SCIENCE AND SCIENTISTS
Introduction
There are many reasons for sending spacecraft to the planets, but in the
final analysis, we send our robot messengers across the vastness of
space for the sake of scientific exploration. Science and exploration
have always gone hand in hand, whether in the transcontinental journey
of Lewis and Clark or the Pacific voyages of Captain James Cook. In this
century, as exploration has become more and more dependent on advances
in technology, the scientific element has attained increased prominence.
The greatest legacy of the NASA Planetary Program is the knowledge it
has provided of the other worlds that share our corner of the universe.
Despite the central role of science in motivating missions to the
planets, specific scientific considerations are not dominant during most
of the development of a mission such as Voyager. The problem of building
a spacecraft and getting it to the outer solar system is too demanding.
Of the ton of mass in a Voyager spacecraft, the scientific instruments
make up only 115 kilograms (about eleven percent). Similarly, the cost
of these instruments amounts to only about 10 percent of the cost of the
spacecraft and launch vehicle. Unless the launch and operation of the
spacecraft are nearly perfect, there can be no scientific return in any
case; even the most sophisticated package of scientific instruments will
not tell much about Jupiter if, following launch, it rests at the bottom
of the Atlantic Ocean. But it is equally true that the ultimate purpose
of the mission is scientific discovery, and NASA makes every effort to
ensure that the very best instruments are flown and that a broad
scientific community is given the opportunity to participate in each
mission.
A decade before the 1977 launch, many astronomers and space scientists
began their involvement with the Voyager mission through participation
in study groups convened by NASA and by the National Academy of
Sciences. They came primarily from universities, but also in significant
numbers from NASA laboratories, from industry, and from abroad. In 1971
the Outer Planets Grand Tours mission definition group carried out a
one-year final study of the mission that was to become Voyager. A
competition was held in 1972 to select the Voyager flight instruments
and science teams, and a third review stage followed a year later to
confirm this selection. Out of this process emerged eleven science
investigations, with which more than one hundred scientists were
associated. In this chapter we look at the instruments and the persons
who designed them for this challenging task.
Direct and Remote Measurements
The diverse measurements made by Voyager of a planet and its environment
can be divided into two broad categories, usually called direct or _in
situ_ measurements and remote sensing measurements. A direct measurement
involves the analysis of the immediate environment of the spacecraft;
remote measurements can be made by analyzing radiation from distant
objects.
The direct measurement instruments on Voyager measure cosmic ray
particles, low energy charged particles, magnetic fields, plasma
particles, and plasma waves. Their activity began immediately after
launch, monitoring the Earth environment and then interplanetary space
until the magnetosphere of Jupiter was reached a few days before the
actual flyby. Long after the flybys of Jupiter and Saturn are completed,
particles and fields data can continue to be acquired by the Voyagers as
they speed outward into previously unexplored regions of space.
The remote sensing investigations are essentially astronomical in
nature, measuring the light reflected from or emitted by the planet and
its satellites. On Voyager, however, these instruments far outstrip
their terrestrial counterparts in capability. Primarily, they derive
their advantage from their proximity to what they observe—at its
closest, Voyager was more than a thousand times nearer to Jupiter than
are Earth-based telescopes; and for the still closer satellite
encounters, Voyager was nearly ten thousand times nearer than
astronomers on Earth. In addition, Voyager provided perspectives, such
as views of the night side of Jupiter, that are impossible from Earth.
Finally, these instruments could exploit the full spectrum of
electromagnetic radiation without concern for the opacity of the
terrestrial atmosphere, which restricts ground-based astronomers to
certain spectral windows and blocks all observation at other
wavelengths.
Five of the remote sensing instruments—the two TV cameras, the infrared
spectrometer, the ultraviolet spectrometer, and the photopolarimeter—are
mounted together on a scan platform. This platform can be pointed to
almost any direction in space, allowing exact targeting of the
observations.
One remote sensing instrument, the planetary radio astronomy receiver,
is not on the scan platform. It measures long-wave radio emission
without requiring special pointing.
