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Houston, We've Had A Problem

v1.1.1 / chapter 24 of 26 / 01 apr 13 / greg goebel / public domain

* The first two manned Moon landings had been great successes, but the third attempt nearly ended in disaster, with Apollo 13 suffering a catastrophic failure on the outbound leg of the journey and making it back home only by the hardest. It was an inspiring cliffhanger adventure story that everyone involved would have been glad to have missed.

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[24.1] THE ASTRONAUTS LEARN GEOLOGY
[24.2] APOLLO 13 OUT ON A LIMB
[24.3] AROUND THE MOON
[24.4] THE LONG ROAD HOME

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[24.1] THE ASTRONAUTS LEARN GEOLOGY

* Apollo 11 and Apollo 12 were basically technology-validation missions and had performed only minimal exploration and research. The next "Apollo 13", with a crew consisting of Jim Lovell, Fred Mattingly, and Fred Haise, would be the first real Apollo science mission.

To Gene Shoemaker, Jack Schmitt, and other geologists with an interest in Apollo, the next mission represented an opportunity to press their agenda. Shoemaker had become so disgusted with the lack of official interest in doing science on Apollo that in the summer he quit his NASA advisory position, and complained publicly about the agency's indifference. Jack Schmitt, never short on opinions, argued with Shoemaker over the resignation, telling him that he would have more influence over the program if he stayed with the agency. In reality, Shoemaker's resignation and public complaints stung NASA management enough to rouse them over the science issue.

Schmitt was still on the inside to carry on. He had, after all, a shot at going to the Moon, and no geologist in such a position would have given it up without thinking it over for a long time. Things might have frustrating while he plugged away in the astronaut corps, where he perceived with good reason that he wasn't quite a first-class citizen, but it was still the opportunity of a lifetime. He could swallow his position a bit better than Shoemaker and some of the other geologists because he was close enough to the practical details to admit, however reluctantly, that the first person on the Moon would have to be a test pilot, not a scientist. The first Moon missions had to be technology shakedowns, and getting things to work properly and safely would have to be the first thing on the agenda.

On the positive side for the geologists, Schmitt knew the astronauts were all the best and the brightest, whose test pilot work environment had given them a keen ability to pick up and act on details. Neil Armstrong had demonstrated a serious interest in the rudimentary geological training he had been given, and had provided some astute observations during his own moonwalk. Encouraged, Schmitt and some of his geology buddies had trained Conrad and Bean in preparation for the Apollo 12 mission, and the two astronauts had seemed very willing students.

However, Conrad and Bean were reluctant to say much about lunar geology during their moonwalks. Schmitt suspected that this was partly due to the fact that Aldrin had made a geologically naive comment or two during his outing on the Moon, and some of the stuffier geologists on the program had sneered. Conrad and Bean were just concerned with looking silly. Besides, geology wasn't the real point of the Apollo 12 mission anyway. NASA management had dictated that geology was to be an important element of the Apollo 13 mission, and that gave Schmitt a lever. But how could he get the best leverage?

Most of the training in geology provided to the astronauts had been dull and routine, at a level not much higher than high-school geology. A more effective approach was required. In the summer of 1969, Schmitt called Shoemaker and discussed the issue with him. Shoemaker suggested that Schmitt contact a mutual acquaintance: Dr. Leon T. Silver of Caltech, who had been one of Schmitt's professors. Lee Silver was not only a prestigious geologist in his own right, he was also regarded as an outstanding teacher, very sharp and full of enthusiasm. Silver, who was one of the geological team examining the Moon rocks, was excited about the idea of training the astronauts. He came out to Florida and had a meeting in a coffee shop with Jim Lovell and Fred Haise. Silver proposed that Lovell, Haise, and the backup Apollo 13 lander crew John Young and Charlie Duke, all go out to California and go through a field geology training session. Lovell was skeptical, but agreed to the idea: "We'll give you this one trip, and if it works out, we'll see about doing it again."

