When President Kennedy declared in 1961 that the United States would land a man on the Moon before the end of the decade, he did not specify where on the Moon that landing would occur. This was not a trivial detail. The Moon’s surface is 14.6 million square miles of craters, mountains, boulders, slopes, and dust. Most of it would have been lethal for a landing attempt. Choosing the right spot was one of the most complex problems NASA faced, requiring years of photography, mapping, scientific analysis, and engineering calculation.
The story of how Apollo landing sites were selected is a story about the intersection of geology, engineering, orbital mechanics, and risk management. Every site had to satisfy dozens of constraints simultaneously. A spot that was scientifically fascinating might be too dangerous to reach. A spot that was perfectly safe might teach us nothing new about the Moon. The final selections were compromises, shaped by the competing demands of science, engineering, and politics.
The Constraints
NASA’s landing site selection criteria were brutally demanding. The requirements fell into three categories: safety, reachability, and scientific value.
Safety
The Lunar Module (LM) needed a flat, smooth surface to land on. A slope of more than two degrees risked tipping the spacecraft. Boulders larger than about two feet could damage the landing legs or prevent a stable touchdown. Deep craters could swallow the LM entirely. And the surface had to be firm enough to support the spacecraft’s weight without the landing pads sinking into dust.
The problem was that in the early 1960s, nobody knew what the lunar surface was actually like at the scale that mattered for landing. Telescopes could resolve features down to about a kilometer, but the LM needed a clear area measured in tens of meters. The gap between what could be seen from Earth and what needed to be known on the ground was enormous.
Reachability
The landing site had to be reachable given the orbital mechanics of the Apollo trajectory. The spacecraft approached the Moon from a specific direction, entered a specific orbit, and the LM separated and descended from that orbit. This meant that only sites within a narrow band near the lunar equator were accessible for the early missions. Sites at high latitudes or on the far side of the Moon were unreachable with the available fuel and trajectory options.
The site also had to be illuminated by the Sun at a low angle (between 5 and 14 degrees above the horizon) at the planned time of landing. This was not about lighting for photography. At low sun angles, shadows cast by boulders, craters, and slopes are long and visible, allowing the crew to judge the terrain during the final approach. At high sun angles, the surface appears uniformly bright and featureless, making it impossible to spot hazards. At zero sun angle (directly overhead), there are no shadows at all.
This sun angle requirement, combined with the orbital constraints, meant that each mission had only a few days per month when a given site was reachable under acceptable lighting conditions. The launch window for each Apollo mission was determined largely by this interplay of orbital mechanics and solar illumination.
Scientific Value
NASA’s scientists wanted landing sites that would maximize the geological information returned from each mission. Different sites offered access to different types of lunar terrain: mare basalts (the dark, flat plains formed by ancient volcanic flows), highland materials (the lighter, more heavily cratered ancient crust), and specific features like rilles, domes, and rays from impact craters.
For the first landing, however, scientific interest was subordinated almost entirely to safety. The primary goal of Apollo 11 was to prove that a crewed lunar landing was possible. Everything else was secondary.
Mapping the Moon
Before landing sites could be selected, the Moon had to be mapped at far higher resolution than Earth-based telescopes could provide. NASA accomplished this through a series of robotic precursor missions in the 1960s.
The Ranger program (1961 to 1965) sent spacecraft on collision courses with the Moon, photographing the surface at ever-increasing resolution during the final minutes before impact. Ranger 7, 8, and 9 returned thousands of images showing surface details as small as a few meters across. These images revealed for the first time the true character of the lunar surface at landing-relevant scales.
The Lunar Orbiter program (1966 to 1967) placed five spacecraft in lunar orbit, systematically photographing potential landing sites at resolutions down to about one meter. The Lunar Orbiter images were the primary data source for Apollo landing site selection. They revealed that the lunar surface was more varied and more rugged than expected, but that sufficiently flat areas could be found within the equatorial zone.
The Surveyor program (1966 to 1968) landed seven robotic spacecraft on the Moon, five of them successfully. The Surveyors tested whether the surface could support a spacecraft (answering the long-standing question of whether the Moon was covered in deep, soft dust), photographed the terrain from ground level, and analyzed the chemical composition of the soil. Surveyor data confirmed that the mare surfaces were firm enough for a landing.
