Future Human Exploration Challenges: An Overview
KeywordsHuman space flight Biomedical Countermeasures Resilience Complexity
Human Space Exploration beyond low Earth orbit could include a return to the moon, exploration of Mars, and stay in cis-lunar space and at Lagrangian points, as well as exploration of asteroids.
Humans have been flying into space and back for well over 50 years. Crews are now routinely sent to the International Space Station (ISS) for durations of approximately 6 months, and in one case (as of 2019), two people have lived and worked on board for almost a year (340 days). A substantial body of work now exists on the human health and performance effects of these flights in low Earth orbit (LEO). Although challenges remain, the major risks to humans from flights of this duration and distance have likely been identified, and countermeasures and technologies are in place or under study to minimize these risks. A significantly smaller number of people have flown to the moon, and only 12 astronauts have walked on the lunar surface. Although those flights involved living and working at significantly greater distances from Earth, they were relatively brief. Crews returned safe and generally healthy (except for some pulmonary disturbances), even though a period of postflight recovery was necessary as with all space flights of more than a few days in duration.
The other articles in this section outline the risks associated with human space travel to certain specific and challenging destinations. Some of the risks to human health and performance are common to all of these destinations, but are more acute in some cases due to duration of the journey and exposure to specific hazards. The major biomedical issues common to all destinations and mission types considered here include space radiation exposure, life support including breathing air contamination, pressure-suit performance and decompression sickness, physiological and cognitive effects and their monitoring, crew autonomy, and medical concerns. This is a subset of the larger set of issues that NASA has currently identified as the primary risks to humans during long-duration space flights, which additionally include behavioral and inter-personal issues, medication toxicity and effectiveness, protection of crew on return to Earth or following the landing on a planetary body with substantial mass which includes possible injury from landing loads and orthostatic intolerance, exposure to celestial or planetary dust, kidney stones, vision changes, sleep disturbances, food viability, and adequate nutrition. Other features that are common to several destinations but are exacerbated by mission length include microbiome and immune-system alterations, habitability issues, decreases in bone density, changes in cardiac and sensorimotor function, and decreases in muscle and aerobic capacity.
A Mars mission encompasses all of these risks. The other missions can serve as exploration destinations in their own right but also as proving grounds for more ambitious missions – the mission to Mars being the paradigmatic example. NASA, with the support of its international partners, is working on many of these concerns already, with research occurring on ISS as well as in ground-based analog facilities and laboratories. NASA’s Human Research Program (HRP) is organized around mitigating these major risks in order to enable exploration-class space flights.
High-altitude balloon flights have not only contributed significant important biomedical information that is relevant to space flight but they continue to be of use as a scientific destination for atmospheric and Earth exploration and as a test bed for many space-relevant concerns. The relatively easy access to space-like conditions – compared to space flight itself – permits repeated testing at significantly reduced cost, allowing for incremental refinements in equipment and procedures. See “Overview of Balloon Flights and Their Biomedical Impact on Human Spaceflight,” this volume.
These flights are particularly useful for addressing issues related to life support, given the challenging environment which includes rapid changes in atmospheric composition during ascent and descent, and the need for adequate suit mobility to conduct operational tasks. Contingency operations related to emergency egress from a spacecraft are also readily amenable to study with balloon flights.
A spacecraft can be placed at a Lagrange point and remain there, as in LEO, with little or no propulsion expenditure. As these points in the Earth-moon system are in deep space (cis-lunar or trans-lunar), they provide an opportunity to investigate many of the biomedical issues that would arise in a lunar mission (similar distance) but without the relative safety of the lunar surface for resources or radiation protection. The locations of Lagrange points also permit relatively rapid return to Earth.
A major concern for such a mission is radiation, since the spacecraft and crew will be well beyond the outermost limits of the Earth’s magnetosphere. Thus a major aspect of the research conducted in this mission would involve characterizing the effects of galactic cosmic rays and solar particle events on the human body, information that will be critical in planning missions to Mars. Other concerns involve semi-autonomous crew operations and long-term effects of microgravity (0 g). The first mission to Mars will require advancements in radiation protection, medical support, space suits, and propulsion, all of which can be tested in proving-ground missions to Lagrange points.
