United States Spacecraft (Partial list, excluding commercial and Defense Department spacecraft) Vanguard Explorer Interplanetary Monitoring Platform (IMP) Orbiting Geophysical Orbiting Solar Observatory Orbiting Astronomical High Energy Astronomical Biosatellite Pegasus Echo Relay Syncom Applications Technology Tiros ESSA Nimbus ERTS Synchronous-orbit Meteorological Satellite (SMS) PAGEOS GEOS LAGEOS Earth Satellites International Geophysical Year, Explorer class. Small satellite for near-earth missions. Explorer-class satellite to explore cislunar and lunar space. Observatory-class satellite for geophysical research. Observatory-class satellite for solar studies. Observatory-class satellite for stellar astronomy. Very heavy, observatory-class satellite for studying shortwave Observatory-class, recoverable satellite for life sciences. Applications satellite. Large metallized sphere for passive Active communications satellite. Active communications satellite in synchronous orbit. Platform for a variety of applications technology researches, particularly in synchronous orbit. Large Explorer-class weather satellite. Operational version of Tiros. Observatory-class weather satellite. Earth Resources Technology Satellite. Applications satellite devoted to earth resources research. Applications satellite in synchronous orbit for meteorological research. Passive geodetic satellite. Active geodetic satellite with flashing lights and radio instrumentation. Geodetic satellite with corner reflectors for use with laser beams from the ground. Pioneer Mariner Viking Voyager Lunar soft lander. Lunar satellite for photography of the moon and lunar Manned lunar lander with manned lunar orbiter. Explorer-class interplanetary probe. Observatory-class planetary and interplanetary probe. Observatory-class planetary orbiter plus lander. Observatory-class planetary and interplanetary probe for outer planet studies. (Spacecraft of the 1970s, not the Voyager of the 1960s that was displaced from the program by Viking.) Sputnik Luna Vostok Voskhod Soyuz Salyut Cosmos Venus (Venera) Polyot Table 4 Soviet Spacecraft Geophysical research satellite. Unmanned lunar orbiter, lander, and return missions. First Soviet manned spacecraft. Adaptation of Vostok to accommodate two and three cos monauts. Two- or three-man spacecraft, with working compartment. Catchall name for variety of research and test spacecraft. Earth satellite with onboard propulsion for changing orbits. Zond Meteor Intercosmos Oreol Mars Prognoz Raduga Ekran Ariel Alouette ISIS San Marco French (FR-1) ESRO AZUR Skynet NATO CAS (Eole) Lunar and deep-space probe. Communications satellite in 12-hour orbit with low perigee and with apogee near synchronous-orbit altitude. Weather satellite. Soviet international satellite. Scientific satellite for upper atmosphere and auroral studies. Unmanned Martian probe. Satellite to study solar plasma fluxes. Geosynchronous communications satellite. Television broadcasting satellite. Other Foreign Spacecraft, Launched with U.S. Cooperation Barium Ion Cloud ANIK Aeros ANS INTASAT Helios Symphonie COS United Kingdom satellite for geophysical and astronomical Canadian satellite for ionospheric research. French satellite for ionospheric research. European Space Research Organization satellite for particles and fields investigations. German satellite for particles and fields research. United Kingdom satellite for communications. NATO satellite for communications. French satellite for data collection and meteorology. German satellite for geophysical research. Canadian geosynchronous satellite for communications. German geophysical satellite. Netherlands satellite for ultraviolet and x-ray astronomy. Spanish satellite for ionospheric research. German deep-space probe for interplanetary and solar studies inside the orbit of Mercury. French-German satellite for communications. European Space Agency satellite for study of cosmic gamma rays. distinguish between sounding rockets and space probes, a limit of one earth's radius was arbitrarily set on the altitude of a sounding rocket.24 The sounding rocket was really both launch vehicle and payload combined. Only rarely was the payload separated from the flying rocket, and when this did happen, the payload still traversed an up-and-down trajectory alongside that of its launching rocket. Sounding rockets, which were the only high-altitude research vehicles capable of exceeding balloon ceilings before the launching of Sputnik, continued through the 1970s to be important in the U.S. space program, being launched at the rate of about 100 a year. They provided the best means of obtaining vertical cross sections of atmospheric properties up to satellite altitudes and also were inexpensive devices for trying out new instrumentation or making exploratory measurements of phenomena to be studied in detail later with more expensive spacecraft. Their relatively low cost and the speed with which a sounding rocket experiment could be prepared and carried out also made sounding