(Background, above) Voyager, one of the most remarkable robotic space probes to travel through the solar system, sent back a wealth of scientific data about several of our neighboring planets. As it sped toward interstellar space after its close encounter with Neptune, it also took a "family portrait" of our planetary system. This diagram depicts the positions at which numerous photographs were taken, superposed on a sketch of the solar system. The inserts show some of the actual images radioed back to Earth.

LEARNING GOALS

Studying this chapter will enable you to:

Describe the overall scale and structure of the solar system.

Summarize the basic differences between the terrestrial and the jovian planets.

Identify and describe the major nonplanetary components of the solar system.

Describe some of the spacecraft missions that have contributed significantly to our knowledge of the solar system.

Discuss the importance of comparative planetology to solar-system studies.

Looking up at the nighttime sky, we see an almost bewildering array of stars. Thousands are visible to the naked eye, and even a small telescope increases the number we can observe into the millions. Because astronomers can see and study so many stars, they have come to recognize patterns in their properties and have classified the stars accordingly. Ultimately, this classification has led to an understanding of the birth, maturity, and death of stars everywhere. In studying the planets, we are not so fortunate. We are aware of only a single planetary system--our own solar system--and each of its nine planets differs significantly from all the others. We don't have tens of thousands of Earthlike planets to compare, so we cannot perform the statistical analyses that have taught us so much about the stars. Every piece of information we can glean about the structure and history of the planets and smaller bodies orbiting our Sun plays a potentially vital role in helping us understand not just our own solar system, but countless undiscovered planetary systems throughout the Galaxy.

In less than a single generation, we have learned more about our solar system--the Sun and everything that orbits it--than in all the centuries that went before. By studying the planets--the major bodies that orbit the Sun and reflect its light--and their moons--which orbit the planets--astronomers have gained a richer outlook on our own home in space. Instruments aboard unmanned robots have taken close-up photographs of the planets and their moons and in some cases have made on-site measurements. The discoveries of the past few decades have revolutionized our understanding not only of our cosmic neighborhood but also its history, for our solar system is filled with clues to its own origin and evolution.

As we describe the solar system in the next few chapters, we will use the powerful and still emerging perspective of comparative planetology--comparing and contrasting the properties of the diverse worlds we encounter--to understand better the conditions under which planets form. Having made some important stops along the way, we will conclude our tour in Chapter 10 with a look at the modern theory of how our planetary system came into being and how it developed over time.

Before we start our journey, let us pause for a moment to consider the solar system as a whole, to try to place the planets in perspective and to see what patterns we can discern in their orbits and properties. We will begin by making a short historical survey of our cosmic neighborhood.

The Greeks and other astronomers of old were aware of five planets in the nighttime sky--Mercury, Venus, Mars, Jupiter, and Saturn. We saw in Chapter 2 how observations of the apparently erratic motions of those wanderers across the celestial sphere ultimately led to the Copernican revolution and the birth of our modern view of the cosmos. In addition to the Sun and the Moon, the ancients also knew of two other types of heavenly objects that were clearly neither stars nor planets.

Comets appear as long, wispy strands of light in the night sky that remain visible for periods of up to several weeks, then slowly fade from view. Meteors, or "shooting stars," are sudden bright streaks of light that flash across the sky, usually vanishing less than a second after they first appear. While these transient phenomena were familiar to ancient astronomers, their role in the "big picture" of the solar system was not understood until much later.

With the invention of the telescope, more detailed observations of the known planets could be made. Galileo Galilei was the first to capitalize on this new technology (his simple telescope is shown in Figure 6.1). His discovery of the phases of Venus and of four moons around Jupiter early in the seventeenth century played a large part in changing forever humankind's vision of the universe. From that time on, Earth was considered a planet like all the others.

Figure 6.1 The telescope with which Galileo made his first observations was simple, but its influence on astronomy was immeasurable.

With continuing technological advances, knowledge of the solar system improved rapidly. Astronomers began discovering objects invisible to the unaided human eye. By the end of the nineteenth century, astronomers had found Saturn's rings (1659), Uranus (1758), Neptune (1846), many planetary moons, and the first of asteroids, "minor planets" that orbit the Sun mostly in a broad band, the asteroid belt, lying between Mars and Jupiter. Ceres, the largest asteroid and the first to be sighted, was discovered in 1801. A large telescope of midnineteenth-century vintage is shown in Figure 6.2.

