DESTINATION MOON: A History of the Lunar Orbiter Program
The Second Mission
[253] Less than three months elapsed between the launch of the first Orbiter and that of Lunar Orbiter II. On November 6, 1966 the second mission began, with the launch of the spacecraft at 23:21 GMT. The cislunar transit went as planned, with no trouble in the Canopus star tracker. One reason for success was that the solar panels and parts of the antenna booms had been painted black to reduce the surface area which could reflect light. A small midcourse correction was made approximately 44 hours after launch, and the initial high lunar orbit was established after 92.5 hours of [254] cislunar transit time. The orbital parameters were: apolune, 1,850 kilometers; perilune, 196 kilometers. The Deep Space Network tracked Lunar Orbiter II for several days to obtain data for a more accurate analysis of the lunar gravitational effects on the spacecraft. After 33 orbits the spacecraft was transferred to the photographic orbit with a perilune of 49-7 kilometers.47
On November 18 Lunar Orbiter II commenced its photographic work. The photo subsystem performed well during all phases of the mission and covered each of 13 primary and 17 secondary sites as planned. Only Secondary Site II S-10.2 had to be rescheduled in the photographic plan, to avoid operating the spacecraft on batteries during photography, a procedure which would have violated a design restriction and resulted in a power shortage.
Several changes bad been made in the photo subsystem of Lunar Orbiter II as a result of the first Orbiter mission:
1. The addition of an integrating circuit in the focal-plane-shutter control circuits to ensure that an output signal represented a valid command pulse (containing amplitude and duration) and was not caused by an electrical transient.
2. The addition of a filter on the 20-volt line to minimize electromagnetic interferences and possible triggering of photo subsystem circuits.
3. [255] The platen clamping spring tension was increased to ensure immobility of the film during exposure, improve film flatness, and maintain focus.
4. Reseau marks were pre-exposed on the spacecraft film in a specific pattern to assist in compensating for any non-linearities in the optical-mechanical scanner.48
The medium- and high-resolution photography was excellent in quality and indicated that the operation of the photo subsystem during exposure, processing, and readout was very good for the first portion of the film.
On November 20 Lunar Orbiter II photographed the impact point of Ranger VIII (Site II P-5).49 On November 23 it recorded one of the most spectacular pictures of the lunar surface. The picture was taken as a result of the threat of Bimat stick and the need to move new film and Bimat onto the processor drum at regular intervals. A certain amount of the film would be wasted if no exposure were made and a choice arose as to the use of this "film-set" frame. One mission ground rule called for the frames to be used to take pictures of any areas in the Apollo zone of interest, should the spacecraft be over one at the time. On the other hand, Douglas Lloyd of Bellcomm, [256] Inc., had suggested during mission planning that this particular "film-set" frame be used to take a photograph of the crater Copernicus when the spaceraft passed due south of it at a distance of 240 kilometers and a vertical altitude of 45 kilometers above the lunar surface. Twice his suggestion was turned down by NASA officials because of the Apollo ground rule. However, upon Lloyd's third suggestion Program officials consented, and the decision to make the picture came during actual mission operations.
The Lunar Orbiter's camera made a telephoto exposure through the 610 mm lens of the crater from a long, low, oblique angle to the lunar surface when lighting conditions were optimum for best contrast. The resultant picture revealed geographic and topographic features of the central portion of this 100-kilometer-wide crater which had never before been discerned. Dominating the center of the photographic frame were mountains rising over 300 meters from the crater floor. Behind them a ledge of bedrock and the crater's rim could be seen. Behind all of this the Gay-Lussac Promontory in the Carpathian Mountains towered 1,000 meters above the lunar surface on the horizon.
This and the oblique pictures of the Marius Hills and Reiner Gamma proved to be extremely valuable to the photo grammetrists, astrogeologists, and other scientists connected with the Lunar Orbiter and Apollo programs. The nation's [257] news media described the Copernicus picture as it one of the great pictures of the century."50
Lunar Orbiter II ended its photographic acquisition on November 26, 1966, and flight controllers concluded the readout on December 7. Only one setback marred an otherwise unqualified success. The traveling-wave-tube amplifier (TWTA) failed on the final day of readout, and half of the photographs of secondary Site II S-1 were not obtained. This area was located at 41.1° east longitude and 3.2° north latitude in Mare Tranquillitatis.51 However, priority readout of the wide-angle photo coverage of this site had previously been conducted, minimizing the seriousness of the loss.
The spacecraft's twenty micrometeoroid detectors recorded three impacts during nineteen days of the mission. These bits did not affect the performance of the spacecraft. Lunar Orbiter I had registered no hits, and program scientists believed that the Lunar Orbiter II hits may have been the result of the annual Leonid meteor shower.52
[258] Lunar Orbiter II demonstrated its ability to obtain high-quality oblique photography of the near and far side of the Moon. It also obtained experimental convergent stereo telephoto pictures of one site, demonstrating the ability of the photographic subsystem to employ the stereo technique. Moreover, it showed that not all crater rays on the lunar surface were necessarily heavily cratered but that the Copernicus-Kepler region was unfit for landing sites. These achievements attested to the accuracy and precision with which the flight controllers were able to position the spacecraft for photographing specific objectives.53
Finally, the problem of overheating which had made more attitude control maneuvers necessary during the mission of the first Lunar Orbiter was overcome on the second mission. With the addition of a coating of S-13G paint, degradation of the thermal paint on the equipment deck of Lunar Orbiter II was substantially reduced. Thermal control of the spacecraft by planned thermal relief maneuvers was better integrated into the total flight operation plan for the second mission, and the spacecraft performance proved markedly better than that of the first Lunar Orbiter mission.54