Category How Apollo Flew to the Moon

SCIENCE ON THE MOON

The rocks and soil returned by the Apollo 11 crew quickly revealed that not only was Mare Tranquillitatis a basalt plain, it was astonishingly old in comparison to typical terrestrial rocks. 1’he Standing Stones at Calanais happen to be made of gneiss which is among Earth’s oldest rocks at around two to three billion years old. Compare this to 3.6 billion years for the Apollo 11 site. Also of surprise to the geologists was the presence of particles of anorthosite, a mineral rich in aluminium, among the soil samples. Later missions would reveal the importance of this type of rock in decoding the Moon’s history.

Some scientists were none too impressed when Apollo 12 was sent to another mare site merely to prove they could land near a defunct probe. It seemed there was little to distinguish it from the Apollo 11 site but when the samples were returned, its basalts were found to be a half billion years younger, showing that lunar volcanism had been active over an extended period.

Both Apollos 12 and 14, especially the latter, returned samples that came to be described as being KREEPy (K. is the chemical symbol for potassium, P for phosphorus and REE means rare earth elements, and the V makes it an adjective). The importance of KREEP lies in the fact that these elements are not easily incorporated into the crystal lattice of a solidifying rock. Therefore in a large body of magma that is slowly solidifying, the last rock to harden will be rich in KREEP and this clue would become significant as later missions added further evidence to our evolving knowledge of the Moon’s early history.

A LOMG DAY

For their pioneering journey to the surface of the Moon, Armstrong and Aldrin made only a single foray onto the surface before attempting to get some sleep in the uncomfortable confines of the LM. The rendezvous and docking next day were therefore carried out by a crew that were hopefully rested to some extent. As each successive flight became more ambitious and the LM was trusted with a crew for longer periods, the rendezvous and docking day grew increasingly packed. At first, moonwalks of four-hour, and eventually nearly 6-hour duration were shoe-horned into that day. Then by the time two hours had been added for getting into a suit in the morning, plus time to prepare for lift-off, meals and the rendezvous itself, the day became especially long and intense. And it was not as if docking marked the end of the working day.

For mission control, the excessive length of the crew’s day became an issue when Scott and Irwin returned from their highly successful stay at Hadley Base near the eastern rim of the mighty Imbrium Basin. This was one of the very few times when the crew in an Apollo spacecraft and the people in mission control managed to get out of sync with one another, probably because managers in the mission operations control room (MOCR) had a perception of the crew’s tiredness and, in the wake of the Soyuz 11 tragedy only a month earlier, they tvere overly worried about it.

Spacecraft condensation

Prior to re-entry, the crews noticed how the area around the forward hatch up in the CM’s apex tended to cool and attract condensation from the cabin’s atmosphere.

‘"You know, I bet when we splash down out there.’’ said Tom Stafford, “this cold water runs all out in that…”

“Bet you’re right,” interrupted John Young. “That’s probably where all the water comes from.”

“I bet there’ll be water galore," said Stafford.

“Well, a lot of it’s condensing up the hatch, too," said Young. “That’s a good place for it; there ain’t no wires up here. I don’t give a shit if we get ice up here as long as there ain’t no wiring up there. As long as we don’t have to live up there.’’

“Good place to pul your feel up,’’ suggested Stafford.

“If I was designing the spacecraft,” continued Young, ever the hardened engineer, ”I’d make the bastard get the water out of it before it ever starts; but once it’s designed, that’s probably as good a place to have a water separator as anywhere.”

“Did the other spacecraft notice water under there?” asked Stafford.

“I don’t know if they ever noticed ice or not. We’ve got a lot of water up there now, a lot, a lot. Let me get my rag and go up in there and clean it out.”

Small amounts of water were not a problem in the cabin’s electrical system, partly as a result of the Apollo 1 fire. One of the changes made to the spacecraft was that all the electrics had to be hermetically sealed. When Odyssey, the Apollo 13 CM, re­entered, its wiring had been chilled for four days and had gathered condensation that covered every surface. Upon re-entry, large quantities of water rained down on the crew.

Keeping cool

Over the final hour of a mission, as the crew prepared for re-entry, most of the systems in the command module were powered up. Throughout the mission the heat generated by these systems had been absorbed by a water glycol solution not unlike that found in the radiator of an automobile, and then shed to space by the two large radiators on the side of the service module or. if required, the primary and secondary water evaporators in the command module.

