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SPACE BIOLOGY 72 5 Space Biology THE PROBLEMS There are two basic questions to be asked in the area of space biology: (1) does the microgravity environment provide a research tool for studies on developmental and other biological phenomena; and (2) can plants and animals undergo normal development and life cycles in the microgravity environment? Life has developed and evolved for at least 3.5 billion years in a relatively constant one-g environment. It is clear that both plants and animals have evolved mechanisms of detecting and responding to gravity. However, other issues are not as well understood: How do individual cells perceive gravity? What is the threshold of perception? How is the response to gravity mediated? Does gravity play a determinitive role in the early development and long-term evolution of the living organism? Studies of the early development and subsequent life cycles of representative samples of plants and animals in microgravity are of basic importance to the field of developmental biology. They are also essential to our ultimate ability to sustain humans for a year or more on the surface of extraterrestrial bodies or in spaceflight missions of long duration where resupply is not possible, and food must be produced in situ. More specifically, the areas important to study include:
SPACE BIOLOGY 73 1. Plant geotropic responses. The responses of plants to gravity and to light have been studied extensively. Roots grow down, shoots grow up. Spaceflight offers the unique opportunity to remove the gravitational stimulus from the developing plant system and to separate the gravitational input from other environmental stimuli known to influence plant growth (for example, phototropism and the circadian influences of the terrestrial environment). Spaceflight provides the opportunity to distinguish between the various tropic responses and to investigate the mechanisms of stimulus detection and response. 2. Animal systems, particularly embryonic systems (amphibian, fish, bird, mammalian) also have clear responses to gravity. For example, the amphibian egg orients itself with respect to gravity within a few minutes after fertilization, and during that short time the future of the embryo is established in the sense that the dorsal- ventral and anterior-posterior axes are established. Do we conclude therefore that the gravitational input is a required stimulus for the establishment of these axes? Clearly, the removal of gravity is a desirable, even necessary, step toward understanding. The process of bone demineralization seen in humans and animals as a progressive phenomenon occurring during spaceflight is not only a serious medical problem, it raises the question of abnormalities in the development of bones, shells, and special crystals (such as the otoconia of the inner ear) in animals developing in microgravity. The study of such abnormalities should provide insight into the process of biomineralization and demineralization. 3. In addition to the scientific need to study basic plant and animal interactions with gravity, there is a practical need to study the responses of animals, and particularly plants, to the ''normal'' environment expected on future spacecraft. The long-term presence of humans in space (either in interplanetary flight or in an extraterrestrial station) is an integral part of planning for future space explorations (for example, manned Mars missions or bases). When the point is reached where it is no longer cost effective or logistically possible to resupply the spacecraft or habitat with water, atmosphere, and food, ways must be found to recycle all these components in a controlled ecological life support system (CELSS). Experiments in spacecraft aimed at determining which plants and animals are most efficient and best suited for such a system are badly needed. While much work can be and is being done on the ground, flight work is essential. Again, however, the
SPACE BIOLOGY 74 experiments needed require long-term (many months) spaceflight. Ideally, this spaceflight would be of sufficient duration to demonstrate that several generations of plants can flourish (for instance, can soybeans germinate, grow normally, produce an optimum crop of new soybeans for food and new seed for ensuing crops?). All of this biological cycling, plus the development of water and atmosphere recycling equipment, the reclamation of wastes, etc., requires a long-term engineering enterprise. WORK TO DATE Although both plants and animals have been flown on space missions, an adequate data base has not yet been obtained. There are several reasons for this. First, the opportunities for flight have been rare. Second, the environment provided for living material in spaceflight has not yet been optimized. For example: â¢ The threshold of gravity sensing systems in plants and animals is unknown, but there are reasons for believing that, in certain organisms, it is in the range of 10-5 to 10-6 g. It is calculated that, at these gravity levels, sedimentation and thermal convection are no longer operative. These two physical factors are believed to be the principal intracellular phenomena in effecting the gravitational input within the cellâa testable hypothesis at the appropriate microgravity level. Spacecraft to date have not provided that gravity level. Most flights have been in the range of 10-3 to 10-4 g. â¢ In the study of embryonic material in particular, most experiments have of necessity been done with eggs that were fertilized on the ground, well before orbital flight, so that the critical g-sensitive time period immediately after fertilization was spentat one-g. â¢ Most U.S. flights have been too short in duration (a few days) for many observations to be made. Early flights (both U.S. and Russian) have amply demonstrated that the flight of living material is possible. The microgravity environment encountered to date (10-3 to 10-4 g) has not been low enough or long enough to allow us to answer the question of threshold and response mechanisms. Cells seem to develop normally, although growth patterns (particularly of plants) are adversely affected even at these levels. Thus, while considerable
SPACE BIOLOGY 75 experience has been gained and will continue to be gained in orbital flights of days to weeks in duration, much lower gravity levels (10-5 to 10-6 g) and much longer flight duration (months) are needed. FUTURE WORK Continuing flight of the Space Shuttle will provide limited opportunity to expose living systems to microgravity, but it appears that the opportunity to do the critical experiments must wait for the development of spacecraft capable of providing both lower gravity levels and flight times of months to years. The first such capability may well arise with the advent of the Space Stationâin 1995 and beyond. Whether such an environment is provided by a space station, free flyer, or platform, the real requirement for this work is an environment that is well defined in its physical parametersâthat is, long-term, uninterrupted low g. A space station module, if mounted on the center of mass of the space station and not perturbed by human or machine activity, could provide a minimal environmentâ10-5 g. Otherwise, a free flyer or platform may be required. Only flight opportunities such as these will permit a rigorous answer to the problems outlined above. Special Requirements The special requirements needed to ensure successful space biology experiments in a microgravity environment can be listed as follows: 1. A "quiet" laboratory or platform that has an environment of 10-5 to 10-6 g at <1 Hz is needed for this work. This requirement is shared by many materials science activities as well. 2. This environment must remain undisturbed by human or machine activity for long periods of time (many months). Special "damping" procedures may be needed. 3. Periodic sampling is needed, but it should be done in such a way as to leave the remaining material undisturbed. Sampling must obviously be carefully planned and minimized to preclude vibrations and other unwanted gravitational forces. 4. Appropriate incubators and growth chambers are needed for cells, simple organisms, plants, and animals.
SPACE BIOLOGY 76 5. In order to investigate the problem of gravity thresholds in living systems, an on-board centrifuge is needed. This centrifuge should provide a one-g control for microgravity experiments, as well as the capability to explore a range of gravities between 10-5 and one- g, in order to study CELSS candidates and gravity thresholds for certain phenomena.