Multimedia Mania/abacus-multimedia-mania.iso/dp/0115/01159.txt

Text File | 1993-07-27 | 17KB | 378 lines

$Unique_ID{bob01159}
$Pretitle{}
$Title{Pioneer
Chapter 4: Part 1 - Pioneer Science at New Frontiers}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{saturn
jupiter
solar
particles
magnetic
wind
field
measured
spacecraft
fields
see
pictures
see
figures
}
$Date{1980}
$Log{See Plasma Analyzer*0115901.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 4: Part 1 - Pioneer Science at New Frontiers

     The scientific payloads of Pioneers 10 and 11 for the missions to Jupiter
and Saturn were virtually identical.  They were designed to gather new
knowledge about interplanetary space beyond Mars and about the Jovian and
Saturnian systems.  Several of the science instruments measured particles,
fields, and radiation, while an imaging photopolarimeter provided spin-can
imaging and analysis of scattered light.  Additionally, the radio signals from
the spacecraft were used to measure the gravitational fields of Jupiter and
Saturn and their major satellites and to investigate the atmospheres of the
two planets.  The Pioneers performed several experiments in interplanetary
space between Earth and the target planets and beyond:

     Mapped the magnetic field in interplanetary space.

     Determined how the solar wind changed with distance from the Sun and,
during the flight of Pioneer 11 to Saturn, how the solar wind changed high
above the plane of the ecliptic.

     Measured solar and galactic cosmic ray particles.

     Studied interactions among the interplanetary magnetic field, the solar
wind, and cosmic rays.

     Searched for a transition region of the heliosphere - the region where
the influence of the Sun on interplanetary space ends - in two opposite
directions.

     Measured the amount of neutral atomic hydrogen in interplanetary space
and near Jupiter and Saturn.

     Ascertained the distribution of dust particles in interplanetary space in
the outer Solar System.

     Determined the sizes, masses, fluxes, and velocities of small particles
in the asteroid belt, thus providing information on the probability of damage
by such particles to spacecraft passing through this region.

     Searched for gravitational radiation impinging on the Solar System by
means of its effects on the spacecraft's velocity as revealed on the microwave
radio link between Earth and the spacecraft.

     Searched for the gravitational effects of large undiscovered objects
orbiting the Sun.

     Within the systems of Jupiter and Saturn, the Pioneer spacecraft
performed a more involved series of experiments:

     Mapped the magnetic fields of the planets their intensity, direction, and
structure.

     Determined how many electrons and protons of various energies were
distributed along the trajectory of each spacecraft through the planetary
magnetospheres and, for Saturn, determined how the rings affect the
distribution of those particles; for both planets, determined how the
particles were affected by the satellites.

     Searched for auroras in the polar atmospheres of the two planets.

     Obtained information to help interpret the observed characteristics of
the two main types of radio waves from Jupiter - decimetric and decametric.

     Mapped how the two planets interact with the solar wind.

     Measured the temperature of the atmospheres of the two planets and of
those of some of their larger satellites.

     Determined the structure of the upper atmospheres of Jupiter and Saturn,
where molecules are ionized by solar radiation to produce an ionosphere.

     Mapped the thermal structure of the two planets by measuring their
infrared radiation, and deduced how much more heat each planet radiates into
space than it absorbs from the Sun.

     Obtained spin-can images of Jupiter and Saturn in two colors during the
encounter sequences, and close-up images of special planetary features (the
Great Red Spot and polar regions of Jupiter and the rings of Saturn); made
polarimetry measurements of Jupiter and Saturn and of some of their large
satellites.

     Probed the upper atmospheres of Jupiter and Saturn with S-band radio
waves at occultation; similarly, probed the Galilean satellite Io to establish
whether it has an atmosphere.

     Investigated, at relatively close range, several of the Galilean
satellites of Jupiter, and Saturn's largest satellite, Titan, by spin-scan
imaging and other measurements to determine their sizes and other physical
characteristics.

     Determined the shape of the external gravitational fields of Jupiter and
Saturn and inferred the internal mass distribution and structure of those
fields.

     Determined more precisely the masses of Jupiter and its Galilean
satellites, and the masses of Saturn, the rings of Saturn, and of the
Saturnian satellites, Rhea, Iapetus, and Titan, by accurate observations of
the effects of their gravitational fields on the motion of the spacecraft.

     Provided information to calculate with greater precision the orbits and
ephemerides of Jupiter and its Galilean satellites, and of Saturn and Titan.

     Determined the maximum radiation dosage for planning future missions.

     Eleven scientific instruments and experiments were selected from more
than 150 proposals submitted to NASA Headquarters in response to the original
request for proposed onboard experiments for the original Pioneer mission to
Jupiter.  These instruments, and two noninstrumented experiments, are
described in the following subsections.  In addition, a high-field
magnetometer was later selected for Pioneer 11 to ensure adequate coverage of
higher magnetic field strengths that might be encountered.  This instrument -
a flux-gate magnetometer - is also described.

