CHAPTER 20

CONTINUING HARVEST: THE BROADENING FIELD OF SPACE SCIENCE

As the decade of the 1960s neared its end, space science had become a firmly established activity. While the past had been immensely productive, world bent to the tasks that lay ahead. A steady stream of results poured the future promised much more and thousands of scientists around the into the literature; universities illustrated courses in the earth sciences, physics, and astronomy with examples and problems from space research, and a few offered courses devoted entirely to space science. For their dissertations graduate students worked with their professors on challenging space science problems. With the loss of that air of novelty and the spectacular that had originally diverted attention from the purposefulness of the researchers, the field had achieved a routineness that equated to respectability among scientists.

Maturity underlay the field’s hard-earned respectability. Starting about 1964, in addition to the individual research articles published in the scientific journals, more comprehensive professional treatments of the kind that characterizes an established, active field of research began to appear.¹ It is interesting, for example, to compare the book Science in Space published in 1960 with the second edition of Introduction to Space Science issued in 1968.² The matter-of-fact tone of the latter, which discussed what space science had already done and was doing for numerous disciplines, contrasts with the promotional tone of the former, which could only treat the potential of space science, what rockets and spacecraft might do for various scientific disciplines.

SPACE SCIENCE AS INTEGRATING FORCE

The breadth of the field as it evolved was impressive. Among the disciplines to which space techniques were making important contributions were geodesy, meteorology, atmospheric and ionospheric physics, magnetospheric research, lunar and planetary science, solar studies, galactic astronomy, relativity and cosmology, and a number of the life sciences. The assured role of space science in so many disciplines in the late 1960s was a source of considerable satisfaction to those who had pioneered the field, an ample justification of their early expectations.

But more significant was the strong coherence that had begun to develop among certain groups of space science disciplines. Perhaps the most profound impact of space science in its first decade was that exerted upon the earth sciences. Sounding rockets made it possible to measure atmospheric parameters and incident solar radiations at hitherto inaccessible altitudes and thus to solve problems of the atmosphere and ionosphere not previously tractable. Satellites added a perspective and a precision to geodesy not attainable with purely ground-based techniques. The improved precision laid a foundation for establishing a single worldwide geodetic network essential to cartographers who wished to position different geographic features accurately relative to each other. The new perspective gave clearer insights into the structure and gravitational field of the earth. These examples illustrate one of several ways in which space science was affecting the earth sciences; that is, making it possible to solve a number of previously insoluble problems.

Following James Van Allen’s discovery of the earth’s radiation belts one and the growing realization over the ensuing years that these were but aspect of a tremendously complex magnetosphere surrounding the earth, magnetospheric research blossomed into a vigorous new phase of geophysical research. This was a second way in which space science contributed to the earth sciences, opening up new areas of research.

But probably the most significant impact of space methods on geoscience was to exert a powerful integrating influence by breaking the field loose from a preoccupation with a single planet. When spacecraft made it possible to explore and investigate the moon and planets close at hand, among the most applicable techniques were those of the earth sciences, particularly those of geology, geophysics, and geochemistry on the one hand and of meteorology and upper atmospheric research on the other. No longer restricted to only one body of the solar system, scientists could begin to develop comparative planetology. Insights acquired from centuries of terrestrial research could be brought to bear on the investigation of the moon and planets, while new insights acquired from the study of the other planets could be turned back on the earth. Delving more deeply into the subject, one could hope to discern how the evolution of the planets and their satellites from the original solar nebula-it being generally accepted as and that the bodies of the solar system did originate in the cloud of gas and dust left over from the formation of the sun-could account for their similarities and differences.

The wide range of problems served to draw together workers from a number of disciplines. Astronomers found themselves working with geoscientists who came to dominate the field of planetary studies that had once been the sole purview of the astronomers. Physicists found in the interplanetary medium and planetary magnetospheres a tremendous natural laboratory in which they could study magnetohydrodynamics free from the constraints encountered in the ground-based laboratory. Also known as hydromagnetics, this field was an extension of the discipline of hydrodynamics to fluids that were electrically charged (plasmas), particularly their interactions with embedded and external magnetic fields. The scientific importance of the field stemmed from the realization that immeasurably more of the matter in the universe was in the plasma state than in the solid, liquid, and gaseous states of our everyday experience. An outstanding practical value lay in the fact that magnetohydrodynamics was central to all schemes to develop nuclear fusion as a power source. Physicists also found the opportunity to conduct experiments on the scale of the solar system attractive for the study of relativity, and many of them began to devise definitive tests of the esoteric theories that were in existence. It is safe to say that this interdisciplinary partnership was a valuable stimulation to science in general.

The expanding perspective derived from space science was, in the author’s view, the most important contribution of space methods to science in the first decade and a half of NASA’s existence. While it was natural for individual scientists to concentrate attention on their individual problems, to those who took the time to assess progress across the board, the growing perspective was clearly evident even in the early years of the program. In a talk before the American Physical Society in April 1965, the author addressed himself to the growing impact of space on geophysics, which even then appeared much as described above.³ NASA managers in their presentations to the Congress began to emphasize the important perspectives afforded by space science. As a case in point, the spring 1967 defense of the NASA authorization request for fiscal 1968 described space science as embracing (1) exploration of the solar system and (2) investigation of the universe.⁴ Gathering the different space science disciplines into these two areas was not simply a matter of convenience. Rather it reflected a growing recognition of the broadening perspective of the subject, a point that was further developed by Leonard Jaffe and the author in a paper published in Science the following July.⁵ At the time it was much easier to treat of the impact of space science on the earth sciences, which already offered many examples. While it would probably take a number of decades to achieve a thorough development of the field of comparative planetology, with an appreciable number of missions to the moon and planets behind and more in prospect, the powerful new perspectives available to the geoscientists were quite clear.

As for astronomy-the investigation of the universe-the deeper significance of the impact of space science on the discipline appeared to be unfolding more slowly. To be sure, the most obvious benefit-that of making it possible for the astronomer to observe all wavelengths that reached the top of the atmosphere, instead of being limited to only those that could reach the ground-began to accrue with the earliest sounding rockets that photographed the sun’s spectrum in the hitherto hidden ultraviolet wavelengths. This benefit grew steadily with each additional sounding rocket or satellite providing observations of the sun and galaxy in ultraviolet, x-ray, and gamma-ray wavelengths. The value of these previously unobtainable data was inestimable. But in the long run, a deeper, more significant impact of space methods on astronomy could be expected, as Prof. Leo Goldberg and others pointed out: the advent of a much more powerful means of working between theory and experiment than had ever existed before.

At one time the author tried to persuade the House Subcommittee on Space Science and Applications that, as far as the origin and evolution of natural objects were concerned, the scientist knew more about the stars than about the earth. The statement was intentionally phrased in a provocative fashion to get attention, which it did. The Congressmen reacted immediately in disbelief, and it took quite a bit of discussion to develop the point, which went as follows.

Certainly men living on the earth, as they do, had been able to amass volumes and volumes of data on the earth’s atmosphere, oceans, rocks, and minerals of a kind and in a detail that could not be assembled for a remote star. But, when it came to the question of just when, where, and how the earth formed and began to evolve many billions of years ago, the scientist was limited to a study of just one planet-the earth itself. From an investigation of that one body and whatever he could decipher of its origin and evolution, he had to try to discern the general processes that entered into the birth and evolution of planets in general. Only in such a broad context could the scientist feel satisfied that he really understood any individual case. Having only the earth to study, he was greatly hampered.

For the stars, however, the astronomer had the galaxy containing 100 billion stars to observe, and billions of other galaxies of comparable size. In that vast array the astronomer could find, for any object he might want to study, examples at any stage of evolution from birth to demise. With such a display before him in the heavens, the astronomer could proceed to develop a theory of stellar formation and evolution and test the theory against what he observed. In such an interplay between theory and observation the theorists did develop a remarkable explanation of the birth, evolution, and demise of stars.⁶ So, in this sense, the astronomer could claim to understand more about the stars than the earth scientist did about the earth.

But there was a shortcoming in this theoretical process. The theory was based on observations of those wavelengths that could reach the ground-mostly visible, with a little in the ultraviolet and infrared, and after World War II critical observations in some radio wavelengths. Yet hat very theory predicted that vitally important stellar phenomena would be manifested in the emission of wavelengths that the astronomer could not yet see. The early formation of a star from a cloud of gas and dust that was beginning to aggregate into a ball would be revealed primarily in the infrared as gravitational pressures caused the material to heat up. At the other end of the spectrum very hot stars would be emitting mostly in wavelengths shorter than the visible, presumably mostly in the ultraviolet. Little attention was paid to x-rays or gamma rays, yet among the most important discoveries of space science have been x-ray emissions from the sun and more than a hundred stellar sources.⁷ Here is where the most profound impact of space science upon astronomy could be expected in the decades ahead. just as the new-found ability to study other planetary bodies than the earth immeasurably broadened the perspectives of the earth sciences, so the ability of the astronomer to observe in all the wavelengths that reached the vicinity of the earth could be expected to strengthen the interplay between theory and experiment in the field of astronomy. By the 1970s the process had already begun, but the full power would doubtless have to wait until astronomers had the benefit of a variety of satellites more powerful than the solar and astronomical satellites of the first decade. In addition to large, precise optical telescopes, which one naturally thought of in the 1950s and early 1960s, there would also have to be specially instrumented spacecraft to pursue the new field of “high-energy astronomy" which leapt into prominence with the early discovery of x-ray sources. One would also need both infrared and radio telescopes in orbit. In short, to make the most of the opportunity that had burst upon the astronomical community, there would have to be established in orbit a rather complete facility consisting not just of a single instrument, but of a set of instruments ranging across the whole observable spectrum.

