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On Mars:
Exploration of the Red Planet. 1958-1978
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- VIKING LANDER: BUILDING A
COMPLEX SPACECRAFT
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- [243] The Viking lander represented a
careful melding of the demands imposed by the scientific mission
and the high degree of reliability required of the spacecraft
subsystems. Weight and volume considerations affected the size of
each subsystem. After the Voyager program with plans for an 11
500-kilogram spacecraft was abandoned in 1967, a follow-on study
concluded that a spacecraft weighing 3700 kilograms could he
transported to Mars by a Titan-Centaur-class launch vehicle. The
lander and its flight capsule would account for more than a third
of this weight (1195 kilograms). At the start of the mission, the
orbiter and lander would be housed in a 4.3-meter shroud atop the
Titan-Centaur. The landed spacecraft would be 3 meters at its
widest point and 2 meters tall from the footpads to the tip of the
large disk S-band high-gain antenna. While weight and volume
limitations helped to shape the Viking lander, data about Martian
atmospheric pressure obtained during the Mariner 69 mission were
also influential.
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- Mariner 69's occultation experiment
indicated that the atmospheric pressure at the surface of Mars
ranged from 4 to 20 millibars, rather than 80 millibars as
estimated earlier. This information had a definite impact on the
aerodynamic shape of the Mars entry vehicle being designed, since
weight and diameter would influence the craft's braking ability.
Langley engineers had determined that aerodynamic braking was the
only practical method for slowing down a lander as large as Viking
for a soft touchdown. The entry vehicle would have a diameter of
3.5 meters, an acceptable ballistic coefficient that would help
ensure Viking's safe landing on Mars.
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- Since electrical power requirements were
thought of in terms of the weight that the power apparatus would
add to the spacecraft, the design engineers sought creative means
for getting maximum results from a minimum amount of power.
Low-power integrated circuits were used extensively both to
conserve energy and to keep the package small. In addition, power
switching techniques were devised to reduce energy requirements.
As John D. Goodlette, deputy project director at Martin Marietta,
noted, the design rule was "turn off unneeded consumers."
1 When power had to be used, the equipment was
designed with multiple power levels, or states, so [244] that
only the minimum power required to achieve the immediate function
would he consumed.
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- Once separated from the orbiter with its
700-watt solar panels, only 70 watts of
radioisotope-thermoelectric-generated power would support the long
mission on the surface. Because of this limitation on landed
power, the radio transmitters could be used only sparingly, a
factor that in turn controlled the amount of data that could be
sent to Earth.
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- The Viking lander was a highly automated
spacecraft for a number of reasons. Since there was only a
20-minute one-way communications opportunity between Earth and
Mars during the landings, control of the lander from Earth from
separation to touchdown was not practical. The entire function of
navigation-from obtaining an inertial reference to locating a
local surface reference-had to be accomplished by the onboard
computer. After landing, the spacecraft would be out of direct
communication with Earth for about half of each Martian day. And
because of electrical power limits, the communications between
lander and mission control in California would amount to only a
short time each day. The lander, therefore, had to be capable of
carrying out its mission unattended by Earth. Mission specialists
could send the lander new assignments or modify preprogrammed
ones, but for the most part the craft was on its own as it did its
day-to-day work.
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