Orbital investigations

With Ranger, JPL had seized the initiative in the development of space probes for missions in deep space. In May 1960 it took on a much more adventurous project to develop two related spacecraft: one to enter lunar orbit to conduct mapping and the other to land. Unfortunately, the development of the powerful Centaur stage to dispatch these new probes proved protracted. In coming to terms with Kennedy’s time scale for Apollo, NASA cancelled JPL’s mapper, and instructed the Langley Research Center to build a lightweight orbiter capable of being dispatched by the Atlas-Agena. This new spacecraft was not to be a global mapper, it was simply to chart predetermined areas as potential Apollo landing sites. This unimaginatively named Lunar Orbiter project was initiated in August 1963. Although Ranger had yet to prove itself, it was apparent that developing an orbiter would not just be a matter of fitting a motor to insert Ranger into lunar orbit. While JPL’s television camera package was ideal for documenting a 20-minute plunging dive that would result in the destruction of the craft, it was capable of providing the required high surface resolution only in its final few seconds, by which time its field of view was extremely constrained. To survey wide areas with such resolution from an altitude of 35 nautical miles, Lunar Orbiter would expose film that would be developed, scanned and transmitted back to Earth. Furthermore, since the orbiter had to be lightweight, the camera could not be heavily shielded from radiation in space, and very fine-grained ‘slow’ film was employed, which in turn necessitated long exposures and a mechanism to enable the camera to compensate for the spacecraft’s motion. A twin-lens system was used, with the images from a wide lens providing the context for those from a narrower lens. In December 1963, at the same time as it cancelled the follow-on Rangers, NASA awarded the contract for Lunar Orbiter to Boeing; in effect, the budget was transferred. As with Ranger, Lunar Orbiter would fly as a 3-axis stabilised platform, but a spacecraft’s configuration is intimately related to its payload and although it was possible to use many off-the-shelf systems, the orbiter was necessarily very different from its predecessor. The budget allowed for five operational spacecraft, plus a spare for engineering trials. It was expected that three successful flights would be sufficient to survey all the candidate sites for the first Apollo landing, which was as far ahead as the agency was thinking at the time. To achieve this, Langley wrote three flight plans, designated ‘A’, ‘B’ and ‘C’.

Lunar Orbiter was to fly an almost equatorial elliptical orbit with a 35-nautical – mile perilune on the near side timed to enable the spacecraft to expose its pictures at a low Sun angle to highlight the surface relief, and let the perilune point drift to more western longitudes to follow the sunrise terminator. After 10 days the perilune point would have travelled the length of the equatorial zone in which the targets were located, documenting each under ideal illumination. Furthermore, because the zone extended 80 nautical miles to each side of the equator, it was necessary to tilt the trajectory. In fact, it was decided to adopt an inclination of 11 degrees for the first two missions, with the perilune of the first south of the equator and the perilune of the second north of the equator, and then incline the orbit of the third mission as required to fill in gaps and make follow-up studies.

Lunar Orbiter 1 was launched on 10 August 1966, and entered lunar orbit on 14 August. A motion compensator fault smeared the pictures from the narrow lens. Although the flight controllers considered raising the perilune in order to reduce the smearing and map the entire Apollo zone with a resolution of about 80 feet, it was decided to remain at low level and document the designated targets using the wide lens. On 29 August the spacecraft photographed the ninth target on its list, processed its film and transmitted the results, thereby completing its primary mission. It transmitted telemetry for a further two months to enable the degradation of its systems to be monitored, and was then de-orbited to clear the radio frequencies for its successor, which began its program on 18 November. In addition to inspecting the remaining 11 candidate targets, Lunar Orbiter 2 was able to snap a number of secondary sites which, while of no immediate interest to the Apollo planners, were of ‘scientific’ interest and possible candidates for later missions. Lunar Orbiter 2 completed its photography on 26 November. In addition to its own targets, it had taken high-resolution images of the most interesting sites photographed at medium resolution by its predecessor. Site ‘A3’ (now labelled 2P-6) was confirmed to be promising and (of the new targets) 2P-2 was deemed to be suitable. In addition to photographing the most promising targets from different angles in order to permit stereoscopic analysis of the topography, Lunar Orbiter 3 charted the routes that an Apollo spacecraft might fly to approach these sites. The US Geological Survey (USGS) produced terrain maps for the Apollo planners.

With the primary objective of the project achieved by the first three spacecraft, NASA released the remaining spacecraft to the scientists, who opted to fly them in near-polar orbits at higher altitudes in order to conduct more general mapping, particularly of the far side of the Moon which permanently faces away from Earth. Even after they had finished imaging, this series of spacecraft provided insight into the lunar interior. Although Lunar Orbiter 1 was de-orbited prior to the arrival of its successor, it was noted that its orbit was being perturbed, indicating that the Moon’s gravitational field was uneven. To study this phenomenon, the subsequent spacecraft were not de-orbited until their attitude-control propellant was almost exhausted, and by virtue of flying vehicles in both equatorial and polar orbits it was possible to chart the field in sufficient detail to infer that the dark plains in the circular basins were the loci of the most intense gravity. The discovery of these ‘mascons’ (i. e. the excess of mass concentrated in these basins) was fortunate, as otherwise their perturbations of the Apollo mission would have come as a surprise.