The only substitute for gravity is acceleration. Anyone who has ever been accelerated has experienced artificial gravity. Probably the first to experience it in significant strength and duration were fighter pilots. In 1917, the U.S. Army Air Service organized a Medical Research Board, "to investigate all conditions which affect the efficiency of pilots" . The chief concern in 1917 was the effect of altitude and reduced barometric pressure, but the aerobatic biplane pilots of World War I certainly experienced variable gravity as well.
The concept of artificial gravity as such - the concept of deliberate acceleration for the sole purpose of replacing natural gravity in space flight - dates back at least as far as Tsiolkovsky's early work in the 1890's. Nevertheless, serious research into the human factors of artificial gravity did not begin until the space race of the 1950's.
The only acceleration that can be sustained without continuous energy input is centripetal acceleration - that is, rotation. Consequently, rotation is the focus of virtually all artificial-gravity research. Research into hyper gravity is typically more concerned with quantity (number of g's) than quality (centripetal versus linear), but even there the method employed is often centrifugation.
Artificial-gravity research has been conducted primarily on the ground, in centrifuges and slow-rotation rooms. For studying motion sickness, researchers use cross-coupled rotation and simulated rotation to provoke visual-vestibular conflicts. The experience of pilots "pulling g's" in high-performance aircraft has also been both source and incentive for artificial-gravity research. No human has ever experienced artificial gravity in its pure rotational form, free of Earth's normal 1-g influence. Rats have - in a centrifuge on a Soviet satellite in 1977. A space-based centrifuge large enough for human experimentation remains an indefinite goal.
Early space station designers relied on intuition and educated guesswork to specify the rotational parameters for artificial gravity. Different priorities - different choices of dependent and independent variables - led to different solutions. In 1916, Tsiolkovsky predicted that a small radius and a large angular velocity would produce "a very interesting effect", which he preferred to avoid. He suggested a radius of 50 meters and a maximum angular velocity of 2 rotations per minute, even though this would yield less than 1/4 g. In 1952, von Braun was concerned with minimizing structural mass, and proposed that "for a number of reasons, it may be advantageous not to produce one full g." He suggested 1/3 g at a radius of 125 feet, requiring slightly less than 3 rotations per minute. In 1960, Kramer and Byers of the Lockheed Aircraft Corporation were most concerned with the ratio of Coriolis to centripetal acceleration. For a given radius, the way to minimize this ratio is to maximize the angular velocity. They suggested more than 7.5 rotations per minute at a radius of 49 feet to yield 1 g. Fred A. Payne of North American Aviation assumed that the head-to-foot gravity gradient should not exceed 15 percent. He concluded that the radius should be at least 40 feet, but refrained from any suggestion regarding rotation rate or gravity level.
In 1960, Carl C. Clark and James D. Hardy, of the Navy's Aviation Medical Acceleration Laboratory (US Naval Air Development Center, Johnsville, Pennsylvania), published "Gravity Problems in Manned Space Stations" in the Proceedings of the Manned Space Stations Symposium . Like Kramer and Byers, they were concerned about "Coriolis acceleration effects", but Clark and Hardy concentrated on the problems of cross-coupled rotations. Clark subjected himself to 24 hours of 2-g acceleration on a centrifuge rotating at 1 radian/second (9.55 rotations per minute), and noted that "illusions of body and visual field angular motions are generated which are approximately specified in magnitude and direction by the vector product of the angular velocity of the rotating system and the angular velocity of the head." The threshold for the onset of these illusions was 0.06 radians2/second2; the threshold of nausea was 0.6 radians2/second2 . Normal head rotations may occur at up to 5 radians/second. Therefore, Clark and Hardy concluded, the angular velocity of a rotating space station should not exceed 0.01 radians/second (that is, 0.06 ÷ 5), or about 0.1 rotations per minute. This is much less than the 7.5 rotations per minute proposed by Kramer and Byers. Clark and Hardy proposed to stay completely below the threshold of illusions, although the threshold of nausea was ten times higher. They acknowledged that this might not be practical: at 0.01 radians/second, a 1-g station would need a radius of 320,000 feet. Nevertheless, they felt that higher angular velocities would require special crew preparations:
It is appropriate to train the astronauts to recognize these angular illusions and by training reduce the consequences of the possible disorientation. These sensations might be reduced or eliminated by surgical operation on the labyrinth, a procedure we do not recommend for humans.
