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Apollo Missions - Landing Site Selection

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Project Apollo: The Beginnings | Mission Planning | Landing Site Selection | Earthbound Support Systems
Astronaut Selection and Training | The Saturn V | The Saturn 1B | The Apollo Spacecraft
Guidance and Navigation | Command and Service Modules | The Lunar Module
Assembling and Launching | Pathfinders | The Early Missions | Apollo 11, The First Landing
The Intermediate Missions | Apollo 15 Exploration | Apollo 16 Exploration | Apollo 17 Exploration
Skylab and Apollo-Soyuz | Conclusion

In 1961, Homer Newell, deputy director of NASA's space science office asked chemist Harold Urey of the University of California at San Diego to recommend parameters for the selection of scientific landing sites.

Urey produced a list that he thought would yield the maximum return of scientific data from the limited time that missions were likely to spend on the lunar surface. However, the choice of the first Apollo landing sites was, of necessity, governed by operational criteria rather than the collection of scientific data. NASA regarded as its prime objective to achieve a landing and return, thereby fulfilling the Kennedy challenge. The scientific aspects of the missions would become more important later.

In March 1962, the American Telephone and Telegraph Company established, at NASA's request, a subsidiary company, Bellcomm Inc, to advise NASA's Office of Manned Space Flight on matters relating to the communications network and to provide technical support on aspects of the Apollo program. Bellcomm was to eventually lead the co-ordination of the landing site selection procedures and create the concept of the Apollo Landing Zone. NASA established an Apollo Site Selection Board in July 1965 to select specific landing sites for individual missions from recommendations made by Bellcomm's Group for Lunar Exploration Planning.

For the first landings at least, safety constraints dictated that they would have to be on relatively flat terrain which would provide a broad expanse of smooth surface to allow a margin of error for under or overshoot when descending to the target site. The target area would be an ellipse of 12 miles in length by 3 miles broad. Safety also required an outward trajectory that would allow the spacecraft to loop around the rear of the moon and slingshot back towards earth in the event of the failure of the spacecraft's main engine. This 'free return' trajectory would place the spacecraft over the equatorial region of the moon as they arrived in lunar orbit and was also the most fuel economic option. This orbital track would place the landing site in a narrow corridor straddling the lunar equator to the north and south by approximately five degrees.

The landing attempt would have to take place just a few hours after the site had been illuminated at lunar sunrise. The sun's height above the horizon would have to be high enough to illuminate the surface, but not so high as to wash out all relief provided by contrasting shadow. The optimum angle for site illumination was estimated to be just after the local lunar dawn with the sun between 5 and 13 degrees above the horizon. The approach to the landing needed to be from the east, as this would allow the crew to view the landing area with the sun illuminating the site from behind them during this most critical phase of the mission, rather than with it in their eyes.

The prime sites would also be on the eastern side of the moon's visible face to earth. In the event of a scrub of the launch and with the recycle time of the Saturn V being nearly 48 hours, it would mean that the sun's height above the landing site's horizon would be too high and wash out much of the contrasting shadow. If the launch were to be delayed a more westerly back-up site would allow the landing attempt to be shifted as the dividing terminator line between light and dark moved across the face of the moon by some 12 degrees each earth day.

The eastern and western bounds of possible landing areas were limited by navigational constraints. For the final descent the height of the spacecraft's orbit and its position over the eastern limbs of the lunar surface had to be calculated from observations on prominent lunar features to within 1500 feet. The Lunar Module's guidance system required realignment with these measurements just prior to commencing the final descent and that information also had to be relayed back to mission control for confirmation. This would enable them to update their computers to confirm the position of the spacecraft and track its descent. As communication between the spacecraft and mission control could not take place until the spacecraft had rounded the eastern limb of the moon and come out of the moon's radio shadow, this effectively limited the eastern boundary of possible sites to 40 degrees east, as anything further towards the eastern limb would not provide sufficient time to update the computers. The 40-degree mark was arbitrarily extended towards the western limb to cover the only other mare landing sites on the western side. This gave a strip of lunar terrain approximately 185 miles wide centred on the lunar equator and 1,500 miles long which became known as the Apollo Landing Zone .

