Microgravity Science Primer

CONTENTS
The Microgravity Environment of Orbiting Spacecraft
Biotechnology
Protein Crystal Growth
Mammalian Cell and Tissue Culture
Fundamental Biotechnology

We experience many manifestations of gravity on a day to day basis. If we drop something, it falls toward Earth. If we release a rock in a container of water, the rock settles to the bottom of the container. We experience other effects of gravity regularly, although we may not think of gravity as playing a role.

Consider what happens when a container of water is heated from below. As the water on the bottom is heated by conduction through the container, it becomes less dense than the unheated, cooler water. Because of gravity, the cooler, more dense water sinks to the bottom of the container and the heated water rises to the top due to buoyancy. A circulation pattern is produced that mixes the hot water with the colder water. This is an example of buoyancy driven (or gravity driven) convection. The convection causes the water to be heated more quickly and uniformly than if it were heated by conduction alone. This is the same density driven convection process to which we refer when we state matter-of-factly that "hot air rises."

In addition to mixing, density differences can also cause things to differentially settle through a process called sedimentation. In this process, the more dense components of mixtures of immiscible fluids or solid particles in fluids settle to the bottom of a container due to gravity. If you fill a bucket with very wet mud, and then leave the bucket sitting on the ground, over time the more dense soil particles will sink to the bottom of the bucket due to gravity, leaving a layer of water on top. When you pick up a bottle of Italian salad dressing from the grocery store shelf, you see several different layers in the bottle. The dense solids have settled to the bottom, the vinegar forms a middle layer, and the least dense oil is on top.

Gravity can also mask some phenomena that scientists wish to study. An example is the process of diffusion. Diffusion is the intermingling of solids, liquids, and gases due to differences in composition. Such intermingling occurs in many situations, but diffusion effects can be easily hidden by stronger convective mixing. As an example, imagine a large room in which all air circulation systems are turned off and in which a group of women are spaced ten feet apart standing in a line. If an open container of ammonia were placed in front of the first woman in line and each woman raised her hand when she smelled the ammonia, it would take a considerable amount of time before everyone raised her hand. Also, the hand raising would occur sequentially along the line from closest to the ammonia to furthest from the ammonia. If the same experiment were performed with a fan circulating air in the room, the hands would be raised more quickly, and not necessarily in the same order. In the latter case, mixing of the ammonia gas with the air in the room is due to both diffusion and convection (forced convection due to the fan) and the effects of the two processes cannot be easily separated. In a similar manner, buoyancy driven convection can mask diffusive mixing of components in scientific experiments.

Some behavior of liquids can also be masked by gravity. If you pour a liquid into a container on Earth, the liquid conforms to the bottom of the container due to gravity. Depending on the shape of the container and on the properties of the container and the liquid, some of the liquid may creep up the walls or become depressed along the walls due to the interrelated phenomena of surface tension, adhesion, cohesion, and capillarity. The resulting curved surface may be familiar to anyone who has measured water in a small diameter glass container (the water cups upward) or has looked at the level of mercury in a glass thermometer (the mercury cups downward). The distance the contact line between the liquid and the container moves up or down the container wall is affected by gravity.

Experiments performed on Earth often take advantage of the effects of gravity discussed. For many experiments, however, these effects tend to make the execution of experiments or the analysis of experimental results difficult and sometimes even impossible. Therefore, many researchers design experiments to be performed under microgravity conditions. The different scientific research areas that are studied in microgravity include biotechnology, combustion science, fluid physics, fundamental physics, and materials science. Each of these areas, or disciplines, is discussed below. The discipline is defined, some of the specific effects of gravity that illustrate the benefits of microgravity research are discussed, and some examples of current research are presented. In addition, a brief discussion of the microgravity environment of orbiting spacecraft is provided as is an introduction to the application of microgravity research to the exploration and development of space.