As for the life sciences, space appeared able to contribute in a variety of ways. One could expose biological specimens-including the crews of manned spacecraft-to the environment of space and observe what happened. But the biologists agreed that the most significant contribution of space science to their discipline could well be in exobiology-the study of extraterrestrial life and the chemical evolution of planets.⁸ This subject was subsumed under the study of the solar system, since the evolutional histories of the planets, the kinds of conditions they developed on their surfaces and in their atmospheres, would have much to do with whether life formed on the planet, or with how far a lifeless planet moved toward the formation of life.

Thus, as one moved into the 1970s, although space scientists could take much satisfaction in the wide variety of individual disciplines to which they had been able to contribute, it was the new perspective that brought groups of disciplines together in a common endeavor that was most important.

EXPLORATTON OF THE SOLAR SYSTEM

By the end of the 1960s study of the solar system extended from Earth to the nearest planets, Mars and Venus; and men had landed on the moon. In the next two years the Apollo astronauts made a searching exploration of the moon, after which Skylab crews turned attention toward Earth and the sun. During the early 1970s unmanned spacecraft also added Mercury, the asteroids, and Jupiter with some of its satellites to the list of objects in the solar system that scientists’ instruments had been able to reach and observe, and had begun the long trek to Saturn and the outer planets.

Geodesy

Around Earth, satellite geodesy continued to advance steadily. As shown in chapter 11, the first half-dozen years of geodetic research using space techniques were largely years of preparation for the ultimate coup, that of establishing a worldwide geodetic net referred to a common reference ellipsoid. Year by year geodesists moved steadily toward that goal. By 1970 such a reference system was in use, at least among the principal experts, and positions relative to the common reference could be given to 15 meters or better. As for Earth’s gravitational field, good estimates had been obtained for coefficients for the various harmonics in the spherical harmonic expansion of the geopotential up to at least the 19th degree. Geodesists were quick to point out that, in a little more than a decade, scientists had increased the quantitative knowledge of global positioning and of the size and shape of Earth by 10 times, and knowledge of the gravity field by 100 times-the same order of improvement as had been achieved in the previous 200 years.⁹

By the mid-1970s another order of magnitude had been realized in positioning techniques, and one could begin to zero in on accuracies sufficient to match variations in mean sea-level height (centimeters) and the very slow movements of tectonic plate motions and continental drift (centimeters per year). Using a combination of satellite techniques, observations on quasars and pulsars with a method called very long baseline interferometry, and extremely accurate clocks (1 part in 10¹⁶) that might be developed with superconducting cavities, one could aspire to positional accuracies of several centimeters relative to the reference ellipsoid.

But, as anticipated, geodesy did not remain Earthbound, although first extension to another planetary body was more by chance than otherwise. Tracking of Lunar Orbiters, five of which were put into orbit of the moon between 10 August 1966 and 1 August 1967, showed a peculiar perturbation of the orbital motion over a number of the circular maria of the moon. Analysis soon indicated that these disturbances were probably caused by unusual concentrations of mass in the maria basins. These mascons, as they came to be called, became one of the many puzzles in connection with the evolution of the moon that scientists had to try to explain. Some suggested that they were caused by heavy metallic material from slow-moving iron meteorites that had gouged out the basins where the mascons are now found. Or they might be due to lavas of different densities that filled the basins with the material now observed in the maria. They could be plugs of denser material from a lunar mantle that was shoved upward after the basins had been formed by impacting meteorites. Those making the above suggestions considered the mascons as strong evidence that the moon was quite rigid. But there were other opinions, as John O’Keefe points out:

This [a rigid moon] is more or less how Urey saw it. What most of the rest of us saw was that the moon was in imperfect isostatic equilibrium like the earth. Apart from the mascons, there was isostatic equilibrium. Kaula pointed out that the earth, like the moon, has important deviations from isostasy, like the Hawaiian mass. Urey … overestimated the significance of the “mascons" as indicating a difference between the earth and the moon. He maintained that the moon was more rigid than the earth right up to the time when actual measurements showed that in fact it is less rigid.¹⁰

Whatever the mascons might turn out to be, however, they were an exciting and clearly significant discovery of the first extension of geodesy into the rest of the solar system.

Atmospheric and Ionospheric Studies

Like geodesy, upper atmospheric research and ionospheric physics continued to build upon the groundwork established earlier (chapter 6). Indeed, to the nonexpert the research and results could easily appear to be more of the same. But to the expert progress in the field was nothing short of phenomenal. Questions that had been uppermost in the experimenter’s mind a decade before were quickly answered. It was shown that above 200 kilometers the neutral atmosphere became essentially isothermal, varying with the solar cycle from around 600 kelvins at sunspot minimum to somewhat more than 2000 K at times of very high solar activity. As had been expected the lighter gases, helium and hydrogen were found to predominate above 600 or 700 km, the outermost portions being largely hydrogen. Above 1000 km the positive ions He⁺ and H⁺ of helium and atomic hydrogen exceeded the concentrations of atomic oxygen 0⁺, which was the major ionic constituent of the F2 region from 150 km to above 800 km.¹¹ At considerably lower altitudes, the D region of the ionosphere was found to be surprisingly complex. In 1965 experimenters found that D-region ionization below 80 km consisted primarily of hydrated protons. Some years later it was shown that negative ions in the D region tended to form complex hydrated clusters.¹² Most important, the various solar radiations responsible for the ionization of the different layers were completely mapped. As Herbert Friedman of the Naval Research Laboratory put it in 1974:

The solar spectrum is now known with high resolution over the full electromagnetic range. Spectroheliograms in all wavelengths have revealed the spatial structure over the disk [of the sun] and in the higher levels of the corona. We may conclude that the major features of the electromagnetic inputs to and interactions with the ionosphere are well understood.¹³

One of the most significant findings was that the upper atmosphere and ionosphere could not be considered to be in or even near equilibrium, previously a common assumption. For example, temperatures of the electrons in the upper ionosphere were found to be appreciably higher than the temperature of the ambient neutral gas, while the ion temperatures were intermediate between the two.¹⁴ A continuing round of dynamic processes characterized these regions, as solar radiations and the day-night cycle set in motion a complex chain of chemical and physical reactions. Also, in the light of space discoveries, the ionosphere was seen to be a part of the much greater magnetosphere within which one could identify a plasmapause, inside of which the hot plasma composing the ionosphere rotated with Earth and outside of which particle radiations, no longer co rotating with Earth, exhibited convective motions induced by the solar wind.¹⁵

But while such detail may be of interest as answering questions that a decade before still puzzled researchers, their true significance lay in the fact that by the 1970s all known major problems of the high atmosphere and ionosphere had a satisfactory explanation based on sound observational data. From then on research in the upper atmosphere and ionosphere could be regarded as largely a mopping-up operation, the investigation of the finer details of what was going on.

Meteorology

As for meteorology, the story was somewhat different. Sounding rockets provided the means for making measurements at all levels within the atmosphere, and satellites furnished worldwide imaging of cloud systems plus observations of atmospheric radiations and temperature profiles. These data amplified by orders of magnitude the amount of information available to the meteorologist, filling in enormous gaps that had existed over the oceans arid uninhabited land areas. But most of the impact of these data was on the forecasting of weather and climatic trends, where their contribution was of inestimable value. In a decade and a half of giant strides in practical meteorology brought about in part by space methods, nothing of a revolutionary nature was contributed to the science of the lower atmosphere. In the mid-1970s atmospheric scientists still had to admit that there had been no breakthroughs attributable to space observations, although a wealth of new information was available and was under continuing intensive study. Most researchers were, however, optimistic that in the years ahead space data would share with ground-based, balloon, and aircraft measurements in leading ultimately to the breakthroughs in that understanding of the atmosphere needed to provide long-term forecasts of both weather and climate and to predict accurately the place and time of occurrence of severe storms.

If there was no general breakthrough, there were several intriguing contributions from space research. For one thing, as with other areas of the earth sciences, the perspective afforded by satellite imaging was a great stimulus to research. The ability to see and assimilate with ease the distribution and kinds of clouds, the location and nature of weather disturbances, the distribution of vorticity, etc., gave the researcher a new handle on his subject. As one result, tropical meteorology, once regarded as a dead-end field, sprang to life; and scientists began to develop new insights into the relations between the tropics and mid-latitudes.

Most of the sun’s radiation lies in the visible wavelengths, which, along with some infrared and ultraviolet, control Earth’s weather. This portion of the sun’s spectrum is remarkably constant over time, although the question of just how constant remains open, and is one of the problems that space methods may help to solve. The short wavelengths, on the other hand, are extremely variable, their changes causing enormous variations within the high atmosphere. A natural question, then, was whether these upper atmospheric and ionospheric changes might not be related to meteorological changes. But, although sudden warnings of the stratosphere appeared to be associated with solar ultraviolet radiations, the general view was that this portion of the solar spectrum, containing less than one one-millionth of the energy carried in the visible wavelengths, could hardly have any significant effect. Yet, after more than a decade of space research, intriguing hints of relationships between upper atmospheric and meteorological activity began to appear.¹⁶ For example, particles-and-fields research had shown that the interplanetary medium around the sun was divided into sectors in some of which magnetic fields were directed away from the sun, while in others the magnetic fields were pointed generally toward the sun (fig. 46). The two kinds of sectors alternated with each other in going around the sun.¹⁷ Quite remarkably the boundaries between these sectors appeared to be associated with changes in atmospheric vorticity. Here was a phenomenon that could have a profound significance and the existence of which lent substance to the question of magnetospheric and upper atmospheric relationships to meteorology.

