Monte Carlo method. Monte Carlo (MC) is a stochastic method, exploiting random numbers to (a) “generate” an initial particles’ “cocktail”; (b) track them in arbitrary geometries; (c) accumulate the contribution of each track to a statistical estimator of the desired physical observables [59]. Step-by-step particles’ transport is simulated according to the statistical model of their interactions. Quantities (such as step lengths, event type, energy losses, and deflections) are sampled via generation of random values according to a given probability distribution. Indeed, in MC codes, the MC method deals with sampling from suitable stochastic distributions, with large samplings allowing to solve the integrations of multidimensional integrals.


In the context of space environment, the main interest is in high-energy particles whose scattering is generally low-angle. Therefore, it is reasonable to approximate multiple scatterings by a single continuous step, taking into account overall energy loss and direction change. This approach is known as the condensed-history technique. For example, ionization and excitation energy losses are described as continuous processes, i.e., they are continuously distributed along a particle step, if the loss is lower than a chosen threshold, together with their fluctuations.


Several MC codes are used nowadays throughout the world, such as Geant4 [60], FLUKA [61], and PHITS [62]. MC codes provide a detailed treatment of the three-dimensional transport of ions and neutral particles (see Chap. 4).


An overview of the different steps for calculation of the radiation environment of a celestial body is given in Fig. 10.10.

A flowchart depicts primary particles in deep space plus atmosphere and regolith model lead to particle transport and surface particle spectra of G C Rs and S E Ps.

Fig. 10.10
Scheme for Monte Carlo (MC) calculations of the radiation environment at a planet/celestial body, here in particular Mars. GCRs galactic cosmic rays, SEPs solar energetic particles, p+ protons, He2+ ions helium ions


A flowchart depicts primary particles in deep space plus atmosphere and regolith model lead to particle transport and surface particle spectra of G C Rs and S E Ps.


The input spectrum for GCRs can be chosen among different existing models that account for the variations of GCR particle fluxes due to variations in solar activity and in the large-scale heliospheric magnetic field throughout the solar cycle. The ISO 15390 model (ISO-15390 2004) [63] accounts for solar cycle variations in the GCR intensities on the basis of 12-month averages of the sunspot number. Changes in the large-scale heliospheric magnetic field are usually taken proportional to the corresponding changes in the Sun’s magnetic field, considering also solar cycle. More accurate models describe the spectra of GCR beyond the heliospheric modulation region. The CREME96 [64] and its updated version CREME2009 (https://creme.isde.vanderbilt.edu/) are based on a semi-empirical model [65] where the particle spectrum is calculated as a product of a function describing the LIS and a function describing the modulation according to solar activity. GCR particle spectra are described in the energy range from 10 to 105 MeV/nucleon, from H up to Ni nuclei from the year 1760 to present. The Badhwar–O’Neill 2010 (BON2010) [66] uses, instead of an empirical description of the modulated GCR.


As in the CREME model, a physical approach to describe the GCR propagation in the heliosphere due to diffusion, convection, and adiabatic deceleration. The BON2010 model exploits data from the International Sunspot Number (ISN) and considers time lag of GCR flux relative to the solar activity. The ISN is calibrated with GCR measurements from the Advanced Composition Explorer (ACE) and the Interplanetary Monitoring Platform-8 (IMP-8). The Burger-Usoskin model [67] is limited to GCR He and H ions assuming a constant ratio of the two types of ions. The reconstruction of the modulation parameter is based on neutron monitor count rates. The DLR model by Matthia et al. [68] describes the GCRs spectra of nuclei based on a single parameter, which is derived from measurements of the ACE spacecraft and from Oulu neutron monitor count rates for different solar modulation conditions.


SEP proton spectra are often considered from historical events, then parameterized by double power law fits in kinetic energy to event-accumulated integral fluence measured by the Geostationary Operational Environment Satellites and/or ground-based neutron monitor data [69].


Input spectra for both GCR and SEP events can often be retrieved via user-friendly tools, such as the SPENVIS online tool (https://www.spenvis.oma.be/) that is actually a collection of modules that allow for calculations of the radiation environment and radiation-induced effects via MC simulations in Geant4, or the On-Line Tool for the Assessment of Radiation in Space (OLTARIS) which operates on top of the deterministic code HZETRN (https://oltaris.nasa.gov/).


For Earth, more than 99.99% of its atmosphere’s mass is contained in the lower atmospheric layers below about 100 km. This region is mainly composed of N2, O2, and Ar which account for about 75%, 23%, and 1.3% by mass, respectively. The exact mass fraction of each constituent depends on the altitude. The water content in the atmosphere is highly variable but small, with the hydrogen fraction only reaching the order of 10–5% even in cloudy conditions [70]. Composition, density, temperature, and pressure vertical profiles can be obtained, for example, from the empirical atmospheric model 1 NRLMSISE-00 [71], which includes total mass density from satellite accelerometers and from orbit determination covering 1981–1997. For Mars, vertical profiles for pressure, density, temperature, and chemical composition of the atmosphere are often constructed exploting databases like MCD (Mars Climate Database http://www-mars.lmd.jussieu.fr) [46, 49]. Data can be extracted for specific locations, a specific day/night time, and season. The surface elevation and topology are extracted from the Mars Orbiter Laser Altimeter (MOLA) aboard Mars Global Surveyor. The fields (temperature, wind, density, pressure, radiative fluxes, etc.) are stored on a 5° × 5°, longitude-latitude grid from the surface to 120 km (and above) are averaged and stored 12 times a day, for 12 Martian “seasons.”


For Earth, the soil is often considered to consist of 50%Vol solids (of which 75%Vol SiO2 and 25%Vol Al2O3) and a scalable amount of H2O. Studies show that the neutron environment strongly depends on soil moisture (and air humidity) [72]. The composition of the surface and subsurface of Mars can either be chosen to model specific scenarios, for example, a default basaltic composition (SiO2 51.2%, Fe2O3 9.3%, H2O 7.4%) [73] or more/less hydrated compositions to study the possibility of underground shielding habitats [49], or it can be taken from data from the Gamma Ray Spectrometer aboard Mars Odyssey [46]. The dosimetric quantities at the Martian surface do not depend strongly on the regolith composition, although some differences due to hydration and Fe-content can affect neutrons and gamma rays spectra [49].


MC particle transport codes strongly rely on the availability of physics models and database of cross sections. A schematic view of the downward and upward main particles that need to be considered is shown in Fig. 10.11. In the open source Geant4 code [60], hadronic models are: (1) data-driven, which mainly deals with the detailed transport of low-energy neutrons and isotope production, (2) parametrized models which include fission, capture, elastic, and inelastic scattering reactions; (3) theoretical models for high energies, above several 10–100 MeV, where experimental cross-section data are scarce. For electromagnetic physics, the basic processes for electrons, positrons, photons, and ions, such as Compton scattering, photoelectric effect, pair production, muon-pair production for photons, ionization, δ-electron production, Bremsstrahlung, Čerenkov radiation, and annihilation, are considered. Additionally, processes involving the atomic shell structure such as Rayleigh scattering are also considered. Special process classes handle muon interactions like Bremsstrahlung, capture, and annihilation. Multiple scattering models provide corrections for path lengths and lateral displacements of multiple scattered charged particles. In order to decrease the computational time and resources, a certain production cutoff in the range is set for electrons, positrons, and photons, which is translated to energy below which the particle then loses its remaining kinetic energy continuously along the track and no secondary particles are produced.

A diagram depicts two different downpours of primary particles, such as proton, neutron, electron, and gamma rays.

Fig. 10.11
Schematic view of the particle showers (main particles are plotted here) generated in the downward propagation of primary GCRs particles through the Martian atmosphere and of the backscattered particles [74]


A diagram depicts two different downpours of primary particles, such as proton, neutron, electron, and gamma rays.


In principle, the proper approach to calculate the absorbed dose and dose equivalent rates is to use. Such standardized phantom has been defined by the International Commission on Radiation Units (ICRU) and it is given by the ICRU sphere, a 30 cm-diameter sphere with a density of 1 g/cm3 and a mass composition of 76.2% O, 11.1% C, 10.1% H, and 2.6% N, which reflects the composition of tissue. Still, in recent times more human-like phantoms have been used [75]. However, such complexity is not always necessary, and sometimes other spheres of water or water slabs have been used [76].


Apart from running the MC (or deterministic) codes in standalone mode, several tools such as the previously mentioned SPENVIS online system (https://www.spenvis.oma.be/), OLTARIS (https://oltaris.nasa.gov/), and the EXPACS/PARMA code (https://phits.jaea.go.jp/expacs/) based on PHITS can be used to run a combination of the steps described above, resulting in a punctual estimation of doses at a specific location on a body or altitude in an atmosphere or in radiation maps covering several regions.


For human exploration of Mars and other bodies, the quantities of interest are the absorbed dose corrected by the relative biological effectiveness (RBE) factor (to estimate the risk for acute effects or death due to high doses for Solar Energetic Particle events) and the Effective Dose and Dose Equivalent to respectively estimate the risks to long-term effects induced by exposure to GCRs and to compare with measurements from radiation detectors. Space Agencies implement the ALARA principle [77] which ensures that mission operations are designed to keep the radiation risks as low as reasonably achievable. Although the different agencies use common limits for deterministic effects on the ISS, different career radiation exposure limits (for stochastic effects) for astronauts in LEO missions exist and no specific limits for interplanetary missions are issued (only those for LEO exist).


Harmonization of risk models requires improvements in modeling radiation sources, in the accuracy of radiation transport codes, and the development of new realistic quality factors based on the features of the variegated radiation field in Space.


As already mentioned in Chap. 2, the approach commonly used for estimating risk from high linear energy transfer (high-LET) radiations is based on multiplying the induced absorbed dose (in units of gray) by a so-called quality factor, or RBE factor (always greater than one, usually below 20) representing the enhancement of effectiveness of the high-LET radiation. Such increased effectiveness comes from available evidence on the RBE of the radiations from both laboratory and theoretical studies (Sects. 10.4 and 10.5). As previously shown, RBE varies with LET. It depends also on other factors and may be different, e.g., for particular chromosome aberrations, mutations, or different tumor types. Also, RBE may vary in different biological systems. Furthermore, low-LET dose response is usually nonlinear while high-LET response tends to be more linear.