[Illustration: The fully deployed Voyager spacecraft is capable of a
wide variety of direct and remote sensing measurements. The
instruments and their objectives were selected many years before the
first Jupiter encounter. Because of the exploratory nature of the
Voyager mission, every effort was made to fly versatile instruments
that could yield valuable results no matter what the nature of the
Jovian system. [P-18811AC]]
Narrow angle TV
Wide angle TV
Steerable platform
Science instrument boom
Plasma detector
Cosmic ray detector
High-gain antenna
Low-gain antenna
Extendable magnetometer boom
Photopolarimeter
Low energy charged particles detector
Infrared interferometer spectrometer and radiometer
Ultraviolet spectrometer
Thrusters
Star trackers
Radioisotope thermoelectric generators
Science instrument calibration panel and shunt radiator
Thrusters
Planetary radio astronomy and plasma wave antenna
[Illustration: The science instrument boom supports the plasma
particle detector, the cosmic ray detector, and the low energy
charged particle detector. These instruments began collecting data
immediately after launch, monitoring the Earth environment and then
interplanetary space until the magnetosphere of Jupiter was reached
a few days before the actual flyby. [353-2992BC]]
A final Voyager investigation did not fit into this pattern of direct
versus remote sensing instruments. In fact, it required no special
instrument at all. This investigation deals with radio science, and it
utilizes the regular communications link between the spacecraft and
Earth to derive the masses of Jupiter and its satellites, to probe the
atmosphere of Jupiter, and to study properties of the interplanetary
medium.
[Illustration: The Voyager Imaging Science Team]
Imaging
The eyes of Voyager are in its imaging system. Two television cameras,
each with a set of color filters, look at the planets and their
satellites and transmit thousands of detailed pictures to Earth. The
imaging system is probably the most versatile and therefore the most
truly exploratory of the Voyager instruments. No matter what is out
there, the imaging system will let us see it and hence, we hope, begin
to understand its nature.
VOYAGER SCIENCE INVESTIGATIONS
Project Scientist: E. C. Stone, Caltech
Investigation Principal Investigator Primary Objectives at
or Team Leader Jupiter
Imaging science B. A. Smith, U. Arizona High resolution
reconnaissance over large
phase angles; measurement
of atmospheric dynamics;
determination of geologic
structure of satellites;
search for rings and new
satellites.
Infrared radiation R. A. Hanel, NASA Determination of
(IRIS) Goddard atmospheric composition,
thermal structure, and
dynamics; satellite surface
composition and thermal
properties.
Ultraviolet A. L. Broadfoot, Kitt Measurement of upper
spectroscopy Peak Observatory atmospheric composition and
structure; auroral
processes; distribution of
ions and neutral atoms in
the Jovian system.
Photopolarimetry C. F. Lillie/C. W. Measurement of atmospheric
Hord, U. Colorado aerosols; satellite surface
texture and sodium cloud.
Planetary radio J. W. Warwick, U. Determination of
astronomy Colorado polarization and spectra of
radio frequency emissions;
Io radio modulation
process; plasma densities.
Magnetic fields N. F. Ness, NASA Goddard Measurement of plasma
electron densities;
wave-particle interactions;
low-frequency wave
emissions.
Plasma particles H. S. Bridge, MIT Measurement of
magnetospheric ion and
electron distribution;
solar wind interaction with
Jupiter; ions from
satellites.
Plasma waves F. L. Scarf, TRW Measurement of plasma
electron densities;
wave-particle interactions;
low-frequency wave
emissions.
Low energy charged S. M. Krimigis, Johns Measurement of the
particles Hopkins U. distribution, composition,
and flow of energetic ions
and electrons;
satellite-energetic
particle interactions.
Cosmic ray particles R. E. Vogt, Caltech Measurement of the
distribution, composition,
and flow of high energy
trapped nuclei; energetic
electron spectra.
Radio science V. R. Eshleman, Measurement of atmospheric
Stanford U. and ionospheric structure,
constituents, and dynamics;
satellite masses.
Unlike other Voyager instruments, the imaging system is not the result
of a competition among proposals submitted by groups of scientists. NASA
assigned the development of the cameras directly to JPL, to be
integrated from the beginning with the design of the Voyager spacecraft
and its subsystems. The members of the Voyager Imaging Science Team were
selected, as individuals, on the basis of the scientific studies they
proposed to carry out. Initially, ten members of the Imaging Science
Team were selected, but by the time of the Jupiter encounters, the team
had been expanded to 22 scientists.
