With the historic landing of Apollo 11 on the Moon, the summer of 1969 was quite memorable for me as a budding seven-year-old space enthusiast. And among my arsenal of reference material for that event, I had a map of the Moon prepared by the National Geographic Society which had been released earlier in February 1969. While I had seen maps of the near side of the Moon for use by amateur astronomers at that stage and had seen a smattering of images of the lunar farside in books and magazines, this National Geographic map of the Moon was the first time I got to see the entire lunar surface rendered in one cartographic product.

This map of the Moon, as well as the more detailed versions used by NASA scientists to select the landing sites for Apollo 11 and subsequent missions, were made using photographs acquired by the sometimes underappreciated series of missions that were part of NASA’s Lunar Orbiter program. As its name implies, this series of spacecraft was designed to orbit the Moon. But unlike the earlier Soviet and American lunar missions which briefly imaged limited parts of the lunar surface, the primary objective of the Lunar Orbiter series was to map most, if not all, of the lunar surface with emphasis on the equatorial region of the lunar near side which was the prime location for the first Apollo Moon landings. Along with the earlier Ranger impact photography missions and the contemporary Surveyor unmanned lunar landings, the Lunar Orbiter missions were among the necessary precursors for the Apollo Moon landings.

The Origins of Lunar Orbiter

Ironically, the very first probes the US launched towards the Moon back in 1958 were intended to be lunar orbiters. Originally part of a USAF program, these first three Pioneer missions attempted to send a simple 38-kilogram spacecraft close enough to the Moon so that they could fire a solid rocket motor to enter lunar orbit. Unfortunately they all failed to reach the Moon due to problems with their Thor-Able launch vehicles (see “Pioneer 1: NASA First Space Mission“). A follow up program using a trio of much more advanced spin-stabilized Pioneer lunar orbiters launched during 1959 and 1960 using the more capable Atlas-Able rocket likewise suffered from launch vehicle problems and were even less successful (see “NASA’s Forgotten Lunar Program”). Afterwards, NASA turned its attention towards developing a new series of lunar orbiters with the goal of mapping the Moon in support of its new Apollo lunar landing program.

Initially the task of building a lunar mapper was part of the Surveyor program managed by the Jet Propulsion Laboratory (JPL). While the “Surveyor A” spacecraft was designed to soft land on the Moon, the “Surveyor B” variant would be placed into a 100-kilometer high lunar orbit to perform television reconnaissance of the Moon’s surface as well as perform other measurements of the lunar environment for a period of six months (see “Surveyor 1: America’s First Lunar Landing”). On January 19, 1961, Hughes Aircraft (whose space division is now part of Boeing) received the contract to build Surveyor with the goal of launching the first Surveyor lander in 1963.

Unfortunately, the ambitious Surveyor program fell far behind its original schedule over the course of the next two years as did the development of the temperamental Atlas-Centaur which was needed to launch this advanced one-ton spacecraft (see “50 Years Ago Today: The Launch of Atlas-Centaur 5”). Coupled with a string of failures with JPL’s Ranger program attempting to hard-land a seismometer package on the lunar surface during 1962 (see “NASA’s First Moon Lander”), there were concerns among many in NASA’s management that JPL would not be able to deliver the promised lunar mapper in time to support the Apollo program. In the end it was decided to restructure the Surveyor program to concentrate just on the lunar landing mission. Responsibility for the lunar mapper was transferred from JPL and given to NASA’s Langley Research Center in August 1963 with JPL relegated to a support role.

Langley issued a request for proposals on August 30, 1963 for a spacecraft which would acquire one-meter class images of potential Apollo landing sites between 45° east and west longitude and within 5° of the lunar equator. In addition, the new spacecraft would use the already-available Atlas-Agena D launch vehicle along with a new and larger launch shroud based on the UNIPAC design NASA was developing. The Atlas SLV-3, built by General Dynamics, was a “Standard Launch Vehicle” derived from the Atlas ICBM while the Agena D, built by Lockheed Space and Missile Company (which, after decades of corporate mergers, is now part of the aerospace giant Lockheed Martin along with what was General Dynamics Space Systems Division), was the latest version of the adaptable Agena upper stage designed for use with the Thor, Atlas, and Titan IIIB launch vehicles. NASA had already chosen the Atlas-Agena D to launch its pair of Mariner Mars 1964 spacecraft because of the ongoing issues with the Atlas-Centaur (see “50 Years Ago Today: The Launch of Mariner 3”). In December 1963, NASA selected the bid by Boeing and on May 10, 1964, the company received an $80 million contract to build five spacecraft as part of a $200 million (or $1.6 billion in today’s money) Lunar Orbiter program.

