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The Biological Effects of Radiation to Humans on a Manned Mission to Mars: Dose and Risk Determination during the trip and during the stay

The Biological Effects of Radiation to Humans on a Manned Mission to Mars: Dose and Risk Determination during the trip and during the stay

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The Biological Effects of Radiation to Humans on a Manned Mission to Mars The Biological Effects of Radiation to Humans on a Manned Mission to Mars Document Transcript

  • Daniel L Maierhafer Clemson University Phone 864 656 7890 342 Computer Court Telex 864 656 7890 Anderson, SC 29678 Fax 864 656 7890 The Biological Effects of Radiation to Humans on a Manned Mission to Mars Dose and Risk Determination during the trip and during the stay
  • Table of Contents I. INTRODUCTION........................................................................................................................................2 II. DEFINITION OF TERMS.........................................................................................................................2 III. NATURAL SOURCES OF RADIATION IN SPACE..............................................................................3 A. SOLAR ENERGETIC PARTICLES (SEP) OR PROTON FLARES......................................................................................3 1. Ordinary Proton Flares..........................................................................................................................4 2. Very Large Proton Flares.......................................................................................................................4 B. GALACTIC COSMIC RAYS (GCR).....................................................................................................................5 1. Solar Maxima and Minima.....................................................................................................................6 C. TRAPPED PARTICLES IN RADIATION BELT............................................................................................................7 IV. MAXIMUM PERMISSIBLE DOSE FOR SPACE TRAVEL.................................................................7 A. AMERICAN NCRP GUIDELINES FOR ASTRONAUTS................................................................................................8 B. RUSSIAN GOST GUIDELINES FOR COSMONAUTS..................................................................................................8 V. EXPECTED RADIATION DOSE ON REPRESENTATIVE MISSION SCENARIOS..........................8 A. CONJUNCTION...............................................................................................................................................9 1. Equivalent Dose During Solar Minima...................................................................................................9 2. Equivalent Dose During Solar Maxima.................................................................................................10 B. OPPOSITION.................................................................................................................................................10 1. Equivalent Dose During Solar Minima..................................................................................................10 2. Equivalent Dose During Solar Maxima.................................................................................................11 VI. RADIATION DOSE RATE ON THE VOYAGE TO MARS................................................................11 A. NOMINAL SHIELDING REQUIREMENTS...............................................................................................................11 B. EXTRA SHIELDING REQUIREMENTS FOR SOLAR FLARES.........................................................................................12 VII. RADIATION DOSE RATE ON THE SURFACE OF MARS..............................................................13 A. SHORT DURATION MISSION DOSE...................................................................................................................13 B. LONG DURATION MISSION DOSE.....................................................................................................................13 C. NATURAL MARTIAN ATMOSPHERIC SHIELDING....................................................................................................14 VIII. ANTHROPOGENIC RADIATION SOURCES AND SHIELDING..................................................14 A. NUCLEAR POWERED ROVER...........................................................................................................................14 1. Maximum Permissible Radiation Dose..................................................................................................15 2. Shielding Materials and Design............................................................................................................15 IX. TOTAL PREDICTED MISSION DOSE AND EFFECTS.....................................................................16 X. CONCLUSIONS.......................................................................................................................................18 XI. WORKS CITED......................................................................................................................................19
  • I.Introduction This paper will discuss the anticipated sources and doses of radiation during a manned mission to Mars. The reasons to go to Mars are many: Mars contains all the necessary resources to sustain life and also to develop a civilization; although there is very little liquid water on the surface of Mars, there are large frozen water reserves underneath the polar caps which have only to be melted; and deuterium, currently valued at $10,000/kg is five times more common on Mars than it is on Earth (Zubrin). President Bush officially proposed a manned mission to Mars as early as 1989 when he called for a Space Exploration Initiative. The answer was in the form of a plan called the “90 day report.” The plan called for the construction of large assembly areas in orbit around Earth or the Moon. Large ships were required to haul propellant to Mars for the trip back. NASA’s Johnson Space Center estimated the price tag of this mission at $450 billion. When this cost estimate of $450 billion reached congress and leaked to the public, the program was all but eliminated … until recently. A new plan called “Mars Direct”, proposed by Dr. Robert Zubrin has generated renewed interest in a manned mission to Mars. The projected cost is $20-$50 billion depending on whom one asks (Zubrin). This new plan utilizes the resources on Mars itself to provide self-sufficiency, and is attainable with today’s technology. So onward to Mars, and don’t forget your radiation shield so you don’t fry on the way. II.Definition of Terms Absorbed Dose, Gray (D) – Energy absorbed per unit mass of irradiated material ALSPE - Anomalously Large Solar Particle Event Aphelion - The point where an object's orbit is farthest from the Sun AU – Astronomical Unit, the average distance from the sun to the earth BFO – Blood Forming Organ Conjunction – Two or more planets are directly opposite each other with the sun between them. Density Thickness (td) - Density of a material multiplied by its thickness Equivalent Dose, Sievert (H) – A quantitative measure of the biological effects of radiation exposure GCR – Galactic Cosmic Rays 2
  • GOST - Gosstandart (Russian Radiation Protection Committee) NCRP - National Council on Radiation Protection and Measurement Opposition – Two or more planets are in line on the same side of the sun. Perihelion - The point where an object's orbit is closest to the Sun SEP – Solar Energetic Particles (Particles ejected by Solar Flares) Solar Minima/Maxima - Solar Flare activity is at its lowest/highest III.Natural Sources of Radiation in Space Once outside of the protective shielding of the earth’s atmosphere and magnetosphere, an astronaut will receive a dose of radiation from several sources. These sources include Solar Energetic Particles from Solar Flares, Galactic Cosmic Rays, and Trapped Particles in the Radiation Belt. A.Solar Energetic Particles (SEP) or Proton Flares “Solar flares are tremendous explosions on the surface of the Sun. In a matter of just a few minutes they heat material to many millions of degrees and release as much energy as a billion megatons of TNT. They occur near sunspots, usually along the dividing line (neutral line) between areas of oppositely directed magnetic fields.” (Hathaway) The sun has a natural cycle with periods of low and high solar flare activity. During periods of high solar flare activity such events as radio communications may be disrupted temporarily, and the Aurora Borealis (Northern Lights) can be seen in the night sky. The GCR particle flux, discussed later, is affected by solar flare activity. The flux is lowest during times of high solar flare activity (solar maxima) and highest during times of low solar flare activity (solar minima). Solar Flares are classified according to their X-Ray energy output. The Space Environment Services Center classifies solar flares according to the peak burst intensity (I) in the 0.1 to 0.8 nm spectral band as shown in Table 1 (Spaceweather.com): 3
