Viking
The Viking Program produced two NASA space probe missions to the planet Mars, Viking I and Viking II. Each probe consisted of an orbiter and lander.
The Viking I mission launched August 20, 1975 and arrived June 19, 1976. Viking I released its lander, which landed on the martian surface at Chryse Planitia on July 20, 1976. The Viking I Orbiter was powered down by NASA on August 17, 1980. The Viking I lander, powered by a radioisotope thermoelectric generator, continued transmitting until contact was lost on November 13, 1982.
The Viking II mission (nearly identical to Viking I) launched on September 9, 1975 and arrived August 7, 1976. The Viking II lander was released and touched down at Utopia Planitia on September 3, 1976. The Viking II orbiter was powered down by NASA on July 25, 1978. The Viking II lander continued transmitting until contact was lost on April 11, 1980.
Each Viking orbiter obtained high resolution photographs of the martian surface, and each lander was equipped to obtain high resolution photographs, spectroscopic and meteorological data. Additionally, each lander was equipped with a sample arm to retrieve soil samples for testing by three experimental packages:
Viking Labeled Release life experiment Viking Gas Exchange life experiment Viking Pyrolytic Release life experiment
The Labeled Release experiment produced an initial positive for all soil samples for both landers. The Gas Exchange and Pyrolytic Release experiments produced negative results. This discrepancy was officially explained as due to the presence of a chemically reactive agent in the martian soil rather than the presence of living microorganisms. However, substantiated challenges to the Gas Exchange experiment results based on its limited sensitivity have left open the possibility that the Viking landers detected extant life on Mars.
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