Photographs from the first three Lunar Orbiters enabled scientists to pick Apollo landing sites, including Apollo 11's Sea of Tranquility base.
Lunar Orbiter 3 was the final Lunar Orbiter mission to study potential Apollo landing sites, although its mission was focused on “site confirmation” rather than “site search.” For the mission, the spacecraft’s orbital inclination was increased to 21 degrees to ensure photography both north and south of the lunar equator.
Building on the stereo photography taken by Lunar Orbiter 2, the third mission was focused on making two “footprints” of the same area on two successive orbits. Lunar Orbiter 3 was also designed to obtain precision trajectory information for defining the lunar gravitational field, to measure micrometeoroid flux, and to measure radiation dosage levels around the Moon.
The spacecraft arrived in lunar orbit after a 9 minute 2.5 second engine burn on Feb. 8, 1967 after a single mid-course correction. Initial orbital parameters were 131 x 1,120 miles (210.2 × 1,801.9 kilometers) at 20.93 degree inclination.
About four days later, the spacecraft entered its operational 34 x 1148-mile (55 × 1,847-kilometer) orbit at 20.9 degree inclination. At the time, Lunar Orbiter 2 was still in operation around the Moon, thus providing key experience for NASA’s ability to track and communicate with two simultaneous spacecraft around the Moon.
Lunar Orbiter 3 began its photographic mission on Feb. 15, 1967. The spacecraft exposed 211 (out of a possible 212) frames of pictures by the time that imaging concluded on Feb. 23, 1967. Soon, the spacecraft began to “read out” the images to the ground, but this activity suddenly stopped on March 4, 1967 due to a problem with the film advance mechanism in the readout section of the imaging system. As such, only 182 images were returned -- 72 others were never read out to the ground.
Despite the glitch, Lunar Orbiter 3 wholly fulfilled its original mission objectives, returning images of 6 million square miles (15.5 million square kilometers) of the near side and 251,000 square miles (650,000 square kilometers) of the far side. One of its images showed the Surveyor 2 lander on the lunar surface.
On Aug. 30, 1967, ground controllers commanded the vehicle to circularize its orbit to 99 miles (160 kilometers) in order to simulate an Apollo trajectory.
Later, on Oct. 9, 1967, the probe was intentionally crashed onto the lunar surface at 14 degrees 36 minutes north latitude and 91 degrees 42 minutes west longitude.
The photographs from the first three Lunar Orbiters allowed NASA scientists to pick eight preliminary landing sites for Apollo by early April 1967, including site 2 in the Sea of Tranquility where Apollo 11 would land and site 5 in the Ocean of Storms where Apollo 12 (and also Surveyor 3) would disembark.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.