[Illustration: Bradford A. Smith, imaging science Team Leader]
The Team Leader is Bradford A. Smith, a professor in the Department of
Planetary Science at the University of Arizona. Smith was involved in
imaging science on several previous missions, including Mariners 6 and 7
and Viking. He was also active in ground-based photography of Jupiter at
both New Mexico State University and University of Arizona, and he is a
member of the team developing a planetary camera for the Space
Telescope, scheduled for operation in Earth orbit in the mid-1980s.
Originally, the Deputy Team Leader was Geoffrey A. Briggs, a young
British-born physicist from JPL. However, in 1977 Briggs took a position
at NASA Headquarters in Washington and was replaced by geologist
Laurence A. Soderblom of the U.S. Geological Survey in Flagstaff,
Arizona. An energetic and articulate scientist, Soderblom, with his
interest in satellite geology, complemented Smith, whose personal
scientific interests are directed more toward the atmosphere of Jupiter.
The objectives of imaging involved multicolor photography of Jupiter and
its satellites. Both wide- and narrow-angle cameras were needed to
obtain the highest possible resolution while retaining the capability to
study global-scale features on Jupiter and the satellites. In normal
photographic terms, both cameras used telephoto lenses. For the
wide-angle camera, a focal length of 200 millimeters was selected,
giving a field of view of about 3 degrees. This field is similar to that
obtained with a 400-millimeter telephoto lens on a 35-millimeter camera.
The narrow-angle Voyager camera has a focal length of 1500 millimeters
and field of view of 0.4 degrees. The camera optics are combinations of
mirrors and lenses, designed for extreme stability of focus and for
freedom from distortion.
Each camera has a rotating filter wheel that can be used to select the
color of the light that reaches the camera. For the wide-angle camera,
these colors are clear, violet, blue, green, orange, and three special
bands for selective observation in sodium light (589-nanometer
wavelength), and in methane spectral lines at 541 nanometers and 618
nanometers. For the narrow-angle camera, the filters are clear,
ultraviolet, violet, blue, green, and orange. To create a color picture,
the cameras are commanded to take, in rapid succession, pictures of the
same area in blue, green, and orange light. These three pictures can
then be reconstructed on Earth into a “true” color image. Other
combinations of colors are used to investigate particular scientific
problems and to determine the spectrum of sunlight reflected from
features on Jupiter and its satellites.
The detector in the cameras is not photographic film but the surface of
a selenium-sulfur vidicon television tube, 11 millimeters square. Unlike
most commercial TV cameras, these tubes are designed for slow-scan
readout, providing 48 seconds to acquire each picture. The shutter speed
can be varied from a fraction of a second (for Jupiter and Io) to many
minutes (for searches for faint features, such as aurorae on the night
side of Jupiter).
Each picture consists of many numbers, each of which represents the
brightness of a single picture element, or pixel, on the image. There
are 640 000 pixels in each Voyager image, representing a square image of
800 × 800 points. This information content is more than twice that of an
ordinary television picture, which has only 520 lines. For each pixel,
eight binary numbers are required to specify the brightness; the total
information in a single image is thus 8 × 640 000 = 5 120 000 bits. Even
at a transmission of one frame per 48 seconds, the “bit rate” is more
than 100 000 bits per second. For comparison, the bit rate from the
first spacecraft to Mars (Mariner 4) was about 10 bits per second,
requiring a week to transmit 21 pictures, with a total information
content equivalent to a single Voyager picture. Altogether, Voyager took
nearly 20 000 pictures at each Jupiter encounter, representing 10¹¹—a
hundred billion—bits of information.
[Illustration: Rudolph Hanel, infrared spectrometer Principal
Investigator]
Infrared Spectrometer
The infrared investigation on Voyager is based on one of the most
sophisticated instruments ever flown to another planet. In the past,
most infrared instruments on planetary spacecraft measured at only a few
wavelengths, but Voyager carries a true spectrometer, capable of
measuring at nearly 2000 separate wavelengths, covering the spectrum
from 4 to 50 micrometers.
