1

Introduction

Imagine what might be accomplished if science could effectively grow plants for life support away from Earth, manufacture better materials from lunar or martian resources, and reduce risks of extended space exploration by humans or by automated machines. The ability to escape the confines of Earth has been achieved in only the most recent moments of life on this planet. This exciting capability makes it possible to ask both new and persistent questions about how the Earth environment has shaped the development and functioning of organisms throughout time, as well as how organisms will adapt and respond to novel space environments in the future. Despite the many advancements made on Earth with present understanding of physical laws, the vast majority of the universe remains unexplored by human beings and is unknown to humans firsthand. Exploration of that universe over greater distances and durations and by more humans requires scientific understanding of the impact of that space on fundamental biological and physical systems, systems that until now were confined to Earth. It is the role of the NASA Division of Biological and Physical Sciences (BPS Division) and its science community to conduct this research. That division manages and pursues biological and physical sciences (BPS) that enable the exploration of space, and also pursues unique opportunities to advance BPS generally by utilizing unique features of the space environment.

Many important scientific questions can be explored and answered only in the space environment, including questions that interrogate the nature of dark matter and dark energy, or that probe the behavior of engineered and living materials when critical interactions are no longer concealed or constrained by Earth’s gravitational force. Studying the nature of gravity itself through experiments on Earth’s surface is limited. In contrast, experiments performed in space present opportunities to examine a wide section of variable gravitational potentials. Additionally, experiments conducted in the space environment can leverage much larger distances to reveal fundamental science, which is often constrained by the diameter of Earth and by seismic activity. Extreme features like these and others, including different and significant radiation and chemical exposures, create conditions not attainable on Earth.

Thus, research conducted in space can significantly open the observation window, allowing scientists and engineers to measure fundamental biological and physical processes and mechanisms that can only be theorized today. In addition to providing inspiration and perspective (Figure 1-1), the capacity to expand scientific discovery to the Moon and on the way to Mars tangibly benefits life on Earth as well as exploration of space.

As an illustrative example, advancements in space science have the potential to address very down-to-Earth issues. Degenerative pathologies associated with aging range from dementias and frailty to cardiovascular disease and cancer. Currently, the annual cost of dealing with age-related medical issues owing to muscle wasting and bone loss alone is in the billions of dollars for the United States (Van Houtven et al. 2008). Muscle wasting and bone loss problems are encountered by biological organisms living in the space environment at a significantly accelerated timescale, making spaceflight an interesting model for aging. Engineered materials can also undergo accelerated aging in the space environment, although that same term is taken to mean physical or chemical changes in the material structure with attendant reduction in properties, on exposure to other features of the space environment such as ultraviolet (UV) radiation or wide cycles in temperature (ESA 2020). Therefore, studies conducted in space to better understand the underlying biological and physical processes will have profound implications and impacts on the health and well-being of societies on Earth.

Biological and physical sciences also benefit space exploration. For example, a sustained lunar presence is just one of NASA’s exploration objectives. The Artemis program (Figure 1-2) is a series of complex missions to provide a foundation for research and knowledge accumulation and to expand capabilities to extend human exploration to the Moon and beyond to Mars. Sustained human exploration beyond low Earth orbit (LEO) presents science objectives and challenges not yet approached through the era of the International Space Station (ISS). Human crews of varied age and background will grow older during transit and perhaps while living and working in lunar or martian stations. Other biological organisms including microbes and plants and animals on those missions will eventually reproduce over several generations, thus elevating the need for science to consider longer durations in space and advancing biophysical approaches to create or monitor nutrients. Likewise, precise clocks

and positioning systems and communications for space navigation all stem from physical science advances. Many of the underlying science studies underpinning those exploration necessities may have seemed esoteric before the use cases of the Global Positioning System (GPS) and the like became so ubiquitous as to seem ever present.

NASA is key to U.S. leadership of and participation in the diverse space research community, including access to the space environment for research discoveries and translation through many flight platforms. With the sunset of the U.S. space shuttle program, the ISS took over as the major science platform in LEO. (See Box 1-1.) The ISS directly serves NASA research as well as the research needs of international partners. To expand access, the NASA Authorization Act of 2005 created the ISS National Laboratory (ISSNL). The ISSNL enables access to space research and development (R&D) access to a broad range of commercial, academic, and government users. The ISSNL—chiefly as a manager, promoter, and broker of research onboard the ISS—has engaged with other U.S. and international government agencies, industrial sectors, and financial markets to fuel the future growth of space-based research. These public, private, and public–private partnerships have resulted in impactful new scientific discoveries, and use-inspired innovation in terrestrial applications (ISSNL 2018). For example, optical fibers are used to transmit data, and tiny defects formed during processing reduce the fibers’ performance to slow transmission speed and degrade information. In 2018, microgravity conditions at the ISS made it possible

to produce optical fibers from a special fluoride glass, avoiding the formation of microstructural defects that currently plague such processing on Earth.

Applied research such as this optical fiber processing example is often described as supporting a practical application, where the research outcome is tangible and includes a new material, component, or device. Fundamental research, in contrast, can be described as generating new knowledge of how the universe works. Fundamental biological and physical sciences in space identify the pieces necessary to eventually mitigate risks, reduce costs, and improve efficiencies of future developments. Over the past decade, fundamental biological science has made great strides in understanding how microbes and plant systems grow and replicate differently in microgravity environments, and such knowledge can advance long-term life support systems for space exploration. Basic physical science research on understanding fluid behavior in low gravity has been extended to applications of three-dimensional (3D) printing, metal alloy solidification, novel cooling systems, and maintenance of plants on spacecraft. Increasingly, the ISS has also housed a number of innovative laboratories for critical fundamental investigations. For example, in 2018, NASA launched the Cold Atom Laboratory (CAL) to enable macroscopic observation of quantum properties of atoms.

Science onboard the ISS has been dependent on international and commercial missions to send cargo to the ISS (upmass) and return cargo to Earth for any analysis (downmass). The ISS is collaboratively managed by a group of international partners and physically consists of two main sections: the Russian Orbital Segment (ROS), which is operated by Roscosmos (the Russian federal space agency), and the U.S. Orbital Segment (USOS), which is operated by NASA, the Canadian Space Agency (CSA), the European Space Agency (ESA), and the Japan Aerospace Exploration Agency (JAXA). In addition, ground facilities in the United States, France, Netherlands, Germany, Russia, Kazakhstan, and Japan support launch, operations, and payload services for the ISS (Joseph and Wood 2019). Visiting crew and cargo vehicles (governmental and commercial) originate from Kazakhstan (Russian Soyuz and Progress spacecraft), Japan, and the United States. As of October 2022, at least 263 individuals from 20 countries have visited the ISS, with the United States having the largest number of unique visitors, at 161.¹

___________________

¹ NASA, “Visitors to the Station by Country,” https://www.nasa.gov/feature/visitors-to-the-station-by-country, accessed October 2022.

Hardware and laboratories inside the ISS support life and experiments in a highly controlled and habitable built space environment, and facilities outside the station provide platforms for testing equipment and performance in the extreme conditions of space.

On December 31, 2021, the White House announced a decision to extend operations of the ISS through 2030, and then announced the National Low Earth Orbit Research and Development Strategy. The ISS international partners are working with their respective governments to extend agreements through 2030. This extension will allow continued science on the ISS while new CLDs come to fruition (Gatens 2022). However, because this extension does not span the targeted duration of this decadal survey through 2032, and BPS transitions will need to be planned and prioritized ahead of ISS retirement, such non-ISS capabilities are a major concern within the period of this survey. (See Box 1-1.)

BIOLOGICAL AND PHYSICAL SCIENCES IN SPACE

Investigating biological and physical systems and related phenomena under the extreme conditions associated with the space environment improves understanding of how such systems function, both in the space environment and in the more familiar conditions on Earth. Figure 1-3 illustrates the conceptually broad swath of scientific expertise within biological and physical sciences, collectively referred to as BPS, in space science research. From an Earth-focused research organization perspective such as a university or industry laboratory, each of these hexagons could represent a single department or division—defined by shared terminology, experimental practices, and research infrastructure. As Figure 1-3 moves from left to right, the scientific underpinnings traverse from the purely biological systems and empirical data-rich studies to the purely physical concepts described by Newtonian laws

or Maxwellian equations or quantum theories. On Earth, these research communities and their funding sponsors may not interact much or share a common framework for scientific inquiry, beyond the expectation of reproducible and ethically reported results. Increasingly, however, interdisciplinary research blurs these boundaries to achieve impressive impact, using physical concepts to predict biological cells’ motion or using complex biological fluids to illustrate a physical phase separation. Indeed, these scientific communities are coupled specifically in NASA’s strategic plan and BPS Division, as well as in the first decadal survey by this intellectually diverse community (NRC 2011).

Development of innovative technology and tools is essential for conducting experiments in extreme space environments across BPS disciplines. The constraints associated with limited available resources, crew time, and accommodation on a spacecraft or space station in turn demand considerable attention to automated and intelligent systems that can also provide the ability for remote monitoring and real-time control of experiments. There is also a need for integrated modules for easily interchanging experiments in existing systems. Resource limitations also motivate a well-coordinated ground-based research program for experiment development, and analysis of samples returned to Earth when possible. Resource limitations also drive a need for systems to be made in space to reduce the need for transporting equipment and samples, as well as to enable longer mission durations without resupply. Similarly, needs drive improvements in space analysis and monitoring capabilities, including the development of sensors, cameras, fluorescent detectors, and microscopy, as well as the ability to control experiments from Earth and improving computational capabilities through enhanced data storage and transmission.

It is essential that the technology and tools perform as expected or needed in the space environment and that space laboratory equipment interact well with human scientists where appropriate. Simply put, technologies need to generate reliable and reproducible data and be manipulable for human interactions. With increasing science in space, and increasing numbers of astronauts in space, further iterative technology designs are likely to be needed. (See further discussion of particular technologies developed for BPS experiments over the past decade in Chapter 3.)

SPACE, SPACEFLIGHT, AND PLANETARY ENVIRONMENTS

The ISS has been a premier destination for conducting BPS research for more than 20 years, perhaps most notably examining the effects of gravity on organisms, combustion, fluids, and materials. However, the influence of microgravity is only one of many extraordinary features of the extreme environment onboard spaceflight vehicles, including the ISS. (See Box 1-2.) In addition to being its own system of closed life support and attendant vehicular mechanisms, the ISS travels at a speed of approximately 17,000 mph in LEO at about 250 miles above the surface of Earth. The ISS experiences 16 light/dark cycles every 24 hours, as well as a host of chronic and combined stressors, including space radiation exposure,² confinement and isolation, and a closed environment (Afshinnekoo et al. 2020). Although the ISS and its inhabitants are partially protected by Earth’s magnetosphere, they are still exposed to an average dose of approximately 100–200 milliSievert (mSv) per year (Cucinotta and Durante 2006), a dose range 2–4 times higher than the annual limit for radiation workers on Earth (50 mSv/yr). Once completely outside the protection of Earth’s magnetosphere on exploration missions to the Moon and Mars, radiation doses will be higher still. For example, astronauts on future missions of about 3 years to and from Mars will experience effective doses approaching and even exceeding 1,000 mSv (1.0 Sv) (Zeitlin et al. 2013).

