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