8/13/2019 Space Radiation Dose Estimates on the Surface of Mars

1/2

VOL.27, NO. 4 J.SPACECRAFT JULY-AUGUST 1990

Space Radiation DoseEstimates on the Surface of MarsLisa C. Simonsen,* John E.Nealy,t

LawrenceW .Tdwnsend,t and John W*WilsontNASA Langley ResearchCenter,Hampton, Virginia 23665

AbstractAFUTURE goal of the U.S. space program is a com-mitment to the manned exploration and habitation ofMars.1 An important consideration of such missions is theexposureof crewmembersto the damagingeffects ofionizingradiation from high-energy galactic cosmic ray fluxes an dsolar proto n flares. The crewwillencounter th emost harmfulradiation environment in transit to Mars from which theymust be adequately protected. However, once on theplanet'ssurface, the Martian environm ent should provide a significantamount of protection from free-space radiative fluxes.In current Mars scenario descriptions, the crew flight timeto Mars is estimated to be any where from 7 months to over ayear each wa y, with stay timeson thesurface ranging from 20days to 2 years.1 To maintain dose levels below establishedastronaut limits, dose estimates need to be d etermined for theentire mission length. With extended crew durations on thesurfacean ticipated, the characterization of the Mars radiationenvironment is important in assessing all radiation protectionrequirements. This synopsis2 focuses on theprobable dosesincurred bysurface inhabitants from th e transport of galacticcosmic raysand solar protons through th e Mars atmosphere.

ContentsDose estimates are predicted using the galactic cosmic ray(OCR) flux-energy distribution for the minim um of the solaractivitycycleor thetimeof maximumOCR flux asprescribedby the Naval Research Laboratory CREME model.3 It isbelieved that these GCR flux intensities do not varysignificantly w ithin the area of the solar system occupied byth e terrestrial planets; thus, this model is applied directly toMars. For the solar flares, the fluence (time-integrated)spectra at 1A.U.are used for the three largest flares observedin th e last half-century, which include th e events of August1972, November 1960, and February 1956.4In the vicinity of

Mars (approximately 1.5A.U.), the fluence from these flaresisexpected to beless; however, there isstill much discussionabout the dependence of th e flare's radial dispersion withdistance. There fore, for the flare calculations inthis analysis,th e free-space fluence-energy spectra at 1 A.U . ha ve beenconservatively applied to Mars.The Mars atmosphere provides protection from galacticcosmic rays and solar flares with the amount of protectiondepending on the composition an d structure of the

ReceivedNov. 15,1989; synopticreceivedJan.15,1990.Full paperavailable from National Technical Information Service, Springfield,VA 22151, by title, at the standard price (available on request).Copyright 1990by the American Institute of Aeronautics andAstronautics,Inc.No copyright isassertedin the UnitedStatesunderTitle 17,U.S.Code.TheU.S.Governmenthas aroyalty-free licenseto exercise all rights under thecopyright claimed herein for Govern-mentalpurposes.Allotherrightsare reserved by thecopyrightowner.*AerospaceEngineer, Space Systems Division.fSenior Research Scientists,SpaceSystems Division.

atmosphere and the crew member 's altitude. In this analysis,the composition of the atmosphere is assumed to be 100%carbon dioxide. The Committee on Space Research hasdeveloped warm-high and cool-low density models of theatmosphere structure.5 The low-density model and thehigh-density model assume surface pressures of 5.9 and 7.8rrib, respectively.The amount of protection provided by theatmosphere, in the vertical direction, at various altitudes isshown in Table 1. Dose predictions at altitudes up to 12 kmare included in the analysis because of the great deal oftopographical relief present on the Mars surface.The NASALangley Research Center nucleon and heavy-iontransport computer codes ar eused to predict thepropagationand interactions of the free-space nucleons and heavy ionsthrough the Mars atm osphere. For large solar flare radiation,the Baryon Transport code, B R YNTR N,6 is used. For thegalactic cosmic rays, an existing heavy-ion code is integratedwith the BR YN TR N code to include the transport of high-ehergyheavy ionsup to atomic number28 .7'8Both codes solvethe fundamental Boltzmann transport equation in the one-dimensional,or straightahead, approximation form:

E )dEfk > j J E

where the quantity to be evaluated (x , E ) is the f lux;ofparticlesof typej having energyEat spatial location x. Thesolution methodology of this integro-differential equationmay be described as a combined analytical-numerical tech-nique with the accuracy of this numerical method determinedto be within 1% of the exact benchmark solutions.9T hedatarequired for solution consist of the stopping powerS,in vari-ous media, the macroscopic total nuclear cross sections /*/,and the differential nuclear cross sections ojk. The presentcode formulation is considered an interim version becausesome features of the transport interaction phenomena have yetto be incorporated; how ever, it is believed that the presentversion does provide a reasonable estimate of cosmic raypropagation fluxes and the corresponding dose predictions.The absorbed dose due to energy deposition at a givenlocation by allparticlescan berelatedto the biological systemdamage by introducing the quality factor Q, as specified bythe International Commission on Radiological Protection.10Thus, th edose-equivalent (rem)values//usedt o specify radi-

Table 1 M ars atmospheric protection in the verticaldirectionAltitude,km

04812

Low-density model,g CO2/cm 2161175

High-densitymodel,gCO2/c m 22216118

353

8/13/2019 Space Radiation Dose Estimates on the Surface of Mars

2/2

354 SIMONSEN,NEALY,TOWNSEND AND WILSON J . SPACECRAFT

Table 2 Integrated skin dose-equivalents r em ) for the ars a t m osp h e r emodels_________

Alti tude, kmIntegrated

Galacticcosmic raySolar flareevent 8/72Solar flareevent 11/60Solar flareevent 2/56

dose, remhigh-densitylow-densityhigh-densitylow-densityhigh-densitylow-densityhigh-densitylow-density

011.313.23.99.06.49.79.2

11.0

413.415.99.521.9

10.015.111.113.4

815.818.921.146.214.813.313.316.2

1218.622.442.882.621.115.915.919.1

Ta ble 3 Integrated BFO dose-equivalents r em ) for theM a r sa t m osp h e r e mode lsAltitude, km

IntegratedGalacticcosmic ra ySolar flareevent8/72Solar flareevent 11/60Solar flareevent2/56

dose, remhigh-densitylow-densityhigh-densitylow-densityhigh-densitylow-densityhigh-densitylow-density

010.511.92.24.65.07. 38.59.9

412.013.84.89.97.510.8

10.011.8

813.715.89.518.5

10.614.811.713.6

1215.618.017.430.314.419.113.415.3

ation exposure limits ar e computed as the following:H(x) = Qj(E)Sj(E)*j(x, E) dE o

Maximumdose-equivalent limits for U.S. astronautshavebeen recommended by the National Council on RadiationProtectiona ndMeasurement (NCRP).11 Forhigh-energyradi-ation fromgalactic cosmicraysand largesolar flareions,thedose delivered to the vital organs, referred to as the blood-forming-organ (BFO)dose,is themostimportantwithregardtolatentcarcinogeniceffects. Thisisusuallycomputed as thedose incurred at a 5 cmdepth in human tissue, simulated inthis analysis by 5 cm ofwater.T he 30-dayskinand BFO limitsforU.S. astronautsare presently 150and2 5rem, respectively.The annuallimitsfo r skinand BFO doses are 300 and 50rem,respectively. Thetotalcareerlimit for the skinis 600rem,an dthe total career BFO dose limit varies between 100 and 400rem, depending on age and gender.The surfacedosesa t various altitudesin theatmospherea redeterminedfrom th ecomputed propagationdatafor theOCRand solar flareprotons.The dosimetricvaluesa t agiventargetpoint are computed fo r carbon dioxide absorber amountsalong slant paths in the atmosphere. In these calculations, aspherically concentric atmosphere is assumed. For a giventarget point , the absorber amounts and the correspondingdosimetricquantities areevaluatedfor zenithangles between0and 90 deg in 5 degincrements.T hetotaldosesarethenfoundby numericalintegration with respect to the solid angle at thetarget point.Integrated dose-equivalent calculationsweremade fo rboththe high-density an d low-density atmosphere models at alti-tudes of 0, 4, 8, and 12 km. The correspondingskin and BFOdoseestimatesa reshownin Tables 2 and 3. AtotalyearlyskinaridBFO dose may beconservativelyestimated as the sum ofthe annual OCR doseand the dose due to onelarge flare. Atan altitude of 0 km,such an estimated skindose-equivalent is21-24rern/y r and an estimated BFO dose-equivalent is19-22rem/yr .At analtitudeof 12 km, anestimatedskindose-equiv-alent is61-105rem /y r and an estimated BFO dose-equivalentis33-48rem/yr. Thesedose predictionsimplythat theatmo-sphere of Mars m ay provide shielding sufficient to maintainthe annualskinand BFO doselevelsbelowthe current 300 and50rem/yr U.S.astronautlimits.The 30-day limits areimpor-tant when considering th e doses incurred from a solar flareevent. The maximum estimated skin dose-equivalent incurredfrom a flare event is 11 rem at the surface and 83 rem at analtitude of 12 km. Thesevalues are wellbelow thecurrent 150rem astronaut 30-day limit. The maximum estimated BFOdose-equivalent incurred from a flare event is 10 rem at thesurface and 30 rem at an altitude of 12 km. The 30-day BFOdose-equivalent isexceeded at an altitudeof 12 km. However,as seen inTable3,theAugust 1972flareis rapidlyattenuatedby matter , and a fewg/cm2 of additional shielding shouldreduce the anticipated dose below this limit. These dose pre-dictionsimplythat theatmosphereofMarsm ayalsoprovide

