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Beyond the Atmosphere: Early Years of Space Science

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CHAPTER 10

 

SPACECRAFT

 

 

 

[140] What the United States could do with its launch capabilities was revealed by the spacecraft the country built and launched. As with the many rockets and rocket stages, one could easily become totally immersed in the subject of spacecraft, which displayed a bewildering variety of shapes, sizes, and purposes. A partial listing of U.S. and foreign spacecraft is given in tables 3 and 4.²³ As with launch vehicles, it is not necessary to know all the spacecraft in intimate detail to understand their role in the space science program. A few general concepts suffice. By dividing spacecraft into a few representative classes, one can understand their various functions better.

 

First, there was the sounding rocket, a rocket instrumented for high-altitude research and fired along a vertical or nearly vertical trajectory. To....

 

 

[141] Table 3

United States Spacecraft

(Partial list, excluding commercial and Defense Department spacecraft)

Earth Satellites
Vanguard	International Geophysical Year, Explorer class.
Explorer	Small satellite for near-earth missions.
Interplanetary Monitoring Platform (IMP)	Explorer-class satellite to explore cislunar and lunar space.
Orbiting Geophysical Observatory (OGO)	Observatory-class satellite for geophysical research.
Orbiting Astronomical Observatory (OAO)	Observatory-class satellite for solar studies.
High Energy Astronomical Observatory (HEAO)	Observatory-class satellite for stellar astronomy.
High Energy Astronomical Observatory (HEAO)	Very heavy, observatory-class satellite for studying shortwave and high-energy phenomena in cosmos.
Biosatellite	Observatory-class, recoverable satellite for life sciences.
Pegasus	Observatory-class satellite for micrometeoroid studies.
Echo	Applications satellite. Large metallized sphere for passive communications studies.
Relay	Active communications satellite.
Syncom	Active communications satellite in synchronous orbit.
Applications Technology Satellite (ATS)	Platform for a variety of applications technology researches, particularly in synchronous orbit.
Tiros	Large Explorer-class weather satellite.
ESSA	Operational version of Tiros.
Nimbus	Observatory-class weather satellite.
ERTS	Earth Resources Technology Satellite. Applications satellite devoted to earth resources research.
Synchronous-orbit Meteorological Satellite (SMS)	Applications satellite in synchronous orbit for meteorological research.
PAGEOS	Passive geodetic satellite.
GEOS	Active geodetic satellite with flashing lights and radio instrumentation.
LAGEOS	Geodetic satellite with comer reflectors for use with laser beams from the ground.
[142] Mercury	One-man satellite.
Gemini	Two-man satellite.
Apollo (earth-orbiting)	Three-man satellite.
Space Probes
Ranger	Lunar hard lander.
Surveyor	Lunar soft lander.
Lunar Orbiter	Lunar satellite for photography of the moon and lunar environment studies.
Apollo lunar module with command module	Manned lunar lander with manned lunar orbiter.
Pioneer	Explorer-class interplanetary probe.
Mariner	Observatory-class planetary and interplanetary probe.
Viking	Observatory-class planetary orbiter plus lander.
Voyager	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.)

 

