Health threat from cosmic radiation during a manned mission to Mars
Alexandra D Bloshenko, Jasmin M. Robinson, Rafael A. Colon, Luis A. Anchordoqui
HHealth threat from cosmic radiation during mannedmissions to Mars
Alexandra D Bloshenko * Department of Physics & Astronomy, Lehman College, CUNY, NY 10468, USA
Jasmin M. Robinson
Department of Physics & Astronomy, Lehman College, CUNY, NY 10468, USA
Rafael A. Colon
Weill Cornell Medicine, Cornell University, NY 10065, USA
Luis A. Anchordoqui
Department of Physics & Astronomy, Lehman College, CUNY, NY 10468, USA
Cosmic radiation is a critical factor for astronauts’ safety in the context of evaluatingthe prospect of future space exploration. The Radiation Assessment Detector (RAD) onboard the Curiosity Rover launched by the Mars Scientific Laboratory mission collectedvaluable data to model the energetic particle radiation environment inside a spacecraftduring travel from Earth to Mars, and is currently doing the same on the surface ofMars itself. The Martian Radiation Experiment (MARIE) on board the Mars Odysseysatellite provides estimates of the absorbed radiation dose in the Martian orbit, whichare predicted to be similar to the radiation dose on Mars’ surface. In combination, thesedata provide a reliable assessment of the radiation hazards for a manned mission to Mars.Using data from RAD and MARIE we reexamine the risks for a crew on a manned flightto Mars and discuss recent developments in space exploration. * Speaker. © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ a r X i v : . [ phy s i c s . pop - ph ] D ec ealth threat from cosmic radiation during a manned mission to Mars Alexandra D Bloshenko
1. Introduction
Space – the final frontier. It has almost become widely accepted that it is our destiny toexplore strange new worlds, to seek out new life and new civilizations, to boldly go whereno one has gone before. However, humanity will have to find new ways to adapt to thecold harsh environments of outer space if it is to ever become a space-faring civilization.One of the greatest threats to long term human survival during spaceflight is theexposure to high energy radiation. There are two primary types of radiation that humansare exposed to in space: solar flares and Galactic cosmic rays (GCRs) [1]. Solar flaresare bursts of low-energy protons and are relatively easy to shield from. GCRs, on theother hand, are extrasolar, high-energy protons and atomic nuclei that damage DNA.Unlike solar flares, they are continuous and cannot be adequately sheltered from. Theycan induce double stranded breaks in DNA via linear tracks of deposited energy. TheGCR flux fills the interplanetary space and is made up of about 85% hydrogen nuclei, 14%helium nuclei, and around 1% high-energy and highly charged ( Z >
2) nuclei referred toas HZE particles [2]. Though there are fewer HZE particles than there are hydrogen andhelium nuclei, they possess significantly higher ionizing power, greater penetration power,and a greater potential for the radiation-induced damage to DNA. Therefore, determiningthe absorbed radiation dosage of GCRs that an astronaut would receive during a mannedmission to Mars is necessary prior to any such mission. In this paper we reexamine theradiation sources and risks for a crew on a manned flight to Mars.One important factor of our study is the duration of the trip to Mars. There areshort duration (roughly 600 days with 30-day Mars stay time) or long stay time (about550 days with 180-day transfer to Mars and 180-day inbound transfer) missions [3]. Theusual benchmark here is the so-called Hohmann transfer, which is an elliptical orbit usedto transfer between the two circular planet orbits using the lowest possible amount ofpropellant [4]. The launch from Earth for a successful Hohmann transfer must occurwhen Earth is at perihelion and the landing takes place when Mars is at aphelion. Whenlaunched within the proper window, a spacecraft will reach Mars’ orbit just as the planetmoves to the same point. Astronauts would travel from Earth to Mars, wait until the nextsynchronized alignment between the planets (about 460 days), and start the return trip toEarth. Kepler’s 3rd law,(orbital period / yr) = (Hohmann orbit semi-major axis / AU) , (1.1)provides a way to estimate the time-scale for the trip to Mars. Since the mean Earth-Sundistance is D (cid:12)−⊕ = D (cid:12)− ♀ (cid:39) .
