NASA Technical Memorandum 104782

Spaceflight Radiation Health Programat the Lyndon B. Johnson Space Center

A. Steve Johnson

KRUG Life Sciences, Inc.Houston, TX

Gautam D. Badhwar

Michael J. GolightlyAlva C. HardyAndrei Konradi

Tracy Chui-hsu Yang

Lyndon B. Johnson Space CenterHouston, TX

National Aeronautics and

Space Administration

https://ntrs.nasa.gov/search.jsp?R=19940017390 2020-04-06T04:21:03+00:00Z

A large solar prominence photographed by astronauts aboard Skylab

ii

CONTENTS

Section Page

INTRODUCTION ................................................................................................................. 1

ENVIRONMENTAL CHALLENGES ................................................................................. 1

Trapped Radiation ........................................................................................................... 2Galactic Cosmic Radiation .............................................................................................. 4

Solar Particle Events ........................................................................................................ 6

Manmade Sources ............................................................................................................ 7

Low Earth Environment .................................................................................................. 7

RADIATION PROTECTION ADMINISTRATION: LEGAL REQUIREMENTS ........... 8

RESEARCH AND DEVELOPMENT .................................................................................. 10

Advanced Radiation Monitors ......................................................................................... 10

Flight Experiment Support .............................................................................................. 12Environmental Models ..................................................................................................... 12

Health Risk Assessment .................................................................................................. 12

MISSION SUPPORT: CREW EXPOSURE MONITORING ............................................. 13

SUPPORTING ORGANIZATIONS ..................................................................................... 13

EXPLORATION OF MOON/MARS .................................................................................... 14

SUMMARY ........................................................................................................................... 14

DEFINITIONS ...................................................................................................................... 14

BIBLIOGRAPHY .................................................................................................................. 15

°°°

111

SPACEFLIGHT RADIATION HEALTH PROGRAM

AT LYNDON B. JOHNSON SPACE CENTER

INTRODUCTION

Early in the evolution of the spaceprogram, radiation was recognized as a hazardto humans traveling in space. Monitoring ofcrew radiation exposures was initiated duringProject Mercury and has continued through thecurrent Shuttle Program.

The space radiation environment is signifi-cantly different from that found terrestrially.

Space radiation consists primarily of high-energy charged particles, such as protons,alpha and heavier particles, originating fromseveral sources, including galactic cosmicradiation, energetic solar particles from solarflares and trapped radiation belts. Some ofthese high-energy particles inflict greater bio-logical damage than that resulting from typicalterrestrial radiation hazards. Crew exposurescan easily exceed exposures routinely receivedby terrestrial radiation workers. Increasedknowledge of the composition of the environ-ment and of the biological effects of spaceradiation is required to assess health risks toastronaut crews.

Future expeditions into interplanetaryspace will place crews at increased risk ofexposure compared to the current shortduration low Earth orbit (LEO) missions.Long duration exploratory class missions willnot benefit from the protection from galacticcosmic radiation (GCR) and energetic solarflare protons afforded by the Earth's geo-magnetic field. Astronaut exposure to GCRwithin an unprotected or thinly shielded space-craft during solar minimum is sufficient toexceed current astronaut exposure guidelines.Energetic solar particle events (SPEs) presentan additional source of risk. These events are

unpredictable in nature and potentially lifethreatening to an inadequately protected crew.Unshielded exposure to large solar particleevents may lead to serious, acute healtheffects. To minimize the risk to the crew, the

magnitude and dynamics of the potential

radiation environment must be considered

when performing spacecraft and missiondesign.

For terrestrial radiation workers, additional

protection against radiation exposure can beprovided through increased shielding.Additional shielding against space radiationexposure may not be practical or efficient.Galactic cosmic radiation is extremelypenetrating. Dose equivalent exposure ratesbehind thin shielding are reduced rapidly at

first, but they plateau with increasingthickness. Thus, thicker shields become less

efficient. The additional mass added purelyfor reducing radiation exposures becomes asubstantial mass penalty for transportationvehicles and therefore may dramaticallyincrease mission cost.

The Johnson Space Center (JSC) leadsthe research and development activities thataddress space radiation exposure and itseffects on the health of astronaut crews. To

assess these risks, increased knowledge of theenvironmental composition and the biologicaleffects of space radiation is needed. Work atJSC covers a broad range of activity, begin-ning with the quantification of astronaut radia-tion exposures and extending to fundamentalresearch into the biological effects of high-energy particle radiation exposure. In addi-tion, the Spaceflight Radiation Health Programis working to achieve balance between therequirements for operational flexibility and theneed to minimize crew radiation exposures.

