With Skylab's flight activities completed, emphasis shifted to reducing and interpreting the vast amount of data collected. This task will continue for several years, both because of the great amount of data collected and the need to correlate it with other data obtained simultaneously through observations made by Earth-based scientific teams.
The multidisciplinary scientific program involved over 100 experiments devoted to observations of the Sun, studies in stellar astronomy, medical experiments to study man's adaptability to long-duration zero-gravity exposure, studies of Earth resources, materials processing, and the conduct of a series of scientific experiments proposed by high school science students. The results of this program constitute a legacy to mankind, the value of which will increase as evaluation of the data produced by the program continues.
Man's Adaptability to Long-Duration Spaceflights
Since the earliest days of the manned spaceflight program, there had been a continuing concern expressed regarding man's ability to live and operate efficiently during extended spaceflights. Previous studies of man exposed to zero gravity observed a consistent loss of body fluid; a small, but repeated loss in bone calcium and muscle mass; and a reduction in the ability of blood vessels to actively distribute blood to the various parts of the body following return to an Earth gravity condition. These effects always disappeared a few days after the astronauts' return to Earth and showed no consistent correlation with the time spent in space. Interestingly, similar effects have been observed in individuals confined to prolonged bed rest on Earth.
The Skylab biomedical program was a study of normal, healthy men and their reactions to an environment in which the influence of gravity was absent. Skylab, with its three, increasingly longer, manned periods, provided an excellent opportunity for the study of the importance of gravity to man's physiological functions. It provided a means for evaluating medical phenomena under prolonged zero-gravity conditions using rigorous evaluation techniques. Moreover, it provided an opportunity to evaluate psychological effects of prolonged periods of weightlessness, an important consideration if even longer manned spaceflights are to be undertaken.
Studies and investigations undertaken included: the effect of gravity on nutritional requirements and the attendant gain or loss of the body's biochemical constituents; the role of gravity in man's metabolic effectiveness in doing mechanical work; the effects of long exposure to zero gravity on the heart and blood vessels and measurement of the response of the circulatory system to various workloads; the behavior of the blood cells, body fluid compartments, body immunity, and the like when gravity is absent. They also included the role....
Astronauts Weitz (above) and Conrad (left) use the bicycle ergometer for metabolic activity experiments and exercise.
....of gravity on man's psychomotor efficiency and the performance of useful tasks; the responses of the human vestibular system in the absence of gravity; the influence of zero-gravity environment and the more rapid day/night cycles on the normal human rhythms of sleep and wakefulness.
All 16 of the biomedical experiments were carried out, and man's adaptation to zero gravity was obtained for the first time. Medical experts have essentially agreed that man can, indeed, function effectively for long periods of time in zero gravity and then return to Earth without experiencing adverse effects. Joe Kerwin, a doctor of medicine, and a commander in the Naval Medical Corps, stated: "It was a continuous and pleasant surprise to me to find out how easy it was to live in zero g, and how good we felt."
Throughout the Skylab mission, crewmen found  the environment of weightlessness thoroughly stimulating. They were able to work long days without tiring excessively, and they worked with an efficiency that far exceeded expectations. They did find that their new environment demanded that they take much more exercise than required on Earth. And exercise in space required some interesting adjustments. For example, the design of the bicycle braking system created the requirement for the astronaut to deliver a forceful upstroke. To remain seated firmly as they exercised, they used an arm restraint and sometimes placed a pad between the top of their heads and the ceiling.
Surprising Results From Vestibular Function Tests
The purpose of the human vestibular (inner ear) function test was to examine the effect of weightlessness on man's sensitivity and susceptibility to motion and rotation, and his perception of orientation. Tests before, during, and after the flights were carried out in a chair that was rotated by a motor at its base while the crewman being tested performed nodding and sidewise head movements until he approached a feeling of stomach uneasiness. An attempt was made to reach the same level of symptoms in each test.
In ground tests conducted before each flight, crewmen rotated at between 10 and 20 revolutions per minute and executed 50 to 75 head movements before experiencing stomach awareness. In the zero-gravity environment, however, crewmen rotated at the maximum rate of 30 revolutions per minute and carried out 150 head movements with few or no symptoms of illness. After returning to Earth, they retained this apparent immunity to motion sensitivity for several weeks.
