Deep Space Missions Research Articles

Bringing Interventional Radiology to Mars!

At present, astronauts on space missions can get medical assistant from Earth. In the future, deep space missions such as missions to Mars will delay communication with physicians on Earth, making it impossible to get immediate support in urgent medical situations. On the spaceship, a polyvalent physician-astronaut could mainly perform small surgery and traumatology procedures. Interventional Radiology (IR) allows minimally invasive interventions and requires small devices. In these conditions of space constrains, IR presents significant benefits. To guarantee the technical realization of specific medical interventions during deep space missions, a team composed of interventional radiologists and space engineers, is developing the IR toolbox. The development of the toolbox intents to minimize the volume/weight of medical devices and to ensure the safety requirements for the crew. New scenarios of IR interventions have been developed to adapt the interventions to the spatial context, making possible the treatment of pathologies that are otherwise, on Earth, optimally treated surgically. Interventional radiology has a major role to play in the management of acute medical problems which may occur in the future story of deep space missions to the Moon, and further to Mars.

Homo sapiens—A Species Not Designed for Space Flight: Health Risks in Low Earth Orbit and Beyond, Including Potential Risks When Traveling beyond the Geomagnetic Field of Earth

Homo sapiens and their predecessors evolved in the context of the boundary conditions of Earth, including a 1 g gravity and a geomagnetic field (GMF). These variables, plus others, led to complex organisms that evolved under a defined set of conditions and define how humans will respond to space flight, a circumstance that could not have been anticipated by evolution. Over the past ~60 years, space flight and living in low Earth orbit (LEO) have revealed that astronauts are impacted to varying degrees by such new environments. In addition, it has been noted that astronauts are quite heterogeneous in their response patterns, indicating that such variation is either silent if one remained on Earth, or the heterogeneity unknowingly contributes to disease development during aging or in response to insults. With the planned mission to deep space, humans will now be exposed to further risks from radiation when traveling beyond the influence of the GMF, as well as other potential risks that are associated with the actual loss of the GMF on the astronauts, their microbiomes, and growing food sources. Experimental studies with model systems have revealed that hypogravity conditions can influence a variety biological and physiological systems, and thus the loss of the GMF may have unanticipated consequences to astronauts' systems, such as those that are electrical in nature (i.e., the cardiovascular system and central neural systems). As astronauts have been shown to be heterogeneous in their responses to LEO, they may require personalized countermeasures, while others may not be good candidates for deep-space missions if effective countermeasures cannot be developed for long-duration missions. This review will discuss several of the physiological and neural systems that are affected and how the emerging variables may influence astronaut health and functioning.

The Effects of Galactic Cosmic Rays on the Central Nervous System: From Negative to Unexpectedly Positive Effects That Astronauts May Encounter.

Galactic cosmic rays (GCR) pose a serious threat to astronauts' health during deep space missions. The possible functional alterations of the central nervous system (CNS) under GCR exposure can be critical for mission success. Despite the obvious negative effects of ionizing radiation, a number of neutral or even positive effects of GCR irradiation on CNS functions were revealed in ground-based experiments with rodents and primates. This review is focused on the GCR exposure effects on emotional state and cognition, emphasizing positive effects and their potential mechanisms. We integrate these data with GCR effects on adult neurogenesis and pathological protein aggregation, forming a complete picture. We conclude that GCR exposure causes multidirectional effects on cognition, which may be associated with emotional state alterations. However, the irradiation in space-related doses either has no effect or has performance enhancing effects in solving high-level cognition tasks and tasks with a high level of motivation. We suppose the model of neurotransmission changes after irradiation, although the molecular mechanisms of this phenomenon are not fully understood.

Mass optimization of a recompression supercritical nitrous oxide and helium power system for space exploration

