Table 5 Examples of Investigations in Explorer-Class Missions Cloud-cover studies. Subject of Investigation Survey of earth's magnetic field and lower edge of radiation belts. Charged particle radiations in space. Interplanetary magnetic field near earth; particle radiations. Magnetospheric studies. Spacecraft Vanguard 1 Satellite geodesy. Gamma rays from space. Explorer 12 Explorer 38 Radio astronomy. Explorer 42 Pioneer 5 Ariel 1 Ariel 2 Alouette 1 San Marco 1 ESRO 2B Micrometeoroids. Charged particles and magnetic fields in magnetosphere. Atmospheric composition. Charged particles and magnetic fields in cislunar space. Geodesy by radio tracking methods. Catalog and study of celestial x-ray sources. Interplanetary charged particles and magnetic fields. Ionospheric and solar research. Atmospheric research and radio astronomy. Charge densities in upper ionosphere and radiation belt studies. Cosmic rays and radiations from sun. one spacecraft, so that a variety of related phenomena could be observed simultaneously and correlated. As with the launch vehicles, it proved impossible to find a single spacecraft carrier that would serve all needs, although some attempts were made in this direction. One spoke of building a standardized satellite, to effect economies and improve reliability. When conceived, the Orbiting Geophysical Observatory was described as a "streetcar satellite," whose continuing use would so reduce the preparation time for an experiment that researchers could get their equipment aboard at the last minute—like catching a streetcar or bus-to follow up on some recent space science discovery. Initial reaction to the streetcar concept was positive, and the idea had the blessing of the Space Science Panel of the President's Science Ad visory Committee.28 But the problems of serving so many experimenters on one spacecraft defeated the objective. For each observatory a great deal of tailoring was required, compromises had to be worked out on orbits, orientation, placement of instruments, magnetic cleanliness of the spacecraft, allocation of telemetering capacity, and operating time. Use of a common electrical power supply invited electrical interference among different experiments, and often an offending experimenter was required to provide his own power. Ionospheric and radiation belt phenomena were fundamentally related to the earth's magnetic field, making it important to measure ions, radiation particles, and magnetic fields simultaneously. But the various measuring instruments could easily interfere with each other unless care was taken. Those who wished to determine atmospheric composition at spacecraft altitudes required that the satellite and other instruments not contaminate the natural atmosphere with gases brought up from the ground-another difficult problem when large numbers of experiments were being conducted simultaneously. Such problems defeated the efforts to produce standardized satellites in the same sense as standardized autos and auto parts or standardized home appliances. Nevertheless a considerable amount of uniformity was achieved. The basic structure, housekeeping, and orientation systems of the solar observatories were essentially the same from spacecraft to spacecraft. Even the geophysical observatories, with all the tailoring that they required, had much in common with each other. More important, the technology on which spacecraft were based acquired over the years a certain amount of standardization. In this sense even the Explorers were standardized. Certainly no more varied looking group of satellites could be assembled than those of figure 15, which shows a large number of the Explorer satellites. Yet they were all cousins, stemming from a common, rather straightforward technology. When an engineer started out to design an Explorer-class satellite, he had pretty much in mind the kinds of structure, temperature control, tracking and telemetering devices, and antenna systems he might use. He was familiar with the kinds of vacuum, thermal, and vibration tests the spacecraft would have to pass to be approved for flight. To be sure, the technology advanced over the years as better components and materials became available, and improved housekeeping equipment was devised. But the family relationship remained. The steady, though gradual, change in Explorer technology did make the later Explorers considerably more capable than earlier ones. For example, Explorer 35 launched on 19 July 1967, weighing 104 kg and operating from an orbit of the moon, could far outperform the first several Explorers which weighed only tens of kilograms. Yet in the evolution of the series, any given Explorer was quite similar in its technology to the immediately preceding one. The same point is illustrated by the solar observatories. These also changed gradually over the years. In August 1969 the sixth solar observatory, weighing 290 kg, went into orbit. Though looking a great deal like the first observatory, launched on 7 March 1962 and weighing 200 kg, OSO 6 was more versatile than earlier ones, having the capability to point two telescopes at the sun to study in detail ultraviolet and x-ray spectra at any point on the solar disk. Like the earth satellites, space probes-which were spacecraft sent away from earth into deep space-fall into several classes. The analogy is very close. Akin to the Explorer satellites were the Pioneer space probes (fig. 23). These were modest-sized vehicles, ranging from around 40 kg in the first models to the 260-kg weights of Pioneer 10 and 11 sent to Jupiter in 1972 and 1973. They were spin-stabilized and instrumented to investigate the interplanetary medium and the environs of a planet as the spacecraft flew by. During the 1960s a number of Pioneers revolving like little planets about the sun provided a great deal of information on the solar wind and magnetic fields in space. When on the far side of the sun from the earth, Pioneer radio signals were carefully observed to find effects of the sun's gravity predicted by the general theory of relativity. On 3 December 1973 Pioneer 10 passed Jupiter at 130000 km from the planet's surface-taking pictures, measuring its radiations, and