IN ROMAN AND GREEK MYTHOLOGY the god Jupiter was accepted as the most powerful and capricious ruler of the heavens; no wonder ancient astronomers gave the same name to the planet that year after year so brilliantly rules the night sky. After the Sun and the Moon, Jupiter is, indeed, the most spectacular object in the sky. Although Venus is at times brighter it cannot ride the midnight sky as does Jupiter.
Today's astronomers acknowledge Jupiter as being perhaps the most important planet of the Solar System. It is the largest and most massive. After the Sun-the star about which all bodies of the Solar System revolve -Jupiter contains two-thirds of the matter in the Solar System. Orbiting the Sun at an average distance of 779 million km (484 million mi.), Jupiter is some 5.2 times as far away as Earth.
Cuneiforms of the Babylonian epic Enuma Elish or Tablets of Creation refer to Jupiter in the Fifth Tablet as the marker of the signs of the Zodiac . . . "He (Marduk - the Creator) founded the station of Nibir (Jupiter) to determine their bounds. . ." To the Babylonians, Nibir was the special name for Jupiter when the planet appeared directly opposite to the Sun and thus shone high and brightly in the midnight sky over the fertile valley of the Euphrates. Since Jupiter travels around its orbit once in almost 12 years, the planet each year moves eastward to occupy the next constellation of the Zodiac. Also, as a result of the relative motions of Earth and Jupiter around the Sun, the faster moving Earth overtakes Jupiter and thereby causes the planet each year to trace out a third of the Zodiacal constellation, i.e., 10 degrees of arc, in a westward, or retrograde, direction relative to the stars (Figure 1-1).
Planets of the Solar System consist of two types: small, dense, inner planets with solid surfaces-Mercury, Venus, Earth with its Moon, and Mars-and large, mainly gaseous, outer planets- Jupiter, Saturn, Uranus, and Neptune, with some satellites as big as the smaller inner planets. Pluto, the outermost known planet, cannot be observed well enough from Earth to be accurately classified, though it is believed to be more like the inner than the outer planets in size.
Between the orbits of Mars and Jupiter, like a transition zone dividing the inner from the outer Solar System, is a wide belt of asteroids, or minor planets, the largest of which, Ceres, is only 1022 km (635 mi.) in diameter. Most asteroids are smaller and many seem to be irregularly shaped.
The first decade of space exploration concentrated on the inner Solar System (Figure 1-2), but at the beginning of the second decade scientists and space technologists started to look at missions to the outer planets. The old fascination of mankind, brilliant Jupiter, became the target for the first mission beyond Mars.
 Dominant Position of Jupiter
Jupiter is an unusual planet by terrestrial standards, both in size and composition. Only slightly denser than water, Jupiter is 317.8 times more massive than Earth. Secondary only to the Sun itself, the giant planet dominates the Solar System (Figure 1-3). Its gravity affects the orbits of other planets and may have prevented the asteroids from coalescing into a planet. Many comets are pulled by Jupiter into distorted orbits, and some of the short period comets appear to have become controlled by Jupiter so that their orbits have their most distant points from the Sun about the distance of the orbit of the giant planet.
Although Jupiter is big (Figure 1-4), it is not big enough to have become a second sun, being too small for its own weight to raise its central temperature high enough for a nuclear reaction to be triggered in its core. However, had Jupiter been 60 to 100 times its present size, our Solar System might have become a binary star system, like so many other stellar systems; and nighttime would have been infrequent on Earth. As it is, Jupiter emits several times more energy than it receives....
....from the Sun, energy probably derived from continued cooling of the planet following its primordial gravitational collapse eons ago when the Solar System formed. A continuing gravitational collapse at a present rate of 1 millimeter per year could alternatively provide the observed heat output from Jupiter.
