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#1 2017-10-29 19:55:40

SpaceNut
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Radiation amount type risk mitigation

5 Mars Mission Radiation Shield Ideas Win NASA Challenge

radiation-space-danger-130529b-02.jpg

Lockheed's Prototype Habitat Plans for NASA's Lunar Orbiting Deep Space Gateway Lockheed Martin was one of them, and this week the company released some details on plans for their full-scale prototype, which they hope to complete over the next 18 months.

Lockheed-NextSTEP-habitat-1-of-1.jpg

“The deep space gateway would have a power bus, a small habitat to extend crew time, docking capability, an airlock, and serviced by logistics modules to enable research,” says NASA.
“The propulsion system on the gateway mainly uses high power electric propulsion for station keeping and the ability to transfer among a family of orbits in the lunar vicinity.
The three primary elements of the gateway, the power and propulsion bus and habitat module, and a small logistics module(s), would take advantage of the cargo capacity of SLS and crewed deep space capability of Orion.
An airlock can further augment the capabilities of the gateway and can fly on a subsequent exploration mission.
Building the deep space gateway will allow engineers to develop new skills and test new technologies that have evolved since the assembly of the ISS.

Deep-Space-Gateway-illustration-1-of-1.jpg

The other 5 companies selected for prototypes are:

    Bigelow Aerospace of Las Vegas, Nevada
    Boeing of Pasadena, Texas
    Orbital ATK of Dulles, Virginia
    Sierra Nevada Corporation’s Space Systems of Louisville, Colorado
    NanoRacks of Webster, Texas

RADIATION MITIGATION TECHNOLOGIES

Radiation Hazard for Deep Space Human Exploratio

Evaluating Shielding Approaches to Reduce Space Radiation Cancer Risks

Deep Space Mission Radiation Shielding Optimization

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#2 2017-10-30 11:06:18

GW Johnson
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Re: Radiation amount type risk mitigation

The .66-ish sieverts in space and .2 to .3 sieverts on Mars is GCR exposure.  That's the steady drizzle.  The occasional solar flare is many orders of magnitude higher,  very directional,  and rather short duration (hours to a day or so).

I don't speak "sieverts,  I speak REM,  which I think differ by a factor of 100.  At Earth's distance from the sun,  GCR varies from 24 to 60 REM per year, more-or-less sinusoidally,  with an approximate 11 year period matching the sunspot cycle.  Lots of sunspot activity reduces the GCR flux,  and vice versa.  Probably a variable solar wind effect.

NASA's own astronaut exposure rules (off their radiation website) are 50 REM max in any one year,  25 REM max in any one month,  and a career limit that varies with age and gender,  but peaks at 400 REM total over the career.  Supposedly these rules were developed to limit late in life cancer risks to 3% above normal.  That actually compares quite well with the 5.5% increased risk associated with .6 sievert (60 REM) I see quoted above. 

I don't see much of a problem for exploration crews,  who generally will be willing to take the 3-6% increased risk,  fuzzy as those criteria really (and fundamentally) are.  Long-term base crews will get limited by the career limit to rather short stays,  I think.  Not very many years,  for sure. 

The real problem is colonists,  especially children.  Those exposures at 3-6% increased risk are around factor 10 too high for long-term exposures,  and factor 100 too high for children.  That means significant regolith cover is required long-term,  and not just for solar flare protection. 

A solar flare recently smacked Mars dead nuts on,  and lit up an aurora all over the planet.  I don't recall seeing doses reported,  although I'd bet Curiosity measured them.  Really big flares can be 10,000+ REM per hour!  The Martian atmosphere only reduces that exposure by a factor of 2 to 3,  if the GCR reduction is any guide.  Solar flare stuff is easier to shield than GCR,  so thick regolith cover should work for that,  especially if it has a little ice in it. 

The problem with too-thick is secondary scatter radiation.  I have no figures for what is too thick,  though. 

The NASA radiation website is where I got the 15-20 cm of water figure that would protect adequately from a solar flare,  and reduces the GCR a little bit,  without any secondary shower effects.  Their curves indicated effectiveness out to double those figures.

It would appear to me that what Curiosity uncovered during its trip to Mars and since while on Mars is pretty consistent with the effects and the exposure rules posted years ago on the NASA radiation website.  This stuff really isn't news,  just confirmation.  But some seem to be misusing it as a scare tactic not to go. 