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Table 4

Foreign Spacecraft

Soviet Spacecraft
Sputnik	Geophysical research satellite.
Luna	Unmanned lunar orbiter, lander, and return missions.
Vostok	First Soviet manned spacecraft.
Voskhod	Adaptation of Vostok to accommodate two and three cosmonauts.
Soyuz	Two- or three-man spacecraft, with working compartment.
Salyut	Earth-orbiting space station for prolonged occupancy and revistation by cosmonauts
Cosmos	Catchall name for variety of research and test spacecraft.
Venus (Venera)	Unmanned Venus probe.
Polyot	Earth satellite with onboard propulsion for changing orbits.
Elektron	Radiation belt satellite, launched in pairs.
Zond	Lunar and deep-space probe.
Molniya	Communications satellite in 12-hour orbit with low perigee and with apogee near synchronous-orbit altitude.
Meteor	Weather satellite.
Intercosmos	Soviet international satellite.
Oreol	Scientific satellite for upper atmosphere and auroral studies.
Mars	Unmanned Martian probe.
Prognoz	Satellite to study solar plasma fluxes.
Raduga	Geosynchronous communication satellite.
Ekran	Television broadcasting satellite.
Other Foreign Spacecraft, Launched with U.S. Cooperation
Ariel	United Kingdom satellite for geophysical and astronomical research.
Alouette	Canadian satellite for ionospheric research.
Isis	Canadian satellite for ionospheric research.
San Marco	Italian satellite for geophysical research.
French (FR-1)	French satellite for ionospheric research.
ESRO	European Space Research.
ASUR	German satellite for particles and fields research.
Skynet	United Kingdom satellite for communications.
NATO	NATO satellite for communications.
CAS (Eole)	French satellite for data collection and meteorology.
Barium Ion Cloud	German satellite for geophysical research.
ANIK	Canadian geosynchronous satellite for communications.
Aeros	German geophysical satellite.
ANS	Netherlands satellite for ultraviolet and x-ray astronomy.
INTASAT	Spanish satellite for ionospheric research.
Helios	German deep-space probe for interplanetary and solar studies inside the orbit of Mercury.
Symphonie	French-German satellite for communications.
COS	European Space Agency satellite for study of cosmic gamma rays.

 

 

[144] ....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 [145] 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 reference 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....

 

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[146-147] (MISSING PICTURES)

Figure 15. Explorer satellites. The great variety is impressive, but underlying that variety was a common technology.

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[148]

 

 

Figure 16. Orbiting Solar Observatory. OSO was the first observatory-class satellite built by NASA. In the photo above (fig.16), OSO 3 is prepared for tests before launch into orbit in 1967.

 

 

 

Figure 17. Orbiting Geophysical Observatory, a very capable spacecraft. Although once dubbed the "streetcar satellite," OGO never acquired the ease of preparation hoped for it. OGO 4's booms, antennas, and solar panels are folded in the photo above, for vacuum chamber tests and launch in 1967. OSO was the first observatory-class satellite built by NASA. In the photo above (fig.17), OSO 3 is prepared for tests before launch into orbit in 1967.

 

 

 

Figure 18. Orbiting Astronomical Observatory. OAO was the most complex and difficult of the observatory-class satellites initially undertaken by NASA. Above (fig.18), solar panels are mounted on OAO 3 in Preparation for its 1972 launch.

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[149] ...drift away, the greater force of gravity on it would pull that end back toward the earth again.

 

Observatory-size spacecraft were also large enough to carry additional rockets which, when fired against the direction of the satellite's motion, could return the spacecraft to earth, where its equipment and records could be recovered. Once on its way down, the spacecraft would have to be protected against heat generated by friction of the atmosphere, for which purpose retrorockets-that is, rockets fired in the direction of motion to slow the spacecraft-or parachutes, or a combination of the two, could be used.

 

Finally, there were the manned spacecraft (figs. 19-22). These were even larger than the unmanned observatories, since, in addition to housekeeping equipment and instruments, the spacecraft had to be completely maneuverable to return the astronaut to earth after the mission and also had to afford a suitable environment for the crew.

 

All three classes of satellites had their uses. The manned spacecraft introduced the element of exploration-of personal, on-the-spot investigation-into the program. With men aboard it became possible to extend laboratory research into the environment of space, as was done dramatically in Skylab and the Apollo-Soyuz Test Project.²⁶ When Space Shuttle development was begun in the early 1970s, one motivation was to make it relatively easy for researchers to perform experiments in space laboratories.

