Planetary and Space Science 63–64 (2012) 123–132

Contents lists available at ScienceDirect

Planetary and Space Science

0032-06

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/pss

Overview of energetic particle hazards during prospective mannedmissions to Mars

Susan McKenna-Lawlor a,n, P. Gonc-alves b, A. Keating b, G. Reitz c, D. Matthia c

a Space Technology Ireland, Maynooth, Co. Kildare, Irelandb Laboratorio de Instrumentac- ~ao e Fısica Experimental de Partıculas (LIP), Av. Elias Garcia 14-1, 1000-149 Lisboa, Portugalc German Aerospace Center, Institute of Aerospace Medicine, Linder Hohe, 51147 Koln, Germany

a r t i c l e i n f o

Article history:

Received 22 January 2011

Received in revised form

21 June 2011

Accepted 26 June 2011Available online 8 July 2011

Keywords:

Mars

Galactic cosmic radiation

Solar energetic particles

Manned missions

33/$ - see front matter & 2011 Elsevier Ltd. A

016/j.pss.2011.06.017

esponding author. Tel.: þ353 1 6286788; fax

ail address: stil@nuim.ie (S. McKenna-Lawlor

a b s t r a c t

A scenario for an initial manned mission to Mars involves transits through the Van Allen Radiation

Belts, a 30 day ‘short surface stay’ and a 400 day Cruise Phase (to/from the planet). The contribution to

the total dose incurred through transiting the belts is relatively small and manageable. Estimates of the

particle radiation hazard incurred during a 30 day stay on the surface (using ESA’s Mars Energetic

Radiation Environment Models dMEREM and e MEREM) indicate that the dose is not expected to be

particularly challenging health-wise due to the shielding effect provided by the Martian atmosphere

and the body of the planet. This is in accord with estimations obtained using the Langley HZETRN code.

Estimates of GCR exposure in free space during the minimum phase of Solar Cycle 23 determined using

the CREME2009 model are in reasonable agreement with published results obtained using HZETRN

(which they exceed by about 10%). The Cruise Phase poses a significant radiation problem due to the

cumulative effects of isotropic Galactic Cosmic Radiation over 400 days. The occurrence during this

period of a large Solar Energetic Particle (SEP) event, especially if it has a hard energy spectrum, could

be catastrophic health wise to the crew. Such particle events are rare but they are not currently

predictable. An overview of mitigating strategies currently under development to meet the radiation

challenge is provided and it is shown that the health problem posed by energetic particle radiation is

presently unresolved.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Manned missions to Mars will combine a Cruise Phase to/fromthe planet (the duration of which will depend on the launchwindow selected) with a surface stay. Table 1 provides severalpreliminary scenarios proposed by NASA for manned Marsexploration.

In the present paper, an overview is provided of the particleradiation hazard expected to be incurred during a manned missionin consequence of passages through the Van Allen radiation belts,a surface stay of 30 days and a 400 day Cruise Phase. In Section 2the prime sources of particle radiation potentially encounteredduring such a mission are briefly described. In Sections 3 and 4estimates made by McKenna-Lawlor et al. (in press) of particleprecipitation and dose levels at three prospective Martian landingsites during a 30 day surface stay under solar maximum and solarminimum conditions (predicted using the European SpaceAgency’s MEREM models described in Gonc-alves et al., in press)are presented. These estimates are shown (despite inherentroughness in the comparisons) to be in reasonable agreement

ll rights reserved.

: þ353 1 6286470.

).

with complementary results obtained using the High-charge(Z) and Energy (HZE) Transport code (HZETRN) developed at NASALangley (Wilson et al., 1991).

In Section 5 the results of a simulation of the isotropic GalacticCosmic Ray radiation potentially encountered during the mini-mum phase of Solar Cycle 23 during a 400 day Cruise Phaseto/from Mars (determined for particle energies in the rangebetween 1 MeV/n and 100 GeV/n using the CREME2009 Model(Tylka et al., 1997)) are presented. These results are comparedwith complementary data obtained using HZETRN with whichthey are in reasonable agreement.

In Section 6 the effect of the occurrence during the cruisephase of a, hard spectrum, Solar Energetic Particle (SEP) event isdiscussed. In Section 7 the ongoing development of mitigatingstrategies to protect prospective crew members from the deleter-ious effects of energetic particle radiation are outlined. Section 8contains general conclusions.

2. Prime sources of the radiation hazard en route to/returningfrom Mars

The main natural sources of particle radiation encountered bypersonnel aboard a spacecraft en route to/residing at/returning

Table 1Early scenarios for manned Mars exploration.

Scenario Duration

(Days)

Deep space

(Days)

Surface stay

(Days)

Mars swingby 600 600 0

Mars short surface stay 430 400 30

Mars long surface stay 1000 400 600

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132124

from Mars are the Van Allen Belts, Galactic Cosmic Radiation(GCR) and Solar Energetic Particle (SEP) Events.

2.1. The Van Allen Belts

The Van Allen Belts, which are made up of particles trapped inthe Earth’s magnetic field, are defined by the intensity of particlescapable of penetrating a specific shield (� 1 g cm�2 of alumi-nium). The inner belt extends from about 400 km to an equatorialradial distance of �12,000 km. The outer belt extends fromapproximately 12,000–60,000 km. There is considerable overlapbetween the two principal belts and, due to solar related dynamicchanges in the interplanetary medium, the outer one is charac-terized by its complex, time-variable structure (Gussenhovenet al., 1996). The belts, which are crescent shaped, span onlyabout 401 of Earth latitude, 201 above and below the magneticequator (i.e. they do not extend as far as the poles). Theirmaximum cross section is above the magnetic equator and theintensity of the radiation is in each case greater at belt centre thanat its outer edges (Sawyer and Vette, 1976).

