Lunar Orbiter program

• 1. Introduction

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  The Lunar Orbiter program sent five uncrewed spacecraft around the Moon in 1966 and 1967, and every one of them succeeded. The United States launched them to map the Moon's surface, so that NASA could choose landing sites for Apollo. Along the way they returned the first photographs ever taken from lunar orbit. How does a spacecraft develop its own film while circling another world? Why did classified spy cameras become a worry for NASA? What did the orbiters learn that no camera could show? And why did people begin restoring their pictures from original analog recordings in 2007?

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• 2. Targets and Orbits

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  Twenty candidate landing sites for crewed missions were the first targets. Earth-based observations had picked them out, and the first three Lunar Orbiters were assigned to photograph them. Those three flew low-inclination orbits. The fourth and fifth missions pursued wider scientific goals from high-altitude polar orbits. Lunar Orbiter 4 photographed the entire nearside along with nine percent of the far side. Lunar Orbiter 5 completed the far side. It also captured 36 preselected areas at a medium resolution of 20 m and a high resolution of 2 m.

  Atlas-Agena-D launch vehicles carried all five spacecraft away from Earth. By the end, 99 percent of the lunar surface had been mapped from photographs with a resolution of 60 m or better. That coverage depended on an unusual camera that developed its film while still in orbit.

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• 3. Camera and Film

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  Eight photographic subsystems were built by Kodak for the program, and five of them flew on the missions of 1966 and 1967. Each camera had two lenses that exposed their frames at the same moment on one roll of film. The wide-angle lens was an 80 mm f/2.8 Xenotar made by Schneider Kreuznach of West Germany. It produced the medium-resolution images. The high-resolution lens was a 610 mm f/5.6 Panoramic lens made by the Pacific Optical Company. Because the two lens axes coincided, each high-resolution frame sat at the center of a medium-resolution frame.

  An electro-optical sensor estimated the spacecraft's velocity as it passed over the surface. The film was moved during each exposure to cancel that motion. The camera used Kodak Bimat film, which was developed onboard in a semidry process. A photomultiplier then scanned the finished images, and the spacecraft sent them to Earth as analog video.

  Receiving stations on Earth converted the video back onto film. That film was shipped to Kodak in Rochester for final processing and printing. Altogether the orbiters returned 2180 high-resolution frames and 882 medium-resolution frames. The design was adapted from the SAMOS E-1 reconnaissance camera, which Kodak had built for a short-lived USAF project in near-realtime satellite imaging. The NRO gave permission for the adaptation, and its involvement did not end there.

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• 4. Spy Cameras Offered

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  Spare cameras from the KH-7 GAMBIT program were the Air Force's original offer to NASA. Then authorities grew worried about the security of the classified hardware. Photographs of the Moon might reveal the cameras' resolution. Some proposals suggested that NASA withhold the orbital parameters of the Lunar Orbiter probes, since altitude would let outsiders calculate that resolution. In the end, NASA's existing camera systems proved adequate for the mission, even though their resolution was lower.

  The NRO and NASA also cooperated on a backup called the Lunar Mapping and Survey System, or LM&SS. It was based on the KH-7 reconnaissance satellite. It would have replaced the Lunar Module in the Saturn V, and Apollo astronauts would have operated it remotely from lunar orbit. NASA canceled the project in the summer of 1967, after the complete success of the Lunar Orbiters.

  The camera still needed a spacecraft built around it. NASA announced the Boeing-Eastman Kodak proposal on the 20th of December 1963.

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• 5. Anatomy of a Spacecraft

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  The main bus was a truncated cone, 1.65 m tall and 1.5 m across at its base. Three decks, held up by trusses and an arch, made up the body. The equipment deck at the bottom held the battery, transponder and flight programmer. It also carried the inertial reference unit, the Canopus star tracker, the command decoder and the multiplex encoder. The traveling-wave tube amplifier and the photographic system were housed there too. Four solar panels extended from this deck, spanning 3.72 m. A high-gain antenna rode on a 1.32 m boom, and a low-gain antenna on a 2.08 m boom.

  The middle deck held the velocity control engine along with its propellant, oxidizer and pressurization tanks. Sun sensors and micrometeoroid detectors were there as well. The top deck was a heat shield that protected the spacecraft when the engine fired, with the engine nozzle poking through its center. Four attitude control thrusters were mounted around its perimeter.