However, by design and a mere 15 minutes before re-entry, most of the elaborate systems for dissipating the spacecraft’s excess heat were about to be cast away along with the rest of the discarded service module, so a special provision had to be made to manage the heat generated within the command module during the half hour between separation and splashdown. Shortly before separation, a ‘chill-down’ process was begun, where both radiators and the primary and secondary water evaporators were used to cool the vvater/glycol to around 5 C. This didn’t cool the cabin, which remained at about 24 C, but it prepared the coolant to absorb large amounts of heat from the electronics. This took advantage of the fact that water has by far the highest heat capacity of the common liquids. Although the total amount of heat that could be absorbed by the coolant was still quite limited, it was sufficient to last from entry to splashdown. The water/glycol within the command module was only used to cool the spacecraft’s electronics. No attempt was made to actively cool the exterior during the fiery plunge through the atmosphere, the heatshield being more than adequate to protect the structure.

One system that did not require to be cooled, but to be heated, was the command module reaction control system and its thrusters. These RCS thrusters had been exposed to the cold of space or the heat of the Sun for up to 12 days. Heaters ensured that they were all warm enough before they were operated for the first time.

HOW NOT TO CRASH INTO THE MOON Part III

If a 24-sccond burn made around the Moon’s far side could lower the spacecraft’s near-side altitude from 300 kilometres down to only about 15 kilometres, it is easy to appreciate that an overburn of only a few seconds would so reduce the altitude that an impact with the surface could become a real possibility. The resultant precautions involved in the DOI burn are especially understandable in view of the fact that there was considerable uncertainty about the Moon’s precise shape, especially with regard to the more northerly regions that would later be overllown by two of the J-missions. Apollos 15 and 17, where some of the mountains reach four or five kilometres above the surrounding terrain.

As with all burns, the amount of delta-v was monitored by the crew via the DSKY. For a DOI burn, the typical delta-v was 210 feet per second (64 metres per second), which was the amount by which their velocity along the. v axis had to drop. One second of burn accounted for about 10 feet per second. As the burn progressed, they would see this value decrease towards zero. If the computer did not shut the engine down at the expected time, the crew had to promptly terminate the burn manually. They would then consult the DSKY to see if there was any overburn. The rules were that if they had slowed a mere 2.2 feet per second (0.67 metre per second) more than planned, they should immediately use their RCS thrusters to regain this speed. If they had overshot by as much as 10 feet per second (three metres per second), they were to turn the spacecraft around 180 degrees and regain the lost speed by firing a burp of the SPS.

Whatever the result of the DOI manoeuvre, once the crew were happy with it, they began to prepare for a possible bail-out burn. This was in case some other sign, in particular radio tracking from Earth, were to suggest they were at risk of impacting the ground. If so. they had at most an hour before the unthinkable would occur, and because they were over the far side at the time of the DOI burn, they would have no confirmation one way or the other for half of that time. The exquisite accuracy of radio tracking could only be brought to bear after AOS. with less than half an hour remaining to any theoretical impact. Therefore, w? hile the tracking stations measured their trajectory, the crew waited for a call from mission control to confirm that their orbit wasn’t going to spray them across some near-side mountain at over 5,000 kilometres per hour. This never became a real threat, and the crew’s and mission control felt confident enough with their hardware and procedures to view the bail-out burn as little more than a formality, but, in the NASA way, they were prepared for it.

Landing point designator – a head-up display

In the LPD, the engineers had devised a simple but powerful and ingenious way to tell the commander where P64 was taking them. It was as basic a device as you could hope to find in a high-tech spacecraft, though its operation depended on what was then one of the world’s most sophisticated small computers. It consisted of nothing more than vertical lines carefully scribed onto the inner and outer panes of the commander’s forward-facing w indow.

The lines calibrated the commander’s line of sight, as measured from a line directly forwards from his eye, downwards in degrees. To use it properly, he merely positioned himself in such a way that the two sets of lines were perfectly superimposed, which meant that he was in the proper position and their sight lines were valid. As the computer flew’ the LM to a landing, it displayed an angle on the DSKY that represented the line of sight to the expected landing site. The commander looked past the markings towards the surface and noted the terrain in front of him that coincided with the stated angle. That, at least, defined a downrange coordinate. Simultaneously, the computer would yaw the LM left or right so the line itself defined a lateral coordinate. The combination of the two pointed to the designated landing site. This lightweight but elegant solution also allowed him to redesignate the landing site by nudging his hand controller left, right, back or forward, and P64 would then aim the LM for the new target.

Immediately Conrad had his angle, he looked out his window to see w here it was aimed. “Hey, there it is!” he called excitedly as he recognised an arc of craters and, just before them, the Snowman. “There it is! Son-of-a-Gun! Right dow n the middle of the road!”

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The commander’s window in a LM simulator with the LPD scribe marks clearly visible on the window panes. (Courtesy Frank O’Brien)

“Outstanding!” said Bean who then began feeding LPD angles to his commander. "42 degrees, Pete.”

“Hey, it’s targeted right for the centre of the crater!” enthused Conrad. “I can’t believe it!”

“Amazing!” agreed Bean. “Fantastic! 42 degrees, babe.”