ENVIRONMENTAL CHALLENGES

The radiation environment may beclassified into three sources of radiation:

trapped radiation, GCR, and SPE. Eachsource is characterized by a differentcomposition of particle energies and fluence.Many of these sources are modulated duringthe course of the solar cycle.

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Figure 1. Trapped radiation belts as a function of energy and distance from the Earth.

Most manned spaceflight missions havebeen conducted within the protection of theEarth's magnetic field. The geomagnetic fieldshields the crews from large SPEs and a sig-nificant portion of the GCR. For the LEOmissions, which have typified the U.S.manned space program, the largest fraction ofthe radiation exposure received has resultedfrom passage through a region known as theSouth Atlantic Anomaly (SAA). The remain-der of the exposure is attributed to high-energygalactic cosmic radiation. During the Apollolunar missions, astronauts traversed the

trapped radiation belts into the unprotectedrealm of free space outside the geomagneto-sphere. These excursions into cislunar spaceplaced the astronauts at risk of receiving life-threatening radiation exposures if a large SPEwere to occur. Fortunately, no major solarproton events occurred during these missions.

Trapped Radiation

The Earth's magnetic field is responsiblefor the formation of the trapped radiation beltsthat surround the Earth. Electrons and protons

are trapped in regions starting above theatmosphere and extending to a distance of 10to 12 Earth radii. Charged particles travelthrough these zones, spiralling around the

magnetic field lines and oscillating back andforth between "mirror points" located inopposite hemispheres. Figure 1 illustrates thedistribution of high and low energy protonsand electrons around the Earth.

The magnetic axis of Earth is tiltedapproximately 11 degrees from the spin axisand is slightly offset from the center of theEarth. As a result of the shift and tilt of the

magnetic field, the trapped proton belts extenddown to the atmosphere in the SAA regionlocated over South America and the South

Atlantic Ocean. Low inclination flights typi-

pcally transit a portion of the SAA during sixor seven consecutive orbits each day. Figure2 shows the exposure rate as a function oforbital position for a 28.5-degree orbit. TheSAA is the primary source of radiationexposure for the Shuttle and the proposedSpace Station Freedom.

The proton spectra and fluence are strongfunctions of altitude. At the higher altitudes,the greater portion of crew exposures isreceived during transits through the SAA as aresult of greater trapped proton fluence levels.At lower altitudes, the protons in the SAAinteract with the residual atmosphere. Someof the protons are lost and contribute to ananisotropic distribution of protons. A greaterthan two factor difference exists between the

proton flux from the east compared to the flux

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Figure 2. Absorbed dose to crew for 28.5 degree inclination Space Shuttle flights.

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Figure 3. Absorbed dose rate as a function of position for a 28.5-degree inclination Shuttle flight.

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Figure 4. Dose rate measured within the Orbiter using the RME-III proportional counter duringSTS-28. Contours represent dose rates within the South Atlantic Anomaly.

from the west. The anisotrophy in particleflux will be an important factor for SpaceStation.

In addition to altitude, the integrated doseis a function of orbital inclination and solar

cycle. The dose received by the crew duringlow inclination (28.5 degree) LEO is illu-strated in figure 3. The Space Station isplanned to operate at this orbital inclination at200 nautical miles (400 km). Increases in

solar activity expand the atmosphere andincrease the losses of protons in LEO.Therefore, trapped radiation doses in LEOdecrease during solar maximum and increase

during solar minimum.Trajectories of low inclination flights do

not pass the regions of maximum intensitieswithin the SAA. Although high inclinationflights pass through the SAA maximumintensity regions, less time is spent in the SAAthan in low inclination flights. Thus crews inhigh inclination flights receive less netexposure to trapped radiation than in lowinclination flights for a given altitude. This is

illustrated in figure 4, which depicts thelocation of the SAA as measured by internalequipment on the Shuttle. Low inclinationflights will not transit the SAA south of 28.5degrees south latitude. High inclination flightstransit between north and south 58 degrees.

Galactic Cosmic Radiation

GCR originates from outside the solarsystem. It consists of ionized charged atomicnuclei from hydrogen (87%) and helium(12%) to uranium (trace) and are characterized

by extremely large kinetic energies (up toseveral thousand GeV per atomic mass unit(amu)). The integral flux of four isotopes atsolar minimum conditions is depicted infigure 5. These particles are distributedisotropically and found in relatively lowfluence.

