yee@trident.arc.nasa.gov (Peter E. Yee) (09/30/89)
115,000 mph, fast enough to jet from Los Angeles to New York in 90
seconds. Deceleration to about Mach 1 -- the speed of sound -- should take
just a few minutes. At maximum deceleration as the craft slows from
115,000 mph to 100 mph, it will be hurtling against a force 350 times
Earth's gravity. The incandescent shock wave ahead of the probe will be as
bright as the sun and reach searing temperatures of up to 28,000 degrees
Fahrenheit. After the aerodynamic braking has slowed the probe, it will
drop its heat shields and deploy its parachute. This will allow the probe to
float down about 125 miles through the clouds, passing from a pressure of
1/10th that on Earth's surface to about 25 Earth atmospheres.
About 4 minutes after probe entry into JupiterUs atmosphere, a pilot
chute deploys and explosive nuts shoot off the top section of the probe's
protective shell. As the cover whips away, it pulls out and opens the main
parachute attached to the inner capsule. What remains of the probe's outer
shell, with its massive heat shield, falls away as the parachute slows the
instrument module.
From there on, suspended from the main parachute, the probe's capsule
with its activated instruments floats downward toward the bright clouds
below.
The probe will pass through the white cirrus clouds of ammonia crystals
- the highest cloud deck. Beneath this ammonia layer probably lie
reddish-brown clouds of ammonium hydrosulfides. Once past this layer, the
probe is expected to reach thick water clouds. This lowest cloud layer may
act as a buffer between the uniformly mixed regions below and the
turbulent swirl of gases above.
Jupiter's atmosphere is primarily hydrogen and helium. For most of its
descent through Jupiter's three main cloud layers, the probe will be
immersed in gases at or below room temperature. However, it may
encounter hurricane winds up to 200 mph and lightning and heavy rain at the
base of the water clouds believed to exist on the planet. Eventually, the
probe will sink below these clouds, where rising pressure and temperature
will destroy it. The probe's active life in Jupiter's atmosphere is expected
to be about 75 minutes in length. The probe batteries are not expected to
last beyond this point, and the relaying orbiter will move out of reach.
To understand this huge gas planet, scientists must find out about its
chemical components and the dynamics of its atmosphere. So far,
scientific data are limited to a two-dimensional view (pictures of the
planet's cloud tops) of a three-dimensional process (Jupiter's weather). But
to explore such phenomena as the planet's incredible coloring, the Great Red
Spot and the swirling shapes and high-speed motion of its topmost clouds,
scientists must penetrate Jupiter's visible surface and investigate the
atmosphere concealed in the deep-lying layers below.
A set of six scientific instruments on the probe will measure, among
other things, the radiation field near Jupiter, the temperature, pressure,
density and composition of the planet's atmosphere from its first faint
outer traces to the hot, murky hydrogen atmosphere 100 miles below the
cloud tops. All of the information will be gathered during the probe's
descent on an 8-foot parachute. Probe data will be sent to the Galileo
Orbiter 133,000 miles overhead then relayed across the half billion miles
to Deep Space Network stations on Earth.
To return its science, the probe relay radio aboard the orbiter must
automatically acquire the probe signal below within 50 seconds, with a
success probability of 99.5 percent. It must reacquire the signal
immediately should it become lost.
To survive the heat and pressure of entry, the probe spacecraft is
composed of two separate units: an inner capsule containing the scientific
instruments, encased in a virtually impenetrable outer shell. The probe
weighs 750 pounds. The outer shell is almost all heat shield material.
The Orbiter at Jupiter
After releasing the probe, the orbiter will use its main engine to go into
orbit around Jupiter. This orbit, the first of 10 planned, will have a period
of about 8 months. A close flyby of Ganymede in July 1996 will shorten the
orbit, and each time the Galileo orbiter returns to the inner zone of
satellites, it will make a gravity-assist close pass over one or another of
the satellites, changing Galileo's orbit while making close observations.
These satellite encounters will be at altitudes as close as 125 miles above
their surfaces. Throughout the 22-month orbital phase, Galileo will
continue observing the planet and the satellites and continue gathering data
on the magnetospheric environment.
SCIENTIFIC ACTIVITIES
Galileo's scientific experiments will be carried out by more than 100
scientists from six nations. Except for the radio science investigation,
these are supported by dedicated instruments on the Galileo orbiter and
probe. NASA has appointed 15 interdisciplinary scientists whose studies
include data from more than one Galileo instrument.
The instruments aboard the probe will measure the temperatures and
pressure of Jupiter's atmosphere at varying altitudes and determine its
chemical composition including major and minor constituents (such as
hydrogen, helium, ammonia, methane, and water) and the ratio of hydrogen
to helium. Jupiter is thought to have a bulk composition similar to that of
the primitive solar nebula from which it was formed. Precise
determination of the ratio of hydrogen to helium would provide an
important factual check of the Big Bang theory of the genesis of the
universe.
