Lunar Orbiter program

The Lunar Orbiter program was a series of five unmanned lunar orbiter missions launched by the United States from 1966 through 1967. Intended to help select Apollo landing sites by mapping the Moon's surface, they provided the first photographs from lunar orbit.

All five missions were successful, and 99% of the Moon was mapped from photographs taken with a resolution of 60 meters (200 ft) or better. The first three missions were dedicated to imaging 20 potential manned lunar landing sites, selected based on Earth-based observations. These were flown at low-inclination orbits. The fourth and fifth missions were devoted to broader scientific objectives and were flown in high-altitude polar orbits. Lunar Orbiter 4 photographed the entire nearside and 9% of the far side, and Lunar Orbiter 5 completed the far side coverage and acquired medium (20 m (66 ft)) and high (2 m (6 ft 7 in)) resolution images of 36 pre-selected areas. All Lunar Orbiter craft were launched by an Atlas-Agena D launch vehicle.

Lunar Orbiter 1

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 to 29, 1966, and readout occurred through September 14, 1966.

Lunar Orbiter 4

Lunar Orbiter 4 was an unmanned US spacecraft, part of the Lunar Orbiter Program, designed to orbit the Moon, after the three previous orbiters had completed the required needs for Apollo mapping and site selection. It was given a more general objective, to "perform a broad systematic photographic survey of lunar surface features in order to increase the scientific knowledge of their nature, origin, and processes, and to serve as a basis for selecting sites for more detailed scientific study by subsequent orbital and landing missions". It was also equipped to collect selenodetic, radiation intensity, and micrometeoroid impact data. The spacecraft was placed in a cislunar trajectory and injected into an elliptical near polar high lunar orbit for data acquisition. The orbit was 2,706 by 6,111 kilometres (1,681 mi × 3,797 mi) with an inclination of 85.5 degrees and a period of 12 hours.