Editing Apollo 13 (section)

=== Looping around the Moon ===
[[File:Direct Abort Trajectory - Lunar Landing Symposium, MSC Jun66.jpg|upright=1.2|thumb|This depiction of a direct abort (from a 1966 planning report) contemplates returning from a point much earlier in the mission, and closer to Earth, than where the Apollo&nbsp;13 accident occurred.]]
[[File:NASA-Apollo13-ViewsOfMoon-20200224.webm|thumb|upright=1.18|NASA – Apollo 13 Lunar Mission – Views Of The Moon (2:24)]]
The lunar module had charged batteries and full oxygen tanks for use on the lunar surface, so Kranz directed that the astronauts power up the LM and use it as a "lifeboat"<ref name = "Cass 1" />{{snd}}a scenario anticipated but considered unlikely.{{sfn|Lovell & Kluger|2000|pp=83–87}} Procedures for using the LM in this way had been developed by LM flight controllers after a training simulation for Apollo 10 in which the LM was needed for survival, but could not be powered up in time.<ref name = "Cass 2" /> Had Apollo&nbsp;13's accident occurred on the return voyage, with the LM already jettisoned, the astronauts would have died,<ref name="LM-ALSEP">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1970-029C|title=Apollo 13 Lunar Module/ALSEP|website=NASA Space Science Data Coordinated Archive|access-date=October 31, 2009}}</ref> as they would have following an explosion in lunar orbit, including one while Lovell and Haise walked on the Moon.<ref name = "Yahoo 50th" />

A key decision was the choice of return path. A "direct abort" would use the SM's main engine (the [[Apollo command and service module#Service propulsion system|Service Propulsion System]] or SPS) to return before reaching the Moon. However, the accident could have damaged the SPS, and the fuel cells would have to last at least another hour to meet its power requirements, so Kranz instead decided on a longer route: the spacecraft would swing around the Moon before heading back to Earth. Apollo&nbsp;13 was on the hybrid trajectory which was to take it to Fra Mauro; it now needed to be brought back to a free return. The LM's [[Descent propulsion system|Descent Propulsion System]] (DPS), although not as powerful as the SPS, could do this, but new software for Mission Control's computers needed to be written by technicians as it had never been contemplated that the CSM/LM spacecraft would have to be maneuvered from the LM. As the CM was being shut down, Lovell copied down its guidance system's orientation information and performed hand calculations to transfer it to the LM's guidance system, which had been turned off; at his request Mission Control checked his figures.<ref name = "Cass 2">{{cite web|last=Cass|first=Stephen|title=Houston, we have a solution, part 2|url=https://spectrum.ieee.org/apollo-13-we-have-a-solution-part-2|date=April 1, 2005|website=IEEE|access-date=August 31, 2019}}</ref>{{sfn|Chaikin|1995|pp=297–298}} At 61:29:43.49 the DPS burn of 34.23&nbsp;seconds took Apollo&nbsp;13 back to a free return trajectory.{{sfn|Orloff & Harland|2006|p=369}}

[[File:Mare Moscoviense AS13-60-8648.jpg|thumb|The Apollo&nbsp;13 crew photographed the Moon out of the [[Apollo Lunar Module|Lunar Module]].]]
The change would get Apollo&nbsp;13 back to Earth in about four days' time{{snd}}though with splashdown in the [[Indian Ocean]], where NASA had few recovery forces. Jerry Bostick and other [[Flight controller#FDO|Flight Dynamics Officers]] (FIDOs) were anxious both to shorten the travel time and to move splashdown to the [[Pacific Ocean]], where the main recovery forces were located. One option would shave 36 hours off the return time, but required jettisoning the SM; this would expose the CM's heat shield to space during the return journey, something for which it had not been designed. The FIDOs also proposed other solutions. After a meeting involving NASA officials and engineers, the senior individual present, [[Johnson Space Center|Manned Spaceflight Center]] director [[Robert R. Gilruth]], decided on a burn using the DPS, that would save 12&nbsp;hours and land Apollo&nbsp;13 in the Pacific. This "PC+2" burn would take place two hours after [[pericynthion]], the closest approach to the Moon.<ref name = "Cass 2" />  At pericynthion, Apollo&nbsp;13 set the record (per the ''Guinness Book of World Records''), which still stands, for the furthest distance from Earth by a crewed spacecraft: {{convert|400171|km|mi nmi|sp=us}} from Earth at 7:21&nbsp;pm EST, April 14 (00:21:00&nbsp;UTC April 15).{{sfn|Glenday|2010|p=13}}{{NoteTag|The record was set because the Moon was nearly at [[Apsis|its furthest from Earth]] during the mission. Apollo&nbsp;13's unique free return trajectory caused it to go approximately {{convert|100|km|-1|sp=us}} further from the [[Far side (Moon)|lunar far side]] than other Apollo lunar missions, but this was a minor contribution to the record.{{sfn|Adamo|2009|p=37}} A reconstruction of the trajectory by astrodynamicist Daniel Adamo in 2009 records the furthest distance as {{convert|400046|km|sp=us}} at 7:34&nbsp;pm&nbsp;EST (00:34:13&nbsp;UTC). Apollo 10 holds the record for second-furthest at a distance of {{convert|399806|km|sp=us}}.{{sfn|Adamo|2009|p=41}}}}

While preparing for the burn, the crew was told that the S-IVB had impacted the Moon as planned, leading Lovell to quip, "Well, at least something worked on this flight."<ref name = "journal leaving" />{{sfn|Cooper|2013|pp=84–86}} Kranz's White team of mission controllers, who had spent most of their time supporting other teams and developing the procedures urgently needed to get the astronauts home, took their consoles for the PC+2 procedure.{{sfn|Houston, Heflin & Aaron|2015|pp=221–222}} Normally, the accuracy of such a burn could be assured by checking the alignment Lovell had transferred to the LM's computer against the position of one of the stars astronauts used for navigation, but the light glinting off the many pieces of debris accompanying the spacecraft made that impractical. The astronauts accordingly used the one star available whose position could not be obscured{{snd}}the Sun. Houston also informed them that the Moon would be centered in the commander's window of the LM as they made the burn, which was almost perfect{{snd}}less than 0.3 meters (1 foot) per second off.<ref name = "journal leaving" /> The burn, at 79:27:38.95, lasted four minutes and 23&nbsp;seconds.{{sfn|Orloff & Harland|2006|p=391}} The crew then shut down most LM systems to conserve consumables.<ref name = "journal leaving">{{cite web|work=Apollo Lunar Flight Journal|url=https://history.nasa.gov/afj/ap13fj/13day4-leaving-moon.html|title=Day 4: Leaving the Moon|date=February 17, 2017|access-date=September 7, 2019}}</ref>