Category How Apollo Flew to the Moon

By July of 1969, NASA had done about as much as they could to prepare for the Moon landing. On the flight of Apollo 10 two months earlier, Tom Stafford and Eugene Cernan had taken their LM Snoopy into the descent orbit but had gone no further before returning to John Young in the CSM Charlie Brown.

Where Snoopy had feared to wander, Eagle swooped in. Although the first landing attempt, flown by Neil Armstrong and Buzz Aldrin, would be ultimately successful, it was by no means a straightforward descent. Landing on the Moon was a 12-minute rocket ride from orbit with a starting speed of nearly 6,000 kilometres per hour leading to a gentle touchdown on a terrain where no prepared ground awaited the LM. In that short time, a plethora of problems were served up to the crew of Eagle that would have curled the toes of everyone involved had it merely been a simulation. The fact that they all occurred on the actual landing attempt in full view of the world, yet were successfully handled by the mission control team and the crew, is testament to their professionalism, and to the power of exhaustive simulation as a means of properly preparing people for the challenges they may face.

Programs and phases

Planners broke the descent into three parts with each controlled by a dedicated program in the computer. The first was the braking phase, when most of the spacecraft’s orbital speed was countered by the thrust of the descent engine. This was the domain of Program 63 which began 10 minutes before the powered descent. It included the engine’s ignition and continued for the first nine minutes or so of the nominally 12-minute burn while the computer worked to take the crew to a point in space known as high gate, typically 2,200 metres in altitude and about seven kilometres from the landing site. At the start of the braking phase, the LM flew with its engine pointing against the direction of travel. Then as the burn

W. D. Woods, How Apollo Flew to the Moon, Springer Praxis Books,

DOI 10.1007/978-1-4419-7179-1 10. © Springer Science+Business Media. LLC 2011

progressed, the spacecraft gradually tilted a little more upright. At high gate, P64 took over.

P64 handled the approach phase of the descent. When the program assumed control, its first action was to pitch the LM further towards an upright attitude in order to enable the crew to see the landscape ahead. The point to which the computer was taking them was just on the near side of the horizon. They then flew in a manner roughly similar to a helicopter, but with the LM carefully balanced on top of the engine’s exhaust with the computer still in full control of where it was going. P64 included a method of informing the commander of where the computer was taking them, but if he deemed this to be unsuitable, then with a nudge of his controls he could instruct the computer to move the aim point. P64 was targeted to take the LM to a point about 30 metres above the surface and about five metres from the landing site. Prior to reaching this point, the crew would reach low gate, about 200 metres altitude and 600 metres short of landing.

As low gate approached, the commander was faced with a range of options. If he was completely satisfied with the job the computer was doing, he could allow it to automatically move on to P65, which could complete the landing. No commander ever allowed that, although it is said that Jim Lovell had intended to if Apollo 13 had reached this point. These competitive ex-test pilots, many of them experienced at landing on aircraft carriers, were happier to have some degree of control and steer the LM, and they all selected P66 before reaching low gate. P66 continuously throttled the engine to control their rate of descent, and the commander could adjust this rate as conditions warranted. At the same time, he assumed manual control of the LM’s attitude, which allowed him to steer the ship to a site of his own choosing. One other program, P67, was available to the commander, which gave him full manual control of the spacecraft, both the attitude and the throttle setting, but this

“Go for the pro”: the landing begins 287

option was never used. Both P65 and P67 were dropped from later versions of the LM software.

As if Apollo ll’s descent wasn’t exciting enough, later analysis demonstrated that Eagle’s software had harboured a problem that could have forced an abort. In fact, Apollo 12 flew with the same problem coded into its software. Alhough it was most prominent during the final few metres before touchdown, when the commander was flying in P66, it could have afflicted any phase of the descent.

The story of the problem, as told by Don Eyles, came to light when an engineer, Clini Tillman, ran a simulation of a descent. Tillman worked for the LM’s manufacturer, Grumman. This gave him the ability to run his simulation using real hardware rather than having to mathematically model the operation of the engine. As a result, faulty assumptions in the model were avoided.

