By L. J. Kosofsky and Farouk El-Baz, NASA SP-200, 1970
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The Lunar Orbiter program was conceived, together with the Ranger and Surveyor programs, with the primary objective of providing information essential for a successful manned Apollo lunar landing. The Lunar Orbiter program comprised five missions, all of which were successful. As the primary objectives for the Apollo program were essentially accomplished on completion of the third mission, the fourth and fifth missions were devoted largely to broader, scientific objectivesphotography of the entire lunar nearside during Mission IV and photography of 36 areas of particular scientific interest on the nearside during Mission V. Photography of the farside during the five missions resulted in an accumulated coverage of more than 99 percent of that hemisphere. The detail visible in the farside coverage generally exceeds that previously attained by Earth-based photographs of the nearside; in some areas objects as small as 30 meters are detectable.
Initiated in early 1964, the Lunar Orbiter program included the design, development, and utilization of a complex automated spacecraft technology to support the acquisition of detailed photographs of the lunar surface from circumlunar orbit. The five spacecraft were launched at 3-month intervals between August 10, 1966, and August 1, 1967.
In addition to its photographic accomplishments, the program provided information on the size and shape of the Moon and the major irregularities of its gravitational field. This selenodetic information was derived from the tracking data. Micrometeoroid and radiation detectors, mounted on the spacecraft for operational purposes, monitored those aspects of the lunar environment.
Photographs for Apollo (Missions I, II, and III) The primary objective of the Lunar Orbiter program was to locate smooth, level areas on the Moon’s nearside and to confirm their suitability as manned landing sites for the Apollo program. To accomplish this, photographic coverage at a ground resolution of 1 meter was required of areas within 5″ of the equator between longitudes 45″ E and 45″ W-the zone of primary interest to the Apollo program.
Twenty potential landing sites, selected on the basis of Earth observations, were photographed during the site search missions of Lunar Orbiters I and 11. Lunar Orbiter I11 rephotographed 12 of the most promising of these areas during its site-confirmation mission. Following analysis of these photographs, consideration was further reduced to the eight most promising areas.
However, to make the final selection of the candidate Apollo landing sites, additional photographs of various types were required at all but three of these areas. These photographs, obtained during Mission V, provided sufficient data to permit the final site selection and mapping.
Approximately three-fourths of the film supply of the first three missions was used to photograph areas of interest to the Apollo program primarily. The remainder could not be used for such areas because of operational film-handling requirements; i.e., the film could not remain stationary in the camera for long periods of time lest it deteriorate. Photographs taken when the areas of primary interest were not in view, referred to as film-set frames, were expended in a variety of ways and for several reasons. In the Mission I flight plan, such sites were selected, in real time, for diagnostic tests of certain spacecraft malfunctions, for reconnaissance of potential photo sites for subsequent missions, and for photography of the Moon’s farside.
To minimize operational demands on Mission I. nearly all of the film-set exposures were made using conventional maneuvers of the spacecraft. This resulted in near-vertical photographs for all but two areas covered during Mission I. Two oblique photographs of the Moon’s farside, with the Earth in the background, were taken while the spacecraft was passing behind the Moon’s eastern limb. The scene provided in these oblique views and the flawless execution by the spacecraft of every single maneuver command during Mission I prompted more rigorous performance demands for the following missions.
Specific plans to use the film-set frames were included in the preflight design of Missions I1 and 111. Several outstanding oblique photographs were obtained on both the nearside and the farside. The Mission I1 oblique view of the crater Copernicus IS a notable example.
Because the area of Apollo interest was near the equator and I-meter resolution was required, photographs taken during Missions I, 11, and 111 were made from low inclination, close-in orbits. This restricted the north and south latitude range for photography and also the total area coverage obtained. These restrictions were relaxed for the final two missions. Photographs of General Scientific Interest (Missions JV and V) The flight plans for Missions IV and V incorporated nearpolar circumlunar orbits from which virtually any part of the Moon could be photographed. Mission IV photography, conducted from high-altitude orbit, was a broad. systematic survey of the entire nearside. Most of this photography contains detail down to 60 meters ground resolution, and the remainder is no coarser than 150 meters. The east and west limbs and both polar regions were covered in near-vertical views.
The primary objective of Mission V was to obtain closeup photographs of geologically interesting features in 36 selected areas of the Moon’s nearside. In addition, some photographs were taken to complete the Apollo requirements and to complete the coverage of the lunar farside. All of these objectives were attained with the faultless execution of Mission V, completing the Lunar Orbiter program.
TYPICAL MISSION PROFILE
Each mission started with the launch from Cape Kennedy by an Atlas-Agena D launch vehicle. Following separation from the Atlas, the Agena engine put the spacecraft into Earth orbit. After a coasting period, a second burn of the Agena engine, before separating from it, placed the spacecraft on a translunar course. A small correction in the trajectory was accomplished by a short burn of the spacecraft’s velocity-control engine some 20 to 30 hours after leaving Earth orbit. Upon arriving at the lunar encounter point, after some 90 hours, the spacecraft velocity was reduced by a retrofiring of its engine, leaving it in an elliptical posigrade lunar orbit. Injection into orbit was scheduled in all cases within a few days of new Moon, and an orbit was established with perilune (point of closest approach) near the equator on the Moon’s eastern limb.
After being tracked in this orbit for several days, the spacecraft was slowed again, lowering the orbital perilune to the desired photographic altitude. The perilune altitude was about 46 kilometers on the first three missions, lo00 kilometers on Mission IV, and 100 kilometers on Mission V.
The placement of the orbital plane had to be such that, when the Moon’s rotation brought the selected sites beneath the perilune, the site would be properly illuminated for photography (Le., have the Sun 10″ to 30″ above the local horizon). In general, the sites on the Moon’s nearside were photographed with sunrise illumination and the farside areas with sunset illumination.
The picture-taking phase was completed about the time of the full Moon, and the entire photographic mission ended with the readout and transmission of the last of the photographic data in about 30 days. The spacecraft remained in orbit and was tracked for extended periods to provide additional lunar gravitational and other environmental data. The longest lifetime in orbit was 335 days, for Lunar Orbiter 11. All the spacecraft except Lunar Orbit IV were deliberately crashed onto the Moon, by a final burn of the velocity-control engine, to ensure that they would not interfere with communication between the Earth and later spacecraft. Communication with Lunar Orbiter IV was lost after 70 days in orbit, about 3 months before it is believed to have crashed.