Editing Apollo 10 (section)

===Training and preparation===
[[File:Two members of the Apollo 10 prime crew participate in simulation activity.jpg|thumb|left|Stafford (right) and Cernan in the lunar module simulator, April 1969]]
Apollo 10, the "F" mission or dress rehearsal for the lunar landing, had as its primary objectives to demonstrate crew, space vehicle, and mission support facilities performance during a crewed mission to lunar orbit, and to evaluate the performance of the lunar module there. In addition, it was to attempt photography of Apollo Landing Site 2 (ALS-2) in the [[Sea of Tranquillity]], the contemplated landing site for Apollo 11.{{sfn|Orloff & Harland|pp=256, 285}} According to Stafford, <blockquote>Our flight was to take the first lunar module to the moon. We would take the lunar module, go down to within about ten miles above the moon, nine miles above the mountains, radar map, photo map, pick out the first landing site, do the first rendezvous around the moon, pick out some future landing sites, and come home.{{sfn|French & Burgess|pp=1337–1338}}</blockquote>

Apollo 10 was to adhere as closely as possible to the plans for Apollo 11, including its trajectory to and from lunar orbit, the timeline of mission events, and even the angle of the Sun at ALS-2. However, no landing was to be attempted.{{sfn|Press Kit|p=2}} ALS-1, given that number because it was the furthest to the east of the candidate sites,{{efn|The five candidate sites for the first lunar landing, ALS-1 through ALS-5, were numbered from the easternmost to the westernmost. See {{harvnb|Press Kit|p=37}}}} and also located in the Sea of Tranquility, had been extensively photographed by Apollo 8 astronauts; at the suggestion of scientist-astronaut [[Harrison Schmitt]], the launch of Apollo 10 had been postponed a day so ALS-2 could be photographed under proper conditions. ALS-2 was chosen as the lunar landing site since it was relatively smooth and of scientific interest, while ALS-1 was deemed too far to the east.{{sfn|Wilhelms|pp=189–192}} Thus, when Apollo 10's launch date was announced on January 10, 1969, it was shifted from its placeholder date of May 1 to May 17, rather than to May 16. On March 17, 1969, the launch was slipped one day to May 18, to allow for a better view of ALS-3, to the west of ALS-2.<ref name="drew" /> Another deviation from the plans for Apollo 11 was that Apollo 10 was to spend an additional day in lunar orbit once the CSM and LM rendezvoused; this was to allow time for additional testing of the LM's systems, as well as for photography of possible future Apollo landing sites.{{sfn|Press Kit|pp=6, 8}}

The Apollo 10 astronauts undertook five hours of formal training for each hour of the mission's eight-day duration. This was in addition to the normal mission preparations such as technical briefings, pilot meetings, and study. They took part in the testing of the CSM at the [[Downey, California|Downey]], California, facility of its manufacturer, [[North American Rockwell]], and of the LM at [[Grumman]] in [[Bethpage, New York|Bethpage]], New York. They visited [[Cambridge, Massachusetts|Cambridge]], Massachusetts, for briefings on the [[Apollo Guidance Computer]] at the [[MIT Instrumentation Lab|Massachusetts Institute of Technology Instrumentation Laboratory]]. They each spent more than 300 hours in simulators of the CM or LM at the [[Manned Spacecraft Center]] (MSC) in Houston and at [[Kennedy Space Center]] (KSC) in Florida. To train for the high-acceleration conditions they would experience in returning to Earth's atmosphere, they endured MSC's centrifuge.{{sfn|Press Kit|p=65}}