The Microgravity Environment of Orbiting Spacecraft

While freefall reduces the effects of gravity, being in an orbiting laboratory introduces other accelerations that cause effects that are indistinguishable from those due to gravity. When a spacecraft is in orbit around Earth, the orbit is actually defined by the path of the center of mass of the spacecraft around the center of Earth. Any object in a location other than on the line traversed by the center of mass of the spacecraft is actually in a different orbit around Earth. Because of this, all objects not attached to the spacecraft move relative to the orbiter center of mass. Other relative motions of unattached objects are related to aerodynamic drag on the vehicle and spacecraft rotations. A spacecraft in low-Earth orbit experiences some amount of drag due to interactions with the atmosphere. An object within the vehicle, however, is protected from the atmosphere by the spacecraft itself and does not experience the same deceleration that the vehicle does. The floating object and spacecraft therefore are moving relative to each other. Similarly, rotation of the spacecraft due to orbital motion causes a force to act on objects fixed to the vehicle but not on objects freely floating within it. On average for the Space Shuttles, the quasi-steady accelerations, resulting from the sources discussed above (position in the spacecraft, aerodynamic drag, and vehicle rotation) are on the order of 1x10 ^-6 g, but vary with time due to variations in the atmospheric density around Earth and due to changes in Shuttle orientation.

In addition to these quasi-steady accelerations, many operations on spacecraft cause vibrations of the vehicle and the payloads (experiment apparatus). These vibrations are often referred to as g-jitter because their effects are similar to those that would be caused by a time-varying gravitational field. Typical sources for vibrations are experiment and spacecraft fans and pumps, motion of centrifuges, and thruster firings. With a crew onboard to conduct experiments, additional vibrations can result from crew activities.

The combined acceleration levels that result from the quasi-steady and vibratory contributions are generally referred to as the microgravity environment of the spacecraft. On the Space Shuttles, the types of vibration-causing operations discussed above tend to create a cumulative background microgravity environment of about 1x10 ^-4 g, considering contributions for all frequencies below 250 Hz.

Biotechnology

Biotechnology is an applied biological science that involves the research, manipulation, and manufacturing of biological molecules, tissues, and living organisms. With a critical and expanding role in health, agriculture, and environmental protection, biotechnology is expected to have a significant impact on our economy and our lives in the next century. Microgravity research focuses on three principal area--protein crystal growth, mammalian cell and tissue culture, and fundamental biotechnology.

Gravity significantly influences attempts to grow protein crystals and mammalian cell tissue on Earth. Initial research indicates that protein crystals grown in microgravity can yield substantially better structural information than can be obtained from crystals grown on Earth. Proteins consist of thousand--or in the case of viruses, millions--of atoms, which are weakly bound together, forming large molecules. On Earth, buoyancy-induced convection and sedimentation may inhibit crystal growth. In microgravity, convection and sedimentation are significantly reduced, allowing for the creation of structurally better and larger crystals.

The absence of sedimentation means that protein crystals do not sink to the bottom of their growth container as they do on Earth. Consequently, they are not as likely to be affected by other crystals growing in the solution. Because convective flows are also greatly reduced in microgravity, crystals grow in a much more quiescent environment, which may be responsible for the improved structural order of space-grown crystals. Knowledge gained from studying the process of protein crystal growth under microgravity conditions will have implications for protein crystal growth experiments on Earth.

Research also shows that mammalian cells--particularly normal cell--are sensitive to conditions found in ground-based facilities used to culture (grow) them. Fluid flows caused by gravity can separate the cells from each other, severely limiting the number of cells that will aggregate (come and stay together). But tissue samples grown in microgravity are much larger and more representative of the way in which tissues are actually produced inside the human body. This suggests that better control of the stresses exerted on cells and tissues can play an important role in their culture. These stresses are greatly reduced in microgravity.

Protein Crystal Growth

The human body contains over 100,000 different proteins. These proteins play important roles in the everyday functions of the body, such as the transport of oxygen and chemicals in the blood, the formation of the major components of muscle and skin, and the fighting of disease. Researchers in this area seek to determine the structures of these proteins, to understand how a protein's structure affects its function, and ultimately to design drugs that intercede in protein activities (penicillin is a well-known example of a drug that works by blocking a protein's function). Determining protein structure is the key to the design and development of effective drugs.