Schmitt and the four other astronauts flew out to California in late September 1969, where they packed up into a Caltech carryall with Silver and a Caltech research assistant to drive off into California's Orocopia Mountains. The astronauts were there on their own time and money, but they found it a good investment. They picked up on Silver's enthusiasm immediately, and for eight days they lived, ate, and breathed geology from sunup to sundown. The astronauts were the best, sharp and competitive, and soaked it up as fast as Silver could feed it to them.

When it was done, all the astronauts agreed that the trip had been valuable, and Lovell said that further field trips should be performed on a regular basis. Later field trips took them to Meteor Crater in Arizona and the volcanic rock beds of Iceland.

The indifference of the Apollo astronauts to lunar science had just turned the corner, and from that time on they would compete with each other to increase their knowledge and skills. Even Ken Mattingly, who would stay in the CSM while Lovell and Haise went down to the lunar surface, got into the geoscience competition.

Farouk el Baz, an Egyptian-born geologist who had worked on the selection of the Apollo landing sites, asked Mattingly if he would attend a briefing on lunar geology. Mattingly, like Lovell, was skeptical, but said he would give it a shot. The geologist came down to Florida from Washington DC and set up a detail map of the Moon based on Lunar Orbiter photographs, with the orbital path of Apollo 13 outlined, in a conference room. Mattingly showed up with Lovell and Haise, and Farouk gave them a geologically-oriented tour of what there would be to see from lunar orbit.

Farouk was articulate, enthusiastic, and lively, and he gradually won over the astronauts. The meeting lasted over twice as long as scheduled, and soon he and Mattingly were conducting regular training sessions in advance of the launch. He was affectionately nicknamed "the Pharaoh".

The astronauts had come a long way from the days when they looked down on the scientists. Jim Lovell took the US Navy's motto, "Ex Tridens Scientia (From The Seas, Knowledge)" and modified it as the motto for Apollo 13: "Ex Luna Scientia". Haise was even more enthusiastic than Lovell, honestly feeling that scientific research was the reason he was flying to the Moon. He liked the drilling gear so much the others called him "The Drilling Fool".

BACK_TO_TOP

[24.2] APOLLO 13 OUT ON A LIMB

* In fact, Mattingly was scrubbed from the mission. The backup LEM pilot, Charlie Duke, had caught German measles and Mattingly had been around him. Mattingly had never had the disease before, had no immunity to it, and so the doctors worried that he might fall ill during the flight. Lovell pleaded for Mattingly, but the doctors were unmoved; he was replaced by his backup, Jack Swigert. Due to some bungle, Mattingly found out he was replaced from a news broadcast on his car radio. He didn't complain, but Deke Slayton handed him a few discreet favors to soften the sting.

The sessions Mattingly had got from Farouk el Baz were now a waste, but Lovell and Haise were still primed from Lee Silver's geology boot camp. Apollo 13 was launched on 11 April 1970. The trip to Earth orbit was uneventful, and after checkout the spacecraft boosted out of Earth orbit for the Moon.

* Everything went fine for two days. On the evening of Monday, 13 April, the crew broadcast a tour of their command module, "Odyssey", and LEM, "Aquarius". Lovell played master of ceremonies for the broadcast and was enjoying himself. The broadcast was taped, since public interest in the flight was mild and the networks didn't want to preempt their regular programming. The only people who saw it live were NASA staff and the astronauts' families.

This was Lovell's third space mission and his second trip to the Moon. Frank Borman had asked him before the launch if he was sick of all the training grind, but Lovell was feeling in prime form. However, he knew he would have to wait for several years for a fourth spaceflight, and it wouldn't be fair to the long line of rookies waiting to fly their first mission. Some of the other astronauts might just have shrugged and said "screw them", but Lovell wasn't like that. Apollo 13 would be his final space mission.

Everything was going well so far, and the biggest thing Jack Swigert was worrying about was that in the rush to get ready for the mission he had failed to file his Federal income tax forms, and of course he wouldn't be back before the filing deadline, 15 April. He was the first bachelor astronaut to go on a space mission and there was nobody to take care of it for him, so he had chatted with Mission Control about somebody filing a request for an extension for him. However, soon that would be the least of his worries; he would have an iron-clad excuse for not making the deadline.