As space scientists were getting a firm grip on the physics and chemistry of Earth’s upper atmosphere, their attention was simultaneously be drawn toward the planets. What was known about planetary atmospheres had come from the efforts of a small, select group of scientists, mostly astronomers.¹⁸ Even more remote from the astronomers than Earth’s upper atmosphere had been from the geophysicists, the atmospheres of Mars, Venus, and the other planets taxed the investigators’ ingenuity. Gross uncertainties often existed in their estimates of atmospheric properties. As with Earth’s upper atmosphere, measurements from space probes promised to eliminate or reduce many of the uncertainties.

The Soviet Venera spacecraft in 1970 and 1972 removed any doubt that the ground-level pressure of Venus’s atmosphere was about 100 times that of Earth, and the Jet Propulsion Laboratory’s Mariners showed that Mars’s atmosphere was roughly one percent that of Earth. The atmospheres of both Venus and Mars were established as primarily carbon dioxide, as had been concluded from ground-based observations. As radio astronomers had already shown, the surface of Venus was confirmed to be in the vicinity of 700 K and fairly steady, while that of Mars was somewhat colder than Earth’s and varied appreciably with the seasons. Whereas both Earth and Mars rotated rapidly with approximately the same periods, ground-based radar measurements showed Venus to turn very slowly-once every 243 days-in the opposite direction to that of the other planets.¹⁹

From the point of view of comparative planetology, the relations between Venus, Earth, and Mars were ideal. Earth was clearly intermediate between the two others in many respects, and many scientists felt that a detailed study of all three should be of special benefit in understanding Earth. An example of the kind of interplay that was possible was furnished by the study of the role of halogens in the atmosphere of Venus. The investigations led to the suspicion that chlorine produced in Earth’s stratosphere from the exhausts of Space Shuttle launches or from freon used at the ground in aerosol sprays might dangerously deplete the ozone layer, which was known to shield Earth’s surface from lethal ultraviolet rays of the sun. In a similar manner, when Venus’s atmosphere was found to exhibit a single circulation cell, global in extent,²⁰ it was recognized that a careful study of this special example could yield important insights into the terrestrial atmosphere, where numerous circulation cells interact on a rapidly rotating globe. The clouds on Venus had long been a mystery, in which stratospheric aerosols now appeared to play a key role. The unraveling of the precise role of aerosols in the Venus atmosphere would certainly benefit studies of chemical contamination of Earth’s atmosphere. At the other end of the scale, the role of dust storms in the thin Martian atmosphere could lend an important additional perspective to the role of dust in modifying Earth’s climate. On a much grander scale, as Pioneer spacecraft passed by Jupiter in 1973 and 1974 it was learned that the famous red spot ²¹ was a huge hurricane large enough to engulf three Earths. What might be learned from the Jupiter hurricane about atmospheric dynamics that could be applied to the case of Earth remained to be seen.

The first planetary ionosphere other than Earth’s to be detected experimentally was that of Mars. By observing the influence of the planet’s atmosphere on radio signals from Mariner 4 as those signals traversed the planet’s atmosphere just before the spacecraft was occulted by the planet, it was possible to obtain an estimate of electron density as a function of altitude. The ionosphere, which was observed at a period of minimum sunspot activity, was somewhat less developed than expected from terrestrial analogies. Later, at times of greater activity, Mariner 6, 7, and 9 revealed a slightly more intense ionosphere, showing a noticeable dependence on the solar cycle. The same sort of occultation experiment on Mariner 5 (1967) gave electron density profiles for Venus, both dayside and nightside. The nightside ionosphere was almost two orders of magnitude less intense than the daytime ionosphere, which showed a distinctly higher electron density than that of Mars.²² Jupiter was shown to have a well-developed ionosphere.²³

Thus, during the 1960s, while satisfactory answers were being obtained for all the known, major problems of Earth’s high atmosphere, a good start was made on the investigation of the atmospheres and ionospheres of other planets.

Magnetospheric Physics

A genuine product of the space age, magnetospheric research also moved on apace. During the first half-dozen years the principal task was for the experimenters to produce an accurate description of the magnetosphere, although once the existence of the radiation belts, a terrestrial magnetosphere, and the solar wind had been revealed by actual observation in space, a host of theorists vied with each other to devise explanations. Indeed, as was pointed out in chapter 11, Eugene Parker’s seminal paper on the solar wind antedated the detection of the wind and met with a great deal of flak until his critics were silenced by the space observations. The situation was typical for science; often many competing theories produce a continuing argument which no one can win until specific measurements become available to weed out those theories that don’t fit the data. After the initial years of discovery and survey, however, the main action shifted to the theorists,²⁴ although the experimentalists continued to amass additional data from both satellites and space probes.²⁵

At the end of the 1960s the theorists could explain many features of sun-earth relations, the interplanetary medium, and the magnetosphere, but a large number of fundamental questions remained to be resolved. To the layman a schematic picture of the magnetosphere drawn at the end of the decade might look much like that of figure 35 (chapter 11), produced a half-dozen years earlier. But the expert would read into that diagram a new collection of rather subtle questions that still had to be answered before one could claim to have a thorough understanding of magnetospheric physics.²⁶ The initial reaction to the discovery that Earth’s magnetic field generated a huge bow shock in the rapidly moving solar wind was to apply hydrodynamical theory. The general shape and position of the bow shock and the magnetopause could be understood from magnetohydrodynamical principles. Also, it appeared that one might explain the sudden commencement and initial phase of magnetic storms in a straightforward way. But the theory could not explain why the solar wind appeared to apply a surface drag to Earth’s field, pulling some of the field lines out into the long geomagnetic tail that was a spectacular aspect of the magnetosphere. Instead one would expect the magnetospheric cavity to close off in a teardrop shape.

To resolve some of these difficulties attention turned to the idea that the solar wind was a collisionless plasma, and the bow shock a collisionless phenomenon. Since under this assumption particle- to-particle collisions would be negligible, one had to seek the cause of the bow shock in cooperative field effects, such as interactions between electrostatic fields of the charged particles and magnetic field components perpendicular to the direction in which the gas velocity changed. As the 1970s opened, a great deal of study was going into collisionless shocks, particularly turbulent shocks, which observations showed Earth’s bow shock to be.

Other problems that required attention were the wide range of geomagnetic activity, the acceleration of particles within the magnetosphere, the production of the auroras, and the formation of the geomagnetic tail. In connection with these matters, the idea that parallel and opposite magnetic fields might merge and annihilate each other aroused stormy debate. One could point to cases in which such a process might be important. For example, if the field in the solar wind had a southward component when it struck Earth’s magnetic field at the nose of the magnetosphere, where the terrestrial field would be northward, merging and annihilation might take place. Or merging could occur when field lines in the interplanetary medium happened to be essentially parallel to field lines in the magnetospheric tail.

With such problems the magnetospheric physicists had an agenda that would keep them amply occupied during the 1970s. Moreover, early in that decade they got their first look at another planetary magnetosphere-that of Jupiter.²⁷ As the fascinating complexity of Earth’s magnetosphere and its important role in sun-earth relationships had unfolded, physicists had immediately thought of the possibility of other planetary magnetospheres. It was known from observation of Jovian radiations that Jupiter had a very strong magnetic field.²⁸ As a consequence no one doubted that the first spacecraft to reach the giant of the solar system would encounter a well defined magnetosphere.

Until instruments could be sent to the nearer planets, the question remained open as to whether they also had magnetospheres. The first Soviet Luniks showed that the moon had very little magnetic field, an observation that was confirmed repeatedly in later U.S. missions.²⁹ Although some magnetism was found in the lunar crust, it became quite clear that the moon did not have a poloidal field such its the dipolar field of Earth. Accordingly there wits no lunar magnetosphere analogous to that of Earth. Similarly the Mariner that flew by Venus in 1962 could detect no magnetic field, nor could Mariner 4 when it reached Mars in 1965.³⁰ But, surprisingly, Mercury when reached in 1974 turned out to have an appreciable field.³¹

These circumstances provided the space scientists an opportunity for comparative studies in sun-planetary relations, all opportunity that in the early 1970s was still largely unexploited. As has already been shown, Earth provided the case of a planet with a sizable atmosphere and a strong magnetosphere, both of which were involved in intricate ways in the processes by which the sun exerted its influence on the planet. At the other extreme the moon provided the case of a body with neither magnetosphere nor atmosphere, so that solar radiations impinged directly upon the lunar surface. For Venus the sun’s particle radiations struck the atmosphere directly, unmodified by a magnetosphere; but the extremely dense atmosphere shielded the planet’s surface completely from these radiations. Mars, with its thin atmosphere but also without a magnetosphere was intermediate between the moon and Venus. Mercury, on the other hand, provided an example of a planet with a magnetosphere but no atmosphere. How such a magnetosphere would differ from Earth’s, which was continually interacting with the planet’s atmosphere and ionosphere, was an interesting subject for investigation, one that doubtless would be explored over the years ahead.³²

In December 1973 Pioneer 10 reached Jupiter, followed a year late Pioneer 11. Their instruments revealed a huge magnetosphere reaching 7.6 million kilometers into space. Within the magnetosphere were radiation belts ten thousand to a million times as intense as those of Earth. Jupiter’s magnetosphere reached well beyond the orbits of its four largest satellites, those observed first by Galileo centuries ago. The innermost satellite, Io, appeared to interact strongly with the magnetospheric radiations.³³ But to pursue these fascinating investigations any further would go beyond the scope of this book.³⁴

Planetology

The final topic of this section concerns the planetary bodies themselves. While investigation of Earth’s atmosphere, ionosphere, and magnetosphere-and related solar studies-were naturally the first areas of research in the space science program, they had only limited appeal to the layman. As exciting as these challenges were to the researchers, the average person could hardly relate to himself the magnetohydrodynamic concepts, terrestrial ring currents, or complex photochemical reactions in the ionosphere. But the investigation of the moon and planets was different. Here in pictures one could see landscapes and clouds-often strange, to be sure, but landscapes and clouds nevertheless. One could envision spacecraft orbiting a planet or landing on its surface and Could identify personally with astronauts stepping onto the bleak and desolate moon. As a consequence NASA had little difficulty in capturing and holding a widespread interest in this aspect of the space science program.