542 AU, the Hohmannorbit semi-major axis = ( D (cid:12)−⊕ + D (cid:12)− ♀ ) / (cid:39) .
262 AU. Therefore, the interplanetary travelwill take 259 days each way, yielding a total mission time of 978 days. We will use thistime-scale to estimate the radiation hazards for a manned mission to Mars.Before proceeding, we pause to specify and establish the relationship between the fourdi ff erent units used to measure the amount of radiation absorbed by an object or person,known as the “absorbed dose.” Such an absorbed dosed reflects the amount of energy thatradioactive sources deposit in materials through which they pass. The radiation absorbed1 ealth threat from cosmic radiation during a manned mission to Mars Alexandra D Bloshenko dose (rad) is the amount of energy (from any type of ionizing radiation) deposited in anymedium (e.g., water, tissue, air). An absorbed dose of 1 rad means that 1 gram of materialabsorbed 100 erg of energy as a result of exposure to radiation. The related internationalsystem unit is the gray (Gy), where 1 Gy is equivalent to 100 rad. Another relevant unit isthe Sievert (Sv), which is an ionizing radiation dose that measures the amount of energyabsorbed by a human body per unit mass (J / kg). This biological unit relates to the previousunits according to: 1 Sv = =
100 rad. Finally, the fourth important unit to define isthe Roentgen equivalent man (rem), which represents the dosage in rads that will causethe same amount of biological injury as one rad of X-rays or γ -rays: 1 Sv =
100 rem.
2. Radiation hazards on space missions
Figure 1:
Longitudinal development of a 100 GeVproton shower on lead as a function of depth X . High-energy ( (cid:38)
GeV) protons andnuclei penetrating the spacecraft will besubjected to di ff erent atomic and nuclearprocesses. These relativistic particleswill su ff er inelastic collisions that pro-duce, on average, a number of fast sec-ondary hadrons. Some of these hadrons(protons, neutrons, charged pions) willhave further nuclear collisions, resultingin a hadronic cascade. Neutral pions,however, will decay almost instantly andtheir decay products ( γ -pairs) can initiateelectromagnetic showers, which are sus-tained by: (i) electron-positron pair pro-duction by photons; (ii) Bremsstrahlunglosses of electrons and positrons. Asthe particle energy decreases, other pro-cesses become dominant (Compton scat-tering of photons, photoelectric e ff ect,ionization losses of charged particles).Some of the charged pions and otherhadrons will also decay and producemuons. Besides the production of fasthadrons, nuclear collisions also generatelower-energy (MeV) neutrons, protons, light ions (alpha particles) and gamma rays, whichare emitted during the de-excitation of target nuclei. The protons and light ions will mainlyrange out because of ionizing losses. The neutrons, however, can be very penetrating andfully capable of depositing a damaging dose deep inside tissue. As an illustration of allthese processes, in Fig. 1 we show the longitudinal development of a simulated 100 GeVproton shower on lead, using the program FLUKA [5].2 ealth threat from cosmic radiation during a manned mission to Mars Alexandra D Bloshenko
The overwhelming health risk comes from GCRs. Those particles would strike thespacecraft to initiate hadronic cascades. The thicker the material they pass through, themore secondary particles there will be in the cascade. In condensed matter (liquid or solid)the shower of a 1 GeV proton continues to grow until it reaches about 200 g / cm , which isthe equivalent of about 2 meters of water. After this point the shower begins to attenuate.The attenuation is exponential, with a 1 / e attenuation length of about 200 g / cm . Theattenuation length in the Earth’s atmosphere is somewhat smaller, roughly 140 g / cm . Ina gas, like in the Earth’s atmosphere, the dependency of the average depth of the peakparticle intensity X max on energy can be parametrized as (cid:104) X max (cid:105) = a + b log ( E / GeV), where E is the energy of the primary particle. For simulated γ -ray showers, a =
