Ionizing space radiation safety for the crew is one of the goals of biomedical support of lunar missions. The results of dose estimations at the International Space Station and experimental data analysis, as well as the modeling of anticipated doses beyond Earth’s magnetosphere, advocate for the acceptability of ~1.5-month missions provided that the existing dose limits are not exceeded.
At present, space agencies of some countries carry out the projects aimed at launching crewed flights beyond the low Earth orbit: the flights to asteroids, circumlunar missions and, later on, the stay of the crew on the lunar surface, followed by establishment of an astronaut-tended lunar base. One of the main precautions taken to ensure the safety of astronauts in interplanetary missions is biomedical radiation safety assurance under the exposure to the natural cosmic ionizing radiation.
Manned space missions (MSMs) require the adequate monitoring, account and prognosis of exposure to cosmic radiation as it can have negative radiobiological effects on crew both during the flight (short-term effects) and long after the mission is completed . The works [2–4] demonstrate the possibility of short-term acute effects associated with the impairment of cognitive functions in astronauts under the exposure to heavy charged particles (HCPs) of galactic cosmic radiation, which needs to be considered separately while planning long-term and long-distance flights beyond Earth’s magnetosphere, where radiation is not attenuated by the geomagnetic field.
Despite the fact that the flights of unmanned spacecrafts (SCs) and astronauts to the Moon have been performed more than once, radiation effects for all stages of lunar missions are insufficiently addressed in Russian scientific literature. The estimates and approaches showing “exorbitant” doses of radiation for the astronauts of Apollo missions, which entirely exclude the possibility of such missions, continues to be discussed in some modern publications (e.g., ).
The present work was aimed at formulating the main features of radiation exposure in lunar missions and the basic approaches to radiation safety assurance in manned SC and on the lunar surface, which later would allow specification of the optimal scenarios of such missions, taking into consideration the radiation and other crucial factors of space flights.
Radiation Conditions of Orbital and Lunar Missions
Cosmic radiation is an unavoidable factor of SF, and its effect on humans cannot be completely eliminated due to protection provided by a spacecraft hull, construction materials, or any other known protection measures because of the existing weight, energy consumption and other limits of manned space systems .
The radiation exposure of the crew in lunar missions is determined by the effects of:
—Galactic cosmic rays (GCR) at all stages of the mission.
—Electrons and protons of the natural Earth’s radiation belts (ERB) during short-term crossing of ERB while flying to the Moon and back.
—Solar proton events (SPE) with the generation of solar cosmic rays (SCRs), mainly during the flights beyond Earth’s magnetosphere, on the circumlunar orbits and lunar surface.
—Secondary radiation generated by the high-energy particles of GCR, ERB and SCRs interacting with SC constructive elements and materials, moon soil, as well as directly in astronaut’s body.
The radiation safety standards that are currently in force in Russia were developed for long-term near-Earth flights . At present, there are no other radiation safety standards elaborated specially for lunar or interplanetary missions both in Russia and in other countries taking part in manned missions. The review, comparison and analysis of modern standards liming the effects of radiation in space missions are presented in . With all the variety of approaches to standardization in space missions, the following two peculiarities can be distinguished:
—The impossibility of using ground-based approaches to ensuring radiation safety and its standards  to set the standards of radiation exposure in space for a specified period of mission (a month, a year, etc.). It is associated with much higher radiation levels in orbital missions compared to dose-related effects on the staff in nuclear industry  even in an unperturbed radiation field.
Taking into consideration the results of long-term observations, it should be noted that the radiation dose aboard the orbital space station at a typical orbital altitude of 400 km and an inclination of 51.6° is approximately 200-fold higher than the average natural ionizing radiation background on the Earth and, accordingly, irradiation of humans under normal terrestrial conditions. For example, after a one-year mission on the ISS at an average radiation dose of 0.6 mSv/day [10, 11], an astronaut is exposed to a dose of 220 mSv/year, which is 10-fold higher than the maximum permissible dose of radiation for a nuclear worker (20 mSv/year on the average ). However, the above annual dose for astronauts during orbital flights (~220 mSv/year) is permissible in terms of radiation safety, because modern Russian standards limit the dose to 500 mSv/year for hematopoietic organs of astronauts .
