$Unique_ID{bob01155} $Pretitle{} $Title{Pioneer Chapter 1: Pioneer To The Giant Planets} $Subtitle{} $Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric} $Affiliation{Ames Research Center;University Of Iowa;Science Writer} $Subject{jupiter saturn earth planets system rings spacecraft planet solar ring} $Date{1980} $Log{} Title: Pioneer Book: Pioneer: First To Jupiter, Saturn, And Beyond Author: Fimmel, Richard O.;Allen, James Van;Burgess, Eric Affiliation: Ames Research Center;University Of Iowa;Science Writer Date: 1980 Chapter 1: Pioneer To The Giant Planets The space age began in 1957 when the first artificial satellite orbited Earth 31 years after the flight of Robert H. Goddard's first liquid propellant rocket. Within only another 22 years, spacecraft of the National Aeronautics and Space Administration had explored all the planets of our Solar System that were known to mankind before the invention of the telescope. On September 1, 1979, Pioneer 11 climaxed these 22 years of space exploration by reaching Saturn after journeying through space for 6-1/2 years over a distance of 3.2 billion kilometers. On its approach to Saturn, Pioneer found that the giant planet has a magnetic field and a complex magnetosphere buffeted by the solar wind. These discoveries added to earlier ones made by Pioneer about the magnetic field and magnetosphere of Jupiter. Pioneer revealed unique and spectacular views of Saturn and its ring system. If an observer had been onboard the spacecraft as it swept by this mighty ringed planet, the view would have been spectacular. Behind the spacecraft, the Sun had shrunk to only about one-ninth the size it appears on Earth. Ahead, Saturn appeared as a rotating, flattened globe with banded patterns. The spacecraft approached the planet from above while the south side of the rings were illuminated by the Sun. This view of the magnificent ring system is one never seen from Earth. The usually bright rings appeared dark and the usually dark gaps between them were bright. This vantage point provided scientists with much new information about the structure of these rings - rings that had been regarded as unique until recently when much less spectacular rings were discovered around the planets Jupiter and Uranus. At a speed approaching 114,000 km/hr, the spacecraft rushed toward Saturn. Over the several days before closest approach, the detailed structure of the rings gradually became clearer. One by one, the bright specks of Saturn's family of small satellites and the globe of the huge satellite Titan appeared. As Pioneer moved closer to Saturn, the most critical period of the mission was at hand - when the spacecraft crossed the plane of the rings and hurtled beneath them for its closest approach to within 21,400 km of the cloud tops of Saturn. There was no way of knowing from Earth whether ring particles were present inside the bright visible rings. Even a relatively few particles could destroy the spacecraft. The polar areas of Saturn were by this time visible and the belts of weather systems at lower latitudes wer more distinct, although still much less clearly defined than the weather systems of Jupiter observed by Pioneer several years before. Evidence of jet streams in the upper atmosphere began also to appear. The shadow of the rings on the planet clearly showed the divisions in the ring system. At just under 1 million kilometers from Saturn and 4 hr before closest approach, Pioneer discovered a faint narrow ring outside the bright A ring. About the same time, the spacecraft's imaging system revealed a new inner satellite of Saturn. Data from the charged particle experiments showed unusual variations that indicated the presence of another previously unknown small satellite. During the next few hours, the planet and its rings expanded to fill the spacecraft's field of view as it hurtled toward the plane of the ring system. The mission controllers and monitors on Earth anxiously wondered whether the spacecraft would survive to complete its mission. At 10:36 a.m., September 1, 1979, the spacecraft plunged through the ring plane. Not until 86 min later - the time for radio waves traveling at 300,000 km/sec to reach Earth from Saturn - did the message reach the controllers back on Earth that the spacecraft was undamaged. Pioneer was continuing its mission. Back on the spacecraft, a magnificent view of the fully illuminated rings unfolded. The rings stretched overhead in great curved sheets as Pioneer rushed along beneath them. Pioneer moved so fast and was so close to the rings that images could not be obtained for transmission to Earth. Hurtling beneath the rings and over the cloudtops, the spacecraft recorded unique details of the alien environment near this giant planet. Then Pioneer plunged back through the ring plane, again without damage to the spacecraft. The encounter with Saturn was an unqualified success. As Pioneer sped away from Saturn, the view was of a crescent-shaped planet crossed by dark rings and their shadow bands. Ahead was the large satellite Titan, which Pioneer would pass at a distance of almost 354,000 km on September 2. As it did so, the