EURADOS project on the impact of the proposed ICRU operational dose quantities.

Following the publication of the joint The International Commissions on Radiation Units and Measurements (ICRU) and on Radiological Protection (ICRP) report on new operational quantities for radiation protection, the European Dosimetry Group (EURADOS) have carried out an initial evaluation. The EURADOS report analyses the impact that the new quantities will have on: radiation protection practice; calibration and reference fields; European and national regulation; international standards and, especially, dosemeter and instrument design. The task group included experienced scientists drawn from across the various EURADOS working groups.

A Lunar Microbial Survival Model for Predicting the Forward Contamination of the Moon.

The surface conditions on the Moon are extremely harsh with high doses of ultraviolet (UV) irradiation (26.8 W · m-2 UVC/UVB), wide temperature extremes (-171°C to 140°C), low pressure (10-10 Pa), and high levels of ionizing radiation. External spacecraft surfaces on the Moon are generally >100°C during daylight hours and can reach as high as 140°C at local noon. A Lunar Microbial Survival (LMS) model was developed that estimated (1) the total viable bioburden of all spacecraft landed on the Moon as ∼4.57 × 1010 microbial cells/spores at contact, (2) the inactivation kinetics of Bacillus subtilis spores to vacuum as approaching -2 logs per 2107 days, (3) the inactivation of spores on external surfaces due to concomitant low-pressure and high-temperature conditions as -6 logs per 8 h for local noon conditions, and (4) the ionizing radiation by solar wind particles as approaching -3 logs per lunation on external surfaces only. When the biocidal factors of solar UV, vacuum, high-temperature, and ionizing radiation were combined into an integrated LMS model, a -231 log reduction in viable bioburden was predicted for external spacecraft surfaces per lunation at the equator. Results indicate that external surfaces of landed or crashed spacecraft are unlikely to harbor viable spores after only one lunation, that shallow internal surfaces will be sterilized due to the interactive effects of vacuum and thermal cycling from solar irradiation, and that deep internal surfaces would be affected only by vacuum with a degradation rate of -0.02 logs per lunation.

Limits of Life and the Habitability of Mars: The ESA Space Experiment BIOMEX on the ISS.

BIOMEX (BIOlogy and Mars EXperiment) is an ESA/Roscosmos space exposure experiment housed within the exposure facility EXPOSE-R2 outside the Zvezda module on the International Space Station (ISS). The design of the multiuser facility supports-among others-the BIOMEX investigations into the stability and level of degradation of space-exposed biosignatures such as pigments, secondary metabolites, and cell surfaces in contact with a terrestrial and Mars analog mineral environment. In parallel, analysis on the viability of the investigated organisms has provided relevant data for evaluation of the habitability of Mars, for the limits of life, and for the likelihood of an interplanetary transfer of life (theory of lithopanspermia). In this project, lichens, archaea, bacteria, cyanobacteria, snow/permafrost algae, meristematic black fungi, and bryophytes from alpine and polar habitats were embedded, grown, and cultured on a mixture of martian and lunar regolith analogs or other terrestrial minerals. The organisms and regolith analogs and terrestrial mineral mixtures were then exposed to space and to simulated Mars-like conditions by way of the EXPOSE-R2 facility. In this special issue, we present the first set of data obtained in reference to our investigation into the habitability of Mars and limits of life. This project was initiated and implemented by the BIOMEX group, an international and interdisciplinary consortium of 30 institutes in 12 countries on 3 continents. Preflight tests for sample selection, results from ground-based simulation experiments, and the space experiments themselves are presented and include a complete overview of the scientific processes required for this space experiment and postflight analysis. The presented BIOMEX concept could be scaled up to future exposure experiments on the Moon and will serve as a pretest in low Earth orbit.

Increased core body temperature in astronauts during long-duration space missions.