For each source of cosmic radiation (GCR, ERB, SCR), the energy spectrum of particles has its own specific shape, and radiation exposure depends not only on this shape but also on other space flight factors such as duration, flight path, solar activity (SA), the material and thickness of bumper shields. The computer versions of empirical models of the particle fluxes of cosmic radiation fields, as well as the model of interaction between these particles and the material of protective shields, were developed for taking into account all the above circumstances and for the quantitative assessment of radiation exposure of astronauts and technical systems.
The major difference between lunar and orbital missions (the distance to the Earth and the flight beyond its magnetosphere) in terms of radiation exposure is as follows:
—The increased contribution to the total dose of GCR.
—The higher probability of a substantial increase in the dose of SCR.
—The contribution of ERB to the total dose depends on a particular flight path of spacecraft (SC) and eventually on the time of crossing the most hazardous areas: the inner proton and outer electron belts.
Now let us consistently examine the effects of these factors on the radiation situation in the near-Earth space.
The Effect of ERB
The flight path of lunar missions inevitably crosses ERB on the way to Moon and back. The radiation situation in ERB is described, e.g., in the most widely used models AE8/AP8 [12, 13]. The dose received by the crew while crossing ERB strongly depends on the chosen flight path and on the time SC spends in ERB. The major particle fluxes of ERB are concentrated in the equatorial region (approximately ±30° of latitude), beginning with the altitudes of about 500 km and higher, and descend only in the area of the South Atlantic Anomaly (SAA). The radiation situation on the SC orbit depends on its inclination. At the same time, SC flying in the equatorial plane or on a slightly inclined orbit (e.g., geostationary (GSO)) is continuously exposed to the ERB particle fluxes. When orbital inclination increases, the time of SC exposure to intensive ERB particle fluxes decreases, which leads to the periodical and short-term (compared to the orbital period) increase in the ERB particle fluxes in the parts of the orbit crossing the equator. In all other parts of the orbit, the fluxe ERB particles depends on both orbital inclination and altitude. For example, the fluxes ERB particles affecting the crew of the International Space Station (ISS) increase only in short parts of the orbit above SAA and are almost absent in other parts. The same spike pattern of exposure to the ERB particle fluxes was shown for SC on highly elliptical orbits (HEO).
Figures 1 and 2 show the energy spectra of the average electron and proton fluxes of ERB, respectively, for typical orbits . As seen in the Figures, the electron flux on GSO (in the outer ERB) is maximal in the entire energy interval, while the proton flux consists of only low-energy protons (below 1 MeV). In the years with the maximum SA, the electron and proton fluxes on GSO almost do not vary with the changes in SA. On the orbits with lower parts, the proton flux increases and the electron flux decreases along with the decreasing SA .
The ERB particle fluxes existing only in Earth’s magnetosphere have an effect on SC for a short period of time on parking escape orbits, which is much less than the time it stays beyond the Earth’s magnetosphere. The particular doses from ERB particles depend on the scenario of the Earth-escape trajectory of SC.
This can be exemplified by the estimates  of radiation doses in the period from October 22, 2008, to August 31, 2009, obtained directly on board the Indian lunar probe Chandrayaan-1 equipped with a Bulgarian semiconductor dosimeter. In view of the fact that it takes ~2 h, i.e., approximately ~1/10 days, to go beyond the magnetosphere (at the second cosmic (escape) velocity) in manned lunar mission, the dose in this sector of the flight was determined as ~1 mSv as a conservative estimate. This is a relatively low dose compared to the total one anticipated for the entire period of lunar mission. However, it should be taken into account that, in case of the SC trajectory with the parking near-Earth orbit at low altitudes, the dose from ERB will be considerably higher and will be determined by the time SC stays on this orbit. In addition, it is not improbable that SCR can penetrate Earth’s magnetosphere at the initial stage of mission (see below), resulting in the exposure of SC crew to higher radiation doses.
The Effect of GCR
In the station module on the near-Earth orbit at an altitude of 400 km under unperturbed conditions (without SCR), the contribution of GCR is about 50% of the total dose from all sources of cosmic radiation, i.e., 0.3 mSv/day .
