Both NASA Viking missions used a combination of orbiter and lander to explore Mars in unprecedented detail.

Sept. 9, 1975: Launch

Aug. 7, 1976: Viking 2 entered orbit around Mars

Sept. 3, 1976: Lander touches down safely on the surface of Mars

July 24, 1978: End of orbiter operations

April 12, 1980: End of lander mission

The Viking-A spacecraft was scheduled to be launched first but ended up being launched second due to a problem with its batteries. It was renamed Viking 2.

After a successful launch and a course correction Sept. 19, 1975, Viking 2 entered orbit around Mars nearly a year after launch Aug. 7, 1976. Initial orbital parameters were 933 × 22,200 miles (1,502 × 35,728 kilometers) inclined at 55.6 degrees.

As with Viking 1, photographs of the original landing site indicated rough terrain, prompting mission planners to select a different site at Utopia Planitia near the edge of the polar ice cap where water was located and where there was a better chance of finding signs of life.

The lander separated from the orbiter without incident at 20:19 UT Sept. 3, 1976, and after atmospheric entry, landed safely at 22:37:50 UT, about 4,000 miles (6,460 kilometers) from the Viking 1 landing site. Touchdown coordinates were 47.968 degrees north latitude and 225.71 degrees west longitude.

Photographs of the area showed a rockier, flatter site than that of Viking 1. The lander was in fact tilted 8.5 degrees to the west. Panoramic views of the landscape showed a terrain different from that of Viking 1, with much less definition and very little in the way of horizon features. Because of the lack of general topographical references on the ground, imagery from the orbiters was unable to precisely locate the lander.

The biology experiments with scooped up soil collected on three occasions (beginning Sept. 12, 1976) produced similar results to its twin: inconclusive on whether life exists or ever has existed on the surface of Mars. Scientists believed that Martian soil contained reactants created by the ultraviolet bombardment of the soil that could produce characteristics of living organisms in Earth soil.

On Nov. 16, 1976, NASA announced that both Viking 1 and Viking 2 missions had successfully accomplished their mission goals and announced an extended mission that continued until May 1978 followed by a "Continuation Mission" until July 1979.

The orbiter continued its successful imaging mission, coming within 17 miles (28 kilometers) of the Martian Moon Deimos in May 1977.

A series of leaks prompted termination of the Viking 2 orbiter operations July 24, 1978, while the Viking 2 lander continued to transmit data until April 12, 1980.

In July 2001, the Viking 2 lander was renamed the Gerald Soffen Memorial Station after Gerald Soffen (1926-2000), the NASA Project Scientist for Viking who had died recently.

In total, the two Viking orbiters returned 52,663 images of Mars and mapped about 97% of the surface at a resolution of about 980 feet (300 meters) resolution.

The Viking landers returned 4,500 photos of the two landing sites.