At times it seems that we have overused various superlative labels to the point where they have lost their meaning. While this trend may seem to be more common today as various media outlets vie for attention in an increasingly competitive international market, it is actually something that has been with us to one degree or another for decades. This was certainly true during the early years of the Space Age as one superlative after another was used as ever more amazing goals in spaceflight were achieved. One of these early examples is largely forgotten today: an oblique image of the famous crater called Copernicus taken by NASA’s Lunar Orbiter 2 spacecraft which a half a century ago was dubbed by some in the press as “the picture of the century”.

The Lunar Orbiter Spacecraft

NASA’s Lunar Orbiter project was started in August 1963 under the responsibility of NASA’s Langley Research Center with its first mission, Lunar Orbiter 1, launched on August 10, 1966 (for details on the early history of the Lunar Orbiter program through the mission of Lunar Orbiter 1, see “Lunar Orbiter 1: America’s First Lunar Satellite”). Lunar Orbiter was designed for a single task: orbit the Moon and take medium to high-resolution images of the lunar surface in order to identify and characterize potential Apollo landing sites located in a zone within five degrees of the equator and ranging from 45° E to 45° W longitude. In order to avoid the ongoing issues with the development of the Atlas-Centaur that was to launch NASA’s one-ton Surveyor lunar lander being built by the Jet Propulsion Laboratory (see “Surveyor 1: America’s First Lunar Landing”), Lunar Orbiter was sized to use the then-new but readily available Atlas-Agena D rocket.

The 385-kilogram, three-axis stabilized spacecraft was designed around a 66-kilogram photographic package built by Eastman-Kodak. Based on Kodak’s previously classified reconnaissance satellite work for the Department of Defense, this subsystem was housed in an ellipsoidal aluminum alloy shell pressurized with dry nitrogen at 120 millibars. Viewing through a quartz window in the side of the shell were a wide-angle 80 mm focal length, f/4.5 lens and a 610 mm focal length, f/5.6 narrow angle lens which would provide medium and high-resolution views of the lunar surface, respectively. These lenses simultaneously produced a pair of images on a single roll of 70 mm Kodak SO-243 high-contrast, fine grain aerial mapping film using exposures of 1/25th, 1/50th, or 1/100th of a second.

About 80 meters of film were carried aboard Lunar Orbiter, allowing as many as 212 high and medium-resolution image pairs to be taken. The 610 mm lens was also used by an electro-optic velocity/height sensor that slowly moved the photographic film during an exposure as part of a motion compensation system to reduce the effects of image smearing caused by spacecraft orbital motion. During its month-long photography mission in a nominal 45 by 1,850-kilometer mapping orbit, the best resolution for the narrow and wide-angle images was expected to be one and 8 meters, respectively.

The exposed film was developed as the photographs were taken using Bimat Transfer Film, which employed spools of a webbing impregnated with the appropriate developing and fixing chemicals that would come into contact with all parts of the exposed film for at least 3½ minutes. The process was similar to that employed by Polaroid instant cameras of that era. Since the photographs could be taken faster than they could be processed, a set of takeup reels were included, allowing up to 21 image pairs to be stored. Once the images were taken and the film was developed, the images were scanned by a 5 micron wide beam of high intensity light at a resolution equivalent of 287 lines per millimeter.

A photomultiplier tube detected the light beam, whose intensity was altered by the film’s image density, and the appropriate electronics converted this signal into a form to be transmitted back to Earth. Each image pair could be transmitted in 43 minutes when both the Earth tracking station and the Sun were visible. The scanned photographs were the equivalent of a 8,360 by 9,880 pixel image for the wide-angle and a 8,360 by 33,288 pixels for the narrow-angle views. The effective storage capacity of this photographic system was the equivalent of several tens of gigabytes of data compared to 615 kilobyte storage capacity of the then state-of-the-art digital magnetic tape recorder employed by the imaging system on Mariner 4 during its historic flyby of Mars in July 1965 (see “Mariner 4 to Mars”). This was one of the reasons why Lunar Orbiter employed a photographic imaging system instead of a digital television system. If time between imaging sessions permitted, photographs could be scanned shortly after they were developed as part of a priority readout sequence to verify system performance. Otherwise, the photographs would all be scanned in the reverse order they were taken after all of the film had been exposed and transmitted back to Earth.

