# Presentation: 208089

## Radiation on Planetary Surfaces

**Radiation on Planetary Surfaces**

**M. S. Clowdsley****1****, G. DeAngelis****2****, J. W. Wilson****1****, F. F. Badavi****3****, and R. C. Singleterry****1**

- 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

## 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)

## 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

## Planetary Surface Material and Atmosphere 

- (Simonsen et al.)

- Mars Induced Fields

- GCR ion

- Diffuse
- neutrons

- High energy
- particles

## 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

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

## Lunar Surface “Worst Case SPE” Environment

## Dose Equivalent on Lunar Surface Due to GCR

## Mars Surface GCR Environments

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

## Mars Surface Neutrons

## Mars Surface “Worst Case SPE” Environment

## Dose Equivalent on Mars Surface Due to GCR

## Mars Surface Mapping

- Charged Ions – 1977 Solar Minimum

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

## Mars Surface Mapping

- Neutrons – 1977 Solar Minimum

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

## 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

## 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:

-                   **CO****2****           %      95.32**
**       ****N****2****             %      02.70**
**      ****Ar             %      01.60**
**       ****O****2****            %      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:

-                    **SiO****2****          %      44.2**
**      ****Fe****2****O****3****        %      16.8**
**      ****Al****2****O****3****        %      08.8**
**      ****CaO          %      06.6**
**      ****MgO         %      06.2**
**       ****SO****3****          %      05.5**
**     ****Na****2****O         %      02.5**
**      ****TiO****2****         %      01.0**
**     **

## Model for Mars Surface

- The composition, different with respect to the regolith (**e.g. CO****2**** ice, H****2****O 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

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

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

## Sample ISS Calculations

**Sample ISS Calculations**

- Ray-trace Mesh

- Directional Dose

**Directional **
**Dose distributions**

**Dose Maps**

**Notes:**
A CAD model of ISS was converted into a radiation shielding model and ported into the CAVE automatic virtual environment.  An external view is shown top left and an internal view on the right.  A person in the CAVE can choose any point, say the center of one of the dose spheres shown.  A ray tracing is then performed with rays going outward through every intersection of the mesh shown bottom left.  Along each ray the amount of shielding material is then calculated.  Dose equivalent versus shielding thickness tables are then used to create the directional dose sphere.  This sphere can show the user in a moment where more shielding is needed.

## 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

## Exposure Limits for LEO Operations (NCRP 98)

- Exposure Limits for LEO Operations (NCRP 98)

- 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.”

## Proposed Exposure Limits for LEO 

- 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.”

**Notes:**
This slide shows the current exposure limits for LEO operations.  Once again, it is important to note that no limits have been set for missions beyond LEO.  We believe that limits for lunar missions will be set in the near future and our best guess is that they will be similar to these limits with the possible addition of a 95% confidence being applied to the requirement that the excess fatal cancer risk must stay below 3%.

The limits in Table 1 were developed out of a concern that the quality factor used to calculate dose equivalent was designed using late cancer induction data and might not be accurately predicting short term deterministic effects.  For this reason, the relative biological effect (RBE) numbers for different types of radiation were developed for deterministic effects and incorporated into the gray equivalent formula.

Note that there are 30 day, annual, and career limits to eyes and skin and 30 day and annual limits for blood forming organ (BFO) exposure.  The career limit to BFO, however, is replaced by whole body effective dose limits which are dependant on astronaut age and gender.  

## Possible Exposure Limits for Lunar Missions(NASA-STD-3000  Vol. VIII - Feb. 1, 2005 Draft)

- 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.”