Category How Apollo Flew to the Moon

The lunar module carried two important radar systems that were tested prior to landing. The first checkout was for the rendezvous radar while the CSM was still nearby. This radar worked in conjunction with a transponder on the CSM to give the crew’ and the LM computer information about how far away the CSM was, how fast it was approaching and in what direction it was located. Although there were backup methods for the spacecraft to rendezvous, this radar was an important primary component for bringing the two spacecraft together. Its dish antenna wras attached to a 2-axis mount that permitted pan and tilt movement. When it started operating, it sw ept the view in front of the LM, looking for the CSM until a return signal from the transponder was found. The receiving horn was split into four so that if the dish were not exactly borcsighied on the CSM, the received signal would be stronger in one of the horns. The electronics could then operate to aim the antenna until all four horns received an equal strength signal. The angle of the dish then represented the direction to the CSM. The information from the radar wras factored in along with knowledge of the LM’s state vector and orbit to derive all the necessary information needed by the crew to make appropriate rendezvous manoeuvres.

Whereas the first three landings had been on open, if rugged sites, the approach taken by Falcon on Apollo 15 took the LM between a pair of mountains. This made the experience of landing somewhat different, especially during P64’s regime.

“Falcon, Houston,” said Ed Mitchell in mission control. “We expect you may be a little south of the site; 3,000 feet.” By that, he meant that their flight path, travelling

east to west, seemed to be taking them to a point one kilometre south of their intended target. When they started P64, Scott would have to steer to the right to get them back on track.

“Okay. Coming up on 8,000,” said Irwin as they passed 2,500 metres altitude. Even before P64, he had begun to concentrate on keeping his commander up to date. This was one of two events that biased Scott’s estimation of where they were going to land. The other concerned what he and Irwin saw out to the left prior to P64. “I looked out the window, and I could see Mount Hadley Delta. We seemed to be floating across Hadley Delta and my impression at the time was that we were way long because I could see the mountain out the window and we were still probably

10,0 to 11,000 feet high.” Scott then approached pitchover thinking he was going to land long and south, which was worrying because several kilometres to the west of the intended landing point was Hadley Rille, and he didn’t want to come down in its canyon. Actually, they were at about 2,750 metres, and the 3,350-metre mountain was towering 600 metres above them.

In later years, Irwin discussed the moment of pitchover. “We’re not looking down as we come over the mountains,” he chuckled. “We’re looking straight up until we get down to around 6,000 feet [1,800 metres] and we pitch forward about 30 degrees and, at that point, we could look forward and see where we were. We could see the mountains. 1 was startled because, out the window, 1 could see Mount Hadley Delta which towered about 6.000 or 7,000 feet above us. And we never had that type of presentation in the simulator." Landing simulations had used a small TV camera flying over a plaster model of the surface to present the crew with the view they would get out of their window’s. It included the impressive rille that would be in front of them, but not the mountains to either side of the approach track.

"When we pitched over." continued Irw’in, "I could see the mountain that tow’ered above us out Dave’s window. I’m sure it startled Dave, too, because we wanted to know, you know, were we coming in to the right place? Fortunately, the rille was there and it was such a beautiful landmark that we knew’ we were coming in to the right area. But we’d never had that side view in any of our simulations. It was just the front view, a level plain with the canyon. And it would have been very impressive to be able to look out as we were skimming over the mountains with about 6,000-foot terrain clearance. At that speed it would have been really spectacular, like a low-level pass as we came over the mountains down into the valley.’’

"7.000 feet. P64!" called Irwin as they passed 2.150 metres altitude.

"Okay." said Scott.

"We have LPD." said Irwin as an angle appeared on the DSKY.

Scott finally saw’ wiiere he was going. Ilis impression of landing long had been wrong, but mission control’s southerly estimation was correct. "LPD. Coming right," he said as he began a series of redesignations to move the computer’s targeting north to where they had planned to touch down.