However, for radiation protection purposes, the use of RBE for low-dose exposure to radiation with different LET was superseded by the adoption of radiation weighting factor, wR, by the International Commission on Radiological Protection (ICRP) [78], to convert absorbed dose (measured in Gy) to equivalent dose (measured in Sv) in a tissue and to effective dose (measured also in Sv) in the body. ICRP recommends wR = 1 for photons of all energies, electrons, and leptons. The value wR = 2 is recommended for protons and charged pions, and wR = 20 for α-particles, heavy charged particles, and fission fragments [78] (see Table 10.2). However, the adoption of specific values for such weighting factors, based on the judgment from the available data on RBE, was accompanied by a recognition of the simplistic description and of the limited accuracy that the systematic application of this set of values for wR would have brought. Thus, quality factors, Q(LET), defined as a continuous function of the LET of the radiation, were later introduced in order to give broadly similar results for measured radiation fields [78] (see Table 10.2). Such quality factors are nowadays used in the risk assessment model by the European Space Agency and were also used in the previous risk assessment model by NASA.Table 10.2
Radiation weighting factors and quality factors

Radiation type

Radiation weighting factor (wR)

Quality factor (Q(LET))

Photons

1

Electrons and muons

1

Protons and charged pions

2

Alpha particles, fission fragments, heavy ions

20

Neutrons

A continuous function of neutron energy

 For LET < 10 keV/μm
 For 10 ≤ LET ≤ 100 keV/μm
 For LET > 100 keV/μm

1
Q = 0.32L–2.2
Q = 300L–1/2


Nevertheless, this specification of Q in terms of the LET alone suffers from the limitations already highlighted in Chap. 1, about the fact that the sole LET cannot fully describe the effectiveness of radiation in inducing biological damage. Indeed, even simply from the perspectives of the first-stage radiation-induced effects, without mentioning the complex dependencies of the RBE on phenomena related to the chemical and biological steps, it remains the fact particles with different charge and different velocity may have the same LET and still inducing different final biological effects. The variation in the effectiveness of radiation in inducing different final biological effects has thus its root in the differences in track structures between particles that have the same LET but different charge and velocity, as highlighted in Chap. 1. Differences can be particularly large for the HZE particles encountered in space, methods used on Earth are inadequate for space travel, as, among other reasons, the ICRP radiation quality description does not represent HZE radiobiology correctly.


The key difference between (a) the quality factor used by NASA [79] for the projection of risk from space exposures and (b) the quality factor recommended by the ICRP (Q(LET)) for operational radiation protection on Earth is consideration of track structure (Box 10.2).


(a)
The Boltzmann equation describes the transport of radiation in matter; it can be solved via analytical (deterministic) or via numerical (Monte Carlo) methods.
 (b)
The different steps for setting up a calculation of the radiation environment are input radiation spectra, definition of the parameters describing the atmosphere, with dependence on the altitude, definition of the regolith composition, definition of the physics model to be used according to the different energy ranges, definition of the target where the scoring of the absorbed dose will be done.


The Boltzmann equation describes the transport of radiation in matter; it can be solved via analytical (deterministic) or via numerical (Monte Carlo) methods.


The different steps for setting up a calculation of the radiation environment are input radiation spectra, definition of the parameters describing the atmosphere, with dependence on the altitude, definition of the regolith composition, definition of the physics model to be used according to the different energy ranges, definition of the target where the scoring of the absorbed dose will be done.


The space environment is hostile to the health of astronauts in several ways. The confinement in the restricted space of spacecraft for shorter or longer periods exposes the crew to sometimes severe behavioral problems. Microgravity can lead to osteoporosis, a modification of the electrolyte compartments, sarcopenia, cardiac arrhythmias, dysthemeral rhythm disorganization, vestibular deconditioning, relative immunosuppression, and postural hypotension on return [80]. Finally, the space radiation environment is very different and much more hostile than that encountered on Earth. Add a temperature amplitude of 300 °C on the spacecraft’s surface and the almost absolute vacuum conditions that astronauts must consider during extravehicular excursions. Finally, let us point out the disturbances secondary to the return to the ground: neurological, vestibular, cardiovascular reconditioning, etc.


The constant flux of galactic cosmic rays (GCR) causes astronauts’ chronic low-dose whole-body exposure during space missions. The primary GCR particles interact with the spacecraft hull, so that astronauts are—like patients—exposed to secondary radiation from nuclear interactions between the incident radiation and the shielding of the spacecraft. Due to mass limitations for launching spacecraft, complete shielding of GCR is not feasible. Compared to an astronaut suit for extravehicular activities, the shielding of the spacecraft by aluminum and other materials strongly reduces the skin dose and also, but to a much lower extent, the whole-body dose. On the microscopic level, due to the physical characteristics of particle radiation, very high doses can be reached, leading to permanent damage (see Sect. 10.4).


In LEO, traversal of the SAA of the inner radiation belt contributes to the accumulated dose during, e.g., a mission on the ISS. Human phantom experiments on the ISS (MATROSHKA experiment series) allowed the quantification of the effective dose rate which was 690–720 μSv/day during extravehicular activities and lower inside the ISS amounting to 550–570 μSv/day [81, 82]. Therefore, astronauts accumulate effective doses of around 100 mSv during a 6-months ISS mission. The variations of the accumulated dose depend on solar activity and the flight altitude of ISS, with higher doses during lower solar activity and increasing flight altitude. For a 1000-day Mars mission, a total effective dose of galactic cosmic radiation of about 1 Sv is expected [83, 84], which is quite considerable and exceeds terrestrial lifetime radiation exposure limits, which amount to 400 mSv in the European Union. Risks of cancer and degenerative diseases are associated with this chronic GCR exposure (Fig. 10.12).

A diagram of an astronaut lists the radiogenic acute effects in central nervous system, immune system, and skin, and chronic effects in central nervous system, eye, and overall risk.

Fig. 10.12
Possible health effects of space radiation exposure


A diagram of an astronaut lists the radiogenic acute effects in central nervous system, immune system, and skin, and chronic effects in central nervous system, eye, and overall risk.


Solar Particle Events (SPE) emanating from the Sun (Sect. 10.3.1.2) result in increased proton fluxes that may reach the spacecraft or a celestial body surface. In LEO, protection by the Earth’s magnetic field is still sufficient to protect from deadly SPE, but in free space or on planets or moons without magnetic field and atmosphere, high doses might be accumulated within hours or days in situations of insufficient shielding, e.g., in a spacesuit. Above a certain threshold, acute effects will occur (Fig. 10.12). In contrast to GCR, shielding of SPE protons is feasible in special compartments of the spacecraft, which can be surrounded by more material. Astronauts can protect themselves from an SPE in such a radiation shelter until the proton flux normalizes.


Deterministic effects appear for acute global exposures classified as medium, high, and very high (0.2 to more than 10 Sv) by UNSCEAR [85].


Under exceptional conditions of insufficient shielding during spaceflight, the exposure to mostly protons during a large solar particle event (SPE), the whole-body dose can reach several Gy or the skin dose even tens of Gy and thereby cause the acute radiation syndrome (ARS, see Chap. 2, Sect. 2.7.2). Such situations in the event of a solar flare of exceptional intensity can occur in LEO in areas of weakness of the Van Allen belts, extravehicular exit, and exit on extraterrestrial soil in a spacesuit or an insufficiently shielded vehicle. The total dose is delivered over a short period of time: generally, instantaneously but by definition over less than 4 days.


The acute effects affect rapidly renewing tissues which are particularly radiosensitive (bone marrow, digestive epithelium, germ cells, skin). The classic “radiation sickness” or prodromal syndrome (headache, dizziness, nausea, bone marrow hypoplasia) occurs for an exposure of 0.5–1 Gy. A dose of 3–4 Gy kills 50% of exposed individuals in 1 month [86]. Unlike the desired partial exposure of patients undergoing radiotherapy, solar flares are unpredictable, which seriously complicates mission planning for astronauts.


For several decades, NASA has collected data concerning acute and chronic morbidity and mortality in US astronauts in the NASA’s Longitudinal Study of Astronaut Health [87]. One main aim is to determine whether astronauts’ occupational space radiation exposure is associated with an increased risk of cancer or other diseases. The cohort is made up of 312 astronauts selected by NASA since 1959. Employees at the NASA Johnson Space Center in Houston, Texas, served as the control group. In January 2003, just before the explosion of the Columbia shuttle, 29 deaths (9.3%) were counted in the group of astronauts versus 17 (1.8%) in the control group. Note 20 accidental deaths among astronauts (versus 2 in the matched group). No other cause reached the threshold of significance.


Compared to the control group at matched age, astronauts had a higher specific mortality rate (SMR) from cancer. This difference was not significant. However, both groups had a lower specific mortality rate than the general population. Fourteen cases of cancer have been described in astronauts (not counting 33 cases of non-melanoma skin cancer), which represents a relative risk of 1.59 compared to the Air Force pairings but of 0.54 compared to the cohort of NCI (general population), which ultimately remains insignificant. A later study found that standardized mortality rates for astronauts were significantly below US white male population rates [88].


During a Mars exploration mission, each cell nucleus of an astronaut would be crossed by a proton or a secondary electron every 2 days, and by a heavier ion every month [89]. Due to their strong ionizing power, these ions appear to be the main vector of carcinogenic risk despite their low fluence.


The interval between irradiation and tumor appearance has been shown in rats to be shortened compared to conventional radiation [90, 91]; fewer events would be needed in the promotion of carcinogenesis induced by high-LET particles. Particle mass, energy, and charge can influence the cancer risk of an HZE particle.


The linear no-threshold (LNT) model used to predict the risk of cancer mortality in astronauts sent on interplanetary missions relies on data from atomic bomb survivors extrapolated to this particular population, to these types of particles, and to the dose rates encountered in the space environment. Though nearly universally used by public bodies to assess cancer risk, LNT is far from being a scientific consensus and its application for low dose rates is rather controversial—see Chap. 2. For cancer risk estimation, age at exposure, attained age, sex- and tissue-specific mortality and incidence, and latency has to be considered. Also, an important question is whether the additional cancer risk induced by space radiation exposure is independent of other cancerogenic events (excess absolute risk, EAR), or whether the risk depends on other cancer risks (excess relative risk, ERR).


