§ 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 :
In brief, at 1.0 RPM even highly susceptible subjects were symptom-free, or nearly so. At 3.0 RPM subjects experienced symptoms but were not significantly handicapped. At 5.4 RPM, only subjects with low susceptibility performed well and by the second day were almost free from symptoms. At 10 RPM, however, adaptation presented a challenging but interesting problem. Even pilots without a history of air sickness did not fully adapt in a period of twelve days.
"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.)
The full text of two of D. Hall's papers, a 1997 paper and the abovementioned 1995 SSI paper, can be found here and here.
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.
- fluid redistribution: Bodily fluids shift from the lower extremities toward the head. This precipitates many of the problems described below [1, 2].
- fluid loss: The brain interprets the increase of fluid in the cephalic area as an increase in total fluid volume. In response, it activates excretory mechanisms. This compounds calcium loss and bone demineralization. Blood volume may decrease by 10 percent, which contributes to cardiovascular deconditioning. Space crew members must beware of dehydration [1, 3].
- electrolyte imbalances: Changes in fluid distribution lead to imbalances in potassium and sodium and disturb the autonomic regulatory system [2, 3].
- cardiovascular changes: An increase of fluid in the thoracic area leads initially to increases in left ventricular volume and cardiac output. As the body seeks a new equilibrium, fluid is excreted, the left ventricle shrinks and cardiac output decreases. Upon return to gravity, fluid is pulled back into the lower extremities and cardiac output falls to subnormal levels. It may take several weeks for fluid volume, peripheral resistance, cardiac size and cardiac output to return to normal [3, 4, 5].
- red blood cell loss: Blood samples taken before and after American and Soviet flights have indicated a loss of as much as 0.5 liters of red blood cells. Scientists are investigating the possibility that weightlessness causes a change in splenic function that results in premature destruction of red blood cells. In animal studies there is some evidence of loss through microhemorrhages in muscle tissue as well [5, 6].
- muscle damage: Muscles atrophy from lack of use. Contractile proteins are lost and tissue shrinks. Muscle loss may be accompanied by a change in muscle type: rats exposed to weightlessness show an increase in the amount of "fast-twitch" white fiber relative to the bulkier "slow-twitch" red fiber. In 1987, rats exposed to 12.5 days of weightlessness showed a loss of 40 percent of their muscle mass and "serious damage" in 4 to 7 percent of their muscle fibers. The affected fibers were swollen and had been invaded by white blood cells. Blood vessels had broken and red blood cells had entered the muscle. Half the muscles had damaged nerve endings. The damage may have resulted from factors other than simple disuse, in particular: stress, poor nutrition, and reduced circulation -- all of which are compounded by weightlessness; and radiation exposure -- which is independent of weightlessness. There is concern that damaged blood supply to muscle may adversely affect the blood supply to bone as well [4, 5, 6, 7].
- bone damage: Bone tissue is deposited where needed and resorbed where not needed. This process is regulated by the piezoelectric behavior of bone tissue under stress. Because the mechanical demands on bones are greatly reduced in micro gravity, they essentially dissolve. While cortical bone may regenerate, loss of trabecular bone may be irreversible. Diet and exercise have been only partially effective in reducing the damage. Short periods of high-load strength training may be more effective than long endurance exercise on the treadmills and bicycles commonly used in orbit. Evidence suggests that the loss occurs primarily in the weight-bearing bones of the legs and spine. Non-weight-bearing bones, such as the skull and fingers, do not seem to be affected [1,2,3,4,5,6,8,9,10,11,12,13, 14,15].
- hypercalcemia: Fluid loss and bone demineralization conspire to increase the concentration of calcium in the blood, with a consequent increase in the risk of developing urinary stones [1, 4].
- immune system changes: There is an increase in neutrophil concentration, decreases in eosinophils, monocytes and B-cells, a rise in steroid hormones and damage to T-cells. In 1983 aboard Spacelab I, when human lymphocyte cultures were exposed in vitro to concanavalin A, the T-cells were activated at only 3 percent of the rate of similarly treated cultures on Earth. Loss of T-cell function may hamper the body's resistance to cancer -- a danger exacerbated by the high-radiation environment of space [1, 3, 5, 16].
- interference with medical procedures: Fluid redistribution affects the way drugs are taken up by the body, with important consequences for space pharmacology. Bacterial cell membranes become thicker and less permeable, reducing the effectiveness of antibiotics. Space surgery will also be greatly affected: organs will drift, blood will not pool, and transfusions will require mechanical assistance [1, 3, 17].
- vertigo and spatial disorientation: Without a stable gravitational reference, crew members experience arbitrary and unexpected changes in their sense of verticality. Rooms that are thoroughly familiar when viewed in one orientation may become unfamiliar when viewed from a different up-down reference. Skylab astronaut Ed Gibson reported a sharp transition in the familiarity of the wardroom when rotated approximately 45 degrees from the "normal" vertical attitude in which he had trained. There is evidence that, in adapting to weightlessness, the brain comes to rely more on visual cues and less on other senses of motion or position. In orbit, Skylab astronauts lost the sense of where objects were located relative to their bodies when they could not actually see the objects. After returning home, one of them fell down in his own house when the lights went out unexpectedly [4, 18].
- space adaptation syndrome: About half of all astronauts and cosmonauts are afflicted. Symptoms include nausea, vomiting, anorexia, headache, malaise, drowsiness, lethargy, pallor and sweating. Susceptibility to Earth-bound motion sickness does not correlate with susceptibility to space sickness. The sickness usually subsides in 1 to 3 days [4,5].
- loss of exercise capacity: This may be due to decreased motivation as well as physiological changes. Cosmonaut Valeriy Ryumin wrote in his memoirs: "On the ground, [exercise] was a pleasure, but [in space] we had to force ourselves to do it. Besides being simple hard work, it was also boring and monotonous." Weightlessness also makes it clumsy: equipment such as treadmills, bicycles and rowing machines must be festooned with restraints. Perspiration doesn't drip but simply accumulates. Skylab astronauts described disgusting pools of sweat half an inch deep sloshing around on their breastbones. Clothing becomes saturated [1, 19].
- degraded sense of smell and taste: The increase of fluids in the head causes stuffiness similar to a head cold. Foods take on an aura of sameness and there is a craving for spices and strong flavorings such as horseradish, mustard and taco sauce [1, 4].
- weight loss: Fluid loss, lack of exercise and diminished appetite result in weight loss. Space travelers tend not to eat enough. Meals and exercise must be planned to prevent excessive loss [1, 19].
- flatulence: Digestive gas cannot "rise" toward the mouth and is more likely to pass through the other end of the digestive tract -- in the words of Skylab crewman-doctor Joe Kerwin: "very effectively with great volume and frequency" .
- facial distortion: The face becomes puffy and expressions become difficult to read, especially when viewed sideways or upside down. Voice pitch and tone are affected and speech becomes more nasal .
- changes in posture and stature: The neutral body posture approaches the fetal position. The spine tends to lengthen. Each of the Skylab astronauts gained an inch or more of height, which adversely affected the fit of their space suits [1, 20].
- changes in coordination: Earth-normal coordination unconsciously compensates for self-weight. In weightlessness, the muscular effort required to reach for and grab an object is reduced. Hence, there is a tendency to reach too "high" .
"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."
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