Tillman noticed that the commanded thrust was varying in an apparently random, stepped fashion that came to be known as throttle castellation, after the similarity of its waveform to castle battlements. The variation was only slight but when Tillman dug into the stored telemetry readings from Apollo 11 and 12. he discovered that during P66 these variations were not only present, they were unduly large and hinted at some intrinsic instability in the system as a whole. One aspect of the problem was uncovered by Eyles’s colleague. Allan Klumpp. Know’n as IMU bob. it came about because the accelerometers at the centre of the IMU did not reside at the centre of mass of the LM. So when the LM underwent significant rotation, the 1MIJ sensed a component of that rotation as being vehicle velocity, which it was not. When this fed into the thrust calculations, it caused a small degree of instability.

A more profound cause of throttle castellation was related to the time taken for the descent engine to respond to commands to change thrust so called ‘throttle lag’. As described by Lyles, the paperwork for the engine stated that its lag time was 0.3 seconds and it was his task to compensate for this in software. To determine the best compensation value to use. Hyles carried out simulations to model the performance of the engine. lie saw how unstable the throttle command was with no compensation, then how compensating for 0.1 seconds helped a lot and how compensation for 0.2 seconds of lag essentially eliminated the instability. He therefore programmed the flight software with compensation for 0.2 seconds of throttle lag instead of 0.3 seconds.

Now it turned out that the compensation was trying to hit a sweet spot and that overcompensation could also induce throttle instability. It also transpired that the documentation for the engine was wrong. The engine was still evolving and by the time it was installed in the LM. it could react to throttle commands in a mere 0.075 seconds. It was later demonstrated that if Hyles had followed the paperwork and programmed for 0.3 seconds compensation, the throttle would have been wildly unstable. Any further source of instability; for example the I MU bob problem and the mission would have had to have been aborted. As it was. the first two flights to the Moon landed with throttles that were barely stable.

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.

Although the Moon appeared to be a very dead world to anyone who looked at it. scientists wondered if some traces of volcanism were still spluttering in some corner of the globe. Tantalisingly, some telescopic observers had reported seeing ’emissions’ in the form of brief glows and hazes, which kept alive hopes of finding extant activity. The alpha-particle spectrometer was designed to look for indications of such activity.

Lunar rock samples from earlier missions were found to contain traces of uranium and thorium, two elements which, through their radioactivity, decay to form gaseous radon-222 and radon-220 among other elements. The alpha-particle spectrometer could detect these substances from lunar orbit by their emission of alpha-particle radiation essentially the nuclei of helium atoms as they further decayed and. by inference, locate areas of possible volcanism or other features that might cause the concentration of uranium and thorium to vary. Any emissions from the Moon of gases such as carbon dioxide and water vapour would also be
detectable as they would be expected to include a small amount of decay­ing radon gas.

The major result to come from this instrument was that there is a small degree of outgassing of radon at various locations on the Moon, especially in the vicinity of the prominent crater Aristarchus – a result confirmed a generation later by the Lunar Prospector probe. Interestingly, Aristarchus, which is also one of the brightest places on the Moon, was the locale for some of the reported emanations seen by telescopic observers. These tentative indications of possible ongoing lunar activity should be seen in the light of studies of a crater, Lichtenberg, on the western side of Oceanus Procel – larum. This crater exhibits a ray system that is believed to be just less than a billion years old, which is quite young by lunar standards. Yet, on a world where most of the basalt is much older, a distinctive dark lava flow has obliterated much of its southern ray system. From this evidence, and as far as is known, the final gasps of lunar volcanism occurred about 800 million years ago. To put this into a terrestrial context, this is 300 million years before complex multicellular life appeared on Earth.

The detection of radon gas, particularly at Aristarchus, is best explained by the effect of the huge impact that formed the Imbrium Basin, within which Mare Imbrium now lies. The current magma ocean theory of the Moon’s early evolution not only explains the richness of aluminium in the upland regions of the Moon, but also predicts that, as the magma ocean cooled, the last vestiges of lava to solidify would have been rich in the KREEP elements that would have found it difficult to become part of the rock’s crystal lattice. Geologists now believe that the violence of the Imbrium impact event nearly four billion years ago was enough to punch through the crust and excavate KREEPy rocks to the surface. A lot of this slightly radioactive rock was covered by the lava flows that drowned the western portion of the Imbrium Basin over three billion years ago. Then half a billion years ago the impact that formed Aristarchus drilled through the layers of basalt to re-excavate KREEPy material.