Table 10.3 summarizes the LNT-estimated carcinogenic risk under different exposure conditions. The confidence interval includes epidemiological, physical, and biological uncertainties. The maximum acceptable risk for an astronaut dying from cancer is typically set at 3% [50].Table 10.3
Doses and LNT-based estimates for cancer mortality risk following space missions

Absorbed dose (Gy)

Effective dose (Sv)

Risk of death by cancer (%) [IC95%]

Male 40 y.o.

Female 40 y.o.

Moon Mission (180 days)

0.06

0.17

0.68 [0.20–2.40]

0.82 [0.24–3.00]

Mars Orbit Mission (600 days)

0.37

1.03

4.00 [1.00–13.50]

4.90 [1.40–16.20]

Mars Mission (1000 days)

0.42

1.07

4.20 [1.30–13.60]

5.10 [1.60–16.40]


Besides the calculated increased cancer risk for astronauts, cataracts might be triggered or promoted by space radiation exposure. Astronauts exposed to a dose of more than 8 mSv exhibit earlier and more frequent cataracts (in a study that identified 295 astronauts paired with as many US Air Force pilots) [92].


Due to the densely distributed ionizations around a heavy ion’s path through a cell nucleus, severe DNA damage (Sect. 10.5.3) possibly leading to chromosomal aberrations (Sect. 10.5.2) can be induced. Therefore, chromosome damage induced in vivo was identified early as a sensitive biodosimeter [93, 94] that integrates radiation exposure in quality and quantity and also the individual radiosensitivity [95]. Peripheral blood lymphocytes are accessible by venipuncture and the chromosomal aberration test can be performed with these cells before and after flight.


In order to determine the effects of space radiation on astronauts, chromosomal aberrations were quantified already in Gemini astronauts before and after the spaceflight [96]. In some astronauts, a small increase was observed after the flight which did not correlate with flight duration (1–14 days), extravehicular activities, or diagnostic radioisotope injections [96]. Missions with a duration of up to 3 weeks did not result in an increase of the aberrations above background; after missions of 6 months or longer, a rise was clearly observed [95, 97–104], but dose estimation based on the cytogenetic analysis varied strongly [95]. Here, the inter-individual variability of the translocations’ half-life in peripheral blood lymphocytes has to be considered [105]. Also, the basal aberration frequency and the reaction toward ionizing radiation varies from individual to individual [106–108]. Furthermore, the effects of multiple space missions might not be additive [109, 110]. Prediction of dicentrics frequencies for a Mars mission assume values 10–40× above background in peripheral lymphocytes [111].


For detection of reciprocal translocations, multicolor fluorescence in situ hybridization (mFISH) was first applied to members of the Mir-18 crew [112]. In search of a specific marker of heavy ion exposure, complex chromosome interchanges were suggested and analyzed in blood lymphocytes of astronauts [113, 114]. High-resolution multicolor banding (mBAND) of chromosome 5 can visualize intrachromosomal exchanges—long-term missions to the ISS did not increase this parameter [115]. Such inversions were only recently found in three astronauts during a 6-months ISS mission [116]. Complex chromosomal rearrangements occur very rarely in astronauts therefore their use as biomarker is limited [93]. Over the years, different cytogenetic or chromosomal signatures that allow reconstruction of absorbed dose and radiation quality were suggested, such as insertions [117], inversions [118], and complex chromosome interchanges, but up to now, no consensus for a biomarker of exposure to high-LET radiation has been reached [119] (see Sect. 8.7).


The relevance of the telomere elongation that was first observed during the 1-year ISS mission and its fast shortening after return to Earth [120], which was now also found during 6-months missions [116], for assessment of space radiation risk is currently unclear. The telomere changes are considered as an integrative biomarker for effects of the spaceflight environment [121].


Before the first human went to space, in 1952, Professor Cornelius A. Tobias made the famous prediction that cosmic radiation can cause unusual light sensations by interaction with the visual system. The Apollo-11 astronaut Edwin (Buzz) Aldrin was first reported to have perceived light flashes during the Moon mission [122]. This initiated a series of investigations already during the following Apollo missions [123], and later on Mir, Skylab, Apollo-Soyuz Test Project (ASTP), Shuttle missions, and on the ISS. They started with observation sessions and nuclear emulsion plates (Apollo light flash moving emulsion detector, ALFMED). The observations were later combined with sophisticated particle detectors in the Silicon Eye (SilEye-1 and -2) experiments on Mir [124], and Alteino-SilEye-3 and Anomalous Long-Term Effects on Astronauts (ALTEA) experiments on ISS, which included also an electroencephalograph.


The observations of the Apollo astronauts resulted in an average event rate of one light flash event in ~3 min [123]. In LEO, when passing through the SAA, the light flash rates are very high [125], and outside the SAA, light flash frequency is higher in the polar parts of the orbit than in equatorial latitudes [126]. The number of light flashes perceived in LEO varies on average between one every minute up to one every 7 min on Mir [127] or every 20 min [128, 129] dependent on the orbital height, the inclination, the shielding of the spacecraft and solar activity [130].


So, in conclusion, contrarily to the usual statement that we have no senses to perceive ionizing radiation, when closing their eyes, most space travelers can “see” the exposure to galactic cosmic rays and trapped radiation as mostly colorless light flashes or phosphenes in the form of spots, stars, streaks, or diffuse clouds of light [125]. About 15–20 min of dark adaptation is required [123] so that they are usually perceived before falling asleep.


This light flash phenomenon is explained by a visual sensation that is produced by the interaction of highly energetic heavy ions with the retina of the eye [131, 132] or possibly with visual centers in the brain or the optic nerve after penetration of the spacecraft walls and the eye or head. The interaction might be direct or indirect via Cherenkov radiation in vitreous humor which is emitted as light when the charged particle passes through it with a velocity higher than the speed of light in the vitreous humor [133]. The probability of a heavy ion to cause a light flash has been estimated to be around 1%—with increasing probability with increasing LET—and for protons to be below 0.001% in LEO [127]. A deleterious effect of the flashes on vision is not suspected, but some astronauts report that their sleep was disturbed by light flashes.


Ionizing radiation, which exists primarily in the form of high-energy, charged particles make up space radiation. The radiation environment in space is characterized by a high complexity due to different sources and a higher number of particle species, and a broad energy range. Galactic cosmic radiation (GCR), solar particle events (SPE), and, in LEO, trapped radiation are the naturally occurring sources of space radiation.


The exposure to GCR occurs at a low dose rate on the organismal level, but strong cellular effects might be triggered in case of a “hit” by an energetic particle, especially high Z and high energy (HZE) particles or heavy ions. HZE particles make up only 1% of GCR therefore only small hit frequencies are expected in the human body that could be responsible for late effects [134]. First evidence of biological effects of HZE particles was found in mice after a high-altitude balloon flight when the coat of black mice locally turned grey [135]. Single particle effects on different dormant biological systems under spaceflight conditions were proven by means of the Biostack experiments on the Apollo-16 and -17 missions [10, 136]. In this experimental system, biological systems and detector foils were stacked onto each other to allow assignment of heavy ion hits to the biological systems. Heavy ion hits were detected in plastic foils (cellulose nitrate, polycarbonate), silver chloride crystals, and nuclear emulsions. The biological systems were immobilized on the foils with water-soluble polyvinyl alcohol and included Bacillus subtilis spores, seeds of the thale cress Arabidopsis thaliana, roots of the field bean Vicia faba, eggs of the brine shrimp Artemia salina, insect eggs (stick insect, Carausius morosus and rice weevil, Tribolium confusum), and protozoa cysts (Colpoda cucullus). The outgrowth of B. subtilis after germination was significantly reduced after an HZE particle hit [137, 138]. During the development of brine shrimp eggs that were hit by a single particle, abnormalities appeared at the extremities, the thorax, and the abdomen [139] and the eggs showed the most sensitive reaction toward HZE particles compared to the other biological systems in Biostack [137, 140]. Developmental abnormalities were also found in hit insect eggs [141]. The total dose for the Biostack experiments was quite low (5.8–7.5 mGy), and ~0.03 mGy was allocated to the HZE particles, whereby it has to be considered that the local dose in a hit cell can be much higher than the total dose.


These experiments were continued in LEO using the Free Flyer Biostack Experiment (LDEF—Long Duration Exposure Facility) [142], EURECA—European Retrievable Carrier [143–146], and the biosatellites COSMOS 1887 and 2004 [147, 148] and refined, so that synergistic effects of HZE particle hits and microgravity in the developmental disorders of C. morosus were revealed.


These intriguing results showing strong deleterious effects of single particle traversals and even an enhancement by other spaceflight environmental effects initiated a multitude of biological experiments in space and at heavy ion accelerators (see Sect. 10.9) in order to quantify the biological effectiveness of HZE particles, to understand the underlying mechanisms and to develop countermeasures. A variety of experimental models are used for these experiments (see Sects. 10.5–10.7, and 10.8.2). The uncertainties in risk assessment for cancer and non-cancer effects in the central nervous system and other organ systems for astronauts are still unacceptably high therefore further investigations into the biological effects of HZE particles are necessary. The experimental approaches shown in Box 10.3 below take the low dose rate but strong biological effects in case of a particle hit into account.


Natural GCR exposure
Correlation of biological effects with single particle hits by combination of biological model and detector foil, e.g., Biostack; can be combined with 1xg reference centrifuge to determine contribution microgravity effects

Correlation of light flashes with HZE particles that traverse astronauts’ eyes

Dose accumulation over weeks or months by storing dormant or freeze-dried or deep-frozen cells or small organisms in space, subsequent reactivation and measurement of radiation damage or response

Determination of spaceflight effects by exposure of, e.g., fruit flies, rodents, or other organisms on satellites or high-altitude balloons
Exposure to selected HZE particles
Exposure of a variety of biological systems at heavy ion accelerators or microbeam facilities to selected heavy ions (singe particle at defined energy or mixture of particles of defined energies) and analysis of the biological response


Correlation of biological effects with single particle hits by combination of biological model and detector foil, e.g., Biostack; can be combined with 1xg reference centrifuge to determine contribution microgravity effects


Correlation of light flashes with HZE particles that traverse astronauts’ eyes


Dose accumulation over weeks or months by storing dormant or freeze-dried or deep-frozen cells or small organisms in space, subsequent reactivation and measurement of radiation damage or response


Determination of spaceflight effects by exposure of, e.g., fruit flies, rodents, or other organisms on satellites or high-altitude balloons


Exposure of a variety of biological systems at heavy ion accelerators or microbeam facilities to selected heavy ions (singe particle at defined energy or mixture of particles of defined energies) and analysis of the biological response


As described in Chap. 2, radiation quality is an important factor influencing the cell death response. It can affect the extent and mode of cell death. A stronger cell killing of human cells by alpha particles with an LET up to 100 keV/μm was already observed in the 1960s [149], indicating an RBE for cell killing up to 7. Since then, survival data after heavy ion exposure were collected for many mammalian cell types including primary cells and tumor cell lines using the colony forming ability (CFA) test which is described in Chap. 2. This was less driven by space radiation research but by tumor therapy research to identify suitable ions and to determine the cell killing RBE for treatment planning. The shoulder observed in the dose response curves for cell killing by low-LET radiation disappears in high-LET survival curves, resulting in purely exponential dose–effect relationships and indicating the lack of repair capacity after heavy ion exposure [150] (Fig. 10.13).

