PIONEERS TO JUPITER
Reaching for the Outer Planets
Since the beginning of the Space Age, scientists had dreamed of sending
probes to Jupiter and its family of satellites. Initially, robot
spacecraft were limited to studying the Earth and its Moon. In 1962,
however, the first true interplanetary explorer, Mariner 2, succeeded in
escaping the Earth-Moon system and crossing 100 million kilometers of
space to encounter Venus, studying Earth’s sister planet at close range
using half a dozen scientific instruments. By the mid 1960s a U.S.
planetary spacecraft had also flown to Mars, there had been a second
flyby of Venus, and an ambitious program was under way for two more
flybys of Mars in 1969, followed by a Mars orbiter in 1971. Based on
this success with the inner planets, NASA scientists and engineers began
to plan seriously to meet the challenge of the outer solar system.
Not only Jupiter, but Saturn and even Uranus and Neptune, were
considered as possible targets. However, the distances between the outer
planets are so vast that many years of flight would be required for a
spacecraft to reach them, even using the most powerful rocket boosters
then contemplated. If a cautious exploration program were followed,
investigating one planet at a time before designing the next mission, it
would be well into the twenty-first century before even a first
reconnaissance of the solar system could be achieved. A way to bridge
the space between planets in a more efficient, economical manner was
needed.
In the late 1960s celestial mechanicians—scientists who study the
motions of planets and spacecraft—began to solve problems posed by the
immensity of the outer solar system. If a spacecraft is aimed to fly
close to a planet in just the right way, it can be accelerated by the
gravity of the planet to higher speeds than could ever be obtained by
direct launch from Earth. If a second, more distant planet is in the
correct alignment, the gravity boost given by the first encounter can
speed the craft on to the second. Jupiter, with its huge size and strong
gravitational pull, could be used as the fulcrum for a series of
missions to Saturn, Uranus, Neptune, and even distant Pluto. In
addition, the early 1980s would offer an exceptional opportunity, one
repeated only about once every two centuries. At that time, all four
giant planets would be in approximate alignment, so that gravity-assist
maneuvers could be done sequentially. A single spacecraft, after being
boosted from Jupiter to Saturn, could use the acceleration of Saturn to
continue to Uranus, and in turn could be accelerated all the way out to
Neptune. Such an ambitious, multiplanet mission was named the Grand
Tour.
The first essential step in the Grand Tour was a flyby of Jupiter.
However, this planet is ten times farther away from Earth than Venus or
Mars. In addition, there were two potentially lethal hazards that had
not been faced before in interplanetary flights: the asteroid belt and
the Jovian magnetosphere.
The first danger was presented by the many thousands of asteroids that
occupy a belt between the orbits of Mars and Jupiter. The largest
asteroid, Ceres, was discovered in 1801 and was initially thought to be
the “missing planet” sometimes hypothesized as lying between Jupiter and
Mars. However, Ceres is only 1000 kilometers in diameter, too small to
deserve the title of planet. Hundreds more of these minor planets were
discovered during the nineteenth century, and by the 1960s more than
3000 had well-determined orbits. Most were only a few tens of kilometers
in diameter, and astronomers estimated that 50 000 existed that were 1
kilometer or more in diameter. Any spacecraft to Jupiter would have to
cross this congested region of space.
Even 50 000 minor bodies spread through the volume of space occupied by
the asteroid belt would present little direct danger, although a chance
collision with an uncatalogued object was always possible. Much more
serious was the possibility that these larger objects were accompanied
by large amounts of debris, from the size of boulders down to
microscopic dust, that were undetectable from Earth. Collisions with
pebble-sized stones could easily destroy a spacecraft. The only way to
evaluate this danger was to go there and find out how much small debris
was present.
A second danger was posed by Jupiter itself. In order to use the gravity
boost of Jupiter to speed on to another planet, a spacecraft would have
to fly rather close to the giant. But this would mean passing right
through the regions of energetic charged particles surrounding the
planet. Some estimates of the number and energy of these particles
indicated that the delicate electronic brains of a spacecraft would be
damaged before it could penetrate this region. Again, only by going
there could the danger be evaluated properly.