In addition to training (and instead of surgery), the authors suggested a variety of innovations to minimize head movement:
The head might be supported in a frame with wheels to the floor, allowing motions but with angular rate dampers. Servo prism glasses and wall mirrors might prove helpful to minimize head motions. The space crew would probably be most comfortable by staying in one place, with necessary equipment automatically brought to them as desired.
They concluded that more human experience in weightlessness was needed in order to determine the requirement - if any - for artificial gravity.
They ended with a caveat: their recommendation of 0.1 rpm for the space station was extrapolated from the observations of a single individual in a ground-based centrifuge rotating at nearly 10 rpm - a hundred times their own recommended rate. They assumed that the thresholds of angular illusions and nausea were directly proportional to the vector product of the angular velocities of the centrifuge and the head, but they did not test this hypothesis with other subjects or other centrifuge velocities.
At about this same time (1960), Ashton Graybiel and colleagues began their long series of experiments in the 15-foot-diameter "slow rotation room" at the Naval Aerospace Medical Research Laboratory (Pensacola, Florida). Graybiel summarized their findings in a paper for the 1975 Princeton Conference . Slow rotation rooms offer several advantages over typical centrifuges, particularly in their habitability and in the freedom of movement they afford. Subjects have remained in the rotating room for over a month - not strapped into a seat, but moving about, eating, sleeping, and performing various tasks. Graybiel confirmed that, in general, higher rotation rates resulted in more severe symptoms and slower adaptation, though individuals varied in their ability to adapt.
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.
At 10 rpm, subjects had to restrict their head movements for several days to avoid severe nausea. Even after the nausea subsided, drowsiness and fatigue persisted throughout the twelve days. Biochemical measurements revealed an increase in the plasma level of the enzyme lactic dehydrogenase.
The experimenters also looked for direction-specific adaptation effects. There was concern that such effects might pose a serious problem when making a sudden transition between rotating and non-rotating sections of a space station. In one series of experiments, the rotation of the room was increased over a period of about 40 minutes from 0 to 6 rpm counterclockwise in 1-rpm increments. Subjects were asked to execute a number of standard head movements at each increment. At the end of this "initial adaptation schedule", the room was de-rotated in one step. Four variations of the experiment were used to assess the adaptation effects - the subjects were asked to repeat the same set of head movements: either at 0 rpm, or while stepping up to 6 rpm clockwise; either immediately, or after a delay of 1 to 24 hours. Graybiel reported that "acquisition of long term adaptation effects in a room rotating counterclockwise is associated not only with the acquisition of long-term adaptation to clockwise rotation, but is also associated with no loss of adaptation to the stationary environment. There is, however, one 'complication' that must be taken into account, namely, that short term adaptation effects are also acquired and these are direction-specific." The direction-specific adaptation effects decay exponentially, and vanish within a few hours.
