As work on the Apollo program continued to accelerate as 1967 began, the process of identifying suitable lunar landing sites was also well underway thanks to the successes of NASA’s first two Lunar Orbiter missions in the second half of 1966 (see “Lunar Orbiter 1: America’s First Lunar Satellite” and “Lunar Orbiter 2 and the ‘Picture of the Century’”). After reviewing the photographs already in hand, it was decided that just one more successful mission was required so that the program’s primary objectives could be met. The responsibility of finishing this important phase of the preparations for the Apollo lunar landing missions fell to Lunar Orbiter 3.

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 by its prime contractor, Boeing, 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 take up 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.

Preparing Lunar Orbiter 3

Despite the issues encountered during the Lunar Orbiter 1 and 2 missions, the majority of the missions’ lunar mapping objectives were met. For the third mission, designated Lunar Orbiter C before launch, the goal was to concentrate on confirming potential Apollo and Surveyor landing sites initially surveyed by the first two missions. Essentially, Lunar Orbiter C was considered a site reconnaissance mission unlike the earlier flights which were mapping missions. All together, Lunar Orbiter C was scheduled to photograph 12 potential Apollo landing sites and 32 additional sites of scientific interest as secondary targets including the landing site of Surveyor 1. Originally this site was to be photographed by Lunar Orbiter 1 but the failure of the its motion compensation system prevented it from taking the high resolution images needed to spot the lander on the lunar surface. The plan for Lunar Orbiter C called for photography to be performed during nearly every orbit until all 212 frames of film had been exposed. Readout of the last images was expected to be finished 32 days after launch completing the photographic part of the mission.

For the Lunar Orbiter C mission, the inclination of the lunar orbit was raised from the 12° used by the earlier missions to 21°. This would allow Lunar Orbiter C to obtain stereo imagery of selected areas observed by the previous mission by photographing the same area during successive orbits for stereoscopic viewing. While the need to tilt the camera’s view out of the vertical to support stereo photography would degrade the resolution of the narrow angle images from one meter down to 2½ or 3 meters, the information obtained about the topographic relief of the sites more than made up for this loss in performance. The higher inclination orbit would also allow the lumpy lunar gravitational field to be mapped more accurately aiding navigation of future lunar missions including Apollo. In further support of the Apollo program, Lunar Orbiter C would serve as a target for a test of the Manned Spaceflight Tracking Network.

Spacecraft No. 6, which was to be the primary spacecraft for the Lunar Orbiter C mission, originally arrived at Cape Kennedy (which reverted back to Cape Canaveral in 1973) on August 26, 1966 to serve as the backup for the Lunar Orbiter 2 mission. After the successful launch of the second Lunar Orbiter mission on November 6, Spacecraft No. 6 was free to be employed on the third mission and was placed into storage. On November 21, Spacecraft No. 7 arrived at the Cape to serve as the backup for the new mission.

On January 2, 1967, Spacecraft No. 6 was removed from storage to begin preparations for its mission. In addition to functional testing and repairs made to faulty or suspect systems, a number of modifications were also made in response to the problems uncovered during the previous mission. Notably, the traveling wave tube amplifier (TWTA) of the high power transmitter used to return photographs back to Earth was replaced and modified by Boeing engineers to remove excess heat buildup more effectively. The TWTA on Lunar Orbiter 2 unexpectedly failed during the final readout of that mission’s photographs and the change would lower the component’s temperature and increase its lifetime. Kodak also made modifications to the photographic subsystem to reduce the incidence of photographic defects caused by bubbles and manufacturing irregularities in the Bimat webbing used to develop the film. On January 5, the photographic subsystem was installed in Spacecraft No. 6.

A Launch Readiness Review held on January 17, 1967 found both spacecraft ready for the Lunar Orbiter C mission and No. 6 was selected for launch scheduled for the evening of February 2, local time. After further testing, the spacecraft’s thermal blankets were installed on January 23. During subsequent testing it was discovered that one of the micrometeoroid detectors had been punctured but the decision was made not to replace it and proceed with launch preparations. The 385-kilogram spacecraft was encapsulated in its launch shroud on January 25 and moved to Launch Complex 13 (LC-13).