Other probe experiments will determine the location and structure of
Jupiter's clouds, the existence and nature of its lightning, and the amount
of heat radiating from the planet compared to the heat absorbed from
sunlight.
In addition, measurements will be made of Jupiter's numerous radio
emissions and of the high-energy particles trapped in the planet's
innermost magnetic field. These measurements for Galileo will be made
within a distance of 26,000 miles from Jupiter's cloud tops, far closer than
the previous closest approach to Jupiter by Pioneer 11. The probe also will
determine vertical wind shears using Doppler radio measurements made of
probe motions from the radio receiver aboard the orbiter.
Jupiter appears to radiate about twice as much energy as it receives
from the sun and the resulting convection currents from Jupiter's internal
heat source towards its cooler polar regions could explain some of the
planet's unusual weather patterns.
Jupiter is over 11 times the diameter of Earth and spins about two and
one-half times faster -- a jovian day is only 10 hours long. A point on the
equator of Jupiter's visible surface races along at 28,000 mph. This rapid
spin may account for many of the bizarre circulation patterns observed on
the planet.
Spacecraft Scientific Activities
The Galileo mission and systems were designed to investigate three
broad aspects of the Jupiter system: the planet's atmosphere, the satellites
and the magnetosphere. The spacecraft is in three segments to focus on
these areas: the atmospheric probe; a non-spinning section of the orbiter
carrying cameras and other remote sensors; and the spinning main section
of the orbiter spacecraft which includes the propulsion module, the
communications antennas, main computers and most support systems as
well as the fields and particles instruments, which sense and measure the
environment directly as the spacecraft flies through it.
Probe Scientific Activities
The probe will enter the atmosphere about 6 degrees north of the
equator. The probe weighs just under 750 pounds and includes a
deceleration module to slow and protect the descent module, which carries
out the scientific mission.
The deceleration module consists of an aeroshell and an aft cover
designed to block the heat generated by slowing from the probe's arrival
speed of about 115,000 miles per hour to subsonic speed in less than 2
minutes. After the covers are released, the descent module deploys its
8-foot parachute and its instruments, the control and data system, and the
radio-relay transmitter go to work.
Operating at 128 bits per second, the dual L-band transmitters send
nearly identical streams of scientific data to the orbiter. The probe's relay
radio aboard the orbiter will have two redundant receivers that process
probe science data, plus radio science and engineering data for
transmission to the orbiter communications system. Minimum received
signal strength is 31 dBm. The receivers also measure signal strength and
Doppler shift as part of the experiments for measuring wind speeds and
atmospheric absorption of radio signals.
Probe electronics are powered by long-life, high-discharge-rate 34-volt
lithium batteries, which remain dormant for more than 5 years during the
journey to Jupiter. The batteries have an estimated capacity of about 18
amp-hours on arrival at Jupiter.
Orbiter Scientific Activities
The orbiter, in addition to delivering the probe to Jupiter and relaying
probe data to Earth, will support all the scientific investigations of Venus,
the Earth and moon, asteroids and the interplanetary medium, Jupiter's
satellites and magnetosphere, and observation of the giant planet itself.
The orbiter weighs about 5,200 pounds including about 2,400 pounds of
rocket propellant to be expended in some 30 relatively small maneuvers
during the long gravity-assisted flight to Jupiter, the large thrust
maneuver which puts the craft into its Jupiter orbit, and the 30 or so trim
maneuvers planned for the satellite tour phase.
The retropropulsion module consists of 12 10-newton thrusters, a single
400-newton engine, and the fuel, oxidizer, and pressurizing-gas tanks,
tubing, valves and control equipment. (A thrust of 10 newtons would
support a weight of about 2.2 pounds at Earth's surface). The propulsion
system was developed and built by Messerschmitt-Bolkow-Blohm and
provided by the Federal Republic of Germany.
The orbiter's maximum communications rate is 134 kilobits per second
(the equivalent of about one black-and-white image per minute); there are
other data rates, down to 10 bits per second, for transmitting engineering
data under poor conditions. The spacecraft transmitters operate at S-band
and X-band (2295 and 8415 megahertz) frequencies between Earth and on
L-band between the probe.
The high-gain antenna is a 16-foot umbrella-like reflector unfurled
after the first Earth flyby. Two low-gain antennas (one pointed forward
and one aft, both mounted on the spinning section) are provided to support
communications during the Earth-Venus-Earth leg of the flight and
whenever the main antenna is not deployed and pointed at Earth. The despun
section of the orbiter carries a radio relay antenna for receiving the probe's
data transmissions.
