Category How Apollo Flew to the Moon


The descent orbit of the final four missions was a particularly exciting affair as the spacecraft gently descended from its 110-kilometre high point over the far side to skim across the near-side mountain tops with a clearance of barely 15 kilometres. The northerly paths taken by Apollos 15 and 17 over the near side were especially notable for the spectacular ride they offered the crews. As they descended from their apolune, these spacecraft passed over Mare Crisium then Mare Serenitatis. On Apollo 15, Mare Serenitatis was already lit by the morning Sun and the mountains on its western shore rose like a wall ahead as they descended across the its smooth basalt plain, so huge, its curvature was readily apparent. Capcom Karl Ilenize, who must have been imagining the approaching peaks, jokingly enquired about their safely. "Fifteen, does it look like you are going to clear the mountain range ahead?" Irwin replied, “Karl, we’ve all got our eyes closed. We’re pulling our feet up.’’ "Open your eyes. That’s like going to the Grand Canyon and not looking.” This range also formed the eastern margin of the great Mare Imbrium. It was within an embayment seated among these peaks that David Scott and Jim Irwin would eventually land.

On Apollo 17, Jack Schmitt found his calling as a teller of stories of the Moon. There was little room in this geologist’s mind for gushing wonderment at the stark beauty of Luna’s ancient surface. No. As soon as the spacecraft had emerged from behind the Moon after LOI and he had completed his report on the SPS propellant utilisation, he began to bend the ear of Capcom Gordon Fullerton with a running commentary of the terrain below, breaking off at one point to remark, "One little minor problem, Gordy, is that we’re breathing so hard that the windows are fogging up on the inside for a change.”

It was little wonder. The only trained scientist to reach the Moon was going to give a master class in observational geology, but coming over Mare Crisium he was just getting warmed up. "Oh, boy, there is Picard [Crater] – or Peirce, one of the two. Okay, Gordy, all those dark and light albedo changes around Picard and Peirce are not obvious at this particular angle yet. There’s some hint of them.”

"Roger,” confirmed Fullerton.

Schmitt stuttered on as the TV camera broadcast the view to Earth. "The rim – Is there one farther south of Peirce? Which – is it far – Is the one farthest – Picard, yes. Picard, I think, is the one I’m looking at. Yes, it is. Yes, and I can see Peirce now just behind the rendezvous radar.”

Jack Schmitt had been trained by NASA to fly jets as a condition of being an


The Apennine Bench Formation at the southeast margin of Mare Imbrium.

astronaut. However, he simply did not think like a pilot, for pilots are trained to stay off the radio unless there is something operationally important to say. and this was the case for most crews. However, Schmitt’s natural tendency, honed by years of scientific observation, was to describe. And this he did in spadefuls. Even during their first near-side pass, as they passed over the night hemisphere of the Moon, he made use of the cool, dim Earlhlight that illuminated the landscape below. “I’ve got a visual on Eratosthenes and Copernicus. ‘I’hey are obviously different-age craters in this light. You can see the ray patterns in Copernicus moderately well. You can even tell that they do cross Eratosthenes. Stadius shows up as a very clear dark area to the southwest of Eratosthenes.”

Later in the flight, he had an opportunity to observe one of the Moon’s most distinctive craters, Archimedes, located in the middle of Mare Imbrium. Archimedes is important to lunar geology because it is part of a series of lunar features that allowed geologists like Schmitt to apply the principle of superposition to construct a stratigraphic history of the region. Simply stated, this principle holds that the realitive ages of features can be deduced by observing which features overlie others. Archimedes is flooded with the lavas that also filled the Imbrium Basin so it is older than the most recent lava outpouring. To its south and southeast is a light-toned patch called the Apennine Bench Formation (the Apennines being the mountain range that forms the southeastern rim of the Imbrium Basin). Schmitt referred to this feature simply as the Imbrium Bench. It is evident that it predates Archimedes because we can see damage from the crater’s formation across its surface, finally, the Bench seems to be a sheet of a different kind of lava that formed soon after the creation of the Imbrium Basin itself. Schmitt told all this to Capcom Gordon Fullerton.

“This is one of the first opportunities that I’ve had to look closely at Archimedes, which is one of those craters that, in the early days of the lunar mapping programme, helped to establish some of the fundamental age relationships between the various units that were visible in the Earth-based photography.”

History lesson over, he began his description: “In this particular case, it related to the sequence of events that created Imbrium, cratered it. and then flooded it with mare. And Archimedes is a completely closed circle as a crater, and it is filled with mare. And it, in itself, is superimposed on one of the main benches of the Imbrium crater. Now7, to have mare filling that crater and actually filling all the depressions of approximately the same level in the vicinity of a large mare region, it’s one of the things that’s suggested to many people that rather than single sources for mare lavas, you have a multitude of sources in a very fractured lunar crust. The ultimate source in depth, though, is still certainly a subject for controversy. Some of the ridge and valley structure of the Archimedes impact blanket is not covered by mare and extends to the southeast out onto the Imbrium Bench. That was also one of the pieces of evidence used in those early days of photogeologic mapping of the Moon. You’ll have to excuse the reminiscing. Gordy.’’

On and on he went, before and after his visit to the surface, providing lunar scientists with a journal of geological observations to stand for all time as the sights of the first scientist to visit the Moon.