After the mission, Conrad talked about this moment when their plans for an accurate landing came good. “For the first couple of seconds, I had no recognition of where we were, although the visibility was excellent. It was almost like a black-and – white painting. The shadows were extremely black, illustrating the craters; and, all of a sudden, when I oriented myself down about the 40-degree line in the LPD, our five – crater chain and the Snowman stood out like a sore thumb.”

Sampling the regolith

One of the most ubiquitous activities carried out on the lunar surface was geological sampling. Each successive mission returned more rock mass than the previous and in total, the amount of rock and soil brought to Earth by the Apollo missions was 382 kilograms well over a third of a tonne. But of greater importance than the sheer mass of material is the fact that the majority of it was carefully selected by trained human eye and brain and then painstakingly documented as it was sampled, especially during the J-missions. Those samples are now among the most highly prized pieces of material on Earth.

There are actually other sources of lunar rock available to scientists. Three Soviet spacecraft successfully gathered 0.326 kg of soil from various sites including a core

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The gnomon next to a patch of orange soil discovered by Jack Schmitt at Shorty Crater. (NASA) "

sample from below the surface. In more recent years, a class of meteorite has been shown to have originated from the Moon, blasted into space by an impact event to eventually pass through Earth’s atmosphere and reach the surface. None of these secondary sources have the provenance of Apollo’s documented samples.

Once a desirable rock or maybe an interesting patch of soil had been identified, the two crewmen began a practised sequence of tasks to properly gain the sample along with as much contextual information as was possible in the brief time available. The normal procedure began with a gnomon being placed on the opposite side of the sample from the Sun – in the down-Sun position. The gnomon was a small tripod arrangement with a central staff that maintained true vertical, as explained by Jack Schmitt: “The gnomon gave you the local vertical, a 40-centimetre scale, a shadow which gave you azimuth, and also had a greyscale and three international colour references for photometric calibration.”

Next, a crewman took two photographs of the sample prior to it being moved. These were taken with Sun shining across the sample so that its shape stood out and were therefore called cross-Sun images. Because he took a step to one side between exposures, they constituted a stereo pair which would allow the sample’s topography to be determined back on Earth. At about the same time, the second crewman took a down-Sun image towards the gnomon which recorded the intrinsic colour and tone of the sample in the same frame as the calibration card. At some point in the sequence, another shot was taken with some notable feature in the background. “We would take a ‘locator’ back to something like the rover,” explained Schmitt years

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A stereo pair of an Apollo 17 sample at Van Serg Crater. This pair of images has been arranged for cross-eyed viewing. (NASA)

later, "just so that there was something in the picture that could be used to work back to where the sample had been taken. We’d just turn around and take a picture of the rover or of the horizon. Originally it had to do with the LM being in the picture as a ‘locator’ because, from the geometry of the lunar module and knowing how it had landed, you could work back along a ray to where you were and, as well, get a distance based on the size of the lunar module.”

For photography, NASA’s crews came to favour the Hasselblad camera after Wally Schirra took one with him on his Mercury flight in 1962. For use on the lunar surface, NASA worked with the Hasselblad Company to produce a specialised version of their professional SLR camera. This took square images on 70-mm-wide thin-base film designed to maximise the capacity of the film magazines. A battery – powered autowinder was added and the normally black finish on these cameras was changed to a reflective silver to minimise the absorption and emission of heat as it was moved in and out of the Sun’s rays. Crews could hold the camera with a special handle or chest mount it on the front of their RCU.

Researchers were keen to extract as much scientific information as possible from the resulting images so changes were made for the purposes of photogrammetry, the measurement of objects using photographs. A 60-mm lens, mildly wide-angle for the format, was specially designed to give accurate image geometry. Flight lenses were individually calibrated so that their image geometry was well understood. And since the plastic base of photographic film is prone to thermal expansion and contraction, the cameras included a means of marking a known geometry within the image at the moment of every exposure. A glass screen called a Reseau plate, upon which were inscribed a series of crosses at 1-centimetre spacing, was added directly in front of the film. When a picture was taken, the crosses left their imprint in the image for a future researcher to use when measuring angles. These crosses now adorn some of the 20th century’s most iconic images and perhaps the greater imprint they leave is in the eyes of graphic designers who use them as a motif to represent space travel and science. To mitigate the build up of static electricity and resultant sparks that might affect the image, the Rescau plate had an extremely thin layer of gold applied, thin enough to pass light.

With initial photography out of the way, the sampling itself could begin. The crews carried a selection of tools to take samples, particularly a scoop and a pair of tongs. While one crewman lifted the sample, the other took a numbered bag from a dispenser, often attached to their camera, and held it in position to allow the rock or soil to be dropped in. ЛИ the while, there wfas a verbal description which ended with the bag number being called out so a geologist in the science baek room could cross­reference it on return to Earth.