The exploration of the moon and planets began with the Soviet Luna flights in 1959. From that time on, every year at least one mission to the moon or a planet was attempted by the United States or the Soviet Union. The American assault on the moon began with Pioneers, followed by Rangers, then the soft-landing Surveyors. In the summer of 1966 the first of five Lunar Orbiters began the task of mapping almost the entire surface of the moon. Even an Explorer was injected into lunar orbit to study the space environment around the moon. The climax was reached when the Apollo missions began manned exploration of the moon with the orbital flight of Apollo 8 in December 1968 and the first manned landing in July 1969. While Apollo was in progress the Soviet Union conducted a series of sophisticated unmanned lunar missions that included circumlunar flights of Zond spacecraft, which were successfully recovered with pictures they had taken of the moon. More advanced Luna spacecraft soft-landed on the moon, carrying roving vehicles to investigate the lunar surface in situ and radio the information to Soviet stations on Earth, and in some cases to send samples of lunar soil to Earth for investigation in the laboratory. The success of the Soviet unmanned rovers and sampling missions sparked an intense debate between the scientists and NASA, many of the scientists feeling that the unmanned approach to the study of the moon was the wiser, and by far the more economical.

Criticism was blunted, however, by the tremendous success of Apollo. The astronauts brought back hundreds of kilograms of lunar rocks and soil from six different locations, the analysis and study of which quickly engaged the attention of hundreds of scientists throughout the United States and around the world. In addition to collecting lunar samples, the astronauts also set up nuclear-powered geophysical laboratories instrumented with seismometers, magnetometers, plasma and pressure gauges, instruments to measure the flow of heat from the moon’s interior, and laser corner reflectors for geodetic measurements from Earth. The geophysical stations operated for many years after the astronauts had left, radioing back volumes of information on the moon’s environment and its seismicity. Twice satellites were left behind in lunar orbit for lunar geodesy and to make extended chemical analyses of the lunar surface material from observations of the short-wavelength radiations of the moon.

The United States claimed the first success in planetary exploration when Mariner 2 , launched in the summer of 1962, passed by Venus the following December, probing the clouds, estimating planetary temperatures, measuring the charged particle environment of the planet, and looking for a magnetic field.³⁵ In 1965 Mariner 4 flew by Mars to take 21 pictures, covering about one percent of the planet’s surface. Then Mariner 5 visited Venus, this time getting substantially more data on the atmosphere, including estimates of the ionosphere. It was back to Mars in 1969 with Mariner 6 and 7, which returned some 200 pictures of the surface along with a variety of other measurements. Mariner 9 went into orbit around the Red Planet in November 1971 at a time when the planet was almost completely obscured by a global dust storm. During the next month and a half the spacecraft monitored the clearing of the dust storm, which itself provided much interesting information about the planet. After that the spacecraft’s cameras were devoted to the first complete mapping ever achieved of another planet-Mariner 9 returned 7329 pictures of Mars and its two satellites, which permitted drawing up complete topographical maps showing the true nature of the markings that had so long puzzled astronomers.³⁶

During the 1960s the Soviet Union had also set its sights on the planets. In fact, its attempts appreciably outnumbered those of the United States, since the USSR seized virtually every favorable opportunity to try a launching. But early Soviet planetary endeavors were about as dismal as America’s early lunar tries. Not until a Venera spacecraft in 1970 succeeded in penetrating the atmosphere of Venus, returning data on the composition and structure of the atmosphere, did fortune smile on these planetary attempts. In December 1970 Venera 7 landed and returned data from the surface of Venus; Venera 8 followed suit in July 1972, for 50 minutes sending back surface data and analyses of the soil of Venus. Less fortunate, however, were the two Soviet Mars landers which, like Mariner 9, also arrived at the planet during the great dust storm of 1971. The storm that provided the Mariner with an unexpected opportunity to observe the dynamics of the planet’s atmosphere may have been the cause of the Soviet spacecraft’s failure to land successfully.³⁷

In November 1973, Mariner 10 left on a journey that would take it first by Venus and then on to Mercury, where the spacecraft arrived in March 1974 taking pictures and making a variety of other measurements. Having completed its first Mercury mission, Mariner 10 was redirected by briefly firing its rockets so that the spacecraft would visit Mercury again in September of 1974. By visiting Mercury several times, Mariner 10 provided the scientists with the equivalent of several planetary missions for little more than the price of one.³⁸ Also, with the visit to Mercury, scientists at long last had close looks at all of the inner planets of the solar system, including the two satellites of Mars.

The result of all these space probe missions was the accumulation of volumes of data on the moon and near planets, illuminated with thousands of highly detailed pictures. The photo resolutions exceeded by orders of magnitude what had been possible through telescopes. When Rangers crashed into the moon the closeup pictures sent back just before the impact were a thousandfold more detailed than the best telescopic pictures previously available (fig. 47). After landing on the surface with its television cameras, Surveyor afforded another thousandfold increase in resolution, revealing the granular structure of the lunar soil and a considerable amount of information on the texture of lunar rocks (fig. 48). In the laboratory Apollo samples put the moon’s surface under the microscope, as it were (fig. 49). As for the planets no detail at all had been available before on the surface characteristics of distant Mercury or clouded Venus. Some Mariner 10 pictures afforded better resolution for Mercury than Earthbased telescopes had previously given for the moon (figs. 50-51). Ultraviolet photos of Venus from passing spacecraft showed a great deal of structure in the atmospheric circulation that was hitherto unobservable (fig. 52), while radar measurements from Earth penetrated the clouds to reveal a rough, cratered topography.³⁹ For Mars, the indistinct markings observable from Earth were replaced with sufficient detail to show craters, volcanoes, rifts, flow channels, apparently alluvial deposits, sand dunes, and structure in the ice caps (figs. 53, 54-55, 56, 57, 58, 59, 60). Added to such pictures, data on planetary radiations in the infrared and ultraviolet; surface temperatures; atmospheric temperatures, pressures, and composition (when there was an atmosphere); and charge densities in the ionosphere (when there was an ionosphere)-this wealth of information completely revitalized the field of planetary studies, which had long been quiescent for lack of new data. By the early 1970s comparative planetology was well under way, although one must hasten to add that the task ahead of understanding the origin and evolution of the planets was one of decades, not merely months or years.

Nevertheless, progress was rapid. Much was learned about the mineralogy and petrology of the moon, and by extrapolation probably about the other terrestrial planets. Radioactive dating of lunar specimens led to the conclusion that the moon is probably some 4.6 billion years old, an age consistent with the ages of meteorites and the presumed age of Earth. The moon was found to be highly differentiated; that is, the lunar materials, through total or partial melting, had separated into different collections of minerals and rock types. The maria were mainly basalt, similar to but significantly different from the rocks of the ocean basins on Earth. In contrast the lunar highlands were rich in anorthosite, a rock consisting mainly of the feldspar calcium aluminum silicate. Both maria and highlands were much cratered-as could already be seen from Earth-and one could now see that the crater sizes extended down to the very small, showing that the moon had been bombarded by very small particles as well as by very large objects. The entire surface was covered with fine fragments and soil, broken rocks and rubble-crustal material chopped up by the cratering process. A considerable amount of glass was found, some of it in coatings splashed onto other rocks, much of it in the form of tiny glass beads of a variety of colors dispersed through the soil.