investigating some of its satellitesand then receded along a trajectory that would eventually carry it out of the solar system. A year later Pioneer 11 also flew by Jupiter, sending back more data on the giant planet and its satellites, leaving the planet on a course that would carry the spacecraft in September 1979 to the vicinity of the second largest planet, Saturn, famed for its rings. These missions into deep space give some idea of Pioneer's usefulness to the space scientist. As with Explorer, Pioneer technology also advanced steadily through the years, so that in the early 1970s it was possible to plan to use Pioneer spacecraft to carry orbiters and atmospheric probes to Venus in the late 1970s.29 Analogous to the observatory satellites were the larger lunar and planetary probes: Ranger, Surveyor, Lunar Orbiter, Mariner, and Viking (figs. 24-28). Like the observatories, these spacecraft were maneuverable and, using a celestial reference system, could be accurately oriented in space. In addition to small jets for orienting the spacecraft, they also carried a larger rocket that could be fired to alter the trajectory so as to keep the craft on course to its intended target. Lunar Orbiter carried enough auxiliary propulsion to place it in a lunar orbit, from which the spacecraft obtained a series of photographs of the moon. These were later used to produce maps of the moon's surface and to aid in planning Apollo missions. Several Mariners carried enough propulsion to place them in orbit about Mars. Surveyor used retrorockets to slow the spacecraft for a soft landing on the moon, after which remotely controlled instruments televised and investigated the surrounding landscape. Viking combined both the orbiting and landing capability, the main vehicle first going into orbit of Mars, after which a portion of the spacecraft separated and was forced by rockets to descend to the Martian surface. Deep-space probes had to overcome problems additional to those encountered by earth satellites. For example, a Martian probe took about two-thirds of a year to get to its destination. Pioneer 10 and 11 required almost two years to fly to Jupiter, and had to survive those two years in the environment of space to accomplish their assigned missions. If an earth satellite operated properly for a few months, the experimenters would have a few months worth of data for their trouble; but if a planetary probe operated for only a few months, they would get no planetary data at all. Of course, one also made interplanetary measurements on planetary flightsobserving the solar wind, magnetic fields in space, dust, meteor streams, and cosmic rays-but on planetary missions these were secondary objectives. Another requirement for the great distances traveled by interplanetary and planetary spacecraft was more powerful radio communications systems. The antenna had to be pointed toward the earth. If omnidirectional or wide-angle antennas were used, the power requirements went up, sometimes prohibitively. If, to conserve power, narrow-beam antennas were used, they exacerbated the antenna-pointing requirement. Finally, spacecraft that flew toward the sun had to be protected against overheating by solar radiations, while those that flew away toward the outer solar system had to be protected against freezing. With these larger space probes one could plan in the course of time to investigate all the planets and major satellites of the solar system, and the asteroids and comets. Spacecraft could be placed in orbit around other bodies, as was done with Lunar Orbiter around the moon and Mariner around Mars. Landers could place instrumented laboratories on the surfaces of other bodies, as did Surveyor and the Soviet Luna on the moon, Viking on Mars, and Soviet Venus probes on Venus. It was even possible to deposit roving laboratories on those bodies, as the Soviet Union did with Lunokhod on the moon, or retrieve samples of material from them as did Luna 16 and 17.30 Finally, there were the manned space probes (fig. 29). During the 1960s these consisted solely of the Apollo-Lunar Module combinations that the American astronauts flew to the moon. As with the manned satellites, these provided the added dimension of manned exploration. The successful Apollo missions yielded such a wealth of scientific data as to soften at last the years-long lament of the scientific community over the tremendous expense of the Apollo program. Of course, operation of these spacecraft required auxiliary equipment and systems. Out of the Minitrack tracking and telemetering network of the International Geophysical Year grew a versatile satellite network for issuing instructions to satellites, determining their orbits, and receiving telemetered information.31 To work with the deep-space probes, the Jet Propulsion Laboratory established a deep-space network using 26-m and 64-m parabolic antennas at three stations spaced roughly equally around the world in longitude, so that a distant space probe could at all times be viewed from at least one of the stations.32 For manned spaceflight a special network was linked to the Johnson Space Center in Houston.33 As needed these were supported by tracking-telemetering ships and aircraft furnished by the Navy and the Air Force. The rockets and spacecraft were a sine qua non of the space program and of space science. It is not surprising, therefore, that most of NASA's activity and resources went into the creation and operation of these vehicles. The scientific researches themselves, while not inexpensive, required only a fraction of what the tools-the spacecraft and launch vehicles-cost. Since the tools were where most of the money was going, Congress spent a great deal of time probing the budgets for them, and NASA managers became accustomed to thinking of their programs in terms of launch vehicles and spacecraft. One would speak of Ranger and Mariner programs, and of the Polar Orbiting Geophysical Observatory program and the Orbiting Solar Observatory program-or rather, to the distress of those to whom acronyms are anathema, of the POGO and OSO programs. From time to time scientists would chide NASA on this habit, pointing out that as far as space |