component was to demonstrate that humans could live in space for relatively long durations while conducting a variety of tasks, including performance of extravehicular activities, and return to Earth without physiological deficits in performance capability. The outcomes of these missions revealed several critical findings, including the following: (1) there were clear-cut deficiencies in cardiovascular functions when exercise was performed by the astronauts at the same absolute intensity before and after the mission; (2) losses in muscle mass and strength as well as in bone density were documented; and (3) loss of movement fidelity, including post-flight balance instabilities, was observed (in the absence of vision abnormalities). It is now known that these alterations were hallmarks of prolonged exposure to the spaceflight environment. Even though the sample size was small, the observations were sufficiently robust to be reported in peer-reviewed scientific journals. Overall, the Skylab program laid the foundation for future microgravity studies of greater breadth and depth.

Dedicated Life Sciences Missions on the Shuttle

In 1978, NASA released a formal research announcement calling for research proposals for the laboratory facility of the space shuttle-based space transport system (STS). The unique feature of this announcement was that the research was to be rigidly peer-reviewed by two different panels: one focusing on scientific merit and the other examining the feasibility of conducting the study with the available facility and equipment infrastructure of the STS laboratory. A similar review model was used in the physical sciences. The first life sciences mission was set for the mid-1980s but was postponed until 1991 following the explosion of the space shuttle Challenger, after which shuttle missions were suspended until NASA deemed that it was safe to continue the shuttle program.

The access to space afforded by the STS missions provided for a broad portfolio of life sciences experimentation aimed at assessing the effects of microgravity and spaceflight on biological responses. Of particular note, however, are the three dedicated Space Life Sciences (SLS) missions that were flown in the decade of the 1990s: SLS-1 in 1991 lasted 9 days; SLS-2 in 1993 lasted 14 days; and Neurolab, a dedicated mission for the neurosciences in 1998, lasted 16 days. Within these three missions was a wide scope of experiments, ranging from plant and cell biology studies to complementary human and animal projects. The human studies were enhanced by NASA astronauts and payload scientists, who not only conducted the research as surrogate investigators but also served as the subjects for the composite human science package.

The research topics investigated across the three missions were broad, covering all the physiological systems discussed in this report. A unique feature of these missions was that laboratories for the animal studies were established at both the launch and the landing sites so that ground-based analyses could be conducted in close proximity to take-off and, especially, at landing to minimize physiological alterations occurring during the recovery period. In Neurolab, as with SLS-2, animal subjects were studied in detail during actual spaceflight. Some of the animal specimens were acquired during flight and then compared to animal samples obtained following landing. This was a major accomplishment because all animal studies prior to SLS-2 were performed during varying time intervals after landing, making it impossible to separate in the various experiments the effects of landing from the effects of the spaceflight environment itself.

An additional unique feature of the SLS and Neurolab missions was the synergy established within the international community of investigators and agencies. For example, in the Neurolab mission all of the ground-based research prior to flight was funded by several institutes within the National Institutes of Health, especially the National Institute of Neurological Disorders and Stroke. Investigators from the Japanese Aerospace Exploration Agency and the European Space Agency were also involved.

Although the life sciences program had its roots in issues of crew health and safety, fundamental biology also grew to be a substantive part of the program, particularly in the area of plant biology. For example, research on the effects of loss of convection on root zone hypoxia showed the impact of spaceflight on plant metabolism, and comprehensive gene expression studies revealed genome-wide effects of spaceflight on gene expression patterns.

In many ways, the dedicated flight program in space life sciences served as an important model of how flight-based research can be integrated across (1) project science disciplines, (2) national and international space research programs, and (3) national and international funding agencies. There have been repeated calls within the life sciences community to recapture the synergy that was present in the Neurolab mission.

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