Humans' core body temperature (CBT) is strictly controlled within a narrow range. Various studies dealt with the impact of physical activity, clothing, and environmental factors on CBT regulation under terrestrial conditions. However, the effects of weightlessness on human thermoregulation are not well understood. Specifically, studies, investigating the effects of long-duration spaceflight on CBT at rest and during exercise are clearly lacking. We here show that during exercise CBT rises higher and faster in space than on Earth. Moreover, we observed for the first time a sustained increased astronauts' CBT also under resting conditions. This increase of about 1 °C developed gradually over 2.5 months and was associated with augmented concentrations of interleukin-1 receptor antagonist, a key anti-inflammatory protein. Since even minor increases in CBT can impair physical and cognitive performance, both findings have a considerable impact on astronauts' health and well-being during future long-term spaceflights. Moreover, our findings also pinpoint crucial physiological challenges for spacefaring civilizations, and raise questions about the assumption of a thermoregulatory set point in humans, and our evolutionary ability to adapt to climate changes on Earth.

The radiation environment on the surface of Mars - Summary of model calculations and comparison to RAD data.

The radiation environment at the Martian surface is, apart from occasional solar energetic particle events, dominated by galactic cosmic radiation, secondary particles produced in their interaction with the Martian atmosphere and albedo particles from the Martian regolith. The highly energetic primary cosmic radiation consists mainly of fully ionized nuclei creating a complex radiation field at the Martian surface. This complex field, its formation and its potential health risk posed to astronauts on future manned missions to Mars can only be fully understood using a combination of measurements and model calculations. In this work the outcome of a workshop held in June 2016 in Boulder, CO, USA is presented: experimental results from the Radiation Assessment Detector of the Mars Science Laboratory are compared to model results from GEANT4, HETC-HEDS, HZETRN, MCNP6, and PHITS. Charged and neutral particle spectra and dose rates measured between 15 November 2015 and 15 January 2016 and model results calculated for this time period are investigated.

The charged particle radiation environment on Mars measured by MSL/RAD from November 15, 2015 to January 15, 2016.

The Radiation Assessment Detector (RAD) on board the Mars Science Laboratory (MSL) Curiosity rover has been measuring the radiation environment in Gale crater on Mars since August, 2012. These first in-situ measurements provide an important data set for assessing the radiation-associated health risks for future manned missions to Mars. Mainly, the radiation field on the Martian surface stems from Galactic Cosmic Rays (GCRs) and secondary particles created by the GCRs' interactions with the Martian atmosphere and soil. RAD is capable of measuring differential particle fluxes for lower-energy ions and isotopes of hydrogen and helium (up to hundreds of MeV/nuc). Additionally, RAD also measures integral particle fluxes for higher energies of these ions. Besides providing insight on the current Martian radiation environment, these fluxes also present an essential input for particle transport codes that are used to model the radiation to be encountered during future manned missions to Mars. Comparing simulation results with actual ground-truth measurements helps to validate these transport codes and identify potential areas of improvements in the underlying physics of these codes. At the First Mars Radiation Modeling Workshop (June 2016 in Boulder, CO), different groups of modelers were asked to calculate the Martian surface radiation environment for the time of November 15, 2015 to January 15, 2016. These model results can then be compared with in-situ measurements of MSL/RAD conducted during the same time frame. In this publication, we focus on presenting the charged particle fluxes measured by RAD between November 15, 2015 and January 15, 2016, providing the necessary data set for the comparison to model outputs from the modeling workshop. We also compare the fluxes to initial GCR intensities, as well as to RAD measurements from an earlier time period (August 2012 to January 2013). Furthermore, we describe how changes and updates in RAD on board processing and the on ground analysis tools effect and improve the flux calculations. An in-depth comparison of modeling results from the workshop and RAD fluxes of this publication is presented elsewhere in this issue (Matthiä et al., 2017).

Charged particle spectra measured during the transit to Mars with the Mars Science Laboratory Radiation Assessment Detector (MSL/RAD).