Although the ISS has been the premier platform for significant advancement and success of space science, it has several unique and unavoidable constraints for research execution and progress, including limited crew time and constraints and logistics complexities that limit science delivery to the ISS and sample return from the ISS. Ground-based experiments and non-ISS flight platforms remain appropriate and necessary for many questions and preliminary testing prior to spaceflight. Planned vehicle developments will also provide new

___________________

² The space radiation environment includes high-energy protons produced from sporadic solar particle events (SPEs) and galactic cosmic rays (GCRs), which consist of electrons and positrons (2 percent), protons (85 percent), helium nuclei (12 percent), and heavier, more densely ionizing and damaging particles known as high atomic number and energy (HZE)-charged particles (1 percent) (Durante and Cucinotta 2008).

opportunities and platforms for conducting BPS research. (See Figure 1-4.) BPS science will therefore engage commercial and government ground-based facilities, including commercial sub-orbital vehicles, CLDs, the lunar orbit (Gateway), the lunar surface (Commercial Lunar Payload Services, Human Landing System), and Mars transit vehicles.

OVERVIEW OF RELEVANT DISCIPLINES

NASA’s Division of Biological and Physical Sciences (BPS Division) engages a broad range of research expertise and practices. This disciplinary breadth fosters interrelationships on a mechanistic level that inform scientific needs. In particular, there is strong overlap between many physical and biological phenomena, especially in the area of interfaces, fluids, phase separation, self-assembly, and colloids.

BPS disciplines support and interact with other major biological programs at NASA, including the Human Research Program (HRP). The relationships between HRP and BPS communities is discussed throughout this report: BPS provides fundamental science that impacts humans as well as other life-forms and natural or engineered systems, and HRP deals directly with astronaut health and safety, including so-called human factors research.

Biological Sciences in Space and Planetary Environments

Science has made great strides in understanding how life evolves and persists on Earth while under the constant influence of, selective pressure resulting from, and protection of its physical environment. Much research in space-related biological sciences pursues identifying the health risks and possible mechanisms for reducing such risks of the space environment on biological systems of animals, plants, and microbes. It also examines the hazards and benefits of interactions within and between organisms in the closed environments of space vehicles. An important objective is to understand, at a fundamental level, how the extreme environment of space influences biological systems. Biological sciences in space can be classified as human and animal biology, plant biology, and microbial biology. This nomenclature is useful for organized discussion and programmatic efficiency. However, it is important to recognize that many of the underlying biological processes

and the physical phenomena driving these processes are shared among all organisms. Also, humans, animals, plants, and microbes will interact with each other both intentionally and inadvertently in spaceflight environments—as they do on Earth.

Human and Animal Biology

Spaceflight poses numerous health challenges to human and animal systems, including alterations in cardiovascular, muscle, bone, vestibular, vision, brain, and likely all organ systems, which have far-reaching effects from the level of whole body to individual genes (Bloomfield et al. 2016; English et al. 2019; Garrett-Bakelman et al. 2019; Van Ombergen et al. 2017). Deleterious health effects are impacted by several factors, including duration of spaceflight, radiation exposure, behavioral aspects, living in confined and highly controlled spaces, and temporal alterations in hormonal and circadian rhythms (Goel et al. 2014; Kennedy et al. 2014; Mark et al. 2014). These effects are further complicated by heterogenous responses among individual crew members to spaceflight and implemented countermeasures (Fitts et al. 2010; Mark et al. 2014; Moore et al. 2014; Trappe et al. 2009).

Current knowledge about the deleterious health effects of extended exposure to the space environment comes primarily from a limited number of studies on astronauts on the ISS (Delp et al. 2016; Elgart et al.

2018; Garrett-Bakelman et al. 2019; Reynolds et al. 2019), and a number of studies done on animal and cell model systems while aboard the ISS (and earlier on the space shuttle), or through ground-based analogues of simulated microgravity, hypergravity, and space radiation. The NASA Twins Study represented the most comprehensive evaluation of human health effects associated with long-duration spaceflight ever conducted (Garrett-Bakelman et al. 2019). These findings highlight potential implications for improved understanding of aging and appropriate countermeasures—both in space and on Earth. Furthermore, individual differences in response were observed, highlighting the importance of personalized, genotype-specific approaches for evaluating effects of spaceflight, particularly as the number and diversity of space travelers increases (Xu et al. 2022; Willey et al. 2021).

Owing to the limitations associated with obtaining astronaut samples, a key research focus of space health in the past decade was investigating biological effects of space environmental factors using in vivo animal models or in vitro/ex vivo human cell models. The majority of studies in animal cells and model systems in altered gravity environments focused on function of bone (Sarkar and Pampaloni 2022), muscle (Juhl et al. 2021; Moosavi et al. 2021), nervous tissue (Kohn and Ritzmann 2018) including the eye (Khossravi and Hargens 2021; Paez et al. 2020), and the immune system (Akiyama et al. 2020; Ludtka et al. 2021).

Although a number of studies have been performed using mammalian models, including humans, the numbers of individual animals in these studies are relatively low and relatively few have been precisely replicated. Thus, it will be important to establish facilities for larger-scale studies that include sufficient replicate experiments for statistically robust conclusions and that include combined features of the space environment. These approaches may also include greater reliance on so-called tissue chips that replicate some features of the physiological microenvironment and require contributions from BPS to harness in the space environment. In the next decade, human space research is expected to emphasize a systems biology, multi-omics approach to understand how the space environment impacts the interplay between complex biochemical networks that regulate human biological systems as a whole, and the interspecific interactions between humans and microorganisms.