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.

On leaving the protection of Earth’s magnetic field, many crews, beginning with

Apollo 11, mentioned occasional brief flashes that would appear in their vision irrespective of whether their eyes were open or closed. On Apollo 12 Conrad noted the same thing. The Apollo 14 crew made a basic study of the phenomenon after the cancellation of a mid-course correction manoeuvre that left them with time on their hands. The Apollo 15 crew had some time set aside specifically to further investigate the phenomenon whereby the crew would sit in various positions in the cabin wearing blindfolds for an hour.

“I would say 90 per cent were of what I’d call a point source of light,” explained Scott at the end of their first experimental period. "And to give you an analogy, you might picture yourself sitting high in the stands of a darkened arena, and you look across at the other side and somebody shoots a flashbulb or something, and that would be what I’d call a typical flash of intensity five on a scale one to five.”

Worden added to Scott’s description: "Most of the light flashes seem to be of the order of flashcubes or maybe starbursts that you’ve seen in the summertime. I saw very few streaks or radial paths of light. They all seem to be just point sources of light.”

The next two missions took the study further by having one crewman wear a film- based particle detector, the Apollo light flash moving emulsion detector (ALFMED) while he described the flashes that he saw. Though it was attributed to cosmic rays passing through the head and interacting with the human visual system, the results of these small-scale experiments were inconclusive. Long-term dedicated experiments

on board the space stations Mir and the ISS showed that they could also be detected in Barth orbit.

The crews carried out other experiments, both scientific and technological, during their coast home. The sensors built into the SIM bays of the J-mission CSMs could no longer look at the Moon but opportunities were taken to aim them at celestial objects. For example, just prior to Apollo 15. the Uhuru x-ray astronomy satellite had discovered a strong x-ray source called Cygnus X-l. The x – ray spectrometer in the SIM bay was therefore brought to bear on it to help to characterise its emissions.

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.

The landing site REFSMMAT was another of the many frames of reference used during an Apollo flight. It was carefully chosen to aid a landing crew by having their attitude displays, the FDAIs. or 8-balls, give readings that would make sense to a pilot as he approached the lunar surface. This frame of reference was defined as being the orientation of the landing site with respect to the stars at the predicted time of landing. The actual orientation of the landing site, of course, continuously changed as the Moon rotated on its axis and only matched the landing site REFSMMAT at one moment in time. This coincidence of the two was known as the ’REFSMMAT 00 Lime’ and therefore this time represented the intended moment of landing.

When properly aligned to this REFSMMA T, the platform’s, v axis would be parallel to a vertical line running from the centre of the Moon out through the landing site position. Its 2 axis would be tangential to the landing site yet parallel with the GSM’s orbital plane and thus with the LM’s approach path, pointed in the direction of flight. Use of this frame of reference meant that if the LM landed at the planned time and place, w-as in a fully upright attitude and was pointed forward, then its FDAI display should show 0 degrees in all axes.

Apollo ll’s troubles began as they came around the Moon’s eastern limb. There had been a major change to the configuration of the lunar module since Apollo 10 had rehearsed the descent orbit. Plume deflectors had been added around the descent stage to protect it from the blast of hot gas from the RCS thrusters and these were now interfering with the radiation pattern of the steerable high-gain antenna. Worse, Armstrong was flying with the windows facing the Moon to gain timings relating to his orbit. This meant that the steerable antenna had to peer past the LM structure. The diagrams that indicated the resultant restrictions and which angles the steerable antenna could use were in error. At acquisition of signal after Eagle had entered the descent orbit, mission control found that not only did this interference make voice communication with the crew difficult, it interrupted the engineering telemetry with which flight controllers would soon make a decision on whether to proceed with the landing.

To try and alleviate the problem, Charlie Duke in mission control passed on a recommendation from Pete Conrad, who was sitting close by, that they yaw the LM right by 10 degrees. Enough data did get through for the Go/no-Go decision to be made positively, though in the event, it had to be relayed via Mike Collins in the command module.

"Okay.” continued Aldrin. "75 feet, and it’s looking good. Down a half, six forward.” They were 23 metres up and almost hovering.

’’Sixty seconds,” called Duke as mission control continued their countdown to the land-or-abort call.

’’60 feet, down 2’A,” called Aldrin. “Two forward. Two forward. That’s good.”

Armstrong had found his spot and was taking the LM down. Like all the commanders, he wanted to land with the LM still moving gently forward so that he could always see where he was going. It was felt unwise to land going backward as it posed the risk of falling into a crater or striking a boulder that could not be seen.

“40 feet, down 2 A,” said Aldrin. “Picking up some dust.”

This was new. This was wdien the people in and around mission control realised that this was for real. No one had ever thought to mention it during the great many simulations they had run. The descent engine’s exhaust plume was blowing a substantial blanket of flying dust that wafted around the small stones scattered across the landing site. They were in a new environment and already discovering new things.

Jack Ciarman later related the phenomenal impact Aldrin’s words had. "We’d watched hundreds of landings in simulation, and they are very real.” Up to this point, and because he was used to the apparent reality of the simulations, Garman had not fully appreciated the fact that what was happening was no simulation.

Then Aldrin mentioned the dust. "And we’d never heard that before.’’ recalled Garman. "It’s one of those, ‘Oh, this is the real thing, isn’t it?’ I mean, you know it’s the real thing, but it’s going like clockwork, even with problems. We always had a problem during descent. A problem happens, you solve the problem, you go on. no sweat. Then Buzz Aldrin says, ’We’ve got dust now’.’ My god, this is the real thing. And you can’t do anything, of course. You’re just sitting down there. You’re a spectator now. Awesome. Awesome.”

On Apollo 15. as Falcon was being brought down onto the plain at Hadley. Scott thought the dust seemed completely enveloping. From his perspective, it was like flying in a fog. "At about 50 to 60 feet [15 to 18 metres], the total view’ outside was obscured by dust. It was completely IFR." Scott was comparing the experience to instrument flight rules, a mode of aircraft flying that pilots adopt when the weather closes in and restricts their visibility. He therefore had to take his attention from the view outside and use the displays in front of him.

As Young brought Orion down the final few metres on Apollo 16, Duke talked him through the dust.

“Okay, down at three [feet per second], 50 feet, down at four." They seemed to be dropping faster. "Give me one click up," advised Duke. Young operated the ROD switch and temporarily found he was hovering above a blanket of flying dust. "Come on. let her down. You levelled off," said Duke. “Let her on down. Okay, six per cent. Plenty fat." They had no problem with propellant.

“We did hover for a short period of time there,” commented Young after the flight, "at about 40 feet [12 metres] off the ground, and the [horizontal velocity] rates were practically zero and there was blowing dust. Vou could still see the rocks all the way to the ground, the surface features, even the craters, which really surprised me."

As Young brought the LM down, he just barely missed a 25-mcirc crater and landed with Orion’s rear footpad right on its edge. It was only when he and Duke got outside and could sec all around the LM that they realised how close they had come to being dangerously tilted over.

“I’m glad you weren’t 10 feet [further back], said Duke. "Whew me!"

“W’e were going forward," said Young, thankfully.

“Yeah, we were landing going forward."

Back on Apollo 11, Armstrong was only 10 metres above the surface and Aldrin was still feeding him data. “Thirty feet, 2 ‘A down." By this time, and since 70 metres altitude, Aldrin could view’ the LM’s shadow when he glanced up as it moved across the landscape. Since they always landed with a low, morning Sun behind them, the approaching shadow could be a useful tool to help to judge the final few metres. However, Armstrong could not see it because he was flying with the LM yawed to the left and the way his window was heavily recessed severely limited his field of view to the right.

“Four forward. Four forward," continued Aldrin. "Drifting to the right a little. 20 feet, down a half."

“Thirty seconds." called Duke. For all their telemetry, the flight controllers simply did not have the situational awareness that the crew’ enjoyed. With only 30 seconds remaining before the land-or-abort call, mission control was beginning to hold its collective breath.

“Drifting forward just a little bit," said Aldrin. coaching his commander down.

"That’s good."

Just then, one of the probes attached to three of the LM’s footpads struck the surface and lit an indicator in the cabin. Their footpads were less than two metres above the surface. "Contact light,” called Aldrin.

Immediately, the pair began a rehearsed series of tasks to turn Eagle from a flying machine to a home on the Moon.