The Lunar Orbiter Spacecraft

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. The 385-kilogram, three-axis stabilized spacecraft was designed around a 66-kilogram photographic package built by Eastman-Kodak. A photographic system, where the exposed film was developed onboard and subsequently scanned for transmission to Earth, was chosen over a television-based system originally envisioned for Surveyor B because of the former’s superior resolution and image storage capacity. Soviet engineers adopted a similar approach with the photo-television systems used in their early lunar and planetary probes for precisely the same reasons.

Lunar Orbiter’s photographic subsystem, based on Kodak’s previously classified reconnaissance satellite work for the Department of Defense, 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 15 to 30 day-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”).

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.

Data were returned via a boom-mounted, 92-centimeter diameter high-gain dish antenna. A ten-watt S-band transmitter used this to transmit the images back to Earth. A low-gain antenna, dedicated to a one-half watt transmitter, was also mounted on the equipment deck opposite the high-gain antenna. This antenna was used to return engineering telemetry. 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.

Preparing for the First Mission

For the first Lunar Orbiter mission, the primary objective was to secure medium and high resolution images covering 30,000 and 10,000 square kilometers, respectively, from an elliptical 45 by 1,850-kilometer mapping orbit inclined 12° to the lunar equator. Based on recommendations drawn up from various sources including NASA’s Apollo and Surveyor programs, nine primary and seven backup sites near the lunar equator were selected by a committee for close scrutiny to help scientists establish a “calibration” of the lunar surface’s major terrain types. The targets included potential Apollo landing sites as well as that of Surveyor 1 which landed on the lunar surface on June 2, 1966. Surveyor’s mast-mounted solar panel and high gain antenna would be purposely oriented to cast the largest shadow possible in hopes that it would be spotted in the high resolution photographs.

The first piece of mission hardware to arrive at Cape Kennedy for this first mission was Atlas SLV-3 number 5801 on February 18, 1966. The Atlas was erected on the pad at Launch Complex 13 (LC-13) on March 10 for the start of a series of prelaunch checks. But as preparations continued towards a tentative July 11 launch date, NASA’s hopes for being first to orbit the Moon had been dashed on April 3 when the Soviet Luna 10 probe successfully entered lunar orbit (see “Luna 10: The First Lunar Satellite”). Although Luna 10 did not carry an imaging system, it did return a wealth of scientific information about the Moon and its environment as well as transmit the Communist Party anthem, Internationale, during a session of the 23rd Congress of the Communist Party then taking place.

As Luna 10 was performing its propaganda function for the 23rd Congress on April 4, the Agena D second stage, number 6630, arrived at Cape Kennedy to start its prelaunch checkout. As preparations for launch continued, delivery of the photographic subsystem for the first Lunar Orbiter to be launch, spacecraft number 4 designated “Lunar Orbiter A” before launch, was delayed as Kodak struggled to finish the unit. Between these delays and scheduling conflicts with NASA’s Deep Space Network which was supporting the Surveyor 1 lunar lander mission, the launch of Lunar Orbiter A was pushed back to August 9.

These ongoing delays with Lunar Orbiter A gave another spacecraft a chance to become the first American spacecraft to orbit the Moon. That distinction fell to a little-known spacecraft built and operated by NASA’s Goddard Space Flight Center (GSFC) called Explorer 33 (see “AIMP: The Forgotten Lunar Orbiters“). This spacecraft was the fourth in their Interplanetary Monitoring Platform (IMP) series. Starting with the launch of Explorer 18 on November 1963, this program’s goal was to place satellites, loaded with particle and fields instrumentation, into highly eccentric orbits in order to study the Earth’s magnetosphere and its interaction with the Sun-dominated interplanetary environment.

