Artificial gravity will be considered as an alternative to currently inadequate countermeasures for long-duration space flight, of the type required to explore Mars.

If we are to commit to a spinning vehicle, the physiological questions get down to two types-one for a full-time rotating habitat and one for intermittent stimulation-a kind of gravity gym.

For full-time rotation, the first question is what level of acceleration at foot level is the minimum required to maintain normal function? Various means of achieving the artificial gravity will be considered, but we initially just concentrate on the size and speed needed to give sufficient centripetal acceleration. We can be reasonably certain that 1 g will suffice, but is it needed? Will a half-g do? Or can we avoid deconditioning by spinning continuously at a level of 0.38 gs to match the Martian gravity?

We are likely to achieve the earliest answers with experiments on animals. We already know that rats which have been centrifuged during their space flight don't show the major deterioration in bone, muscle, and cardiovascular response seen by their free-floating brethren-but that is only at 1 g. The current International Space Station plans include a module for a centrifuge to carry up to eight modules for rodents, fish, and eggs but will not accommodate the primates which many feel are needed to adequately model human responses.

Intermittent artificial gravity stimulation, on the other hand, presents a number of potential advantages. As part of our normal circadian rhythm, the very g-dependent processes which result in fluid loss and bone deconditioning are probably turned off during our normal sleeping hours. On the other hand, extended periods of bed rest produce effects on the skeleton, muscles, and cardiovascular system which are similar to those occurring in space. This simulation of space flight is made more accurate if the bed rest is conducted with a six-degree head-down tilt to accelerate the shift of fluid toward the head and if the subject lies partially immersed, although dry, in a high-tech waterbed.

A combination of short-duration and long-term studies in ground centrifuges and rotating rooms can be useful in answering many of the key questions concerning the application period, frequency, and intensity of centripetal acceleration. They will also be useful in determining the seriousness of the problem of dual adaptation-to the rotating and non-rotating environments.

Importantly, they may shed light on the physiological importance of a gravity gradient across the body if one is to truly proceed with rotators having a radius comparable to the subject's height. These ground gravitational physiology studies are essential for effective use and interpretation of the space variable-gravity centrifugation tests to be mentioned.

Even if the main physiological issues are adequately addressed by a rotating space vehicle, important human factors issues remain. The specific constraints of artificial gravity remain at the crux of the research in terms of a practical solution to interplanetary travel.

If we are right about the ability of astronauts to adapt and maintain the adaptation, it may be possible to achieve a measure of protection from an intermittent artificial g exposure with a radius as low as 4 m, rotating at 10 rpm to produce about half a g at foot level.

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