DESTINATION MOON: A History of the Lunar Orbiter Program
The Fourth Orbiter Mission
[273] Last minute tests did not reveal any problems of a magnitude serious enough to delay a launch, and on May 4 Lunar Orbiter IV rode into space atop its Atlas-Agena D launch vehicle at 18:25 Eastern Daylight Time (EDT) from Launch Complex 13 at Cape Kennedy on an azimuth of 100.80. About thirty minutes after liftoff the Agena injected the spacecraft into a cislunar trajectory. Early tracking data indicated that it was on course, and the first midcourse maneuver was scheduled for 13:00 EDT on May 5.9
Early in Lunar Orbiter IV's journey to the Moon the Canopus star tracker experienced difficulty acquiring Canopus. Glint from the Sun and earthshine probably were [274] the causes of this trouble. The star tracker did lock onto a celestial body, but flight controllers were not sure if it had acquired Canopus or the planet Jupiter, which was also in its field of view. Program operators planned to correct this situation by staging a roll reference maneuver during the first midcourse correction.10
Passing through the Van Allen Belt, Lunar Orbiter IV experienced a higher dose of radiation than had the previous Orbiters: 5.5 rads recorded by the radiation dosimeter for the film supply cassette versus 0.75 rads on earlier Orbiters. However, the dosimeter for the camera storage loopers registered 0.0 rads when it was turned on after the spacecraft had traversed the Van Allen Belt.11
Shortly after noon EDT on May 5 Lunar Orbiter IV executed the planned midcourse maneuver to line the spacecraft up with the aiming point before deboost into orbit around the Moon. At 11:08 EDT on May 8 the spacecraft's rocket burn deboosted the Orbiter into an initial near-polar orbit around the Moon, with 6,111-kilometer apolune, 2,706-kilometer perilune, 85.5° inclination to the lunar equator, and 12.01-hour period of orbit.12
[275] All subsystems performed well and within acceptable temperature limits up to this point. Flight controllers at the Deep Space Network facilities commanded the spacecraft to scan the Goldstone Test Film at 7:30 p.m. EDT on May 9 in order to check the readout and communications subsystem. The DSN stations at Goldstone, California, and Woomera, Australia, read out the film and received data of excellent quality. The TWTA onboard the spacecraft had been turned on for readout and would remain on for the duration of the mission. The spacecraft would execute thermal control maneuvers to suppress any overheating tendency of the TWTA during the mission. Readings of the radiation dosimeters for the film storage cassette continued to stand at 5.5 rads, while the dosimeter for the storage loopers indicated a change from 0.0 to 0.5 rads. Ground control attributed this to background radiation from space, which did not threaten the film.13
In its sixth orbit around the Moon Lunar Orbiter IV began its first photographic pass at 11:46 a.m. EDT on May 11. As the spacecraft sped from south to north the photo subsystem exposed five sets of four frames each at intervals ranging from 30 to 40 minutes. At the high altitude, image-motion compensation did not enter into the photographic [276] process. Passing over the vicinity of the lunar north pole, the spacecraft dropped out of sight and radio contact with Earth. How could it conduct farside photography without direct communication with flight controllers? The key to the Orbiter IV farside photography as well as to all farside photography of the five Lunar Orbiter missions was the Flight Programmer, previously discussed.
Originally Boeing had designed the Programmer for a command storage capacity of sixteen hours, twice the length of time in which any of the DSN ground receiving stations would be out of line-of-sight communications with the spacecraft. This represented a safety margin of eight hours, should one of the stations fail to acquire the spacecraft. The storage capacity mean that flight programmers could store commands to be executed up to sixteen hours following storage without any further command from Earth. Thus, during the periods when the spacecraft was out of sight of the Earth, it was already programmed to conduct photography of the lunar far side.14
Heading south from the north pole Lunar Orbiter IV took one frame of the Moon's far side as it reached apolune (6,111.3 kilometers). By 8:40 p.m. EDT May 11, it had exposed a total of 27 frames, and flight controllers commanded the readout of this photography to begin. The [277] first high- and medium-resolution pictures turned out excellently.15
Despite this apparent success, the spacecraft had already developed a serious problem which threatened to jeopardize the whole mission. Telemetry data indicated that after the second set of four frames had been exposed, the camera thermal door failed to close until ground control had sent additional commands to close it. After the third set of four frames had been made, spacecraft telemetry did not confirm if the door had opened sufficiently. Flight controllers initiated a preliminary corrective action by commanding the door to open far enough in advance of the fourth set's exposure time to allow for additional commands if required.