Tests of cardiovascular (blood circulating) response and exercise tolerance were usually conducted sequentially. A characteristic of cardiovascular deconditioning is the partial failure of reflexes in the venous blood vessels resulting in excessive pooling of the blood in the legs when standing erect in a gravity field. When excessive pooling takes place, the rate of blood flow through the heart and lungs is reduced, causing a reduction in pulse pressure, elevation of pulse rate, and eventual reduction of the average pressure. This results in a reduced flow of blood to the brain and other tissues, and it often results in dizziness or fainting.
Tests of the cardiovascular system in zero gravity were conducted in a lower body negative pressure experiment. The crewman, instrumented to record his blood pressure, vectorcardiogram, and calf circumference measurements, was sealed up to his waist in a cylinder. A partial vacuum was then drawn on his lower limbs and torso, producing blood pooling in his legs. The pressure reduction was such that it challenged the cardiovascular system in a manner comparable to the man standing erect on Earth. Following this test, the crewman moved to the bicycle ergometer for a carefully prescribed program of exercise.
The lower body negative pressure experiment became a significant challenge in orbit, although it had never been a problem on Earth. This was apparently the result of reduced total blood volume in orbit and pooling in the legs during the periods of negative pressure. Some tests were terminated before reaching the desired pressure reduction, because of the approach of fainting.
With one exception, manual coordination functions posed no problem. Reaching or handling objects was entirely normal, as long as visual contact could be maintained. However, in the sleep compartment, with all the lights out, crewmen found it almost impossible to reach out and touch the light switch located less than 2 feet away.
"The result was not just a near miss," according to Owen Garriott, "we found that our hands might first encounter a locker as much as 45 degrees away from the correct direction. Although I tried to 'practice' this move on a number of occasions, I still could not do it well after 2 months."
All crewmen reported that there were no psychological problems associated with the lack of clearly established "vertical" or "horizontal" orientations. Crewmen easily adjusted their impression of "up" to conform to body orientation. This capability was evidenced repeatedly as the flight crews performed repairs and carried out experiments with increasing effectiveness as the mission progressed.
One of the more intriguing results of the program was the change in outlook, "almost of a spiritual nature," of the crewmen, observations which had also been expressed by earlier space explorers.
In the view of Edward Gibson, "Being up here and being able to see the stars and look back at the Earth and see your own Sun as a star makes  you . . . realize the universe is quite big, and just the number of possible combinations ... which can create life enters your mind and makes it seem much more likely."
And Bill Pogue commented, "I now have a new orientation ... of almost a spiritual nature. My attitude toward life and toward my family is going to change. When I see people, I try to see them as operating human beings and try to fit myself into a human situation instead of trying to operate like a machine."
A Detailed Study of the Sun
As an observing station in orbit, well above the constraining influence of the Earth's atmosphere, Skylab attracted the interest of astronomers and space physicists from the time it was first conceived.
The Sun is the center of the solar system. Its radiations and emissions affect everything within that system, from planets such as Earth and Mars to comets and spacecraft. Its atmosphere extends out into space in a steady stream called solar wind and engulfs the entire solar system. It is a giant laboratory in space in which physical processes can be studied on a scale not producible in an Earth laboratory. Many discoveries have resulted from studying the Sun, including the lightweight element helium. Many of the high-energy processes of modern astrophysics have counterparts on the Sun where they can be studied in sufficient detail to arrive at an understanding of the mechanisms involved. The Sun serves as a testbed of the theory of gravitation, because it has the strongest gravitational pull of any celestial object near enough for detailed study.
Riddles presented by the Sun include its loss of mass, through the solar wind, radiations, and perhaps even dust; its spin-down through solar wind drag; its failure to produce the nuclear reaction products called "neutrinos," which could confirm or disprove the theory of nuclear energy production in its interior; its temperature structure with a 10 000°F inner atmosphere supporting an outer atmosphere of millions of degrees; the mysterious structures in its atmosphere including sunspots, polar regions, and streamers.
 The flow of solar energy is not steady. Some solar emissions are highly energetic and can cause important effects on Earth, including the interruption of shortwave radio communication, scrambled telegraphic messages, and other phenomena.
Solar flares set off magnetic storms which cause many curious effects. For example, oil exploration teams making electromagnetic measurements deep in wells find their highly sensitive electrical instrumentation inoperable due to interference from magnetic storms. Commercial power systems have been blacked out by high-voltage surges caused by magnetic storms.
There is also increasing evidence leading meteorologists to conclude that solar activity profoundly influences our weather and climate.
Skylab's orbiting solar observatory was designed for the conduct of unique experiments and observations which would help man advance his knowledge about the Sun and the myriad greater and lesser stars beyond.