Electrical power is provided to Galileo's equipment by two radioisotope
thermoelectric generators. Heat produced by natural radioactive decay of
plutonium 238 dioxide is converted to approximately 500 watts of
electricity (570 watts at launch, 480 at the end of the mission) to operate
the orbiter equipment for its 8-year active period. This is the same type of
power source used by the Voyager and Pioneer Jupiter spacecraft in their
long outer-planet missions, by the Viking lander spacecraft on Mars and the
lunar scientific packages left on the Moon.
Most spacecraft are stabilized in flight either by spinning around a
major axis or by maintaining a fixed orientation in space, referenced to the
sun and another star. Galileo represents a hybrid of these techniques, with
a spinning section rotating ordinarily at 3 rpm and a "despun" section which
is counter-rotated to provide a fixed orientation for cameras and other
remote sensors.
Instruments that measure fields and particles, together with the main
antenna, the power supply, the propulsion module, most of the computers
and control electronics, are mounted on the spinning section. The
instruments include magnetometer sensors mounted on a 36-foot boom to
escape interference from the spacecraft; a plasma instrument detecting
low-energy charged particles and a plasma-wave detector to study waves
generated in planetary magnetospheres and by lightning discharges; a
high-energy particle detector; and a detector of cosmic and Jovian dust.
The despun section carries instruments and other equipment whose
operation depends on a fixed orientation in space. The instruments include
the camera system; the near-infrared mapping spectrometer to make
multispectral images for atmosphere and surface chemical analysis; the
ultraviolet spectrometer to study gases and ionized gases; and the
photopolarimeter radiometer to measure radiant and reflected energy. The
camera system is expected to obtain images of Jupiter's satellites at
resolutions from 20 to 1,000 times better than Voyager's best.
This section also carries a dish antenna to track the probe in Jupiter's
atmosphere and pick up its signals for relay to Earth. The probe is carried
on the despun section, and before it is released, the whole spacecraft is
spun up briefly to 10 rpm in order to spin-stabilize the probe.
The Galileo spacecraft will carry out its complex operations, including
maneuvers, scientific observations and communications, in response to
stored sequences which are interpreted and executed by various on-board
computers. These sequences are sent up to the orbiter periodically through
the Deep Space Network in the form of command loads.
GROUND SYSTEMS
Galileo communicates with Earth via NASA's Deep Space Network (DSN),
which has a complex of large antennas with receivers and transmitters
located in the California desert, another in Australia and a third in Spain,
linked to a network control center at NASAUs Jet Propulsion Laboratory in
Pasadena, Calif. The spacecraft receives commands, sends science and
engineering data, and is tracked by Doppler and ranging measurements
through this network.
At JPL, about 275 scientists, engineers and technicians, will be supporting
the mission at launch, increasing to nearly 400 for Jupiter operations
including support from the German retropropulsion team at their control
center in the FGR. Their responsibilities include spacecraft command,
interpreting engineering and scientific data from Galileo to understand its
performance, and analyzing navigation data from the DSN. The controllers
use a set of complex computer programs to help them control the
spacecraft and interpret the data.
Because the time delay in radio signals from Earth to Jupiter and back is
more than an hour, the Galileo spacecraft was designed to operate from
programs sent to it in advance and stored in spacecraft memory. A single
master sequence program can cover 4 weeks of quiet operations between
planetary and satellite encounters. During busy Jupiter operations, one
program covers only a few days. Actual spacecraft tasks are carried out by
several subsystems and scientific instruments, many of which work from
their own computers controlled by the main sequence.
Designing these sequences is a complex process balancing the desire to
make certain scientific observations with the need to safeguard the
spacecraft and mission. The sequence design process itself is supported by
software programs, for example, which display to the scientist maps of the
instrument coverage on the surface of an approaching satellite for a given
spacecraft orientation and trajectory. Notwithstanding these aids, a
typical 3-day satellite encounter may take efforts spread over many
months to design, check and recheck. The controllers also use software
designed to check the command sequence further against flight rules and
constraints.
The spacecraft regularly reports its status and health through an
extensive set of engineering measurements. Interpreting these data into
trends and averting or working around equipment failures is a major task
for the mission operations team. Conclusions from this activity become an
important input, along with scientific plans, to the sequence design
process. This too is supported by computer programs written and used in
the mission support area.
Navigation is the process of estimating, from radio range and Doppler
measurements, the position and velocity of the spacecraft to predict its
flight path and design course-correcting maneuvers. These calculations
must be done with computer support. The Galileo mission, with its complex
gravity-assist flight to Jupiter and 10 gravity-assist satellite encounters
in the Jovian system, is extremely dependent on consistently accurate
navigation.
In addition to the programs that directly operate the spacecraft and are
periodically transmitted to it, the mission operations team uses software
amounting to 650,000 lines of programming code in the sequence design
process; 1,615,000 lines in the telemetry interpretation; and 550,000 lines
of code in navigation. These must all be written, checked, tested, used in
mission simulations and, in many cases, revised before the mission can
begin.