Magnetic Fields

     Magnetic fields permeate the plasma of electrically charged particles in
interplanetary space as it spreads out from the Sun across the Solar System.
Before the Pioneer missions to Jupiter and Saturn, these effects had been
observed and measured only to the orbit of Mars.  Scientists were still
uncertain about many specific details concerning the interplanetary medium and
particularly the configuration of the magnetic field beyond the orbit of Mars
to the outer regions of the Solar System.  The outer boundaries of the Sun's
influence were, and still are, vague, and interactions between the plasma and
the magnetic fields of the Solar System and those of the nearby interstellar
medium still puzzle scientists. Pioneers 10 and 11 will continue to explore
the regions beyond the orbit of Saturn and will provide data that will help
define the transition region of the solar influence, or the heliopause.

     Of even greater importance was the objective of measuring the detailed
magnetic fields of Jupiter and Saturn and the configurations throughout the
magnetospheres of these planets.

Principal investigator:

Edward J. Smith
Jet Propulsion Laboratory, Pasadena, California

Coinvestigators:

Palmer Dyal and David S. Colburn
Ames Research Center, NASA, Moffett Field, California

Charles P. Sonett
University of Arizona, Tucson, Arizona

Douglas E. Jones
Brigham Young University, Provo, Utah

Paul J. Coleman, Jr.
University of California at Los Angeles

Leverett Davis, Jr.
California Institute of Technology, Pasadena, California

     This experiment used a sensitive magnetometer mounted on the tip of a
lightweight boom extending 6.6 m (21.5 ft) from the center of the spacecraft
to reduce the effects of even the minute amount of residual magnetic field of
the spacecraft and to help balance the spin-stabilized Pioneer spacecraft. The
helium vector magnetometer measured the fine structure of the interplanetary
field, mapped the fields of Jupiter and Saturn, and provided field
measurements to evaluate the interaction of the solar wind with the two
planets.  The magnetometer operated in any one of eight ranges, the lowest of
which covered magnetic field strengths from +0.01 to +4.0 gamma (1 gamma =
10^-5 gauss); the highest measured field strengths up to +140,000 gamma (i.e.,
+1.4 gauss).  (The surface field of Earth is about 0.5 gauss.)  The ranges
were selected by ground command or automatically by the instrument as it
reached the limits of a given range.

     The sensor for the magnetometer consisted of a cell filled with helium
that was excited by electrical discharge at radio frequencies and by infrared
optical pumping.  Changes in helium absorption caused by magnetic fields
passing through the magnetometer were measured by an infrared optical
detector.

Flux-Gate Magnetometer Experiment

     Pioneer 11 carried another instrument for measuring the magnetic field, a
flux-gate magnetometer.  This instrument was designed to measure the intense
planetary fields of Jupiter and Saturn and to extend the measuring capability
of the spacecraft beyond the range provided by the helium vector magnetometer.
The scientific objectives were to study the intrinsic magnetic fields of
Jupiter and Saturn by carrying out measurements during the closest approach
phases of the Pioneer 11 mission.  The knowledge acquired allowed a
comprehensive study of the general problem of how planets, including Earth,
generate their magnetic fields, and a determination of the detailed geometry
of their inner magnetospheres.

Principal investigator:

Mario H. Acuna
Goddard Space Flight Center, NASA, Greenbelt, Maryland

Coinvestigator:

Norman F. Hess
Goddard Space Flight Center, NASA

     The instrument, mounted on the main body of the spacecraft, used two
magnetic ring cores that were driven to saturation by associated oscillators
at a frequency of 8 kHz.  The presence of an external magnetic field created
an imbalance in the sensors which was detected by four coil windings; the coil
windings were oriented perpendicular to each other.  The instrument had a
single dynamic range with a compressed response that provided a maximum field
measurement capability of +10 gauss (0.001 tesla) and a resolution of +0.05
gauss for external fields of less than 2 gauss.

Interplanetary Solar Wind and Heliosphere

     The solar wind consists of streams of protons, electrons, and some helium
nuclei emitted by the Sun in all directions.  Particles in the solar wind
affect electrical and communication systems on Earth and may give rise to
long-term weather cycles.  This wind was unknown until spacecraft began to
explore space beyond Earth's magnetosphere less than 20 years ago.  Some of
the charged particles of the solar wind become trapped in radiation belts by
Earth's magnetic field.  They also account for the aurora borealis, the aurora
australis, and other phenomena that baffled scientists until the radiation
belts were discovered by experiments carried out by Earth satellites.

     The behavior of the solar wind at great distances from the Sun could only
be conjectured before the flight of Pioneer 10 to the outer planets.  Until
the Pioneer 10 mission, instruments on spacecraft had measured the wind only
as far as the orbit of Mars.  And virtually nothing was known of the
interaction of the solar wind with Jupiter and Saturn or about the effects of
Saturn's rings on the wind.

