Friday, October 5, 2018

Space Radiation Risks: GCR vs SFE

I got my data on risks and passive shielding from NASA’s own site and documents.  See

http://srag.jsc.nasa.gov/Publications/TM104782/techmemo.htm, titled Spaceflight Radiation Health Program at JSC (no cited reference newer than 1992).

or go to http://exrocketman.blogspot.com and click on year 2012 then May 2,  for the article titled “Space Travel Radiation Risks”.  That article,  which I wrote and posted,  abstracts the most relevant information from the NASA source for the questions at hand. 

Two Risks:

There is galactic cosmic radiation (GCR) and there are solar flare events (SFE).  GCR is a very sparse trickle of extremely high-energy particles (mostly protons) that are very hard to passively shield,  and which can induce secondary showers of other dangerous particles in some materials.  The risk is modulated by the solar wind which varies with the solar activity level.

SFE radiation is mostly proton and heavier particles emitted from the sun sporadically,  from eruptions on its surface.  These come in very concentrated but brief events,  comprising much lower-energy particles that are far easier to passively shield. 

GCR:  Roentgen equivalent man (REM) per year  = 42 + 18 sin(360o (t, year)/ (11 year)),  max 60 at solar min,  min 24 at solar max.  Solar max is max sunspot activity,  with more frequent eruptions.  Although,  eruptions can occur throughout the cycle.

SFE: from 1968 to 1970,  events every month or so ranging from 2 REM to 50 REM accumulated during each event;  from 1970 to 1972,  events about every 6 months ranging from about 50 REM to about 100 REM accumulated during each event;  and in 1972 right between Apollo 16 and Apollo 17,  one event right at 5000 accumulated REM.  There was one event of about 5 REM during Apollo 16.  Figure 1 is a plot of SFE events during the Apollo program,  direct from the NASA document’s Figure 10.  The quoted numbers are for somebody outside a spacecraft wearing only a spacesuit,  per NASA’s figure.

By way of comparison,  the Earthly natural background in the US is near 300 milli-REM (0.30 REM) per year. Worldwide is not significantly different. 

A rough rule of thumb:  500 REM accumulated in a short time (hours or days) is considered lethal to 100% of those so exposed. 

NASA’s Astronaut Exposure Rules:

These vary with the affected organ (eyes,  skin,  and 5 cm inside the body as representative of blood-forming organs or “BFO”),  but the lowest values are for 5 cm inside.  NASA’s exposure rules limit exposure to 50 REM accumulated in any one year,  25 REM in any single month,  and a career limit that varies with age and gender,  but peaks at 400 REM accumulated over an entire career. 


These are illustrated with Tables 1 and 2 lifted from the NASA document,  and presented here as Figures 2 and 3,  respectively.  Use the equations in Figure 3 to calculate career limits.  These exposure limitations represent approximately twice the exposures nuclear workers are allowed to face,  with a single-digit percentage increase in cancer risk expected. 

 Figure 1 – SFE Events During Apollo,  from the NASA Document’s Figure 10

 Figure 2 – NASA Time Interval Exposure Limits

 Figure 3 – NASA Career Exposure Limits (Use the Equations)


Effectiveness of Shielding Materials:

These divide into the effects of aluminum,  water,  and hydrogen,  which fairly well bounds a lot of possible materials.  The true risk here is the SFE event, of a very large magnitude,  such as the 5000 REM 1972 event.  That’s outside in nothing but a space suit.  Inside the command module,  the effect of the spacecraft structure reduces the exposure to 500 REM.  That’s a 10:1 reduction for the spacecraft hull (remember,  SFE particles are lower-energy and far easier to shield passively).

Based on the NASA document’s Figure 9,  given here as Figure 4,  the plot for the 1972 event reduces 500 REM inside the command module to 20 REM at 20 g/cm2 of aluminum shielding added to the effects of the spacecraft hull.  Use the density of aluminum (2.7 g/cm3) to find the actual physical thickness of this aluminum to be 7.4 cm.  You would want an aluminum shield that thick or thicker to survive a 1972-magnitude SFE event,  and stay barely within the month exposure limit.

The effect of the various materials as passive shielding for GCR is given in Figure 5,  which is lifted from the NASA document’s Figure 6.  In terms of shield mass per unit area of hull,  hydrogen is the most effective,  and aluminum the least,  with water in between.  Note how the curves flatten at larger masses per unit area,  leading to stronger differences in the amount of shield material required.

Here we ignore the shielding effect of the spacecraft hull structure as negligible against the more energetic radiation.  For 20 g/cm2 (7.4 cm thick) aluminum,  60 REM/year reduces to 40 REM/year,  which is well within the annual limit.  You get the same exposure at only 10 g/cm2 water at 1 g/cm3,  which is 10 cm thick,  and 3 g/cm2 hydrogen,  which at .07 g/cm3 is some 43 cm thick. 

