Flight physicians were almost unanimous in expressing forebodings about the effect of weightlessness on man's physical and mental performance. Some feared that the body organs depended on sustained gravity and would not function if deprived of the customary gravitational force. Others worried over the combined effects of acceleration, weightlessness, and the heavy deceleration during atmospheric entry. Still other experts were concerned especially about perception and equilibrium. For example, Heinz Haber and Otto Gauer, another émigré German physician who joined the Air Force aeromedical program, noted that the brain receives signals on the position, direction, and support of the body from four mechanisms - pressure on the nerves and organs, muscle tone, posture, and the labyrinth of the inner ear. They theorized that these four mechanisms might give conflicting signals in the weightless state and that such disturbances "may deeply affect the autonomic nervous functions and ultimately produce a very severe sensation of succumbence associated with an absolute incapacity to act."10
The basic difficulty retarding the study of weightlessness was the impossibility of duplicating the exact condition on Earth. The X-15, considered by many in the mid-fifties to be the penultimate step to manned orbital flight, progressed slowly and would fly too late to shed much light on the problem of zero g for Project Mercury. By the fall of 1958, however, when the newly formed National  Aeronautics and Space Administration undertook to orbit a manned satellite, American aeromedical researchers had been studying the gravity-free condition intensively for some eight years.
The best but most expensive device for zero-g experimentation was the sounding rocket. For several years, beginning with the V-2 firings from White Sands, parachutes for nose cones containing rocket-launched animals invariably failed to open and the subjects were killed on impact. The first successful recovery came in September 1951, when an instrumented monkey and 11 mice survived an Aerobee launch to 236,000 feet from Holloman Air Force Base, New Mexico. The last of three Aerobee shots at Holloman, in May 1952, like the previous experiments, carried a camera on board to photograph two mice and two monkeys under acceleration, weightlessness, and deceleration. An Air Force aeromedical team, headed by James P. Henry, a physician who later would direct the Mercury animal program, and young Captain David G. Simons, found no adverse effects on the animals.11
For the next six years the priority military ballistic missile program almost monopolized rocket development in the United States. Medical experimentation employing live test subjects launched to high altitudes by rockets came to a virtual standstill. By contrast, during the same period from 1952 to 1957, researchers in the Soviet Union carried out numerous animal rocket flights, with dogs of the Pavlovian sort being their favorite passengers. By late 1957, when the Soviets sent the dog Laika into orbit aboard Sputnik II, the peak altitude of their vertical launches of animals was nearly 300 miles, and the Russian scientists had perfected a technique for catapulting animals from nose cones and recovering them with parachutes. Apparently the Russians also were able to measure a wider range of physiological reactions than their American counterparts.12
During the six-year hiatus in animal rocket experimentation in this country, investigators had to resort to the aircraft, "the oldest aeromedical laboratory," for studying the weightless phenomenon.13 In 1950, Fritz and Heinz Haber, of the Air Force School of Aviation Medicine, had considered various ways of simulating zero g for medical experiments. Discarding the free fall and the elevator ride, the Habers concluded that the best technique involved an airplane flight along a vertical parabola, or "Keplerian trajectory." If properly executed, such a maneuver could provide as much as 35 seconds of zero g and a somewhat longer period of subgravity, a condition wherein the body is under only partial gravitational stress.14 During the summer and fall of 1951 test pilots A. Scott Crossfield of NACA and Charles E. Yeager of the Air Force tried out the technique, flying a number of Keplerian trajectories in jet interceptors. Up to 20 seconds of weightlessness resulted from some of these flights. Crossfield reported initial "befuddlement" during zero g but no serious loss of muscle coordination, while Yeager described a sensation of falling and in one instance of spinning and feeling "lost in space." The latter sensation the physicians and psychologists called "disorientation."15
 The Habers' technique and these early experiments with it represented a promising beginning, but as one Air Force aeromedical specialist pointed out, "The results of these flights were inconclusive in many respects."16 An enormous amount of work remained before students of weightlessness could do much generalizing about this greatest anomaly of space flight.
In 1953 a small group comprising the Space Biology Branch of the Aeromedical Field Laboratory at Holloman Air Force Base inaugurated an ambitious program of parabolic flights to continue the investigations of weightless flight that had halted with the termination of the Aerobee animal launches in the spring of 1952. Supervised by Major David G. Simons, a physician who acted as test subject on many occasions, the Holloman studies for two years utilized T-33 and F-89 jet aircraft. Late in 1955, after Captain Grover J. D. Schock came to the field laboratory as task scientist, the standard tool for zero g research became the F-94C, which offered a longer parabola than other aircraft and thus a longer period of weightlessness. In the summer of 1958 the Air Force canceled all zero-g research at Holloman, and the coterie of scientists broke up. Colonel John P. Stapp, head of the field laboratory, and Simons went elsewhere, while Schock turned his attention to other research projects.17
For three years before the termination of the Holloman flight program, students of zero g at the School of Aviation Medicine had duplicated and even surpassed the investigations being carried out in New Mexico. Although sponsored by the Department of Space Medicine, the program carried out at Randolph Air Force Base was actually directed by Siegfried Gerathewohl, who was not a member of the department. Gerathewohl and his colleagues began their studies with the T-33 jet trainer, but like their counterparts in New Mexico, they soon turned to the superior F-94C. Major Herbert D. Stallings, a Randolph physician, estimated that by April 1958 he had flown more than 4,000 zero-g trajectories and compiled about 37 hours of weightless flight.18
Gerathewohl, Simons, Schock, and the other scientists at Randolph and Holloman tried to get as great a variety of information as possible during the 30 to 40 seconds of weightlessness and subgravity produced by the F-94C flights. They carried out numerous eye-hand coordination tests, for example, wherein a subject tried to make crosses in a pattern or hit a target with a metal stylus. Subjects usually missed their mark in the first moments of zero g or subgravity, but most of them improved their performance with their cumulative experience. The Air Force scientists also studied eating and drinking, bladder function, and disorientation after awakening during weightlessness; the functions at zero g of various animals, especially cats, whose vestibular organs had been removed; and the phenomenon called the "oculo-agravic illusion," wherein luminous objects seen in the dark appear to move upward during weightlessness.19
At the Wright Air Development Center, in Ohio, a team of researchers headed by Major Edward L. Brown picked up the experimental program discontinued at Holloman in mid-1958, except that they used the relatively slow, propeller-driven  C-131 transport in their studies. A parabola in a C-131 gave only 10 to 15 seconds of weightlessness, but the spacious interior of the cargo carrier made it possible to observe the reactions of several subjects simultaneously, including their coordination and locomotion and even their ability to walk along the ceiling while wearing shoes with magnetic soles.20
In general the aeromedical specialists at Randolph, Holloman, and Wright-Patterson - as well as those in more modest programs at the Navy School of Aviation Medicine, Pensacola, Florida, and at the NACA Lewis Flight Propulsion Laboratory in Cleveland - found that the principal problems of weightless flight seemed solvable. Eating and drinking at zero g were not troublesome when squeeze bottles and tubes were used, and urination presented no real difficulty. Some subjects suffered nausea, disorientation, loss of coordination, and other disturbances, but the majority reported that after they adjusted to the condition they found it "pleasant" and had a feeling of "well-being."21 As early as 1955, Simons concluded that weightlessness produced no abnormalities with regard to heart rate and arterial and venous blood pressure, while Henry, Simons' colleague in the Aerobee animal experiments, prophesied, "In the skilled pilot weightlessness will probably have very little significance."22 And in 1959, about a year after Project Mercury got underway, Gerathewohl remarked that "the majority of flying personnel enjoy the exposure to the subgravity state in our controlled experiments. We have reason to believe that even longer periods of absolute weightlessness can be tolerated if the crew is properly conditioned and equipped."23
10 Otto Gauer and Heinz Haber, "Man under Gravity-Free Conditions," in German Aviation Medicine, World War II, I, 641-643.
11 David Bushnell, "History of Research in Subgravity and Zero-G at the Air Force Missile Development Center, 1948-1958," Air Force Missile Development Center, 1959, 3-7; David G. Simons, "Use of V-2 Rocket to Convey Primate to Upper Atmosphere," Air Force Tech. Report 5821, Air Materiel Command, Wright-Patterson Air Force Base, Ohio, May 1959; James P. Henry et al., "Animal Studies of the Subgravity State During Rocket Flight," Journal of Aviation Medicine, XXIII (Oct. 1952), 421-423; David G. Simons, interview, San Antonio, April 24, 1964. A breezy popular account of the Holloman shots with V-2s and Aerobees is Lloyd Mallan, Men, Rockets, and Space Rats (Rev. ed., New York, 1962), 84-116.
12 Andrei G. Kousnetzov, "Some Results of Biological Experiments in Rockets and Sputnik II," Journal of Aviation Medicine, XXIX (Nov. 1958), 781-784; S. M. Polovskov and B. A. Mirtov, "Study of the Upper Atmosphere by Means of Rockets at the U.S.S.R. Academy of Sciences," and A. V. Pokrovskii, "Study of the Vital Activity of Animals during Rocket Flights in the Upper Atmosphere," in F. J. Kreiger, ed., A Casebook on Soviet Astronautics (2 vols., Santa Monica, Calif., 1956-1959), II, 151-172; Siegfried J. Gerathewohl, Principles of Bioastronautics (Englewood Cliffs, N.J., 1964), 91-95.
13 Harald J. von Beckh, "Human Reactions during Flight to Acceleration Preceded by or Followed by Weightlessness," Aerospace Medicine, XXX (June 1959), 391.
14 Fritz Haber and Heinz Haber, "Possible Methods of Producing the Gravity-Free State for Medical Research," Journal of Aviation Medicine, XXI (Oct. 1950), 395-400.
15 Allen C. Fisher, "Aviation Medicine on the Threshold of Space," National Geographic, CVI (Aug. 1955), 257; Siegfried J. Gerathewohl, "Weightlessness," in Gantz, ed., Man in Space, 115.
17 Bushnell, "Research in Subgravity and Zero-G at the Air Force Missile Development Center," 12-17; James H. Hanrahan and David Bushnell, Space Biology: The Human Factors in Space Flight (New York, 1960), 137, 139-140; Green Peyton and Jean Evans, "History, Aerospace Medical Division, Air Force Systems Command: Reorganization, 1 November 1961-30 June 1962," Hist. Publication 62-180, Brooks Air Force Base, Tex., 1962, 93-97.
18 Hanrahan and Bushnell, Space Biology, 139-140; "USAF School Simulates Living in Space," Aviation Week, LXVIII (Jan. 27, 1958), 49-51; Siegfried J. Gerathewohl, Oskar L. Ritter, and Herbert D. Stallings, "Producing the Weightless State in Jet Aircraft," Report 57-143, Air Force School of Aviation Medicine, Aug. 1957.
19 Gerathewohl, Principles of Bioastronautics, 211-234; Siegfried J. Gerathewohl and Herbert D. Stallings, "Experiments during Weightlessness: A Study of the Oculo-Agravic Illusion," Journal of Aviation Medicine, XXIX (July 1958), 504-515; Julian E. Ward, Willard R. Hawkins, and Herbert D. Stallings, "Physiologic Response to Subgravity I: Mechanics of Nourishment and Deglutition of Solids and Liquids," Journal of Aviation Medicine, XXX (March 1959), 151-154, and "Physiologic Response to Subgravity II: Initiation of Micturition," Aerospace Medicine, XXX (Aug. 1959), 572-575.
20 "They Float through the Air," Astronautics, IV (Feb. 1959), 42; Hanrahan and Bushnell, Space Biology, 147-148; William Leavitt, "The Weird World of Weightlessness," Air Force, XLII (April 1959), 113.
21 Gerathewohl, "Weightlessness," 108-132 . The Navy School of Aviation Medicine experiments also employed parabolic aircraft flights, while the Lewis studies featured use of a tall drop-tower.
22 David G. Simons, "Review of Biological Effects of Subgravity and Weightlessness," Jet Propulsion, XXV (May 1955), 209-211; Fisher, "Aviation Medicine on the Threshold of Space," 257.
23 Gerathewohl, "Weightlessness," 132. See also Oskar L. Ritter and Siegfried J. Gerathewohl, "The Concepts of Weight and Stress in Human Flight," paper no. 58-154, Air Force School of Aviation Medicine, June 26, 1958.