Beyond Earth’s magnetosphere (at a distance of more than 10 terrestrial radiuses from Earth), the GCR dose rate is several times higher compared to that on the ISS orbit. Such increase is determined by two factors: the absence of the “shadowing” effect of Earth in half of the total solid angle and the absence of protective effect of the magnetosphere. Due to these factors, the GRC dose rate in the interplanetary space (and on the trajectory of flight to Moon), according to the available data of measurements with a Lulin-MO dosimeter in the ExoMars planetary probe is already 1.5 to 1.8 mSv/day , i.e., five to six-fold higher than the contribution of GCR on the near-Earth orbits.
The dose rate in the near-Moon space, with allowance for 30% shielding by Moon from GCR (for low-orbital near-Moon missions, the shielding effect can be up to 50%), is taken as 1.2 mSv/day.
At present, the most commonly used model of GCR for calculating the radiation stress is the international standard of ISO , the result of a consensus between experts working in this field. This model makes it possible to predict the fluxes of GCR particles (from protons to uranium) depending on SA, the measure of which is the smoothed (±6 months) average monthly Wolf numbers. It can be noted that many authors use the traditional model [19, 20], which also takes into account SA variations by using the less obvious, so-called solar modulation parameter determined by neutron monitor data. The recently published new model of GCR  takes into account the modern experimental data on GCR fluxes in the interplanetary medium.
During an 11-year solar cycle, the GCR particle fluxes (protons and nuclei of chemical elements) vary from the maximum in the years of the minimum SA to the minimum in the years of the maximum SA (modulation of GCR fluxes). On the GSO (as well as in the entire outer ERB), the GCR flux of particles almost completely coincides with the flux of particles in the interplanetary space. On the low orbits, there is a considerable decrease in the number of GCR particles due to their deflection by the Earth’s magnetic field.
Figure 3 shows, as an example, the characteristic energy spectra of the GCR flux of protons and their time variations, which are typical of the fluxes of all GCR nuclei in the interplanetary medium . The characteristic feature of GCR energy spectra is the presence of a modulation peak at the energies of hundreds of MeV, which is determined by the effect of plasma fluxes and magnetic fields of the Sun with the amplitude depending on the phase of SA.
In order to estimate radiation exposure while planning long-term space missions, we need to know the level of SA determined by the Wolf numbers (W) which, according to the models of GCR fluxes, determine the amplitude of modulation of their fluxes. It can be done on the basis of the SA prediction models. It should be noted, however, that currently there are no accurate SA prediction models. Hence, it seems necessary to use models referred to as “extreme,” i.e., describing GCR modulation at the lowest predicted values of SA in future. Enhanced GCR fluxes can be expected precisely at the lowest levels of SA. One of such models of changes in GCR fluxes  based on SA forecast, which was used in the work , demonstrates a monotonous increase in GCR fluxes in future (Fig. 4). This would certainly lead to an increase in radiation doses from GCR for future lunar missions, which can be up to 20% and more depending on a particular time interval of the mission.
The Effect of SCR
The existing models of SCR use different approaches to describing their characteristics. The most of SCR models take into account the probabilistic nature of the particle fluxes, either only protons (see, e.g., [24–26]) or also fluences and peak fluxes of heavy ions [27, 28] during episodic solar events. The models of the fluxes of energetic charged particles of SCR are statistically probabilistic in nature, i.e., determine the fluence value of protons of preset energy for a certain rather long time interval (1 year, etc.), which can be exceeded with a predetermined probability. Radiation doses from SCR during space missions are calculated by the international standard actively used as a model , which allows obtaining the probability of nonexceedance of the preset dose from SCR over a given time interval, e.g., 1%. This means that the specified dose from SCR will not be exceeded for 99 out of 100 such missions.
The existing differences between SCR models are mainly due to description of the effect of SA on the frequency of solar events, as well as the shapes of particle energy spectra. These differences between the models of SCR particle fluxes developed over more than a 30-year period of observations obviously influence the estimates of radiation risk for the crews of near-Earth and especially interplanetary missions.
As an example, Fig. 5 shows the frequency distribution functions of SCR events over more than a 30-year period (from 1974 to 2005) of their observations in the interplanetary medium on SC . Such a great difference between model representations of the frequency distribution function of SCR in case of their large fluxes is due to the limited statistics of observations of strong SCR events, the so-called Ground Level Enhancement (GLE), on ground-based neutron monitors, on the one hand, and to insufficiently accurate determination of their spectral characteristics, which are model-dependent for this recording technique, on the other hand. At high SCR fluences, there is a significant uncertainty due to the lack of experimental data, which reduces the accuracy of model radiation dose estimates.