* Nobody knew it at the time, and the details wouldn't be sorted out until much later, but the Apollo service module had a built-in booby trap. Although the SPS engine used storable propellants, the service module still had two sets of cryogenic liquid oxygen and hydrogen tanks to drive the fuel cell power system, with the oxygen also being provided to the crew. The Apollo command module had a small gaseous oxygen tank, but that was simply to tide the crew over after releasing the service module before reentry.

The cryo tanks had heater systems to prevent their contents from freezing solid, with a thermostat system to turn off the heaters when the tanks threatened to overheat. Unfortunately, there had been a change in design specs during development of the tank system and a single component of the thermostat system hadn't been upgraded to keep pace.

Although the design work was reviewed, it was just too tiny a thing for anyone to notice. Its effects were by no means tiny, however. If the tank overheated, the thermostat system would try to turn off the heater elements and only end up frying itself while the tank continued to overheat. Since nobody ever really imagined that a tank full of slushy semi-frozen oxygen would get too warm, there was no system to provide a warning that the tank temperature was redlining.

This booby trap had been in every Apollo spacecraft flown to that time and done no harm. However, oxygen tank two of Apollo 13 had been dropped during manufacturing. It was inspected and seemed to be all right, but during a preflight rehearsal for the launch it proved difficult to void its contents. The matter was brought up for discussion, the problem didn't seem to be serious, and replacing the tank would set the launch schedule back for a lunar month until the Moon came back into the proper alignment. Jim Lovell was consulted and agreed that the tank didn't need to be swapped.

The flaw in the tank was not in itself a hazard; what made it dangerous was the flaw in the heater thermostat system. The techs decided to use the heater for an extended period of time to make sure balky oxygen tank two was completely voided. It heated up until the thermostat tripped and promptly fried itself. The tank became hotter than an oven and burned teflon insulation off the wiring inside the tank. When the tank was refilled with oxygen for the actual flight, there was still teflon debris in the tank, along with bare electrical wiring. The booby trap was now armed and dangerous.

Another feature of the tanks was that they had stirrers to help keep the contents from solidifying, and part of the crew routine was to turn on the stirrers on a periodic basis. At a few minutes after 9:00 PM Houston time, Mission Control requested that Swigert perform a "cryo stir", and he dutifully turned on the stirrers.

* While teflon is normally a thoroughly nonflammable material, the fragments in oxygen tank two were in a pressurized oxygen atmosphere, which will make almost anything burn. No more than a few breaths after Swigert turned on the stirrers, at about 9:07 PM, about 55 hours 55 minutes after launch, the bare wiring struck a spark, igniting the debris and blowing off the top of the tank, in turn blowing the side panel off the service module.

The crew heard a muffled BANG and the spacecraft shuddered. Then the main system alarm went on, as well as indicators showing an electrical power system failure. Swigert called home: "OK, Houston, we've had a problem."

The CAPCOM, astronaut Jack Lousma, replied: "This is Houston. Say again please?"

Lovell shot back: "Houston, we've had a problem." Mission Control people monitoring telemetry data from the spacecraft had, to their shock, observed oxygen tank two abruptly cease to operate, which meant that its associated fuel cell system was gone as well. It seemed initially that it might just be an instrumentation problem, even though the explosion suggested otherwise; Lovell suspected they might have been hit by a piece of space junk of some sort. They tried to dog down the hatch into the LEM, thinking the lander might have been compromised by the accident, but the hatch proved balky. The LEM seemed intact so they stopped fussing with the hatch; they had an emergency that needed immediate attention.

Whatever had gone wrong seemed to be shoving the spacecraft off course, and then the crew observed a cloud of gas growing around them, obviously being vented by the damaged service module. The cloud was then observed by the small telescope operated at the Houston MSC to visually track the flight. This was no mere instrumentation problem, this was big trouble, though it seemed at the moment that things weren't desperate. The Moon landing was obviously off, but there was another set of tanks and another fuel cell system that would get Apollo 13 back home. A news bulletin went out on the US TV networks that announced there had been an accident on Apollo 13, but that the astronauts "were in no immediate danger".