'c,I_ MJnlmurn

Hydrogen _-"_ M_amken

[ _ _d--...

Energy, MeV/n

Figure 5. Integral GCR fluence as a functionof solar cycle.

The effect of solar activity on the integralGCR flux is also shown in figure 5. Duringperiods of solar maximum activity, theinterplanetary magnetic field generated by theSun provides some protection to the innersolar system, decreasing GCR integralintensity. Higher energy particles are notappreciably attenuated; lower energy fluence issignificantly reduced. The integral GCR doserate in free space is approximately a factor of2.5 higher at solar minimum than at solar

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maximum. During solar minimum, theunshielded dose to the blood-forming organs(BFO) is approximately 60 rem/year.

Due to its high energy, GCR is verypenetrating. Thin to moderate shielding iseffective in reducing the projected equivalentdose rate, but as shield thickness increases,

shield effectiveness drops. This is the resultof the production of a large number ofsecondary products, including neutrons fromnuclear interactions between the GCR and

shield nuclei. Figure 6 illustrates the annual

BFO dose equivalent as a function of differentshield materials.

The geomagnetic field also deflects manyof the lower energy GCR components. Thisprotection is primarily a function of latitude, asshown in figure 7. Compared to lowinclination orbits, higher inclination orbits areexposed to increased GCR levels as thespacecraft transits the higher latitudes. In theproposed Space Station orbit, the magneticfield provides a factor of 10 reduction in totalGCR exposure relative to the free spaceenvironment. During geomagnetic storms,higher GCR exposures may be experienced atlower latitudes. Little increase in GCR isrealized as the altitude increases.

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GEOMAGNETIC LATITUDE (deo)

Figure 6. Shielding effectiveness of GCR atsolar minimum.

Figure 7. GCR dose rate as a function oflatitude and solar cycle.

SOLAR PARTICLE EVENTS

SPEs are injections of energetic electrons,protons, alpha particles and heavier particles,into interplanetary space during solar flares.During periods of maximum solar activity, thefrequency and intensity of solar flaresincrease. Most flares do not present asignificant hazard because they are either toosmall to inject significant numbers of energeticsolar particles, or they occur at solarlongitudinal positions that are unfavorable forthe direct transfer of particles to the Earthalong interplanetary magnetic field lines.However, flares or rapid sequences of largeflares that are orders of magnitude greater inintensity than most flares are of particularconcern for generating very large, energeticSPEs. These solar proton events generallyoccur only once or twice a solar cycle.However, during the 22nd solar cycle, fourcomparable, extremely large flares occurred ina 4-month period. The October 19, 1989,event was the largest.

Each solar particle event is characterizedby the total number of particles and the particleenergy spectrum. The spectra of three of thelargest proton events are depicted in figure 8.The intensity and spectral distribution of SPEshave a significant impact upon shieldeffectiveness. Figure 9 represents theshielding effectiveness for these large SPEs.

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Figure 8. Spectra of three of the largest solarproton events.

Several factors make accurate prediction ofSPEs difficult. First, solar flares occur

without much warning. The magnitude andintensity of a flare are difficult to determineuntil the event is in progress. The directionalemission of particles from the Sun furthercomplicates predictions. Since SPEs arerelatively directional, SPEs sensed by aterrestrial network may not threaten a Martiantransit mission, and conversely, a flareinjection of energetic solar particles thatthreatens a Martian transit mission may notproduce particles at Earth.

SPEs pose the greatest threat to

unprotected crews in polar, geostationary, orinterplanetary orbits. To date, the greatestthreat of significant exposures to astronautsexisted during the Apollo Program. Figure 10

illustrates the variation in timing andmagnitude of SPEs that occurred during thecourse of the Apollo Program. The calculateddose for crewmembers in the command

module, in the lunar module, or in a spacesuit performing EVA is represented for eachflare. As seen in the figure, it is onlyfortuitous that no significant SPEs occurredduring the lunar missions.

Fortunately, most SPEs are relativelyshort-lived (less than 1 to 2 days), whichallows for relatively small volume "stormshelters" to be feasible. To minimize

exposure, the crew would be restricted to thestorm shelter during the most intense portion

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Figure 9. Shielding effectiveness for three ofthe largest solar proton events.

of the SPE, which may last for several hours.

Storm shelters with shielding of 20 g/cm 2 or

more of water equivalent material will providesufficient protection for the crew.

Manmade Sources

Additional environmental hazards may bepresent from the use of manmade sources.These hazards may be in the form of exposureresulting from medical investigations,radioisotopic power generators, or smallsources for experiments. Lunar and Martianmissions may include either nuclear reactorsfor power or propulsion purposes that willcontribute to crew radiation health concerns.

Low Earth Orbit Environment

Current missions are restricted to LEO.

Figure 11 shows data taken during the highinclination STS-28 flight and illustrates the

contributions of the three natural sources of

radiation. The GCR component variescyclically with maximums at the extremenorthern or southern portion of the orbitaltrack. Minimums correspond to transits of thegeomagnetic equator, where the spacecraftexperiences the maximum geomagneticprotection from GCR. At periodic intervalslarge spikes in the exposure rates areencountered, which correspond to passagesthrough the SAA. The largest spikes arepassages through the regions of peak SAAintensity; smaller peaks represent passagethrough the fringes of the SAA. The effects ofa solar particle event measured in LEO are a

unique feature of these data. Peaks in the doserate attributed to the SPE occur at the extreme

northern or southern portions of the orbitaltrack. While the GCR and SAA componentsshown in the figure are "typical" for highinclination flights, the effects of SPEs on doserates will depend upon a variety of parameters.

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Figure 10. Solar proton events during the Apollo Program.

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Figure 11. Measured dose rate vs. time for a high inclination Shuttle flight (STS-28). A largesolar proton event occurred toward the end of the mission.

RADIATION PROTECTIONADMINISTRATION: LEGAL

REQUIREMENTS

Astronauts have been classified as

radiation workers; therefore, a program mustexist to protect them from excessive radiationexposure. The Presidential Executive Order12196 requires that all Federal agencies,including NASA, comply with OccupationalSafety and Health Administration (OSHA)regulations related to ionizing radiationexposure. While NASA is required to followOSHA regulations, no OSHA standards existfor spaceflight. Terrestrial radiation exposureguidelines provided in the Code of FederalRegulations (29 CFR 1910.96) are toorestrictive for space activities, and, therefore,have been judged inappropriate. NASA canestablish supplementary standards forappropriate control of radiation for astronauts

in accordance with 29 CFR 1960.18.

Implementation of the supplementary standardrequires that (1) its use applies to a limitedpopulation, (2) detailed flight crew exposurerecords are maintained, (3) preflight hazardassessment/ appraisal is performed, (4)planned exposures be kept as low asreasonably achievable, (5) operational proce-dures and flight rules are maintained tominimize the chance of excessive exposure,and (6) manmade onboard radiation exposurecomplies with 29 CFR 1910.96, except wherethe NASA mission/objectives cannot beaccomplished otherwise.

NASA hasadoptedtherecommendationsthat the National Council on RadiationProtection and Measurements(NCRP) pre-sented in its Report 98, "Guidance onRadiationReceivedin SpaceActivities" (July1989) as the basis for the supplementarystandard for spaceflight crew radiationexposures.Themaximumexposurelimits arepresentedin Tables1 and2. While monthlyandannuallimits primarily existto preventtheshort-termphysiologicaleffectsof exposure,careerlimits exist to contain radiation riskwithin a 3% increased lifetime cancermortality. Therecommendationsof theNCRPapply to activities in LEO, such as SpaceStation. Astronautexposurelimits aregreaterthanthoseof terrestrialradiationworkers.

Recentinformation from reevaluationof

atomicbomb survivor dataandother sources

has provided impetus for further examinationof the acceptable limits of astronaut radiationexposure. Preliminary recommendations fromthe NCRP evaluation of the new data suggestthat even lower career limits for astronauts

may be warranted.During spaceflight, crew exposures are

monitored using passive dosimeters. A newgeneration of radiation instrumentation isbeing developed to assist in interpreting thecrew radiation exposures. Exposure frommedical procedures and experiments are alsodetermined for each astronaut. Records of

exposure both to medical examinations andspaceflight are documented as part of theSpaceflight Radiation Health ProtectionProgram.

TABLE 1.

