Lunar Orbiter 5: Filling the Gaps in the Maps

A half of a century after the fact, it is difficult to imagine the excitement during 1967 as NASA continued to work through its backlog of scheduled robotic missions to the Moon in preparation for the first Apollo missions. The launch pace seemed to reach a fever pitch during the summer of that year with the launch of NASA’s Surveyor 4 lunar landing mission on July 14 (see “Surveyor 4: The Impact of a Low Probability Event”) followed five days later by the second Anchored Interplanetary Monitoring Platform known as Explorer 35 which measured radiation and other characteristics of the space environment from lunar orbit (see “AIMP: The Forgotten Lunar Orbiters”). As the beginning of August 1967 approached, NASA was preparing to launch its third mission to the Moon in just 18 days: Lunar Orbiter 5 which would finish the task of mapping the Moon in preparation for the Apollo Moon landings just a couple of years away.

 

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 1/25^th, 1/50^th, or 1/100^th 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. Later flights which would spend more time in sunlight would also employ small quartz mirrors on the underside to reflect away the heat. 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 micrometeoroid 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 E

With the primary program objective of mapping potential Apollo lunar landing sites met with the completion of the Lunar Orbiter 3 mission (see “Lunar Orbiter 3: Preparing for Apollo”), the remaining two flights of the series were free to address other objectives of a more scientific nature. Lunar Orbiter 4 was placed into a relatively high, 12-hour polar orbit with the goal of performing a systematic photographic survey of almost the entire lunar surface in order to increase our scientific knowledge of the nature, origin and processes that shaped its surface (see “Mapping the Moon: The Mission of Lunar Orbiter 4”). Combined with images from the earlier missions, about 99% of the nearside and 60% of the farside were photographed well enough to be mapped. The primary goal of the fifth Lunar Orbiter mission, designated “Lunar Orbiter E” before launch, was to photograph selected areas on the near and far sides of the Moon of scientific interest as well as perform supplemental photography of proposed Apollo landing sites. Like the earlier missions, secondary objectives 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.

A working group formed inside of the Lunar Orbiter program started work in March 1967 on a tentative list of targets to photograph during the Lunar Orbiter E mission with the final list approved on June 14 following a series of meetings. During the beginning of its operations in lunar orbit, Lunar Orbiter E would fill in a large gap in coverage of the farside ranging from 104° to 143° W longitude. Although much of this region had been originally photographed at comparatively low resolution during the Soviet Zond 3 lunar flyby of July 20, 1965 (see “The Soviet Zond Missions of 1963-65: Planetary Probe Test Flights”), higher resolution photographs of this area acquired by Lunar Orbiter 4 in mid-May 1967 were lost due to a light leak which exposed and ruin the film before it could be processed. Lunar Orbiter E would acquire photographs to fill this coverage gap during high apolune passes over the farside early in the mission.

The majority of the targets for the Lunar Orbiter E mission were confined to the nearside and would be photographed during low perilune overpasses. A total of 51 targets were selected including additional Apollo landing site photography, landing sites of high scientific interest for future Surveyor and later Apollo missions as well as photographs of areas of high scientific interest. Out of the 212 frames of film available, 44 were to be devoted to Apollo-related targets while the balance of 168 frames would be used on scientific targets including completing farside coverage.

Unlike the earlier Lunar Orbiters which were placed into low inclination orbits, Lunar Orbiter E would be placed into a near-polar orbit with an inclination of 85° in order to photograph the desired targets under lighting conditions which maximize the visibility of surface features. Initially, Lunar Orbiter E would be placed into a 200 by 6,050 kilometer orbit with a period of about 8½ hours and the perilune or low point over the lunar equator so that nearside photography would have about the same resolution north and south of the equator. The apolune, whose height was similar to that of the Lunar Orbiter 4 mapping mission, would allow photography of the gaps in farside coverage with a resolution as good as 1.2 to 1.6 kilometers. In order to align the orbit properly with respect to the terminator, Lunar Orbiter E would perform an out of plane maneuver during orbit insertion.