To deal with ihc mountainous terrain around the Hadley landing site, planners had made a change to the approach phase of the descent. Instead of making a shallow approach of only about a 12-dcgree angle to the ground as the previous missions had done. Falcon came in on a much steeper 25-degree approach path.

"Four-zero." called Irwin as he began feeding Scott with constant updates of the LPD angle on the window and their altitude.

"5.000 feet. 39. 39. 38. 39."

"4.000 feet. 40. 41. 45. 47. 52."

"3.000 feet. 52. 52. 51. 50. 47. 47."

"2.000 feet. 42."

"Okay. 1 got a good spot." said Scott once he had decided where he was going to set it down.

LUNA COGNITA

The Moon – that inconstant orb; a glorious bright light in our night sky; an ancient vehicle for human myths and deities; and now a world become known. For thousands of years before the rise of the scientific method, humans gazed at Earth’s one natural satellite, wondered at its nature and worked it into their stories as they struggled to understand their universe and its impact upon them. It was only with the invention of the telescope that the true nature of our satellite world began to be revealed.

Luna’s face

When casually viewed from Earth, the Moon exhibits a mottling of dark grey patches against a lighter grey landscape. What was not realised until Apollo’s rocks were returned was that these features are a window into the earliest era of the solar system. We now know that the light grey areas, rough and heavily cratered, form an extremely ancient highland terrain that dates beyond four billion years ago. The dark patches, called mare (pronounced hnaa-ray’, plural maria), are great plains of basalt that solidified from immense effusions of the Moon’s particularly runny lava that flowed more than three billion years ago. In many cases, these outpourings of molten rock filled large circular basins that had been excavated some time previously by cataclysmic impacts. Peppered across its face are bright sprays of material that emanate from some of the craters. These are rays of ejecta – shocked and pulverised rock thrown out from more recent high-speed collisions by somewhat smaller bodies. Given immense time, these rays will fade and darken to match their surroundings.

Apollo 13 saw a serious push to use the CSM as an orbiting science platform from which to reconnoitre the lunar surface. The primary tool for this was the Hycon lunar topographic camera: a monster instrument modified from an aerial reconnaissance camera, whose 467-millimetre-focal-length lens peered out through the round hatch window and exposed square 114-millimetre negatives. The Hycon had an unhappy career in Apollo. It lay unused on board the Apollo 13 command module while the crew struggled to reach home after their mission was aborted. It was sent once more on Apollo 14. Once alone in his domain, and having made the circularisation burn to take the Apollo 14 CSM Kitty Hawk into a 110-kilometre orbit, Stu Roosa began a photographic pass that was to have included the Descartes region where scientists were considering sending a future mission. After about 200 exposures, the camera failed, never to work again despite the best troubleshooting efforts of Roosa and mission control.

The SI. VI bay

Lunar science from orbit really got into its stride with the J-missions. just as surface exploration had. In particular, one of the six sectors in the cylindrical service module which had been largely empty on previous missions, gained a bay of instruments and cameras that could be trained on the lunar surface for the five or so days that the CSM spent in orbit. This scientific instrument module, or SIM bay, was operated by the CMP from the time the spacecraft entered orbit until the service module was jettisoned shortly before re-entry.

Each example of the SIM bay that flew carried two cameras: a mapping camera and a panoramic camera, both of which were heavily derived from aerial and space reconnaissance cameras that were classified at the time. Each bay also included a suite of sensors, some of which were deployed out on the ends of retractable booms. These could divine the mineral composition of the lunar soil by sampling its various emissions. On Apollos 15 and 16. a tiny satellite was ejected just before the crew left to come home. It monitored the particles and fields mound the Moon for up to a year.