The problem with “over-killing” the shielding is the secondary shower of dangerous particles created by the high-energy GCR particles.  This gets to be a problem if you make the shield too thick,  and it simply doesn’t happen with the lower-energy SFE particles.  That makes passive shielding a real trade-off for design purposes.  This is less a problem with spacecraft design,  and more of a problem with surface habitation design and construction.  The temptation would be to pile too much regolith atop the roof,  causing the secondary scatter problem.  

 Figure 4 – Effectiveness of Aluminum Against SFE Events,  from NASA Document Figure 9

Figure 5 – Effectiveness of 3 Materials Against GCR,  From NASA Document Figure 6


My own recommendation would be to use 15-20 g/cm2 water,  some 15-20 cm thick,  which would reduce 60 REM/year GCR to about 30 REM/year.  Against the GCR,  the same protection obtains at some 50 g/cm2 aluminum,  which is about 18.5 cm thick.  These are similar thicknesses (15-20 cm water vs 18-19 cm aluminum),  but very different masses per unit area:  15-20 g/cm2 water vs 50 g/cm2 aluminum.  Water is simply the lighter shield for the same effect, by about a factor of 2.5 to 3.   

We don’t have anything but aluminum to look at in Figure 4 the for the SFE event.  At the same 50 g/cm2 aluminum that looks good against GCR,  the 500 REM SFE event,  as measured inside the command module,  gets reduced to a 2 REM/event exposure.  Assuming the same attenuation ratio and mass per unit area ratio between aluminum and water that we saw with GCR in Figure 5,  then we should see the same 500 REM to 2 REM reduction of SFE radiation that 50 g/cm2 of aluminum provides,  with only 15-20 g/cm2 water,  which is 15-20 cm thick.  That’s an assumption requiring verification

Lessons for Spacecraft Design:

Water is the best shielding material,  because it is the lightest,  while providing practical thicknesses,  unlike hydrogen.  My best guess is that storable propellants should resemble water in their shielding properties.  They are light molecules made of light atoms,  like water,  and have densities far more comparable to water than to liquid hydrogen.

The recommended water shield is 15-20 g/cm2 (15-20 cm thickness),  which could be water,  wastewater,  or even frozen food.  It could also be storable propellants like the hydrazines and NTO oxidizer.  Increase the thicknesses for wastewater,  ice,  or frozen food:  a good guess is a factor of 1.5 to 2 increase in thickness over straight water.

Against GCR and ignoring any effects of the spacecraft hull,  15-20 cm of water should reduce 60 REM/year of GCR to something near 30 REM/year.  This might,  or might not,  be practical for the entire spacecraft,  but putting water in one form or another around the sleeping quarters might be,  in addition to a designated radiation shelter space.  

There is some benefit of the spacecraft hull reducing SFE from a 5000 REM/event outside the hull to a 500 REM/event inside the hull.  Since that is still a lethal dose,  further shielding is absolutely required!  That same 15-20 cm of water should reduce an inside-the-hull 500 REM/event to something nearer 2 REM/event,  which is well within the monthly limit,  even if multiple such events occur spaced rapidly together.  This gives us a lot of margin in the case of an event far larger than the 1972 SFE event.

Any spacecraft design should incorporate its flight control station within the designated-shelter radiation shielding so that critical maneuvers may be flown regardless of the solar weather.  Shielding about the sleeping quarters is also recommended for purposes of reducing GCR exposure. 

By using the shadow-shield effect,  this kind of shielding might be obtained with a combination of water/wastewater/frozen food items located about the sleeping quarters and flight control station,  combined with propellant tanks for the next burn,  that are docked about the periphery of these regions,  outside the spacecraft hull.  These are all items you already must have,  anyway,  so that extra shielding mass is not added to the design.  See Figure 6 for a concept sketch.

 Figure 6 – Concept for Incorporating Propellants and Water-Based Materials as a Shadow-Shield


Exposures Calculated for a Mars Mission at 60 REM/Year GCR with Three 1972-Class SFE Events:

The mission is 9-month transit/13-month near (or on) Mars/9-month transit.  One SFE event occurs during each transit,  and the other occurs while the crew is on or near Mars.  Calculations are made with and without the sleeping quarters shielded,  for 1/3 of clock time during each day.  Shielding about the sleeping quarters and the designated shelter is spacecraft hull plus 15-20 cm water-equivalent for SFE,  just 15-20 cm water-equivalent for GCR.  How this might actually be done was shown conceptually in Figure 6 above.

The first radiation exposure year is 3 months pre-mission on Earth,  then 9 months in transit to Mars.  The second radiation-exposure year is 12 months on Mars.  The third radiation-exposure year is one month on Mars,  9 months in transit to Earth,  and 2 months post-mission on Earth.  Earthly exposure is at the 0.3 REM/year rate.

