Multimedia Mania/abacus-multimedia-mania.iso/dp/0115/01155.txt

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$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.