Figure 6.2 By the mid-nineteenth century, telescopes had improved enormously in both size and quality. Shown here is the telescope built and used by Irish nobleman and amateur astronomer the Earl of Rosse.

The twentieth century has brought continued improvements in optical telescopes. One more planet (Pluto) has been discovered, along with three more ring systems, dozens of moons, and thousands of asteroids. The century has also seen the rise of both nonoptical astronomy--especially radio and infrared--and spacecraft exploration, both making vitally important contributions to the field of planetary science. Astronauts have carried out experiments on the Moon (see Figure 6.3), and numerous unmanned probes have left Earth and traveled to all but one of the other planets. Figure 6.4 shows the 1989 launch of Galileo, which was carried in the cargo bay of the space shuttle Atlantis.

Figure 6.3 An Apollo astronaut doing some lunar geology-- prospecting near a huge boulder near the Mare Serenitatus.

Figure 6.4 The launch of the space probe Galileo on a mission to explore in detail the moons and the atmosphere of Jupiter.

As currently explored, we know that our solar system contains 1 star (the Sun), 9 planets, 63 moons (at last count), 6 asteroids larger than 300 km in diameter, more than 4000 smaller (but well-studied) asteroids, myriad comets a few kilometers in diameter, and countless meteoroids less than a meter across. The list may grow as we continue to explore our neighborhood. The near-void between all these objects is termed interplanetary space.

Astronomical Ruler

Back By terrestrial standards, the solar system is immense. The distance from the Sun to Pluto is about 40 A.U., almost a million times the radius of Earth and roughly 15,000 times the distance from Earth to the Moon. Despite its vast extent, though, the entire solar system lies very close to its parent Sun, astronomically speaking. Even the diameter of Pluto's orbit is less than 1/1000 of a light year, and the next nearest star is several light years distant from the Sun.

The planet closest to the Sun is Mercury. Moving outward, we encounter in turn Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. In Chapter 2 we saw the basic properties of the planets' orbits. Their paths are all ellipses, with the Sun near one focus. Most orbits have low eccentricities, the exceptions being the innermost and the outermost worlds, Mercury and Pluto. We can reasonably think of most planets' orbits as circles centered on the Sun. Figure 6.5 is an artist's rendition of the planetary system as future generations of space voyagers might perceive it from a distant vantage point.

Figure 6.5 Might future space voyagers travel far enough from Earth to gain this perspective on our solar system? Except for Mercury and Pluto, the orbits of the planets lie nearly in the same plane. As we move out from the Sun, the distance between the orbits of the planets increases. The entire solar system spans nearly 80 A.U.

All the planets orbit the Sun in nearly the same plane as Earth (the ecliptic plane). Again, Mercury and Pluto deviate somewhat from this rule--their orbital planes lie at 7° and 17° to the ecliptic, respectively. Still, we can think of the solar system as being quite flat. Its "thickness" perpendicular to the plane of the ecliptic is less than 1/50 the diameter of Pluto's orbit. If we were to view the planets' orbits from a vantage point in the ecliptic plane about 50 A.U. from the Sun, only Pluto's orbit would be noticeably tilted. All planets orbit the Sun in the same direction--counterclockwise, seen from above Earth's North Pole. Figure 6.6 is a photograph of planets Mercury, Venus, Mars, and Jupiter taken during the July 1991 solar eclipse. These four planets happen to be visible in this one photograph in large part because their orbits lie nearly in the same plane.

Figure 6.6 Taken from Hawaii during the July 1991 eclipse of the Sun, this single photograph shows four planets--Mercury, Venus, Mars, and Jupiter. Because they all orbit in nearly the same plane, it is possible for them all to appear (by chance) in the same region of the sky, as seen from Earth.

The planetary orbits are not evenly spaced: The orbits get farther and farther apart as we move farther out from the Sun. Nevertheless, there is a certain regularity in their spacing. In the eighteenth century a fairly simple rule, now known as the Titius-Bode law (see Interlude 6-1), seemed to "predict" the radii of the planetary orbits remarkably well. Even the asteroid belt between Mars and Jupiter had a place in the scheme, which excited great interest among astronomers and numerologists alike. There is apparently no simple explanation for this empirical "law." Today, it is regarded more as a curiosity than as a fundamental property of the solar system.

The solar system presents us with a sense of orderly motion. The planets move nearly in a plane, on almost concentric elliptical paths, in the same direction around the Sun, at steadily increasing orbital intervals. However, the individual properties of the planets themselves are less regular. Some selected properties of various solar system objects are presented in Table 6-1.

Table 6-1 lists, for each object, its distance from the Sun, the length of its year, its mass and radius, the number of moons known to orbit it, its escape velocity, its rotation period, and finally its density--a very important property of any object, to which we will return in more detail in a moment. In most cases, all these quantities can be determined by straightforward methods already described in this book. As we saw in Chapter 2, the distance of each planet from the Sun is known from Kepler's laws once the scale of the solar system is set by radar-ranging on Venus. The masses of planets with moons may be calculated by application of Newton's laws of motion and gravity, again as described in Chapter 2, just by observing the moons' orbits around the planets. The sizes of those orbits, like the sizes of the planets themselves, are found by measuring their angular sizes and then applying elementary geometry, as we saw in Chapter 1.

The masses of Mercury and Venus, as well as our own Moon and the asteroid Ceres, are a little harder to determine accurately because they have no natural satellites of their own. Nevertheless, it is possible to measure their masses by careful observations of their influence on other planets or nearby bodies. Mercury and Venus produce small but measurable effects on each other's orbits, as well as that of the Earth. The Moon causes small "wobbles" in Earth's motion as the two bodies orbit their common center of mass. Only in the case of Ceres is the mass poorly known, primarily because that asteroid's gravity is so slight. All the techniques necessary for determining mass were available to astronomers well over a century ago. Today, the masses of most of the objects in Table 6-1 have been very accurately measured through their gravitational interaction with artificial satellites and space probes launched from Earth.

Once a planet's mass and radius are known, its escape velocity--the minimum speed required for an object to escape forever from its gravitational grasp--can be calculated from the formula given in Chapter 2. As explained in the More Precisely feature, escape velocity is the primary factor in determining the composition, and even the existence, of a planet's atmosphere. A planet's rotation period is determined simply by watching surface features appear and disappear again as the planet rotates. For some planets this is difficult to do, as their surfaces are hard to see or may even be nonexistent! Venus's surface is completely obscured by clouds, while Jupiter, Saturn, Uranus, and Neptune have no solid surfaces at all--their atmospheres simply thicken, and eventually become liquid, as we descend deeper and deeper below the visible clouds. We will describe the methods used to measure the rotation periods of these planets in later chapters.

The final column in Table 6-1 lists a property called density. This is a measure of the "compactness" of the matter within an object. It is computed by dividing the object's mass (in kilograms, say) by its volume (in cubic meters, for example). Dividing Earth's mass (determined by observations of the Moon's orbit) by its volume (which we know because we know Earth's radius--see the More Precisely feature on p. 25), we obtain an average density of approximately 5500 kg/m3. On average, then, there are about 5500 kilograms of Earth matter in every cubic meter of Earth volume. For comparison, the density of ordinary water is 1000 kg/m3, rocks on the Earth's surface have densities in the range 2000­3000 kg/m3, and iron has a density of some 8000 kg/m3. Earth's atmosphere has a density of only a few kilograms per cubic meter. Because most astronomers are more familiar with the CGS units of density (grams per cubic centimeter--g/cm3--where 1 kg/m3 = 1000 g/cm3), Table 6-1 lists density in both SI and CGS units.

A clear distinction can be drawn between the inner and the outer members of our planetary system based on densities and other physical properties. In short, the inner planets--Mercury, Venus, Earth, and Mars--are small, dense, and rocky in composition. The outer worlds--Jupiter, Saturn, Uranus, and Neptune (but not Pluto)--are large, of low density, and gaseous in nature.

Because their physical and chemical properties are somewhat similar to Earth's, the four innermost planets--Mercury, Venus, Earth, and Mars--are often called the terrestrial planets. (The word terrestrial derives from the Latin word terra, meaning "land," or "earth.") The larger, outer planets--Jupiter, Saturn, Uranus, and Neptune--are often labeled the jovian planets because of their physical and chemical resemblance to Jupiter. (The word jovian comes from Jove, another name for the Roman god Jupiter.) The jovian worlds are all much larger than Earth and quite different from it in both composition and structure.

