Table 1.

Mission attributes that drive risk to the human system and examples of how the risk drivers affect risk from specific mission parameter.

Time: Exposure to spaceflight hazards increases with mission duration14
Gravity Environment	Exposure to a gravity environment that is less than Earth-normal begins a process of adaptation; some of these adaptations create issues because human bodies have evolved to function in a 1 G environment. Increasing duration of exposure leads to increasing deconditioning.
Radiation Environment	Risk from exposure to space radiation is both duration-dependent and intensity dependent and may have in-mission or long-term health impacts.
Isolation and Confinement	As the period of isolation during a space mission increases, the risk of psychological, physical, and mental health issues increases.
Hostile Closed Environment	Perturbations in vehicle or spacesuit conditions (air quality, temperature, accelerations, movement restriction, etc.) can result in illness, injury, or inability to perform critical tasks, and risk increases over time of exposure. Examples include launch and landing loads, CO2 levels in the spacesuit and vehicle, and amount of time spent in a hot or cold environment due to insufficient capability of the environmental control and life support system.
Distance from Earth: Distance from Earth affects the energy and cost associated with mass delivery as well as communications and logistical factors
Communications Delay	As distance from Earth increases, communication lags will delay ground support to crews and operations will shift from real-time support to greater crew autonomy, implementation of intelligent support software, and store-and-forward communications22,23.
Time to Definitive Care (Evacuation Time)	As distance from Earth increases, the time required to deliver medical care increases. Medical evacuation timeframes must be considered as drivers of health risk for crews. In particular, for Mars DRMs, medical evacuation will not be possible and this shifts the risk posture for crews3,4.
Consumables Resupply	As distance from Earth increases, design and operational system trades are likely to target the mass and volume needed for food, pharmaceuticals, medical equipment, and consumables. If resupply is possible, the risk of interruption of the supply chain becomes greater with greater distance from Earth. For Mars DRMs, no resupply options will be available and pre-supply options have severe disadvantages due to shelf life5,7.
Vehicle Resource Constraints: The limitations on mass, power, and volume will be determined by the mission goals and attributes. Different mission types will carry different risk postures based on the total available mass, power, volume, and data bandwidth that can be traded among vehicle systems.
Vehicle Habitable Volume and Capability	The levels of risk from hazards such as isolation and confinement and closed/toxic environments is heavily dependent on net habitable volume, which is different from total resource volume. Limited habitable volume may result in the restriction or exclusion of private crew quarters and amenities that can help offset behavioral and interpersonal issues. Decrements in individual and team performance are expected as capabilities and countermeasures are sacrificed.
Crew Selection and Assignment	Medical and behavioral profiles of crewmembers must be understood, formalized into standards, and accommodated in mission planning. If crews are composed of a mix of government and private crews, this may result in more medical and behavioral risk because it is unclear if commercial and private astronauts will undergo the same selection procedures (both medical and psychological screening) and the same teaming evaluations as NASA astronauts and other government-sponsored astronauts24–27.
High Risk Activities: Certain missions will require tasks and activities that pose greater risk to both crew and mission
Extravehicular Activities (EVAs)	Increasing the number of EVAs increases the likelihood of decompression sickness and suit- or activity—related injuries28–31. If crews cannot shelter effectively during a solar particle event during an EVA, this may affect the likelihood of acute radiation sickness32,33.
Beyond Low Earth Orbit	Travel outside Earth’s magnetic sphere increases the radiation exposure of crews beyond that which has been experienced in low Earth orbit33,34. In the case of EVA activity this can result in increased damage from solar particle events and may limit mission activity.
Orbital Mechanics: Orbital mechanics may extend the time required to return a sick or injured crewmember to the Earth for definitive care, so NASA may be forced to prioritize between mission objectives and loss of crew life/permanent disability to astronauts for some types of missions. A realistic assessment of the probability of a medical condition occurring, the complexity of resources needed to treat these conditions, and the potential futility of treatment for some severe medical conditions should drive prioritization that ensures a reasonable match between medical need and medical capability35.