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On Mars:
Exploration of the Red Planet. 1958-1978
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- MARINER MARS 69
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- [156] Born in the winter of 1965, Mariner
Mars 69 was supposed to be only a modest improvement over
Mariner 4 . Early plans for a 1969 orbiter and hard-lander
mission had been scrapped, and in its place a flyby craft had been
substituted that would approach Mars at a distance of about 3200
kilometers, rather than the 13 800-kilometer pass made by
Mariner 4 in 1965. 2 The 1969 spacecraft would also carry more weight
(384 kilograms) than earlier Mariners (Mariner 2 -203 kg.
Mariner 4 -261 kg), because of the performance capability of
its Atlas-Centaur launch vehicle. (Detailed information on the
Mariner flights is given in Appendix C.) Building on Project Ranger and Project Mariner
experience, JPL engineers borrowed a number of fundamental mission
and systems features for use with Mariner Mars 69. The most
important of these was three-axis stabilization (roll, pitch, and
yaw), provided by gyroscopes and celestial sensors, switching
amplifiers, and cold-gas jets. This attitude control system
permitted orientation [157] of the solar panels and thermal
shields, which provided temperature control, relative to the sun.
The high-gain communications antenna could be aimed toward Earth
to improve communications, and the scientific instruments could be
directed toward the objects of their study. The attitude control
system also permitted the craft to be maneuvered more precisely.
3 Other characteristics of the Mariner spacecraft
included an extensive ground command capability and a large number
of engineering and scientific telemetry measurements. The ground
command capability was used primarily as a backup to the onboard
central sequencer, a mini-computer that also reacted to commands
from Earth.
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- Mariner Mars 69 followed the general
design pattern of Mariner 4
. The central body was octagonal
with a magnesium framework (127-centimeter diagonal, 46-centimeter
depth). with electronic assemblies and onboard propulsion system
fitted into the equipment bays on all sides. Four hinged solar
panels radiated from the body. On the side of the spacecraft
opposite the solar panels was a platform for mounting the
television camera, an infrared radiometer, an ultraviolet
spectrometer, and an infrared spectrometer. The omnidirectional
antenna and the fixed, high-gain, reflector antenna were attached
on the side generally oriented toward the sun. Ground stations
could communicate with the spacecraft continuously for tracking
and the return of scientific data. Images would be stored by an
onboard tape recorder for relay to Earth at a reduced play-back
rate, since the cameras necessarily acquired imaging data at a
rate much higher than the telemetry channel could
accommodate.
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- As they worked on early Mariner and Ranger
spacecraft, specialists at JPL had also evolved systems for
tracking and controlling spacecraft from Earth, recognizing the
requirement for a highly sensitive, steerable antenna (radio
telescope) for communication with deep space probes. For
continuous long-range coverage, a network of three stations, about
equidistant in longitude, was normally sufficient. The first
stations were at Goldstone, California; Johannesburg, South
Africa; and Woomera, Australia. By the time Mariner 69 was ready
to fly, there were eight 26-meter radio antennas and one 64-meter
antenna in the Deep Space Network. Signals from the Space Flight
Operations Facility at JPL were directed to the spacecraft by the
appropriate ground station. 4
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- As first established, Mariner Mars 69 had
three objectives. The primary goal was to fly spacecraft by Mars
to investigate that planet, establishing the basis for future
experiments, especially those related to the search for
extraterrestrial life. While exploiting existing technology,
Mariner 69 engineers also hoped to develop new technology
necessary for future missions. A tentatively approved objective to
investigate certain aspects of the solar system was dropped from
consideration by NASA Headquarters managers in April 1966. Mariner
69 would concentrate its efforts on Mars-related science.
Experiment proposals were solicited and received by the Space
Science Board, which acted as an advisory body to the NASA Office
of Space Science [158] and Applications. As had been proposed
several times before, an atmospheric entry probe was suggested,
but it was also rejected as before, because it would have
significantly increased both the time required to develop the
craft and the budget for the project. Scientific payload selection
was announced on 26 May 1966.