Finally, beyond the outermost gas giant lies one more small world, frozen and mysterious. Pluto doesn't fit well into either planetary category. It might once have been a moon of Neptune, or perhaps even a large cometlike body and not originally a planet at all.

Back The four terrestrial planets are close to the Sun in astronomical terms, all being within 1.5 A.U. of their parent star. All are small and of low mass, and all have generally rocky composition and solid surfaces. Beyond that, however, the similarities end. When we take into account the differing compression of their interiors by the weight of the overlying layers (greatest for Earth, least for Mercury), we find that the average uncompressed densities of the terrestrial worlds--that is, the densities in the absence of any compression--decrease steadily as we move farther from the Sun. This indicates that their overall compositions differ significantly. In addition, the planets' present-day surface conditions are quite distinct. All but Mercury have atmospheres, but the atmospheres are about as dissimilar as we could imagine, ranging from a near-perfect vacuum on Mercury to a hot, dense inferno on Venus. Earth alone has oxygen in its atmosphere (as well as liquid water on its surface). Earth and Mars spin at roughly the same rate--one rotation every 24 (Earth) hours--but Mercury and Venus both take months to rotate just once. Earth and Mars have Moons, but Mercury and Venus do not. Finding the common threads in the evolution of four such worlds is no simple task! Comparative planetology will be our indispensable guide as we proceed through the coming chapters.

For all the differences among the terrestrial worlds, they still seem very similar when compared with the jovian planets. Perhaps the simplest way to express the major differences between the terrestrial and jovian worlds is to say that the jovian planets are everything the terrestrial planets are not: The terrestrial worlds lie close together, near the Sun; the jovian worlds are widely spaced through the outer solar system. The terrestrial worlds are small, dense, and rocky; the jovian worlds are large and gaseous, being made up predominantly of hydrogen and helium (the lightest elements), which are quite rare on the inner planets for reasons to be described in the More Precisely feature on p. 136. The terrestrial worlds have solid surfaces; the jovian worlds have none (their dense atmospheres thicken with depth, eventually merging with their liquid interiors). The terrestrial worlds have weak magnetic fields, if any; the jovian worlds all have strong magnetic fields. The terrestrial worlds have only three moons among them; the jovian worlds each have many moons, no two of them alike and none of them like our own. In addition, all the jovian planets have rings, a feature unknown on the inner planets. Despite their greater size, the jovian worlds all rotate much faster than any terrestrial planet.

Figure 6.7 presents a diagram of the sizes of the planets relative to the Sun. The Sun is clearly the largest object in our solar system. It has over 1000 times the mass of the next largest object, the planet Jupiter. The Sun in fact contains about 99.9 percent of all solar system material. The terrestrial planets--including our home planet--are insignificant in comparison. Table 6-2 compares and contrasts the key properties of the terrestrial and jovian worlds.

Terrestrial Planet Part I

Figure 6.7 Diagram, drawn to scale, of the relative sizes of the planets and our Sun. Notice how much larger the jovian planets are than the Earth and the other terrestrials and how much larger still is the Sun.

Back In the vast space among the nine known major planets move countless chunks of rock and ice, all orbiting the Sun, many of them on highly eccentric paths. This final component of the solar system is the collection of interplanetary matter--debris that ranges in size from the relatively large asteroids, through the smaller comets and even smaller meteoroids, down to the smallest grains of interplanetary dust that litter our cosmic environment. Larger bodies collide and break apart into smaller bodies. In turn, smaller bodies collide and are ground into dust, which eventually settles into the Sun or is swept away by the solar wind, a stream of energetic charged particles that continually flows outward from the Sun. The dust is generally quite difficult to detect in visible light, but studies in the infrared range reveal that interplanetary space contains surprisingly large amounts of it. Our solar system is an extremely good vacuum by terrestrial standards, but positively dirty by the standards of interstellar or intergalactic space.

Asteroids (Figure 6.8a) and meteoroids are generally rocky, a little like the outer layers of the terrestrial planets. Their total mass is much less than that of Earth's Moon. They are important because many of them are made of material that has scarcely changed since the early days of the solar system. In addition, they often conveniently deliver themselves right to our doorstep (in the form of meteorites), allowing us to study them in detail without having to fetch them from space.