The Pioneer Jupiter Mission
In 1969 the U.S. Congress approved the Pioneer Jupiter Mission to
provide a reconnaissance of interplanetary space between Earth and
Jupiter and a first close look at the giant planet itself. The Project
was assigned by NASA to the Ames Research Center in Mountain View,
California. The primary objectives were defined by NASA:
Explore the interplanetary medium beyond the orbit of Mars.
Investigate the nature of the asteroid belt, assessing possible
hazards to missions to the outer planets.
Explore the environment of Jupiter, including its inner magnetosphere.
The Pioneer spacecraft was designed for economy and reliability, based
on previous experience at Ames with Pioneers 6 through 9, all of which
had proven themselves by years of successful measurement of the
interplanetary medium near the Earth. Unlike the Mariner class of
spacecraft being used to investigate Venus and Mars, the Pioneer craft
rotated continuously around an axis pointed toward the Earth. This
spinning design was extremely stable, like the wheels of a fast-moving
bicycle, and required less elaborate guidance than a nonspinning craft.
In addition, the spin provided an ideal base for measurements of
energetic particles and magnetic fields in space, since the motion of
the spacecraft itself swept the viewing direction around the sky and
allowed data to be acquired rapidly from many different directions. The
only major disadvantage of a spinning spacecraft is that it does not
allow a stabilized platform on which to mount cameras or other
instruments that require exact pointing. Thus the spacecraft design was
optimized for measurements of particles and fields in interplanetary
space and in the Jovian magnetosphere, but had limited capability for
observations of the planet and its satellites.
As finally assembled, the Pioneer Jupiter spacecraft had a mass of 258
kilograms. One hundred forty watts of electrical power at Jupiter were
supplied by four radioisotope thermoelectric generators (RTGs), which
turned heat from the radioactive decay of plutonium into electricity.
The launch vehicle for the flight to Jupiter was an Atlas-Centaur
rocket, equipped with an additional solid-propellant third stage. This
powerful rocket could accelerate the spacecraft to a speed of 51 500
kilometers per hour, sufficient to escape the Earth and make the
billion-kilometer trip to Jupiter in just over two years. The specific
scientific investigations to be carried out on Pioneer were selected
competitively in 1969 from proposals submitted by scientists from U.S.
universities, industry, and NASA laboratories, and also from abroad.
Eleven separate instruments would be flown, in addition to two
experiments that would make use of the spacecraft itself.
Three complete Pioneer spacecraft, with their payloads of 25 kilograms
of scientific instruments, were built: one as a test vehicle and two for
launch to Jupiter. One of these—the test vehicle—is now on display at
the National Air and Space Museum in Washington. The first opportunity
to launch—the opening of the “launch window”—was on February 27, 1972.
However, it was not until shortly after dark on March 2 that all systems
were ready, and Pioneer 10 began its historic trip to Jupiter.
Pioneer 10 was the first human artifact launched with sufficient energy
to escape the solar system entirely. Fittingly, the craft carried a
message designed for any possible alien astronauts who might, in the
distant future, find the derelict Pioneer in the vastness of
interstellar space. A small plaque fastened to the spacecraft told the
time and planet from which it had been launched, and carried a symbolic
greeting from humanity to the cosmos.
[Illustration: Two identical Pioneer spacecraft were designed and
fabricated by TRW Systems Group at their Redondo Beach, California
facility. Each weighed only about 260 kilograms, yet carried eleven
highly sophisticated instruments capable of operating unattended for
many years in space. Data systems on board controlled the
instrumentation, received and processed commands, and transmitted
information across the vast distance to Earth.]
The second Pioneer was to wait more than a year before launch. By
following far behind Pioneer 10, its trajectory—in particular how deeply
it penetrated the radiation belts during Jupiter flyby—could be
modified, depending on the fate of the first spacecraft. At dusk on
April 5, 1973, Pioneer 11 blasted from the launch pad at Cape Canaveral
and followed its predecessor on the long, lonely journey into the outer
solar system.