Even as the bulletin was being broadcast, it was becoming obvious to the people in Mission Control that the astronauts were in grave danger. The explosion of oxygen tank 2 had also damaged gas lines from oxygen tank one, and it was gradually venting its contents into space, contributing to the growing cloud outside the spacecraft. Although the crew didn't know the specific details of what was happening, they knew that very soon all oxygen and power from the service module would be completely gone.

The crew of Apollo 13 was outward bound, a long way from home, on a dying spacecraft. There had been rumors for a long time that the astronauts were issued cyanide pills on their missions, but the astronauts knew that was a joke: they'd fight to survive as long as they could, and if things got completely hopeless they could simply vent the spacecraft and get it over with. Later that night, a reporter would ask Chris Kraft to assess the magnitude of the problem. Kraft thought it over for a good time and then answered: "I would say that this is about as serious a situation as we've ever had in manned space flight." Anybody familiar with the "don't panic" mentality of Mission Control immediately understood the gravity of the crisis.

BACK_TO_TOP

[24.3] AROUND THE MOON

* Back in Mission Control, Gene Kranz and his White Team had been caught flat-footed by the emergency and were trying to wrestle with the issue. Flight director Glynn Lunney of the Black Team came up and took over their shift, but Kranz and the White Team didn't go home. Kranz, who was the "boss of bosses" for Flight Control, reorganized the schedule so that the other three teams -- Lunney's Black Team, Gerry Griffin's Gold Team, and Milt Windler's Maroon Team -- would monitor the mission while the White Team became the "Tiger Team", remaining offline to focus on analyzing the problems and coming up with solutions.

With Odyssey a lost cause, the only hope was to use the systems on the LEM, Aquarius, to keep the crew alive and get them back home. This notion had been vaguely considered on earlier missions and so required no flash of insight, but working out the details was another thing, particularly given the absolute urgency of the situation. Decisions had to be made immediately. Delay could well be fatal.

First, Aquarius had to be put into operational condition. This was normally a lengthy process, but to the extent that Odyssey was running at all, it was just on battery power, and that had to be conserved to use the command module for Earth reentry. Mission Control was able to step Fred Haise through the absolutely minimal procedure needed to get the LEM running as fast as possible. At 10:50 PM, 57:37 hours into the mission, the crew was logged as having transferred to Aquarius.

Another immediate problem was ensuring that Apollo 13 was on an Earth-return trajectory. By default, it was not. Apollo 8, 10, and 11 had flown to the Moon on a trajectory that would have shot them around the Moon and sent them back to Earth on a figure-eight path without major course corrections, but the Moon landing target for Apollo 12 and 13 required a trajectory that would cause them to miss the Earth by a wide margin.

Using Odyssey's engine to perform the course correction was out of the question, and so the LEM's descent engine had to be used instead. It had the thrust to do it and Apollo 9 had performed some experiments with using the LEM to shove the CSM around, but it was still very dodgy. Navigation was a particular problem. Apollo 13 was surrounded by a cloud of gas and debris miles wide that made shooting the stars impossible. The LEM had an inertial navigation system to keep it on track, but the reaction from the ruptured oxygen tank, which continued to leak for an extended period of time, tended to push Apollo 13 off course. If the spacecraft was pushed too far off, the inertial navigation system would lose lock and Apollo 13 would be effectively lost in space.

Jim Lovell had to clumsily use the reaction thrusters on Aquarius to keep on trajectory, a trick he hadn't practiced in the wildest training simulations. As he prepared for the descent engine burn, a ground controller asked him: "How do you like this sim?" Lovell replied: "It's a beauty!"

