Radiation on Planetary Surfaces

1
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, Virginia October
16-20, 2005
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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

3
Free Space Radiation Environment

• Galactic Cosmic Rays (GCR)
• Made up of heavy ions as well as alpha particles
  and protons
• Modeled using the Badhwar-ONeill 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

4
Free Space GCR Environments at 1 AU
1977 Solar Minimum (solid) 1990 Solar Maximum
(dashed)
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Solar Sunspot Numbers and Deep River Neutron
Monitor Count Rates
(Measured and Predicted)
Deep River Neutron Monitor
Solar Sunspot Number
6
Free Space Solar Particle Event Proton Spectra at
1 AU
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Planetary Surface Material and Atmosphere
Mars Induced Fields
GCR ion
High energy particles
Diffuse neutrons
(Simonsen et al.)
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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

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Lunar Surface GCR Environments
1977 Solar Minimum (solid) 1990 Solar Maximum
(dashed)
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Lunar Surface Worst Case SPE Environment
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Dose Equivalent on Lunar Surface Due to GCR
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Mars Surface GCR Environments
1977 Solar Minimum (solid) 1990 Solar Maximum
(dashed)
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Mars Surface Neutrons
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Mars Surface Worst Case SPE Environment
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Dose Equivalent on Mars Surface Due to GCR
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Mars Surface Mapping
Charged Ions 1977 Solar Minimum
from Space Ionizing Radiation Environment and
Shielding Tools (SIREST) web site
http//sirest.larc.nasa.gov
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Mars Surface Mapping
Neutrons 1977 Solar Minimum
from Space Ionizing Radiation Environment and
Shielding Tools (SIREST) web site
http//sirest.larc.nasa.gov
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Mars Surface Mapping
Low Energy Neutrons 1977 Solar Minimum
from Space Ionizing Radiation Environment and
Shielding Tools (SIREST) web site
http//sirest.larc.nasa.gov
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Mars Surface Environment
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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

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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.

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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
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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

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Callisto Surface GCR Environments
1977 Solar Minimum (solid) 1990 Solar Maximum
(dashed)
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Dose Equivalent Rate on Callisto Due to GCR for
Jan. 1, 2047
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Sample ISS Calculations
Directional Dose distributions
Dose Maps
Ray-trace Mesh
Directional Dose
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Conclusions

• Surface radiation calculations have been
  performed for the Earths 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

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Exposure Limits for LEO Operations (NCRP 98)
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 LdL and
Q(L) is a quality factor.
Based on 3 excess career fatal cancer risk
ALARA In addition to the above limits,
radiation exposure must be kept as low as
reasonably achievable.
Note limits not yet defined for missions beyond
LEO
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Proposed Exposure Limits for LEO Operations
(NCRP 132)

• 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
ALARA In addition to the above limits,
radiation exposure must be kept as low as
reasonably achievable.
Note limits not yet defined for missions beyond
LEO
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Possible Exposure Limits for Lunar
Missions(NASA-STD-3000 Vol. VIII - Feb. 1, 2005
Draft)
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.