The typical radiation doses from SCR can be illustrated by the measurements made on the Mir orbital space station in September–October 1989, when the daily dose in the station module increased tenfold on average due to a series of strong SCR events [31, 32], reaching the estimated value of 6 mSv. Such events are no more frequent than once per the 11-year cycle of solar activity.
Beyond the magnetosphere, with the multiplication factor of its attenuation effect in the dose equivalent for such unique events taken as 100 , the dose is accordingly found to be 600 mSv during the flight beyond the Earth’s magnetosphere. It should be noted that in the ultimately “worst” scenario of SCR events beyond the Earth’s magnetosphere, with a probability much less than 1%, doses can also be higher than the selected value of 600 mSv. However, for relatively short-term lunar missions (compared to Mars missions, etc.), the probability of exceedance of this value with the chosen approach can be taken as close to zero.
To determine the values of equivalent doses, we need to know the elementary composition of space particles. Figure 6 demonstrates the spectra of protons and heavier particles of one of the recorded typical GLE . At high energies, the data on protons are obtained from ground-based measurements of neutron monitors, which give highly approximate estimates of proton spectra but cannot estimate the content of heavier nuclei. It should be particularly emphasized that it is important to know the elementary composition of SCR particles at the energies close to the maximum for accelerated SCR particles. The increase in the proportion of heavy particles in this energy region (which cannot be excluded from the analysis of approximations of the spectra of particular SCR components shown in this figure) should lead to an increase in equivalent dose stresses for biological objects. The lack of experimental data on the protons in the high energy region do not give any definitive answer as to what is the ratio between the fluxes of protons and heavier nuclei; however, there is an apparent tendency of the relative abundance of heavy particles to increase. Therefore, it should be emphasized that further experimental studies of SCR from strong solar flares are needed to improve the knowledge of real dose stresses during space missions.
The Combined Effect of SCR and GCR
In the interplanetary space beyond Earth’s magnetosphere, where lunar SC spends the most part of time, there are only GCR and SCR fluxes. Moreover, the GCR fluxes of particles exist constantly in outer space, whereas the SCR fluxes of particles are episodic. Thus, GCR models determine the values of the GCR fluxes of particles for a given period of time, while SCT models establish the upper limit for the SCR fluxes of particles, which can be exceeded with given probability in the given period of time.
Table 1 and Fig. 7 presented in  estimate the radiation hazard in the interplanetary space by the annual values of absorbed and equivalent doses expected for circumlunar missions of SC, which have been calculated by the SCR and GCR models standardized in Russia [25, 35], as well as by the international standard for GCR . For SPE, the limiting values of absorbed dose were estimated for the exceedance probability of 1% (i.e., the estimated values can be exceeded in one out of 100 missions). All values were obtained for the years of the minimum and maximum SA depending on different spherical hull thickness, with allowance for secondary radiation (neutrons), and should naturally be corrected for the real configuration of SC, flight trajectory and duration.
Table 1 graphically demonstrates the ratio between the expected contributions of GCR and SCR fluxes of particles to the doses. The doses from GCR begin to dominate at a hull thickness of more than 10 g/cm2. However, in case of strong (but rare) solar events, the doses of radiation from SCR (such as GLE) will exceed the doses from GCR.
The differences between SCR models are mainly in description of the SA effect on their frequency, as well as the shape of energy spectra of the particles. These differences in the models of SCR fluxes of particles should naturally influence the estimates of radiation hazard for the crews of near-Earth and especially interplanetary missions.
In the study , the equivalent dose was estimated on the basis of GCR and SCR models [18, 25], which leads to a conclusion that the lunar surface-stay time for humans in the period of maximum SA should be shorter than in the period of minimum SA at the equal levels of radiation hazard. This result is shown in Fig. 8 of the study . For example, with the probability of SCR occurrence at a level of 3–5%, the maximum lunar surface-stay time for humans with the given shield thickness (10 g/cm2) is no more than 1.5 months in the SA maximum and about a year in the SA minimum. At a greater thickness, the contribution from GCR and SCR to the total dose will increase and decrease, respectively.
