The first five chapters laid a foundation for our exploration of the cosmos. One important aspect of that foundation was the consideration of the structure or processes of science. Our discussion focused on first, the terms in which a scientific argument may be cast, second, the underlying preconceptions or themes in science, and third, what constitutes a scientific explanation of our experiences with such natural phenomena as motion and light. Scientific explanation is expressed in Newton's theory of motion and in Maxwell's, Planck's, Einstein's, and Bohr's theory of a dual nature for light. Although Newton's theory of motion is not the final word on the subject of motion, for we have yet to consider Einstein's relativity concept, Newtonian theory has been abundantly successful in predicting what has been found regarding motion in the Solar System all the way from the motion of planets to the launching of spacecraft to survey distant parts of the system. Likewise, the wave and photon concepts of light provide an abundant basis for interpreting light coming either to directly to the Earth or first to a wandering spacecraft which relays its finding on to the Earth. From this diagnoses of light we have gained an understanding of the physical nature of self-luminous sources like the Sun or reflecting bodies such as planets.
For purposes of study, the Solar System, which is where our conquest of space began, is an awkward mixture of bodies. The Sun is a star--a very different kind of body from planets, or satellites, or asteroids, or comets; even the planets are not simply larger or smaller versions of the same type of body. Consequently, since the Sun is the only star in the Solar System, we shall delay its consideration until we discuss stars. And in Chapters 6 to 11, we will briefly touch on the other occupants of the Solar System, asking what they are like, how they do or do not resemble each other, reasons for their being as they are, and what reasonable scenarios exist to account for their evolution from primal matter to the bodies we recognize today.
With the advent of the space age, astronomers have been able to use aircraft, balloons, rockets, and now primarily satellites and orbiting observatories to extend their vision of the Solar System and even beyond to the Universe by going above part or all of the Earth's veiling atmosphere. Amazed at what the space-platformed telescope has revealed through radiation in the ultraviolet, X-ray, gamma-ray, and infrared regions, astronomers now vigorously pursue knowledge with more advanced spacecraft which will continue to revolutionize our understanding of the Solar System. The conquest has only begun and it is unlikely that it will ever end.
6.1. Components of the Solar System
As long as humanity has been aware of the sky and the objects in it, various cultures have recognized the five bright planets--Mercury, Venus, Mars, Jupiter, and Saturn--as something other than stars. By Newton's time, there had unfolded a train of thought leading to the realization that the Earth was but another of the Sun's planets, probably not terribly unlike the other five. The number of known planets did not change until the invention of the telescope, after which Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930. The discovery of the Solar System's minor bodies, primarily after the invention of the telescope, broadened our conceptual understanding of the Solar System from the Greek's concept of seven wandering bodies to a far more complex set of objects with relationships existing among them that had ever before been envision. By the turn of this century, the full extent of the Solar System was beginning to be realized, but it was the beginning of the space age a mere thirty years ago that truly revealed the Solar System for what it is.
The dominant body in the Solar System is the Sun; it is the source of the gravitational bonds holding the Solar System together. The Sun is also the source of radiant energy that powers so many physical processes going on throughout the Solar System. But as stated, the Sun is a star and is therefore quite unlike any other Solar System body.
Like other stars, the Sun, Figure 1, is a gas from center to surface possessing a radius over 100 times greater and a mass over 300,000 times greater than that of Earth. The Sun generates deep within its hot interior the radiant energy that it radiates from its surface. The Sun's family of planets intercepts only a minute fraction of this radiation flooding the Solar System.
In addition to the steady emission of radiant energy, there are numerous transient phenomena occurring in the Sun's atmosphere, such as sunspots, flares, and prominences. Associated with these is a flow of subatomic particles and magnetic fields out through the orbital planes of the planets. This solar wind of particles impinges on the planets and their magnetic fields, producing a variety of phenomena, such as Earth's aurora (northern and southern lights). Thus the range of interaction between the Sun and its planets is far more complex than just gravitational.
After the Sun, the nine planets as a group, shown in Figure 2, contain the next largest fraction of the Solar System's mass. The variation in mass among the nine planets, however, is immense with Jupiter being almost 120,000 times the mass of Pluto, the smallest planet, and over 300 times the mass of even our Earth.
Although we believe the planets had a common origin in time, some 4.6 billion years ago, they currently display significant chemical, physical, and geologic differences. Such diversity stems primarily from their different masses and distances from the Sun at the time of their formation. During that formative period, these factors determined the ability of the fledgling planet to retain matter and further they defined the chemical composition of that matter.
In spite of their differences, we know of sufficient chemical and physical similarities among planets so that we can divide them into two categories, the Terrestrial planets and the Jovian planets. The mean properties of these two groups are summarized in Table 1. The Terrestrial planets, consisting of Mercury, Venus, Earth, and Mars (Figure 3), occupy the inner Solar System and are composed mostly of iron and its oxides and silicates. Since the Moon is not significantly smaller than Mercury, it can conveniently be included as one of the Terrestrials. Although like the Moon in size, Pluto does not have a composition like that of the Terrestrial planets, but possesses one more like that of comets. Thus it does not belong to the Terrestrials, and it is not like the Jovian planets either.