All that being said,  then where is the radiation shelter in the Gateway design?  Not the GCR, that's an accepted risk and a stay limiter.  I mean the 10,000+ REM/hour shelter!  Like a surface nuclear explosion's fallout,  that is.  500 REM accumulated in a short interval like minutes,  hours,  or even a few days is the "fatal dose" figure.  They'll kill a crew without one.

GW

Last edited by GW Johnson (2017-10-30 11:14:21)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#3 2017-11-05 12:27:01

kbd512
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Re: Radiation amount type risk mitigation

Oldfart1939,

NASA can make this a useful program any time they want by imposing delivery deadlines and making payment contingent upon delivery of an agreed upon product.

GW,

The GCR reduction of the Martian atmosphere is not a guide for lower energy radiation reduction.  Virtually all GCR's are in the GeV range versus SPE's which typically range from less than 1MeV to 100MeV.  And yes, I'm aware that a few major solar events produced energies into the low GeV range.  GCR is nearly always at least one order of magnitude higher than most of the more energetic SPE's and generally two or even three orders of magnitude higher than SPE's.  That'd be why a water wall less than a foot thick can attenuate SPE's, but only marginally reduces GCR dose rates until there are meters of water between the people taking the dose and the source.

There's no such thing as "too thick", either.  The thicker the shielding, the lower the dose.  There is such a thing as secondary particle showers from the thin Aluminum pressure vessels commonly used, much like there are secondary projectiles created from spalling when a round impacts an armored plate and fails to penetrate.  The solution in Zvezda was to use a PE liner inside the Aluminum module, so that module serves as the radiation shelter for ISS and the dose rates from that recent event you mentioned were high enough that the crew had to stay there briefly.

I think NASA should pursue using non-structural BNNT liners instead of BNNT pressure vessels.  It's the most volume and mass efficient solution we've come up with and it works as well as or better than any other practical solution to the problem.  NASA is trying to "go for the gold" and make the BNNT a structural material instead of accepting the mass penalty that comes with a liner inside the pressure vessel.  I think that's a project for another day.  The "best" is the enemy of the "good".  A non-structural BNNT is "good enough" to get the job done and it's fire retardant.

All we need to do is solution the major problems acceptably well and leave absolute perfection for future projects.  There is no radiation problem that existing materials can't adequately protect against, but someone at NASA is going to have to accept that perfection is not achievable in the short term if they actually want to do this mission in our lifetimes.

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#4 2017-11-06 11:46:46

GW Johnson
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Re: Radiation amount type risk mitigation

I'm just guessing because radiation shielding is outside my experience,  but the secondary shower effect seems to be associated with higher molecular weight atoms. 

If that's true,  then regolith cover over a Mars habitat might produce some secondary shower effects from the metal atoms in the minerals making up the rock particles.  Adding ice into the regolith (in other words,  wetting it down in sandbags and letting it freeze in place) should help reduce that with more hydrogen content. 

In space,  most any low molecular weight material should shield solar flare events well,  and reduce the GCR a little.  I know virtually nothing about nanotube technology (I think one kind of those,  perhaps boron nitride,  is what Kbd512 means when he says BNNT),  but there's water (in multiple forms that even includes frozen food),  a variety of plastics,  and even the storable propellant materials (any of the hydrazines,  NTO,  and even IRFNA). 

Your radiation shield does NOT have to be a liner inside your pressure shell,  it could be tanks clustered around outside your pressure shell. That only-inside-the-shell-liner notion is an unnecessary thinking box that I see operating all too often.   I know enough to have heard of a "shadow shield".  They work,  too.

Kbd512 is quite right:  we just need things that work in order to go,  they do NOT have to be utterly optimized.  Columbus's 3 ships were not the very best available,  but they worked.  Sailing by latitude only was not the best way to navigate,  but it sort of worked until naval-quality timepieces were invented almost 4 centuries later.

If you accept that there are more ways to launch stuff than just SLS with its high cost and infrequent launch rate,  then the min-thrown-weight constraint drops away.  Orbital assembly and an orbit-to-orbit transport for the crew starts to look really good once again (it first looked really good in the 1950's).  If the life support system isn't "perfect",  then just pack more supplies.  It's heavier,  but so what?  Unless you only fly on SLS,  that is.

GW

Last edited by GW Johnson (2017-11-06 11:49:39)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#5 2017-11-06 12:08:57

RobertDyck
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Re: Radiation amount type risk mitigation

GW Johnson wrote:

...secondary shower effect seems to be associated with higher molecular weight atoms. 

