How does the absence of gravity affect the human organism? The experience during free fall shows that a state of weightlessness lasting only a short time is not dangerous to one's health. Whether this would be true in the case of long-lasting weightlessness, however, cannot be predicted with certainty because this condition has not been experienced by anyone. Nevertheless, it may be assumed with a high probability, at least in a physiological sense, because all bodily functions occur through muscular or osmotic forces not requiring the help of gravity. Actually, all vital processes of the body have been shown to be completely independent of the orientation of the body and function just as well in a standing, a prone, or any other position of the body.
Only during very long periods in a weightless state could some injury be experienced, perhaps by the fact that important muscle groups would atrophy due to continual lack of use and, therefore, fail in their function when life is again operating under normal gravitational conditions (e.g, following the return to Earth). However, it is probable that these effects could be counteracted successfully by systematic muscular exercises; besides, it might be possible to make allowance for these conditions by means of appropriate technical precautions, as we will see later.
Apparently, the only organ affected by the absence of gravity is the organ of equilibrium in the inner ear. However, it will no longer required in the same sense as usual, because the concept of equilibrium after all ceases to exist in the weightless state. In every position of the body, we have then the same feeling: "up" and "down" lose their usual meaning (related to the environment); floor, ceiling and walls of a room are no longer different from one another.
However, in the beginning at least, the impression of this entirely unusual condition may cause a strongly negative psychological effect. Added to this is the effect that is directly exercised on the nervous system by the weightless state. The most important sensations related to this effect are as follows: the previously discussed effect on the organ of equilibrium, cessation of the perception of a supporting pressure against the body, and certain changes in the feelings in the muscles and joints.
However, this complex of feelings is known to us so far only from the free fall state because, as already discussed, we can experience freedom from gravity under terrestrial conditions only during falling; involuntarily, we will, therefore, feel anxiety related to the falling, as well as other psychological states aroused by this unusual situation during a cessation of the feeling of gravity, when the lack of gravity is not even caused by falling, but in another way (such as, in the space station by the effect of centrifugal force).
In any event, it can be expected based on previous experiences (pilots, ski jumpers, etc.) that it will be possible through adaptation to be able easily to tolerate the weightless state even in a psychological sense. Adapting occurs that much sooner, the more one is familiar with the fact that "weightless" and "falling" need not be related to one another. It can even be assumed that anxiety is altogether absent during a gradual release from the feeling of gravity.
Oberth has addressed all of these issues in depth. By evaluating his results, they can be summarized as follows: while weightlessness could certainly be tolerated over a long time, although perhaps not indefinitely, without significant harm in a physical sense, this cannot be stated with certainty in a psychological sense, but can be assumed as probable none the less. The course of the psychological impressions apparently would more or less be the following: in the beginning at least during a rapid, abrupt occurrence of the absence of gravity anxiety; the brain and senses are functioning extremely intensively, all thoughts are strongly factual and are quickly comprehended with a penetrating logic; time appears to move more slowly; and a unique insensitivity to pains and feelings of displeasure appear. Later, these phenomena subside, and only a certain feeling of elevated vitality and physical fitness remain, perhaps similar to that experienced after taking a stimulant; until finally after a longer period of adaptation, the psychological state possibly becomes entirely normal.