Twelve scientists, led by Principal Investigator Rudolph Hanel of the
NASA Goddard Space Flight Center at Greenbelt, Maryland, proposed this
infrared instrument. Hanel is an acknowledged world leader in infrared
spectroscopy from space. With his co-workers at Goddard, he has
pioneered in adapting the extremely complex art of interferometric
spectroscopy to the rigors of space flight. His spectrometers have made
many studies of the Earth’s atmosphere from meteorological satellites,
and a Hanel interferometer flew successfully to Mars on Mariner 9.
The primary goals of the infrared spectrometer investigation are
directed toward analysis of the composition and structure of the
atmosphere of Jupiter. Among the molecules to be searched for on Jupiter
were hydrogen (H₂), helium (He), methane (CH₄), ammonia (NH₃), phosphine
(PH₃), water (H₂O), carbon monoxide (CO), simple compounds of silicon
and sulfur, and a variety of organic compounds consisting of atoms of
carbon and hydrogen (e.g., C₂H₂, C₂H₄, C₂H₆). In addition to indicating
the abundance of these constituents of the atmosphere, the infrared
spectra also contain information on atmospheric structure, that is, on
the variation of temperature and pressure with altitude. The presence of
clouds or dust layers can also be inferred from the shapes of spectral
lines.
In addition to its spectroscopy of Jupiter, the Voyager instrument could
be used as a heat-measuring device to map the temperatures of both the
satellites and the atmosphere of Jupiter. Particularly interesting for
the satellites are measurements of the surface cooling and heating
rates, since these rates reveal the physical compactness of the surface,
easily distinguishing between rock and sand or dust.
The infrared instrument is a Michelson interferometer at the focus of a
gold-plated telescope of 51-centimeter aperture. The spectrum is not
obtained directly, by moving a prism or grating, but indirectly through
the interference effects of light of different wavelengths: hence its
name, an interferometer. The complex interference pattern generated by
the motion of one of the mirrors in the light path is transmitted to
Earth, where computer analysis is required to transform it into a
recognizable spectrum. The entire instrument, which has a mass of 20
kilograms, is called IRIS, for infrared interferometer spectrometer.
The development of the infrared system for Voyager posed many problems.
IRIS was designed to cover the optimum spectral region for studies of
the atmospheres of Jupiter and Saturn. However, when it was decided in
1974 that an extended Voyager mission might also allow a visit to
Uranus, Hanel and his colleagues realized that their instrument had
serious deficiencies for investigation of that colder and more distant
planet, and they proposed that a modified IRIS (MIRIS) be substituted
for the original design. A crash program was authorized to develop MIRIS
in parallel with IRIS, and in early 1977, as launch approached, it
appeared that the improved instrument would be ready. Problems occurred
during testing, however, and for several weeks in June and July the
decision hung in the balance. Unfortunately, there simply was not enough
time to solve all the problems; both Voyagers were launched carrying the
original IRIS, and the final flight qualification of MIRIS came in
October, about six weeks after launch. With no other missions planned
beyond Saturn, no alternate use for MIRIS has been found; it remains “on
the shelf,” one of the rare cases where a technological gamble by NASA
did not pay off.
[Illustration: Lyle Broadfoot, ultraviolet spectrometer Principal
Investigator]
Ultraviolet Spectrometer
A second spectroscopic instrument on the Voyager scan platform examines
short-wave, or ultraviolet, radiation. The Principal Investigator for
the ultraviolet spectrometer (UVS) is A. Lyle Broadfoot of Kitt Peak
National Observatory in Tucson, Arizona. Broadfoot’s expertise lies in
instrumentation for atmospheric research, and he was previously the
Principal Investigator for a similar ultraviolet instrument on the
Mariner 10 mission to Venus and Mercury. Associated with Broadfoot in
the Voyager investigation are fourteen other scientists from the United
States, Canada, and France.
In the upper atmosphere of a planet, the lighter gases tend to diffuse,
rising above their heavier neighbors, and unusual chemical reactions
take place as the action of sunlight breaks some chemical bonds and
stimulates the formation of others. The study of these tenuous regions
of planetary atmospheres is called aeronomy. Many of the exotic chemical
processes that occur can best be studied by examining radiation of very
short wavelength, and it is to these problems that much of the work of
the UVS team is directed.