The full capabilities of the SIM bay were never brought to bear on the whole lunar surface, largely because the CSM’s orbit was defined by the position of the landing site. This limited the reach of the cameras and sensors to a narrow swathe near the equator. At one time, planners had envisioned an I-mission that would have placed a CSM in a polar orbit for a full month, from which its cameras could have imaged the entire surface at a consistent lighting angle, and its remote-sensing instruments could similarly have sampled the entire Moon. With Apollo in its declining years, funds for such a mission were not forthcoming and the scientists had to wait a generation for comprehensive coverage from advanced probes sent by a number of interested nations.

For the early Apollo missions, NASA settled on the coelliptic rendezvous to carefully reunite the two spacecraft. To best grasp the various elements of this technique, it may help to work backwards from the rendezvous itself and see how its demands determined the orbital dance around the Moon that led up to it.

NASA standardised how it would fly the final part of the rendezvous – what it termed the terminal phase – to control the approach speed of the LM. They wanted to limit the speed to a value that could be dealt with by the RCS thrusters of either spacecraft, ft is worth remembering that by this point in its mission the LM was a very light spacecraft and its thrusters were very responsive. The CSM, on the other hand, was heavy and still had a substantial load of propellant on board. Its thusters were much less effective, should it be called upon to fulfil the rendezvous. Another issue was illumination. Theoretical studies and Gemini experience had demon­strated that the terminal phase should be flown over 130 degrees of the CSM’s orbit, with appropriate approach speeds set throughout this period to maintain control of the situation. Therefore, planners could choose where in the CSM’s orbit they wanted the rendezvous to occur, and with lighting taken into account, work back 130 degrees to define the point where the terminal phase ought to begin. This point would be where the LM would execute the terminal phase initiation (TPI) burn. One huge advantage of this approach was that as the LM rose to meet the CSM. the latter would appear to be stationary against the background stars. In one sense, the two craft would be on intersecting orbits hurtling around the Moon at nearly 6,000 kilometres per hour. But in another, inertial sense that ignored the Moon below, they would be approaching along a straight line that could be drawn out to the stars. This arrangement would allow the crew7 to visually check their progress. Because of the need to be able to see the stars clearly, most of the terminal phase was arranged to occur in the Moon’s shadow7, but the final approach would be made in sunlight for improved visibility. Additionally, opportunities were included during the approach for course corrections, based on data from their radar.

Continuing to work backwards, planners arranged for the LM to spend about 40 minutes in an orbit that was a constant 28 kilometres below7 the CSM’s orbit. An important point to note is that this constant difference in height had to be maintained even if the CSM’s orbit was elliptical. NASA used the term constant delta height (CDII) for this part of the rendezvous trajectory, and as the LM crew7 had to make a burn to shape their orbit to meet this condition, the manoeuvre was obviously known as the CDII burn. The purpose of this part of the flight was to give the crew time to track the CSM and calculate the burn that would be needed at the start of the terminal phase of the rendezvous. If the orbits of both spacecraft leading up to the CDII burn were nearly circular and the errors were small, then it was possible to dispense with this manoeuvre.

The trajectory leading up to the CDII burn was essentially a circular orbit of 84 kilometres altitude which was entered by the coelliptie sequence initiation (CSI) burn. This burn w-as made half an orbit back from where the CDH burn would occur.

As we continue to work backwards, the only section that remains is the time from launch to the CSI burn. Around the time that the CSM passed over the landing site, the LM ascent stage lifted off from the discarded descent stage. The ascent engine burned for about seven minutes, ideally to achieve an orbit with a perilune of 17 kilometres and an apolune of 84 kilometres. Half an orbit after insertion, the spacecraft had coasted to its apolune. at which point the CSI burn would circularise the orbit and the coelliptie rendezvous would begin. Except for the initial ascent, all the burns would be made with the RCS thrusters.

To summarise this sequence chronologically:

• After launch, the LM entered a 17 by 84-kilometre orbit.

• It coasted for half an orbit until it reached apolune. The crew then made the CSI burn to circularise the orbit and then began to track the CSM.

• After another half orbit, they made the CDH burn, if required, to reshape the orbit and have the LM fly 28 kilometres below the CSM’s orbit.