Family of Satellites
Early in the seventeenth century news spread across Europe of an astounding invention by a spectacle-maker, Hans Lippershey of Middelburgh, Holland. Using a convex and a concave lens at opposite ends of a tube, he made remote objects appear nearer. Two men acted on this news and separately constructed telescopes as the new invention was called. Looking at Jupiter they were astounded to discover that the bright planet possessed a system of satellites - an undreamed of condition in the Aristotelean world of Earth-centered philosophy holding sway at that time. In fact, some scientists of that day claimed the luminous objects were defects of the new instrument, not real objects.
The discovery of these satellites of Jupiter (Figure 1-5) is usually accredited to Galilei Galileo, who published the results of an observation made at Padua on January 7, 1610. Some historians claim, however, that it was Simon Marius of Ausbach, Germany, who first observed the Jovian satellites on December 29, 1609; but he did not publish his observation. These satellites were 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.
One of the most important discoveries in physics was made by the Danish astronomer, Ole Roemer, by means of Jupiter's satellites. Astronomers had observed that the eclipses of Jovian satellites occur 16 minutes and 40 seconds late when Jupiter is on the far side of the Sun from the Earth. In 1675, while in Paris, Roemer explained that this delay results from the finite velocity of light. Light traveling across Earth's orbit, when Earth is farthest from Jupiter, takes 16 minutes and 40 seconds to cover the additional distance. He thereby measured the velocity of light as being about 300,000 km ( 186,000 mi.) per second (Figure 1-6).
The Galilean satellites of Jupiter are quite large bodies (Figure 1-7). Two of the satellites, Callisto and Ganymede, are about the size of the planet Mercury, while Io and Europa rival Earth's Moon. All four satellites are easily seen through a pair of field glasses, appearing as star-like objects nearly in a straight line on either side of the disc of the planet because their orbits are viewed almost edgewise from Earth. Some people with acute vision have been able to see the satellites with their unaided eyes-a good test for sharp vision. The best time to do this is when the sky is still faintly light following sunset, before the planet becomes too brilliant in a black sky.
A fifth satellite of Jupiter was not discovered until almost three centuries later - by E. E. Barnard in 1892. Today, Jupiter is known to have at least fourteen satellites-the other ten are much smaller bodies than the four Galilean satellites. The Jovian system thus resembles a miniature solar system, except that the outermost four satellites of Jupiter orbit oppositely to the others, whereas all the planets go around the Sun in the same direction.
Solar Orbit of Jupiter and Appearance in Earth's Skies
Ancient astronomers, observing the motions of planets against the background of stars, called them wandering stars. The word "planet" is derived from the Greek word "wanderer." Today we know that all the planets, including the Earth, move around the Sun in approximately circular orbits. Because Jupiter orbits the Sun outside the orbit of the Earth, it is called a superior planet. As seen from Earth, all superior planets appear to move eastward close to the ecliptic-the apparent yearly path of the Sun relative to the stars, which is the projection of the plane of the Earth's orbit, the ecliptic plane, against the stars.
In their solar orbits, planets move completely around the celestial sphere. Since Jupiter takes 11.86 Earth years to orbit the Sun, it also takes this time to move around the star sphere. So, as viewed from Earth, Jupiter moves along the ecliptic year by year progressively entering each of the Zodiacal constellations, as noted by the ancient Babylonian writers of the Enuma Elish.
When a superior planet is directly opposite to the Sun in the sky it is in opposition (Figure 1-8). Earth is between the Sun and the planet which, at this time, shines its brightest in the southern sky at midnight in the northern hemisphere. The planet is closest to Earth, too. Jupiter comes into opposition every 13 months. Inferior planets Mercury and Venus cannot reach opposition because they are always within Earth's orbit. So they cannot appear in the midnight sky but remain relatively close to the Sun as seen from Earth.
Conjunction occurs when a planet is on the part of its orbit directly behind the Sun, as seen from Earth, and is thus not visible in the night sky. The planet is then most distant from Earth. This is referred to as superior conjunction to differentiate from inferior conjunction when a planet, orbiting within the Earth's orbit (i.e., Venus and Mercury), is between the Earth and Sun and is closest to Earth in its orbit.