EXPOSURE

INTERVAL

30 DAYS

ANNUAL

CAREER

ORGAN SPECIFIC EXPOSURE LIMITS

DEPTH

(5 CM)

25 REM

50 REM

100 TO 400

EYE

(0.3 CM)

100 REM

200

400

SKIN

(0.01 CM)

150 REM

300

600

TABLE 2. CURRENT CAREER EXPOSURE LIMITS BY AGE AND SEX*

SEX

MALE

FEMALE

25

150 REM

100

AGE

35 45

250 REM 325 REM

175 250

55

400 REM

300

*The career depth equivalent dose limit is based upon a maximum 3% lifetimeexcess risk of cancer mortality. The total equivalent dose yielding this risk depends onsex and age at start of exposure. The career equivalent dose limit is approximatelyequal to:

200 + 7.5 (age - 30) rem for males up to 400 rem maximum200 + 7.5 (age - 38) rem for females up to 400 rem maximum.

9

RADIATION HEALTH PROGRAM

QUANTIFY SPACE RADIATIONENVIRONMENT

SPACIAL& TEMPORALVARIATION

SSiVE II ACTIVE I I

WARNING III DEVELOP ISYSTEMS I I IBtOLOGICAq

FOR I II ]DOSIMETRYIFLARESII L __1

RADIOBIOLOGICAL

RESEARCH

BIOLOGICALEFFECTS

COUNTERMEASURES

RADIO-EPIDEMIOLOGICAL

STUDIES

ICHARACTERIZE TRANSPORT

[

SHIELDING & J ENERGYCREW MEMBER BODIES

OPTIMIZE SHIELDING

-

RISK ASSESSMENT & VALIDATION

IASSESS RADIATION IMPACTS ON

SPACECRAFT DESIGN &MISSION PLANNING

Figure 12. Areas of investigation within the Radiation Health Program.

RESEARCH AND DEVELOPMENT

Under the Space Radiation HealthProgram, NASA sponsors many inves-tigations regarding space radiation research.Sponsored research typically fails intoone of three categories: radiation physics,dosimetry, and radiobiology. An annualworkshop for sponsored investigators isprovided each year to discuss the achieve-ments for the past year and direction of theupcoming year. The elements of theresearch program are illustrated in figure12. Efforts at JSC address all three areas.

Advanced Radiation Monitors

Longer duration stays in the spaceradiation environment demand improveddosimetric instrumentation to evaluate thetrue health hazard to crews. The passive

dosimetry currently used for Shuttlemissions provides only part of theinformation needed to quantify the creweffective dose equivalent. Two newinstruments (TEPC and CPDS) are being

developed for use on the Space Station toimprove the crew radiation exposure riskestimation. These real-time instruments

will continuously telemeter radiation data toMission Control for monitoring theradiation environment. Prototypes of thenew instruments are being tested duringShuttle missions. The new Space Stationinstrumentation includes:

Tissue Equivalent Proportional Counter(TEPC) -The TEPC, shown in figure 13, isa gas proportional counter that measures thelinear energy transfer (LET) spectrum ofthe incident radiation in a simulated small

volume of tissue. From the LET spectrum

10

and information provided in Report 60 ofthe International Commission on Radio-

logical Protection (ICRP-60, 1990), theappropriate radiation quality factor can beestimated. The TEPC will be relocatable

into any of the Space Station modules ornodes. The TEPC also is being modifiedand tested for potential flight opportunitieswith the Soviet Space Program. Oneversion is planned for use on the Sovietspace station Mir, and a second version isplanned for inclusion as a radiation

instrument on a probe to Mars. Otherversions of the TEPC are being consideredfor routine use on commercial airline flightsto assess the radiation dose from galacticcosmic radiation to the flight crews.

Figure 13. Tissue equivalent proportionalcounter (TEPC).

Charged Particle Directional Spectrometer(CPDS) - The CPDS will measure the fluxof all trapped, galactic cosmic radiation andsecondary radiation as a function of time,charge, energy, and direction. The CPDS

is a semiconductor energy loss spec-trometer. Incident radiation penetrates astack of semiconductor wafers followed bya Cerenkov detector. The energy lost toeach wafer is quantified and summed.Total particle energy and particle charge arededuced from these measurements. A

60-degree solid angle cone of acceptance ofincoming radiation is achieved using coin-cidence circuitry and instrument geometry.Position sensitive detectors will provide

angular resolution of the incident radiationpath within the detector. Two versions ofthe CPDS are planned: one for intemal useand one for external use. The external

instrument will consist of a single, fixedlocation unit with three independentdetectors oriented in specific directions.The internal unit can be relocated within the

Space Station modules or nodes. Theexternal CPDS will collect data for accurate

mapping of the space radiation environ-ment, building improved crew dose pro-jection tools and serving as a control forrelocatable internal instruments. The inter-

nal CPDS will define the types of radiationinside the Space Station for assessing thepenetration capability of the radiation. Aprototype of the internal instrument isshown in figure 14.