The principal sources of particles in the inner belt are electronsand protons formed during the (in-flight) radioactive decay ofcosmic ray albedo neutrons (generated in nuclear reactionsresulting from collisions between cosmic rays/energetic solarparticles and nuclei in the upper atmosphere). The outer beltconsists mainly of electrons injected from the magneto-tailfollowing geomagnetic storms and subsequently energized inwave-particle interactions.

Standard data bases providing particle fluxes in the belts areavailable in the models of the trapped radiation environment, Ap8for protons and Ae8 for electrons, maintained (Bilitza, 1992) atthe NSSDC (NOAA National Space Sciences Centre), These databases indicate, very generally speaking, that protons with ener-gies 410 MeV have a flux of about 105 cm�2 s�1 between about1.5 and 2.5 Earth radii (9500–16,000 km) while electrons withenergies 41 MeV have a flux of about 106 cm�2 s�1 between1 and 6 Earth radii (about 6300–38,000 km). It should, however,be pointed out, based on results obtained using the CRRESspacecraft, that the NSSDC radiation belt models can lead bothto gross overestimation and gross underestimation of the Earth’sparticle radiation environment, depending on the spacecraft orbitconsidered (Gussenhoven et al., 1996 and references therein).

In the course of the Apollo Moon Landings (Missions 11, 12,14–17), the astronauts were exposed to several radiation sources,including the Van Allen Belts and cosmic rays, as well as toneutrons and other subatomic particles created in high energycollisions between primary particles and spacecraft materials. Nomajor solar particle events occurred during these missions(Bailey, 1975).

By suitably selecting a trajectory that makes a high speedpassage through the less intense areas of the Van Allen belts(of the order of 1.5 h to take into account the gradual slowing ofthe spacecraft and using the conversion coefficients of Pelliccioni(2000) to convert the estimated composite fluxes to an averagedose rate for such a path), a value of about 20 mSv is obtained,assuming no spacecraft shielding. Since the trapped particles havemuch lower energies than galactic cosmic rays and solar energetic

particles, the presence of even a few millimetres of aluminiumshielding would result in a significant reduction in this, estimated,unshielded dose. NASA’s radiation plan for the Apollo lunarmissions required (Modisette et al., 1969) that the Van Allen Beltradiation dose be kept below 10 mSv for a transit to the Moon andthis was, thus, not a significant problem to achieve. In the case ofa manned mission to Mars the radiation encountered in traver-sing the Van Allen Belts would be of the same order of magnitudeand can thus be expected to make a relatively small contributionto the total radiation incurred (see below) during the overallmission.

2.2. Galactic Cosmic Radiation

Galactic Cosmic Rays (GCRs) constitute the nuclei of chemicalelements that were accelerated to very high energies outside thesolar system (generally within the Milky Way galaxy). Thisradiation constitutes an isotropic source of radiation during theCruise Phase to Mars. Although the energies of cosmic rayparticles can reach 1020 eV, most of the deleterious effects withregard to health produced by this radiation are associated withnuclei in the energy range from several hundred MeV/nucleon toa few GeV/nucleon. A conservative value of 10 MeV/nuc wasproposed by Armstrong et al. (in press) to define the thresholdenergy of particles potentially dangerous to humans in space.

GCR radiation at Mars varies as a function of the pertaininglevel of solar activity. At solar maximum enhanced numbers ofparticles and complex interplanetary magnetic fields in the nearMartian environment interact with incoming GCRs and, in thisprocess, remove lower energy particles from the incident radia-tion. In consequence the GCR component in the inner environ-ment of Mars has a higher average energy but a lower fluence atsolar maximum than is the case at solar minimum (Adams et al.,1981). The minimum phase of Solar Cycle 23 was unusuallyprolonged, thereby giving rise in 2009 to the highest GCR doserates recorded in the past twenty five years (Schwadron et al.,2010).

Fig. 1 (a) presents Galactic Cosmic Ray radiation differentialflux spectra (cm�2 s�1 sr�1 (MeV/nuc)�1) pertaining to the mini-mum of Solar Cycle 23 for selected elements (H, He, Li and Fe).Fig. 1 (b) shows corresponding spectra at solar maximum.The differences between these plots are attributed to the scatter-ing and deceleration of lower energy cosmic rays by the magneticfield embedded in the solar wind at the time of solar maximum.

Investigations by Mewaldt et al. (2005), based on measure-ments of Be-10 in polar ice cores and other data sets, indicate thatthe cosmic ray intensity was significantly higher at 1 AU in 1954,and yet higher prior to 1900, than has been the case since thespace era began in 1957 (the estimated radiation levels duringthese earlier periods are estimated to have been up to �1.7 timeshigher than was the case during recent solar minima). See alsoresults for the period 1890–1986 reported by McCracken et al.(2004). These findings are important, since they show that recentcosmic ray studies made at 1 AU did not take place during the‘worst–case’ cosmic ray intensities on record and it is conjectured,based on dendrochronological and other evidence (Bonev et al.,2004), that the high radiation levels formerly present could returnat any time.

