As 1967 began, NASA had ambitious plans for the year in their push to get Apollo to the Moon. But while the tragic loss of the Apollo 1 crew during a pad fire effectively put the brakes on the Apollo program as the cause of the accident was investigated (see “The Future That Never Came: The Unflown Mission of Apollo 1”), work continued on the eight automated precursor missions to the Moon NASA had scheduled for the year.

The first up was Lunar Orbiter 3 which lifted off on February 5, 1967 – just nine days after the Apollo 1 accident. By the end of the month, Lunar Orbiter 3 had completed its photographic reconnaissance of future Apollo and Surveyor lunar landing sites and had started its extended mission (see “Lunar Orbiter 3: Preparing for Apollo”). On April 17, NASA launched Surveyor 3 which landed on the lunar surface three days later for its two-week long investigation of another potential Apollo landing site (see “Surveyor 3: Touching the Face of the Moon”). Just as Surveyor 3 was wrapping up its mission, NASA was already preparing to launch their second Lunar Orbiter mission of the year during the first week of May to continue the agency’s push to the Moon.

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.

Objectives of Lunar Orbiter D

Since the Lunar Orbiter program’s primary objective of mapping potential Apollo landing sites had been effectively met by the conclusion of the Lunar Orbiter 3 mission (see “Lunar Orbiter 3: Preparing for Apollo”), the fourth mission of the series, designated Lunar Orbiter D before launch, was free to pursue other objectives with a focus on lunar science. Based on inputs from NASA’s Surveyor and Lunar Orbiter science team representatives, the primary objective of the Lunar Orbiter D mission was to perform a systematic photographic survey of the lunar surface in order to increase our scientific knowledge of the nature, origin and processes that shaped its surface. This information would be used for the selection of sites for future detailed study from orbit and the ground. Secondary objectives for this mission included improving our knowledge of the Moon’s gravitational field as well as providing measurements of the flux of micrometeoroids and radiation in the vicinity of the Moon.

In order to meet its mission objectives, Lunar Orbiter D would be placed into a 2,520 by 6,290 kilometer orbit with an orbital period of 12 hours and an inclination of 85.5°. This was a much higher orbit than the nominal 45 by 1,850 kilometer mapping orbit with an inclination of 12° employed for the first Lunar Orbiter missions. In order to optimize the lighting for photography, the near-polar orbit of Lunar Orbiter D would be aligned so that its plane would be 10° to 30° from the lunar terminator with the closest point or perilune aligned with the equator on the nearside and the farthest point or apolune over the equator on the farside. From this orbit, contiguous coverage of at least 80% of the nearside would be possible with a resolution of 50 to 100 meters using the high-resolution telephoto system – about an order of magnitude better than was possible with Earth-based telescopic photography. Contiguous coverage of the farside with resolution as good as 1.2 to 1.6 kilometers would also be possible.

The photography sequence to be followed by Lunar Orbiter D was very different than that of the earlier missions. Each orbit would typically include five single-frame imaging sequences during the ascending pass across the nearside to produce a contiguous swath of telephoto images which overlapped with the swath of the previous orbit as the Moon slowly rotated beneath the spacecraft. One of these single frame sequences would alternate coverage of the north or south polar regions on odd and even numbered orbits, respectively. A sixth imaging sequence would take place on even numbered orbits over the lunar farside. In order to orient Lunar Orbiter’s camera properly for each image sequence, the spacecraft would need to perform about 200 camera-pointing attitude changes during the course of its nominal 29-orbit photography mission compared to the 50 typically performed during earlier mapping missions.

With the long-period orbit, which provided nearly continuous visibility of the Earth, as well as the long period between the near and far side photography sessions, the photographs from Lunar Orbiter D would be scanned using the priority readout mode shortly after they had been processed. After the 212-frame film supply had been used up and developed, the normal end-of-the-mission readout mode would be used to scan the photographs in reverse order until all of the photographs that had been missed or experienced problems during the priority readout had been scanned. This procedure was also insurance to minimize any losses from a failure in the film advance motor which had cut short the photographic readout at the end of the Lunar Orbiter 3 imaging mission.

