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GRO, which weighs just over 35,000 pounds (15,876 kilograms),
will be the heaviest NASA science satellite ever deployed by the
Space Shuttle into low-Earth orbit.
GRO is a space-based observatory designed to study the universe
in an invisible, high-energy form of light known as gamma rays.
Although a variety of smaller satellites and high-altitude balloons
have carried instruments to study the universe in gamma-ray light
during the past 30 years, GRO represents a dramatic improvement in
sensitivity, spectral range and resolution.
Gamma-rays, which cannot penetrate the EarthUs atmosphere, are
of interest to scientists because these rays provide a reliable
record of cosmic change and evolution. Their study will yield
unprecedented answers about the structure and dynamics of the Milky
Way Galaxy, the nature of pulsars, quasars, black holes and neutron
stars, as well as clues about the origin and history of the universe
itself.
The four instruments on GRO were selected by NASA to provide
the first comprehensive, coordinated observations of a broad
gamma-ray energy range with much better sensitivity than any
previous mission. The instruments include: the Burst and Transient
Source Experiment (BATSE), the Oriented Scintillation Spectrometer
Experiment (OSSE), the Imaging Compton Telescope (COMPTEL) and the
Energetic Gamma Ray Experiment Telescope (EGRET). During the first
15 months of the mission, an all-sky survey is planned. The
observing program that follows will be guided by the results of this
survey.
The instruments onboard GRO, with sensitivities 10 times
greater than that of earlier instruments, will scan active galaxies
for new information on celestial objects. GRO also can detect the
very high temperature emissions from the vicinity of stellar black
holes, thereby providing evidence for the existence of these exotic
objects. GRO observations of diffuse radiation will not only help
resolve questions relating to the large scale distribution of matter
in the universe, but also about the processes that may have taken
place shortly after the universe began in the theoretical energetic
explosion or "Big Bang.S
GRO is a NASA cooperative program. The Federal Republic of
Germany, with co-investigator support from The Netherlands, the
European Space Agency, the United Kingdom and the United States, has
principal investigator responsibility for COMPTEL. The Federal
Republic of Germany also is furnishing hardware elements and
co-principal investigator support for EGRET.
GAMMA RAY OBSERVATORY SUBSYSTEMS
The Gamma Ray Observatory is the first scientific payload with
a refuelable onboard propulsion system. In addition, GRO provides
the support and protection necessary for the observatory to complete
its mission. The spacecraftUs subsystems include propulsion, power,
controls, electronics, communications and thermal.
Propulsion
The Gamma Ray Observatory has a self-contained propulsion
system that will allow controllers on the ground to keep the GRO
spacecraft at the proper altitude. The propulsion system provides
thrust for orbit altitude change, orbit maintenance, attitude
control and if necessary, controlled reentry. GRO's four propellent
tanks hold 4,200 pounds (1900-kilograms) of hydrazine fuel. The
spacecraft has four 100-pound (45-kilogram) thrusters and isolation
valves. GRO also has four dual thruster modules, each consisting of
two 5-pound (2.2-kilogram) thrusters for attitude control. The fuel
tanks are designed to be refueled by a future Space Shuttle mission,
although no mission is currently planned for this purpose.
Attitude Control and Determination System
The primary purpose of the Attitude Control and Determination
(ACAD) subsystem is to point the GRO instruments to selected
celestial gamma-ray sources and to supply attitude information for
data processing. The ACAD subsystem is a three-axis system made up
of many NASA standard components and other flight-proven hardware.
The system contains sensors that tell GRO where it's pointed and
actuators for vehicle orientation. The primary sensors are the
Fixed-Head Star Trackers and the Inertial Reference Unit. The star
trackers relay information to GRO's onboard computers about the
location of the spacecraft based on the known positions of
pre-programmed guide stars. The Inertial Reference Unit relays
attitude and position information based on the forces of inertia
working in much the same manner as a gyroscope. The primary
actuators are the four Reaction Wheel Assemblies. They rely on the
principle of the spinning flywheel to maintain spacecraft attitude.
Communications and Data Handling
The Communications and Data Handling (CADH) system is based on
the standard NASA modular design used with great success on the
Solar Maximum Mission and Landsats 4 and 5. By using modules,
repair of damaged or defective components is vastly simplified. The
CADH subsystem consists of the CADH module, a 60-inch (152
centimeter) high-gain antenna, two omnidirectional low-gain antennas
and a radio frequency combiner to interface the module with the
antennas.
The CADH includes two second generation Tracking and Data Relay
Satellite System (TDRSS) transponders for both incoming and outgoing
transmissions to TDRSS and for command and telemetry transmissions
to the Shuttle during in-bay and deployment sequences. Two NASA
standard tape recorders are included for data storage. They will be
used to record data for later playback to scientists on the ground.
These playbacks, or data dumps, take place every other orbit at a
rate of 512 kilobytes per second via the high-gain antenna system
and the TDRSS S-band.
GRO also has a sophisticated clock that converts spacecraft
time into universal time and distributes it to each instrument.
Remote Interface Units are distributed throughout the spacecraft to
interface the instruments with other onboard subsystems.
Electrical Power
The ObservatoryUs solar arrays are accordion style,
multi-panel, rigid arrays, deployed by motor-driven rigid booms.
The total power available for the observatory from the solar arrays
is approximately 2000 watts. Two Modular Power System (MPS) modules
condition, regulate and control solar-array power during sunlight
portions of the orbit to satisfy load demands and battery charging.
