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NASA's Viking program consisted of two crewless space missions to Mars, Viking 1 and Viking 2. Each mission had a satellite designed to photograph the surface of Mars from orbit, and to act as a communication relay for the Viking lander that each mission carried.
Viking 1 was launched on August 20, 1975, and the second craft, Viking 2, was launched on September 9, 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.
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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 two 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.
Carl Sagan with a model of the Viking Lander, for scale
The lander consisted of a 6-sided aluminum base with alternate 1.09 m and .56 m long sides, supported on three extended legs attached to the shorter sides. The leg footpads formed the vertices of an equilateral triangle with 2.21 m sides when viewed from above, with the long sides of the base forming a straight line with the two adjoining footpads. Instrumentation was attached to the top of the base, elevated above the surface by the extended legs. Power was provided by two radioisotope thermal generator (RTG) units containing plutonium-238 affixed to opposite sides of the lander base and covered by wind screens. Each generator was 28 cm tall, 58 cm in diameter, had a mass of 13.6 kg and provided 30 W continuous power at 4.4 volts. Four wet cell sealed nickel-cadmium 8 amp-hour, 28 volt rechargeable batteries were also onboard to handle peak power loads.
Propulsion was provided for deorbit by a monopropellant hydrazine (N2H4) rocket with 12 nozzles arranged in four clusters of three that provided 32 N thrust, giving a delta-V of 180 m/s. These nozzles also acted as the control thrusters for translation and rotation of the lander. Terminal descent and landing was achieved by three (one affixed on each long side of the base, separated by 120 degress) monopropellant hydrazine engines. The engines had 18 nozzles to disperse the exhaust and minimize effects on the ground and were throttleable from 276 N to 2667 N. The hydrazine was purified to prevent contamination of the Martian surface. The lander carried 85 kg of propellant at launch, contained in two spherical titanium tanks mounted on opposite sides of the lander beneath the RTG windscreens, giving a total launch mass of 657 kg. Control was achieved through the use of an inertial reference unit, four gyros, an aerodecelerator, a radar altimeter, a terminal descent and landing radar, and the control thrusters.
Viking lander model (NASA)
Communications were accomplished through a 20 W S-band transmitter and two 20 W TWTA's. A 2-axis steerable high-gain parabolic antenna was mounted on a boom near one edge of the lander base. An omnidirectional low-gain S-band antenna also extends from the base. Both these antennae allowed for communication directly with the Earth. A UHF (381 MHz) antenna provided a one-way relay to the orbiter using a 30 W relay radio. Data storage was on a 40 Mbit tape recorder, and the lander computer had a 6000-word memory for command instructions.
The lander carried instruments to achieve the primary scientific objectives of the lander mission: to study the biology, chemical composition (organic and inorganic), meteorology, seismology, magnetic properties, appearance, and physical properties of the martian surface and atmosphere. Two 360-degree cylindrical scan cameras were mounted near one long side of the base. From the center of this side extended the sampler arm, with a collector head, temperature sensor, and magnet on the end. A meteorology boom, holding temperature, wind direction, and wind velocity sensors extended out and up from the top of one of the lander legs. A seismometer, magnet and camera test targets, and magnifying mirror are mounted opposite the cameras, near the high-gain antenna. An interior environmentally controlled compartment held the biology experiment and the gas chromatograph mass spectrometer. The X-ray fluorescence spectrometer was also mounted within the structure. A pressure sensor was attached under the lander body. The scientific payload had a total mass of approximately 91 kg.
The total cost of the Viking project was roughly one billion United States dollars.
The Viking landers contained several experiments to detect the presence of Martian life (at least bacterial life). Prior to the mission, NASA had established criteria to decide if life had been detected. The results of the experiments actually met the criteria, but the NASA scientists decided that a chemical reaction was a more likely explanation than biological activity, and subsequently changed the criteria.
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