The opposite conclusion was made in the work , where GCR and SCR models were the models of GCR  and SCR , respectively. According to the estimates of mission durations given in this work, their maximum possible duration falls on the SA maximum. This controversy is associated with the fundamental divergence between the energy spectra of SCR fluxes of protons in the energy region above 100 MeV in the models used, which in turn leads to the differences in estimated doses behind the protective shields. Nevertheless, it should be noted again that our knowledge of the energy spectra of SCR at high energies is limited (see Fig. 6) and needs further verification. These results will determine the accuracy of dose estimation for particular time spans of SA cycles and thus the estimated duration of lunar missions.
Obviously, the estimates of radiation hazard should be adapted to the real scenario of humans’ stay on Moon, which should take into account the potential changes in shield thickness under different conditions, e.g., while working on lunar surface in a space suit or moving to a more protected module during solar power events.
Thus, it should be admitted that reliable conclusions about the radiation hazard in lunar missions require the correction and modification of the models of particle fluxes in cosmic space, with allowance for space weather factors. In this context, e.g., one should properly take into account the interrelationship between the monitor series of experimental data (including the fluxes of heavy ions) and SA varying both within a single cycle and from one cycle to another.
The space radiation on the lunar surface, in addition to the contribution of GCR, with allowance for shielding by the Moon and stochastic SPE freely reaching the lunar surface, also depends on secondary emissions from lunar soil. The modern computational models [36–39] show more severe radiation levels on the lunar surface compared to those measured by the NASA radiation probe on the surface of Mars . The conservative (i.e., accepted for the sake of radiation safety assurance) estimate of daily average dose on the lunar surface can be taken as 1.2 mSv/day, disregarding the contribution of SCR. According to the data obtained from the Curiosity Mars rover, the average dose intensity on the surface of Mars is 0.7 mSv/day , i.e., comparable with the daily average dose in ISS modules varying from 0.3 to 0.8 mSv/day . According to the estimates, the radiation intensity on the lunar surface exceeds nearly twofold the level recorded on Mars. It is associated with several factors. First, the Moon is closer to the Sun than Mars, and the dose from solar energetic particles on the lunar surface, with allowance for the distance from the Sun, is two to three times higher than on Mars. Second, in contrast to Moon, Mars has a low-density atmosphere (~20 g/cm2 thick), which weakens both galactic and solar radiations. Third, there is water on Mars (in its solid state, ice, as a soil component), which effectively slows down the secondary neutrons emerging after space-particle bombardment of Martian soil, i.e., ice decreases the yield of secondary neutrons on the surface.
Characteristics of Radiation Exposure of the Crew in Some Scenarios of Lunar Missions
The levels of exposure to different sources of cosmic radiation considered above are systematized in Table 2, where the characteristic values of dose stresses for modern manned space missions are presented together with the radiation exposure limits for ground- and space-based facilities. It should be noted that the doses of cosmic radiation given in the table are the averaged conservative estimates of dose stresses in tissue behind an aluminum shield of ~10 g/cm2 intended to be used at the initial stage of planning and selecting the mission scenario, with due regard to the requirements for radiation protection of the crew.
Radiation doses during lunar missions depend on many factors:
—The flight path (short-term (within only a few hours) crossings of the proton (inner) and electron (outer) ERBs are considered; the dose can change by several times depending on the trajectory).
—The phase of the SA cycle (the minimum and maximum SA or the intermediate phase; the dose can change by tens of percent; further, the flights close to the SA minimum are considered, when the GCR doses are maximal and the contribution of SCR can be neglected).
—The protection (mass) of lunar spacecraft (the dose can change by many times for different variants of module protection; the estimates were made for the averaged aluminum shield of 10 g/cm2).
—The occurrence of SPE, including the period of minimum SA (the doses, even though with a low probability, can change tenfold and hundredfold compared to the background doses in the absence of these events);
—The duration and scenario of the mission (Tables 3, 4).
Since currently there are no special standards for radiation safety of the crew in the category of lunar missions, further consideration will be based on the standards for orbital flights . If we assume that the duration of lunar missions should be no more than 1.5 months (according to the above estimate for the most unfavorable space conditions in the SA maximum for the average SC shield of 10 g/cm2 and a several percent radiation hazard), then the dose for an astronaut, according to the data from , should be less than 250 mSv (the stricter standard for a 1-month flight is used instead of recalculation per 1.5 months).
The conservative estimates of radiation doses in different scenarios of lunar missions are the values of dose stresses for astronauts described above and presented in Table 2 (these estimates have been averaged but rather towards overestimation, in order to prevent underestimation of the expected radiation exposure for the sake of radiation safety of the crew).
