The Viking mission used two identical spacecraft, each consisting
of a lander and an orbiter. Launched on August 20, 1975 from the
Kennedy Space Center in Florida, Viking 1 spent nearly a year cruising
to Mars, placed an orbiter in operation around the planet, and landed
on July, 20 1976 on the Chryse Planitia (Golden Plains). Viking 2 was
launched on September 9, 1975 and landed on September 3, 1976. The
Viking project's primary mission ended on November 15, 1976, 11 days
before Mars's superior conjunction (its passage behind the sun),
although the Viking spacecraft continued to operate for six years
after first reaching Mars. The last transmission from the planet
reached Earth on November 11, 1982.
While Viking 1 and 2 were on Mars, this third vehicle was used on
Earth to simulate their behavior and to test their responses to radio
commands. Earlier, it had been used to demonstrate that the landers
could survive the stresses they would encounter during the mission.
Smithsonian Air & Space Museum, Nov 2016