radiation physiology and effects€¦ · lunar regolith shielding for spe 28 –, human integration...

Radiation Physiology and Effects ENAE 697 - Space Human Factors and Life Support

U N I V E R S I T Y O FMARYLAND

Radiation Physiology and Effects• Sources and types of space radiation!• Effects of radiation!• Shielding approaches!• Recent advances in understanding of radiation and

its affects

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© 2015 David L. Akin - All rights reserved http://spacecraft.ssl.umd.edu

http://spacecraft.ssl.umd.edu



The Origin of a Class X1 Solar Flare

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The Earth’s Magnetic Field

Ref: V. L. Pisacane and R. C. Moore, Fundamentals of Space Systems Oxford University Press, 1994

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The Van Allen Radiation Belts


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Cross-section of Van Allen Radiation Belts


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Electron Flux in Low Earth Orbit


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Heavy Ion Flux

Ref: Neville J. Barter, ed., TRW Space Data, TRW Space and Electronics Group, 1999

Background Solar Flare

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Radiation Units• Dose D= absorbed radiation!!!

• Dose equivalent H= effective absorbed radiation!!!!!

• LET = Linear Energy Transfer <KeV/µ m>

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1 Gray = 1Joule

kg= 100 rad = 10, 000

ergs

gm

1 Sievert = 1Joule

kg= 100 rem = 10, 000

ergs

gm

H = DQ rem = RBE � rad



Radiation Quality Factor

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Radiation QX-rays 1

5 MeV γ-rays 0.51 MeV γ-rays 0.7

200 KeV γ-rays 1Electrons 1Protons 2-10

Neutrons 2-10α-particles 10-20

GCR 20+



Radiation in Free Space

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Radiation Dose vs. Orbital Altitude

Ref: Neville J. Barter, ed., TRW Space Data, TRW Space and Electronics Group, 1999

300 mil (7.6 mm) Al shielding

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Dosage Rates from Oct/Nov 2003 SPE

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SPEs in Solar Cycles 19, 20, and 21

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GCR Constituent Species

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Solar Max/Min GCR Proton Flux Ratio

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Radiation Damage to DNA

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Symptomology of Acute Radiation Exposure• “Radiation sickness”: headache, dizziness, malaise,

nausea, vomiting, diarrhea, lowered RBC and WBC counts, irritability, insomnia!

• 50 rem (0.5 Sv)!– Mild symptoms, mostly on first day!– ~100% survival!

• 100-200 rem (1-2 Sv)!– Increase in severity and duration!– 70% incidence of vomiting at 200 rem!– 25%-35% drop in blood cell production!– Mild bleeding, fever, and infection in 4-5 weeks

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Symptomology of Acute Radiation Exposure• 200-350 rem (2-3.5 Sv)!

– Earlier and more severe symptoms!– Moderate bleeding, fever, infection, and diarrhea at 4-5

weeks!• 350-550 rem (3.5-5.5 Sv)!

– Severe symptoms!– Severe and prolonged vomiting - electrolyte imbalances!– 50-90% mortality from damage to hematopoietic system if

untreated

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Symptomology of Acute Radiation Exposure• 550-750 rem (5.5-7.5 Sv)!

– Severe vomiting and nausea on first day!– Total destruction of blood-forming organs!– Untreated survival time 2-3 weeks!

• 750-1000 rem (7.5-10 Sv)!– Survival time ~2 weeks!– Severe nausea and vomiting over first three days!– 75% prostrate by end of first week!

• 1000-2000 rem (10-20 Sv)!– Severe nausea and vomiting in 30 minutes!

• 4500 rem (45 Sv)!– Survival time as short as 32 hrs - 100% in one week

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Long-Term Effects of Radiation Exposure• Radiation carcinogenesis!

– Function of exposure, dosage, LET of radiation!

• Radiation mutagenesis!– Mutations in offspring!– Mouse experiments show doubling in mutation rate at

15-30 rad (acute), 100 rad (chronic) exposures!

• Radiation-induced cataracts!– Observed correlation at 200 rad (acute), 550 rad (chronic)!– Evidence of low onset (25 rad) at high LET

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Radiation Carcinogenesis• Manifestations!

– Myelocytic leukemia!– Cancer of breast, lung, thyroid, and bowel!

• Latency in atomic bomb survivors!– Leukemia: mean 14 yrs, range 5-20 years!– All other cancers: mean 25 years!