Because the orbit of a superior planet is outside the orbit of the Earth, and because the Earth moves fastest, there is a period each year around the date of opposition when a superior planet is being overtaken and appears to move backward - toward the west among the stars in what is termed retrograde motion (Figure 1-9).
Jupiter the Planet
Jupiter measures 133,516 km (82,967 mi.) from pole to pole, compared with Earth's 12,900 km (8,000 mi.). Rotating faster than any other planet in the Solar System, Jupiter turns completely on its axis once in 9 hours 55-1/2 minutes. But the equatorial regions rotate slightly faster than other regions: in 9 hours 50-1/2 minutes. This means that any point on Jupiter's equator moves at 35,400 km (22,000 mi.) per hour compared with 1,600 km (1,000 mi.) per hour for a point on the Earth's equator.
As a consequence of the rapid rotation, the equatorial regions of Jupiter bulge outward under centripetal force to make the equatorial diameter of the visible globe about 9,280 km (5,767 mi.) greater than the polar diameter. Consequently, Jupiter (Figure 1-10) is not a sphere but has an oblate shape, its polar diameter being 94.2 percent of its equatorial diameter. Earth is flattened at the poles but proportionately much less to only 99.66 percent.
Although Jupiter's volume is 1317 times that of Earth, its mass is only just under 318 times Earth's mass. Since Jupiter is much less dense than Earth, it being only one and one-third times as dense as water, it cannot be a solid sphere like the Earth but instead must consist mainly of gas and liquid with possibly a small solid core. At least three-quarters of Jupiter probably consists of the lightest gases, hydrogen and helium; the same gases that are most common in the Sun and the stars. Jupiter is probably more like the Sun in basic composition than like the Earth.
The gases methane and ammonia have been detected in Jupiter's atmosphere and small....
....amounts of other gases such as ethane and acetylene. Other gases may be there but are difficult to detect in measurements made directly from Earth.
Seen through a telescope from Earth, Jupiter presents a magnificent sight, a striped banded disc of turbulent clouds with all the stripes parallel to the planet's equator. Large dusky gray regions cap each pole in an amorphous hood. Dark, brown or gray stripes are called belts; lighter, yellow-white colored bands between the belts are called zones. All the colors are soft, muted, but quite definite. Many of the belts and zones are permanent enough features to be given names (Figure 1-11).
Over the years, colors on Jupiter are observed to change; the zones vary from yellow to white, while the belts vary from gray to reddish brown. The bands fade and darken as well as change color. They may also widen or become narrow and move up and down in latitude, i.e., farther from or closer to the equator.
Some astronomers suggest that the cold tops of the Jovian clouds in the zones consist of ammonia crystals and vapor. Water clouds are also likely but probably form at a level too deep in the atmosphere to be identified from Earth.
A transparent atmosphere rises some 50 to 65 km (30 to 40 mi.) above the cloud tops.
Many smaller features add interesting details to the zones and bands - streaks, wisps, arches, loops, plumes, patches, lumps, spots, festoons. Some are probably knots of clouds. These small features sometimes change form rapidly in the course of days or even of hours. The scale of Jupiter is so vast that even these features are thousands of miles in extent.
The cloud features of Jupiter move around the....
....planet at different rates. For example, a great equatorial current sweeps around the planet at 360 km (225 mi.) per hour faster than regions on either side of it. It represents a 20 degree-wide girdle around the planet. In addition, some astronomers have interpreted observations as showing that the clouds move at different speeds at different altitudes.
In the southern hemisphere of Jupiter is an outstanding long oval feature known as the Great Red Spot (Figure 1-12). At present 24,000 km (15,000 mi.) long, it has at times extended almost 48,000 km (30,000 mi.). The spot has intrigued generations of astronomers since first observed and recorded centuries ago. In 1664, during the reign of Charles II, the astronomer Robert Hooke reported seeing a large red spot on Jupiter, which could have been the first observation of the Great Red Spot. This was, indeed, the first record of a scientific discovery from a government research contract. In 1665, Cassini referred to the marking as the "Eye of Jupiter." The spot appeared and vanished at least eight times between the years 1665 and 1708, and became a strikingly conspicuous red object in 1878. Early in 1883, the Great Red Spot faded to become almost invisible and then became distinct again, only to fade once more at the beginning of the present century.
