Viking program: Viking 1 takes the famous Face on Mars photo.
Viking 1 was the first of two spacecraft, along with Viking 2, each consisting of an orbiter and a lander, sent to Mars as part of NASA's Viking program. The lander touched down on Mars on July 20, 1976, the first successful Mars lander in history. Viking 1 operated on Mars for 2307 days (over 614 years) or 2245 Martian solar days, the longest Mars surface mission until the record was broken by the Opportunity rover on May 19, 2010.
The Viking program consisted of a pair of identical American space probes, Viking 1 and Viking 2, which landed on Mars in 1976. Each spacecraft was composed of two main parts: an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface. The orbiters also served as communication relays for the landers once they touched down.
The Viking program grew from NASA's earlier, even more ambitious, Voyager Mars program, which was not related to the successful Voyager deep space probes of the late 1970s. Viking 1 was launched on August 20, 1975, and the second craft, Viking 2, was launched on September 9, 1975, both riding atop Titan IIIE rockets with Centaur upper stages. Viking 1 entered Mars orbit on June 19, 1976, with Viking 2 following on August 7.
After orbiting Mars for more than a month and returning images used for landing site selection, the orbiters and landers detached; the landers then entered the Martian atmosphere and soft-landed at the sites that had been chosen. The Viking 1 lander touched down on the surface of Mars on July 20, 1976, more than two weeks before Viking 2's arrival in orbit. Viking 2 then successfully soft-landed on September 3. The orbiters continued imaging and performing other scientific operations from orbit while the landers deployed instruments on the surface.
The project cost was roughly $1 billion at the time of launch, equivalent to about 5 billion USD in 2020 dollars. The mission was considered successful and is credited with helping to form most of the body of knowledge about Mars through the late 1990s and early 2000s.