A double line graph of surviving fraction versus dose in gray units plots the fast and slow declining curves for high L E T radiation and low L E T radiation, respectively.

Fig. 10.13
Survival of mammalian cells after exposure to low linear energy transfer (LET) and high-LET radiation. Low-LET radiation includes photons, electrons, positrons, protons, and more. High-LET radiation encompasses heavy ions, and, depending on energy, also He ions and neutrons


A double line graph of surviving fraction versus dose in gray units plots the fast and slow declining curves for high L E T radiation and low L E T radiation, respectively.


Clonogenic cell survival data for more than 1100 experiments comparing the effects of ion irradiation to photon irradiation are available in a database established by the GSI biophysics group [151]. The database is called Particle Irradiation Data Ensemble (PIDE, www.gsi.de/bio-pide). The maximal RBE for cell killing (10% survival level) was observed in the LET range of 100–200 keV/μm with values of 2–7 [151]. This large variation in RBE is explained by the influence of particle species and energy in addition to LET, of cell type and other experimental factors. At LETs above ~200 keV/μm, more energy is deposited in a cell traversed by a particle than is required to kill the cell and more hits per cell cannot produce more cell death as any hit will kill the cell, resulting in a decrease of RBE that is called “overkill effect.”


The clonogenic survival data integrate cell death by various modes such as mitotic catastrophe, apoptosis, necrosis, autophagy, and other mechanisms (see Chap. 2) and permanent cell cycle arrest, possibly accompanied by cellular senescence. As for low-LET radiation, it depends on the cell or tissue type whether a cell population is prone to ionizing radiation-induced apoptosis [152]. Apoptosis might occur at higher rates after high-LET radiation exposure compared to low-LET irradiation with a maximum at a LET of ~100 keV/μm [153].


The consequences of heavy ion-induced cell death for the organism can be that transformation of a heavily damaged cell is prevented thereby protecting from cancer. This effect also limits the number of cells with mutations (see Sect. 10.5.4) or chromosomal aberrations at a LET >200 keV/μm (see Sect. 10.5.2) and cellular transformation (see Sect. 10.5.5). On the other hand, deleterious effects might occur such as depletion of stem cell pools or loss of terminally differentiated cells with no or low regeneration potential that might affect the functionality of a tissue or organ.


For some microorganisms, growth and viability were measured during space missions. A 14-days exposure of Escherichia coli on the Space Shuttle or 140-d exposure on Mir did not result in any differences in viability and mutations frequencies in comparison to ground controls [154, 155]—the same was the case in Saccharomyces cerevisiae [156]. Using repair-deficient E. coli mutants, DNA polymerase, and 3′→5′ exonuclease were identified as the most important enzymes for GCR-induced DNA damage in E. coli [157]. Also, the slime mold Dictyostelium discoideum did not grow differently and did not show differences in the mutation frequency in the spores during a 7-days Shuttle flight [158, 159], but the number of spores per fruiting body was reduced [160].


Chromosomal aberrations are alterations in DNA structure that become microscopically visible after following a chromosome staining protocol [161] (see Chap. 2). They can result from mis-rejoining of DNA ends from ionizing radiation-induced DNA double strand breaks (DSB), from lack of repair leading to terminal deletions and incomplete exchanges or from chromosome mis-segregation [162, 163]. They are exquisitely and quantitatively sensitive to ionizing radiation. Symmetrical resolution of the DNA DSB can lead to chromosomal interchanges resulting in translocations which are usually nonlethal. Asymmetrical resolution produces among other dicentrics (chromosomes with two centromeres) and acentric fragments, mostly contained within micronuclei; also, during the repair process, DNA sections can be lost, producing a deletion [164]. Ionizing radiation can also induce quadriradials (U-type by asymmetrical resolution, X-type by symmetrical resolution). Complex and asymmetric aberrations such as dicentrics usually lead to cell death (lethal aberrations) [163].


They are determined during metaphase or by chemically induced Premature Chromosome Condensation (PCC, see Chap. 2) during interphase [165], usually in lymphocytes or fibroblasts, providing data on a cell-by-cell basis. Their dose–response relationship follows a curvature. While dicentrics and acentric fragments can be detected with a GIEMSA staining, mFISH is required for interchromosomal translocations and mBAND for intrachromosomal translocations (see Sect. 10.3.4). Inversions can be detected by Directional Genomic Hybridization (dGH) [166].


Chromosomal aberrations are of high interest in space radiation biology as they are an early-stage effect and regarded as a surrogate endpoint for cancer risk as many human cancers are linked to them and all “clastogens2” are both mutagenic and carcinogenic.


For carcinogenesis, the surviving cells with chromosomal aberrations are relevant. The fraction of these cells depends on LET, track structure, and fractionation (Box 10.4).


Dose rate

Fractionation

Linear energy transfer (LET)

Track structure

Cell nuclear geometry (e.g., spherical or flat)


Cell nuclear geometry (e.g., spherical or flat)


HZE particles have a very high efficiency in inducing chromosomal aberrations—the RBE in comparison to low-LET radiation was estimated to reach 30–35 during interphase [163, 167, 168]. Furthermore, high-LET α-particles at low fluences (1 track per cell nucleus) were more efficient in inducing complex aberrations in human peripheral blood lymphocytes than X-rays [117]. Complex chromosome aberrations are defined as aberrations that involve three or more breaks in at least two chromosomes. Here, the particle track structure comes into play [169]. Delta rays move out of the primary particle track, producing further ionizations that can induce damage. This damage might interact with other breaks generated by either a separate track or delta rays emanating from it (intratrack action). The range of the delta rays is proportional to the specific energy of its corresponding primary particle. Higher energy particles would have a greater chance of track interaction than their lower energy counterparts because of the longer range of the delta rays where breaks can be close in space and time at high doses and dose rates as they are produced by multiple tracks (intertrack action). The breakpoints induced by delta rays add up to those produced in the primary particle track. Hence, the number of exchange breakpoints and their spatial arrangement are important determinants for the formation of complex exchanges. For example, the number of breakpoints per cell was higher for 56Fe ions (1.1 GeV/n) and α-particles (0.9 MeV/n) in comparison to 137Cs γ-rays. In spherical cell nuclei, one particle traversal is sufficient to produce two breakpoints, e.g., in a lymphocyte [170, 171]. In summary, HZE particles produce more breakpoints per track and more highly complex exchanges compared to low-LET radiation [118] (Fig. 10.14). These complex aberrations partly disappear between the first and second cell division after radiation exposure, but some are transmissible and might be stable through several cell generations.

A diagram of a cell depicts a prominent nucleus with nucleolus, chromatin, and nuclear membrane. The passage of F e 56 ion depicts many breaches in the chromatin material.

Fig. 10.14
As a heavy ion travels through a mammalian cell nucleus, a multiple of ionizations is produced, damaging a chromosome arranged in its nuclear territory several times. Delta rays emanating from the primary track can induce further damage. Therefore, traversal of high-LET radiation through a cell nucleus can produce many breakpoints in chromosomes


A diagram of a cell depicts a prominent nucleus with nucleolus, chromatin, and nuclear membrane. The passage of F e 56 ion depicts many breaches in the chromatin material.


As other radiation qualities, protons, α-particles, and HZE particles can induce various types of DNA damage by direct ionization or indirectly through radiolysis of intracellular water (see Chap. 2). Among base damage, loss of bases, DNA-DNA and DNA-protein crosslinks, single strand breaks (SSBs), and double strand breaks (DSBs), DNA DSBs are the most severe DNA lesion. Unrepaired DNA DSBs are at the center of biological effects such as cell killing and chromosomal aberrations and are trailblazers of the majority of early and late effects induced by ionizing radiation exposure [163, 172, 173].


What makes particle radiation special is the multitude of ionizations localized along the particle’s path through the cell. The spatial distribution of direct DNA damage differs strongly for low- and high-LET radiation, with a diffuse distribution for the former and clusters for the latter. Such clusters of different damage (base lesions, abasic sites, SSB, DSB, etc.) within a few helical turns of DNA are called complex DNA damage (Fig. 10.15) (formerly: multiply damaged sites or clustered DNA damage) [164, 174, 175].

A diagram of double helical structure of D N A depicts a few ionizing events in the passage of electron with low L E T and many ionizing events in the passage of alpha particles with high L E T.

Fig. 10.15
Comparison of ionizations (grey dots) in a DNA molecule that are induced by electrons as an example of low-LET radiation and by a high-LET α-particle. The ionizations produced by the α-particle are located densely along the track, with some secondary electrons (δ rays) generated while traversing the cell. This spatial distribution goes along with a higher probability of simultaneously breaking both DNA strands thereby producing a double strand break (DSB), and also further damage to bases and single strand breaks (SSB) in close proximity which is then called complex DNA damage


A diagram of double helical structure of D N A depicts a few ionizing events in the passage of electron with low L E T and many ionizing events in the passage of alpha particles with high L E T.


Although the contribution of direct action to the biological effectiveness of high-LET radiation is larger than indirect action [176], reactive oxygen species (ROS) generated by radiolysis can also play a part in the overall radiation effects. As the lifetime and diffusion range of ROS are small, only radicals produced in DNA’s vicinity are relevant for DNA damage induction and increase in its complexity. With increasing LET, the contribution of direct effects rises, and the indirect effects drop. Low-LET radiation and endogenous ROS rarely induce complex DNA damage [163].


