Viking 1 Orbiter

Description

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 surface. The fully fueled orbiter-lander pair had a mass of 3527 kg. After separation and landing, the lander had a mass of about 600 kg and the orbiter 900 kg.

Spacecraft and Instrumentation

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. Temperature control was achieved by multilayer insulation, thermally activated louvers, and electrical heaters.

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.

Mission Profile

Following launch and a 10 month cruise to Mars, the orbiter began returning global images of Mars about 5 days before orbit insertion. The Viking 1 Orbiter was inserted into Mars orbit on 19 June 1976 and trimmed to a 1513 x 33,000 km, 24.66 hr site certification orbit on 21 June. Imaging of candidate sites was begun and the landing site was selected based on these pictures. The lander separated from the orbiter on 20 July 08:51 UT and landed at Chryse Planitia at 11:56:06 UT. The orbiter primary mission ended at the beginning of solar conjunction on 5 November 1976. The extended mission commenced on 14 December 1976 after solar conjunction. Operations included close approaches to Phobos in February 1977. The periapsis was reduced to 300 km on 11 March 1977. Minor orbit adjustments were done occasionally over the course of the mission, primarily to change the walk rate - the rate at which the planetocentric longitude changed with each orbit, and the periapsis was raised to 357 km on 20 July 1979. On 7 August 1980 Viking 1 Orbiter was running low on attitude control gas and its orbit was raised from 357 x 33943 km to 320 x 56000 km to prevent impact with Mars and possible contamination until the year 2019. Operations were terminated on 17 August 1980 after 1485 orbits.

The total cost of the Viking project was roughly one billion dollars. 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.