1 Introduction

In the human exploration of space, the exposure to radiation has always been one of the main issues to be faced to protect the health and life of astronauts during missions. A human crew in a space habitat, whether a spacecraft, a space station, a planetary base or a space suit, is exposed to a radiation environment that is quite different from that on Earth, as in the first case high-energy ions constitute the larger part. Primary radiation fields, such as Galactic Cosmic Rays (GCR) and Solar Cosmic Rays, consist mainly of ions ranging from hydrogen to iron and beyond and can deliver significant doses to human tissues also through secondary particles produced by their interaction with the space habitat materials.

Systematic studies of space radiation fields and their interaction with the human body have made a significant advancement in the ’60s during the Apollo missions, the first and so far the only ones to take humans beyond Earth’s magnetic field exposing crews to deep space radiation [1]. Peculiar phenomena due to the specific composition of cosmic radiation fields have been observed and studied, such as the light-flashes. Apollo 11 astronauts first reported light-flashes observation in environment with no light-sources (namely phosphenes), raising the scientific community’s interest. Experimental investigations in space and on ground subsequently indicated that light-flashes were generated by the direct interaction of charged particles with the eye or Central Nervous System (CNS) [2]. The knowledge gained in space radiological protection until today, also through experiments performed on space stations, is being exploited to assess the exposition risk and also to define the necessary countermeasures to protect the health and life of astronauts, constituting the radioprotection foundations of any space manned mission plan.

In particular, NASA has planned missions toward deep space (far from Earth’s magnetic field) to go back to the Moon establishing a colony in the near future and explore Mars in the next two decades. It is known, from the data available, that the mission to Mars would imply a dose equivalent exceeding 1 Sv, with a REID (Risk of Exposure Induced cancer Death) beyond the NASA limit established to be equal to 3% [3].

The integrated multidisciplinary approach, pursued by NASA and other space agencies to protect humans in future missions toward the Moon and Mars, is based on the synergy of different countermeasures, such as passive and active shielding, drugs or nutritional supplements to repair or prevent DNA radiation damages, developed in the fields of biology, pharmacology and physiology and a space-weather forecasting system.

The present work aims to provide a review of the radioprotection in space, highlighting the main differences with that on ground. First, a characterization of space radiation fields in human habitats and their source terms is given. Then, after focusing on the space radioprotection methodological foundations, the integrated multidisciplinary approach is introduced, and in this context, different issues related to human exposures in space and specific problems related to astronautics are highlighted.

2 Radiation fields inside human space habitats

Inside human habitats, the radiation environment is the result of different fields superposition, each one characterized by a large dynamical behavior both in time and space. Such a variability makes difficult any field characterization that is not point-like in space and in time, raising difficulties in assessing effects on exposed human crews.

Radiological protection in space considers radiation fields composed mostly by ions from hydrogen to iron (\(^{56}\textrm{Fe}\)), which can generate secondary neutrons and nuclear fragments via their interaction with the habitat material, further contributing to the total delivered dose to the astronauts.

Each field component shows flux and energy variations as a function of the specific location within the habitat, the relative position of the habitat with respect to the Earth’s magnetic field, but also as a function of time, due to the solar activity modulation induced on galactic radiation component.

Space radiation fields can induce severe damage to the exposed biological tissues, because they can release a large amount of energy over very short distances (locally), making the damage to DNA more likely. This particular characteristic is quantified in radio-biology by the LET (Linear Energy Transfer) variable, defined as the average amount of energy lost per unit track length in matter by a specific type of radiation. High-LET radiation fields, such as those in space, can induce damage more efficiently than low-LET fields (e.g. X-rays), because can cause the same physical effect with smaller delivered doses. In radio-biology, this radiation characteristic is quantified by the RBE (Relative Biological Effectiveness) variable, defined as the ratio between absorbed doses of two kinds of radiation required to produce the same biologic effect. Thus, space radiation fields are characterized by high values of LET and RBE, which makes them one of the main risks to be addressed in future human deep space missions to preserve the health and lives of astronauts. Space radiation fields, their origin, and their physical characteristics are introduced in the next sections.

2.1 Galactic cosmic rays

In the broad spectrum of energy and elemental composition of Galactic Cosmic Rays (see Fig. 1, right panel), the radiation fields of radioprotection interest are those having sufficient high fluxes of particles capable to induce significant biological effects. The part of GCR considered for risk assessment in space consists mainly of ions ranging from hydrogen to iron (\(^{56}\textrm{Fe}\)), exhibiting maximum fluxes at energies of 1–2 GeV/A (where A is the mass number). In such a energy range, the most abundant particles are hydrogen ions (about 90%), followed by helium nuclei (about 8–9%) and heavier ions (about 1–2%) [4]. The extra galactic component, as well as ions beyond iron, are not considered in risk assessment in space, because of the small contribution due to their very low fluxes.