Explorer 33 was to be the first “Anchored” IMP, or AIMP, with the Moon serving as the anchor. From this vantage point, Explorer 33 could continuously monitor the radiation and magnetic field environment from lunar distances, unlike the previous IMPs which would periodically swing back towards Earth in their elongated geocentric orbits. A secondary objective for this Anchored IMP was to study the Moon’s effect on this environment as well as the lunar gravitational field.

The relatively simple 93-kilogram AIMP design employed a 37-kilogram solid retrorocket motor to enter a 1,300 by 6,400-kilometer lunar orbit with an inclination of 175° and an orbital period of about ten hours. Without a secondary propulsion system to correct its trajectory, AIMP had to rely on the accuracy of its Delta E launch vehicle (a descendant of the Thor-Able used to launch the first Pioneer lunar orbiters) to reach the proper point with respect to the Moon to enter orbit – a risky approach adopted to simplify spacecraft design and operations.

On July 1, 1966 Explorer 33 lifted off from LC-17A at Cape Kennedy. As luck would have it, the Delta’s second and third stages worked slightly better than anticipated and imparted an excess velocity of 21.3 meters per second to Explorer 33. Although Delta’s second and third stages worked well within their specifications, this excess velocity was just enough so that Explorer 33 could not enter lunar orbit. Instead, ground controllers fired the tiny Explorer’s rocket motor to place the IMP into an extended 30,550 by 449,174-kilometer Earth orbit where Explorer 33 would conduct an alternate mission similar to previous IMPs. Another attempt to launch an AIMP was scheduled for one year later.

Meanwhile, Kodak finally delivered the lunar photography system to NASA on July 19. As the Agena D was being mated with the Atlas at LC-13 on July 23, work continued on finishing Lunar Orbiter A which was finally certified for launch on July 26. The spacecraft was mated with its now completed Atlas-Agena D number 17 launch vehicle on August 2. With the successful conclusions of testing culminating with a successful simulated launch on August 6, Lunar Orbiter A was finally ready for its mission.

The Lunar Orbiter 1 Mission

The window for the first Lunar Orbiter launch attempt on August 9, 1966 ran from 12:07 to 4:43 PM EDT. After dealing with a series of minor problems, the countdown was halted at T-7 minutes when an issue was encountered with the Atlas’ propellant utilization system. The launch was ultimately scrubbed and a second attempt was scheduled for the following day with the window running from 1:43 to 6:02 PM EDT.

On August 10, the countdown proceeded without incident until a fuel tank pressurization issue with the Agena forced the clock to stop at T-35 minutes. After a 15-minute hold, the countdown resumed and Atlas-Agena D number 17 finally lifted off from LC-13 at 3:26:00.7 PM EDT (19:48:00.7 GMT) sending Lunar Orbiter 1 on its way. After 312 seconds of powered flight, the Atlas shut down and the Agena ignited for a 154.5-second burn which placed the Agena and its payload into a temporary 189 by 202-kilometer parking orbit inclined 29.6° to the Earth’s equator. After coasting for about 33 minutes, the Agena D reignited its engine for 89 seconds placing itself and its payload into a geocentric 194 by 348,524-kilometer orbit that would carry the spacecraft to the vicinity of the Moon. Less than 41 minutes after launch, Lunar Orbiter 1 separated from the spent Agena D for its 92-hour transit to the Moon.

Finally on its way to the Moon, Lunar Orbiter 1 proceeded to unfold its solar panels, deploy its antennas and start the process of orientating itself for its mission. While the spacecraft quickly locked onto the Sun, problems were encountered when Lunar Orbiter’s star sensor was unable to lock onto its second celestial reference, the bright star Canopus, conveniently located near the south ecliptic pole. Troubleshooting from the ground quickly diagnosed the problem as being the result of stray light reflecting off of the spacecraft – an issue that should have been detected during preflight testing.