2.3. Solar energetic particle (SEP) events

Significant solar flares not only generate intense electromag-netic radiation but they can also be associated with the accelera-tion of particles up to several hundred MeV/nucleon (in someinstances up to a few GeV/nucleon). Solar Energetic Particle (SEP)

Fig. 1. (a) Estimated GCR differential flux spectra for selected elements at the

minimum of Solar Cycle 23 and (b) corresponding spectra for the maximum of the

cycle. Both spectra were calculated using the ISO Standard 15390 model.

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132 125

events are mostly composed of protons with, in addition, about10% He and o1% heavier elements.

In a simplified division between types of SEP events (Impul-sive/Gradual) proposed by Cane et al. (1986), Impulsive Eventsare defined to be of relatively short duration (o1 day) and todisplay a high proton content. Although Impulsive Events aremuch more frequent than Gradual Events they do not, becauseof their limited fluences, constitute a serious radiation hazard andwill not be further considered here. Gradual SEP Events on theother hand are of longer duration (days), have higher fluxes,display a wider spread in longitude and are associated with fast,coronal mass ejection-driven shocks. These enhancements typi-cally show a rapid rise in proton fluxes on time scales of tens ofminutes to an hour, and they attain a maximum that is sometimesfollowed by a second, occasionally higher, intensity peak atenergies o50 MeV. This latter peak is recorded when the inter-planetary shock arrives at the observer (Reames, 1999). Theparticle profile then recorded depends on where a particularobserver is located relative to the moving shock source.

Fluxes of flare associated protons with energies 410 MeVpotentially pose a biological hazard at Mars to personnel engagedin extravehicular activity or directly exploring the Martian surface(protons with energies 430 MeV pose a threat to those located inthinly shielded habitats). Although SEPs are more likely to occuraround solar maximum (Feynman et al., 1993, 2002), such events

are at present unpredictable with regard to their times ofoccurrence and it cannot be assumed that SEPs will not occurunder solar minimum conditions.

Three effects were shown by McKenna-Lawlor et al. (in press)to play a significant role with regard to SEP radiation at theMartian surface namely: (a) shadowing by the planet, which cutsoff �50% of SEP primary particle flux; (b) atmospheric attenua-tion, which shields out SEP primaries characterized by relativelylow energies; and (c) backscattering of particles, mostly neutrons,due to the interaction of high energy particles with soil materials.

3. Particle precipitation at the Martian surface during shortsurface stays

The European Space Agency (ESA) and America’s NationalAeronautics and Space Administration organization (NASA) haveeach made a large investment in developing risk assessmentmodels for future manned missions. Influences attributable toseasonal variations in the Martian surface pressure/atmosphericdepth are taken into account in these models.

3.1. ESA’s MEREM models

The Mars Energetic Radiation Environment Models (dMEREMand eMEREM) were developed for ESA to permit assessments tobe made of energetic particle radiation in the close Martianenvironment. The outputs of both models are predictions ofparticle fluence, effective dose (ED) and ambient dose equivalent(ADE) within: the Martian atmosphere; in planetary orbit aboutMars; on the Martian Moons Phobos and Deimos and at specificlocations on the planetary surface and below ground (Gonc-alveset al., in press and contained references). For convenience, ED andADE are described in Appendix 1.

dMEREM calculates radiation environments through makingdetailed Monte Carlo simulations for individual sets of user condi-tions. Simulations are provided of the ionization and nuclearinteractions of all primary and secondary particles (ions, chargedkaons and muons) from 1 TeV/nuc to 100 keV/nuc and of neutronsdown to thermal energies. If required, photons can also be includedin the simulations. A penalty associated with making such detailedcalculations is the time required to produce a result. To overcomethis drawback, an engineering model (eMEREM) was also developedwhich, instead of a Monte Carlo simulation, uses pre-calculatedvalues of the FLUKA radiation transport code to obtain a rapid result.

Both models rely on a pre-processor to define the atmosphericcomposition and density profile at Mars. In addition, dMEREMuses the pre-processor to specify the surface composition anddensity of Mars. Although each model is capable of operating in‘stand alone’ mode, use of the Space Environment Information

System (SPENVIS) provides a means to export the results tovarious already established simulation tools and, thereby, gen-erate (among other things) estimates of effective dose andambient dose equivalents at locations on/near Mars.

3.2. The HZETRN code

Transport of the high energy nucleons and heavy ions makingup Galactic Cosmic Radiation through spacecraft shielding andhuman tissue can be estimated using the High-charge (Z) andEnergy (HZE) Transport code (HZETRN) developed at NASA Lang-ley. A galactic heavy ion transport code (GCRTRN) and a nucleontransport code (BRYNTRN) are both integrated in the HZETRNpackage (Wilson et al., 1991).

HZETRN acts to transport nuclear species with charge numbersbetween 0 and 28 and also their secondary products. The doses

Table 3Ionizing exposure limits at LEO for manned vehicles.

Exposure

Interval

Dose equivalent (mSv)

Blood forming organs (BFOs) Ocular lens Skin

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132126

estimated are based on the linear energy transfer (LET) ofparticles traversing media of interest. Dose equivalent is definedby introducing a quality factor, which relates the biological riskincurred due to ionizing radiation to the damage that would becaused by soft X-rays. The quality factor used is a function ofparticle type and energy (Guetersloh et al., in press).