Due to high output power, long lifetime, high efficiency, and compact structure, the space nuclear power (SNP) system is considered an ideal choice for future large-scale and deep space missions. The system's mass, volume, and thermoelectric conversion efficiency directly affect and determine the system's performance compared with ground nuclear devices. The mass of the space nuclear power system is a vital parameter due to the limitation of cost, volume, and transportation. This study designed and optimized the recompression supercritical with nitrous oxide and helium (N2O–He) Brayton cycle with a small modular reactor for mass. Each Brayton cycle component model was developed to estimate the cycle performance systematically. The mass estimation model of the Brayton system includes a nuclear reactor, radiation shadow shield, Brayton cycle unit, recuperator, and radiator. A small modular reactor was designed to predict the minimum mass reactor suited to the given cycle conditions. The system mass optimization explored the trade-offs between the reactor, radiation shield, Brayton cycle unit, recuperator, and radiator to obtain the minimum mass of the space nuclear energy system to satisfy the operating requirements. Sensitivity parametric investigations are considered to evaluate the effects of crucial decision variables on the whole Brayton cycle mass. Furthermore, multi-objective optimization is performed to find optimum operating parameters to minimize the system mass. The results illustrate that the specific mass of the system is less than 20 kg/kW. The total mass of the whole system is 5605.93 kg using near-term materials, of which the Baryton Rotating Unit, radiator, and shadow shield mass dominate the total mass. The Brayton Rotating Unit, radiator, and radiation shield account for 44.64%, 32.53%, and 14.29%, respectively. It is concluded that after the dual-objective optimization analysis, the mass of the cycle decreased by 18.44%, which used the existing technical materials.

Assessment of the calendar aging of lithium-ion batteries for a long-term—Space missions

Energy availability is a critical challenge for space missions, especially for those missions designed to last many decades. Space satellites have depended on various combinations of radioisotope thermoelectric generators (RGTs), solar arrays, and batteries for power. For deep space missions lasting as long as 50 + years, batteries will also be needed for applications when there is no sunlight and RTGs cannot support peak power demand due to their insufficient specific power. This paper addresses the potential use of lithium-ion batteries for long-term space missions. Using data collected from the literature and internal experiments, a calendar aging model is developed to assess the capacity fade as a function of temperature, state-of-charge and time. The results for various LIB chemistries are used to identify the best candidate chemistries and determine the conditions, with a focus on low temperatures, that can best enable deep space missions.

Equilibrium points of heterogeneous small body in finite element method

ABSTRACT This paper presents a finite element method to search for equilibrium points around a heterogeneous small body, which degenerates from the full two-body problem in finite element form. The gravitational potential, acceleration, gravitational gradient matrix, and the linearized perturbation equation, which are key formulas for solving the equilibrium points and discriminating their stabilities, are interpolated by the tetrahedral model nodes of the small body. The finite element method could capture the complex internal structures of small bodies and provide a uniform and simple formula for various configurations. The method is applied to the Kuiper Belt Object (486958) Arrokoth, which is a contact binary asteroid. Three types of heterogeneous structures are hypothesized to investigate the evolutions of equilibrium points, namely density disparity of two sub-lobes, hardcore structure, and cavity structure. Under the condition of constant Arrokoth mass, the density disparity of the two sub-lobes greatly influences the local gravitational field. It thus has a significant influence on the positions of equilibrium points. The hardcore and cavity structures have less impact on the equilibrium points. These three simulation groups verified that the equilibrium points of a heterogeneous small body are quite different from those of a homogeneous small body. Such investigation of the equilibrium points may give an in-depth understanding of the dynamical environment around the heterogeneous small body, which is significant for future deep-space missions.

Refined MagSail thrust model for preliminary mission design and trajectory optimization

A magnetic sail is an advanced propellantless propulsion system concept for deep space missions, proposed by Dana Andrews and Robert Zubrin about 30 years ago. The thrust is generated by the electromagnetic interaction between the solar wind charged particles and an artificial magnetic field obtained through a large current-carrying coil made of superconducting material. This paper discusses an up-to-date single-loop magnetic sail thrust model based on the numerical and experimental results obtained in the last decade. In particular, a particle model is initially used to evaluate the performance of a reference configuration in terms of thrust components in a body reference frame. Then, some scaling relationships are used to estimate the magnetic sail performance as a function of the design parameters. The proposed thrust model is used to analyze the minimum-time transfer problem between two circular and coplanar heliocentric orbits. Minimum flight times necessary to complete ephemeris-free Earth-Venus and Earth-Mars transfers are calculated as a function of the magnetic sail characteristic size in a parametric way.