significant shielding to helpmaintain30-daydoselevelswithinpermissible astronaut limits.The aforementioned calculated doses are forhumanson thesurface of Mars with their only protection being the carbondioxideatmosphe re. Additional protectionwillbe provided bythe crew's habitat structure, equipment, and consumables.Theatmospherei s predicted to provide sufficient shielding tomaintainincurreddosesbelow NCRP-establishedlimits if theastronauts are not at highaltitudes of ~12 km and if largedoses have no t already been received by the astronauts. Itmust be realized that Mars exploration crews ar e likely toincura substantialdosewhileintransittoM ars, perhapsfromother radiation sources (e.g., nuclear reactors), that will re -duce th e allowable dose that can be received while on thesurface. In this case, it may become necessary to provideadditional shielding, perhaps in the form of a solar flareshelteror byutilizinglocalresources such asMartian regolith.

Referencesaig, M. K., and Lovelace, U.M., StudyR equirements Docu-

ment , FY 1989 Studies, Doc. Z-2.1-002, NASA Office of Explo-ration, March, 1989.2Simonsen,L.C.,Nealy,J.E., Townsend,L. W .,andWilson,J.W ., Radiation Exposure for Manned Mars Surface Missions,N A S ATP-2979, March 1990.3Adams, J. H., Silberberg, R., and Tsao, C. H., Cosmic RayEffects onMicroelectronics,PartI:TheNear-Earth ParticleEnviron-ment, Naval Research Lab., Washington, DC, NRL Memo. Rep.4506-PartI, Aug. 1981.4Wilson,J.W., Environmental Geophysicsand SPS Shielding,Workshop on theRadiationE nvironment of theSatellitePowerSys-tem,edited by W. Schimmerling and S. B .Curtis,Univ. of California,Lawrence BerkeleyLab., Berkeley, CA ,Sept. 1978, pp .33-116.5Smith,R .E.,andWest,G .S., Spacean dPlanetaryEnvironmentCriteriaGuidelines for Use inSpaceVehicle Development, 1982 Revi-sion, (Vol. 1) NASA TM-82478, 1983.6Wilson, J.W., Townsend, L. W., Nealy, J.E., Chun, S. Y.,Hong,B.S.,Buck,W. W.,Lamkiri,S.L.,Ganapol, B. D.,Khan,F.,an d Cucinotta, F. A., BRYNTRN: A BaryonTransport Model,NASATP-2887, March 1989.7Wilson, J. W., and Badavi,F. F., Methods of Galactic HeavyIon Transport, Radiat ion Research, Vol. 108, No. 3, 1986, pp.231-237.8Wilson, J. W ., Townsend, L. W., and Badavi ,F. F., GalacticHZE Propagation Through th eEarth's Atmosphere, Radiat ion R e-search, Vol. 109, No. 2, 1987, pp. 173-183.9Wilson, J. W., Townsend, L. W., A Benchmark for GalacticCosmicR ayTransportCodes, Radiat ionResearch, Vol. 114,No. 2,1988,pp. 201-207.10 Recommendations of the International Commission on Radio-logical Protection, ICRP Publication 26, Pergamon, N ew York ,Jan. 1977.HFry, R .J., an d Nachtwey, D .S., Radiation Protection Guide-lines fo r Space Missions, Health Physics, Vol. 55, No. 2, 1988,pp. 159-164.

Paul F. MizeraAssociate Editor