Risk Management Strategies During Solar Particle Events on Human Missions to the Moon and Mars:
The Myths, the Grail, and the Reality
ByDr. Ronald Turner
Presented at the workshop on
Solar and Space Physics and the Vision for Space ExplorationWintergreen Resort Wintergreen, Virginia
October 18, 2005
ANSERSuite 800
2900 South Quincy StArlington, VA 22206
Outline
• Background
• Systems Approach to Radiation Risk Management
• Conclusions/Observations
The Myths, the Grail, the Reality
• Solar Particle Events are potent killers and mission showstoppers
• SPEs can be adequately mitigated with modest shielding
• We cannot forecast SPEs, and never will
• A far-side solar observatory is a necessary component to an SPE risk mitigation strategy
The Myths, the Grail, the Reality
• A dynamic theory and appropriate observations that enable operationally robust models to forecast SPEs at least 6-12 hours prior to onset...
• ...Contributing to an overall risk mitigation architecture that includes
• Adequate shelter, • Effective radiation monitoring, • Reliable communications, and • Integrated mission planning and operations
concepts • to ensure the safety of astronauts throughout the
various phases of missions planned for the space exploration vision
The Myths, the Grail, the Reality
• There is only one more solar cycle before humans return to the Moon
• Funding will always be limited
• Each component of a risk management strategy must demonstrably contribute to enhanced safety of the astronauts on exploration missions
How Bad Can an SPE Be?Selected Historical Events
Lunar Surface BFO Radiation Dose (cGy)
0.1
1.0
10.0
100.0
1000.0
FEB 56 NOV 60 AUG 72 AUG 89 SEP 89 OCT 89
Cen
tiG
ray
30.0 10.0 5.0 0.3
Differential Fluence Spectra(particles/MeV-cm2)
10 100 1000
109
107
105
103
101
10-1
10-3
MeV
Shielding Thickness(g/cm2 Aluminum)
5% Chance of Vomiting5% Chance of Death10% Chance of Death50% Chance of Death
What is the Worst Case SPE?
• Traditionally the assessment of SPE threat is done by analyzing “worst case” historical examples
• To provide safety factors, the analysis may:
• Increase flux by a factor of two or more
• Use composite historical SPEs:
• Fluence of Aug 72 with the
• Spectral character of Feb 56
• What if the next large SPE is not a simple multiple of Aug 72?
Dose Equivalent Sensitivity to Spectral Character( Aug 72 Example)
Harder Spectra
Softer Spectra
Au
g 7
2 fi
t
50 100 1500 Eo
100
10
1
0.1
No
rmal
ized
Su
rfac
e B
FO
Do
se-E
qu
ival
ent
>60 MeV Fluence
fixed
BFO Dose Equivalent
>30 MeV Fluence
fixed
>10 MeV Fluence
fixed
Slightly harder spectra may increase BFO dose equivalent by a factor of two or more
oEEkeEFlux /)( −=
Dose Equivalent Sensitivity to Spectral Character( Aug 72 Example)
oEEkeEFlux /)( −=
Harder Spectra
Softer Spectra
Au
g 7
2 fi
t
50 100 1500 Eo
100
10
1
0.1
No
rmal
ized
Su
rfac
e S
kin
Do
se-E
qu
ival
ent
Skin Dose Equivalent
>60 MeV Fluence
fixed
>30 MeV Fluence
fixed
>10 MeV Fluence
fixed
Slightly softer spectra may increase skin dose equivalent by an order of magnitude
oEEkeEFlux /)( −=
Potential Elements of an SPE Risk Mitigation Architecture
Detection/Forecast Reduction
Active and passive dosimeters, dose rate monitors
Solar imagers, coronagraphs
In situ particle, plasma monitors
Remote sensing of plasma properties
Data/information communications infrastructure
Forecast models, algorithms
Active and Passive shielding
Reconfigurable shielding
Storm shelters
Pharmacological measures
Prescreening for radiation tolerance
Particle transport, biological impact
models/algorithms
Alert/warning communications infrastructure
Operational procedures, flight rules
Radiation Safety Information Flow
Recommendations to Mission
Commander
Space Environment
Situation Awareness
Space EnvironmentObservations
Data Archive
Space Environment
Models
Exposure Forecast
Dosimetry, Radiation Transport
Models
Exposure Verification,
Validation
Impact and Risk
Analysis
Mission Manifest, Flight Rules, Other Safety
Factors
Crew Exposure
History
Forecasting SPE is a Multidiscipline Challenge
Predict the eruption of a CME
Predict the character of the
CME
Predict the efficiency of the CME to accelerate particles
Predict the particle escape from shock and subsequent transport through heliosphere
One Approach to Radiation
Safety
EVA?
