Editing Apollo 11 (section)

=== Lunar descent ===

[[File:Apollo 11 CSM photographed from Lunar Module (AS11-37-5445).jpg|thumb|upright=1.2|left|[[Command module Columbia|''Columbia'']] in lunar orbit, photographed from [[Lunar Module Eagle|''Eagle'']]|alt=The top of the silvery command module is seen over a grey, cratered lunar surface]]

At 12:52:00 UTC on July 20, Aldrin and Armstrong entered [[Lunar Module Eagle|''Eagle'']], and began the final preparations for lunar descent.{{sfn|Orloff|2000|p=106}} At 17:44:00 ''Eagle'' separated from ''Columbia''.{{sfn|Orloff|2000|p=107}} Collins, alone aboard ''Columbia'', inspected ''Eagle'' as it pirouetted before him to ensure the craft was not damaged, and that the landing gear was correctly deployed.{{sfn|Mission Evaluation Team|1969|p=9}}{{sfn|Collins|Aldrin|1975|p=209}} Armstrong exclaimed: "The ''Eagle'' has wings!"{{sfn|Collins|Aldrin|1975|p=209}}

As the descent began, Armstrong and Aldrin found themselves passing landmarks on the surface two or three seconds early, and reported that they were "long"; they would land miles west of their target point. ''Eagle'' was traveling too fast. The problem could have been [[mass concentration (astronomy)|mascons]]—concen{{shy}}tra{{shy}}tions of high mass in a region or regions of the Moon's crust that contains a [[Gravity anomaly|gravitational anomaly]], potentially altering ''Eagle''{{'s}} trajectory. Flight Director Gene Kranz speculated that it could have resulted from extra air pressure in the docking tunnel, or a result of ''Eagle''{{'}}s pirouette maneuver.{{sfn|Mindell|2008|pp=220–221}}{{sfn|Mission Evaluation Team|1969|p=82}}

Five minutes into the descent burn, and {{convert|6000|ft|m|-2}} above the surface of the Moon, the [[Apollo Guidance Computer|LM guidance computer]] (LGC) distracted the crew with the first of several unexpected 1201 and 1202 program alarms. Inside Mission Control Center, computer engineer [[Jack Garman]] told [[Flight controller#GUIDO|Guidance Officer]] [[Steve Bales]] it was safe to continue the descent, and this was relayed to the crew. The program alarms indicated "executive overflows", meaning the guidance computer could not complete all its tasks in real-time and had to postpone some of them.{{sfn|Collins|Aldrin|1975|pp=210–212}}{{sfn|Hamilton|Hackler|2008|pp=34–43}} [[Margaret Hamilton (software engineer)|Margaret Hamilton]], the Director of Apollo Flight Computer Programming at the [[MIT]] [[Charles Stark Draper Laboratory]] later recalled:
[[File:Apollo 11 Lunar Module Eagle in landing configuration in lunar orbit from the Command and Service Module Columbia.jpg|thumb|upright=1.25|right|[[Lunar Module Eagle|''Eagle'']] in [[lunar orbit]] photographed from [[Command module Columbia|''Columbia'']]]]

{{blockquote|To blame the computer for the Apollo 11 problems is like blaming the person who spots a fire and calls the fire department. Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software's action, in this case, was to eliminate lower priority tasks and re-establish the more important ones. The computer, rather than almost forcing an abort, prevented an abort. If the computer hadn't recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful Moon landing it was.<ref>{{cite magazine |last=Hamilton |first=Margaret H. |author-link=Margaret Hamilton (software engineer) |date=March 1, 1971 |page=13 |title=Computer Got Loaded |journal=[[Datamation]] |type=Letter |issn=0011-6963}}</ref>}}

During the mission, the cause was diagnosed as the rendezvous radar switch being in the wrong position, causing the computer to process data from both the rendezvous and landing radars at the same time.{{sfn|Mission Evaluation Team|1969|pp=190–192}}<ref>{{cite web |url=http://www.hq.nasa.gov/alsj/a11/a11.1201-fm.html |title=Apollo 11: 25 Years Later |last=Martin |first=Fred H. |date=July 1994 |work=Apollo 11 Lunar Surface Journal |publisher=NASA |access-date=June 13, 2013 |archive-date=May 27, 2013 |archive-url=https://web.archive.org/web/20130527012357/http://www.hq.nasa.gov/alsj/a11/a11.1201-fm.html |url-status=live }}</ref> Software engineer [[Don Eyles]] concluded in a 2005 Guidance and Control Conference paper that the problem was due to a hardware design bug previously seen during testing of the first uncrewed LM in [[Apollo 5]]. Having the rendezvous radar on (so it was warmed up in case of an emergency landing abort) should have been irrelevant to the computer, but an electrical phasing mismatch between two parts of the rendezvous radar system could cause the stationary antenna to appear to the computer as dithering back and forth between two positions, depending upon how the hardware randomly powered up. The extra spurious [[cycle stealing]], as the rendezvous radar updated an involuntary counter, caused the computer alarms.<ref>{{cite web |url=http://klabs.org/history/apollo_11_alarms/eyles_2004/eyles_2004.htm |title=Tales from the Lunar Module Guidance Computer |last=Eyles |first=Don |date=February 6, 2004 |work=27th annual Guidance and Control Conference |publisher=[[American Astronautical Society]] |location=Breckenridge, Colorado |access-date=June 13, 2013 |archive-date=February 24, 2021 |archive-url=https://web.archive.org/web/20210224051423/http://klabs.org/history/apollo_11_alarms/eyles_2004/eyles_2004.htm |url-status=live }}</ref>