Designing a novel tomato ideotype for future cultivation in space manned missions

Introduction: Methods for production of fresh, health food are needed in view of long-term, deep-space manned missions. To this end, crops tailored for better performance under non-terrestrial conditions may be obtained by the exploitation of biochemical patterns related to specialized metabolites known to confer protection against environmental challenges and to be beneficial to human health.Methods: In this work, for the first time, MicroTom plants have been engineered specifically for agrospace applications to express PhAN4, a MYB-like transcription factor able to regulate the biosynthesis of anthocyanins that influence tomato genes possibly involved in agrospace-relevant functions. Results:PhAN4 engineering underpinned the genetic background of the dwarf tomato MicroTom while maintaining yield and photosynthetic capacity. PhAN4 expression resulted in the accumulation of anthocyanins and polyphenols, a differential carotenoid profile, increased antioxidant scavenging capacities of fruits compared to the original genotype. Improved ability to counteract ROS generation and to preserve plant protein folding after ex-vivo gamma irradiation was observed.Discussion: These results highlights that the manipulation of specific metabolic pathways is a promising approach to design novel candidate varieties for agrospace applications.

Further Advances for staging orbits of manned lunar exploration mission

With the increasing demand for manned lunar exploration and deep-space exploration missions, the application value of the three-body orbit in cislunar space becomes increasingly prominent due to its special orbital characteristic. The efficient round-trip versions for manned missions of both the nominal fuel-optimal and emergency transfers between NRHO and the Moon are exploited deeply. The collocation method and indirect optimization technique with constraint gradients are utilized in conjunction with the features of the NRHO configuration to increase the algorithm's accuracy and speed. The required initial value and suitable collocation point can be determined quickly based on a global search strategy and Lambert algorithm. To highlight the effectivity of the design scheme and aim at global lunar surface exploration, typical scenarios of lunar sites at different latitudes and longitudes are investigated with various time of flight, which show that the difference of fuel consumption varies from a few kilograms to hundreds of kilograms. Meanwhile, the relations among Moon landing sites, transition time and fuel consumption are analyzed and the advantages and disadvantages of NRHO in lunar exploration are compared. The design results have some reference value for the lunar landing vehicle's deployment in the near-Moon space, as well as the round-trip transfer mission and key parameter selection.

Physiological Challenges of Space Travel and Ground-Based Simulation Possibilities for Monitoring Brain Circulatory Changes: A Rheoencephalography Study

The functional integrity of brain perfusion and oxygen transport profoundly determines mental performance during military flight missions and spaceflight. Presently, at the selection phase of pilot candidates, there are no screening methods to evaluate cerebral circulation and its autonomous regulation (AR), meanwhile the pilot information processing capacity could be insufficient in dangerous flight situations with high mental workload or during high “head-to-foot” G loads. On-board ISS (International Space Station) and during deep-space missions circulatory changes can be evolved in the opposite direction due to the microgravity: blood shift toward the head-neck region can increase ICP (Intracranial Pressure) and tenfold increase of carbon-dioxide concentration can provoke complaints and disturbances in eye and brain blood circulation (Space Associated Neuro-Ocular Syndrome – SANS). The alteration of brain perfusion dynamics and oxygen utilisation was investigated on the head-down tilting table (HDT) test and in the hypobaric (low-pressure) chamber. We registered the brain regional pulse wave changes by the bioimpedance (Rheoencephalography – REG) on 19 volunteers in rest and after the breath-holding manoeuvre. We found that during the head-down tilt (HDT) position, the amplitude of the second peak of the REG pulse wave increased, like the ICP pulse wave, being an unfavourable sign for intracranial pressure increase in clinical cases. Manual readings resulted in significant differences during HDT between the female (P = 0.0007) and male (P < 0.0001) groups. With automated analysis, the increase in REG P2 wave was significant, and the ratio was 4/5 (80%) for women and 10/14 (71%) for men. The newly written automatic program script was able to detect this in 92% of the cases. The calculated values detected the state of cerebral circulatory autoregulation and the identity between the male and female groups. Based on this result and previous REG correlation studies, it can be concluded that REG could be used to monitor fighter pilots, astronauts, and neurocritical care patients in real-time as emergency alert in the transitory cessation of brain perfusion.