Plant Biology

The space environment provides unique opportunities for fundamental plant sciences, as distinct from applied studies that may be concerned with cultivation of plants for life support in space. The study of green plants in microgravity and partial gravity environments encountered in space enables the interrogation of scientific questions that cannot be investigated on Earth, such as the isolation and study of mechanical effects on plant cells (Sampathkumar et al. 2014), environmental responses, and plant growth. Because plants are rooted in place and depend on sunlight for photosynthesis, environmental cues play a relatively larger role in the regulation of plant developmental processes compared to other organisms. Experiments conducted in microgravity allow for disentangling the interaction between light and gravity (Millar et al. 2010; Vandenbrink et al. 2016), which are primary cues directing plant growth on Earth (Hangarter 1997).

The space environment also poses unique challenges to plant growth and development, with issues related to water delivery, air composition and circulation, lighting and temperature, as well as altered pressure and radiation. Because plants are vital components of bioregenerative life support systems, the successful propagation of plants in space is essential for the success of future space missions (Wheeler 2017). This requires a fundamental understanding of plant adaptations to spaceflight and extraterrestrial environments.

Moving beyond LEO, the need to grow plants for lunar and Mars missions will require new types of plant growth facilities, optimizing plant growth, and selecting or engineering traits optimized for growth on supplemented lunar or martian regolith and under altered gravitational conditions and within the complex microbiome of space vehicles and habitats.

Microbial Biology

Microorganisms and their associated viruses have the potential to be present in all areas of a spacecraft or extraterrestrial habitat where plants, humans, and animal may coexist, in addition to the human gut microbiome.

As such, understanding of space microbial biology will impact the success of current and future missions. Ongoing culture-based isolations and characterizations are being performed to monitor microbiological conditions on spacecraft (Castro et al. 2004), and the establishment of culture collections of these isolates remains a key resource for studies related to microbial control and microbial evolution (Mora et al. 2016; Yang et al. 2016). There is ongoing development of monitoring technology to enhance analysis of organisms during spaceflight and reduce the need for samples to be collected and analyzed on Earth (Khodadad et al. 2021). Culture-independent studies are becoming more widely used to describe microbial communities associated with spacecraft (Be et al. 2017; Haines et al. 2019). In clinical settings, sequenced-based identification has an advantage in that identification is quite rapid, with results available within a few hours, as compared to 2–3 days for culture-based identification (Schmoch et al. 2021). DNA sequencing has been demonstrated on the ISS (Castro-Wallace et al. 2017), and ongoing developments in sequencing and other technologies enable some underlying biological mechanisms to be identified. While -omics and other technological developments enable considerable experimental data to be acquired, it is vitally important that these data become available and are used to identify underlying biological processes and other scientific principles, rather than being merely descriptive catalogs of data (Biteen et al. 2016; Thompson et al. 2017).

Microorganisms can also impact spacecraft materials. The water recovery system recycles crew urine and humidity from breath condensate (Yang et al. 2018; Zea et al. 2020). As such, it would receive a regular introduction of microorganisms. Bacteria readily attach to surfaces and form adherent microbial populations encased in an extracellular polymer layer, biofilms, that can potentially coat, clog, and foul surfaces. When growing as biofilms, such microorganism communities become highly tolerant to antimicrobial agents and are thus very difficult to control (Orazi and O’Toole 2019).

On Earth, microbial communities carry out functions essential for life, including cycling of carbon and other nutrients, degradation of pollutants, biosynthesis of valuable materials, and support of plant growth (NASEM 2019). It is important to know how conditions in space influence microbial communities and the beneficial services they provide. At a fundamental level, studies are needed to determine the impact of reduced gravity and increased radiation on microbial community interactions. Systems biology and multi-omics approaches are applicable to decipher mechanisms underlying metabolic and other interactions between community members. This research would eventually entail increasing system complexity: from model or synthetic microbial communities (SynComs) in liquid cultures, toward SynComs in and on solid human-made materials, and eventually to soil-associated and plant-associated microbiomes.

Biophysics in Space and Planetary Environments

Biophysics is a rapidly developing area of space science research for the BPS community. It is an inherently interdisciplinary, bridging field that seeks to understand and describe biological phenomena through application of the theories and approaches of physics (Limbert 2017; Rapin et al. 2021).

Biophysics is sometimes treated as a discipline within the physical sciences, and other times treated as a discipline within quantitative, applied life sciences. From the latter perspective, alternative terms may be used including physical biology, chemomechanics, and mechanobiology. This decadal survey does not dwell on those distinctions in terminology, but instead aims to use the accumulated expertise of this community to address KSQs at the interface of biological systems and physical principles.

Many new developments in biophysics are deeply connected to progress in other experimental and computational fields. In the past decade, expansion of experimental tools such as single-molecule fluorescence techniques and atomic force microscopy, both products of advancements in biophysics, has made biological systems far more accessible to quantitative experiments. Engineering advances contribute to biophysics, and provide the current capacity to weigh, deform, and sort individual cells without biochemically (e.g., fluorescently) labeling them; such biophysical attributes can be correlated with cell function (Lee et al. 2014; Stockslager et al. 2021) through advanced data analytics. Thus, all of this progress on Earth and in space-based research is driven by a complex interplay of biology, chemistry, engineering, data science, and physics (NASEM 2022).