Figure 14. Charged particle directionalspectrometer (CPDS).

Passive Dosimetry - Improved passive dos-imetry, similar to that currently used on theShuttle, will be used on Space Station.These include personal passive dosimetryfor monitoring the absorbed dose to eachastronaut and passive area monitors, whichwill be located at specific locations through-out the Space Station to measure theabsorbed dose distribution.

11

Flight Experiment Support

Radiation flight experiments arefrequently flown as part of the ShuttleDetailed Supplementary Objectives (DSOs)Program. These experiments include flighttesting the radiation monitor prototype unitsas well as other monitoring devices.

Data from Space Station prototypespectrometers also have been used to assistinvestigations into Single Event Upsets(SEUs) in Shuttle computers. SEUs areevents resulting from charged particlesinteracting within semiconductor devicesthat generate enough charge to change thestate of flip-flop circuits, such as computermemory. These are known as "soft" upsetsand can be corrected by reloading programs

or data. "Hard" upsets result from physicaldamage to a circuit element from theincident radiation and are permanent.

Some medical procedures use radio-active tracers to track physiological changesas crewmembers adapt to the weightlessenvironment. These hazards are reviewed

by the Human Research Procedure andProtocol Committee. Crew doses are esti-

mated for the procedures and documentedin their health records.

Environmental Models

Current models of the trapped radiationenvironment (AP8, AE8) are over 20 yearsold. These are static models and do not

take into account the dynamics that canoccur within the trapped radiation belts.The Space Station platform will providecontinuous coverage over many years,enabling data to be collected that willimprove both the models and projections ofcrew doses. Data from the Space Stationactive monitors will be used to update andverify models of the trapped radiationenvironment as well as the cosmic radiation

environment. Improvements will beincorporated in the models to accommodatetransients as well as to correct for the

physical shift of the SAA. Data gatheredfrom prototype development flights havebeen used to provide a better understandingin the current structure of the LEO trappedradiation hazard.

Health Risk Assessment

The penetration of radiation within thebody is being studied. Human equivalentdensimetric phantoms are used to analyzethe dose at different depths within thebody. This information is correlated withtransport codes and human anatomicalcomputer models. These evaluations willlead to the ability of performing organ-specific dose estimates and of estimatingthe net increased risk of cancer for thecrew.

Fundamental research into the effects of

high-LET radiation also is performed atJSC. A variety of cell tissue cultures areirradiated with both on-site (gammaradiation, Co-60) and off-site (proton andheavy particle accelerators) sources. Thetransformation toward cancer within these

samples is monitored, and the correlationof observed biological effects with receiveddose is determined. These types of studiesare pivotal in the development of exposurestandards for astronauts. Research in this

arena may lead to the development of abiological dosimeter and/or counter-measures. Biological dosimeters will usebiological media directly to measurebiologically effective doses instead ofrelying on extrapolation from physicaldosimeter measurements.

Figure 15. Space Shuttle Dosimetry.

12

MISSION SUPPORT: CREWEXPOSURE MONITORING

Monitoring astronaut radiation exposureshas been a key requirement for spaceflightsince Project Mercury. Current SpaceShuttle operations require a variety of acti-vities. Preflight activities include projectingmission doses and reviewing of crew healthrecords. Dosimetry, shown in figure 15,for the crew and Orbiter are prepared andshipped to the Kennedy Spaceflight Centerfor integration into the Orbiter. Duringeach mission, continuous radiologicalsupport and space environment monitoringare provided from within the MissionControl Center. Postflight, crew dosimetryis retrieved and analyzed and crewexposure records are updated. This pro-cess is depicted in figure 16.