The observations are also sensitive to special processes in the Jovian
atmosphere resulting from the magnetosphere. At the very top of the
atmosphere, energetic charged particles interacting with the atmospheric
molecules produce ultraviolet aurorae that indicate the location and
nature of the bombarding electrons and ions. Emissions from atoms in the
extended gas clouds around Io and possibly around other satellites were
also expected to be observable.
The ultraviolet instrument is a relatively straightforward optical
spectrometer, adapted for use in space. A diffraction grating disperses
the light into a spectrum, and the other optical elements are
gold-coated mirrors. The field of view is rectangular, about 0.1 degrees
× 0.9 degrees. An array of sensitive electronic detectors provides
simultaneous measurements in 128 wavelength channels over a wavelength
range from 50 to 170 nanometers.
Photopolarimeter
The fourth scan-platform instrument is designed to measure the
brightness and polarization of light with high precision. Unlike the
imaging system, however, it can look at only a single point (one pixel)
at a time; thus it sacrifices spatial capability in favor of precision
of measurement. The prime objectives of this photopolarimeter are
related to study of the clouds of Jupiter.
During the development stage for this instrument, the Principal
Investigator was Charles F. Lillie of the Laboratory for Atmospheric and
Space Physics of the University of Colorado at Boulder. Later, the
Principal Investigator’s role was assumed by Lillie’s colleague, Charles
W. Hord, a physicist with considerable experience in space missions,
primarily with ultraviolet instruments. Four other scientists are
Co-Investigators on the photopolarimeter.
The instrument itself is essentially a small Cassegrain reflecting
telescope of 15-centimeter aperture. Two consecutive filter wheels
select the color and polarization of the light, which is then detected
and measured by a photomultiplier tube. The entire photopolarimeter
weighs just 2.5 kilograms.
In spite of their apparent simplicity, however, the Voyager
photopolarimeters were plagued with troubles almost from the moment of
launch. A succession of mechanical failures on Voyager 1 led to the
sticking first of the polarization wheel and then of the filter wheel.
Even when repeated commands from Earth managed to free the wheels, their
behavior was erratic. Ultimately an electronic failure also took place,
leading to the reluctant decision to shut down the Voyager 1
photopolarimeter. On Voyager 2, similar mechanical problems greatly
restricted the ability of the instrument to carry out its observations.
The photopolarimeter thus was unable to contribute its share to
unraveling the mysteries of Jupiter and its satellites.
[Illustration: Charles W. Hord, photopolarimeter Principal
Investigator]
[Illustration: James W. Warwick, planetary radio astronomy Principal
Investigator]
Planetary Radio Astronomy
The final Voyager remote sensing instrument is designed to measure radio
emission from Jupiter and Saturn over a wide range of frequencies. These
emissions, which sound like hiss or static if played through an audio
receiver, result from interactions of charged particles in the
magnetospheres and ionospheres of the giant planets. The planetary radio
astronomy (PRA) Principal Investigator is James W. Warwick, an
astronomer in the Department of Astro-Geophysics of the University of
Colorado at Boulder. Eleven colleagues from the United States and France
participate as Co-Investigators. Warwick has been studying Jupiter
longer than any other Voyager Principal Investigator. He has monitored
its radio emissions since the 1960s, and he played a central role in the
discovery that Io influenced these emissions.
Jupiter emits many kinds of radio radiation, ranging from bland thermal
emission at short (centimeter) wavelengths, to synchrotron emission from
energetic electrons at intermediate (decimeter) wavelengths, to erratic,
extremely intense bursts at long (meter and decameter) wavelengths. The
origin of these latter, nonthermal emissions constitutes one of the
major unsolved problems of the Jovian system, and, more generally, of
the physics of plasmas. One of the most important advantages of the
Voyager PRA for studying Jovian radio emission lies in its proximity to
Jupiter and hence its ability to locate the sources of different kinds
of radio bursts.
The PRA instrument consists of an antenna and a radio receiver. The
antenna is made up of two thin metal poles, each 10 meters long,
extended from the spacecraft after launch at an angle of 90 degrees to
each other. These are electrically connected to two receivers of
extremely high sensitivity and broad frequency response: from 1.2
kilohertz to 40.5 megahertz. The PRA can operate in a number of modes,
depending on the measurements desired. At its lowest level of activity,
it monitors intensity in all 198 bands and transmits the data at 266
bits per second. In its highest mode, where searches are made for
variations with very short time scales, the data rate goes up to 108 000
bits per second, essentially the same as that required by imaging. In
fact, the high-rate PRA data actually use an imaging frame as a display
form, and occasionally throughout the mission an unfamiliar looking
“image” was transmitted that was actually a block of PRA data—a portrait
of electrical events in the Jovian atmosphere and magnetosphere that
only Warwick and his colleagues could interpret.