30 Days 250 1000 1500

Annual 500 2000 3000

Career 1000–4000a 4000 5000

a Varies with gender and age at initial exposure.

Table 4Comparison of estimates of radiation incurred at the Martian surface during a 30

day stay calculated using the MEREM models (at 3 different landing sites) and

using the HZETRN code (Simonsen and Nealy, 1993).

30 Day stay on surface—solar

minimum GCR induced dose

Dose equivalent (mSv)

Skin BFO

Ionizing exposure limits at LEO for manned vehicles 1500 250

dMEREM—Viking 1/Phoenix/Mawrtha 21.0/20.3/14.4

eMEREM—Viking 1/Phoenix/Mawrtha 21.2/16.0/21.0

HZETRN (Simonsen and Nealy, 1993) 9.3–10.8 8.6–9.8

a Ambient dose equivalent.

4. Determination of the radiation dose potentially incurredduring a short surface (30 day) stay at the Martian surface

The MEREM models were utilized for the first time by McKenna-Lawlor et al. (in press) to determine the radiation doses potentiallyincurred due to Galactic Cosmic Radiation during a Short SurfaceStay (30 days) during Solar Cycle 23 at three candidate landing siteson Mars under solar minimum and solar maximum conditions. Therisk incurred due to Galactic Cosmic Radiation was exacerbated inthese calculations due to the assumed occurrence of a large SEPevent at solar minimum and at solar maximum, each underconditions of good magnetic connectivity.

The SEP events used in the calculations were not among those,relatively few, solar events that displayed a hard spectrum (seebelow) and the MEREM simulations showed that all the incidentprimaries concerned disappeared during their transit through theMartian atmosphere. Thus, only secondaries successfully reachedthe Martian surface. GCR primaries on the other hand, because oftheir harder spectra, were less degraded when transiting theplanetary atmosphere and could reach ground level.

On combining the GCR contribution over 30 days due toprotons at each of the selected (Viking/Phoenix/Mawrth) Martiansites with the individual proton contributions provided by theSEPs (each of which endured for 14 days), a total of 10.5 mSv wasestimated to have arrived at the Viking site during solar minimumand 6.02 mSv during solar maximum. These results are comparedin Table 2 with complementary estimates for a 30 day periodobtained using the HZETRN code (De Angelis et al., 2007).

The estimations indicate that, despite ongoing GCR incidenceand the occurrence in each phase of a typical SEP, the atmosphereof Mars would have provided sufficient shielding during theproposed 30 day stay to maintain annual dose levels for skinand blood forming organs (BFO) at the Martian surface well belowthe currently adopted (NASA) ionizing radiation exposure limitslisted in Table 3. Over a significantly longer stay, the cumulativeeffects of the GCR component would become a matter of concern.

Simonsen et al. (1990) and Simonsen and Nealy (1991)estimated using the HZETRN code that the total yearly dose atthe Martian surface (estimated to be the sum of the annual GCRdose and the dose due to a large flare in February, 1956), wouldhave provided a skin dose equivalent of between 21 and 24 cSvper year and a BFO dose equivalent of between 19 and 22 cSv peryear (the ranges quoted are due to the employment of a high-density as well as a low-density atmospheric model in makingthese estimations). The predictions indicate that the atmosphereof Mars would have provided sufficient shielding to maintain the

Table 2Comparison of estimates of radiation incurred at the Martian surface during 14

days calculated using the MEREM Models (McKenna-Lawlor et al., in press) and

using the HZETRN models (De Angelis et al., 2007).

Regolith soil type Dose equivalent (mSv)

Solar minimum Solar maximum

dMEREM—Viking 1 10.5 6.0

eMEREM—Viking 1 13.8 8.9

HZETRN (De Angelis et al., 2007) 11.2 4.5

annual skin and BFO dose levels below the current annual doselimits employed by NASA. In a further study Simonsen and Nealy(1993) investigated a sequence of large SEP events that occurredduring 1989 and it was demonstrated in the case of an event inOctober 1989 which produced the highest surface dose, while alsotaking into account the GCR background, that the radiation limitfor astronauts would, again, not have been exceeded.

In the above studies the crew on Mars were considered to haveas their only protection the carbon dioxide atmosphere of theplanet. Atmospheric attenuation was taken into account but notthe interaction of particles with the top layer of the Martiansoil—which leads to backscattering of neutrons and other sec-ondary particles (McKenna-Lawlor et al., in press). It can, there-fore, be expected that the ADE values predicted by the MEREMmodels, which take into account this backscattered population,would be significantly higher than those reported by Simonsenand Nealy (1993). This effect is apparent in the results displayedin Table 4, which presents the dose incurred due to GCR over a 30day stay on the Martian surface as computed using HZETRN,dMEREM and eMEREM for each prospective landing site undersolar minimum conditions. It is noted that differences seen in thetable between the dMEREM and eMEREM predictions are dis-cussed in McKenna-Lawlor et al. (in press).

Closer comparisons between the MEREM and the LangleyHZETRN model calculations will require matching of the Martianatmospheric models and estimates of the soil compositionemployed in each case, as well as specification of the pertainingbackground GCR level (having regard to the solar cycle phase/solar cycle number concerned). It can, however, already bededuced using the results obtained on employing both methodol-ogies, that the atmosphere of Mars should provide sufficientshielding from GCR and SEP radiation to maintain the annualskin and BFO dose levels below current ionizing radiation expo-sure limits during short surface (30 day) stays.