Missions to near-Earth objects (NEOs) incorporate some of the key hazards that will exist in almost any exploration-class space flight: lack of a rapid return capability, the need for crew autonomy, and communication delays of up to several minutes. In addition, the crew will likely be small (perhaps four), and the duration will approach that of a presumed Mars mission (180–450 days). These missions will have so many operational concerns, including extended EVAs with novel tasks in new settings, that a large amount of dedicated biomedical research may be unlikely.
Key biomedical challenges encountered during missions to NEOs will be those related to deep-space radiation (including shielding and countermeasures), EVA life support and suit mobility, and behavioral and psychological issues due to remoteness and confinement. Of special note will be a range of issues related to crew autonomy, including the ability to deal with medical emergencies, and human-robotic interaction issues such as trust in automation and proper design of semiautonomous operations.
The moon is a particularly interesting and relevant destination to pursue space biomedical research because extended lunar surface stays can provide substantial opportunities for experimentation, research, and the development of operations and technology that would be appropriate for more ambitious missions. Thus a lunar mission can serve as a test bed and laboratory for the future, in addition to the moon being a destination of intrinsic value. Extended stays on the moon would mimic some of the conditions of a Mars mission, which could make it a useful proving ground. This is in distinction to some of the other destinations considered here (balloons, NEOs), where the operational issues are likely to take the majority, if not all, of available mission resources.
Thus some of the issues germane to a lunar flight are the same as those inherent to a Mars journey, although they are much less acute due to the relatively close proximity to Earth and hence the relative ease of return. Key concerns for a moon mission, assuming an extended stay on the surface, include: long-term effects of reduced gravity, exposure to dust from the surface soil, life support including food and other consumables, and deep-space radiation. There also exists, as noted, the potential for significant biomedical research which might include animal experiments. One point worth noting is that any beneficial effects of lunar gravity on the physiological deconditioning that takes place in microgravity (bone, muscle, cardiovascular, fluid shifts) could readily be assessed. This aspect is important since we do not know if such exposure to lunar or planetary hypo-g (<1 g) will constitute a sufficient countermeasure to these deconditioning effects or if specific exercises and other countermeasures will still be needed (as they are now on ISS). This is of great relevance to an eventual Mars mission. The lunar gravity (1/6 g) lies approximately half way between zero-g, on orbit, and Martian gravity (3/8 g). Finally, extended stays on the moon might reveal new space biomedical issues, given that astronauts could operate for long periods of time in the novel environment, which would also be especially helpful for planning of Mars missions.
Lunar missions would include partial autonomy, in that distance precludes a rapid return to Earth in an emergency. Still, communication delays are negligible which would permit close coordination and assistance from ground control personnel. Of course, more general testing and development of operations and hardware can also be carried out on the moon, encompassing issues of automation and autonomy.
A mission to Mars is, arguably, the ultimate exploration-class space flight, at least for the foreseeable future. The duration (close to 3 years) and distance from Earth place such a mission in a category all its own. The range of biomedical concerns and hazards encompasses all that has been discussed above and more.
Many of the specific characteristics of a Mars mission contribute to the extreme biomedical hazards. Whether the exploration is to the surface of Mars or to one of its moons, Deimos or Phobos, the challenges will be similar. Trajectory dynamics and propulsion limitations preclude an early return to Earth: once the mission proceeds on its interplanery trajectory, it will go to completion regardless of possible crew health and medical issues. Resupply of consumables will not be possible – food and medications will need to be prepositioned or carried along with the crew. Normal operations and maintenance will consume a substantial part of the crew’s time. Communication with Earth will occur at a low rate with long delays, up to 20 min each way. Therefore, crews will have significant autonomy in dealing not only with nominal operations but also with contingencies such as medical problems. Added to these will be the environmental hazards of decreased gravity while on the surface, isolation and confinement, high radiation levels, and altered light-dark cycles. While many, if not most, of these are concerns for all space flights, they are especially acute for a Mars mission.
As a consequence of the hazards of a Mars mission, there is a wide range of human health and performance concerns, covering almost every physiological system in the body. These are reflected in the set of HRP risks outlined above and comprehensively detailed in NASA’s Human Research Roadmap (https://humanresearchroadmap.nasa.gov/). As is the case for lunar missions, exercise may be a lesser concern on Mars, at least during surface operations when partial g loading is present. The need for dedicated exercise besides what is provided by normal operations has not been established. Special concerns include those areas of human health and performance that could be exacerbated by a small number of people living together in close confines for a long period of time, such as alterations in the gut microbiome, behavioral and psychological issues including interpersonal interactions, and issues of habitability and personal space. Other critical areas arise due to the complexity of the mission, the range of tasks to be performed (expected and unanticipated), and the long period between ground training and in-mission task performance. Some of these include proper training procedures, automation and human-robotic interaction, human-computer interfaces, and remote operations.