In space, biological systems ranging from unicellular microbes to individual mammalian tissue cells to entire humans adapt in multiple ways to the altered influences or cues of the space environment. Biophysics aims to bring insight into underlying cellular and molecular mechanisms, or signals and responses, that contribute to well-documented phenomena, including the formation of biofilms, mechanical force generation via a cell’s motor proteins along its cytoskeleton such as muscle cell contraction, migration of cells during tissue development or cancer metastasis, and a range of reduced-gravity-related physiological effects on biofluid flow in a plant stem or a human artery. As discussed further in Chapter 2, gravitational forces have increasingly dominant effects on biological assemblies and organized systems that are typically greater than 1 mm in length; systems of smaller length and mass scales are dominated typically by viscous and electrostatic forces rather than gravitational forces.

Physical Sciences in Space and Planetary Environments

Some important questions about the physical world can be answered only in the space environment, leveraging unique conditions such as low gravity and low pressure. For example, a wide range of physical sciences experiments, from investigating the nature of dark matter and dark energy to the behavior of complex materials, are affected by Earth’s gravity. Feeble but key interactions in many experiments, ranging from those conducted on soft matter to those on ultracold atoms, are hidden by effects of Earth’s gravity. Access to the space environment enables not just elegant but otherwise impossible research that opens up new insights on how to steer light, control magnetic effects at the nanoscale, and process quantum information (Ballantine and Ruostekoski 2020).

Physical sciences also concern the search for precise understanding of what happens to physical systems and processes in the space environment, and how to use both Earth-based and off-Earth materials effectively in space. Such knowledge of how to design, process, and use the solid, liquid, and gaseous materials that define the built environment of space travel and habitation is critical for sustaining long-term space exploration. Physical sciences in space also advance understanding of physical systems to improve life on Earth, including how to adhere or separate or recycle materials. Fundamental research across a wide range of disciplines and settings contributes to the robust physical sciences program. The primary relevant disciplines within the physical sciences include materials sciences such as physical metallurgy, complex fluids and soft matter, fluid physics, combustion science, and fundamental physics. In the United States, it is as common to find researchers in these disciplines who are trained as engineers as those who are educated as physicists or applied mathematicians. As noted for the biological science kingdoms, these physical science discipline references are useful for organized discussion and programmatic efficiency, even though the separation between domains is seldom stark and many phenomena in each domain are driven by common physical processes across some of these domains.

Materials

Materials science and engineering is a multi-disciplinary field that connects the processing, internal structure, and properties of materials to predict and control key functional properties of those materials. By understanding the thermodynamics and kinetics by which changes in material composition, phase, and structure can be engineered, materials scientists have developed the aluminum alloys and polymer composites of aircraft fuselages as well as the semiconducting thin films in the chips that fill the aircraft computers and the specialty textiles that comprise wearable sensors. Practically, materials in the space environment need to be able to function well when exposed for long times to different temperatures and pressures, altered gravity and other potentials, and radiation. Materials need to protect the payload of devices, equipment, and life-forms from harmful effects. At the same time, the space environment is a unique laboratory for making new materials and studying materials properties unhampered by typical terrestrial conditions. Both advancing materials for future space exploration and manufacturing materials within the space environment require knowledge of the behavior, microstructures, and fundamental properties of materials in extreme environments and the mechanisms governing their formation, transformation, damage, and destruction.

Advances in materials science in the past decade have highlighted the synthesis of new materials comprising metals, ceramics, and/or polymers fabricated as thin films, composites, and nanomaterials. Concurrent advances in

materials engineering experiments and theory have afforded more accurate control and measurement of their thermodynamic, structural, and functional properties. There have been tremendous advances in computation, including density functional theory, molecular dynamics, and, more recently, machine learning and artificial intelligence to predict properties and processing approaches to make such materials. The theoretical prediction of materials’ properties could continually be linked to experiment. Experiments in space, where one can separate the effect of gravity from that of other drivers, offers new opportunities for fundamental understanding of the driving forces for chemical reactions, melting, crystallization, phase separation, and microstructure development. Furthermore, there are opportunities to synthesize new high-purity materials with controlled topologies and microstructures, including high-value semiconductors and pharmaceuticals. Last, additive manufacturing approaches including 3D and 4D (time-variable) printing of materials and material composites affords new possibilities in processing and prototyping.

For sustainable operation on the Moon or Mars, local resources could be utilized to create the built environment; it is prohibitively expensive and time consuming to bring all needed engineering materials from Earth. Mining, transport, processing, and manufacturing could be adapted to the local space environment. To do this successfully, science will need to be integrated with engineering to ensure that appropriate processes are proposed, developed, and tested. Likewise, there needs to be an active interface between engineered materials and biological materials and systems, whether in considering materials that interface with humans in medical devices or materials that augment human potential in soft robotics.

In addition to the mechanically stiff and hard crystalline materials comprising most metallic and ceramic load-bearing structures on Earth and in the ISS, the space environment includes the familiar glassy or amorphous stiff materials (e.g., borosilicate glass) and the more easily deformable materials including colloidal suspensions and gels that comprise our adhesives, pharmaceutical capsules, and personal hygiene products. These materials, alternatively called complex fluids or soft matter, are potentially useful in space; their formation in low-gravity environments may lead to new microstructures and properties. While study of materials naturally includes the study of soft matter, this subset is described next because the communities that conduct research on this topic can be described variously as materials scientists, physicists, and plasma scientists.