4.1. Model validation

It is necessary that validation of the predictions of the variousmodels be secured before any manned mission to Mars takes

Fig. 3. Quality Factor of GCR versus depth in a water sphere in interplanetary

space during solar minimum (exposure maximum) in 2009.

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132 127

place. It is already planned in this regard that a RadiationAssessment Detector (RAD), which will be carried aboard NASA’sMars Science Laboratory (scheduled for launch in 2011), will beused to obtain proton, heavy ion and neutron spectra, as well asassociated dose measurements, onboard a rover that will explorethe Martian surface (Hassler et al., 2009). Also, a Solar EnergeticParticle (SEP) experiment scheduled for launch on the MavenMission in 2013 will study from orbit ongoing interactionsbetween the Martian atmosphere and incoming SEPs.

Over a longer term it is planned that a biochip using antibodiesas a molecular recognition tool will be tested aboard the Inter-national Space Station with a view to later mounting it on a MarsRover to monitor the biological effects of cosmic rays (Le Postollecet al., 2009). In addition, tissue equivalent spherical and anthro-morphic phantoms are currently under development and test toverify methods for calculating radiation transmission throughvarious substances and tissues, with a view to later deploying anetwork of such phantoms at sites at different altitudes on theMartian surface which are characterized by different levels ofhydration.

5. GCR exposure in free space

5.1. GCR exposure in free space estimated using CREME2009

Monte Carlo simulations using GEANT4 (Agostinelli et al.,2003) were performed in order to estimate the radiation exposuredue to Galactic Cosmic Radiation in interplanetary space. Forthese calculations the irradiation of a water sphere (radius 25 cm)with atomic nuclei of elements from Hydrogen to Iron havingenergies between 1 MeV/n and 100 GeV/n was simulated. Theenergy spectrum of the primary particles was obtained using theCREME2009 model for the middle of the year 2009, when theintensity of the GCR flux peaked close to the end of Solar Cycle 23.The CREME2009 model is an update of the Cosmic Ray effects on

Micro-Electronics code (Adams et al., 1981; Tsao et al., 1984; Tylkaet al., 1997), which encompasses a widely used suite of programsfor evaluating (among other things) radiation effects aboardspacecraft. The prime deficiency of earlier versions of the codeis that it gave an inaccurate description of solar cycle variations ingalactic cosmic rays and needed, in particular, to be updated totake into account the conditions that pertained during 2009.

The GCR spectra provided by CREME2009 describe the condi-tions outside the magnetosphere of the Earth in interplanetaryspace. Fig. 2 presents resulting estimates of the dose and dose

Fig. 2. Total absorbed dose and total dose equivalent in a water sphere from GCR

in interplanetary space during solar minimum (exposure maximum) in 2009.

equivalent due to Galactic Cosmic Radiation in free space, as wellas the quality factor (Fig. 3) versus depth in a water sphere. Depthis expressed as the area density (g/cm2) of the shielding material,i.e. the density integrated over the thickness of the shielding. Forwater (density r¼1 g/cm3) the numerical value is equivalent tothe shielding thickness in cm. For materials with higher densitythe shielding thickness would be less, and vice versa. Theshielding effect is related to the mass of the traversed materialrather than to its geometrical thickness, which makes areadensity a preferred parameter. In this context the thickness ofan aluminium (r¼2.7 g/cm3) layer would be only one third thatof water with the same area density. The values shown in Fig. 2,which correspond to the dose in a thin spherical shell of thesphere, can be regarded as providing an estimate of the skin dosebehind a certain amount of water shielding (or alternatively of thedose behind a certain amount of water shielding plus the selfshielding provided by the human body).

While the absorbed dose stays nearly constant with depth (itincreases only slightly from 0.35 to 0.4 mGy/d), the dose equiva-lent is strongly reduced (from about 1.5–1.6 mSv/d at the, outer-most, unshielded region to around 0.9 mSv/d within the innerparts of the sphere). The slight increase in the absorbed dose isrelated to the production of secondary particles and to thefragmentation of primary nuclei. The drop in dose equivalentand the corresponding decrease in the quality factor are con-sequences of the absorption of lower energy nuclei (which arecharacterized by having high biological effectiveness and highquality factors).

From Fig. 2, the skin dose incurred at a shielding of 1 g/cm2 isestimated to be 1.3–1.4 mSv/d (47–51 cSv/y) with a quality factorof Q¼3.7–4. This result is in very reasonable agreement with thepredictions of Schwadron et al. (2010) made using the HZETRNcode, see Section 5.2, although they exceed the latter by about10%. Schwadron et al. (2010) estimated that the skin dose behindshielding of 1 g/cm2 aluminium was around 40–45 cSv/y in near-Earth interplanetary space in 2009 (top panel in Fig. 6).

The eye lens is located at the surface of the body and isconsequently exposed to an amount of radiation similar to thatincurred by the skin. Estimation of the dose absorbed by the bloodforming organs (BFO) is somewhat more complicated to deter-mine. If we assume that the average depth of the BFO in thehuman body is of the order of 10 g/cm2 (including 1 g/cm2

shielding) we can estimate from Fig. 2 that the dose equivalentof the BFO is around 0.8 mSv/d (33 cSv/y), which again is about

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132128

10% higher than the result obtained by Schwadron et al. (2010).Further, these values are identical to the expected skin dosebehind a shielding of about 10 g/cm2 water (estimated fromFig. 2) as opposed to the prediction by Schwadron et al. (2010)shown in Fig. 6, third panel.