Two-man sampling w’as found to be much more productive than one crewman trying to work single-handedly but on occasion, perhaps when one crewman was occupied by another task, the second could usefully spend his time w orking alone. Schmitt often found himself in this position: "With two of us working together, bagging samples was fairly easy, but it was a lot harder solo. You had to hold the bag in one hand, and somehow or another get your scoop out over it so that you could dump something in it. And it was not easy, because you’re moving your arms against the pressure in the suit while gripping both the bag and the scoop."

After three days, Schmitt became quite adept at solo sampling, but not before he came a cropper at a small crater halfway through their second EVA. As Cernan worked to obtain a core sample. Schmitt Look samples at the crater’s edge, gathering soil and rock fragments in a scoop and. with some difficulty, pouring them into sample bags. When he finished with the scoop, he would rest it against his legs while using his two hands to manipulate the bag. Often the scoop would fall to the ground which forced him to bend one knee and lean down to retrieve it. Each sample bag went into a large sample collection bag that he sat on the ground beside him. As he turned at the end of this effort, he inadvertently knocked the bag over and spilled the smaller sample bags across the surface.

"Aaaah!” he cried, dropping to the surface on his hands and knees to gather his samples. "You don’t mind a little dirt here and there, do you, gang?’’

"No," replied Bob Parker in mission control.

Schmitt’s next problem was getting back up from his position kneeling on the outer slope of the crater. He brought his torso upright then straightened his legs. As he successfully got to his feet, the bag slipped from his grasp and impulsively, he leaned over to retrieve it. only to bring his centre of mass too far forward. Gravity took control and pulled him face-down into the dust once again as his legs Hailed uselessly off the surface. It Look him a while to return to a kneeling position from which he could regain his feet.

Mission control watched Schmitt’s pirouettes and spills with a mixture and bemusement and concern. "Hey, Gene, would you go over and help Twinkletoes, please?"

Cernan looked across. "Want some help. Jack? I’ll be there.’’

“No! I don’t need any help.” said Schmitt, annoyed at his display in front of the TV camera. “I just need belter bags.”

Schmitt checked his camera lens was clean and finished up at the crater. As they prepared to drive off. Parker had one further message for him. “Be advised that the switchboard here has been lit up by calls from the Houston Ballet Foundation requesting your services for next season."

“I should hope so,” replied Schmitt at which point he adopted a mock ballet pose, hopping on one leg with the other stretched out behind him. After two hops, he promptly fell on his face again. "How’s that?"

The little crater where Jack Schmitt fell, frolicked and performed his little dancing stunt will forever be known to researchers as Ballet Crater.

FINAL AOS

Ed Mitchell, LMP on Apollo 14, once wrote. "Preparing for a burn is a serious business, and before each one. Slu [Roosa] would announce, ‘It s sweaty palms lime again, gentlemen."’ The TEI burn tvas the one where mission control sweated more than usual, and that of Apollo 8 on Christmas Eve of 1968 was viewed with greater apprehension than any other, simply because it was the first. Its CSM was only the second Block II Apollo spacecraft to have flown in space, and they had sent it and its living human cargo all the way around ihc Moon. While ihc engineers had complete confidence in the reliability of the SPS engine, there was always a deep fear that, somewhere in ihc system, human frailly would cause a problem. In the MOCR, a clock counted down to the moment w’hen, if the burn had gone well, the spacecraft should come around the limb. The Earth station at Honeysuckle Creek in Australia was mosi favoured and its 26-mcirc anienna listened carefully.

The time for acquisition of signal (AOS) arrived, and almost immediately, engineers ai Honeysuckle reported a Unified S-band radio signal coming from the spacecraft.

"Apollo 8, Houston," Capcom Ken Mattingly called out to the crew as the engineers in Australia worked to lock ihc great dish’s receivers and transmitters onto the spacecraft.

"Apollo 8. Houston. Apollo 8, Houston,” continued Mattingly.

"Apollo 8. Houston. Apollo 8, Houston.”

"Houston, Apollo 8. Over.” called Jim Lovell from the speeding spacecraft.

"Hello. Apollo 8. Loud and clear.” replied Mattingly, speaking on behalf of all at mission control, all of them relieved that they had pulled off the most daring part of the flight.

"Roger.” said Lovell. Then, with the holiday period in mind. "Please be informed, there is a Santa Claus.”

"Thai’s affirmative,” agreed Mattingly. "You are the best ones to know.”

Soon after CSM Charlie Brown appeared on its way home after TEI on Apollo 10, commander Tom Stafford, wiio tvas an enthusiastic proponent of television from Apollo, turned the spacecraft around to aim their colour TV camera at the receding Moon. One of his impressions when seeing the entire ball of the Moon in one view’ was: "It’s a good thing we came in backwards at night lime where we couldn’t see it, because if we came in from this angle, you’d really have to shut your eyes.”