Space Station operations will followsimilar functional responsibilities. One ofthe most significant differences will be the

RADIATION HEALTH PROGRAM SUPPORT

FOR SPACE SHUTTLE MISSIONS

MISSION IDENTIFICATION

ALTITUDE, INCLINATION, CREW ASSIGNMENTDURATION

REVIEW CREWPROJECT MISSION DOSE EXPOSURE RECORDS

__E DICA L AN_SPACEFLIGHT

ASSESS HEALTH RISK

PREPARE DOSlMETRY

SHIP TO / I FLIGHT SURGEON

PRE-FUGHT REVIEWCAPE

1 ;SPACEFLIGHT FLIGHT SUPPORT FLIGHT SURGEON

MISSION CONTROL _ MI_ON CONTROL

RETRIEVEDOSIMETRY

POST-FLIGHT

-_......_OST-FLIGHT ANALYSISOF DOSIMETRY

1UPDATE MI88_ON UPDATE CREW

DOSE INFORMATION _ EXPOSURE RECORDS

HEALTH RISK ANALYSIS

REPORTS TO CREWS

FLIGHT SURGEON

RADIOLOGICAL HEALTH OFFICER

E SOLAR SYSTEM [ MEDICAL SCIENCES

XPLORATION DIVISION 1 , DIVISION ]

Figure 16. Space Shuttle support activities.

utility of real-time active radiationmonitoring on the Space Station. Thisfeature will provide significant improve-ment to operations by providing exposuremonitoring during missions.

SUPPORTING ORGANIZATIONS

The Spaceflight Radiation HealthProtection Program is administered fromwithin the Space and Life SciencesDirectorate. The general responsibilities ofthe respective divisions are summarizedbelow.

Medical Sciences Division

• Support the mission Flight Surgeons inadvising the Flight Director duringradiation contingencies.

• Maintain astronaut health records

including documentation of bothmission and medical radiation exposurehistories.

• Provide health risk analysis.• Support payload safety reviews for

Shuttle spaceflight experiments.• Establish radiation health requirements

for manned spaceflight.• Provide integration support to Space

Station; Develop radiological opera-tional support training programs.

• Conduct and administer fundamental

research into the biological effects ofspace radiation.

Solar System Exploration Division

• Provide real-time operational radiolog-ical support during manned missions.

• Provide operational dosimetry supportfor crews and manned vehicles.

• Provide preflight mission crew andequipment exposure projections.

• Improve and develop engineering toolsused for space radiation exposureanalysis.

• Develop advanced radiation monitoringequipment.

13

EXPLORATION OF MOON/MARS SUMMARY

As NASA plans for future missions tothe Moon or Mars, radiation exposureremains one of the limiting technologicalissues. Random solar proton events wereavoided during Apollo missions (Fig. 17)because of short mission duration. Future

missions will involve longer stays, makingsimple avoidance less practical. Lunarstays of up to 6 months and round tripduration of 3 years for Martian missions are

being considered. Accurate prediction ofsolar particle events still is not possibletoday. In addition, the biological effectfrom long-term exposure to high energygalactic cosmic radiation is not wellunderstood. Current exposure guidelineswill be exceeded unless adequate shieldingis provided.

Nuclear technology may be used toenhance these missions, adding to theradiation protection concerns. Nuclearpower plants on the lunar surface may berequired to overcome the energy storagechallenges resulting from 2-week durationlunar nights. Substantial savings in traveltime and/or in propellant launched into orbitfor Martian missions may necessitate theuse of nuclear thermal propulsion.Although nuclear sources increase radiationrisk to astronauts, overall mission

reliability, success, and safety may beincreased. JSC will be actively involved inthese issues to ensure crew health.

Figure 17. Apollo 17 astronaut on Moonwith Lunar Rover.

JSC is the focal point within NASA forcrew radiation health protection. Acomprehensive range of activities by JSCengineers and scientists, ranging frommeasuring biological effects to physicalenvironmental model development, is beingconducted to improve the quantitativeunderstanding of risks to astronauts due toradiation exposure during spaceflight.Special instrumentation provides the meansto quantify crew doses. Operationalsupport for current missions and projectedsupport for future exploration class mis-sions will continue to be the function of the

Spaceflight Radiation Health ProtectionProgram at JSC.

DEFINITIONS

Several terms are frequently used todescribe or quantify radiation exposure andare defined as follows:

RAD - The absorbed dose, RAD, is the

amount of energy absorbed fromradiation per mass of material (1 RAD =100 ergs/g). The SI unit for absorbeddose is the Gray (1 Gray (Gy) = 100Rad).

LET - Linear Energy Transfer quantifies theamount of energy deposited per unitlength of particle track. This factorincreases with the square of the chargeand is inversely proportional to theenergy of the radiation particle.

QUALITY FACTOR - Is a function of theparticle LET, which is determined bythe charge and energy of radiationparticles. This factor accounts for thedifferences in biological effectivenessof different particles and is used toconvert an absorbed dose into a dose

equivalent. Currently, values for thequality factor may range from 1 to 20.Quality factor values as high as 100may be deemed appropriate asadditional research continues.