After a day or two of tracking in its initial lunar orbit, Lunar Orbiter E would use its velocity control engine to lower its perilune to 100 kilometers in preparation for the near side photography which would dominate the planned two weeks of actual photography. Although this was higher than the usual 45 kilometer perilune altitude flown by the first three mapping missions, this higher orbit allowed wider coverage of target areas with a resolution as good as two meters as well as provide better stereo imaging from successive orbits. After the initial farside photography was completed, Lunar Orbiter E would lower its apolune to 1,500 kilometers to begin photography in earnest of the nearside targets in a faster three hour and 11 minute orbit.

In order to get Lunar Orbiter E into the desired orbit around the Moon, three daily launch windows were available at the beginning of August 1967 using the increasingly reliable Atlas-Agena D rocket employed by the earlier missions. The first on August 1 opened at 4:09 PM EDT and closed at 8:00 PM. Slightly shorter windows opened about 1¾ hours later on each of the next two days. Photography was scheduled to start on August 6 and continue for every orbit thereafter until the film supply was exhausted on August 19. While selected photographs would be scanned at every opportunity after they were developed using the priority scan mode, the final readout of all of the photographs would be completed by August 27. Lunar Orbiter E would then begin its extended mission which would include exercises with NASA’s new Apollo tracking network.

The Lunar Orbiter E mission was flown using Spacecraft No. 3 which had originally arrived at Cape Kennedy (which reverted back to its original name of Cape Canaveral in 1973) on March 10, 1967 to serve as the backup to Spacecraft No. 7 slated for the Lunar Orbiter 4 mission. As Spacecraft No. 3 was being prepared for storage following the successful launch of Lunar Orbiter 4 on May 4, technicians discovered that there were out-of-tolerance leaks in the bladders of the propulsion system’s oxidizer tanks. This required that the spacecraft be returned to Boeing in Seattle so that the bladders could be replaced.

Spacecraft No. 3 was returned to Cape Kennedy to begin prelaunch preparation on June 23. Other modification made to the spacecraft prior to launch included a new sensor to indicate when the photographic  subsystem’s thermal door was closed and a modified set of light baffles to avoid the problems encountered during the Lunar Orbiter 4 mission. An additional 72 2.5-centimeter square quartz solar reflectors were added to the underside of the spacecraft deck to help maintain thermal control along with test coupons for thermal coating degradation studies.

In the mean time, Atlas SLV-3 number 5805 and Agena D number 6634 arrived at Cape Kennedy on May 27, 1967 from their prime contractors, General Dynamics and Lockheed, respectively. On June 6, the Atlas was erected on the pad at Launch Complex 13 (LC-13) to begin preparations for launch. After an Atlas tanking test on June 28, the Agena D was added to the stack three weeks later. The Lunar Orbiter enclosed in its nose fairing was integrated with its launch vehicle on July 25. Following a successful countdown rehearsal on July 29, all was set to launch the last Lunar Orbiter mission.

 

The Last Mission

The countdown for the first launch window on August 1, 1967 encountered a couple of issues resulting in two hours and 24 minutes in unscheduled holds. The first was a faulty velocity meter on the Agena which required replacing and then there was a hold at T-90 minutes as severe thunderstorms passed through the area – a common occurrence late in the afternoon during Florida summers. After the weather hold, the countdown proceeded without any major issues until the Atlas-Agena D lifted off at 6:33:00.352 PM EDT (22:33:00.352 GMT) in a light rain.

A near-nominal performance by Atlas 5805 ended with its sustainer engine shutting down only 0.7 seconds late 288.6 seconds after launch. With the first burn of primary propulsion system of the Agena 6634 lasting 154.3 seconds, the upper stage and its Moon-bound payload had been placed into a temporary 176.6 by 196.3 kilometer orbit with an inclination of 31.1° just eight minutes and 44 seconds after launch. After coasting for about 22½ minutes, the Agena’s main engine reignited for a 87.5-second burn to place what was now called Lunar Orbiter 5 on a trajectory to the Moon.