The Mars Science Laboratory (MSL) started its 253-day cruise to Mars on November 26, 2011. During cruise the Radiation Assessment Detector (RAD), situated on board the Curiosity rover, conducted measurements of the energetic-particle radiation environment inside the spacecraft. This environment consists mainly of galactic cosmic rays (GCRs), as well as secondary particles created by interactions of these GCRs with the spacecraft. The RAD measurements can serve as a proxy for the radiation environment a human crew would encounter during a transit to Mars, for a given part of the solar cycle, assuming that a crewed vehicle would have comparable shielding. The measurements of radiological quantities made by RAD are important in themselves, and, the same data set allow for detailed analysis of GCR-induced particle spectra inside the spacecraft. This provides important inputs for the evaluation of current transport models used to model the free-space (and spacecraft) radiation environment for different spacecraft shielding and different times in the solar cycle. Changes in these conditions can lead to significantly different radiation fields and, thus, potential health risks, emphasizing the need for validated transport codes. Here, we present the first measurements of charged particle fluxes inside a spacecraft during the transit from Earth to Mars. Using data obtained during the last two month of the cruise to Mars (June 11-July 14, 2012), we have derived detailed energy spectra for low-Z particles stopping in the instrument's detectors, as well as integral fluxes for penetrating particles with higher energies. Furthermore, we analyze the temporal changes in measured proton fluxes during quiet solar periods (i.e., when no solar energetic particle events occurred) over the duration of the transit (December 9, 2011-July 14, 2012) and correlate them with changing heliospheric conditions.

Radiation Measurements Performed with Active Detectors Relevant for Human Space Exploration.

A reliable radiation risk assessment in space is a mandatory step for the development of countermeasures and long-duration mission planning in human spaceflight. Research in radiobiology provides information about possible risks linked to radiation. In addition, for a meaningful risk evaluation, the radiation exposure has to be assessed to a sufficient level of accuracy. Consequently, both the radiation models predicting the risks and the measurements used to validate such models must have an equivalent precision. Corresponding measurements can be performed both with passive and active devices. The former is easier to handle, cheaper, lighter, and smaller but they measure neither the time dependence of the radiation environment nor some of the details useful for a comprehensive radiation risk assessment. Active detectors provide most of these details and have been extensively used in the International Space Station. To easily access such an amount of data, a single point access is becoming essential. This review presents an ongoing work on the development of a tool that allows obtaining information about all relevant measurements performed with active detectors providing reliable inputs for radiation model validation.

Space experiment "Cellular Responses to Radiation in Space (CellRad)": Hardware and biological system tests.

One factor contributing to the high uncertainty in radiation risk assessment for long-term space missions is the insufficient knowledge about possible interactions of radiation with other spaceflight environmental factors. Such factors, e.g. microgravity, have to be considered as possibly additive or even synergistic factors in cancerogenesis. Regarding the effects of microgravity on signal transduction, it cannot be excluded that microgravity alters the cellular response to cosmic radiation, which comprises a complex network of signaling pathways. The purpose of the experiment "Cellular Responses to Radiation in Space" (CellRad, formerly CERASP) is to study the effects of combined exposure to microgravity, radiation and general space flight conditions on mammalian cells, in particular Human Embryonic Kidney (HEK) cells that are stably transfected with different plasmids allowing monitoring of proliferation and the Nuclear Factor κB (NF-κB) pathway by means of fluorescent proteins. The cells will be seeded on ground in multiwell plate units (MPUs), transported to the ISS, and irradiated by an artificial radiation source after an adaptation period at 0 × g and 1 × g. After different incubation periods, the cells will be fixed by pumping a formaldehyde solution into the MPUs. Ground control samples will be treated in the same way. For implementation of CellRad in the Biolab on the International Space Station (ISS), tests of the hardware and the biological systems were performed. The sequence of different steps in MPU fabrication (cutting, drilling, cleaning, growth surface coating, and sterilization) was optimized in order to reach full biocompatibility. Different coatings of the foil used as growth surface revealed that coating with 0.1 mg/ml poly-D-lysine supports cell attachment better than collagen type I. The tests of prototype hardware (Science Model) proved its full functionality for automated medium change, irradiation and fixation of cells. Exposure of HEK cells to the ß-rays emitted by the radiation source dose-dependently decreased cell growth and increased NF-κB activation. The signal of the fluorescent proteins after formaldehyde fixation was stable for at least six months after fixation, allowing storage of the MPUs after fixation for several months before the transport back to Earth and evaluation of the fluorescence intensity. In conclusion, these tests show the feasibility of CellRad on the ISS with the currently available transport mechanisms.

Constitutive expression of tdTomato protein as a cytotoxicity and proliferation marker for space radiation biology.