The spot was likened to something floating in the atmosphere of Jupiter; early astronomers suggested that it was a raft or an island, since over the centuries the spot drifted around the planet relative to the average movement of the clouds. Sometimes cloud currents have swept around it as though the spot itself were a vortex in the atmosphere. Some scientists postulated that the Great Red Spot represents a column of gas, the center of an enormous whirlpool-like mass of gas rising from deep in the planet to the top of the atmosphere and anchored in some way to the surface far below.
That the Great Red Spot is a hurricane-like structure a fantastic grouping of "thunderstorms" was suggested from recent astronomical investigations prior to the Pioneer mission to Jupiter. Photographs to detect methane revealed that the Great Red Spot is the highest cloud structure on Jupiter and thus implied that the marking might have some internal energy source to push it above the other cloud layers. This would be unlikely if it were a floating mass such as an island, but could be explained by its consisting of a large grouping of thunderstorms-rising air masses.
On Jupiter there are also white spots which are more short lived than the Great Red Spot. They seem to be atmospheric storms, too, and become quite bright for relatively short periods of time (Figure 1-13). These white spots also move relative to the nearby cloud systems.
Jupiter emits three different types of radio waves. These are not like the signals that carry programs on Earth radios but are more akin to the sferics (static or "noise") that interfere with a program when lightning flashes or electric motors are run nearby. The radio noise reaching Earth from Jupiter is greater than that from any other extraterrestrial source except the Sun. The three types are called thermal, decimetric, and decametric radiation.
The thermal radiation is at wavelengths less than a few centimeters. Decimetric radio waves are from a few centimeters to tens of centimeters in length. Decametric refers to radio waves with wavelengths of tens of meters (Figure 1-14).
Thermal radio waves are produced by molecules moving about in the atmosphere of Jupiter. Decimetric radio waves are produced by electrons moving about - oscillating - above the atmosphere. Decametric radio waves are produced by electrical discharges, like lightning flashes, in the upper atmosphere of Jupiter.
 Scientists observed that the decametric radio signals from Jupiter appeared to be linked in some mysterious way to the orbital motion of Jupiter's closest big satellite, Io. Bursts of electrical energy, somehow triggered by Io, are equivalent to billions of simultaneous lightning flashes on Earth.
Observations of the decimetric radio waves from Jupiter caused scientists to conclude that the planet possesses radiation belts similar to those of Earth in which charged particles are trapped and move under the influence of an intense magnetic field. From the intensity of the radiation it was also concluded that Jupiter's magnetic field must be many times stronger than Earth's field. Thus Jupiter and the Earth are the only two planets of the Solar System known to have strong magnetic fields.
The magnetic field of Jupiter traps protons (nuclei of hydrogen atoms) and electrons that flow through interplanetary space from the Sun and are referred to as the solar wind. These trapped, electrically charged particles move backward and forward across the equator of the planet, forming radiation belts.
The electrons, oscillating along the lines of force of the magnetic field, generate radio waves in a similar fashion to electrons caused to oscillate within the antenna of a radio transmitter.
Jupiter is internally quite different from the inner planets (Figure 1-15).
Astronomers generally agree to a basic internal structure of Jupiter, although they differ in detail and interpretation. The average temperature on the top of the cloud layer is very low by terrestrial standards, probably about 150 degrees Kelvin ( - 189° F). Below the cloud tops the temperature rises steadily. The topmost regions consist of supercold ammonia crystals, ammonia droplets, and ammonia vapor. As temperature rises with depth into the atmosphere, there may be ice crystals, water droplets, and water vapor present. Estimates of the total depth of the Jovian atmosphere vary enormously, from 9 5 to 5,800 km (60 to 3,600 mi.) before a "surface" would be reached. This "surface," however, may be a gradual transition from gaseous to liquid hydrogen rather than a sharp interface between gas and liquid or a solid surface. Modern theories suggest a very deep atmosphere at the bottom of which the pressure, exerted by the weight of all the gas above, is enormous, reaching millions of times Earth's 14 pounds per square inch sea level pressure.