Landing radar

On Earth, an aircraft’s altitude is conventionally determined by measurement of the external pressure. This makes use of the fact that as altitude is gained the atmosphere gradually thins in a well-understood manner. On the Moon there is essentially no atmosphere, so another method had to be devised to determine how high the LM was above the lunar surface. This was particularly important given the fact that there are few’ clues a pilot can use to determine speed or altitude by eye. There are no trees, roads or houses; no haze to give a sense of depth or distance. Most remarkably, there


The rendezvous radar antenna on Apollo 9’s LM Spider. (NASA)

is little variation of topography as one descends from high to low altitudes. Just a pock-marked landscape of large craters overlaid with small craters, peppered with even smaller craters. Not that a pilot could manually fly the LM to the surface. The tight margins involved ruled that out. Good altitude information was needed by the computer so that it could fly an efficient path to the surface.

The way the lunar module made sense of its altitude above this landscape was by directing radio signals at it from an antenna mounted on the underside of the descent stage. One beam used Doppler techniques to determine the altitude and vertical velocity of the LM. Three more beams directed in a splayed pattern yielded the spacecraft’s horizontal velocity, again using the Doppler effect. Combined, the radar’s electronics could supply three-dimensional velocity information and altitude to the computer.

Whereas the initial landing missions flew over smooth terrain on their way to the touchdown, the later missions approached their landing sites over mountainous landscapes. To deal with this, the computer had a simplified model of the terrain profile added to its programming to compensate for the natural changes in height that would be encountered by the LM on its planned ground track. The computer also took account of the antenna’s slant angle; that is, its angle away from true vertical in which it was pointed at any moment. The data derived from the radar was not only used by the computer in its control of the descent. It also drove the tapemeter and cross-pointer displays for the crew.

The antenna operated in one of two positions, depending on the flight mode of


Diagram of the landing radar antenna. (NASA)

the LM. Throughout most of the descent, the LM was flying on its back, with the crew looking up into a black sky. In this mode, the landing radar antenna was in its ‘descent’ position, angled 24 degrees from the LM’s vertical axis. For the final phase of the landing after pitch-over, when the LM adopted a more upright attitude, the landing radar moved to its ‘hover’ position to aim in a direction parallel to the spacecraft’s v axis and therefore pointing more or less straight down.

Looking out of the window

Responsibilities in the LM were tightly defined, especially during the approach and final phase. The commander wanted to keep his eyes out of the window, watching where the spacecraft was going. The LMP, on the other hand, generally had to keep his attention inside the cabin. His responsibility w’as to vocally feed the relevant information the commander would require at a given point in the descent. The details of this w’ere worked out by each crew individually over the months of training and simulation.

As Orion descended, Charlie Duke managed to steal a little time looking out at the landing site as they reached P64.

“Pitchover,” he shouted. “Iley, there it is. Gator, Lone Star. Right on!” These were craters around the landing site that he and Young had named when drawing up their map. Being on the right side of the westward-flying spacecraft, he was able to see the northern half of the site.

“Call me the things, Charlie," said Young, bringing Duke’s attention back into the cabin to call out the LPD angles.

“Okay. 40, 38 degrees.”

Young was delighted with the way the LPD worked, as he recounted after the mission. “I think the LPD was perfect. I don’t have any gripes there whatsoever. When we pitched over, we were north and long and you could see that. I was just letting the LM float in there until I could see where it was going."

As they came in. Duke Look further opportunities to glance outside where he recognised more craters. "Palmetto and Dot; North Ray.” he called out to Young. “Looks like we’re going to be able to make it, John. There’s not too many blocks up there.” He was thinking about how easy it would be for Young to drive the lunar rover around the site. It had not been possible to infer this from the limited orbital imagery.

The road to knowledge

On the Isle of Lewis, part of an archipelago off Scotland’s west coast, ancient peoples constructed an arrangement of huge stones which survives to this day near

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

DOI 10.1007/978-1-4419-7179-1 12. © Springer Science+Business Media. LLC 2011


The Standing Stones at Calanais, Scotland, perhaps an ancient lunar computer.


Подпись:the village of Calanais (pronounced ‘callan – ish’). This impressive 5,000-year-old monu­ment is believed by some to have been a means of predicting the more subtle motions of the Moon across the sky over an 18-year cycle. If true, then its Neolithic builders seem to have imbued the Moon with a spiritual significance that caused them to devote major resources to its construction.

Down the centuries and all across the planet, peoples have portrayed the Moon as a deity. The Greeks associated its changing appearance with three goddesses; Selene, He­cate and Artemis. The Romans likewise looked to Luna and Diana. Despite this deification of the Moon, one Greek philosopher, Hip­parchus, was able to determine its distance and size by clever interpretation of naked-eye observations. It would be nearly 1,500 years before Europeans learned to stand on the shoulders of Hipparchus by applying scientific principles to the gaining of knowledge.

Four hundred years ago, Galileo Galilei acquired an early version of the telescope and turned it towards the Moon. His written descriptions and drawings reveal that he saw

it not as a perfect celestial body that merely reflected the imperfect landscape of Earth. as some of his contemporaries believed, but as a world in its own right, with plains, highland areas and ranges of mountains. Although he. like others, called the dark areas ‘seas’, his perception was sufficiently developed to suggest that they were just as likely to be dry plains.

As the telescope and its use increased in sophistication, a series of maps were drawn by ever more capable selcnographcrs. notably by Giovanni Riecioli who instituted the scheme of nomenclature that is used today and which has gradually evolved to name most of the large features that can be seen from Earth. The finest maps of the pre-photographic age were drawn by Wilhelm Beer and Johann Madler in Germany using a telescope equipped with a micrometer to measure features using cartographic techniques. After its invention in the mid-1800s, photography became the staple medium of lunar research and good atlases were produced to show the near side with oblique lighting that displayed lunar topography well.

