Editing Apollo 13 (section)

=== Accident ===
{{For|the cause of the accident|Apollo 13#Investigation and response}}

About six and a half minutes after the TV broadcast{{snd}}approaching 56:00:00{{snd}}Apollo&nbsp;13 was about {{convert|180000|nmi|mi km}} from Earth.<ref name = "journal Houston">{{cite web|work=Apollo Lunar Flight Journal|url=https://history.nasa.gov/afj/ap13fj/08day3-problem.html|title=Day 3: 'Houston, we've had a problem'|date=May 30, 2017|access-date=August 18, 2019|last1=Woods|first1=W. David|last2=Kemppanen|first2=Johannes|last3=Turhanov|first3=Alexander|last4=Waugh|first4=Lennox J.}}</ref> Haise was completing the shutdown of the LM after testing its systems while Lovell stowed the TV camera. [[Jack Lousma]], the [[Flight controller#CAPCOM|CAPCOM]], sent minor instructions to Swigert, including changing the [[orientation (geometry)|attitude]] of the craft to facilitate photography of [[Comet Bennett]].<ref name = "journal Houston" />{{sfn|Chaikin|1995|p=292}}

The pressure sensor in one of the SM's oxygen tanks had earlier appeared to be malfunctioning, so [[Seymour Liebergot|Sy Liebergot]] (the [[Flight controller#Electrical, environmental, and consumables manager (EECOM)|EECOM]], in charge of monitoring the CSM's electrical system) requested that the stirring fans in the tanks be activated. Normally this was done once daily; a stir would [[Thermal destratification|destratify]] the contents of the tanks, making the pressure readings more accurate.<ref name = "journal Houston" /> The Flight Director, Kranz, had Liebergot wait a few minutes for the crew to settle down after the telecast,{{sfn|Houston, Heflin & Aaron|2015|p=207}} then Lousma relayed the request to Swigert, who activated the switches controlling the fans,<ref name = "journal Houston" /> and after a few seconds turned them off again.{{sfn|Chaikin|1995|p=292}}

{{Listen|filename=Apollo13-wehaveaproblem.ogg|title=Houston, we've had a problem.|description=Swigert and Lovell reporting the incident on April 14, 1970 [2:59] "[[Houston, we have a problem|Houston, we've had a problem]]"|format=[[Ogg]]}}

Ninety-five seconds after Swigert activated those switches,{{sfn|Houston, Heflin & Aaron|2015|p=207}} the astronauts heard a "pretty large bang", accompanied by fluctuations in electrical power and the firing of the attitude control thrusters.{{sfn|Orloff & Harland|2006|p=368}}{{sfn|Orloff|2000|pp=152–157}} Communications and telemetry to Earth were lost for 1.8&nbsp;seconds, until the system automatically corrected by switching the [[directional antenna|high-gain]] [[Unified S-band|S-band]] antenna, used for translunar communications, from narrow-beam to wide-beam mode.{{sfn|Accident report|p=4-44}} The accident happened at 55:54:53 (03:08 UTC on April 14; 10:08 PM EST, April 13). Swigert reported 26&nbsp;seconds later, [[Houston, we have a problem|"Okay, Houston, we've had a problem here,"]] echoed at 55:55:42 by Lovell, "Houston, we've had a problem. We've had a Main&nbsp;B Bus undervolt."<ref name = "journal Houston" /> William Fenner was the [[Flight controller|guidance officer]] (GUIDO) who was the first to report a problem in the control room to Kranz.<ref name = "journal Houston" />

Lovell's initial thought on hearing the noise was that Haise had activated the LM's cabin-repressurization valve, which also produced a bang (Haise enjoyed doing so to startle his crewmates), but Lovell could see that Haise had no idea what had happened. Swigert initially thought that a [[meteoroid]] might have struck the LM, but he and Lovell quickly realized there was no leak.{{sfn|Chaikin|1995|p=293}} The "Main Bus&nbsp;B undervolt" meant that there was insufficient [[voltage]] produced by the SM's three [[fuel cell]]s (fueled by hydrogen and oxygen piped from their respective tanks) to the second of the SM's two electric power distribution systems. Almost everything in the CSM required power. Although the [[Bus (computing)|bus]] momentarily returned to normal status, soon both buses A and B were short on voltage. Haise checked the status of the fuel cells and found that two of them were dead. Mission rules forbade entering lunar orbit unless all fuel cells were operational.{{sfn|Chaikin|1995|pp=293–294}}

In the minutes after the accident, there were several unusual readings, showing that tank{{nbsp}}2 was empty and tank{{nbsp}}1's pressure slowly falling, that the [[Apollo Guidance Computer|computer on the spacecraft]] had reset and that the high-gain antenna was not working. Liebergot initially missed the worrying signs from tank{{nbsp}}2 following the stir, as he was focusing on tank{{nbsp}}1, believing that its reading would be a good guide to what was present in tank{{nbsp}}2, as did controllers supporting him in the "back room".  When Kranz questioned Liebergot on this, he initially responded that there might be false readings due to an instrumentation problem; he was often teased about that in the years to come.<ref name = "Cass 1">{{cite web|last=Cass|first=Stephen|title=Apollo 13, we have a solution|url=https://spectrum.ieee.org/apollo-13-we-have-a-solution|date=April 1, 2005|access-date=October 13, 2022|website=IEEE}}</ref> Lovell, looking out the window, reported "a gas of some sort" venting into space, making it clear that there was a serious problem.{{sfn|Houston, Heflin & Aaron|2015|p=215}}

Since the fuel cells needed oxygen to operate, when Oxygen Tank{{nbsp}}1 ran dry, the remaining fuel cell would shut down, meaning the CSM's only significant sources of power and oxygen would be the CM's batteries and its oxygen "surge tank". These would be needed for the final hours of the mission, but the remaining fuel cell, already starved for oxygen, was drawing from the surge tank. Kranz ordered the surge tank isolated, saving its oxygen, but this meant that the remaining fuel cell would die within two hours, as the oxygen in tank{{nbsp}}1 was consumed or leaked away.{{sfn|Chaikin|1995|pp=293–294}} The volume surrounding the spacecraft was filled with myriad small bits of debris from the accident, complicating any efforts to use the stars for navigation.{{sfn|Chaikin|1995|p=299}} The mission's goal became simply getting the astronauts back to Earth alive.<ref name = "Cass 2" />