At 35 minutes and 33 seconds after launch, Lunar Orbiter 5 separated from its spent upper stage and proceeded to unfold its appendages and look for its first attitude reference, the Sun. Lunar Orbiter 5 was acquired by NASA’s DSS-41 tracking station in Woomera, Australia 23:23:17 GMT on August 1 and finally locked its star sensor on Canopus 19:00 GMT the next day. Meanwhile about ten minutes after releasing its payload, the spent Agena 6634 turned to perform a retromaneuver in order to move safely away from Lunar Orbiter 5 and enter an extended 9,380 by 369,831 kilometer geocentric orbit. With a period of ten days, this new orbit would miss the Moon by 25,317 kilometers.

Tracking of Lunar Orbiter 5 showed that it was travelling on a trajectory that was about 7,000 kilometers from the intended aim point. A midcourse correction with a delta-v of 29.75 meters per second was performed 31 hours and 27 minutes after launch to set Lunar Orbiter 5 on course for a point 5,707 kilometers above the Moon’s south pole. At 16:48:54.4 GMT on August 5, 1967, the spacecraft turned and fired its velocity control engine for 508 seconds for a total delta-v of 643 meters per second that was 13.79° out of plane in order to achieve the proper orientation of the initial orbit. After a transit of 90 hours and 15 minutes, Lunar Orbiter 5 had entered a 195 by 6,028 kilometer orbit with an inclination of 85.0°. About an hour and a half after entering orbit, 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, the photographic mission was set to begin.

Lunar Orbiter 5 acquired its first photograph at 11:22 GMT on August 6 during Orbit 2 to start its photographic survey of the farside. At 08:43:48.7 GMT the next day, Lunar Orbiter fired its velocity control engine at the apolune of Orbit 4 for a delta-v of 15.97 meter per second which dropped its perilune to 101 kilometers in preparation for the photographic survey of the nearside targets. But before that phase of the mission began, ground controllers decided to repurpose a previously planned “film set” (where the film was advanced to avoid the Bimat sticking after the photography subsystem was inactive for too long) to instead photograph the Earth. Lunar Orbiter 5 snapped a telephoto photograph of the Earth at 09:05:00 GMT on August 8 as the spacecraft approached apolune near the end of Orbit 7. Unlike earlier images of the Earth taken by Lunar Orbiters, this one showed a nearly “full Earth” with features of the eastern hemisphere clearly visible over the distance of 361,740 kilometers.

A second orbit transfer maneuver at 05:08 GMT on August 9 during the perilune pass of Orbit 10 lowered the apolune and placed Lunar Orbiter 5 into its final 99 by 1,499 kilometer orbit for the balance of its photographic mission. Photography of the nearside targets began on Orbit 15. During the 74 revolutions in this final mapping orbit, Lunar Orbiter 5 executed 63 separate imaging sequences to photograph a total of 41 nearside and 13 farside targets. The final 213^th frame was exposed at 21:40 GMT on August 18. But before the command could be given to cut the Bimat, telemetry indicated that it had run out leaving the last two frames undeveloped. A later investigation revealed that the Bimat supply was shorted by 1½ meters resulting in its premature exhaustion.

Final readout of the photographs was started at 04:30 GMT on August 19 and continued without any problems with four photographs typically being returned every orbit. The readout was completed at 05:38 GMT on August 27 ending the photographic portion of the mission. When combined with the images from the other Lunar Orbiter missions, about 99% of the lunar surface had been mapped with only a thin strip near the south pole representing the largest unphotographed region. This collection of photographs would form the basis of the definitive lunar maps for the next quarter of a century.

In addition to providing new imagery of potential landing sites for the initial Apollo lunar landing missions, Lunar Orbiter 5 also managed to survey sites which would be visited by later missions. These sites included Fra Mauro (visited by Apollo 14 in February 1971), Hadley Rille (Apollo 15 in July 1971), Taurus-Littrow (Apollo 17 in December 1972) and Tycho (Surveyor 7 in January 1968 – see “Surveyor 7: Mission to Tycho “) among many others of scientific importance.