Reading through the record imprinted on the lunar surface one could bit by bit piece together the course of the moon’s evolution. Contrary to expectations of many, before the unmanned and manned space missions showed differently, the present lunar surface was not a virginal record of conditions that existed at the time of the moon’s formation. Instead one could discern a continual, sometimes violent process of evolution. Either at the time of formation or very shortly thereafter the moon’s crust was molten to a considerable depth. Presumably during this phase the differentiation of lunar materials took place, producing the light-colored, feldsparrich highlands. After solidifying about 4.4 billion years ago, the highlands were bombarded for hundreds of millions of years, probably by material left over from the formation of the moon and planets. This process produced the cratered topography of the highlands still visible today. Between 4.1 and 3.9 billion years ago the bombardment of the moon became cataclysmically violent, with asteroid-sized meteorites gouging out the great basins, hundreds and even thousands of kilometers across (fig. 61). Then, as radioactive dating of lunar materials showed, between 3.8 and 3.1 billion years ago a series of eruptions of basaltic lavas filled the basins to form the maria, or dark regions of the moon clearly visible from Earth. By 3 billion years ago the violent evolution of the moon had come to an end. Cratering and “gardening" of the lunar surface continued, but at about the rates observed today, most of the impacting particles being micrometeor and grain sized, occasionally cobble sized, with the very large impacts exceedingly rare.⁴⁰

The moon observed by Apollo instruments was very quiet. The lack of any substantial organized magnetic field suggested that there was no molten core, but the presence of magnetism in lunar rocks indicated that there might have been a core at some time in the past-on the assumption that a liquid iron core was required to generate the lunar field that magnetized the lunar rocks. Seismometer data showed that the moon was at least partly molten below 800 km and suggested a small, possibly iron, core a few hundred km in radius. The energy released by moonquakes detected by the Apollo seismometers was nine orders of magnitude-a billion times-less than that released by earthquakes over a similar period of time, which, however, seemed consistent with the relative sizes of Earth and moon. The seismic data yielded the picture of a lunar crust 50 to 60 km thick, four times the average thickness of Earth’s crust, underlain by a mantle solid at the top but partially melted toward the bottom (fig. 62). The very thick crust could account for the lack of any recent volcanic activity.⁴¹

Before these investigations it was common to think of the moon as a small body and to suppose that small bodies would remain cold and essentially unchanged throughout their histories. Now it was clear that bodies the size of the moon, and even smaller ones, whether formed in the molten state or melted after formation, undergo a substantial evolution. This conclusion was borne out further by data from the other planets. Mercury appeared even more cratered than the moon. There was widespread evidence of lava flows on the planet. Large cracks and long scarps were visible in the Mariner 10 pictures. There was no doubt that Mercury underwent a great deal of evolution after its formation. ⁴²

The evidence of activity on Mars was even more striking. After a period of discouragement for scientists when the Mariner 4 pictures appeared to show a dead, moonlike planet, the pictures of Mariner 6 and 7 revived interest, and those of Mariner 9 aroused excitement. Those pictures showed huge volcanoes, one of them-Olympus Mons-twice the diameter of the largest known volcanic structure on Earth, namely, the big island of Hawaii. A great deal of the Martian surface was cratered, indicating an ordeal of bombardment like that experienced by the moon and Mercury. Some of the surface was smooth and featureless, indicating a process of filling in as with blowing, drifting sands. A huge rift, comparable in length to the width of the United States, indicated considerable tectonic activity (fig. 56). Numerous channels hundreds of kilometers long looked as though they might have been produced by flowing water (fig. 57). Consistent with this observation were the frequent formations that looked like alluvial fans produced by the deltas of terrestrial rivers or sedimentary deposits of meandering streams (fig. 58). While the variable frost cover observed in the north and south polar caps was shown to be solid carbon dioxide, a substantial base of frozen water was also found. Various estimates suggested that a considerable amount of water must have outgassed from the planet over time, and if past conditions on the planet were just right there could have been ponds and rivers. But the question of the role of water in the evolution of the planet remained unsolved.⁴³

It was very clear that Mars is an active planet, by no means dead, as some had prematurely concluded. Some investigators thought they could detect in the marked differences between the cratered highlands of the planet and the volcanic provinces the suggestion of an incipient separation into individual tectonic plates as on Earth, a picture not generally accepted by students of Mars. Paul Lowman, however, was led to conclude that the evidence was piling up that earthlike planetary bodies would follow similar courses of evolution.⁴⁴ In the larger bodies like Earth, the rates of evolution would be faster and the duration longer than for the smaller planets. Of the inner planets, Earth, still vigorously active, was most advanced in its evolutionary course (fig. 63). Venus might be in a comparable stage, but no life evolved there to convert the carbon dioxide atmosphere to one with large amounts of oxygen. Mars was following the course taken by Earth, but was well behind, only now approaching the tectonic plate stage. The moon and Mercury had long since run the course of their evolution, which terminated well before a tectonic plate stage.

This picture, although consistent with much of the data, could hardly be regarded as more than tentative. It would have to pass the test of further observations and measurement, and stiff debate. But one satisfying feature was the emphasis the theory gave to the kinship of the planets with each other. As theorists had pointed out, if the planets did form from the material of a solar nebula left over after the creation of the sun, then their individual characteristics should depend to a considerable extent on their distances from the sun (fig. 64). Near the sun, where the nebular material would be heated to rather high temperatures by the sun’s radiations, one could expect to find planets composed primarily 9f materials that condense at high temperatures, the silicates and other rock-forming minerals. Moreover, the densities of the planets could be estimated according to distance from the sun by considering what compounds were likely to form at the temperature to be expected at Mercury’s distance, which at the distance of Venus, which in the vicinity of Earth, etc. From what was known of the inner planets, they did indeed fit such a picture.

As for the outer planets, one would expect them to consist of large quantities of the lighter substances-hydrogen, helium, ammonia, methane-which could condense out of the solar cloud only at the low temperatures that would exist so far from the sun. Qualitatively, the outer planets also fitted this picture, but quantitatively there were discrepancies. To develop the true state of affairs in proper perspective, an intensive investigation of the outer planets was called for, and was on the agenda for the 1970s and 1980s. The investigation got off to an exciting start with the visit of Pioneer 10 to Jupiter in 1973. It was clear that an exciting period in planetary exploration lay ahead as scientists began to amass data on the atmospheres, ionospheres, and magnetospheres of these strange worlds. While these planets themselves would be quite different from the terrestrial planets, their satellites could be expected to resemble the latter in many ways. Moreover, as many persons pointed out, the satellite systems of Jupiter and Saturn might turn out to be very much like miniature solar systems, particularly the satellites that formed along with the parent planet rather than being captured later. Supporting this view was the early discovery from the Pioneer observations that the four regular satellites of Jupiter decreased in density with increasing distance from the planet, as though they had formed from a cloud of gas and dust that was hotter near the planet than it was farther away.⁴⁵ The opportunities for important research seemed endless.

Finally there was the question of extraterrestrial life. Space research on fundamental biology was early divided into two areas: (1) the study of terrestrial life forms under the conditions of space and spaceflight, and (2) exobiology. The former was discussed briefly in chapter 16. The latter came to mean the search for extraterrestrial life and its study in comparison with Earth life. Most scientists considered the chance of finding life elsewhere in the solar system to be minute, but it was universally agreed that the discovery of such life would be a tremendously important event. Thus, while recognizing the unlikelihood of finding extraterrestrial life, many considered that the potential implications offset the small chance of finding any, and accordingly devoted considerable time to studying in the laboratory the chemical and biological processes that seemed most likely to have been part of the formation of life. They sought out Earth forms that could live under extremely harsh conditions-like arid deserts, the brines of the Great Salt Lake, or the bitter cold Antarctic-and paid special attention to them. And they devised experiments to probe the Martian soil for the kinds of life forms deemed most likely to be there.

But, after a decade and a half, the problem of life on other planets remained open. No life was found on the moon, nor was there any evidence that life had ever existed there. A careful search was made for carbon, since Earth life is carbon-based. In lunar samples a few hundred parts per million were found, but most of this carbon was brought in by the solar wind. Of the few tens of parts per million that were native to the moon, none appeared to derive from life processes.⁴⁶

Nor was any life found on Mars, even though the two Viking spacecraft with their samplers and automated laboratories were set down in 1976 in areas where once there might have been quite a bit of water.⁴⁷ Still, the subject could hardly be called closed. If life had been found, that would have settled the question. But that life was not found in two tiny spots on Mars did not prove that there was no life on the planet. So, although the first attempts were disappointing, it could be assumed that future missions to Mars would pursue the question further. Nor would it be likely that exobiological research would be confined to the Red Planet. At the very least, one would expect that, as scientists studied the chemical evolution of the planets and their satellites, they would keep the question of the formation and evolution of life in mind.

While the foregoing has touched upon but a few of the results accruing from the exploration of the solar system, still the reader should be able to derive some insight into the impact that space science was having upon the earth and planetary sciences. For one thing the study of the solar system was revitalized after a long period of relative inactivity. Second, lunar and planetary science became an important aspect of geoscience, attracting large numbers of researchers. Third, the new perspective afforded by space observations gave an immeasurable boost to comparative planetology, a field that made great strides during its first 15 years. Nevertheless, no one doubted that in the mid-1970s comparative planetology still looked forward to its most productive years.

All of which leads to the usual question. If space science was having such a profound impact on Earth and planetary sciences, was space science producing a scientific revolution in the field? In the broad sense, no. But, viewed strictly from within the discipline there were indeed numerous revolutionary changes. Much new information was accumulated, permitting the theorist to deal in a realistic way with topics about which one could only speculate before. Many had to relinquish pet ideas about the nature of the lunar surface or the markings on Mars. Proponents of a cold moon were faced with incontrovertible evidence of extensive lunar melting. No pristine lunar surface was to be found; instead a substantial evolution had marked the moon’s first one and a half billion years. Far from being an inert planet, Mars turned out to be highly active. Of course, in different aspects of the subject many investigators had been on the right track. Noted astronomers R. B. Baldwin and E. J. Opik had correctly anticipated that many of the features of Mars were due to craters.⁴⁸ Gerard Kuiper had been sure that volcanism was important on the moon, as he explained many times to the author. Thomas Gold had been certain that the lunar surface would contain a great deal of fine dust. Yet, no one had succeeded in putting the separate pieces together in satisfactory fashion. Thus, the most revolutionary aspect of space science contributions to the earth and planetary sciences was probably in helping to develop an integrated picture of the moon and near planets. This was an enormous expansion of horizons, an expansion that could be expected to continue with each new planetary mission.

