Launch Date/Time: 1975-08-20 at 21:22 UTC
On-orbit dry mass: 883.00 kg
Nominal Power Output: 620.00 W
Manufacturer: Martin Marietta for NASA
Launch vehicle: Titan III-Centaur

The primary objectives of the Viking orbiters were to transport the landers to Mars, perform reconnaissance to locate and certify landing sites, act as a communications relays for the landers, and to perform their own scientific investigations. 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 30-amp-hr batteries.

The main propulsion unit was mounted above the orbiter bus. Propulsion was furnished by a bipropellant (monomethyl hydrazine and nitrogen tetroxide) liquid-fueled rocket engine which could be gimballed up to 9 degrees. The engine was capable of 1323 N thrust, translating to a delta-V of 1480 m/s. Attitude control was achieved by 12 small compressed-nitrogen jets. An acquisition Sun sensor, a cruise Sun sensor, a Canopus star tracker and an inertial reference unit consisting of 6 gyroscopes allowed three-axis stabilization. Two accelerometers were also on board. Communications were accomplished through a 20-W S-band (2.3 GHz) transmitter and 2 20-W TWTA's. An X-band (8.4 GHz) downlink was also added specifically for radio science and to conduct communications experiments. Uplink was via S-band (2.1 GHz). A 2-axis steerable high-gain parabolic dish antenna with a diameter of approximately 1.5 m was attached at one edge of the orbiter base, and a fixed low-gain antenna extended from the top of the bus. Two tape recorders were each capable of storing 1280 Mbits. A 381 MHz relay radio was also available.

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.

For a detailed description of the Viking mission and experiments, see "Scientific Results of the Viking Project," J. Geophys. Res., v. 82, n. 28, 1977.

Source: Viking Project Information - NSSDC

References:

1. Snyder, C. W., The missions of the Viking Orbiters, J. Geophys. Res., 82, No. 28, 3971-3983, Sep. 1977.
2. Soffen, G. A., Scientific results of the Viking mission, Science, 194, No. 4271, 1274-1276, Dec. 1976.
3. Soffen, G. A., The Viking project, J. Geophys. Res., 82, No. 28, 3959-3970, Sep. 1977.
4. Soffen, G. A., and Snyder, C. W., First Viking mission to Mars, Science, 193, 759-766, Aug. 1976.
5. Tolson, R. H., Blackshear, W. T., and Anderson, S. G., Orbit and position determination for Mars Orbiters and Landers, J. Spacecr. Rockets, 7, No. 9, 1095-1100, Sept. 1970.

Wednesday, 31-Dec-1969 18:00:00 CST
CSR/TSGC Team Web