The Jovian planets (Figure 4) consisting of Jupiter, Saturn, Uranus, and Neptune, define the outer Solar System with all being farther from the Sun than the Terrestrial planets. They are also larger than the Terrestrials and are composed primarily of the light elements hydrogen and helium; the most abundant elements in the Universe. Jupiter and Saturn apparently have a chemical composition somewhat like that of the Sun, while Uranus and Neptune seem to have relatively more carbon, nitrogen, and oxygen than hydrogen and helium. Specific physical properties for the Terrestrials can be found in Table 8.1, for the Jovians in Table 9.1, and for Pluto in Table 9.4. The important point is not to mentally visualize the planets as being either the same size or type of bodies for they are very far from being a category of uniform bodies.
Satellites of the two planetary groups are as distinctive in their physical structure as are their parent groups. There is also considerable variation in the sizes of the objects that we label as satellites. For example two of Jupiter's satellites, Ganymede and Callisto, and one of Saturn's satellites, Titan, are as large as or larger than Mercury, and as we mentioned our Moon is not all that much smaller than Mercury either. To verify this, see Table 10.1 which lists information on some of the larger satellites. Thus it is not size or physical similarities that define satellites as a category, but it is their dynamical relationship to bodies we identify as planets, along with the historical tradition of having always done so.
One of the most exciting developments in planetary research has been the discovery of ring systems for Uranus and Jupiter. Saturn's rings had been discovered with the introduction of the telescope into astronomy. Rings are actually individual, small solid bodies in orbit about a planet usually in its equatorial plane. They are thus very small satellites of the planet. Uranus's and Jupiter's rings do not contain as many tiny satellites as do Saturn's, so they are much fainter and have managed to escape detection until recently. A faint, patchy ring system has been reported for Neptune, but its nature is still much in question.
Unlike planets and their satellites, comets are icy fragments impregnated with rocky matter coming from deep space well beyond the orbits of the planets. Nevertheless, they are members of the Sun's family of Solar System bodies. Their icy composition is apparently characteristic of many bodies in the outer Solar System, including most of the Jovian satellites. As comets come into the inner Solar System drawn by the Sun, they evaporate in the intense solar radiation strewing rocky bits and pieces along their orbital path. This material forms much of the meteoric material.
Asteroids, or minor planets, are small, rocky bodies which display as much diversity in size as do planets and satellites. And those asteroids in the inner Solar System near the orbit of Mars which have been study for chemical composition, suggest again a variation such that there is no close uniformity in structure among them either. In recent years, the term asteroid has been expanded to include small objects, presumably not comets, located in the outer Solar System. It is unlikely, however, that their physical makeup is like that of asteroids in the inner Solar System.
Meteoroids range in size from irregular solid bodies, called meteorites when they strike the Earth's surface, to tiny particles, called meteors if they merely flash through our atmosphere. As we go down the scale in size, the number of meteoroids increases rapidly. All the meteoroids are satellites of the Sun and are moving in orbits that vary widely in their characteristics. They are composed of rocky material and are apparently derived primarily from asteroids and comets.
The interplanetary medium is composed primarily of gas particles--mostly protons and electrons--that are ejected from the Sun's atmosphere. These subatomic particles form the solar wind mentioned above. Some dust is there too, most being cometary debris. Despite huge numbers of gas and dust particles, interplanetary space has fewer bits of matter and is a better vacuum than can be made in a terrestrial laboratory.
6.2. Studying the Solar System
Today astronomers do very little naked-eye study with telescopes of planets or the minor Solar System bodies. The primary use of a telescope, regardless of whether it is here on Earth or located in a spacecraft, is as a camera for taking direct photographs, or for recording spectra, or for photometric measurements. This is because these techniques produce permanent records that can be studied as needed. In addition, using radiation detectors such as photographic emulsions or photoelectric devices makes quantitative studies possible, something generally not possible with naked-eye observations. As a reminder, photometry means making measurements of the amount of radiant energy a body gives off, while spectroscopy is the analysis of light by separating composite light into its component colors.
Photometric measurements provide information about the nature of reflecting materials, such as clouds in a planet's atmosphere or surface features, spectroscopy provide clues to the chemical composition of a planet's surface, clouds, and atmosphere. Using the spectrum of sunlight reflected from the planet by either its atmosphere or surface, may reveal a planet's atmospheric constituents by the absorption lines or bands that are superimposed on the solar spectrum, as illustrated in Figure 5.
In studying the radiation from planets, or for that matter and astronomical body, we divide the radiation into two categories, thermal and nonthermal radiation. Thermal radiation, which is due to the fact that a body is hot, can be studied from the ultraviolet to the far infrared region with today's modern radiation detectors. Thermal radiation is blackbody radiation (discussed in Section 14.3.1) which possesses a continuous spectrum and is the product of the random thermal motion of particles that compose the outer portions of planets, satellites, asteroids, or any other astronomical body. Because of the very low temperature of planets compared to stars, thermal radiation coming from a planet is situated primarily in the infrared portion of the electromagnetic spectrum. Such infrared data provides important information on surface and atmospheric temperatures and, indirectly, chemical composition.
Nonthermal radiation is radiation due to physical processes other than that involved in producing thermal radiation. That is, it owes its explanation to some other fact than that the body is hot. For example, light produced in lightning is nonthermal radiation. Both thermal radiation and nonthermal radiation emitted by planets can in some cases be observed with radio telescopes.
Since we cannot directly determine the chemical composition of the deep interior of even the Earth, how is it possible to think that we know the composition of planets and other Solar System bodies? Above we mentioned ways of obtaining evidence as to the compositions of atmospheres and surface layers, but that is far from believing one knows the composition throughout the body. As you already know, the Earth's atmosphere is made up of nitrogen and oxygen, while the surface layers of the Earth are rich in silicon, oxygen, and aluminum. Thus is it possible that the main body of the Earth may have a composition that is different from either its atmosphere or surface? We believe that, yes, it is indeed different, and we have arrived at that conclusion not by means of experimentation, although some types of experimental results are important to the argument, but primarily by means of a theoretical argument. The argument proceeds along the following lines.
Whatever composition the planets had at their birth has clearly not changed over the span of their lives, since no significant influx of new material from outside has been added to the planets' masses and no known processes are at work to change their original composition into something else, as there are in the case of stars.
What then was the original composition of the planets, and is it likely that it is not the same for each planet? As the Solar System formed, it is probable that the Sun was well along in the process and consequently reasonably hot by the time the planets began to form. Thus the temperature of the matter from which the planets formed was higher closer to the Sun, probably 2000 K or so, and declined rapidly outward to about 100 K. Since matter should be solid or solidlike to coalesce to form a planet, we can divide the chemical compounds most probably present at the time of the formation of the Solar System into three broad groups on the basis of the ease with which they vaporize (Table 2).
The first group, called the gaseous materials, consists of those elements that are gases at temperatures above a few tens of kelvins, such as hydrogen and helium. Next are the icy materials, such as methane, ammonia, carbon dioxide, and water (containing such light elements as carbon, nitrogen, and oxygen besides hydrogen), which do not vaporize until the temperature is over a couple of hundred degrees. Finally, there are rocky materials, such as iron, magnesium, and their oxides, sulfides, and silicates, which remain solids until the temperature is several thousand degrees. Hence close to the Sun, where the Terrestrial planets formed, the rocky materials would have been the only ones not in a gaseous form. Iron and the elements near it in the periodic table (Appendix 4) should dominate the compositions of the Terrestrial planets, as they seem to do, while lighter elements, such as hydrogen, helium, carbon, nitrogen, and oxygen, should be the principal constituents of the Jovian planets, as they seem to be.
This form of argument in science has proven to be most fruitful since it provides many opportunities to predict and then to verify or not those predictions through observations.
6.3. History of the Space Conquest
The space programs of the United States, the Soviet Union, Western European nations, and Japan have made possible an incredible expansion of our knowledge about the Solar System. Yet no escape from Earth to uncover this knowledge would have been possible without the rocket; an old idea that has made our repeated conquests of gravity an integral part of modern live.
Many individuals have innovated, built, and flown rockets since the Chinese first experimented with them in the thirteenth century. Blackpowder rockets were tried in medieval warfare, but this and later attempts to use rockets militarily produced no great successes prior to Newton. His theory of motion laid the foundation for flight with balloons, airplanes, and rockets. For in 1783, not long after Newton's death, the first sustained flight using balloons was achieved. But it was not until 1903 that powered flight was accomplished, and in fact not until the 1920s that air transportation arrived as part of the modern technological society.
Even before the development of airplanes, visions of flights to the Moon can be found in literature. Jules Verne (1828-1905) published in 1865 one of the most celebrated of these in his From the Earth to the Moon. In 1898, H. G. Wells (1866-1946) published The War of the Worlds about a Martian invasion of Earth and later in 1901 The First Men in the Moon.
These and other science fiction stories inspired a generation of young futurists, such as the Russian school teacher Konstantin Tsiolkovsky (1857-1935). His publication of Exploration of Cosmic Space with Reactive Devices in 1903, a technical paper proposing the concept of boosting scientific payloads into space with rockets, can be said to be the beginning of the space age. During succeeding years Tsiolkovsky enlarged his vision seeing numerous human colonies around the Earth "like the rings of Saturn," where people would tap solar energy to live and work, and finally "to move farther away from Earth and become an independent planet--a satellite of the Sun and a brother of Earth."
The 1920s in the Soviet Union, Germany, and America were the springtime of rocket pioneering. In 1919 the American rocket experimenter, Robert Goddard (1882-1945), a physics professor at Clark University in Massachusetts, published his first treatise on rocketry. Goddard in 1926 became the first to experiment with liquid-fueled rockets. Tsiolkovksy's and Goddard's efforts to use rockets as a peaceful scientific aid in exploring space were mostly ignored in their native countries. Another visionary of the time was the Rumanian-German physicist Hermann Oberth (1894-198?), who in 1923 predicted that rockets could be built that would overcome Earth's gravity and eventually transport machines and men into space. Although not alone in their efforts, Tsiolkovsky, Goddard, and Oberth were by far the most significant pioneers whose stubborn persistence pushed open the doors in their respective countries to usher in the space age.
Oberth's work spawned a new generation of rocket enthusiasts in Germany including Wernher von Braun (1912-1975) whose leadership of Germany's rocket development program in World War II lead to the V-2 ballistic missile. With the development of the V-2, the rocket had arrived as a booster system capable of launching a meaningful scientific payload into the upper atmosphere. At the end of the war, the United States acquired both production examples of the V-2, as well as members of the German rocket development teams including von Braun. With the German V-2 and the rocket development team, a rocket testing facility was established at the White Sands Proving Grounds in New Mexico, so that we could gain experience in handling and firing ballistic missiles. It was hoped that this experience would lead to improvements in our own rocket technology. In 1946 the United States launched the first of 67 instrumented V-2 rockets from the White Sands range. Over a year later, the first Soviet V-2 roared away from its launch site at Kapustin Yar. Modified V-2s and V-2 spinoffs served as the basis of both the American and Soviet rocket programs as well as being the inspiration in both countries for a new generation of medium-ranged ballistic missiles. Today rocket technology has advanced far beyond the V-2 to the giant boosters that are the work horses of the American, Soviet, and Western European space programs (Figure 6) lifting massive payloads into orbit.
As a byproduct, those early V-2 rocket experiments added immeasurable to our knowledge of the little-explored upper atmosphere when interested scientists were invited to take part and conduct experiments in upper atmospheric research and extraterrestrial phenomena. On October 24, 1946, for example, a V-2 rocket carried a small ultraviolet spectrograph to a height of 100 km. During its ascent, a camera made a running record of the never-before photographed near ultraviolet portion of the solar spectrum.
A rocket engine operates on the principle of action and reaction, as expressed in Newton's third law. To illustrate this principle consider a balloon that has been inflated (Figure 7), which is in essence a toy rocket. By pinching the balloon's neck, the air inside is prevented from escaping. The pressure exerted by a gas, such as air inside a balloon, is the result of many collisions by gas molecules with the molecules composing the walls of its container. Inflating a balloon compresses the air inside thereby increasing slightly its density and temperature, and as a consequence the air inside has a higher pressure than the air outside which forces the balloon to expand. Because the internal air pressure is equally balanced in all directions, there is no tendency for the balloon to move. But if you release the neck, air rushes out causing the internal pressure to become unbalanced (action), being less in the direction of the opening, which by Newton's third law causes the deflating balloon to move in a direction opposite (reaction) to that of the escaping air.
In a rocket engine, a fuel, such as liquid hydrogen, and an oxidizer, such as liquid oxygen, are pumped into the engine's combustion chamber. There they mix and are ignited converting potential energy stored in the chemical bonds in the molecules of the fuel into kinetic energy of random thermal motion. That is the exhaust gases are heated which in turn increases the pressure they exert against the walls of the combustion chamber. Because the hot exhaust gases can freely rush out the back of the engine through an exhaust nozzle (action), there is an opposite but equal force exerted on the rocket body (reaction). Thus the force of the chemical reaction produces a thrust that accelerates the rocket upward away from the Earth's surface. As the burning continues, the rocket accelerates until it reaches its maximum velocity at burnout.
Rockets are used to lift satellites above the Earth's surface to a point at which they can be injected into orbit. The type of orbit achieved is determined by the injection velocity and the angle the injection makes with the horizontal at the Earth's surface, as shown in Figure 8.
To see how this works, let us go through an argument due essentially to Newton. Assume that from the top of an extremely tall mountain, we will attempt to inject a ball into Earth orbit by throwing it parallel to the surface of the Earth. If we throw the ball giving it a small velocity, the ball's initially horizontal path will bend downward as Earth's gravity accelerates it toward the center of the Earth, so that it eventually strikes the surface. The greater the velocity we give to the ball, however, the farther around the curvature of the Earth it will go before striking the Earth's surface.
We can imagine an initial injection velocity large enough so that as the Earth's centripetal acceleration bends the ball's path round the Earth, it just misses the Earth's surface 180o from where we launch it. Such a path will carry the ball back to the point from which it was launched, and thus it is orbiting the Earth in an elliptical orbit. There is a yet larger injection velocity for the ball for which it will maintain roughly the same distance above the Earth's surface, so that it orbits the Earth in a circular orbit. The velocity at which the ball will orbit the Earth in a circular path is called the circular velocity, and for a launch point near Earth's surface the circular velocity is about 8 km/s; it is less farther from the Earth.
Injection velocities less than the circular velocity result in elliptical orbits most of which intersect the Earth's surface as shown in Figure 8. For these elliptical orbits, the launch point is the apogee point of the orbit, or the most distant point from the center of the ellipse, and the perigee point of the orbit, or the closest point to the center of the ellipse, is located 180o from the launch point. For injection velocities greater than the circular velocity, the orbit is also an ellipse, but with perigee being the launch point and apogee being located 180o from launch.
It strikes one as almost intuitive that, as the injection velocity grows larger than the circular velocity, an injection velocity must be reached at which the ball escapes the Earth's gravitational attraction altogether. Such a velocity is known as the escape velocity. Near the surface the escape velocity is about 11 km/s, and it also decreases the farther away from the surface. Launching our ball from the top of the mountain with an injection velocity equal to the escape velocity causes the ball to follow a parabolic orbit away from Earth, and velocities greater than the escape velocity result in hyperbolic orbits.
The launching of a satellite into Earth orbit is analogous to launching the ball. The rocket carries the satellite to the appropriate altitude where it is injected into orbit. If the final speed of the rocket at burn out is not large enough or the direction of motion is not correct for the desired orbit, a small rocket attached to the satellite container can be used to change the direction of the velocity and to boost the satellite to the appropriate speed. After the injection rocket has served its purpose, it and the satellite container drop away leaving the satellite in the desired orbit.
On October 4, 1957, Sputnik 1, launched by the Soviet Union as the world's first artificial satellite, blazed across the sky announcing the opening of the space age for the inhabitants of Earth. Sputnik 2, launched on November 3, 1957, carried a small dog named Laika as part of a biological experiment. The United States did not launched its first satellite, Explorer 1, until January 31, 1958, even though we had been working on developing a satellite program since the early 1950s in connection with the International Geophysical Year in 1958. However until 1958, a year after the Soviets' Sputnik 1, our country had no official space organization. Neither did the US have a well formulated national policy to guide us in exploring space. Prior to 1958, our space activities had been directed mostly by the armed services in connection with national security.
On April 2, 1958, President Eisenhower proposed that the 85th Congress enacted legislation establishing the National Aeronautics and Space Administration or NASA for short. Eisenhower signed the bill into law on July 29, 1958. One primary objective of the agency was "to acquire scientific knowledge on the environment of our Solar System and Galaxy which will lead to a better understanding of the physical universe and man's role in it." This objective has and continues to guide NASA's programs even today.
In December 1963 the General Assembly of the United Nations unanimously approved a resolution declaring that space is open and free to all states for exploration, that nothing in space can be owned by any one country, and exploration of space should be directed toward bettering the human race. These views were formalized in 1967 in the Outer Space Treaty signed by much of the UN membership. In addition to this treaty, four other UN treaties are currently in force and have been signed by most of the membership. To further discussions in space law and international cooperation, the International Institute of Space Law has been established.
For the first two decades of the space age, space exploration was conducted almost entirely through the space programs of the United States and the Soviet Union. Technical capability in space exploration is rapidly proliferating now, and more and more nations are developing their own space programs. For example, China, India, Japan, and the European Space Agency have developed launch facilities and do indeed launch scientific satellites on a reasonably regular basis. The aerospace industries in Australia, Canada, France, Germany, Italy, and the United Kingdom are substantial and capable of most facets of space exploration. Other nations such as Brazil and Indonesia have expressed the desire to build their own launch facilities, and will undoubtedly be launching satellites in the next century.
It is difficult in a few short paragraphs to do justice to the drama, excitement, and intellectual achievements of the space program. We have all become somewhat blase' about its accomplishments. But we should remember that only about 50 years have elapsed between the discovery of Pluto and close-up views of Jupiter and Saturn by Voyager 1 and 2 (Figure 9). It has been a little less than 30 years since the first Venus probes by the Soviet Union in 1961 and by the United States in 1962 propelled astronomy into the realm of physical exploration of the planets. US spacecraft, with names like Mariner, Viking, Pioneer, and Voyager, and Russian spacecraft, with names like Venera, Zond, and Mars, have now visited all five naked-eye planets and Voyager 2 has even given us close-up views of the first of the "discovered" planets, Uranus. To help visualize the chronology of events in our exploration of space, Table 3 lists some of the milestones in space exploration.
In addition to these accomplishments, we have landed spacecraft on Mars and Venus and have carried out experiments on their surfaces (Figure 13). We have brought back samples from the Moon and could do so from Venus, Mars, asteroids, and comets, if we so desired. We have photographed and mapped the surface of Mars and with an orbiting radar system have mapped the surface of Venus through its veiling clouds (Figure 8.?). In 1986 the Soviet Union orbited a permanently manned space station, known as Mir, to carry out a variety of scientific tasks. There are now plans for the US to build and orbit about the Earth in the 1990s a permanently staffed space station (Figure 16) from which future space voyages could be launched to again explore the Moon and to initiate manned exploration of Mars.
But, what were the reasons that propelled the United States into space exploration? Of course, the pursuit of knowledge was one. And that goal has been more than abundantly met. In fact the growth of knowledge concerning the Solar System has been overwhelming, leading many observers of science to refer to the last 20 years of Solar System astronomy as its golden age. Unfortunately, to many people, knowledge for its own sake is not a sufficient rationale for the expenditures that have been made in the space program. Basically, there are four dominant reasons: vision of the future, knowledge, applications, and national prestige. We human beings are visionaries; we have always been explorers and will undoubtedly continue to chart the unknown right on through the Solar System to the stars someday.
Concrete planning in space exploration, including planetary exploration, can be said to have begun in the period 1957-1959. The first coherent plan for planetary exploration was produced by NASA's Jet Propulsion Laboratory in 1959. This plan contained the lunar exploration programs Ranger, a hard-landing spacecraft, and Surveyor, a soft-landing craft. Then in 1961 came plans for the Apollo manned-landing program and the lunar-orbiter photomapping program. Plans for more ambitious ventures followed that ultimately lead us to want to explore all the Solar System. Some of these plans have been executed, but others have been dropped for various reasons. The results of many of these missions will be discussed in later chapters.
What can be considered a strategy for planetary exploration began to emerge in the 1960s. It was not, of course, conceived fully developed but has evolved from NASA-sponsored groups of scientist and engineers, mostly from universities, and from the scientists composing the Committee on Planetary and Lunar Exploration of the National Academy of Sciences Space Science Board. The basic concept is that of a four-stage program, each successive stage building on its predecessor. The four stages are reconnaissance, exploration, intensive study, and utilization.
Reconnaissance is the first effort to characterize a planet, whereas exploration is the attempt to determine much more about the physical state of a planet and to derive some understanding of physical processes that have shaped and are shaping its evolution. The effort in the intensive-study phase is to identify those scientific questions which have been revealed by reconnaissance and exploration and which are capable of being pursued further. These kinds of studies are of course those done from onsite inspection (not necessarily manned). Only the Moon and Mars have been subjected to intensive study--and this still in a very limited fashion. The final phase, utilization, hopes to yield scientific data only as a by-product. Although there are proposals and plans for establishing a permanent base or bases on the Moon, which would include some for mining and manufacturing, no efforts are underway. Beyond the Moon, there are no plans, that go beyond the talking stage, for utilization of any Solar System body.
6.4. The Conquest in the Ultraviolet, X-Ray, and Gamma-Ray Regions
Although much useful and important observational work remains to be done from ground-based observatories, an increasing portion of future astronomical research will be carried out from platforms outside most of the veil that is the Earth's atmosphere. Up to the middle of this century, nearly all our knowledge about the cosmos had come from studying the visible light of astronomical objects, and the visible and radio windows still constitute our most readily accessible sources of information. Yet much of today's research is centered on the invisible regions of the electromagnetic spectrum, which do not penetrate the Earth's atmosphere (Figure 4.1). To explore these regions--the infrared, the ultraviolet, the X-ray, and the gamma-ray wavelengths--new techniques and equipment are being developed, which must be flown in spacecraft above the atmosphere.
Ultraviolet observations from space began, as mentioned above, in 1946 with the recording of the solar ultraviolet spectrum. The ultraviolet portion of the electromagnetic spectrum has been divided by astronomers into three segments, more or less derived from the time in which serious research into them began. First there is the ground-based ultraviolet, from 4000 A to the atmospheric cutoff at 3000 A. Next is the far ultraviolet, from 3000 to 1000 A. And last is the extreme ultraviolet, from 1000 to 100 A. One reason why the ultraviolet region is important to astronomers is that a number of elements, or certain stages of ionization of some elements, have their emission and absorption spectrum primarily in the ultraviolet and not the visible region. Thus for us to gain any information about these elements in celestial bodies we must work in the ultraviolet from above the Earth's atmosphere.
Telescopes, analyzing instruments, and radiation detectors for ultraviolet research are basically the same kinds of instrument used in visible and infrared observations. The principal difference is that a number of types of glass are not transparent to ultraviolet photons but are highly absorbing. Therefore, special materials must be used for lenses and entrance windows into such instruments. And since ultraviolet-sensitive film cannot be retrieved from an orbiting satellite, photoelectric devices have been the primary radiation detectors, so that data could be radioed back to ground stations.