The 9 month transit is 0.75 year.  Without any shielding effects at all,  45 REM are accumulated during transit for the year in which transit occurs. 

If there is sleeping quarters shielding,  its presence cuts the GCR to a rate of 30 REM/year while sleeping. Then based on clock times,  a 2/3-1/3 split occurs between the 60 and 30 REM rates:  that is a rate of 50 REM/year applied to a 9 month transit.  Thus the crew accumulates 37.5 REM during the transit,  which goes toward the total accumulated exposure during the year in which the transit takes place.

While on or near Mars,  the planet blocks half the spherical “sky”,  for a net in-space unshielded GCR rate,  assumed unattenuated by the planet’s atmosphere,  of 30 REM/year,  accumulated during each year spent on Mars.  The stay is 13 months,  so without sleeping quarters shielding,  30 REM counts toward the first full year on Mars,  and one month’s worth (2.5 REM) counts toward the second year on Mars and in-transit home.   

If there is shielding about the sleeping quarters on Mars,  the same 2/3-1/3 split applies to rates of 30 and 15 REM/year,  reducing the effective exposure rate to 25 REM/year.  In that case,  the year on Mars accumulates 25 REM,  and the 13th month accumulates 2.1 REM. 

Add 2 REM to the accumulation in any one month for SFE events during the transits and during the stay on Mars.  There must be a designated radiation shelter for SFE events,  even while on Mars,  or the exposures could easily be lethal.  That assumes no attenuation of the radiation by Mars’s thin atmosphere. 

The result is depicted graphically in Figure 7.  Again,  shielding is 15-20 cm of water-equivalent.  


Figure 7 – Radiation Profiles for 15-20 cm Water Shield,  with or without Sleeping Quarters Shield


These results show a marginal yearly exposure during year 3,  at just barely under the 50 REM annual limit,  for the case of no sleeping quarters shielding.  With sleeping quarters shielding,  this reduces well under the limit.  Worst case monthly exposures are well under the limit for both cases.  Two such missions will approach career limits,  in either case. 

Note that if there is no shielded place for SFE events,  then during any event in the same class as the 1972 event,  the exposure will be fatal at 500 REM received over a matter of hours.    There simply must be a solar flare shelter somewhere.  This is true during the transits and on Mars.

Note also that during times when the GCR is under 60 REM/year out in space,  exposures inside the ship (either case) will be much lower.  It is only the worst-case 60 REM/year space environment that is analyzed here.

Note also that it is the shorter-than-a-year transit time that reduces in-transit unshielded exposure to 45 accumulated REM!  Extending the transit time by using repeated aerobraking passes to capture at Mars,  instead of a one-time rocket burn,  will quickly violate the annual exposure limit!  The same thing applies to electric propulsion using spiral-out/spiral-in flight plans.  (Not to mention exposure times passing through the Van Allen Belts at Earth.)

Once the unshielded one-way flight time exceeds 10 months,  the annual exposure limit gets exceeded in a max GCR year.  Once that long-transit situation obtains,  you must shield the entire habitable volume of the ship,  not just a designated shelter and perhaps the sleeping quarters. 

Finally,  if the entire habitable volume of the spacecraft could be shielded at the 15-20 cm water-equivalent level,  exposures would be cut essentially in half,  to only around 30-something REM per radiation-exposure year in the transits,  and 15 REM/year on Mars,  even in a 60 REM/year part of the solar cycle.  Year 1 would be 32.08 REM,  year 2 would be 17 REM,  and year 3 would be 33.3 REM,  for a mission total of 83.4 REM.  Three,  or possibly even 4,  such missions in 60 REM GCR years might be feasible,  before hitting career exposure limits.  

Final Comments:

First,  this kind of radiation shielding will inevitably prove to be absolutely necessary,  but it will look nothing at all like what we have ever before done with our spacecraft designs.  When “they” show you spacecraft design concepts that look like what we have done before,  you already know that ”they” have not thought this problem through!

Second,  the shield design concept shown in Figure 6 above is entirely compatible with a “long” ship design that is spun end-over-end like a rigid baton for artificial gravity.  A design like that is also entirely unlike anything we have ever before done,  but it is rather well-understood from an engineering viewpoint,  and would require far less technology development and demonstration than any sort of cable-connected spin gravity design.

Third,  it is quite evident that worst-case GCR risks are slight over-exposure for late-in-life cancer,  while the SFE risks really are lethal doses leading to an ugly death within hours.  Thus,  when “they” point to GCR as the radiation risk that precludes humans going into deep space,  you already know that (1) they are lying for nothing but fear-mongering purposes,  and (2) “they” are truly ignorant of the real radiation risk.  Such claims are simply not credible.

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