When Columbia similarly reappeared on time after Apollo ll’s TEI burn. Duke was ready to quiz the crew.

"Hello Apollo 11. Houston. How did it go? Over.”

Collins cheerily replied, "Time to open up the LRL doors. Charlie.” The crew

The Moon’s far side from Apollo 15 as it departed for Earth. Jenner is top right with its central peak, and Yallis Schrodinger is the gash near the bottom. (NASA)

were now officially in quarantine and were destined to spend most of the next three weeks isolated in the Lunar Receiving Laboratory in Houston.

"Roger,” said Duke. "We got you coming home. It’s well stocked.” Armstrong provided the details of the bum and then praised their trusty SPS engine. "That was a beautiful bum. They don’t come any finer.”

David Scott concurred with how well the SPS worked on Apollo 15: "What a smooth bum that one was. Just can’t beat these rocket engines for travelling.” On his mission, and all the J-missions, it was customary to adjust the spacecraft’s attitude so that the mapping camera could photograph the retreating Moon, and perhaps image more of the polar regions which had been relatively poorly covered by the Lunar Orbiters. Each succeeding exposure showed the Moon receding further and further into the darkness of space.

Apollo 16’s view of the receding Moon taken by its mapping camera. At first, only the far side was visible, but gradually, the eastern mare came into view. (NASA)

As Alan Bean watched the stark lunar globe move away from Yankee Clipper on Apollo 12, he and his crewmates were struck by the unreality of their situation. "This Moon is just this white ball right out in the middle of a big black void, and there just doesn’t seem to be any rhyme or reason why we are here, or why it’s sitting out there. All the time we were in lunar orbit we were discussing this thing – how unreal it looked. And it is amazing to us to fly around it as it is. When you just think about going to the Moon, it is very, very unreal to be there. It’s really getting small in a hurry. It’s just sort of unreal to look outside. It is almost like a photograph moving away from you. It doesn’t seem possible it can be a whole sphere that you were orbiting a couple of hours ago.”

When the CSM left Earth, the service module’s tanks were loaded with 18.5 tonnes of propellant. By the time it was on its way back, the majority of this had

been consumed. What remained had been kept aside as a contingency in case the CSM had to make manoeuvres to rcseue a stricken LM in lunar orbit. For the Apollo 8 flight, with no LM to transport to lunar orbit, the tanks were still a quarter full after TEI. while for Apollo 11, which did have a heavy LM. only an eighth remained. Not all of this remaining propellant was usable. By Apollo 17, the planners had become more knowledgeable about the spacecraft and its capabilities and felt confident to plan the mission such that, after TEI, only four per cent of usable propellant remained in its tanks.

A crewman’s favourite sight: red and white

With only 3.000 metres of altitude remaining, another barometric sw itch operated to fire mortars that deployed three pilot chutes into the smooth air stream, which in turn pulled the three main parachutes out from their bays around the tunnel. These were a welcome sight to the crews and became familiar to the public as the impressive 25-metre red-and-white canopies that featured clearly on colour television coverage of an Apollo’s return to Earth.

Both the main and drogue chutes were deployed in a reefed condition; that is. they were inhibited from inflating properly for the first 10 seconds by a line that ran around the edge of the canopy in order to reduce the mechanical shock of their deployment. A timed pyrotechnic device eventually cut the reefing line to allow the canopies to fully open.

“Going to free fall.-’ called Conrad as the drogue chutes disappeared.

“There go the mains!” yelled Gordon when he saw1 them replaced by the three glorious main parachutes.

“Hang on,” said Conrad. “We’ve got all three. A good show.-’

“They’re not dereefed yet,– warned Gordon. They couldn’t slow – enough until at least two canopies were fully inflated.

“There they go,” said Bean. "They’re dereefed.”

“A couple of them are,” said Gordon. “One of them isn’t yet. There they go,” as the last reefing cord let go. “Hello, Houston; Apollo 12,” he yelled to mission control. “Three gorgeous, beautiful chutes, and we’re at 8,000 feet on the way down in great shape.”

When things are occurring rapidly all around, events can appear to happen in slow motion. Collins was watching the deployment of the parachutes intently. “It seemed to me there was quite a bit of delay before they dereefed. All three chutes were stable and all were reefed and they kept staying that way until I was just about the point where I was getting worried about whether they were ever going to dereef; then they did.”

The fully deployed main parachutes rapidly slowed the spacecraft’s descent to just

8.5 metres per second.

While the service module had been attached, spacecraft communications on the VHF system had used two scimitar antennae mounted in semicircular housings on either side of that module. For VHF communication with the recovery forces, two small antennae stored beneath the apex cover popped up automatically soon after the main parachutes had been deployed. To use them, the crew had to manually switch the output of the VHF electronics across to the ‘Recovery’ position.