It is obvious that the differences between the skin dose and thedose incurred by the internal organs are reduced when theshielding is increased (Figs. 2 and 6). The contributions ofdifferent GCR nuclei to the total dose and to the dose equivalentare illustrated in Figs. 4 and 5. The absorbed dose is stronglydominated by the contribution of hydrogen (E45–65%) andhelium (E20%) for shielding between 0 and 10 g/cm2 (seeFig. 4). Heavier ions (especially C, N, O, Ne, Mg, Si, and Fe) onlycontribute between 15% (at 10 g/cm2) and 35% (at 0 g/cm2). Thesituation is, however, significantly different regarding the doseequivalent, shown in Fig. 5. Due to their higher biological effec-tiveness and quality factor, the contribution made in this case byheavier nuclei is considerably higher, especially when the amount

Fig. 4. Contribution of different GCR nuclei (1oZo26) to the total dose behind

different amounts of shielding during solar minimum (exposure maximum)

in 2009.

Fig. 5. Contribution of different GCR nuclei (1oZo26) to the total dose

equivalent behind different amounts of shielding during solar minimum (exposure

maximum) in 2009.

of shielding is low. Next to hydrogen, which contributes between15% (at 0 g/cm2) and 40% (at 10 g/cm2), iron is the second mostimportant component of GCR (15–20%) with regard to the totaldose equivalent. The contribution of helium (10–15%) is slightlyhigher than, but comparable with, those of oxygen, magnesiumand silicon.

5.2. GCR exposure in free space estimated using HZETRN

A GCR model developed by Badhwar and O’Neill (1996), whichwas validated by Saganti et al. (2006) using in situ measurementsmade by the MARIE instrument aboard Mars Odyssey, was usedby Schwadron et al. (2010) to model how the species dependentGCR flux varied with time during Solar Cycle 23. The procedurefollowed was based on fitting measured differential energyspectra to stationary solutions of the Fokker-Planck equation,parameterized by an assumed form of the diffusion coefficientthat varied as a function of the interplanetary modulationpotential. Thereafter, the HZETRN Transport Code was applied,using a 1-D analytical formulation of the Boltzmann transportequation, to describe the transport of GCRs. Weak modulation ofgalactic cosmic radiation during the minimum of Solar Cycle 23was confirmed by this study. For further details see Schwadronet al. (2010).

Fig. 6 shows the estimated dose equivalent rates from GCR infree space (exposure over 4p steradians) behind different levels ofAl shielding from 1975 to 2010 for different particle species, asdetermined by Schwadron et al. (2010). These rates take intoaccount quality factors and relative biological effectiveness fac-tors for the different species making up the radiation. The plots

Fig. 6. Dose Equivalent Rates from GCRs in open space behind different levels

(1–100 g/cm2) of Al shielding, while taking into account quality factors and

relative biological effectiveness factors (following Schwadron et al., 2010).

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132 129

show for instance that, behind 0.3 g/cm2 of Al, dose rates ofbetween 25 and 35 cSv/y are expected.

Career dose limits for space personnel quoted in NASA Tech-nical Standard, 2007 are:

F

52 cSv for a 25 year old male;37 cSv for a 25 year old female;72 cSv for a 35 year old male55 cSv for a 35 year old female.

These figures indicate that the dose level estimated bySchwadron et al. (2010), and also shown in Section 5.1, to beincurred over a year in interplanetary space near Earth,approaches the dose limits defined for 25 year old personnel.

6. Cruise phase exposures to SEPs

Simonsen and Nealy (1991, 1993) pointed out that anomalouslyhard solar proton events are relatively rare (‘hard’ events are thosein which the particle fluence rates decrease slowly with increasingenergy). The largest events observed up to 1975 were recorded inassociation with flares in November, 1949; February, 1956; Novem-ber, 1960 and August, 1972. Fig. 7 shows integrated fluence spectrafor three of these events, following Wilson (1978). Thereafter, nounusually large flares occurred during Solar Cycle 21 (1975–1986)but, in the next cycle, six large events were recorded in the intervalAugust–December, 1989. Of these, the magnitude of an SEP inOctober 1989 was considered to be of the same order as thatrecorded in August 1972.

The Langley cosmic ray and nucleon transport codes were usedby Simonsen et al. (1990) to quantify the transport and attenua-tion profiles of three of these SEPs (in August, 1972; November,1960 and February, 1956) and it was estimated that theseenhancements would have delivered to unshielded crews BFOdoses of �411, 110 and 62 cSv, respectively. These values aresignificantly in excess of the 30-day limit of 25 cSv (Table 3). Atthe present time such anomalous events cannot be predicted orguarded against.

ig. 7. Proton fluences of three major solar events (following Wilson, 1978).

7. Mitigating strategies

Strategies to reduce the total mission dose predicted to beincurred during a mission to Mars include optimization of thelaunch window and the development of faster, more efficientmeans of propulsion than are presently available. The topic of theradiation dose pertaining inside spacecraft protected by differentkinds of shielding is also widely considered (e.g. Dudkin andPotapov, 1992; Gonc-alves et al., in press; Petrov et al., in press)and the investigation of possibilities to modify the design ofpresent-day spacecraft so as to ensure their greater reliability inharsh radiation environments, while also providing better protec-tion from radiation for the crew, are ongoing (e.g. Parker, 2005;Daly et al., 2011; Petrov et al., in press).

