Chapter II
Physiology

You have to be careful what you invent, because it also invents you.

- Kerry Feldman

It's no longer a question of staying healthy.  It's a question of finding a sickness you like.

- Jackie Mason

The study of human response to gravity and acceleration has been concurrent with - but distinct from - the engineering studies described in Chapter 1.  Physiology, psychology, anthropometry, and ergonomics have always been important concerns in aerospace system design.  Engineers and mission planners attempt to accommodate the corporal needs of crew members, based on current knowledge of the human body and its response to novel circumstances.  Debriefing of crew members after actual missions, as well as deliberate experiments and simulations conducted both in space and on the ground, contribute to the knowledge base for the next iteration of design.  Astronauts have always been both experimenters and experiments.

The sole purpose of artificial gravity in spacecraft design is to maintain the health and effectiveness of the crew.  Nevertheless, the requirement for artificial gravity has always been uncertain.  In 1952, Dr. Heinz Haber, of the Air Force School of Aviation Medicine, wrote [1]:

Some believe that weightlessness will be entirely harmless; others have gone so far as to predict that man can survive only a few minutes without gravity.  This latter point of view, in the opinion of top experts, is almost certainly wrong.

Much has been learned in the past thirty years.  Top experts now know that man can survive for up to a year without gravity.  They also know that gravity deprivation causes progressive, debilitating, physiological changes, and that extraordinary countermeasures are required to prevent permanent injury.

Because provisions for artificial gravity add so much complexity to spacecraft design - with regard to mass distribution, structural strength, propulsion, spin-despin interfaces, ingress-egress, and so on - engineers and mission planners are reluctant to adopt it.  Furthermore, scientists are still uncertain whether artificial gravity would be an effective countermeasure, how much would be required, and whether it would bring its own undesirable side effects.  Therefore, gravity-related physiological research has taken two parallel courses: