§ 6.13 Artificial Gravity and the Effects of Zero Gravity on Humans
Zero gravity has many effects on the human body, some of which lead to significant health concerns. It is clear that it would be much healthier for crews to provide artificial gravity for long duration space habitation. This means rotating the habitat to produce artificial gravity by the centrifugal (centripetal) force.
Deleterious effects of zero gravity on astronauts to date are well documented. Because this is a long topic, and a topic of frequent inquiry, we have started a separate page - the PERMANENT page on the adverse effects of weightlessness.
One issue regarding space settlements rotating for artificial gravity is the beginning of the "comfort zone" as regards the radius of the rotating structure. For example, if we want to connect two fuel tanks by a cable and rotate them to produce artificial gravity as strong as Earth's gravity, how far should we put them apart?
Some people ask how much artificial gravity we need in order to stay healthy and live in space for the rest of our lives. We could assume Earth-normal gravity and design accordingly, but less might be found to be acceptable. There's literature on this but it's not covered here yet.
For very small habitats, rotating them to produce artificial gravity results in some very noticible differences with real gravity due to the coriolis effect. When you drop an object, it does not fall straight now, but falls by a curve (according to the perspective of the person inside the rotating habitat). Likewise for objects bouncing up. When you stand up, your upper body will find itself significantly leaned over if you are in a small habitat rotating fast. For larger habitats, these effects are diluted to where they are humanly unnoticable. If we want artificial gravity in spacecraft or small habitats (including industrial ones) and strive for a most economical design, then we need to understand the significance of rotation on humans. The analogy to the comfort of sailors on ships at sea is appropriate. Large, steel hulled ships are more comfortable than small, fiberglass hulled ships.
Based on experiments on people in centrifuges and slow rotation rooms, it appears that the minimum radius for an artificial gravity habitat is about 20 meters (i.e., diameter 40 meters). This is not very long. Secondly, the maximum rotation rate appears to be around 4 revolutions per minute.
If a gravity of about one third Earth's is permissable, then a short radius habitat may be comfortable.
The main reason for lowering radius would be simply economics in an early space habitat in that lower radius means less material needed, including designs for stress. However, in a scenario using asteroidal or lunar material whereby the costs of material in orbit is much lower, we will probably opt for larger habitats and perhaps even Earth-normal gravity.
There are numerous technical designs for small spacecraft with artificial gravity, e.g., for missions to Mars. Space stations in low Earth orbit to date have not used artificial gravity for several reasons: so that they could be smaller and cheaper; many of the experiments to be conducted by the station were in microgravity (where gravity is undesirable), and docking systems are simpler when the station is not rotating.
For connecting spent fuel tanks to produce a space station situated in orbit, we can just put a long cable between them and rotate the structure.
People in space will start to move away from an entirely "up vs. down" sense of reference, and start to integrate the circular elements into their frame of reference as opposed to rectangular elements on Earth.
A few recent papers at the SSI/AIAA Princeton conferences on space habitats using artificial gravity, by architect Theodore Hall, are worth reading. His first two papers in up through 1993 (ref.) analyze the physics and the human comfort zone of artificial gravity (basing the latter on references to experiments by previous researchers). Dr. Hall's third conference paper in 1995 (ref.) applies the factors of the circular living environment to the architectural design of habitats in orbital space, with emphasis on psychology, perceptions and ergonomics.
Dr. Hall has prepared the following synopsis for PERMANENT:
Artificial gravity and the comfort zone
"Much of the research into the human factors of rotating habitats is twenty or thirty years old. Since the 1960s, several authors have published guidelines for comfort in artificial gravity, including graphs of the hypothetical "comfort zone". The zone is bounded by values of acceleration, head-to-foot acceleration gradient, rotation rate, and tangential velocity. Individually, these graphs depict the comfort boundaries as precise mathematical functions. Only when studied collectively do they reveal the uncertainties [23, 25, 26, 27, 28, 29].