Astronomers first began using X-ray detectors in balloons and rockets and in a few unmanned satellites during the 1960s. X-ray astronomers divide "their" portion of the electromagnetic spectrum into two categories: soft X-rays, from about 10 to 100 A, and the more penetrating hard X-rays, from approximately 0.1 to 10 A. Both X-rays and gamma rays are emitted by regions of space characterized by very high temperatures, low density, and high-speed subatomic particles--that is, wherever there are extreme conditions involving nuclear and atomic reactions. The observed radiation is in part thermal radiation but mostly nonthermal radiation.
Most people are aware that X-rays are more penetrating than visible light, since they pass through the human body when making X-ray pictures for medical diagnoses. In this great penetrating power lies the difficulty in making telescopes that focus X-rays, analyzing instruments, and radiation detectors for X-rays, for glass lenses and mirrors do not refract or reflect X-rays impinging directly on them. However, if X-rays strike a smooth surface at a very shallow angle, less than a couple of degrees, they will reflect off the surface. This phenomenon has been used successfully to design an all-grazing-incident reflector that focuses X-rays as an optical telescope focuses visible light. Such an X-ray telescope was flown as the heart of the Einstein Observatory satellite, which is shown in Figure 11.
At the focus of the X-ray telescope is the radiation detector, just as in an optical telescope. Although certain photographic emulsions are sensitive to X-rays so as to produce X-ray pictures, photoelectric devices are the primary radiation detectors, since as with ultraviolet studies film can not be easily retrieved from satellites. For hard X-rays special crystalline materials will absorb X-rays, converting their energy into visible photons that can be detected with photoelectric devices. And there are solid silicon detectors whose ability to conduct electric charges is influenced by their absorption of X-ray photons.
Gamma-ray astronomy is a comparatively new field of study that has burgeoned in the last decade or so from modest beginnings, employing balloons and rockets, to today's major endeavor using highly sophisticated satellites. Like that of X-rays, the great penetrating power of gamma-ray photons makes observation and detection different from those for visible photons. Gamma-ray photons carry the highest energy of any photon. The primary gamma-ray detector used for space astronomy is a crystalline material that absorbs the gamma-ray photon, converting its energy to a flash of visible light. The visible photons can then be detected by a photoelectric device.
The Universe is highly transparent to gamma rays because of their great penetrating power. One would therefore expect them to be capable of carrying the imprint of their origin from far-distant places because they pass so easily through interstellar matter. Thus the unique penetration of gamma rays reveals directional and temporal information about their origin in regions that are too dense for visible photons and even X-rays to penetrate. Gamma rays serve as a probe to provide us with new insights into the structure of the cosmos.
6.5. The Future for Space Exploration
The past for planetary and Solar System exploration has been spectacular, both from a scientific perspective and for its inspirational value. Such exploration has not been accomplished without hard work, some grief, false starts, false hopes, and quick modifications. This occurs in part because our methods for establishing goals, planning, and funding through the federal government are not really geared to a time scale for planetary exploration, which can involve 10 to 15 years from conception to launch. In this regard, the problems in planetary exploration are not much different from those in other areas of science, in that the length of time between conception of a scientific experiment and its execution grows steadily longer. It is difficult to carry out complicated scientific programs that span many years when our commitment to such efforts waxes and wanes affecting the funding levels.
High-altitude aircraft and balloons are the least expensive way of investigating invisible extraterrestrial radiation. Jet aircraft can ascend to about 15 km, while balloons are useful up to about 30 km, above which only 5 percent of the atmosphere remains. Although their flights are short compared with balloon flights, lasting for minutes instead of hours, rockets can reach altitudes five times higher than balloons can. Artificial satellites cost much more than rocket flights, but satellites can continuously monitor events over different regions of the electromagnetic spectrum for long periods of time, an advantage that outweighs their additional cost.
Until about 1981, the means of getting satellites off the surface of the Earth have been rockets. The advent of the Space Shuttle (figure 12) has now provided another way to launch satellites. Space Shuttle is a true aerospace launch vehicle in that it takes off like a rocket, maneuvers in Earth orbit as do other spacecraft, but lands like an airplane. Consequently, this launch vehicle is reusable and is the first generation of the aerospace plane. Several large satellites can be carried into orbit in the Shuttle's cargo bay; when the Shuttle is in orbit, the satellites can be lifted out by a controllable arm and placed in their respective orbits. This also means that satellites can be retrieved from orbit and brought back to the Earth's surface or serviced and returned to orbit, as was done for the Solar Maximum Mission satellite by the astronauts aboard Space Shuttle Challenger. Observations and experiments can also be carried out in the Shuttle's cargo bay, where the equipment is manipulated by an astronaut. Space Shuttle gives us the capability of carrying pieces of immense spacecraft, including a Space Station that is under construction (Figure 16), into orbit to be assembled there. Its versatility signals a new generation of possibilities for space exploration.
