1. AN OUTPOST IN SPACE
"Unlimited vacuum of outer space and the absence of gravitational forces in an orbiting satellite," said Professor Hermann Oberth 1 in 1923, "make an Earth-circling spacecraft an ideal site for the observation of stars, the Moon, the Sun, the planets, and particularly our Earth." Barely 35 years later, these prophetic words began to transform into reality. Today, hundreds of instruments already have observed the sky and the Earth from satellite orbits. More than 80 astronauts and cosmonauts have seen our earth from vantage points above the atmosphere and from as far away as the moon. Impressive discoveries were made with unmanned probes and satellites; among them X-rays from the sun and numerous other stellar objects; the radiation belts around the Earth, the magnetosphere,2 the solar wind, and the geocorona, not to speak of the vast improvement of our capabilities for meteorological and other Earth-oriented observations. Apollo proved the usefulness and efficiency of man in space as a pilot and navigator, a technician, a troubleshooter, and particularly as an observer and a research scientist.
Our first projects in space provided us with a harvest of scientific understanding and of technological experience beyond any expectations. The value of the space program for the enrichment of life on Earth has become manifest in three distinct ways: space flight presents a new frontier of exploration; it opens the doors to a huge source of scientific knowledge; and it offers direct help in many of our earthly needs. Early pioneers of space flight, anticipating these benefits from Earth-orbiting outposts, have often suggested that long-time stations in orbit be established. These stations would be equipped with instruments to observe the sky and the Earth, with receivers and transmitters for global communications, and with processing machinery which utilizes the state of weightlessness; they would be manned by astronauts who use, operate, and service the instruments with an efficiency and a flexibility not attainable with automated equipment. An orbiting station would operate for years, while its crew members would be rotated every few weeks or months. As space flight began to evolve about 25 years ago, interest and work concentrated at first on high altitude rocket probes,  and then on small automated satellites. Plans for manned orbiting stations had to wait. However, with Apollo, large Earth-to-orbit transportation systems and life support facilities were developed. Instruments for the observation of astronomical objects, of the space environment, and of the Earth became available from numerous science and applications projects. Solarelectric power supplies, communication equipment, data systems, and attitude control systems were developed and operated from the early 1960's on. Conditions for the development of a manned orbiting space station, therefore, become favorable during the past decade, and plans for a first station in orbit began to materialize early in the sixties. Project work was started a few years later. Named "Skylab" in 1970, the first manned space station of the U.S. will be ready for launch in May, 1973. In its large and comfortable interior, three-man crews will conduct an exciting program of experiments and observations. Several of the experiments were developed by scientists in foreign countries. NASA has invited world-wide participation in the analysis and interpretation of Skylab data.
2. OPERATION ABOVE THE EARTH'S ATMOSPHERE
The shell of atmospheric gases which surrounds the earth as a source of oxygen and carbon dioxide for animals and plants, and as a protection against the dangerous radiations and the temperature extremes of space, sets a narrow limit to Earth-bound observations of celestial objects because it allows only a small portion of the total wavelength spectrum to reach instruments near the Earth's surface. The light-scattering effect of the atmosphere generates a background in the viewing field of an Earth-bound telescope which sets a lower limit of about the 24th magnitude to the star images which can be recorded from Earth. Furthermore, even the finest telescopes on the ground can provide only limited optical resolution because of the image-blurring effect of the atmosphere. Telescopes outside the Earth's atmosphere are not subject to these limitations. Several solar telescopes on Skylab will view the sun with a clarity and resolution, and in ultraviolet and X-ray regions of the spectrum, which would be impossible from the Earth's surface. These observations will substantially increase our knowledge of the Sun, of its mechanisms to convert and to transport energy, of the violent outbursts of radiations and particles in solar flares, of the strange behavior of hot plasmas, and of the many ways in which the Sun influences our' weather, our environment, and, in fact, our lives on Earth. Skylab's stellar cameras will greatly expand the capabilities of Earth-based observatories by extending the range of measurements far into the ultraviolet regions which are inaccessible from Earth. Cosmic ray particles which would not be able to penetrate the Earth's atmosphere will be recorded during Skylab's flight with emulsion plates and plastic detectors; meteoritic particles will be collected on ultra-clean surfaces. Special optical sensors will observe the airglow in the highest layers of the atmosphere, and also the "Gegenschein" 3 which is probably caused by a reflection of light from dust clouds orbiting the Sun.
