The Moon is the next stepping stone for human space exploration, and several nations have announced plans for its exploration by humans. Space radiation exposure is one of the major risks for astronauts’ health as chronic exposure to galactic cosmic rays (GCRs) may have late health effects such as induction of cataract, cancer, or degenerative diseases of the central nervous system or other organ systems. Moreover, exposure to large solar particle events (SPEs) in a situation with insufficient shielding may cause severe acute effects. The exposure to GCR is inevitable but generally contributes a low dose rate compared to the sporadic, unpredictable, but sometimes very intense SPEs in which Solar Energetic Particles (SEPs) are accelerated close to the Sun by solar flares (Radio Blackouts; large eruptions of electromagnetic radiation from the Sun) and Coronal Mass Ejections (CMEs; large expulsions of plasma and magnetic field from the Sun’s corona). The nucleonic component of GCR consists mainly of protons (~87%), helium (~12%), and heavier nuclei (~1%). These nuclei have very high energy and are therefore highly penetrating. Heavy nuclei are the nuclei of ordinary atoms of high atomic number whose electrons have been stripped away yielding a very heavy, highly charged particle. Because of their single charge, protons are only weakly ionizing, and helium nuclei are four times more ionizing per nucleus. The remaining 1% of nuclei are high (H) atomic number (Z) and energy (E) elements (HZE ions; the high-energy nuclei component of galactic cosmic rays which have an electric charge greater than +2) that contribute to radiation damage disproportionally according to the square of their nuclear charge, Z, resulting in very dense ionization along their tracks. Because of nuclear fragmentation and other complex interactions with matter, their exact effects on humans are uncertain but may be considerable.

During a stay on the Moon, humans are exposed to elevated radiation levels due to the lack of substantial atmospheric and magnetic shielding compared to the Earth’s surface. The absence of magnetic and atmospheric shielding allows cosmic rays of all energies to impinge on the lunar surface. Besides the continuous exposure to galactic cosmic rays (GCR), which increases the risk of cancer mortality, exposure through particles emitted in sudden nonpredictable solar particle events (SPE) may occur. SPEs show an enormous variability in particle flux, i.e. the rate of transfer of particles through a unit area, and energy spectra, i.e. the number of particles or intensity of a particle beam as a function of particle energy, and have the potential to expose space crew to life-threatening doses. On Earth, the contribution to the annual terrestrial dose of natural ionizing radiation of 2.4 mSv (millisieverts; 1 sievert (Sv) = 1,000 millisieverts (mSv)) by cosmic radiation is about 1/6 (0.345 mSv/year), whereas the annual exposure caused by GCR on the lunar surface is roughly 380 mSv (solar minimum) and 110 mSv (solar maximum). Radiation is measured using the unit sievert (a derived unit of ionizing radiation dose), which quantifies the amount absorbed by human tissues. The analysis of worst-case scenarios has indicated that SPE may lead to an exposure of about 1 Sv. The only efficient measure to reduce radiation exposure is the provision of radiation shelters. Measurements on the lunar surface performed during the Apollo missions cover only a small energy band for thermal neutrons and are not sufficient to estimate the exposure.

Recent Measurements of Space Radiations

Though the Apollo missions of the 1960s and 1970s proved it was safe for people to spend a few days on the lunar surface, NASA did not take daily radiation measurements that would help scientists quantify just how long crews could stay. This question was resolved recently after a Chinese-German team published in the journal Science Advances the results of an experiment carried out by China’s Chang’E 4 lander in 2019.

Astronauts on the Moon will encounter radiation between two and three times more intense than experienced on the International Space Station (ISS) and 200 times more intense than levels on Earth. According to Robert Wimmer-Schweingruber from the University of Kiel, who is a co-author of the published research results says that you could only stay on the Moon for two months. That takes into account the radiation you’d encounter on the week-long trip to and from the Moon as well. Currently, NASA hopes to have humans back on the Moon in 2024, and the goal is to move toward a long-term presence after that. The agency is even laying the groundwork for a space station that would remain in lunar orbit to provide crews easy access to the surface. However, extended forays to the Moon will apparently require new shielding technologies. Artemis-1 would test out all the critical systems. Artemis-2, which will repeat the trip around the Moon with astronauts. Artemis-3 will become the first mission to send astronauts to the lunar surface. But deploying the US military’s space branch on the Moon is not going to happen anytime soon and may take a few decades.

Radiation Protection During Extravehicular Activity

The SPE shelter will have to comfortably host astronauts from several hours up to a few days. It should be equipped with an airlock system through which the astronauts on extravehicular activities can quickly come back. Ideally, this system should have the capability to be docked with the surface transport vehicle that the crew will use. Since at the time of an SPE or high solar activity an astronaut can be far from the base, a solution must be proposed for surface transport vehicle radiation protection also. This protection is only a short-term solution and the astronauts are required to return to the base and shelter immediately if an alert is received.