Probes, Ranger, Surveyor and Lunar Orbiter

It had long been realised that the composition of the lunar surface on which the astronauts were expected to touch down was an unknown quantity. Speculation abounded as visual observation and probing by radar could not resolve whether the surface would be firm enough or smooth enough to be suitable for a landing attempt. Visual observations by telescope could not resolve objects below 60 feet in size while Radar sounding indicated that the surface could be many feet deep in loose dust into which a spacecraft might sink without trace.

These questions needed to be resolved before a landing could be attempted. NASA joined two unmanned probe projects then under development with the Jet Propulsion Laboratory in Pasadena. Originally intended as a deep space probes both Ranger and Surveyor were sent to the moon between July 1964 and January 1968 and finally resolved the nature of the lunar surface with close up photography and sampling. NASA also commissioned a lunar orbiting satellite, prosaically named Lunar Orbiter, which between August 1966 and August 1967 photographed the Apollo Landing Zone from orbit and went on to map almost the complete lunar surface. From the information gained from these probes, a comprehensive picture could be built up to assess the risks that would be taken on a manned landing.

USGS Geology Team Tasks

By 1963 NASA had established their own team of scientists based at its headquarters in Washington and were able to draw on experts from major institutions in the fields of astronomy, chemistry, physics and geology. One of the major influences on mission planning came from its association with the United States Geological Survey and its head of the embryonic Astrogeologic Studies Group, Gene M Shoemaker1.

Shoemaker, whose previous lunar studies had already provided an insight into lunar topography and composition, would become the principle imaging scientist for the Ranger and Surveyor projects and would later provide a major contribution to Apollo mission planning and landing site selection. In 1963 he reached an agreement to establish a staff of USGS geologists to work with NASA's own headquarters team. They were to support Apollo's geological investigation of the moon and provide training in geology for the astronauts.

The Selected Sites

Working in conjunction with Bellcomm and the USGS, NASA used the data from the probes and ground based observations to shortlist five possible prime sites which could be used for the initial landing attempts. All were on the broad mare expanses within the ALZ. They were:

  • ALS1 and ALS2 in the Mare Tranquillitatis (Sea of Tranquillity). ALS1 was finally discounted as it was too far east. ALS2 was the final selection for the first landing attempt and eventually became Apollo 11's 'Tranquillity Base'.

  • ALS3 in the Sinus Medii. (Centre Bay) Just dead centre of the Moon's visible face. This site was to be used as a back up for ALS1 and ALS2 in the event of a 48-hour launch delay.

  • ALS4 and ALS5. in the Oceanus Procellerum, (Ocean of Storms). ALS4 was most favourable for launches during winter months and ALS5 during the Summer.

Two more sites were added to the ALZ in 1968. Both had previously been discounted in the initial selection, but were resurrected to demonstrate a 'pinpoint' landing near one of the previously successful Surveyor landers once the first manned landing had been accomplished. It also had the benefit of being able to sample a second mare, which was thought to be younger and would prove that not all maria had been created at the same time. Success with a pinpoint landing here would also open the way to landings in other locations where an over or undershoot would be impossible, notably in the mountainous highland regions. They were:

  • ALS6 in Procellarum, The Surveyor 1 site inside the Flamsteed Ring.

  • ALS7 in Procellarum, The Surveyor 3 site which eventually became the Apollo 12 landing site. ALS5 could also be used as a recycle site.

Once confidence had been gained and Apollo 12 demonstrated that a landing could be put down on the intended spot with a high degree of accuracy, other sites were selected for their scientific interest.

  • Apollo 13 and Apollo 14 were targeted on the Fra Mauro region, a highland area just within the ALZ.

  • Apollo 15 was sent far to the north on the very edge of the Imbrium Basin.

  • Apollo 16 went south of the ALZ to the supposedly volcanic Descartes Highlands.

  • Apollo 17 went north to Taurus-Littrow on the edge of Mare Serenitatis (The Sea of Serenity).

1Shoemaker was an expert in impact crater studies on earth and would later discover a comet which smashed into Jupiter. On his death, he was cremated and his ashes were sent to the moon and scattered about Meteor Crater in Arizona.

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