This large difference in terms of fluxes of light and heavy ions becomes smaller considering the dose and even smaller when the dose equivalent is considered (see Fig. 1-left panel).

Different acceleration mechanisms have been proposed to explain the high kinetic energy values observed in cosmic ions: they can be accelerated by collisions with magnetic field disuniformities in the moving interstellar medium (plasma clouds, second-order Fermi mechanism), by shock waves propagating at supersonic speeds in the interstellar medium (Supernovae Remnant, first-order Fermi mechanism), or directly by cosmic accelerators such as Active Galactic Nuclei (AGN) and Gamma-Ray Bursts (bottom-up mechanisms) [5]. When entering the heliosphere, galactic cosmic rays are modulated by solar activity, which has an 11-year half-period. Solar modulation of GCR is a physical phenomenon created by the complex and time-varying combination of different physical mechanisms, which contribute to the variation of intensity and shape of the GCR spectrum inside the heliosphere. During the maximum of solar activity, the heliosphere expands as a result of the increased solar wind flux, which also transports the sun’s magnetic field. In this case, the lower energy components of GCR spectrum are deflected outward the Solar System, while the penetrating part undergoes diffusion, convection, drift and energy losses phenomena, resulting in a net flux decrease [6]. In contrast, during periods of solar activity minima, the galactic component of radiation in the Solar System exhibits a maximum.

Fig. 1
figure 1

Left: Percentage contribution to the fluence (green), delivered dose (blue), and dose equivalent (red, color online) in function of the ion atomic number (source of original image [7]). Right: Galactic and extra galactic fluxes in function of energy for different particles (source of original image [8]). Superimposed (in blue, color online) the energy range of radioprotection interest

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2.2 Solar cosmic rays

The Solar component of Cosmic Rays consists essentially of protons and \(\alpha\) particles accelerated from the Sun photosphere and corona in the so-called Solar Particle Events (SPEs). These Solar Energetic Particles (SEPs) are generated by two distinct physical phenomena: Solar Flares and Coronal Mass Ejections (CMEs) [9].

In the case of a flare-driven event, protons and \(\alpha\) particles normally present on the Sun’s surface and above are accelerated by a flare, which is an explosive phenomenon on the Sun photosphere induced by magnetic lines reconnection in the solar plasma. For particularly powerful flares (class M or X), the phenomenon can interest also the Earth, with integrated proton fluxes that can reach \(10^{11}~\text {protons}/\text {cm}^2\) and typical energy of hundreds MeV (see Fig. 2, right). Flare-driven SPEs are pulse-type phenomena [10], with a typical time duration of a few hours [11].

Coronal Mass Ejections are originated from solar prominences, in which protons and \(\alpha\) particles (solar plasma) are trapped in a magnetic flux tube in the corona region. In the case of magnetic reconnection phenomenon (which can also generate a flare on the photosphere), the trapped plasma can be released in the heliosphere at velocities ranging from 200 to 2000 km/s, forming a CME. This release of material and magnetic field originates a shock-wave in the heliosphere which interacts with the solar wind, and accelerates particles to hundreds of MeV/A (see Fig. 2, left) [12]. A CME is a gradual event that can take a few days to reach the maximum in terms of particle flux [10].

Fig. 2
figure 2

Left: Shock wavefront generated in heliosphere by material release during a CME (source of original image [11]). Right: Typical proton fluence spectra for some high-intensity SPEs (source of original image [12]). The superimposed blue line (color online) marks the part of the spectrum shielded by a typical spacecraft shielding

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2.3 Trapped particles

Charged particles, such as protons and electrons, are trapped by Earth’s magnetic field in toroidal regions surrounding the planet, the Van Allen belts. Two radiation belts are identified, an inner containing protons and an outer containing electrons, generally distinguished in terms of altitude and latitudinal position. Protons trapped in the innermost band (between 1000 and 12000 km in height) travel spiraling along magnetic lines from the south to the north magnetic pole and vice-versa, with energies that can exceed 100 MeV. In particular protons, with energies greater than 50 MeV, come from neutrons produced by the interaction of cosmic rays with the uppermost layers of the atmosphere, which decay passing through this region. The electrons are trapped in the outermost belt (from 13000 to 65000 km in height) and have energies up to 10 MeV [13]. Trapped particles in belts are not one of the main risk for deep space manned missions, but they can still deliver non-negligible doses to astronauts in space stations, especially the inner belt because of the repeated passage through the South Atlantic Anomaly (SAA). The SAA is a region where the Earth’s magnetic field is weaker because the dipole field is offset from the center of an ideal magnetic dipole by about 500 km. SAA develops above part of Brazil and a large part of the South Atlantic, at about \(200~\textrm{km}\) of altitude, intersecting the Low Earth Orbit (LEO) at which the space stations generally fly (about 400 km) (see Fig. 3, left).