Engineers wisely allowed a generous margin by designing the main parachutes to enable the CM to land safely with only two inflated canopies. This precaution was

The Apollo 15 CM descends with one of its three main parachutes uninflated. (NASA)

justified when one of the canopies that should have been lowering Endeavour. the Apollo 15 CM to the ocean, failed and uselessly streamed beside its two functioning counterparts. The impact speed only rose from 8.5 to just less than 10 metres per second. Apollo 15‘s CMP Л1 Worden noted that all three chutes had inflated properly when first deployed so blame was put on the crew s next task, their propellant dump.

The propellant tanks for the RCS thrusters still contained much highly noxious propellant, especially hydrazine fuel. As such hazardous substances could not be on board when swimmers were clambering all over the spacecraft after splashdown, the excess was dumped by firing all their thrusters until the tanks were depleted as the spacecraft descended on its three main parachutes. Before doing so. the crew – closed the cabin pressure relief valve to prevent RCS fumes from entering the cabin, and instead, released fresh oxygen from the surge tank into the cabin. When Endeavour’s thrusters fired, its oxidiser tanks had emptied before its fuel tanks so that for a few seconds, unburnt hydrazine was leaving the engines. As hydrazine can burn in air, it has been blamed for damaging the parachute. On subsequent flights, engineers biased the propellant load towards the oxidiser and altered the liming of the burn to try to avoid the problem.

The timing of Apollo 8’s arrival meant that it re-entered just before dawn over the recovery site, so when the RCS tanks started emptying as the spacecraft descended on its main parachutes, the crew were treated to a sight which, though spectacular, was somewhat worrying. ‘The ride on the mains was very smooth,’’ said Borman afterwards, "and we could not. of course, see the mains because of the darkness until we started dumping the fuel. When we dumped the fuel, we got a good chute check, but there was so much fire and brimstone around those risers that we were really glad to see the fuel dump stop.”

Once the RCS propellant tanks had been emptied, the system’s plumbing was purged with helium gas to drive out as much trace propellant as possible.

At 1,000 metres altitude, with the RCS dump completed, the cabin pressure relief valve was reset to its dump position, which allowed the cabin’s air pressure to fully equalise wfith the outside atmosphere. It was finally closed 250 metres up, to prevent water entering the cabin at impact. For a short Lime, the spacecraft would be partially submerged when it hit the water and there was a good chance that it might be upside-down for a few minutes. The parachutes suspended the command module at an angle of 27.5 degrees to the horizontal with the main hatch facing upwards. This caused the hull to hit the water ‘toe first’, in a fashion that spread the final deceleration over the longest possible time. Also, the periphery of the CM structure was formed by shaped ribs. Those opposite the hatch, where the spacecraft would contact the water first, were designed to be crushable to help to reduce the force of impaet. They were primarily intended for the undesirable contingency of a land impact but could deform to help to reduce the shock of a conventional sea landing.

The moment of Apollo 15’s splashdown. (NASA)

Dead band

The next stage of the LM checks required the crew to think about the concept of the dead band, which is another of those curious terms in spaceflight where a simple concept lay behind opaque jargon.

Apollo was one of the first applications of a digital Пу-by-wirc system whereby control of a vehicle was placed in the hands of a computer. In the Apollo guidance computer, programmers included a scries of algorithms that would fire the RCS jets as necessary to bring the spacecraft to a desired attitude with respect to the IMU platform, and hold it there. These algorithms were called the digital autopilot (DAP). However, the gimbals around the platform were able to measure angular errors in the spacecraft’s attitude to an accuracy of hundredths of a degree, and to have constantly corrected the slightest drift to such a Light tolerance would have made the spacecraft seesaw backwards and forwards as the jets incessantly fought to maintain the ideal attitude, wasting propellant in the process. Instead, a range of attitude error around the ideal was deemed acceptable and the thrusters did not fire within this band; they were said to be ’dead’. This error band, the dead band, could be set to be cither a half or five degrees from ideal, depending on how accurately the spacecraft had to be pointed. A narrow er dead band used more RCS fuel, because the thrusters tended to fire more often when the spacecraft drifted beyond the permissible deviation.

While still docked, the commander gave the LM’s RCS system a checkout, first by using the computer to Lest that the hand controls were producing the commands expected of them, the so-called ‘cold-fire’ checks; and then by firing all 16 thrusters for short periods in a ‘hot-fire’ test. Prior to carrying out these tests, he had to ensure that his crcwmatc in the command module had the CSM’s digital autopilot set for a wide, five-degree dead band. That is, although overall the thruster firings – fore aft, left/right. etc. should be neutral, they would briefly rotate the entire CSM/LM stack by a few degrees, and it would have been a waste of propellant if the thrusters on the service module had to battle to restore attitude.

