Lunar Orbiter 4

Courtesy of NASA's National Space Science Data Center

Launch Date: 1967-05-04
On-orbit dry mass: 385.60 kg. (850 lb.)

Description

The Lunar Orbiter 4 spacecraft was designed primarily to photograph smooth areas of the lunar surface for selection and verification of safe landing sites for the Surveyor and Apollo missions. It was also equipped to collect selenodetic, radiation intensity, and micrometeoroid impact data. The spacecraft was placed in a cislunar trajectory and injected into an elliptical lunar orbit for data acquisition. It was stabilized in a three-axis orientation by using the sun and the star Canopus as primary angular references. A three-axis inertial system provided stabilization during maneuvers and when the sun and Canopus were occulted by the Moon. Communications were maintained by an S-band system which utilized a directional and an omnidirectional antenna. The spacecraft acquired photographic data from May 11 to 26, 1967.

Lunar Photographic Studies

This experiment consisted of a dual-lens camera system designed to satisfy the primary mission objective of providing photogaphic information for mapping and for the evaluation of Apollo and Surveyor landing sites. An 80-millimeter lens system was used to obtain medium-resolution (MR) photos, and a 610-millimeter lens system was used for high-resolution (HR) photos. The two separate lens, shutter, and platen systems utilized the same film supply and recorded imagery simultaneously in adjacent areas on 70-millimeter film. Continual automatic sequences of photos were obtained. At an altitude of 2,700 kilometers (1,670 miles), which was approximately the perilune height, the system photographed over 85 percent of the lunar surface. At apolune, on the moon's farside at about 6,110-kilometer (3,800-mile) altitude, the areas photographed were correspondingly larger. The film was bimat processed on board and optically scanned, and the resulting video signal was telemetered to ground stations. Film density readout was accomplished by a high-intensity light beam focused to a 6.5-micron-diameter spot on the spacecraft film. The spot scanner swept 2.67 millimeters in the long dimension of the spacecraft film. This process was repeated 286 times for each millimeter of film scanned. The raster was composed of 2.67- by 65-millimeter scan lines along the film. The video signal received at the ground station was recorded on magnetic tape and also fed to ground reconstruction equipment (GRE), which reproduced the portion of the image contained in one raster on a 35-millimeter film positive framelet. Over 26 framelets were required for a complete MR photograph and 86 for a complete HR image. Fogging of the window rendered some photographs unusable. Otherwise, experiment performance was nominal until the final readout on June 1, 1967. A detailed description of the experiment, a bibliography, and indexes of all the available Lunar Orbiter 1 through 5 photos are contained in the report, 'Lunar Orbiter photographic data,' NSSDC 69-05, June 1969.

Selenodesy Experiment

The instrumentation for this experiment included a power source, an omnidirectional antenna, and a transponder to obtain information for determining the gravitational field and physical properties of the moon. High-frequency radio signals were received by the spacecraft from earth tracking stations and retransmitted to the stations to provide doppler frequency measurements. The telemetry data were processed in real time on an IBM 7044 computer in conjunction with an IBM 7094 computer. They were then displayed on 100-wpm teletype machines, X-Y plotters, and bulk printers for analysis. Data coverage was continuous while the spacecraft was visible from earth. Information was acquired during the cislunar, the photo ellipse, and the extended mission (from end of the photographic mission to July 11, 1967) phases. Doppler, ranging, hour angle points, and declination angle points data were accumulated during tracking. The quality of recorded data ranged from good to excellent.

Meteoroid Detectors

Twenty 0.025-millimeter beryllium copper pressurized cell detectors were used to provide direct measurements in the near-lunar environment of the rate of penetration by micrometeoroids. The detectors were arranged on the periphery of the tank deck. Each cell was a helium pressurized semicylinder with a pressure sensitive switch that remained closed until pressure was released by puncture of the cell's surface. Meteoroid hits were recorded by discrete telemetry channel state changes. The total exposed area of the detectors was 0.282 square meters, and the effective area after shielding by other components was 0.186 square meters. Two micrometeoroid hits were recorded during the photographic mission phase of the flight.

Cesium Iodioe Dosimeters

The principal purpose of the Lunar Orbiter radiation measuring systems was to monitor, in real time, particle fluxes that would damage processed film in case of major solar cosmic-ray events. This would make it possible for the mission control to minimize darkening of the film by operational maneuvers. A secondary purpose was to acquire a maximum amount of information on radiations on the way to the moon and near the moon. The sensor system consisted of two separately monitored thin cesium iodide scintillators (2-Pi solid angle acceptance) that were positioned and shielded in the same way as the film in the cassette and in the loopers. The shielding thickness of the cassette and cassette dosimeter was 2 gm/sq centimeters aluminum. The shielding of the loopers and the looper dosimeter was 0.17 gm/sq centimeters aluminum. These shielding thicknesses also corresponded approximately to the thickness of the Apollo module wall and of a space suit. In the case of protons at verticle incidence, particles with energy greater than 40 and 11 MeV penetrated 2 and 0.17 gm/sq centimeters, respectively.

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Calvin J. Hamilton