Science investigators are located at JPL or other university laboratories
and linked by computers. From any of these locations, the scientists can be
involved in developing the sequences affecting their experiments and, in
some cases, in helping to change preplanned sequences to follow up on
unexpected discoveries with second looks and confirming observations.
JUPITER'S SYSTEM
Jupiter is the largest and fastest-spinning planet in the solar system.
Its radius is more than 11 times Earth's, and its mass is 318 times that of
our planet. Named for the chief of the Roman gods, Jupiter contains more
mass than all the other planets combined. It is made mostly of light
elements, principally hydrogen and helium. Its atmosphere and clouds are
deep and dense, and a significant amount of energy is emitted from its
interior.
The earliest Earth-based telescopic observations showed bands and
spots in Jupiter's atmosphere. One storm system, the Red Spot, has been
seen to persist over three centuries.
Atmospheric forms and dynamics were observed in increasing detail
with the Pioneer and Voyager flyby spacecraft, and Earth-based infrared
astronomers have recently studied the nature and vertical dynamics of
deeper clouds.
Sixteen satellites are known. The four largest, discovered by the Italian
scientist Galileo Galilei in 1610, are the size of small planets. The
innermost of these, Io, has active sulfurous volcanoes, discovered by
Voyager 1 and further observed by Voyager 2 and Earth-based infrared
astronomy. Io and Europa are about the size and density of Earth's moon (3
to 4 times the density of water) and probably rocky inside. Ganymede and
Callisto, further out from Jupiter, are the size of Mercury but less than
twice as dense as water. Their cratered surfaces look icy in Voyager
images, and they may be composed partly of ice or water.
Of the other satellites, eight (probably captured asteroids) orbit
irregularly far from the planet, and four (three discovered by the Voyager
mission in 1979) are close to the planet. Voyager also discovered a thin
ring system at Jupiter in 1979.
Jupiter has the strongest planetary magnetic field known. The resulting
magnetosphere is a huge teardrop-shaped, plasma-filled cavity in the solar
wind pointing away from the sun. JupiterUs magnetosphere is the largest
single entity in our solar system, measuring more than 14 times the
diameter of the sun. The inner part of the magnetic field is doughnut-
shaped, but farther out it flattens into a disk. The magnetic poles are
offset and tilted relative to Jupiter's axis of rotation, so the field appears
to wobble with Jupiter's rotation (just under 10 hours), sweeping up and
down across the inner satellites and making waves throughout the
magnetosphere.
WHY JUPITER INVESTIGATIONS ARE IMPORTANT
With a thin skin of turbulent winds and brilliant, swift-moving clouds,
the huge sphere of Jupiter is a vast sea of liquid hydrogen and helium.
Jupiter's composition (about 88 percent hydrogen and 11 percent helium
with small amounts of methane, ammonia and water) is thought to resemble
the makeup of the solar nebula, the cloud of gas and dust from which the
sun and planets formed. Scientists believe Jupiter holds important clues to
conditions in the early solar system and the process of planet formation.
Jupiter may also provide insights into the formation of the universe
itself. Since it resembles the interstellar gas and dust that are thought to
have been created in the "Big Bang," studies of Jupiter may help scientists
calibrate models of the beginning of the universe.
Though starlike in composition, Jupiter is too small to generate
temperatures high enough to ignite nuclear fusion, the process that powers
the stars. Some scientists believe that the sun and Jupiter began as
unequal partners in a binary star system. (If a double star system had
developed, it is unlikely life could have arisen in the solar system.) While
in a sense a "failed star," Jupiter is almost as large as a planet can be. If
it contained more mass, it would not have grown larger, but would have shrunk
from compression by its own gravity. If it were 100 times more massive,
thermonuclear reactions would ignite, and Jupiter would be a star.
For a brief period after its formation, Jupiter was much hotter, more
luminous, and about 10 times larger than it is now, scientists believe. Soon
after accretion (the condensation of a gas and dust cloud into a planet), its
brightness dropped from about one percent of the Sun's to about one
billionth -- a decline of ten million times.
In its present state Jupiter emits about twice as much heat as it
receives from the Sun. The loss of this heat -- residual energy left over
from the compressive heat of accretion -- means that Jupiter is cooling and
losing energy at a tremendously rapid rate. Temperatures in Jupiter's core,
which were about 90,000 degrees Fahrenheit in the planet's hot, early
phase, are now about 54,000 degrees Fahrenheit, 100 times hotter than any
terrestrial surface, but 500 times cooler than the temperature at the
center of the sun. Temperatures on Jupiter now range from 54,000 degrees
Fahrenheit at the core to minus 248 degrees Fahrenheit at the top of the
cloud banks.