Let us give an example of dose estimation based on the data from Table 2 for some typical scenarios of lunar missions.
Short-term circumlunar mission:
—1 day on the near-Earth orbit;
—1 day of the flight from Earth to Moon;
—7 days on the near-Moon orbit;
—1 day of the travel back to the near-Earth orbit;
—1 day on the near-Earth orbit before landing.
Dose calculation for this case is given in Table 3.
The dose of 13.2 mSv received by this scenario is much lower than the above-mentioned standard per 1 month (250 mSv). Thus, the requirements for flight radiation safety can be satisfied. The dose received during an 11-day circumlunar mission is equivalent to 13.2 mSv/0.6 ≈ 22 days of the ISS orbital flight with respect to the total dose of cosmic ionizing radiation.
Provided that the time on the near-Moon orbit would be increased to 1.5 months in the selected scenario, the total dose for such mission, taking into account the flight from the Earth to the Moon and back, will be 58.8 mSv, i.e., much less than 250 mSv, which will be equivalent to ≈98 days of the ISS orbital flight with respect to the total dose of cosmic ionizing radiation.
The short-term stay on the lunar surface (according to the scenario of Apollo missions by NASA):
—1 day on the near-Earth orbit;
—1 day of the flight from Earth to Moon;
—about 1 day on the near-Moon orbit;
—3 days on Moon’s surface;
—1 day of the travel back to the near-Earth orbit;
—1 day on the near-Earth orbit before landing.
Dose calculation for this case is given in Table 4.
The dose of 9.6 mSv received by this scenario is also much lower than the standard for 1 month (250 mSv). The dose received during an 8-day lunar mission is equivalent to ≈16 days of the ISS orbital flight with respect to the total dose of cosmic ionizing radiation.
As is known from published sources , during the Apollo lunar missions in 1969–1972, radiation doses were measured with individual dosimeters, which did not allow determining the equivalent doses (and the quality factor of cosmic radiation) and taking into account the contribution of the entire energy spectrum of ionizing radiations to the dose. In the Apollo missions of 6 to 12.5 days, the total doses measured over the entire mission were within a range from 1.6 to 11.4 mGy and the daily average radiation intensities were in the range from 0.22 to 1.27 mGy/day. The averaged quality factor of GCR (QF = 3.5) gives equivalent doses in the range from 6 to 40 mSv, which are comparable with the dose level of ~10 mSv for such missions that we have calculated (see Table 4).
It should be noted that the estimates of radiation intensity in Tables 3 and 4 were obtained without taking into account the contribution of SCR to the dose over the period of mission under consideration. The relatively low levels of radiation encountered by the astronauts of Apollo missions are explained by exceptionally favorable space weather during their flights to Moon. However, it is known that, e.g., during the strong SPE in August 1972, the total dose on the way to Moon in the common weakly protected module of SC would be ten times higher for each member of the crew, i.e., as reported in the work , more than 500 mSv (according to some data, up to 4 Sv). In other words, if the mission was not delayed, within several days the crew would receive a dose exceeding the current annual limit established for ISS.
However, delayed negative effects were observed in the astronauts of the Apollo mission in spite of relatively low radiation doses , probably due to the combined effect of various factors of such SF such as: zero gravity, cosmic radiation and more than a thousand-fold reduction in magnetic field compared to terrestrial conditions (hypomagnetic environment).
Methods of, and Main Approaches to, Radiation Protection in Lunar Missions
Modern means of protection do not prevent the exposure to GCR radiation during SF. The energies of GCR particles are so high that the available protective shield of SC quite insignificantly reduces the doses of GCR. For example, in the most protected module of the space station, GCR dose is by no more than 10–20% weaker compared to the least protected module. In addition, modern computational methods with the involvement of refined models of radiation transfer within a substance, based on the data of experiments in accelerators, have shown that further increase in the thickness and mass of aluminum protective shield will lead only to an increase in the equivalent dose of GCR due to formation of secondary particles in the substance of the shield . In case of GCR, the optimal thickness of aluminum shield is ~30 g/cm2; until this thickness has been reached, the equivalent dose of GCR decreases (though slowly) and then begins to increase with further increase in the thickness of protective shield.