spacecraft obtained the first images of the largest satellite in our Solar System. With Titan shrinking into a starlike object behind it, and having explored the two giants, Jupiter and Saturn, for the first time, Pioneer still had not completed its mission. The spacecraft headed out of our Solar System toward the distant stars, seeking information about interplanetary space to the orbit of Pluto and beyond. During the first decade of space exploration, scientists concentrated on the inner Solar System, but at the beginning of the second decade scientists and space technologists began to consider missions to the outer planets, particularly to the gaseous giants Jupiter and Saturn. These two planets are perhaps the most important in the Solar System because, after the Sun, they contain most of the matter in the Solar System. Jupiter alone accounts for over two-thirds of the planetary of the Solar System. Both giant planets are unusual by terrestrial standards the density of Jupiter is only slightly greater than water while that of Saturn is sufficiently low that the planet would float in water. Jupiter's mass is 317.8 times that of Earth. Its gravity affects the orbits of other planets and may have prevented the asteroids from coalescing into a planet. Jupiter's gravitational force pulls many comets into distorted orbits; some short-period comets appear to have become controlled by Jupiter so that their orbits at their most distant points from the Sun are about the distance of the Jovian orbit. Saturn also has collected a family of comets. Despite their size, Jupiter and Saturn were not large enough to become stars. Their masses were insufficient to raise internal temperatures high enough to trigger nuclear reactions in their cores. However, had they been some 100 times larger, the Solar System might have been a triple star system, and nighttime would have been infrequent on Earth. As it is, both giant planets emit more energy than they absorb from the Sun, energy that is probably generated as these planets continue to cool following their primordial gravitational collapse eons ago soon after the Solar System formed. In 1608, spectacle-maker, Hans Lippershey of Middleburgh, Holland, invented an astounding instrument. He happened to pick up two lenses and looked through them, discovering that objects viewed through the lenses appeared nearer. He experimented further with a convex and a concave lens at the opposite end of a tube. His spyglass engendered considerable excitement and word of his on spread across Europe. Two men, Galileo and Simon Marius, using the idea of Lippershey's spyglass, constructed a device - which came to be known as a telescope in about 1611 - and trained it on the heavens. The discovery of the satellites of Jupiter is usually credited to Galileo, who published "The Starry Messenger" relating the results of observations he made at Padua on January 7, 1610. Galileo made a staggering number of observations at that time - "great, unusual, and remarkable spectacles, a host of stars." The Starry Messenger described what Galileo considered the most important discovery of all, the moons of Jupiter. Some historians, however, claim that Simon Marius of Auerbach, Germany, first discovered the Jovian satellites on December 29, 1609, but he did not publish his observations. Both men, looking at Jupiter, were astounded to discover that the bright planet possessed a system of satellites - an undreamed of condition in the Aristotelian philosophy of an Earth-centered universe then holding sway over most human thought. In fact, some scientists of that day claimed the luminous objects were defects of the new instrument, not real objects. These satellites Are later given the names Io, Europa, Ganymede, and Callisto by Marius, but are often referred to as the Galilean satellites. Today the satellites are frequently identified by the Roman numerals I, II, III, and IV, respectively. In 1675, Ole Roemer, observing Jupiter's satellites, made one of the most important discoveries in physics. He noted that the eclipses of Jovian satellites occur progressively later as the Earth moves away from Jupiter and progressively earlier as it moves toward Jupiter. He explained that this effect is evidence for the finite velocity of light. Light traveling across Earth's orbit, when Earth is farthest from Jupiter, takes 16 min and 40 sec to cover the additional distance. From this, he estimated the velocity of light to be about 300,000 km/sec (186,000 mps). The Jovian satellites are quite large - Callisto and Ganymede are about the size of the planet Mercury, while Io and Europa are larger than Earth's Moon. Viewed through a pair of good field glasses, all four satellites appear as starlike objects nearly in a straight line on either side of the disk of the planet because their orbits are viewed almost edgewise from Earth. These satellites have been sighted without the use of a telescope. The best viewing time is when the sky is faintly light following sunset, before the planet becomes overpoweringly brilliant in a black sky. Almost three centuries later, in 1892, E. E. Barnard discovered a fifth