Complex Fluids and Soft Matter

A class of materials that has characteristics of both solids and fluids is referred to as soft condensed matter, or simply soft matter. Soft matter has been described as “everything squishy,” and is a term used to denote materials that flow readily under low applied forces, including gravitational forces on Earth; they are also termed complex fluids. Soft matter includes research areas such as polymer flow, colloids, gels, foams, liquid crystals, complex/dusty plasmas, and granular media.

The lack of buoyancy in the microgravity environment eliminates natural convection and sedimentation, which allows for study of 3D systems, as opposed to 2D and planar systems. Slower chemical reactions as well as reorientation of molecular arrangements that are not possible in a normal gravity environment where density-driven flows disrupt these processes can be studied in the space environment. For example, the liquid content in foams in a terrestrial environment drain from within the intercellular network (Langevin and Vignes-Adler 2014), but in microgravity, the coarsening mechanism is substantially different, because surface tension forces dominate the redistribution of liquid. Extensional rheological measurements of polymers without the fluid “sagging” become possible in a microgravity environment (Hall et al. 2009). In the absence of gravity, attractive forces in granular media may become dominant and allow for other packing arrangements of the material as well as easier dispersion. (See Figure 1-6.)

Knowledge gained about soft matter in the space environment can improve many Earth-bound applications ranging from stability of colloidal solutions, gels, and foams in the consumer market; extrusion processes for polymers; the use of liquid crystals in electronics; and the extraction, transport, and reacting of various raw minerals into usable commodities.

Utilizing soft matter in productive ways will be important as space exploration on other planetary bodies develops, especially with regard to handling granular media. Several areas of research will facilitate these activities

and mitigate the risks. In situ resource utilization (ISRU) will also rely on extraction, transport, and processing of various types of lunar soil or regolith (Nangle et al. 2020). Designing safe and effective means of collecting, handling, and processing these materials in this environment will require a thorough understanding of handling granular media. Conversely, mitigating the effect of fine particles of granular media, or dust, to prevent it from adhering to photovoltaic solar cells and thermal radiators and diminishing their performance is necessary for missions that last weeks to months. Last, understanding the processes associated with 3D printing to manufacture delicate biocompatible structures or replacement parts from soft matter is also being investigated in both the microgravity and partial gravity environments (Nangle et al. 2020).

Fluid Physics

The study of single-phase fluids and their behaviors in space is critical to a wide range of functions in the space environment. These include controlling liquid fuels in ways that enable spaceflight and keep crew members safe (Fester et al. 1975; Hansen et al. 2020); recycling air and water and supporting food production within spacecraft (Meyer and Schneider 2018); power production (Bennett et al. 2020) and temperature control (Sarraf and Anderson 2007); and understanding fluid-like behaviors of lunar (Agui and Wilkinson 2010) and martian surface materials.

The initial, and continuing, purpose of such research is to develop and validate techniques for controlling the position and motion of fluids in the space environment based on their physical properties, with particular emphasis on understanding the effects of reduced gravity on fluid behaviors.

Fluid research has been conducted using drop towers (less than 5 seconds) (Masica and Salzman 1967; Wollman et al. 2016) and parabolic flight aircraft (less than 20 seconds) (Dukler et al. 1988; Konishi et al. 2015b; Raj et al. 2009) for behavior that quickly transitions from normal gravity to the reduced gravity environment. Suborbital capabilities such as sounding rockets (Zell et al. 1984) and commercially available reusable rocket systems (Collicott and Alexeenko 2019) extend the possible duration of experiments (up to about 2–3 minutes). Fluid research aboard orbital spacecraft occurred as early as Scott Carpenter’s Mercury 7 flight (Nussle et al. 1963). Fluids experiments were conducted on many space shuttle flights (Lee et al. 1997) and continue to be conducted aboard the ISS (Chatterjee et al. 2013; Motil et al. 2021). Most of these investigations have been conducted in the low-gravity environment ranging from 10⁻⁶ to 10⁻²g’s. Research in a partial gravity, similar to lunar or martian gravity, has been conducted aboard aircraft flying through a modified parabolic arc (Hurlbert et al. 2004). Centrifuges in drop towers and the ISS also have been used to generate a partial gravity environment, but compensation for spinning fluid motion generated by the centrifuge is required for these environments (Ferkul 2017). Fluid and gas behaviors in a high-gravity rotating environment (giant planets) also have their peculiarities (Bouchet and Sommeria 2002; Dowling 2020).

Gravity affects both the shape of the vapor–liquid interface in bubbles and droplets and the flows of gas and liquid along that interface, thus impacting mixing within each phase and between phases as well as the mass and heat transfer through the interface. As a result, the length, velocity, and timescales of fluid behavior are dramatically altered by moving to a reduced gravity environment. These alterations make detailed scientific investigation of fluid behavior possible. On Earth, heavier objects such as solid crystals fall through liquids, while lighter phases like gas bubbles rise. This motion can affect the shape and size of the bubble and crystal, as well as the rate at which the crystal forms (or dissolves). In the space environment, the gravitational pull causing this motion is reduced. However, other physical mechanisms become more prominent and can induce motion and consequently affect the shape of bubbles, droplets, and crystals in different ways. For example, in space, the dominance of capillary effects is significantly increased, resulting in larger bubbles during boiling. Temperature or concentration differences along a bubble surface can cause bubbles to “swim” toward regions of warmer liquids, whereas the motion of growing crystals allows mixing that increases the rate of growth and can lead to shapes and internal structures that are different than if there was no flow. In addition, as a crystal falls to the bottom of a terrestrial vessel, it stops its growth along the bottom surface and has limited growth along its sides until it merges with neighboring crystals. Different crystal orientations and dimensions are possible in microgravity, as the crystal growth is not blocked by any surfaces or support structure, and the mixing is non-existent.