Application of shaken lattice interferometry based sensors to space navigation

High–sensitivity shaken lattice interferometry (SLI) based sensors have the potential to provide deep space missions with the ability to precisely measure non–gravitational perturbing forces. This work considers the simulation of the OSIRIS-REx mission navigation in the vicinity of Bennu with the addition of measurements from onboard SLI–based accelerometers. The simulation is performed in the Jet Propulsion Laboratory’s (JPL) Mission Analysis, Operations and Navigation Toolkit (MONTE) and incorporates OSIRIS-REx reconstructed trajectory and attitude data from the Navigation and Ancillary Information Facility (NAIF) database. The use of the reconstructed data from NAIF provides realistic true dynamical errors and JPL’s MONTE software allows for a high–fidelity simulation of an integrated trajectory for the filter. The navigation performance and reduction of tracking and complex modelling enabled by the onboard SLI–based sensor are presented for two orbital phases of the OSIRIS–REx mission. Overall, the results show that the addition of SLI–based accelerometer measurements improves navigation performance, when compared to a radiometric tracking only configuration. In addition, results demonstrate that highly–precise accelerometer measurements can effectively replace at least one day of DSN passes over a three–day period, thereby reducing tracking requirements. Furthermore, it is shown that lower–fidelity surface force modeling and parameter estimation is required when using onboard SLI–based accelerometers.

Redefining low Earth orbit as a parking orbit for flexible and economical Earth departure in deep space missions

There are a number of deep space probes that are currently in operation with diverse destinations and objectives. For example, the Japanese Hayabusa2 and the U.S. OSIRIS-REx missions are both sample returns, targeting different near-Earth asteroids. Europe’s ExoMars and the U.S. Perseverance are orbiting and roving Mars as precursors of future manned explorations. Conventionally, deep space missions require dedicated launch vehicles for each mission. The interplanetary Earth departure trajectory from the low Earth orbit (LEO) usually lacks flexibility and efficiency. Furthermore, innovative and reusable launch systems have been researched and developed by multiple organizations including private sector organizations such as SpaceX and Blue Origin. It is expected that the unit cost per launch weight to LEO be significantly reduced by rideshare mass transportation executed by using reusable mega launchers in the near future. This study aims to fill the transportation gap between LEO and deep space by realizing a flexible and economical interplanetary Earth departure without sacrificing the arbitrariness of LEO, target V-infinity vector, and target Earth departure epoch. Thus, the one-revolution Earth free-return orbit (1rEFRO) and the consequent Earth gravity assist (EGA) are introduced to separate the velocity increment and direction adjustment. The planetary free-return and EGAs are common in interplanetary missions; however, a comprehensive study on the flexibility, economic efficiency, and arbitrariness of the sequence (1rEFRO + EGA) originating from LEO was not explicitly found. After describing the necessary coordinate frames, LEO’s orbital elements, 1rEFRO, and the terms ‘flexibility’ and ‘economic efficiency’ are defined in Section 2. Then in Section 3, the two-body-based preliminary orbit design method is proposed and formulated. Section 4 aims to reveal LEO’s comprehensiveness as efficient parking orbits when adopting the 1rEFRO + EGA sequence, using the newly proposed “ΣVEt LEO i-Ω diagram”. Section 5 describes a detailed orbit design constructed based on multi-body propagation and optimization to confirm the feasibility, flexibility, and economics of the solution and the usefulness of the initial solution given by the preliminary design method formulated in Section 3.

Evaluating the effects of low-dose simulated galactic cosmic rays on murine hippocampal-dependent cognitive performance.

Space exploration has advanced substantially over recent decades and plans to increase the duration of deep space missions are in preparation. One of the primary health concerns is potential damage to the central nervous system (CNS), resulting in loss of cognitive abilities and function. The majority of ground-based research on space radiation-induced health risks has been conducted using single particle simulations, which do not effectively model real-world scenarios. Thus, to improve the safety of space missions, we must expand our understanding of the effects of simulated galactic cosmic rays (GCRs) on the CNS. To assess the effects of low-dose GCR, we subjected 6-month-old male BALB/c mice to 50 cGy 5-beam simplified GCR spectrum (1H, 28Si, 4He, 16O, and 56Fe) whole-body irradiation at the NASA Space Radiation Laboratory. Animals were tested for cognitive performance with Y-maze and Morris water maze tests 3 months after irradiation. Irradiated animals had impaired short-term memory and lacked spatial memory retention on day 5 of the probe trial. Glial cell analysis by flow cytometry showed no significant changes in oligodendrocytes, astrocytes, microglia or neural precursor cells (NPC's) between the sham group and GCR group. Bone marrow cytogenetic data showed a significant increase in the frequency of chromosomal aberrations after GCR exposure. Finally, tandem mass tag proteomics identified 3,639 proteins, 113 of which were differentially expressed when comparing sham versus GCR exposure (fold change > 1.5; p < 0.05). Our data suggest exposure to low-dose GCR induces cognitive deficits by impairing short-term memory and spatial memory retention.