  • Table 1: Classification of Solar Flares Clas Peak Intensity, I (W/m^2) s B I < 10.0E-06 C 10.0E-06 < = I < 10.0E-05 M 10.0E-05 <= I < 10.0E-04 X I >= 10.0E-04 1.Ordinary Proton Flares Ordinary proton flares are B, C, M, and X class flares that happen throughout the day at random intervals. Currently there is no way to predict them. Looking at Figure 1, their equivalent dose at 1 AU from the sun varies from 0.1 to 140 cSv with no shielding. Figure 1. Equivalent Dose vs. Absorber Density Thickness for Ordinary Proton Flare 2.Very Large Proton Flares Very Large Proton Solar Flares tend to occur at the ascending and descending portion of the solar cycle, which has a time span of approximately 11 years (Pissarenko). Figure 2 shows the equivalent dose from these events is 140 to 1400 cSv, and is more than all the ordinary proton flares shown on Figure 1 combined. Clearly, it is wise to schedule a mission to avoid very large proton flares. 4
  • Figure 2. Equivalent Dose vs. Absorber Density Thickness for Very Large Proton Flare B.Galactic Cosmic Rays (GCR) Galactic Cosmic Rays contain a variety of charged particles whose origin is a region outside of our solar system extending isotropically nearly 100 AU from the sun. GCR consist of many types of particles. The majority of the equivalent dose comes from H, He, C, Ne, Si, and Fe Nuclei (Badhwar). The equivalent dose comes from this large spectrum of poly-energetic particles. “Calculated spectra for the GCR environment … provided by NASA Langley Research Center indicate that behind a shielding consisting of 10 g/cm2 of Aluminum followed by 2.5 g/cm2 of Red Bone Marrow (a typical vehicle environment) some 64% of the dose results from exposure to protons (H nuclei). This is followed in decreasing order by He (15%), O (4.4%), C (3.2%), and Fe (1.9%). Figure 3 shows a plot of f (fraction of dose delivered by a particle with atomic number Z), Q (Quality or Weighting Factor), fQ, and QLET (Quality Factor dependent only on Linear Energy Transfer and independent of particle type) vs. Z (atomic number), (Zaider). 5
  • Figure 3. The variation of f, Q, fQ, and QLET with the atomic number (Z) of the particle in the GCR flux. From Figure 3, you can see that the majority of particles from the GCR have low atomic numbers (Z=1 to 2), and a quality factor (QLET) of approximately 1. In this case, shown in Equation 1, Equivalent Dose (H) = Absorbed Dose (D), because the damage done by the particles is unity. Equation 1: H (Sv) = D (Gy) * Q (Gy/Sv) Note that the QLET goes quite high with a particle Z around 20 (Ca), but the GCR flux does not contain many of these particles (f is low). 1.Solar Maxima and Minima The interplanetary magnetic field, generated by the sun, is strongest at solar maxima. Therefore, more of the charged intergalactic particles (which are a part of GCR) are deflected during solar maxima than during solar minima. A solar minimum occurs during the first and last few years of the 11-year solar cycle (Badhwar). Figure 4 shows the difference between Equivalent Dose Rates from solar maxima to solar minima in Low Earth Orbit. From Figure 4, the Equivalent Dose Rate can be estimated at Solar Minima to be 113 cSv/yr, and at Solar Maxima to be 45 cSv/yr. Clearly it is preferable if a manned mission could completely start and finish its transit 6
  • during solar maxima. The SEP is greater during this time, but the dose rate drops more rapidly with increased shielding (See Fig. 1, 2, and 4) Figure 4. Equivalent Dose vs. Absorber Density Thickness for GCR Another study estimates the Equivalent Dose at Solar minima with 5 g/cm^2 of Al (representing habitable space) to be between 37 – 72 cSv/year (Pissarenko). During Solar maxima the Equivalent Dose with 5 g/cm^2 of Al is estimated to be 28 cSv/year. This number will be used in calculations because it deals with galactic and solar cosmic radiation, whereas the previous number deals with radiation in Low Earth Orbit. The majority of the Mars Mission will not be spent in Low Earth Orbit. C.Trapped Particles in Radiation Belt Trapped particles in the Radiation Belts consist mainly of protons and electrons and should only be a factor for manned flights in low earth orbit. The Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX) Satellite has discovered belts of deuterons and tritons, but they contribute less than 1% of the total flux. Electron belts also do not contribute much to the dose rate for low altitude (< 700 km) orbits (Badhwar). Since the Manned Mars Mission will not be spending much time in low earth orbit, we will neglect the equivalent dose given by these protons. IV.Maximum Permissible Dose for Space Travel Several organizations have developed radiation dose limit guidelines to help mission planners with the task of determining shielding requirements and mission duration. The most sensitive organ to radiation is the BFO, hence protection of the BFO dictates radiation-shielding requirements (Striepe, et. al.). 7