98 g / cm and b =
83 g / cm , whereas for proton primaries a =
111 g / cm and b =
74 g / cm [6]. Whetherpions decay or initiate more interactions accounts for the attenuation di ff erence betweencondensed matter and gas.In the outer heliosphere, beyond about 100 AU, the slowing of solar wind is thoughtto form a large magnetic barrier that shields out (cid:38)
90% of the GCR radiation present ininterstellar space at energies below roughly 100 MeV [7]. Because this reduction is solarge, even a very small change in the shielding e ffi ciency can have a large impact on theradiation environment in the solar system. However, because these regions have neverbeen directly sampled or observed, there is great uncertainty about the physics of outerheliospheric shielding and its sensitivity to changes in the solar wind output and thelocal interstellar medium. A small fraction of GCRs penetrate into the heliosphere andpropagate toward the Sun and planets. These residual GCRs are modulated by the solarwind’s magnetic field in the inner heliosphere [8, 9]. The GCR intensity is at its lowestduring the peak of solar activity. This is because the enhanced solar activity sweeps awaythe low energy part of the GCR spectrum. The e ff ect is statistically significant and morethan compensates for solar generated cosmic rays during solar maximum.Focusing now directly on Mars, the red planet lost its magnetosphere 4 billion yearsago and thus the solar wind interacts directly with the martian ionosphere, lowering theatmospheric density by stripping away atoms from the outer layer. The atmosphere ofMars consists of about 95% carbon dioxide, 3% nitrogen, 1.6% argon and contains tracesof oxygen, water, and methane [10]. The scale height ( ≡ vertical distance over which thedensity and pressure fall by a factor of 1 / e ) of the Mars atmosphere is about 10.8 km, whichis higher than Earth’s (8.5 km); the surface gravity of Mars is only about 38% of Earth’s,an e ff ect o ff set by both the lower temperature and 50% higher average molecular weightof the atmosphere of Mars. The atmospheric pressure on the surface today ranges froma low of 0.030 kPa on Olympus Mons to over 1.155 kPa in Hellas Planitia, with a meanpressure at the surface level of 0.60 kPa (which is only 0.6% of that of the Earth 101.3 kPa).The highest atmospheric density on Mars is equal to the density found 35 km above theEarth’s surface. However, the atmosphere of Mars is just thick enough to produce plentyof dangerous secondaries (including neutrons). The radiation would be reduced to Earth-like intensities for facilities located at a depth of about 2 ,
000 g / cm beneath the martiansurface.In our analysis we use data from the Radiation Assessment Detector (RAD) on board3 ealth threat from cosmic radiation during a manned mission to Mars Alexandra D Bloshenko :
00 PMPIA03479.jpg 3,000 × :
57 PMPIA03480.jpg 3,000 × Figure 2:
Estimated radiation dose from GCRs on the martian surface, from MARIE orbital radia-tion data and MOLA laser altimetry. The lower the altitude, the lower the expected dose, becausethe atmosphere provides shielding. The global map of Mars on the left panel shows estimatesfor amounts of high-energy particle cosmic radiation reaching the surface of the planet. Colorsin the map refer to the estimated average number of times per year each cell nucleus in a humanthere would be hit by a high-energy cosmic ray particle. The range is generally from two hits, amoderate risk level, to eight hits, a high risk level. The global map of Mars on the right panelshows the estimated radiation dose from GCRs reaching the surface. The colors in the map refer tothe estimated annual dose equivalent in rems. The range varies from 10 rem / yr to 20 rem / yr. Thisfigure is courtesy of NASA / JPL / Johnson Space Center.