Such great pressure could convert hydrogen into a special form in which it behaves like a metal: it readily conducts both heat and electricity as metals do. So beneath a sea of liquid hydrogen could be a shell of metallic hydrogen (probably liquid because of the high temperature) surrounding a small internal core consisting of rocky material and other metals; somewhat the same as the composition of the inner planets, including the Earth. Jupiter's core has been estimated as ten times the mass of the Earth. However, the existence of such a rocky core is still widely debated among planetologists.
Near the center of Jupiter, the temperature might be tens of thousands of degrees and could account for Jupiter radiating into space 2.3 times as much energy as the planet receives from the Sun.
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 their own gravity. It is believed that after the Sun itself condensed from a primordial nebula, planets of different sizes formed from different concentrations of matter present at various distances from the Sun. Electrical and magnetic forces in the gas clouds or gravitational collapse of the proto solar cloud probably thrust the condensing planets into orbits around the central Sun. Those planets that started to aggregate early scooped up more matter than those which started later and had less free material to collect. Mass distribution in the cloud probably had a lot to do with the resultant masses of the planets.
 Scientific experiments made by space probes that photographed the inner planets and their satellites, coupled with geological evidence on Earth and radar probing to the surface of Venus, indicate that the terrestrial planets have been highly cratered, and this cratering presents evidence of the final stages of planetary accretion (Figure 1-16). On Earth, subsequent changes to the surface through internal heat, plate tectonics, and weathering obliterated nearly all evidence of impact cratering.
Much of the primordial gas was hydrogen the most common material in the universe which consists of a proton and an orbital electron. The Sun, for example, is nearly all hydrogen, as are the stars. Astronomers have also discovered vast clouds of hydrogen in the spaces between the stars.
While it is most probable that the Earth and the other inner planets were never able to attract much hydrogen, they may have possessed some hydrogen in their atmospheres for a relatively short time on the scale of planetary development. Hydrogen atmospheres of the inner planets could have been lost by massive eruptions on the Sun during its early development. Also, the closeness of the terrestrial type planets to the Sun, coupled with their....
...relatively small gravities, allowed hydrogen to escape into space. But the cooler Jupiter, 565 million km (350 million mi.) beyond Mars, with additionally a much stronger gravity, holds hydrogen in tremendous quantities. So probably do the other large planets: Saturn, Uranus, and Neptune.
Knowledge about these complex atmospheres may help our understanding of Earth's more simple atmosphere. Already the study of dust storms in Mars' very thin, dry atmosphere, and the circulation patterns in Venus' very dense atmosphere, is helping meteorologists understand the dynamics of planetary atmospheres in general.
At some level in the deep atmosphere of Jupiter the temperature should equal that on Earth. At this level ammonia crystals could become liquid ammonia droplets. Water could condense too. Such droplets could rain from the clouds, sometimes frozen into snows of water and ammonia. But the drops and snowflakes could never fall to the surface as they do on Earth. Instead, at warm lower regions of the deep atmosphere, they would probably evaporate and rise back into the clouds.
Such a circulation pattern, somewhat analogous to those that build up violent thunderstorms and tornadoes in Earth's atmosphere (Figure 1-17), would probably give rise to endless violent turbulence in the Jovian atmosphere; more violent by far than the thunderstorms of Earth. Accompanying electrical discharges would probably make Earth's lightning flashes mere sparks by comparison. Thus, vertical movements in the atmosphere of Jupiter may provide examples of the most violent storms imaginable. At the same time jet circulations in the  cloud bands and zones may be analogous to Earth's major atmospheric patterns such as the trade winds, tropical convergences and jet streams.