 

At the other end of the spectrum, Explorer-class satellites permitted scientists to perform a wide range of space science experiments, as shown in table 5. To the scientists, the lower costs meant that more of the funds available could be put into the scientific research itself. Also, the effort required to put an experiment into an Explorer was considerably less than that required for an observatory-one or two years as opposed to many years. When a discovery was made, an experimenter could more quickly follow up with new experiments on sounding rockets and Explorers than he could using observatories. Moreover, not having to fit his instruments alongside those of many other experimenters-with all the problems of electrical, radio, and other kinds of interference-the experimenter could exercise greater control over his experiment. These advantages account for the unvarying insistence of the scientific community that NASA continue to provide sounding rockets and small satellites. Whenever larger projects appeared to threaten the funding of the smaller ones, the scientific community rose in defense of the smaller. Over this issue the scientists came as near to unanimity as they ever did.

 

On the other hand, some investigations required a greater capability than the Explorers afforded. Astronomy experiments that required large telescopes and precision pointing are one example. Bioscience experiments that required the recovery of specimens after exposure to the environment of space-as in the Biosatellite-are another.²⁷ The Orbiting Geophysical Observatory afforded the means of conducting 20 or more experiments on...

 

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[150]

 

Figures 19-22. Manned spacecraft. Mercury (left) and Gemini (below [right]) laid the groundwork for Apollo (lower left) while Skylab (lower right) grew out of the Apollo technology.

 

 

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[151] Table 5

Examples of Investigations in Explorer-Class Missions

Spacecraft	Subject of Investigation
Vanguard 1	Satellite geodesy.
Vanguard 2	Cloud-cover studies.
Vanguard 3	Survey of earth's magnetic field and lower edge of radiation belts.
Explorer 1	Charged particle radiations in space.
Explorer 6	Radiation belt; meteorology.
Explorer 7	Energetic particles; micrometeoroids.
Explorer 8	Ionosphere; atmospheric composition.
Explorer 9	Atmospheric pressures and densities.
Explorer 10	Interplanetary magnetic field near earth; particle radiations.
Explorer 11	Gamma rays from space.
Explorer 12	Magnetospheric studies.
Explorer 13	Micrometeoroids.
Explorer 14	Charged particles and magnetic fields in magnetosphere.
Explorer 16	Micrometeoroids.
Explorer 17	Atmospheric composition.
Explorer 18	Charged particles and magnetic fields in cislunar space.
Explorer 20	Probe of topside of ionosphere.
Explorer 27	Geodesy by radio tracking methods.
Explorer 30	Solar x-ray studies.
Explorer 38	Radio astronomy.
Explorer 42	Catalog and study of celestial x-ray sources.
Pioneer 5	Interplanetary charged particles and magnetic fields.
Ariel 1	Ionospheric and solar research.
Ariel 2	Atmospheric research and radio astronomy.
Alouette 1	Charge densities in upper ionosphere and radiation belt studies.
San Marco 1	Atmospheric physics.
ESRO 2B	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 [152] Advisory 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 [153] 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 130,000 km from the planet's surface-taking pictures, measuring its radiations, and investigating some of its satellites-and then receded along a trajectory that would eventually carry it out of...

 

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Figure 23. Pioneer. The earliest of the U.S. space probes provided the kind of flexibility for the deep-space investigator that Explorers provided for earth-satellite missions. Pioneer 8 in the photo is prepared for December 1967 launch into orbit of the sun.

 

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[154] ....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 flights-observing 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 [155] 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....

 

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[156-157]

 

Figures 24-28. Lunar and planetary probes. These were the deep-space analogs of the observatory-class satellites: Ranger, above left; Surveyor, above right, on the lunar surface; Lunar Orbiter, lower right; [157] Mariner, opposite at top [below, left] ; Viking, lower opposite [below right]

 

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....science was concerned, the program was to investigate the magnetosphere, to probe the origin and evolution of the moon and planets, to understand the solar processes, etc. NASA managers agreed, of course, and indeed from the very outset NASA people set forth the scientific objectives, not the space hardware, as the purpose of the space science program.³⁴ But the shorthand was too convenient, and the practice persisted even among the scientists.

 

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