Mainly uniform in composition, Jupiter's structure is determined by
gradations in temperature and pressure. Deep in Jupiter's interior there is
thought to be a small rocky core, comprising about four percent of the
planet's mass. This "small" core (about the size of 10 Earths) is surrounded
by a 25,000-mile-thick layer of liquid metallic hydrogen. (Metallic
hydrogen is liquid, but sufficiently compressed to behave as metal.)
Motions of this liquid "metal" are the source of the planet's enormous
magnetic field. This field is created by the same dynamo effect found in
the metallic cores of Earth and other planets.
At the outer limit of the metallic hydrogen layer, pressures equal three
million times that of Earth's atmosphere and the temperature has cooled to
19,000 degrees Fahrenheit.
Surrounding the central metallic hydrogen region is an outer shell of
"liquid" molecular hydrogen. Huge pressures compress Jupiter's gaseous
hydrogen until, at this level, it behaves like a liquid. The liquid hydrogen
layer extends upward for about 15,000 miles. Then it gradually becomes
gaseous. This transition region between liquid and gas marks, in a sense,
where the solid and liquid planet ends and its atmosphere begins.
From here, Jupiter's atmosphere extends up for 600 more miles, but only
in the top 50 miles are found the brilliant bands of clouds for which Jupiter
is known. The tops of these bands are colored bright yellow, red and orange
from traces of phosphorous and sulfur. Five or six of these bands,
counterflowing east and west, encircle the planet in each hemisphere. At
one point near Jupiter's equator, east winds of 220 mph blow right next to
west winds of 110 mph. At boundaries of these bands, rapid changes in
wind speed and direction create large areas of turbulence and shear. These
are the same forces that create tornados here on Earth. On Jupiter, these
"baroclinic instabilities" are major phenomena, creating chaotic, swirling
winds and spiral features such as White Ovals.
The brightest cloud banks, known as zones, are believed to be higher,
cooler areas where gases are ascending. The darker bands, called belts, are
thought to be warmer, cloudier regions of descent.
The top cloud layer consists of white cirrus clouds of ammonia crystals,
at a pressure six-tenths that of Earth's atmosphere at sea level (.6 bar).
Beneath this layer, at a pressure of about two Earth atmospheres (2 bars)
and a temperature of near minus 160 degrees Fahrenheit, a reddish-brown
cloud of ammonium hydrosulfide is predicted.
At a pressure of about 6 bars, there are believed to be clouds of water
and ice. However, recent Earth-based spectroscopic studies suggest that
there may be less water on Jupiter than expected. While scientists
previously believed Jupiter and the sun would have similar proportions of
water, recent work indicates there may be 100 times less water on Jupiter
than if it had a solar mixture of elements. If this is the case, there may be
only a thin layer of water-ice at the 6 bar level.
However, Jupiter's cloud structure, except for the highest layer of
ammonia crystals, remains uncertain. The height of the lower clouds is
still theoretical -- clouds are predicted to lie at the temperature levels
where their assumed constituents are expected to condense. The Galileo
probe will make the first direct observations of Jupiter's lower atmosphere
and clouds, providing crucial information.
The forces driving Jupiter's fast-moving winds are not well understood
yet. The classical explanation holds that strong currents are created by
convection of heat from Jupiter's hot interior to the cooler polar regions,
much as winds and ocean currents are driven on Earth, from equator to
poles. But temperature differences do not fully explain wind velocities
that can reach 265 mph. An alternative theory is that pressure differences,
due to changes in the thermodynamic state of hydrogen at high and low
temperatures, set up the wind jets.
Jupiter's rapid rotation rate is thought to have effects on wind velocity
and to produce some of Jupiter's bizarre circulation patterns, including
many spiral features. These rotational effects are known as
manifestations of the Coriolis force. Coriolis force is what determines the
spin direction of weather systems. It basically means that on the surface
of a sphere (a planet), a parcel of gas farther from the poles has a higher
rotational velocity around the planet than a parcel closer to the poles. As
gases then move north or south, interacting parcels with different
velocities produce vortices (whirlpools). This may account for some of
Jupiter's circular surface features.
Jupiter spins faster than any planet in the solar system. Though 11 times
Earth's diameter, Jupiter spins more than twice as fast (once in 10 hours),
giving gases on the surface extremely high rates of travel -- 22,000 mph at
the equator, compared with 1000 mph for air at Earth's equator. Jupiter's
rapid spin also causes this gas and liquid planet to flatten markedly at the
poles and bulge at the equator.
