Saturday, April 11, 2015

Radiation Risks for Mars Trip

I used NASA’s own data and criteria,  obtained as they published it,  on the internet.  There are two types to worry about:  (1) galactic cosmic radiation (GCR),  and (2) major solar flare events (SFE).  These data (and shielding material effectiveness data) are available in both text and graphical form at http://exrocketman.blogspot.com,  in an article titled “Space Travel Radiation Risks”,  dated 5-2-12.  There is a by-date navigation tool on the left side of the website page. 

Update 4-12-15 in the conclusions section below.

The NASA data were obtained from http://srag.jsc.nasa.gov/Publications/TM104782/techmemo.htm, titled Spaceflight Radiation Health Program at JSC  

Update 4-15-15 in the conclusions below.

The Nature of the Problem

GCR is a slow but varying drizzle of really high energy particles for which ready-to-use shielding technologies are relatively ineffective.  If you have too much passive shielding material,  the secondary particle shower effects can negate the value of your shield. 

The Van Allen belts provide shielding for much of this,  which is why astronauts in low Earth orbit have relatively little risk.  Earth’s atmosphere is a really good shield,  so that GCR contributes only a little to the natural background for people on the surface. 

When the sun is most active,  GCR is minimized at around 25 REM per year (“roentgen equivalent man”,  a unit of measure that also factors in the different damage caused by the different types of radiation).  When the sun is quiet,  GCR maximizes at around 60 REM per year.  This varies with the sunspot cycle.  Figures are for Earth’s vicinity in the solar system,  which should not be drastically different at Mars. 

GCR,  REM/year = 42.5 + 17.5 sin (2 π (t, yr) / T)  where T = 11 years approximately,  but it does vary

SFE are relatively random in occurrence and strength.  Between 1968 and 1971,  several events occurred,  all under 100 REM accumulated over the course of a few hours.  Very large events occurred in February 1956,  November 1960,  and August 1972,  that last being between the Apollo 16 and 17 missions to the moon.  That 1972 event accumulated about 3400 REM over the course of several hours,  far beyond a lethal dose for an unprotected person. 

Allowable Doses

Lethal doses start at about 500 REM whole body over a short interval (hours to days).  Allowable exposures are far lower.  Different figures are published for skin,  eye,  and 5 cm inside the body.  The 5 cm deep exposures are the smallest allowable,  and thus the most conservative to use. 

The other issue to consider is timing:  how quickly does the dose accumulate?  NASA uses a short-term 30 day basis,  an annual (1-year) basis,  and a career limit (accumulated over multiple years).  The career limits depend upon age and gender.  These are for astronauts,  and are higher than for civilians on the ground.  They correspond to an estimated 3% higher incidence of cancer late in life,  compared to ordinary civilians. 


30 day short term NASA limit (5 cm)  =  25 REM whole body
Annual (1 year) NASA limit (5 cm)      =  50 REM whole body
Career limit for males: 200 + 7.5(age – 30)  REM; 400 REM max at estimated age 57
Career limit for females: 200 + 7.5(age – 38)  REM; 400 REM max at estimated age 65

Mars Trip Exposures

With the kinds of spacecraft we can build at this time in history,  a trip to Mars will be 6 to 8.5 months one-way,  with about a year or more at the planet waiting for the orbits to be “right” for a return.  The trip home is also 6 to 8.5 months one way.  That’s a 2 to 3 year mission.  For comparison,  missions to the moon were at most 2 weeks. 

Mars has a very thin atmosphere.  Even so,  it is a fairly effective shield against the radiation threats.  Protection varies a lot with thicker or thinner Martian “air” from place to place,  but it does cut the threat by at least half.  So,  that year or so spent at Mars,  could be at most half-exposure,  if spent on the surface.  If spent in orbit,  the exposure there is full space exposure:  Mars has no analog to the Van Allen belts that help shield Earth.  For very low orbits,  there is a shielding effect from the presence of the adjacent planet,  but to be conservative,  I neglect that effect. 