The launch vehicle for Lunar Orbiter was an Atlas-Agena D rocket. The first stage for this mission consisted of a General Dynamics Atlas SLV-3 rocket serial number 5803 while the second stage was a Lockheed Agena D serial number 6632. The configuration of the launch vehicle was essentially identical to those used on the previous two Lunar Orbiter missions save for new light weight engine boots used on the Atlas SLV-3. After Spacecraft No. 6 was mated to the launch vehicle at LC-13 on January 26, preflight checks were performed culminating in a successful countdown rehearsal on January 31. The Lunar Orbiter C mission was now ready to get underway.

The Mission

The original February 2, 1967 launch window starting at 7:22:09 PM EST (00:22:09 GMT on February 3) was missed due to problems with the ground power supply at LC-13. The affected system was replaced and the countdown recycled for launch on the evening of February 4, local time. Despite some issues encountered during the countdown, Lunar Orbiter 3 successfully lifted off near the beginning of its 107-minute launch window at 8:17:01 PM EST (01:17:01 GMT on February 5). A nearly perfect performance by the Atlas SLV-3 resulted in the rocket’s sustainer engine shutting down just a fraction of a second later than planned 288 seconds after launch followed by the ignition of the Agena’s main engine. After a burn of just under 156 seconds, Agena number 6632 with Lunar Orbiter 3 still attached had been placed into a temporary 179.4 by 201.9 kilometer Earth parking orbit.

After coasting for nine minutes and 38 seconds in low orbit, the Agena’s engine was reignited for an 88.7-second burn which placed Lunar Orbiter 3 on a trajectory that would reach the Moon after a 92-hour transit. With its job completed, Agena number 6632 separated from Lunar Orbiter 3 and performed a retro maneuver to deflect itself safely away from the spacecraft and the Moon. Afterwards, Agena 6632 was tracked in a 24,611 by 428,662 kilometer geocentric orbit which reached the vicinity of the Moon six hours after Lunar Orbiter 3 some 17,000 kilometers beyond the lunar capture radius. In the mean time, Lunar Orbiter 3 unfolded its solar panels and antennas and settled in for its cruise to the Moon.

Early tracking indicated that Lunar Orbiter 3 would require at least one course correction in order to hit its target point for lunar orbit insertion. At 15:00:00 GMT on February 6, the receding spacecraft fired its velocity control engine for 4.4 seconds for a delta-v of 4.4 meters per second. This small burn moved the spacecraft’s aim point by 835 kilometers and pushed out its arrival time by about 19 minutes. Lunar Orbiter 3 was now on course to the Moon with a closest approach distance of 862 kilometers above the surface. On this same day, ground controllers began to simultaneously track both Lunar Orbiter 3 as well as its predecessor already in lunar orbit for three months demonstrating the ability to track two spacecraft in the vicinity of the Moon at the same time. The following day it was decided that a second course correction scheduled for 70 hours after launch would not be required.

At 21:38:38 GMT on February 8, 1967, Lunar Orbiter 3 started turning out of its cruise attitude and aligned its velocity control engine for lunar orbit insertion. For this maneuver, a plane change of 13° from the approach trajectory was required not only to get the final orbit inclination up to 21° but also properly align the orbit’s low point or perilune with respect to the mission’s target areas to begin the photography mission a week later. At 21:54:19 GMT, Lunar Orbiter’s engine ignited for a 542.5-second burn for a total delta-v of 704.3 meters per second. Lunar Orbiter 3 was now in a 210 by 1,802 kilometer orbit with an inclination of 20.9° – not far from the nominal 200 by 1,850 kilometer initial orbit that was desired.

After tracking the spacecraft and checking its systems, the velocity control engine was ignited for a third time on February 12 at 18:13:27 GMT. This 33.7 second burn changed the spacecraft’s velocity by 50.7 meters per second in order to take up a 55 by 1,847 kilometer orbit which was expected to dip as low as 48 kilometers during the course of the mapping mission as it evolved over time. Two days later, Lunar Orbiter 3 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. With the successful completion of this test, Lunar Orbiter 3 was ready to start its reconnaissance mission.