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- By mid-1966, the design of the mission and
the spacecraft was well under way. Money was the problem faced by
N. William Cunningham, program manager at headquarters, and Harris
M. Schurmeier, project manager at JPL, and their Mariner 69 team.
Successive budget cuts each fiscal year forced the team to defer
delivery of certain parts and components, which repeatedly
required the engineers to reschedule the assembly and testing of
the spacecraft. The budget reductions also forced the deletion of
some spare parts and tests and led to several mission design
changes. Despite financial constraints, the Mariner project staff
was able to expand the scope and effectiveness of the spacecraft.
An increase in mission science, for example, affected the
planetary encounter phase of the mission. JPL specialists
developed an improved telemetry transmission system that would
return information at a higher rate than previously possible,
increasing the overall volume of scientific return substantially.
Since scientists would be using their instruments more frequently,
the central control computer and sequencer through which ground
controllers talked to the science instruments and manipulated the
instrument scan platform would experience greater demand.
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- As early as September 1966 at the second
project quarterly review, it became apparent that the 1969 mission
was going to be much more than just a repeat of the
Mariner 4 flight. The instrument scan platform alone had
grown in weight from 9 kilograms to 59. Throughout 1967 and 1968,
as work progressed on the spacecraft and Earth-based systems,
Schurmeier reported to NASA Headquarters that experimenters would
be able to take more pictures of the Martian surface with the
Mariner 69 equipment than previously anticipated. The accumulated
improvements in telecommunications-increased telemetry data rates,
expanded communications network, and better computer
processing-would lead to a rate of data transmission 2000 times
better than anything they had received before.
5 For the scientists associated with the television
experiment, this was exciting news. Instead of taking only 8
television pictures during the last day of the spacecraft's
approach to Mars, Robert B. Leighton and his colleagues on the
television experiment team could gather some 160 images, starting
two or three days before encounter with the planet. These approach
pictures of the entire planet would bridge the gap between photos
taken from Earth and closer images gathered by Mariner 69 craft as
they passed by Mars. 6
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- Engineers and technicians at JPL assembled
components supplied by about a dozen subcontractors into four
spacecraft-a proof-test model (PTM), two flight craft (M69-3 and
M69-4), and one assembled set of spares (M69-2). While the
proof-test model would never fly, it was a very important [159]
part of the 1969 project because it had to endure simulated
conditions worse than any that were expected during the flight to
Mars. The other three units were tested more gently on the
vibration table to rehearse the launch and in the thermal-vacuum
space-simulation chamber to practice the mission through deep
space.
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- Following several visits to the test bench
and much rebuilding and repairing, the craft were pronounced ready
for their voyage. While the proof-test model remained behind in
Pasadena to continue its service as a test article, the other
three craft were sent to the Kennedy Space Center during December
1968 and January 1969. All went well with the preflight checks of
Mariner F and Mariner G (preflight designations) until about 10
days before the scheduled launch. On 14 February while the
Atlas-Centaur- Mariner F vehicle was standing on the pad
undergoing unfueled simulation of launch, the Atlas began to
collapse like a punctured tire. Most of the structural strength of
the Atlas is provided by the pressure in its fuel tanks. While
this balloon-like structure saves a great deal of weight, it means
that the pressure must be maintained at a constant level. On this
day, a faulty relay switch had opened the main valves, permitting
the pressurizing gases to escape. As the Atlas began to sag on its
launch tower, two alert ground crewmen sprinted to the scene and
shut off manual valves inside the launch vehicle. Pumps restored
tank pressure, and the big rocket resumed its original shape. The
terrible scar in the thin stainless steel skin of the Atlas made
it clear, however, that another launch vehicle would have to be
used in its place.
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- The Centaur and Mariner components were
unharmed, and on 18 February KSC personnel moved the Mariner F
craft and the Centaur upper stage to the Atlas originally
scheduled for Mariner G. Six days later, 24 February,
Mariner 6 began its journey to Mars. After being mated to a
new Atlas shipped from San Diego by General Dynamics/Convair, the
second Mariner 69 craft was launched on 27 March.
7 As Mariner 6
and
7 were en route, another group of
JPL specialists was at work preparing for the next mission to
Mars.
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