14

REM - The biological equivalent dose,

REM (Roentgen equivalent man), is theabsorbed dose adjusted for biologicaleffectiveness of the particular type ofradiation. It is the product of theabsorbed dose and quality factor. The

SI unit for biological equivalent dose isSieverts (1 Sievert (Sv) = 100 Rem).

Rem = Rad X Quality Factor

Sievert = Gray X Quality Factor

eV - The kinetic energy of charged particlesis measured in units of electron Volts

(eV). Multiples of eV are frequentlyused:

keV = 1000 eV, MeV = 1,000,000 eV,and GeV = 1,000,000,000 eV.

BIBLIOGRAPHY

Benton, E.V., Editor, "Space Radiation,"Nuclear Tracks and Radiation

Measurements, Vol. 20, No. 1,

Pergamon Press, New York, January,1992.

Benton, E.V. and J.H. Adams Jr., Editors"Galactic Cosmic Radiation: Constraints

on Space Exploration," Nuclear Tracksand Radiation Measurements, Vol. 20,

No. 3, Pergamon Press, New York,July, 1992.

Johnson, A.S., G.D. Badhwar and C. Yang,"Space Station Radiation Dosimetry andHealth Risk Assessment," 23rd ICES,SAE 932212, July 1993.

McCormack, P.D., C.E. Swenberg and H.

BiJcker, editors, Terrestrial Space

Radiation and Its Biological Effects,NATO ASI Series, Series A: LifeSciences, Vol. 154, Plenum Press, NewYork, 1988.

Nachtwey, D.S. and T.C. Yang,Radiological Health Risks for

Exploratory Class Missions in Space,Acta Astronautica, 23: 227-231, 1991.

NASA Life Sciences Strategic Planning

Study Committee, "Exploring the LivingUniverse, A Strategy for Space LifeSciences," Final Report, F.C. Robbins,Chairperson, NASA, June 1988.

Robbins, D.E. and T.C. Yang, "Radiationand Radiobiology," Chapter 20, in Space

Physiology and Medicine, A.Nicogossian, C. Huntoon and S. Pooleds., third edition, Lea and Febiger, Inc.,Philadelphia, PA, 1993.

Wilson, J.W., L.W. Townsend, W.

Shimmerling, G. Khandelwal, F. Khan,J. Nealy, F. Cucinotta, L. Simonsen, J.Shinn and J. Norbury, "TransportMethods and Interactions for SpaceRadiations," NASA Reference

Publication 1257 (NASA-RP-1257),December 1991.

15

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4mTITLEANDSUBTITLESpaceflight Radiation Health Program at the Lyndon B. JohnsonSpace Center

6. AUTHOR(S)

A. Steve Johnson*, Gautam O. Badhwar, Michael J. Gollghtly,

Alva C. Hardy, Andrea Konradi, Tracy Chui-hsu Yang

7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES)Lyndon B. Johnson Space Center

Houston, TX 77058

9. SPONSORING/MONITORINGAGENCYNAME(S)ANDADDRESS(ES)National Aeronautics and Space AdministrationWashington, D.C. 20546-001

5, FUNDING NUMBERS

8. PERFORMING ORGANIZATIONREPORT NUMBER

S-742

10. SPONSORING / MONITORINGAGENCY REPORT NUMBER

NASA TM-104782

11SUPFLEMENTARYNOTES*KRUG Life Sciences, Inc.

Houston, Texas 77058

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Springfield, VA 22161 (703) 487-4600Subject Category: 52

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13 ABSTRACT (Maximum 200 words)

The Johnson Space Center leads the research and development activities that address thehealth effects of space radiation exposure to astronaut crews. Increased knowledge ofthe composition of the environment and of the biological effects of space radiation isrequired to assess health risks to astronaut crews. The activities at the Johnson SpaceCenter range from quantification of astronaut exposures to fundamental research into thebiological effects resulting from exposure to high energy parttcle radiation. TheSpaceflight Radiation Health Program seeks to balance the requlrements for operationalflexibility with the requirement to minimize crew radiation exposures. The componentsof the space radiation environment are characterized. Current and future radiationmonitoring instrumentation is described. Radiation health rlsk activities are describedfor current Shuttle operations and for research development program activities to shapefuture analysis of health risk.

14. SUBJECFTERMSExtraterrestrial Radiation, Radiation Dosage, Biological Effects,Radiation Measurement

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uncl assi fted

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