Researchers have successfully made yarn out of boron nitride nanotubes (BNNTs), so it’s flexible enough to be woven into the fabric of space suits, providing astronauts with significant radiation protection even while they’re performing spacewalks in transit or out on the harsh Martian surface. Though hydrogenated BNNTs are still in development and testing, they have the potential to be one of our key structural and shielding materials in spacecraft, habitats, vehicles, and spacesuits that will be used on Mars.

The FLARE Suit is being studied in the intra-vehicular environment as a supplement to already existing shielding provided by the spacecraft’s structure, but extravehicular activities in space and on other planets can be considered. It consists of a bladder suit that is to be filled with water when needed, the water is already present on any human-carrying spacecraft. The suit can be deployed within a few minutes, be very lightweight at launch due to the resource utilization of onboard water, and does not use a lot of material compared to a fully shielded module since it is fitted to the individual human body.

Passive Radiation Shielding

Passive space radiation shielding consists of placing some sort of physical material in between a person and the source of radiation. Its main advantage over other forms of radiation shielding is its ability to shield against any form of radiation, be it positively charged, negatively charged, or neutral, and it is widely employed in Earth-based shielding applications since weight is not an issue. For space applications, however, every kilogram of mass has a significant impact on the mission cost and feasibility. While dense materials or thick layers of material are great at attenuating the energy levels of incoming radiation, they come with a significant amount of mass. Additionally, in Earth-based scenarios, engineers usually need to shield a radioactive item and prevent radiation from leaking out. In space-based scenarios, however, engineers seek to keep radiation from getting into the area where the astronauts are located. For long-duration space missions, the livable area that the astronauts occupy can become quite a bit larger than most Earth-based radiation sources. Thus, not only is mass an issue in space, but the volume enclosed by the shield is much larger and therefore requires even more shielding material. This can get prohibitively heavy when used as the sole method of radiation protection for long-duration space flights.

Active Radiation Shielding

Active space radiation shielding is inspired by the Earth’s magnetic field, which serves both to deflect and to trap portions of the incoming space radiation. Many spacecraft designers have proposed innovative designs that form large electromagnetic fields around a spacecraft, in order to mimic the protection of Earth’s magnetosphere. None of these designs have been tested in the space environment, but represent the future directions that technology must take in order to have a safe method of interplanetary space travel. Since the field is still under development, a multitude of suggested approaches exist:

1. Electrostatic Shielding: This approach creates an electric field around an astronaut’s habitat, with the negative potential facing outwards to slow down negatively charged radiation. Engineering trade-offs to consider when designing such a system include the dielectric breakdown strength of the electrostatic material, the maximum voltage capabilities of the power supply, and the mechanical limits of the support structure in comparison to the internal coulomb forces generated by the charged components of the shield. Finally, there are no known major physiological issues associated with humans in large electrostatic fields, but further investigations are required in order to verify astronaut safety with a sufficient degree of certainty.
2. Magnetic Shielding. Magnetic shielding consists of forming a large magnetic field around the spacecraft, usually through the use of superconducting solenoids. Unlike with electric fields, there are known and suspected physiological effects of moving within a strong magnetic field. In order to use this approach for space radiation shielding, the design must allow for a habitable region without significant magnetic field strengths. Usually, this is done by using a torus-shaped design that has a shielded region internal to the torus. These layouts allow for a small region between the solenoids that is free of magnetic fields, while still generating a magnetic field that is comparable to an ideal dipole at large distances. Charged particles are either deflected by the magnetic field or trapped along the magnetic field lines, well before they approach the internal shielded region of the torus.
3. Plasma Shielding. Plasma shielding is a field of ongoing research, fundamentally consisting of a mass of ionized particles that are entrapped by electromagnetic fields, swirling around a spacecraft enclosure and serving to deflect or ensnare charged particles. The protection is threefold: first, an electrostatic field with a positive potential repels positively charged radiation. Next, a magnetic field is added to ensnare the negatively charged particles that are drawn to the positive potential. Finally, these negatively charged particles would be drawn towards the positively charged surface, which could neutralize the surface; thus, a passive current-absorbing shield is placed at the magnetic poles, to absorb the negatively charged particles before they impact the positive surface. While the system is more complex, it leverages the strengths of passive, electrostatic, and magnetic shielding, and combines them into a highly effective solution.

Lava Tubes as Radiation Shield

In 2009, deep holes in the Moon’s Marius Hills, Mare Tranquillitatis, and Mare Ingenii were discovered by cameras onboard SELENE, a lunar polar orbiter. The vertical holes are considered to be constructed by the collapse of the roof on lava tubes. More recently radar echo patterns acquired by the SELENE lunar radar sounder suggested the existence of intact lava tubes at the Marius Hills. One possible lava tube was estimated to be 50 km in length.