2.4 Secondary radiation fields

The radiation fields described above, commonly named ’primary radiation fields’, generate also secondary radiation fields by interacting with materials of space habitat. When primary protons and ions interact with matter a wide range of particles are generated via nuclear reactions, in the development of hadronic shower. However, for radioprotection purposes, only neutrons and nuclear fragments are typically considered among these secondary particles, since these two components deliver the larger part of the dose to astronauts [14].

Considering neutrons, it is important to remind that their half-life is about 10 min and, because of their instability, they cannot exist freely in the primary radiation space fields. Rather, neutrons are created only as secondary particles alongside nuclear fragments by the ablation and fragmentation of heavier nuclei [15], two physical phenomena at the basis of development of the hadronic shower. Measurements on the ISS indicate that neutrons can reach a few hundred MeV energies [16], but the largest flux occurs at a few eV of kinetic energy [17] (see Fig. 3, right).

Fig. 3
figure 3

Left: Cartoon that shows schematically the Van Allen radiation belts and typical altitudes of LEO (Low Earth Orbit) and GEO (Geostationary Orbit) orbits. Right: Neutron spectra measured onboard the International Space Station (source of original image [17])

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3 Radiation risk assessment in space

The risk assessment paradigms in space and on ground are significantly different because of the peculiar characteristics of the two considered radiation environments. The first difference concerns the radiation fields: in human space habitats, there is an exposure to mostly medium and high-LET particle radiation, which can transfer energy locally along the track in non-uniform patterns, creating correlated damages to organized targets (e.g. cells) [18]. ’Terrestrial’ radiation fields have a major component of low-LET radiation (photons) able to generate isotropic damage to organized targets in uniform patterns. A medium and high-LET component (\(\alpha\), \(\beta\), neutron) can be added to the photon part on Earth, with kinetic energies lower than the space ones.

This strong difference in terms of radiation fields brings an increased probability of stochastic effects, and deterministic effects cannot be ruled out for exposure in space. For this reason, some concepts of the quantities used in radiological protection on Earth must be reconsidered for space. The equivalent (\(\mathrm {H_T}\)) and effective dose (\(\textrm{E}\)) introduced by ICRU [19, 20] and ICRP [21] to evaluate exposition in working activity on Earth are defined from the absorbed dose averaged over tissue or organ weighted by appropriate dimensionless factors describing the risk as a function of the type of radiation (\(\mathrm {w_R}\)) and the radiosensitivity of the tissue (\(\mathrm {w_T}\)). These quantities cannot be used in the evaluation of exposures to space fields because the weighting factors \(\mathrm {w_R}\) and \(\mathrm {w_T}\) used in their definition are not representative of radiation fields and of the population exposed. Following the ICRP 103 definition [21], \(\mathrm {w_R}\) is a dimensionless factor by which the organ or tissue absorbed dose is multiplied to reflect the higher biological effectiveness of high-LET radiations compared with low-LET radiations. It is used to derive the equivalent dose from the absorbed dose averaged over a tissue or organ. Considering the space radiation fields, \(\mathrm {w_R}\) does not discriminate on ions RBE basis, because the factor is always 20 for any nuclear fragment. In this way, there is no distinction in terms of hazard, for example, between a \(^{56}\textrm{Fe}\) and a hydrogen ion with the same nucleon-normalized kinetic energy.

The tissue weighting factor \(\mathrm {w_T}\) is defined by ICRP as the factor by which the dose equivalent in a tissue or organ T is weighted to represent the relative contribution of that tissue or organ to the total health detriment resulting from uniform irradiation of the body. It is used to assess the effective dose on ground, but is not directly applicable in space, because \(\mathrm {w_T}\) is not representative of astronauts’ tissue sensitivity to cosmic radiation. Values of \(\mathrm {w_T}\) are calculated from statistics accumulated on Earth on very large cohorts, especially individuals exposed to the nuclear warhead detonations on Hiroshima and Nagasaki [22]. These values cannot be related to astronauts’ exposure in space, because they are referred to an Asiatic population of the ’40 s exposed to a radiation field profoundly different from those in space. Astronauts crews belong to different ethnic groups (Caucasian, African, Asian), and as it is well known, different populations show different incidences of specific cancer development [23]. At the same time, the small number of astronauts who have participated in space missions from Gagarin onward does not provide sufficient statistics to define new tissue weighting factors for spaceflight. However, the effective dose is sometimes used to study the exposure of astronauts in space habitats. In this case, the \(\mathrm {w_T}\) factors are calculated with specific mathematical models or simulations [24].