satellite of Jupiter. Of the 14 Jovian satellites known today, 10 are much smaller than the 4 Galilean satellites. The Jovian system thus resembles a small solar system, except that the orbits of its four outermost satellites are traversed in the opposite sense to that of the other satellites, whereas all the planets orbit the Sun in the same sense, counterclockwise when viewed from the north ecliptic pole. Saturn and its rings and satellites resemble a small solar system as well. But Saturn's satellites did not intrigue astronomers so much as other strange, unusual appendages during the years following the discovery of the telescope. In 1610, Galileo was mystified by two appendages, one on either side of Saturn. He was even more mystified when a few years later he could no longer see them. Many years later, Christian Huygens, who mastered the art of grinding telescope lenses with higher precision than his contemporaries, observed a thin ring surrounding the planet inclined at a considerable angle to the plane of the ecliptic, sometimes seen open and bright and at other times invisible from Earth when viewed edge-on. At this same time, 1655, Huygens discovered a satellite of Saturn - Titan. Astronomers had identified two bright rings of Saturn. But for many years, the nature of Saturn's rings remained an enigma. In June 1838, the astronomer Galleo observed that from the inner ring a veil extended across half the dark space separating it from the planet. It was not until 1850 that Bond in the United States and Dawes in England showed that this effect was caused by a faint third "crepe" ring. Still the nature of Saturn's rings remained a mystery. Laplace and Herschel thought the rings were solid. In 1848, Edward Roche suggested they were probably small particles; Bond, in 1851, thought they must be fluid. Not until 1857 did James Clerk Maxwell, the Scottish physicist, prove mathematically that the rings consist of particles orbiting Saturn so closely crowded together as to appear as a continuous mass. By the end of the 17th century, astronomers had discovered five satellites of Saturn, all but Titan being smaller than the Galilean satellites of Jupiter. In 1789, Herschel, with a new reflecting telescope, observed two more satellites. In the next century two more were discovered - at the time of the Pioneer missions to the giant planets nine known satellites orbited Saturn and two more were suspected. Solar Orbits of the Giant Planets Ancient astronomers observing the motions of the planets against a background of stars called them wandering stars. The word "planet" is derived from the Greek word "wanderer." All the planets, including Earth, orbit the Sun in near circular paths. Jupiter and Saturn orbit the Sun outside the orbit of Earth - they are called superior planets. As seen from Earth they appear to move eastward on the average, nearly along the ecliptic. The ecliptic is the apparent yearly path of the Sun relative to the stars, which is the projection of the plane of Earth's orbit, the ecliptic plane, against the background of stars. Jupiter takes 11.86 Earth-years to orbit the Sun, Saturn, 29.46 years. So, as viewed from Earth, Jupiter and Saturn move along close to the ecliptic year by year progressively passing through the 12 zodiacal constellations. When a superior planet is opposite the Sun in the sky, the planet is nearest Earth - and is in opposition. Consequently, the planet appears brightest at this time. At midnight it shines in the southern sky of the Northern Hemisphere, or in the northern sky of the Southern Hemisphere. Jupiter is in opposition every 13.1 months, Saturn every 12.4 months. A planet is in conjunction when it lies in nearly the same direction as the Sun as seen from Earth. At this time, the planet is not visible in the night sky and is then most distant from Earth. Because the orbit of a superior planet is outside Earth's orbit, and because Earth moves faster, each year around the date of opposition, a superior planet is "overtaken" by Earth and the planet appears to move backward relative to the background of stars - toward the west - in "retrograde motion." Jupiter as Observed from Earth From pole to pole, Jupiter measures 134,000 km (83,270 miles) compared with Earth's 12,700 km (7,890 miles). Jupiter turns on its axis faster than any other planet in our Solar System, once every 9 hr 55.5 min. Its equatorial regions rotate slightly faster than other regions, in 9 hr 50.5 min. Such rapid rotation has flattened the poles, and at its equator, Jupiter bulges to about 8,200 km (5,095 miles) greater than its polar diameter. Although Jupiter's volume is 1317 times that of Earth, its mass is just under 318 times Earth's mass. Scientists have long known that Jupiter is not a solid body like Earth but consists mainly of gas and liquid with possibly a small rocky core (which is also liquid). By the 1950's, scientists realized that Jupiter's composition, predominantly hydrogen and helium, more closely resembles that of the Sun than of Earth. The sight of Jupiter, seen from Earth through a telesccope, is magnificent - stripes and bands of turbulent clouds