During eclipse periods, Nicad batteries supply the spacecraft power.
The batteries also supplement solar-array power during periods of
peak power. Each MPS can receive power from external sources during
ground operations and while in the Shuttle payload bay.
Thermal Subsystems
The thermal control of the observatoryUs subsystems and
instruments is accomplished by coatings, blankets, louvers,
radiators and heaters. The instruments are thermally isolated from
each other and the spacecraft structure to reduce temperature.
The COMPTEL instrument uses a heat pipe system that transfers
heat to a remote radiator providing active cooling for the
instrument. The other instruments have passive thermal designs.
GRO uses three types of heaters, each having redundant
thermostats and heater elements. Operational heater circuits are
adequate for normal orbital operations. Make-up heaters replace the
power of an instrument or component when it is turned off in orbit.
Space Shuttle auxiliary heaters are used to maintain temperatures
while GRO is in the payload bay.
GRO SCIENCE INSTRUMENTS
Gamma rays are a form of light that cannot penetrate the
Earth's atmosphere or be seen by the human eye. Gamma rays have the
highest energies of any type of light radiation. Since high-energy
processes tend to produce high-energy radiation, gamma rays are
emitted by some of the most exotic structures in our universe --
supernovae, neutron stars, black holes and quasars. The study of
gamma rays offers a window into the inner workings of these and
other fascinating objects, providing insights unattainable from the
study of any other form of radiation.
Although the four instruments on GRO are essentially telescopes
for seeing gamma-ray light, they do not look like ordinary
telescopes. Instead, the GRO instruments observe gamma rays
indirectly, by monitoring flashes of visible light, called
scintillations, that occur when gamma rays strike the detectors
(made of liquid or crystal materials) built into the instruments.
GRO's instruments are much larger and much more sensitive than
any gamma-ray instrument ever flown in space. Size is crucial for
gamma-ray astronomy. Because gamma rays are detected when they
interact with matter, the number of gamma-ray events recorded is
directly related to the mass of the detector. With the small number
of gamma rays emanating from celestial sources, large instruments
are needed to detect a significant number of photons in a reasonable
amount of time.
The gamma rays emitted from celestial objects span a wide range
of energies. The most energetic gamma rays to be studied by GRO
have energies some 1 million times greater than the weakest. This
is a far greater range in energy than that spanned by visible light,
and no single instrument yet devised can detect gamma rays
throughout this range. GRO's four instruments together span the
gamma-ray range from about 20,000 to 30 billion electron volts (eV).
Each of the four instruments has a unique design and is specialized
for particular types of observations.
Burst and Transient Source Experiment (BATSE)
The Burst and Transient Source Experiment (BATSE) was developed
by scientists and engineers at Marshall Space Flight Center,
Huntsville, Ala., to continuously monitor a large segment of the sky
for detection and measurement of short, intense bursts and other
transient sources of gamma rays. BATSE consists of 8 identical
detectors, with one detector located at each corner of the
spacecraft to give it a very wide field of view. BATSE works in the
low-energy part of the gamma-ray range (20,000 to 2 million eV) in
which bursts are expected. Once BATSE discovers a burst of gamma
rays, it can signal the other three instruments to study the source
in more detail. Dr. Gerald Fishman of Marshall is the principal
investigator.
Oriented Scintillation Spectrometer Experiment (OSSE)
The Naval Research Laboratory (NRL), Washington, D.C., designed
the Oriented Scintillation Spectrometer Experiment (OSSE) to detect
nuclear-line radiation and emissions associated with low energy
gamma-ray sources (100,000 to 10 million eV). OSSE is sensitive to
the spectral signature of radioactive elements. This enables OSSE
to study supernovae and novae which are believed to be the sites
where the heavy elements are created. These elements are the basis
for life as we know it. OSSE also will provide insight into various
types of science targets, such as neutron stars, black holes,
pulsars and quasars. Dr. James Kurfess of the NRL is the principal
investigator.
Imaging Compton Telescope (COMPTEL)
The Imaging Compton Telescope (COMPTEL), developed as a
cooperative effort by the Federal Republic of Germany, The
Netherlands, the European Space Agency and the United States, is
designed for observations at moderate gamma-ray energies (1 to 30
million eV). Because COMPTEL has a wide field of view (though not
as wide as BATSE) and can locate gamma ray sources, one of its
primary functions will be to produce a detailed map of the sky as
seen in moderate gamma rays. Dr. Volker Schoenfelder of the Max
Planck Institute, Germany, is the principal investigator.
Energetic Gamma Ray Experiment Telescope (EGRET)
The Energetic Gamma Ray Experiment Telescope (EGRET) is between
10 and 20 times larger and more sensitive than any high energy,
gamma-ray telescope previously flown in space. The mission of
EGRET, a joint effort by scientists and engineers at NASA's Goddard
Space Flight Center (GSFC), Greenbelt, Md.; Stanford University,
Stanford, Calif.; Max Planck Institute, Germany; and Grumman
Aerospace Corp., Bethpage, N.Y., is to search the cosmos for high
energy gamma-rays. One of its primary missions will be to generate
a map of the sky as seen in high-energy gamma rays, complementing
the map produced by COMPTEL. Another will be to discover and monitor
gamma-ray emissions from pulsars. GoddardUs Dr. Carl Fichtel is the
principal investigator.