The radiation risk assessment for long-term space missions requires knowledge on the biological effectiveness of different space radiation components, e.g. heavy ions, on the interaction of radiation and other space environmental factors such as microgravity, and on the physical and biological dose distribution in the human body. Space experiments and ground-based experiments at heavy ion accelerators require fast and reliable test systems with an easy readout for different endpoints. In order to determine the effect of different radiation qualities on cellular proliferation and the biological depth dose distribution after heavy ion exposure, a stable human cell line expressing a novel fluorescent protein was established and characterized. tdTomato, a red fluorescent protein of the new generation with fast maturation and high fluorescence intensity, was selected as reporter of cell proliferation. Human embryonic kidney (HEK/293) cells were stably transfected with a plasmid encoding tdTomato under the control of the constitutively active cytomegalovirus (CMV) promoter (ptdTomato-N1). The stably transfected cell line was named HEK-ptdTomato-N1 8. This cytotoxicity biosensor was tested by ionizing radiation (X-rays and accelerated heavy ions) exposure. As biological endpoints, the proliferation kinetics and the cell density reached 100 h after irradiation reflected by constitutive expression of the tdTomato were investigated. Both were reduced dose-dependently after radiation exposure. Finally, the cell line was used for biological weighting of heavy ions of different linear energy transfer (LET) as space-relevant radiation quality. The relative biological effectiveness of accelerated heavy ions in reducing cellular proliferation peaked at an LET of 91 keV/µm. The results of this study demonstrate that the HEK-ptdTomato-N1 reporter cell line can be used as a fast and reliable biosensor system for detection of cytotoxic damage caused by ionizing radiation.

NUNDO: a numerical model of a human torso phantom and its application to effective dose equivalent calculations for astronauts at the ISS.

The health effects of cosmic radiation on astronauts need to be precisely quantified and controlled. This task is important not only in perspective of the increasing human presence at the International Space Station (ISS), but also for the preparation of safe human missions beyond low earth orbit. From a radiation protection point of view, the baseline quantity for radiation risk assessment in space is the effective dose equivalent. The present work reports the first successful attempt of the experimental determination of the effective dose equivalent in space, both for extra-vehicular activity (EVA) and intra-vehicular activity (IVA). This was achieved using the anthropomorphic torso phantom RANDO(®) equipped with more than 6,000 passive thermoluminescent detectors and plastic nuclear track detectors, which have been exposed to cosmic radiation inside the European Space Agency MATROSHKA facility both outside and inside the ISS. In order to calculate the effective dose equivalent, a numerical model of the RANDO(®) phantom, based on computer tomography scans of the actual phantom, was developed. It was found that the effective dose equivalent rate during an EVA approaches 700 µSv/d, while during an IVA about 20 % lower values were observed. It is shown that the individual dose based on a personal dosimeter reading for an astronaut during IVA results in an overestimate of the effective dose equivalent of about 15 %, whereas under an EVA conditions the overestimate is more than 200 %. A personal dosemeter can therefore deliver quite good exposure records during IVA, but may overestimate the effective dose equivalent received during an EVA considerably.

Imaging of nuclear factor κB activation induced by ionizing radiation in human embryonic kidney (HEK) cells.

Ionizing radiation modulates several signaling pathways resulting in transcription factor activation. Nuclear factor kappa B (NF-κB) is one of the most important transcription factors that respond to changes in the environment of a mammalian cell. NF-κB plays a key role not only in inflammation and immune regulation but also in cellular radiation response. In response to DNA damage, NF-κB might inhibit apoptosis and promote carcinogenesis. Our previous studies showed that ionizing radiation is very effective in inducing biological damages. Therefore, it is important to understand the radiation-induced NF-κB signaling cascade. The current study aims to improve existing mammalian cell-based reporter assays for NF-κB activation by the use of DD-tdTomato which is a destabilized variant of red fluorescent protein tdTomato. It is demonstrated that exposure of recombinant human embryonic kidney cells (HEK/293 transfected with a reporter constructs containing NF-κB binding sites in its promoter) to ionizing radiation induces NF-κB-dependent DD-tdTomato expression. Using this reporter assays, NF-κB signaling in mammalian cells was monitored by flow cytometry and fluorescence microscopy. Activation of NF-κB by the canonical pathway was found to be quicker than by the genotoxin- and stress-induced pathway. X-rays activate NF-κB in HEK cells in a dose-dependent manner, and the extent of NF-κB activation is higher as compared to camptothecin.