Perhaps the biggest problem with ground-based rotating room experiments is that the artificial gravity does not replace Earth gravity, but is merely added to it. The axis of rotation is vertical, and the subjects generally remain vertical as well. In a space station, free of Earth's influence, the axis of rotation would be horizontal relative to the crew, and the cylindrical surface would be the floor rather than the wall. Thus, head motions must be measured relative to the room's rotation axis rather than the subject's body axis. To explore the significance of axis orientation, Graybiel and colleagues conducted an experiment in which the subjects were supported horizontally, in custom-fitted articulated fiberglass molds riding on air bearings. This allowed the subjects to walk along the cylindrical "wall" (now the "floor") while the room was rotated at 4 rpm. Some subjects began the experiment in the horizontal mode and switched to vertical in the middle of the experiment; others began in the vertical mode and switched to horizontal. Graybiel reported that susceptibility to motion sickness was not only similar in the two modes, but also appeared to be "transferred" between them. However, he noted a distinction between motion sickness and ataxia (postural disequilibrium): adaptation acquired in the horizontal mode provided freedom from motion sickness but did not prevent ataxia on change to the vertical mode. "If these findings are substantial, different mechanisms underlie the acquisition and restoration of postural equilibrium in a slow rotation room than motion sickness."
The rotating room experiments conducted by Graybiel and others have provided much of the habitability data adopted by subsequent artificial gravity engineering studies. A 1989 article in Final Frontier describes a layman's experience in the rotating room at the Ashton Graybiel Spatial Orientation Laboratory at Brandeis University (Waltham, Massachusetts) .
Similar experiments were conducted during the 1960's in the Rotating Space Station Simulator at the NASA Langley Research Center. The 40-foot-diameter simulator was rotated at angular velocities between 3 and 10.5 rpm, yielding centripetal accelerations between 0.05 and 0.75 g at the rim. A cable suspension system supported the test subjects horizontally, allowing them to walk around the curved surface of the simulator and to "climb" a ladder to a "higher" deck at a smaller radius. Walking in the direction of rotation at gravity levels between 0.167 and 0.3 g was judged to be the most comfortable. Above 0.3 g, subjects reported sensations of leg and body heaviness, and these became quite disturbing above 0.5 g. (These sensations may have been related to the large gravity gradient, which would be 30% for a 6-foot person at a 20-foot rotational radius.) This simulator was dismantled following the NASA agency-wide reduction in force of 1971 [77, 78].
In 1962, Paul R. Hill and Emanuel Schnitzer, of the Langley Research Center, published "Rotating Manned Space Stations" in the journal Astronautics . They included a chart that identified a comfort zone for artificial gravity with the following boundaries:
The minimum centripetal acceleration of 0.035 g was not explained; the maximum of 1 g was assumed to be "ample". The minimum rim speed was chosen to be large relative to a person's walking speed, so that walking would not change apparent weight by more than 15 percent; but a simple calculation shows that, with a rim speed of only 20 feet per second, one would have to walk very slowly - less than 1 mile per hour - to stay within the 15 percent limit . The maximum angular velocity of 4 rotations per minute was based on centrifuge experience and was intended to avoid vestibular disturbances - though it is 40 times the limit recommended by Clark and Hardy. (Hill and Schnitzer cited Clark and Hardy, but their comfort criteria seem closer to Graybiel.)
Astronautics was published monthly by the American Rocket Society and had a large circulation within the aerospace profession . Perhaps that explains the persistence of the comfort chart presented by Hill and Schnitzer: as recently as 1987, it was reproduced in several technical papers [82, 83, 84], virtually unchanged since 1962. That says much about the rate of progress in artificial-gravity research. Other comfort charts with other boundaries have been proposed in the succeeding thirty years (examples follow), but, for whatever reason, they have not been as influential or as widely circulated.
In 1968, in the proceedings of the "Manned Laboratories in Space" symposium, Robert R. Gilruth, of the NASA Manned Spacecraft Center, published a very different chart , with different comfort boundaries:
The lower boundary of 0.3 g was labeled "mobility limit". "Parabolic airplane flights have indicated that most of the problems of locomotion and fluid transfer are overcome by gravity as low as three-tenths g." The upper boundary of 0.9 g was labeled simply "upper gravity boundary", without further explanation. The upper boundary of 6 rotations per minute was labeled "canal sickness limit", referring to the disturbances occurring in the semicircular canals of the inner ear at higher rotation rates. Ground-based tests had indicated that "man can tolerate [up to 6 rotations per minute] without serious problems of adjustment."
