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During July and August 1973, the U.S.S.R. launched four space- 
craft to Mars. Mars 4 passed by the planet in February 1974. Mars 5 
is in an elliptical orbit with closest approach at about 1700 km. It 
carries two multispectral scanners with 12 mm focal length, and two 
phototelevision cameras with focal length of 50 and 350 mm, on-board 
photo processing, and opto-electronic transmission. The cameras can 
resolve approximately 700 m and 100 m on the surface. Pictures are 
quite similar to those obtained by Mariner 9 (30). Mars 6 was a 
lander but it is not transmitting data. Mars 7 released a descent 
capsule, but it apparently missed the planet. 
Of consummate interest at this time is the Viking mission to 
Mars (31). The Viking I spacecraft was launched on August 20, 1975, 
and arrived at Mars on June 19, 1976. Viking II was launched seven 
weeks later and is expected to arrive in Mars orbit on August 7. The 
Viking spacecraft actually contains two major components - an orbiter 
and a lander. The lander is enclosed in a biological shield which is 
attached to the base of the orbiter. At the appropriate time, the 
two components will be separated to conduct their individual missions. 
The orbiter science platform contains three instruments - an infrared 
thermomapper, two identical TV cameras, and a water vapor mapper. The 
cameras have a telescopic optical system with focal length 475 mm and 
aperture f/3.5 (32). The field of view is a little bit more than 1°. 
The target area on the Vidicon Sensor is 14.0 x 12.5 mm. There are 
1180 picture elements in each of 1,056 scan lines. From the nominal 
1500 km altitude, a pixel subtends approximately 40 m on the surface 
of Mars. The cameras also contain filter wheels permitting multiband 
photography from which color composites can be assembled. Photographic 
sequences will start when the spacecraft is 500,000 km from the planet. 
Pictures from several spectral bands will be digitally combined to 
produce full color views of the planet during approach. 
After arrival at. the planet the spacecraft is placed into an 
elliptical orbit with periapsis of 1500 km and apoapsis of 32,000 km. 
The period of this orbit is 24.6 hours which is equivalent to one 
Martian day. Therefore each periapsis passage will be over approxi- 
mately the same area and with the same illumination conditions. The 
pointing of the two camera systems is offset, and the timing of the 
camera frames is offset by one-half frame time. The combined effect 
is to produce two parallel swaths of pictures as the motion of the 
spacecraft carries it across the surface of Mars. Each swath will 
contain 10 pictures from each of the two cameras. For the early 
passes, there will actually be four picture-taking sequences over the 
same area. For the first sequence the cameras will be inclined 25° 
forward, the second pass will be 100 forward, the third 10° aft, and 
the fourth 250 aft. When these pictures are received on Earth, they 
will be digitally processed to remove distortion and enhance contrast 
Images will be selected and measured and an analytical block triangu- 
lation solution will be performed, using spacecraft position and 
attitude data as a constraint. Following the triangulation, terrain 
statistics will be extracted, mosaics compiled, and contoured maps