NASA SP-441: VIKING ORBITER VIEWS OF MARS

 

- The Viking Mission -

 

[3] PROJECT VIKING was begun by the National Aeronautics and Space Administration on November 15, 1968. The main objectives of the project were to achieve a soft landing on the surface of Mars and to relay scientific data back to Earth. Scientific goals for the mission were established in response to recommendations of the Space Science Board, a NASA advisory panel. Foremost among their recommendations was that the first Mars lander mission should emphasize life detection experiments to answer the question about the possibility of life on the planet. In consideration of these goals and the everpresent constraints of funds and spacecraft capability, the following investigations and associated instruments were established:

 

Investigations

Instruments

Orbiter imaging

Two vidicon cameras

Water vapor mapping

Infrared spectrometer

Thermal mapping

Infrared radiometer

Entry science

Ionospheric properties

Retarding potential analyzer

Atmospheric composition

Mass spectrometer

Atmospheric structure

Pressure, temperature and acceleration sensors

Lander imaging

Two facsimile cameras

Biological analyses

Three separate experiments, gas exchange, labelled release, and pyrolytic release, were included to test different biologic models.

Metabolism

Growth

Photosynthesis

Molecular analysis

Gas chromatograph mass spectrometer

Organic compounds

Atmospheric composition

Mass spectrometer

Inorganic analysis

X-ray fluorescence spectrometer

Meteorology

Pressure, temperature, and wind velocity sensors

Seismology

Three-axis seismometer

Magnetic properties

Magnets on sampler and a camera test chart, observed by cameras

Physical properties

Various engineering sensors

Radio science

Celestial mechanics

Orbiter and lander radio and radar systems

Atmospheric properties

Test of general relativity

 

[4] The main functions of the orbiter cameras, whose pictures are displayed in this book, were to aid in the selection of safe, landing sites, to establish the geologic and dynamic environments in which the lander experiments were performed, and to add to our knowledge of the evolution of the Martian surface and the dynamics of its atmosphere.

 

Mariner 4, launched in 1964, weighed only 262 kg and carried only a small science payload. As it flew by Mars at a minimum range of slightly less than 10 000 km, it returned 21 pictures of the planet. An airbrush mosaic of two of the better Mariner 4 frames is shown here. The resolution is approximately one kilometer, about 150 times better than Earth-based photographs. The pictures show a cratered surface that superficially resembles the Moon, a result that was somewhat disappointing in view of some of the more extreme expectations. Certainly, no canals were observed.

 

In 1969, twin spacecraft, Mariner 6 and Mariner 7, were sent to examine Mars. Like Mariner 4, these spacecraft were flybys, and between them they acquired 201 images that were a blend of low-resolution, wide- area frames and nested, high-resolution frames. A typical image pair is shown in the frames taken south of Sinus Sabaeus. The highest resolution achieved was approximately 500 meters. Again, most of the area photographed resembled the Moon, and craters dominated the landscape. Only a few pictures displayed nonlunar-like features, such as chaotic and featureless terrains, that might have hinted at what remained to be discovered. In fact, the two spacecraft had passed over only those parts of the planet that retain an ancient cratered surface, and they had missed the parts of the planet that have younger and more diverse geological features.

 

Mariner 9, the final predecessor to Viking, eventually revealed the extraordinary diversity of the planet's surface. Arriving at Mars in November 1971, Mariner 9 was the first spacecraft to go into orbit about another planet. It took more than 7300 images of Mars, covered the entire surface at a resolution ranging from 1 to 3 km, and covered selected areas at resolutions down to 100 meters. The typical Mariner 9 image included here was taken in the same area as the Mariner 4 images. Mariner 9 operated for almost a year, nearly four times the minimum mission requirement, before running out of attitude control gas. Along with the images, extensive data were obtained on the atmosphere, surface temperature, and global weather patterns.

 

The images from Mariner 9 were the most exciting ever obtained in planetary exploration, revealing giant canyons and volcanoes, large channels (possibly cut by liquid water), and puzzling features that defied all geological < explanations. The stage was set for the Viking missions.

 

Viking 1 was launched from Kennedy Space Center at Cape Canaveral on August 20, 1975, and arrived at Mars on June 19, 1976. Initially, the spacecraft was put into a Mars-synchronous elliptical orbit with a period of 24.66 hours, an apoapsis of 33 000 km, and a periapsis of 1513 km. During the first month, Viking 1 was used exclusively to search for and certify a safe landing site for Viking Lander 1. After the lander touched down on Mars on [5] July 20, 1976, (the seventh anniversary of the first manned lunar landing) the orbiter began systematically imaging the surface. Its highly elliptical orbit was particularly suited for studying the surface because it allowed a mix of close-up, detailed views at periapsis and long-range, synoptic views near or at apoapsis.

 

Table 1 is a chronology of the orbit of Viking Orbiter 1. Two events merit further description. On February 12, 1977, the orbit was changed to permit a flyby of Phobos, the larger, inner Martian moon. At closest approach, the orbiter flew within 90 km of the surface of Phobos. On March 11, 1977, the periapsis of Viking Orbiter 1 was lowered to 300 km from the Martian surface. At this low periapsis, surface features as small as 20 meters across could be identified, while the ability to acquire lower resolution and greater areal coverage away from periapsis was retained. At the beginning of this decade, Viking Orbiter l had taken more than 30 000 pictures of the planet and was still operational.