====Lunar landing capability====
{| class="wikitable plainrowheaders floatright"
|+ Comparison of LM weights
|-
! scope="col" rowspan=2 | Component
! scope="col" colspan=2 | Apollo 10 LM-4
! scope="col" colspan=2 | Apollo 11 LM-5
|-
! scope="col" | {{abbr|lb|pound (mass)}}
! scope="col" | {{abbr|kg|kilogram}}
! scope="col" | lb
! scope="col" | kg
|-
! scope="row" | Descent stage dry{{sfn|Orloff|2004|pp=276–277}}
| {{cvt|4,703|lb|kg|disp=table}} || {{cvt|4,483|lb|kg|disp=table}}
|-
! scope="row" | Descent stage propellant{{sfn|Orloff|2004|p=295}}
| {{cvt|18,219|lb|kg|disp=table}} || {{cvt|18,184|lb|kg|disp=table}}
|- style="font-weight: bold; background-color: #EAECF0;"
! scope="row" style="font-weight: bold;" | Descent stage subtotal
| {{cvt|22,922|lb|kg|disp=table}} || {{cvt|22,667|lb|kg|disp=table}}
|-
! scope="row" | Ascent stage dry{{sfn|Orloff|2004|pp=276–277}}
| style="background-color: #9f9" {{cvt|4,781|lb|kg|disp=table}} || {{cvt|4,804|lb|kg|disp=table}}
|-
! scope="row" | Ascent stage propellant{{sfn|Orloff|2004|p=296}}
| style="background-color: #f99;" {{cvt|2,631|lb|kg|disp=table}} || {{cvt|5,238|lb|kg|disp=table}}
|- style="font-weight: bold; background-color:#EAECF0;"
! scope="row" style="font-weight: bold;" | Ascent stage subtotal
| {{cvt|7,412|lb|kg|disp=table}} || {{cvt|10,042|lb|kg|disp=table}}
|-
! scope="row" | Equipment
| {{cvt|401|lb|kg|disp=table}} || {{cvt|569|lb|kg|disp=table}}
|- style="font-weight: bold; background-color:#EAECF0;"
! scope="row" style="font-weight: bold;" | Total{{sfn|Orloff|2004|pp=276–277}}
| {{cvt|30,735|lb|kg|disp=table}} || {{cvt|33,278|lb|kg|disp=table}}
|}

While Apollo 10 was meant to follow the procedures of a lunar landing mission to the point of powered descent, Apollo 10's LM was not capable of landing and returning to lunar orbit. The ascent stage was loaded with the amount of [[aerozine 50|fuel]] and [[nitrogen tetroxide|oxidizer]] it would have had remaining if it had lifted off from the surface and reached the altitude at which the Apollo 10 ascent stage fired; this was only about half the total amount required for lift off and rendezvous with the CSM. The mission-loaded LM weighed {{convert|30735|lb|kg|order=flip}}, compared to {{convert|33278|lb|kg|order=flip}} for the Apollo 11 LM, which made the first landing.{{sfn|Orloff|2004|pp=276–277}} Additionally, the software necessary to guide the LM to a landing was not available at the time of Apollo 10.<ref name="Hamish" />

Craig Nelson wrote in his book ''Rocket Men'' that NASA took special precautions to ensure Stafford and Cernan would not attempt to make the first landing. Nelson quoted Cernan as saying "A lot of people thought about the kind of people we were: 'Don't give those guys an opportunity to land, 'cause they might!' So the ascent module, the part we lifted off the lunar surface with, was short-fueled. The fuel tanks weren't full. So had we literally tried to land on the Moon, we couldn't have gotten off."<ref>[[#Nelson|Nelson 2009]], p. 14</ref> Mueller, NASA's Associate Administrator for Manned Space Flight, stated, <blockquote>There had been some speculation about whether or not the crew might have landed, having gotten so close. They might have wanted to, but it was impossible for that lunar module to land. It was an early design that was too heavy for a lunar landing, or, to be more precise, too heavy to be able to complete the ascent back to the command module. It was a test module, for the dress rehearsal only, and that was the way it was used.<ref>{{Cite web |date=April 9, 2019 |title=Apollo 10 |url=https://solarsystem.nasa.gov/missions/apollo-10/in-depth/ |url-status=dead |archive-url=https://web.archive.org/web/20220804181858/https://solarsystem.nasa.gov/missions/apollo-10/in-depth/ |archive-date=August 4, 2022 |access-date=June 19, 2022 |publisher=NASA Science}}</ref></blockquote>