parallel the planet's equator. Dusky amorphous areas cover each pole. The darker stripes or "belts" and the lighter bands between these belts called "zones" are permanent enough to be given names. The colors of Jupiter appear soft and muted, yet quite definite. They change at different times the zones vary from yellowish to white, the belts from gray to reddish brown. The intensity of the bands changes, fading and darkening. The bands also widen or become narrow and move up or down in latitude. Streaks, wisps, arches, loops, plumes, patches, lumps, spots, and festoons embellish the zones and bands. Astronomers have suggested that these smaller features are clusters of clouds, and that others are zones of turbulence between jet streams moving at different speeds. These small features are observed to change during the course of a day, sometimes within hours. Cloud formations move around Jupiter at different rates. A great equatorial current, 200 wide, sweeps around Jupiter at 360 km/hr (225 mph) faster than surrounding regions. In the southern hemisphere of Jupiter, a huge oval feature has intrigued astronomers since it was first observed in 1664 by the astronomer Robert Hooke. This Great Red Spot is now about 24,000 km (15,000 miles) long, but at times has extended to almost 48,000 km (30,000 miles). The spot has, on occasion, faded almost completely. Many scientists have speculated on this marking, describing it as a high mountain peak or an island floating in the clouds. Small and less persistent red spots have been seen from time to time as well as relatively short-lived white spots. After the Sun, Jupiter is the strongest source of radio signals in the Solar System. Three types of radiation received on Earth are emitted from Jupiter - thermal, decimetric, and decarnetric. Thermal radio waves are produced by agitated molecules in the Jovian atmosphere. Decimetric radio waves are produced by electrons spiraling around lines of force in the planet's magnetic field. Decametric radio waves are produced by some remarkable type of electrical instability. Scientists have observed that decametric radiation is linked in some way to the orbital motion of Io, the closest large satellite of Jupiter. From observations of decimetric radio waves, scientists concluded that Jupiter has a magnetic field and radiation belts similar to Earth's belts within which charged particles are trapped and spiral around magnetic field lines. Because of the intensity of these radiation belts, scientists calculated that Jupiter's magnetic field is many times stronger than that of Earth. Saturn Observed from Earth From pole to pole, Saturn measures 107,000 km (66,490 miles). The planet spins on its axis once in about 10 hr 40 min. Saturn's equatorial regions rotate slightly faster than other regions, in 10 hr 14 min. This rapid rotation has flattened the poles and has bulged Saturn's equatorial diameter to about 12,000 km (7,456 miles) greater than the polar diameter. Although Saturn's volume is 755 times that of Earth, its mass is just under 95.2 times Earth's mass. Saturn is not a solid body like Earth but is similar to Jupiter, consisting mainly of gas and liquid with possibly a small rocky (liquid) core. Like Jupiter, Saturn is composed predominantly of hydrogen and helium and is therefore more like the Sun than Earth. However, there is a striking difference between Jupiter and Saturn - the density of Saturn is only about half the mean density of Jupiter and about one-eighth that of Earth. Viewed through a telescope from Earth, Saturn is a spectacular sight - a dull, flattened globe surrounded by bright rings extending to a diameter of 274,200 km (170,400 miles). Faint bands on the globe suggest stripes of cloud paralleling the planet's equator. Large darker regions cover each pole, with dark stripes or "belts" and lighter bands between the belts called "zones." The colors of Saturn are softer and more muted than those of Jupiter, varying from pale yellow to brownish yellow. The contrast between belts and zones is much less striking than on Jupiter. Observations from Earth have revealed many interesting details. Of course, Saturn is much farther away and, viewed through a telescope, appears as a disk whose diameter is less than half that of Jupiter. Light-colored spots have been observed on Saturn from time to time, but they do not last as long as those on Jupiter nor are their colors so intense. No feature of Saturn compares with Jupiter's Great Red Spot. In fact, only 10 conspicuous spots have been observed on Saturn during 300 years of telescopic observations from Earth. From observations of such spots, astronomers have determined that clouds move around Saturn in 10 hr 37 min at 400 north and south latitudes, some 23 min longer than clouds at the equator. The variations in cloud speeds are believed to cause turbulence between belts and zones as on Jupiter. Since no nonthermal radiation had been detected before the Pioneer mission, whether Saturn possessed a magnetic field could not be proved from radio data. However, because Saturn resembles Jupiter and spins rapidly on its axis, scientists thought it likely