A study of the manned space flights and laboratory observations to date
suggests that during long periods of weightlessness, some physiological
difficulties may arise which may produce serious effects on human
performance. Although recent experience gives no grounds for expecting
insuperable difficulties, neither the quantity nor quality of the
available observations permits the conclusion that long-term exposure to
weightlessness will _not_ have serious consequences. The critical role
to be played by the astronaut demands that every effort be made to
identify in advance those phenomena which may affect performance, and to
study their qualitative and quantitative relationships so that proper
precautions can be taken.
Lawton ([ref.197]), in reviewing the literature on prolonged
weightlessness, found few instances in which physiological function was
truly gravity dependent. He stated that the physiological systems likely
to be most affected by weightlessness were the musculoskeletal system,
the cardiovascular system, and the equilibrium senses. Subsequent
experience proved this to be the case. McCally and Lawton ([ref.198])
analyzed the data from experiments since 1961 and concluded that much
more basic laboratory work is necessary. Studies using immobilization,
immersion, and cabin-confinement techniques were recommended approaches
toward simulating weightlessness.
Much of the difficulty in obtaining precise information of anticipated
problems arises from a lack of knowledge of normal mammalian physiology.
Many of these deficiencies can be remedied in the laboratory. In
space-flight development, however, two distinct investigational
approaches can be adopted. The first of these may be characterized as
empirical and incremental; that is, the capabilities of the astronaut
are explored in successive flights involving relatively modest increases
in difficulty or severity of the environmental conditions. In this way
it is hoped to ascertain the human limitations without running too great
a risk. The second approach can be described as fundamental: determining
by a series of controlled experiments the effects of exposure to
space-flight conditions upon comparative mammalian physiology, with
emphasis on man. A fundamental understanding of the observed effects
would be sought so that predictions for new situations and possible ways
to control them could be made with confidence.