The photographic subsystem was mounted on a 1.4-meter diameter equipment deck located at the base of the 2.0-meter tall, roughly conical-shaped spacecraft. Also mounted on this deck were a Canopus star sensor, five Sun sensors, and an inertial reference unit all used to determine Lunar Orbiter’s attitude to an accuracy of ±0.2°. A flight programmer possessed a 128-word memory that was able to control spacecraft activities for 16 hours worth of photography work. Under the control of this unit, the photographic system could be programmed to take groups of four, eight, or sixteen photographs in a variety of patterns of selected sites during each orbital pass. Depending on the latitude of the target area and the inclination of Lunar Orbiter’s orbit, the rotation of the Moon would allow overlapping coverage on successive orbits.

Data were returned via a boom-mounted, 92-centimeter diameter high-gain dish antenna. A ten-watt S-band transmitter used this antenna to transmit the images back to Earth. A low-gain antenna, dedicated to a half watt transmitter, was also mounted on the equipment deck opposite the high-gain antenna. This antenna was used to return engineering telemetry and non-photographic science data. Four solar panels, spanning a total of 5.2 meters, were also mounted here to provide the orbiter with 375 watts of electrical power. When the spacecraft was in shadow, power was provided by nickel-cadmium batteries recharged by the solar panels.

Mounted on an open truss frame above the equipment deck was the upper structural module. This unit housed the velocity control engine used to place Lunar Orbiter in orbit as well as trim that orbit once there. This engine, based on the Apollo attitude control thruster, produced 445 newtons of thrust using the hypergolic propellants hydrazine and nitrogen tetroxide. These propellants were stored in tanks also located in the upper structural module. Eight nitrogen gas jets mounted at the top of the spacecraft provided attitude control. For temperature control, the entire spacecraft was shrouded in aluminized Mylar-Dacron thermal blankets. The underside of the equipment deck, which would normally face the Sun, was covered with a white thermal paint. These measures were expected to maintain the temperatures of the orbiter’s systems between 2° and 29° C.

The only instruments other than the photographic subsystem carried by Lunar Orbiter were a ring of twenty pressurized meteoroid detectors and a pair of dosimeters to assess any radiation hazards to manned spacecraft in the near-lunar environment. By monitoring the orbital changes of the spacecraft, the mass distribution of the Moon could also be mapped. This knowledge would be essential for the pinpoint accuracy needed for the Apollo landing missions. While the photographic portion of the mission was expected to last no more than one month, these other investigations would employ the spacecraft for up to one year.

Getting Lunar Orbiter 2 On Its Way

With the completion of the largely successful photographic portion of the mission of Lunar Orbiter 1 at the end of August 1966, mission planners worked with representatives from NASA’s Apollo and Surveyor programs to determine the targets of interest for the second mission in the series initially designated “Lunar Orbiter B”. A total of 13 primary sites in the northern half of the equatorial zone of interest to Apollo mission planners were chosen for close scrutiny. The 12° inclination chosen for this mission’s nominal 50 by 1,850-kilometer mapping orbit would allow overlapping coverage of each site on successive orbits as the Moon slowly rotated beneath Lunar Orbiter B. It was anticipated that 184 frames out of the photographic subsystems 211-frame film supply would be used on these targets. Among these targets was the impact site of NASA’s Ranger 8 mission which reached the Moon on February 20, 1965 (see “50 Years Ago Today: The Launch of Ranger 8”).

In order to help keep the film supply moving and prevent the Bimat webbing from sticking to the film and damaging the negatives, photographs had to be acquired every four to eight hours even when one of the primary targets were not in view. During these periods, 17 secondary targets would be photographed with Lunar Orbiter’s remaining 27 frames of film including wide area views of the lunar far side as well as selected oblique views across the lunar landscape towards the horizon. While originally Lunar Orbiter was to acquire all of its photographs looking more or less straight down onto the Moon, a couple of oblique imaging sessions performed by Lunar Orbiter 1 to capture the Earth rising over the lunar horizon were found to have scientific as well as public relations value.

Although the Lunar Orbiter 1 mission was considered successful, it did encounter a number of issues which had to be corrected for the Lunar Orbiter B mission. Among the worst of these was the failure of motion compensation system on Lunar Orbiter 1 which blurred the highest resolution images taken close to perilune (i.e. the closest point to the Moon of a lunar orbit). The shutter triggering circuit on Lunar Orbiter B was altered to make it less susceptible to electronic noise which was believed to be the source of the problem. Lunar Orbiter 1 also experienced problems with stray light confusing its star sensor preventing it from properly locking onto its guide star, Canopus, located near the southern ecliptic pole. The outer end of the low gain antenna as well as the edges and backsides of the solar panels were coated in an antireflective black paint to reduce stray light reaching the sensor. Finally, Lunar Orbiter 1 also experienced a number of issues with thermal control largely due to a faster than expected degradation of the paint coating the underside of the spacecraft’s equipment deck. Lunar Orbiter B would employ a new thermal paint to resolve this problem. Also included on the underside were an instrumented mirror and three metal coupons coated with various types of paint meant to evaluate their usefulness in case the new paint also did not perform as well as required.