At first thought Jupiter might be considered an inhospitable planet on which life could not survive. This need not necessarily be so. Since there are probably liquid water droplets in an atmosphere of hydrogen, methane and ammonia, Jupiter may provide the same kind of primordial "soup" in which scientists currently believe that life originated on Earth.
Life has been described as an unexplained ability to organize inanimate matter into a living system that perceives, reacts to, and evolves to cope with changes to the physical environment that threaten to destroy its organization. In 1953, a mixture of hydrogen, methane, ammonia, and water vapor the kind of atmosphere Jupiter still retains today and many scientists believe Earth possessed soon after its formation was bombarded in a laboratory with electrical discharges. These were passed through the gas mixture to simulate the effects of bolts of lightning. The electrical energy bound together some of the simple gas molecules into more complex molecules of carbon, hydrogen, nitrogen, and oxygen of the type believed to be the building blocks for living systems (Figure 1-18).
 At some point in Earth's history, postulated at about 3.5 billion years ago, something organized the complex carbon-based molecules of Earth's oceans and atmosphere into living systems which were then able to make copies of themselves to reproduce. It is theorized that from then on, by slight changes to subsequent copies, biological evolution produced all the living creatures of Earth, including Man.
The big question is: Has life evolved in the atmosphere of Jupiter? It is known that the temperature may be right at lower elevations in the Jovian atmosphere. It is known that the gas mixture may be suitable. It is known that electrical discharges probably take place. Jupiter could hold a key to the evolution of life, and this key may be found if unmanned probes are sent to the Jovian atmosphere later this century. Such probes are technologically possible today as a result of experience gained with the Pioneer flyby of Jupiter and probes to other planets.
Why a mission to Jupiter?
The question of beginnings has always intrigued mankind. How did something appear from nothing and become the physical universe? Man is still far from having satisfactory answers even as to how the Solar System condensed from charged atoms, energetic molecules, and electromagnetic forces of some primeval nebula. How did the various planets evolve their unique differences? How did life originate and flourish on Earth, a planet so different from all the others?
It is not easy to find answers here on Earth since this planet can be studied only in its present stage of evolution, a single frame in the long motion picture of Earth's history as an astronomical body. The single picture does not provide enough information for scientists to be really sure about Earth's past let alone its future. However, other planets may pass through evolutionary history at different rates, and some, such as the Moon and Mercury,...
....have "fossilized" so that they preserve the ancient record of planetary evolution.
It is not possible to study planets in very great detail by use of telescopes on Earth; all the planets are much too far away and, in addition, observations are limited by the screening and distorting effects of the Earth's atmosphere (Figure 1-19). Since planetary probes have been dispatched, astronomers have undoubtedly learned more about the planets during the last ten years than in all the previous centuries of observation from Earth.
Knowledge about these other planets is important to our understanding of our own planet, its....
...past and its future. Such knowledge and understanding might be vital to the long-term survival of the human species if people are to adapt to inevitable natural and man-caused changes to the Earth's environment. Mankind might be able to predict long-term changes to the terrestrial environment and prepare for them.
In many respects, Jupiter provides a model of what is taking place in the universe at large. Many processes on Jupiter may be similar to those in stars before their nuclear reactions begin. And the great turmoil in Jupiter's processes, coupled with the high speed of planetary rotation, provides an extreme model for the study of jet streams and weather in quieter planetary atmospheres such as the Earth's.
The satellites of Jupiter represent a veritable Solar System in miniature, even to the densities of the satellites, like the planets, decreasing with distance from the central body. Thus, their formation may have paralleled the formation of the Solar System. Astronomers are questioning whether these satellites are Earth-like planetary bodies, or more like giant snowballs. The four outermost satellites, Andrastea, Pan, Poseidon, and Hades, move around Jupiter in a counter direction to most of the Jovian satellites. They could be captured asteroids. Examination of the surfaces of the Jovian satellites by space probes may reveal differences that will throw light upon their origin. So far only the four large Jovian satellites have been seen at close hand, as described later in this book.
The outer Solar System is relatively unknown to Man. Saturn (Figure 1-20), the next planet beyond Jupiter, never approaches closer than 1250 million km (780 million mi.) of Earth; while Uranus, the next planet, is almost one billion miles farther away.
Saturn will not be reached by a spacecraft until Pioneer 11 flies by it in September 1979.
Figure 1-21. The gravity of Jupiter, coupled
with the planet's orbital motion, can be used in a slingshot
technique to speed spacecraft to the outer planets. But first NASA
had to find out if the environment of Jupiter could be penetrated
without causing the spacecraft to fail.
Yet these big planets of the Solar System are probably of great importance to developing a full understanding of the system's origin. Since they are so distant, they require that spacecraft travel very fast to reach them in reasonable times. Unfortunately, launch vehicles cannot boost spacecraft of practical size to the necessary high velocities. However, by using the gravitational field and orbital motion of Jupiter in a slingshot technique, spacecraft can be swung into more energetic paths to carry them relatively quickly to the outer planets (Figure 1 -21).
Jupiter thus provides a means to explore the outer Solar System. But there is a problem: Jupiter's strong magnetic field traps charged particles in radiation belts that extend out from the planet a greater distance than from Earth to Moon. Without exploring these radiation belts, scientists could not be sure the belts would not damage any spacecraft using Jupiter as a gravity slingshot to the outer planets. If the radiation belts proved to be a serious hazard, the exploration of the outer Solar System might have to wait until more energetic propulsion systems than chemical rockets could be developed, perhaps several decades hence.
Although scientists can tell from the radio waves emitted by the Jovian radiation belts approximately how many electrons are trapped in the belts' they have no way of knowing from Earth how many high energy protons are trapped there, and it is especially the protons that do the damage. The only way to find out is to send a spacecraft to Jupiter to penetrate the radiation belts and measure the protons on the spot and this has been done by the two Pioneers.
Such a mission to Jupiter poses many technical challenges. It extends Man's exploration of the Solar System to a new scale 800 million km (half a billion mi.) to Jupiter compared with only 65 million km (40 million mi.) to Mars. The vast....
.....distance presents problems of communications; not only the diminution of the radio signals, but also the time delay in information traveling to Earth from the spacecraft and the equal time delay for radio commands from Earth to reach the spacecraft (Figure 1-22). This delay makes it necessary for controllers on Earth to become skilled in flying the spacecraft 90 minutes out of step with the spacecraft itself at the distance of Jupiter. Everything has to be planned well in advance with no opportunity to react to and correct for any hazards caused by unknowns.
Additionally, because of the great distance traveled from the Sun itself, the sunlight at the distance of Jupiter has an intensity of only one twenty-seventh of that at Earth's distance from the Sun (Figure 1-23). The normal method of supplying electrical power in space by converting sunlight to electricity cannot be used. A spacecraft bound for Jupiter has to carry a nuclear energy source to generate electricity. Also the spacecraft must fly through space for several years before reaching its objective. So new levels of high reliability are mandatory. Moreover, the high velocities needed to reach Jupiter call for a lightweight spacecraft, thereby demanding lightweight design of the spacecraft and all its components and scientific instruments.
Finally, between Mars and Jupiter is the asteroid belt (Figure 1-24), which some theories suggested may be a 280-million-km (175-million-mi.) wide zone of abrasive dust that might seriously damage any spacecraft trying to cross it.
Such were the obstacles. But the opportunity to explore the outer Solar System beyond the orbit of Mars beckons strongly, challenging the ingenuity of space technologists. The National Aeronautics and Space Administration accepted the challenge in a double-pronged exploratory program: two spacecraft, Pioneers F and G, were planned to make the assault on Jupiter. Their mission was a journey into the unknown territory of space, truly a pioneer odyssey for an encounter with a giant to open the outer Solar System for mankind. Thus began to unfold early in 1970, the story of an incredible journey to the planet Jupiter and beyond; a mission to the most spectacular object in the night skies of Earth, an object that has not only held the attention of mankind since time immemorial, but also offers a doorway to the outer Solar System.