DESTINATION MOON: A History of the Lunar Orbiter Program
The Third Orbiter Mission
[259] The third mission differed slightly from the first two because it concentrated its photography on Apollo and Surveyor site confirmation instead of site search. To permit confirmation photography of sites both north and south of the lunar equator the spacecraft's orbital inclination was increased to 21°. The convergent stereo photography of Mission II had proved successful and potentially useful to the Apollo and Surveyor programs. It consisted of making two "footprints" of the same area on two successive orbits. To accomplish this at the higher orbital inclination, the camera would necessarily be tilted during one of the two sequences. Resolution of a convergent stereo picture pair was slightly degraded because of the camera tilt, and a loss of one-meter to two-and-one-half-meter, or perhaps three-meter, resolution resulted.55
The Air Force Aeronautical Chart and Information Center (ACIC) and the Army Map Service bad evaluated the Mission II convergent stereo photography and had concluded that "this type of photography increases the topographic knowledge that can be obtained concerning potential landing sites."56 [260] The Lunar Orbiter Project Office at Langley planned to include more convergent stereo coverage on Mission III as a result of the ACIC and Army Map Service (since January 1970, U.S. Army TOPOCOM) evaluations.
On November 15, 1966,a technical interchange meeting convened at the Jet Propulsion Laboratory to assess the various methods of calibrating the Lunar Orbiter's 610 mm high-resolution camera for the new photographic tasks. Precise geometric calibration was mandatory if stereo photography was to be conducted successfully on the three remaining missions. The calibration was to be done at the photographic subsystem level, and the members of the meeting determined the method to use.57 Leon J. Kosofsky coordinated the calibration activities.
Although primarily a reconnaissance photographic system, rather than a mapping system, the Lunar Orbiter photo subsystem was upgraded after Mission I. The Aeronautical Chart and Information Center and the Army Map Service had previously argued that the use of reseau marks on the camera film or a grid on the camera lens would greatly facilitate the utilization of photographic data for purposes of lunar mapping. Langley accepted the idea of pre-exposing reseau marks on the camera film for Mission II and all subsequent [261] missions.
On January 5 the photo subsystem for Spacecraft 6 (the third flight spacecraft) was installed, and Boeing conducted the functional check-out with the Deep Space Instrumentation Facility. The spacecraft's inertial reference unit (IRU) was taken out, tested, and reinstalled and the actuator for solar panel 3 was replaced. Retesting at Hangar S was accomplished by January 13 in preparation for mating with the launch vehicle.58
Meanwhile, on January 5 the Ad Hoc Surveyor/Orbiter Utilization Committee of OSSA bad approved the plan for the third Lunar Orbiter mission:
Mission III is primarily designed to photograph promising areas that have been identified by screening Lunar Orbiter I and II photographs and for which additional data is needed to confirm their adequacy as Apollo and/or Surveyor landing sites. In addition Mission III will provide photography of broad scientific interest as did Missions I and II.59
The mission would also obtain precision trajectory information to be used in improving the definition of the lunar gravitational field and measurements of micrometeoroid flux and of radiation dosage levels in near-lunar environment for use in evaluating the spacecraft's performance. [262] Finally Lunar Orbiter III would serve as a target for the Manned Space Flight Tracking Network and the Orbit Determination Program.60
The Launch Readiness Review for Lunar Orbiter III and for the back-up (Spacecraft 7) was held at the Eastern Test Range facilities on January 17. Both Orbiters were found to be ready for launch, and personnel working with Spacecraft 6 proceeded with the preparations for that event. The tentative date for launch was February 3.61
Boeing and Eastman Kodak were attempting to resolve the problems which had caused minor film processing defects on the first two Missions. Manufacturing irregularities and bubbles in the Bimat bad been the chief causes of these defects. As it turned out, localized Bimat processing defects continued to appear on some photographs from all five missions, despite attempts to correct the condition. Still unresolved as the third launch approached was the failure of the TWTA aboard Lunar Orbiter II. However, Boeing engineers were modifying this component so that excess heat build-up could be removed during the flight, thus prolonging the tube's lifetime. Readout times would also be reduced in the event of a heat build-up, and flight controllers would [263] monitor the flow of electrical current through the traveling wave-tube amplifier, since program scientists considered any irregularities in the flow to be an indication of pending trouble in it.62
