Viking 1 Orbiter Mission Profile
Following launch and a 10 month cruise to Mars, the orbiter began returning global images of Mars about 5 days before orbit insertion. The Viking 1 Orbiter 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 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 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 the year 2019. Operations were terminated on 17 August 1980 after 1485 orbits.