Relative Dynamics and Modern Control Strategies for Rendezvous in Libration Point Orbits

Deep space missions are recently gaining increasing interest from space agencies and industry, their maximum exponent being the establishment of a permanent station in cis-lunar orbit within this decade. To that end, autonomous rendezvous and docking in multi-body dynamical environments have been defined as crucial technologies to expand and maintain human space activities beyond near Earth orbit. Based on analytical and numerical formulations of the relative dynamics in the Circular Restricted Three Body Problem (CR3BP), a family of optimal, linear and nonlinear, continuous and impulsive, guidance and control techniques are developed for the design of end-to-end rendezvous trajectories between co-orbiting spacecraft in this multi-body dynamical environment. To this end, several modern control techniques are effectively designed and adapted to this problem, with particular emphasis on the design of low cost rendezvous manoeuvres. Finally, the designed hybrid rendezvous strategies, combining both discrete and continuous control techniques, are effectively tested and validated under several start-to-end deep space testbench mission scenarios, where their performance is compared and quantitatively assessed with a set of performance indices.

A Study of Autonomous Small Satellite Constellations for Disaster Management and Deep Space Strategy

The complex and dynamic space environment is both exciting and challenging in this NewSpace era. In particular, Low-Earth Orbits are being realigned and reinvented for various purposes using suitable technological advancement. This paper is focused on the major parameters that can be analyzed to attain orbit control and autonomy of small-satellite constellations for real-time applications. By applying industry experience to graduate research, this work addresses the related concerns in a realistic manner. Currently, global small-satellite constellation solutions are too expensive and inaccessible for many nations to help in their data reception requirements. This issue was addressed, and some of the main aspects relating to low-cost and high-benefit technical synthesis, in addition to utilization for deep-space missions, were also discussed in detail. In conclusion, this paper demonstrated a strategic approach to optimize the coverage and performance, and reduce the cost of small-satellite constellations, compared with present day constellations, allowing the data to be relayed faster and with precision. This will benefit the industry in the development of low-cost constellations, and effectively assist in disaster management and deep-space communication relays. Autonomous orbit selection and navigation can be established for better path alignment for satellites to efficiently propagate and deliver the required data.

State Estimation in Spacecraft Power Distribution Systems Using Compound Current Sensors

The autonomous operation of the electric power system of a spacecraft is a key enabling functionality for deep-space missions. This functionality is informed by the real-time physical status of its power system, which is determined by processing the inputs obtained from various voltage and current sensors distributed throughout the electric network. In particular, it is important to have an accurate estimate of the current in each branch of the power system to detect overload conditions. However, voltage and current measurements can have an error due to noise and sensor faults. In this work, the focus is on ensuring accurate estimation of the voltages and currents in the presence of such faults. To this end, a method is proposed to optimally design a network of compound current sensors that enhances the ability to reliably identify faulty or biased current sensors. The proposed sensor fault detection method is validated using simulation results.

In-situ measurement of triple junction solar cells under low intensity and low temperature (LILT) for deep space mission

1 MeV electron radiation has been conducted to study the radiation hardness of GaInP/GaAs/Ge triple junction (TJ) solar cells. The characterization has been carried out under the conditions of (1) low light intensity and room temperature (LIRT), (2) low light intensity and low temperature (LILT) and (3) high radiation low intensity and low temperature (HRLILT). In LIRT, Jsc is linear with the light intensity, and Voc is linear with the logarithmic change of the intensity. In LILT, the results show that Jsc gradually decreases with the temperature from 300 K to 15 K. Voc increases linearly with the temperature from 300 K to 100 K. While Voc increases sub-linearly with the decreasing temperature under 100 K. The temperature coefficient of Voc (αVoc) of the TJ solar cells in this work are similar under different light intensities. In HRLILT, the results show that the degradation degree of Jsc caused by radiation is related to the light intensity, and more pronounced degradation occurs at lower light intensity. Moreover, TJ solar cells before and after irradiation are with a conversion efficiency of 42.0% and 34.4%, respectively (@0.01sun, 15 K). Promisingly, neither “flat spot” (FS) effects nor shunts effects are observed in this experiment.

