Photo: NASA
I was inspired by the 50th anniversary of the Moon landing on July 16 and our focus on mapping this month to look into imagery of the Moon.
Only recently have we learned that the lunar orbiters that photographed the Moon in the 1960s sent back images that were stunningly high resolution (HR), even by today’s standards. The actual resolution was presumably kept secret because the imaging technology was also used in our Cold War spy satellites.
Under the Lunar Orbiter Program, satellites took photographs of the Moon’s surface to identify suitable landing sites for the Apollo Program. Managed by the Langley Research Center, five Lunar Orbiters were successfully flown in 1966 and 1967, mapping 99% of the Moon’s surface with a resolution of 60 meters or better.
The first three missions were dedicated to imaging 20 potential landing sites, and 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 95% of the farside, and Lunar Orbiter 5 completed the farside coverage and acquired medium (20-meter) and high (2-meter) resolution images of 36 pre-selected areas.
In that pre-digital era, the Lunar Orbiters had an ingenious imaging system, which consisted of a dual-lens camera, a film processing unit, a readout scanner and film-handling apparatus. Both lenses, a 610-mm narrow angle HR lens and an 80-mm wide-angle medium resolution (MR) lens, placed their frame exposures on a single roll of 70-mm film.
The axes of the two cameras were coincident so the area imaged in the HR frames were centered within the MR frame areas.
The film was moved during exposure to compensate for spacecraft velocity, which was estimated by an electric-optical sensor. The film was then processed, scanned, and the images transmitted back to Earth. Based on these images, the NASA Apollo Site Selection Board would name five candidate landing sites in February 1968.
Through the dedication of volunteers, the images have all been digitized. The entire Lunar Orbiter atlas is online.