With most of the systems shut down, the temperature in the spacecraft was falling rapidly. Although the three astronauts might have kept the temperature bearable in the LEM with their body heat, they couldn't close the balky hatch into the command module, and the total space was too big to keep warm. Water was starting to condense on the walls of the LEM. Although the wiring had been designed to be as waterproof as reasonably possible, there was still the risk that dripping high humidity would short out vital electrical systems.

* The Apollo 13 crew was lucky, if such a term could be used under the circumstances, that the accident had happened when it did. Once they'd entered lunar orbit and gone down to the surface in Aquarius, they would not have had the LEM's resources to help them get back home. In any case, the initial course correction, a short burn, took place as planned at 2:43 AM on 14 April, 61:30 hours into the mission. The crew fired the thrusters to perform ullage, using the slight acceleration to drive fuel into the LEM's descent engine, and then burned for 30 seconds. Even though the crew was blinded by the fog of venting oxygen, the burn was perfect, and no adjustment burns were required. It was the first bit of good news since the crisis began. On their current track, they would splashdown in about 91:30 hours, or a little under four days.

Now that the immediate concerns had been taken care of, it was time to deal with the longer-range concerns, particularly consumables. Although there had been worries that the LEM didn't have enough oxygen to keep the three astronauts alive until Earthfall, as it turned out Aquarius had plenty of oxygen. Two moonwalks had been scheduled, and that meant venting the LEM completely and then repressurizing it; there was more than enough oxygen to keep the three astronauts alive for four days.

However, there was another problem: the accumulation of carbon dioxide. Aquarius used canisters of lithium hydroxide (LiOH) to "scrub" carbon dioxide out of the air, so that it would not slowly accumulate to toxic levels, but there were only five small scrubber canisters on board the LEM and they weren't enough to get the crew home alive. There were of course more scrubber canisters on board Odyssey, but they were much bigger and couldn't be plugged into the LEM's environmental control system. Some way, no matter how crude, had to be figured out to "haywire" Odyssey's lithium hydroxide canisters into Aquarius.

There was also the problem of electrical power. The LEM had enough battery power to supply 55 amps for up to two days, and with the Earth about four days away that meant power consumption would need to be cut to 24 amps. However, there was another power issue that wasn't directly related to battery capacity. The LEM's electronic systems were water-cooled, and the lander's water supply was also used as drinking water by the crew. There was plenty of water on board the command module, but once Odyssey was powered-down that water would freeze up and become unavailable. Water turned out to be in shorter supply than battery capacity, and so power consumption had to be cut down to 17 amps. There was a bit of margin on the cooling system, however; the Apollo 11 crew had performed an experiment with their LEM, Eagle, before discarding it in lunar orbit, turning on its systems while shutting down the water supply. Eagle had lived for about eight hours after being cast away.

Flight planners actually managed to get it down to 12 amps, which was fortunate because yet another demand arose on the LEM's power system. Odyssey had been forced to run off its batteries for a time before it was powered down. The batteries weren't big, since they only needed to keep the command module running during the reentry phase, and they had to be topped off before reentry. That meant recharging them from the LEM's batteries.

* Reentry was several days away and the crew was still busy trying to stay on top of the situation. They had to make sure the inertial navigation system was still on track, but performing a star check through the gas cloud dogging the spacecraft was impossible. There was one star they could check on, though: the Sun. It wouldn't provide a very good check, but it was better than nothing. The astronauts were a bit skeptical when Mission Control suggested they try it, but they went through the procedure and got their check.

They passed around the lunar farside, losing contact with Earth at 6:15 PM on Tuesday, 77:03 hours into the mission, passing over the Moon's surface at a closest approach of about 224 kilometers (139 miles). Once they were in the shadow and the Sun wasn't illuminating the gas cloud around the them, they could actually see the stars, but they didn't run a star check. Mission Control felt the spacecraft was on track within acceptable limits and didn't want to waste thruster fuel playing games; besides, the Apollo was moving fast and there wouldn't be much time before they emerged into sunlight again.