Cell cycle delay in murine pre-osteoblasts is more pronounced after exposure to high-LET compared to low-LET radiation.

Space radiation contains a complex mixture of particles comprised primarily of protons and high-energy heavy ions. Radiation risk is considered one of the major health risks for astronauts who embark on both orbital and interplanetary space missions. Ionizing radiation dose-dependently kills cells, damages genetic material, and disturbs cell differentiation and function. The immediate response to ionizing radiation-induced DNA damage is stimulation of DNA repair machinery and activation of cell cycle regulatory checkpoints. To date, little is known about cell cycle regulation after exposure to space-relevant radiation, especially regarding bone-forming osteoblasts. Here, we assessed cell cycle regulation in the osteoblastic cell line OCT-1 after exposure to various types of space-relevant radiation. The relative biological effectiveness (RBE) of ionizing radiation was investigated regarding the biological endpoint of cellular survival ability. Cell cycle progression was examined following radiation exposure resulting in different RBE values calculated for a cellular survival level of 1 %. Our findings indicate that radiation with a linear energy transfer (LET) of 150 keV/µm was most effective in inducing reproductive cell killing by causing cell cycle arrest. Expression analyses indicated that cells exposed to ionizing radiation exhibited significantly up-regulated p21(CDKN1A) gene expression. In conclusion, our findings suggest that cell cycle regulation is more sensitive to high-LET radiation than cell survival, which is not solely regulated through elevated CDKN1A expression.

Resistance of Bacillus subtilis spore DNA to lethal ionizing radiation damage relies primarily on spore core components and DNA repair, with minor effects of oxygen radical detoxification.

The roles of various core components, including α/ß/γ-type small acid-soluble spore proteins (SASP), dipicolinic acid (DPA), core water content, and DNA repair by apurinic/apyrimidinic (AP) endonucleases or nonhomologous end joining (NHEJ), in Bacillus subtilis spore resistance to different types of ionizing radiation including X rays, protons, and high-energy charged iron ions have been studied. Spores deficient in DNA repair by NHEJ or AP endonucleases, the oxidative stress response, or protection by major α/ß-type SASP, DPA, and decreased core water content were significantly more sensitive to ionizing radiation than wild-type spores, with highest sensitivity to high-energy-charged iron ions. DNA repair via NHEJ and AP endonucleases appears to be the most important mechanism for spore resistance to ionizing radiation, whereas oxygen radical detoxification via the MrgA-mediated oxidative stress response or KatX catalase activity plays only a very minor role. Synergistic radioprotective effects of α/ß-type but not γ-type SASP were also identified, indicating that α/ß-type SASP's binding to spore DNA is important in preventing DNA damage due to reactive oxygen species generated by ionizing radiation.

Bacillus subtilis RecA and its accessory factors, RecF, RecO, RecR and RecX, are required for spore resistance to DNA double-strand break.

Bacillus subtilis RecA is important for spore resistance to DNA damage, even though spores contain a single non-replicating genome. We report that inactivation of RecA or its accessory factors, RecF, RecO, RecR and RecX, drastically reduce survival of mature dormant spores to ultrahigh vacuum desiccation and ionizing radiation that induce single strand (ss) DNA nicks and double-strand breaks (DSBs). The presence of non-cleavable LexA renders spores less sensitive to DSBs, and spores impaired in DSB recognition or end-processing show sensitivities to X-rays similar to wild-type. In vitro RecA cannot compete with SsbA for nucleation onto ssDNA in the presence of ATP. RecO is sufficient, at least in vitro, to overcome SsbA inhibition and stimulate RecA polymerization on SsbA-coated ssDNA. In the presence of SsbA, RecA slightly affects DNA replication in vitro, but addition of RecO facilitates RecA-mediated inhibition of DNA synthesis. We propose that repairing of the DNA lesions generates a replication stress to germinating spores, and the RecA·ssDNA filament might act by preventing potentially dangerous forms of DNA repair occurring during replication. RecA might stabilize a stalled fork or prevent or promote dissolution of reversed forks rather than its cleavage that should require end-processing.

The MATROSHKA experiment: results and comparison from extravehicular activity (MTR-1) and intravehicular activity (MTR-2A/2B) exposure.