If that's true,  then regolith cover over a Mars habitat might produce some secondary shower effects from the metal atoms in the minerals making up the rock particles.  Adding ice into the regolith (in other words,  wetting it down in sandbags and letting it freeze in place) should help reduce that with more hydrogen content.

Documents from the team for the MARIE instrument on Mars Odyssey state most heavy ion Galactic Cosmic Radiation is blocked by Mars atmosphere. At a high altitude location such as where Odyssey landed, 90% is blocked from reaching the surface. At a low altitude location such as where Spirit or Curiosity landed, 98% is blocked. This already includes radiation blocking from the half of the sphere beneath your feet protected by the planet itself. So the best protection from heavier molecular weight atoms is just to get down to the surface; don't stay in space.

As for shielding from the 50% of medium weight ions and the small remainder of heavy ions that get through, you're right. 'nuf said.

Last edited by RobertDyck (2017-11-06 12:14:04)

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#6 2017-11-06 14:54:23

3015
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Re: Radiation amount type risk mitigation

RobertDyck, GW is referring to high molecular weight atoms in the shielding material. I didn't know about those results from MARIE though, thanks for sharing that.

GW, you are indeed correct about secondary radiation from regolith. This paper simulated radiation dose with different levels of regolith shielding (results are in a table on the last page) and found that absorbed dose actually increased with the first 10 cm of shielding.

Last edited by 3015 (2017-11-06 14:54:47)

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#7 2017-11-07 13:28:31

Oldfart1939
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Re: Radiation amount type risk mitigation

Just recall that linear polyethylene is a good radiation moderator. Plastics are well-suited as radiation shielding products

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#8 2017-11-07 13:49:46

Oldfart1939
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Re: Radiation amount type risk mitigation

The key point here is we can never eliminate the radiation hazard, but we CAN attenuate the dosages. This can be done several ways on the Mars surface. One is through piling regolith on top of the habitats, regardless of the type of structure finally utilized. That gives some 12 Earth hours of reduced exposure while sleeping and eating, in addition to inside work tasks. Then lets say we only do exploration outside the hab on a staggered work schedule for the Marsonauts. Only 4 outside while 3 remain inside on any given Sol. Then use a rotating schedule for exposure. That gives a 4 on 3 off work schedule for a seven person crew. That calculates to only 28.6% of unshielded exposure time. Of course, suits incorporating substantial Boron Nanotube shielding for the body core and head will also moderate the effects of GCR.

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#9 2017-11-07 15:53:48

RobertDyck
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Re: Radiation amount type risk mitigation

In the Mars Homestead Project, phase 1, we did a calculation. If the permanent habitat has at least 2.4 metres of regolith piled on top to practically eliminate radiation, then "Marsonauts" would have to be restricted to 40 hours outside in a spacesuit per 7 Mars solar days (sols). That's a work week, so I don't think anyone would object. The result would reduce radiation exposure to that of a US nuclear reactor worker. If a greenhouse has simple glass windows or plastic film, no regolith, then time in the greenhouse counts as time outside. Again, 40 hours per week is not restrictive.

A science mission won't have 2.4 metres of regolith. It'll have a light-weight hull with sandbags filled with regolith. Still, radiation on the surface of Mars is half that of ISS, and that's without sandbags. With sandbags, Mars astronauts will be fine.

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#10 2017-11-07 17:50:14

louis
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Re: Radiation amount type risk mitigation

I see very little need for Mars pioneers to step outside and expose themselves to radiation.  Outside the home habs they will spend nearly all their time in well-shielded rovers I think, where necessary directing robot rovers, while in visual contact. I think there may be a call for some geologists to get really close up, depending on what we find on Mars.

RobertDyck wrote:

In the Mars Homestead Project, phase 1, we did a calculation. If the permanent habitat has at least 2.4 metres of regolith piled on top to practically eliminate radiation, then "Marsonauts" would have to be restricted to 40 hours outside in a spacesuit per 7 Mars solar days (sols). That's a work week, so I don't think anyone would object. The result would reduce radiation exposure to that of a US nuclear reactor worker. If a greenhouse has simple glass windows or plastic film, no regolith, then time in the greenhouse counts as time outside. Again, 40 hours per week is not restrictive.

A science mission won't have 2.4 metres of regolith. It'll have a light-weight hull with sandbags filled with regolith. Still, radiation on the surface of Mars is half that of ISS, and that's without sandbags. With sandbags, Mars astronauts will be fine.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#11 2017-11-07 19:31:26

RobertDyck
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Re: Radiation amount type risk mitigation

If you obsess about radiation, nobody will go.