The main purpose in growing protein crystals is to advance our knowledge of biological molecular structures. Researchers can use microgravity to help overcome a significant stumbling block in the determination of molecular structures: the difficulty of growing crystals suitable for structural analysis. Scientists use X-ray diffraction to determine the three-dimensional molecular structure of a protein. They can calculate the location of the atoms that make up the protein based on the intensity and position of the spots formed by the diffracted X-rays. From high resolution diffraction data, scientists can describe a protein's structure on a molecular scale and determine the parts of the protein that are important to its functions. Using computer analysis, scientists can create and manipulate three-dimensional models of the protein and examine the intricacies of its structure to create a drug that "fits" into a protein's active site, like inserting a key into a lock to "turn off" the protein's function. But X-ray diffraction requires large, homogeneous crystals (about the size of a grain of table salt) for analysis. Unfortunately, crystals grown in Earth's gravity often have internal defects that make analysis by X-ray diffraction difficult or impossible. Space Shuttle missions have shown that crystals of some proteins (and other complex biological molecules such as viruses) grown on orbit are larger and have fewer defects than those grown on Earth. The improved data from the space-grown crystals significantly enhance scientists' understanding of the protein's structure and this information can be used to support structure-based drug design.

Scientists strive for a better understanding of the fundamental mechanisms by which proteins form crystals. A central goal of microgravity protein crystal growth experiments is to determine the basic science that controls how proteins interact and order themselves during the process of crystallization. To accomplish this goal, NASA has brought together scientists from the protein crystallography community, traditional crystal growers, and other physical scientists to form a multidisciplinary team in order to address the problems in a comprehensive manner.

Mammalian Cell and Tissue Culture

Mammalian cell tissue culturing is a major area of research for the biotechnology community. Tissue culturing is one of the basic tools of medical research and is key to developing future medical technologies such as ex vivo (outside of the body) therapy design and tissue transplantation. To date, medical science has been unable to fully culture human tissue to the mature states of differentiation found in the body.

The study of normal and cancerous mammalian tissue growth holds enormous promise for applications in medicine. However, conventional static tissue culture methods form flat sheets of growing cells (due to their settling on the bottom of the container) that differ in appearance and function from their three-dimensional counterparts growing in a living body. In an effort to enhance three-dimensional tissue formation, scientists have developed a ground-based facility for cell and tissue culture called a bioreactor. This instrument cultures cells in a slowly rotating horizontal cylinder, which produces lower stress levels on the growing cells than previous Earth-based experimental environments. The continuous rotation of the cylinder allows the sample to escape much of the influence of gravity, but because the bioreactor environment tends to be rather passive, it is sometimes difficult for the growing tissue to find the fresh media (food supply) it needs to survive.

Another reason normal mammalian cells are sensitive to growth conditions found in standard bioreactors is that fluid flow causes shear forces that discourage cell aggregation. This limits both the development of the tissue and the degree to which it possesses structures and functions similar to those found in the human body. Tissue cultures of the size that can be grown in these bioreactors allow tests of new treatments on cultures grown from cells from the patient rather than on patients themselves. In the future, this technology will enable quicker, more thorough testing of larger numbers of drugs and treatments. Ultimately, the bioreactor is expected to produce even better results when used in a microgravity environment.

In cooperation with the medical community, the bioreactor design is being used to prepare better models of human colon, prostate, breast, and ovarian tumors. Cells grown in conventional culture systems may not differentiate to form a tumor typical of cancer. In the bioreactor, however, these tumors grow into specimens that resemble the original tumor. Similar results have been observed with normal human tissues as well. Cartilage, bone marrow, heart muscle, skeletal muscle, pancreatic islet cells, liver cells, and kidney cells are examples of the normal tissues currently being grown in rotating bioreactors by investigators. In addition, laboratory models of heart and kidney diseases, as well as viral infections (including Norwalk virus and Human Immunodeficiency Virus (HIV)) are currently being developed using a modified NASA bioreactor experiment design with slight variations in experimental technique and some adjustments to hardware. Continued use of the bioreactor can improve our knowledge of normal and cancerous tissue development. NASA is beginning to explore the possibility of culturing tissues in microgravity, where even greater reduction in stresses on growing tissue samples may allow much larger tissue masses to develop. A bioreactor is in use on the Russian Space Station Mir in preparation for the International Space Station.

Fundamental Biotechnology

Electrophoresis has been studied on a dozen Space Shuttle flights and has led to additional research in fluid physics in the area of electrohydrodynamics. Phase partitioning experiments, which use interfacial energy (the energy change associated with the contact between two different materials) as the means of separation, have flown on six missions.

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