• During a 40-minute coast, further tracking determined the details of the burn that would begin the terminal phase.

• The TPI burn placed the LM on an intercept trajectory that was, at least for a short period, essentially a transfer orbit. It raised the orbit’s apolune slightly higher than the CSM’s altitude so that it would intercept its target over 130 degrees of orbital travel.

As the LM rose to meet the CSM during the terminal phase, the rendezvous radar on the LM continually worked with the transponder on the CSM to determine their separation and their rate of closure. The crew monitored these values using the PGNS and the AGS, searching for any hint that they might be deviating from their preferred trajectory – which was a straight line in terms of inertial space, this being implied by the fact that they held the target fixed against the stars. There were two opportunities for mid-course corrections, then the commander made a series of braking burns using his thrusters to bring their closing speed to zero as the LM pulled up alongside the CSM, thereby never reaching the notional apolune of the orbit of which the start of the terminal phase was a short arc.

On Apollo 12, Pete Conrad, like all the other Apollo commanders, did most of the actual flying as he monitored Intrepids return to orbit by watching the PGNS and making burns based on its results. Meantime, to his right, A1 Bean was never really given the chance to pilot anything, which was normal on an Apollo mission. Despite being the lunar module pilot, his role was more that of a flight cngincer/co – pilot, although in extreme situations he could take over using the controls that were provided at his station. His chief task was to operate the AGS in case this backup guidance system had to be brought in to control the spacecraft. Like the PGNS. it generated numbers that reflected its determination of their trajectory, which he compared to the answers coming from the primary system. At any point in the rendezvous, usually after they completed an important step, the LMP could update the AGS’s knowledge of where they w’ere, with that from the PGNS assuming, that is, that everyone w-as happy with the performance of the PGNS. The AGS w’ould then continue to determine its independent trajectory from that point onwards.

As a test in Apollo 12. Bean w’as to try and operate the AGS entirely separately from the PGNS to see if it were possible. Unfortunately, a misskey on the run up to the CSI burn meant that he had to make one update from the PGNS after which he resumed the test. Bean found it to be quite exhausting: “After CSI, we realigned the AGS to the PGNS. Then I made all the AGS marks after that just as we’d planned to do, and got solutions that all compared very favourably. This show’s that the AGS w’ould do the job, would get solutions, w’hich we. of course, suspected anyhow-. But the whole point is that you dona want to use the AGS as the normal rendezvous mode. It requires that every two or three minutes, you make a lot of entries in the AGS. It requires that you point the spacecraft exactly at the command module, which takes time and effort. The LMP is working continually and isn’t able to sit back and think through exactly what’s going on in the rest of the spacecraft.”

Bean wished his role in operating the AGS could have been less manual. Of all the LM crewmen, this man. who w-as to become an accomplished artist, perhaps deserved more than others a little time to absorb the experience. During his debrief, he related how Conrad had been sensitive to his needs: "I continued to work to input the data into the AGS until the second mid-course [in the terminal phase] when Pete said, Tley. why don’t you quit working and sit back and enjoy the flight?’ I got to thinking about it later and that was the first Lime I’d really looked out to see what was going on. The rest of the time I’d just been w’orking my fanny off trying to get all those marks into the AGS. and that’s not the way you want to fly a spacecraft.”

Years later, he told how Conrad had offered him the controls over the far side of the Moon. Out of earshot of mission control. Bean experienced how: the light spacecraft, with its main tanks nearly empty, responded keenly to every impulse from the thrusters. At last, a lunar module pilot had been allowed to pilot a lunar module.

The step-by-step coelliptic rendezvous took nearly two orbits to fulfil but it gave crew’s plenty of Lime to Lake optical and radar measurements of the angle and distance to their quarry, to evaluate their progress and to calculate burns that minimised the risk of errors placing them into a dangerous orbit. It also permitted greater flexibility if the CSM had to perform a rescue. For this possibility, and as a

backup, the CMP in the passive CSM was kept busy making his own determinations of their orbit in permanent readiness for a LM abort.