It is anticipated that, through optimum scheduling, significantamounts of crew time can be spent in an onboard sheltered areaduring the Cruise Phase. Also that, through implementing amission plan based on predicting the arrival at the spacecraft ofpotentially dangerous levels of particle radiation, the bestonboard response to such emergencies can be quickly made. Inthis regard, methodologies that involve (inter alia) the use of:artificial intelligence, locally-weighted learning or Bayesian infer-ence, to infer from early SEP development the course of futuredose build up are under investigation to improve space weatherpredictions.

At the present time, two categories of numerical simulationmodels are used in predicting solar shock arrival times at Earthnamely (a) models based on fundamental physics principals,such as the Hakamada-Akasofu-Fry version 2 (HAFv.2) model(Fry et al., 2003; McKenna-Lawlor et al., 2006), which is now usedby the U.S. Airforce for space weather predictions and (b) moredetailed models that employ fully three dimensional magnetohy-drodynamic theory for predictive purposes (see accounts of theSpace Weather Modelling Framework/SWMF in Groth et al., 2000;Gombosi et al., 2002 ; Toth et al., 2005).

HAFv.2 has already been extended (McKenna-Lawlor et al.,2005, 2008) to predict the arrival of solar disturbances at Marsand Venus. SWMF comprises a coupled sequence of MHD simula-tion models which starts at the Sun and extends to the thermo-sphere of the Earth. Work on developing presently missingmodels of the photosphere and chromosphere (which are impor-tant for the understanding of SEP events) is ongoing. Another Sun-to-Earth system of coupled models, also based on fully 3D MHDtheory (the Hybrid Heliospheric Modelling System/HHMS), wasdeveloped by Detman et al. (2006). Yet other modelling efforts arelisted in Wu et al. (2008). Validation of these various models andimprovements in their accuracy for operational purposes is workin progress. A set of recommendations for observational andmodelling support of Mars missions is contained in Armstronget al. (in press).

A study (named HUMEX) commissioned by ESA (summarizedin Horneck et al., 2006) to study the survivability and adaptationof humans during long duration missions to Mars, identifiedradiation health risks as a major critical item in implementingsuch a journey. For an account of radiation health impacts onhumans in space see Reitz (in press). It was recommended inHUMEX, in order to reduce the inherent health risk, that(a) careful planning of mission duration, timing and operations(b) surrounding crew habitats with necessary absorbing materialand (c) increasing the initial resistance of personnel to thedeleterious consequences of exposure, be implemented. Anaccount of the potential use by crew members of prophylacticradio-protective drugs and possibilities to control phytochemicalantioxidants in the body through dietary choices is considered inClancy et al. (2005). In HUMEX it was also pointed out thatresearch using robotic missions (including orbiters and landing

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132130

vehicles), is a necessary precursor to manned flight in order toincrease current knowledge of the Martian environment.

A report prepared for NASA to provide radiation protectionresearch recommendations for ‘missions beyond Low Earth Orbit’appears in NCRP Report 153 (2006). This document providesresearch recommendations that cover broad areas including: thespace radiation environment; radiation physics and transport; radia-tion dosimetry; radiation biology and radiation risk assessment.

Another way to address the health safety issue is the con-fidence level approach pioneered by Cucinotta et al. (2001a,2001b). In this method, cancer risk projections are taken to be aproduct of many biological and physical factors, each of which hasa differential rate of uncertainty due to current lack of data andknowledge. Initial Monte Carlo sampling from subjective errordistributions that represented the lack of knowledge in eachfactor was used to quantify the overall uncertainty in riskprojections in the particular case of a mission to Mars undertakenat solar minimum. The number of days in space where career doselimits less than the limiting 3% excess cancer mortality specifiedin NCRP Report 98 (1989) could be assured at a 95% confidencelevel was, thereby, found to only be of the order of 100 days (doselimits correspond to an average career duration of 10 years, withthe doses assumed to be spread evenly over a career). Approachesto reduce the uncertainties involved and mitigate the inherentrisks are presently ongoing within the scientific community.

Against this general background, ongoing studies that attemptto illuminate the basic physics underlying the presence ofhazardous particle sources in interplanetary space are carriedout at many centres. Among these are investigations of thevariations in GCR radiation over a range of time scales (e.g.Gushchina et al., 2009; Schwadron et al., 2010) as well as studiesof the characteristics and frequency of occurrence of solar flarescharacterized by hard energy spectra (e.g. Townsend et al., 2006;Smart et al., 2006) In parallel, space programs already in train/under development at ESA, NASA and other space agencies seek toprovide a predictive understanding of space weather conditions inthe interplanetary medium.

Although the above outlined interdisciplinary approach hasalready enabled a substantial improvement to be made in presentday understanding of the problems underlying the realization of,even a relatively short, manned excursion to Mars, the serioushealth risks inherent in implementing such a venture still pose anunresolved problem.

