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As we all know in 1969 Apollo 11 landing made history but, why did it land specifically at the sea of tranquility also known as Mare Tranquilitatis. Probably they would have chosen this because of flatness and smoothness of surface but are there any other technical reasons behind it? Moreover what was the temperature of the site at the time of landing? As the lit part of moon is quite hot and the dark part is much colder than normal temperature we human come across in daily life.

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The set of landing sites considered for all the Apollo missions was driven by a desire to sample a wide variety of lunar geology, but the order for them was chosen to make the first landings easiest and safest from a technical standpoint.

The scientific rationale for site selection is described in detail in an appendix to the NASA historical document Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions. In summary:

The short list of landing sites for the first 10 lunar exploration missions should include:

  • two types of mare material, "older" or eastern and "younger" or western;

  • regional stratigraphic units, such as blanket (ejecta) deposits around mare basins;

  • various types and sizes of impact craters in the maria and in the highlands;

  • morphological manifestations of volcanism in the maria and in the highlands; and

  • areas that may give clues to the nature and extent of processes other than impact and volcanism, which may have acted on the lunar surface.

An Apollo 11 summary gives some technical criteria for landing site selection:

The original sites were located on the visible side of the Moon, within 45° east and west of the Moon’s center and 5° north and south of its equator. The final site choices were based on the following factors:

Smoothness: Relatively few craters and boulders.

Approach: No large hills, high cliffs, or deep craters that could cause 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 delayed.

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

Slope: Less than 2° slope in the approach path and landing area.

I believe the propellant requirements, recycling, and free return constraints all pointed to sites close to the lunar equator, where the (slow) rotation of the moon would not take the landing site too far from the ground-track of the orbiting command module. To rendezvous from a landing site at high lunar latitudes would require an expensive plane-change maneuver.

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    $\begingroup$ So, it wasn't that the name sounded good in the Tourist Brochures ? $\endgroup$ – Criggie May 18 at 7:31
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    $\begingroup$ Could you explain "recycling"? I take it that means "if the launch is delayed, it wouldn't prevent us using the site", but I can't quite make the connection. Why are some sites more "recyclable" than others in the event of delay? $\endgroup$ – Steve Bennett May 18 at 9:16
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    $\begingroup$ @SteveBennett Presumably linked to the listed benefits of landing near the equator - a location was chosen such that those benefits would not substantially come and go over time, due to orbital mechanics or something else, adding timing constraints. I'm not sure of the specifics, but by analogy I'm picturing an elevator that stops and lets you on, versus an elevator that just travels up and down and you have to jump on it at precisely the right time if you want a lift. In the former case, you can "recycle" your original landing plan a few hours or days later, as it's still viable. $\endgroup$ – Asteroids With Wings May 18 at 15:03
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    $\begingroup$ @Criggie: But just about all of the lunar "seas" and "oceans" have names that would sound good in tourist brochures: bbc.co.uk/programmes/articles/5gdrKwHtXhRkcq0xHDdhqvj/… Might want to avoid Mare Crisium (Sea of Crises), though :-) $\endgroup$ – jamesqf May 18 at 16:47
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    $\begingroup$ @SteveBennett Yes, as discussed here. Sun low in the east during landing kept the sun out of the crew's eyes, provided sharp shadows to help visualize the shape of the terrain, and let the commander judge altitude by the LM shadow in front of him in the last seconds of landing. $\endgroup$ – Russell Borogove May 19 at 1:32
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It's best to go directly to the team who chose the landing sites.

Selection of Apollo lunar landing sites was contracted to Bellcomm, a joint venture between AT&T Bell Labs and Western Electric. The team was headed by Farouk El-Baz. Their recommendations were then approved by NASA administration. An entire issue (30.3 Mb, 176 pp., 7 appendices) of the Bell System Technical Journal was devoted to a single article entitled "Where on the Moon? An Apollo Systems Engineering Problem".

