Suzy B Software 2/Suzy B Software CD-ROM 2 (1994).iso/nasa/provikng/provikng.txt

Text File | 1995-05-02 | 10.9 KB | 190 lines

FACT SHEET: PROJECT VIKING

Viking was the culmination of a series of missions to explore the planet Mars;
they began in 1964 with Mariner 4, and continued with the Mariner 6 and 7
flybys in 1969, and the Mariner 9 orbital mission in 1971 and 1972.

Viking was designed to orbit Mars and to land and operate on the planet's
surface. Two identical spacecraft, each consisting of a lander and an orbiter,
were built.

NASA's Langley Research Center in Hampton, Virginia, had management
responsibility for the Viking project from its inception in 1968 until April 1,
1978, when the Jet Propulsion Laboratory assumed the task. Martin Marietta
Aerospace in Denver, Colorado, developed the landers. NASA's Lewis Research
Center in Cleveland, Ohio, had responsibility for the Titan-Centaur launch
vehicles. JPL's initial assignment was development of the orbiters, tracking
and data acquisition, and the Mission Control and Computing Center.

NASA launched both spacecraft from Cape Canaveral, Florida -- Viking 1 on
August 20, 1975, and Viking 2 on September 9, 1975. The landers were sterilized
before launch to prevent contamination of Mars with organisms from Earth. The
spacecraft spent nearly a year cruising to Mars. Viking 1 reached Mars orbit
June 19, 1976; Viking 2 began orbiting Mars August 7, 1976.

After studying orbiter photos, the Viking site certification team considered
the original landing site for Viking 1 unsafe. The team examined nearby sites,
and Viking 1 landed on Mars July 20, 1976, on the western slope of Chryse
Planitia (the Plains of Gold) at 22.3 degrees N latitude, 48.0 degrees
longitude.

The site certification team also decided the planned landing site for Viking 2
was unsafe after it examined high-resolution photos. Certification of a new
landing site took place in time for a Mars landing September 3, 1976, at Utopia
Planitia, at 47.7 degrees N latitude, and 48.0 degrees longitude.

The Viking mission was planned to continue for 90 days after landing. Each
orbiter and lander operated far beyond its design lifetime. Viking Orbiter 1
exceeded four years of active flight operations in Mars orbit.

The Viking project's primary mission ended November 15, 1976, 11 days before
Mars's superior conjunction (its passage behind the Sun). After conjunction, in
mid-December 1976, controllers reestablished telemetry and command operations,
and began extended-mission operations.

The first spacecraft to cease functioning was Viking Orbiter 2 on July 25,
1978; the spacecraft had used all the gas in its attitude-control system, which
kept the craft's solar panels pointed at the Sun to power the orbiter. When the
spacecraft drifted off the Sun line, the controllers at JPL sent commands to
shut off power to Viking Orbiter 2's transmitter.

Viking Orbiter 1 began to run short of attitude-control gas in 1978, but
through careful planning to conserve the remaining supply, engineers found it
possible to continue acquiring science data at a reduced level for another two
years. The gas supply was finally exhausted and Viking Orbiter 1's electrical
power was commanded off on August 7, 1980, after 1,489 orbits of Mars.

The last data from Viking Lander 2 arrived at Earth on April 11, 1980. Lander 1
made its final transmission to Earth Nov. 11, 1982. Controllers at JPL tried
unsuccessfully for another six and one-half months to regain contact with
Viking Lander 1. The overall mission came to an end May 21, 1983.

With a single exception -- the seismic instruments --the science instruments
acquired more data than expected. The seismometer on Viking Lander 1 would not
work after landing, and the seismometer on Viking Lander 2 detected only one
event that may have been seismic. Nevertheless, it provided data on wind
velocity at the landing site to supplement information from the meteorology
experiment, and showed that Mars has very low seismic background.

The three biology experiments discovered unexpected and enigmatic chemical
activity in the Martian soil, but provided no clear evidence for the presence
of living microorganisms in soil near the landing sites. According to mission
biologists, Mars is self-sterilizing. They believe the combination of solar
ultraviolet radiation that saturates the surface, the extreme dryness of the
soil and the oxidizing nature of the soil chemistry prevent the formation of
living organisms in the Martian soil. The question of life on Mars at some time
in the distant past remains open.

The landers' gas chromatograph/mass spectrometer instruments found no sign of
organic chemistry at either landing site, but they did provide a precise and
definitive analysis of the composition of the Martian atmosphere and found
previously undetected trace elements. The X-ray fluorescence spectrometers
measured elemental composition of the Martian soil.

Viking measured physical and magnetic properties of the soil. As the landers
descended toward the surface they also measured composition and physical
properties of the Martian upper atmosphere.