• Overall marginal cancer risk!– 70-165 deaths/million people/rem/year!– 100,000 people exposed to 10 rem (acute) -> 800

additional deaths (20,000 natural cancer deaths) - 4%

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NASA Radiation Dose Limits

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Density of Common Shielding Materials

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0

2

4

6

8

10

12

Polyethyle

neWate

rGr/E

p

Acrylic

s

AluminumLea

d



Comparative Thickness of Shields (Al=1)

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0

1

2

3

Polyethyle

neWate

rGr/E

p

Acrylic

s

AluminumLea

d



Comparative Mass for Shielding (Al=1)

25

0

1

2

3

4

5

Polyeth

ylene

Water

Gr/Ep

Acrylics

Aluminu

mLe

ad



Effective Dose Based on Shielding

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Francis A. Cucinotta, Myung-Hee Y. Kim, and Lei Ren, Managing Lunar and Mars Mission Radiation Risks Part I: Cancer Risks, Uncertainties, and Shielding Effectiveness NASA/TP-2005-213164, July, 2005



Shielding Materials Effect on GCR

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–, Human Integration Design Handbook, NASA SP-2010-3407, Jan. 2010



Lunar Regolith Shielding for SPE

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Mars Regolith Shielding Effectiveness

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Radiation Exposure Induced Deaths

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Francis A. Cucinotta, Myung-Hee Y. Kim, and Lei Ren, Managing Lunar and Mars Mission Radiation Risks Part I: Cancer Risks, Uncertainties, and Shielding Effectiveness NASA/TP-2005-213164, July, 2005

National Aeronautics and Space Administration

1

!

What’s New in Space Radiation Research for Exploration?

!Francis A. Cucinotta

NASA, Lyndon B. Johnson Space Center !

Presented to Future In-‐Space Operations (FISO) May 18, 2011

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The Space Radiation Problem

Space radiation is comprised of high-‐energy protons and heavy ions (HZE’s) and secondary protons, neutrons, and heavy ions produced in shielding – Unique damage to biomolecules,

cells, and tissues occurs from HZE ions

– No human data to estimate risk – Expt. models must be applied or

developed to estimate cancer, and other risks

– Shielding has excessive costs and will not eliminate galactic cosmic rays (GCR)

!

Single HZE ions in cells And DNA breaks

Single HZE ions in photo-‐emulsions Leaving visible images



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Executive Summary

• Estimating space radiation risks carries large uncertainties that preclude setting exposure limits and evaluating many mitigation measures !

• NASA needs to close the knowledge gap on a broad-‐range of biological questions before radiation protection goals can be met for exploration !

• The Human Research Program (HRP), Space Radiation Program Element (SRP) led by JSC is committed to solving the space radiation problem for exploration


Space Radiation Environments

• Galactic cosmic rays (GCR) penetrating protons and heavy nuclei -‐ a biological science challenge – shielding is not effective – large biological uncertainties limits ability to

evaluate risks and effectiveness of mitigations

!• Solar Particle Events (SPE) largely medium

energy protons – a shielding, operational, and risk assessment challenge – shielding is effective; optimization needed

to reduce weight – improved understanding of radiobiology

needed to perform optimization – accurate event alert and responses is

essential for crew safety

4

GCR a continuum of ionizing radiation types

Solar particle events and the 11-‐yr solar cycle

GCR Charge Number0 5 10 15 20 25 30

% C

ontr

ibut

ion

0.001

0.01

0.1

1

10

100Fluence (F)Dose = F x LETDose Eq = Dose x QF


5

Space Safety Requirements

• Congress has chartered the National Council on Radiation Protection (NCRP) to guide Federal agencies on radiation limits and procedures – NCRP guides NASA on astronaut dose

limits • Crew safety

– limit of 3% fatal cancer risk – prevent radiation sickness during mission – new exploration requirements limit brain

and heart disease risks from space radiation

• Mission and Vehicle Requirements – shielding, dosimetry, and

countermeasures • NASA programs must follow the ALARA principle to ensure astronauts do not approach dose limits

Cell fusion caused by radiation

Fe+TGFβ

γ

TGFβ

Fe

γ +TGFβ

Sham

Space Radiation in breast cancer formation


6

Categories of Radiation Risk

Four categories of risk of concern to NASA:

– Carcinogenesis (morbidity and mortality risk)

– Acute and Late Central Nervous System (CNS) risks

✓ immediate or late functional changes

– Chronic & Degenerative Tissue Risks

✓ cataracts, heart-‐disease, etc.