For my figures here,  I assume two transits at 8.5 months each,  and 13 months at Mars,  half of it in orbit,  half on the surface (two alternating surface crews for visits lasting a week to a month).  That’s a 2.5 year (30 month) mission.  That should be a reasonably realistic,  yet very conservative,  exposure estimate.  I will further assume GCR to be the maximum 60 REM per year,  and I will assume one lethal SFE,  and two 100-REM SFE’s,  all spaced months apart. 

The lethal SFE absolutely requires me to provide a radiation shelter of some kind,  but most of the time,  I assume my astronauts are outside of it.  I do assume they take shelter for any SFE,  not just the lethal ones. 

The first year’s exposure to GCR is 8.5 months full exposure,  plus 3.5 months at reduced exposure,  accounting for time on the surface.  The second year’s exposure is 9.5 months at reduced exposure accounting for surface time,  plus 2.5 months exposure at full in-space strength to start the voyage home.  The third year is just 6 months’ in-space exposure during the voyage home. 

I assumed one SFE in each exposure year.  There are two minor events each at 100 REM accumulated (unshielded),  plus one major event at 3400 REM (unshielded,  comparable to the August 1972 event).  It is the major event that we have to worry about from a shielding standpoint,  and it could happen in any of the three exposure years.  So I looked at all three possible cases. 

Shielding Data

The NASA data that I obtained addresses two scenarios:  (1) thicknesses of aluminum,  water,  and liquid hydrogen required to reduce exposure to GCR,  and (2) mass per unit area of aluminum required to attenuate the three largest known SFE events (which includes the August 1972 event,  the largest of all).  One can convert mass per unit area (g/cm2) of shielding to its thickness (cm) by dividing by the density (g/cm3). 

Looking at NASA’s GCR shielding effects data,  it is easy to see that the best shielding agent is liquid hydrogen,   with aluminum the worst of the three.  Their other plot shows 15 g/cm2 aluminum provides attenuation of the three large SFE events down to about 25 REM,  for shield knockdown factor of 0.0074 and an aluminum thickness of about 5.5 cm.  About 50 g/cm2 (18.5 cm) aluminum knocks the dose from the August 1972 event down to around 2.5 REM,  a 10-fold improvement. 

You can also see from the GCR shielding data that the thickness of water that is required is around 80% or less than the thickness of aluminum required,  for the same effect.  I used 80% as conservative.  The plots say that for 60 REM/year GCR,  that to hit 50 REM/year,  you need 7-8 cm aluminum,  4-5 cm water,  or about 2 cm liquid hydrogen.  The same curves say that 15 cm aluminum knocks 60 REM down to 42 REM,  that 15 cm water knocks it down to 33 REM,  and that 15 cm liquid hydrogen knocks it down to 14 REM.  And,  1 cm aluminum knocks 60 REM down to about 59 REM,  1 cm water knocks it down to 56 REM,  and 1 cm liquid hydrogen knocks it down to 50 REM. 

I’d rather use water supplies,  wastewater awaiting treatment,  and water-bearing foodstuffs as the shielding material.  Aluminum is likely to be there in shell structures of thin sheet,  not plates several cm thick,  in any practical vehicle design.  I rather doubt the wisdom of having liquid hydrogen in close proximity to the crew.  So,  water it is. 

I’d recommend placing the shielding around the flight control station,  so that maneuvers can be made even if an SFE hits at the time the maneuver is required.  If that’s not enough radiation shielding,  the next priority is shielding around the sleeping quarters. 

Mission Accumulation Calculations – Flight Control Station as Radiation Shelter

I assumed no shielding effects for GCR,  other than time spent on the surface at half exposure.  I went to 50 g/cm2 aluminum equivalent (as 15 cm water) to reduce some serious violations of NASA’s annual limit of 50 REM.  This assumes no shielding about the sleeping quarters.  Here is what I found,  dependent upon which year gets the major SFE:


Maj SFE ....year 1...year 2...year 3...total
3rd...............56..........48.........33........136
2nd..............56..........51.........30........136
1st ..............58..........48..........30........136

In all three cases,  year 1 exceeds the nominal annual maximum of 50 REM by some margin.  If the major SFE hits during year 2,  that year also exceeds the standard by a trivial amount.  The worst exposure for year 1 is the case with the major SFE hitting during year 1.  Yet,  with 15 cm of water shield available,  we do not exceed the limit by a huge amount.  This does indicate that we should place shielding around the sleeping quarters,  which is one third of the day cycle while in space. 

None of the individual months come close to the 25 REM in one month limit,  at this level of shielding (15 cm water equivalent to 50 g/cm2 aluminum).  If we were to reduce the shielding to 15 g/cm2 aluminum equivalent,  we would violate the monthly limit in that month when the major SFE occurs.  So we really do need the thicker shielding. 

The mission totals are actually rather modest,  at 136 REM no matter when the major SFE occurs.  Using NASA’s equations and the assumptions of astronauts who are 25 years old (min credible) to 50 years old (more typical),  I get the following career limit accumulated REM:

 Male......age 25...age 30...age 40...age 50
 ..............163........200.......275........350         
 Female..age 25...age 30...age 40...age 50
 ..............103.......140........215.......290         

It is fairly clear that females should be older than 30 to make this trip.  Younger astronauts up to about age 30-something if male,  40-something female,  should not make this trip but once,  without violating career exposure,  as long as the accumulations prior to the trip are negligible.  Older astronauts might make this trip twice,  if prior accumulations were near zero. 

Mission Accumulation Calculations – Flight Control Station and Sleeping Quarters as Radiation Shelter

What I assumed for this comes direct from NASA’s data for GCR shielding effects.  At 15 cm water shield thickness,  their curves say 60 REM per year is reduced to 33 REM per year.  If we put that 15 cm of water around the sleeping quarters in addition to the flight control station,  then (1) we have more SFE shelter space available,  and (2) we reduce GCR exposure by the time spent sleeping,  while in space.  I applied this to transits,  but not to time spent in Mars orbit,  just to be conservative.  I also used 8 hours out of every 24 spent in sleeping quarters. 

Major SFE....year 1...year 2...year 3...total
3rd.................49.........46.........28.........124
2nd................49.........49.........26.........124
1st.................52.........46.........26.........124


That brings the annual exposures into compliance with the 50 REM limit,  unless the major SFE occurs in the first year,  and then the deviation is rather small.  Remember,  my numbers are conservative!  It is fairly likely that the 52 REM that I show in year 1 for a year-1 major SFE is really under the 50 REM limit. 


Note also that the lower mission accumulation makes it feasible to fly female astronauts as young as about age 27.  And,  the age beyond which two such trips become feasible is now very clearly 30-something male,  and 40-something female. 

Conclusions

For the sake of safety-of-flight,  I’d say we must shield the flight control station so that critical maneuvers may be flown no matter the solar weather.  But adding shielding to the sleeping quarters to reduce GCR exposure in peak years makes a big difference to annual and mission accumulations.  I would very strenuously recommend shielding both zones!

Note that in min GCR years,  this entire radiation exposure question becomes far less relevant (25 REM per year GCR versus 60 REM/year).  In max GCR years,  the toughest issues are yearly exposure limits,  and the monthly limit for the month in which the major SFE occurs.  Only young female astronauts run into career limit exposures for one such flight.  Older astronauts could make two flights without violating career limits.  

Mission durations to the main asteroid belt would likely be around twice these Mars mission durations.  With the flight control plus sleeping quarters shielding recommended here,  it seems likely that older astronauts could make a main belt trip without violating any radiation exposure limits.  I have not investigated that to be sure,  though. 

Finally, these numbers show that radiation exposure is not a credible excuse for not sending men to Mars,  even in a peak GCR year.  The shielding requirements to stay within exposure limits are actually rather modest at 15 cm water thickness equivalent around only two zones in the vehicle. 

Update 4-15-15:  my own recommendation would be to use a bit more water shielding at 20 cm thickness,  not just 15.  That lets even very young females make the trip safely,  and all astronauts with low career accumulations could make such a trip twice.  A reminder:  water,  wastewater,  and non-freeze dried foodstuffs all qualify as "water shielding".  Use them all.  

But without it,  you will likely violate the limits from GCR exposure (a dead certainty in a peak GCR year),  and you will very likely kill a crew from exposure to an SFE event.

There is nothing as expensive as a dead crew. 