The energy spectrum of SCR is milder compared to GCR and, therefore, the protection from SCR protons during strong SPEs on the near-Moon orbits can be provided by using a special radiation shelter or a module with a relatively higher mass thickness that would effectively reduce SCR doses.
The proper materials for additional radiation protection from GCR and SCR are polyethylene, plastic (carbon fiber reinforced polymer, fiberglass, organic plastic, etc.), and composite materials. The advantage of these materials is their lesser capacity to generate secondary radiation, including neutrons, because they mainly consist of relatively light carbon and hydrogen atoms. As a result, such materials prevent an increase in the equivalent dose with further increase in shield thickness as it occurs in aluminum, which is the main protective material in modern SCs.
Another promising substance for additional protection from cosmic radiation is water. Water can be used for radiation protection if water tanks are placed close to radiation shelters, so that it would be possible to considerably decrease the additional mass of SC intended for protection against cosmic radiation. The example of efficient use of water for additional protection is the Protective Shutter set in the compartment of the ISS service module, which is filled with several layers of personal hygiene means of astronauts: waterlogged tissues and towels put into polyethylene bags before being used for the purpose intended [45, 46]; thereat, the radiation level in the compartment decreases by 20–30%. Protective Shutter is effectively used to reduce both ERB doses during an orbital flight and SCR doses during a flight beyond the magnetosphere. In addition, for further enhancement of radiation protection for the crew it is recommended to use the local (zonal) and individual means of anti radiation protection (such as shutters, rugs, helmets made of special materials, etc.). This approach requires further calculations and experiments under SF conditions with due regard to the ergonomics of using the proposed means of protection by the crew.
Habitable stations on the lunar surface must be equipped with a shelter: a radiation protected accommodation where lunar base inhabitants could wait till SPE is over. Such shelters could be natural lunar caves or inflatable lunar modules covered with a regolith layer. In the depth of lunar soil, the flux of secondary neutrons from the GCR particle fluxes makes the major contribution to the equivalent dose and, hence, the construction of inhabited modules protected by planetary soil matter should be treated with caution. According to the estimates [36, 38], the thickness of regolith used for radiation protection must be more than ~1 m to provide effective absorption of secondary charged particles, including neutrons. The most favorable areas for establishing lunar bases are those with lunar soil containing water in its solid state (ice), which effectively slows down secondary neutrons.
During the flights on the near-Moon orbits, the field of cosmic ionizing radiation is characterized by the complex composition varying within a broad range and abrupt fluctuations in intensity, setting up stricter requirements for dosimetry and radiation monitoring equipment and for the methods of predicting the changes in the radiation situation.
The on-board dosimetry systems become the most crucial part of radiation safety assurance for the crew. These systems should not only measure the radiation fields and send data to the Earth, but also assess the radiation situation and provide real-time information for the crew to go timely to the radiation shelter. The necessary experience of creating, developing and operating the equipment of on-board radiation monitoring system and individual radiation dosimetry devices has been gained from exploitation of the Russian Orbital Segment of the ISS [1, 47, 48].
In order to determine the maximum permissible radiation exposure during flights beyond Earth’s magnetosphere, it is necessary to calculate (based on the initial data on particular mission) the absorbed and equivalent doses from different sources of cosmic radiation, to investigate the pattern of dose stress distribution over astronauts’ bodies with time and to assess with the maximum accuracy various radiobiological effects resulting in work capacity and health impairment in astronauts during the flight, as well as to assess the risk of health impairment long after the flight.
Since the existing radiation safety standards have been developed for orbital flights, there is a need to develop and approve new standards for missions beyond the Earth’s magnetosphere, taking into account the specific characteristics of radiation during such flights. In lunar missions, special attention should be focused on the exposure to GCR radiation, with taking into consideration the above-mentioned probability of acute effects of HCP being part of GCR [2–4]. It should be noted that it is precisely HCP that make the major contribution to the equivalent dose of GCR (up to 80%) .
However, in addition to the doses received under conditions of exposure of biological objects to HCP, much attention is currently paid to the study of biological effects of these particles at the level of cellular structures, as well as on the state of the central nervous system. The latter factor can lead to the changes in behavioral reactions and affect human memory and mental health, etc. Due to the fact that GCR doses on the low orbits (ISS, etc.) are approximately 5–6 times weaker due to shielding by the Earth and the effect of the geomagnetic field, the modeling of GCR effect on astronauts during a 1-year flight in the interplanetary space will require an ~5-year stay on board the near-Earth station. Thus far, astronauts have never yet achieved such flight durations, even when summing up the durations of several missions, and the radiobiological effect of GCR during long-term space missions has been studied insufficiently, even taking into consideration the experience of long-term flights on the ISS.