Table 1.- Viking Orbiter 1 Chronology

Date

Revolution

Event

June 19 1976

0

Mars orbit insertion

June 21, 1976

2

Trim to planned site-certification orbit

July 9, 1976

19

Orbit trim to move westward

July 14, 1976

24

Synchronous orbit over landing site

July 20, 1976

30

VL-1 landing et 1153:06 UTC.

Aug. 3, 1976

43

Minor orbit trim to maintain synchronization over VL-1

Sept. 3, 1976

75

VL-2 landing

Sept. 11, 1976

82

Decrease of orbit period to begin eastward walk

Sept. 20, 1976

92

Orbit trim to permit synchronization over VL-2

Sept. 24, 1976

96

Synchronous orbit over VL-2

Jan. 22, 1977

213

Period change to approach Phobos

Feb. 4, 1977

227

Orbit synchronization with Phobos period

Feb. 12, 1977

235

Precise correction to Phobos synchronization

March 11, 1977

263

Reduction of periapsis to 300 km

March 24, 1977

278

Adjustment of orbit period to 23.5 hours

May 15, 1977

331

Small Phobos-avoidance maneuver

July 1, 1977

379

Adjustment of orbit period to 24.0 hours

Dec 2, 1978

898

Adjustment of orbit period to 24.85 hours: beginning slow walk around planet

May 19, 1978

1061

Adjustment of orbit period to 25.0 hours: acceleration of walk rate

July 20,1979

1120

Raising of periapsis to 357 km; adjustment of orbit period to 24.8 hours: and slowing of walk rate

 

[6] Viking 2 was launched September 9, 1975, and arrived at Mars on August 7, 1976. Like its predecessor, Viking 2 spent nearly a month after arrival finding and certifying a landing site for its lander. Table 2 is a chronology of Viking Orbiter 2. A major difference in the orbit of this spacecraft compared to that of Viking Orbiter 1 is its high inclination, which allowed Viking Orbiter 2 to observe the complex, enigmatic polar regions at relatively close range. Later in its mission, Viking Orbiter 2 flew by Deimos, the smaller of the two Martian moons, at a distance of only 22 km. Spectacular pictures allowing features as small as a compact car were taken. Viking Orbiter 2 returned nearly 16 000 pictures of Mars and its satellites before it was powered down on July 25, 1978.

Table 2.- Viking Orbiter 2 Chronology

Date

Revolution

Event

Aug. 7, 1976

0

Mars orbit insertion

Aug. 9, 1976

2

Period and altitude adjustment; beginning of westward walk

Aug. 14, 1976

6

Increase of period to increase walk rate

Aug. 25, 1976

16

Decrease of walk rate to proceed to landing site

Aug. 27, 1976

18

Synchronous orbit over landing site

Sept. 3, 1976

25

VL-2 landing at 2237:50 UTC

Sept. 30, 1976

51

Change of orbit plane to 75° inclination and beginning of westward walk

Dec. 20, 1976

123

Lowering of periapsis to 800 km and increasing of inclination to 80

March 2, 1977

189

Synchronous orbit over VL-2

April 18, 1977

235

Period change: 13 revolutions equals 12 Mars days

Sept. 25, 1977

404

Change of orbit period to approach Deimos

Oct. 9, 1977

418

Orbit synchronization with Deimos

Oct. 23, 1977

432

Change of orbit period to 24.0 hours and lowering of periapsis to 300 km

July 25, 1978

706

Powered down

 


[7] Mariner 4 Mosaic of Southern Hemisphere Ancient Cratered Terrain. This mosaic of two Mariner 4 frames suggests a lunar-like surface and shows no traces of the canals and oases of early observers nor the volcanoes, canyons, and channels revealed by later missions. The major surprises to come from Mars remained hidden. [Mariner 4 frames 15-16A; 47° S, 141° W]

multiple frame photo of mars


one low and one high resolution photo taken from the Mariner 6

[8] Mariner 6 Frames South of Sinus Sabaeus. (a) A low resolution frame from Mariner 6 shows the large craters Wislicenus (left) and Flaguergues (right). (b) This is a high-resolution frame of a small portion of the northeast rim of Flaguergues and the adjoining terrain. Fortuitous placing of all the Mariner 6 and 7 frames within the oldest terrain of Mars was such that none of the more spectacular younger features was observed. [Mariner 6 frames (a) 6N21, (b) 6N22; 15° S, 345° W]


[9] Mariner 9 Frame of Southern Hemisphere Ancient Cratered Terrain. A typical Mariner 9 image in the same area as shown for Mariner 4 confirms lunar-like appearance of most the southern hemisphere of Mars. Mariner 9 made many discoveries about Mars, including channels, giant volcanoes, and a giant rift valley, which set the stage for the Viking missions. [ Mariner 9 frame 115A18/32, DAS time 05706928; 49° S, 141° W]

Mariner 9 photo of Mars


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