The question that most intrigued lunar scientists concerned the origin of craters – circular landforms that appeared ubiquitous on the Moon and whose sizes ranged from many hundreds of kilometres down to the limits of detection. Craters could be found on Earth, although their size seemed to be limited to a few kilometres at most, and all were associated with volcanoes. Many tried to bend the volcano hypothesis to explain the origin of lunar craters but it was a geologist. Grove Karl Gilbert, who postulated accurately that the major process responsible for the lunar landscape was impact, sometimes on an utterly cataclysmic scale. Volcanism did occur and was responsible for laying down the vast mare plains, but there are no volcanic edifices on the Moon that would rival a Mount Fuji, Mount Kilimanjaro. Mount Vesuvius or Mount Etna. Instead lunar volcanism produced low mounds with small vents at their summits.

Gilbert’s work was not fully acknowledged by the lunar science community for decades, but this difficulty in accepting the new occurs in science far more often than many people realise. Too many scientists were wedded to their grand imaginings of massive volcanic events to grasp how time and a rocky rain from space could form such consistent structures. They reasoned that if craters came from falling rocks, they should arrive from many angles and produce elongated craters. Lunar craters were notable by their circularity.

Generally, astronomers don’t like the Moon. Its shine swamps the feeble light they are trying to capture from immensely distant objects. However, they often use the Moon as a handy target when new telescopes are being tested. Thus, in the second decade of the twentieth century, an exquisite photograph of the impressive crater Copernicus was taken during tests of the 2.54-metre Hooker Telescope at Mount Wilson. At the turn of the 1960s. Eugene Shoemaker used this photograph to carry out an elegant study of crater morphology. His investigation finally drove home the importance of impact as the prime sculptor of the Moon’s face. The study was coupled with findings from ballistic trials that demonstrated how the extremely violent explosions that resulted from cosmic impacts would produce circular craters for all but the most oblique impacts. Related studies of the manner in which rock becomes shocked by impact enabled sites of terrestrial impacts to be identified. One


The face of the Moon, its major features and the Apollo landing sites.

significant product of this knowledge was the dawning realisation that impact is still reworking not only the surface of the Moon, but also the surface of Earth.

As the space age developed and Cold War politics aimed the United States to the Moon, lunar scientists found themselves with an undreamt-of opportunity to extend their discipline which, up to that point, had accomplished about as much as could be achieved using Earth-bound photographs made blurry by the roiling atmosphere. NASA was aware of the bureaucratic danger of justifying its existence only on Kennedy’s political whim, so it turned to science as a valid, long-term rationale for flying men to the Moon. Although the primary driver for Apollo was international prestige and technical supremacy, science would give the crew of the first mission something useful to do after planting the national flag, and it would then go on to underpin the missions that followed.

The mapping camera

The SIM bay’s mapping camera was really two cameras in one package, with a third instrument included to aid interpretation of the imagery. It was based on a wide-field camera designed in the 1960s as part of the then-secret Corona reconnaissance satellite programme to provide context images of target sites. The main instrument was the metric camera, a conventional photographic imager with a 76-millimetre lens that took wide-angle, often spectacular, square images on 127-millimetre wide film with a maximum resolution on the lunar surface of about 20 metres. Like the surface Hasselblads, the camera had a Reseau plate to imprint tiny crosses onto the image to allow researchers to compensate for changes in the film’s geometry over time. A large cassette carried over 450 metres of film which was sufficient for at least 2,500 images.


The Apollo 15 CSM Endeavour showing its SIM bay. (NASA)

When using such imagery for mapping purposes, it was vital to know the direction in which the camera was pointed. This information was supplied by the associated stellar camera. At the same time as a frame was being exposed by the metric camera, another was taken of the stars looking to the side. Since researchers knew the precise angle between the axes of the two optical systems, they could deduce exactly where in space the metric camera was aimed. To facilitate the sideways view of the stellar camera, the entire mapping camera system was mounted on a track that allowed it to extend out of the service module bay.

The third part of the mapping camera system, the laser altimeter, determined the distance between the camera and the lunar surface to an accuracy of about one metre. This worked by sending extremely brief pulses of laser light to the surface along a line parallel to the metric camera’s axis. A detector then received the pulse reflected by the surface and accurately timed its return in order to obtain the distance. Altitude information was sent to Earth by telemetry and, when carried out simultaneously with a metric camera frame, was photographically coded onto the


Crater Alphonsus as photographed by Apollo 16’s mapping camera. (NASA)

film. Both the stellar camera and the laser altimeter could continue operating while over the Moon’s darkened hemisphere – that is, the stellar camera served to locate the terrain sampled by the laser pulses in the darkness.

Direct rendezvous

Not to be confused with the previously mentioned ‘direct ascent’ method, direct rendezvous (also known as short rendezvous) had the LM enter the terminal phase at the point where the CSI burn would normally occur. It relied on the confidence that had been gained in the spacecraft, radars and guidance systems over repeated flights. It also took advantage of the fact that although lift-off had to occur at exactly the right time, a missed launch would only require them to wait for the CSM to come around again on its next orbit.

Direct rendezvous began with a launch and insertion into orbit that was identical to the coelliptic method. As they rose on the ascent engine’s flame, both the crew and mission control analysed the numbers coming from the spacecraft’s two computers, checking that its performance was within the range expected. On insertion into lunar orbit, mission control could advise them of which computer, in their opinion, had measured the ascent more accurately. The crew then knew which numbers to watch as they fired the RCS thrusters to compensate for deviations in the ascent engine’s performance. If their ascent had not been sufficienty accurate to support this direct technique, the crew had the option to undertake the rendezvous using the longer but more forgiving coelliptic method.