rockets useful for graduate research where the student needed to complete a project in a reasonable amount of time to support his dissertation. But not only students found the sounding rocket attractive. Many professional space scientists continued to favor sounding rockets for much of their research, as opposed to the more complicated, more expensive, and more demanding satellites.25 Through the years there was a steady demand on NASA for sounding rockets, and the agency was frequently urged to increase its budget in this area, even though by 1965 the budget had risen to $19 million a year, an order of magnitude more than had been spent per year on such research during the days of the Upper Atmosphere Rocket Research Panel. Yet satellites were required for many space experiments, particularly for long-duration observations above the earth's atmosphere. For these, many different satellites were devised, as may be seen from table 3. Generally these spacecraft could be divided into three classes: Explorers, observatories, and manned spacecraft. The simplest were the Explorers, whose weights usually ranged from less than 50 kilograms to several hundred (fig. 15). Each was devoted either to a single experiment or to a small collection of related experiments. Explorers were either unstabilized or used fairly simple techniques for obtaining a rough degree of stability. If the spacecraft were spun around a suitable axis, that axis would maintain its general direction in space. Long booms could be used to generate a gravitational torque on the satellite, keeping a chosen side toward the ground. Explorers usually used battery power-supplies, sometimes replenished by energy from solar cells. Explorers were much simpler than observatory satellites. The observatory class-which included the Orbiting Solar Observatory (fig. 16), the Orbiting Geophysical Observatory (fig. 17), and the Orbiting Astronomical Observatory (fig. 18)-consisted of very heavy, complex, accurately stabilized spacecraft. Weights ranged from several hundreds of kilograms to tons. The much greater size and weight of the observatories permitted more scientific payload and more sophisticated instrumentation. They could devote a considerable weight to what was called "housekeeping" equipment— power supplies, temperature control, and tracking and telemetering equipment. As with the Orbiting Geophysical Observatory, there might be provision for special operations such as erecting booms to hold instruments like magnetometers at a distance from the main body of the spacecraft, which otherwise might influence the measurements to be made. The Orbiting Astronomical Observatory and the Nimbus meteorological satellite were equipped with large paddles, covered with solar cells, which could be unfolded and kept facing the sun to furnish power for the spacecraft and its instruments. As a rule observatories carried elaborate systems for maintaining a desired orientation in space, so that scientific instruments could be pointed in a chosen direction. This was especially true of the astronomical satellites, whose instruments measured radiations from selected celestial bodies. The Orbiting Astronomical Observatory, for example, used specially designed star trackers which, by fixing on several chosen stars in widely different directions, could establish a reference frame for the spacecraft. With this refrence frame as a guide, the observatory could be slewed around to any chosen direction. Once properly oriented, the spacecraft could be held fixed to within a minute of arc, and telescopes within the satellite could be trained for long periods of time on their observational targets with an accuracy of fractions of a second of arc. A variety of schemes provided the ability to alter and then maintain spacecraft orientation. Most frequently used were small intermittent jets, developed either by small chemical rockets or by releasing high-pressure gas-nitrogen, for example-through small nozzles. Other methods were also used, often supplementing the jets. Electric current passing through loops of wire mounted in the spacecraft would develop magnetic fields which, reacting against the magnetic field of the earth, would exert a torque on the satellite that could be used to alter the orientation. Rapidly spinning wheels-gyroscopes-that stubbornly resist efforts to change their orientation in space, could also help point and stabilize spacecraft. Most of the structure of the Orbiting Solar Observatory rotated to provide gyroscopic stabilization. Finally, if the mass of a spacecraft was distributed much as the material in a dumbbell, the earth's gravity could keep the satellite pointed toward the earth's surface, as in Applications Technology Satellites. Since the earth's gravitational field varies inversely with the square of the distance from the center of the earth, the end of such a spacecraft nearer the earth would experience a greater pull of gravity than would the end farther away. Thus, whenever the end facing the earth tended to |