The coelliptic rendezvous used for Apollos 10, 11 and 12 took nearly four hours to execute, which was a significant amount of time in a spacecraft whose total working life was itself measured in mere hours. When the advanced missions added a six-hour moonwalk to the last surface day, such a rendezvous would keep the crew awake for nearly 24 hours. In the hiatus imposed by the Apollo 13 incident, Scott and Irwin experimented in the simulators to see whether they could shorten the rendezvous, and the arrangement that they devised, which removed an entire orbit lasting two hours, was successfully demonstrated by the Apollo 14 mission.

The coast back from the Moon could be something of an anticlimax, particularly during the early Moon flights. The main purpose of the mission had been achieved, most of the danger had been successfully negotiated, and if the crew and mission control could keep the CSM working well, a safe return was likely. This was a chance for the crew to rest a little, and an opportunity to reflect on their successes and, perhaps, some of the problems they had encountered. There would often be a TV show or two beamed to the masses, and an interplanetary press conference for the world’s journalists. But everyone involved knew that danger could lie in the unguarded moment and at no time did the flight controllers drop their attention, even as the crew slept.

ft would be a mistake to think that nothing happened on the way home, although duties were certainly much lighter. There was no lunar module to take up the surface crew‘s time and some of the housekeeping duties around the command module could be shared among all three crewmembers. Some flights were lucky enough to witness interesting astronomical events during their return; others had various small science and technology experiments that made use of the very rare and expensive time during which NASA had people in space. The J-missions. in particular, had a heavier workload during their coast home because they had a bay full of science instruments in the service module, and while there was no Moon nearby for them to sense and sniff, they could be used for a little pathfinding astronomy.

As was done before all major manoeuvres performed during the flight, and with over four hours remaining to the landing. Capcom read up a PAD to the crew – a large list of numbers and notes that, in this case, defined the parameters of re-entry for the crew and the computer. This list included cheeks to be made of their attitude; and the times, angles and velocities to be expected at various points along their trajectory. Much of the information was to be fed into the computer and the EMS so that this equipment could be properly initialised prior to entry.

In April 1972, rookie astronaut and Capcom Henry Ilartsfield made a call to Casper, the returning CSM of Apollo 16. to pass up a preliminary version of their re-

entry details. They were only 4.5 hours away from splashdown yet they were still 63,000 kilometres out. “Apollo 16, Houston. Have an entry PAD for you.”

“Okay. Go ahead with the PAD,” replied mission commander John Young.

Hartsfield then launched into the long, monotonous, yet precise string of digits and comments that would bring the crew safely to Earth. “Okay, MidPac; 000 153 000 2900632 267; minus 0071, minus 15618…” As with previous PADs, there was no punctuation. Confu­sion was only avoided by the pro forma sheet onto which the informa­tion was copied, along with the crew’s experienced expectation of what each number was likely to be.

Hartsfield continued: “069 36196 650 10458 36276 2902332 0027 Noun 69 is N/A; 400 0202 0016 0333 0743; sextant star 25 1515 262; boresight N/A; lift vector UP. Use non-exit EMS; RET for 90K, 0606; RET mains, 0829; RET landing, 1321; constant-g entry, roll right; Moon – set, 2902026; EMS entry, reverse bank angle at 20,000 feet per sec­

spacecraft to the atmosphere in the attitude it would naturally adopt with its biased centre of mass in their current heads-down attitude.

After a journey lasting up to 13 days, and having taken men further from Earth than they would go for at least another two generations, an Apollo lunar mission ended with a hefty thump on the surface of the ocean. With luck, the spacecraft would catch the tip of the descending swell which would soften its impact. Not so for the crew of Apollo 12. Luck went against them, and especially against the skull of LMP A1 Bean when rough waves and bad timing created a very hard impact, as Conrad explained afterwards: “We really hit flatter than a pancake, and it was a tremendous impact. Much greater than anything I’d experienced in Gemini. The 16-millimetre camera, which was on the bracket, whistled off and clanked A1 on the head to the tune of six stitches. It cold-cocked him, which is why we were in Stable II.”