‘’Shut down,” said Armstrong.

“Okay. Engine stop.” replied Aldrin.

By the lime Armstrong got the engine stopped, they had already settled onto the surface. No harm came to the engine, but he was struck by the unexpected way the lunar dust behaved in the exhaust gases in front of him. In an interview 32 years later, he talked about this surprising phenomenon: “I was absolutely dumbfounded when I shut the engine off. They just raced out over the horizon and instantaneously disappeared, just like it had been shut off for a week. That was remarkable. I’d never seen that. I’d never seen anything like that. And logic says, yes. that’s the way it ought to be there, but I hadn’t thought about it and 1 was surprised.”

On later flights, the commander was spring-loaded to stop the engine as soon as the probes touched the surface, particularly on the. l-missions where the longer engine nozzle provided only 30 centimetres of clearance to level ground.

“АСА out of detent,” was Aldrin’s next item, vvhieh referred to the controller in Armstrong’s hand. As they touched down, the LM adopted whatever attitude the surface dictated. However, the RCS thrusters w’ere still busily firing in a futile attempt to restore their previous attitude. By moving the stick, known as the attitude control assembly (АСА) out of its central position, Armstrong made the system think that the current attitude was also the desired attitude, and thereby stopped the jets from firing.

“Out of Detent. Auto.” said Armstrong.

“Mode control, both auto. Descent engine command override, off. Engine arm, off. 413 is in.” Aldrin’s litany of checklist instructions ended with an entry into the AGS, their secondary guidance system. Aldrin was entering a number into address 413 that told the machine they had landed and that it should take note of their current attitude in case they had to abort from it. The body-mounted gyros of the AGS were prone to drift and were unlikely to provide an accurate attitude by the time an abort might be called.

“We copy you dowm. Eagle." said Duke, spokesperson for an anxious mission control and a waiting Earth. Armstrong was not yet finished with the checklist.

“Engine arm is off,” he responded to Aldrin." Then to the w orld, he announced. "Houston, Tranquillity Base here. The Eagle has landed.”

Duke tvas caught by the moment and by Armstrong’s sudden change of call sign, despite having been forewarned. “Roger. Twan… Tranquillity. We copy you on the ground. You got a bunch of guys about to turn blue. We’re breathing again. Thanks a lot."

In the view of the public, the defining moment of the event would be when a human footprint deformed the lunar dust. This would have a human dimension; there would be a personal link to the hearts of all people who left footprints on Earth and a sense of the frailty of a mere human stepping out on a hostile alien world. The moonwalk would be the pinnacle of the whole achievement, itself a supremely difficult accomplishment.

Neil Armstrong didn’t view a moonwalk as being particularly difficult not in comparison to the rigours of gelling Eagle onlo the Moon in one piece. Speaking in 2001 to NASA historians, he weighed up how difficult the landing had been. ’‘The most difficult part from my perspective, and the one that gave me the most pause, was the final descent to landing. 1 hat was far and away the most complex pari of the flight. The systems were very heavily loaded at that time. The unknowns were rampant. The systems in this mode had only been tested on Harth and never in the real environment. There were just a thousand things to worry about in the final descent. It was hardest for the system and it was hardest for the crews to complete ihai part of the flight successfully.’’

Then somewhat humourously, he tried to enumerate the difficulty of the landing as compared to an excursion outside. “Walking around on the surface, you know, on a ten scale, was one. and I thought that the lunar descent on a ten scale was probably a thirteen.”

A final and eloquent perspective on the moment of touchdown comes from Jim Scotti, a planetary scientist who once asked Gene Cernan about the sounds he could hear as he landed on the Moon. Scotti wanted to know about the noise produced by the engineering that surrounded Cernan; pumps, switches, thrusters, and that big descent engine. The answer he got was not what he expected. Jim picks up the story. "What he heard in the moments after landing was… silence! You sec, before landing, he was so engrossed in the activity that he heard Jack calling out numbers and the occasional call from Houston and everything else blended into the background because he was so focused on the task of landing. At touchdown, however, the spacecraft fell silent and mission control was staying quiet to try not to interfere with what they expected was the final moments of touchdown. And Gene added: ‘And the guy standing next to me was struck silent staring out the window looking at the surface and he sure wasn’t saying anything!’”