The spacecraft assigned to the Lunar Orbiter B mission, Spacecraft No. 5, arrived at Cape Kennedy on June 10, 1966 to serve as the backup for what would become Lunar Orbiter 1 after its successful launch on August 10. No longer needed to support the first mission, Spacecraft No. 5 went into temporary storage at the Cape on July 30. Following the arrival of Spacecraft No. 6 on August 26 to serve as the backup for the Lunar Orbiter B mission, No. 5 was removed from storage four days later to begin modifications and retesting. The photographic subsystem arrived from Kodak on September 4 for installation into Spacecraft No. 5.

In the mean time, the various components of the Atlas-Agena 18 launch vehicle also began to arrive for the Lunar Orbiter B mission. On August 29, Agena number 6631 arrived at the Cape followed the next day by Atlas SLV-3 number 5802. The Atlas SLV-3 was erected on the pad at Launch Complex 13 (LC-13) on September 13 to begin its prelaunch preparation and testing. On October 25, the Agena D was added to the stack with the 390-kilogram Lunar Orbiter B spacecraft encapsulated inside its fairing mated with the launch vehicle six days later.

The first launch attempt was scheduled to take place on November 6, 1966 during a launch window running from 5:58 to 8:35 PM EST. Launch windows were available for the next four days 15 to 85 minutes later on each successive day. Lunar Orbiter B would enter its initial orbit around the Moon about 92 hours after launch. After dropping its perilune to 50 kilometers, Lunar Orbiter B would begin its photography mission on November 18 starting with its eastern-most targets not long after local sunrise to enhance the visibility of surface features. Over the next week, Lunar Orbiter B would work its way westward to image the other primary sites as the Moon rotated beneath it to finish its photographic mission on November 25. With the photographs developed, Lunar Orbiter would then begin scanning all of its images and transmitting them back to Earth. This transmission was expected to be completed on December 13 after which Lunar Orbiter B would continue its non-photographic mission in orbit around the Moon mapping its gravity field as well as monitoring the flux of micrometeorites and radiation.

The Lunar Orbiter 2 Mission

The countdown for the first launch attempt of the Lunar Orbiter B mission started at T-538 minutes on the morning of November 6, 1966. Aside from a couple of minor issues, the countdown was uneventful with Atlas-Agena 18 lifting off from LC-13 at 6:21:00.2 PM EST (23:21:00.2 GMT). A nominal performance by the Atlas SLV-3 followed by an initial 155-second burn of Agena number 6631 placed the upper stage and its payload into a temporary 160-kilometer parking orbit. After coasting for 11 minutes and 17 seconds, the Agena D reignited its engine for 88 seconds to send what was now called Lunar Orbiter 2 on its way to the Moon. Some 24 minutes and 12 seconds after liftoff, Lunar Orbiter 2 separated from its spent final stage 165 seconds after the Agena’s engine shutdown and started unfolding its solar arrays and antennas.

NASA’s Deep Space Network acquired Lunar Orbiter 2 about 51 minutes after launch and started its tracking operations. Unlike its predecessor, Lunar Orbiter 2 quickly aligned itself with the Sun and Canopus to start its cruise to the Moon. After the spacecraft had receded to a distance of 265,485 kilometers from the Earth, Lunar Orbiter 2 was commanded to fire its velocity control engine for 18 seconds at 19:30:00 GMT on November 8 about 44 hours after launch. This changed the spacecraft’s velocity by 21 meters per second and set Lunar Orbiter on course for a point 2,732 kilometers above the lunar surface for orbit insertion.

About 92 hours after launch, Lunar Orbiter 2 was quickly approaching the Moon and began preparations to enter lunar orbit. At 20:26:37.3 GMT on November 10, the spacecraft’s velocity control engine was ignited to enter lunar orbit. After firing for 611.6 seconds, Lunar Orbiter’s velocity had been cut by 830 meters per second to enter an initial 196.3 by 1871.3 kilometer orbit with an inclination of 11.97° – quite close to the desired 200 by 1,850 kilometer orbit with an 11.95° inclination. Including the Soviet Luna 12 orbiter which had arrived 16 days earlier for its lunar photography mission, the Moon once again had two operating spacecraft in orbit (see “Mapping the Moon: The Soviet Luna 11 & 12 Missions”). While in this initial orbit, Lunar Orbiter’s photographic subsystem was commanded to readout and transmit its “Goldstone test film”. This was a pre-exposed leader of the film supply that was meant to provide an end-to-end test of not only the spacecraft’s photographic and transmission subsystems, but also the receiving and image reconstruction hardware back on the Earth.

