NASA's Viking Project found a place in history when it became the first
U.S. mission to land a spacecraft successfully on the surface of Mars.
Two identical spacecraft, each consisting of a lander and an orbiter, were
built. Each orbiter-lander pair flew together and entered Mars orbit; the
landers then separated and descended to the planet's surface.
The Viking 1 Lander touched down on the western slope of Chryse Planitia
(the Plains of Gold) on July 20, 1976, while the Viking 2 lander settled down
at Utopia Planitia on September 3, 1976.
Besides taking photographs and collecting other science data on the
Martian surface, the two landers conducted three biology experiments
designed to look for possible signs of life. These experiments discovered
unexpected and enigmatic chemical activity in the Martian soil, but
provided no clear evidence for the presence of living microorganisms in
soil near the landing sites. According to scientists, Mars is
self-sterilizing. They believe the combination of solar ultraviolet
radiation that saturates the surface, the extreme dryness of the soil and
the oxidizing nature of the soil chemistry prevent the formation of living
organisms in the Martian soil.
The Viking mission was planned to continue for 90 days after landing.
Each orbiter and lander operated far beyond its design lifetime. Viking
Orbiter 1 functioned until July 25, 1978, while Viking Orbiter 2 continued
for four years and 1,489 orbits of Mars, concluding its mission August 7,
1980. Because of the variations in available sunlight, both landers were
powered by radioisotope thermoelectric generators -- devices that create
electricity from heat given off by the natural decay of plutonium. That
power source allowed long-term science investigations that otherwise would
not have been possible. The last data from Viking Lander 2 arrived at
Earth on April 11, 1980. Viking Lander 1 made its final transmission to
Earth November 11, 1982.