that Saturn has a magnetic field, but a flyby spacecraft would be needed to establish its presence and to measure its strength. The ring system of Saturn is a fascinating spectacle. These rings have divisions that were not thought to be empty space but regions where there were smaller numbers of particles. The most prominent division discovered by Cassini in 1675 is called Cassini's division. From Earth-based observations, astronomers believed it was about 6,000 km (3,730 miles) wide. It separates the two main bright rings - A for the outermost and B for the next inner visible ring - that comprise the bright visible system. The B ring is the brighter - a golden yellow ring with a brighter rim near its outer circumference which stands out in high contrast against Cassini's division. The A ring is silvery, not so bright as the B ring. As discovered by Encke, the A ring also has a less clearly defined gap about one-fourth the width of Cassini's division. A faint inner "crepe" ring, or C ring, has a milky transparency against the blackness of space or, when seen against the globe of the planet, it appears to be a dusky veil. The rings, being so thin, virtually disappear when seen edge on. These rings are estimated to be only 2 km thick. Some observers claim to have observed faint rings inside the C ring and outside the A ring - an innermost D ring and an outermost E ring. Whether rings existed beyond the visible rings was an important consideration to scientists planning spacecraft trajectories to fly by Saturn. Much of the evidence concerning Saturn's rings was conflicting. Both the composition and size of particles in the rings were disputed for many years. Just before the Pioneer mission, there were many speculative theories about the composition of the rings. One of these suggested that the rings consisted of ice or ice-coated rocks with diameters of at least 5 cm (2 in.) but not greater than several meters. Planetary Interiors Astronomers believe that the interiors of Jupiter and Saturn are very similar to each other but quite different from those of the terrestrial planets such as Earth. It is believed that the giant planets consist of shells of increasing density. The outermost shell, the hydrogen atmosphere, has some helium and traces of heavier gases such as methane, ammonia, and water vapor. At depth within this atmosphere, the pressure becomes so great that the hydrogen liquefies. The next lower shell is of liquid hydrogen. Much deeper, the pressure is so great that liquid hydrogen becomes a special form called liquid metallic hydrogen, which behaves as a metal and readily conducts heat and electricity. Convective motions within this shell of metallic hydrogen could be responsible for the magnetic field of Jupiter and would be expected to produce a magnetic field for Saturn as well. Deep within each planet, highly compressed volatiles such as water, ammonia, and methane might surround a liquid metallic core with a mass perhaps 10 to 20 times greater than that of Earth. Each core might be extremely hot, exceeding the temperature at the surface of the Sun, because of the tremendous pressures at the planetary cores. Some models of planetary interiors suggest that, for Saturn, helium might separate from hydrogen to form another shell just above the core. Planetary Evolution The planets of the Solar System probably formed four to five billion years ago when hosts of small rocky particles and clouds of gas were drawn together by gravity. It is believed that as the Sun condensed from a primordial nebula, planets formed from concentrations of matter at various distances from the Sun. One speculation is that the planets that began early to aggregate material scooped up more matter than those planets that started later and had less free material to collect. The distribution of mass in the clouds probably contributed greatly to the resultant masses of the planets. Photographs taken by spacecraft of the inner planets and their satellites, coupled with geological evidence on Earth and radar probing of the Venusian surface, show that the crusts of the terrestrial planets are densely cratered by many impacts. This cratering presents evidence of the final stages of planetary accretion. On Earth, subsequent changes to the surface through internal heat, plate tectonics, and weathering obliterated nearly all evidence of impact cratering. While such cratering would not, of course, have taken place on the gaseous giants, cratering on their satellites may offer clues about the distribution of the matter that impacted their surfaces. Much of the primordial gas was hydrogen, the most common material in the Universe. The Sun, for example, is nearly all hydrogen, as are the stars. Vast clouds of hydrogen fill the spaces between the stars. Earth and the other inner planets may have possessed some hydrogen in their atmospheres for a very short time in the scale of planetary development. Energetic eruptions on the Sun during its early development may have swept hydrogen from the inner Solar System, depleting hydrogen from the atmospheres of the inner planets. The atmospheres of the outer planets still