In the article on radiation dose at the lunar surface and in lava tubes published in the Journal of Radiological Protection, the researchers modeled a lava tube as a radiation shield. The thick ceiling of a lava tube offers significant shielding against HZE particles (i.e. heavy ions) and micrometeorites (i.e. tiny meteoroid that has survived entry through the Earth’s atmosphere). In addition, the temperature fluctuation in a lava tube is much smaller than that observed for the lunar surface. A lava tube can be expected to be a promising candidate site to host a lunar base.

In-situ Resource Utilization (ISRU): Regolith for Radiation Shielding

The work on utilization of the Moon regolith for radiation protection and thermal insulation in permanent lunar habitats balances out the benefits of regolith utilization as the main material for protection against galactic cosmic rays (GCR) and solar particle events (SPE) on the one hand, and for an adequate thermal environment inside the habitat on the other. Such an approach aims to outline the potential merit of regolith utilization in habitats from a double point of view, ultimately allowing the definition of specifications for the architects designing the construction of future habitats on the Moon.

Studies continuously evaluate the use of loose regolith, either through piling it up on top of an inflatable habitat, 3D printing (so-called additive manufacturing), sintering (also known as frittage), or melting (also called fusion). The baseline of such designs relies on multilayer structures, using complementary materials, such as polyethylene bags and aluminum foils, to contribute to the wall’s stress resistance and thermal protection. Polyethylene and aluminum are used as standard materials in space exploration. In a recent study, properties to further enhance radiation protection such as regolith compaction and multilayer materials are evaluated. This work outlines different options for regolith compaction and utilization which scopes out the architectural liberty in habitat construction. The available compression techniques and the amount of compaction that will be possible to make in situ will largely define the shape of future lunar habitats. The added value of this paper is in the proposal of a reasonably achievable protective layer from radiation which also satisfies the thermal insulation of the habitat. Radiation protection can take several forms, mainly divided into passive and active with electrostatic, magnetic, and plasma shielding. This work studies passive shielding which is based on the ability of a chosen material to stop or decelerate charged particles and attenuate the dose accumulated in the target placed behind the shield. For human outposts on the Moon, passive shielding from radiation also plays the role of structural support and protection from micrometeoroid impacts which altogether may require a habitat to be covered with dense or thick material. The in-situ resource utilization (ISRU; using space-based resources for deep space exploration) is key in achieving this in order to reduce transportation of materials from the Earth and mission costs. Regolith is abundant on the surface of the Moon and it may serve as the main material for protecting habitats from the space environment. When processed, i.e., molten, sintered, mixed with binders, loose regolith can become a useful solid material. This concept relies on the development of ISRU technologies.

Following the As Low As Reasonably Achievable (ALARA) principle of NASA, additional materials such as aluminum, polyethylene, Kevlar, etc., are repeatedly investigated in literature to determine how their utilization may optimize the radiation protection of habitats and in deep space. Hydrogen-rich materials, e.g., water and polyethylene, rank high among others because they have more nuclei in the path of the particles for the same shield thickness in mass per unit area. This helps to break up the heavy nuclei. These materials contain few neutrons which leads to fewer secondary neutrons and as neutrons are a big concern for the effective dose, it is an important factor in radiation protection. In this respect, hydrogen (H1; a neutral hydrogen atom has one proton and one electron) is the best option because it contains no neutrons (also called “protium”). Furthermore, nuclei with lower atomic mass have a smaller atomic charge, which makes them less effective in producing secondary emissions (i.e. ejection of electrons from a solid that is bombarded by a beam of charged particles).

Planetary Extravehicular Activity (EVA) Protection Against Radiations

The risk of radiation exposure in both deep space and on a planetary surface is primarily mission-dependent. Space radiation includes solar particle events (SPEs) and galactic cosmic radiation (GCR), and consequences of exposure fall into four major categories:

1. Carcinogenesis,
2. Degenerative tissue risk,
3. Acute and late risks to the central nervous system, and
4. Acute radiation syndrome (ARS; also known as radiation sickness or radiation poisoning).

Highly energetic heavy ions, or HZE charged particles, are also hazardous to astronauts and their equipment, justifying the need to protect crew members and their critical electronic components. Researchers at NASA have also reviewed current ARS biomathematical models and recommended the utilization of onboard dosimeter input for estimating both radiation doses to organs and the most probable outcomes. Planetary EVAs should be planned around solar activity, but not all SPEs and GCRs are predictable, and so carcinogenesis risk mitigation is necessary for lunar visit/habitation, deep space journey/habitation, and planetary missions. The EMU’s material layup includes a Thermal Micrometeoroid Garment (the outer layer of a space suit; orthofabric composed of Teflon/Nomex/Kevlar, Reinforced Aluminized Mylar (also known as BoPET; Biaxially-oriented polyethylene terephthalate), and neoprene-coated nylon (i.e. a fabric that is coated on both sides) ripstop; holds up well against the harsh space conditions), Dacron polyester, urethane-coated nylon ripstop, and a liquid cooling and ventilation garment (LCVG).