The detection of GCR-induced DNA damage succeeded in HeLa cells during the Shuttle and Mir missions [177–179]. In human lymphoblastoid cells that were stored at –80 °C for several months on ISS—in total 134 days at an average dose rate of 0.7 mSv/day, one particle track per 100 cells was detected by means of immunofluorescence staining of γH2AX after return to the ground (see below) [180]. Such tracks were also observed in human fibroblasts that were cultivated for 14 days on the ISS [181].


Various DNA damage repair pathways ensure genome integrity and stability in uni- and multicellular organisms. The current understanding is that multiple repair pathways have to be coordinated to repair complex DNA damage making it very challenging, that short fragments might be lost during repair and that multiple breakpoints in the DNA ribose-phosphate backbone can favor complex genomic rearrangements [164, 182]. The damage might still persist at DNA replication because of repair delays that were observed after HZE particle exposure. If repair of complex lesions is completed, its fidelity might be lower when compared to simple DNA damage [183–185]. After 56Fe ion (1 GeV/n) exposure, 14% of the damage remained unrepaired compared to 5% after γ-ray or α-particle exposure [171]. In vivo, persistent DNA DSBs were found even 1 month after exposure to iron ions [186]. Growth arrest, cell death, or senescence are possible consequences of such unrepaired DNA damage [164], while mutations and chromosomal aberrations are key steps in cellular transformation and tumorigenesis.


DSBs are mainly repaired by nonhomologous end joining (NHEJ) and homologous recombination (HR) in eukaryotes (see Chap. 2). DNA DSB repair follows biphasic kinetics with a faster velocity in the beginning and lower speed at later timepoints. The phosphorylated form of the histone variant H2AX (γH2AX) [187, 188] as a marker of DNA DSB is often applied to microscopically visualize DSB induced by high-LET radiation exposure, sometimes in combination with antibodies binding to 53BP1 or other DSB repair proteins or to oxidative base damage [189]. After immunofluorescence staining, fluorescent foci indicate γH2AX and 53BP1 accumulation around DNA DSB. Ground-based experiments performed at heavy ion accelerators allow quantification of DNA damage induction and DNA repair by one ion with a specific energy or, since lately, several ion species with specific energies hitting the cells from one direction (see Sect. 10.10.4). They are usually performed additionally with low-LET radiation for comparison.


For example, in human fibroblasts, repair of DSB induced by carbon ions was slower than those induced by proton or helium ion irradiation and the size of the repair foci increased with increasing LET [190]. Larger repair foci that persist longer are a common finding when exposure to heavy ions and X-rays are compared [191, 192]. One day after exposure to 1 GeV/n iron ions, 30–40% of the 53BP1 and γH2AX foci still remained indicating the extent of residual damage [193]. The slow repair kinetics and incompleteness of repair of DNA damage induced by high-LET radiation [190, 191, 194] are consistent findings of experiments with mammalian cells at heavy ion accelerators. Also, there are some hints that high-LET radiation inhibits c-NHEJ and shifts toward error-prone alternative nonhomologous end joining repair and microhomology-mediated end joining, resulting in a lowered fidelity of repair for days or weeks [195]. Other studies have shown that the repair of complex DNA requires DNA resection for processing at the DNA ends in G1 and G2 cells and forces the pathway choice toward resection-dependent HR [196, 197].


As mentioned above, to repair complex DNA damage, other repair pathways might be involved such as base excision repair (BER) and/or nucleotide excision repair NER [198]. Oxidative base damage such as 8-oxoguanine can be restituted by BER starting with damage recognition and removal by a DNA glycosylase and final steps by polymerase and ligase proteins [172]. NER is responsible for the repair of larger helix-distorting lesions.


In summary, DNA damage complexity increases with increasing LET, resulting in less effective DNA repair, a higher rate of residual lesions, genomic instability, and enhanced cell killing [174].


The results of the Biostack experiments raised the question of whether microgravity or other spaceflight environmental factors affect DNA repair processes, as explained hereinafter. The advanced Biostack experiments included an inflight 1g control on a centrifuge, allowing the separation of effects of microgravity and of all other environmental factors. In this experiment, eggs of the stick insect Carausius morosus were exposed in space and the HZE particle hits were traced back to the eggs by means of particle track detector foils. Back on Earth, the insects were allowed to hatch. When the eggs were hit by an HZE particle under microgravity, more abnormalities were observed compared to hits during centrifugation at 1g, indicating additive or even synergistic damaging effects of cosmic radiation and microgravity [144].


Therefore, DNA repair and radiation response under microgravity were examined in further spaceflight experiments using a 1g centrifuge inflight control and in ground-based simulation using clinostats or random positioning machines. For determining subtle differences in DNA repair capacity or kinetics, a high level of DNA damage has to be induced. For this purpose, the dose rates of GCR in LEO on the Space Shuttle or on ISS are too low; therefore, DNA damage has to be induced by irradiation on ground in a metabolically inactive state, by irradiation in space using an artificial radiation source brought in LEO or by incubation with chemicals. When radiation damage was already induced on ground using a radiation source, cooled cells were brought to space and activated there to repair their DNA under microgravity [199, 200]. Alternatively, DNA DSB were induced by bleomycin [201] or restriction enzymes [202] during spaceflight. Often, no or only small interactions were found [203, 204]. In yeast however DNA DSB repair was delayed under microgravity, suggesting additive effects of radiation and microgravity [205, 206]. In human fibroblasts and Bacillus subtilis, microgravity did not influence the repair of DNA SSB and DSB [200, 207]. Also, ligase activity [204] and DNA replication [208] were not affected. The expression of genes involved in the DNA damage response was altered under microgravity [209–211]. Besides these gene expression changes, a growth-stimulating effect of microgravity was observed in many ground-based and space experiments that might contribute to the microgravity effects on the DNA damage response [209]. In ground-based experiments, limitations of various microgravity simulators have to be considered [212], especially the generation of shear forces [213] as possible confounders. For microorganisms, such as bacteria, it has also to be considered whether they are motile because of, e.g., flagella or not, and the effect of microgravity can be most likely attributed to changes in the medium surrounding the microbes [214].


Animal experiments addressing the question of DNA repair under spaceflight conditions are scarce. After a 14-day spaceflight, the level of the tumor suppressor p53, which acts as a transcription factor in the DNA damage response, was increased in the muscle of mice compared to ground control mice [179]. Experiments with the nematode Caenorhabditis elegans during the Shenzhou-8 mission revealed changes in the expression of four microRNAs and of 4.2% of the genes involved in the DNA damage response after 16.5 days of microgravity when compared to the inflight 1g control [215]. Hindlimb unloading is used in rodent models to simulate on ground the head-ward fluid shift that occurs in microgravity. After 21 days of hindlimb unloading and low-dose irradiation of mice, some genes involved in DNA repair, chromatin organization, and cell cycle were differentially expressed in the spleen compared to control mice [216].


Space experiments are the only way to unambiguously identify the effects of real microgravity on biological systems, here the enzymatic repair of radiation-induced DNA damages. The opportunities to perform experiments with actively metabolizing organisms in space are rare and usually have a long lead time from the acceptance of an experiment proposal to the execution of the experiment in space.


The Biolab facility in the Columbus module of the ISS provides many possibilities for biological experiments on microorganisms, cells, tissue cultures, small plants, and small invertebrates in LEO (https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Columbus/Biolab). However, experiments on the ISS are subjected to limitations such as up- and download mass, up- and download temperature conditions, availability of a suitable facility in space, data downlink, number of sample replicates, appropriate control experiments in space and on ground. The Biolab facility will be used for LUX-in-Space (ESA AO LSRA-2014-026, Team Coordinator: P. Rettberg), the first space experiment where the whole series of events from DNA damage induction in metabolically active cells to the different steps of enzymatic repair reactions will take place in real microgravity and the repair kinetics will be monitored by optical measurements in situ. The effects of microgravity will be clearly separated from other spaceflight factors by comparison with parallel samples on an onboard 1g centrifuge in the Biolab facility and in a parallel ground control experiment with identical samples in flight-identical hardware. Due to safety issues, ESA decided to apply UV radiation for DNA damage induction. It causes defined types of DNA damage, e.g., cyclobutane pyrimidine dimers, which are among those also induced by ionizing radiation. Bacteria serve as model organisms possessing the same type of nucleotide excision repair as all other living organisms including humans. The capability of bacterial cells to counteract radiation damage by activating genes involved in DNA repair will be assessed using a bioluminescent reporter gene operon under the control of the SOS regulon, known as the SOS LUX assay. The DNA repair kinetics will be followed by bioluminescence and optical density measurements. For the space experiment, TripleLux Part C preparatory work was already performed successfully to adapt the SOS LUX assay to the space conditions provided by the Biolab facility on the ISS. This experiment was canceled later by ESA due to a lack of available resources at that time and it is a predecessor of LUX-in-Space [217, 218]. The launch of LUX-in-Space is scheduled for 2023/2024.


Biosentinel will be the first deep-space experiment investigating the repair of DNA damage induced by space radiation (Principal Investigator: Sharmila Bhattacharya). It is a further development of NASA’s biological CubeSats, small satellites with different payloads that were already flown successfully in LEO. Biosentinel will first follow a trajectory of cis-lunar flyby and, for 6–12 months, enter a heliocentric orbit. The organism under investigation is the budding yeast Saccharomyces cerevisiae. These eukaryotic cells are robust, desiccation resistant, were already flown in space before, and have similarities to cells of higher organisms such as humans. Cells from a radiation-resistant yeast wildtype strain and a radiation-sensitive Δrad51D mutant will be uploaded in a dry form. After different periods of time, during which the cells will accumulate radiation-induced DNA damage, the cells will be activated by the addition of nutrient medium and their growth and metabolic activity will be measured optically. In parallel, another Biosentinel payload will be flown in the ISS, in addition to the corresponding ground reference experiment. The launch is scheduled for 2022 as a secondary payload of NASA’s Artemis-1 mission [219, 220].


Mutations as a deleterious outcome of erroneous repair of space radiation-induced DNA damage are of special interest in radiation risk assessment as they can initiate the multi-step carcinogenic process [163, 182] and they can be responsible for genetic effects in the offspring if they occur in the germline. Mutations can be detected in cells that survived irradiation and are, as chromosomal aberrations, late endpoints of radiation-induced DNA damage. For improving space radiation risk assessment, the dependence of mutation induction by radiation of different linear energy transfer (LET) was examined in different biological systems: Mutation induction by heavy ions was determined in many organisms including bacteria (E. coli, B. subtilis), yeast (S. cerevisiae), Neurospora, Drosophila, C. elegans, M. musculus, plants, and mammalian cell systems including human fibroblasts and lymphoid cells. These were mostly ground-based experiments at heavy ion accelerators.


