The Lunar Orbiter 1 robotic (unmanned) spacecraft, part of the Lunar Orbiter Program, was the first American spacecraft to orbit the Moon. It was designed primarily to photograph smooth areas of the lunar surface for selection and verification of safe landing sites for the Surveyor and Apollo missions. It was also equipped to collect selenodetic, radiation intensity, and micrometeoroid impact data.
The spacecraft was placed in an Earth parking orbit on August 10, 1966 at 19:31 (UTC). The Trans lunar injection burn occurred at 20:04 (UTC). The spacecraft experienced a temporary failure of the Canopus star tracker (probably due to stray sunlight) and overheating during its cruise to the Moon. The star tracker problem was resolved by navigating using the Moon as a reference and the overheating was abated by orienting the spacecraft 36 degrees off-Sun to lower the temperature.
Lunar Orbiter 1 was injected into an elliptical near-equatorial lunar orbit 92.1 hours after launch. The initial orbit was 189.1 by 1,866.8 kilometres (117.5 mi × 1,160.0 mi) and had a period of 3 hours 37 minutes and an inclination of 12.2 degrees. On August 21, perilune was dropped to 58 kilometres (36 mi) and on August 25 to 40.5 kilometres (25.2 mi). The spacecraft acquired photographic data from August 18–29, 1966, and readout occurred through September 14, 1966.
A total of 42 high resolution and 187 medium resolution frames were taken and transmitted to Earth covering over 5 million square km of the Moon’s surface, accomplishing about 75% of the intended mission, although a number of the early high-res photos showed severe smearing. It also took the first two pictures of the Earth ever from the distance of the Moon. Accurate data were acquired from all other experiments throughout the mission.