Two identical spacecrafts know as Viking 1 & 2 were designed to orbit Mars and send a probe that would land on the planet's surface. The Viking program was a continuation of the Mariner series explorer satellites. Viking 1 was launched on August 20, 1975 and its sister craft Viking 2 went into space on September 9, 1975. It took these spacecrafts almost a year to travel to Mars, they went into orbit on June 19, 1976 and August 7, 1976. The spacecrafts continued to function long past their planned 90-day mission. Viking 2 made its final transmission on April 11, 1980 and the last data from the Viking 1 lander was received on November 11, 1982.
Each spacecraft was composed of two distinct sections: the orbiter and the landing probe. The orbiter's job was to first find a suitable place for the probe to land, collect atmospheric data and to relay all information from itself and from the landing probe back to scientists on Earth.
The landing probes were designed much like the lunar landing Surveyors of the 1960's. As the lander descended to the surface of Mars it fired three retro-engines to slow its decent and then turned off those engines in order to minimize contamination of the surface. The lander then unfolded its three spring loaded legs to ensure a soft touchdown. Once on the surface the probe was powered by the heat given off by the natural radioactive decay of plutonium dioxide which is converted into electricity much like a mini nuclear generator. It communicated with scientists on Earth using a large dish-shaped high gain antenna and with the Viking orbiter through smaller antennas. The lander was outfitted with two cameras, extendable scoops and tools to dig in the soil, and meteorological equipment which was mounted on an extendable boom. Also, on board the lander was a mini laboratory used to analyze the samples that it collected from the surface.
The orbiters mapped out almost the entire surface of Mars in great detail. Scientist could identify volcanoes, lava plains, cratered areas, deep canyons, and evidence that water once flowed on the planet's surface from these pictures. These pictures far surpassed those taken by the early Mariner missions. To learn more about the surface of Mars that was revealed to us by Voyager go to the section entitled the "Surface of Mars".
As the lander descended to the surface of Mars through the thin atmosphere composed mainly of carbon dioxide, it found traces of nitrogen that have been previously undetected. One of the landing probe's objectives included several experiments designed to test for life on Mars. Although the landing modules found no indications of organic compounds, three test that were run on the Martian soil returned unexpected chemical activity. Even though scientists could not explain the puzzling results no clear evidence of living microorganisms was found. This does not rule out the possibility that life may have once existed on Mars in the distant past.