Yes
No
Yes
No
Consider GCR Radiation
Environment
Is MissionWithin Limits?
Is MissionWithin Limits?
Increase Habitat
Shielding
Increase Habitat
Shielding
Model Mission
Exposure
Model Mission
Exposure
GCR
YesNo YesNo
Consider Worst Case SPE
Model Mission
Exposure
Model Mission
Exposure
Is MissionWithin Limits?
Is MissionWithin Limits?
Add/ Increase Storm ShelterAdd/ Increase Storm Shelter
SPEShielding is the Main Defense
against Radiation
Surface Operations are Rule-Driven
• Astronaut activities are managed against a set of “Flight Rules”
• These Rules define the overall Concept of Operations (CONOPS)
• CONOPS should reflect the best science available to the mission planners
• Translation of research to operations is not trivial and needs thoughtful scientist input
Converting Science to Operations
ChallengesUnder what conditions, and with what probability, would
SPEs be significant under modest shielding
How can NASA ensure that astronauts are protected during EVA or surface excursions
• How far should astronauts be permitted to travel away from a “safe haven”
• Overly-restrictive rules limit the science that can be accomplished• Too-lenient rules put the astronauts at risk
• Under what conditions must they abort an excursion; With how much urgency?
• Based on what observations?• Based on what forecasts?
Example
MissionOperations
SRAG and Flight Surgeon
Space Weather Forecast Center
Outlook/Warning/
AlertImpact/Options
Concept of Surface OperationsDosimeter data
Instructions to astronauts
Climatology
Nowcast
Forecast
Environment
TransportCode
Flight Plan
Flight Rules
Limits
Models andAnalysis Input
Radiation Risk Management Architecture Elements
Solar Imager (s)
Heliosphere Monitor(s)
Particle Environment
Monitor(s)
Spacecraft
Habitat Rover Suit
Shielding
Dose/Dose Rate Monitors
Communications
Radiation Risk Mitigation Objective
NASA will establish radiation limits
Any mission must be designed to ensure that radiation exposures do not become comparable to these
radiation limits
Top Level Requirement
Reduce the impact of the radiation environment enough to achieve the top level requirement
Forecast the radiation environment with adequate timeliness to take appropriate actions
System Level Requirements
Radiation Risk Management Investment StrategyStep One: Strategic Decisions
Radiation Limits:• Lifetime• Annual• 30-Day• Peak Dose Rate?
Radiation Risk Management Strategy:• Cope and Avoid• Anticipate and React
Biological EffectsIncluding Uncertainty
Risk Philosophy
Radiation Risk Management Investment StrategyStep Two: Mission Design Concept
Mission Architecture Elements• Spacecraft• Habitat• Rover• Suit (space and surface)
Radiation Architecture Elements• Shielding• Dosimeters• Concept of surface operations• Space weather architecture
Radiation Risk Management Investment StrategyStep Three: Transit Phase Shielding Analysis
Mission Limits
Biological EffectsIncluding
Uncertainty
Risk Philosophy
Anticipated Exposure
Including Uncertainty
Nuclear Cross Section Database
Shielding Studies
SPE Worst Case
SPE Climatology
GCR Models
Design Reference
Mission
In Situ Validation
Transport Code
Development
Biological Weighting
Factors
Dose Estimate
Spacecraft Shielding• Mass• Distribution• Composition
Transport Analysis
Including Uncertainty Peak Dose Rate
Estimate
Final Mission Design
Within Limits?
Yes
NoModify Shielding
Radiation Risk Management Investment StrategyStep Four: Surface Operations Concept Development
Shielding Analysis for Habitat, Rover, Suits
Baseline Space Weather Nowcast/Forecast Elements
Integrated Surface Operations Plan
Dose Estimate
Peak Dose Rate
Estimate
Final Concept of
Surface Operations
ALARA?