Visible at the top of Jupiter's atmosphere are eye-catching features
such as the famous Great Red Spot and the exotic White Ovals, Brown
Barges and White Plumes. The Great Red Spot, which is 25,000 miles wide
and large enough to swallow three Earths, is an enormous oval eddy of
swirling gases. It is driven by two counter-flowing jet streams, which
pass, one on each side of it, moving in opposite directions, each with speeds
of 100-200 mph. The Great Red Spot was first discovered in 1664, by the
British scientist Roger Hook, using Galileo's telescope. In the three
centuries since, the huge vortex has remained constant in latitude in
Jupiter's southern equatorial belt. Because of its stable position,
astronomers once thought it might be a volcano.
Another past theory compared the Great Red Spot to a gigantic hurricane.
However, the GRS rotates anti-cyclonically while hurricanes are cyclonic
features (counterclockwise in the northern hemisphere, clockwise in the
southern) -- and the dynamics of the Great Red Spot appear unrelated to
moisture.
The Great Red Spot most closely resembles an enormous tornado, a huge
vortex that sucks in smaller vortices. The Coriolis effect created by
Jupiter's fast spin, appears to be the key to the dynamics that drive the
spot.
The source of the Great Red Spot's color remains a mystery. Many
scientists now believe it to be caused by phosphorus, but its spectral line
does not quite match that of phosphorus. The GRS may be the largest in a
whole array of spiral phenomena with similar dynamics. About a dozen
white ovals, circulation patterns resembling the GRS, exist in the southern
latitudes of Jupiter and appear to be driven by the same forces. Scientists
do not know why these ovals are white.
Scientists believe the brown barges, which appear like dark patches on
the planet, are holes in the upper clouds, through which the reddish-brown
lower cloud layer may be glimpsed. The equatorial plumes, or white
plumes, may be a type of wispy cirrus anvil cloud.
GALILEO MANAGEMENT
The Galileo Project is managed for NASA's Office of Space Science and
Applications by the NASA Jet Propulsion Laboratory, Pasadena, Calif. This
responsibility includes designing, building, testing, operating and tracking
Galileo. NASA's Ames Research Center, Moffett Field, Calif. is responsible
for the atmosphere probe, which was built by Hughes Aircraft Company, El
Segundo, Calif.
The probe project and science teams will be stationed at Ames during
pre-mission, mission operations, and data reduction periods. Team
members will be at Jet Propulsion Laboratory for probe entry.
The Federal Republic of Germany has furnished the orbiter's
retropropulsion module and is participating in the scientific investigations.
The radioisotope thermoelectric generators were designed and built for the
U.S. Department of Energy by the General Electric Company.
STS-34 INERTIAL UPPER STAGE (IUS-19)
The Inertial Upper Stage (IUS) will again be used with the Space Shuttle,
this time to transport NASA's Galileo spacecraft out of Earth's orbit to
Jupiter, a 2.5-billion-mile journey.
The IUS has been used previously to place three Tracking and Data Relay
Satellites in geostationary orbit as well as to inject the Magellan
spacecraft into its interplanetary trajectory to Venus. In addition, the IUS
has been selected by the agency for the Ulysses solar polar orbit mission.
After 2 1/2 years of competition, Boeing Aerospace Co., Seattle, was
selected in August 1976 to begin preliminary design of the IUS. The IUS
was developed and built under contract to the Air Force Systems Command's
Space Systems Division. The Space Systems Division is executive agent for
all Department of Defense activities pertaining to the Space Shuttle
system. NASA, through the Marshall Space Flight Center, Huntsville, Ala.,
purchases the IUS through the Air Force and manages the integration
activities of the upper stage to NASA spacecraft.
Specifications
IUS-19, to be used on mission STS-34, is a two-stage vehicle weighing
approximately 32,500 lbs. Each stage has a solid rocket motor (SRM),
preferred over liquid-fueled engines because of SRM's relative simplicity,
high reliability, low cost and safety.
The IUS is 17 ft. long and 9.25 ft. in diameter. It consists of an aft
skirt, an aft stage SRM generating approximately 42,000 lbs. of thrust, an
interstage, a forward-stage SRM generating approximately 18,000 lbs. of
thrust, and an equipment support section.
Airborne Support Equipment
The IUS Airborne Support Equipment (ASE) is the mechanical, avionics
and structural equipment located in the orbiter. The ASE supports the IUS
and the Galileo in the orbiter payload bay and elevates the combination for
final checkout and deployment from the orbiter.
The IUS ASE consists of the structure, electromechanical mechanisms,
batteries, electronics and cabling to support the Galileo/IUS. These ASE
subsystems enable the deployment of the combined vehicle; provide,
distribute and/or control electrical power to the IUS and spacecraft;
provide plumbing to cool the radioisotope thermoelectric generator (RTG)
aboard Galileo; and serve as communication paths between the IUS and/or
spacecraft and the orbiter.
IUS Structure
The IUS structure is capable of supporting loads generated internally and
also by the cantilevered spacecraft during orbiter operations and the IUS
free flight. It is made of aluminum skin-stringer construction, with
longerons and ring frames.
