A problem of not knowing the ground
Before astronauts could land on the Moon, someone needed to know what the surface looked like at close range. Telescopes on Earth could not show details much smaller than a kilometre or so, and the early Ranger probes had sent back pictures only during their final plunge. A landing needed smooth, firm ground and a safe approach path, and nobody could be sure that such places existed in useful numbers.
The answer was Lunar Orbiter, a series of five uncrewed spacecraft launched between August 1966 and August 1967. Their job was to circle the Moon and photograph it, and they were built for NASA by Boeing. Each was sent on an Atlas-Agena rocket.
How the cameras worked
The spacecraft did not carry digital cameras, which did not exist in a usable form. Instead each carried a photographic system with film, based on equipment from Eastman Kodak. The film was exposed, developed on board with a processing system, and then scanned in narrow strips. The strips were radioed to Earth and reassembled into pictures by ground teams.
The system had two lenses, one giving wide coverage at moderate detail and another giving much finer detail over a smaller area. This meant the mission could provide both an overview and close looks at promising spots.
The five missions
The programme was planned in two parts. The first three flights focused on candidate sites near the lunar equator, since those were the places Apollo spacecraft could reach most easily. The final two used different orbits to map more widely.
- Lunar Orbiter 1 (August 1966) photographed possible landing sites and also took the first picture of the Earth as seen from the vicinity of the Moon, an image that attracted wide public attention.
- Lunar Orbiter 2 (November 1966) continued the survey of sites. It returned a dramatic oblique view of the crater Copernicus that became famous.
- Lunar Orbiter 3 (February 1967) completed the study of candidate sites. It also photographed the Surveyor 1 craft sitting on the lunar surface.
- Lunar Orbiter 4 (May 1967) was placed into a near-polar orbit and photographed much of the near side of the Moon at moderate detail.
- Lunar Orbiter 5 (August 1967) also flew a polar orbit. It photographed sites of scientific interest, filled gaps, and covered the far side.
Lunar Orbiter 1 is a 1966 mission and Lunar Orbiter 2 also flew in 1966, so only the last three belong strictly to 1967. All five are usually discussed together because they formed a single programme.
Choosing landing sites
The most practical result was a list of places where the ground looked acceptable. Planners wanted terrain with few large craters and boulders, gentle slopes, and a safe approach corridor for the descending lunar module. Photographs from the first three missions allowed NASA to narrow a list of candidates to a smaller set of sites in the equatorial belt. One of them, in the Sea of Tranquility, was later selected for the first landing, in July 1969.
Photographs alone could not fully answer whether the surface would hold a spacecraft. For that, the Surveyor landers, such as Surveyor 3, which dug into the soil in April 1967, supplied the evidence. The two programmes were designed to complement each other, with the orbiters mapping from above and the landers testing from the ground.
Gravity as well as pictures
The spacecraft’s tracking produced an unexpected finding. By watching how the orbits changed, scientists detected local variations in the Moon’s gravity. Some large circular basins seemed to have concentrations of mass beneath them, later called mascons. This mattered to Apollo because it affected orbits over time, and it showed that low lunar orbits were less stable than first assumed.
The end of the programme
At the end of each mission the spacecraft were generally sent to crash on the surface, partly so their radio signals would not interfere with later missions. By the time the last was finished, the programme had photographed the great majority of the lunar surface, with reports typically putting the figure above 99 percent at some level of resolution. The exact coverage at each level of detail varied.
Why it matters
Lunar Orbiter was less dramatic than a crewed launch, but it supplied the map. It also gave the public some of its first clear views of the Moon as a place, and the sight of Earth rising over the lunar horizon in those pictures prefigured images from later Apollo flights. In a year when crewed American flight was grounded after the Apollo 1 fire, Lunar Orbiter was one of the ways the programme kept moving, and its photographs were still being studied when astronauts set off in 1969.