NASA and Boeing engineers began immediately to analyze the problem. The danger of the thermal door's failing in the closed position and making all further photography impossible forced flight controllers to fly the spacecraft with the door open. The open door created a danger of light leakage, which could fog portions of the film. Flight controllers had to strike a delicate balance between prohibiting light leaks and preventing the temperature within the subsystem from dropping below the dew point of the gas [278] which pressurized it. Too low a temperature could cause moisture condensation on the camera lens window and thus reduce the contrast and resolution of the photographs. Maintaining a balance between these two conditions led to extra attitude control maneuvers.16
The danger of light leakage revealed itself early on May 13 during the readout of the exposures which the spacecraft had made since ground control had initiated contingency measures to cope with the camera thermal door problem. Portions of the photographs were light struck. NASA engineers deduced the mishap by comparing readout results of film that had been kept in the spacecraft's camera storage looper for one half hour with film that had been there five hours and longer. The quality of the exposures declined with the length of time the film had been in the looper before readout.17
Lunar Orbiter Program personnel from Langley, Boeing, and Eastman Kodak attempted to solve the problem of the door. Flight controllers devised and executed several tests to assess its reliability. These showed that the door could be partially closed, then reopened. Further tests placed the spacecraft in several orientations to the Sun with the door [279] partially closed. Ground control monitored the thermal response of the camera lens window and commanded the spacecraft to take photographs. On May 16 these photographs were read out, and they indicated that light leaks had ceased. Program officials concluded that their procedures were effective. However, the low contrast of some pictures indicated probable fogging of the lens window due to moisture condensation at lower temperatures. Ground control maneuvered the spacecraft to raise the temperature of the lens window on orbit 14 and subsequent orbits.18
As of May 19 Scherer could report to NASA Administrator James E. Webb that the Langley/Boeing flight operations team had the photographic fogging problem under control. The team had established the following subjective grading system for Orbiter IV pictures: 1) excellent quality, 2) light fogging, 3) heavy fogging, and 4) blank. The most recent high-resolution photographs fell into the first or second categories, with most being graded excellent. A preliminary analysis of the photographic coverage during the first 60° of lunar longitude arc indicated that 64% of this area had been covered by grade 1 or 2 photography.19
Early on Saturday morning, May 20, ground control [280] picked up an anomaly during readout. The readout drive mechanism turned off in a normal manner without being commanded to do s Ground control restarted it, but after scanning a short segment of film it stopped abruptly. Throughout the day this start-stop situation repeated itself; the distance scanned varied from 5 to 30 centimeters. Langley and Boeing engineers suspected the readout encoder was falsely indicating a full readout looper. They began to analyze the problem while primary readout proceeded. Pictures obtained through readout proved that the new operational procedures for the camera thermal door continued to be effective, and no change in photography schedules was necessary at that time.20
By 8:00 a.m. EDT on May 25 Lunar Orbiter IV was in its thirty-fourth orbit around the Moon and had photographed its surface as far as the 100° west meridian. Ground control had recovered photographs up to about the 75° west meridian. The sector from 90° east to 45° east meridian, which the Orbiter had first photographed, had been photographed again from apolune because fogging had degraded the quality of the perilune pictures. While photography proceeded well, flight controllers believed that they had brought the premature [281] termination of readout under control. They used a repetitive series of commands to prevent the noisy encoder from stopping readout until commanded to do so.21
Between May 21 and May 25, while problems with the thermal door and the readout encoder were being resolved, Lunar Orbiter IV experienced increased radiation dosage from solar flare particle events. Trutz Foelsche, primary investigator for the Lunar Orbiter radiation experiment, was able to make preliminary conclusions about the potential hazards to Lunar Orbiter IV based upon early data which the Space Flight Operations Facility had obtained from the spacecraft's two dosimeters. On May 21 a solar particle event had produced low-energy protons whose energy levels did not exceed 20 Mev. Since they had little energy these protons would hardly affect the camera film. Moreover, he concluded, the May 21 event was much less serious than the event of September 2, 1966, which Lunar Orbiter II had encountered, and the Orbiter had experienced no film fogging.22
[282] On the thirty-fifth orbit around the Moon Lunar Orbiter IV experienced worsening readout difficulties. These brought a quick decision to cut the Bimat to escape the high probability that the Bimat would stick to the film, thus ending the photographic mission. At this time the photographic subsystem had exposed and processed 163 frames. Ground control successfully commanded Lunar Orbiter IV to cut the Bimat, but final readout presented more problems.23
The erroneous encoder signals hindered film transport from the take-up spool considerably, and ground control had to improvise a non-standard procedure to get around this condition. Sending false picture-taking commands, mission controllers inched the film towards the take-up spool and then moved short segments of film back through the readout gate. Using this procedure they successfully recovered 13 additional frames at the end of the film which might otherwise have remained between the processor and the readout looper. Then ground control sent commands to the spacecraft to apply tension throughout the film system. Following this the system responded normally to readout operations. Only 30 of the 163 frames which had been exposed remained to be recovered. NASA ground stations completed final readout on June 1.24
[283] Lunar Orbiter IV photography had covered 99% of the Moon's near side at a resolution exceeding by ten times the best Earth-based telescopic photography. This coverage revealed significant, heretofore unknown, geological detail in the polar and limb regions of the Moon. Unofficially the Orbiter IV photography increased to 80% the coverage of the far side of the Moon obtained during the first four Orbiter missions. These accomplishments attested to the high degree of organization in the flight operations of the fourth mission in the face of the problems that had been encountered.25
Its photographic mission ended, Lunar Orbiter IV proceeded into its extended mission. Program officials planned to change the spacecraft's orbit so that it would approximate that planned for Lunar Orbiter V. The additional information which ground control could obtain about the Moon's gravitational environment by tracking Lunar Orbiter IV and analyzing the telemetry data would prove valuable in planning the final Orbiter mission. In addition ground stations continued to track the second and third Orbiters. Lunar Orbiter II, launched in November 1966, was moving [284] closer to the Moon's surface on an inevitable collision course. Program Officials planned to raise its orbit, thus extending its lifetime. Lunar Orbiter III would undergo a plane change in its orbit in addition to having it raised. The change would provide new data on the lunar gravitational field for use in further mission planning and in the Apollo Program.26