Once established on their initial elliptic orbit, they had about 40 minutes until the TPI burn would start the approach to their crewmate. Like musicians in an orchestra, each playing their own instrument, all the players in the Apollo ensemble struck up a flurry of tracking activity: the LM crew on their radar, their guidance computer and their backup computer with its own instruments; the CMP on his VHF transponder, his sextant and another computer; and mission control with tracking stations around Earth chorusing on large computers in Houston. They all carefully, and repeatedly, measured the flights of two spacecraft hurtling around the Moon. Like a band rising to a perfectly harmonised chord, they each derived solutions for the upcoming TPI burn and compared them. If the commander could see that all the solutions, including his. were converging towards a common answer, then, with confidence that his own systems were working well, he would choose the solution generated by the PGNS. The TPI burn for a direct rendezvous was relatively large because it had to turn their 17-kilometre perilune into a 113-kilometre apolune, so it was made using the ascent engine. Any residual velocity that had to be made up could be achieved with the RCS afterwards.

On Apollo 15. Ed Mitchell let Falcons crew know that they should go ahead and burn their TPI manoeuvre. ”Falcon; Houston. You’re Go for an APS 1 PI. You have 180 feet [per second] available."

"Roger. Understand. Go for the APS TPI, thank you."

This w as a measure of how tight the propellant margins were with the APS. Prior to lift-off, Falcon’s tanks had sufficient propellant to change their speed by 2.130 metres per second overall. Mitchell w-as telling them that, as far as mission control could tell, only 2.5 per cent of that capability remained, w hich was enough for a 6.5- second burn. In the event, their TPI burn required only 2.6 seconds, and by using the ascent engine they saved wear and tear on the RCS thrusters that might be needed in case of problems prior to docking with the CSM.

As was typical for Apollo, it was not considered enough to have the PGNS. the AGS, a guy in the CSM and folk back on Earth all working to find a solution to the size and direction of the TPI burn. NASA’s mentality for such a critical operation as rendezvous was to give the crew options wherever possible, so for a fifth attempt at the answer, the LM crew7 carried a set of charts with which, if everything else failed, they could derive a solution for TPI and reach the CSM safely. Scott explained how they worked: "In simple terms you needed range, range-rate, angle and time. The equations allowed you to draw a curve on a chart which wras a nominal curve. At certain points, you would have a known range, range-rate and angle to the target. What you did on the charts was. at the specified time, to look at the range, range-rate and angle to the target and match that with

the nominal. If it didn’t match, you would change the range, range-rate or angle by cranking in a correction off another chart. You can lose communications with the ground; you can lose the PGNS and the AGS and still do the rendezvous because all you need is a watch and the COAS and the radar. A grease pencil on the window was fine too. The COAS goes out, you mark the window with a grease pencil. Works! That’s the beauty of the equations. They were elegant and just beautiful because you could rendezvous with just nothing. But you had to practice a lot and you had to get the feel of it because you knew just about where you were and it would compute TPI and you do the burn and you are on your way. Unless you purposely screwed it up. you’d gel there. I mean you had to make an effort to screw it up. It’s beautiful. That’s why we had all the confidence in this stuff. The confidence is based on the fact that it was set up right by these guys that wrote these very elegant equations that went into the computer – but they gave you a manual backup that you could do on a piece of paper.” So contrary to early fears, lunar rendezvous proved to be a straightforward procedure once the problem had been studied in depth and thoroughly practised.

The TPI burn usually occurred over the Moon’s far side, out of communication with Earth. As much of the subsequent approach was also out of sight of Earth, the crew’ relied on regular measurements by both spacecraft of their separation distance, their rate of closure and their angle with respect to each other. Solutions for possible mid-course corrections were compared and burned with the RCS jets. Their approach was cross-checked on charts, and the target viewed against the background of stars to check for any apparent movement.

Homeward activities

For the three days that the CSM fell to Earth, the crew performed the housekeeping chores required to keep a multimillion-dollar machine purring along safely. Lithium hydroxide canisters were changed regularly to remove carbon dioxide from the air, the fuel cells were purged to remove contaminants from their reactive surfaces, and the general-purpose batteries were recharged after busy periods.

Crews would often indulge in a little Moon and Earth photography to use up the spare film in their magazines there was. after all, no point returning it unexposed. However, for most of the journey, neither world was particularly photogenic unless very long lenses were used, which, except for 500-nnn lenses carried on Apollos 12 and 13, they did not possess. Because of this, the Moon tended to be well photographed as they departed, and due to the timing and geometry of the solar system when the flights occurred, it was usually nearing its full phase. Conversely, Earth often appeared as an increasingly thin crescent that, for some flights, led to the spacecraft entering and then exiting Earth’s shadow.


"We’re getting a spectacular view at eclipse,” said Dick Gordon as Apollo 12 approached Earth. "We’re using the Sun filter for the G&N optics, looking through, and it’s unbelievable.”

It was four hours before splashdown and he was astonished at the celestial spectacle that was unfolding through the hatch window’ as Yankee Clipper began to enter Earth’s shadow, and the planet’s limb gradually ate away at the Sun. To protect his eyes from the glare, he was using a strong filter normally used in the sextant.

"It’s noi quite a straight line." Gordon continued, "but it’s certainly a large, large disk right now. Looks quite a bit different than when you see the Moon eclipse the Sun.”

The timing of this event had been known well in advance and the flight plan had it marked, but no one, not even the crew’, had realised just what a feast for the eyes it would be. Now they were desperate to know what camera settings to use to try to capture the scene.