The Orlan-DM spacesuit, however, incorporates several layers of polyethylene for radiation dose reduction. Polyethylene merges a high level of hydrogenation, is affordable, machines well, and is the material effectiveness standard. Kevlar, which provides shielding from debris, also exhibits radiation shielding properties. The Personal Radiation Shielding for interplanetary missions (PERSEO) project, led by the Italian Space Agency, developed a simulated radiation protection garment filled with water to shield crew member’s organs during SPEs. The water can then be recycled to optimize the use of available resources. The simulated prototype reduced radiation dose levels by 40% to blood-forming organs and the gastrointestinal tract (also called digestive tract). Although the system was designed for IVA (IntraVehicular Activity), the concept can scale to EVA. Another IVA concept transferable to EVA is the FLARE Suit, proposed by a researcher at the KTH Royal Institute of Technology.

It consists of multiple bladders that can quickly be filled with salt water, which act as shielding against neutrons. Another potential radiation countermeasure may be the inclusion of hydrogenated Boron Nitride Nanotubes (BNNTs) in either the surface lander module or the spacesuit itself. Compared with carbon (which serves as one of the base elements in polyethylene), both boron and nitrogen have greater neutron absorptions capabilities, with boron having one of the largest out of all the elements. Initial testing of BNNT within the context of long-duration spaceflight applications has been conducted at MIT. Results from this study indicate that hydrogen-enriched BNNT is comparable to polyethylene in terms of SPE shielding effectiveness (90.0% and 90.1%, respectively), but shows more of an improvement with respect to GCR radiation shielding (23.2% for hydrogenated BNNT, 16.7% for polyethylene). Radiation shielding alone may not be enough, thus alternative radiation protection methods could also be necessary. These include biological countermeasures against radiation such as antioxidants (such as Vitamin C), Neulasta (a bone marrow stimulant), topically applied steroid creams, antibiotics, and tardigrade DNA (injected into tissue).

Active radiation shielding, such as electrostatic and magnetic shielding, should also be researched for possible countermeasures against radiation damage. Electrostatic shielding operates on the principle of creating a lens of gossamer membrane (gossamer, meaning thin and light) structures to deflect incoming GCR via multiple charged spheres in certain orientations. Magnetic shielding through either a superconducting solenoid around the spacecraft or mini-magnetospheres (and toroidal magnetic fields) deployed further from the spacecraft could deflect radiation below certain thresholds. Both may be applicable to protecting crew members during EVA operations and reduce the likelihood of radiation damage. However, these concepts are both at a low technology readiness level (TRL) and extensive research and development are still required in order for these concepts to be implemented on a spacecraft.

Biological Countermeasures

It is important to explore various prospects of improving human radioresistance using recent advances in biotechnology. These would include the possibility of genetic modifications to humans utilizing breakthrough technologies in gene editing in combination with the current knowledge of molecular pathways counteracting radiation-induced DNA damage, as well as other possible therapies, such as regenerative medicine, low-dose radio-adaptation, the use of deuterated organic compounds, hypostasis (considerable slowdown of all the vital processes in the body) or a combination thereof.

In principle, ionizing radiation interacts along charged particle tracks with biological molecules such as DNA. The process is largely stochastic and can damage DNA via direct interactions (e.g. ionization and excitation) or via indirect interactions such as through the production of reactive oxygen species (ROS) as a result of radiolysis of water molecules, i.e. the decomposition of water molecules due to ionizing radiation.

Radioresistance denotes the capacity for organisms to protect against, repair, and remove molecular, cellular, and tissue damage caused by ionizing radiation. It is a quality that varies greatly in terms of effectiveness between different organisms. For instance, it is well-known that certain organisms are remarkably resistant to the damaging effects of radiation. The bacterium Deinococcus radiodurans, for instance, possess error-free DNA repair mechanisms and can withstand doses as high as 7 kGy (KiloGray; radiation unit of measure).

Similarly, tardigrades can withstand doses as high as 5 kGy, though doses exceeding 1 kGy render them sterile. Initially, the remarkable radio-resistance of tardigrades was thought to be the result of their anhydrobiotic (i.e. dehydrated) state reducing the effective concentration of hydrolyzable water. However, subsequent studies have found that hydrated tardigrades were more radio-resistant to both gamma-rays and heavy ions than anhydrobiotic specimens (the median lethal dose being 5 kGy for gamma-rays and 6.2 kGy for heavy ions in hydrated animals compared to 4.4 kGy for gamma-rays and 5.2 kGy for heavy ions anhydrobiotic specimens). By comparison, the human median lethal dose is around 0.004 kGy.

All eukaryotic organisms have evolved against a backdrop of constant exposure to endogenous and exogenous mutagens, and as such have developed robust cellular mechanisms for DNA repair and protection against DNA damage.