hold hydrogen. Jupiter and Saturn are thought to be predominantly hydrogen, and Uranus and Neptune are also believed to contain much of this very light gas. Knowledge of the complex atmospheres of the outer planets should be helpful in understanding Earth's early atmosphere. From studies of duststorms in the thin, dry atmosphere of Mars and circulation patterns in the dense, hot atmosphere of Venus, meteorologists have gained a better understanding of planetary atmospheres in general. Information about the atmospheres of the giant planets is expected to add to this body of understanding. At some level in the deep atmospheres of Jupiter and Saturn, the temperature should equal that on Earth. At this level ammonia crystals become liquid ammonia droplets and water condenses. These droplets would rain from the clouds, sometimes frozen into snows of water and ammonia. But the drops and snowflakes would never fall to a solid surface as on Earth. Instead, at the warm lower regions of the deep atmospheres of Jupiter and Saturn, these droplets would evaporate and return to the clouds. Such a circulation pattern, somewhat analogous to those that create violent thunderstorms and tornadoes in Earth's atmosphere, would probably cause endless violent turbulence in the atmospheres of the giant planets, far more violent than Earth's thunderstorms. The electrical discharges that would accompany such turbulence would make Earth's flashes of lightning mere sparks by comparison. Thus vertical turmoil in the atmospheres of Jupiter and Saturn may provide examples of the most violent storms imaginable. Jet circulations in the cloud bands of these giants may be analogous to Earth's major atmospheric patterns such as the trade winds, tropical convergences, and jet streams. It was long thought that Jupiter and Saturn might be inhospitable planets on which life could not survive. But since there are probably liquid water droplets in an atmosphere of hydrogen, methane, and ammonia, the atmospheres of the giant planets may provide the same primordial "soup" from which it has been suggested that life originated on Earth. Life has been described as an unexplained ability to organize nonliving matter into a continuing system that perceives, reacts to, and evolves to cope with changes in the physical environment which threaten to destroy its organization. In 1953 a mixture of hydrogen, methane, ammonia, and water vapor - components of the atmospheres of Jupiter and Saturn - was bombarded by electrical discharges to simulate the effects of bolts of lightning. As a result, some of the gas molecules combined into more complex molecules of the type believed to be the building blocks of living systems. At some point in Earth's history, postulated as being about 3.5 to 4 billion years ago, highly complex, carbon-based molecules became organized into living systems that were able to replicate themselves - to reproduce. It is theorized that from this beginning through developmental changes in this biological chain, all the living creatures on Earth evolved. But has life evolved in the atmospheres of Jupiter and Saturn? It is known that, on these planets, the temperature may be right and the gas mixture suitable, and that electrical discharges occur. Although the Pioneer spacecraft were not intended to search for evidence of such evolutionary processes, such missions could be precursors to more sophisticated ones, perhaps probing deep into the atmospheres of these intriguing planets. Mission Objectives The question of beginnings has always intrigued man. There are no satisfactory answers to how the Solar System condensed from charged atoms, energetic molecules, and electromagnetic forces of some primeval nebula. Nor is it known how the various planets evolved in their unique way. And more significant to man - how did life originate and flourish on Earth, a planet so different from all the others? Earth itself reveals few answers because our planet can be studied only in its recent stages of evolution, a very short period in the long history of Earth as an astronomical body. From the available information, scientists cannot be sure about Earth's past, let alone its future. However, other planets have passed through evolutionary phases at different rates and some, such as the Moon and Mercury, have "fossilized" their ancient record of planetary evolution. But planets are much too far away to be studied in great detail by use of telescopes on Earth. Also, observations are limited by the screening and distorting effects of Earth's atmosphere. With the planetary probes, astronomers have learned more about the planets during the first years of the space age than in all the previous centuries of observations from Earth. From this knowledge, man has gained a better understanding of our planet Earth, its past and its future. Such knowledge and understanding are vital to the survival of all living species, for man must protect his tenuous environment while adapting to inevitable natural and man-made changes. In many respects, the giant planets Jupiter and Saturn provide models of what is taking place in the entire Universe. Many