The commander also ensured that telemetry from the LM was being sent to mission control at a high bit rate. This maximised the number of engineering parameters that could be received while the health of the RCS was checked.

Almost like a third eye on the forehead of the LM’s face, another dish sprouted from the ungainly LM cabin, preferring a direction that faced forward. This was the antenna for the rendezvous radar, one of the subsystems that allowed the ascent stage to seek, find and follow the CSM as it chased the mothership around the Moon during the rendezvous. The radar operated in conjunction with a transponder on board the CSM so that the LM’s computer could tell how far apart they w ere, and in which direction. As the commander put it through a self-test routine, the final steps towards undocking were completed.

Breaking the link

Perhaps il was a bitter sweet moment for the command module pilot as he watched his crewmates leave for the Moon. There would probably be some relief that the mission had reached this point, and increasingly looked like it was going to be successful. At the same time, there might have been a deep longing for an opportunity to take a ride to the surface knowing that there were only a few kilometres between him and a moonwalk. But for all the CMPs, there was terror in the knowledge that it required only one or two of many possible failures to occur, and he might have to light his SPS engine and return to Earth alone, as a marked man, having left his crewmates on the Moon.

Alan Bean found Dick Gordon’s reaction to sending his crewmates away on Apollo 12 remarkably sanguine. Gordon and Conrad had flown together on Gemini 11, although their friendship went back to Patuxent River Naval Air Station where they were both test pilots and good buddies. Although Gordon’s friendship with the likeable and super-competent Conrad had helped to seal his place on Gemini, the experience subsequently gained prevented him from taking a ride to the Moon’s surface. Deke Slayton, who decided Apollo’s crewing arrangements, generally gave command to the most experienced in a crew, and that was Conrad. At the Lime, he preferred the astronaut in charge of the command module Lo have had experience of rendezvous in ease the CSM had to rescue an ailing LM. Thus Gordon got to fly the CSM solo. That left Bean, the rookie and the third member of this friendly crew-, with a ride to the moondust. Twenty-three years later. Bean completed a painting that imagined Gordon being down on the surface with his two buddies. In his notes on that work, Bean stated. "Dick was the more experienced astronaut, yet I got the prize assignment. In the three years of training preceding our mission, he never once said. Tt’s not fair, I wish I could w? alk on the Moon too.’ 1 do not have his unwavering discipline or strength of character.”

First moments

"Tm at the foot of the ladder.” Neil Armstrong brought a quiet coolness to the moments before he took humankind’s first step on the Moon. "The LM footpads are only depressed in the surface about one or two inches, although the surface appears to be very, very fine grained, as you get close to it. It’s almost like a powder. [The] ground mass is very fine.”

Armstrong was not telling science anything it did not already know. Previous unmanned probes and objective theorising by lunar geologists had established that the lunar surface would be finely powdered, beat up from an incessant rain of objects over extremely long time periods. But, first and foremost, Armstrong was an engineer and test pilot and one of the best in the business. What test pilots do is observe and describe in physical terms, and that was exactly what he was going to bring to this endeavour.

“I’m going to step off the LM now.”

With his right hand holding onto the ladder. Armstrong placed his left foot onto the dust of Marc Tranquillitatis. "That’s one small step for [a] man; one giant leap for mankind.’’

With the moment appropriately marked, Armstrong continued onto the surface and tentatively began to adapt to moving around in the weak gravity field. He also returned to his descriptive roots. "Yes. the surface is fine and powdery. I can kick it up loosely with my toe. It does adhere in fine layers, like powdered charcoal, to the sole and sides of my boots.”

In the years leading up to this moment, one scientist had attracted the attention of the press, ever hungry for a story, by suggesting that the LM or an astronaut would be swallowed up by a great depth of dust which, he theorised, would have taken on a Tairy-castlc’ structure. Thomas Gold’s theory was based on interpretations of radar observations which showed that the surface consisted of very loose material, which is indeed an accurate description of the Lop few millimetres. However. Gold took this observation and wove it into a tale of great seas filled with electrostatically supported dust. In fact, the large size of the LM footpads is attributed to his influence. Gold continued to provide reporters with a yarn of possible catastrophe even after unmanned Surveyor landing craft had successfully touched down using footpads designed to impart the same pressure as the LM pads. These spacecraft also returned images of boulders resting on the surface and orbiting spacecraft had imaged great swathes of ejected blocks from large craters that had clearly not sunk into the dust. But of this period, geologist Don Wilhelms wrote, "One would think that the presence of all this dust-free blocky material would have weakened the Gold-dust theory, but no amount of data can shake a theoretician deeply committed to his ideas.”