The two landers continuously monitored weather at the landing sites. Weather in
the Martian midsummer was repetitious, but in other seasons it became variable
and more interesting. Cyclic variations appeared in weather patterns (probably
the passage of alternating cyclones and anticyclones). Atmospheric temperatures
at the southern landing site (Viking Lander 1) were as high as -14 degrees C
(+7 degrees F) at midday, and the predawn summer temperature was -77 degrees C
(-107 F). In contrast, the diurnal temperatures at the northern landing site
(Viking Lander 2) during midwinter dust storms varied as little as 4 degrees C
(7 degrees F) on some days. The lowest predawn temperature was -120 degrees C
(-184 F), about the frost point of carbon dioxide. A thin layer of water frost
covered the ground around Viking Lander 2 each winter.

Barometric pressure varies at each landing site on a semiannual basis, because
carbon dioxide, the major constituent of the atmosphere, freezes out to form an
immense polar cap, alternately at each pole. The carbon dioxide forms a great
cover of snow and then evaporates again with the coming of spring in each
hemisphere. When the southern cap was largest, the mean daily pressure observed
by Viking Lander 1 was as low as 6.8 millibars; at other times of the year it
was as high as 9.0 millibars. The pressures at the Viking Lander 2 site were
7.3 and 10.8 millibars. (For comparison, the surface pressure on Earth at sea
level is about 1,000 millibars.)

Martian winds generally blow more slowly than expected. Scientists had expected
them to reach speeds of several hundred miles an hour from observing global
dust storms, but neither lander recorded gusts over 120 kilometers (74 miles)
an hour, and average velocities were considerably lower. Nevertheless, the
orbiters observed more than a dozen small dust storms. During the first
southern summer, two global dust storms occurred, about four Earth months
apart. Both storms obscured the Sun at the landing sites for a time and hid
most of the planet's surface from the orbiters' cameras. The strong winds that
caused the storms blew in the southern hemisphere.

Photographs from the landers and orbiters surpassed expectations in quality and
quality. The total exceeded 4,500 from the landers and 52,000 from the
orbiters. The landers provided the first close-up look at the surface,
monitored variations in atmospheric opacity over several Martian years, and
determined the mean size of the atmospheric aerosols. The orbiter cameras
observed new and often puzzling terrain and provided clearer detail on known
features, including some color and stereo observations. Viking's orbiters
mapped 97 percent of the Martian surface.

The infrared thermal mappers and the atmospheric water detectors on the
orbiters acquired data almost daily, observing the planet at low and high
resolution. The massive quantity of data from the two instruments will require
considerable time for analysis and understanding of the global meteorology of
Mars. Viking also definitively determined that the residual north polar ice cap
(that survives the northern summer) is water ice, rather than frozen carbon
dioxide (dry ice) as once believed.

Analysis of radio signals from the landers and the orbiters -- including
Doppler, ranging and occultation data, and the signal strength of the
lander-to-orbiter relay link --provided a variety of valuable information.

Other significant discoveries of the Viking mission include:

* The Martian surface is a type of iron-rich clay that contains a highly
oxidizing substance that releases oxygen when it is wetted.

* The surface contains no organic molecules that were detectable at the
parts-per-billion level -- less, in fact, than soil samples returned from the
Moon by Apollo astronauts.

* Nitrogen, never before detected, is a significant component of the Martian
atmosphere, and enrichment of the heavier isotopes of nitrogen and argon
relative to the lighter isotopes implies that atmospheric density was much
greater than in the distant past.

* Changes in the Martian surface occur extremely slowly, at least at the Viking
landing sites. Only a few small changes took place during the mission
lifetime.

* The greatest concentration of water vapor in the atmosphere is near the edge
of the north polar cap in midsummer. From summer to fall, peak concentration
moves toward the equator, with a 30 percent decrease in peak abundance. In
southern summer, the planet is dry, probably also an effect of the dust
storms.

The density of both of Mars's satellites is low --about two grams per cubic
centimeter -- implying that they originated as asteroids captured by Mars's
gravity. The surface of Phobos is marked with two families of parallel
striations, probably fractures caused by a large impact that may nearly have
broken Phobos apart.

* Measurements of the round-trip time for radio signals between Earth and the
Viking spacecraft, made while Mars was beyond the Sun (near the solar
conjunctions), have determined delay of the signals caused by the Sun's
gravitational field. The result confirms Albert Einstein's prediction to an
estimated accuracy of 0.1 percent -- 20 times greater than any other test.

* Atmospheric pressure varies by 30 percent during the Martian year because
carbon dioxide condenses and sublimes at the polar caps.

* The permanent north cap is water ice; the southern cap probably retains some
carbon dioxide ice through the summer.

* Water vapor is relatively abundant only in the far north during the summer,
but subsurface water (permafrost) covers much if not all of the planet.

* Northern and southern hemispheres are drastically different climatically,
because of the global dust storms that originate in the south in summer.

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