"Anybody down there know what we can set the camera at to use the Sun filter on it?’’ asked A1 Bean. "To take a couple of shots of this eclipse right through it?’’

"Stand by and w’e’ll check.” replied Paul Weitz. final Capcom for the mission.

"They’d better hustle,” said Bean, seeing how quickly things were changing. Still the glare of the Sun was drowning out the scene that was to unfold. "You cannot see the Earth at all when you just shield your hand from the Sun and look where the Earth should be. It’s not there at all."

Soon the accelerating spacecraft was moving into Earth’s shadow’. "Fantastic sight,” called Bean. "What we see now is that the Sun is almost completely eclipsed, and what it’s done is illuminated the entire atmosphere all the way around the Earth.” They were now about 60.000 kilometres from Earth and the planet was growing rapidly in their window, its limb aglow’ with the hues and tones of a 40.000- kilometre ring that comprised the largest single sunset ever witnessed by humans. "It really looks pretty. You can’t sec the Earth. It’s black, just like space.”

Pete Conrad took up the commentary: "You can’t see any features on it. All you can see is this sort of purple-blue, orange, some shades of violet, completely around the Earth. It has blues and pinks in it, but instead of being banded, it’s segmented, which is very peculiar; I don’t understand why. It may be the difference between over the landmasses and water or something.”

"Roger. Pete. Understand." replied Weitz.

"About a quarter of the Earth is pure blue,’’ Conrad clarified, "and then it becomes pink to about 20 degrees of arc; and then it turns back to blue again. And it’s blue all the way around the bottom to where it turns pink again, and then it turns blue again.”

Apollo 12’s unusual and spectacular perspective of the approaching Earth as it eclipses

the Sun. This frame is from 16-mrn movie coverage of the event. (NASA)

"It’s a heck of a time to be without any 70-millimetre colour film, I’ll tell you,” bemoaned Bean, referring to the Hasselblad film. "But I know how to get it on a 16- millimetre camera.” The magazines for their movie camera were the only remaining source of unexposed colour film. “It looks like this is going to have an illuminated atmosphere, probably the whole time it’s eclipsed. The Sun is set, but it’s so close to the limb that that bright light is being channelled through the atmosphere, and so if you look at it with a naked eye you can’t tell if the Sun is set yet. Through the smoked glass, you can see that it’s no longer a disk there, but you just see a bright white line the diameter of the Sun.”

Gordon was running out of words to describe the view: "This is really spectacular. Have you got any more adjectives for spectacular? I’d like to use some if you have.”

"No. We’ll put somebody to work on that, too,” replied Weitz in jest.

It is often said that when humans went to explore the Moon, what they really discovered was Earth. This was literally coming true for the crew of Apollo 12. Once the Sun was behind Earth, their eyes could adapt to the darkness and allow detail to become visible across the night-time hemisphere, only now it was illuminated by sunlight reflected from the full Moon.

"This has got to be the most spectacular sight of the whole flight.” Bean was also running short of adjectives. “Now that the Sun’s behind the Earth, we can see clouds on the dark part of the Earth; and. of course, the Earth’s still defined by this thin blue-and-red segmented band. It’s a little bit thicker down where the Sun just set than it is at the other one, but it is really a fantastic sight. The clouds appear sort of pinkish grey, and they’re scattered all the way around the Earth.”

Gordon began to see further detail on Earth’s dark face. "Say. Houston. It’s very interesting. We can see lightning and the thunderstorms down there on the Earth. You can see it quite clearly, flashing from wherever we are.”

"Yes. They look sort of just like fireflies down there blinking off and on.” added Bean. They were now’ less than 50.000 kilometres out and approaching the height of the geostationary communications satellites.

"We’re starting to look out for these synchronous satellites now’.” said Bean. "We’ve been looking ahead.”

"Sure hate to run into one up here.” added Conrad.

"Yes. It could ruin your day," agreed Weitz.

In fact, their trajectory was not in the plane of the equator and so there was no risk of a collision.

As Earth’s sunset lightshow continued, the crew fished out their monocular to get a closer look.

"We’re better night-adapted now’.” said Conrad, "and by golly, we can see India, and we can sec the Red Sea, and we can see the Indian Ocean quite clearly. It’s amazing how we can see. for that matter. We can see Burma and the clouds going around the coastline of Burma, and we can sec Africa and the Gulf of Aqaba. We can also distinguish the lights of large towns with our naked eye, just barely, and by using the monocular, we can confirm that that’s what w’e’re seeing.”

Conrad was getting into his stride.

"There’s a couple of ripdoozer thunderstorms down there that are really, really letting go. There seems to be a weather system out there, and it s got thunderstorms all the way along it. Venus is just below the Earth, and we can see Venus quite clearly. This is really a sight to behold, to see it at night-time like this. And looking at the airglow with the monocular is – Boy, there is another sight now that is not like being in Earth orbit whatsoever. It’s a bright red. next to the Earth, and then it’s got a green band in it, and then it’s got a blue band.”

"Would you say these colour bands encircle the Earth now’, Pete?” asked Weitz. "Yes,” replied Conrad. "But it’s not the same all the way around. What I’m seeing is sunrise, really. This is about 40 degrees from the Sun, and there’s a bright red band, and then a sort of a light green band that’s very thin, and then a blue one which must be all of the atmosphere."

The crew’ of Apollo 12 were deeply struck by what they had seen and made a point about it after the flight. "We all w’ere caught with our pants down.” said Conrad during the debriefing. "We should have had good camera settings and film available for that because it w’as certainly a spectacular sight.”