[Illustration: Norman F. Ness, magnetometer Principal Investigator]
[Illustration: Voyager’s 13-meter-long magnetometer boom is shown
fully extended. In space, under zero gravity conditions, the
triangular epoxy glass mast spirals from its housing and provides a
rigid support for two magnetometer instruments—one at the end of the
boom and another at about the midpoint. [260-181]]
Magnetometer
The first of the direct sensing instruments to be discussed is the
magnetometer, designed to measure the magnetic fields surrounding the
spacecraft. Such measurements can be interpreted to yield the intrinsic
fields of Jupiter and its satellites and to characterize, in conjunction
with data from particle and plasma instruments, the processes taking
place in the magnetosphere of the planet. The Principal Investigator for
this instrument is Norman F. Ness of the NASA Goddard Space Flight
Center. Ness is an intense, competitive scientist with a great deal of
previous experience in spacecraft magnetometers, primarily on board
Earth satellites. Ness is also the only Voyager Principal Investigator
who has previous experience at Jupiter; he was Principal Investigator on
one of the two magnetometer instruments flown on Pioneer 11. For the
Voyager investigation, Ness is joined by four colleagues from Goddard
and one from Germany.
The magnetometer instrument consists of two systems: a high-field
magnetometer and a low-field magnetometer. Each system contains two
identical three-axis magnetometers that measure the intensity and
direction of the magnetic field. The low-field system requires isolation
from magnetic fields induced by electric circuits in the spacecraft
itself. To achieve this isolation, it is mounted on the largest
component of Voyager—a 13-meter boom, about as long as the width of a
typical city house lot. This boom was coiled tightly in a canister
during launch; later, when the package was opened, it uncurled and
extended automatically.
By using two magnetometers, Ness and his colleagues are able to correct
for the residual artificial magnetic fields that reach even 13 meters
from the main spacecraft. The dynamic range extends from a maximum field
of 20 gauss down to 2 × 10⁻⁸ gauss—a factor of one billion. The fields
can be measured as frequently as 17 times per second. The total mass,
exclusive of the 13-meter boom, is 5.6 kilograms.
[Illustration: Herbert S. Bridge, plasma particle Principal
Investigator]
Plasma Particles
Plasma is the term given to a “gas” of charged particles; the electrons
and protons are separate, yet there are equal numbers of each, producing
a zero net charge. If the velocities of the electrons and protons are
less than about 0.1 percent of the speed of light, they can be measured
by the Voyager plasma instrument; if their energies are higher, they are
measured by one of the other two particle instruments—the low energy
charged particle (LECP) detector or the cosmic ray detector. The plasma
instrument, like the magnetometer, was designed to provide basic data on
the particles and fields environment of Voyager.
The Principal Investigator for the plasma instrument is Herbert S.
Bridge of the Massachusetts Institute of Technology in Cambridge,
Massachusetts. An experienced space physicist, Bridge has flown similar
instruments on many Earth satellites and planetary probes. He also holds
the position of Director of the Laboratory for Space Experiments at MIT.
On the Voyager experiment, he is joined by one German and ten U.S.
Co-Investigators.
The objectives of the plasma investigation are directed toward study of
both the interplanetary medium and the Jovian magnetosphere. At Jupiter,
Bridge expected to determine the plasma populations and processes in the
inner and outer regions of the magnetosphere and in the plasma tail that
extends beyond Jupiter, much as a comet tail is blown outward by the
solar wind. The densities and temperature of electrons were measured,
and their origins determined: Some originate near Jupiter and diffuse
outward through the magnetosphere; others derive from the solar wind.