The mechanisms of cellular protection against DNA damage and mutation and of DNA repair can be categorized as immediate and adaptive defense mechanisms. Immediate defense mechanisms include:

• The production of antioxidants and reactive oxygen species (ROS) scavengers that neutralize ROS produced by oxygen metabolism, ionizing radiation, and UV radiation,
• DNA repair, including direct reversal repair, base excision repair (BER), nucleotide excision repair (NER) and mismatch repair for single-strand modifications, and non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), homologous recombination for double-strand breaks (DSBs), and interstrand crosslink repair for modifications affecting both strands,
• Elimination of damaged cells via apoptosis, and
• Proliferative arrest and replicative senescence.

Hibernation

Recently, a new idea, hibernation, has been proposed as possible mitigation against radiation. Hibernation is a state of reduced metabolism used by many mammals to survive periods of scarcity of resources. During the hibernation period, animals go through a series of extreme physiological adaptations. Among these is a reduction in food intake, and the most important adaptation, as shown by several studies on acute high-dose low-linear energy transfer (low-LET) irradiation, is that animals increase their radioresistance, one of the main advantages of hibernation.

Torpor and hibernation are natural physiological processes. Torpor refers to a period of metabolic suppression with a duration from a few hours to several weeks. The state of torpor is probably older in evolutionary terms and was likely a survival strategy of protomammals. Hibernation is a more elaborate behavior, structured in many long bouts of torpor separated by brief interbout arousals (IBA). The scope of these arousals is still unknown. (Evidence suggests that interbout arousals function to maintain or rebalance carbon and nitrogen homeostasis.) During hibernation, the animal undergoes a series of profound physiological changes. (Remarkably, in most hibernators, hibernation is interspersed by short periods of less than a day, called interbout arousals, during which animals quickly (<90 min) increase their metabolism and return to euthermic T_b levels while restoring their main physiological functions.) Recently, the neurons and neuronal circuits that are involved in controlling hibernation have become evident. The first artificial method capable of bringing a non-hibernator (rat) into what is now called synthetic torpor was developed using microinjections of the GABA-A receptor agonist muscimol (temporarily inactivates neurons by mimicking the inhibitory neural effects of GABA) into the brainstem region of the raphe pallidus (RPa) of a rat. This synthetic torpor was shown to increase the radioprotection of organs such as the liver and testis four hours after X-ray irradiation. Here, we discuss the possible mechanisms underlying this fascinating physiological process.

The human metabolism is finely organized and coordinated because of its complex machinery. It becomes a challenge when we have to adjust to or survive in extreme conditions. Astronauts require a high level of physical fitness to perform their mission. The need to be well trained and in good health compared with other occupations is fundamental. However, they still cannot escape from the risks of radiation-induced carcinogenesis. Radiation carcinogenesis is a slow process. Normal living cells damaged by ionizing radiation start a progressive genotypic change, in turn causing a drastic change in their phenotype. Regular cell cycle control points, cell contact inhibition, and apoptosis-programmed death are lost, and cells become malignant. An epidemiological study showed that leukemia has been linked with external radiation exposure in Japanese atomic bomb survivors and medically exposed persons, and skin cancer is linked with radiation exposure in radiologists. Even though astronauts are exposed to much lower doses, there are still not many studies defining the risk of carcinogenesis. Other epidemiological studies show that cancer initiation processes dominate radiation risk after exposure in young people, and radiation could promote preexisting malignant cells after exposure at later ages. This means that if astronauts have an unknown preexisting condition, space flight could promote cancer later in life.

Although hibernators can be found naturally, there are still many things to be discovered about hibernation. Why are hibernators more radioresistant during their inactive state than in their active state? How can they overcome inactivity problems due to prolonged immobility, such as the loss of muscle tone and bone calcium? Although artificially induced torpor in rats was successfully done and showed increased radioresistance, the intriguing questions evade direct answers due to the limitations of currently available experimental preparations, techniques, and data. Hibernation is no longer just a phenomenon that affects a few animal species globally. Perhaps, thanks to the in-depth study of the hibernator phenotype, it can become a new tool to improve the quality of life and radiation protection in future space missions.

Smaller, Lighter Electronics Radiation Shielding

Researchers at North Carolina State University have developed a new technique for shielding electronics in military and space exploration technology from ionizing radiation. The new approach is more cost-effective than existing techniques, and the secret ingredient is rust. The approach can be used to maintain the same level of radiation shielding and reduce the weight by 30% or more, or one could maintain the same weight and improve shielding by 30% or more – compared to the most widely used shielding techniques. Either way, the approach reduces the volume of space taken up by shielding.

Ionizing radiation can cause significant problems for electronic devices. To protect against this, devices that may be exposed to radiation, such as devices used in spacecraft, incorporate radiation shielding. Weight is a significant factor in designing aerospace technologies, and the shielding most commonly found in aerospace devices consists of putting an aluminum box around any sensitive technologies. This has been viewed as providing the best tradeoff between a shield’s weight and the protection it provides.