INVESTIGATION OF THE UNIVERSE

As space scientists were busily altering the complexion of solar system research, space methods were also profoundly affecting the investigation of the universe. Here space science could contribute in a number of ways to solar physics, galactic and metagalactic astronomy, and cosmology, including a search for gravitational waves, observations to determine whether the strength of gravity was changing with time, and studies of the nature of relativity. But the contribution of space techniques to these areas was qualitatively different from those in the planetary sciences. Whereas rockets and spacecraft could carry instruments and sometimes the observers themselves to the moon and planets to observe the phenomena of interest close at hand, this was not possible in astronomy. The stars and galaxies would remain as remote as before, and even the sun would continue to be a distant object extremely difficult to approach even with automated spacecraft because of the tremendous heat and destructive radiation.

The connection between the scientist and the objects of study would continue to be the various radiations coming from the observed to the observer. But rockets and satellites would increase the variety of radiations that the scientist could study by lifting telescopes and other instruments above Earth’s atmosphere, which was transparent only in the visible and some of the radio wavelengths. This extension of the observable spectrum proved to be as fruitful to the prober of the universe as were the lunar and planetary probes to the student of the solar system.

As stated in chapter 6, rocket astronomy began in 1946 when sounding rockets were outfitted with spectrographs to record the spectrum of the sun in hitherto hidden ultraviolet wavelengths. In 1948 x-ray fluxes were detected in the upper atmosphere, after which rocket investigations of the sun ranged over both ultraviolet and x-ray wavelengths. Inevitably experimenters turned their instruments on the skies, and when they did various group found some celestial fluxes that might have been x-rays, but the real ultraviolet sources were found. In 1956 the Naval Research Laboratory significance of x-rays for astronomy had to await more sensitive instruments that did not become available until the early 1960s.

In the meantime sounding rocket research on the sun’s radiations moved on apace. Investigators from a number of institutions continued to amass detail on the sun’s spectrum in the near and far ultraviolet, which was important in understanding the quiet sun and normal sun-earth relationships. But the real excitement proved to be with the x-rays. It was these, rather than the ultraviolet wavelengths, that came into prominence with high solar activity. When satellites came into being, they were put to use in making long-term, detailed measurements of the sun’s spectrum in all wavelengths. On 7 March 1962 the first of NASA’s Orbiting Solar Observatories went into orbit, to be followed by a series with steadily improving instrumentation. The Naval Research Laboratory built and launched a series of Solrad satellites, intentionally less complex than the OSOs, to provide a continuous monitoring of the sun in key wavelengths. But, while satellites came into prominence in the 1960s, sounding rockets, some of them launched at times of solar eclipse, continued to yield important results. In fact, some scientists felt that the most significant work on the sun came from sounding rockets rather than from the far more expensive satellites.

NASA’s Orbiting Solar Observatories continued into the 1970s, the first one of the decade being OSO 7, launched on 29 September 1971. An important event for solar research was the launching of Skylab in 1973. In this space laboratory astronauts studied the sun intensively using a special telescope mount built for the purpose. Although the high cost of Skylab’s solar mission in dollars and time to prepare and conduct the experiments was distressing to many of the scientists, nevertheless the results were extremely important for solar physics, some of them providing solutions to long unsolved problems.

Sounding rocket experiments were also fruitful in stellar astronomy. Perhaps the most significant event in rocket and satellite astronomy occurred when American Science and Engineering experimenters, with an Aerobee rocket flown on 12 June 1962, discovered the first x-ray sources outside the solar system to be clearly identified as such. As will be seen later, this discovery proved to be of profound significance to modern astronomy. During the 1960s sounding rockets continued to search for and gather information on these strange sources, but progress was slow. Long term observations with more precise instruments were needed, a, need NASA was much too slow in supplying. The breakthrough came with the launching of NASA’s first Small Astronomy Satellite, Explorer 42, on 12 December 1970.

During the 1960s the course of ultraviolet astronomy from satellites also proceeded slowly. The principal satellite designed for such studies, NASA’s Orbiting Astronomical Observatory, proved difficult to bring into being, and it wasn’t until the end of the decade that OAO 2 (7 December 1968), the first successful astronomical observatory, went into orbit. It took another four years to get OAO 3 aloft (21 August 1972). From OAO observations the Smithsonian Astrophysical Observatory compiled the first complete ultraviolet map of the sky, issuing the results in the form of a catalog for use by astronomers.⁴⁹ With OAO 3, which had been named Copernicus in honor of that dauntless pioneer in scientific thought, Princeton University experimenters obtained a number of significant results. They showed that, while hydrogen in the interstellar medium was almost entirely in atomic form, most interstellar clouds had an abundance of neutral hydrogen molecules, the relative abundance being consistent with a balance between the catalytic formation of H₂ on grains of material and the competing dissociation of the gas by absorption of light. Much of the galactic disk was found to be occupied by a hot coronal gas at half a million kelvins, with a hydrogen density of one particle per liter. The Princeton workers also observed that the relative abundance of the different chemical elements in the interstellar gas was what would be expected if, starting with a mixture of elements in the ratios found in the overall cosmic abundance, the materials of high-condensation temperatures had already condensed out to form small solid particles or dust grains. Finally, flowing from most very hot stars were stellar winds of some thousands of kilometers per second.

Other experiments were also on the OAOs, some of them concerning x-rays and gamma rays, and the Orbiting Astronomical Observatories were clearly proving very fruitful. Yet one could detect the feeling that OAO was a bit out of step. The satellite had been sufficiently difficult to construct that it had delayed satellite optical (visible plus ultraviolet) astronomy for about a decade, whereas a series of cheaper, simpler satellites could have kept research moving while work on a larger instrument proceeded. Also, now that it had come, OAO was well behind both existing telescope technology and current needs. For most of the problems of greatest concern to the optical astronomers, a larger aperture (2.5 to 3 m), more precise telescope was required. As agitation developed for the construction of a Space Shuttle it was quickly realized that one of the things that the Shuttle could do ideally was to launch such an instrument into orbit and service it throughout the years. The launching of such a telescope became one of the prime scientific missions for the Shuttle.

But in the 1970s the circumstances surrounding astronomy had changed. Whereas in 1959 and 1960 the most important tasks for satellites had appeared to be in ultraviolet astronomy, in the late 1960s and early 1970s both ground-based and space research had changed the picture. Now the high-energy end of the spectrum-x-rays and gamma rays-was the center of attention for many astronomers, particularly for the large number of physicists who had moved into the field of astronomy. As a result a number of NASA’s sounding rockets and small astronomy satellites were devoted to this area of research. In addition the agency began to plan for outfitting and launching a series of multiton satellites for x-ray and gamma-ray astronomy-to be called High Energy Astronomical Observatories. The importance of this work in the eyes of scientists was shown by the fact that Britain, the Netherlands, and the European Space Agency all instrumented satellites of their own for high-energy astronomy work.

The result of all the research with sounding rockets and satellites was an outpouring of data, not obtainable from the ground, at a time when ground-based astronomy was each year turning up new, exciting, often unexplainable discoveries. The quasars had extremely large red shifts in their spectra, suggesting that they were among the most remote of objects observed, yet if they were as remote as indicated, then they would have to be emitting energy at rates that defied explanation. Strange galaxies appeared to have violent nuclei, emitting unexplainable quantities of energy. The discovery of pulsars introduced the neutron star to the scene. Radio galaxies gave evidence of cataclysmic explosions in their centers. The rocket and satellite could not have appeared at a more propitious time.

As with the exploration of the solar system, the flood of new data and information was far beyond what could be covered in a brief summary like the present one. To keep within bounds, it is necessary to illustrate the impact of space science upon the field by means of selected examples. Two will be given: x-ray astronomy and some of the contributions of space science to solar physics

X-ray Astronomy

Once stars are born the major part of their evolution can be followed in the optical wavelengths-that is, the visible and ultraviolet. For this reason most attention was directed at launching space astronomy in the direction of ultraviolet studies. Very little thought was given to the higher energy wavelengths, although these were proving to be extremely informative about the sun, particularly about solar activity. But there were a few who thought that one ought to look for celestial x-ray sources. Perhaps the most insistent was Bruno Rossi, professor of physics at the Massachusetts Institute of Technology. As Rossi later said, while he had not been in a position to predict specific phenomena like the x-ray sources that were eventually discovered, he had had “a subconscious trust…; in the inexhaustible wealth of nature, a wealth that goes far beyond the imagination of man.”⁵⁰ Moreover, there was a very compelling reason to try to look at the universe in the very short wavelengths. Much has been made of the fact that sounding rockets and satellites gave experimenters their first opportunity to look at all the wavelengths that reached the top of the atmosphere. But, like the atmosphere, interstellar space also had its windows and opacities, and not all wavelengths emitted in the depths of space could reach the vicinity of Earth. While the interstellar medium was quite transparent to wavelengths all the way from radio waves through much of the near ultraviolet, at and below 1216 Å absorption by hydrogen, the most abundant gas in space, cut off radiations from distant objects. Farther into the ultraviolet, absorption decreased again, rising once more when the absorption lines of helium, also abundant in space, were encountered in the far ultraviolet. Not until the x-ray region could one again see (with instruments, not with the eyes) deep into the galaxy. The existence of that window in the spectrum was an important reason for sounding out the possibilities of x-ray astronomy. One could study the universe only in the wavelengths that were available for observation, and all known windows ought to be exploited.

Rossi urged his ideas upon Martin Annis and Riccardo Giacconi of American Science and Engineering, who were enthusiastically receptive.⁵¹ Quick calculations showed that x-ray intensities one might expect from galactic sources would be well below the detection limit for existing instruments-doubtless explaining why Naval Research Laboratory searches for x-ray sources had not found any. In 1960 NASA provided support to Giacconi and his colleagues and they prepared to fly sufficiently sensitive instruments in sounding rockets.