On November 15 at 22:58:25 GMT, the velocity control engine was fired again during the Orbit 33 for 17.4 seconds to change the velocity of Lunar Orbiter 2 by 28 meters per second and drop its perilune to 50.5 kilometers in preparation for its mapping mission. The photography mission finally started on November 18 during the 52nd orbit with a 16-frame sequence of the first primary target. The mapping mission proceeded essentially as planned for the next 50 orbits with Lunar Orbiter 2 observing all 13 primary and 17 secondary targets by the scheduled end of photography on November 25. During Orbit 105, the command was sent to cut the Bimat webbing and two orbits later, final readout of Lunar Orbiter’s 211 photographs began.

The readout of the photographs taken by Lunar Orbiter 2 continued until Orbit 179 on December 7 when the traveling wave tube amplifier of the high power transmitter failed to turn on when commanded. This failure occurred with eight photographs remaining to be scanned. Fortunately, priority readouts of portions of several of these missed images transmitted not long after they were developed meant that 98.5% of all the photographs secured by Lunar Orbiter 2 were transmitted back to Earth before the 10-watt transmitter failure. Engineering and non-photographic science data would be returned to Earth via the low-power transmitter for the balance of the mission of Lunar Orbiter 2.

The Lunar Orbiter 2 mission had successfully met all of its primary objectives returning a wealth of new high resolution images of the lunar surface. As scientists and mission planners examined these images in detail to choose landing sites for the Apollo and upcoming unmanned Surveyor missions, the press and public were most impressed with one of the oblique images taken by Lunar Orbiter 2 of the prominent 100-kilometer crater called Copernicus. Douglas Lloyd, who worked for the NASA contractor Bellcomm, Inc., had twice suggested taking this photograph but his request was turned down because it would interfere with acquiring images of potential Apollo landing sites. On the third try, Lloyd’s suggestion was finally approved by program officials.

The oblique image was taken at about 23:05 GMT on November 23 from an altitude of 45.9 kilometers above the lunar surface and 240 kilometers south of the famous crater. The photograph for the first time revealed the Moon to be a real place – a dramatic three-dimensional landscape with the three-kilometer deep crater sporting cliffs up to 900 meters high and a series of central peaks scattered over 16 kilometers reaching 450 to 600 meters above the crater floor. The spectacular view was enthusiastically dubbed by some in the press as the “picture of the century”. While this photograph of Copernicus was incredible, it would prove to be only one of many outstanding images that would be taken during the course of dozens of NASA lunar and planetary missions over the 34 years left in the 20th Century which could compete for the ultimate title, “Picture of the Century”.

With its photography mission completed, Lunar Orbiter 2 fired its engine for 62 seconds to increase its orbit inclination to 17.5° on December 8. This allowed the spacecraft to be tracked over a wider swath of the equatorial region to map the Moon’s gravitational field as well as provide ground controllers with experience operating in higher inclination orbits. During its mission, Lunar Orbiter 2 detected only three micrometeorite strikes and radiation measurements showed them to be at safe levels for astronauts. A brief three-second burn of the velocity control engine on April 14, 1967 shortened the spacecraft’s orbital period by 65 seconds reducing the total time the ageing spacecraft would spend in darkness during the total lunar eclipse on April 24.

With its systems deteriorating, Lunar Orbiter 2 fired its engine one last time on October 11, 1967 to decrease its speed by 71 meters per second deliberately crashing the spacecraft out of view of the Earth somewhere about 4°S, 98°E on the lunar far side in the vicinity of the southern rim of the crater today known as Wyld. This eliminated the possibility that unintended transmissions from Lunar Orbiter 2 could interfere with its successors. But as this mission was winding down, preparations were underway to launch Lunar Orbiter C which would finish mapping potential lunar landing sites of highest priority for the Apollo and Surveyor programs.

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Related Reading

“Lunar Orbiter 1: America’s First Lunar Satellite”, Drew Ex Machina, August 14, 2016, [Post]

“Mapping the Moon: The Soviet Luna 11 & 12 Missions”, Drew Ex Machina, October 22, 2016, [Post]

General References

Bruce K. Byers, Destination Moon: A History of the Lunar Orbiter Program, NASA TM X-3487, NASA History Office, 1977

“Lunar Orbiter B”, NASA Press Release 66-286, November 4, 1966

“Lunar Orbiter 2”, TRW Space Log, pp. 43-45, Vol. 6, No. 4, Winter 1966-67

“Lunar Orbiter II: Photographic Mission Summary”, NASA CR-883, October 1967