Astronauts working and living in space are exposed to considerably higher doses and different qualities of ionizing radiation than people on Earth. The multilateral MATROSHKA (MTR) experiment, coordinated by the German Aerospace Center, represents the most comprehensive effort to date in radiation protection dosimetry in space using an anthropomorphic upper-torso phantom used for radiotherapy treatment planning. The anthropomorphic upper-torso phantom maps the radiation distribution as a simulated human body installed outside (MTR-1) and inside different compartments (MTR-2A: Pirs; MTR-2B: Zvezda) of the Russian Segment of the International Space Station. Thermoluminescence dosimeters arranged in a 2.54 cm orthogonal grid, at the site of vital organs and on the surface of the phantom allow for visualization of the absorbed dose distribution with superior spatial resolution. These results should help improve the estimation of radiation risks for long-term human space exploration and support benchmarking of radiation transport codes.

Utilization of low-pressure plasma to inactivate bacterial spores on stainless steel screws.

A special focus area of planetary protection is the monitoring, control, and reduction of microbial contaminations that are detected on spacecraft components and hardware during and after assembly. In this study, wild-type spores of Bacillus pumilus SAFR-032 (a persistent spacecraft assembly facility isolate) and the laboratory model organism B. subtilis 168 were used to study the effects of low-pressure plasma, with hydrogen alone and in combination with oxygen and evaporated hydrogen peroxide as a process gas, on spore survival, which was determined by a colony formation assay. Spores of B. pumilus SAFR-032 and B. subtilis 168 were deposited with an aseptic technique onto the surface of stainless steel screws to simulate a spore-contaminated spacecraft hardware component, and were subsequently exposed to different plasmas and hydrogen peroxide conditions in a very high frequency capacitively coupled plasma reactor (VHF-CCP) to reduce the spore burden. Spores of the spacecraft isolate B. pumilus SAFR-032 were significantly more resistant to plasma treatment than spores of B. subtilis 168. The use of low-pressure plasma with an additional treatment of evaporated hydrogen peroxide also led to an enhanced spore inactivation that surpassed either single treatment when applied alone, which indicates the potential application of this method as a fast and suitable way to reduce spore-contaminated spacecraft hardware components for planetary protection purposes.

Protective role of spore structural components in determining Bacillus subtilis spore resistance to simulated mars surface conditions.

Spores of wild-type and mutant Bacillus subtilis strains lacking various structural components were exposed to simulated Martian atmospheric and UV irradiation conditions. Spore survival and mutagenesis were strongly dependent on the functionality of all of the structural components, with small acid-soluble spore proteins, coat layers, and dipicolinic acid as key protectants.

Multifactorial resistance of Bacillus subtilis spores to high-energy proton radiation: role of spore structural components and the homologous recombination and non-homologous end joining DNA repair pathways.

The space environment contains high-energy charged particles (e.g., protons, neutrons, electrons, α-particles, heavy ions) emitted by the Sun and galactic sources or trapped in the radiation belts. Protons constitute the majority (87%) of high-energy charged particles. Spores of Bacillus species are one of the model systems used for astro- and radiobiological studies. In this study, spores of different Bacillus subtilis strains were used to study the effects of high energetic proton irradiation on spore survival. Spores of the wild-type B. subtilis strain [mutants deficient in the homologous recombination (HR) and non-homologous end joining (NHEJ) DNA repair pathways and mutants deficient in various spore structural components such as dipicolinic acid (DPA), α/ß-type small, acid-soluble spore protein (SASP) formation, spore coats, pigmentation, or spore core water content] were irradiated as air-dried multilayers on spacecraft-qualified aluminum coupons with 218 MeV protons [with a linear energy transfer (LET) of 0.4 keV/µm] to various final doses up to 2500 Gy. Spores deficient in NHEJ- and HR-mediated DNA repair were significantly more sensitive to proton radiation than wild-type spores, indicating that both HR and NHEJ DNA repair pathways are needed for spore survival. Spores lacking DPA, α/ß-type SASP, or with increased core water content were also significantly more sensitive to proton radiation, whereas the resistance of spores lacking pigmentation or spore coats was essentially identical to that of the wild-type spores. Our results indicate that α/ß-type SASP, core water content, and DPA play an important role in spore resistance to high-energy proton irradiation, suggesting their essential function as radioprotectants of the spore interior.