"I feel very strongly about this,” added Gordon. "I think that someone, the crew’ as much as anyone, really dropped the ball on this. We knew’ this was going to occur before flight and we mentioned it. It was a very poorly handled phenomenon w:e all knew about before the flight.”

The Apollo 15 crew witnessed a different type of eclipse during their return flight. It was not the spacecraft that entered Earth’s shadow, it was the Moon. This was a lunar eclipse and it was visible across half of the world. However, unlike those on Earth who were watching from within the cone of the shadow, the crew of Endeavour had the benefit of a side-on view of the entire spectacle. While they coasted to Earth, the Moon continued in its orbit such that two days into the coast, they were substantially to one side of the Moon-Earth line. When the Moon passed into the shadow’, they had a perspective on the event that has never been repeated.

When Earth’s shadow erosscs the Moon’s disk, viewers on Earth can sec the arc of our planet projected onto the lunar surface, which is classic proof that the w’orld is round, familiar even to the ancient Greeks. But from Endeavours position, well off to the side, the spherical shape of the Moon altered the apparent line of the Earth’s shadow’ such that, at the start of the eclipse, the curve of the shadow’ was more than cancelled out. Two hours later, when the Moon exited the shadow-, its shape reinforced the curve of the Earth and produced a very strong crescent effect.

Once the Moon had completely entered the umbra, it was no longer lit directly by the Sun. Ilow’ever, had Scott and Irw’in been standing at Hadley Base at this time (and thankfully, they w’eren’t), they could have looked up at the Earth and seen a similar awe-inspiring sight as their buddies had seen on Apollo 12 of 40,000 kilometres of sunset and sunrise all around the globe forming a ring of gold in the sky. Unfortunately, their rover’s TV camera had long since stopped working after a circuit breaker in its pow’er supply had opened in the heat of the lunar day. During that moment of eclipse, shared by half a world and the occupants of Endeavour, this golden ring turned the eclipsed Moon a dark, copper-brown colour.

Irw’in described w’hat he could see: "Right now’ the Moon varies from a very pale orange to a good deep burnt orange on one side and a very gradual change. It certainly is pretty.”

"Very good.” replied Karl Henize. himself an astronomer as w’ell as an astronaut. "It sounds like a beautiful view’ from up there. You’ve seen a lunar eclipse of the Moon twice as big as anyone else has ever seen such an eclipse.”

"That was very interesting.” said Irw’in. "It’d be a great place for somebody like you to come up and use your trained eye to interpret all this and understand it.”

"Sounds like it w’ould be fun. someday,” agreed Henize w’istfully.


A brief recap of the REFSMMAT might now be appropriate. The computer’s idea of the direction in which the spacecraft was pointed was always given with respect to the orientation of the stabilised platform inside the IMU. However, to make any sense, the platform itself had to be aligned to some known reference – one that was related to the universe around the spacecraft. The REFSMMAT numbers defined such an orientation in space to which the guidance platform could be aligned. Flight controllers could choose this orientation arbitrarily to suit the mission’s operational needs.

For re-entry, an orientation was chosen that would help the crew to make sense of their 8-ball displays, essentially turning them into artificial horizons that would show attitude relative to the ground below. It was based on the point of entry interface, whereby the v axis was aligned along the azimuth of their flight path but parallel to the horizontal plane. The г axis was parallel to a vertical line at the point of entry interface. In other words, at entry interface, the г axis would be pointing towards the Earth’s centre. By default, the у axis was aimed to the right of the flight path. The upshot of this arrangement was that at entry interface, if the heatshield was presented exactly forward and the crew heads-down, the FDAI display would read 0 degrees for roll, 180 degrees for pitch and 0 degrees for yaw. In this way, the display was much easier to interpret. Of course, for re-entry, the heatshield did not face exactly forward but was tilted towards Earth so that the FDAI would display 153*.

Returning to the entry PAD, the next two items were related to the first of many checks of their attitude and trajectory. This check would be made at 290 hours 06 minutes 32 seconds into the mission, 17 minutes prior to hitting the atmosphere. It did not require any fancy instru­ments. All that was required was for the crewman in the left couch, in this case CMP Ken Mattingly, to look out of the rendezvous window in front of him and see if Earth’s horizon aligned with a set of angle marks inscribed on the window. The spacecraft’s pitch angle at this time should be 267 degrees. A leeway of 5 degrees was allowed.

The next two items referred to the latitude and longitude of the planned point of splashdown.

Apollo 16 was targeted to land 00.71 degrees south of the equator
and 156.18 degrees west, a point about 2,200 kilometres south of Hawaii, near Christmas Island. In the event, it landed about 4.5 kilometres west of this point.

Even though the approach into the atmosphere was quite shallow, the crews had to sustain quite high deceleration forces as the CM rammed into the air. Most returning crews endured a peak force of about six g. The Apollo 16 crew were warned to expect a peak force of 06.9 g. Records show’ that their deceleration peaked at 7.19 g, the highest for any Apollo crew.

When the spacecraft reached entry interface, the arbitrary altitude where re-entry was said to begin, it was expected to be at the extraordinary velocity of 36,196 feet per second or slightly over 11 kilometres per second (11.033 kilometres per second to be exact). At this point. Retro believed the flight path would form an angle of 6.50 degrees to the horizontal which was considered ideal. Subsequent analysis showed that the flight path angle was actually 6.55 degrees, well within the 1-degree tolerance allowed for re-entry.

The spacecraft was expected to travel 1,045.8 nautical miles from the time the guidance system sensed 0.05-g switch until landing on the ocean, a distance of nearly

2.0 kilometres.