At the spacecraft's speed, the Moon could swing the it back around towards Earth, but couldn't capture it. Haise and Swigert were impressed by the view of the Moon rolling beneath them and got out cameras to take pictures, but Lovell sat back, folded his arms, and asked them politely what they thought they were doing. The two looked at each other, put the cameras away, and got back to work.

The astronauts reacquired contact with Houston after 25 minutes of silence. They prepared for the next burn, which would speed up their return to Earth. The burn took place at 8:40 PM, 79:28 hours into the mission, a little under a full day since the accident, with the LEM engine burning for four and a half minutes, half of that under full throttle. That would shave off ten hours of flight time, with landfall scheduled for about 63:30 hours later, a little over two and half days.

The burn went like clockwork. By now, not only were NASA and the Apollo contractors fully mobilized to deal with the emergency, news reports had focused the public's attention on it as well. The entire cliffhanger was now playing itself out in front of an audience of hundreds of millions of people.

The LEM's descent engine still had fuel for another burn if it was needed, but the engine had a "clock" that was running out. It was a restartable engine, to be sure, but it had never been designed to operate over an extended period of time. The storable fuels were pressure-fed using helium stored in liquefied form in a pressure tank, and as the helium vaporized the pressure in the tank rose. This was expected, and the tank had been designed with a relief disk that would give way when the pressure reached a certain level. Once the disk gave way, the engine would be dead. This event was predictable to the extent that Mission Control knew minimum and maximum times for it.

Another concern was that systems on the "dark side" of the spacecraft would freeze up and be inoperable when they were actually needed. The trick was to put the spacecraft assembly into a slow roll so it would be "cooked" by the Sun regularly, like a chicken on a spit. Trying to set up the roll properly using the LEM's thruster system was tricky, particularly since the crew was exhausted, having had no rest since the beginning of the emergency.

Deke Slayton got into a short confrontation with Gene Kranz, insisting that the astronauts needed some rest, but Kranz insisted back that it would be fatal if spacecraft systems froze up while the crew got some sack time. Chris Kraft was also worried about the exhaustion of the astronauts, though Max Faget, who was in Mission Control, agreed with Kranz's judgement. Kranz went quiet for a moment, thought things over, and simply told them that the roll would be performed, adding that he would see to it that the astronauts got some sack time after that. Kraft was technically Kranz's superior, but Kraft himself had set up the rule that the flight director called the shots; nobody challenged or could effectively challenge Kranz's authority. The spacecraft was put into a slow roll after some difficulty, and then reduced to minimum power.

BACK_TO_TOP

[24.4] THE LONG ROAD HOME

* The crew set up a sleep schedule, allowing each crewman to catch a few hours of rest in Odyssey module before taking his turn on watch. That was the theory; in practice, it was very hard to get any sleep in the increasingly frigid command module. No sweatshirts or the like had been packed for the trip, and the sleeping bags were simply constraints to keep the astronauts from floating off while they caught some sack time. Fred Haise, the first to take a break, found that if he lay very still a layer of warm air would accumulate around his body. In zero gravity, it weighed no less than the colder air and didn't circulate.

Carbon dioxide was continuing to accumulate in the cabin atmosphere, but by that time a fix for the scrubber problem was being tested. Interestingly, although nobody had imagined any scenario as drastic as the one that had put Apollo 13 on the edge of destruction, almost every trick the mission controllers implemented had been investigated individually as part of simulations for earlier Apollo shots. A similar issue had been considered during a simulation for the Apollo 8 mission, and spacecraft engineers had suggested improvising an adapter a plastic bag and other odds and ends on the spacecraft, held together by duct tape, which was always carried on the Apollo capsules to secure trash bags and as a universal "fix-it" tool.

A little after 3:30 AM on Wednesday, 15 April, about 86:30 hours into the mission, the crew began to collect the parts for the adapter and piece them together. Before "dawn" of 15 April a command module LiOH module was plugged into the LEM system, and the CO2 count, which had been edging up towards the critical level, began to edge back down again. Within six hours, the carbon dioxide concentrations were close to unmeasurable. By early afternoon on Wednesday, Apollo 13 was out of the grip of the Moon's gravity and being pulled at an increasing rate towards the Earth. It was literally "all downhill" from there.