As engineers considered various solutions to the star sensor issue, they were able to roll the spacecraft to lock onto the bright Moon to serve as a temporary attitude reference. With the attitude stabilized, Lunar Orbiter 1 fired its engine for 32 seconds some 24.7 hours after launch to ensure the spacecraft would arrive within 80 kilometers of its intended aim point relative to the Moon. After this maneuver, Lunar Orbiter started overheating forcing ground controllers to reorient the spacecraft the spacecraft 36° off of the Sun for 8½ hours in an effort to cool the equipment deck and stem the degradation of its thermal paint. While work-arounds were eventually found, ground controllers would continue to struggle with the star sensor and thermal control issues for the rest of the mission.

Just under 92 hour after launch at 15:22:56 GMT on August 14, ground controllers began transmitting a series of commands to place their spacecraft into lunar orbit. Lunar Orbiter 1 fired its main engine for 579 seconds cutting its velocity by 790 meters per second to enter a 189 by 1867-kilometer orbit around the Moon with an inclination of 12.2° at 15:43 GMT. After eight attempts over as many years, the United States had finally placed a satellite into orbit around the Moon.

The following day, during its sixth orbit around the Moon, Lunar Orbiter 1 began 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. Despite the ongoing thermal control issues that threatened the photographic film supply, which were solved by periodically turning the equipment deck slightly away from the Sun, the test went well.

Finally on August 18, Lunar Orbiter 1 began its first photography session during its 26th orbit acquiring one batch of 16 frames in the high resolution mode and another four frames in the medium resolution mode. These first images were then developed and transmitted back to Earth five hours later. While the medium resolution images were of excellent quality, the high resolution images were smeared because of a failure in the motion compensation system used to correct for spacecraft orbital motion. Subsequent test images acquired over the next two days for a total of 20 photographs found that electrical interference was generating spurious signals which caused the camera shutter to trip out of sync with the motion of the film.

At 9:49:58 GMT on August 21, Lunar Orbiter 1 fired its engine for a third time to trim its orbit to 58 by 1,855 kilometers with an inclination of 12.3° in preparation for the formal start of its mapping mission. The perilune was lowered again on August 25 to an altitude of 40 kilometers in a last ditch attempt to get the motion compensation system to work properly. While the wide angle images looked fine, the high resolution images were still hopelessly smeared. Ultimately the problem could not be resolved resulting in the loss of the mission’s highest resolution images. The narrow angle images were usable only when the motion compensation system was turned off and the spacecraft was at higher altitudes closer to apolune.

Early on in the mission, it was discovered that the Bimat webbing was sticking to the film damaging the photographs in places as a result. Originally the plan was to advance the Bimat at least once every 15 hours which meant that at least two images had to be acquired every four orbits. Because of the sticking issue, it was decided to expose some photographs during every orbit instead. Along with the extra images acquired to diagnose the high resolution image smearing issues, this necessitated a change in the photography plans with three of the primary targets getting half the originally planned coverage.

But the need to take additional images during each orbit also opened up some opportunities for Lunar Orbiter’s camera. Program managers wanted to turn Lunar Orbiter away from its normal nadir-viewing attitude during one of its passes behind the Moon and capture the Earth rising above the lunar horizon. Making this maneuver away from the normal vertical orientation while out of touch with ground controllers was risky and opened the possibility, however remote, of losing Lunar Orbiter. The plan was opposed by the conservative Boeing engineers but NASA program officials thought the move was worth the risk. On the responsibility of NASA in case the mission were lost, the photograph was taken at 16:35 GMT on August 23 during the 16th orbit. Lunar Orbiter 1 perfectly executed its commands to acquire the first-ever picture of the Earth as seen from the Moon and returned to its normal orientation afterwards. The feat was repeated with a similar imaging geometry during the 26th orbit a day and a half later. In addition to the public relation value of the “Earthrise” photographs, scientists found the out-of-the-vertical views of the lunar surface to be useful as well. Plans for more oblique photography would be included in future Lunar Orbiter missions as a means of minimizing the Bimat sticking issues.

On August 29, 1966, Lunar Orbiter 1 finished its photography mission when it exhausted its film supply. Readout of the 205 exposed photographs began the next day and was completed on September 16 when the spacecraft began its extended mission. Despite the failure of the motion compensation system, not to mention the star sensor and thermal control issues, 75% of the objectives were met and the mission was deemed a success.