Another velocity value of 36,276 represented, in feet per second, how fast they believed the spacecraft w’ould be travelling when the 0.05-g point was reached. As well as going into the computer, this number and the previous item went into the EMS, which allowed them to choose to monitor either velocity to be lost or distance still to travel. Note that the value given here was slightly higher than that quoted for entry interface. This demonstrated that mission control expected the spacecraft to gather yet more speed between the arbitrary point of entry interface and the estimated 0.05-g event.

Mission control expected the spacecraft to reach entry interface at a time of 290:23:32 since launch. It was then expected that 27 seconds would elapse between entry interface and the 0.05-g event being sensed. The actual time would depend on the local atmospheric conditions.

The next item concerned a computer entry, Noun 69, that was not applicable, or. Y A to this PAD. As entry commenced, the computer ran Program 64 and its display was set to show’ the contents of Noun 74. This had three numbers derived from the primary guidance system which told the crew: (a) how much drag they were experiencing (i. e. the g-force to which re-entry was subjecting them), (b) their current inertial velocity, not taking into account the rotation of the Earth, and (c) the angle their flight path made with the horizon. If the entry profile required the spacecraft to skip out of the atmosphere for a time, the computer was programmed to move to Program 65, whereupon the display w’ould show Noun 69. which again displayed drag and velocity, this Lime relating to the skip-out flight path. The PAD form had spaces in which mission control could advise the crew of the expected maximum and minimum values for drag and velocity for this phase of the entry, but on Apollo 16 these w’ere not needed. Of all the Apollos that returned from the Moon, only Apollo 11 had to deal with this skip-out condition and that was to extend their flight path and avoid a storm.

During part of the entry, the spacecraft was flown in a trajectory that

maintained a constant, g-force. In all cases, this was set at 4.00 g. As the spacecraft continued to slow, there came a point when its velocity was equal to that of an orbiting object. In other words, on reaching this speed, the spacecraft no longer had enough momentum to return to space on a long-duration elliptical orbit that would have threatened the crew. Also, since the spacecraft would be in the atmosphere already at this point, drag would continue to reduce its velocity, and a landing somewhere on Earth was assured. The next item in the PAD informed the crew that they were expected to reach this safe milestone 2 minutes 2 seconds after entry interface.

The extreme heat generated by the shock of compression ionised the air and created a sheath of plasma that effectively blocked radio signals to and from the spacecraft. This period, known as blackout, was expected to begin at 00:16 or 16 seconds after entry interface and continue until 3 minutes 33 seconds after entry interface.

Once the spacecraft had shed most of its speed, the Earth landing system (ELS) would convert it from a 6-tonne lump falling through the atmosphere to a gently descending vehicle returning explorers from the Moon. The first part of this system was a pair of drogue chutes which the Apollo 16 crew could expect to be deployed 7 minutes 43 seconds after entry interface.

The Apollo 16 entry PAD continued with six further items that were all concerned with two methods of ensuring that the command module’s attitude was correct just prior to re-entry. The first of these would use the sextant to aim at a particular star. The second, not used on Apollo 16, was a similar thing using the COAS mounted in a window. Apollo 16’s sextant star check said that two minutes prior to entry interface, the crew should expect to see sextant star 2.5, Acrux – which is the major star in the southern constellation Crux – through the eyepiece when their sextant was aimed to a shaft angle of 151.5 degrees and a trunnion angle of 26.2 degrees.

The final entry on the form used for the PAD told the crew that the lift force generated by the shape of the hurtling spacecraft would be ‘up’ at least at entry interface. This was another way of saying that they should be in a ‘fcci-up, hcads – down’ attitude as they approach the atmosphere. This information was passed on to the computer by entering a ‘1′ in the right place in its memory.

A number of additional comments were read to the crew at the end of the PAD. The first concerned the scroll pattern to be used to monitor the entry on the EMS. The appropriate pattern was one that was calibrated for the type of non-exiting re­entry they planned to make. In other words, since there was no intention to exit the atmosphere on a long, skipping re-entry, they should line up the scroll pattern at the start of the non-exit EMS pattern.

Next were three Limes, stated with respect to entry interface, for important milestones in the latter part of the flight. The first {RET for 90K) was for the command module’s descent to an altitude of 90,000 feet, or 27.4 kilometres, which was expected 6 minutes 6 seconds after entry interface. The spacecraft’s tltree main parachutes (RET mains) were expected to be deployed 8 minutes 29 seconds after entry interface while the entire re-entry, from entry interface to splashdown {RET landing) on the Pacific Ocean, was expected to take 13 minutes 21 seconds.

During the constant-g entry phase of the flight, the spacecraft had to roll to aim the lift force it generated up or down to maintain a constant deceleration. Mission control included a note to say that if the crew needed to do this manually, perhaps through equipment failure, they should roll right.

As the crew returned from their lunar exploration, the view out of their windows afforded them a view’ of the Moon setting behind Earth’s horizon. The precise time it did so w-as calculated to the second and mission control informed the crew they could expect Moonset at 290:20:26 – that is. at 290 hours 20 minutes 26 seconds into the mission. This item was more of a confidence booster. If Moonset didn’t occur on time, there was nothing they could do about it. The CM was on its own and had no propulsion to correct any error in their trajectory prior to entry.

The final item concerned a detail of the re-entry in ease the crew had to fly the profile manually. Manual re-entry w’as flown by following the cues provided by the EMS (EMS entry) with which the CMP controlled the g-forces as the spacecraft’s velocity fell. Mission control’s note was to reverse their angle of bank as their velocity passed 20.000 feet per second.