Since Lunar Orbiter D would spend most of its time exposed to the Sun, modifications were made to the power subsystem charge controller to decrease the current charging the battery by half in order to lower the heat load it produced as well as help limit battery overcharging. In addition, about 20% of the underside of the equipment deck was covered with 2.5-centimeter square quartz mirrors to help reflect more sunlight away from the spacecraft to keep temperatures down. Additional instrumented test coupons covered with various thermal coatings were also carried to test their properties during spaceflight. The maximum allowable pressure in the nitrogen gas tanks used for attitude control was also increased by 6.5% in order to provide more gas to support the greater number of attitude changes required by this mission.

The Lunar Orbiter D mission had four launch windows available during the first week of May 1967. The first ran from 4:57 PM to 8:10 PM EDT on May 4. The following three days had launch widows with about the same opening time which increased in duration to just over four hours on the evening of May 7. During its transit to the Moon, Lunar Orbiter D was scheduled to make two midcourse maneuvers. Since the Atlas-Agena D launch vehicle for the Lunar Orbiter D mission was programmed to place the spacecraft into a lunar orbit with an inclination of 21° like that of Lunar Orbiter 3 (and there was insufficient time to change the guidance program after the high-inclination mission had been approved), Lunar Orbiter D would make a large maneuver to change the plane of its outbound trajectory to ensure that it would take up the required high-inclination orbit with the desired orientation upon reaching the Moon.

After performing a second course correction maneuver if it were deemed necessary, Lunar Orbiter D would fire its velocity control engine about 89 hours after launch at a distance of 3,780 kilometers from the Moon to enter a high 2,520 by 6,290 kilometer orbit. Regardless of the actual launch date, photography was scheduled to begin on May 11 and then proceed for the following 28 orbits. After the final readout of the photographs, Lunar Orbiter D would start its extended mission of up to 11 months mapping the Moon’s gravitational field as well as continuing to monitor the flux of micrometeoroids and radiation.

Spacecraft No. 7, which was to be the primary spacecraft for the Lunar Orbiter D mission, originally arrived at Cape Kennedy (which reverted back to Cape Canaveral in 1973) on November 21, 1966 to serve as the backup for the Lunar Orbiter 3 mission. After the successful launch of the third Lunar Orbiter mission on February 8, 1967, Spacecraft No. 7 was placed into storage until it was needed for the Lunar Orbiter D mission. On March 10, Spacecraft No. 3 arrived at the Cape to serve as the backup in case a problem was found with the primary spacecraft. On March 23, 1967, Spacecraft No. 7 was removed from storage to begin preparations for its mission.

In the mean time, the components of the mission’s Atlas-Agena D launch vehicle began to arrive at the Cape on March 1, 1967 with the delivery of the Lockheed Agena D serial number 6633. The General Dynamics Atlas SLV-3 serial number 5804 arrived the following day and was erected on the pad at Launch Complex 13 (LC-13) on March 13. As testing and preparation of the launch vehicle stages were proceeding, there were concerns raised by the failure of the Atlas-Agena D to place NASA’s ATS 2 satellite into the proper orbit. The problem appeared to be the result of a failure of the Agena D to reignite due to a valve issue. The suspect valve on Agena 6633 was replaced during the course of flight preparations and the stage was mated to its Atlas booster on April 28. Spacecraft No. 7, which had been encapsulated inside of its launch fairing and fueled for flight, was added to the stack the following day. With the successful completion the last preflight tests on May 2, Lunar Orbiter D was ready to attempt a launch during the first window on May 4.