  • A.American NCRP Guidelines for Astronauts The NCRP has developed a 30-day, annual, and career recommended set of guidelines for exposure to skin, eyes, and Blood-forming organs (BFO). These NCRP Guidelines are shown in Table 2 (Striepe, et. al.) Table 2: NCRP Equivalent Dose Limit Guidelines (cSv) Time of Ski Ey BFO Exposure n e 30-day 150 100 25 Annual 300 200 50 Career 600 400 100-400* * = Depending on Age B.Russian GOST Guidelines for Cosmonauts The Russian annual permissible Equivalent Dose limit for long duration manned space missions is 59.25 cSv (Pissarenko). V.Expected Radiation Dose on Representative Mission Scenarios As one would expect, mission launch scenarios have been developed years in advance of the actual launch. They have been developed by government agencies such as NASA, or visionary individuals such as Robert Zubrin. “In current Mars scenario descriptions, the crew flight time to Mars is estimated to be anywhere from 7 months to over a year each way, with stay times on the surface ranging form 20 days to 2 years.” (Simonsen, et. al). There are two favored time windows for transit between Earth and Mars. One is during conjunction and one is during opposition. These mission scenarios have a slightly different equivalent radiation dose between them. Table 3 shows a summary of some important data between the two missions (Zubrin). 8
  • Table 3: Transit Window Flight Data of Mars Missions A.Conjunction The German mathematician W. Hohmann discovered in 1925, that the best time window to conserve energy when traveling from Earth to Mars is when the two planets are in conjunction. This will minimize the course corrections required to depart or rendezvous with either. The Hohmann conjunction mission gives a travel time of about 258 days one way from Earth to Mars. This may be acceptable for a payload mission, but this is a little long for a manned mission to Mars. Luckily, it is possible to speed up the Hohmann conjunction mission by burning a little extra propellant and this reduces the transit time to 180 days one way (Zubrin). 1.Equivalent Dose During Solar Minima The Equivalent Dose for 180 days at Solar Minima (from Section B.1) is calculated to be:  0.37 Sv  1 yr  180dy  Minimum:       ( 2trip ) = 36.47cSv   yr  365.25dy  trip   0.72Sv  1 yr  180dy  Maximum:       ( 2trip ) = 70.97cSv   yr  365.25dy  trip  9
  • 2.Equivalent Dose During Solar Maxima The Equivalent Dose for 180 days at Solar Maxima (from Section B.1) is calculated to be:  0.28Sv  1 yr  180dy   yr  365.25dy  trip ( 2trip ) = 27.60cSv         B.Opposition In an opposition type mission, the outbound leg of the mission (from Earth to Mars) is similar to a conjunction mission. However, the inbound leg requires a gravity assist by Venus for the required course correction. Because of the higher ∆v (change in velocity) requirements, this mission is expected to have a launch mass double that of the conjunction mission (Zubrin). Table 3 shows some other disadvantages for this type of mission. The inbound transit time is much higher than the outbound transit time. From this knowledge, it can be assumed that the radiation exposure due to GCR and SEP’s will be greater also. In transit on this type of mission, the distance from the sun varies greatly from a perihelion of less than 0.72 AU to an aphelion of 1.5 AU. Therefore, the radiation dose rate will differ also, and cannot be based on Earth’s 1 AU orbit without compensation (Striepe, et. al). The mission would be more efficient if the crew stayed on the surface of Mars for one year because a “Quick Return” flight trajectory must receive a gravitational boost by swinging past Venus, where the radiation from the sun is twice that relative to the position of the earth (Zubrin). Because of the longer time in space, and the closer travel to the sun, there will be an increased absorbed dose with this mission. Some mission planners refer to this mission type as a Venus “fryby.” 1.Equivalent Dose During Solar Minima The Equivalent Dose for 610 days roundtrip at Solar Minima (from Section B.1) is calculated to be:  0.37 Sv  1 yr  Minimum:     ( 610dy ) = 61.80cSv   yr  365.25dy   0.72 Sv  1 yr  Maximum:     ( 610dy ) = 120.25cSv   yr  365.25dy  10
