For countless millennia, the Moon has captivated the fascination of humanity, but only in the last few decades has our species had the technological ability and scientific knowledge to visit and explore our closest celestial neighbor. In May 1961, President John F. Kennedy committed the United States to landing a man on the lunar surface, and returning him safely to Earth, before the decade’s end. However, in order to do so, the systems necessary to deliver that man to the Moon had to be conceived and perfected. Moreover, the very nature of the Moon itself—from the consistency and load-bearing characteristics of its regolith to the appropriateness of its rugged terrain to support a heavy spacecraft and human explorers—was acutely unknown.
At the midpoint of the decade, between August 1966 and January 1968, five unmanned Lunar Orbiter missions were launched and operated by NASA, successfully mapping 99 percent of the surface at resolutions of better than 200 feet (60 meters). In doing so, they provided vital data which would enable the space agency to ultimately select a series of five potential landing zones for the first Apollo explorers. And the first of those five trailblazers, Lunar Orbiter-1, completed its 80-day mission by impacting the Moon on 29 October 1966, 50 years ago, tomorrow. Not only was it a huge success, but it also marked the first time that the United States had ever placed one of its own spacecraft into lunar orbit.
Although plans for an in-situ examination of the Moon had been under serious discussion since the late 1950s, it was not until the months after Kennedy’s decision that the seeds of the Lunar Orbiter program were sown. Originally, it was hoped that modifications could be made to another pair of spacecraft families—Ranger and Surveyor—to fulfill this requirement, but neither could meet the level of mapping precision necessary to select Apollo landing sites. Specifically, the orbiter would need to record objects in the region of 147.6 feet (45 meters) in diameter across the entire lunar surface, with enhanced resolution of 14.7 feet (4.5 meters) in the areas of primary interest and still better resolution down to just 4 feet (1.2 meters) at landing spots.
In December 1963, NASA Administrator Jim Webb announced the selection of Boeing to develop the Lunar Orbiter. The three-axis-stabilized spacecraft weighed about 848 pounds (385 kg) and assumed the form of a truncated cone, measuring 5 feet (1.5 meters) across its base and standing 5.6 feet (1.7 meters) tall. At its base, it boasted a quartet of power-producing solar arrays and high-gain and low-gain antennas. Lunar Orbiter’s lowermost segment housed the nickel-cadmium batteries, transponders, flight programmer, photographic system, Inertial Reference Unit (IRU), Canopus star-tracker, command decoder, multiplex encoder, and Traveling-Wave-Tube Amplifier (TWTA). The spacecraft’s middle “deck” accommodated the velocity control engine, with a thrust of 100 pounds (45.3 kg), together with propellant tanks, a coarse Sun-sensor, and micrometeoroid detectors. Finally, the uppermost deck carried four attitude control thrusters and a heat shield to guard against the exhaust from the velocity control engine.
However, the key role of Lunar Orbiter was to acquire imagery of the Moon’s surface. Eastman Kodak provided a scaled-down Air Force photographic system, featuring a twin-lens camera for simultaneous imagery at high and medium resolution. “On a single mission,” noted Bruce Byers in Destination Moon, the official NASA history of the Lunar Orbiter program, “the orbiter could photograph a greater area of the lunar surface and also obtain more detailed photographic data than any other proposed system. Moreover, if loss of the use of one lens occurred … the whole photographic mission would not be ruined.”
In essence, the photographic system could provide imagery of areas up to 3,000 square miles (8,000 square km) at high resolution, which was four times better than NASA had requested. Drawing its heritage from the military, it was miniaturized in size and weight to fit aboard its launch vehicle, an Atlas-Agena D booster. “Film from a supply reel passed through a focal plane optical imaging system and controlled exposures were made,” wrote Byers. “Once past the shutter, the film underwent a semi-dry chemical developing process and then entered a storage chamber. From here it could be extracted upon command from the ground for scanning by a flying-spot scanner and then passed on a take-up reel.” Eastman Kodak also incorporated its Bimat process, which eliminated the need to use “wet” chemicals on the film. “Instead,” Byers continued, “a film-like processing material was briefly laminated to the exposed film to develop and fix the negative image … Once the film had been developed and fixed, the Bimat material separated from the film and wound onto a storage spool.”