The Lunar Orbiter 4 Mission

On the morning of May 4, 1967 the countdown for the launch of Lunar Orbiter D started at the T-8 hours and 50 minutes mark and proceeded smoothly. During the planned hold at T-60 minutes, engineers reprogrammed Lunar Orbiter D to delay the opening of its solar panels by 40 seconds to two minutes after separation from the Agena D in order to further minimize the risk the panels might come into contact with the spent stage. At 6:25:00.571 PM EDT (22:25:00.571 GMT), the Atlas-Agena D lifted off from LC-13 to start NASA’s fourth Lunar Orbiter mission.

After a nearly perfect 289.4-second burn of the Atlas, the fairing protecting the Moon-bound payload was jettisoned and the Agena D separated from the spent booster. After a wait of 76.9 seconds following the cutoff of the Atlas’ sustainer engine, the main engine of Agena 6633 ignited for its first burn of 152 seconds to place itself and Lunar Orbiter D into a temporary 177.8 by 192.6 kilometer Earth parking orbit. After coasting for 20 minutes and 43 seconds, Agena 6633 reignited its engine without incident for its second scheduled burn. After 87.4 seconds, the Agena’s engine shutdown and 164 seconds later, what was now called Lunar Orbiter 4 separated from the spent upper stage on its way to the Moon. The Agena D turned and performed a minor deflection maneuver to move it safely away from the spacecraft and the Moon.

Following separation, Lunar Orbiter 4 dutifully unfolded its appendages and proceeded to locate its first attitude reference, the Sun, with final acquisition about 58 minutes after launch. While the first attempt of the star tracker to locate the craft’s second attitude reference had failed possibly due to interference from stray light, by 8:26 GMT on May 5 Lunar Orbiter 4 had successfully locked onto Canopus. In the mean time, radiation sensors noted that the spacecraft had received a higher dose of radiation during its passage through the Van Allen belts than had earlier missions. The dosimeter monitoring the film supply recorded an exposure of 5.5 rads compared to 0.75 rads typical of earlier missions. Despite the higher radiation exposure, the film supply was not adversely affected.

After initial tracking determined the trajectory of the receding spacecraft, Lunar Orbiter 4 ignited its velocity control engine at 16:45 GMT for a 52.3-second course correction burn. The maneuver with a large delta-v of 60.85 meters per second successfully altered the trajectory of Lunar Orbiter 4 to pass within 53 kilometers of the desired aim point over the Moon’s south pole so that it could enter the required high altitude polar orbit without the need of the second scheduled midcourse maneuver. Finally, 88 hours and 44 minutes after launch at 15:09 GMT on May 8, the velocity control engine on Lunar Orbiter 4 ignited again for a burn of 501.7 seconds. The resulting delta-v of 659.6 meters per second placed Lunar Orbiter 4 into a 2,706 by 6,114 kilometer orbit with an inclination of 85.48° and a period just one minute longer than the desired value of 12 hours.

After entering orbit, ground controllers checked the performance of the spacecraft’s various systems in preparation of the photography mission. On Orbit 3 and again on Orbit 5, Lunar Orbiter’s photographic subsystem readout what was called the “Goldstone test film” – the 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 testing, Lunar Orbiter 4 started photography at 15:46 GMT on May 11, 1967 during Orbit 6. In order to move the film leader splice through the photographic subsystem in a reasonable time, this first orbit consisted of five four-frame sequences with 8 seconds between frames. Later orbits would use the planned five single-frame sequences during nearside overpasses.

Telemetry being returned to Earth indicated that the thermal door which covered the camera optics between sessions had failed to open during the third photo sequence of Orbit 6 and the first of Orbit 7. As a result of changes made to the command sequences to ensure that this would not happen again, the spacecraft was oriented so that oblique rays from the Sun would keep the camera lenses warm enough to prevent condensation on the window and optics. Unfortunately, this allowed sunlight to leak past the camera’s baffles and strike the exposed but unprocessed film. The problem was not detected until fogging and local image degradation was noted during initial readout of the photographs. To solve the light leakage issue, the pitch attitude was altered slightly. In order to take care of the condensation on the window and optics, real time commands were sent to partially close the thermal door to shield the window from the cold of space and help maintain temperatures. By Orbit 13, all the condensation affecting the photographic subsystem had evaporated.