Figure 6.8 (a) Asteroids, like meteoroids, are generally composed of rocky material. This asteroid, Gaspra, is about 20 km long; it was photographed by the Galileo spacecraft on its way to Jupiter. (b) Comet West, seen as it approached the Sun in 1976. Most comets are composed largely of ice, and so tend to be relatively fragile. Shortly after this photograph was taken, the comet split into several fragments.

The comets are quite distinct from the other small bodies in the solar system. They are icy rather than rocky--in fact, they are quite similar in composition to some of the icy moons of the outer planets--but they too represent truly ancient material. Since their formation long ago, along with the rest of the solar system, most comets have probably not changed or interacted with anything at all. Comets striking Earth's atmosphere do not reach the surface intact, so we do not have actual samples of cometary material. However, they do vaporize and emit radiation as their highly elongated orbits take them near the Sun (Figure 6.8b). We can, therefore, determine their makeup by spectroscopic study of the radiation they emit as they are destroyed. In this way we can obtain information on what the solar system was like soon after its birth.

Galileo Arrives at Jupiter

Back Since the 1960s, there have been dozens of unmanned space missions to the other planets in the solar system. All the planets but Pluto have been visited and probed at close range by U.S. or Soviet craft. The impact of these missions on our understanding of our planetary system has been nothing short of revolutionary. In the next few chapters, we will see many examples of the marvelous images radioed back to Earth. Here, we focus on just a few of these remarkable technological achievements.

THE MARINER 10 FLYBYS OF MERCURY

In 1974, the U.S. spacecraft Mariner 10 came within 10,000 km of the surface of Mercury, sending back high-resolution images of the planet. These photographs, which showed surface features as small as 150 m across, revolutionized our knowledge of the planet. For the first time, we saw Mercury as a heavily cratered world, in many ways reminding us of our own Moon.

Mariner 10 (see Figure 6.9) was launched from Earth in November 1973 and was placed in an eccentric 176-day orbit about the Sun, aided by a gravitational assist (see Interlude 6­2) from the planet Venus. In that orbit, Mariner 10's nearest point to the sun (perihelion) is close to Mercury's path, and its farthest point away (aphelion) lies between the orbits of Venus and Earth. The 176-day period is exactly two Mercury years, so the spacecraft revisits Mercury roughly every 6 months. However, only on the first three encounters--in March 1974, September 1974, and March 1975--did the spacecraft return data. After that, the craft's supply of maneuvering fuel was exhausted. In total, over 4000 photographs, covering about 45 percent of the planet's surface, were radioed back to Earth during the mission's active lifetime. The remaining 55 percent of Mercury is still unexplored.

Figure 6.9 The path of the Mariner 10 probe to Mercury included a gravitational boost from Venus. The spacecraft (inset) returned data from March 1974 until March 1975 providing astronomers with a wealth of information on the planet Mercury.

EXPLORATION OF VENUS

In all, some 20 spacecraft have visited Venus since the 1970s, far more than have spied on any other planet. The Soviet space program took the lead role in exploring Venus's atmosphere and surface, while American spacecraft have performed extensive radar mapping of the planet from orbit. The American Mariner 2 and Mariner 5 missions passed within 35,000 km of the planet in 1962 and 1967, while Mariner 10 grazed Venus at a distance of 6000 km en route to Mercury.

During roughly the same period, the Soviet Venera (derived from the Russian word for Venus) program got under way, and the Soviet Venera 4 through Venera 12 probes parachuted into the planet's atmosphere between 1967 and 1978. The early Venera probes were destroyed by enormous atmospheric pressures before reaching the surface. Then, in 1970, Venera 7 (sketched in Figure 6.10) became the first spacecraft to soft-land on the planet. During the 23 minutes it survived on the surface, it radioed back information on atmospheric pressure and temperature. Since that time, a number of Venera landers have transmitted photographs of the surface back to Earth and have analyzed the atmosphere and the soil. None of them survived for more than an hour in the planet's hot, dense atmosphere. The data they sent back make up the entirety of our direct knowledge of Venus's surface. In 1983, the Venera 15 and Venera 16 orbiters sent back detailed radar maps (at about 2 km resolution) of large portions of Venus's northern hemisphere.

Figure 6.10 One of the Soviet Venera landers that reached the surface of Venus. The design was essentially similar for all the surface missions. Note the heavily armored construction, necessary to withstand the harsh conditions on the planet's surface.