Apollo 11: Tranquility Base
For the first crewed landing, NASA selected a site in the Sea of Tranquility (Mare Tranquillitatis), a broad, flat basaltic plain in the Moon’s equatorial region. The site, designated Landing Site 2, was chosen primarily for its safety: Lunar Orbiter images showed it to be one of the smoothest, flattest areas within the accessible zone.
Even so, the landing was not straightforward. As the LM descended on July 20, 1969, commander Neil Armstrong realized that the automatic guidance system was steering toward a boulder-strewn crater field. He took manual control and flew the LM horizontally over the hazards, searching for a clear spot, finally landing with less than 25 seconds of fuel remaining. The site he chose, named Tranquility Base, was about four miles from the planned target.
Armstrong’s improvised landing demonstrated both the necessity of human judgment and the limitations of pre-mission site selection. No amount of orbital photography could guarantee a safe landing; the final decision always rested with the crew.
Later Missions: Increasing Ambition
With the success of Apollo 11 proving that landing was possible, subsequent missions ventured to progressively more challenging and scientifically rewarding sites.
Apollo 12 (November 1969) landed in the Ocean of Storms, within walking distance of the Surveyor 3 spacecraft that had landed two and a half years earlier. The precision of the landing (the LM touched down just 600 feet from Surveyor 3) demonstrated that pinpoint landings were achievable.
Apollo 14 (February 1971) landed near Fra Mauro crater, a site chosen for its geological interest. The Fra Mauro formation is believed to consist of material ejected by the impact that created the enormous Imbrium basin, making it a window into the Moon’s deep interior.
Apollo 15 (July 1971) was the first of the “J missions,” designed for extended surface exploration. It landed at Hadley Rille, a sinuous channel on the edge of the Apennine Mountains. The site combined a volcanic rille with mountain slopes, offering access to multiple geological features. Apollo 15 was the first mission to carry the Lunar Roving Vehicle, which allowed the crew to explore a much larger area.
Apollo 16 (April 1972) targeted the Descartes highlands, the first landing in the lunar highlands rather than the mare plains. The site was chosen to sample the ancient crustal material that forms the Moon’s original surface.
Apollo 17 (December 1972), the final Apollo mission, landed at Taurus-Littrow, a valley between massive mountain blocks on the edge of the Sea of Serenity. The site offered access to both ancient highland material and younger volcanic deposits, maximizing the geological return from the last crewed mission to the Moon.
The Trajectory That Got Them There
Selecting a landing site was only the first step. Getting there required computing a trajectory from Earth to lunar orbit with extraordinary precision. The translunar trajectory had to account for the gravitational fields of the Earth, Moon, and Sun; the rotation of the Earth; the orbital motion of the Moon; and the specific location of the landing site on the lunar surface.
Kronecker Wallis’s Apollo Translunar Trajectory Plotting Chart reproduces the original Apollo 11 trajectory chart from June 23, 1969, showing the planned translunar and transearth flight paths. The chart is a visual summary of the orbital mechanics that carried Armstrong, Aldrin, and Collins from the Earth to the Moon and back: the curves, velocities, and timing that had to be calculated with precision measured in fractions of a second.
For those interested in the emergency procedures that saved Apollo 13 when its landing was aborted, the Apollo 13 LM Systems Activation Checklist reproduces the crucial document that Commander James Lovell annotated by hand during the spacecraft’s emergency return, including his calculations of the descent angle that brought the crew safely back to Earth.
Choosing the Unknown
The selection of Apollo landing sites was an exercise in making decisions under radical uncertainty. NASA had to choose specific points on a world that no human had ever visited, using photographs taken from orbit, and guarantee that a spacecraft could land safely, that the crew could work on the surface, and that the science would justify the risk. The fact that six missions landed successfully (Apollo 13’s failure occurred en route, not at the landing site) is a testament to the thoroughness of the selection process and the skill of the crews.
The Moon is now better mapped than most of Earth’s ocean floor. Future missions, including NASA’s Artemis program, will land near the lunar south pole, in regions that the Apollo program could not reach. But the methods of site selection remain fundamentally the same: photograph, analyze, model, debate, and choose. The Moon does not make it easy. It never did.
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