  A total of 10,856 n/p solar cells in four arrays produced 375 W. That power ran the spacecraft directly and also charged a 12 A·h nickel-cadmium battery. The battery covered the brief periods of occultation when no sunlight reached the panels. For major maneuvers, a gimballed 100 lbf hypergolic rocket motor from Marquardt Corp. did the work. Four 1 lbf nitrogen gas jets handled three-axis stabilization and attitude control. For navigation the craft used five Sun sensors, the Canopus star sensor and an inertial navigation system.

  Two transmitters handled communications, both operating in the S band at about 2295 MHz. A 10 W transmitter and a directional high-gain antenna, one meter across, sent the photographs. A 0.5 W transmitter and an omnidirectional low-gain antenna covered all other communications. A multilayer blanket of aluminized Mylar and Dacron wrapped the main bus, helped by special paint, insulation and small heaters. The first of these machines left the ground on the 10th of August 1966.

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• 6. Earth From Orbit

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  In August 1966, Lunar Orbiter 1 photographed an Earth-rise over the lunar surface. That image began the first pictures ever taken of Earth as a whole. The first full picture of the whole Earth came from Lunar Orbiter 5 on the 8th of August 1967. The same spacecraft took a second whole-Earth photo on the 10th of November 1967. At that point Lunar Orbiter 5 had less than three months of flight left.

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• 7. Beyond the Photographs

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  Twenty-two impacts were recorded by the micrometeoroid experiments aboard the orbiters. They showed that the average micrometeoroid flux near the Moon was about two orders of magnitude greater than in interplanetary space. It was slightly less than the flux in the near-Earth environment. Radiation experiments confirmed that Apollo hardware would shield astronauts from average and greater-than-average short-term exposure to solar particle events.

  Three orbiters, numbers 2, 3 and 5, were tracked simultaneously from August through October 1967. That tracking tested the Manned Space Flight Network stations and the Apollo Orbit Determination Program, and it was judged successful. Doppler tracking of all five orbiters also allowed the Moon's gravitational field to be mapped. It revealed mass concentrations, called mascons, which are gravitational highs. They sat in the centers of some of the lunar maria, but not all of them.

  NASA Langley Research Center managed the program, at a total cost of roughly $200 million. Each orbiter was eventually commanded to crash on the Moon before its attitude control fuel ran out. That way none would become a navigational or communications hazard for later Apollo flights. Lunar Orbiter 1 struck on the 29th of October 1966. Lunar Orbiter 5 was the last, hitting the Moon on the 31st of January 1968.

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• 8. Rescuing the Archive

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  Each exposure produced two photographs, one from the 80 mm lens and one from the 610 mm lens. Because of their large size, the high-resolution frames were divided into three sub-frames. The images reached Earth as strips of analog data, which were written to magnetic tape and to film. The film was used to build hand-made mosaics. Large-format prints, 16 x 20 in, were made from those mosaics and sent to NASA libraries called Regional Planetary Information Facilities. The views had very high spatial resolution and covered much of the Moon. They also suffered from "venetian blind" striping, missing or duplicated data, and frequent saturation effects. Because the photos were taken at low to moderate Sun angles, they remain especially useful for studying the shapes of lunar topographic features.

  Bowker and Hughes published the most definitive atlas of the photographs in 1971. It held 675 photographic plates with approximately global coverage of the Moon. When that atlas went out of print, the Lunar and Planetary Institute scanned the large-format prints. The results went online as the Digital Lunar Orbiter Photographic Atlas of the Moon.

  In 2000, the Astrogeology Research Program of the US Geological Survey in Flagstaff, Arizona received NASA funding for the Lunar Orbiter Digitization Project. The team scanned archival positive film strips at 25 micrometer resolution, aiming for a global mosaic of the Moon. The digitally constructed frames were map-projected and free of the original stripes. Only about 15% of the available frames were scanned, including data from missions III, IV and V. The project also created frames of very high resolution images for sites of scientific interest, such as the Apollo 12 landing site, the Marius Hills and the Sulpicius Gallus rille.

  The Lunar Orbiter Image Recovery Project began in 2007. It converted the images directly from the original Ampex FR-900 analog video recordings, which gave vastly better resolution than the 1960s releases. The first restored images appeared in late 2008. Almost all of the Lunar Orbiter images had been recovered and were being processed for submission to NASA's Planetary Data System.

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