Following its launch on 20 August 1975 and a 10 month cruise to Mars, the Viking 1 Orbiter began returning global images of Mars about 5 days before orbit insertion. It was inserted into Mars orbit on 19 June 1976 and trimmed to a 1513 x 33,000 km, 24.66 hr site certification orbit on 21 June. Imaging of candidate sites was begun and the landing site was selected based on these pictures. The lander separated from the orbiter on 20 July 08:51 UT and landed at Chryse Planitia at 11:56:06 UT. (The landing had been planned for the US Bicentennial on July 4, but was delayed until a suitable landing site was located.) The lander collected the first-ever samples taken from the surface Mars with its robot arm on 28 July. Meanwhile, the orbiter continued photographing Mars. On 25 July 1976, it photographed a mesa in the Cydonia region resembling a humanoid face. The orbiter primary mission ended at the beginning of solar conjunction on 5 November 1976. The extended mission commenced on 14 December 1976 after solar conjunction. Operations included close approaches to Phobos in February 1977. The periapsis was reduced to 300 km on 11 March 1977. Minor orbit adjustments were done occasionally over the course of the mission, primarily to change the walk rate - the rate at which the planetocentric longitude changed with each orbit, and the periapsis was raised to 357 km on 20 July 1979. On 7 August 1980, the Viking 1 Orbiter was running low on attitude control gas and its orbit was raised from 357 x 33943 km to 320 x 56000 km to prevent impact with Mars and possible contamination until at least the year 2019. Orbital operations were terminated on 17 August 1980 after 1485 orbits, and communications with the lander were terminated on 13 November 1982.

See also Viking 1 Lander

https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1975-075A