Finally, the chapter includes a discussion of the comprehensive, long-range approach to clinical research that NASA needs to consider implementing to best protect human health and safety during long-duration space travel. Historically, NASA has faced difficulty in conducting clinical research in space medicine. One problem is the small numbers of research subjects (astronauts) available for study. The overriding reason, however, is that microgravity cannot be duplicated on Earth; it can only be approximated. The terrestrial means of research on bone mineral density loss in microgravity are bed rest, immersion in water, or immobilization. All have their own disadvantages. The opening era of the permanent presence of humans in Earth orbit on the ISS in October 2000, however, provides an enduring test bed that will eventually help provide an understanding of human physiology in microgravity.
Faced with the necessity to maintain astronauts’ health during periods of exposure to microgravity and other extreme conditions of spaceflight, NASA has pursued the development of preventive and counteracting measures (i.e., countermeasures) to guard against or reverse the potential pathophysiological effects of space travel. A variety of countermeasures have been used in longer-duration spaceflights (Mikhailov et al., 1984; Bungo et al., 1985; Greenleaf et al., 1989; Fortney, 1991; Arbeille et al., 1992; Cavanagh et al., 1992; Charles and Lathers, 1994; Hargens, 1994; Convertino, 1996b). The American and Russian space programs use different strategies. Some examples of countermeasures that had been developed as of 2000 include subcutaneous injections of erythropoietin to prevent decreases in erythrocyte mass and vigorous in-flight exercise regimens to reduce loss of bone mineral density. So far, countermeasures appear to be largely ineffective, but the data are sparse (Bungo et al., 1985; Buckey et al., 1996b; Convertino et al., 1997; Lane and Schoeller, 2000).
NASA’s general approach to the development of countermeasures was presented to the IOM committee at the Johnson Space Center (Paloski, 2000; Sawin, 2000). The rationale (Figure 2–1) outlined a number of steps that have been incorporated into NASA’s Countermeasure Evaluation and Validation Project, which can be summarized as follows:
conduct research to understand the basic nature of the physiological problem,