 

The Extended Mission

With the primary mission completed, ground controllers placed Lunar Orbiter 5 into its extended mission configuration at 02:00 GMT on August 28, 1967 during Orbit 152. Lunar Orbiter still had 2.5 kilograms of nitrogen gas out of its initial supply of 3.7 kilograms left for attitude control and sufficient propellant for a 93 meter per second change in velocity giving it plenty of consumables for its extended mission. During the course of its extended mission, Lunar Orbiter 5 continued monitoring the flux of radiation (which was at safe levels for humans) and micrometeorites (which only detected one hit). Tracking also helped to refine the model for the Moon’s gravitational field as well as exercise the Apollo tracking stations for upcoming missions.

On October 10, 1967 the velocity control engine was fired for 41 seconds to adjust the spacecraft’s orbit to 200 by 1,986 kilometers to help decrease the amount of time it spent in darkness during an eclipse on October 18. One last novel exercise performed using Lunar Orbiter 5 before its consumable were exhausted was an attempt to spot it visually while in orbit. With sunlight reflecting off of the small quartz mirrors mounted on the underside of the orbiter, Lunar Orbiter 5 was expected to appear as bright as a 6^th magnitude star, assuming that the reflectors were perfectly aligned. An initial attempt by seven astronomical observatories to observe the orbiter on January 18, 1968 failed. A second attempt on January 21 was more successful. At 11:50 GMT, ground controllers reoriented Lunar Orbiter 5 so that it would reflect sunlight back towards the Earth as the spacecraft approached apolune. Between 12:24 and 13:01 GMT, Lunar Orbiter 5 was commanded to execute a series of six small pitch and yaw maneuvers to make the cone of reflected sunlight pass over the Earth in hopes that it would be observed. The spacecraft was successfully spotted by the 1.55-meter reflector at the Catalina Observatory operated by the University of Arizona’s Lunar and Planetary Laboratory using a series of five and ten-second photographic exposures. Lunar Orbiter 5 appeared as a streak about as bright as a 12^th magnitude star above the bright limb of the Moon.

Lunar Orbiter 5 ended its mission on January 31, 1968 after 1,201 orbits of the Moon. It was commanded to fire its velocity control engine one last time for 18.9 seconds at apolune to cut its speed by 28.6 meters per second. The spacecraft purposely crashed into the lunar surface near the equator at 70° W longitude somewhere just to the southwest of the rim of the crater called Hevelius. While there were problems encountered during each mission, all five of the Lunar Orbiter missions launched within the course of just twelve months had successfully met their objectives for an unprecedented 100% success rate – a welcomed change after the series of failures NASA had experienced with the early Pioneer and Ranger missions to the Moon. And with this success under their belts, NASA was one step closer to landing astronauts on the surface of the Moon before the end of the decade.

 

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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.

 

 

Here is another excellent NASA documentary from 1967 on Lunar Orbiter and other early missions to photograph the surface of the Moon entitled Assignment: Shoot the Moon:

 

 

Related Reading

All of the articles on NASA’s Lunar Orbiter program published here on Drew Ex Machina can be found on the Lunar Orbiter Program page.

 

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

Fifth and Final Lunar Orbiter Launch Planned, NASA Press Release 67-192, July 27, 1967

Atlas/Agena-24 Lunar Orbiter-5 Flash Flight Report, KSC TR-562, August 4, 1967

Lunar Orbiter V Photographic Mission Summary, NASA CR-1095, July 1968

Lunar Orbiter V System Performance, NASA CR-66608, January 4, 1968

“First Photographs from Earth of a Lunar Satellite”, Sky & Telescope, Vol. 35, No. 4, pp 220-221, April 1968

Flight Performance of Atlas-Agena Launch Vehicles in Support of the Lunar Orbiter Missions, III, IV and V, NASA TM X-1859, August 1969