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#12 2017-11-08 15:00:30

Terraformer
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Re: Radiation amount type risk mitigation

Adding shielding to a rover means adding mass to it. A rover cannot be anywhere near as well shielded as a static hab.

The Martian atmosphere already provides 200kg of shielding per square metre. Any awning that can be put up and move easily isn't going to improve that much.

Given that most of their time is going to be spent in the hab anyway, including a lot of work (analysing samples, maintaining equipment, doing experiments etc), I don't think radiation on Mars is going to be much of a problem.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#13 2017-11-09 10:45:14

louis
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Re: Radiation amount type risk mitigation

Good points.

It's not mass we have to ferry to Mars in any case...it might add something like, say, 20% to energy usage by the rover but I really don't think that is a major issue on Mars where energy for transport will be plentiful.

If it means we are reducing radiation exposure to manageable levels, then it would be a definite plus.


elderflower wrote:

Your rover might be better for a bit of mass. It would improve traction in some circumstances in the low gravity environment. Also rovers have to carry some water and other consumables to allow for return to base or rescue with a recycling system fault.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#14 2017-11-09 11:33:15

Antius
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From: Cumbria, UK
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Re: Radiation amount type risk mitigation

Terraformer wrote:

Adding shielding to a rover means adding mass to it. A rover cannot be anywhere near as well shielded as a static hab.

The Martian atmosphere already provides 200kg of shielding per square metre. Any awning that can be put up and move easily isn't going to improve that much.

Given that most of their time is going to be spent in the hab anyway, including a lot of work (analysing samples, maintaining equipment, doing experiments etc), I don't think radiation on Mars is going to be much of a problem.

I wonder if it would be practical to engineer a roof arrangement for a vehicle that would allow dirt to be shovelled onto the roof?  It could be shovelled on when the vehicle stops and removed before it sets off again.  If the crew drive for 8 hours per day on average, spend 8 hours asleep and 4 hours awake and inside the vehicle, a roof dirt layer could cut radiation dose somewhere between a third and half.

Regarding galactic cosmic rays - I wonder how much of the surface dose results from secondary particles generated in the Martian atmosphere?  These would presumably have lower energy but higher flux.  For lower energy charged particle radiation, a magnetic field would be an effective means of deflecting radiation and reducing dose rate.  A magnetic field protecting a base doesn't need to be mobile and a big iron core electromagnet might be an acceptable solution.  A vehicle could generate magnetic shielding whilst the engine was running and the crew could assemble a temporary dirt shield for those periods where the vehicle was not moving.

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#15 2017-11-09 21:18:19

RobertDyck
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Re: Radiation amount type risk mitigation

Antius wrote:

I wonder if it would be practical to engineer a roof arrangement for a vehicle that would allow dirt to be shovelled onto the roof?

Short answer: No

Antius wrote:

For lower energy charged particle radiation, a magnetic field would be an effective means of deflecting radiation and reducing dose rate.  A magnetic field protecting a base doesn't need to be mobile and a big iron core electromagnet might be an acceptable solution.  A vehicle could generate magnetic shielding whilst the engine was running and the crew could assemble a temporary dirt shield for those periods where the vehicle was not moving.

To deflect radiation, you need a magnetic field that extends kilometres away. GCR travels at 99.6% the speed of light. Deflecting that requires extreme distance because of the extreme speed. Solar wind speed is 250 to 750 km/s, speed of light in vacuum is 299,792.458 km/s. Solar wind can be deflected at less distance, but still requires a lot. Mini-Magnetosphere extends kilometres from the generator, it can deflect solar wind in vacuum so would be highly effective for an interplanetary spacecraft. But the thin plasma of a Mini-Magnetosphere cannot form in an atmosphere; not even a thin atmosphere such as Mars. A strong magnetic field could deflect solar radiation, but to be effective that would require a mountain with iron to extend the magnetic field to the required distance.

Bottom line: radiation on Mars is half that of ISS. Stop obsessing.

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#16 2017-11-09 21:44:16

Oldfart1939
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Re: Radiation amount type risk mitigation

This thread was not about the surface of Mars, but the unwarranted radiation exposure in the totally unshielded Deep Space Gateway. The surface of Mars is quite benign by comparison. The Deep Space Gateway (DSG) is a potential death trap; not so the surface of Mars.