The fundamental science of fluids impacts other science in space. Fluids provide cooling to science instruments aboard space telescopes and spacecraft to prevent overheating of electronics and motors. The ability to prepare chemical compounds in space by transferring reagents to sample chambers such as pipetting reagents into sample vials for analysis or storage safely without releasing stray droplets is required for crew safety. Material science investigations require controlling bubbles for solidification or crystallization studies. In addition to conducting successful plant biology research in space, plants need the correct balance of water, nutrients, and air to their roots to prevent them from succumbing to situations of drought or flooding.

Combustion

Combustion science involves fluid mechanics, heat transfer, phase change, transport phenomena, thermodynamics and chemistry. The major products of combustion are carbon dioxide and water, but that is an oversimplification of a much more complex physicochemical process. For even the simplest fuels there are a large number of intermediate reactions and species. In addition, flow motion, molecular diffusion, and thermal radiation are coupled with the reaction processes (Turns and Haworth 2021). Gravity has profound effects on most flames (Law and Faeth 1994).

Altered gravity environments, including those experienced in spaceflight or on the Moon or Mars, create significant challenges in combustion, whether in an unwanted fire in a spacecraft or an extra-terrestrial combustor.

The lack of a buoyant flow, however, also creates a unique opportunity for researchers—the ability to perform fundamental studies of combustion phenomena without the complicating and dominating influence of the buoyancy-induced flow. Over the past 3 decades, combustion and fire researchers have exploited the reduced-gravity environment in ground-based and space-based facilities to study fundamental combustion problems to unlock and discover aspects of combustion that have eluded researchers owing to the pervasive influence of buoyancy-induced flows in typical terrestrial laboratory studies. Entirely new combustion systems, such as so-called sofballs (Ronney 1998) and cool diffusion flames (Ju 2021), have been developed. Such understanding may extend to improved prediction and management of terrestrial wildfires, in that the large length scales and temperature differences suggest that some aspects of these fires are suited to more fundamental study in environments such as space, where buoyant forces are reduced.

Analysis of combustion in microgravity can improve the understanding of fundamental combustion physics, leading to more efficient and less polluting terrestrial combustion devices such as engines and burners (Ju 2021). Currently, 86 percent of the world’s energy demand is met through combustion (Ritchie et al. 2022). With movement toward hydrocarbon fuels from greener or carbon-neutral feedstocks (as opposed to conventional petroleum-based fuels), future engines will be required to simultaneously increase efficiency and reduce emissions. These objectives can be met only through a fundamental, predictive understanding of combustion physics that does not currently exist (Ju 2021).

Combustion science within the space environment also informs spacecraft fire safety, leading to improved material qualification, fire detection, and fire suppression in spacecraft and outposts (Guibaud et al. 2022). NASA’s future exploration goals are just beginning with the start of the Artemis program. The cabin atmospheres in the Artemis program will have lower cabin pressure with an increased ambient oxygen concentration (to minimize the required prebreathing time for extravehicular activities). Because fires are very sensitive to ambient oxygen mole fraction, reactants and materials, diluents, pressure, and gravitational forces, fire safety practices deemed safe in the microgravity environment of the ISS (with a cabin atmosphere identical to air on Earth) may no longer offer the same factor of safety (or be completely inapplicable) for future exploration missions. Last, combustion within space environments may be utilized beyond propulsion, such as in supercritical water oxidation for waste disposal, or for ISRU processes including the synthesis of cementitious materials.

While there have been dozens of microgravity combustion and fire studies in drop facilities, parabolic aircraft, and spacecraft (Kono et al. 1996; Sun et al. 2020), as well as the ISS (Dietrich et al. 2014), those studies have been limited in terms of flame and fire sizes, diagnostics, pressures, and the effects of lunar and martian gravity.

Fundamental and Quantum Physics

The goals of fundamental physics research are to discover and explore the physical laws governing matter, space, and time. Such understanding can help predict physical behaviors in space and on Earth. Present physical theories (e.g., general relativity, the standard model of elementary particles and fields, and quantum mechanics) provide an excellent description as well as prediction of many physical phenomena. However, most (more than 95 percent) of the universe’s composition is unexplained—a “dark matter” and “dark energy” of unknown origin. Many other puzzles, such as obvious matter–antimatter asymmetry, the near-horizon geometry of black holes, and the physics of the early universe prior to recombination, remain unanswered.

Some of the most important questions of modern physics can be answered only in the unique environment of space. Space provides a unique platform of exploration away from Earth’s gravity and access to a variety of variable gravitational potentials and enables experiments needing baseline distances beyond Earth scale. The Moon provides a unique combination of very low seismic backgrounds, vacuum, and a permanent cryogenic environment. Other solar system objects, including asteroids, could provide access to unique fundamental physics explorations.

Deployment of high-precision optical clocks in space and demonstration of space-to-Earth and space-to-space optical time transfer are the next steps toward new physics discoveries enabled by space. Space optical clocks will also greatly advance relativistic geodesy (measurements of Earth’s gravitational potential for Earth science applications), enable autonomous spacecraft navigation throughout the solar system, provide links for

high-precision optical clocks on Earth, and dramatically expand the capabilities of very long baseline interferometry (VLBI). Applications of other quantum technologies and corresponding R&D, including atomic interferometers, solid-state-defect magnetometry, quantum gases, quantum memories, entanglement sources, and others, will leverage fundamental and applied advances enabled by quantum devices in space. In particular, optical atomic clocks and atom interferometers have been proposed for novel future gravitational wave detectors in wavelength ranges not accessible by either Earth-based detectors or the Laser Interferometer Space Antenna (LISA), providing a powerful new window on diverse phenomena, including ultra-massive black holes at the center of most galaxies and the physics of the early universe.