Following an initial assessment of the photographs and other data returned by Lunar Orbiter 1, the project’s science team presented their initial results during a press conference at Langley on October 6. The images returned confirmed that the lunar surface was capable of supporting a lander due to the presence of large boulders in various areas. Although the failure to secure high-resolution images meant that Surveyor 1 could not be spotted from orbit as originally planned, medium-resolution images of its landing area showed it to have 20% fewer craters than other lunar maria, making it a good candidate of a future manned landing. A total of eleven sites on the lunar farside were photographed from high altitude at lower resolution. Despite their comparatively lower resolution, these images were superior in quality to those returned by the Soviet Luna 3 and Zond 3 probes in 1959 and 1965, respectively, and confirmed the observation that this region of the Moon was almost completely devoid of large maria that dominate the familiar lunar near side.

During Lunar Orbiter‘s entire time in lunar orbit, not a single micrometeoroid impact was recorded, compared to the four that would be expected if the experiment were conducted in Earth orbit. The measured radiation dose was as predicted before the flight and was deemed not to be a problem for the upcoming Apollo missions. Tracking of the evolution of spacecraft’s orbit allowed the shape of the Moon to be determined better than ever before. Like the Earth, the Moon turned out to be slightly pear shaped with the pointed end at the north pole. In addition, there were hints of irregularities in the Moon’s gravitational field that had also been detected by the Soviet Luna 10. This was the first indication of the presence of mass concentrations or mascons beneath some of the largest impact basins on the lunar surface. Future missions with more inclined orbits would be required to properly map these.

By October 28, 1966, the systems on Lunar Orbiter 1 had degraded significantly due in part to the ongoing thermal control issues. Overheating had resulted in significant deterioration of the batteries and other power supply components as well as the flight programmer, the transponder and the inertial guidance system used to maintain attitude. In addition, the extra attitude changes required to keep the spacecraft cool during the last 2½ months had used far more nitrogen gas than planned. At best, only two to five weeks of gas was left. Fearing that a system failure would leave them unable to shut off Lunar Orbiter’s transmitter thus jeopardizing the upcoming mission of Lunar Orbiter B, the decision was made to purposely crash Lunar Orbiter 1 into the lunar surface to dispose of the craft.

On October 29, Lunar Orbiter 1, after completing 577 orbits, fired its main engine one last time for 97 seconds. The spacecraft dropped from lunar orbit and crashed on the far side of the Moon at 6.7° north latitude, 162° east longitude at about 13:29 GMT. The mission of the first successful American lunar satellite was completed and the way was clear for the launch of the second Lunar Orbiter the following week. With the first batch of clear images of potential landing sites in hand, NASA was one step closer to its goal of landing men on the Moon before the end of the decade.

Follow Drew Ex Machina on Facebook.

Related Video

Here is a NASA film, The Lunar Orbiter: A Spacecraft to Advance Lunar Exploration, summarizing what NASA hope to accomplish with the Lunar Orbiter Program.

Related Reading

“Surveyor 1: America’s First Lunar Landing”, Drew Ex Machina, May 30, 2016 [Post]

“Luna 10: The First Lunar Satellite”, Drew Ex Machina, March 31, 2016 [Post]

“AIMP: The Forgotten Lunar Orbiters”, Drew Ex Machina, July 28, 2017 [Post]

“NASA’s Forgotten Lunar Program”, Drew Ex Machina, September 27, 2015 [Post]

General References

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

Michael M. Mirabito, The Exploration of Outer Space with Cameras, McFarland, 1983

Raymond N. Watts, Jr., “Lunar Orbiter Surveys the Moon”, Sky & Telescope, pp. 192-196, Vol. 32, No. 4, October 1966

“Lunar Orbiter A”, NASA Press Release 66-195, July 29, 1966

“Lunar Orbiter A Flash Flight Report”, TR-422, Kennedy Space Center, August 15, 1966

“Lunar Orbiter 1”, TRW Space Log, pp. 40-43, Vol. 6, No. 4, Winter 1966-67

“Lunar Orbiter 1 Preliminary Results”, NASA SP-197, 1969