8. General conclusions

�

The European Space Agency (ESA) and America’s NationalAeronautics and Space Administration (NASA) have eachdeveloped risk assessment models for future, long durationmanned missions. ESA’s Mars Energetic Radiation Environ-ment Models (dMEREM and eMEREM) allow, inter alia, anassessment to be made of the energetic particle radiationenvironment potentially present at, and close to, Mars underdifferent interplanetary conditions. The transport of highenergy nucleons and heavy ions through spacecraft shieldingand human tissue can be estimated using the High-charge(Z) and Energy (HZE) Transport code (HZETRN) developed atNASA Langley. � Results obtained by MEREM and HZETRN with regard to

particle doses incurred at the Martian surface are shown tobe in reasonable agreement given the inherent differencesinvolved.

� For (30 day) surface stays, the dose predicted to be incurred on

Mars, even if a hard SEP event is assumed to take place, isindicated by the models not to exceed pertaining guideline

annual limits. The longer is a surface stay however, the greaterwill be the cumulative risk due to background galactic cosmicradiation.

� Surface measurements at Mars combined with contempora-

neous orbital measurements of the incident flux are nowrequired to validate the model predictions before mannedmissions can be launched. Since the surface flux is anticipatedto vary with subsurface composition and with altitude, thesesurface measurements should be made using a range of sites atdifferent altitudes, utilizing locations characterized by differ-ent hydration levels.

� GCR exposure in free space during the minimum of Solar Cycle

23 was estimated using the CREME2009 model together withthe GEANT4 Monte Carlo code and compared with publishedresults obtained using the HZETRN code. These results arein reasonable agreement although those obtained usingCREME2009 exceed those obtained using HZETRN by about 10%.

� The dose incurred during a 400 day Cruise Phase due to GCR is

estimated to be hazardous and the models indicate that careerlimit values currently adopted by NASA for space personnelare approached in this regard.

� The hazard due to background isotropic GCR radiation during

the Cruise Phase would be further increased if levels recordedprior to 1957 return.

� It cannot be excluded, whether a manned mission takes place

under solar minimum or solar maximum conditions, that aparticle associated flare characterized by a hard spectrummight take place during the transit to/from Mars. Such solarevents tend to occur at, or just after, solar maximum but arepresently unpredictable.

� The occurrence during the mission Cruise Phase of a major SEP

event characterized by a hard spectrum could deliver a lethaldose of radiation.

� Many mitigating strategies for crew protection are currently

under consideration but the health problem posed by ener-getic particle radiation is as yet unresolved.

Appendix A

A.1 Radiological quantities

The risks associated with radiation exposure cannot be mea-sured directly. What is measured instead is the absorbed dose Din a given tissue. The unit of dose is the Grey (1 Gy¼1 J ofradiation energy absorbed per kilogram of tissue). The absorbeddose is related to the energy deposited by energetic particles in amaterial, predominantly due to ionization energy loss. A measureof the energy transferred to a material as an ionizing particletravels through it is commonly referred to as Linear Energy

Transfer (LET).

A.2 The equivalent dose

The practical measure of radiation exposure is the equivalentdose (H), which attempts to account for the varying biologicaleffects of different types of radiation by appropriately assigning tothem a radiation weighting factor wR. The equivalent dose, HT, inan individual tissue or organ T, is given by

HT ¼X

R

wRDT ,R

where DT,R is the average absorbed dose from radiation R in thetissue or organ T concerned and wR is the radiation weightingfactor of radiation R, which varies according to particle type andenergy. The Equivalent Dose is measured in Sievert (Sv).

S. McKenna-Lawlor et al. / Planetary and Space Science 63–64 (2012) 123–132 131

A.3 The effective dose

The effective dose (ED) is the sum of the equivalent doses in alltissues and organs of the body, weighted by an organ/tissueweighting factor such that

ED¼X

T

wT HT ,

where wT is the weighting factor of tissue T and HT is theequivalent dose for that tissue. Both the radiation weightingfactors and the tissue weighting factors are available in ICRP(1996).

A.4 Ambient dose equivalent

In the area of radiological protection, two types of quantitiesare utilized: protection quantities and operational quantities.Protection quantities are recommended by the International

Commission on Radiological Protection (ICRP) and include organ/tissue equivalent dose (HT) and effective dose (ED). Conversioncoefficients for use in radiological protection against externalradiation are discussed in ICRP-74 (ICRP, 1996). Operationalquantities are defined by the International Commission on Radia-

tion Units and Measurement (ICRU) and aim at providing estimatesof protection quantities. For measurement purposes the opera-tional quantities, which are defined in terms of the quality factor(Q) are: Ambient Dose Equivalent; Directional Dose Equivalentand Personal Dose Equivalent. Operational quantities are morestable than protection quantities because they are independent oftissue weighting factors.

For strongly penetrating radiation the appropriate operationalquantity for region monitoring is the Ambient Dose Equivalent. Inthis connection ICRU Report 39 (1985) defined the ICRU Spherewhich is used as a reference phantom when defining doseequivalent quantities (the ICRU sphere is a 30 cm diameter spheremade of tissue equivalent material: density 1 g/cm3; mass com-position 76.2% oxygen; 11.1% carbon; 10.1% hydrogen and 2.6%nitrogen.) The Ambient Dose Equivalent at a location of interest ina radiation field is defined to be the dose equivalent which wouldbe generated in an oriented and expanded radiation field at adepth of 10 mm on the radius of an ICRU Sphere, oriented so as tobe opposite to the direction of the incident radiation. This ismeasured in Sievert (Sv).

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