Yes
NoAdjust Surface Operations Plan
Metrics affecting “Reasonable”
• Cost
• Probability of mission success
• Operational flexibility
• Implicit risk in other areas
ALARA:As Low As Reasonably Achievable
Solar Imager (s)
Heliosphere Monitor(s)
Particle Environment
Monitor(s)Dose andDose Rate Monitor(s)
Communications
Radiation Risk Management Investment Strategy Baseline Space Weather Nowcast/Forecast Elements
Climatology
Physical Models
?Nowcast
Forecast
Baseline Space
Weather Architecture
Yes
NoAdjust
Architecture
Robust
Timely
Comple
te
Relia
ble
Metrics Affecting “Performance”
• Cost
• Accuracy/Precision
• Timeliness
• Reliability
• Availability
Radiation Risk Management Investment Strategy SW Architecture Investment Strategy
ThreeTwoOne
ThreeTwoOne
ThreeTwoOne
ThreeTwoOne
Three
Two
One
Three
Two
One
Dosimeter
particle monitor
plasma monitor
solar imager
nowcast/forecast
LunarMars
Express
ThreeTwoOne
ThreeTwoOne
Products
A Hard Lesson for Scientists to Learn
Intuitively:
BETTERBETTER IS THE ENEMY OF GOOD ENOUGHGOOD ENOUGH
MORE IS BETTER
However:
What is “Good Enough”
• What metrics are appropriate for trade-off studies?• Minimizing Biological Impact (by some quantification scheme)?• Maximizing Operational Flexibility?• Minimizing Total System Cost?• Maximizing Probability of Mission Success?
• How do you effectively create an interdisciplinary team?• Spacecraft Designers• Operators• Biologists• Physicists• Human Factors Engineers
• How do you ensure communication between team members?
• “If I already have enough shielding for a worst case SPE, why do I need a forecast?”
• “If I could give you a perfect 3-hour forecast, would you do anything different?”
Only One More Solar Cycleto Learn What We Must Learn
2000 2010 2020 2030
Solar Cycle 24
Solar DynamicsSolar Dynamics
SentinelsSentinels
Return to the MoonReturn to the Moon
On to MarsOn to Mars
ObservatoryObservatory
Human Mission DesignHuman Mission Design
STEREOSTEREO
SOHOSOHO
ACEACE
Observations• Improve Climatology
– Probability of exceeding event thresholds– Distribution of spectral hardness– Probability of multiple or correlated events
• Create Extreme Events Catalogue– Community consensus on contents– Characterize temporal evolution– Spectral character to high energy– Include uncertainties– Composite worst case event
• Develop and Validate Transport Codes– Agree to standard test cases/benchmarks– Validate in situ as well as in laboratory
• Develop Reliable “All Clear” Forecasts– Multiple time ranges (six hours, one day, one week)
Conclusions
• Important time for radiation protection, with advances underway in physics, biology, and the complexity of missions
• Need for quantification of benefits beyond ALARA• Need for operators, biologists, physicists, and
others to work together to define optimal system approach
• Time is right to lay the groundwork for a new paradigm– From: Cope and Avoid– To: Anticipate and React
Backup Slides
Space Weather Contributions toSupport the Moon, Mars, and Beyond Vision
Better understanding of Solar Dynamics Improved Forecasting of Coronal Mass Ejections
Improved forecasting of SPEsBetter understanding of Heliospheric Dynamics
Improved Forecasting of Solar Wind profiles Improved forecasting of SPEs
Better understanding of SPEs Improved design of habitats and shelters Higher confidence in mission planning
Better forecasts of SPE evolution after on-set Higher confidence in exposure forecast
Implementation of more flexible flight rules Reduced period of uncertainty
Greater EVA scheduling flexibility Less down-time of susceptible electronics
Prediction of SPEs before on-set Higher confidence in exposure forecast
Greater mission schedule assurance Less down-time of susceptible electronics
Prediction of “all clear” periods Higher confidence in exposure forecast
Greater EVA scheduling flexibility Greater mission schedule assurance
Improved Safety and Enhanced Mission Assurance