  • 2.Equivalent Dose During Solar Maxima The Equivalent Dose for 610 days roundtrip at Solar Maxima (from Section B.1) is calculated to be:  0.28Sv  1 yr   yr  365.25dy ( 610dy ) = 46.76cSv       VI.Radiation Dose Rate on the Voyage to Mars A.Nominal Shielding Requirements Due to the long duration of time spent in space, the astronauts should have shielding available to protect them from GCR and Solar Flares. It is cost-prohibitive and impossible to shield the entire living quarters from all GCR and Solar Flare flux. Current plans call for two levels of shielding: The Habitat Section and the Radiation Shelter. These function just as they are written. The crew normally spends their time in the larger habitat section, but when a solar flare is predicted, they move to a smaller radiation shelter to protect themselves from the increased dose. It should be noted that judicious placement of food and water will add additional shielding protection to the astronauts. If the sleeping quarters were in the radiation shelter, that would reduce the dose to the astronauts even more. Table 4 shows calculated shielding density thickness for mission scenarios using different types of rocket engines (Dudkin, et. al.). These calculations have been done using a set of state standards for space flight developed in the USSR. The following list gives the definition for the acronyms in this table. LRE - Liquid Propellant Rocket Engine NRE – Nuclear Rocket Engine NERE (a~0.0001) – Low Thrust Nuclear Electric Rocket Engine NERE (a~0.001) – Medium Thrust Nuclear Electric Rocket Engine A spacecraft with a nuclear rocket engine requires more shielding than one with a liquid propellant rocket engine to protect the astronauts from the additional radiation emitted by the engine. The shielding requirements for the Low Thrust NERE are higher than the Medium Thrust NERE because of the longer stay within the Earth’s radiation belts as the engine provides thrust and the spacecraft gains speed. 11
  • Table 4: Shielding Thickness in g/cm2 during a 1-2 year flight to Mars depending on Solar Activity Period and Rocket Engine Type Solar Activity Shield Type LRE NRE NERE NERE (a~0.0001) Period (a~0.001) Minimum Radiation - - 21.2 – 14.3 119.0 – 101.0 Shelter Habitat Section 2.5 – 2.6 – 8.3 9.2 – 11.1 3.4 3.5 Intermediate Radiation 11 11 17.1 – 12.5 105.3 – 84.2 Shelter Habitat Section 1 1 5 – 4.8 6.8 – 8.0 Maximum Radiation 11 11 11 92.4 – 70.8 Shelter Habitat Section 1 1 1 1 Since no nuclear rocket engines are currently under development for space travel, (although many are in the concept and prototype stage) the radiation shielding values for the LRE will be used. Table 4, implies that the crew can meet the Russian guidelines with no solar flare activity and a very small amount of radiation shielding of 3.4 g/cm2. This corresponds to a 3.4 cm thick wall of water at room temperature, or a 2.89 cm thick wall of plexiglass (See Equation 2). t d ( g / cm 2 ) Equation 2: t (cm) = ρ ( g / cm 3 ) In one study, twelve mission departure dates have been thoroughly investigated with respect to travel time and radiation exposure. Six of the twelve missions studied can satisfy ICRP guidelines with no time spent in a storm shelter except during a solar flare event. If 50% of the crew’s time is spent in a storm shelter of 25-g/cm2 density thickness, then 11 out of 12 missions studied can meet the BFO guidelines (Striepe, et. al). B.Extra Shielding Requirements for Solar Flares If the shielding in the Habitat Section is made to be as thin as Table 4 implies, then during a solar flare, the crew must enter the Radiation Shelter and stay there for a short time until it passes. From Table 4, the additional shielding requirements for the radiation shelter are 11 g/cm2, which corresponds to 11 cm thick wall of water at room 12
  • temperature, or 9.33 cm thickness of plexiglass. One way to accomplish this would be to store the drinking water as a cylindrical wall outside of the radiation shelter. VII.Radiation Dose Rate on the Surface of Mars Now that the crew has arrived on Mars, they can use the Martian planet as radiation shielding. Simonsen states, “The crew will encounter the most harmful radiation environment in transit to Mars from which they must be adequately protected. However, once on the planets surface, the Martian environment should provide a significant amount of protection from free space radiative fluxes (Simonsen).” The astronauts will receive only half of the GCR flux that they were receiving in space because the planet shields them from the other half. A.Short Duration Mission Dose The stay time on Mars for short duration missions is defined as less than 100 days, with a less than 2-year total mission duration (Striepe). At an altitude of 0 KM, the estimated skin Equivalent Dose is 0.21-0.24 Sv/year (21-24 rem/year). Estimated BFO Equivalent Dose is 0.19-0.22 Sv/year (19-22 rem/year) (Simonsen). Calculating the skin and BFO equivalent dose on the surface of Mars for a short duration mission yields:  0.24 Sv  1 yr  Skin Equivalent Dose:    (100dy ) = 6.58cSv  yr  365.25dy     0.22 Sv  1 yr  BFO Equivalent Dose:     (100dy ) = 6.03cSv   yr  365.25dy  B.Long Duration Mission Dose The long duration mission will be the most efficient from a time perspective. It is better than a short duration mission because it allows the astronauts to accomplish more, and the nation to get more value for it’s funding. From Table 3, the long duration mission can be defined as 550 days. The skin and BFO equivalent dose on the surface of Mars for a long duration mission is as follows:  0.24 Sv  1 yr  Skin Equivalent Dose:     ( 550dy ) = 0.3614 Sv   yr  365.25dy  13