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#17 2017-11-10 11:04:52

SpaceNut
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Re: Radiation amount type risk mitigation

The space shields being put forth are plasma, rf field, magnetic ect for space travel along with water, bnnt blankets ect but until we built it we can only test simulate it  in a chamber....

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#18 2017-11-12 12:17:45

kbd512
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Re: Radiation amount type risk mitigation

Apart from the mini-magnetosphere and BNNT, the H2, H2O, and HDPE liners have all been extensively tested and proven effective at stopping SPE's and GCR's.  The magnetosphere obviously works well at a planetary scale, but testing of a miniature facsimile is ongoing.  The H2, H2O, and HDPE liners have all been repeatedly tested in space.  The results have been quantified in exacting detail.  The BNNT samples tested thus far have been proven to provide most of the protection that HDPE provides with a very low mass penalty.  For equivalent mass or volume, BNNT is the clear winner.  NASA is stuck on this idea of all-plastic or all-composite pressure vessels.  While that sort of thing actually exists, any cursory examination of fatigue life of those vessels indicates that it just barely works in a reliably consistent manner.  In contrast, metallic alloy pressure vessels are the most well tested and proven forms of pressure vessels in existence.  The metal alloy pressure vessels produce secondary radiation effects, but the liners were also designed to attenuate the effects of those less energetic events.

Incidentally, testing radiation protection materials in a test chamber works quite well.  Radiation of a given type and emission rate is whatever it is and it doesn't matter if it's tested in a vacuum chamber or in space.  Radiation attenuation does not vary because the sample is larger or smaller in dimension, although accurate measurement can be a problem if the sample is too small.  If sample size greatly affected results, then nuclear reactors would have had to wait for powerful supercomputers.  Fortunately, the only thing that matters is the material in question, thus how it behaves when it absorbs types and intensities of radiation, and how thick it is.  SPE and GCR radiation dose rates vary wildly, so an extra measure of protection is warranted to cover the "fudge factor" of measurements and dose rates, but even then the min/max radiation levels expected are very well quantified at this point from the plethora of instrumented probes we've sent to Mars.

I don't believe we should throw our hands up and declare a snow day.  The passive shielding is mandatory, irrespective of what active solutions become available since the electrical power for those devices can always fail, so the liner design and thickness should be sufficient to attenuate the radiation dose to the levels mandated by OSHA for astronauts.

The surface of a planetary body or low orbit is generally the best possible place for humans to be once they get there, as it nearly always blocks 50% of the potential radiation dose that would be received in interplanetary space.  From a radiation does rate perspective, even being on the surface of a small asteroid would be preferable to being in a spacecraft located in interplanetary space, away from any shadow shielding that a massive object can provide.

If any of these new "wonder materials" overcome their cost and complexity of manufacturing challenges to serve as structural materials for pressure vessels, then after at least a decade or so of test and evaluation, we can think about incorporating them into future spacecraft designs.  Right now, we have things that just work and we need to use them in whatever way they work best.

The DSG is not "a joke" if NASA designs it to be the interplanetary transport vehicle / miniature space station it needs to be.  Right now, we're not going anywhere because Orion and SLS are consuming all the funding for the design of this DSG vehicle, closed loop life support, radiation protection, advanced electrical power generation and storage, and upper stages for orbital transfer.  Giant capsules and rockets are legacy technology.  We now have reusable rockets and capsules and those technologies are well tested at this point.  Take the technologies that were developed for Orion or SLS and apply them to the components actually required to do these lunar and Mars missions.

A great deal of knowledge about how to design and fabricate a mass-optimized aluminum pressure vessel that can withstand extreme aerodynamic forces was gleaned from the first Orion test.  Perhaps one or two more tests can confirm what was learned.  Apply that knowledge to both the DSG pressure vessels and landers to create commodity pressure vessel spacecraft capable of being spun for AG using in deep space or landing on Mars.  Apply the knowledge about BNNT to radiation protection liners.  Apply the advances in solar panel and Lithium-ion battery design to the electrical power bus for the DSG.  Use the new generation of life support equipment to maintain a livable environment for the astronauts.  Someone just has to decide to do these things and move forward with what they currently have to work with.

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#19 2017-11-28 10:09:42

Lake Matthew Team - Cole
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Re: Radiation amount type risk mitigation

GW Johnson wrote:

The .66-ish sieverts in space and .2 to .3 sieverts on Mars is GCR exposure.  That's the steady drizzle.  The occasional solar flare is many orders of magnitude higher,  very directional,  and rather short duration (hours to a day or so).