Armstrong was demolishing such worries once and for all. "I only go in a small fraction of an inch, maybe an eighth of an inch, but I can see the footprints of my boots and the treads in the fine, sandy particles."

Always aware that a problem could cause the EVA to be terminated at any Lime, Armstrong’s initial moments on the surface were carefully planned. He had a short moment to ensure he would have no difficulty moving around, then he and Aldrin used a looped strap to send a Hasselblad camera down from the cabin. The hmar equipment conveyor (LEC) was NASA’s reply to a fear that it might be difficult and time-consuming to carry items up and down the ladder, particularly boxes of rock samples. The LEC w-as discarded after Apollo 12 as crews came to better understand the ease with which heavy loads could be handled on the Moon. With the camera attached to a bracket on his chest mounted RCU. Armstrong proceeded to take a series of overlapping pictures that could later be merged into a panoramic view of the landing gear. Then, after a reminder from Bruce McCandless in Houston, he used a scoop to gather a contingency sample of the soil near Eagle.

"Looks like it’s a little difficult to dig through the initial crust,” noted Aldrin from Eagle’s cabin.

"This is very interesting,” said Armstrong. "It’s a very soft surface, but here and there where I plug with the contingency sample collector, I run into a very hard surface. But it appears to be a very cohesive material of the same sort.” Armstrong

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Neil Armstrong practises using the lunar equipment conveyor three months before Apollo ll’s flight. (NASA) "

had discovered what is, to people used to Earth, an odd property of the soil. This part of Mare Tranquillitatis had seen essentially no volcanic activity for well in excess of three billion years. The only large scale processing of the rock had been by countless impacts, and the vast majority of these were small. Each had helped to pulverise the surface into a layer of dust and boulders about five metres thick called the regolith and each had served to shake the subsoil until it became extremely well compacted yet unconsolidated material. So while the top few centimetres were loose, the subsurface seemed hard and unyielding.

It was then Aldrin’s opportunity to climb down the ladder as Armstrong photographed him. “Now I want to back up and partially close the hatch, making sure not to lock it on my way out.”

“A particularly good thought,” laughed Armstrong.

“That’s our home for the next couple of hours and we want to take good care of it.” The checklist had called for the hatch to be partially closed, probably to prevent the shaded interior radiating its heat into space.

Ever the test pilot, Aldrin continued to narrate his descent of the ladder. “It’s a very simple matter to hop down from one step to the next.”

“Yes. I found I could be very comfortable, and walking is also very comfortable. You’ve got three more steps and then a long one.”

Aldrin practised leaping between the ladder and the footpad, then, before he stepped onto the surface, he turned to take in the landscape.

“Beautiful view!”

‘‘Isn’t that something!” said Armstrong. “Magnificent sight out here.”

“Magnificent desolation.”

Snow on the Moon

As A1 Bean waited to follow’ Pete Conrad out of Intrepid’s cabin, he moved to his window to adjust a movie camera that would record their work on the surface. Just after Conrad had taken a contingency sample of lunar soil, both men heard a warning in their headsets.

“Uh-oh, did I hear a tone?” said Conrad as he tested his mobility on the surface.

“Yeah; I’ve got an H20 A,” said Bean.

“You do?”

“Yeah. I wonder why?”

The ‘A’ flag and tone was telling Bean that his cooling system wras failing. For a few’ minutes he tried to troubleshoot the balky PLSS as Conrad got started on tasks around the LM.

“Okay. I think I know what happened. Houston.” said Bean as he spotted the cause. After the flight, he explained the circumstances: "1 happened to glance down and noticed the door was closed. I realised what had happened. The outgassing of my sublimator had closed the door, with the result that I didn’t have a good vacuum inside the cabin anymore. I quickly dove to the floor and threw back the hatch. The minute I did. a lot of ice and snow went out the hatch.”

“What did you just do, Al?” asked Conrad when he saw’ resultant display.

“Man, I just figured it out.”

“You sure did. You just blew water out the front of the cabin.” Then correcting himself, “Ice crystals.”

“That’s what had happened to the PLSS. The door had swung shut, […] and probably bothered the sublimator. ‘cause it wasn’t in a good vacuum anymore. So 1 opened the door and it’s probably going to start working in a minute.”

“I should hope so. laughed Conrad. "When you opened the door, that thing shot iceballs straight out the hatch.”

The first time David Scott opened Falcon s forward hatch, he treated Jitn Irwin to a similar display of ice particles visible out of his window. “It’s blowing ice crystals out the front hatch,” laughed Irwin. “It’s really beautiful. You should see the trajectory on them.”

“I bet they’re Паї, aren’t they, Jim?” asked Capeom Joe Allen. “The trajectories?” Allen wras a scientist by training and as soon as he heard about the flying crystals, he immediately began to think about how they would move.

“Very flat. Joe,” answered Irwin to Allen’s query.