According to section 1.3 of the article, all landing sites had to meet these basic criteria:

  1. There had to be communications with Earth during descent, surface exploration, and ascent. This eliminated the far side of the moon.
  2. The landing site had to be within the latitudes for which the Saturn and Apollo spacecrafts were capable. In particular for Apollo 11, it required a free-return trajectory:

    The first landing mision was flown with a requirement on the translunar trajectory such that, if the main spacecraft engine had become inoperative following translunar injection (the start of the coasting flight from Earth to Moon), the spacecraft would have swung around the Moon and returned to the Earth with acceptable re-entry conditions, requiring only minor trajectory corrections using the Command and Service Module (CSM) attitude control system. The consequence of using this type of trajectory was that the surface area accessible for landing was confined to a region close to the lunar equator, the Apollo zone (Fig. 3).

  3. Landing sites needed adequate lighting. You ask about temperature; it wasn't directly considered, but lighting has a major effect on that.

    Another significant constraint was that associated with lunar lighting (Appendix A), which was complicated by the fact that the Moon exhibits very little color variation or contrast. The best lighting conditions occurred when the Sun was low enough on the horizon to reveal rough terrain by shadowing, but not so low that the landing area was within shadow; in addition, the Sun needed to be behind the astronauts in order to avoid glare. The net result was a requirement for landing in the early lunar morning such that any given landing site had the proper lighting only one day per month.

  4. Once propellants were loaded, that spacecraft had to be used within 110 days. Thus, if a launch was to be delayed, another site needed to be available within that launch window:

    In addition, the lifetime of the space vehicle propulsion subsystems after initial propellant loading was about 110 days, allowing, at best, only three monthly launch periods. Thus, it was desirable to provide as many launch opportunities as possible to insure flight of the hardware during the system's lifetime. For the first lunar landing mission, multiple launch opportunities for a given month were provided by using several sites separated in longitude. Additional launch opportunities were provided for later missions by planning to launch one day early and waiting in lunar orbit, or landing a day later than nominal (tolerating a higher Sun at landing).

  5. Terrain roughness and clearance was considered:

    Another important consideration in the selection of sites was the terrain roughness at the site and along the approach path to be followed by the Lunar Module (LM) during descent to the lunar surface. Adequate terrain clearance and the interaction between the terrain and the landing radar/descent guidance system were important for safety, as well as landing accuracy. Obviously, there had to be a suficiently large, smooth area, adequate for landing, at any site selected.

  6. Finally, the known geology of the moon was considered. This is described in section 2.1 of the paper.

By 1965, Belcomm had established the "Apollo Zone" on the moon. Rangers VII, VIII, and IX; Surveyors I, III, V, and VI; and Lunar Orbiters I through V photographed the lunar surface throughout the Apollo Zone. From these photographs, Bellcomm determined 32 possible landing sites. The next step was to score and enumerate these 32 sites by their "landability".

For the first moon landing, the list of 32 sites were narrowed down:

From the original 32 sites, eight sites that best met the above constraints were selected. In January of 1968 these were further reduced to five sites (Fig. 3). Within these five sites, target landing areas (ellipses of various sizes representing landing error probabilities) were carefully chosen to minimize the landing hazards for the LM. Approach path landmarks and terrain features within the ellipses were identified for pilot recognition. A relief model of Apollo Site 3 (in Sinus Medii, near the center of the lunar disc) was constructed at a scale of 1:2000 for use with the LM simulators to practice final approach procedures. Map packages at a scale of 1:5000 were prepared for all the sites and included photographs and geological maps of the entire ellipse.

Section 2.2.2 describes the final choice for Apollo 11. Apollo 10's practice flight was also targeted to the same landing site.

The Apollo Site Selection Board recommended Apollo Site 2 in Mare Tranquilitatis (Sea of Tranquility) as the prime site for Apollo 11 (Fig. 3). Apollo Site 3 in Sinus Medii (Central Bay) and Site 5 in Oceanus Procelarum (Ocean of Storms) were selected as backups in case a launch hold and recycle occurred (Fig. 3). These recommendations were approved by the Associate Administrator for Manned Space Flight. In May of 1969, Apollo 10 flew a preview mission of the Apollo 11 flight, descending to within 15 kilometers of Apollo Site 2. The astronauts reported that it appeared acceptable for landing.

Note that the scale model used in the simulator (Sinus Medii) ended up being one the back-up landing sites. The other back-up landing site ("Ocean of Storms") would have been a cool name for a landing site, too!

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