I don't speak "sieverts,  I speak REM,  which I think differ by a factor of 100.  At Earth's distance from the sun,  GCR varies from 24 to 60 REM per year, more-or-less sinusoidally,  with an approximate 11 year period matching the sunspot cycle.  Lots of sunspot activity reduces the GCR flux,  and vice versa.  Probably a variable solar wind effect.

NASA's own astronaut exposure rules (off their radiation website) are 50 REM max in any one year,  25 REM max in any one month,  and a career limit that varies with age and gender,  but peaks at 400 REM total over the career.  Supposedly these rules were developed to limit late in life cancer risks to 3% above normal.  That actually compares quite well with the 5.5% increased risk associated with .6 sievert (60 REM) I see quoted above. 

I don't see much of a problem for exploration crews,  who generally will be willing to take the 3-6% increased risk,  fuzzy as those criteria really (and fundamentally) are.  Long-term base crews will get limited by the career limit to rather short stays,  I think.  Not very many years,  for sure. 

The real problem is colonists,  especially children.  Those exposures at 3-6% increased risk are around factor 10 too high for long-term exposures,  and factor 100 too high for children.  That means significant regolith cover is required long-term,  and not just for solar flare protection. 

A solar flare recently smacked Mars dead nuts on,  and lit up an aurora all over the planet.  I don't recall seeing doses reported,  although I'd bet Curiosity measured them.  Really big flares can be 10,000+ REM per hour!  The Martian atmosphere only reduces that exposure by a factor of 2 to 3,  if the GCR reduction is any guide.  Solar flare stuff is easier to shield than GCR,  so thick regolith cover should work for that,  especially if it has a little ice in it. 

The problem with too-thick is secondary scatter radiation.  I have no figures for what is too thick,  though. 

The NASA radiation website is where I got the 15-20 cm of water figure that would protect adequately from a solar flare,  and reduces the GCR a little bit,  without any secondary shower effects.  Their curves indicated effectiveness out to double those figures.

It would appear to me that what Curiosity uncovered during its trip to Mars and since while on Mars is pretty consistent with the effects and the exposure rules posted years ago on the NASA radiation website.  This stuff really isn't news,  just confirmation.  But some seem to be misusing it as a scare tactic not to go. 

All that being said,  then where is the radiation shelter in the Gateway design?  Not the GCR, that's an accepted risk and a stay limiter.  I mean the 10,000+ REM/hour shelter!  Like a surface nuclear explosion's fallout,  that is.  500 REM accumulated in a short interval like minutes,  hours,  or even a few days is the "fatal dose" figure.  They'll kill a crew without one.

GW

Good (data) points to keep in mind, surely.  Maybe you'd have some thoughts on improvements to a water shield:

Omaha_Trail_Shielding.png

SpaceX spaceship, Omaha Trail water shielding in blue.

To set a target for in-transit protection on the Omaha Shield's Omaha Trail, we aimed for an additional 90% reduction in flare proton flux.  To reach that target, we added 130 t of Deimos ISRU water, distributed around the quarters of a 2017 SpaceX ITS spaceship.  That gives 1 m+ water shielding.  This is far more than the 15-20 cm shielding you cited, but it seems necessary for 90% reduction because water is a poor shield against energetic protons when the shield thickness is under 20 cm.

But of course that's a lot of water, and flight efficiencies improve if shield mass can be reduced.  Glad for any thoughts on that.

One thought:  Adding a thin high-Z layer

A thin layer of high-Z material could be integrated into the water shield, either serving as the vessel's inner surface or else standing within the water volume.  Pound-for-pound, water is hard to beat as a flight-safe proton shield.  However two-layer or graded-Z shielding can improve performance in some cases, as in Atwell et al. 2013 Fig. 3.  But even there, the inferred benefit to a thick water shield is negligible, as seen in Fig. 5. 

Long story short:  it's not obvious that any thin high-Z shielding layer will offer a big cut in the proposed water mass.

Last edited by Lake Matthew Team - Cole (2017-11-28 10:48:20)

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#20 2017-11-28 13:22:39

RobertDyck
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Re: Radiation amount type risk mitigation

Lake Matthew Team - Cole wrote:

A thin layer of high-Z material could be integrated into the water shield

Do you understand why that's a bad idea? First, you're using obscure terminology. 'Z' is simply the last letter of the alphabet, each field of science or engineering uses it for something different. In the field of radiation shielding, it's usually used to refer to atomic number. That is number of protons in the nucleus, or positive static charge of the nucleus. If you mean atomic number, then say atomic number.