– Acute Radiation Risks – sickness or death

Differences in biological damage of heavy nuclei in space with x-‐rays, limits Earth-‐based data on health effects for space applications

– New knowledge on risks must be obtained

Lens changes in cataracts

First experiments for leukemia induction with GCR

cataracts


Space Radiation Health Risks

• NASA limits acceptable levels of risks of astronauts to a 3% Risk of Exposure Induced Death (REID) from cancer – PEL requirement to be below 95% Confidence Interval (C.I.) for cancer

risk protects against uncertainties in risk projection models – Estimates of number of days to be within a 95% C.I. are used to assess:

• Safe mission lengths • Crew selection criteria such as Age, Gender and Prior Exposure • Mitigations such as Shielding or Biological Countermeasure Requirements !

• Non-‐cancer risks are not well defined – Potential for late non-‐cancer mortality risks (Heart and CNS) on long-‐

term exploration missions confounds assessments of Acceptable Risk, which includes only cancer at this time

– Additionally, the NCRP recommends that limits for non-‐cancer morbidity risks be based on avoiding any clinically significant effect • Research in cells and murine models are not conclusive regarding clinical

significance of space radiation exposure to the astronaut's CNS • Need appropriate animal model to assess clinical significance

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• Retinal flashes observed by astronauts suggests single heavy nuclei can disrupt brain function. ― Central nervous system (CNS) damage by x-‐

rays is not observed except at very high doses

• In-‐flight cognitive changes and late effects similar to Alzheimer’s disease are a concern for GCR.

• NASA research in cells and mouse/rat models has increased concern for CNS Risks – Over 90 CNS journal publications supported

by NASA since 2000 – Studies have quantified rate of neuronal

degeneration, oxidative stress, apoptosis, inflammation, and changes in dopamine function related to late CNS risks

– Cognitive tests in rats/mice show detriments at doses as low as 10 mGy (1 rad)

• Large hurdle remains to establish significance in humans

Reduction in number of neurons (neurodegeneration) for increasing Iron doses in mouse hippocampus

CNS Risks from Galactic Cosmic Rays (GCR)


Radiation and Non-‐Cancer Effects

• Early Acute risks are very unlikely: – Low or modest dose-‐rates for SPE’s insufficient

for risk of early death – SPE doses are greatly reduced by tissue or

vehicle shielding • Radiation induced Late Non-‐Cancer risks are

well known at high doses and recently a concern at doses below 1 Sv (100 rem) – Significant Heart disease in Japanese Survivors

and several patient and Reactor Worker Studies

– Dose threshold is possible making risk unlikely for ISS Missions(<0.2 Sv) ; however a concern for Mars or lunar missions due to higher GCR and SPE dose

– Qualitative differences between GCR and gamma-‐rays are a major concern

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Control Iron NucleiVasculature Damage by GCR

NASA Space Radiation Laboratory• A $34-‐million facility, is located at DOE’s Brookhaven National Laboratory is managed by NASA’s Johnson Space Center. It is one of the few places in the world that can simulate heavy ions in space. • New joint DoE-‐NASA Electron beam injector source (EBIS) for 2009 increases space simulation capability • $9 M Annual operations cost

Beam port

RFQ Linac

EBIS SC solenoid

Dipoles – preparing

EBIS Construction


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Major Sources of UncertaintyNational Aeronautics and Space Administration

• Radiation quality effects on biological damage –Qualitative and quantitative differences between space radiation compared to x-‐rays or gamma-‐rays

• Dependence of risk on dose-‐rates in space – Biology of repair, cell & tissue regulation

• Predicting solar events – Temporal and size predictions

• Extrapolation from experimental data to humans • Individual radiation-‐sensitivity – Genetic, dietary and “healthy worker” effects

Durante & Cucinotta, Nature Rev. Cancer (2008)

(%) Fatal Cancer Risk0 3 6 9 12 15

Pro

babi

lity

0.000

0.003

0.006

0.009

0.012

0.015

Distribution aluminumDistribution polyethyleneDistribution Liq. Hydrogen (H2) E(alum) = 0.87 Sv E(poly) = 0.77 SvE(H2) = 0.43 SvR(alum) = 3.2 [1.0,10.5] (%)R(poly) = 2.9 [0.94, 9.2] (%)R(H2) = 1.6 [0.52, 5.1] (%)