"With regard to the rotation rate, perhaps the most enlightening commentary on human adaptation was published by Graybiel in 1977 :
"The comfort graphs described above are succinct summaries of abstract mathematical relationships, but they do nothing to convey the look and feel of artificial gravity. Consequently, there has been a tendency in many design concepts to treat any point within the comfort zone as "essentially terrestrial", although that has not been the criterion for defining the zone. The defining criterion has been "mitigation of symptoms", and authors differ as to the boundary values that satisfy it. This suggests that the comfort boundaries are fuzzier than the individual studies imply. Comfort may be influenced by task requirements and environmental design considerations beyond the basic rotational parameters.
"Perhaps a more intuitive way to compare artificial-gravity environments with each other as well as with Earth is to observe the behavior of free-falling objects. Figure 1 shows, for Earth-normal gravity, the trajectory of a ball when launched from the floor with an initial velocity of 2 meters per second, and when dropped from an initial height of 2 meters. Of course, both trajectories are straight up and down. The "hop" reaches a maximum height of 0.204 meters, indicated by a short horizontal line. The "drop" is marked by dots at 0.1-second intervals.
The figure left shows Earth-normal gravity -- the ball goes straight up and down.
In an artificial gravity system, the ball trajectory is not straight up and down, but curves relative to the observer. The larger the habitat, or the longer the cable in a tethered habitat, the less curve there is.
Hence, in the second figure, the five "hop and drop" diagrams correspond to five different sizes of habitat and rates of rotation, corresponding to a typical comfort chart for artificial gravity, after that of Hill and Schnitzer - one for each boundary point of the comfort zone. The twisting of the free-fall trajectories in artificial gravity reveals the distortion of the gravity itself.
"Evidently, the comfort zone encompasses a wide range of environments, many of them substantially non-terrestrial. Conformance to the comfort zone does not guarantee an Earth-normal gravity environment, nor does it sanction "essentially terrestrial" design. "
(The above quoted text and figures on the preceding page are copyright © 1997 by Dr. Theodore W. Hall. Interspersed, unquoted material was written by Mark Prado.)
How much artificial gravity do we need?
Many researchers think that one-third Earth-normal gravity is sufficient to prevent practically all the significant biological changes associated with zero gravity. However, we don't know for sure because we haven't put humans into artificial gravity situations and studied the effects. What we do know for sure is that artificial gravity prevents physiological changes associated with zero gravity.
Humans adapt very well to space. However, there's a lot we don't know about the long term effects of weightlessness on humans. We can, however, eliminate that concern entirely by using artificial gravity with rotating space habitats.
Adverse effects of weightlessness
The entire following text is extracted from a paper by Dr. Theodore W. Hall entitled "Artificial Gravity and the Architecture of Orbital Habitats", and is Copyright © 1997 by Theodore W. Hall, All Rights Reserved. Reprinted by PERMANENT with permission.
"It is ironic that, having gone to great expense to escape Earth gravity, it may be necessary to incur the additional expense of simulating gravity in orbit. Before opting for artificial gravity, it is worth reviewing the consequences of long-term exposure to weightlessness.
"Many of these changes do not pose problems as long as the crew remains in a weightless environment. Trouble ensues upon the return to life with gravity. The rapid deceleration during reentry is especially stressful as the apparent gravity grows from zero to more than one "g" in a matter of minutes. In 1984, after a 237-day mission, Soviet cosmonauts felt that if they had stayed in space much longer they might not have survived reentry . In 1987, in the later stages of his 326-day mission, Yuri Romanenko was highly fatigued, both physically and mentally. His work day was reduced to 4.5 hours while his sleep period was extended to 9 hours and daily exercise on a bicycle and treadmill consumed 2.5 hours. At the end of the mission, the Soviets implemented the unusual procedure of sending up a "safety pilot" to escort Romanenko back to Earth .
"Soviet cosmonauts Vladimir Titov and Moussa Manarov broke the one-year barrier when they completed a 366-day mission on 21 December 1988. Subsequent Russian missions have surpassed that. These long-duration space flights are extraordinary. They are milestones of human endurance. They are not models for space commercialization."