The risk assessment in space is based on the dose equivalent (\(\textrm{H}\)), an operational quantity defined by multiplying the absorbed dose at the point of interest in tissue for the radiation quality factor Q. According to [25], Q is a dimensionless parameter, with a ’point-wise’ definition:

$$\begin{aligned} Q = \frac{1}{D} \int _{L=0}^{\infty } Q(L)D_L \,\textrm{d}L \end{aligned}$$
(1)

where D is the absorbed dose at the point of interest of tissue, \(D_L=\textrm{d}D/\textrm{d}L\) is the dose distribution in terms of unrestricted LET, and Q(L) is the quality factor trend as a function of LET (see Eq. 2). The integration is performed over all values of L in order to consider all charged particles excluding the secondary electrons produced in the particle-matter interaction. The Q(L) function is defined as:

$$\begin{aligned} Q(L)= {\left\{ \begin{array}{ll} ~~~~~~~~~~~~~~~ 1 &{} L<10~\mathrm {keV}~\mu {\text {m}}^{-1}\\ 0.32~L-2.210~\mathrm {keV}~\mu {\text{m}}^{-1} ~\le &{} L\le 100~\mathrm{keV}~\mu {\text{m}}^{-1}\\ ~~~~~~~~~~~300/\sqrt{L} &{} L\ge 100~\mathrm {keV}~\mu {\text{m}}^{-1} \end{array}\right. } \end{aligned}$$
(2)

Depending on LET, the factor Q is representative of different RBE of interacting ions, allowing discrimination between high-LET and low-LET radiation. On the other hand, dose equivalent H is evaluated along the particle track, so it is not a reliable indicator of overall exposure for the entire body. For this reason, generalizations of the quality factor have been proposed. One of these, function of \(\textrm{Z}\) and \(\textrm{E}\) expressed in terms \(Z^{*2}/\beta ^2\) and LET, [26, 27] is:

$$\begin{aligned} Q(Z,E) = (1-P(Z,E))+6.24 \left( \frac{\Sigma _0}{\alpha _{\gamma }L}\right) P(Z,E) \end{aligned}$$
(3)

where the function P(Z, E) is defined as follows:

$$\begin{aligned} P(Z,E) = \left( 1- e^{-{\textrm{Z}^{*2}}/{\mathrm {k \beta }^2}}\right) ^m \end{aligned}$$
(4)

Parameters k, m, and \(\Sigma _0\) are extrapolated from the fit of radio-biological experiments data, while \(\alpha _{\gamma }\) parameter for low-LET radiation is estimated from epidemiological data available for gamma expositions. The variable \(Z^{*}\) is the effective charge of the particle, which includes corrections for low-energy particles [28]. Considering Eq. (3), L is not an independent variable besides Z and E, but for a given Z and E, the L value is fixed. The factor Q(Z,E) and the dose equivalent as a function of \(\textrm{Z}\) and \(\textrm{E}\) are shown in Fig. 4.

Attempts are also being made to quantify tissue damage due to radiation exposure in space, see for example [29].

Fig. 4
figure 4

Left: Quality factor Q(Z,E) behavior as a function of kinetic energy and Z of cosmic ions. Right: dose equivalent evaluated with quality factor Q(Z,E) for GCR fluence at solar minimum (source of original image [28])

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Fig. 5
figure 5

Dose to astronauts from Mercury program to missions onboard the International Space Station (ISS) (source of original image [24]). Badge dose (blue dots, color online) is the absorbed dose measured by badge dosimeters in mGy (see text). Effective dose (white dots, in mSv) presented in the reference has been calculated using a gender-specific \(w_T\) averaged over typical astronauts age (30–60 years). Biodosimetry results (in mGy-equivalent, red dots, color online) are related to the assessment of the chromosomal damage in lymphocyte cells using the multicolor fluorescence in situ hybridization (FISH) technique [30]

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Besides scientific experiments performed with active detectors to measure and characterize radiation fields in space habitats, the dose to astronauts is measured with personal dosimeters. Thermoluminescent dosimeters (TLD) are used for low-LET radiation (\(<10~\mathrm {keV\mu m}^{-1}\)) and trace dosimeters for high-LET radiation (\(>10~\mathrm {keV\mu m}^{-1}\)) (see Fig. 5) [31]. Also personal active dosimeters are used onboard the ISS, which allow a real-time personal monitoring of the crew [32]. Biodosimetry is also used to assess doses to astronauts: a blood sample of astronauts, once back on Earth, is taken, and the number of specific chromosomal aberrations in lymphocytes is counted. Such number is proportional to the absorbed dose (for more details see [33]).