3D printing of conical centrifuge system for mineral particle separation

Establishing a lunar base and refueling station for future deep space missions depends on available resources, such as oxygen and raw metals, that will need to be extracted from extraterrestrial sources. Beneficiation represents the first step in the extraction process and therefore plays a critical role in the success of deep space missions. Current beneficiation processes require heavy prefabricated devices that cannot be manufactured in space, such as on the surface of the Moon, making them expensive to transport. In this work, a polymer conical centrifuge system was explicitly designed to be fabricated on the lunar surface through fused deposition modeling (FDM) three-dimensional (3D) printing, in which molten polymer filaments are deposited layer-by-layer to construct complex 3D structures. A mechanics model was developed to unveil the separation mechanism of this system. Polylactic acid cones were fabricated and tested to analyze the effects of cone geometries and surface roughness on separation efficiency. Additionally, cone geometries do not significantly affect surface roughness, but layer height can be used to control the surface roughness of the printed cones directly. Peak values of fine particle concentration, experimental recovery, and theoretical recovery were measured as 74.2%, 53.5%, and 45.6%, respectively, when using the conical centrifuge system during fine particle separation, validating the effectiveness of the proposed system and separation method.

Track Structure-Based Simulations on DNA Damage Induced by Diverse Isotopes.

Diverse isotopes such as 2H, 3He, 10Be, 11C and 14C occur in nuclear reactions in ion beam radiotherapy, in cosmic ray shielding, or are intentionally accelerated in dating techniques. However, only a few studies have specifically addressed the biological effects of diverse isotopes and were limited to energies of several MeV/u. A database of simulations with the PARTRAC biophysical tool is presented for H, He, Li, Be, B and C isotopes at energies from 0.5 GeV/u down to stopping. The doses deposited to a cell nucleus and also the yields per unit dose of single- and double-strand breaks and their clusters induced in cellular DNA are predicted to vary among diverse isotopes of the same element at energies &lt; 1 MeV/u, especially for isotopes of H and He. The results may affect the risk estimates for astronauts in deep space missions or the models of biological effectiveness of ion beams and indicate that radiation protection in 14C or 10Be dating techniques may be based on knowledge gathered with 12C or 9Be.

Late effects of heavy-ion space radiation on splenocyte subpopulations and NK cytotoxic function

Introduction: With current goals of increased space exploration and travel to Mars, there has been great interest in understanding the long-term effects of high atomic number, high energy (HZE) ion exposure on various organ systems and the immune system. Little is known about late effects on the immune system after HZE exposure. Therefore, our objective was to determine how natural killer (NK) cell populations were affected in geriatric mice that were exposed to HZE particles during middle-age, thereby representing elderly retired astronauts that undertook deep space missions.Methods: 10 month old male CBA/CaJ mice were whole-body irradiated: sham (control); 150-cGy gamma-rays (delivered in 1 fraction); 40-cGy 1-GeV/nu 28Si14+ ions (delivered in 3 fractions); 40-cGy 1-GeV/nu 16O8+ ions (1 fraction); and 40-cGy 1-GeV/nu 16O8+ ions (3 fractions). The mice were sacrificed 1–1.5 yr post-exposure, and the spleens harvested. Splenocyte effector (E) cells were harvested and added to 51Cr-labeled Yac-1 target (T) cells in E:T ratios of 12:1, 25:1, 50:1, and 100:1. NK cytotoxicity was measured with 51Cr release. In addition, 2 million splenocytes were aliquoted and stained with a seven-antibody cocktail, and flow cytometry was used to determine the percentage of NK, B lymphocytes, and T lymphocytes in the splenocyte population.Results: Mice exposed to either a single fraction of 150-cGy gamma rays or 40-cGy 16O8+ ions in 3 fractions were found to have significant decreases in NK cytotoxicity of approximately 30% and 25%, respectively. No significant differences were observed in NK cytotoxicity for 40-cGy 16O8+ ions delivered in 1 fraction, or 40-cGy 28Si14+ ions delivered in 3 fractions. No significant differences were observed in the percentage of spleen cells that were NK (%NK) amongst the groups.Conclusion: Fractionated HZE ion exposure has the potential to affect the innate arm of the immune system long after exposure, leading to decreases in NK cell function. Therefore, protective countermeasures may need to be considered to decrease the risk of reduced long-term immune function in elderly retired astronauts that undertook deep space missions.