  •  0.22Sv  1 yr  BFO Equivalent Dose:     ( 550dy ) = 0.3313Sv   yr  365.25dy  C.Natural Martian Atmospheric shielding The Martian atmosphere, although a thin layer of CO2, provides additional shielding capability. It varies from a density thickness of 16 g/cm2 CO2 vertically, to a density thickness of 59.6 g/cm2 at large zenith angles (Striepe). VIII.Anthropogenic Radiation Sources and Shielding A.Nuclear Powered Rover Once on Mars, the astronaut will require a rover to carry heavy items and increase range. A small nuclear reactor is the only viable alternative to power a manned Mars rover whose electric power requirements are a few tens of KW’s (Morley, et. al.). The United States Department of Defense, Department of Energy, and NASA have funded the development of a small 500 kW nuclear reactor utilizing the Stirling cycle of energy conversion. This reactor is designated the SP-100/ Stirling Cycle Space Nuclear Power System and is designed to be fitted on board a manned Mars Rover. Figure 5 shows a picture of the rover, which is made up of 4 jointed sections: The Primary Control Vehicle (PCV), the Experimental Unit, the Storage and Supply Car, and the Reactor Car. Figure 5. Layout of a train-type manned Mars rover powered by an SP-100 type reactor 14
  • 1.Maximum Permissible Radiation Dose For the safety of the astronauts and to comply with the NCRP guidelines for a 1-year stay on the Martian surface, the maximum permissible dose from the reactor to an astronaut is calculated as follows: Table 5: Equivalent Dose Calculations for Allowable Reactor Exposure from a Nuclear Powered Rover Description Calculation Equivalent Dose Max. Annual Equivalent Dose Guideline for BFO By definition 500 mSv/yr per NCRP Guidelines Mission duration 2 years enroute, + 1 year on surface = 3 years Allowable Mission Dose 3 years * (500 mSv/year) 1.5 Sv Anticipated Dose per transit due to Natural 2 directions * (450 mSv/transit) 900 mSv Sources Anticipated Dose on surface due to natural By definition 100 mSv sources Possible Worst Case ALSPE Dose Dose for transit in one direction 200 mSv only Max Anticipated Mission Dose (900 + 100 + 200) mSv 1.2 Sv Equivalent Dose Margin (1.5 – 1.2) Sv 300 mSv From Table 5, the maximum permissible dose from the rover’s reactor must not exceed 300 mSv/yr, which is the equivalent dose margin (Morley, et. al.). The reactor will require shielding to protect the astronauts from radiation during operation. Infinite shielding would be the safest, but the expected cost of launching mass to the Martian surface is estimated to be $1M USD / kg (Morley, et. al.). At this price, the total mass of the rover, including the shielding is an important design constraint. Therefore, the most efficient shield must be developed with the lightest materials. 2.Shielding Materials and Design The primary types of radiation given off by a nuclear reactor are gamma rays and neutrons. The most effective way to lower the kinetic energy of a neutron is an elastic collision with a particle of the same mass, for example the proton in a hydrogen nuclei. The most effective shield against gamma rays is a high Z material (material with a high atomic number). For this goal, the shield design for the reactor consists of two layers of shielding. Each layer consists of a Tungsten (T) layer followed by a Lithium Hydroxide (LiH) layer. The Tungsten (182T) layer is a strong absorber of neutrons in 15
  • an (n,γ) reaction. The LiH matrix decreases the number of neutrons reaching the tungsten layer, which decrease the number of secondary gammas produced by (n,γ) reactions with Tungsten (Morley, et. al.). Figure 6 shows the one-dimensional layout used to calculate the thickness of the shielding required. The 3-cm thickness of aluminum represents the walls of the rover cars and the equipment stored between the reactor and the astronauts. The 10-cm thick wall of water represents wastewater and consumables stored in the PCV. Figure 6. One-Dimensional Layout of man-rated shield for a Mars rover vehicle powered by an SP-100 type reactor A cylindrical shield consisting of a total of 63 cm of LiH layered with two-10 cm layers of Tungsten will protect the crew in the PCV at a distance of 25 meters without exceeding the maximum permissible dose rate of 0.034 mSv/hr (300 mSv/yr) (Morley, et. al.). IX.Total Predicted Mission Dose and Effects Simonsen states that, “A total yearly skin and BFO Dose may be conservatively estimated as the sum of the annual GCR dose and the dose due to one large flare. (Simonsen)” Using this information, and the Equivalent Dose Rates found above, the additional Risk of Fatal Cancer can be calculated using Equation 3. The probability of contracting a fatal cancer due to radiation exposure is ρ = 0.05/Sv. Note that the spontaneous risk of contracting a fatal cancer is 0.2, or 20%. Table 6 summarizes the anticipated total Equivalent Dose and Stochastic Risk Factor for the Fast Hohmann Conjunction Mission. Equation 3: R = ρ(1/Sv) * H (Sv) 16
