The Mission of Microgravity and Physical Sciences Research at NASA (2001)

Earth-orbiting laboratories make it possible to employ a near-zero-gravity environment to carry out systematic and careful investigations of new physical phenomena. The Microgravity Research Division (recently renamed the Physical Sciences Division [PSD]) in NASA’s Office of Biological and Physical Research (formerly the Office of Life and Microgravity Sciences and Applications) has played the central role in sponsoring research programs that take advantage of this near-zero-gravity environment. In addition to flight experiments, the PSD has also sponsored a host of ground-based studies that either support or complement the flight experiments or have the potential to develop into future flight projects.

In the near-zero-gravity or near-weightless environment, buoyancy-driven effects are greatly reduced. These conditions are the prerequisite for the many investigations supported by the PSD in materials science, fluid physics, combustion science, and fundamental physics. By studying phenomena that are masked on Earth by buoyancy-driven convection or pressure gradients, many of the constraints and complexities that are intrinsic to earthbound measurements are removed. Thus, NASA’s microgravity research has resulted in insights into many physical processes that would have been difficult, or impossible, to obtain using other approaches, and a considerable body of expertise has been built up in each of the current program areas discussed briefly below. A more detailed description of these research programs can be found in a previous report of this committee (NRC, 1995).

Fluids are ubiquitous in nature and in many industrial processes. Fluid motions are responsible for most transport and mixing that occur in the environment, in industrial processes, in vehicles, and in living organisms. Scientists studying basic problems from chaotic systems to the dynamics of stars also turn to fluid physics for their models. The goal of much of the fluid physics program is to comprehend the fundamental physical phenomena underlying flows observed in nature. Fluid physics also has a crucial role in the space program in support of the effort to develop new technologies or to adapt existing technologies. The fluid physics program encompasses five major research areas: interfacial phenomena, biological fluid dynamics, dynamics and instabilities, complex fluids, and multiphase flows and phase change. Interfacial phenomena include research directed at understanding capillary phenomena and the dynamics of fluids at contact lines that occur, for example, at solid-liquid-gas trijunctions. Biological fluid dynamics is a new area of emphasis and focuses on the underlying fluid physics and transport phenomena in biological and physiological systems. The study of dynamics and instabilities encompasses research topics ranging from fluid mechanics of star formation and Earth’s interior to the dynamics of electrically charged fluids. Complex fluids currently under investigation include fluids as diverse as colloids, foams, and granular aggregates. Multiphase flows and phase change involve investigations in two-phase flows, such as gas-liquid systems, in which gravity has a controlling influence on the flows due to the large density difference between the phases. The research in many of these areas is of relevance to the human exploration and development of space (HEDS) effort. For example, multiphase fluid flow experiments performed in microgravity are important for applications such as spacecraft thermal management, environment control, human life support, and power and propulsion systems.

Materials science plays a key role in virtually all aspects of the nation’s economy. While it is clear that the structure of a material determines its properties, the ability to produce a certain structure, and hence materials properties, is not at hand. Thus, a central goal of any materials science research program is to understand at a fundamental level the relationship between a material’s process history, microstructure, and resulting properties. The absence of, or greatly reduced, buoyancy-driven convection and sedimentation effects enable special opportunities for the studies of materials processing that are not possible in normal gravity. The materials systems being investigated include electronic and photonic materials, glasses and ceramics, metals and alloys, polymers, and more recently, biomaterials. Common to these materials systems are the phenomena that form the key microgravity research themes: (1) nucleation and metastable states, (2) prediction and control of microstructures, (3) interfacial and phase separation phenomena, (4) transport phenomena, and (5) crystal growth and defect control. Examples of microgravity materials research include the verification of long-standing theories of dendrite formation, the precision measurement of liquid diffusion coefficients, the use of containerless processing to understand the formation of special materials such as yttria-doped glasses used for optical fibers or undercooled metal alloys, the growth of infrared sensor materials, and the space-based studies of liquid-phase sintering. Additionally, work is under way to improve the radiation resistance of shielding materials for use in the human exploration of space.

