Radiation on Planetary Surfaces

M. S. Clowdsley1, G. DeAngelis2, J. W. Wilson1, F. F. Badavi3, and R. C. Singleterry1

1 NASA Langley Research Center, Hampton, VA 2Old Dominion University, Norfolk, VA

3Christopher Newport University, Newport News, VA

Solar and Space Physics and the Vision for Space Exploration Meeting

Wintergreen, VirginiaOctober 16-20, 2005

Outline• Requirements for Evaluating Risk Due to

Radiation on Planetary Surfaces– Description of the free space radiation environment near the planet

(types of particles and their energy spectra) – Model of planetary magnetic field (if one exists)– Models of planetary surface material and atmosphere (if planet has

an atmosphere) – Radiation transport code or codes – Guideline defining how much of each type of radiation is too

much

• Examples Calculations– The Moon– Mars– Callisto

• Conclusions

Free Space Radiation Environment

• Galactic Cosmic Rays (GCR) – Made up of heavy ions as well as alpha particles and protons – Modeled using the Badhwar-O’Neill formulation– Modulated by the solar wind

• Vary with the solar cycle• Dependant on distance from the sun

• Solar Particle Events (SPE)– Made up of a large number of particles, mostly protons– Correspond to large coronal mass ejections

• Large SPE rare• Last only a few hours• Could result in fatality

Free Space GCR Environments at 1 AU

1977 Solar Minimum (solid)1990 Solar Maximum (dashed)

Energy (MeV/amu)

Particle

Flu

ence

(#particles/cm

2-M

eV/a

mu-y

ear)

10-2 10-1 100 101 102 103 104 105 10610-3

10-2

10-1

100

101

102

103

104

105

106

Z=1

Z=2

3Z10

11Z20

21Z28

Solar Sunspot Numbers and Deep River Neutron Monitor Count

Rates(Measured and Predicted)

Deep River Neutron Monitor

Solar Sunspot Number

Free Space Solar Particle Event Proton Spectra at 1 AU

Energy (MeV/amu)

Particle

Fluence

(#particles/cm

2-M

eV/a

mu)

10-2 10-1 100 101 102 103 104103

104

105

106

107

108

109

1010

1011

1012

Worst Case SPEFeb. 1956Aug. 1972Sept. 1989

Planetary Surface Material and Atmosphere

(Simonsen et al.)

Mars Induced Fields

GCR ion

Diffuseneutrons

High energyparticles

Radiation Transport Codes• Monte Carlo Codes: MCNPX, HETC, FLUKA, TIGRE

– Accurately model the transport of neutrons, protons, and other light ions (and electrons in the case of TIGRE)

– GCR ions being added – Require large amounts of computer time

• Deterministic Codes: HZETRN, GRNTRN, Electron Transport Code (Nealy et. al.) – Accurately model the transport of neutrons, protons, light ions,

and GCR (and electrons in the case of the electron transport code)

– Provide rapid transport calculations

Lunar Surface GCR Environments

Energy (MeV/amu)

Particle

Fluence

(#particles/cm

2-M

eV/a

mu-y

ear)

10-2 10-1 100 101 102 103 104 105 10610-4

10-2

100

102

104

106

108

1010

Z=1

Z=0

Z=2

3Z1011Z20

21Z28

1977 Solar Minimum (solid)1990 Solar Maximum (dashed)

Lunar Surface “Worst Case SPE” Environment

Energy (MeV/amu)

Particle

Fluence

(#particles/cm

2-M

eV/a

mu)

10-2 10-1 100 101 102 103 104102

103

104

105

106

107

108

109

1010

1011

Z=1

Z=0

Dose Equivalent on Lunar Surface Due to GCR

Sphere Thickness (g/cm2)

AnnualB

FO

Dose

Equivale

nt(c

Sv)

0 25 50 75 1000

5

10

15

20

25

30

35

40AL2219 - 1977 min.Polyethylene - 1977 min.H Nanofibers - 1977 min.Liquid Hydrogen - 1977 min.AL2219 - 1990 max.Polyethylene - 1990 max.H Nanofibers - 1990 max.Liquid Hydrogen - 1990 max.

Mars Surface GCR Environments

Energy (MeV/amu)

Particle

Fluence

(#particles/cm

2-M

eV/a

mu-y

ear)

10-2 10-1 100 101 102 103 104 105 10610-4

10-2

100

102

104

106

108

1010

Z=1

Z=0Z=2

3Z10

11Z20

21Z28

1977 Solar Minimum (solid)1990 Solar Maximum (dashed)

Mars Surface Neutrons

Mars Surface “Worst Case SPE” Environment

Energy (MeV/amu)

Particle

Flu

ence

(#particles/cm

2-M

eV/a

mu)

10-2 10-1 100 101 102 103 104100

101

102

103

104

105

106

107

108

109

Z=1

Z=0Z=2

Dose Equivalent on Mars Surface Due to GCR

Sphere Thickness (g/cm2)

AnnualB

FO

Dose

Equivale

nt(c

Sv)

0 25 50 75 1000

5

10

15

20

25

30

35

40AL2219 - 1977 min.Polyethylene - 1977 min.H Nanofibers - 1977 min.Liquid Hydrogen - 1977 min.AL2219 - 1990 max.Polyethylene - 1990 max.H Nanofibers - 1990 max.Liquid Hydrogen - 1990 max.