Flight to Jupiter
Within a few hours of launch, each Pioneer spacecraft shed the shroud
that had protected it and unfurled booms supporting the science
instruments and the RTG power generators. After each craft had been
carefully tracked and precise orbits calculated, small onboard rockets
were commanded to fire to correct its trajectory for exactly the desired
flyby at Jupiter. Pioneer 10 was targeted to fly by the planet at a
minimum distance of 3 Jupiter radii (R_J) from the center, or 2 R_J
(about 140 000 kilometers) above the clouds. This close passage, inside
the orbit of Io, allowed the craft to pass behind both Io and Jupiter as
seen from Earth, so that its radio beam could probe both the planet and
its innermost large satellite. Pioneer 11 was intended to fly even
closer to Jupiter, but the exact targeting options were held open until
after the Pioneer 10 encounter.
On Pioneer 10, all instruments appeared to be working well as the craft
passed the orbit of Mars in June 1972, just 97 days after launch. At
this point, as it headed into unexplored space, it truly became a
pioneer. In mid-July it began to enter the asteroid belt, and scientists
and engineers anxiously watched for signs of increasing particulate
matter.
Pioneer 10 carried two instruments designed to measure small particles
in space. One, with an effective area of about 0.6 square meters,
measured the direct impact of dust grains as small as one-billionth of a
gram. The other looked for larger, more distant grains by measuring
sunlight reflected from them. To the surprise of many, there was little
increase in the rate of dust impacts recorded as the craft penetrated
more and more deeply into the belt. At about 400 million kilometers from
the Sun, near the middle of the belt, there appeared to be an increase
in the number of larger particles detected optically, but not to a level
that posed any hazard. In February 1973 the spacecraft emerged unscathed
from the asteroid belt, having demonstrated that the much-feared
concentration of small debris in the belt did not exist. The pathway was
open to the outer solar system!
On November 26, 1973, the long-awaited encounter with Jupiter began. On
that date, at a distance of 6.4 million kilometers from the planet,
instruments on board Pioneer 10 detected a sudden change in the
interplanetary medium as the spacecraft crossed the point—the bow
shock—at which the magnetic presence of Jupiter first becomes evident.
At the bow shock, the energetic particles of the solar wind are suddenly
slowed as they approach Jupiter. At noon the next day, Pioneer 10
entered the Jovian magnetosphere at a distance of 96 R_J from the
planet.
As the spacecraft hurtled inward toward regions of increasing magnetic
field strength and charged plasma particles, the instruments designed to
look at Jupiter began to play their role. A simple line-scan camera that
could build up an image from many individual brightness scans (like a
newspaper picture transmitted by wire) obtained its first pictures of
the planet, and ultraviolet and infrared photometers prepared to observe
it also. By December 2, when the spacecraft had crossed the orbit of
Callisto, the outermost of the large Galilean satellites, the line-scan
images were nearly equal in quality to the best telescopic photos taken
previously, and as each hour passed they improved in resolution. Near
closest approach, Pioneer 10 transmitted partial frames of Jupiter that
represented a threefold improvement over any Earth-based pictures ever
taken.
[Illustration: The asteroid belt is a region between Mars and
Jupiter that is populated by thousands of minor planets, most only a
few kilometers in diameter. Before 1970, some theorists suggested
that large quantities of abrasive dust might damage spacecraft
passing through the asteroid belt. Pioneer 10 proved that this
danger was not present, thus opening the way to the outer solar
system.]
Earth at encounter
Earth at launch
Jupiter at encounter
Jupiter at launch
Asteroid belt
Tension increased as the spacecraft plunged deeper into the radiation
belts of Jupiter. Would it survive the blast of x-rays and gamma-rays
induced in every part of the craft by the electrons and ions trapped by
the magnetic field of Jupiter? Several of the instruments measuring the
charged particles climbed to full scale and saturated. Others neared
their limits but, as anxious scientists watched the data being sent
back, the levels flattened off. Meanwhile, the spacecraft itself began
to feel the effects of the radiation, and occasional spurious commands
were generated. Several planned high-resolution images of Jupiter and
its satellites were lost because of these false signals. But again the
system stabilized, and no more problems occurred as, just past noon on
December 3, 1973, Pioneer 10 reached its closest point to Jupiter, 130
000 kilometers above the Jovian cloud tops. Pioneer had passed its most
demanding test with flying colors, and at a news conference at Ames,
NASA Planetary Program Director Robert Kraemer pronounced the mission
“100 percent successful.” He added, “We sent Pioneer off to tweak a
dragon’s tail, and it did that and more. It gave it a really good yank,
and it managed to survive.” The Project Science Chief pronounced it “the
most exciting day of my life,” and most of the hundreds of scientists
and engineers who participated in the encounter probably agreed with
him.
Pioneer 11 continued to follow steadily, emerging from the asteroid belt
in March 1974. Based on the performance and findings of Pioneer 10, it
was decided to send Pioneer 11 still closer to Jupiter, but on a more
inclined trajectory. On April 19 thrusters on the spacecraft fired to
move the Pioneer 11 aimpoint just 34 000 kilometers above the clouds of
Jupiter.
In using the Pioneer 10 data to assess the hazard to Pioneer 11,
scientists had to consider three aspects of the charged particle
environment. First was the energy distribution of the particles: The
most energetic presented the most danger. Second was the flux, the rate
at which particles struck the craft. Third was the total radiation dose.
One can make an analogy with a boxing match. The energy distribution
tells you how hard the blows of your opponent are. The flux is a measure
of how many times a minute he hits you, and the total dose measures how
many blows land. The spacecraft reacts just like a boxer; the crucial
question is how much total dose it absorbs. Enough radiation blows, and
the system is knocked out. The trajectory chosen for Pioneer 11 resulted
in higher flux, since the craft probed more deeply into Jupiter’s inner
magnetosphere than had Pioneer 10. But by moving at a high angle across
the equatorial regions where the flux is highest, the total dose could
be kept below that experienced by Pioneer 10.
Pioneer 11 entered the Jovian magnetosphere on November 26, 1974, just a
year after its predecessor. Closest approach took place on December 2.
As with Pioneer 10, the radiation dose taxed the spacecraft to its
limit. Again spurious commands were issued, this time affecting the
infrared radiometer more than the imaging system. The craft flew at high
latitude over the north polar region of Jupiter, an area never seen from
Earth, and returned several excellent high-resolution pictures. Once
more the little Pioneers had succeeded against the odds in opening the
way to the giant planets.
[Illustration: Pioneer 10 and 11 encounters with Jupiter are shown
as viewed from the celestial North Pole. Pioneer 10 swung around the
giant planet in the counterclockwise direction, while Pioneer 11
followed a clockwise approach. In this view, Jupiter rotates
counterclockwise.]
Pioneer 11
Pioneer 10
Callisto
Ganymede
Europa
Io
Amalthea
Following their encounters with Jupiter, both Pioneer spacecraft
returned to their normal routine of measuring the interplanetary medium.
Pioneer 10 had gained speed from the gravity field of Jupiter and became
the first craft to achieve the velocity needed to escape from the solar
system. Pioneer 11, however, had used the pull of Jupiter to bend its
trajectory inward, aiming it across the solar system toward Saturn.
Following the successes at Jupiter, NASA announced that Pioneer 11 would
be targeted for a close flyby of Saturn five years later, which was
successfully carried out in September 1979. In early 1980, far beyond
their design lifetimes, both spacecraft were still performing
beautifully.
[Illustration: In this view of Jupiter, the Great Red Spot is
prominent and the shadow of Io traverses the planetary disk. The
gross morphology of the belts and zones, with structures showing
turbulence and convective cells in the middle latitudes, is clearly
seen. The small white spots surrounded by dark rings, seen mainly in
the southern hemisphere, indicate regions of intense vertical
convective activity, somewhat similar to cumulonimbus or
thunderclouds.]
Jupiter Results
The scientific results of the Pioneer flybys of Jupiter were many and
varied. As is always the case, some old questions were answered and new
problems were raised by the spacecraft data. Highlights of these results
are summarized below.
Photographs of Jupiter.
The line-scan imaging systems of Pioneer returned some remarkable
pictures of the planet during the two encounters, showing individual
features as small as 500 kilometers across. In addition, the Pioneers
were able to look at Jupiter from angles never observable from Earth.
One of the discoveries made from these pictures was the great variety of
cloud structures near the boundaries between the light zones and dark
belts. Many individual cloud patterns suggested rising and falling air.
The convoluted swirls evident in these regions appeared to be the result
of dynamic motions; unfortunately, with only the few “snapshots”
obtained during the hours of the flyby, the actual motions of these
clouds could not be followed.
Thermal Emission.
In the year the Pioneer Project was begun, astronomers on Earth had
measured that Jupiter emitted more heat than it absorbed from the Sun.
From the Earth these measurements could be made only of the sunlit part
of the planet; neither the night side nor the poles could be seen. One
of the main objectives of the Pioneer flybys was to determine the heat
budget accurately from temperature measurements at many points on both
the sunlit and the night sides.
The Pioneer data confirmed the presence of a heat source in Jupiter and
supplied a quantitative estimate of its magnitude. The global effective
temperature was found to be -148° C, to a precision of ±3 degrees. This
temperature implies that Jupiter radiates 1.9 times as much heat as it
receives from the Sun. The corresponding internal heat source is 10¹⁷
watts. Surprisingly, the poles were as warm as the equator; apparently,
the atmosphere is very efficient at transferring solar heat absorbed
near the equator up to high latitudes, or perhaps the internal component
of the heat comes preferentially from the polar regions.
Helium in the Atmosphere.
The Pioneer infrared experiment made the first measurement of the amount
of helium on Jupiter. The ratio of the number of helium atoms to the
number of hydrogen atoms was found to be He/H₂ = 0.14 ± 0.08. This is
consistent with the known solar ratio of He/H₂ = 0.11. Measurements of
helium in the upper atmosphere were also made by the ultraviolet
experiment.
[Illustration: One of the best Pioneer images of Jupiter was
obtained at a range of 545 000 kilometers by Pioneer 11. Structure
within the Great Red Spot and the surrounding belts and zones can be
seen. There was much less turbulent cloud activity round the spot at
the time of the Pioneer flybys than was seen five years later by the
Voyager cameras.]
[Illustration: Pioneer 10 confirmed theoretical models of Jupiter
that suggest the planet is nearly all liquid, with a very small core
and an extremely deep atmosphere. The liquid interior seethes with
internal heat energy, which is transferred from deep within the
planet to its outer regions. The temperature at the center may be 30
000 K. Since the temperature at the cloud tops is around -123° C,
there is a large range of temperatures within the planet.]
Distance (km)
Visible clouds
Hydrogen gas
Cloud tops
Ammonia crystals
Ammonium hydrosulfide crystals
Ice crystals
Water droplets
-123° C
Transparent atmosphere
Hydrogen/Helium gas
70 000
Transition zone
1980° C
60 000
Liquid hydrogen
50 000
11 000° C
Transition zone
3 million atmospheres pressure
40 000
Liquid metallic hydrogen
30 000
20 000
Possible “sea” of helium
10 000
30 000 K
Possible solid core
0
PIONEER SCIENCE INVESTIGATIONS
Project Scientist: J. H. Wolfe, NASA Ames
Investigation Principal Investigator Primary Objectives
Magnetic fields E. J. Smith, JPL Measurement of the magnetic
field of Jupiter and
determination of the
structure of the
magnetosphere.
Magnetic fields N. F. Ness, NASA Measurement of the magnetic
(Pioneer 11 only) Goddard field of Jupiter and
determination of the
structure of the
magnetosphere.
Plasma analyzer J. H. Wolfe, NASA Ames Measurement of low-energy
electrons and ions,
determination of the
structure of the
magnetosphere.
Charged particle J. A. Simpson, U. Determination of the
composition Chicago number, energy, and
composition of energetic
charged particles in the
Jovian magnetosphere.
Cosmic ray energy F. B. McDonald, NASA Measurement of number and
spectra Goddard energy of very high
energy charged particles
in space.
Jovian charged J. A. Van Allen, U. Measurement of number and
particles Iowa energy distribution of
energetic charged
particles and
determination of
magnetospheric structure.
Jovian trapped R. Walker Fillius, UC Measurement of number and
radiation San Diego energy distribution of
energetic charged
particles and
determination of
magnetospheric structure.
Asteroid-meteoroid R. K. Soberman, Observation of solid
astronomy General Electric particles (dust and
larger) in the vicinity
of the spacecraft.
Meteoroid detection W. H. Kinard, NASA Detection of very small
Langley solid particles that
strike the spacecraft.
Celestial mechanics J. D. Anderson, JPL Measurement of the masses
of Jupiter and the
Galilean satellites with
high precision.
Ultraviolet photometry D. L. Judge, U. Measurement of ultraviolet
Southern California emissions of the Jovian
atmosphere and from
circumsatellite gas
clouds.
Imaging T. Gehrels, U. Arizona Reconnaissance imaging of
photopolarimetry Jupiter and its
satellites; study of
atmospheric dynamics.
Jovian infrared G. Münch, Caltech Measurement of Jovian
thermal structure temperature and heat
budget; determination of
helium to hydrogen ratio.
S-Band occultation A. J. Kliore, JPL Probes of structure of
Jovian atmosphere and
ionosphere.
Atmospheric Structure.
Several Pioneer investigations yielded information on the variation of
atmospheric temperature and pressure in the regions above the ammonia
clouds. Near the equator, at a level where the atmospheric pressure is
the same as that on the surface of the Earth (1 bar), the temperature is
-108° C. About 150 kilometers higher, where the pressure drops to 0.1
bar, is the minimum atmospheric temperature of about -165° C. Above this
point the temperature rises again, reaching about -123° C near a
pressure level of 0.03 bar. Presumably this temperature rise is due to
absorption of sunlight by a thin haze of dust particles in the upper
atmosphere of Jupiter.
Internal Structure.
The measurements of the amount of helium, of the gravitational field,
and of the size of the internal heat source on Jupiter greatly clarified
scientists’ understanding of the deep interior of the planet.
Calculations showed that the core of Jupiter must be so hot that
hydrogen cannot become solid, but must remain a fluid throughout the
interior. Even at great depths, therefore, Jupiter does not have a solid
surface. The theory that the Great Red Spot was the result of
interactions with a surface feature below the clouds thus became
untenable. Whatever its exact nature, the Red Spot must be a strictly
atmospheric phenomenon.
Magnetic Field.
Pioneer data showed that the magnetic field of Jupiter has a dipolar
nature, like that of the Earth, but 2000 times stronger. The calculated
surface fields measured about 4 gauss, compared to a field of about 0.5
gauss on the Earth. The axis of the magnetic field was tilted 11 degrees
with respect to the rotation axis, and it was offset by about 10 000
kilometers (0.1 R_J) from the center of the planet.
[Illustration: Pioneers 10 and 11 did not obtain very detailed
pictures of the satellites of Jupiter. The best view was of
Ganymede, which showed a surface of contrasting light and dark spots
of unknown nature.]
[Illustration: A less detailed image of Europa clearly reveals the
illuminated crescent but supplies little information about the
surface of this ice-covered satellite.]
Satellite Atmospheres.
Two experiments yielded exciting new information on possible atmospheres
of the Galilean satellites. First was an occultation, in which the
Pioneer 10 spacecraft was targeted to pass behind Io as seen from Earth.
At the moments just before the spacecraft disappeared, and just after
reemergence from behind the satellite, the radio signal was influenced
by a thin layer of ionized gas, in which the electrons had been stripped
from the atoms by absorbed sunlight or by other processes. The
ionosphere thus discovered had a peak density of about 60 000 electrons
per cubic centimeter. In addition, a very extended far-ultraviolet glow,
probably due to atomic hydrogen, was found near the orbit of Io by the
ultraviolet photometer.
Masses of Jupiter and Its Satellites.
Precise radio tracking of the Pioneers as they coasted past Jupiter and
its satellites revealed that Jupiter is about one percent heavier than
had been anticipated, and several satellites were found to have masses
that differ by more than ten percent from values determined previously.
These improvements in knowledge of the masses were required to achieve
the close satellite flybys being planned for later missions.
[Illustration: The Pioneer spacecraft carried this plaque on the
journey beyond the solar system, bearing data that tell where and
when the human species lived and that convey details of our
biological form. When Pioneer 10 flew by Jupiter it acquired
sufficient kinetic energy to carry it completely out of the solar
system. Some time between one and ten billion years from now, the
probe may pass through the planetary system of a remote stellar
neighbor, one of whose planets may have evolved intelligent life. If
the spacecraft is detected and then inspected, Pioneer’s message
will reach across the eons to communicate its greeting.]
The Inner Magnetosphere.
A great deal of the scientific emphasis of the Pioneer missions was
directed at characterizing the particles and fields in the inner
magnetosphere, the region in which charged particles are trapped in
stable orbits. The Pioneers found that it extended to about 25 R_J, well
beyond the orbit of Callisto. Within this region, instruments on the
spacecraft recorded the numbers and energy of electrons, protons, and
ions. The electrons reached a maximum concentration near 3 R_J, and
their numbers remained almost constant from there in toward the planet.
The maximum concentration of protons observed by Pioneer 10 was at 3.4
R_J, a little inside the orbit of Io. Pioneer 11 penetrated deeper and
found another maximum, about twenty times higher, at 1.9 R_J; at this
distance, 10 million energetic protons hit each square centimeter of the
spacecraft every second. It was believed that the gap between these two
peaks was due to tiny Amalthea, the innermost satellite, which orbits at
2.5 R_J. Apparently this satellite sweeps up the particles as it circles
Jupiter. Another large dip in the proton distribution was attributed to
sweeping by Io, with smaller effects seen near the orbits of Europa and
Ganymede. There was an additional small effect at 1.8 R_J, later found
by Voyager to be due to Jupiter’s ring and its fourteenth satellite.
The Outer Magnetosphere.
From about 25 R_J outward to its boundary near 100 R_J, the Jovian
magnetosphere is a complex and dynamic place. Beyond about 60 R_J, both
Pioneers found the boundary to be highly unstable, apparently blown in
and out by variations in the pressure of the solar wind, which consists
of charged particles flowing outward from the Sun. In this region
concentrations of Jovian particles are sometimes seen that rival the
inner magnetosphere in intensity. Between 60 R_J and 25 R_J, a region
sometimes called the middle magnetosphere, the particle motions are more
ordered, and for the most part electrons and protons are carried along
with the planet’s rotation by its magnetic field. Near the equatorial
plane, the flow of these particles produces an electric current circling
the planet, and this current in turn generates its own magnetic field,
which approaches in strength that of Jupiter itself. Occasionally the
outer magnetosphere collapses down to about 60 R_J, and energetic
particles are squirted from the middle region into space; these bursts
of Jovian particles can sometimes be detected as far away as Earth.
At the same time that Pioneer scientists were analyzing their results
and developing new concepts of the Jupiter system, a new team of
investigators had been selected for the next mission to Jupiter. From
about 1975 on, attention shifted from Pioneer to its successor—Voyager.
[Illustration: The Voyager spacecraft are among the most
sophisticated, automatic, and independent robots ever sent to
explore the planets. Each craft has a mass of one ton and is
dominated by the 3.7-meter-diameter white antenna used for radio
communication with Earth. Here Voyager undergoes final tests in a
space simulator chamber. [373-7162AC]]