The new technique relies on mixing oxidized metal powder – rust – into a polymer and then incorporating it into a common conformal coating on the relevant electronics. Metal oxide powder offers less shielding than a metal powder would, but oxides are less toxic and don’t pose electromagnetic challenges that could interfere with a device’s operation. Radiation transport calculations show that the inclusion of the metal oxide powder provides shielding comparable to a conventional shield. At low energies, the metal oxide powder reduces both gamma radiation to the electronics by a factor of 300 and the neutron radiation damage by 225%.

At the same time, the coating is less bulky than a shielding box and in computational simulations, the worst performance of the oxide coating still absorbed 30% more radiation than a conventional shield of the same weight. On top of that, the oxide particulate is much less expensive than the same amount of pure metal. This could potentially reduce the need for conventional shielding materials on space-based electronics. The researchers are continuing to test and fine-tune their shielding technique for use in various applications. They are now looking for industry partners to help them develop the technology for commercial use.

Protection of Materials from Space Radiation

High-energy charged particles or high-energy photons in a space radiation environment can cause temporary damage or permanent failure of spacecraft materials or devices, including single-particle effects, total ionization dose effects, displacement damage effects, surface charge, and discharge effect, internal charging effect, ultraviolet radiation effect, etc. Radiation damage to spacecraft caused by space radiation environment mainly includes ionization damage and displacement damage. Ionization damage refers to the ionization of target atoms and the excitation of extranuclear electrons in the material caused by incident particles, thereby forming electron-hole pairs in the material, resulting in severe degradation of semiconductor device performance, causing single-particle effect and total dose effect; and high-energy photons. The action of the polymer material causes the chemical bond of the polymer material to break and form new bonds, degrading its physical and chemical properties. Displacement damage refers to the interaction of incident particles with atoms in the material and exchange of kinetic energy, causing the target atoms in the material to leave the original position to form interstitial atoms and create vacancies. A displaced atom may collide with other atoms multiple times, creating a shift chain. The vacancies or interstitial atoms generated by atomic displacement usually have strong reactive electrical properties and are trapping traps for carriers or carriers in semiconductors.

According to the action time of the radiation damage effect, it can be divided into long-term effects and transient effects. Long-term effects are long-term changes or degradations in the performance of materials or devices. Transient effects are those that can alter or degrade the properties of a material or device in a short period of time, and the effects of recovery or interruption will soon occur.

Radiation Effects on Electronics

One of the biggest challenges of using electronics in space applications is that integrated circuits are generally not tolerant to cosmic radiation. There are various types of radiation effects that impact integrated circuits’ functionality, performance, and life. The most energetic particle populations found in the heliosphere are the constant flux of Galactic Cosmic Rays (GCRs) and the sporadic SEP events. One of the major sources of radiation damages to spacecraft is attributed to penetrating high-energy solar protons. Radiation effects on electrical and electronic space systems and materials are usually considered in terms of cumulative radiation damages and Single Event Effects (SEE). As mentioned previously, ionization of target atoms results in severe degradation of semiconductor device performance.

Solar energetic particles are also responsible for a number of specific effects caused by direct ionization from single particles traversing through the active volume of an electronic device. These can trigger currents within the device leading to non-destructive as well as potentially destructive effects — which are grouped under the heading SEE (Single Event Effect) — such as:

• SEU (Single Event Upset): single bit flip in a digital memory or flip-flop;
• SEL (Single Event Latch-up): triggering of a parasitic PNPN thyristor structure in a device, which can destroy the component, affecting mainly CMOS structure;
• SEB (Single Event Burnout): triggering of a vertical n-channel transistor or power NPN transistor accompanied by regenerative feedback which has destructive impact; affecting mainly power MOSFETs.

A single-event upset (SEU) is a change of state caused by one single ionizing particle (ions, electrons, photons…) striking a sensitive node in a microelectronic device, such as in a microprocessor, semiconductor memory, or power transistors. The state change is a result of the free charge created by ionization in or close to an important node of a logic element (e.g. memory “bit”). The error in device output or operation caused as a result of the strike is called an SEU or a soft error.

In space, high-energy ionizing particles exist as part of the natural background, referred to as galactic cosmic rays (GCR). Solar particle events and high-energy protons trapped in the Earth’s magnetosphere (Van Allen radiation belts) exacerbate this problem. The high energies associated with the phenomenon in the space particle environment generally render increased spacecraft shielding useless in terms of eliminating SEU and catastrophic single-event phenomena (e.g. destructive latch-up). Secondary atmospheric neutrons generated by cosmic rays can also have sufficiently high energy for producing SEUs in electronics on aircraft flights over the poles or at high altitudes. Trace amounts of radioactive elements in chip packages also lead to SEUs.