Atoms with high atomic number also have high atomic mass. That's a problem, and something we've moved away from. The reason is heavy ion radiation of any sort (high energy or low) and medium ion radiation with high energy both cause secondary radiation on impact with anything with high atomic mass. Both the radiation particle and material from shielding split apart. Each heavy particle (radiation and shielding) becomes multiple smaller particles, each with lower energy. The shower of secondary radiation is far worse than the original radiation. A high calibre bullet becomes a shotgun blast. The shotgun pellets are smaller, lower mass, lower velocity, but there's a lot more of them. If you're at close range so you get impacted with most of the shot, it actually does more damage than the high calibre bullet. Same with radiation.

"High-Z" material is good for shielding against neutron radiation and gamma radiation. There isn't any neutron radiation in space. Any neutron radiation from stars outside our solar system are so far, the radiation takes so much time, that the half-life of a free neutron causes it to decay into a proton and electron, long before it reaches our solar system. Our Sun had nuclear fusion in its core, but no high energy fusion near the surface. I used to think neutron radiation would decay in transit from Sun to Earth, but nuclear physicists tell me the neutrons either are absorbed or decay before they leave the surface of the Sun. Some fancy 21st century instruments have used this, they detect neutron radiation created as secondary radiation from the impact of solar radiation with the surface of the planetary body (Moon or Mars), and measure what comes back. Most of that neutron radiation is absorbed by Mars atmosphere, but enough gets back to low orbit that instruments can measure the difference between high energy vs low energy neutrons. Of course, the Moon doesn't have an atmosphere, so the instrument works even better there; but Lunar Prospector had to be in low Lunar orbit to work. Bottom line: no neutron radiation in interplanetary space.

Lead or other "high-Z" material has been used for radiation shielding against nuclear bombs or nuclear reactors because they produce so much neutron radiation. Space is different, it has proton radiation and particle radiation consisting of atomic nuclei. Stopping that without causing more radiation than you're blocking requires "low-Z" material, preferably hydrogen.

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#21 2017-11-28 20:20:19

SpaceNut
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From: New Hampshire
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Posts: 21,910

Re: Radiation amount type risk mitigation

poly ether ether keytone fused sandwich of materials sounds like the bigelow inflatables to which show they are about the same as the aluminum can...as its just layered materials made to adhere to each other to form a flexible or ridgid item.

polyether ether ketone Chemical chain composition C21H18O3 Molecular Formula

One-dimensional analyses conducted by Wesley effectively demonstrated that a low/high/low-Z layering illustrated in, provides a 60% mass savings to achieve the same radiation attenuation levels as a single layer of aluminum.

https://www.calpaclab.com/polyetherethe … ity-chart/

https://en.wikipedia.org/wiki/Polyether_ether_ketone

PEEK is a semicrystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained to high temperatures.

Space is cold so high temperatiure plastic will be brittle...but would work to hold the water....

Another materials is a blanket of Boron Nitride Nanotubes

So to get the water amount down such that we will have that mass for payload we will need more than 1 type of shielding method....

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#22 2017-11-29 12:53:12

Lake Matthew Team - Cole
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Registered: 2016-12-21
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Re: Radiation amount type risk mitigation

Antius wrote:

Regarding galactic cosmic rays - I wonder how much of the surface dose results from secondary particles generated in the Martian atmosphere?  These would presumably have lower energy but higher flux.  For lower energy charged particle radiation, a magnetic field would be an effective means of deflecting radiation and reducing dose rate.  A magnetic field protecting a base doesn't need to be mobile and a big iron core electromagnet might be an acceptable solution.  A vehicle could generate magnetic shielding whilst the engine was running and the crew could assemble a temporary dirt shield for those periods where the vehicle was not moving.

re: cosmic ray particles at Mars:  Yes, for example the proton flux peak is at lower energy at the base of the martian atmosphere than at the top.  Still, with a peak around 100 MeV, many of those lower-energy protons do still require robust shielding.

Omaha Field

If feasible, a magnetostatic shielding field would liberate surface operations considerably -- getting rid of the troublesome "dirt shield", for example.  Our "Omaha Field" proposal deflects protons near the martian surface, to ~1 GeV.  This catches more than half of GCR protons and nearly all solar flare protons.  (Max shielding in light green.)

Fig10.png

Suspended superconducting cables can deliver the required MA current for effective shielding, efficiently.  Our suggested field design makes use of them.

