Mars Observer was launched September 25, 1992 from Cape Canaveral
on a Titan III built by Martin Marietta Corporation, with an
upper Transfer Orbit Stage from Orbital Sciences Corporation.
Flight controllers lost contact with the spacecraft on August
21, 1993, effectively terminating the mission.
For most Mars Observer experiments, data were to be collected by
instruments on the spacecraft. Those data were then to be
relayed via the telemetry system to stations of the NASA Deep
Space Network (DSN) on the ground. Radio Science experiments
(such as radio tracking of the spacecraft and bistatic radio
scattering experiments) required that DSN hardware also
participate in data acquisition. The following sections provide
an overview first of the spacecraft and then of the DSN ground
system as both supported Mars Observer science activities.
Instrument Host Overview - Spacecraft
=====================================
The Mars Observer spacecraft provided a stabilized,
nadir-oriented platform for continuous observations of Mars by
an advanced set of science instruments.However, communication
with the spacecraft was lost three days before it was scheduled
to enter Mars orbit in late August 1993. Only data from
Earth-Mars cruise and a very small amount of distant-encounter
Mars data were obtained from Mars Observer.
The spacecraft contract was originally won by the RCA
Astro-Space Division (ASD), which subsequently became the
General Electric Astro-Space Division when the two companies
merged. The selection of RCA was intended to make maximum use
of existing designs and technologies from DMSP/TIROS weather
satellites and Satcom-K communication satellites. General
Electric sold ASD to Martin Marietta Corporation at about the
time Mars Observer was launched; Martin Marietta was the prime
contractor when the spacecraft was lost.
The dimensions of the rectangular bus were 2.1 x 1.5 x 1.1 m in
the x-y-z dimensions. When fully deployed, the six-panel solar
array was 7.0 x 3.7 meters, and would have developed over a
kilowatt of power at Mars. The mass of the spacecraft was
about 1028 kg, including 166 kg of payload but excluding the
approximately 1346 kg of propellant required, most of which was
intended for orbit insertion and orbit circularization at Mars.
The propellant also included 63 kg of hydrazine, which would
have been used for spacecraft control during mapping.
At launch (25 September 1992), the solar panels and high-gain
antenna were folded against the rectangular bus and the two
science booms (Magnetometer and Gamma-Ray Spectrometer) were
retracted. During early cruise the partially-deployed
spacecraft was stabilized in a controlled 0.01-rpm roll about
the y-axis; communications were conducted with Earth via a
low-gain antenna. For trajectory correction and orbit
insertion maneuvers the spacecraft was oriented under three-
axis control and then returned to cruise attitude.
In its mapping orbit, Mars Observer was to be controlled in
three axes, using its horizon sensors to point the science
instruments on the +Z face toward the nadir. The six-panel
solar-array was to be fully deployed on a boom to track the Sun
during each orbit, and the high-gain antenna boom was to be
fully deployed to track the Earth around each orbit.
Spacecraft pointing control was provided by four reaction
wheels. Attitude information during mapping was to be provided
by a Mars horizon sensor that defined the nadir direction, a
star-mapper for inertial attitude, gyros and accelerometers for
measuring angular rates and linear accelerations, and multiple
sun sensors. The spacecraft was required to maintain adequate
pointing control and to provide sufficient telemetry to allow
reconstruction of the pointing after each orbit. The telemetry
stream provided data from the sensors sufficient to
characterize nadir and high-gain antenna pointing to within +/-
3 mrad (per axis, 3 sigma) and boom-mounted science instrument
pointing to within +/- 25 mrad (per axis, 3 sigma).
Two independent propulsion systems were provided. All major
maneuvers, both in cruise and during orbit insertion, were to
be accomplished by a hypergolic, bipropellant system. A
hydrazine system was available for orbit trim maneuvers during
the mapping period and some attitude control functions,
including unloading of the momentum wheels. The hydrazine
thrusters were chosen to minimize contamination of the
instruments during the mapping period.
Most instruments were rigidly mounted on a nadir-pointing
spacecraft panel and, in general, provided simultaneous views
of the same nadir area. No movable scan platform was provided;
the spacecraft was to be continuously pointed toward nadir,
rotating at the orbital rate. Those instruments that required
scanning or multiple fields of view were constructed with
internal scanning mechanisms. The Gamma Ray Spectrometer and
Magnetometer sensor assemblies were mounted on individual booms
on the spacecraft. The steerable high-gain antenna (used for
radio science) was mounted on a third boom.
For more information regarding the Mars Observer spacecraft,
see [ALBEE&PALLUCONI1990]
Instrument Host Overview - DSN
==============================
The Mars Observer Radio Science investigations utilized
instrumentation with elements both on the spacecraft and at the
NASA Deep Space Network (DSN). Much of this is shared
equipment, being used for routine telecommunications as well as
for Radio Science.
The Deep Space Network is a telecommunications facility managed
by the Jet Propulsion Laboratory of the California Institute of
Technology for the U.S. National Aeronautics and Space
Administration.
The primary function of the DSN is to provide two-way
communications between the Earth and spacecraft exploring the
solar system. To carry out this function the DSN is equipped
with high-power transmitters, low-noise amplifiers and
receivers, and appropriate monitoring and control systems.
The DSN consists of three complexes situated at approximately
equally spaced longitudinal intervals around the globe at
Goldstone (near Barstow, California), Robledo (near Madrid,
Spain), and Tidbinbilla (near Canberra, Australia). Two of the
complexes are located in the northern hemisphere while the
third is in the southern hemisphere.
The network comprises four subnets, each of which includes one
antenna at each complex. The four subnets are defined
according to the properties of their respective antennas: 70-m
diameter, standard 34-m diameter, high-efficiency 34-m
diameter, and 26-m diameter.
These DSN complexes, in conjunction with telecommunications
subsystems onboard planetary spacecraft, constitute the major
elements of instrumentation for radio science investigations.
For more information see [ASMAR&RENZETTI1993].