The modeling of GCR under terrestrial conditions is a difficult (and rather expensive) task, because it is necessary to have a set of accelerated beams of charged heavy nuclei of several types in a rather broad energy range with subsequent chronic radiation exposure of biological objects [49, 50]. Nevertheless, it should be assumed that further studies in this field can lead to substantial adjustment of radiation hazard in space flights towards its increase.
(1) In view of the radiation factor, the flights to Moon are far more dangerous than the near-Earth orbital flights. Due to the absence of protection provided by Earth’s magnetosphere, the doses from strong SPEs can be tens and hundreds times higher than those recorded on the ISS orbit. The flights to the Moon in such periods of radiation perturbations place special requirements and demands on SPE forecasting during the flight and on the operational activities of the Radiation Safety Service for Manned Space Flights.
(2) In contrast to the deterministic models of GCR, the statistical models of SCR (including the model of the Research Institute of Nuclear Physics, Moscow State University) calculate the predicted flux of particles (with allowance for their energy spectra) that will be generated by numerous individual SCR events within the estimated period of time and can be exceeded with a preset probability. Apparently, in such predictions aimed at protecting the flight crew and equipment from radiation, this probability should be set at a level of no more than several percent. Similar to the SCR particle fluxes, the dose per lunar mission calculated for a specified period of time is a random value. The question arising in this context concerns the radiation hazard for flight crews and for personnel of planetary stations as a probability of the dose received in the predefined time period exceeding the maximum permissible dose. Or, to put it another way: what should be the duration of space mission for the received dose not to exceed the maximum permissible dose with a preset probability (or the level of reliability).
(3) The estimates based on the SCR model of the Research Institute of Nuclear Physics, Moscow State University, show that at the predefined level of reliability of 97% (the probability of exceedance of maximum permissible doses of radiation below 3%), the expected stay time on the near-Moon orbit or on the lunar surface for humans protected with a 10-g/cm2 aluminum shield should be no more than 1.5 months in the SA maximum and 1 year in the SA minimum.
(4) For controlling the designed level of reliability in lunar missions, lunar spacecrafts should be equipped with local (on astronaut’s body) and/or additional (on the less protected wall of a module) protective equipment with a specially selected protective material. The prototypes of such protective equipment and the experience of flight support in hazardous periods have been already worked through by the experts of the Radiation Safety Service for Manned Space Flights on the ISS.
(5) The contribution to the dose of heavy charged particles during the flights to the Moon is several times higher than during the flights of the same duration on the ISS orbit; hence, the probability of negative effects of GCR on the cognitive functions of astronauts increases, leading to mistakes in operator performance directly during the flight. Further studies in this field are necessary in view of the high uncertainty of radiobiological results obtained in the experiments with animals in accelerators.
(6) For lunar missions, it is necessary to take into account the joint effect of zero gravity, radiation and low magnetic field (hypomagnetic conditions), which is currently understudied.
(7) The manned lunar flights with an acceptable level of reliability with respect to radiation safety are possible but require further studies and the development of special technical and organizational measures and means for radiation (and, in a broader sense, medical) safety assurance for flight crews.
(8) In view of the lack of knowledge about radiobiological effects of HCPs from GCR and strong SCR events, at the first stage it seems reasonable to limit the duration of lunar missions to 1.5 months. Later on, the duration of lunar missions can be extended as more data on the medical support, methods and means of radiation protection are accumulated.
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The study was performed as part of the Program for Basic Research of the Russian Academy of Sciences, project no. 63.2.
COMPLIANCE WITH ETHICAL STANDARDSThe authors declare that they have no conflict of interest. This article does not contain any studies involving animals or human participants performed by any of the authors.
Translated by E.V. Makeeva
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Orlov, O.I., Panasiuk, M.I. & Shurshakov, V.A. Radiation Factor in Lunar Missions. Hum Physiol 46, 709–721 (2020). https://doi.org/10.1134/S0362119720070117
- lunar mission
- ionizing radiation
- dose measurement
- radiation safety