* That didn't mean everything was going perfectly. The spacecraft was being nudged off course by some mysterious force. It would turn out later that this force was from tiny wisps of steam emitted into space by the LEM's cooling system. It wasn't much of a force, it wouldn't have even been noticed in the hop from lunar orbit to the Moon's surface and back up again, but in the long fall to Earth it made a dangerous difference. The LEM's descent engine could be used for another course correction, as long as it remained operational, but the pressure was building in the helium tank and its time was running out.

Then, at 2:23 PM, 97:10 hours into the mission, the crew felt another explosion, a relatively small one compared to that which had begun the crisis, but it still got everyone's attention. Telemetry showed that one of LEM's four batteries was fading out rapidly. In fact the battery had exploded, but the matter was quickly understood. The batteries generated hydrogen and oxygen in the course of their operation, and there were places in the battery housing where the two gases could accumulate, to then combine violently if a spark occurred. Everyone finally exhaled when it became obvious that the battery was still online, if dodgy, and that the rest of the electrical system was working perfectly.

They would need every bit of juice available. Much to the surprise of the crew, at 7:00 PM Mission Control asked them to power-up Odyssey. Lovell said to Swigert: "You following this?" Swigert replied: "Sure. I'm just assuming it's a mistake." Actually, Mission Control was worried about the state of the command module's systems after their stint in temperatures not that far above freezing, and wanted to make sure everything was still working. The crew powered up Odyssey, telemetry readings were taken back in Houston, and the command module was promptly powered back down again.

The crew then began preparation for a course correction burn, a short one, about 14 seconds long. The inertial guidance system of the LEM had been shut down with most of the rest of the systems, but as Apollo 13 neared Earth precision guidance became less and less necessary, and the proper orientation for the burn was obtained by simply eyeballing the Earth and Sun through an optical sight. The burn took place at 10:31 PM on Wednesday evening, about 105:18 hours into the mission, with about a day and a half left until Earthfall.

A little over three hours later, the pressure relief disk on the LEM's helium tank finally popped. The descent engine had done its duty and would do no more.

* After the final burn of the descent engine, back in Mission Control Kranz told the Tiger Team to knock off for the night and get a bit of rest. He had to repeat the command three times, in an increasingly louder and more authoritative voice, to get their attention, concluding: "I expect every one of you to knock off for six hours, and I don't want to see you back here until morning."

Back on course, Apollo 13's crew had to simply endure the cold and damp. They remained functional, though they were increasingly exhausted. They were drinking little water, partly because it was in limited supply and partly because they didn't want to have to perform urine dumps, which would have ever so subtly pushed Apollo 13 off course. When they urinated, they did so in such bags as they could scrape up. All three of the astronauts were dehydrated, and during the night, with no fluids to flush out his kidneys, Fred Haise found himself running a fever from a kidney infection. He tried to conceal it from the others.

The astronauts kept busy through the night arranging the power transfer from the LEM to the command module, and working out the procedures relayed up from Mission Control on getting Odyssey back into shape for reentry. In the dark hours of the morning, they powered the LEM back up in preparation for the final tasks of the mission. It made conditions substantially warmer, if still cool.

One task was another course correction. Apollo 13 was still drifting off course, due to the steam from the LEM's cooling system, and though nobody knew what was causing the drift everyone knew it had to be corrected. At 6:52 AM on Friday, 17 April, 137:40 hours into the mission, the crew fired the LEM's thrusters on a short burn to put the spacecraft back on track. Once again on course, they would hopefully land at about noon.

Things were moving quickly now. Less than a half hour later, at 7:14 AM Jack Swigert cast off Apollo 13's crippled service module to prepare for reentry. As the module floated away, they were able to see what had happened to it, taking pictures for accident analysis once they were back on the ground. Lovell told Mission Control: "There's one whole side of that spacecraft missing! Look out there, would you!" The CAPCOM, astronaut Joe Kerwin, on hearing the excited descriptions of the damaged service module, replied: "Well, James, if you can't take better care of a spacecraft than that, we might not give you another one."