Curiosity Rover launched by the Mars Scientific Laboratory (MSL) mission [11–13] andthe Martian Radiation Environment Experiment (MARIE) on board the Mars Odysseysatellite [14]. The RAD data collection instrument gathered valuable data to model theenergetic particle radiation environment inside a spacecraft during travel from Earth toMars [11,12], and is currently doing the same on the surface of Mars itself [13]. The MARIEdata collection instrument provides estimates of the absorbed radiation dose in the martianorbit, which are predicted to be similar to the absorbed radiation dose on the surface ofMars [14]. Variations of the GCR absorbed radiation dose rate during the transfer betweenEarth and Mars range from 1.75 to 3.0 mSv / day [11, 12]. The GCR absorbed radiation doseon the surface of Mars measured by the RAD instrument is 0.21 mGy / day [13]. The GCRabsorbed radiation dose in the Mar’s atmosphere (collected by the MARIE instrument)is 0.25 mGy / day [14]. In Fig. 2 we show the estimated radiation dose from GCRs on themartian surface, using data from MARIE and the Mars Orbiter Laser Altimeter (MOLA)(which was one of five instruments on the Mars Global Surveyor spacecraft that operatedin Mars orbit from September 1997 to November 2006 [15]). The areas of Mars that areexpected to have the lowest levels of cosmic radiation are at the lowest points of elevation,where there is more atmosphere above them, and regions with a localized magnetic field.All in all, using these measurements we estimate that the absorbed radiation doseduring a round trip to Mars for a Hohmann transfer would be between 906 mSv and1 ,
554 mSv, whereas the absorbed radiation dose during the stay on Mars would be between97 mSv and 115 mSv. 4 ealth threat from cosmic radiation during a manned mission to Mars
Alexandra D Bloshenko
3. Astronaut career radiation dose limits and prospects for space exploration
Health risks from exposure to radiation on Earth in the form of X-ray or γ -ray radiationare well known. The primary concern is the development of cancer due to radiationinduced mutations in DNA. DNA strand and tissue degradation, immunological changes,cataracts, and damage to the central nervous system are all serious risks that must alsobe mitigated in preparation for a manned mission to Mars. These health risks are likelyamplified in space, where astronauts will face exposure to much higher energy radiationin the form of GCRs.To protect astronauts from the dangerous e ff ects of high-energy radiation, the NASAspace-program has determined a set of career radiation exposure limits for astronauts.Career radiation exposure limits vary by age and are lower for younger astronauts. Thelogic behind this is that younger astronauts have a longer life to live and therefore a greaterchance of developing subsequent health problems in their lifetime. Career radiationexposure limits also vary by sex and are lower for female astronauts. This is due toa number of factors, including a generally higher risk of radiation induced cancer infemales.The NCRP Report No. 98 (NCRP 1989) made recommendations pertaining to astro-nauts involved in space shuttle missions and the orbiting space station. The NCRP ReportNo. 132 (NCRP 2000) made similar recommendations for radiation protection of astro-nauts. However, the longer follow-up of the atomic bomb survivors and the reevaluationof survivor organ doses by the DS86 dosimetry system, used to recommend dose to riskconversion factors, led to substantial increases in risk estimates [16]. The predictions ofminor increased risk factors in the NCRP 2000 report are also supported by Mir spacestation microdosimetry measurements [17]. A few years after the publication of the NCRPReport No. 132, a reevaluation of atomic bomb survivor organ doses with longer follow-up of the cohort led to further increases in risk estimates [18]. In the last decade, NASAhas deviated from the ground-based approach in which radiation weighting factors areadopted to assign equal risk quality factors for all types of cancer. In addition, the NCRPrecommendation for use of sex and age at exposure-dependent limits was extended to con-sider a small specialized group of the population: a never-smoker model, which lowersradiation risk estimates by about 20% compared to estimates for a U.S. average popula-tion. The NSCR-2012 model yields another increment in risk estimates, with substantiallyweaker dependence on the age factor [19]. In Table 1 we show a comparison between thee ff ective doses in the recent NSCR-2012 model and the values from NCRP Reports No. 98and No. 132 [20]. (GCR solar modulation gives a ∼
5% e ff ect [20].)Setting side by side the most recent risk factors given in Table 1 and the absorbed doseestimates of Sec. 2 it is straightforward to see that (with current shielding technology) theHohmann transfer is not suitable for a round trip to Mars. However, a long stay missionof about 550 days, with 900-day mission duration [3] would be feasible. Indeed, this isalso consistent with the recent NASA twins multidimensional analysis, which allowedcomparison of the impact of the spaceflight environment on one twin’s human healthonboard the International Space Station for 340 days (his twin served as a genetically5 ealth threat from cosmic radiation during a manned mission to Mars Alexandra D Bloshenko matched ground control) to the simultaneous impact of the Earth environment [21]. Thedata suggest that human health can be sustained over a year-long spaceflight.
Table 1:
Comparison of e ff ective dose limits. The last row corresponds to never-smokers. Sex Female MaleAge (yr) 25 35 45 55 25 35 45 55NCRP 1989 1 .