4 Human space missions: exposures, risks and their mitigation

NASA, in collaboration with ESA and other space agencies such as JAXA and CSA, is planning a series of manned missions to come back to the Moon after 50 years after the last Apollo mission and toward Mars (within the next 20 years). One of the main showstoppers is the crew’s exposure to the hostile radiation environment of deep space for long periods. Data collected in the Apollo program (at present the only one with human crews traveling in deep space) and in unmanned space missions indicate that the highest exposures are those measured during the transit phases to celestial bodies. The MSL-RAD detector (Mars Science Lab-Radiation Assessment Detector) [34], mounted on the Curiosity rover, measured a dose equivalent rate of \(1.84\pm 0.30 ~\mathrm {mSv/day}\) during the cruise toward the red planet, of which only 5 percent can be attributed to solar events. This value drops to \(0.70\pm 0.17~\mathrm {mSv/day}\) on the planet, more than a factor two less than the cruise value. This reduction is due both to a geometric reason, since the planet screens half of the sky, and to the thin atmosphere of the red planet. The CRaTER (Cosmic Ray Telescope for the Effects of Radiation) detector, housed in the science payload of the Lunar Reconnaissance Orbiter (LRO) orbiting around the moon, has shown that measured ion fluxes are comparable to those recorded by MSL-RAD during the cruise to Mars [35]. Data collected on the surface of the moon by the Chinese Lunar Lander Neutrons and Dosimetry (LND) experiment indicated a radiation environment with an exposure rate of about 1 mSv/day [36]. For deep space missions, which may last from 100 to 1000 days, the integrated dose to astronauts can be therefore prohibitive, reaching 1 Sv in the case of Mars missions. For this mission has been calculated a Risk of Exposure-Induced Death (REID) that exceeds the 3%, the limit value adopted by NASA to discriminate unacceptable risks. The REID is calculated considering a certain population not exposed to radiation and its baseline level of cancer deaths, and quantifies the risk increase with respect to this baseline, not accounting for cancer deaths that would occur anyway, but shifted to an earlier age due to radiation exposure [37]. Limiting the risk (in terms of REID) is one of the main differences between radioprotection in space and on ground, where limits are set on equivalent and effective dose. The REID concept allows to overcome the dose equivalent assessment limitations, evaluating the risk of cancer or other effects induced by radiation considering the whole body exposure. Moreover, the REID considers age, sex, and professional career (number of flights, age at first flight) of astronauts. This constitutes a first step toward a risk model based on the individual characteristics. In order to mitigate this risk, space agencies propose the application of the integrated multidisciplinary approach. It is based on different countermeasures developed in different scientific and technical fields, such as mission planning, passive shielding, active shielding, molecular biology, pharmacology, physiology, and space-weather forecasting.

4.1 Mission planning

The planning of a mission in deep space, in terms of spacecraft trajectory as well as the time period, is one of the aspects that can largely contribute to limit the crew exposure. Each mission to deep space involves the Van Allen belts crossing by the spacecraft. Although it is well known today that they do not contribute significantly to the astronauts’ total dose because of the limited time to cross the regions (less than 10 mSv/year for launch and return, about 1.5 % of the dose from GCR in the cruise phase and about 5 % of the dose to the ISS crew from repeated SAA passages [38]), during the Apollo program they were considered a hazard for the crew [39]. For this reason, the flight paths of the Apollo spacecrafts, at the present the only manned missions in deep space, were designed to cross the radiation belt outside the geo-equator at high-latitude, in order to exploit their conformation, where particle fluxes decrease as latitude increases [40].

In addition, the choice of the best time period in terms of solar activity plays a fundamental role in dose limitation for deep space missions, due to the solar modulation of the galactic cosmic ray fluxes.