Dividends From the Solar Observation Program
The solar observatory, with its eight high-precision instruments, was operated principally from the control console in the docking adapter by an astronaut who directly initiated instrument observing sequences. During periods when the astronauts were on board but not present at the console, limited operation of several of the instruments was possible by ground command. In addition, three instruments could be operated from the ground during the unmanned periods.
The Sun was supposedly in the quiet period of its 11-year cycle. But solar observations quickly revealed that even during its quiet phase, it was extremely active, as evidenced by the appearance, frequency, and distortions produced by large transients in the corona. Large "magnetic loops" observed by Skylab's telescopes expanded through the corona at a velocity of 900 000 miles per hour. During planning for the Skylab mission, astronomers had expressed the hope that transient events of some sort might be observed on two or three occasions; over 60 events of this type were seen, usually associated with eruptive prominences in the chromosphere. After a major transient, the coronal structure was completely altered within the volume of the event and a new structure was formed which persisted for weeks or longer.
Observations of the Sun's lower corona and the...
....chromosphere revealed dramatic details of this extremely active region between the dense photosphere and the very tenuous outer corona. X-ray photographs of the Sun showed numerous "bright spots," with diameters of about 600 miles and lifetimes of only a few hours. Other X-ray studies showed "coronal holes," from which X-ray emission was virtually absent and which were believed to represent regions in which the magnetic field lines extend far out into the interplanetary medium. The holes could be the source of the solar wind, whose relationship to the interplanetary magnetic field is undergoing further intensive study, along with the possible relationship between the interplanetary magnetic-field structure and the Earth's weather.
 Studies of Comet Kohoutek
An unplanned bonus resulted during the Skylab mission. Skylab planning was modified to include observations of the new comet Kohoutek, since calculations showed that it would closely approach the Sun about Christmas.
Kohoutek became the best observed comet in history, and its observations contributed a wealth of new knowledge about comets. The third crew took a series of photographs of the comet in visible light with a small hand-held camera. Other photographs made with a French ultraviolet stellar camera gave information useful for determining the gas-to-dust ratio in the comet's coma. After perihelion (the comet's closest approach to the Sun) photographs taken through the coronagraph clearly showed the antitail of the comet, a spike apparently protruding from the side of the comet toward the Sun.
"The comet's got a spike and a tail," Astronaut Gibson gleefully confirmed during an extravehicular activity, "The spike is very evident. It is not 180 degrees out, but more like 160 degrees. It is yellow and orange . . . just like a flame. It seems to be the same distance out as the tail, and there is a diffuse amount of material which goes out and joins up with the tail."
Gibson made a further valuable contribution to these observations through a series of sketches.
The Skylab observations of the comet further proved the great flexibility and adaptability of a manned spacecraft. Even though the Kohoutek project was introduced and prepared on short notice, it was worked into the Skylab mission plan without disrupting other areas.
A Broad View of the Earth
With its precise instruments, the observational capability of its crew, and its coverage of three-quarters of the Earth's surface, Skylab offered an Earth observation capability never before available. In traversing this area, Skylab passed over each point once every 5 days, so that time-based variations on Earth were easily recorded.
The more than 40 000 photographs made of the Earth and the thousands of observations recorded on miles of magnetic tape provided a mass of data, already of great value to those involved in improvements of agriculture and forestry, geological applications, studies of the oceans, coastal zones, shoals and bays, and continental water resources, investigations of atmospheric phenomena, regional planning and development, mapping and further development of remote sensing techniques.
Materials Processing Studies
The zero-gravity condition existing in Skylab made it possible to perform operations in materials processing that would have been impossible or extremely difficult on Earth. When the opportunity for elaborate and controlled materials processing experiments on Skylab arose, experimenters were quick to prepare a program of studies. Major experiments included crystal formation from vapors; alloy formation from components of different densities; homogeneity of dopant distribution in semiconductors; diffusion in liquid metals; and solidification of molten metals. All of these studies required controlled heating in a specially provided furnace, and subsequent cooling of samples.