After one failure, the group succeeded in getting an Aerobee rocket to an altitude of 230 kilometers, on 12 June 1962. Although the planned objective of the flight was to look for solar-induced x-radiation from the moon, that objective was completely eclipsed by the excitement of detecting an object in the sky that was apparently emitting x-rays at a rate many, many orders of magnitude greater than the sun. The sheer intensity of the source gave one pause, and the experimenters spent a considerable time reviewing their results before announcing them in late summer. In two additional flights, October 1962 and June 1963, Giacconi’s group was able to confirm the original findings and discover additional sources, again detecting a strong isolated source near the center of the galaxy.⁵² There was also a diffuse isotropic background which one supposed could be extragalactic in origin.

Within a few months of Giacconi’s original announcement, the Naval Research Laboratory experimenters had confirmed the existence of these sources by independent observations. In April 1963 the NRL group made an important contribution by pinpointing the source near the galactic center more precisely. Since it lay in the constellation Scorpio, the source was named Sco X-1. NRL also detected a somewhat weaker source, about 1/10 the strength of Sco X-1, in the general vicinity of the Crab Nebula.⁵³ During the rest of the decade additional sources were discovered until by 1970 several dozen were known. Efforts were made to discover just what these strange objects were. In particular, it was felt especially important to identify the sources with objects already known in the visible or radio wavelengths. The reason for wanting to tie x-ray sources in with known objects was quite simple. It seemed clear that the extremely intense x-radiation from these sources had to be connected with the basic energetics of the objects. It could be very important then to compare the x-radiation with that in other wavelengths, a comparison that might better reveal the true nature of the phenomenon.

As an illustration consider the Crab Nebula.⁵⁴ It is believed to be the remnant of a supernova explosion. The material of the nebula consists of the debris ejected by the exploding star into the surrounding space. At present this debris, which is expanding at about 1000 kilometers per second, fills a roughly ellipsoidal volume with the major diameter about six light-years-where a light-year is 9500 trillion km. Radiation from the Crab Nebula was observed to be strongly polarized across the whole spectrum, probably resulting from electrons revolving about the lines of force of a magnetic field. At the densities that appeared to exist in the cloud, the lower-energy electrons that would produce the radio wavelengths observed could last for thousands of years, but those responsible for optical wave-lengths would be depleted in a few hundred years and those producing the x-rays in less than a year. Hence there had to be a continuous resupplying of energy to maintain the observed radiations.

This remained a mystery until the discovery of a pulsar in the Crab. The end product of a supernova explosion was expected to be a very dense neutron star, in which a mass comparable to that of the sun would be compressed into a ball about 10 kin in diameter. There were reasons to believe that the pulsations from which a pulsar took its name were generated by the rapid spinning of the star. When it was noted that the period of the Crab pulsar was lengthening at the rate of about one part in 2000 per year, a possible source for the resupply of energy to the nebula leapt to mind. The slowdown of the neutron star’s rotation corresponded to a considerable amount of energy, and calculations soon showed that the amount was adequate to provide the energy being released by the nebular gases. One speculation had it that the spinning magnetic field of the pulsar accelerated gases to relativistic speeds at which they could escape from the star’s magnetosphere into the nebula, carrying their newly acquired energy with them.

This example illustrates the importance of being able to observe and visualize the object that was radiating the x-rays. As a consequence there was a continuing effort to identify the other x-ray sources with known objects. But, except for Sco X-1 the efforts were unsuccessful. As for Sco X-1, in an experiment in March 1966, Giacconi’s group showed that the angular size of the source could not exceed 20 arcseconds. The new positional data were communicated to workers at the Tokyo Observatory and at Mt. Wilson and Palomar Observatories. On 17-18 June 1966 the Tokyo observers found a blue star of 13th magnitude, which further study identified with the x-ray source. This result confirmed that stars existed that emitted 1000 times as much power in x-rays as in the visible wavelengths. As Giacconi wrote: “Sco X-1 was a type of stellar object radically different from any previously known and whose existence had not and could not be foreseen on the basis of observations in the visible and radio.”⁵⁵

But that was as far as one appeared able to go with the sounding rockets. Longer-term observations with very sensitive detectors were required to advance the field further. These were supplied by Explorer 42. Launched from an Italian platform in the Indian Ocean off the coast of Kenya on 12 December 1970, Kenyan Independence Day, the satellite was at once named Uhuru, the Swahili word for independence. Uhuru provided the breakthrough astronomers were looking for. When issued sometime afterward, the Uhuru catalog listed 161 x-ray sources. It was clear that the strongest sources had to have x-ray luminosities at least 1000 times the luminosity of the sun. Even the weakest x-ray source was 20 times more intense than the meter-band radiation from the strongest radio source. Thirty-four sources had been identified with known objects. Of the sources known to lie within the galaxy, one was at the galactic center, seven were supernova remnants, and six were binary stars. Outside the galaxy were-sources associated with ordinary galaxies, giant radio galaxies, Seyfert galaxies, galactic clusters, and quasars. And much of the diffuse background x-radiation had been shown to come from outside the local galaxy.⁵⁶

One of the most exciting results from x-ray astronomy came from the realization that some at least of the sources were binary stars-two stars revolving around each other (fig. 65)-in which one of the companions was a very dense star, either (1) a white dwarf, a mass comparable to that of the sun compressed into a sphere about the size of Earth; or (2) a neutron star, of mass exceeding 1.4 solar masses, compressed into a ball about 10 km in radius; or (3) a black hole in space. A white dwarf is the end product of a star about the size of the sun, after it has used up its nuclear fuel and can no longer avoid collapsing under gravity to a planetary size. The neutron star is the end product of a massive star that, after burning up its nuclear, fuel, undergoes a violent implosion caused by gravity, following which a great deal of the star’s material rebounds from the implosion to be blown out in a supernova explosion, leaving behind an extremely dense object consisting of neutrons. But if the residual mass after the supernova explosion is greater than a certain critical value, the gravitational contraction of the star does not stop even at the neutron star stage. Instead the star continues to contract indefinitely, pulling the matter tighter and tighter together until the object disappears into a deep gravitational well out of which neither matter nor electromagnetic radiation can escape because of the intense gravitational fields there. Hence the name “black hole.”

The binary nature of some of the x-ray objects could be deduced from the doppler shifts in the light from the ordinary companion, the shift being toward the blue as the star moved toward the observer in its orbit, and toward the red as the star moved away. If the stars eclipsed each other the binary nature would show up in a periodical disappearance of the x-rays when the emitter was hidden by the other star and reappearance when the emitter emerged from eclipse.

After careful study astronomers finally concluded that the x-rays were generated by material from the ordinary companion’s being pulled into the gravitational well of the degenerate star. If the gravitational attraction were sufficiently strong, then the gas would be accelerated to such velocities that the gas would emit x-rays as particles collided. It seemed that white dwarfs would not provide sufficient gravity to accomplish this, so one was left with the conclusion that the degenerate companion in binary x-ray sources was either a neutron star or a black hole in space. In most cases it appeared that the companion was a neutron star, but the source Cyg X-1, in the constellation Cygnus, could be a black hole. If so, it was the first such object to be detected in the universe.⁵⁷

The possibility that a black hole had at last been discovered emphasized the fundamental importance to astronomy of the new field of x-ray and gamma-ray astronomy. Gradually scientists had begun to talk about their work as high-energy astronomy, not only because they were working at the high-energy end of the wavelength spectrum, but more significantly because their observations were showing that throughout the universe extremely violent events were rather common, involving enormous quantities of energy and tremendous rates of energy production. And among these energetic events were those occurring during the last stages of a star’s evolution, stages in which neutron stars and black holes were created, with intense x-ray emissions. Speculating on the philosophical implications, Giacconi showed excitement:

The existence of a black hole in the X-ray binary Cyg X- I has profound implications for all of astronomy. Once one such object is shown to exist, then this immediately raises the possibility that many more may be present in all kinds of different astrophysical settings. Super-massive black holes may exist at the center of active galaxies ... and explain the very large energy emission from objects such as quasars. Small black holes of masses [very much smaller than the mass of the sun] may have been created at the instant of the primeval explosionæ In black holes matter has returned to condition similar to the primordial state from which the Universe was created. The potential scientific and intellectual returns from this research are clearly staggering.⁵⁸

Should one then conclude that rocket and satellite astronomy had by the early 1970s generated a scientific revolution in the field of astronomy? The answer may well be yes, although many of the strange concepts that were being dealt with had been considered decades before.⁵⁹ In any event, it is probably too early to make the case. Certainly these topics, concerning the interplay of energy and matter on a cosmological scale, are fundamental; and if anywhere in the space program one might expect a scientific revolution to emerge, it would be here. But it should also be noted that if any such revolution is to arise, it would almost certainly come from a cooperation between ground-based and space astronomy.

Solar Physics

The sun was a most important target of space science investigations for at least two reasons. First, the sun’s radiation supports life on Earth and controls the behavior of the atmosphere. For meteorology it was important to know the sun’s spectrum in the visible, infrared, and near-ultraviolet wavelengths. To understand the various physical processes occurring in the upper atmosphere, a detailed knowledge of the solar spectrum in the ultraviolet and x-ray wavelengths was essential. The reader will recall the overriding importance that S. K. Mitra, in his 1947 assessment of major upper-atmospheric problems, gave to learning about the electromagnetic radiations from the sun (pp. 59-60). For this reason many sounding rocket experimenters devoted much of their time to photographing and analyzing the solar spectrum both within and beyond the atmosphere. Some of their work before the creation of NASA was discussed in chapter 6. Finally, with the discovery of the magnetosphere and the solar wind the importance of the particle radiations from the sun for sun-earth relationships became apparent, a topic that was treated in chapter 11. Thus, solar physics was of central importance in the exploration of the solar system.