Fig. 6
figure 6

Left: Solar activity (above) and normalized Galactic Cosmic Rays counts (below). The two illustrated phenomena are in counter-phase (source of original image [41]). Right: Result of computational models for Galactic cosmic rays fluence spectrum during the 1977 solar minimum (above) and the 1981 maximum (below) (source of original image [42]). The blue line (color online) marks the part of the spectrum shielded by a typical spacecraft shielding

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GCR deliver to the crew the larger part of the dose equivalent with respect to the solar radiation, as shown by MSL-RAD detector data (see Sect. 4), because of their higher radiation quality factor Q, the isotropic irradiation of the spacecraft as well as the low shielding efficiency of spacecraft material for these high-energy ions. The intensity and shape of the GCR differential fluxes vary depending on the conditions of the heliospheric environment, as clearly visible in Fig. 6, right. In particular, during periods of maximum solar activity, the component of galactic fields decreases as the solar particle field increases [41], as shown in Fig. 6, left. Different mathematical models have been developed to estimate the difference in terms of GCR fluence or flux during solar minima and maxima. In Fig. 6, right, are shown the results of computational models for Galactic cosmic rays fluence spectrum during the 1977 solar minimum (above) and the 1981 maximum (below) [42, 43]. In the model, during the solar maximum, the GCR fluence decreases by a factor 10 and at the same time, there is a shift of the peak of the spectrum toward higher energies, resulting in a flux hardening. This factor is reduced to 2 considering the absorbed dose measured on Moon (radiation environment similar to deep space) or less on ISS and Mars [44]. Considering the hardening of GCR flux during solar maxima (see Fig. 6, right), this difference can further vary in terms of dose equivalent.

Supposing a reduction factor equal 2 also in terms of GCR dose equivalent, values registered by MSL-RAD during the cruise phase toward Mars (performed in between the 2008 solar minimum and the 2014 solar maximum) could be about halved for this major component [34]. While bearing this in mind, making a thoughtful selection of the flight window could prove to be a viable strategy to reduce the delivered dose to the crew if no large SPEs, whose occurrence frequency is higher during solar maxima, hit the spacecraft. Solar particles have a LET lower than GCR for the same nucleon-normalized kinetic energy, resulting in a lower quality factor that compensates for the higher fluxes in the dose equivalent calculation. Additionally, there is a lower probability of a single solar event hitting the spacecraft during its development, and part of the solar radiation is blocked by typical habitat shielding (protons up to about 20 MeV). However, it is important to consider the increasing number of SPEs during a solar maximum when conducting a trade-off risk assessment.

In addition, radiation hardness of the equipment and computers, which will play a key role in the missions, must be considered: solar events, with high proton fluxes, can induce cumulative effects in electronics, as well as changes in the logic state of micrometer size transistors (SEU—Single Event Upset) [45]. Since missions will depend on computers and electronic equipment, their protection from radiation damage must be appropriately studied.

4.2 Passive shielding

Exposure to space radiation can be mitigated by using passive shielding, which has to be effective against the space radiation fields described in Sect. 2. Considering an ion passing through the matter, it loses energy both via electromagnetic interaction processes as well as via nuclear interaction processes. Nuclear fragmentation processes, such as abrasion and ablation of nuclei, give a net increase of dose equivalent downstream of the shielding material due to the creation of charged fragments and neutrons with energies less than the initial impinging particle and consequently higher values of radiation quality factor Q. While the energy loss due to electromagnetic processes depends on the shielding material via the ratio Z/A of the Bethe-Block equation (that range from 1 for the hydrogen to 0.4 for lead, and it is almost 0.5 for stables nuclides in between), the nuclear fragmentation process has a cross section proportional to \(\textrm{A}^{1/3}\) (Bradt-Peters formula [46]). An effective material to shield from space radiation must limit the development of the hadronic showers that increase the dose equivalent to the crew. Considering these functional trends, light materials perform best in space because they limit nuclear interaction and nuclei fragmentation. The shielding efficiency of a given material is the highest for medium-thickness shields, but it lowers as the thickness increases. At larger material thickness, the passing-through ion finds more scattering centers (nuclei) to interact via nuclear processes, resulting in an enhanced probability to have fragmentation or ablation. Low-A materials are generally lightweight and therefore particularly suitable in astronautics: currently, hydrogen-rich materials are used in human space habitats (theoretically the best shielding material is liquid hydrogen). In particular, Polyethylene and Kevlar (see Fig. 7) are already used in space habitat also for purposes other than radiation shielding, or water, proposed to fill radiation safety vests [47] or to surround habitat safety vaults in case of solar events.