Lunar Orbiter III lifted off of Pad 13 at the Eastern Test Range at 01:17 Greenwich Mean Time on February 5, 1967. (The February 3 launch window had been canceled because of problems encountered in the ground power-supply system at Launch Complex 13.) Despite numerous pre-launch problems the liftoff was successfully accomplished on a flight azimuth of 80.8° at the start of the February 5 launch window. Ground control placed the Agena-spacecraft combination in a parking orbit for approximately ten minutes before injecting it into a cislunar trajectory.63
Following injection the spacecraft separated from the Agena, deployed its solar panels and antennas, and acquired the Sun as an attitude reference. Seven hours into the mission flight controllers commanded Lunar Orbiter III to turn on its Canopus star tracker and give a star map before Canopus acquisition. It executed this command successfully. On Monday, February 6, at 37 hours into the mission [264] the Space Flight Operations Facility tracking Lunar Orbiter III commanded a midcourse correction maneuver to adjust the spacecraft's cislunar trajectory in order to hit the preplanned aiming point for deboost into lunar orbit, As on previous missions, the midcourse maneuver was so accurate that no second maneuver was required.64
At 4:54.p.m. Eastern Standard Time on February 8 Lunar Orbiter III fired its 100-pound-thrust rocket engine for 9 minutes, 2.5 seconds to decelerate the spacecraft into its initial orbit. The parameters were: apolune, 1,801.9 kilometers; perilune, 210.2 kilometers; inclination, 20.93°; period of orbit, 3 hours 25 minutes.65 Ground control tracked the spacecraft in the initial orbit for approximately four days (25 orbits) to obtain data for analysis of the lunar gravitational effect. Following this the spacecraft was transferred to a new orbit with a low perilune of 55 kilometers and an apolune of 1,847 kilometers.66 Inclination to the lunar equator was 20.9°.67
As Lunar Orbiter III had executed its deboost maneuver Lunar Orbiter II was still in orbit around the Moon. On February 6 ground control began tracking both spacecraft [265] simultaneously, thus demonstrating its ability to track two spacecraft in different orbits around the Moon at the same time. This exercise greatly extended the usefulness of each mission by providing simultaneous telemetry on the two orbiting spacecraft. Monitoring showed that all Lunar Orbiter II subsystems were operating normally.68
Lunar Orbiter III began its photographic mission on February 15 over primary Site III P-1 at 35°15" east longitude, 2°5" north latitude, near the crater Maskelyne F in the southeastern region of Mare Tranquillitatis. The first readout in the primary mode revealed photographs of excellent quality. A solar flare occurred at 12:54 p.m. EST on February 13. Though it had a high amount of optical activity, there was little of the proton activity that could have presented a danger to the film on board the spacecraft.69 The first readout revealed no fogging of the film and indicated that all subsystems were working normally.
The film advance mechanism in the readout section of the photo subsystem of Lunar Orbiter III began to show erratic behavior even during the mission's photographic phase. Because of this, program officials decided to begin final readout earlier than planned. Ground control at the [266] DSN decided not to photograph secondary Site S-32, an oblique shot of the Grimaldi crater area. A total of 211 out of 212 planned frames had been exposed when, at 1:36 a.m. EST on February 23, flight controllers commanded the spacecraft to cut the Bimat, closing out the photographic portion of the third mission. By March 1, readout had been completed for 114 frames of photography, or 54% of the total. Film advance through the readout gate was intermittently hampered during this time, but no no photography was lost.70
Then suddenly on March 4 readout ceased. Of the 211 frames, 72 still remained to be read out, but the worst had happened. The film advance motor had burned out, and the 72 frames remained on the take-up reel. Program engineers concluded that an inexplicable electrical transient had scrambled the photo system's logic, causing the motor to run out of control. Nonetheless, 75% of the photographic data bad been transmitted to Earth before this failure. The decision to begin readout earlier than planned had proved very prudent indeed.71
Mission III photography displayed the finest overall quality thus far obtained in the program. The quality was due in [267] part to the use of more diversified photographic procedures, including the use of precisely oriented camera axis over a wide range of tilt angles and azimuth. The exposure sequencing modes were varied and used more extensively. Relaxation of earlier photographic constraints, higher orbit inclination and extended stereoscopic photography resulted in greater coverage over a wider range-of latitude and successful photography under extreme illumination conditions.72
Among other important sites Lunar Orbiter III photographed the Surveyor I landing area, permitting the location and identification of the spacecraft on the Moon's surface in Telephoto Frame 194 of Site III Pl2a.73 This and other accomplishments proved the reliability, accuracy, and versatility of the spacecraft in its lunar exploration mission and gave program officials the confidence to attempt more complex precision photography on the two remaining missions.