen the legs which are normally physiologically weakened under microgravity and low gravity environments.,Within pressurized habitats on the Moon an Mars, heavily weighted vest could be worn throughout the day, providing exercise for the leg muscles when standing, walking, running and jumping.

However, children and infants who are born on the Moon and Mars  may also have to wear weighted vest on a regular basis soon after they are born if their bodies are to grow and develop properly on such low gravity worlds. But  it would appear that humans should be able to live and reproduce on low gravity planets and moons such as the Moon and Mars if they wear the appropriate clothing (weight vest or weight packs) while periodically experiencing hypergravity on a short armed centrifuge.

Of course, Homo sapiens is a species that it use to modifying  its clothing and its habitats in order to survive in more hostile environmental. That's why human ancestors were able to radiate from the tropical regions of Africa into the wintery weather of  Europe, Northern Asia, and eventually North America-- especially during the Earth's glacial periods.

Links and References

Bone Loss and Human Adaptation to Lunar Gravity

 Effects of artificial gravity during bed rest on bone metabolism in humans

 How Much Gravity Is Needed to Establish the Perceptual Upright?

Impacts of Altered Gravity on Male and Female Reproductive Health

 Detrimental Effects of Microgravity on Mouse Preimplantation Development In Vitro

 Morphological and Morphometric Study on the Effect of Simulated Microgravity on Rat Testis

Pioneering and Commercial Advantages of Permanent Outpost on the Moon and Mars

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

Thursday, September 18, 2014

Spent Fuel and the Thorium Solution

Thorium deposits in North America (Credit:USGS)
by Marcel Williams

Humanity currently exist in a global energy economy that is dominated by fossil fuels. And the combustion of fossil fuels by our industrial civilization has  created atmospheric conditions with an ever increasing  CO2 (carbon dioxide) content. The carbon dioxide in the Earth's atmosphere is now higher than it has ever been in the history of the human species. In fact, it is higher than in the entire history of our genus, Homo, which first emerged in sub-Saharan Africa more than 2.5 million years ago.

At approximately 400 parts per million, current CO2 levels in the Earth's atmosphere may be as high as they were during the Pliocene Epoch when sea levels may have been 10 to 40 meters higher than they are today. And as long as we continue to use fossil fuels, the CO2 content in the Earth's atmosphere is likely to reach levels not seen since the Earth was devoid of polar ice caps altogether which could eventually raise global sea levels above 60 meters.

So thanks to the humongous energy needs of our modern civilization, future generations face the possibility of living in a much warmer world with substantially higher sea levels. Rising sea levels  could eventually flood most of the world's coastal areas including some of the world's major cities.

Altruistically,  our current civilization should be trying to create a better tomorrow for future generations.  Unfortunately, there are global economic interest that  have tens of  trillions of dollars invested in the fossil fuel economy. And their priorities are to make near term profits-- even at the expense of humanity's long term environmental and economic future. Of course, America's capitalist system exist within a government of the people, by the people, and for the people. So within a democratic republic, free people have the ultimate responsibility to make sure that our civilization doesn't wreck the environment for future generations. 

While there are viable technological alternatives to the fossil fuel economy, there are many who actually fear the-- best technological solution-- to the problem of global warming and  the deposition of excess CO2 in the Earth's atmosphere: nuclear energy.

Commercial nuclear power is the principal carbon free producer of electricity in the United States, producing more than three times as much carbon free electricity as hydroelectricity,  six times as much as wind, and more than 100 times as much carbon free electricity as solar. And this is in spite the fact that the United States pretty much halted the building of new nuclear power plants in the US for more than thirty years.

Commercial nuclear energy is also the safest form of electricity production ever created. Even if you include the accidents at  Chernobyl, Fukushima, and Three Mile Island, nuclear energy production is still substantially safer than using coal, natural gas, hydroelectricity, solar, or wind.

Energy Mortality Rate (deaths per trillion kilowatt hours)

US Coal -------- 15,000

Natural Gas ------ 4000

Hydroelectric ---- 1400

Solar (rooftop) ---- 440

Wind ---------------- 150

Nuclear ---------------90

Although the United States has more commercial nuclear reactors in operation than any other nation on Earth, the construction of new reactors in the US still lags well behind  China and Russia and even behind India, and Europe. While the next generation of small centrally mass produced nuclear reactors  should be available for commercial service in the US by the  2020's, the future domestic demand for such reactors by US utilities is still clouded by the fact that there is still no long term solution to the political problem of spent fuel which is often referred to as nuclear waste. 

Number of nuclear reactors currently (9/18/2014) under construction by nation: 

CHINA------------------------------------------     27
RUSSIA-----------------------------------------     10
INDIA--------------------------------------------     6
KOREA, REPUBLIC OF ---------------------    5
JAPAN ---------------------------------------------2 
PAKISTAN -------------------------------------    2
SLOVAKIA -------------------------------------    2
TAIWAN ------------------------------------------ 2
UKRAINE---------------------------------------     2
UNITED ARAB EMIRATES-----------------     2
FRANCE -----------------------------------------    1  
ARGENTINA-----------------------------------     1   
BELARUS ---------------------------------------    1  
BRAZIL ------------------------------------------    1  
FINLAND ----------------------------------------    1         
What to do with the spent fuel once its removed from commercial nuclear reactors  is one of the most difficult political obstacles hampering the approval and construction of new nuclear reactors in the US.   Within some American States, it is even illegal to build  new nuclear reactors  until there is a permanent repository or another long term solution to the problem of nuclear waste.

