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Human Health in Space

Jennifer Fogarty

Chief Scientist, Translational Research Institute for Space Health, Baylor College of Medicine Director, Human Health and Performance, Sophic Synergistics, LLC, Houston, TX, USA

1.1 Introduction

The human is capable of surviving in ordinary and extreme environments on every continent on Earth. This robustness indicates that humans adapt to various diverse environments and use resources to endure conditions that challenge our survival. The most extreme and austere environments that humans have inhabited are low-Earth orbit, lunar orbit, and the lunar surface otherwise known as spaceflight. Being prepared to enable survival and performance during spaceflight requires an understanding and assessment of the natural course of human health as we age and are exposed to environmental and biological stressors, specifically spaceflight stressors such as microgravity, isolation and confinement, closed loop environmental systems, and space radiation.

Human health and performance form a continuum and data used to quantify and describe each requires context to be adequately interpreted for risk assessment to provide a rational foundation for space exploration medical and countermeasure systems [1-4]. This concept is especially important in the extreme environment of spaceflight where small or unanticipated changes may have critical impacts to the individual and the mission. Loss of crew (LOC) and loss of mission (LOM) are common endpoints that are used to assess the severity of acute and off-nominal events. All mission planning and space vehicle development from the time of conception are designed to avoid or mitigate LOC or LOM events and parallels the occupational health model (engineer out the exposure, provide personal protective equipment for unavoidable exposures, and perform surveillance for untoward effects for early intervention) used to manage the human system risk. This is challenging given the independent and intersecting components of the spaceflight environment, vehicle design, and missions requirements. This challenge is compounded during human spaceflight due to the human changing over time in response to the natural course of their biology (aging?) and exposure to the spaceflight environment that includes novel stressors such as microgravity, extreme isolation and confinement, and space radiation (chronic galactic cosmic rays and acute solar particle events).

Establishing a comprehensive understanding and record of an individual's baseline health is extremely important to risk assessment and risk acceptance processes of human spaceflight. Each human spaceflight participant represents a unique combination of genetics and environmental exposures including lifestyle factors (diet, smoking, alcohol consumption, exercise training, etc.) that contribute to the risk of disease and impaired function that might affect the mission, LOC, or LOM. However, requirements that drive decisions about vehicle design, inclusion of large countermeasure equipment (treadmill, cycle ergometer, resistive exercise, lower body negative pressure (LBNP), etc.), and mission objectives are determined long before specific participants are identified. This temporal chasm between knowing an individual's risk and needs versus locking-in a vehicle design and capability has been addressed by using an epidemiological approach to human spaceflight requirements. This approach has been refined over time and adjusted for healthier populations based on updates to terrestrial medical and public health standards and practices. It allows the development of requirements based on a generic representation of humans as an average and standard deviation with occasional allowance for accommodations of ranges such as that associated with anthropometrics, and a risk management approach that uses current evidence for both the likelihood and consequence of a medical event. In addition, historical human spaceflight and dedicated research data are used to assess the effect of exposure to spaceflight specific stressors on the risk of a medical event or a performance decrement. The implication of this process is that participants that do not fall into an acceptable risk posture based on the requirements that were used to generate design and capability of the vehicle may not be permitted to fly on a mission. Since we are entering the era of commercial spaceflight and opening access to spaceflight to a broader population, this undesirable and restrictive potential outcome can be mitigated by actions on the human, the vehicle, and the mission. The human risk can be affected by aggressive risk factor and disease management that may shift the risk benefit assessment in favor of prophylactic surgery or use of additional pharmaceuticals, but each action must have a thorough assessment to understand the inadvertent introduction of risk. The vehicle and mission can mitigate risk by accommodating specific medical and countermeasure hardware, custom protocols for existing hardware, providing personalized pharmaceuticals in custom kits, and adjusting mission duration so that pre-flight screening and treatment can effectively mitigate the medical risk during the mission. This approach is human system integration (HSI) and requires significant communication, evaluation of data, and integration by many specialists from the medical, engineering, and risk management communities.

1.2 Preflight Determination of Health and Performance Risk

The approach to assess health and medical risk is rooted in preventive medicine's clinical standard of care and focuses on gathering multi-system quality evidence, characterization of health risk factors and presence of disease, comparison to relevant populations (terrestrial and previous spaceflight) to assess probability of medical event, and quantification of the health and medical risk for the mission. Assessing performance begins with understanding the health of the individual and collecting fitness metrics, aerobic capacity, power output, endurance, etc. that can be used to characterize and benchmark functional performance and provide a comparison for in-mission assessment to detect decrements and develop an action plan if an unacceptable threshold is reached. Actual evaluation of mission-relevant performance that includes physical capabilities and executive function is done by the flight operations community during mission training and includes intra-vehicular activity (IVA; telerobotic, vehicle maintenance and repair, emergency procedures, medical officer functions, etc.) and extra-vehicular activity (EVA; microgravity and planetary) objectives to establish fitness for duty and a baseline for comparison during the mission. Establishing the pre-flight health and performance baseline provides evidence to calculate risk and also the ability to estimate the impact of changes over the course of the mission. The duration of a mission plays crucial role in estimating risk since many medical conditions have a limited window of prediction based on current screening techniques. This principle is illustrated by the terrestrial medical guidance for annual exams and regular screening tests such as blood work to assess high-density lipids (HDL) and low-density lipids (LDL), prostate specific antigen, fasting blood glucose, mammography, colonoscopy, and pap smears. Screening in-line with these recommendations allows for early and pre-symptomatic disease identification and treatment often remediating the pathological process and reducing the risk of a medical outcome to an acceptable baseline level. Most human spaceflight missions have not exceeded the recommended terrestrial screening durations and coupled with intensive preventive medicine the spaceflight community has been confident that most critical medical events of a natural origin are very unlikely to occur during the mission. [1, 2].

Exploration class missions require mission durations, three years or more, that are outside the prediction window of current medical screening and will require additional screening tools such as precision medicine [31] or more robust in-mission diagnostic and interventional capability to manage medical risk [32]. Missions that do not require extreme durations but have high tempo EVAs, such as lunar exploration, represent additional medical risk due to injuries associated with repetitive activities in the suit, use of heavy equipment, and lunar dust exposure. Therefore, pre-flight the overall health and performance risk for a mission is a complex matrix of medical event probability for each crewmember based on pre-existing risk factors and health risks induced by the operational environment. The element of human spaceflight that has been very challenging to include in the quantitative health and performance risk assessment is how the hazards of spaceflight (altered gravity, closed environment, isolation and confinement, and space radiation) influence how the human changes overtime in response to those unique exposures and how those changes alter the probability of a health and performance risk ultimately influencing LOC and LOM. [3, 4].

1.3 Physiologic Responses to Spaceflight

Human response to these hazards ranges from adaptive to maladaptive and has been documented through evaluation of data from medical operations and dedicated research performed during spaceflight and terrestrial analog missions. The interpretation...