After their flights, crews were often asked to talk about the non-scicntifie aspects of flying to the Moon; the meaning, symbolism and the raw human emotion behind it. Typical replies have said that they were too busy for such things. Some have suggested that for such answers. NASA should have sent artists and poets. A few crewmen have pointed out that had artists and poets comprised the crews, they would never have returned alive. The sheer degree of detail involved in flying the Apollo spacecraft explains why NASA preferred highly proficient pilots as astronauts.


The crew released their restraints and began to power down the spacecraft while the recovery forces swung into action. A typical recovery force had five helicopters deployed from a US Navy aircraft carrier that was stationed at the spacecraft’s aim point. As spacecraft crews and their support teams gradually improved their re-entry guidance and landing became more accurate, recovery planners began to worry that the CM might make a hard landing on the carrier itsellj so for later missions, the carrier stationed itself a few kilometres to one side of the aim point. Each of the five helicopters had a specific task in the recovery. One was intended to photograph and later televise the splashdown from close quarters. Another had little more to do than be a radio relay between the CM, the helicopters and the ship. Two further helicopters had frogmen on board whose task was to drop into the sea and attach a flotation collar around the spacecraft that would both stabilise it and provide a platform for the exiting crewmen. They also recovered the detached parachutes if possible. The fifth helicopter carried a ‘Billy Pugh’ rescue basket with which it would pluck the three crewmen, one by one, off a life raft next to the spacecraft and into the helicopter to be taken to the ship.

Inside the spacecraft, the crew charged a gas-powered counterbalance for the main hatch in preparation for opening. After the Apollo 1 fire, the two-piece inward-opening hatch of the Bloek-I spacecraft was replaced with a single unified

Jack Schmitt exits Apollo 17, helped by a Navy swimmer. (NASA)

hatch that could quickly be opened outwards. A problem with this arrangement was that while sitting upright on Earth, the heavy door had to move upwards against gravity as well as outwards by a crewman weakened by a long period in space and so an ingenious counterbalance arrangement was added that was powered by compressed air bottles.

Mascons: a lumpy Moon

Over a one-year period between 1966 and 1967. five Lunar Orbiicr spacecraft were dispatched to the Moon to carry out comprehensive photo-reconnaissance of its surface largely in support of the Apollo programme. It was mostly through this programme that engineers gained the skills necessary to accurately track an object around the Moon and control its flight path. As they did so. they were surprised to discover that, unlike orbits around Hanh where the gravity field is nearly uniform, the orbits of these spacecraft were being perturbed by regions of higher density in the Moon’s crust that often seemed to be associated with the circular maria. It appeared that events in the Moon’s past had bequeathed it with a gravity field which, to scientists of the late L960s. seemed unexpectedly uneven. Subsequent analysis suggested that the majority of these mass concentrations, or ‘mascons’ as they came to be known, are due to denser mantle material having been brought nearer the surface by the impacts that formed the basins, and, to a lesser extent, by the layers of dense basalt that were extruded onto the surface to fill these basins and form the maria. This ability to determine a body’s sub-structure by analysis of a spacecraft’s flight path would be built upon over succeeding decades as spacecraft were sent to orbit around other bodies in the solar system. It became a powerful technique that would reveal much about their large-scale structures.

Mascons complicated the mission planning process for Apollo because of their profound effects on lunar orbits. Since they are largely on the near side, they accelerate spacecraft slightly when compared to the far side. Over a remarkably short time, and with some help from perturbations due to Earth’s gravity, lunar orbits are modified by being lowered on the near side and raised on the far side until, if no intervention occurs, they cause the spacecraft to impact the lunar surface. As later high – precision gravity maps of the Moon would reveal. Apollos 15 and 17 happened to fly over two of the most intense mascons – those associated with Marc Sercniiatis and Mare Imbrium.

On Apollo 1 5, FIDO compensated for the effects of the mascons by targeting the pcrilunc of the descent orbit a little high in the expectation that while the crew slept, it would drop to about L5 kilometres, the preferred altitude from wilich the LM should begin its descent to the surface. This strategy had worked well on Apollo 14, which flew around the lunar equator, but because the intensity of the Imbrium and Serenitatis mascons had not been appreciated, the perilune of the Apollo 15 orbit descended below 15 kilometres while the crew were asleep. The monitoring flight controllers reckoned that by the time Scott and Irwin were ready to begin their final descent, their perilune would be down to around 10 kilometres, which was much lower than had been trained for. However, with their usual foresight, the planners had inserted a possible adjustment to the orbit into the flight plan in case this happened, and CMP A1 Worden made a small trim manoeuvre to raise the perilune. On Apollo 16. no trim was required. Its equatorial flight path took it away from the strongest mascons and lessened the gradual drop of its perilune which, in any case. FIDO had targeted even higher for the DOI manoeuvre.

For Apollo 17. the lessons from Apollo 15 were applied, and the DOI manoeuvre was split into two parts to deal with the influence of the Imbrium and Screnitaiis mascons. As with the previous flights, the CSM America burned an initial DOI manoeuvre. This took their perilune down to a safe 26.5 kilometres which, overnight, dropped to 22 kilometres. After the separation of the LM Challenger, the two spacecraft went behind the Moon for the last lime before landing. While out of sight of Earth. America returned to its 110-kilometre circular orbit, and Challenger burned the second DOI manoeuvre that dropped its perilune right down to 13 kilometres. Since they only had half an orbit to go before their final descent to the surface, there would not be enough time for mascons to perturb their trajectory.

Constantly learning from their operational experience, flight controllers and planners had become pretty savvy by the end of the Apollo programme. These were important lessons that would be applied to space operations as a whole across succeeding generations of space exploration.