RESEARCH LANDSCAPE

The current BPS research landscape is a highly complex system of government space agencies, federal research agencies, commercial entities, private funders, and a diverse array of researchers across the biological and physical sciences. The complexity of this system over the next 10 years is likely to increase. The government agencies operating the ISS—NASA, ESA, Roscosmos, JAXA, and CSA—are among the many space agencies striving to make substantial progress in space exploration in the coming years. (See Figure 1-7.) With ISS operations expected to expire in 2030 (NASA 2022b) and goals focused on deep-space exploration, government agencies are examining how they can maintain and surpass the research progress made thus far on the ISS. How the launch costs associated

with BPS research are shared in the coming decade among U.S. government agencies and research performers also remains unknown at present. Clarity on that point will affect the pace of research that is not also prioritized by the private sector, including corporate research and philanthropy. For example, will facilities for BPS experiments be made available on private spaceflights? The description of the research landscape here is a summary of the changing opportunities that will affect U.S. leadership of science priorities and avenues for research over the next 10 years. This is not an exhaustive review of all the changes affecting the BPS research community, but it reflects the types of developments under way.

NASA has participated in many international partnerships that have included the bartering of experiment resources such as crew time and facility usage, launch mass, and so on, and enabled sharing the data among principal investigators from many nations. Now, commercial entities have entered the suborbital and LEO environments. NASA is supporting the growth of a robust LEO economy. A number of companies are engaged in crew and cargo transport (e.g., to the ISS), free-flyer operation, and development of new stations to continue the ability for humans to live and work in space. Such companies include Axiom Space, Blue Origin, Boeing, Nanoracks, Northrop Grumman, Sierra Space, and SpaceX. These companies are also likely to operate ground-based replica science facilities (in addition to space platforms) in order to conduct ground controls for space experiments and thus compete for the business of BPS.

The possibilities of expanded LEO destinations and laboratory resources are exciting, but there will be many challenges to consider in terms of the orchestration of research over the next decades. These include uncertainty of the frequency and extent of government-funded research (by NASA and other federal agencies) on these platforms, how unique hardware will be provided on various platforms, data sharing requirements, crew and data protections, broadening of the BPS research talent pipeline, intellectual property rights, export control restrictions, and policy considerations when at least one CLD is in operation (Gatens 2022).

Databases for BPS Research

Development of public databases in the past decade have expanded the opportunities for researchers to mine and utilize data from prior flight investigations to further scientific inquiry. Efforts within NASA to establish and promote use of open databases were supported by a 2013 Executive Order that directed the default practice for government data. These data are used to identify underlying biological and other scientific principles (Biteen et al. 2016; Thompson et al. 2017). There are also biospecimen repositories such as the NASA Biological Institutional Scientific Collection (NBISC) and the Physical Science Informatics (PSI) data repository for physical science experiments performed on the ISS and other flight or space analogs.³ The growth of data repositories has been an extremely positive outcome in support of open science within the BPS community, and is in line with the national open science initiative (NASA 2023b; White House 2023)⁴ and attendant expectations that NASA champion FAIR access principles (Wilkinson et al. 2016) for the BPS research community. Table 1-1 assembles the major data repositories of relevance to BPS research.

Research Platforms and Laboratories

BPS researchers utilize a range of platforms and laboratories to carry out experiments and study space-related phenomena. These include the ISS (see Figure 1-8), drop towers, parabolic flight, balloons, suborbital vehicles, and free-flyers, as well as a number of terrestrial analogs and ground-based laboratories. (See Figure 1-4 and Box 1-3.) A number of these platforms produce microgravity conditions through free fall of an experiment for different time durations: less than 5 seconds in vertical drop towers, less than 30 seconds at the apex of aircraft parabolic flight, up to 3 minutes in sub-orbital vehicles, and 5 to 20 minutes in sounding rockets (Joseph and Wood 2021). Space-like radiation environments can be accessed with high-altitude balloons, with the conditions varying with location in Earth’s magnetic field. These facilities are used for concept development, testing matrix refinement,

___________________

³ This text was modified after the release to sponsor to clarify what is included in the PSI data repository.

⁴ NASA, “2023: Year of Open Science,” https://nasa.github.io/Transform-to-Open-Science/year-of-open-science.

and hardware verification. The committee anticipates that this range of platforms and laboratories will continue to be used and even expand as the BPS community seeks to leverage commercial LEO destinations, developments in free-flyers and ground-based simulators, Artemis mission capabilities, and other platforms beyond LEO. Box 1-3 briefly describes the environment and testing conditions for different platforms in space.

Chapter 2 next summarizes key advances in BPS science over the prior decade. These discoveries and developments—made by a resilient BPS research community that still has much room to broaden its talent pipeline—establish a baseline of knowledge and capabilities that motivate the framework for BPS research themes of the coming decade as outlined in Chapter 3. This baseline of scientific insights highlighted in Chapter 2 also serves as a reference for the key scientific questions of the coming decade described in Chapters 3–5, as well as the research campaign concepts outlined in Chapter 6 and overall strategy described in Chapter 7. These chapters, as well as the associated findings and recommendations, are responsive to the statement of task in Appendix A. Acronyms and abbreviations are listed in Appendix B and defined at the first instance within each chapter. References are listed in a separate section preceding the appendixes.