Mars Surface MappingCharged Ions – 1977 Solar Minimum

from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site http://sirest.larc.nasa.gov

Mars Surface MappingNeutrons – 1977 Solar Minimum

from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site http://sirest.larc.nasa.gov

Mars Surface MappingLow Energy Neutrons – 1977 Solar Minimum

from Space Ionizing Radiation Environment and Shielding Tools (SIREST) web site http://sirest.larc.nasa.gov

Mars Surface Environment

Model for Mars Atmosphere• Atmospheric chemical and isotopic composition

modeled using results from in-situ Viking 1 & 2 Landers measurements for both major and minor components:

CO2 % 95.32

N2 % 02.70

Ar % 01.60

O2 % 00.13

CO % 00.08

Model for Mars Surface• The surface altitude, or better the atmospheric depth for

incoming particles, determined using a model for the Martian topography based on the data provided by the Mars Orbiter Laser Altimeter (MOLA) instrument on board the Mars Global Surveyor (MGS) spacecraft.

• The Mars surface chemical composition model based on an averaging process over the measurements obtained from orbiting spacecraft, namely the Mars 5 with gamma-ray spectroscopy, and from landers at the various landing sites, namely Viking Lander 1, Viling Lander 2, Phobos 2 and Mars Pathfinder missions.

Model for Mars Surface• The adopted Mars surface chemical composition:

SiO2 % 44.2

Fe2O3 % 16.8

Al2O3 % 08.8

CaO % 06.6

MgO % 06.2

SO3 % 05.5

Na2O % 02.5

TiO2 % 01.0

Model for Mars Surface

• The composition, different with respect to the regolith (e.g. CO2 ice, H2O ice), of seasonal and perennial polar

caps has been taken into account by modeling the deposition of the possible volatile inventory over the residual caps, along with its geographical variations all throughout the Martian year, for both the Mars North Pole and South Pole, from results from imaging data of orbiter spacecraft and from groundbased observations

• No 3D time dependent models for the Martians polar caps was previously available for radiation studies

Callisto Surface GCR Environments

Energy (MeV/amu)

Particle

Fluence

(#particles/cm

2-M

eV/a

mu-y

ear)

10-2 10-1 100 101 102 103 104 105 10610-4

10-2

100

102

104

106

108

1010

Z=1

Z=0

Z=2

3Z1011Z20

21Z28

1977 Solar Minimum (solid)1990 Solar Maximum (dashed)

Dose Equivalent Rate on Callisto Due to GCR for Jan. 1, 2047

Sphere Thickness (g/cm2)

BFO

Dose

Equivale

ntRate

(cSv/

day)

0 25 50 75 1000

0.05

0.1

0.15

0.2

0.25

0.3

0.35AL2219PolyethyleneH NanofibersLiquid Hydrogen

Sample ISS Calculations

Ray-trace Mesh Directional Dose

Directional Dose distributions

Dose Maps

Conclusions• Surface radiation calculations have been performed for the

Earth’s moon, Mars, and Callisto• These calculations show that radiation shielding will be an

important consideration in planning of long term missions to these surfaces

• These calculations also demonstrate the large variation in exposure rates due to solar cycle

• The advantages of using shielding materials containing hydrogen were demonstrated

• The ability of the HZETRN code to calculate the radiation environment on the surface of any planet or moon has been demonstrated

Table 1 – Dose Equivalent Limits (Sv)

  BFO Eye Skin

Career Table 2 4.0 6.0

1 Year 0.50 2.0 3.0

30 Day 0.25 1.0 1.5

Exposure Limits for LEO Operations (NCRP 98)

Table 2 – Career Dose Equivalent to BFO Limits (Sv)

Age at Exposure 25 35 45 55

Male 1.5 2.5 3.2 4.0

Female 1.0 1.75 2.5 3.0

Based on 3% excess career fatal cancer risk

Note: limits not yet defined for missions beyond LEO

Limits defined in terms of dose equivalent (H) H = Q(L) DL dL

where DL is the dose (energy absorbed per unit mass) from particles with linear energy transfer between L and L+dL and Q(L) is a quality factor.

ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”

Table 2 – Career Effective Dose Limits (Sv)

Age at Exposure

25 35 45 55

Male 0.7 1.0 1.5 3.0

Female 0.4 0.6 0.9 1.7

Table 1 – Gray Equivalent Limits (Gy-Eq)

  BFO Eye Skin

Career Table 2 4.0 6.0

1 Year 0.50 2.0 3.0

30 Day 0.25 1.0 1.5

New radiation protection quantities• Gray equivalent to BFO, eyes, and skin used to evaluate risk due to deterministic effects

Gy-Eq = i RBEi Di

• Whole body effective dose used to evaluate health risk due to stochastic effects

E= wTHT

Based on 3% excess career fatal cancer risk

Note: limits not yet defined for missions beyond LEO

Proposed Exposure Limits for LEO Operations (NCRP 132)

ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”

Possible Exposure Limits for Lunar Missions

(NASA-STD-3000 Vol. VIII - Feb. 1, 2005 Draft)

Table 1 – Gray Equivalent Limits (Gy-Eq)

  BFO Eye Skin

Career REID 4.0 6.0

1 Year 0.50 2.0 3.0

30 Day 0.25 1.0 1.5

REID:“Occupational radiation exposure is limited to not exceed 3% probability of radiation exposure induced death (REID). NASA will assure that this risk limit is not exceeded at a 95% confidence level using a statistical assessment of the uncertainties in the risk projection calculations…”

ALARA – In addition to the above limits, radiation exposure must be kept “as low as reasonably achievable.”