In the same 1968 symposium proceedings, Theodore J. Gordon and Robert L. Gervais, of the McDonnell-Douglas Astronautics Company, presented yet another comfort chart :
The lower bound of 0.2 g "is required to provide adequate friction for locomotion." The minimum rim speed of 24 feet per second is intended to avoid excessive variations in apparent weight due to walking - similar to the limit presented by Hill and Schnitzer, but twenty percent faster. The upper limit of 6 rotations per minute is again attributed to "canal sickness".
In 1970, at the Fifth Symposium on the Role of the Vestibular Organs in Space Exploration, Ralph W. Stone, of the Langley Research Center, proposed substantially different - and more lenient - limits :
Stone described the characteristics of the gravity environment not in terms of machine dimensions, but rather in terms of human performance. Nevertheless, the limits he suggested for gravity gradient and walking weight-change are clearly less stringent than in earlier proposals (50%, versus 15%). Also, the last condition assumes slower head rotations than assumed by Clark and Hardy (3 radians/second, versus 5), and a much higher tolerance for cross-coupled rotations (2 radians2/second2, versus 0.6). The engineering implications of these criteria - as formulated above - are not immediately apparent. With some calculation, they can be restated as follows :
The relatively low limit on centripetal acceleration (2/3 g) is a consequence of the high tolerance for walking weight-gain (50%) and a desire never to exceed 1 g. Other limits emerge by satisfying several criteria simultaneously. For example, the greatest lower limit on tangential velocity (10.17 m/s) divided by the upper limit on angular velocity (2/3 rad/s) implies a radius of at least 15.25 meters.
The proper application of these criteria is subject to interpretation. For example: when climbing radially at a constant rate, the Coriolis acceleration remains constant, while the centripetal acceleration decreases linearly to zero at the center of rotation. Therefore, the ratio of Coriolis to centripetal acceleration must exceed any finite limit, unless a minimum radius is imposed on the radial motion - which would prohibit motion to the center.
In 1983, at a NASA conference on the Applications of Tethers in Space, D. Bryant Cramer of NASA Headquarters presented the following criteria :
Compared to previous guidelines: the minimum gravity for traction is less; the acceptable gravity gradient is less (6% over 6 feet, assuming 1 g at the floor); and the maximum angular velocity is less. Cramer's Coriolis limit is less than Stone's limit for climbing, but greater for lifting . The progression of guidelines from 1960 to 1980 or so seemed to follow a trend toward increasing tolerances; Cramer's guidelines represent a partial reversal of that trend.
An interesting additional feature of Cramer's comfort chart is a boundary for the structural capacity of a tether. This boundary limits the comfort zone to a structurally feasible region. Cramer indicated that a 10,000 pound cylindrical Kevlar tether could support a 100,000 pound module at 1 g at a radius of up to 20,000 feet, with an angular velocity as low as 0.38 rpm.
In 1987, the NASA Man-System Integration Standards  seemed to move the maximum angular velocity back up to 6 rotations per minute: "Most subjects, without prior experience, can tolerate rotation rates up to 6 rpm in any axis or combination of axes," but "rapidly become sick and disoriented above 6 rpm unless carefully prepared by a graduated program of exposure." The standards also describe human response to very high angular velocities: "Unconsciousness from circulatory effects alone occurs after 3 to 10 seconds in the pitch mode at 160 rpm with the center of rotation at the heart and at 180 rpm with the center of rotation at the iliac crest." Clearly, these criteria relate to raw survivability, not to habitability, performance, or comfort. The standards refrain from making any specific recommendation regarding the appropriate angular velocities, radii, or acceleration levels for artificial gravity. Nevertheless, they do offer the following:
The compartment orientation is chosen to minimize Coriolis accelerations. The workstation orientation is chosen to avoid cross-coupled head rotations. In the recommended arrangement, the crew member can scan a vertical array of displays with up-down head rotations without cross-coupling with the station rotation.
