Apollo requirements

The objective of the Lunar Orbiter series was to reconnoitre possible landing sites for Apollo. As they had insufficient film to search for sites, they concentrated on sites that appeared suitable on the basis of telescopic observations. The Apollo Site Selection Board reduced an initial list of 30 candidates – all located on the near side of the Moon within 45 degrees of the meridian and 5 degrees of the equator – to a short-list of five by applying the following operational factors:

Of which more in chapter 10.

The primary objective of the Surveyor missions was to investigate the dark plains. Five Apollo Landing Sites (ALS) were selected in the equatorial zone for the early Apollo landings (bottom).

• Smoothness: Relatively few craters and boulders.

• Approach: No large hills, high cliffs, or deep craters that could result in incorrect altitude signals to the lunar module landing radar.

• Propellant requirements: Least potential expenditure of spacecraft propellants.

• Recycling: Effective launch preparation recycling if the countdown were to be delayed.

• Free return: Sites within reach of the spacecraft launched on a free-return translunar trajectory.

• Slope: Less than 2-degree slope in the approach path and landing area.

The flight dynamics team insisted that the primary site be located in the eastern hemisphere, in order to allow room further west for one or two suitably lit backup sites in the event of the launch being postponed by several days. As the launch ‘windows’ for a given site occurred only once per month, it was thought better to go for a secondary site several days late than to wait a month for the primary site to present itself again. The time of landing had to be just after local sunrise, as the Sun was required to be low on the horizon to cast sufficient shadows to reveal surface topography. Because the Sun traverses the lunar sky at a rate of 12 degrees per 24 hours, the backup sites were set 12, 24 or 36 degrees apart in lunar longitude to ensure that the illumination would be right for a delayed mission. On the other hand, the primary site could not be too far east, as this would not allow sufficient time after the final limb crossing to perform the navigational checks prior to initiating the powered descent. All sites had to be within 5 degrees of the lunar equator because a higher latitude would require a less propellant-efficient trajectory, and propellant economy was a priority for the first landing mission. Furthermore, the approaches to the sites had to be flat in order not to complicate the task of the landing radar. These operating constraints restricted the first landing to an eastern dark plain near the equator, which put the primary landing site in either the Sea of Tranquility or the Sea of Fertility, the backup site in the Meridian Bay, and the reserves in the Ocean of Storms. However, the Sea of Fertility was too far east to accommodate the final navigational update, which left the Sea of Tranquility, where there were two sites. The trajectory of Apollo 8 had been timed to inspect the easternmost site, ALS-1, called 2P-2 by the Lunar Orbiter team, at ideal illumination.

The time of the landing was determined by the location and the acceptable range of Sun elevation angles. The range of these angles ran from 6 to 14 degrees, east to west. Under these conditions, the visible shadows of craters would aid the crew in recognising topographical features. As the Sun angle approached the descent angle, the mean value of which was 16 degrees, the viewing conditions would be degraded by a ‘washout’ phenomenon in which backward reflectance was high enough to eliminate contrast. Sun angles above the flight path were not as desirable, because shadows would not be readily visible unless the Sun was significantly outside the descent plane. Because lunar sunlight incidence changed about 0.5 degree per hour, the Sun elevation angle restriction defined a 16-hour period, recurring every 29.5 days, when landing at a given site could be attempted. The number of Earth-launch opportunities for a given lunar month was equal to the number of candidate landing sites. The time of launch was primarily determined by the allowable variation in launch pad azimuth. A total launch pad azimuth variation of 34 degrees afforded a launch period of 4 hours 30 minutes. Two launch windows occurred each day. One was available for a translunar injection over the Pacific Ocean, and the other over the Atlantic Ocean. The injection opportunity over the Pacific Ocean was preferred because it usually permitted a daytime launch.