The hypoxanthine guanine phosphoribosyl transferase (HPRT, EC 2.4.2.8) gene (mutations on the single copy X-chromosome in male-derived cells) in human diploid fibroblasts was used in early studies of LET dependency of mutation induction. A maximum of around 7 times more mutations compared to low-LET radiation was observed for helium ions or heavier ions with a LET of 100–300 keV/μm [221]. The number of mutations per single track through a mammalian cell nucleus increases with LET, reaching saturation at around 100 keV/μm [222]. The induction of mutations in the X-linked HPRT locus in Chinese hamster cells by accelerated heavy ions reached a local maximum in the LET range of 80–100 keV/μm [223].


Studies on mutation induction in autosomes became possible by means of AL human-hamster hybrid cells having one copy of human chromosome 11. In these hybrid cells, neutrons of various energies were more efficient in inducing mutations in the a1 locus on chromosome 11 compared to gamma rays; the RBE reached up to 30 at the 0.1% survival level [224]. The autosomal thymidine kinase gene (TK1) locus in human cells allowed investigation of the loss of heterozygosity (LOH) which can occur via deletion or allelic recombination and it revealed a higher peak of mutations at a lower LET (~50–100 keV/μm) compared to the HPRT mutations (up to 15× compared to ~5×). As for other biological endpoints, LET is not the only determinant of the biological efficiency of an HZE particle. The track structure means the energy deposition pattern varies for different ion species of the same LET. Such an effect of ion species was observed for mutation induction at the HPRT locus in human fibroblast-like cells—the RBE for mutation induction determined in this system was between 3.6 and 7 for carbon and neon ion beams in the LET range of 60–120 keV/μm compared to 137Cs gamma rays [225].


Besides mutations observed in the direct hit cells, bystander mutagenesis can contribute to the overall mutation rate after particle exposure as it was observed, for example, after alpha particle exposure [226].


An experiment on the ISS designed to detect mutations in human cells that were induced by natural galactic rays made use of the frozen storage as described in Sect. 10.5.3. Frozen human lymphoblastoid TK6 cells were stored for 134 days in the Kibo module of the ISS and accumulated a dose of 72 mSv. After analysis on ground, a tendency for higher mutation frequency at the TK locus was observed in the flight samples compared to ground control [227]. Earlier experiments on Mir for 40 days with a model system based on Saccharomyces cerevisiae and Escherichia coli also revealed two to threefold higher mutation frequencies in some flight samples compared to ground samples, with a predominance of large deletions that might be caused by high-LET radiation [228].


If mutations occur in tumor suppressor genes and inactivating them, or proto-oncogenes and activating them, cells can be transformed and lose growth control including anchorage-dependent growth. It can be seen as a surrogate marker for the carcinogenic potential of a radiation quality in question. Transformation can only occur in cells that survived the radiation exposure. In vitro, transformation of mammalian cells is determined by their ability to grow anchorage independently in soft agar. The soft agar test was applied to different cell types after exposure to HZE particles at heavy ion accelerators in order to determine their potential for transformation, usually in comparison to low-LET radiation.


Already in the 1980s, it was shown that HZE particles are more effective in transforming mammalian cells than low-LET radiation: In mouse embryonic cells (C3H10T1/2), the effectivity of transformation increased up to 10 with a LET ~200 keV/μm [229] while Hei et al. observed a plateau at LETs of 80–120 keV/μm [230]. In Golden hamster embryo cells, 14N ions (LET 530 keV/μm) and 4He ions (36 and 77 keV/μm) were ~3× more effective in inducing cellular transformation than gamma or X-rays [231]. Later, a maximal RBE for neoplastic transformation was found at a LET of ~100 keV/μm, reaching a maximum of seven [232]. In human bronchial epithelial cells, iron and silicon ions (LET 151 and 44 keV/μm, respectively) were more efficient in inducing transformation than gamma rays from a 137Cs source especially when these cells were oncogenically progressed by stable transfection of mutant oncogenes [233].


Cell cycle arrests play a central role in the DNA damage response of dividing cells. Before the cell enters the next cell cycle phase, e.g., from G1 to S phase or from S to G2/M phase, they allow repair of damaged DNA (Chap. 2). They can therefore protect from cell death, mutations or chromosomal aberrations. Concerning the special radiation qualities present in space that are prone to induce complex DNA damage which might persist longer, stronger, or longer cell cycle arrests might be induced in comparison to low-LET radiation. Early experiments observing mitotic delay by time-lapse microscopic cinematography already gave hints that accelerated neon ions produce a stronger delay compared to Co-60 gamma rays [234]. High-LET radiation produces stronger and more persistent blocks in the G2 phase of the cell cycle than low-LET radiation [235]. In synchronous V79 Chinese hamster cells, the cell cycle delays per particle traversal increased with increasing LET and were primarily due to blocks in S and G2/M phase of the cell cycle [236]. Permanent arrest in the G1 phase can also be induced by high-LET radiation [237]. The relative biological efficiency of heavy charged particles with a LET in the range of 100–330 keV/μm for inducing cell division delays was 3.3–4.4 [236] and the percentage of mitotic cells as indication of an arrest at the early G2/M checkpoint decreased with increasing LET [238]. The cell cycle regulating protein p21 (CDKN1A) accumulates in nuclear foci rapidly after heavy ion exposure of fibroblasts [239]. Besides this, expression levels of cell cycle regulatory proteins might be affected to a higher extent by high-LET radiation compared to low-LET radiation [237], for example, after iron ion exposure p21 expression was much higher compared to gamma rays and persisted 10 days after irradiation [193].


Similar to studies with low-LET radiation, gene expression studies after high-LET radiation developed from a focus on single genes (mRNA and protein level by Northern Blot, RT-PCR, real-time RT-qPCR, Western Blot) to arrays of multiple genes, microarrays [240] and detection of the levels of all mRNAs present in cell populations or even single cells by RNA sequencing. After exposure to ionizing radiation, signal transduction pathways can result in the activation of transcription factors. These transcription factors bind to binding sites in their target genes’ promoters which are specific for them (usually short palindromic DNA motifs) [241]. Also, besides promoter or enhancer activation via transcription factor binding, epigenetic mechanisms can be responsible for (persistent) gene expression changes and are therefore the focus of mechanistic research (see Sect. 10.5.9).


In addition to spaceflight experiments, a huge amount of gene expression data from ground-based exposure to neutrons, protons, and different heavy ions for different experimental model systems exists. NASA GeneLab (https://genelab.nasa.gov/) offers a repository for space-related omics data, among others transcriptomics and proteomics from experiments with model organisms, cells, cell lines, and tissues. Currently, a comprehensive picture of gene expression changes is difficult to paint due to the multiple influencing factors that range from the model system (e.g., gut epithelial cells and human bronchial epithelial cells, tissue, animal model) to the methods, cell cycle phase, radiation qualities, doses, kinetics of exposure, timepoint after exposure, and additional spaceflight environmental factors (such as simulation of microgravity effects by hindlimb unloading). The interpretation of the data is complicated by the fact that in the majority of the heavy ion accelerator experiments, the dose is acutely applied within minutes, while exposure during long-term space missions is protracted over several months.


The emerging view is that heavy ions, especially iron ions are capable to induce a stress response persisting for several weeks in addition to an early transient response. This early response can encompass p38MAPK and TP53 activation and expression of its target genes, whereby the cell cycle regulator gene CDKN1A can also be expressed TP53-independently. In tissues, long-term changes in the expression of genes involved in inflammatory and free-radical scavenging pathways occur after iron ion exposure and these changes involve transcription factors such as signal transducer and activator of transcription 3 (STAT3), GATA binding protein 4 (GATA4), Nuclear Factor κB (NF-κB) and nuclear factor of activated T cells 4 (NFATc4) [242]. In human cells, NF-κB was strongly activated by heavy ions, its activation depended on LET [243] and the expression of several chemo- and cytokines was increased [244].


HZE particles are potent inducers of senescence, more potent than gamma rays. Senescence-associated changes in the tumor microenvironment may induce invasion and stemness of tumor cells. Senolytics can be applied to eliminate senescent cells and thereby deplete senescent stromal cells with tumor supportive roles. Shortening of telomeric sequences can lead to telomere fusions and contributes the chromosome instability after heavy ion exposure [245]. Furthermore, accumulation of short telomeres eventually triggers apoptosis or senescence. Unlike normal somatic cells, germline, stem, and tumor cells avoid the latter through a high expression of telomerase. Due to natural telomere shortening during cell division, telomere length is highly linked to aging [246]. Considering the environmental radiation exposure during spaceflight, with higher levels of HZE particles compared to on Earth, NASA investigated the effect of spaceflight on telomere length in the twin study. The twin study examined molecular- and physiological differences of twin astronauts, one spending a year onboard the ISS and the other on Earth [120]. Telomere lengths of peripheral blood mononuclear cells (PBMCs), collected from peripheral blood samples taken preflight from both twins were of similar length. However, during spaceflight, the space twin’s telomere length increased significantly, while the Earth twin’s telomers remained stable during the study. Once returning to Earth, the increased telomere length diminished within 48 h and the number of short telomeres increased compared to preflight [116]. While an unexpected finding, increased telomere length has recently been associated with other biological functions such as DNA damage response, cell cycle kinetics, and mitochondrial stress [247]. Indeed, chromosome aberrations (inversions and translocations) were more frequent during spaceflight and inversion frequencies of the space twin remained elevated postflight, consistent with ionizing radiation exposure inflight. Furthermore, DNA damage repair pathways were upregulated in several circulating immune cells, suggesting increased genomic instability due to ionizing radiation during spaceflight [121]. Similar results (increased telomere length and chromosomal aberrations) were also seen in astronauts during a 6-month spaceflight mission. While telomerase activity likely is responsible for the increased telomere length inflight, the actual contributing mechanism is still unknown. However, astronauts returning from 1 year and 6 month missions showed elevated telomerase activity upon return to Earth [116].