In spite of early concerns over experiment integration, delays in the fabrication of spacecraft adapter hardware for the Atlas-Agena D, problems with the Lunar Orbiter propellant tanks, and difficulties in the certification of Eastman Kodak’s imaging system, the program entered major development by mid-1965. Final testing was completed early the following year, although the launch date for Lunar Orbiter-1 slipped from early June 1966 into mid-July and eventually into the second week of August. These slips were caused in part by delays in the delivery of Eastman Kodak’s imaging system, coupled with demands imposed on the Deep Space Network (DSN) by the Surveyor-1 lunar lander, which had launched at the end of May. At length, on 25 July, a Flight Readiness Review (FRR) was concluded at Cape Kennedy in Florida and Lunar Orbiter-1 was confirmed as ready to launch, no sooner than 9 August.
At the beginning of the month, the imaging system was installed aboard the spacecraft and Lunar Orbiter-1 was transferred to Launch Complex (LC)-13 at the Cape for integration aboard the Atlas-Agena D. (Interestingly, LC-13 is today operated by SpaceX and has been reconfigured as Landing Zone-1 for its returning Upgraded Falcon 9 first-stages.) The countdown operations on the 9th proceeded normally until T-7 minutes, when an anomaly with the booster’s propellant utilization system forced a 24-hour scrub. The next day met with no such problems and Lunar Orbiter-1 roared aloft at 3:26 p.m. EDT. Within five minutes of leaving the Cape, the spacecraft was boosted into an initial “Earth-parking” orbit, after which the Agena was restarted to deliver Lunar Orbiter-1 onto a four-day voyage through cislunar space to reach the Moon.
Problems were experienced during the journey with the Canopus star-tracker; a particularly worrisome circumstance, in view of the fact that the spacecraft depended upon proper orientation along its yaw, pitch, and roll axes to reach the Moon’s vicinity in the correct attitude to achieve orbit. Work-around procedures were developed, whereby Lunar Orbiter-1 was commanded to establish a roll reference, pointing the Canopus sensor toward the Moon at the point of its mid-course correction maneuver. This proved successful and with the sensor locked onto the Moon, controllers were reasonably confident that it was functioning correctly. The spacecraft also suffered issues of overheating systems during its transit phase, but by the morning of 14 August it was less than 7 miles (10 km) off its planned orbit-insertion point. Commands to inject Lunar Orbiter-1 into a highly-elliptical path around the Moon got underway at 11:22 a.m. EDT.
Operating from an “apolune” of 1,160 miles (1,867 km) and a “perilune” of 117 miles (189 km), America’s first mission in lunar orbit was finally underway. For the next 2.5 months, Lunar Orbiter-1 was exclusively focused upon potential Apollo landing sites. Its first imaging location was part of Mare Smythii, on the easternmost edge of the lunar near side, and high-resolution photographs were acquired every 10 seconds. And despite problems with the imaging system itself, the spacecraft was “deboosted” to a lower altitude of 25 miles (40.5 km) for further photography. By month’s end, images of nine potential Apollo landing sites had been taken, as well as early surveys of the lunar far side. Additionally, Lunar Orbiter-1 returned the first image of Earth, as seen from the vicinity of the Moon.
An extended mission got underway in mid-September, but by the tail end of October it was apparent that the spacecraft was deteriorating. Its attitude control system was depleted of propellant, its overheating battery was rapidly losing power, its transponder was behaving erratically, and the IRU was struggling to maintain stabilization. All told, it was anticipated that the ability of the spacecraft to remain stable would last between two and five weeks. As a result, Lunar Orbiter-1 was deliberately crashed into the far side during its 577th orbit on 29 October 1966. As a result, 50 years ago today, America’s first mission to enter lunar orbit came to a conclusion, with 42 high-resolution and 187 medium-resolution images having been returned, covering 1.93 million square miles (5 million square km) of the Moon. All told, 75 percent of the planned mission objectives were fulfilled.
Lunar Orbiter-1’s pathfinding voyage gave way to four more missions over the course of the following 15 months. The next two spacecraft to be launched—Lunar Orbiter-2 in November 1966 and Lunar Orbiter-3 in February 1967—also focused upon 20 potential Apollo landing sites, whilst the last pair entered high-altitude polar orbits and followed broader scientific objectives. Specifically, Lunar Orbiter-4, which rose from Earth in May 1967, photographed the entire lunar near site, as well as 10 percent of the far side, whilst Lunar Orbiter-5, launched in August 1967, completed far-side coverage and achieved an imaging resolution as fine as 66 feet (20 meters). By the time the final Lunar Orbiter-5 was impacted into the Moon in January 1968, around 99 percent of the surface had been imaged. In a very significant sense, the program contributed enormously to our understanding of our closest celestial neighbor and built confidence as the United States prepared to send the first humans to the Moon.