The United States' Pioneer Venus mission in 1978 placed an orbiter at an altitude of some 150 km above Venus's surface and dispatched a "multiprobe," consisting of five separate instrument packages, into the planet's atmosphere. During their hour-long descent to the surface, the probe returned information on the variation of density, temperature, and chemical composition with altitude in the atmosphere. The orbiter's radar produced images of most of the planet's surface.

The most recent U.S. mission was the Magellan probe (shown in Figure 6.11), which entered orbit around Venus in August 1990. The spacecraft began sending back spectacular data (see Chapter 9) in September 1990. It completed its first 243-day mapping cycle (the time required for Venus to rotate once beneath the probe's orbit) in May 1991. Magellan's spatial resolution was at least 10 times better than the best images previously obtained. It could distinguish objects as small as 120 m across and measure vertical distances to within less than 50 m. The probe covered the entire surface of Venus with unprecedented clarity, rendering all previous data virtually obsolete. Many theories of the processes shaping the planet's surface have been radically altered or abandoned completely because of Magellan.

Figure 6.11 The U.S. Magellan spacecraft is launched from the space shuttle Atlantis in May 1989.

SPACECRAFT EXPLORATION OF MARS

Both NASA and the Soviet (now Russian) space agency have Mars exploration programs that began back in the 1960s. However, the Soviet effort was plagued by a string of technical problems, along with a liberal measure of plain bad luck. As a result, almost all of the detailed planetary data we have on Mars has come from unmanned U.S. probes launched in the 1960s and 1970s.

The first spacecraft to reach the red planet was Mariner 4, which flew by Mars in July 1965. The images sent back by the craft showed large numbers of craters caused by meteoroids impacting the planet's surface, but nothing of the Earthlike terrain some scientists had expected to find. Flybys in 1969 by Mariner 6 and Mariner 7 confirmed these findings, leading to the conclusion that Mars was a geologically dead planet having a heavily cratered, old surface. Studies of Mars received an enormous boost with the arrival in November 1971 of the Mariner 9 orbiter. The craft mapped the entire Martian surface at a resolution of about 1 km, and it rapidly became clear that here was a world far more complex than the dead planet imagined only a year or two previously. Mariner 9's maps revealed vast plains, volcanoes, drainage channels, and canyons. All these features were completely unexpected, given the data provided by the earlier missions. These new findings paved the way for the next step--actual landings on the planet's surface.

The last U.S. spacecraft to visit Mars were the two Viking missions, which reached the planet in mid-1976. Viking 1 and Viking 2 each consisted of two parts. An orbiter mapped the surface at a resolution of about 100 m (about the same as the resolution achieved by Magellan on Venus), and a lander (see Figure 6.12) descended to the surface and performed a wide array of geological and biological experiments. Viking 1 touched down on Mars on July 20, 1976. Viking 2 arrived in September of the same year.

Figure 6.12 A Viking lander, here being tested in the Mojave Desert prior to launch.

By any standards, the mission was a complete success, as the Viking orbiters and landers returned a wealth of data on the Martian surface and atmosphere. In August 1993, the first U.S. probe since Viking--the Mars Observer, which was designed to radio back detailed images of the planet's surface and provide data on the Martian atmosphere--mysteriously fell silent, for reasons that are still unclear, just before entering orbit. As a result, the U.S. Mars program is now on hold. Although most space scientists would agree that more exploration of Mars is needed, they disagree as to the best means of achieving that goal. NASA has ambitious plans for manned missions to Mars. However, the enormous expense of such an undertaking, coupled with the belief of many astronomers that unmanned missions are economically and scientifically preferable to manned missions, make the future of these projects uncertain at best.

MISSIONS TO THE OUTER PLANETS

Two pairs of U.S. spacecraft have revolutionized our knowledge of Jupiter and the jovian planets. The first pair, Pioneer 10 and Pioneer 11, were launched in March 1972 and April 1973, arriving at Jupiter in December 1973 and December 1974. The Pioneer spacecraft took many photographs and made numerous scientific discoveries. Their orbital trajectories also allowed them to observe the polar regions of Jupiter in much greater detail than later missions would achieve. In addition to their many scientific accomplishments, the Pioneer craft also played an important role as "scouts" for the later Voyager missions. The Pioneer series demonstrated that spacecraft could travel the long route from Earth to Jupiter without colliding with debris in the solar system. They also discovered--and survived--the perils of Jupiter's extensive radiation belts (somewhat like Earth's Van Allen belts, but on a much larger scale). In addition, Pioneer 11 used Jupiter's gravity to propel it along the same trajectory to Saturn that the Voyager controllers planned for Voyager 2's visit to Saturn's rings.