In the water, the command module had two stable modes of floating – right-way up, known as ‘Stable Г; or upside-down, or ‘Stable IT, which left the crew hanging uncomfortably in their straps facing downwards. One of Bean’s tasks was to close two circuit breakers mounted by his side to let power reach the pyrotechnic circuits, so that when Conrad threw a switch, guillotines would cut the main parachutes free from the spacecraft. However, as Bean had been temporarily

Apollo 12’s CM in its ‘Stable ІГ attitude soon after landing. (NASA)

knocked unconscious by the dislodged camera, the cutters were not fired and the capsule was pulled over.

"He was out to lunch for about five seconds,” continued Conrad. "Dick was hollering for him to punch in the breakers, and in the meantime, I’d seen this thing whistle off out of the corner of my eye and [Bean] was blankly staring at the instrument panel. I was convinced he was dead over there in the right seat, but he wasn’t, and finally got the breakers in. By that time, we’d gone Stable II, which was no big deal.”

Apollo 8’s CM also ended upside-down. When it hit in the dark, Borman got drenched with a few litres of sea water. "The one item that we were perhaps not expecting was the impact at touchdown,” he explained afterwards. "There was a severe jolt and we got water in through the cabin repress valves even though they were closed. A good deal of water came in the cabin pressure relief valve.” Distracted by the torrent of water that had entered the cabin, he did not release the chutes before they pulled the spacecraft over.

With the parachutes cut free, the SECS pyro system was safed for the last time. Beneath where the parachutes were packed, three flotation bags had been installed which the crew inflated with stored gas to upright an inverted spacecraft. Even if the spacecraft was already floating upright, the bags were inflated in case a freak wave should flip it over.

There seemed to be an evens chance that the CM would end up in the ‘Stable IT position. For Apollo 16, it took a bit longer to get it upright. "It may have taken us four or five minutes to upright,” explained Young. "The centre bag apparently didn’t fully inflate. It’s supposed to be the one that inflates first. But the other two bags were certainly inflated. It uprighted just like normal.”

“I felt a solid jolt.1′ was Collins’s recollection of the Apollo 11 impact. “It was a lot harder than 1 expected.’’ Aldrin had tried to be ready to close the circuit breakers to allow Armstrong to release the chutes as quickly as possible, but the force of the impact foiled him. "It pitched me forward with a little bit of sideways rotation.” he said. "I [had] my fingers quite close to the circuit breaker. The checklist fell, and the pen or pencil, whatever I had. dropped. It didn’t seem as though there was any way of keeping my fingers on the circuit breakers.’’

Once they had the spacecraft upright and stable, a dye was released into the water if required, to aid search and rescue, and the post-landing vent (PLV) could be opened to let fresh air in. Mindful of NASA’s need to keep supposed lunar bugs at bay, Collins, while somewhat sceptical of the fear, tried his best not to leave the vent open. "The big item for us was that we not contaminate the world by leaving the post-landing vent open. We had that underlined and circled in our procedures to close that vent valve prior to popping the circuit breakers on panel 250. I’d like to say for the following crews that they pay attention to that in their training. If you cut the power on panel 250 before you get the vent valve closed, in theory, the whole world gets contaminated, and everybody is mad at you.”

Dick Cordon pointed out how this vent gave the Apollo 12 crew some problems, given that they were floating on heavy seas. "The procedures say. of course, to open the PLV duets. With that rough water out there, when we did, we just took water in through the intakes and that fan just blew it into the spacecraft. After a while, we got tired of getting wet so we just turned the PLV duct off. We just turned it off, and then when we got real warm again, I turned it back on just to let some more air in."

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

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.

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

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.