Equipment Support Section
The top of the equipment support section contains the spacecraft
interface mounting ring and electrical interface connector segment for
mating and integrating the spacecraft with the IUS. Thermal isolation is
provided by a multilayer insulation blanket across the interface between
the IUS and Galileo.
The equipment support section also contains the avionics which provide
guidance, navigation, control, telemetry, command and data management,
reaction control and electrical power. All mission-critical components of
the avionics system, along with thrust vector actuators, reaction control
thrusters, motor igniter and pyrotechnic stage separation equipment are
redundant to assure reliability of better than 98 percent.
IUS Avionics Subsystems
The avionics subsystems consist of the telemetry, tracking and
command subsystems; guidance and navigation subsystem; data
management; thrust vector control; and electrical power subsystems.
These subsystems include all the electronic and electrical hardware used to
perform all computations, signal conditioning, data processing and
formatting associated with navigation, guidance, control, data and
redundancy management. The IUS avionics subsystems also provide the
equipment for communications between the orbiter and ground stations as
well as electrical power distribution.
Attitude control in response to guidance commands is provided by thrust
vectoring during powered flight and by reaction control thrusters while
coasting. Attitude is compared with guidance commands to generate error
signals. During solid motor firing, these commands gimble the IUS's
movable nozzle to provide the desired pitch and yaw control. The IUS's roll
axis thrusters maintain roll control. While coasting, the error signals are
processed in the computer to generate thruster commands to maintain the
vehicle's altitude or to maneuver the vehicle.
The IUS electrical power subsystem consists of avionics batteries, IUS
power distribution units, a power transfer unit, utility batteries, a
pyrotechnic switching unit, an IUS wiring harness and umbilical and staging
connectors. The IUS avionics system provides 5-volt electrical power to
the Galileo/IUS interface connector for use by the spacecraft telemetry
system.
IUS Solid Rocket Motors
The IUS two-stage vehicle uses a large solid rocket motor and a small
solid rocket motor. These motors employ movable nozzles for thrust vector
control. The nozzles provide up to 4 degrees of steering on the large motor
and 7 degrees on the small motor. The large motor is the longest-thrusting
duration SRM ever developed for space, with the capability to thrust as long
as 150 seconds. Mission requirements and constraints (such as weight) can
be met by tailoring the amount of propellant carried. The IUS-19
first-stage motor will carry 21,488 lb. of propellant; the second stage
6,067 lb.
Reaction Control System
The reaction control system controls the Galileo/IUS spacecraft attitude
during coasting, roll control during SRM thrustings, velocity impulses for
accurate orbit injection and the final collision-avoidance maneuver after
separation from the Galileo spacecraft.
As a minimum, the IUS includes one reaction control fuel tank with a
capacity of 120 lb. of hydrazine. Production options are available to add a
second or third tank. However, IUS-19 will require only one tank.
IUS To Spacecraft Interfaces
Galileo is physically attached to the IUS at eight attachment points,
providing substantial load-carrying capability while minimizing the
transfer of heat across the connecting points. Power, command and data
transmission between the two are provided by several IUS interface
connectors. In addition, the IUS provides a multilayer insulation blanket of
aluminized Kapton with polyester net spacers across the Galileo/IUS
interface, along with an aluminized Beta cloth outer layer. All IUS thermal
blankets are vented toward and into the IUS cavity, which in turn is vented
to the orbiter payload bay. There is no gas flow between the spacecraft and
the IUS. The thermal blankets are grounded to the IUS structure to prevent
electrostatic charge buildup.
Flight Sequence
After the orbiter payload bay doors are opened in orbit, the orbiter will
maintain a preselected attitude to keep the payload within thermal
requirements and constraints.
On-orbit predeployment checkout begins, followed by an IUS command link
check and spacecraft communications command check. Orbiter trim
maneuvers are normally performed at this time.
Forward payload restraints will be released and the aft frame of the
airborne-support equipment will tilt the Galileo/IUS to 29 degrees. This
will extend the payload into space just outside the orbiter payload bay,
allowing direct communication with Earth during systems checkout. The
orbiter then will be maneuvered to the deployment attitude. If a problem
has developed within the spacecraft or IUS, the IUS and its payload can be
restowed.
Prior to deployment, the spacecraft electrical power source will be
switched from orbiter power to IUS internal power by the orbiter flight
crew. After verifying that the spacecraft is on IUS internal power and that
all Galileo/IUS predeployment operations have been successfully completed,
a GO/NO-GO decision for deployment will be sent to the crew from ground
support.
