Viking.
Two identical American probes sent to the planet Mars in 1975. Both Vikings 1 and 2 consisted of an orbiter, which remained circling the planet, and a soft lander.
Viking 1 was launched on 9 September 1975 and reached Mars orbit on 19 June 1976. Images were taken to locate a suitable landing site and the landing took place on Chryse Planitia on 20 July 1976. The orbiter's path was adjusted several times in order to obtain close-up images of Mars' satellites, Deimos and Phobos, and to observe different aspects of the martian surface.
Viking 2 was launched on 20 August 1975 and reached Mars orbit on 7 August 1976. The landing took place on Utopia Planitia on 3 September 1976.
The orbiters were equipped with two television cameras, an infrared spectrometer to map the distribution of water vapour and a radiometer for determining temperature distribution. The landers sampled the upper atmosphere during descent, made meteorological measurements and carried out experiments on samples of martian soil. One of the the prime objectives was to test for the presence of organic material which might indicate the existence of life, but nothing incontrovertible was found. Thousands of images were returned from both the orbiters and landers. The whole of the martian surface was mapped to a resolution of 150-300 metres.
The Viking 1 orbiter operated until 7 August 1980, and the Viking 2 orbiter until 25 July 1978. The landers ceased operating in November 1982 and February 1980, respectively. The mission was regarded as very successful, and it had greatly exceeded its expected lifetime.