The  irony in all of this, of course,  is the fact that relative to other electric power producing facilities, nuclear power plants actually create very little toxic waste. A 1000 MWe nuclear power plant only produces about  27 tonnes of spent fuel every year.  That's a quantity that is so small that all of the radioactive material ever produced from the commercial nuclear power industry in the US could be placed in an area the size of a football field only a few meters high. That's it!

A 1000 MWe coal power plant, on the other hand, produces approximately 400,000 tonnes of toxic material every year:  ash from coal power plants that is  contaminated with toxic materials such as  mercury, arsenic, chromium, and cadmium which can contaminate drinking water supplies and damage the human nervous system and other vital  organs. The ash pumped into the atmosphere of a coal power plants also expose surrounding populations to approximately 100 times more background radiation than a nuclear power plant does. Coal power plants, of course, are the primary producers of greenhouse gasses amongst electric power facilities.

But even solar energy produces substantially more toxic waste than commercial nuclear reactors. Per kilowatt of electricity produced,  the toxic materials required to produce rooftop solar panels and the toxic materials contained in the dismantling of solar panels is quantitatively at least 10,000 times that of the toxic materials produced from the nuclear industry. So the toxic waste produced from commercial nuclear power plants is miniscule compared to the toxic waste produced from the solar panel industry.

Ironically, most of the spent fuel produced from a commercial nuclear power plant is actually not waste at all. More than 95% of the fissile and fertile material contained in spent fuel can actually be recycled. This is already been successfully done to a partial degree in countries like France where plutonium is extracted from spent fuel and then mixed with depleted uranium 238. 

But way back in 1982, the  Shippingport Atomic Power Station in Beaver County, Pennsylvania was shut down after utilizing enriched uranium in a blanket of thorium 232 for five years.  In 1987, it was reported that  the core of the light water thorium reactor contained 1.3% more fissile material than it had when it was originally fueled.  This clearly  demonstrated that a light water breeder reactor could produce more fissile  material than it consumed if fissile material was utilized in a blanket of fertile thorium.

So plutonium could be extracted from the spent fuel of Light Water Reactors and mixed with thorium in order to produce carbon free electricity in Light Water Thorium Reactors while burning up the plutonium.   The fissile uranium produced from the conversion of thorium 232 into uranium 233 could then be mixed with the with depleted uranium or reprocessed uranium from spent fuel to produce power in current Light Water Reactors.  Burning plutonium from spent fuel in Light Water Thorium Reactors while utilizing uranium 233 from thorium reactors for reuse in Light Water Uranium Reactors could demonstrate that more than 95% of the material in spent fuel can be recycled. Recycling the fissile material in spent fuel would dramatically reduce the already meager amount of radioactive material that has to be sequestered into nuclear waste site. And this could help to end the prohibition against building new nuclear reactors in some States in the United States.

Countries with the Largest Thorium Reserves (tonnes)

India ......................    846,000
Turkey...................     744,000
Brazil ....................     606,000
Australia ...............    521,000
USA ......................     434,000
Egypt....................      380,000
Norway.................      320,000
Venezuela.............      300,000
Canada.................      172,000
Russia..................       155,000
South Africa........      148,000
China...................      100,000
Greenland..............     86,000
Finland..................      60,000
Sweden..................      50,000
Kazakhstan............     50,000

However, the moderation of neutrons could be reduced if the water content of the thorium reactor were reduced by 25 to 50%. This would allow the reactor to burn the other radioactive waste products  in a solid fuel mix with plutonium and thorium.  That, of course, would completely eliminate the need to bury any spent fuel products created by commercial nuclear reactors.

There's only enough-- terrestrial uranium-- to produce all of the  electricity and synfuels required to power all of  human civilization at current levels for about 15 years. However,  there's more than 4 billion tonnes of uranium in seawater, enough   provide all of the energy needs for human civilization for more than 3600 years. Recycling the spent uranium and might extend this to over 5000 years. Utilizing the plutonium from Uranium Light Water Reactors to power Thorium Light Water Reactors, could power human civilization for 2800 years.

So a uranium and thorium economy could power human civilization at current levels for nearly 8000 years.  Beyond this point, plutonium/uranium breeder reactors would finally be required to continue to power human civilization on Earth by solely using nuclear fission.   

Thorium deposits on the lunar surface (credit:NASA)

However, additional sources of thorium could be mined on the surface of the Moon, a resource that's only a few days away by chemical rockets. Because there is no life on the Moon, thorium could be exploited much more extensively on the lunar surface than on Earth, perhaps to a level that could allow lunar thorium to power a nuclear fuel economy on Earth forever.

Marcel Williams

Links and References

Atmospheric CO2 decline during the Pliocene intensification of Northern Hemisphere glaciations

Departures from eustasy in Pliocene sea-level records

National Geographic: Rising Seas

How Deadly Is Your Kilowatt? We Rank The Killer Energy Sources

Under Construction Reactors
State Restrictions on New Nuclear Power Facility Construction

Radioactive Waste Management

Safely Managing Used Nuclear Fuel

Spent Fuel Transport & Storage

The real waste problem, solar edition

Light Water Breeder Reactor: Adapting A Proven System

How thorium can solve the nuclear waste problem in conventional reactors

The Thorium Dream

The Thorium Alternative

Use of Reprocessed Uranium

Fueling our Nuclear Future 

USGS Map of Thorium Deposits in North America
Thorium Deposits on the Moon

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