Fig. 7
figure 7

Comparative shielding efficiency measurements of Kevlar and polyethylene tiles, measured onboard the International Space Station (ISS) by the ALTEA experiment. Left: Dose-rate and dose equivalent-rate spectra measured downstream polyethylene and Kevlar tiles of different densities. Right: Dose-rate and dose equivalent-rate net values downstream polyethylene and Kevlar tiles at High Latitudes of ISS (source of original image [48])

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The evaluation of material suitability for space radiation shielding is generally carried out in several steps: the first is a numerical simulation, followed by ground tests on accelerators in specialized centers. In case of positive results, the material is characterized in space, usually onboard the International Space Station (see Fig. 7). No significant progresses are expected from materials science and research on new materials in terms of radiation shielding efficiency: materials used today, such as polyethylene, already perform close to optimal. Expectations from this field rely on new materials that perform well as space radiation shielding and that can be used for other purposes as well, such as Kevlar, which is useful for shielding the space habitat also from micro-meteorites.

4.3 Active shielding

Active shielding is based on magnetic fields used to deflect charged particles, according to the Lorentz force (see Fig. 8, right). The radius of curvature is expressed by the well-known gyroradius formula:

$$\begin{aligned} \rho =\frac{\mathrm {m_0}\gamma \beta \textrm{c}}{\textrm{q B}} \end{aligned}$$
(5)

where \(\mathrm {m_0}\) is the particle mass at rest, \(\gamma\) and \(\beta\) are the Lorentz factors, \(\textrm{c}\) is the speed of light, q is the particle charge, and B is the magnetic field strength. Considering the masses and kinetic energies of the ions of the space radiation, the required magnetic field strengths must be very high, on the order of tens of T (tesla). These field values can be reached only using cryogenic superconductors. In order to surround the spacecraft habitat module with such magnetic field, the coils should have large dimensions (several meters) and weight (tens of tons, depending on the design [49]), requiring an assembly procedure in space with potentially prohibitive costs. Several geometries have been proposed, with designs that meet the highest active shielding efficiency and the lower intensity of magnetic field inside the human space habitat, in order to preserve the health of the crew [50]. The effectiveness of this kind of shielding is related to the magnetic rigidity of particles, which expresses the resistance of a charged particle of a given momentum p to be deflected by a magnetic field:

$$\begin{aligned} \chi _B = B\rho = p/q = \frac{m_0 c}{q}\sqrt{\gamma ^2-1} \end{aligned}$$
(6)

GCR ions have magnetic rigidity higher than solar particles, so these kind of shields will be less effective on GCR, part of which cannot be bent-out enough by the field. In this case, they can interact with the large amount of high-Z materials of the coils, starting hadronic showers which can deliver high doses to astronauts in the habitat module. This issue can be partially attenuated using passive shielding below the coils [51]. In addition, there is a considerable hazard for the crew, given by the risk of magnets quenching, a phenomenon potentially explosive that can happen if the coils loose the superconductive state in case of a fault of cryogenics.

Fig. 8
figure 8

left—Sketches of active shielding around living space module, with ’toroidal racetrack’ geometry. Middle and right panels—Principle of the toroidal magnetic shield, with curvature radius as a function of the angle \(\Phi\) with to the toroidal axis (source of image: [49, 52]

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Proposed solutions spans from the coil design in segmented sectors to limit the consequences of quenching, to the addition of supplementary mass in the coil structure to absorb the large magnetic energy stored inside in case of its instantaneous release [52], or considering high-temperature superconductors [53]. Among active radiation shields also electric fields have been proposed. This kind of solution can overcome both the problems of large weight of the structure, using inflatable charged gossamer structures realized with light materials, and the quenching risks related to the coils. In addition, they can be exploited also for the electrostatic-based propulsion system for interplanetary missions [54]. On the other-hand, their shielding efficiency will be always less than the magnetic field one (for the same amount of required energy). Let us consider the electric rigidity of a charged particle, given by the equation:

$$\begin{aligned} \chi _E = E\rho = \frac{\gamma ^2-1}{\gamma } \frac{m_0c^2}{q} \end{aligned}$$
(7)

As can be verified, \(\chi _E\) is a factor \(c\sqrt{\gamma ^2-1}/\gamma =\beta c\) higher than the magnetic rigidity \(\chi _B\), making the use of electric fields to deflect relativistic particles extremely disadvantageous. To obtain the same bending radius (equal to 1 m) given by a magnetic field equal to 3.6 T for a 500 MeV proton, which has an electric rigidity equal to \(830~\textrm{MV}\), an unfeasible electrostatic field of \(830~\textrm{MV}\cdot \textrm{m}\) is required. The only way to overcome the problem is relaxing the condition on bending radius, considering larger values of \(\rho\) (tenth of meters), involving the placement of electrostatic structure far away from the spacecraft [55], a solution difficult to implement for superconducting coils.