But the sun was important also to astronomy, to the investigation of the universe. Although an average star, unspectacular in comparison with many of the strange objects that astronomers were uncovering in their probing of the cosmos, nevertheless it is a star, and it is close by. The next star, Proxima Centauri, is 4.3 light-years (400 trillion kilometers) away, while most of the stars in the galaxy are many tens of thousands of lightyears distant. Stats in other galaxies are millions and even billions of lightyears from earth. So the sun afforded the only opportunity for scientists to study stellar physics with. a model that could be observed in great detail.

Because of its nearness and its importance, astronomers amassed a great deal of data and theory about the sun in the years before rockets and satellites.⁶⁰ What they learned came almost entirely from observations in the visible, with only occasional glimpses from mountain tops and balloons at the shorter wavelengths. But, as space observations showed, much of solar activity, particularly that associated with the sunspot cycle, solar flares, and the corona involved the short wavelengths in essential ways. Sounding rocket and satellite measurements were, accordingly, able to round out the picture in important ways.

To understand the significance of these contributions, a brief summary of the principal features of the sun may be helpful.⁶¹ The visible disk of the sun is called the photosphere (fig. 66). It is a very thin layer of one to several hundred kilometers thickness, from which comes most of the radiation one sees on Earth. The effective temperature of the solar disk is about 5800 K. Above the photosphere lies what may be called the solar atmosphere; below it, the solar interior.

The sun’s energy is generated in the interior, from the nuclear burning of hydrogen to form helium in a central core of about one-fourth the solar radius and one-half the solar mass. Here, at temperatures of 15 X 10⁶ K, some 99 percent of the sun’s radiated energy is released. This energy diffuses outward from the core, colliding repeatedly with the hydrogen and helium of the sun, being absorbed and reemitted many times before reaching the surface. In this process the individual photon energies continually decrease, changing the radiation from gamma rays to x-rays to ultraviolet light and finally to visible light as it emerges from the sun’s surface.

From the core of the sun to near the surface, energy is thus transported mainly by radiation. But toward the surface, between roughly 0.8 and 0.9 of the solar radius, convection becomes the principal mode of transporting energy toward the surface. The existence of the convection zone is evidenced by the mottled appearance of the photosphere in high-resolution photographs. This mottling, or granulation as it is called, consists of cells of about 1800-km diameter which last about 10 minutes on the average and which are thought to be associated with turbulent convection just beneath the photosphere. A larger scale system of surface motion-20 times the of the granulation cells, called supergranulation-is believed to be much more deeply rooted in the convection zone.

Analogous to Earth’s upper atmosphere is the chromosphere, or upper atmosphere, of the sun. The chromosphere overlies the photosphere and is about 2500 km thick. While the density drop across the photosphere is less than an order of magnitude, density in the chromosphere decreases by four orders of magnitude from the top of the photosphere to the top of the chromosphere.

Not long ago, photospheric and chromospheric temperatures were extremely puzzling to astronomers, dropping from about 6600 K at the base of the photosphere to around 4300 at the base of the chromosphere, and then rising through the chromosphere, at first slowly but then very steeply to between 500000 and 1000000 K at the top. This temperature curve posed a problem, for it was assumed that the corona derived its heat from the chromosphere, yet that would imply that heat was flowing from a colder region to a hotter one, contrary to the laws of thermodynamics. As late as 1972 Leo Goldberg, director of Kitt Peak National Observatory, pointed to this phenomenon as “the most important unsolved mystery surrounding the quiet sun.” ⁶²

Above the chromosphere lies the corona, the sun’s exosphere. Here 1000000 K temperatures prevail, and an important problem facing the solar physicist was to explain how the corona gets its energy. Although the corona is extremely hot and very active, its density is so very low that it is not normally visible from the ground, where it is completely obscured by scattered sunlight in the earth’s atmosphere. Only during solar eclipses, with the moon blocking out the sun’s disk, could the astronomer get a good look at the entire corona. One of the benefits of rockets and satellites was to permit carrying coronagraphs above the light-scattering atmosphere where the corona could be seen even in the absence of a solar eclipse.

Much of solar physics concerns the interplay among the different regions of the sun. This interplay, however, can be followed only in terms of its effect upon the radiations emitted from those regions. For this reason, one of the first tasks of the astronomer was to obtain good spectra of the sun and their variation with time. Regions from which radiations of highly ionized atoms came would be hot regions, and temperatures could be estimated. The magnetic field intensities, for example in sunspots, could be estimated from the splitting of lines emitted within the field. If a cooler gas overlay a hotter, similar gas, the cooler gas would absorb some of the light emitted by the hotter one. This would produce reversals in the emission lines of the hotter gas, generating the famous Fraunhofer lines of the solar spectrum discovered in the 19th century. By piecing together information of this kind, the locations of different gases relative to each other and their temperatures could be determined. Changes in magnetic field that occurred in association with solar activity, such as the appearance of solar flares, could be followed. Changes were important, since there were strong indications that magnetic fields were the source of much of the energy in solar flares.

These techniques were, of course, applicable in the visible wavelengths and were employed to the fullest by the ground-based astronomer. The space astronomer simply provided an additional handle on things by furnishing spectral data in the ultraviolet and x-ray wavelengths. And these data began to accumulate from the very earliest sounding rocket flights. Year by year, flight by flight, they were added to until by the end of the decade the solar spectrum was known in great detail from visible through the ultraviolet wavelengths and into the x-rays.⁶³

A powerful technique for study of the sun is that of imaging the sun in a single line; for example, the red line emitted by hydrogen known as hydrogen alpha. In such spectroheliograms, as they are called, one can see the structure and activity of the sun associated with that line. Spectroheliograms taken in hydrogen, calcium, and other lines in the visible have long been an effective too) for the study of solar activity. Members of the Naval Research Laboratory group pioneered the use of this technique in space astronomy, where it was possible to get spectroheliograms in both the ultraviolet and x-rays.⁶⁴ These, taken with photographs in the visible, gave a powerful means of discerning and analyzing active regions on the sun. Sequences of such images taken over many days, or at intervals of 27 days, the solar rotation period, permitted one to follow the evolution of flares and other features on the sun. It was in this sort of imaging that Skylab was particularly fruitful.

During the decade and a half that was climaxed by the Skylab solar observations, solar physics progressed rapidly, advanced by a combination of ground-based and space astronomy. In the shorter wavelengths the sun was found to be extremely patchy (fig. 67), a patchiness that extended into the visible wavelengths as well.⁶⁵ The sequence of events in a solar flare could be followed in wavelengths all the way from x-rays through the ultraviolet and visible into the radio-wave region, and related to motions of electrons and protons associated with the flare.⁶⁶ Contrary to previous expectations fostered by ground-based pictures during solar eclipses, the corona turned out to be not even nearly homogeneous. X-ray images of the corona especially showed a great deal of structure (fig. 67). Quite surprising were large-scale dark regions of the corona-which came to be called dark holes-and hundreds of coronal bright spots. The holes appeared to be devoid of hot matter and to be associated with diverging magnetic field lines of a single polarity. If the magnetic field lines were open, these holes could be a source of particles in the solar wind.⁶⁷

The bright spots were observed to be uniformly distributed over the solar disk. They were typically about 20,000 km in diameter, and of a temperature about 1.6 x 10⁶ K. They appeared to be magnetically confined, and one speculated that they might be an important link in the explanation of the sun’s magnetic field.⁶⁸

The interlocking features of the lower solar atmosphere and the corona visible in satellite images of the sun provided hints as to how the sun might heat the corona to the extreme temperatures that were observed. Gravity waves and acoustical waves might carry energy upward from the convective regions below the photosphere into the corona. This explanation would remove the mystery of the steep temperature curve in the chromosphere. It would not be the chromosphere that was heating the corona in violation of the laws of thermodynamics. On the contrary, the corona, heated by energy from within the sun, would itself be heating the top of the chromosphere.

The importance of rocket and satellite solar astronomy lay in the integrated attack that the researcher could now make in seeking to understand the nearest star, an integrated attack made possible by opening up the window that the earth’s atmosphere had so long kept shut. It was an importance attested to by the large numbers of solar physicists who bent to the task of assimilating the new wealth of data.

By the end of the 1960s the early years of space science were well behind. More than a dozen disciplines and subdisciplines had found sounding rockets and spacecraft to be powerful tools for scientific research. Thousands of investigators turned to these tools to help solve important problems. Moreover, while the disciplines to which the new tools could contribute were many and varied, there was a clearly discernible melding of groups of disciplines into two major fields: the exploration of the solar system and the investigation of the universe. The pursuit of these two main objectives would grow in intensity as space science moved into the 1970s-in spite of fears prevalent in the late 1960s that support for space science was waning. The new decade would witness the scientific missions of Apollo to the moon, the remarkable solar astronomy from Skylab, breakthroughs in x-ray astronomy, and the serious start of a survey of all the important bodies of the solar system. It was eminently clear that space scientists would be important clients of the Space Shuttle, which was intended to introduce a new era in space activities. Because of their accomplishments, the scientists could legitimately ask that the Shuttle be tailored as much to their requirements as to other space needs.