Monday, August 29, 2016

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

 The SLS and the Case for a Reusable Lunar Lander


Anonymous said...

This is NOT Neil Armstrong on the surface of the Moon. It is Edwin "Buzz" Aldrin!

Marcel F. Williams said...

Thanks for the correction!

Engineer-Poet said...

Instead of using water as a shield and throwing it away, why not use hydrogen peroxide and use it as propellant?

Marcel F. Williams said...

I agree that being able to use a radiation shield as propellant during the final trajectory burns would be even more efficient.

But hydrogen peroxide would have to be manufactured from water in orbit making it a lot more expensive to use than simple water.

Piping your shielding into a propellant tank for the final trajectory burns would also be more complex than simply dumping water into space.

As a monopropellant, hydrogen peroxide would have a substantially lower specific impulse relative to a mixture of hydrogen and oxygen propellant.

You'd also have to develop a whole new rocket system instead of using the LOX/LH2 EUS.


Engineer-Poet said...

"hydrogen peroxide would have to be manufactured from water in orbit"

As opposed to making LOX and LH2 from water in orbit?

"Piping your shielding into a propellant tank for the final trajectory burns"

So don't.  Arrange the shield tanks so they're plumbed directly to the engines.

"As a monopropellant, hydrogen peroxide would have a substantially lower specific impulse"

Still higher than water, and H2O2 works fine as an oxidizer for bipropellant engines.  Almost any fuel you'd burn with oxygen will burn in the mixture of O2 and steam from decomposing H2O2.

"You'd also have to develop a whole new rocket system instead of using the LOX/LH2 EUS."

There's a point, but I'd bet it wouldn't be hard to feed the output of an H2O2 gas generator into the LOX feed downstream of the RL10 oxygen pump.  For that matter, you could instead use water to mass-load the RL10 hydrogen feed downstream of the hydrogen turbine.  You'd achieve a higher total delta-V using the water as reaction mass than simply dumping it.

Doug Space said...

It seems to me that polyethylene (or liquid methane) is noticeably better than water at stopping GCRs.

I wouldn't modify the space suits. Rather, I think that they should use a rover in which the cab is shielded. They could remain inside and collect samples via remotely-controlled manipulator son the outside of the vehicle.

Yes, GCRs might hit 1/3rd of neurons to Mars and back. But I believe that is the case of no shielding were used which is not realistic. Crew will routinely take water-containing provisions along with them and the mission designers will take into consideration where the provisions are relative to the crew.

Also, GCRs come in a variety of Z number (i.e. number of protons in the nucleus) and a variety of velocities. I believe that the most common GCRs are single proton ions with most at lower energies and fewer as you go up the energy levels. So, I think that the relevant question is, "What percentage of neurons will be destroyed given reasonable amounts of shielding"?

If I am reading the graph correctly, 20 g/cm2 will reduce GCR radiation by about 62% and I would imagine that the number of GCR stopped before reaching the astronaut's head would be considerably higher that that. But I don't know for sure.

Marcel F. Williams said...

For short trips into space, a couple weeks or less, cosmic radiation is really not a problem-- with the exception of the cell killing heavy nuclei component.

20gm/cm2 is the amount of mass needed to stop nuclie larger than the nucleus of helium atoms.

Water allows you the ability to internally shield astronauts from heavy ions and to easily dump the shielding during the final trajectory burns. This would be more difficult to do with cryogenic fluids. It would also be difficult to dump polyethylene before the final trajectory burns, however, the internal water storage could be within polyethylene pipes or water bags.

For interplanetary journeys, I'd probably increase the water shielding to at least 30 cm which should keep radiation level exposure down to 30 Rem per year during solar minimum conditions combined with a major solar event. I believe that total mission exposure sure be less than 50 Rem. This would allow female astronauts within the 25 to 30 year range to possibly participate in more than one interplanetary mission. I don't think-- a single interplanetary mission-- should be a career ender because of radiation exposure.


Engineer-Poet said...

You can get upwards of 55 μSv/hr on the beach, which people have been doing for centuries with no ill effects.  That's about .5 Sv/yr (50 rem).  If the major issue is heavy nuclei, breaking them up into alphas and lighter stuff should be sufficient.

Marcel F. Williams said...

I agree with you that multiple day, multiple and multiple months journeys into space will mostly be about protecting astronauts from heavy ion exposure.

Five Rem per year is the legal limit for radiation workers on Earth. And in the near future, I believe that should also apply for permanent habitats in space on on the surfaces of other worlds. But that shouldn't be difficult on the Moon and Mars since their is an almost endless supply of regolith.

A community of over 2000 people in Iran, is naturally exposed to 1 to 25 Rem of radiation annually-- with no signs of any deleterious physical or reproductive effects on that population. Of course, we don't know if this community has been adapted to a higher level of radiation due to natural selection.

So I'd be initially conservative with permanent habitats, limiting annual exposure to less than 5 Rem per year within habitats. That, of course, wouldn't include the increased exposure to radiation during interplanetary journeys and when exploring the surface withing vehicles or pressure suits.


Engineer-Poet said...

There's good reason to believe that 5 rem per year as a chronic (not prompt) exposure is not just far below the tolerance dose, it's below the optimum for human health.

There are scientists trying to get exposure limits raised, based on actual scientific information.  The current limits are based on scientific fraud, bought and paid for by the Rockefeller Foundation.

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