 3. ZERO GRAVITY ENVIRONMENT
On an orbiting spacecraft, the force of gravity, omnipresent in our earthly environment, is counteracted by centrifugal force. The resulting force is exactly zero at the center of gravity of the spacecraft. At other points within the spacecraft, the small resulting force varies between plus and minus about one millionth of the Earth's gravitational force at the surface, depending upon the distance and direction from the center of gravity.
The state of weightlessness on Skylab will be utilized by a number of experiments which study the influence of gravitational forces in biological, chemical, physical, and metallurgical processes. The cleavage of living cells may be triggered by a density stratification caused by gravity; cell metabolism may depend on gravitational forces; the content of minerals in the bone structure of animals and man may be influenced by gravity; vestibular functions, pressure distribution within the body, muscle tension, blood circulation, perhaps even the function of endocrine glands should be expected to depend on the force of gravity. The study of these functions under weightlessness will help our understanding of the mechanisms that govern many of the processes in living organisms.
Gravitational forces cause stratification of liquids which contain components of different densities. The desired mixing of certain alloys in the molten state, or of chemical reagents, is often difficult or even impossible on the Earth's surface because of this gravitational separation effect. Under conditions of zero gravity, fluids can be maintained in a state of perfect mixing. Even the mixing of liquid metals with gases, resulting in foams, can be achieved; the production of foam metals with extreme strength-to-weight ratios may be one of the future activities on orbiting space stations. The growth of single crystals from solutions, likewise, is often perturbed by effects attributable to gravity. These perturbing effects will not exist in orbiting laboratories, and single crystals of high purity may be obtainable in much larger sizes under weightlessness than on Earth. Such crystals are of great importance in the manufacturing of semiconductor components for electronic circuits.
4. BROAD VIEW OF THE EARTH'S SURFACE
Skylab will fly over about 75 percent of the Earth's surface. Each of its 93-minute orbital tracks will be repeated every five days. Photographic, infrared, and microwave equipment will provide pictures and measurements of the terrain underneath the spacecraft. The decisive advantage of viewing from a satellite orbit, rather than from airplane altitude, lies in the fact that a satellite sees a large area on Earth at the same time and under the same lighting conditions. Comparison of different parts of an area with regard to cloud cover, plant growth, irrigation, land use, urban expansions, crop conditions, natural resources, air pollution, water management, topographical features, geological formations, and other surface properties will be much easier with the "synoptic" 4 capability of a satellite. As an example, Figure 1....
 .....shows two pictures of the same surface feature, a huge volcanic structure in Mauritania in Central Africa. The first picture (a) is a composite of numerous photographs taken from an airplane; the second (b) was taken from a satellite. The superiority of satellite pictures is obvious. This superiority is further enhanced by the rapidity with which satellite pictures can be taken and transmitted to Earth.
One of the picture-taking systems on the Earth Resources Technology Satellite, launched in July, 1972, is capable of transmitting a photograph of the Earth every 25 seconds, covering a square of 185 by 185 km (115 by 115 statute miles), with a resolution of about 30 meters (100 ft). One of the Skylab cameras will cover squares of 109 km by 109 km (68 by 68 statute miles) with a resolution of 11.5 meters (35 ft). Skylab, during its three mission periods, will take almost 40,000 photographs with its several cameras. In conjunction with aircraft and ground-based observations, these photographs will allow an analysis and assessment of man's interaction with his environment unprecedented in previous studies.
1 Professor Hermann Oberth, Hungarian-born rocket expert and pioneer of space flight, lives in Feucht, West Germany.
2 Scientific and technical terms are explained in the Glossary, page 237.
3 Faint glow in the night sky in a direction opposite to the Sun.
4 "Synoptic" is the capability of seeing several different things at the same time and under the same conditions.