  • Table 6: Total Mission Dose Mission Portion Equivalent Dose to BFO Prob., ρ Risk Factor, R (Sv) (1/Sv) Outbound 0.7097 0.05 0.0355 Inbound 0.7097 0.05 0.0355 On Planet Surface 0.3313 0.05 0.0166 From Nuclear Powered Rover 0.3 0.05 0.015 Totals 2.0507 0.05 0.1026 Total Fatal Cancer Risk (0.2 + N/A N/A 0.3026 R) Note that the Outbound and Inbound Equivalent Dose is calculated using 5 g/cm2 of shielding. This is most certainly too low. This fatal cancer risk can be reduced tremendously with additional shielding on the spacecraft. The astronaut’s total additional risk of fatal cancer caused by this mission is 10.26%. Table 7 shows the Deterministic Effects of Acute Radiation exposures for longer than 1 year. Table 7: Deterministic Effects of Chronic Exposure H Effects (Sv/year) > 0.04 Depression of Blood Forming Organs (BFO) > 0.15 Vision Impairment > 0.4 Temporary Sterility Recall from Table 3 that the inbound and outbound portion of the mission is anticipated to take 180 days, while the stay is 550 days. Using this length of time, the Average Equivalent Dose Rate can be calculated in Table 8. 17
  • Table 8: Chronic Effects of Mars Radiation Exposure Mission Time Time (Years) Dose Ave. Equiv. Dose Chronic Effects Leg (Days) (Sv) Rate (Sv / Yr) Outbound 180 days (180 dy)(1 yr / 365.25 0.7097 1.4401 Temporary dy)=0.4928 Sterility Stay 550 days (550 dy)(1 yr / 365.25 0.6313 0.4192 Temporary dy)=1.5058 Sterility Inbound 180 days (180 dy)(1 yr / 365.25 0.7097 1.4401 Temporary dy)=0.4928 Sterility X.Conclusions From these calculations, it is obvious that more shielding is needed. Also, these are worst- case equivalent dose numbers used in crude algebraic calculations. A different approach using a computer model to simulate dose rates as they change all throughout the voyage should give a more accurate result. However, there is a physical plan, and it is workable with today’s technology. Even though the sterility is temporary, there are most likely double strand DNA breaks that could cause a mutation. After one’s children are all grown up, someone may still choose to go on this mission knowing the consequences. Throughout the history of humanity, there are always people motivated by heroism, exploration, and world conquest. We have all of these bundled up in a 2.5 year time window. 18
  • XI.Works Cited Table of Authorities Badhwar, G.D. (1997). “The Radiation Environment in Low-Earth Orbit.” Radiation Research, S3-S10. Badhwar, G.D., Dudkin, V., Doke, T., Atwell, W. (1998). “Radiation Measurements on the Flight of IML-2” Advances in Space Research: The Official Journal, V22, N4, 485-494. Dudkin, V.E., Kovalev, E.E., Kolomensky, A.V., Sakovich, V.A., Semenov, V.F., Demin, V.P., Benton, E.V. (1992). “Radiation Shielding Estimates for Manned Mars Space Flight.” International Journal of Radiation and Applied Instrumentation, V20, N1, 29-32. Hathaway, David H. (2000 July 17), http://science.msfc.nasa.gov/ssl/pad/solar/flares.htm, Marshall Space Flight Center Morley, N.J., El-Genk, M.S. (1992). “Manned Mars Rover Powered by a Nuclear Reactor: Radiation Shield Analysis.” Nuclear Technology, V99, N2, 188-201. Pissarenko, N.F. (1994). “Radiation Environment due to Galactic and Solar Cosmic Rays during Manned Mission to Mars in the Periods between Maximum and Minimum solar activity cycles.” Advances in Space Research: The official journal, V14, N10, 771-778. Simonsen, L.C., Nealy, J.E., Townsend, L.W. (1990). “Space Radiation Dose Estimates on the Surface of Mars.” Journal of spacecraft and Rockets, V27, N4, 353-354. Spaceweather.com, “X-ray Solar Flare Classification”, http://www.spaceweather.com/glossary/flareclasses.html Striepe, S.A., Nealy, J.E., Simonsen, L.C. (1992). “Radiation exposure predictions for short-duration stay Mars missions.” Journal of Spacecraft and Rockets, V29, N6, 801-807. Zaider, M. (1996). “Microdosimetric-Based Risk Factors for Radiation Received in Space Activities during a trip to Mars.” Health Physics, V70, N6, 845-851. Zubrin, R. (1997). The Case for Mars: The Plan to Settle the Red Planet and why we must. 19