The satellites used for space astronomy can be divided roughly into two broad categories: (1) near-Earth and Solar System explorers, and (2) telescopes housed in satellites that orbit the Earth. The Solar System explorers are the planetary probes (Figure 13), which are exploring the near-Earth environment, the Sun, or the planets by direct sampling or telescopic studies. Their role is to go to a planet, for example, to photograph and analyze from a close flyby, to orbit the planet, or in some cases to land (we shall discuss them in Chapters 8 and 9). For example, the Viking 1 and Viking 2 spacecrafts landed on the surface of Mars. Other examples are the Voyager 1 and Voyager 2 spacecrafts that flew by Jupiter, Saturn, and Uranus, and perhaps will pass close to Neptune in 1990. Examples designed for solar studies are the eight Orbiting Solar Observatories (OSO-1 through OSO-8) launched between 1962 and 1975 for studies in ultraviolet, X-ray, and gamma-ray radiation.
The orbiting space-telescope platforms (Figure 14), however, are principally to allow astronomers to observe deep-space objects in various wavelength regions from above the Earth's atmosphere. These are satellites placed in orbits several hundred kilometers above Earth's surface. For example, OAO-Copernicus, launched in August of 1972, carried an ultraviolet telescope and three small X-ray telescopes. In January of 1978 the International Ultraviolet Explorer or IUE was launched by NASA. This was a joint undertaking by NASA and several western European countries. Its facilities have been used for ultraviolet studies of planets, stars, galaxies, and the interstellar medium. Astronomers conduct their experiments from an elaborate console of controls located at the NASA Goddard Space Flight Center. The three High Energy Astronomy Observatories, including the Einstein Observatory shown in Figure 13, were launched between 1977 and 1979. They were designed specifically to study X-rays, gamma rays, and subatomic particles (called cosmic rays) from different astronomical objects.
Much of our understanding of the nature of the Universe is changing--rapidly and dramatically--because of these space observatories. In Table 4 we offer a summary of the proposed new space observatory programs into the 1990s.
One of the most sophisticated of the observatory satellites, which after nearly a quarter-century of planning and development, NASA expects to place in orbit with Space Shuttle in late 1989. Orbiting at an altitude of about 500 km and inclined by 29o to the Earth's equator, the Hubble Space Telescope (Figure 15) will be enclosed in a cylindrical tube 13 m long and 7.3 m wide. The telescope consists of a 2.4-m reflector and is equipped with accessory instruments including two imaging cameras, faint-object and high-resolution spectrographs, a photometer, and other specialized devices. These analyzing instruments are designed to cover the wavelength range from about 1200 A in the ultraviolet to 12,000 A in the near infrared. Data from the telescope will be radioed in digital (number) form through the NASA Goddard Space Flight Center in Greenbelt, Maryland to the Space Telescope Sciences Institute on the Johns Hopkins University campus for processing.
Out in space no atmospheric absorption or turbulence will distort the images produced by the telescope. Thus the telescope should detect astronomical sources from 10 to 100 times fainter than those visible from the Earth's surface; in terms of distance, this means a faint object can be seven times farther away than could be seen from the surface of the Earth. The Hubble Space Telescope's spatial resolution will be 20 times better than the best Earth-based reflectors, producing stellar images only 0.05 seconds of arc across. With proper maintenance from Space Shuttle, the telescope should operate for at least a decade and probably much longer.
In 1981, President Reagan called for the nation to proceed to build and orbit a permanently manned space station. Space Station (Figure 16), planned for orbiting sometime in the 1990s, is the response to that call. As mentioned above the Soviet Union has had its permanently manned space station, Mir, in orbit since 1986.
As plans now stand, Space Station will be carried into orbit on approximately 12 Shuttle flights there to be assembled over a span of 18 months. Space Station will orbit at an altitude of about 250 km between latitudes 28.5o north and south; it will have a 90-minute period in low-Earth orbit. The station will initially be a 100-m by 90-m structure consisting of four pressurized modules, two for living and two for working, assorted attached pallets for experiments and manufacturing, eight large solar panels for power, communications and propulsion systems and a robotic manipulator system. The initial station will support a crew of 6 individuals at one time with a replacement crew brought on board every 90 days.
Over time both the Russian Mir and the US Space Station will be expanded in size and modified to meet the needs for an increasing range of scientific and technical programs. Programs in astronomy, microgravity, biomedical research, and space manufacturing are all scheduled to be carried out on Space Station. Finally, Space Station will serve as an intermediate facility for future programs concentrating on high-earth orbit facilities, lunar bases, and eventually interplanetary flight to Mars and other planets in the next century.
As of the time of this book was written in early 1988, several studies done for and by NASA, such as Pioneering the Space Frontier: The Report of the National Commission on Space, call for major initiatives in: