Multimedia Mania/abacus-multimedia-mania.iso/dp/0115/01156.txt

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$Unique_ID{bob01156}
$Pretitle{}
$Title{Pioneer
Chapter 2: Part 1 - The Pioneer Jupiter/Saturn Mission}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{spacecraft
pioneer
jupiter
mission
space
launch
solar
planets
first
saturn
see
pictures
see
figures
}
$Date{1980}
$Log{See Atlas-Centaur*0115601.scf
See Data Systems*0115602.scf
}
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 2: Part 1 - The Pioneer Jupiter/Saturn Mission

     The Pioneer program began in 1957 when the Advanced Research Projects
Agency authorized the launching of small unmanned spacecraft toward the Moon.
In March 1958, Secretary of Defense McElroy announced that the United States
Air Force would launch three such probes in an attempt to place a scientific
payload in the vicinity of the Moon.  The first successful Pioneer, launched
0342 EST October 11, 1958, traveled 117,100 km (72,765 miles) from Earth and
returned scientific data for 48 hr.  A magnetometer on board yielded evidence
of complex geomagnetic effects thousands of kilometers from Earth.  Another
Pioneer, launched December 6, 1958, confirmed the existence of two Van Allen
belts of trapped energetic particles.  The first spin-scan image of any
planet, in this case Earth, was obtained by one of these early Pioneers.

     In May 1960, about a year after the National Aeronautics and Space
Administration was formed, an informal study of solar probes was started at
NASA's Ames Research Center.  This study, led by Charles F. Hall, was to
demonstrate the Center's potential for managing a space project.  From this
study, a new concept was presented a small, siple, long-lived spacecraft to
explore the interplanetary medium inside Earth's orbit.  Smith J. DeFrance,
then Center Director, was enthusiastic about this new space work for the
Center.  In September 1960, he organized a formal team; in 1961 and 1962,
Charles Hall and others, including scientists, tried to stimulate interest in
the concept at NASA Headquarters.

     As a result of several presentations made to NASA Headquarters, Edgar M.
Cortright, then Deputy Director of the Office of Space Science, invited Hall
and his team to become involved in an interplanetary Pioneer spacecraft.
Space Technology Laboratories, which had been involved in the first Pioneer
spacecraft for the Air Force, was chosen to determine the feasibility of the
spin-stabilized spacecraft concept they had evolved for the Interplanetary
Pioneer mission; a project approval document was issued in June 1962.  Space
Technology Laboratories was selected over other competitors to build the
spacecraft.  The Pioneer project moved rapidly ahead for a first launch.

     Pioneers 6 through 9, launched by Thor-Deltas in 1965 through 1968,
explored interplanetary spac in a band extending several million kilometers
inside and outside Earth's orbit.  Measurements made by these spacecraft
greatly increased our knowledge of the interplanetary medium and the effects
of solar activity upon the Earth.  New information was gathered about the
solar wind, solar cosmic rays, the structure of the Sun's plasma and magnetic
fields, the physics of particles in space, and the nature of solar flares.
These spacecraft continued to operate in space for many years.  Several of the
scientists involved with the Interplanetary Pioneers had been associated with
the first Pioneers and continued with later Pioneer spacecraft missions to the
outer planets, thereby devoting a major part of their professional lives to
this program.

     During the development of the Interplanetary Pioneers, an important
series of meetings was held as part of the activities of NASA's Lunar and
Planetary Missions Board.  In 1967, an Outer Planets Panel associated with
this Board and chaired by James Van Allen of the University of Iowa
recommended that plans should be made for low-cost exploratory missions to the
outer planets because such missions would make significant contributions to
space science.

     In June 1968, the Space Science Board of the National Academy of Sciences
stated that Jupiter was probably the most interesting planet from a physical
point of view and that it was at that time technically feasible to send space
probes to that planet.  The Board recommended that "Jupiter missions be given
high priority, and that two exploratory probes in the Pioneer class be
launched in 1972 or 1973."

     In June 1969, in a further report, the Lunar and Planetary Missions Board
emphasized the importance of obtaining more information about the outer giant
planets and recommended that a long-term plan be developed to explore the
outer Solar System.  This report endorsed the earlier Space Science Board
studies.

     In previous years, a number of proposals and scientific papers had been
presented about exploration of the outer planets, including missions to
several planets by one spacecraft using gravity assist from some of the
planets.  Several NASA centers and private companies had completed studies
showing that the gravity field of Jupiter combined with the orbital motion of
the planet could accelerate spacecraft to speeds that would enable them to
complete missions to more distant planets in reasonable times and with useful
scientific payloads.

     In March 1967, for example, in a paper presented at the Fifth Goddard
Memorial Symposium in Washington, D.C., several types of outer Solar System
probes were discussed which could explore interplanetary space beyond the
orbit of Mars, the solar wind and its interaction with deep space, and the
Jovian environment.  A Jupiter probe of this type would be accelerated
sufficiently by the large planet to allow the spacecraft to escape completely
from our Solar System into interstellar space.  Such a mission would provide
an opportunity to investigate how far the influence of the solar wind extends
into the outer limits of our Solar System.

     About this time, at the Goddard Space Center, a Galactic Jupiter Probe
was studied as a means to explore solar, interplanetary, and galactic
phenomena to as great a distance from our Sun as possible.  Every 13 months,
for a few weeks, the relative positions of Earth and Jupiter permit a
spacecraft to be launched into a Jupiter-bound trajectory with a minimum
launch energy.  Launch energies required for the remainder of the 13 months
are prohibitive.  The study recommended that two spacecraft be launched to
Jupiter in 1972 and 1973 by Atlas-Centaur launch vehicles.  As each spacecraft
passed through Jupiter's gravity field, it would obtain enough additional
energy to carry it high above the ecliptic plane or to great distances from
our Sun, hopefully into the interstellar medium.  The spacecraft were to be
spin-stabilized and would have an Earth-oriented antenna.  Each spacecraft
would receive its power from radioisotope thermoelectric generators.

     A mission to Jupiter was officially approved by NASA Headquarters in
February 1969, and the program was assigned to the Planetary Programs Office,
Office of Space Science and Applications, NASA Headquarters, Washington, D.C.
The Pioneer Project Office at Ames Research Center, Moffett Field, California,
was selected to manage the project, and TRW Systems Group, Redondo Beach,
California, was awarded a contract to design and fabricate two identical
Pioneer spacecraft for the mission.  In a scientific paper delivered to the
American Astronautical Society's June 1969 meeting in Denver, Colorado, Howard
F. Mathews and Charles F. Hall described the first mission to the outer
planets as "an exciting era of exploration of the outer planets."  The initial
mission plan was to reach Jupiter.  The mission was later extended when it
became apparent that, without any change to its design, one of the spacecraft
could reach Saturn.

     From a consideration of the time needed to build the spacecraft, to
select its scientific experiments, and to build instruments to perform these
experiments, the first feasible launch opportunity for the mission appeared to
be during late February through early March of 1972.  The first spacecraft,
Pioneer F, was scheduled to meet this launch opportunity.  The second
spacecraft, Pioneer G, was to be launched approximately 13 months later,
during the 1973 opportunity.  Before launch, all NASA spacecraft are given
letter designations that are later changed to number designations after a
successful launch - Pioneer F became Pioneer 10 and Pioneer G became Pioneer
11.  The two spacecraft are hereafter referred to by their number
designations.

Planning

     Planning for the Pioneer mission to Jupiter and Saturn required a close
involvement between NASA and industry.  The Pioneer Program was managed at
NASA Headquarters, first by Glenn A. Reiff and then by F. D. Kochendorfer.

     At Ames Research Center, Charles F. Hall became Manager of the Pioneer
Project.  After the Pioneer 11 encounter with Saturn, Richard O. Fimmel became
Project Manager.  The experiments carried by the spacecraft were the
responsibility of Joseph E. Lepetich, and the spacecraft system was the
responsibility of Ralph W. Holtzclaw.  The original Flight Operations Manager
was Robert R. Nunamaker, then later Norman J. Martin.  For the journey of
Pioneer 11 from Jupiter to Saturn and for the encounter with Saturn, Robert P.
Hogan was Flight Director.  For the mission beyond Saturn, Robert W. Jackson
was Flight Director and Dr. John H. Wolfe was Project Scientist; after Saturn
encounter, Palmer Dyal became Project Scientist.  Other members of the team
were: Robert U. Hofstetter, Launch Vehicle and Trajectory Analysis
Coordinator; Richard O. Fimmel, Science Chief; Gilbert A. Schroeder,
Spacecraft Chief, and John W. Dyer, Chief, Mission Analysis.

     The Jet Propulsion Laboratory of the California Institute of Technology,
Pasadena, California, provided tracking and data system support with
originally Alfred J. Siegmeth as the first Pioneer Tracking and Data Systems
Manager and later Richard B. Miller.  Goddard Space Flight Center, Greenbelt,
Maryland, provided worldwide communications to the various stations of the
Deep Space Network.

     Lewis Research Center, Cleveland, Ohio, was responsible for the launch
vehicle system, under the management of D. J. Shramo.  John F. Kennedy Space
Center, Florida, was responsible for launch operations, under J. W. Johnson.

     At TRW Systems Group, Bernard J. O'Brien was Manager of the Pioneer
Project, and William T. Dixon, the Systems Engineer.  At the Atomic Energy
Commission (now part of the Department of Energy, where Harold Jaffe manages
the Isotope Flight Systems Office), B. Rock was Project Engineer for the
SNAP-19 radioisotope thermoelectric generators built by Teledyne Isotopes.
Bendix Field Engineering Corporation, under the management of Walter L.
Natzic, then Thomas S. Goves, and later Patrick J. Barclay, supported the
mission operations system.  The responsibilities of the various individuals
continue into the mission beyond Saturn until communications with the
spacecraft stop as they move toward the outer fringes of the Solar System.

Mission Objectives

     Initially, the objectives of the Pioneer mission to the giant planets, as
defined by NASA, were:

     To explore the interplanetary medium beyond the orbit of Mars.

     To investigate the nature of the asteroid belt from a scientific
standpoint and to assess the belt's possible hazards to missions to the outer
planets.

     To explore the environment of Jupiter.

     When the potential of the spacecraft to explore beyond Jupiter became
clear, the objectives were extended:

     If the firt spacecraft to fly by Jupiter attained its scientific
objectives, the second would be targeted to fly by Jupiter in such a way
that the spacecraft would enter a trajectory that would enable it to
reach Saturn.

     The second spacecraft would then explore the Saturnian environment.

     Ames Research Center was chosen for the mission because of its experience
with earlier spin-stabilized spacecraft that are still exploring our inner
Solar System.

     The new Pioneer was required to utilize proven spacecraft modules of
Pioneers 6 through 9 - it had to be a small, lightweight, magnetically clean,
interplanetary spacecraft.  To propel the 250-kg (550-lb) spacecraft to the
tremendously high velocity needed to enter a transfer trajectory to Jupiter,
the Atlas-Centaur launch vehicle was equipped with an additional
solid-propellant stage.

     A series of planning meetings was held in the late 1960's.  By early
1970, all scientific experiments had been selected:

     Magnetic fields and plasma in interplanetary space and planetary magnetic
fields and trapped radiation in the magnetospheres of the planets were to be
measured.

     Polarimetric measurements and images of Jupiter, possibly Saturn, and of
several satellites were to be taken.

     Compositions of charged particle beams in space were to be determined.

     Cosmic rays were to be recorded.

     Planets were to be observed at ultraviolet and infrared wavelengths.

     Asteroids and meteoroids were to be detected and the distribution of
meteoric dust observed.

     The intensity and distribution of the zodiacal light were to be observed.

     The radio communication signal was to be used to probe the planetary
atmospheres during occultation.

     The radio communications signal would be used to learn about the
planetary masses from analysis of Doppler residuals.

     Principal investigators were selected for all experiments, and contracts
were awarded to build the instruments and conduct these experiments.
(Experiments are more fully described in chapter 4.)

Mission Overview

     The two spacecraft for this mission were identical.  Pioneer 10, the
first, blazed the trail.  If the asteroid belt or the Jovian magnetosphere had
proved hazardous to Pioneer 10, Pioneer 11 would have been the backup
spacecraft.  Initially, Pioneer 11 was launched and targeted to follow the
path of Pioneer 10.  However, the capability existed and it was therefore
planned that Pioneer 11 be retargeted as necessary on its way to Jupiter,
based on the results from Pioneer 10's encounter with Jupiter.  Pioneer 11 was
retargeted to encounter Jupiter in a way that provided it with the capability
of reaching Saturn.

     The launch vehicle boosted each spacecraft in direct ascent, that is,
with no parking orbit, to begin the flight to Jupiter at about 51,500 km/hr
(32,000 mph).  A trip of just under 600 days was the shortest time to Jupiter
within the capabilities of the launch vehicle, and a trip of 748 days was the
longest.

[See Atlas-Centaur: Missions to the giant planets required that the spacecraft
reach the highest launch velocity yet achieved by any man-made object, over
51,500 km/hr (32,000 mph). An Atlas-Centaur launch vehicle was used, equipped
with a third upper stage.]

     Several in-flight maneuvers were to be made during the Pioneer 10 mission
to target the spacecraft so that it would arrive at Jupiter at a time and
position best suited to observe the planet and several of its large
satellites.  For Pioneer 11, the in-flight maneuvers were planned to preserve
the option of continuing the mission to Saturn.

     Pioneers 10 and 11 were designed to be compatible with the launch
vehicle, and their communications systems were designed to be compatible with
the Deep Space Network.  Each Pioneer spacecraft had to provide a thermally
controlled environment for its scientific instruments.  The spacecraft were
also designed to operate reliably in space for many years.  Each carried a
data system to sample the scientific instruments and to transmit scientific
and engineering information to Earth about the "health" of the spacecraft and
its instruments.  The spacecraft also had to be capable of being commanded
from Earth to perform their missions and to change the operating modes of
onboard equipment.

[See Data Systems: Each spacecraft, spin-stabilized in flight, carried various
scientific instruments and a large dish-shaped antenna that would allow
communications over great distances.]

     Each Pioneer's curved path to Jupiter was about 1000 million kilometers
(620 million miles) long, covering about 1600 azimuthally around the Sun as
the spacecraft traveled between the orbits of Earth and Jupiter.  During each
Pioneer's flight to Jupiter, Earth traveled almost twice around the Sun, while
Jupiter moved only about 1/6 of its solar orbit.

     There were options available in selecting the path to Jupiter.  Certain
arrival dates were unsuitable because the sensors on the spacecraft would have
been unable to perform the desired scientific experiments.  Other arrival
dates were unsuitable because they would have clashed with the arrival of
another spacecraft, Mariner 10, at Venus or Mercury and would have caused
conflict in the use of the large 64-m (210-ft) antennas of the Deep Space
Network.

     Pioneer 10 could have been launched from February 25 to March 20, 1972,
to arrive at Jupiter some time between mid-October 1973 and late July 1974.
The arrival of Pioneer had to be timed so that Jupiter and the spacecraft
would not appear too close to the Sun as observed from Earth.  About 300-325
days and 700-725 days after launch, the motions of Earth and the spacecraft
put them on opposite sides of the Sun.  Thus it was impractical for Pioneer 10
to arrive at Jupiter more than 700 days after launch.  During the earlier
passage of the spacecraft behind the Sun, just over 300 days after launch
while the spacecraft was en route to Jupiter, communications with the
spacecraft were interrupted, but not at a critical period of the mission.
Similar options applied to Pioneer 11 for its launching 1 year later.

     There were critical targeting options at Jupiter:  how close should the
spacecraft be allowed to approach the planet, how much should the trajectory
be inclined to Jupiter's equatorial plane, and at what point should the
closest approach be relative to Jupiter's equatorial plane?

     An early decision was made that the encounter trajectory of Pioneer 10
should be one to provide the maximum information about the radiation
environment of Jupiter to the smallest feasible radial distance, even if, by
the spacecraft following such a trajectory, its systems were damaged by
radiation and the mission ended at Jupiter.  Hence, images of Jupiter could
only be assured before closest approach.  An approach trajectory was selected
so as to view a well illuminated planet before encounter and a partially
illuminated crescent planet after the encounter.  At first it seemed desirable
that occultation of the spacecraft by Jupiter should be avoided, but an
occultation was selected because the information it could provide about the
Jovian atmosphere could not be obtained any other way.

     Because Jupiter has radiation belts trapped within its magnetic field,
scientists wanted to know how close a spacecraft could safely approach Jupiter
to take advantage of the gravity slingshot effect without damage to the
spacecraft's electronic and optical equipment.  Obtaining an answer to this
question was one of the primary objectives of the first mission to Jupiter.
In July 1971, scientists at a workshop held at the Jet Propulsion Laboratory
defined the Jovian environment in terms of the best available information.
With slight modifications, this environment was accepted as the design
environment for the Pioneer spacecraft and its scientific instruments.  No one
could be sure that this environment, although based on the very best
observations from Earth, was the actual environment of Jupiter - it was a task
of the Pioneer mission to determine the true Jovian environment.  The
environment of Saturn was considered less hazardous than Jupiter's, at least
from the standpoint of radiation damage.

     A tradeoff for the Pioneer mission was that, although a closer approach
to Jupiter would increase the intensity of radiation encountered, the
spacecraft would fly by Jupiter more quickly and would therefore be exposed to
radiation for less time.  These two factors, which determine the integrated or
total radiation dosage, were carefully considered before the final flyby path
was selected.

     Generally, the mission was designed so that Pioneer 10 would fly by
Jupiter at three times the radius of the planet (referred to as 3RJ), that is,
twice the Jovian radius above the cloudtops.  Although it was possible to
target Pioneer 10 at a closer approach, this trajectory was selected because,
from available information, the spacecraft might have been seriously damaged
by radiation had it been sent closer to the planet.  When the mission was
being planned, the ephemeris of Jupiter was uncertain - to about 2000 km (1250
miles) - but, navigationally, the spacecraft could have been sent to within
3/8 Jupiter radii above the cloud-tops.  Navigation to Jupiter is simplified
somewhat because the intense gravitational pull of the planet provides a
focusing effect.  Such gravity focusing would reduce an aiming error of 1600
km 32 (1000 miles) to an encounter error of 480 km (300 miles).

     After the approach was chosen the electronic equipment and science
sensors were designed to survive the level of radiation expected while Pioneer
10 passed through the radiation belts.

     The amount of propellant carried on Pioneer 10 permitted the time of
arrival at Jupiter to be changed by several days, thereby allowing mission
planners to direct the spacecraft to fly close enough to a Jovian satellite to
obtain a spin-scan image of it or to be occulted by a satellite.

Hazards of the Mission

     In 1800, Johann Elert Bode called a meeting of astronomers at the
Schroter Observatory in Lilienthal, Germany.  He asked them to search for a
planet believed to be orbiting between Mars and Jupiter.  On January 1, 1801,
Giuseppe Piazzi, director of the Observatory of Palermo, Italy, discovered a
small planetary object, 1022 km (635 miles) in diameter, which he named Ceres.
But soon after it was discovered, Ceres, moving along its orbit, was lost in
the glare of the Sun.

     The great mathematician, Friedrich Gauss, developed a theory for
determining the orbits of planetary bodies based on a minimum number of
observations.  He calculated the orbit of Ceres and showed where it would
emerge from the solar glare.  While observing Ceres again in 1802, Heinrich
Olbers discovered a second planetary body smaller than Ceres, measuring only
560 km (348 miles) across, which he named Pallas.  Other surprising
discoveries were made by Karl Ludwig Harding in 1804, Juno, 226 km (141 miles)
in diameter; and by Olbers in 1807, Vesta, 504 km (313 miles) in diameter -
the brightest of these minor planets.  These small planetary bodies, called
"asteroids" by William Herschel, were regarded as fragments of a trans-Martian
planet.

     At least 8 other asteroids larger than Juno are known today, but they
were not discovered until almost 50 years after the discovery of the first
four.  The year 1845 marked the beginning of discoveries of great numbers of
minor planets - today 40,000 to 100,000 such bodies are postulated.  Many have
been discovered photographically.  Most are found between the orbits of Mars
and Jupiter, while others stray closer to or farther from the Sun in more
elliptical orbits.  Several have approached Earth; one at least approaches the
orbit of Mercury and another, that of Saturn.

     The orbits of the larger asteroids have been cataloged, but many of the
asteroids move in unknown orbits.  Although the risk of a spacecraft colliding
with a charted asteroid was negligible, there was no way to estimate how many
particles the size of a grain of sand might be present in the asteroid belt to
collide with the spacecraft and seriously damage it.

     At the beginning of the Pioneer program, scientists did not know whether
the first Pioneer would survive passage through the asteroid belt on its way
to Jupiter.  But before other missions to the outer planets could be
considered, at least one spacecraft had to penetrate this region and survive
the passage.  Another problem had to be faced by the Pioneer mission planners:
how to supply electrical power to the spacecraft at such great distances from
the Sun.  Solar cells were considered during early planning of the mission
because radioisotope power generators had not then been tested over the long
lifetimes required for such a mission and the radiation from them would have a
mildly deleterious effect on certain scientific instruments.  However, since
sunlight at Jupiter carries only 1/27 the energy it does at Earth, very large
arrays of solar cells would be required.  Also, damage to solar cells by the
Jovian radiation belts could be serious.  Therefore, radioisotope
thermoelectric generators were judged to be a better engineering choice and
were adopted for the Pioneer spacecraft.

     Because of the tremendous speed required to carry the spacecraft from
Earth to Jupiter, the payload weight was severely restricted - complicated
onboard computing systems would be much too heavy.  The Pioneer spacecraft had
to be virtually "flown from the ground," despite the long delays in
communicating over the vast distances to Jupiter, Saturn, and beyond.

     The long time span of the mission and the weight limitations imposed on
the spacecraft also required that all spacecraft components be reliable to an
unprecedented degree.  Such a level of reliability was achieved by making the
spacecraft as simple as possible, leaving as much as possible of the
complexity on the ground.  Such vital items as transmitters and receivers were
duplicated, and only systems and components that had been flight-proven on
other spacecraft were used.  Electronic components were "burned in" before
they were assembled on the spacecraft so that components likely to fail were
eliminated before the flight.  The success of the mission depended heavily on
an advanced command, control, and communications system to link Earth-based
computers and human controllers to the spacecraft.