Persistent gene expression and functional changes induced by space radiation exposure could be caused by changes in the epigenome. Changes in the DNA methylation profile and in the histone code encompassing methylation and acetylation of histones could therefore contribute to high-LET carcinogenesis and degenerative diseases and could represent possible prophylactic or therapeutic targets.


For example, in immortalized human bronchial epithelial cells, hypermethylation at CpG sites occurred early after Fe-56 ion exposure and persisted a long time [248]. Long-term epigenetic reprogramming after such exposure was also observed in hematopoietic progenitor and stem cells [249].


High levels of DNA methylating enzymes were also found in the hippocampus of Si-28 ion irradiated mice that developed cognitive impairment [250].


In addition to heavy ion exposure experiments, combined exposure to simulated microgravity and chronic low-dose irradiation or spaceflight experiments using small animals or cell cultures and astronaut data reveal alterations in the methylome and histone modification status after combined exposure to spaceflight environmental factors such as microgravity and space radiation. The lasting imprint of high-LET radiation exposure on the epigenome might allow monitoring the cumulative biological impact of space radiation exposure [248].


The use of small animal models in research is debatable, but still essential to provide general information on cellular and molecular mechanisms, to develop new drugs and treatments. They are mainly used in fundamental scientific research, for the advancement and development of new diagnostic tests and treatment for diseases, for education of researchers as well as in safety assessments of drugs and chemicals.


Animals are a useful research subject for a variety of reasons. Only in living organisms, it is possible to study complex physiological processes. Furthermore, the environment of the experiment can be perfectly controlled (e.g., diet, light, housing, etc.). Also, they have a shorter life cycle so studies can be conducted throughout a whole lifespan or across generations. Animals are biologically very similar to humans and often suffer from similar health problems. In fact, mice share more than 85% of protein-encoding genes with humans—Why Mouse Matters, from the National Human Genome Research Institute (https://www.genome.gov/10001345/importance-of-mouse-genome).


Animal experiments can cause harm to the animal thus ethical review processes have been established around the world [251]. With respect to this, the 3R´s principle by [252] ensure the reduction of animal numbers, refining the test methods to lower the harm to the animal to a minimum and replace animal experiments with alternative methods, when possible (Box 10.5).


The aim of the principle is to improve the treatment of laboratory animals and at the same time advance the quality of scientific studies.
Replacement: Includes methods that avoid or replace the use of animals such as computer/mathematical models (in silico), cell culture models (in vitro), or relative replacement (e.g., invertebrates, such as fruit flies and nematode worms).

Reduction: With improved experimental design, modern imaging, or sharing data and resources, the total number of animals needed can be minimized.

Refinement: Modification in the experiment, which minimize pain, suffering, and distress and allow general improvement of animal welfare (e.g., improvement in the research animal housing conditions, analgesia, and anesthesia for pain relief).


Replacement: Includes methods that avoid or replace the use of animals such as computer/mathematical models (in silico), cell culture models (in vitro), or relative replacement (e.g., invertebrates, such as fruit flies and nematode worms).


Reduction: With improved experimental design, modern imaging, or sharing data and resources, the total number of animals needed can be minimized.


Refinement: Modification in the experiment, which minimize pain, suffering, and distress and allow general improvement of animal welfare (e.g., improvement in the research animal housing conditions, analgesia, and anesthesia for pain relief).


The animals that are most used for terrestrial research are mice, fish, and rats. Since the beginning of space exploration also animals have been used in space programs. Similarly, to how microgravity and cosmic radiation can affect human health, animals are also affected. This is why during an early space mission, at the beginning of 1940, animals were used to investigate various biological processes and the effects of space flights on living organisms. On the 20th of February 1947 the first living organism, fruit flies, were sent to space with the V2 rocket. The dog Laika was the most famous and first mammal which was sent to an orbital spaceflight around the Earth (Fig. 10.16) onboard of the Soviet Spacecraft Sputnik 2 on 3rd November 1957 [253]. Since then, a variety of animals have been sent into space including rodents, ants, cats, monkeys, spiders, and jellyfishe. Nowadays the effect of space conditions on animals, including microgravity and radiation, can also be studied to a certain degree on Earth with the help of clinostats, particle accelerator, and X-ray machines. However, all factors of the complex space environment cannot be simulated simultaneously on Earth.

A photograph of Laika, a Soviet space dog who was one of the first animals in space and the first to orbit the Earth.

Fig. 10.16
On 3 November 1957 Laika was the first living mammal that was sent to space onboard the satellite Sputnik 2


A photograph of Laika, a Soviet space dog who was one of the first animals in space and the first to orbit the Earth.


In case of a large SPE and insufficient shielding, the acute radiation syndrome (ARS, see Chap. 2) might be induced, endangering astronauts’ health and mission success. To understand the pathogenesis of ARS induced by protons and develop therapeutic approaches for space missions, experiments with different animal models including rodents, minipigs, and non-human primates were performed. Whole-body doses up to 2 Gy are expected when astronauts are exposed to large SPE in free space with insufficient shielding. In this dose range, effects on the immune system (see Sect. 10.6.2.3) dominate the syndrome. As the skin dose can be 5–10 higher, the skin might be damaged (see Sect. 10.6.2.2).


Forming the barrier between the outside environment and the inside of the body, the skin is a vital organ. Different skin layers provide the skin with tensile strength and keep a proper barrier function to prevent body water loss, regulate the immune defense and temperature, and protect against ultraviolet damage. The outermost layer, the epidermis, is built mostly out of layers of keratinocytes that differentiate and migrate toward the skin surface. A balance between the proliferation of keratinocytes and shedding of dead cells at the surface of the skin regulates the thickness of the epidermal layer. Below the epidermis lays the dermal skin layer which is mostly composed of connective tissue. Skin’s tensile strength and elasticity are provided by Collagen type I and III, and elastic fibers. Fibroblasts are the major provider synthesizing these proteins. Furthermore, they play a major part in skin wound healing by migrating to the side of the wound, recruiting other cells, and remodeling the extracellular matrix (ECM) to restore the injured skin [254].


The skin receives greatest dose and greatest number of stopping particles, particularly during solar flares [255]. SPE events during EVA could lead to higher skin dose than to internal organs. Furthermore, simulations of SPEs has shown that the total skin dose for astronauts performing EVAs is estimated to be up to 32 Gy (for SPE simulation of August 1972) [31].


Radiation-induced skin injuries can be distinguished by several phases depending on the condition of exposure [256]. Early skin reaction is shown by erythema within a few hours after irradiation. After several weeks, inflammatory damage, erythema, loss of epidermal cells, moist desquamation, hyperpigmentation, edema/hyper-proliferation, and epilation can be observed. Late effects can develop after several months and include dermal atrophy, necrosis, and problems related to the deterioration of the skin vasculature. Skin problems, such as burns and slower wound healing, combined with a deprived immune system increase the risk of infections and hinder recovery from ARS [31].


Because of morphological similarities between (mini) pigs and human skin, these animals have been widely used to better understand the skin reaction to ionizing radiation. Furthermore, rodent models such as mouse, rat, or guinea pig have also been studied for ionizing radiation effects on skin.


Using porcine models, researchers have been able to indicate skin toxicity after exposure to a simulated SPE radiation resembling the energy and fluence profile of a SPE documented in 1989 [257]. Hyperpigmentation of minipig irradiated skin was observed 7 days after irradiation and lasted throughout the entire observation period. These observations were supported by an increase in melanin deposition found in the stratum granulosum. Further observations of increased proliferation, parakeratosis (an accelerated keratinocytic turnover) and increased amount of melanophages, are thought to be an indication of an inflammatory skin response after irradiation.


Other studies exposed minipigs to doses ranging from 5 to 25 Gy of electrons [258]. In agreement with previous mentioned study, a dose-dependent hyperpigmentation of the skin was observed as well as an increase in melanin deposition. Furthermore, in the highest dose exposed group of 25 Gy, skin wounds and ulcers developed 19 days after irradiation on body parts that received the highest dose (tail, ears, and legs). In addition, hair loss in the form of alopecia was observed along the dorsum of these pigs.


Low dose rate exposure of skin to low doses of photons, seem to mostly induce oxidative stress and ECM alterations as observed in a mouse model [259]. Skin gene expression changes related to oxidative stress and extracellular matrix (ECM) have been found after whole-body γ-ray exposure. At low dose rates, genes involved in the formation of reactive oxygen species (ROS) were significantly upregulated at doses of 0.25 Gy. Furthermore, dose rate effects were also found in ECM gene expression profiles. Enhanced expression of genes encoding ECM structural components were found after low dose rate exposure.


The immune system consists of a variety of cells, processes, and chemicals that combine efforts to protect the body from foreign microbes, viruses, cancer cells, and toxins [260].


Dysfunction of the human immune system has been shown during [261] and even after space flight [262]. Among the causes of this immune dysfunction, an altered distribution of the cellular components and altered cytokine profiles [263], as well as cytoskeleton alterations and gene expression dysregulation [264] has been shown in many immune cells. When human lymphocytes are subjected to simulated cosmic radiation in vitro they show chromosomal damage, depending on the type of radiation shielding.


The adverse effects of space radiation on the immune system is one of the major concerns for space flight. The vast majority of the cellular components that constitute the immune system are highly sensitive to ionizing radiation [265]. It is still not clear if space radiation has a synergistic effect in combination with microgravity, principally in long duration missions and in the context of the immune system.


As mentioned, in vitro models have been widely used for studying the effects of space radiation on several cellular types. However, the complexity of most systems—such as the case of the immune system—require approaches that will better mimic physiologic conditions, either in ground-based studies or inflight campaigns. Several animal models that recreate some of the conditions of space flight have been developed for use on Earth. For immunology studies, murine models remain one of the most commonly used small animal model in space radiobiology. Rats exposed to 56-Fe (5 GeV/n) to total doses of 0, 1, 2, and 4 Gy showed a decrease in their lymphocytes, particularly B cells. In another study, mice were irradiated with total (single) doses of 0, 0.5, 2, and 3 Gy with 56-Fe ions. Red blood cell (RBC) counts diminished proportionally to the dose. All three major types of leukocytes also decreased [266].