Open issues concerning the Effects of Galactic Cosmic Rays in Electronic Devices, involving relativistic heavy ions are 2-fold. Firstly there are possible effects in the material that are different for ions of the same LET for higher energy than for lower energy (e.g., single-event effects (SEE) due to nuclear reactions). Then there are new technologies that simply cannot be tested with low energy ions due to the limited range of the ions. To evaluate radiation sensitivity, electronics are tested on earth with various irradiation sources. Cosmic rays (CR) are the most difficult to simulate on earth. CR can have energies up to 1,020 eV, with a flux maximum at around 1 GeV/n. For reasons of cost-effectiveness and availability, the qualification tests on earth are done at accelerators with much lower energies, usually in the 10 MeV/n range.

There is no full-proof way to eliminate the radiation effects on electronic devices aboard space platforms. However, there are steps that if taken can reduce or minimize these effects.

For non-ionizing radiation you should do the following:

• Choose the best route for each trace on your boards,
• Apply good impedance control to improve signal integrity,
• Isolate radiating components and elements on your Printed Circuit Boards (PCBs),
• Use shielding on your boards.

For ionizing radiation you should do the following:

• Use a radiation-hardened enclosure for your electronic device,
• Use radiation-hardened components on your boards.

Mitigation and Precautions

Spacecraft depend upon their onboard electronic devices to ensure safe operations and mission success. Radiation is the most consistent threat to these devices and the PCBs that comprise them. When designing these boards it is imperative to understand their effects and what steps to take to combat them.

The vacuum of space is a favorable environment for tin whiskers, so prohibited materials are a concern. Pure tin, zinc, and cadmium plating are prohibited on IEEE parts and associated hardware in space. These materials are subject to the spontaneous growth of whiskers that can cause electrical shorts. Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin is used as a final finish. Devices with pure tin leads can suffer from the tin whiskers phenomenon that can cause electrical shorts. Using lead-based solder eliminates the risk of shorts occurring when devices are used in high-stress applications. 

Radiation hardening can help protect integrated circuits from ionizing and non-ionizing radiation (in the form of particle radiation and high-energy electromagnetic radiation) damage in space. Radiation-hardened components or “rad-hard components” are designed specifically to resist ionizing radiation effects. Radiation-hardened integrated circuits can function reliably in the space environment, ensuring that the devices they are used in operate properly. Often times the radiation hardening is dependent upon how the device is used in the system and the end-user must implement the device correctly to get the expected radiation tolerance out of the design.

Spacecraft charging can be mitigated by the methods of electron emission and ion reception. Electron emission is the method in which a device pulls (or draws) electrons from the spacecraft ground and ejects them into space, while ion reception is the method in which positive ions arrive at a spacecraft that is negatively charged to neutralize the negative charges. The former method is effective for reducing the negative charge of the spacecraft ground but not effective for dielectric surfaces. As a demerit, the process can lead to differential charging between the dielectric and the conducting ground. The latter method is effective for mitigating negatively charged surfaces (whether dielectric or conductor) and reducing differential charging. However, it has the disadvantage of electroplating the entire spacecraft with extended use. Because each method has the advantage (or disadvantage) over the other, the use of a combination of both types has been recommended. Other mitigation methods include plasma emission, partially conducting paint, polar molecule emission, mirror reflection, and violet irradiation.

Wearable Radiation Shielding

AstroRad is designed like a wearable shield, to protect astronauts from radiation. Lockheed Martin in coordination with StemRad, Inc., is developing this wearable, radiation-shielding vest, made up of a patented hexagonal material and form-fitted to protect astronauts as they travel to the Moon, Mars, and beyond. Wearing AstroRad would make it easier for astronauts to move and work, especially if a radiation storm were to last longer than a few hours.

Adopting this technology for long-duration human spaceflight makes perfect sense because:

• AstroRad can provide astronauts with protected mobility when traveling between different spacecraft elements of the Gateway (or other future space architecture).
• The vest design protects the most susceptible vital organs — like bone marrow, reproductive organs, and lungs — from the harmful effects of radiation.
• Wearable vests take up minimal space. This is important since efficient use of mass is critical for long-duration human spaceflight missions.

NASA’s Radiation Health Program

As we discussed radiation protection is essential for humans to live and work safely in space. To accomplish this challenging task, NASA has developed the Radiation Health Program. The goal of the program is to carry out the human exploration and development of space without exceeding acceptable risk from exposure to ionizing radiation. Legal, moral, and practical considerations require that NASA limit risks incurred by humans living and working in space to acceptable levels. To determine acceptable levels of risk for astronauts, NASA follows the standard radiation protection practices recommended by the U.S. National Academy of Sciences Space Science Board and the U.S. National Council on Radiation Protection and Measurements.