The plasma instrument, with a mass of 9.9 kilograms, was designed to
view in two directions: one toward the Earth and Sun, primarily to study
the solar wind, and the other sideways, looking toward the direction
plasma would flow if it were caught up in the rotating Jovian magnetic
field. If it is desired to look in other directions, the entire
spacecraft must be tipped, a maneuver that was carried out several times
near Jupiter. The detectors directly sense the flow of electrons,
protons, and alpha particles (helium nucleii, made up of two protons and
two neutrons each). Analysis of the energy spectra can also yield data
on positive ions of higher mass.
[Illustration: Frederick L. Scarf, plasma wave Principal
Investigator]
Plasma Waves
The plasma wave investigation on Voyager was a late addition to the
scientific payload. It was selected to broaden the capability of the
mission to study a wide variety of plasma processes. Because of the
electrically charged nature of the plasma, it responds to energy inputs
in ways that ordinary gas cannot. One of these modes of response yields
plasma waves, which are oscillations in density and electric field that
generally cover the audio range of frequencies. Measurement of such
waves characterizes the density and temperature of the local plasma
surrounding the spacecraft, and it also allows remote sensing detection
of distant events from the plasma waves they produce.
The plasma wave Principal Investigator is physicist Frederick L. Scarf
of the TRW Defense and Space Systems Group of Redondo Beach,
California—he is the only Voyager Principal Investigator to come from
industry. Scarf has been associated with many particles and fields
investigations in the terrestrial magnetosphere, although he is better
known as a theorist than as an experimenter. A member of the Space
Science Board of the National Academy of Sciences, he is familiar in
Washington as an eloquent advocate of space physics—the study of
plasma-physical processes in the space environment.
The plasma wave instrument shares with the planetary radio astronomy
investigation a pair of 10-meter-long antennas. Whereas the PRA uses
these as electric antennas to detect radio radiation, the plasma wave
system uses them to detect directly the oscillations in the plasma near
the spacecraft. Waves are measured over a broad frequency range, from 10
hertz (a bit deeper than the lowest bass note we can hear) to 56
kilohertz (about three times higher than the highest pitch to which the
human ear responds). The instrument electronics have a total mass of
only 1.4 kilograms.
Low Energy Charged Particles
Charged particles with energies greater than a few thousand electron
volts are not easily measured by a plasma instrument such as that
designed by Herb Bridge. Instead, these faster moving particles, with
speeds up to a few percent the speed of light, are the subject of a pair
of Voyager instruments called collectively the LECP, or low energy
charged particle instrument. Like the other particles and fields
investigation, the LECP is designed to provide basic data on
plasma-physical processes in the Jovian magnetosphere and the solar
wind, and on their interactions.
The Principal Investigator for the LECP investigation is Stamatios Mike
Krimigis, a Greek-born physicist from Johns Hopkins University. Krimigis
has participated in a number of satellite studies of the terrestrial
magnetosphere, and he now serves as Head of Space Physics and
Instrumentation at the Johns Hopkins Applied Physics Laboratory. He is
joined in this investigation by one German and five U.S.
Co-Investigators.
The LECP instrument consists of two subsystems. The first, called the
low energy magnetospheric particle analyzer, is optimized for
measurement of particles within the Jovian magnetosphere, with high
sensitivity over a broad dynamic range. Measurements of electrons,
protons, and other positive ions can be carried out, determining the
energy and composition of individual particles. The total energy ranges
covered are 10 kiloelectron volts (keV) to 11 million electron volts
(MeV) for electrons and 15 keV to 150 MeV for protons and ions.
[Illustration: Stamatios Mike Krimigis, low energy charged particle
Principal Investigator]
The second LECP subsystem is a low energy charged particle telescope,
designed to operate where the density of charged particles is low, such
as in interplanetary space or the outer magnetosphere of Jupiter. For
protons and positive ions, the energy range is from 50 keV to 40 MeV per
nucleon. The energy and species resolution is again sufficient to
determine the composition, both chemical and isotopic, of many ions
encountered. In order to provide directional discrimination even on a
spacecraft of fixed orientation, both LECP subsystems are mounted on a
moving platform that steps through eight positions in a time that can be
commanded to vary from 48 seconds to 48 minutes. The mass of the
instrument and its platform is 6.7 kilograms.