The two Voyager spacecraft (see Figure 6.13) left Earth in 1977, reaching Jupiter in March (Voyager 1) and July (Voyager 2) of 1979 to study the planet and its major satellites in detail. Each craft carried sophisticated equipment to study the planet's magnetic field, as well as radio, visible-light, and infrared sensors to analyze its reflected and emitted radiation.

Figure 6.13 The Voyager spacecraft. Voyager 1 and Voyager 2 (shown here) were identical.

Both Voyager 1 and Voyager 2 used Jupiter's gravity to send them on to Saturn. Voyager 1 visited Titan, Saturn's largest moon, and so did not come close enough to the planet to receive a gravity-assisted boost to Uranus. However, Voyager 2 went on to visit both Uranus and Neptune in a spectacularly successful "Grand Tour" of the outer planets. The data returned by the two craft are still being analyzed today. Like Pioneer 11, the two Voyager craft are now headed out of the solar system, still sending data as they race toward interstellar space. Figure 6.14 shows the past and present trajectories of the Voyager spacecraft.

Figure 6.14 The paths taken by the two Voyager spacecraft to reach the outer planets. Voyager 1 is now high above the plane of the solar system, having been deflected up and out of the ecliptic plane following its encounter with Saturn. Voyager 2 continued on for a "Grand Tour" of the four jovian planets. It is now outside the orbit of Pluto.

The most recent mission to Jupiter is the U.S. Galileo probe, launched by NASA in 1989 (see Figure 6.4). It arrived at its target in 1995 after a rather roundabout route (shown in Figure 6.15) involving a gravity assist from Venus and two from Earth itself. The mission consists of an orbiter and an atmospheric probe. The probe will descend into the atmosphere of Jupiter, slowed by a heat shield and a parachute, making measurements and chemical analyses as it goes. The orbiter will execute a complex series of gravity-assisted maneuvers through Jupiter's moon system, returning to some moons already studied by Voyager and visiting others for the first time. Scientists around the world eagerly anticipate the results of the mission. Though Galileo's findings may answer many old riddles, they are sure to pose many new ones too.

Figure 6.15 Galileo's path to Jupiter included one flyby of Venus, two of Earth, and two trips through the asteroid belt, before reaching its destination in 1995.

Shortly after launch, Galileo mission controllers discovered that the craft's main antenna, needed to radio its findings back to scientists on Earth, had failed to fully deploy. Despite repeated efforts, the instrument remains jammed in an almost closed position. As a result, all data must be transmitted via a small secondary antenna, greatly reducing the amount of information that can be returned. Nevertheless, with better data-processing techniques, engineers believe that most of the scientific objectives of the mission can still be met. Already, Galileo has provided astronomers with spectacular views of Earth and Moon, as well as the only close-up photographs of asteroids ever obtained.

Back As we proceed through the solar system, we will seek to understand how each planet compares with our own Earth­Moon system, and we will learn what each planet contributes to our knowledge of the solar system as a whole. On the basis of our studies of our own backyard--the Earth and Moon--we will identify some important questions to ask. For example, does the planet have a magnetic field, an atmosphere, or geological activity? Does it have a rocky surface? Does it have a liquid core? But, as we will see, each planet will also present us with new questions and insights of its own. Whatever the answers, the comparison enriches our knowledge of the ways planets work.

As we catalog the similarities and differences among the planets, terrestrial and jovian, what can we learn? To understand the solar system, we must try to answer basic questions such as, "Why did planet X evolve in one way, while planet Y turned out completely different?" "Why are the planets' orbits so orderly, when their individual properties are not?" "What can we learn from the makeup of the debris that orbits among the planets?" Our main problem in finding answers is that we don't have many objects to work with. Thus, we must study the properties of all known planets, moons, and larger asteroids in great detail to determine their common features and their differences and to find the reasons for both. Our goal is to develop a comprehensive theory of the origin and evolution of the solar system that explains all, or at least most, of its observed properties. As we unravel the origin of our own solar system, perhaps we will learn about the origin of planetary systems beyond our own.