Samples had to be selected according to the temperature range offered by the furnace. In one experiment, the front end of a small cylinder of indium antimonide was melted by radiative heat inside a graphite cavity and allowed to resolidify without touching the walls. Under Earth conditions, the semiconductor alloy would have solidified in crystalline form. The crystals, however, would have been limited in homogeneity because of convection currents which disturb uniform crystal growth and cause defects within the lattice structure. The Skylab sample, undisturbed by gravity, crystallized as a very homogeneous single crystal with only very few lattice defects. In fact, the number of lattice defects in the space-grown sample was an order of magnitude smaller than in the best Earth-grown crystals. Even its surface was of a smoothness unobtained in Earth-bound laboratories. This success implies that other semiconductors in all probability will also form crystals of unprecedented homogeneity and size when they are allowed to solidify under zero gravity without wall contact. The availability of such crystals will have a profound effect on the electronics industries.
In another crystallization experiment, germanium selenide was vaporized at the hot end of an ampoule and allowed to condense at the cold end. This process involves a gaseous component, germanium iodide, as transport agent. Under Earth...
....conditions, crystals forming in this vapor deposition process would remain relatively small; they show signs of irregular growth, caused mainly by convective motion in the gaseous medium. A germanium selenide crystal grown on Skylab exceeded in size that of Earth-grown crystals by a factor of 10, and its surface after cleaning proved to be virtually free from defects. This dramatic improvement of crystal quality by elimination of gravity effects is even more surprising since the experiment was only exploratory in nature, without an attempt to optimize thermal gradient and vapor pressure in the ampoule. The growth rate of the crystal was considerably higher than expected for a vapor deposition process governed only by diffusion, without convective currents. The reason for this high deposition rate has not yet been established.
A third experiment involved the recrystallization of indium antimonide, doped with tellurium. A sample of this material was prepared on the ground and partially remelted during orbital flight in such a way that the molten part, attached to the solid part by surface tension and cohesion, did not touch any container walls. When samples of this material solidify under gravity, convection currents caused by thermal gradients segregate the dopant, a process which leads to a layered structure of the....
....crystal. This separation of components results in nonuniform electrical conductivity, and in unpredictable semiconductor properties of the crystal. Solidification under the zero-gravity condition on Skylab produced a material in which the striations had completely disappeared. Measurements showed that the electrical conductivity throughout this part of the crystal, with the exception of narrow regions near the edges, was very uniform. This result showed that without gravitational forces, crystals obtain far greater homogeneity than under Earth conditions; ideal steady-state growth in all three dimensions seems to be achieved under zero-gravity conditions. With special precautions in Earth-bound laboratories, such as carefully controlled temperature profiles or strong magnetic fields, the layering of tellurium-doped indium antimonide crystals can also be suppressed to a substantial extent; however, the uniformity of electric properties in space-grown crystals is still superior to that of Earth-grown crystals.
Several of the materials processing experiments on Skylab concerned the mixing and alloying of components which do not form under Earth conditions because gravity forces separate the components before they solidify. Mixtures of gold and germanium, for example, show very coarse dispersion and even large-scale segregation when solidified in Earth-bound laboratories. Samples of gold-germanium mixtures were melted in the furnace on board Skylab. After solidification under zero gravity, the material showed a very fine and uniform dispersion. At some places within the material, small areas were found which obviously represent a composite not formed under gravity, perhaps a new alloy or a compound. When irradiated with X-rays, these areas emitted characteristic X-ray lines which had not been known before.
Each of the 14 experiments in the Skylab materials processing program was successful in demonstrating some decisive influence of gravity upon processes that are essential in the formation....
....of materials. The results of most experiments substantially exceeded the expectations of the experimenters.
All experiments dealing with materials in space convincingly proved that the weightless environment of orbital flight indeed offers to the materials scientist highly useful tools unavailable on Earth, but full of promise for further experimentation and discovery.
Student Experiments and Science Demonstrations
The student experiments provided a unique opportunity for a large number of high school students to participate in spaceflight activities as well as to pursue areas of scientific research. Although not all experiments worked as expected, they were a successful and very rewarding part of the program. Biologists were further enlightened on the adaptive processes of life in a weightless environment when two spiders wove webs of a quality approaching those they made on Earth. Similarly, another student experiment provided valuable new data on the neutron flux within space.
The science demonstrations performed by the astronauts also proved to be successful as well as educational. Some provided valuable new data on the behavior of liquids in the absence of gravity. Films of the demonstrations have been made available to science teachers as training aids.
Earth-bound Man's Capabilities Extended
Skylab was, essentially, a means of extending man's observational capabilities. It permitted him to make scientific observations of the solar system without interference from Earth's atmosphere. It....
....made possible a broad view of the planet Earth denied him by other available means, and it allowed him to make comparisons and judgments as he viewed earthly features. And it allowed him to escape the gravitational influences felt on Earth to conduct biomedical and space processing experiments, possible only in the space environment.
Man, therefore, became not only the reason for Skylab, but also a vital element in the conduct of the Skylab mission. Astronaut Gibson summed it up reflecting on solar observations:
"The success of the . . . missions depends to a large extent on the scientific knowledge, training, and decision-making capabilities of both the astronauts and the ground support team. The opportunity to exercise scientific judgment during flight and to enhance significantly the value of the data returned arises directly from the nature of the solar observations. We are close enough to the Sun to see much detailed structure in its atmosphere. Because of the complexity of this structure and the wide range that has been observed in its characteristic time for change (from many years down to seconds) a wide variety of observations is possible. Thus, decisions must be made which determine the amount of new and significant information in the returned data.
"The role of the onboard observer can be simply stated. He is presented with television pictures of the Sun at several wavelengths in the electromagnetic spectrum, as well as with other indicators of the state of solar activity. Instruments which are capable of high data-acquisition rates and which can be operated to observe only a small portion of....
 ....the Sun are available for use by the observer. However, he is constrained by limited quantities of photographic film in all but one of the instruments. Hence, the scientific value of the returned data is dependent upon the ability of the onboard observer to make judicious decisions concerning when, at what rate, and from where on the Sun to take data with each instrument."
Alter Ego of the Principal Investigator
All of Skylab's crewmen received several years of flight training. Much of it related to operations of the spacecraft, but about an equal fraction was devoted to experimental objectives in trainers and simulators, on some occasions with a principal investigator working alongside. In a few cases, such training was conducted in the laboratory of the principal investigator. The result was the establishment of a close rapport between flight-crews and the ground-based scientific staff, which greatly increased mutual confidence and the ability to work together to achieve the experimental objectives.
One example of this cooperative effort was the development of scientific shopping lists which were devised after the first manned period. These lists allowed the crewmen to work independently of ground advice in selecting targets and objectives for the solar observations. The lists were originally devised to suggest to the crewmen a variety of short objectives that could be met if an extra 5 or 10 minutes of observing time should become available. The data collected in those intervals were so useful that the scientists began requesting specific allotments of time to be used entirely at the crewman's option. "In this activity," according to Astronaut Garriott, "the crewman truly performed as the alter ego of the solar science community. "
Scientists Laud Skylab Program
Scientists in various disciplines, some of whom had reservations about the program initially, had a great deal of praise for Skylab after its successful mission.
In speaking of experiments in growing crystals aboard Skylab, Harry C. Gatos, Massachusetts Institute of Technology, said, "Understanding solidification processes and achieving crystalline perfection and chemical uniformity will without doubt usher in a new era in materials processing, leading to materials that can be used at their theoretical performance, rather than at hundreds of thousands of times below that."
John T. Shepherd, Director of Research at the Mayo Clinic, Rochester, Minn., reported at the Skylab Medical Experiments Symposium: "The wealth of data which has been accumulated on the adjustments of man to space travel ... truly establish his future role in the exploration of the universe." J. R. Hordinsky, of the Johnson Space Center, provided the following summary: "The medical experiments data analyzed to date and the observations made during and after the flight do not indicate any major medical constraints for continued extension of man's duration in space. . . . The Skylab crewmen demonstrated that man can fully adapt to a weightless environment, perform in an efficient and effective manner, and then readapt to the one-g environment. A major milestone in the manned spaceflight program has been accomplished."
Herbert Friedman, of the E. O. Hulburt Center for Space Research, Naval Research Laboratory, commented upon the completion of the Skylab mission: "The Skylab ATM [Apollo Telescope Mount] observations may upset many longcherished ideas of solar physicists. Achieving finer detail in geometry and spectral resolution than ever before, ATM probed the solar atmosphere from the lowest photospheric level to the far reaches of the corona.... It marks a new revealing of the Sun, and the opening of a neoclassic period of solar studies linked with the much-anticipated shuttle sorties of the 1980's."
Perhaps the most sign)ficant comment concerning man's role in the scientific exploration of space, as demonstrated in Skylab, was that of Leo Goldberg, Director of Kitt Peak National Observatory, who said:
"Many of us had serious doubts about the scientific usefulness of men in space, especially in a mission . . . which was not designed to take advantage of man's capability to repair and maintain equipment in space. But these men performed near miracles in transforming the mission from near ruin to total perfection. By their rigorous preparation and training and enthusiastic devotion to the scientific goals of the mission, they have proven the value of men in space as true scientific partners in space science research."