In recent years, two schools of thought have emerged regarding the best strategy for providing artificial gravity during long space missions. While some advocate continuous rotation in tethered systems with large radii and low angular velocities, others are investigating the potential of intermittent rotation in on-board centrifuges with small radii and high angular velocities.
Antonutto, Capelli, and di Prampero propose a system whereby astronauts work for their gravity - by riding bicycles around the inside of a cylindrical module. The bicycles - running in separate parallel tracks - would be individually counterbalanced and mechanically coupled through differential gearing to keep them moving at the same relative speed but in opposite directions .
In 1988, Peter Diamandis, of the Massachusetts Institute of Technology, described experiments with his "Artificial Gravity Sleeper" - affectionately known as the "Robocot" - at a conference on "Space Station Design and Development" . An article in Final Frontier the following year (1989) provided a layman's account . The device is a rotating bed with a water mattress and a semi-cylindrical plastic cover that serves as a windshield. The occupant lies on his back, with his head at the center of rotation. A lap belt secures him in place and a sleep mask blocks out visual cues of motion while the bed is spun up to 24 rotations per minute, producing 1 g of acceleration at the feet.
David Cardús, of the Baylor College of Medicine, is experimenting with an advanced version that can accommodate up to four test subjects at a time, lying head-to-head like spokes in a wheel, rotating up to 30 times per minute to provide 2 g at the feet. Each of the four beds can also provide up to 6° of head-down tilt. The rotating bed is being tested as both a prevention and a treatment for the deconditioning that results from long exposure to micro gravity or extended bed rest .
The advantage of an on-board centrifuge in lieu of rotating the entire habitat is that it would simplify the design of interplanetary spacecraft and space stations for long-duration habitation. (To avoid imparting a counter-rotation to a space vehicle, rotating bed systems would have to be installed in balanced counter-rotating pairs.) It would also be applicable to the low-gravity surface conditions on the Moon and Mars. The concern is over its efficacy in maintaining health, and its violation of the presumed comfort conditions for artificial gravity. With the head at the center of rotation, the gravity gradient is 100%. The angular velocity is far beyond what has generally been considered tolerable, and suggests that the occupant should remain motionless during the eight-hour sleep period to avoid severe vestibular disturbances.
Understanding the workings of the vestibular system is critical to planning for either micro gravity or artificial gravity. A series of experiments conducted in Spacelab I in 1983 explored the relationships between visual cues, vestibular cues, motor muscle reflexes, and motion sickness. In particular, researchers used nystagmus (rapid reflexive eye movement) in the test subject as an indicator of perceived rotation. In one experiment, a dome with rotating spots was placed over a crew member's head; eye movements indicated whether he perceived the rotation to be in the spots or in himself. In another experiment, hot or cold air (above or below body temperature) was blown into a crew member's ears, setting up convection currents in his semicircular canals; again, eye movements indicated the perceived rotation. This experiment verified a 1914 Nobel-prizewinning theory concerning the workings of the vestibular system. In sum, these experiments supported the "sensory conflict" theory of motion sickness. In micro gravity, as in artificial gravity, motion sickness seems to be triggered by head motion [96, 97, 98, 99].
Much of the research on artificial gravity and rotation would be moot if artificial gravity proved to be ineffective in maintaining space crew fitness. Experiments on the Soviet satellite Cosmos 936 in 1977 provided encouraging results. The life span of rats exposed to centrifugation during 18.5 days of space flight was significantly greater than that of non-centrifuged control animals. Centrifugation reduced hemolysis (red blood cell loss) and preserved bone minerals, structure, and mechanical properties . Experiments on Spacelab D-1 in 1985 discovered that T-cell function - which is severely hampered in micro gravity - is preserved in artificial gravity via centrifugation . Keller, Strauss, and Szpalski predict that artificial gravity of 1/6 g would be "sufficient to preserve bone strength above the fracture risk level" .