Cosmic Rays
The solar system is constantly bombarded by extremely energetic charged
particles. These are called cosmic rays, although they are particles,
not photons—“rays” are only produced when the particles strike
something, such as the molecules of the Earth’s atmosphere, and give up
their energy in a flash of x-rays and gamma-rays. One of the Voyager
instruments is designed to study these galactic cosmic rays,
particularly to look from beyond the orbit of Saturn, where the cosmic
ray particles will be less affected by the solar magnetic field and
solar wind than they are near Earth.
The cosmic ray Principal Investigator is Rochus E. Vogt of the
California Institute of Technology. Vogt has measured cosmic rays from
the ground, from balloons, and from spacecraft for many years. During
1977 and 1978 he served as Chief Scientist at JPL, and then assumed the
job of directing the physics, mathematics, and astronomy programs at
Caltech. Among his six Co-Investigators is Ed Stone, the Voyager Project
Scientist.
Because the cosmic ray instrument was not directed principally toward
measurements of the Jovian system, it is described only briefly. Like
the LECP, it is designed to determine the energy and composition of
individual electrons and positive ions. For electrons, the energy range
is from 3 to 110 MeV, and for ions from 1 to 500 MeV per nucleon; the
corresponding velocities are from about 10 percent to 99 percent of the
speed of light. For the positive ions, composition can be determined for
elements from hydrogen to iron. At Jupiter, this system could be used to
determine the nature of the rare particles accelerated to very high
energies in the Jovian magnetosphere.
Radio Science
The final Voyager science investigation is in the field of radio
science. No special instrument was required for this study; rather, NASA
selected members of a Radio Science Team who proposed investigations
that could be carried out using the already existing spacecraft
telecommunication system.
The radio science Team Leader is Von R. Eshleman of the Center for Radio
Astronomy at Stanford University. Eshleman is a radar physicist who has
been interpreting spacecraft radio occultation data since the first such
probe was carried out when Mariner 4 passed behind Mars in 1964. The
Deputy Team Leader is G. Leonard Tyler, a colleague of Eshleman’s at
Stanford. There are five other radio team members, four of them from
JPL.
The radio science investigations are divided into two groups. The first
deals with the atmosphere of Jupiter. During the Voyager flybys, the
spacecraft passed behind the planet as seen from Earth, and the radio
signal was dimmed by the atmosphere before it was finally extinguished.
During an occultation, the propagation of the radio waves is slowed down
by passage through the neutral atmosphere and is speeded up by passage
through the electrically charged ionosphere. Because of the extreme
stability of the ground-based and spacecraft radio transmitters, it is
possible to measure these shifts in the signal with high precision. The
shifts are proportional to electron density for the ionosphere, and to
gas density for the atmosphere. From a careful study of the interactions
of the transmitted beam with the Jovian atmosphere, Eshleman and his
colleagues can reconstruct a temperature-pressure profile of the
ionosphere and the upper atmosphere of Jupiter. The same approach can be
used to search for tenuous atmospheres on the satellites.
The second area of study is in the field of celestial mechanics. The
frequency stability of the communications system permits measurements of
the speed of the spacecraft, relative to Earth, to a precision of one
part in several million. By careful tracking, gravitational
perturbations on the spacecraft can be detected and used to measure the
gravitational fields, and hence the masses, of Jupiter and its
satellites.
[Illustration: Rochus E. Vogt, cosmic ray Principal Investigator]
[Illustration: Von R. Eshleman, radio science Team Leader]
These scientific instruments and their objectives were selected many
years before the first Jupiter encounter in March 1977. Because Voyager
was an exploratory mission, every effort was made to fly versatile
instruments that could yield valuable results no matter what the nature
of the Jovian system. In addition, the Voyager spacecraft control system
permitted the instruments to receive commands from Earth to adjust their
sensitivities and observing sequences in response to new information. By
the spring of 1977, all the instruments were completed, ready to be
installed in the Voyager spacecraft for testing and launch.
[Illustration: The first picture to capture crescent Earth and
crescent Moon in the same frame was taken by Voyager 1, the
second-launched spacecraft, on September 18, 1977, at a distance of
12 million kilometers from Earth. On the Earth eastern Asia, the
western Pacific, and part of the Arctic can be seen. Since the Moon
is much less reflective than the Earth, JPL image processors
brightened the lunar image by a factor of three to ensure that both
Earth and Moon were visible on this print. [P-19891C]]