One of the most catastrophic events that can occur in the human exploration of space is a large fire. The absence of any safe refuge in space makes the prevention and/or containment of small fires a subject of critical importance to NASA. Microgravity combustion research has been driven in large part by a desire to understand the influence of the microgravity environment on combustion processes known to be of importance in fires on Earth. Microgravity studies of ignition and flame spread over condensed-phase materials have a direct bearing on material screening for fire safety in space. The flammability of materials has been assessed by investigating whether a flame will spread given the velocity and oxygen concentration of the airflow over the materials. Microgravity experiments have demonstrated that at oxygen levels and flow velocities characteristic of space shuttle and International Space Station environments, it is possible for a given material to be relatively more flammable in space than on Earth. This is of critical importance for determining the fire safety margins in space. The combustion program has also focused on studies of how fundamental combustion phenomena behave in the absence of gravity. This work makes possible the observation of phenomena and the verification of theories that are not possible on Earth. Examples include the first measurements of pure diffusion flame shapes, the discovery of “flame balls” (weak spherical stationary flames that can maintain their shape indefinitely with no net fluid motion), and the observation of radiative extinction of burning fuel droplets.

Fundamental physics research employs the microgravity environment to study the basic laws that govern the physical world on all length scales, from the microscopic to the cosmic. Prior to the early 1990s, the fundamental physics program centered on condensed matter physics, in particular the physics of continuous phase transitions or critical phenomena. Since then, it has been broadened to include gravitational physics, high-energy physics, laser cooling and atomic physics, and biological physics. The early emphasis on the study of critical phenomena is an obvious one. By eliminating the pressure gradient in a liquid helium sample in space, it was possible to study the superfluid transition and validate the prediction of the Renormalization Group (RG) theory of critical phenomena to within a billionth of a

kelvin, an improvement of a hundredfold over the most precise earthbound experiment. An important by-product of the low-temperature condensed-matter program is the development of very high precision thermometry techniques, which allows for determination of temperature to 1 part in 100 billion. Ongoing projects in the Fundamental Physics Program include cold atom space clocks, scheduled to fly in the near future. It has been demonstrated recently that laser light can be used to cool a dilute atomic sample to within a few microkelvin of absolute zero. At such temperatures, the mean velocity of the atoms drops from hundreds of meters per second to a few centimeters per second. In this regime, gravity dominates the atomic motion, limiting the time that the atoms can be interrogated. While atomic clocks on Earth have an accuracy of about 1 part in 10¹⁵, in space where the atoms do not “fall,” an ultimate accuracy and stability exceeding 1 part in 10¹⁷ may be possible.

The biotechnology program focuses on two fields: protein crystal growth and cell science. The protein crystal growth work is directed at using microgravity to understand the growth processes of macromolecular crystals and to produce crystals used for molecular structure determination. The cell science work focuses on the effects of a microgravity environment on the fundamental behavior of cells and tissue formation. This program was examined recently by the Task Group for the Evaluation of NASA’s Biotechnology Facility for the International Space Station (NRC, 2000). Thus, this report addresses research in that area only insofar as it is relevant to formulating the PSD mission statement.

The current program has begun to incorporate, to a limited extent, the new areas of nanotechnology, biomolecular physics and chemistry, and cellular biophysics and chemistry. For example, work on protein folding, biodegradable polymers, and techniques for levitation of cells in magnetic field gradients was funded through the most recent Fundamental Physics NASA Research Announcement. There are a few programs centered on nanoparticle formation in the PSD materials science program, such as those studying nanoparticle formation in glasses, formation of nanoparticles from a vapor, and growth of composite nanoparticles. The fluids program funds a small number of efforts in the biofluids area such as capillary-elastic instabilities in lung airways and the effects of mechanical perturbations due to microgravity on the transport properties in the vascular system. The combustion program funds a few studies on the combustion synthesis of carbon nanotubes. These research efforts, however, constitute a very small fraction of the total programs funded by the PSD. It is the intent of the PSD to broaden its research profile in these areas, and some possible opportunities for expansion are discussed later in this report.

National Research Council (NRC), Space Studies Board. 1995. Microgravity Research Opportunities for the 1990s. National Academy Press, Washington, D.C.

National Research Council (NRC), Space Studies Board. 2000. Future Biotechnology Research on the International Space Station. National Academy Press, Washington, D.C.