Viking 2 Orbiter
By Marty McDowell/NASA
The Viking project consisted of launches of two separate spacecraft to
Mars, Viking 1, launched on 20 August 1975, and
Viking 2, launched on 9 September 1975. Each
spacecraft consisted of an orbiter and a lander. After orbiting Mars and
returning images used for landing site selection, the orbiter and lander
detached and the lander entered the martian atmosphere and soft-landed at
the selected site. The orbiters continued imaging and other scientific
operations from orbit while the landers deployed instruments on the
The orbiter, based on the earlier Mariner 9 spacecraft, was an octagon
approximately 2.5 m across. The total launch mass was 2328 kg, of which
1445 kg were propellant and attitude control gas. The eight faces of the
ring-like structure were .4572 m high and were alternately 1.397 and 0.508
m wide. The overall height was 3.29 m from the lander attachment points on
the bottom to the launch vehicle attachment points on top. There were 16
modular compartments, 3 on each of the 4 long faces and one on each short
face. Four solar panel wings extended from the axis of the orbiter, the
distance from tip to tip of two oppositely extended solar panels was 9.75
m. The power was provided by eight 1.57 x 1.23 m solar panels, two on each
wing. The solar panels were made up of a total of 34,800 solar cells and
produced 620 W of power at Mars. Power was also stored in 2 nickel-cadmium
Viking 2 Orbiter image of
Mars. Dune fields are visible within a 50 km
impact crater in the top right corner of the
frame. This area is in the high-latitude south
polar plains. The frame is 150 km wide and north
Image courtesy of NASA.
Scientific instruments for conducting imaging, atmospheric water vapor,
and infrared thermal mapping were enclosed in a temperature controlled,
pointable scan platform extending from the base of the orbiter. The
scientific instrumentation had a total mass of approximately 72 kg. Radio
science investigations were also done using the spacecraft transmitter.
Command processing was done by two identical and independent data
processors, each with a 4096-word memory for storing uplink command
sequences and acquired data.
Following launch and a 333 day cruise to Mars, the Viking 2 Orbiter
began returning global images of Mars prior to orbit insertion. The
orbiter was inserted into a 1500 x 33,000 km, 24.6 hr Mars orbit on 7
August 1976 and trimmed to a 27.3 hr site certification orbit with a
periapsis of 1499 km and an inclination of 55.2 degrees on 9 August.
Imaging of candidate sites was begun and the landing site was selected
based on these pictures and the images returned by the Viking 1 Orbiter.
The lander separated from the orbiter on 3 September 1976 and landed at
Utopia Planitia at 22:37:50 UT.
Normal operations called for the structure connecting the orbiter and
lander (the bioshield) to be ejected after separation, but because of
problems with the separation the bioshield was left attached to the
orbiter. The orbit inclination was raised to 75 degrees on 30 September
1976. The orbiter primary mission ended at the beginning of solar
conjunction on 8 November 1976.
The extended mission commenced on 14 December 1976 after solar
conjunction. On 20 December 1976 the periapsis was lowered to 778 km and
the inclination raised to 80 degrees. Operations included close approaches
to Deimos in October 1977 and the periapsis was lowered to 300 km and the
period changed to 24 hours on 23 October 1977. The orbiter developed a
leak in its propulsion system that vented its attitude control gas. It was
placed in a 302 x 33176 km orbit and turned off on 25 July 1978 after
returning almost 16,000 images in 706 orbits around Mars.
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Courtesy of NASA
Launched: September 9, 1975
Arrival: August 7, 1976
Mission: Mars orbit and mapping.