Sanzari et al. [267] directed a series of radiation experiments using Yucatan minipigs. The animals were exposed to beams comprised of Solar Particle Events (SPE)-like protons, 155 MeV, and electrons, 6 and 12 MeV, with dose profiles that mimic SPE radiation. Their findings suggest that, based on the magnitude of the decrease and the time required to reach the lowest leukocyte counts after irradiation, the proton SPE radiation had more impact on the count than electron SPE radiation, with lymphocytes being the most sensitive type of leukocytes. After proton SPE radiation at skin doses >5 Gy, certain populations of leukocytes (neutrophils) had lasting effects following the irradiation (up to 90 days) [267].


For studying the intricate function of the immune system and how it responds to acute exposures of space radiation, small animal models are essential since they can showcase the network of phenomena. Adding up to the already challenging task of pinpointing the alterations occurring in the irradiated immune system we must find a way of adding the following to the equation: isolation, altered circadian rhythms, psychologic stress, and, of course, altered gravity levels.


Chronic and late effects of space radiation exposure encompass increased cancer risk, early cataract formation, and a possibly increased risk for degenerative diseases of several organ systems such as the cardiovascular and the central nervous system).


Animal models of cancer induction by space radiation play a crucial role in the determination of the radiation risk associated with a space mission.


Firstly, they provide with information about the RBE of different space radiation components such as HZE particles for cancer induction in different organs when compared to a low-LET radiation quality, such as gamma rays or X-rays. The Radiation Quality Factor is derived from the RBE data for cancer induction by HZE particles to scale from gamma radiation to the mixed field of GCR in space radiation cancer risk models. If the RBE is above 1, a higher cancer risk can be assumed for space radiation compared to well-known terrestrial low-LET radiation qualities.


Secondly, experiments with high and low dose rates are the basis to estimate the dose and dose rate effectiveness factor (DDREF) to scale from acute to chronic radiation exposure and thereby account for dose rate effects. As animal experiments with exposure at low dose rates are rarely feasible at heavy ion accelerators because of restricted beam time access, dose-rate effect experiments were performed so far at neutron facilities.


Furthermore, animal models give insight into the mechanisms of cancerogenesis by HZE particle exposure, e.g., the role of non-targeted effects, and thereby allow to identify potential molecular targets for effective countermeasures (Box 10.6).


Inbred mouse strains are produced by at least 20 generations of brother-sister mating and they are traceable to a single founding pair. The individuals of an inbred strain are genetically nearly identical to each other and experimental results are highly reproducible. Examples: CBA mouse (cross of Bagg albino and DBA), C57BL/6 mouse (with black coat), BALB/c (Bagg albino) mouse.

Outbred strains provide genetic diversity and are effectively wildtype in nature with as little inbreeding as possible.

Mating of at least two strains led to the generation of the first filial generation (F1) hybrid mice.


Inbred mouse strains are produced by at least 20 generations of brother-sister mating and they are traceable to a single founding pair. The individuals of an inbred strain are genetically nearly identical to each other and experimental results are highly reproducible. Examples: CBA mouse (cross of Bagg albino and DBA), C57BL/6 mouse (with black coat), BALB/c (Bagg albino) mouse.


Outbred strains provide genetic diversity and are effectively wildtype in nature with as little inbreeding as possible.


Mating of at least two strains led to the generation of the first filial generation (F1) hybrid mice.


The first animal experiment with HZE particles to determine cancer induction by single ion exposure used mice and focused on the induction of tumors of the exocrine Harderian gland which is located between eye and ear [268–271]. In these experiments, tumor prevalence was determined by sacrificing mice at a predetermined timepoint after exposure and the number of mice with tumors was counted or the number of tumors per mouse was counted. As this gland does not exist in humans, other animal models were developed and applied. Two different approaches predominate: either wildtype rodents, e.g., inbred, F1 hybrid, or outbred mice, or genetically altered rodent models are exposed to HZE particles at a heavy ion accelerator. Multiparent outbreeding strategies can reduce the strong effects of the genetic background that limit gene-environment interactions in studies with inbred, genetically homogeneous animals [272]. To consider sex-specific cancer types, optimally, both sexes are included [272]. After whole-body irradiation of wild-type rodents, they were followed up over the lifespan of the animals for tumor induction. Alternatively, rats were followed up by palpation until first tumor (time-to-cancer incidence), with an additional follow-up until death. After necropsy, histology was performed to determine the number and types of cancer, e.g., mammary tumors [273, 274]. Here, high numbers of animals are required to detect the increase of cancer incidence above the background cancer rates.


Therefore, genetically altered mouse models were developed in order to lower the number of mice and to mimic a specific cancer induction and promotion pathway, mostly for lung, gastrointestinal [275, 276] or liver cancer (hepatocellular carcinoma) [277, 278]. Using a genetically radio-sensitized model implies an assumption about the mechanisms of radiation-induced cancerogenesis—genetically engineered mice carry some, but not all mutations, needed to generate cancer. The rationale behind this approach is to consider somatic mutations in cancer genes such as NOTCH1 and TP53 that might be already present in astronauts when they depart for their first space missions as the number of mutations in the epithelium increases with age [279].


In risk models, development of leukemia (leukemogenesis) and induction of solid tumors are considered separately because of different latency periods after radiation exposure and dose–response relationships. Leukemogenesis is highly relevant for space missions because of its short latency in humans. The CBA mouse strain is susceptible to radiation-induced acute myeloid leukemia (AML) [280] which is explained by a deletion in chromosome 2 (PU.1) that can occur 1 month after irradiation. A point mutation in the second copy of the PU.1 gene causes a differentiation block in the myeloid cells which favor autocrine growth stimulation. In this model, the RBE of iron ions for induction of AML was 1, meaning that the risk of AML induction by high-LET iron ions and low-LET radiation is comparable. As only surviving cells can be transformed into a cancer cell, a higher mutation and chromosomal aberration rate induced by HZE particles can be compensated by cell death from collateral damage after an HZE ion traversed a myeloid cell [277]. RBE for other cancer types can be much different as the effectiveness of HZE ions in inducing a specific cancer type depends on the mechanism responsible for the tumorigenesis in that particular cancer. For example, in the same mouse strain that was used for the AML studies, hepatocellular carcinoma (HCC) was induced by HZE particles with an RBE of up to 74 [278].


Concerning solid tumors, a special focus in the studies so far was to evaluate the stage of tumors that can be induced by HZE particles and on detailed studies on lung cancer, gastrointestinal cancer, and brain tumors (Box 10.7).


Pituitary adenoma, osteosarcoma, Harderian gland tumor, soft tissue sarcoma, thyroid adenoma, ovarian Granulosa cell tumor, mammary adenocarcinoma, histiocytic sarcoma, hemangiosarcoma, hepatocellular carcinoma (HCC), pulmonary adenocarcinoma, small cell lung cancer, myeloid leukemia, (thymic) lymphoma (T cell, B cell), brain tumors, e.g., gliomas [272].


The lung has the highest susceptibility to radiation-induced carcinoma incidence and mortality, based on analysis of human populations exposed to radiation (Life Span Study of atomic bomb survivors). A minimum of five genetic changes convert immortalized human lung epithelial cells to malignant tumors. For lung carcinogenesis, BALB/cByJ or C57/BL6 mice or the K-rasLA1 mouse model [281] were used. In C57/BL6 mice, lung tumors occurred in irradiated mice but not in controls and all were adenocarcinomas, with no significant differences between males and females and for dose fractionation (dividing a radiation dose into multiple fractions, see Chap. 4) versus single dose were found. Incidence of lung tumors was higher in high-LET-irradiated mice than in X-ray-irradiated mice, with an RBE above 6 for all investigated HZE particles (Fe, Si, and O ions) [282].


In the pathogenesis of gastrointestinal tumors, for instance, colorectal cancer and hepatocellular carcinoma (HCC), inflammation plays a crucial role. Animal experiments revealed that heavy ion radiation triggers a pro-inflammatory state which can be associated with late colonic tumors. Furthermore, premalignant polyps with mutations in the Adenomatous polyposis coli (APC) gene could be already present in middle-aged astronauts. In the small intestine, the formation of a few polyps and later adenomas and even adenocarcinomas can result from truncation of the APC gene at codon 1638 [283]. Therefore, a mouse model with a chain-terminating mutation by a mutation to a stop codon or a frameshift (see Chap. 4) in one allele of the APC gene was developed for colon cancer research (Apc1638N/+). APC mutant mouse models show a good correlation with carcinogens implicated in human colorectal cancer. Delayed genomic instability in APC1638N/+ mice paves the way to gastrointestinal tumorigenesis. In this model, no evidence for dose-rate effects with HZE particle exposure was found [275], indicating that the carcinogenic potential of HZE particles is independent of the dose rate.


Also, genetically altered mouse models for the formation of brain tumors are used in space radiation research as already experiments from the 1970s indicated that charged particles can induce glioblastomas: Monkeys (Macaca mulatta) irradiated with high-energy protons (55 MeV, penetration depth ~2.5 cm) surviving 2 years or longer developed glioblastomas [284]. Here, the focus is on the loss of tumor suppressors such as cyclin dependent kinase inhibitor 2A (Cdkn2a or Ink4-Arf), phosphatase and tensin homolog (Pten), and TP53 in astrocytes and on oncogene activation (e.g., epidermal growth factor receptor variant III, EGFRvIII) after irradiation. Iron and silicon ions were much more potent tumor inducers in “preinitiated” astrocytes than gamma rays [285].


The animal studies with single beam irradiations show that the efficiency of HZE particles to induce cancer is related to ion energy, LET with a peak RBE below 100 keV/μm, sex of the animals, and depends on the tumor type [275]. The RBE for cancer induction was recently determined to range from 5 to 16 [286], representing a snapshot that will be further updated as not all available data were included. Currently, based on the results of single beam irradiations, multiple beam experiments with up to 33 ion beams are performed at the NASA Space Radiation Laboratory (NSRL) using the GCR simulator in order to understand whether the effects of the different GCR components act in an additive or even in a synergistic manner in cancer induction.


According to recent epidemiological evidence, radiation-induced cataract (see Chap. 2) occurs with a threshold absorbed dose of 0.5 Gy (0–1 Gy) of sparsely ionizing radiation, meaning that a cataract can arise after any ionizing radiation dose no matter how low if the remaining lifespan is long enough for its appearance. The 1 Sv GCR dose to be expected for a 1000-day Mars mission [83, 84] means that even the upper limit of the cataract-induction threshold dose confidence interval will be reached during a human Mars exploration mission. In astronauts, epidemiological data suggest a higher risk for the development of cataracts in case of missions in LEO with high inclination [287].