processes within these planets may be similar to those in stars before nuclear reactions occur. The great turmoil in these processes, particularly those of Jupiter, coupled with the high-speed rotation of these planets, provide an opportunity for scientists to make comparative studies of jet streams and weather in quieter planetary atmospheres such as Earth's. Each satellite system of the two giant planets represents a lesser solar system, even, as for Jupiter, to the densities of the satellite bodies which, like the planets, decrease with distance from the central body. Thus, their formation may have paralleled the formation of the Solar System. The outer reaches of the Solar System were relatively unknown before the Pioneer odyssey to Jupiter and Saturn. Yet these great planets provide valuable information to help us understand the origins of the Solar System. Since they are so distant from the Sun, they require that spacecraft depart Earth's orbit very fast to reach them in a reasonable time. For planets more distant than Jupiter, available launch vehicles cannot boost spacecraft of practical size to the necessary velocities. However, using the gravitational field and orbital motion of Jupiter in a slingshot technique, spacecraft can be hurtled into more energetic paths to carry them to the more distant planets and to escape from our Solar System. But there was a danger: within Jupiter's strong magnetic field, radiation belts extend outward to great distances. These radiation belts must be explored to ascertain whether they will damage spacecraft if Jupiter is to be used as a gravity slingshot to the outer worlds. If these radiation belts prove to be a serious hazard, the exploration of the outer Solar System might have to await the development of more energetic propulsion systems than chemical rockets, perhaps decades hence. Whether Saturn can be used as a slingshot to reach Uranus and Neptune is in doubt until scientists can determine what hazards, if any, are presented by the ring particles outside the bright visible rings of the planet. Although scientists have estimated from radio waves emitted by the Jovian radiation belts approximately how many electrons are trapped there, they have no way of knowing from Earth how many high-energy protons are trapped there - protons of the type that would be especially hazardous to spacecraft. Similarly, scientists cannot determine from Earth whether Saturn's rings extend far beyond the visible rings. Only with a spacecraft that could penetrate the radiation belts of Jupiter and, if it survived this journey, one that could then pass through the ring plane of Saturn near the visible rings, could scientists find out the answers. The mission to Jupiter and Saturn posed many technical challenges. It would extend man's exploration of the Solar System to a new scale - 780 million kilometers (485 million miles) from the Sun to Jupiter and another 650 million kilometers (400 million miles) to Saturn, with a chance to explore interplanetary space far beyond the orbit of Uranus, although not in close proximity to that planet. The vast distances to be covered by the spacecraft presented problems of communications - not only because of the weakness of radio signals but also because of the time delay in information traveling to Earth from the spacecraft and radio commands transmitted from Earth to the spacecraft. This delay required that controllers on Earth become adept at flying the spacecraft 90 min out of step with the spacecraft itself at Jupiter and 170 min out of step at Saturn. Because of the great distance between the Sun and Jupiter, sunlight at Jupiter's orbit is only 1/27 as intense as at Earth's orbit; at the distance of Saturn, sunlight is only 1/90 as intense. Normally, a spacecraft's electrical power is supplied by converting sunlight to electricity. But a spacecraft bound for the outer Solar System must carry a nuclear energy source to generate electricity. Also, since the spacecraft must fly through space for several years before reaching its destination, the demands for a highly reliable spacecraft were more stringent than in previous missions. Moreover, because of the high velocities required to reach Jupiter and Saturn, the spacecraft and its components and scientific instruments had to be lightweight. Additionally, between Mars and Jupiter stretches an asteroid belt which some scientists thought might include abrasive dust, perhaps 280 million kilometers (175 million miles) wide, which might seriously damage a spacecraft crossing it. Despite these obstacles, the opportunity to explore the outer Solar System beyond the orbit of Mars beckoned strongly, challenging the ingenuity of space technologists. The National Aeronautics and Space Administration accepted he challenge in a double-pronged exploratory mission: two spacecraft, Pioneers F and G, were to make the assault. Their journeys into the unknown to explore the far reaches of our Solar System began early in 1972 - incredible journeys to he planets Jupiter and Saturn, two spectacular points of light in the night skies of Earth that have held the attention of mankind for centuries.