Cucinotta et al Radiat Meas (2006)

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Space Radiation Shielding is Well Understood

Radiation Shielding Materials

August 1972 SPE and GCR Solar Min

Shielding Depth, g/cm20 5 10 15 20 25 30 35

Dos

e Eq

uiva

lent

, rem

/yr

1

10

100

1000

10000GCR L. HydrogenGCR PolyethyleneGCR GraphiteGCR AluminumGCR RegolithSPE GraphiteSPE RegolithSPE L. Hydrogen

• NASA has invested in shielding technologies for many years and understanding is nearly complete – Over 1,000 research publications since 1980 – Solar events can be shielded – GCR requires enormous mass to shield

because of high energies and secondary radiation

• Highly accurate predictive codes exist with +15% errors for organ exposure projections – Transport codes – Environmental models – Optimal materials – Topology Design methods

• Knowledge missing is accurate understanding of radiobiology for Exposure to Risk conversion

Confidence Levels for Career Risks on ISSEXAMPLE: 45-yr.-Old Males; GCR and trapped proton exposures

Solar Max

Days on ISS0

(%) C

onfid

ence

tobe

bel

ow c

aree

r lim

it

100Current Uncertainties With Uncertainty Reduction

50

60

70

80

90

250 500 750 1000 250 500 750 1000Days on ISS

Solar Max

Solar MinSolar Min

SAFE ZONE

Value Of Uncertainty Reduction Research: Cost of research to reduce uncertainties much less than cost of shielding in space or reducing mission length


What’s New in Space Radiation Research?

• New Epidemiology data suggests much weaker age dependence on radiation cancer risks – Number 1 Trade variable (Astronaut age) is negated

• Probabilistic risk assessments replace “rads and rem” – New Quality factors and uncertainty assessments

• Galactic cosmic rays (GCR) are much higher concern than Solar particle events – Shielding plays only a small role for GCR

• New health risks of concern from radiation – Heart disease, and Central nervous system (CNS) risks

• Risks estimated to be much smaller for “Never-‐smokers”

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Roles of Select Committees and Radiation Projection Councils

• Select expert panels from the National Academy of Sciences (NAS) and United Nations (UN) update human radio-‐epidemiology based estimates of radiation cancer risks each decade

• These reports form the basis for revised radiation protection standards and policy as recommended by the US National Council on Radiation Protection and Measurements (NCRP) and International Commission on Radiological Protection (ICRP)

• The most recent reports from NAS (BEIR VII) and the UN (UNSCEAR 2006) make important changes to the description of the age dependence of cancer risks, and cancer risks at low dose-‐rates – BEIR VII: Linear dose response with no age at exposure dependence above age 30-‐yr – UNSCEAR model shows similar age dependence for cancer incidence

• These changes will increase risk projections if accepted by NASA

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NASA 2010 Cancer Projection Model

• NASA is developing new approaches to radiation risk assessment: – Probabilistic risk assessment

framework – Tissue specific estimates

• Research focus is on uncertainty reduction – Smaller tolerances are needed as risk

increases, with <50% uncertainty required for Mars mission

• NASA 2010 Model – Updates to Low LET Risk coefficients – Risks for Never-‐Smokers – Track Structure and Fluence based

approach to radiation quality factors • Leukemia Q lower than Solid cancer Q


GCR doses on Mars


Radiation Risks for Never-‐Smokers• More than 90% of Astronauts are never-‐

smokers and remainder are former smokers

• Smoking effects on Risk projections: – Epidemiology data confounded by possible

radiation-‐smoking interactions, and errors documenting tobacco use

– Average U.S. Population used by NCRP Reports 98 and 132

• NASA Model projects a 20 to 40-‐% risk reduction for never-‐smokers compared to U.S. Ave. – Larger decreases are possible if more were

known on Risk Transfer models – Balance between Small Cell and Non-‐Small

Cell Lung Cancer a critical question including high LET effects

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Thun et al., PLoS Med (2008)

Lung cancer in Unexposed

CDC Estimates of Smoking Attributable Cancers

Relative Risk to Never-‐smokers (NS) RR for NS to U.S. Avg

Males Current smokers Former smokers Never-‐smokers RR(NS/U.S.)

Esophagus 6.76 4.46 1 0.27

Stomach 1.96 1.47 1 0.71

Bladder 3.27 2.09 1 0.50

Oral Cavity 10.89 3.4 1 0.23

Lung* 23.26 8.7 1 0.11

Females Current smokers Former smokers Never-‐smokers RR(NS/U.S.)

Esophagus 7.75 2.79 1 0.35

Stomach 1.36 1.32 1 0.85

Bladder 2.22 1.89 1 0.65

Oral Cavity 5.08 2.29 1 0.46

Lung* 12.69 4.53 1 0.23


*Other cancers being considered Colon, leukemia, and liver

Point Estimates: Risk of Exposure Induced Death (REID)

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National Aeronautics and Space Administration %RE

ID per Sv

Fatal lung cancer risks per Sv (per 100 rem)Transfer model impact much larger change than >100 cm of GCR shielding– the 100 Billion Dollar question?

% REID, Females % REID, MalesAge at Exposure 35, y 45, y 55, y 35, y 45, y 55, y

Model Type Model rates Average U.S. Population, 2005Additive BEIR VII 1.20 1.20 1.18 0.65 0.66 0.66

UNSCEAR 1.28 1.27 1.22 0.71 0.71 0.69RERF 1.33 1.34 1.32 0.72 0.73 0.73

Multiplicative BEIR VII 2.88 2.74 2.38 0.95 0.92 0.83UNSCEAR 3.56 3.50 3.23 1.17 1.17 1.11RERF 3.71 4.16 4.21 1.13 1.30 1.37

Mixture BEIR VII 2.04 1.97 2.78 0.80 0.79 0.74UNSCEAR 2.43 2.39 2.23 0.94 0.94 0.89RERF 2.53 2.77 2.78 0.92 1.02 1.05

Never-smokersMultiplicative BEIR VII 0.44 0.41 0.37 0.15 0.15 0.14

UNSCEAR 0.57 0.57 0.54 0.15 0.15 0.14RERF 0.55 0.61 0.66 0.14 0.15 0.16

Mixture BEIR VII 0.85 0.84 0.81 0.40 0.40 0.38UNSCEAR 0.96 0.95 0.91 0.46 0.45 0.42RERF 0.98 1.01 1.02 0.46 0.47 0.45

Generalized Multiplicative

RERF, Generalized Multiplicative for never-smokers

0.39 0.47 0.53 0.16 0.17 0.20


“Safe” days in Space: Uncertainties estimated using subjective PDFs propagated using Monte-‐Carlo techniques

%REID for Males and 95% CI %REID for Females and 95% CIa Avg. U.S. Never-‐Smokers Decrease

(%)Avg. U.S. Never-‐Smokers Decrease

(%)30 2.26 [0.76, 8.11] 1.79 [0.60, 6.42] 21 3.58 [1.15, 12.9] 2.52 [0.81, 9.06] 30

40 2.10 [0.71, 7.33] 1.63 [0.55, 5.69] 22 3.23 [1.03, 11.5] 2.18 [0.70, 7.66] 33

50 1.93 [0.65, 6.75] 1.46 [0.49, 5.11] 24 2.89 [0.88, 10.2] 1.89 [0.60, 6.70] 34

a NASA 2005 NASA 2010 Avg. U.S.

NASA 2010 Never-‐Smokers

Males

35 158 140 (186) 180 (239)45 207 150 (200) 198 (263)55 302 169 (218) 229 (297)

Females

35 129 88 (120) 130 (172)45 173 97 (129) 150 (196)55 259 113 (149) 177 (231)

%REID predictions and 95% CI for never-‐smokers and average U.S. population for 1-‐year in deep space at solar minimum with 20 g/cm2 aluminum shielding:

Maximum Days in Deep Space with 95% Confidence to be below Limits (alternative quality factor errors in parenthesis):


Solar Min and Max Comparison with Proposed NASA Quality Factor (Q) and Tissue Weights (Wt) vs ICRP Quality Factor Definition

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Shielding Materials play little role for GCR

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MaterialE (Sv)

Solar Minimum SPE + Solar Maximum

10 g/cm2

Liquid H2 0.40 0.19Liquid CH4 0.50 0.30Polyethylene 0.52 0.33Water 0.53 0.35Epoxy 0.53 0.36Aluminum 0.57 0.43

20 g/cm2


40 g/cm2


Annual effective dose. Solar max calculations include 1972 Solar Particle Event.


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Solar Particle Event (SPE) Risks


Research studies show that risks of acute death from large SPEs has been over-‐estimated in the past: – Proper evaluation of dose-‐rates, tissue shielding, and proton biological effectiveness show risk is very small

SPE risk remain important for lunar EVA – Radiation sickness if unprotected > 2 hour EVA – Cancer risk is priority for both EVA and IVA

Proper resource management through research: – Probabilistic risk assessment tools for Lunar and Mars Architecture studies – Optimize shielding requirements by improved understanding of proton radiobiology & shielding design tools

– ESMD and SMD collaborations on research to improve SPE alert, monitoring and forecasting

– Biological countermeasure development for proton cancer, and Acute radiation syndromes (if needed)


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SPE Probabilistic Risk Assessment

• Using detailed data base of all SPE’s in space age (1955-‐current) and historical data on Ice-‐core nitrate samples (15th-‐century to current), SRP has developed a probabilistic model of SPE occurrence, size, and frequency – Hazard rate model using Survival

analysis – Non-‐uniform Poisson process

provides high quality fit of all SPE data

• Probabilistic model supports shielding design and resource management goals for Exploration missions

• Department of Defense model estimates various acute risks

0

20

40

60

80

100

120

140

160

2/1/54 2/1/58 2/1/62 2/1/66 2/1/70 2/1/74 2/1/78 2/1/82 2/1/86 2/1/90 2/1/94 2/1/98 2/1/02 2/1/06

Date

λ (t)

SPE Hazard Rate in Space Era

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000 2500 3000 3500 4000

Time, d

P ModelSample

Non-‐Uniform Poisson Process


Acceptable Risk Levels for Exploration Missions

• The NASA Standard of 3% Risk of Exposure Induced Death was set in 1989 by NASA Administrator with OSHA Concurrence under Code of Federal Regulation (CFR 1960)

• NASA has set an identical acceptable risk level for Exploration missions under the OCHMO’s 2006 Permissible Exposure Limits (PEL) – OSHA concurrences on NASA Health policy in Spaceflight dropped in

2004 after discussion with OCHMO • The NCRP recommendation of 3% Limit based on 3 rationales:

– Comparison of fatality rates in less-‐safe Industries made in 1989 – Comparison to risk limits for ground-‐based workers – Recognition of other spaceflight risks

• Fatality rates in less-‐safe industries have improved more than 2-‐fold since 1989 and therefore no longer valid basis; however other 2 rationale from NCRP in 1989 are still valid

26


Acceptable Levels of Risk -‐ continued

• A discussion of higher or lower Acceptable Risk Levels would consider – Over arching Ethical and Safety standards at NASA and in the U.S. – Benefits to Human-‐kind from Exploration missions – Emerging information on possible radiation mortality risks from non-‐

cancer diseases, notably Heart (Stroke and Coronary Heart Disease) and Central Nervous System risks

– The resulting burden for morbidity risks including cancer, cataracts, aging, and other diseases that entail pain, suffering, and economic impacts • Radiation cancer incidence probability approximately Two times higher than cancer death

probability

– Improvements in other areas of safety at NASA, other government agencies and work places since 1989

– Balance between other space flight risks and space radiation risks • NCRP Recommendation is the high risk nature of space missions precludes allowing an

overly large radiation risk to Astronauts

– Impacts on finding solutions through research programs and mission design architectures that result from Acceptable Risk Standards

27


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3% Risk (REID)

6% Risk (REID)

95% CL 90% CL 95% CL 90% CL

Age, y Males

35 140 184 290 36145 150 196 311 392

55 169 219 349 439

Age, y Females

35 88 116 187 232

45 97 128 206 255

55 113 146 234 293

Number of Days in Deep Space At Solar minimum with a 95% or 90% CL to be below 3% or 6% Risk of Cancer Death from Space Radiation (Avg US pop)

3% and 6% Cancer Mortality Risks at 90% to 95% Confidence Levels (CL) (Solar Min at 20 g/cm2 Aluminum)

radiation physiology and effects€¦ · lunar regolith shielding for spe 28 –, human integration...

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