Update 4-12-15:

These calculations really provide an upper bound,  because of the mission architecture assumed,  not just the conservative assumptions made.  That architecture is an orbit-based mission sending down alternating crews to the surface at multiple sites.  Spending all the time at Mars on the surface reduces mission radiation-exposure accumulations still further,  because of the shielding effects of the Martian atmosphere.  There also is "wiggle-room" in year 3 exposures to handle longer trips.  

Calculation Details

These follow in the images of 6 spreadsheet pages.  I did sheets for shielding only the flight control station with major SFE in years 3,  2,  and 1;  and for shielding both the flight control deck and the sleeping quarters,  with major SFE in years 3,  2,and 1.  These are given in Figures 1 through 6. 



 Figure 1 – Shield Only Flight Control Station,  Major SFE in Year 3

 Figure 2 – Shield Only Flight Control Station,  Major SFE in Year 2

 Figure 3 – Shield Only Flight Control Station,  Major SFE in Year 1

 Figure 4 – Shield Flight Control Station and Sleeping Quarters,  Major SFE in Year 3

 Figure 5 – Shield Flight Control Station and Sleeping Quarters,  Major SFE in Year 2

Figure 6 – Shield Flight Control Station and Sleeping Quarters,  Major SFE in Year 1


3 comments:

  1. Thanks for publishing this blog on this very interesting subject. I addressed the problem of cosmic radiation and major solar events in one of my blogs last year. I was a lot more conservative, requiring about 50 cm of water shielding for interplanetary missions to Mars orbit. But I'm starting to become more confident that at least 20 centimeters less might be required. But that would still be more than twice as much water shielding than you're advocating.

    Your article didn't address the problem of potential brain damage from the heavy nuclei component of cosmic radiation. Heavy nuclei can penetrate as far as 10 cm into lunar regolith. Lunar regolith ranges from 1 gm/cm3 in density to 2 gm/cm3 in density. So, conservatively, at least 20 cm of water would be needed to protect the human brain from constant heavy nuclei bombardment during an interplanetary journey. But that's still pretty close to the 15 centimeters of water shielding that you're advocating.

    As far as protecting astronauts from major solar events is concerned, NASA's Simonsen and Nealy 1991 paper notes that during the 1972 solar event , the annual 50 Rem BFO does-equilvalent value is exceeded from water shields that are less than 18gm/cm2. This is when the solar event is combined with cosmic radiation levels during solar minimum conditions. They also note that flare dose is insignificant if the water shields are thicker than 25 to 30 gm/cm2.

    Since these figures do not include the additional inherent radiation shielding from the structural aluminum of the pressurized habitat itself, I would assume that 30 cm of water should be more than adequate to protect astronauts from heavy nuclei and major solar events while normally exposing humans on interplanetary voyages to Mars to less than 25 Rem per year during dollar minimum conditions.

    I assume that during Conjunction Class missions (nearly three years), once astronauts arrive at Mars they will have substantially less cosmic radiation exposure on the Martian surface due to the inherent shielding from the planet's mass, some shielding from the martian atmosphere, and further shielding from regolith shielded habitats on the surface. But even during Opposition Class orbital missions (less than two years) during solar minimum conditions, astronauts should only be expose to less than 50 Rem. And that would allow even a 25 year old woman to be able to join-- a second-- deep space mission to Mars or Venus orbit later in her career.

    Marcel

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    1. Hi Marcel, you seem to know more about this than I do. These numbers posted here suggest 15 cm water, but I'm really staying with my original guess of 20 cm water. Any aluminum is going to be very thin sheet metal, thus essentially zero value as a shield. -- GW

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  2. Here are some excellent resources on cosmic radiation Gary:

    Cosmic Radiation and the New Frontier (tons of references)

    http://newpapyrusmagazine.blogspot.com/2014/03/cosmic-radiation-and-new-frontier.html

    Radiation Protection for the Moon and Mars

    http://www.lpi.usra.edu/lunar/documents/NTRS/collection3/NASA_TP_3079.pdf


    The Lunar Source Book

    http://www.lpi.usra.edu/publications/books/lunar_sourcebook/pdf/LunarSourceBook.pdf

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