Lunar photography started on the mission’s 44th orbit at 10:00:41 GMT on February 15, 1967 with the first primary target site being in Mare Tranquillitatis near the crater Maskelyne F. Initial readout of the first photographs starting the following orbit showed that they were of excellent quality. As Lunar Orbiter continued systematically photographing its primary and secondary targets, however, engineers were growing increasingly concerned about the erratic behavior of the film advance mechanism. If this mechanism failed, the majority of the mission’s photographs would be lost. The decision was made to forego photographing the last secondary target – an oblique view of the crater named Grimaldi – and begin the readout of all the photographs a day early after 211 frames out of the planned 212 were exposed.

On February 23 at 6:36:42 GMT, Lunar Orbiter 3 was commanded to cut its Bimat webbing during orbit 99 and commence with its final readout of the exposed film during the following orbit. Despite problems with the film advance mechanism which intermittently hampered readout, by March 1 Lunar Orbiter 3 had returned 114 frames or 54% of its photographs during the final readout. A major anomaly in the film drive was experienced on March 2 during orbit 149 at 15:45:33 GMT which prevented the readout from starting when commanded. Subsequent attempts to restart the readout resulted in the inadvertent activation of the film advance motor causing it to stall and burnout. In retrospect, the decision to begin the readout early proved to be a good one with 72 of the 211 frames remaining to be readout when the motor failed.

Wrapping up the Lunar Orbiter 3 Mission

Despite the failure of the film advance motor, about 75% of all the photographic data gathered by Lunar Orbiter 3 had been transmitted during priority and final readouts. Even with the loss, the mission had met all of its objectives with 650,000 square kilometers of the Moon’s near side and 15.5 million square kilometers of the far side photographed.

Based on an analysis of the photographs returned by the first three Lunar Orbiter missions, by April 1967 scientists had selected eight preliminary landing sites for the upcoming Apollo missions. One of them, designated Site 2, would eventually become the landing site of the Apollo 11 mission on July 20, 1969. Another, Site 7, would serve as the landing site of NASA’s upcoming Surveyor 3 mission which would subsequently be visited by the Apollo 12 mission in November 1969 (see “Surveyor 3: Touching the Face of the Moon“). Lunar Orbiter 3 also succeeding in photographing Surveyor 1 on the lunar surface on February 21 during orbit 91. In addition to helping to place Surveyor’s surface findings into a broader, regional context, this was the first time that a man made object was imaged on the surface of another world.

With its photographic mission completed, Lunar Orbiter 3 proceeded with the next phase of its extended mission mapping the Moon’s gravitational field. On April 12, 1967 the velocity control engine of Lunar Orbiter 3 was used to place the spacecraft into a 143 by 315 kilometer orbit in part to avoid prolonged periods of darkness during an upcoming lunar eclipse on April 24 (Lunar Orbiter 2, which was still on its extended mission, was likewise commanded to alter its orbit slightly for the same reason two days later). On August 30, Lunar Orbiter 3 fired its engine for 125.5 seconds to circularize its orbit at an altitude of 160 kilometers. This would simulate Apollo’s standard “100-mile” lunar orbit for tracking exercises as well as allow for a more accurate mapping of the Moon’s gravitational field.

A sixth and final burn of the velocity control engine on October 9, 1967 for 32 seconds cut the orbiter’s velocity by 53 meters per second. Lunar Orbiter 3 deliberately crashed into the lunar surface at 14.6° N, 91.7° W near the crater today known as Einstein. With the first three Lunar Orbiters succeeding in meeting the program’s main objective of mapping potential Apollo landing sites, the final two missions were now free to pursue more science-oriented objectives from high-inclination, polar orbits.

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Related Reading

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

“Lunar Orbiter 2 and the ‘Picture of the Century’”, Drew Ex Machina, November 23, 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

L.J. Kosofsky and Farouk El-Baz, The Moon as Viewed by Lunar Orbiter, NASA SP-200, 1970

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

Andrew Wilson, Solar System Log, Janes Publishing, 1987

“Lunar Orbiter 3 Mission System Performance”, CR-66461, Boeing, August 11, 1967