Fig5.png

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#23 2017-11-29 14:46:30

kbd512
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Registered: 2015-01-02
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Re: Radiation amount type risk mitigation

I think a plasma shield is a much lighter way to shield the occupants in the transit vehicles.  The colonists obviously need water, just not 130t of water for a 3 to 6 month cruise, presuming we continue to use water recycling in space.  More importantly, satellites that depart with the transit vehicle could strengthen or enlarge the plasma field on an as-needed basis.  Satellites orbiting Mars Base Camp could provide the same service.  We're talking about small independent vehicles using electric propulsion for station keeping and solar arrays with a couple kW's of power, maybe less.  They'd be deployed around the transit vehicle after TMI and either follow a free return trajectory or spiral into Mars Base Camp for redeployment aboard Earth bound ships.  Combine that with a storm shelter and I think we're set.

Radiation shielding produced by mini-magnetospheres

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#24 2017-11-29 15:29:43

Lake Matthew Team - Cole
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Registered: 2016-12-21
Posts: 119
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Re: Radiation amount type risk mitigation

re: Radiation shielding produced by mini-magnetospheres

aka Sink or Swim in Space

Sure, transit EM shields are one complementary area of research; if an EM system could meet the GeV shielding requirement without crossing mass, power and safety redlines, why not consider it? 

However ISRU water shielding has some big advantages.  It works even when the fuse blows.  Also arguably it doesn't need multi-billion$ R&D to get off the drawing board. 

A better way of looking at it:  The Omaha Trail's ISRU water puts a cost-benefit curve out there.  Now it's a competition:  ISRU water vs. EM shielding.  :-)  If the EM curve can't rise above the water curve and deliver significantly more value for the money, EM transit shielding sinks, so to speak. 

As for M2P2 shielding, it doesn't look as though anyone's bothered with it, after that 2002 publication.  Maybe the issues raised in Metzger et al. 2004 were just too challenging.

Metzger et al. 2004 wrote:

...it seems to us that two problems exist.  First, the magnetic bubble has two poles located at the ends of the spacecraft, and therefore particles that follow the field lines will be directed in at both ends...  Second, it has been learned recently that magnetic structures in the solar wind frequently break the Earth's magnetic field lines and attach themselves to their ends.  This creates a direct thoroughfare of radiation into a "hotspot" region... the bubble might completely break and be swept away... leaving the spacecraft with no shielding at all just at the time it is needed the most.

Last edited by Lake Matthew Team - Cole (2017-11-29 15:38:07)

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#25 2017-11-29 17:40:56

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,367
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Re: Radiation amount type risk mitigation

"Sure, but only for DSG, which isn't in transit to Mars.  Doesn't work for transit, because when it's time to return home from Mars... "Now where'd we put those external tanks?""

Whether that notion is true depends upon how you put your Mars mission together.  If you send return/arrival-home propellant (in dockable tanks) ahead unmanned to Mars orbit,  those are your shield for coming home.  If your manned transport is a reusable orbit-to-orbit transport,  then you can use an Earth departure stage to leave Earth,  and hide behind your Mars arrival propellant tanks for an outbound transit shield for solar flares. 

In Mars orbit,  you detach those arrival tanks,  and dock your Earth arrival tanks around the habitat for a return transit shield.  The Mars arrival tanks go elsewhere on the docked cluster,  and get staged off after that burn.  This architecture presumes you intend to reuse the orbit-to-orbit transport,  which is why there are Earth arrival tanks to use as a radiation shield.  Such a vehicle's cost gets amortized over multiple missions.  Those could be return trips to Mars,  or trips to NEO's,  or trips to asteroids in the main belt. 

As soon as I asked myself "why throw all this stuff away?",  I got to an architecture like that,  which actually dates to the 1950's.  As for landers,  you also send them ahead to Mars orbit,  with their landing propellant supplies.  Great costs savings result if you send all the unmanned stuff ahead with electric propulsion,  because flight times and radiation exposure in the van Allen belts are irrelevant for this unmanned stuff. 

Up front assumptions are not just important,  they can be make-or-break.  There are few today who make good assumptions anymore.

Also:  I really,  really doubt whether there are any usable water resources at either Phobos or Deimos.  These resemble nothing so much as the small asteroids,  which seem to be very dry.  It is the much-larger asteroids that seem to be wet.  Somehow,  that should not be surprising,  since it takes cover with some weight to it to keep ice from subliming away in vacuum. 

GW

Last edited by GW Johnson (2017-11-29 17:45:46)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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