The roadmap of NASA for radiation protection outlines future research directions toward the goal of enhancing human radio-resistance, including upregulation (i.e. the process by which a cell increases its response to a substance or signal from outside the cell to carry out a specific function) of endogenous repair (i.e. the ability of cells to engage in the repair and regeneration process) and radio-protective mechanisms, possible leeways into gene therapy in order to enhance radio-resistance via the translation of exogenous (i.e. growing or originating from outside an organism) and engineered DNA repair and radio-protective mechanisms, the substitution of organic molecules with fortified isoforms (i.e. any of the chemically distinct forms of a protein that perform the same biochemical function, although often at different rates), the coordination of regenerative and ablative technologies, and methods of slowing metabolic activity while preserving cognitive function.  

European Space Agency (ESA) Research on Radiation Prevention

ESA is currently strengthening its initiatives in identifying requirements and promoting research towards optimizing radiation protection for astronauts. ESA supports the development of common risk limits and risk estimations or, in the absence of consensus, the development of ESA methodologies for the purpose of human exploration of space.

ESA has teamed with Germany’s GSI particle accelerator to test potential shielding for astronauts, including lunar and Mars soil. ESA’s two-year project is assessing the most promising materials for shielding future astronauts going to the Moon, the asteroids, or Mars.

GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany is the only facility in Europe capable of simulating the high-energy heavy atomic nuclei found in galactic cosmic radiation. They have assessed materials including aluminum, water, polyethylene plastic, multilayer structures, and simulated Moon and Mars material – the latter on the basis these will be accessible to planetary expeditions. They have also confirmed a new type of hydrogen storage material holds particular promise.

In order to work on the radiation mitigation and prevention strategies for deep space exploration, ESA put in place a Topical Team (TT) to provide expert advice on relevant research to be undertaken. This TT, supported by the Radiation Protection Initiative (RPI) of the Directorate of Human and Robotic Exploration, forms a forum for ESA and non-ESA experts from space-science, biology, epidemiology, medicine, and physics to identify the most pertinent research requirements for improved space radiation protection.

The research requirements identified by the TT broadly fall into three main research areas, namely:

1. improving characterizations of ionizing radiation in space,
2. increasing knowledge on the potential detrimental health effects of space radiation exposure,
3. and better characterizing the associated risks to astronauts.

The relatively short-term aim of providing an optimized approach to guide the occupational radiation health risk assessment for astronauts, in terms of cancer and non-cancer health effects, will enable improvements in effective risk communication between space agency medical operations and astronauts and aid decision-making relevant to human space missions. The TT, therefore, recommended ESA to promote the development of a European Space Radiation Risk Model (ESRRM) based on innovations in risk assessment and uncertainty analysis. Further longer-term improvements to the new ESRRM could be based on developments of innovative radiation transport codes, refined Relative Biological Effectiveness (RBE is defined and calculated as a ratio of the reference radiation dose to the dose of the test radiation) models, and results from nanodosimetry (i.e. the measurement, and application of nanoscale doses) experiments. The ESRRM should be developed to provide cancer and non-cancer radiation risks for crews in realistic beyond-low Earth orbit (BLEO; deep space) missions. A further important TT recommendation was for ESA to support ICRP (the International Commission on Radiological Protection) in future efforts to harmonize international models and dose limits in BLEO in line with RPI and the International Systems Maturation Team – Radiation (ISMT-RAD).

ESA’s initiatives are partly carried out in collaboration with the ISMT-RAD, which is a coordinated activity of ESA with other partner national space agencies (NASA, RSA, JAXA, CSA) to provide radiation protection for their crewmembers. In December 2017, the ISMT-RAD convened at the European Astronaut Centre in Cologne (Germany), to discuss the possibility of adopting an international consensus framework for human risk modeling and limits on astronaut exposures to ionizing radiation for exploration-class space missions. Examples of such missions include a cis-lunar (e.g., Deep Space Gateway), long-duration presence in a free space, habitat located outside the influence of the Earth’s geomagnetic field, or a mission to a planetary surface such as Mars.

Conclusion

The Moon is the most important and nearest celestial body for humankind and naturally, it is also the first step in expanding human activity to space. However, the radiation environment at the lunar surface is hazardous as this natural satellite has effectively neither an atmosphere nor a magnetic field. For the future presence of humans on the Moon, it is necessary in advance to investigate the environmental radiation of the lunar surface which is quite different from that of the Earth’s surface.

The largest contribution to dangerous radiation levels for crews and/or habitants is given by Galactic Cosmic Rays (GCRs), Solar Energetic Particles (SEPs), and their secondary particles, mainly neutrons. Missions requiring long-duration exposure to the deep space radiation environment, in particular to Galactic Cosmic Rays, must have properly designed shielding to protect electronics and humans from adverse effects.

The radiation data are also relevant for future interplanetary missions. Since the Moon has neither a protective magnetic field nor an atmosphere, the radiation field on the Moon’s surface is similar to that in interplanetary space. It is essential to refine our models and thus contribute towards radiation protection for astronauts on future missions.