00 Sv 1 .
75 Sv 2 .
50 Sv 3 .
00 Sv 1 .
50 Sv 2 .
50 Sv 3 .
25 Sv 4 .
00 SvNCRP 2000 0.40 Sv 0.52 Sv 0.75 Sv 1.35 Sv 0.75 Sv 1.00 Sv 1.48 Sv 2.98 SvNSCR 2012 − − − − ff ective at shielding solar flares and 15% more e ff ectiveat shielding GCR radiation than aluminum is. However, while polyethylene plastic couldimprove the shielding, it is not strong enough for load bearing aerospace structural ap-plications, which is why aluminum is primarily used. A new promising material underresearch at NASA is made out of boron, nitrogen, and hydrogen, and is called BoronNitride Nanotubes (BNNT) [22]. Hydrogenated BNNT takes the form of microtubules -an excellent design for increased stability, mechanical strength, and resistance to extremetemperatures - and can also hold a large quantity of hydrogen atoms for even greater shield-ing. Hydrogenated BNNT microtubules can be woven into composites, fabric, yarn, andfilm forms that could be integrated into the spacecraft structure as well as the astronauts’spacesuits. While water is another molecule with a high hydrogen content and potential toabsorb radiation, it is significantly heavier than polyethylene plastic and BNNT, and doesnot possess the necessary strength for structural applications. Utilizing water would re-quire additional energy due to its mass and hence would be more cost intensive. Creatingan electrostatic radiation shield around the spacecraft could also potentially deflect someGCR radiation, but would still leave astronauts exposed to dangerous levels of radiation.Finally, another interesting idea would be to use waste generated during the cruise phaseof the mission to attenuate the GCR cascades.In future missions to Mars, longer / permanent stays on the planet would become areality. Protective measures on the red planet could include constructing an undergroundshelter to reduce the amount of GCR radiation that penetrates into human tissues. Such anunderground shelter would have to be built at ∼ ,
000 g / cm depth for e ff ective shieldingof secondaries produced by the interaction of HZE particles with the atmosphere of Mars.There are several prospective lava tubes on Mars that are located in low lying regions of theplanet and may be viable options for an underground shelter. As previously mentioned,lower elevations are locations with the least amount of cosmic radiation and the hollowlava tubes may extend well below the surface of Mars. Radiation experiments at analog6 ealth threat from cosmic radiation during a manned mission to Mars Alexandra D Bloshenko lava tubes on Earth showed that, on average, the amount of radiation in the interiorof the tubes is 82% lower than on the surface [23]. Given that Mars is a di ff erentiatedterrestrial planet that formed from analogous chondritic materials and that many of thesame magmatic processes that took place on Earth happened on Mars as well, it is possiblethat the interior of lava tubes on Mars could also provide similar shielding from radiation.For example, the radiation level at the Hellas Planitia ( ∼ µ Sv / day) is considerably lessthan in other regions on the surface of Mars ( ∼ µ Sv / day). A radiation exposure of342 µ Sv / day is, however, significantly higher than what humans are annually exposedto on Earth (levels typically range from about 1.5 to 3.5 mSv / yr, but can be up to about50 mSv / yr in some regions of Europe). Three candidate lava tubes identified in the vicinityof Hadriacus Mons (an ancient low-relief volcanic mountain along the northeastern edge ofHellas Planitia) could provide protection from excessive radiation exposure. The absorbeddose inside these natural caverns would be ∼ . µ Sv / day.
4. Conclusions
We have shown that with current technology sending a long stay time manned missionto Mars, with 180-day transfer to Mars and 180-day inbound transfer, could be feasible.Incorporating BNNT shielding into spacecrafts in conjunction with building undergroundshelter structures in lava tubes on Mars might be the key for colonization of the red planet.
Acknowledgments
This work has been supported by the U.S. National Science Foundation (NSF GrantPHY-1620661) and the National Aeronautics and Space Administration (NASA Grant80NSSC18K0464). Any opinions, findings, and conclusions or recommendations ex-pressed in this material are those of the authors and do not necessarily reflect the views ofthe NSF or NASA.
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