Next step was to return to Odyssey and get it powered up. Lovell expected that the dripping wet humidity would short out the command module's circuitry, but everything came online. They found they were drawing two amps more than expected and then discovered the backup gyro system was on. They shut it down and the power got on the level. Flight telemetry was switched on momentarily to allow Mission Control to check on the command module's systems, and once checked out telemetry was shut down again.

By midmorning, it was the turn of Aquarius to be cast off. Lovell used the LEM's thrusters one last time to get the spacecraft into the proper attitude for reentry, and then the lander was abandoned. The hatch was dogged down, leading to a moment's apprehension when the command module's air system began to run aggressively, suggesting there was a leak. After a bit of fussing, it was determined that the air pressure produced by the LEM was lower than that expected by the command module, and it was just bringing the pressure up to spec. It leveled off and everyone breathed more easily.

Aquarius was kicked off at 10:43 AM, with the air in the docking tunnel causing it to pop off softly. It was on a path to fall into the deep sea off of New Zealand; it was still carrying the ALSEP instrument package with its RTG, and the Atomic Energy Commission had called NASA to make sure the RTG landed in the safest possible place.

Odyssey was falling down the reentry corridor rapidly now. One final check was made to make sure it was on course, the crew observing the set of the Moon as they passed over the horizon. They were on target. Lovell told them: "Gentlemen, we are about to reenter. I suggest you get ready for a ride."

Reentry began at 11:53 AM. Communications went out as they always did during a reentry. Four minutes later, CAPCOM Joe Kerwin began to hail the spacecraft, continuing to do so for over another minute while tensions in Mission Control mounted. Finally they heard Swigert: "OK Joe!" Kranz pumped his fist in the air there were cheers in the room, something that almost never happened. Kerwin replied: "OK, we read you Jack."

Odyssey continued its fall, and then Swigert announced: "We got two good drogues!" The parachutes were deploying correctly. The recovery vessel, the assault carrier IWO JIMA, was standing by in the recovery zone; the crew of the vessel saw the capsule coming down, only a few hundred meters away.

Lovell said: "Hang on. If this is anything like Apollo 8, it could be rough." It wasn't; they landed almost softly. Lovell said: "Fellows, we're home." It was 12:07 in the afternoon, after a hair-raising journey lasting 142:55 hours. Swimmers were dropped to help retrieve the crew. They were taken from the command module, still frosty after its trip through space, enjoying the warm Pacific air, to the IWO JIMA, where they could clean up and rest. Against all obstacles, they had made it home safely from beyond the far side of the Moon.

The next day, President Nixon went to Houston and awarded Kranz, Lunney and the other flight directors the Presidential Medal of Freedom. Two weeks later, they followed the astronauts down the streets of Chicago in convertibles for a ticker-tape parade. Much later, Jim Lovell got an unexpected windfall from the mission. He was allowed to keep various "souvenirs" when he retired from NASA, and one was a bindered checklist used to track procedures on the mission. Lovell put it up for auction in 2011, with the expectation that it would sell for $25,000 USD; it actually sold for $388,375 USD.

* The rescue of Apollo 13 was all very inspiring, but no matter how heroic the rescue, it had been a very near thing, and the mission had been a failure. It couldn't help but drag down the flagging Moon program further. Tom Paine became discouraged at the budget cuts inflicted on NASA, and resigned his position as administrator in September 1970. He went back to GE to a vice-presidential position, remaining there until retiring in 1976. In the mid-1980s, he would be appointed chairman of a committee on space strategies for the Reagan Administration, and died of cancer in 1992.

George Low filled in as temporary administrator until the spot was filled by James Fletcher, who took office in January 1971. Fletcher had worked on the Thor-Able and the Pioneer Moon shots at Ramo-Woolridge back in the late 1950s. He would generally spend his time as the agency's boss trying to promote plans for a future beyond Apollo, with very limited success.