Principal investigator:

John H. Wolfe (Jupiter/Saturn)
Ames Research Center, NASA, Moffett Field, California

Aaron Barnes (Post-Saturn)
Ames Research Center, NASA, Moffett Field, California

Coinvestigators:

John Mihalov, H. Collard, and D. D. McKibbin
Ames Research Center, NASA

Louis A. Frank
University of Iowa, Iowa City

Reimar Lust
Max Planck Institut fur Physik und Astrophysik, Garching, Germany

Devrie Intriligator
University of Southern California, Los Angeles

William C. Feldman
Los Alamos Scientific Laboratory, New Mexico

     The Pioneer spacecraft each carried a plasma analyzer to evaluate the
solar wind.  It looked toward the Sun through a hole in each spacecraft's
large dish-shaped antenna.  The solar wind particles entered the plasma
analyzer's apertures between two quadraspherical plates where the direction of
arrival, the energy (speed), and the number of ions and electrons making up
the solar wind were measured.

[See Plasma Analyzer: A lasma analyzer was aimed toward the Sun through a hole
in the large dish antenna of each Pioneer spacecraft; its purpose was to map
the density and energy of the solar wind. (a) Diagram of the plasma analyzer.
(b) Plasma analyzer ready for installation in the spacecraft.]

     A voltage was applied across the quadraspherical plates in a maximum of
64 steps, at a rate of one step/revolution of the spacecraft, to count
particles in discrete energy ranges.  The direction of particle travel was
determined from instrument orientation and by knowing which of the detector
targets the particle struck.  The instrument had a high resolution analyzer
and a medium-resolution analyzer to detect particles of different energy
levels.  The high-resolution analyzer had 26 continuous channel multipliers
(CCM) to measure the number of ions per second with energies from 100 to 8000
eV.  The medium-resolution analyzer had 5 electrometers to count ions in the
energy range from 100 to 18,000 eV and electrons from 1 to 500 eV.

Charged Particle Composition

     The charged particle detector had four measuring systems:  two particle
telescopes that operated in interplanetary space and two that measured the
trapped electrons and protons inside the radiation belts of Jupiter and
Saturn.

Principal investigator:

John A. Simpson
University of Chicago

Coinvestigators:

Joseph J. O'Gallagher
University of Chicago

Anthony J. Tuzzolino and R. Bruce
McKibben University of Chicago

     During the interplanetary phase of the mission, before and after
encounter with Jupiter and Saturn, this experiment sought to identify the
chemical elements hydrogen, helium, lithium, beryllium, boron, carbon,
nitrogen, and oxygen, and to separate hydrogen, deuterium, helium-3, and
helium in an attempt to differentiate between particles emanating from the Sun
and those from the Galaxy.  The instrument was also used to determine how
streams of high-energy particles from the Sun travel through
interplanetary space.

     The main telescope of seven solid-state detectors measured the
composition of cosmic rays from 1 to 500 MeV, and a three-element, low-energy
telescope measured 0.4- to 10-MeV protons and helium nuclei.

     Two new types of sensors were developed to cope with the extremely high
intensities of trapped particles in the Jovian magnetosphere.  A solid-state
electron current detector, operating below -40 C (-40 F), detected those
electrons above 3.3 MeV that generate the decimetric radio waves emitted by
Jupiter and similar electrons in the radiation environment of Saturn.  A
trapped proton detector contained a foil of thorium, the atoms of which
underwent nuclear fission when hit by protons with energies above 35 MeV; the
foil was insensitive to electrons.

Energy Spectra of Cosmic Rays

     The cosmic ray telescope used for this experiment was designed to monitor
solar and galactic cosmic ray particles and to track the high-energy particles
from the Sun.  The instrument could determine which nuclei of the 10 lightest
elements are the cosmic ray particles.  Before saturation by radiation when
near Jupiter and Saturn, the cosmic ray telescope measured high-energy
particles in the radiation belts of these planets.

Principal investigator:

Frank B. McDonald
Goddard Space Flight Center, NASA, Greenbelt, Maryland

Coinvestigators:

Kenneth G. McCracken
Minerals Research Laboratory, North Ryde, Australia

William R. Webber and Edmond C. Roelof
University of New Hampsture, Durham

Bonnard J. Teegarden and James H. Trainor
Goddard Space Flight Center, NASA

     The instrument consisted of three three-element solid-state telescopes.
The high-energy telescope measured the flux of protons between 56 and 800 MeV.
The medium-energy telescope measured protons with energies between 3 and 22
MeV and identified the 10 elements from hydrogen to neon.  The low-energy
telescope measured the flux of electrons between 0.05 and 1 MeV and of protons
between 0.05 and 20 MeV.