Category How Apollo Flew to the Moon

All the power required by the LM for the duration of its separate mission came from batteries. There were four mounted in the descent stage, two in the ascent stage and a fifth battery was added in the descent stage for the J-missions solely to extend the crew’s stay Lime on the surface. Known as the ’lunar battery, it was not used in flight. For the duration of the powered descent, all six major batteries were linked in parallel to power the spacecraft. The batteries for the ascent stage were included in order to power the LM in the event of an abort. Now. on the surface, fewer systems required power so the crew took the ascent batteries offline and reduced the number of online descent batteries. This was when the lunar battery, if present, was brought into use to share the load with combinations of the other four.

Many of the LM’s electronics were designed to run on AC power so a couple of inverters were provided. These electronic devices convert direct current to alternating current but unlike the 50-Н/ or 60-Н/ supplies that most people arc aware of in their homes, these ran at 400 Hz. Control of all these power systems was via two electrical control assemblies (fCAs), one each on the commander’s and LMP’s side of the cabin. Little talkback indicators showed the status of the ECAs to confirm that their switch settings had worked. For the next few’ minutes, the crew w’orked their w’ay through a checklist that set their systems either to a mode suitable for the surface, or pow’ered dowm altogether.

Other tasks at this time included dealing with the multitude of items, large and small, that had to be unstowed and prepared for the first EVA. From back packs to cameras and film magazines, even a brush to keep their lenses clean, they had to be placed in bags that would be taken to the surface or readied for when suiting up would begin.

Among the most remarkable items to be mounted on the rover were those associated with communication, at the centre of which was the lunar communications relay unit (LCRU). This went on the very front of the vehicle between a mast for the TV camera and another for an umbrella-type dish antenna. These turned what was already a uniquely capable vehicle into the most outlandish television outside broadcast unit ever seen.

The LCRU had a number of important functions. It acted as a relay for voice communication between the two crewmen on the surface and Houston, and it passed biomedical telemetry to the Surgeon. It sent telemetry about the state of the astronauts’ back packs and itself to the flight controllers. But its most visible role was to send TV pictures to Earth. It also allowed a flight controller in Houston to remotely operate the camera while the crew got on with their work. An S-band radio link with Earth was provided by either the high-gain dish if TV was to be sent, or a low-gain antenna if only voice and telemetry were required. When the rover halted at a geology station, one of the crew had to manually aim the high-gain antenna before television could be received on Earth and for this, the antenna included a simple sighting arrangement that could be used by a fully suited crewman.

The INCO flight controller in mission control had the job of operating the camera in response to requests by others in the mission control team. In this way, many eyes in Houston could watch what the two crewmen were doing, and it enabled scientists in the mission support room to build up panoramic views of each such site and look
around for interesting rocks for the astronauts to inspect.

As he prepared the rover for their first traverse, David Scott noticed that the newly installed camera was following his every move. "Gee, you’re watching me flounder around out here.” He later recounted the moment he and frwin realised they were not really alone on the Moon.

"You’re suddenly aware of the third person. I remember that, at that moment, I realised for the first time that we were being watched by everybody behind that lens! It was almost like looking through the lens into the control room.”

Although the TV camera used on the rover was among the best flown on Apollo, engineers continued to improve the final picture quality throughout the J-missions. Apollo 15’s camera had no lens hood and as soon as Scott began to drive, significant dust was kicked up by the wheels onto the lens. Then, as INCO panned around their first geology stop, he found that when the Sun caught even a small amount of dust on the lens surface, the scattered light greatly degraded image quality. Though the crew brushed it regularly, even a small amount of residual dust scattered sunlight effectively and affected the picture which required the dust to be brushed off the lens every time they arrived at a new station. A more effective solution had to wait for Apollo 16 with the addition of a lens hood to exclude the Sun whenever possible. For the last two missions of the Apollo programme, further improvements were made by having the TV signals linked to California where a proprietary system enhanced the images before they were returned to Houston for distribution.

The cooling mechanism for the LCRU was similar to that for the batteries in that it used radiators, with a dust cover provided for when the rover was moving. But one interesting difference was that the LCRU contained a quantity of wax that absorbed large amounts of heat as it melted from solid to liquid form. When the covers came off, the heat radiated away and the wax solidified, ready for the next cycle.

Had the Moon been a smooth, featureless body with no variations in its composition

or surface structure, then the expected heating and cooling of its surface would be simple to predict. The temperature of any object in space that does not have its own heat source is a balance between the heat it absorbs from the Sun and any other sources, and the heat it radiates into space. These properties are strongly affected by the thermal conductivity of the surface, its structure, its reflectivity, or albedo and the angle of illumination. Under a vertical Sun, surface temperatures can reach well over 100°C while just before lunar dawn after the 2-week-long night they can fall as low as -180°C. In the permanently shadowed craters at the lunar poles, it can be -230°C. Since different materials in their various forms will heat and cool in different ways, important clues to its nature can be gained by studying the detailed temperature of the lunar surface.

Apollo 17’s infrared scanning radiometer was an early attempt to measure the Moon’s thennal profile by having an infrared sensor pass over the landscape, both night and day, as the spacecraft orbited overhead. The concept is similar to the thermal pictures taken of houses in cold climates to show where warmth from the building is being lost, except that Apollo’s sensor was only a single point, like a one – pixel camera. There was no multi-line or multi-pixel imaging sensor to create a ‘picture’, and images had to be processed from the results of the spot scanning a site over multiple passes, one line per orbit.

The instrument showed how varying rock types within craters could strongly affect the temperature profile of a landscape. For example, at night time, the central area of the crater Kepler proved to be over 30°C warmer than the surrounding mare, perhaps due to exposed rocks at the bottom of the bowl absorbing heat from the daytime and slowly radiating it at night while the surrounding dust chilled quickly. It was hoped that hot spots might be found over the night-time hemisphere indicating a source of volcanism, but none was found.

This radiometer was a forerunner of a later generation of instruments that have provided thermal images of many of the solar system’s worlds. Notable among such instruments was the thermal emission spectrograph which could analyse infrared light to deduce rock types. These were used at Mars, both in orbit and on the surface, to locate rocks that implied a history of running water on the red planet. Apollo was paving the way, but with a much cruder technology.

At first glance, this episode after Apollo 15’s docking might appear to be a comedy of errors by both the crew and MOCR. yet it indicates how. in an environment that is extremely unforgiving, a safe and successful outcome was achieved. Scott
recognised that the separation burn was unsafe (probably because of the extra revolution around the Moon), brought it to the MOCR’s attention and proceeded to carry on, trusting the people on the ground to assess the situation correctly.

This illustrates the close relationship between a crew and mission control. The people in Houston had a very high visibility into the spacecraft, its systems and its trajectory by virtue of telemetry, available computing power and the knowledge and experience of the entire team in the control centre. The crew had a high situational awareness by virtue of having their eyes and ears in situ so to speak. The two sides then worked together to fulfil the mission’s objectives. This was an extension of the aviation model where the pilot in command of an aircraft works with air traffic control to ensure safe travel in what is a very unforgiving medium. In a sense, both are in collaborative control, linked in their common purpose by the air/ground communications loop. Rarely do the two get out of phase and when they do, it is usually down to the quality of communication on this loop.

Gerry Griffin, one of Apollo 15’s flight directors, and later director of the Johnson Space Center, made this very point. "In aviation, pilots don’t control what goes on in the airspace, they control their aircraft by a set of rules and by following instruc­tions from the various control facilities who also operate under certain rules. For sure, the aircraft commander can take any action he or she deems necessary to safely operate the aircraft, including disobeying an instruction from an air traffic control centre, approach control, or a control tower. As soon as the aircraft commander takes that overriding step, he or she will have a lot of explaining to do when they get on the ground, and if they can’t convince the powers-that-be that they took the proper course of action given the conditions, they won’t be flying anymore, or at least, they won’t be flying for a long while. It is no different in the American manned space flight environment. Like the aviation analogy, the commander in Mercury-Gemini-Apollo controlled the spacecraft and could take any step he felt necessary to operate the spacecraft safely and to finish the task at hand in accordance with the flight plan and mission rules. Those of us in the mission control centre [MCC] understood that fully and agreed with it. But when any out-of-the-ordinary situation reached a ‘safe harbour’ or stopping point, the commander or crew was expected to (and always did) work closely with the MCC to proceed on with the mission. Often the ‘next steps’ were ‘directive’ in nature and emanated from MCC.”

One of the important differences from the air traffic analogy is the visibility

mission control has into so many of the spacecraft’s systems. Air traffic control looks at the movements of many aircraft all at once but cannot diagnose an impending technical problem in a single aircraft. Apollo’s mission control, on the other hand, while having no other spacecraft to worry about, could see far more detail about its health than the crew could, and could bring much more brainpower to any situation. Throughout an Apollo flight, there were many times when a decision from mission control was vital to the progression of the mission, typified by the ‘Go/no-Go’ call which became a media catchphrase of the MOCR, for example, before ТІЛ, LOI and the final decision to land on the Moon (PDI), as Griffin explained:

"The MCC had to make sure the ground-based systems and the systems the crew couldn’t see (for example, the S-IVB before TLI, or the command module before PDI) were ready. Then, we unequivocally told them that we were Go, or no-Go. If the MCC and the crew were Go, the crew was expected to carry out the ‘next step’. Of course, similar to the aviation analogy, the commander or crew could hold off perfonning one of those milestone events even after we both had agreed it was Go if they didn’t like something they saw. If they were right, so be it – good catch – but they had better be right.”

The Mission Control Center evolved during the 1960s through the Mercury and Gemini programmes, beginning with Chris Kraft as the model for the flight director. He defined this role, and also that of mission control and the crew. Around him, the very best engineers and specialists were brought together, some in the MOCR, others in outlying rooms and buildings or at contractors’ premises, able to coordinate and run a normal mission, and to react to and troubleshoot an anomalous one.

During Apollo, international TV coverage of the astronauts at their work was often interspersed with a wide-angle shot of the MOCR during periods when no pictures were available from the spacecraft. Images of serious-looking people in shirt and tie, seated at high-tech consoles where screens flickered and lights blinked, became part of

the public mythology of American spaceflight. Л generation later, the imagery of astronauts and mission control turns up in such movies as Contact, Deep Impact and. of necessity, in Apollo 13. But, as is often the case, this is an incomplete image. Gerry Griffin advised on the making of some of these movies and even took a cameo role as a flight controller. But he believes this public imagery needs to be put into perspective.

“The astronauts didn’t run the Apollo programme, neither did the flight controllers, and neither of the two were totally responsible for the success of Apollo. The Apollo programme was run by a very capable bunch of guys-on-the-ground who managed the funding for. and the building of. the flight and ground hardware. These same guys also got the agency enough money to hire the best people in the world at NASA and its contractors to do the work including astronauts and flight controllers. The MCC included all of these guys-on-the-ground, not just the flight controllers from the Flight Control Division at [the Manned Spacecraft Center, later renamed as the Johnson Space Center]. When a decision was made by the entire team on the ground it was always discussed with the crew for their input, adjusted if necessary, then implemented. While the astronauts and flight controllers got most of the visibility in Apollo, we both were actually very small, albeit very important, parts of the programme.”

The most important lesson to take from how Apollo managed to operate so well, was that it represented the epitome of teamwork. The commander and his crew have the situational awareness at the sharp end of the operation, while the personnel at mission control have a far wider knowledge of the context within which the flight is flown. David Scott commented on this in later years:

“Mission control doesn’t have any control over the spacecraft, so it’s “mission advisory’. It’s like air traffic control; they control the airspace but they don’t control the airplane. They advise the airplane and the pilot is accountable for all of his actions and that’s basically the way the system works. Who’s in command and who’s in control and who’s advisory – it’s a team kind of thing. Everybody has to work. You have to balance the situational awareness with the advice from MCC because they have much more data to look at so their advice is invaluable to the situational decision. But it’s not a command. Some people in management and MCC would consider it a command, but that’s OK. The commander, in a situation, sometimes has to override the words that he gets from MCC in order to complete the objectives for w’hich he’s responsible."

Having crews that were mostly derived from military and test flying affected the melding of the crew/controller relationship. These w’ere people who w’ere used to fulfilling orders and getting the job done in association with controllers, yet able to cope at the sharp end of aircraft command in sometimes difficult situations, as Scott explained: “Another mindset of those of us who were flying during those days was that wc had a lot of flight experience alone in airplanes where we had to make decisions. If you don’t have that, then you probably have a more open mindset to MCC’s instructions or advice. In other words, if you haven’t been in these situations where you got bad advice or had to make decisions on your own in flight, then you would rely more totally on MCC because you don’t have this experience of needing to do your own decisions on all the data.”

We will leave it to Gerry Griffin to sum up why NASA’s mission control worked so well: “Simply stated, it was the best flight operations team ever assembled. It was built on respect, trust and teamwork. The flight crews and flight controllers were a tight bunch who trusted each other to do the right thing. The only time it didn’t work, and work extremely well, was Apollo 7; and even at that, the mission turned out to be a huge success.”

Although the Apollo spacecraft was a truly amazing machine for its time, it was. in many ways, extremely marginal. Only by the very careful husbanding of all the on­board consumables could a mission be completed. There was little room for error or excess. However, it was sometimes difficult to measure how much of a resource was being consumed. Hydrogen and oxygen were relatively easy. The tanks had sensors to directly measure quantity, the flow of these two elements to the fuel cells was w ell understood and the pow er produced w as related to their consumption. If power was being produced as expected, then the amount of w’ater available for cooling, food and drinking would be well known. Oxygen flow – for the cabin air supply w as also directly measured.

The propellant for the RCS thrusters presented a greater problem and illustrated some of the indirect techniques used by flight controllers to understand how much remained – for like so many systems on Apollo, the functioning of these little engines was crucial to the correct execution of the flight. Propellant quantity was not directly measured because this is always difficult to do in weightlessness.

Instead, controllers used every form of data available to them to make estimations of usage. The tanks contained the fluids within bladders to ensure that only liquid was expelled to the thrusters. As propellant was consumed, the space between the bladder and the tank walls w’as filled with helium that applied pressure for expulsion, and as this volume increased, the helium pressure reduced. Also, every thruster firing was telemetered to Earth to enable the usage for each quad cluster to be summed. However, techniques such as these gradually accrued errors winch became greatest towards the end of the mission, just when controllers wanted to know the quantities remaining most accurately.

Controllers made use of the fact that each quad cluster had a primary and a secondary pair of tanks – four tanks in all – and that the secondary pair constituted

39.5 per cent of the total available. Therefore, by switching across to the smaller tanks at some point, they got a data point which they knew’ was a precise value for how much propellant remained available to that quad. In order to avoid running the primary tanks empty, this ‘crossover’ was carried out when the total remaining had been calculated to have reached 43 per cent. It provided an accurate measure to work from in continuing to plot the quantities for the remainder of the mission.

Timing their tasks up to and beyond re-entry was important to help the crew’ to coordinate their progress through an increasingly busy checklist. To this end, they set their digital event timer to count up to, and beyond the moment of entry interface.

Their next task was to prepare the EMS by setting it to the starting conditions for re-entry, as read up in the PAD. The CMP moved the scroll to the start of the relevant monitor pattern, aligning the scribe to the velocity that would be expected when 0.05-g was sensed. The total distance for re-entry was entered into the digital display at the bottom of the panel and the dial of the roll indicator was aligned to ensure that its reference matched the GDC and that it would accurately show whether they were orientated feet-up or feet-down.

Then attitude control was transferred to the CM and the thrusters on both rings of the RCS were operated for a briefly to verify their operation. Once complete, attitude control was returned to the SM thrusters.

With just 25 minutes remaining to entry interface, the crew began to prepare the service module for jettison. A valve was closed to cut off the supply of oxygen from the SM tanks which left the CM reliant on the contents of a small tank mounted in its periphery. One of the three fuel cells was shut down to force the batteries to take more of the load and so help to warm them up. ‘I’he service module’s radio systems were switched off. Circuit breakers were pulled to remove power from a number of heaters that kept the radiators, the dump nozzles and the potable water tank warm. Once all these small tasks had been completed, a check was made to ensure that the spacecraft was still in the correct attitude.

Use of the computer was quite intensive during re-entry, especially since, if all was working wrell. it would be the computer that would fly the spacecraft all the way to deployment of the parachutes. This began with Program 61 (P61) which started re­entry navigation by measuring the acceleration acting on the spacecraft. Ii also accepted relevant information from the PAD to allow subsequent programs to control the re-entry. This included their planned impact latitude and longitude and whether they would be entering heads up or down, information that went into Noun 61; their maximum deceleration, their velocity and flight path angle at entry interface went into Noun 60. They then checked the contents of Noun 63, which held their range from the 0.05-g point to the landing site, their velocity at the 0.05-у event and the total duration of re-entry.

When they were happy with the numbers to this point, they pressed ‘Proceed’ and the computer moved to P62 which handled the jettisoning of the service module and placed the command module in the correct attitude for re-eniry. First the CMP had to carry out a horizon check at 17 minutes to go. All he had to do was look out to see where Earth’s horizon appeared with respect to a series of angle markings on the edge of the rendezvous window. It was expected that it would be coincident with a line draw’ll at the 31.7-degree mark. However, because the line of sight was dependent on exactly where the CMP placed his head, a tolerance of 5 degrees was allowed. If the horizon was outside these limits, the rules said that they should assume the G&N to be faulty and steer the ship manually.

Apollo 12 CMP Dick Gordon couldn’t even see the horizon, as he related after the mission: "It was dark and I never was certain that there was a horizon out there.” he told his debriefers. He was not concerned as they had already made so many attitude checks. "It really didn’t make any difference. We had already checked the alignment. We were satisfied with the IMU. We had a boresight star. We had a sextant star check. We knew where we were, and the DAP [digital autopilot] was working properly. We were confident the w:hole time, and I didn’t care whether I made that check or not.”

His commander Pete Conrad pointed out the inconsistency in this approach. ‘We ought to change the rule, because we actually violated the rule."

"Well, we actually picked up the horizon check later on during the entry.” pointed out Gordon. In fact, there was no real necessity for the check to be done 17 minutes prior to entry as the checklist included a graph. "You’ve got the chart of the Harth horizon angles versus time from entry interface and you can check that any time prior to entry interface. It’s a nothing cheek, and you can either do it or not do it. I couldn’t care less."

Continuing to the bottom of the LOI PAD form. Capeom Karl Hcni/c read, "25, 2671, 228; the rest is NA.” This refers to six lines on the form that were concerned with two methods of double checking to ensure that the spacecraft was in the proper attitude for the burn. On this PAD for Apollo 15, only the first was brought into play; it exploited the fact that the spacecraft’s sextant could be aimed precisely at the stars. If the spacecraft had been placed in the correct attitude for the burn, then flight controllers had calculated that a particular star should be visible through the sextant when its shaft and trunnion angles were set to specific values. In this case, the crew’ were to expect Star 25, which is commonly known as Acrux, in the constellation Crux, to be visible in the sextant when the shaft and trunnion angles were preset to 267.1 and 22.8 degrees respectively.

Ilenize indicated that nothing else need be entered on the form by pronouncing it as LNA’ for ‘not applicable’. What was skipped was the boresight star method. This used the crewman optical alignment sight (COAS) – a unit with an illuminated graticule similar to a gunsight that could be mounted in a window and whose aim could be calibrated. It was not required for LOI because the windows would be facing the Moon or the LM.

As many checks as possible were made to the LM within the time available before it was allowed to fly free and continue to the surface. While these were being carried out. the CMP mounted cameras in brackets to monitor the departure of the lander through the CM windows. A television camera would then allow live pictures to be sent to Houston and the world, although on Apollo 11, the opportunity was passed by. A 16-mm movie film of the event would also be taken using the lightweight Maurer cameras carried on all Apollo missions.

These 16-mm cameras have provided much of the best motion documentation of the Apollo flights, yet their official NASA designation implied that history and posterity w7ere far from the thinking behind their inclusion on the trip. NASA called them data acquisition cameras (DACs) and used them in precisely that way – to gather data on the performance of its vehicles. For example, one of the most familiar Apollo scenes is a short shot of a lunar rover being driven around the Apollo 16 landing site at Descartes, w7ith John Young at its controls and a great rooster-tail of dirt rising from its wheels. This wonderful film was shot only because engineers wanted to see how the rover performed in its designed

environment. Since the TV camera was mounted on the rover itself, it was unable to provide such coverage and in any case, a moving rover could not maintain the aim of its dish antenna.

With a DAC mounted in a command module window, the first moments of the lunar module’s flight were recorded, and for Apollo 11, this included a pirouette to allow the landing gear to be inspected. This DAC would also film the LM’s ascent stage as it returned from the Moon. Another DAC mounted in the right-hand lunar module window would film the approaching landscape during the descent, and again as the ascent stage rose from the surface.

The DACs have played their part in distorting the historical record, not in terms of image, but of time. NASA instructed the crews to shoot at frame rates that were often much slower than that normally used for live action. Conventionally, movie film is shot at 24 frames per second and subsequently projected or transferred to TV at about the same rate. The Maurer cameras that were used on Apollo could also shoot at one, six or 12 frames per second, and much of what was shot during the missions used these slower rates to conserve film. When replayed on conventional equipment at the higher frame rate, the recorded events would be portrayed at twice, four times or even 24 times their actual speed. Eventually those recordings found their way into TV and film documentaries and influenced the public’s notion of the Apollo spacecraft. It was only with the advent of advanced video processing technology in later years that careful researchers were able to restore a true sense of the speed of the Apollo films. It was also common to keep a window clear by mounting a camera to one side and have it look through a small mirror, thereby

reversing the shot and further misleading future interpreters of the imagery. In the long run, while the television coverage suffered from the degradation inherent with the technology of the time, the movies shot by the DACs have stood as a clear, high – quality record of the achievements of Apollo.

"Ignition.” announced Armstrong to Aldrin as Apollo 1 l’s LM Eagle began the human species’ first descent to another world.

“Ignition,” repeated Aldrin. “Ten per cent.”

On all the missions, at PDI the DPS engine was initially run at only 10 per cent of its maximum thrust for 26 seconds to give the computer enough time to sense whether the engine’s thrust was acting through the LM’s centre of mass, and if it was not. to move its supporting gimbals until it was. This ability to vector the thrust was not intended to steer the craft. It was too slow for that. Steering was provided by the RCS thrusters which altered the attitude of the entire craft to aim the thrust, leaving the engine’s gimbals to deal with longer-term centre-of-mass shifts.

Aldrin counted up to the end of the low-thrust phase. “24, 25. 26. Throttle up. Looks good!” Propellants poured into the engine as it went to its high-thrust setting.

Starting with Apollo 12, engineers added a modification to the computer’s programming to achieve pinpoint landings. As soon as Intrepid came around the Moon’s limb prior to landing, its velocity was compared with what would be ideal to achieve a pinpoint landing. From this, engineers could calculate the difference between where they wanted to land and where the computer, which believed it was on the right course and unaware of external factors which had perturbed its path, was actually taking them.

“Intrepid, Houston,” called Capcom Gerry Carr only 80 seconds into Apollo 12’s powered descent. “Noun 69, plus 04200. Over.”

“Roger. Copy. Plus 04200," confirmed Bean.

This was the important call that ensured that the LM would land where it was
supposed to, and it was extremely dangerous. Noun 69 held three values that represented an update to the position of the landing site in three dimensions. Changing one of those values by +4,200 feet (1,280 metres) shifted the computer’s idea of where they should land to a point further downrange, thereby fooling it into taking them where they wanted to go. When the crew had punched the number into the DSKY’s register, mission control took a look at the telemetry to verify they had done so correctly before confirming that they could ‘enter’ it into memory. Had the crew inadvertently entered the wrong data, they could easily have sent the LM out of control and been obliged to abort.

“Intrepid, Houston. Go for Enter,” said Carr once he had received word from other flight controllers that the crew had typed the update into the correct field.

“It’s in, babe,” said Bean.

“Intrepid, Houston. Looking good at two,” replied Carr as they passed the 2-minute mark into the burn.

Throughout P63’s regime, the DPS engine had to fire more or less into the direction of travel, especially during the initial minutes. As long as it did so, the LM could make rotational manoeuvres around the engine’s axis. On Apollo 11, the first few minutes of powered descent were flown with the windows, and there­fore the crew, facing towards the surface. Armstrong had a method of using the angle markings on his window to time the passing of land­marks below. Before ignition, it had given him a check of what their perilune altitude was going to be. This used the fact that the closer you orbit a body, the faster the landscape below appears to pass by. Then after the commencement of powered des­cent, he could compare the absolute time a landmark passed with a predicted time. Since they were travelling at about 1.5 kilometres a second, only a few seconds early or late signalled the extent of any miss. It was a simple but powerful techni­que.

“Looking good to us,” Capcom

Charlie Duke informed Apollo 11. "You’re still looking good at three. Coming up. three minutes."

"Okay, we went by the three-minute point early." said Armstrong. “We’re long." He w’as right, because they landed six kilometres further down-range from where they had planned.

Conrad dispensed with the idea of having the windows looking down at the start of PD1 on Apollo 12 since they had other techniques in the wings to determine their approach errors. As soon as they entered the descent orbit over the far side, he placed the LM into the correct windows-up attitude for PDI. As this was an inertial altitude, set with respect to the stars, it was not concerned with the position of the Moon, so a windows-up, engine-first attitude over the near side of the Moon was a windows-down, engine-trailing attitude over the far side, as Pete Conrad explained: “From that inertial attitude, we watched ourselves pass from face down, through local horizontal [i. e. feet down, facing forward], to pitch up at PDI. It gave us an excellent look at the Moon going around.’’ It also gave their steerable antenna a clear view1 of Earth from AOS right through to landing.

As mission control worked towards a final decision to stay, and as the crew got on with their tasks, there was usually some discussion about how close to their target point they had managed to land. Armstrong had known long before touchdowm that he and Aldrin w’ere going to land w’ell past their intended destination. Shortly after arrival at Tranquillity Base, he raised the question with Houston. "The guys that said that we wouldn’t be able to tell precisely where we are. are the winners today.” It was no great surprise, given the hair-raising nature of their descent. "We were a little busy worrying about program alarms and things like that in the part of the descent w’here w’e w’ould normally be picking out our landing spot.” he continued, "and aside from a good look at several of the craters w? e came over in the final descent, I haven’t been able to pick out the things on the horizon as a reference as yet.”

‘;No sw’eat,” reassured Charlie Duke in mission control. "We’ll figure it out.”

It took a long Lime for anyone to figure it out. Throughout the time he spent in orbit alone, Mike Collins never once managed to view’ Eagle through the sextant, which was hardly surprising, given that it had landed six kilometres from its intended site and the plain of Mare Tranquillitatis is a featureless wasteland of crater imposed

upon crater. The best estimate was made by the geological team at mission control who noted how Armstrong had described flying over a large blocky crater to a "fairly level plain with a large number of craters”.

The factors that led to the inaccuracy of the Apollo 11 landing were overcome to guide Pete Conrad to a pinpoint landing on Apollo 12. He knew where he was, but had trouble telling mission control because he was reading the map incorrectly. Four hours later, as Dick Gordon coasted overhead in Yankee Clipper, he had his eye firmly affixed to the sextant eyepiece.

"Houston, I have Snowman.” The familiar pattern of craters stood out when the computer was asked to point the optics at the intended landing site. At first, he confused Intrepid with the Surveyor 3 spacecraft that had arrived 31 months earlier. Then it clicked.

"I have Intrepid. I have Intrepid,” said Gordon excitedly. "He’s on the Surveyor Crater; he’s about a quarter of a Surveyor Crater diameter to the northwest.”

"Roger, Clipper. Well done,” replied Capcom Ed Gibson. Surveyor Crater formed the torso of the Snowman and was where the eponymous spacecraft had been located.

"I’ll tell you, he’s the only thing that casts a shadow down there.”

If the planned walking traverses to geologically interesting sites were to be fulfilled, an accurate landing was mandatory for Apollos 12 and 14. Just 600 metres off could make a destination unreachable. Alan Shepard brought Antares down within a mere 50 meters of his target, the best of the programme, but for the J – missions, only the commander’s bragging rights were in jeopardy by such a miss. Having a rover rendered such inaccuracies moot.