4.4 Molecular biology, pharmacology, physiology

One of the main objective pursued by these disciplines concerns the development of drugs that can repair and/or mitigate cell damage as a result of radiation exposure. Part of the ionizing radiation damages to the DNA double-helix structure is induced by free radicals generated by the interaction of the particle with the cell, inducing the so-called oxidative shock [56]. DNA oxidation leads to its breakdown (single- or double-strand break). The ‘anti-radiation’ drugs are essentially antioxidants, able to eliminate free radicals and at the same time to promote DNA repair by specifically designated enzymes. Among the molecules available today (cysteines, cysteamines), the only clinically approved one is the amifostine \((\mathrm {H_2N(CH_2)_3NH(CH_2)_2SPO_3H_2)}\). This drug allows: (1) the elimination of secondary free radicals generated by radiation exposure, (2) the release of an electron to carbon-centered radicals (chemical repair), (3) the stabilization of DNA repair, (4) the inhibition of cell cycle progression (5) the induction of cell hypoxia [57]. However, amifostine has important adverse reactions that prevent its use except in very rare cases. An antioxidant-rich diet for the crew has been proposed as a preventive countermeasure in place of amifostine [58].

4.5 Space-weather forecasting

Space weather is a branch of space and solar physics dealing with the study of space environment variations within the heliosphere. In particular, space radiation fields, whether of solar or of galactic origin, are one of the subjects studied by this branch, which aim to forecast particle events with a potential impact on the Earth and on its surrounding space. For this reason, results from space-weather studies can play a fundamental role in the multidisciplinary integrated approach, since the forecasting can provide early warning about solar events that may affect the traveling crew. There is not yet a model of the Sun detailed enough to forecast the happening of a solar event (SPE or CME), where it will happen on the sun’s surface, and how it will propagate inside the heliosphere. For this reason, today we speak about nowcasting (forecasting by a few hours) rather than forecasting. The exploitation of nowcasting to give a prompt alarm to the crew travelling in deep space requires some major features, some of which are today only partially met: (1) deep knowledge of SPEs precursors, which can be monitored by a sentinel satellites network; (2) detailed models for SPEs propagation in interplanetary space; (3) measurement of SPEs particles in proximity of space habitat to evaluate the hazard for the crew. Even if today the first point is far to be met, the second and third points of the list are close to be realized. Today exist a network of Sun sentinel satellites (GOES Geostationary Operational Environmental Satellites), which continuously monitors the Sun. In particular, GOES satellites 16 and 17 [59] are equipped with a telescope for observing the solar corona in the extreme ultraviolet (EUV). EUV photons are emitted about 15 h before a CME event can reach the Earth. Measurements in proximity of the space habitat can be carried out using different particle detectors, already used on the ISS to evaluate the dose equivalent [48, 60].

5 Conclusions

This paper presents the state of the art for radiological protection in human space habitats, highlighting the major differences between radioprotection in space and the one on Earth. In particular, it has been emphasized that:

  • Space radiation fields are constituted mostly by ions, which can deliver significant doses to astronauts because their high LET and RBE;

  • This characteristic makes the space a particularly hostile environment for humans, where exposure can become prohibitive for long-duration missions such as those planned toward Moon and Mars;

  • The physical differences between space and terrestrial radiation fields require an approach to risk assessment for astronauts that is quite different from that generally used for exposed workers on ground (limitation on risk versus limitation of dose);

  • Radioprotection quantities defined for exposed workers on ground cannot be directly used for astronauts—specific quantities are defined and used for space radioprotection.

The strategy proposed to mitigate exposure risk in the hostile space radiation environment during deep space missions is based on the synergy of different countermeasures. The integrated multidisciplinary approach proposed by NASA for human exploration of the solar system is based on:

  • Mission planning—solar activity modulation of GCR can be exploited to lower the GCR contribution to astronauts exposure, even if a comparative analysis with the exposure to solar events must be made;

  • Passive shielding—highly hydrogenated light materials are proposed (and currently used) to limit fragmentation processes that can deliver significant doses to astronauts;

  • Active shielding—Magnetic and Electrostatic fields have been proposed to deflect charged particles away from the space habitat, even if there are still technical issues that are difficult to overcome;

  • Solutions from molecular biology, pharmacology and physiology—use of anti-radiation drugs (amifostine) or antioxidant-rich diets in order to repair or preserve cells from the oxidative shock due to radiations are also proposed;

  • Space-weather forecasting—it can be fully exploited in the future to prevent space crew exposure to large solar events.

This approach can theoretically allow the human exploration of Solar System, although continuous progress in research is necessary to overcome some of the issues and limitations that at present do not allow to fully exploit its potential.