When the orbiter flight crew is given a "Go" decision, they will activate
the ordnance that separates the spacecraft's umbilical cables. The crew
then will command the electromechanical tilt actuator to raise the tilt
table to a 58-degree deployment position. The orbiter's RCS thrusters will
be inhibited and an ordnance-separation device initiated to physically
separate the IUS/spacecraft combination from the tilt table.
Six hours, 20 minutes into the mission, compressed springs provide the
force to jettison the IUS/Galileo from the orbiter payload bay at
approximately 6 inches per second. The deployment is normally performed
in the shadow of the orbiter or in Earth eclipse.
The tilt table then will be lowered to minus 6 degrees after IUS and its
spacecraft are deployed. A small orbiter maneuver is made to back away
from IUS/Galileo. Approximately 15 minutes after deployment, the
orbiter's OMS engines will be ignited to move the orbiter away from its
released payload.
After deployment, the IUS/Galileo is controlled by the IUS onboard
computers. Approximately 10 minutes after IUS/Galileo deployment from
the orbiter, the IUS onboard computer will send out signals used by the IUS
and/or Galileo to begin mission sequence events. This signal will also
enable the IUS reaction control system. All subsequent operations will be
sequenced by the IUS computer, from transfer orbit injection through
spacecraft separation and IUS deactivation.
After the RCS has been activated, the IUS will maneuver to the required
thermal attitude and perform any required spacecraft thermal control
maneuvers.
At approximately 45 minutes after deployment from the orbiter, the
ordnance inhibits for the first SRM will be removed. The belly of the
orbiter already will have been oriented towards the IUS/Galileo to protect
orbiter windows from the IUS's plume. The IUS will recompute the first
ignition time and maneuvers necessary to attain the proper attitude for the
first thrusting period. When the proper transfer orbit opportunity is
reached, the IUS computer will send the signal to ignite the first stage
motor 60 minutes after deployment. After firing approximately 150
seconds, the IUS first stage will have expended its propellant and will be
separated from the IUS second stage.
Approximately 140 seconds after first-stage burnout, the second- stage
motor will be ignited, thrusting about 108 seconds. The IUS second stage
then will separate and perform a final collision/contamination avoidance
maneuver before deactivating.
SHUTTLE SOLAR BACKSCATTER ULTRAVIOLET INSTRUMENT
The Shuttle Solar Backscatter Ultraviolet (SSBUV) instrument was
developed by NASA to calibrate similar ozone measuring space-based
instruments on the National Oceanic and Atmospheric Administration's
TIROS satellites (NOAA-9 and -11).
The SSBUV will help scientists solve the problem of data reliability
caused by calibration drift of solar backscatter ultraviolet (SBUV)
instruments on orbiting spacecraft. The SSBUV uses the Space Shuttle's
orbital flight path to assess instrument performance by directly comparing
data from identical instruments aboard the TIROS spacecraft, as the
Shuttle and the satellite pass over the same Earth location within a 1-hour
window. These orbital coincidences can occur 17 times per day.
The SBUV measures the amount and height distribution of ozone in the
upper atmosphere. It does this by measuring incident solar ultraviolet
radiation and ultraviolet radiation backscattered from the Earth's
atmosphere. The SBUV measures these parameters in 12 discrete
wavelength channels in the ultraviolet. Because ozone absorbs in the
ultraviolet, an ozone measurement can be derived from the ratio of
backscatter radiation at different wavelengths, providing an index of the
vertical distribution of ozone in the atmosphere.
Global concern over the depletion of the ozone layer has sparked
increased emphasis on developing and improving ozone measurement
methods and instruments. Accurate, reliable measurements from space are
critical to the detection of ozone trends and for assessing the potential
effects and development of corrective measures.
The SSBUV missions are so important to the support of Earth science
that six additional missions have been added to the Shuttle manifest for
calibrating ozone instruments on future TIROS satellites. In addition, the
dates of the four previously manifested SSBUV flights have been
accelerated.
The SSBUV instrument and its dedicated electronics, power, data and
command systems are mounted in the Shuttle's payload bay in two Get Away
Special canisters, an instrument canister and a support canister. Together,
they weigh approximately 1200 lb. The instrument canister holds the
SSBUV, its specially designed aspect sensors and in-flight calibration
system. A motorized door assembly opens the canister to allow the SSBUV
to view the sun and Earth and closes during the in-flight calibration
sequence.
The support canister contains the power system, data storage and
command decoders. The dedicated power system can operate the SSBUV for
a total of approximately 40 hours.
The SSBUV is managed by NASA's Goddard Space Flight Center, Greenbelt,
Md. Ernest Hilsenrath is the principal investigator.
GROWTH HORMONE CONCENTRATIONS
AND DISTRIBUTION IN PLANTS
The Growth Hormone Concentration and Distribution in Plants (GHCD)
experiment is designed to determine the effects of microgravity on the
concentration, turnover properties, and behavior of the plant growth