Meanwhile, the other instruments on Lunar Orbiter 4 continued to collect data as the photography mission proceeded. A micrometeoroid hit was detected on May 12 and another on the 18th with no effect on the spacecraft noted. Solar flares were reported on May 23 and 25 resulting in a peak radiation exposure rate of 6 rads per hour primarily from low energy plasma. These events produced no noticeable fogging of the film supply.

In order make up for the lost photographs, a plan was developed so that the affected areas were rephotographed during the higher altitude apolune overpass of the nearside on Orbit 29 near the end of the photographic mission. By Orbit 34 on May 25, 199 frames had been taken completing orbital photography. But as the last images were being processed and readout, spurious error signals were making the procedure increasingly difficult and eventually forced the cutting of the Bimat on Orbit 35 after only 196 frames had been developed. Final readout was started on Orbit 41 and was completed during Orbit 46 on June 1, 1967. Changes in the operational procedures allowed all of the significant photographs to be scanned despite the readout issues caused by the spurious error signals.

With its photography mission completed on June 1, Lunar Orbiter 4 had managed to photograph 99% of the nearside of the Moon at a resolution significantly exceeding previous ground-based efforts. The photographs revealed amazing geologic detail on the lunar surface especially in the previously unseen polar regions. About 80% of the lunar farside had also been imaged at lower resolution. The final Lunar Orbiter E mission would be tasked with filling in the blanks in NASA’s lunar maps as well as taking a closer look at some of the more interesting features on the nearside during low perilune passes from polar orbit (see “Lunar Orbiter 5: Filling the Gaps in the Maps“).

The Extended Mission

Lunar Orbiter 4 was now ready for its extended mission but with only 1.6 kilograms of nitrogen gas left for the attitude control system, there would not be enough for the usual 11 month duration. In order to map the variations in the Moon’s gravitational field which would affect the upcoming Lunar Orbiter E mission, the orbit of Lunar Orbiter 4 was lowered in two steps. A 117.9-second burn of the velocity control engine starting at 01:56 GMT on June 5 with a delta-v of 186.7 meters per second dropped the perilune altitude from 2,578 to 74 kilometers. This was followed by a 42.8-second burn at 01:59 GMT on June 8 where the apolune was dropped from 6,084 to 3,952 kilometers as a result of a 70.5 meter per second delta-v. Over the course of the next 18 days, a total of 134 hours of tracking allowed a more accurate lunar gravitational harmonics model to be developed to aid in planning the next Lunar Orbiter mission.

During the course of its extended mission, Lunar Orbiter 4 continued to gather data on the lunar environment as well as engineering data valuable for the last mission in the series. Exercises were also performed with the Apollo tracking network to ensure that its equipment and procedures would work during the upcoming manned lunar missions. Lunar Orbiter 4 was last heard from on July 17, 1967 only 74 days after launch. Attempts to regain contact with the spacecraft continued until the mission was declared officially terminated on August 16. The cause of the failure was never determined. Calculations at the time predicted that the now-silent Lunar Orbiter 4 impacted the lunar surface on October 31 somewhere between 22° and 30° W longitude. In the mean time, NASA continued its push to the Moon with the five remaining unmanned lunar missions scheduled for 1967.

Follow Drew Ex Machina on Facebook.

Related Video

Here is a NASA documentary entitled Close-Up of the Moon: A Look at Lunar Orbiter which provides an excellent summary of the Lunar Orbiter program up to the first half of 1967.

﻿

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]

“Lunar Orbiter 3: Preparing for Apollo”, Drew Ex Machina, February 5, 2017 [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

Lunar Orbiter D: Second Mission in Three Weeks, NASA Press Release 67-101, May 2, 1967

Lunar Orbiter IV: Photographic Mission Summary, NASA CR-1054, June 1968

Lunar Orbiter IV: Extended-Mission Spacecraft Operations and Subsystem Performance, NASA CR-1092, June 1968