No matter which deep-space destination is explored, it is almost certain that robotic systems will play a role. Thus human-robotic integration is an essential enabling technology for human space exploration missions. Given the great technology improvements in this area, and the prospects for continuing advances, robotic systems will be able to take on significant responsibilities. To date, the major space uses of robotic technology have been in fully-robotic probes to the moon and other planets, and in robotic manipulator arms on the US Space Shuttle and ISS. Human-computer interaction is evident in the non-real-time planning and replanning of robotic exploration conducted by the Mars rovers, including Curiosity. In the future, we can expect robots and humans to work more closely together, with each taking on those duties to which it is best suited: robots for initial scouting and especially risky operations, and humans for further exploration where powers of reasoning and observation are critical.
Broader Concerns, Summary, and Conclusion
Although each of the biomedical concerns noted above is significant in its own right and presents serious challenges during exploration missions, the larger challenge is in understanding and addressing integration across conventional disciplines. These discipline areas encompass the different physiological subsystems (White and Averner 2001); the larger system that includes the vehicle, mission design and goals, ground crew, supplies, and training; the destination environment; and many others. Part of this integrative approach is the domain of human-system integration (HSI), which aims to provide solutions that are useful and effective, and that make efficient use of limited resources.
There are obvious areas in which integration across traditional disciplines is necessary. Radiation, for example, has long-term effects on the body that increase the risk of cancer and possibly also cardiovascular disease, but also acute effects on central nervous system (CNS) function and short-term effects on inflammation. Poor task design and training can lead to increased workload and stress, which can have detrimental effects on interpersonal interactions and teamwork. Many factors can lead to impaired or inadequate sleep, which in turn can affect health and performance in many ways. There are many such interactions, which are not yet treated in a systematic manner. All of these become more serious with increased mission duration and distance from Earth, and come to a head with a Mars mission.
These interactions might be considered as a complex network with many interacting parts, and with emergent behaviors and possible failure modes that are not obvious from the properties of the individual components (Barabási 2007; Mindock and Klaus 2014; Newman 2003). These various parts include not only the flight and ground crews, but also vehicle and mission requirements, the requirement for the appropriate amount of autonomy depending on mission phase, and the changing roles of individual crew members during different mission phases. Balancing these factors over the course of the changing circumstances of a long-duration mission entails a broad understanding of these interactions and how to track them.
In light of the many hazards, concerns, risks, and challenges to be faced on future exploration missions, it is natural to ask what are the most critical or important. The issue that is most likely to have the greatest impact is the one that may not have been thought of yet. These missions are exceedingly challenging, and the settings and tasks to be performed will be so novel that it is not possible to identify and mitigate every risk ahead of time. Therefore, it is necessary to create systems – including the crew members themselves – that have a high degree of resilience and flexibility. Crews on these missions must be provided with the physical and mental tools to deal with the unexpected. Such resilience is another aspect of a properly designed complex system, and ensuring it will require significant effort that entails new ways of thinking (Shelhamer 2016).
A ground-based facility that mimics some aspects of space flight (e.g., isolation, confinement, physiological deconditioning) and allows for scientific investigations to occur in a controlled and less-expensive setting
The three dimensional region of space between the Earth and its moon
A system made up of many smaller interacting subsystems, leading to emergent behaviors that cannot be predicted based on the properties of the individual subsystems; the interactions provide the dominant dynamics
Extra-vehicular activity – operations conducted in space by a suited astronaut outside of the spacecraft, including those conducted on the surface of another celestial body
Human space flights beyond low Earth orbit, with the intent of exploring other celestial bodies or locations
Human-systems interaction, an engineering approach in which the combined properties of humans and the (mechanical, electrical, computing) systems that they interact with are specifically recognized and addressed
International Space Station
A point in space where the gravitational attractions of two large bodies provide the centripetal force for a third smaller object, allowing it to orbit with them without expending significant energy.
Low Earth orbit – an orbit of Earth with an altitude between 160 and 2000 km
Near-Earth object – a comet, asteroid, or meteor that is “near” Earth or its orbit (within 45 million km)
The three-dimensional region of space beyond Earth’s moon and its orbit