Armstrong armed the descent propulsion system (DPS) and Aldrin depressed the PROCEED key on the DSKY. As the thrusters provided ullage to settle the fluids in their tanks, Puddy had intended to accurately measure the propellant quantities, but the telemetry was inadequate and he had to resort to subtracting from the initial load the amount estimated to have been used during the DOI burn, which introduced an unfortunate uncertainty into his prediction of the total firing time available to the engine.
“Ignition,” announced Armstrong when the computer decided that Eagle was at the PDI point. “10 percent.”
“Just about on time,” noted Aldrin.
It was just after 3 pm in Houston. Frustrated with the television commentary, Jan Armstrong had retired to her bedroom to listen to the powered descent on the squawk box, with Bill Anders joining her to provide technical exposition. Prior to launch, she had impressed on Slayton that if there was a problem she wanted her squawk box feed to continue. She did not want a repeat of Gemini 8, which Neil had commanded, when her audio had been cut as soon as it was realised the ship was in trouble and, even worse, on going to Mission Control to find out what was going on she had been refused entry.
Columbia was 120 nautical miles behind and above, but it would catch up and by the nominal landing time would be 200 nautical miles farther west, and near to or below the local horizon. Collins’s role was to monitor the link between Eagle and Houston, and be prepared to act should intervention prove necessary.
The mood in the Mission Operations Control Room was intense concentration, and great expectation. The main wall screen showed a plot of the nominal powered descent profile, with a travelling symbol tracing Eagle’s progress. Bales noted that at ignition the radial velocity component had been off by 20 feet per second. Being more than halfway towards the ‘abort limit’, this was concerning. But he reasoned that if it was a navigational issue the magnitude of the error would remain constant because it reflected a failure of the initial conditions, whereas if it was a guidance issue the error would probably increase; time would tell.
When the computer throttled up to 100 per cent 26 seconds into the burn, there was a silent high-frequency vibration and the astronauts’ feet settled onto the floor, leaving them in no doubt that they had a good engine.
“PGNS is holding,’’ Aldrin confirmed for Armstrong, being heard by Houston because he was on VOX.
The 10-degree yaw had helped communications, but because the spacecraft’s attitude was fixed with respect to the surface as it travelled westwards, the line of sight to Earth was changing and the antenna was again being blocked. “Columbia, Houston,’’ Duke called. “We’ve lost them. Tell them to go aft omni.’’ On receiving Collins’s relay, Aldrin opted to override the automatic pointing. He selected Slew mode and specified the pitch and yaw pointing angles appropriate to this phase of the descent profile. The signal improved.
“Eagle, we’ve got you now,’’ Duke called.
“Rate of descent looks good,’’ said Aldrin, speaking to Armstrong.
“Everything’s looking good here,’’ Duke said, by way of an advisory. Noting that Aldrin had the steerable antenna in Slew, Duke passed up a recommendation for how it should be pointed after Eagle had yawed ‘windows up’.
“PGNS good? AGS good?’’ Armstrong prompted Aldrin.
‘‘AGS and PGNS agree very closely,’’ Aldrin confirmed.
The AGS was operating passively, ready for use in the event of PGNS failure. Although (as its name suggests) the AGS was for aborts, if the PGNS were to fail so close to the surface that an abort was deemed risky, the AGS would be used to continue the descent in order to land and then perform an emergency liftoff under more controlled conditions.
‘‘How are you looking, Guidance?’’ Kranz prompted.
The residual in the radial velocity had remained constant, indicating it to be the result of a simple navigational error. ‘‘The residual is still 20 foot per second,’’ Bales replied. ‘‘It looks good.’’
‘‘No change, is what you’re saying?’’ Kranz asked.
‘‘No change,’’ Bales confirmed. ‘‘That’s down track, I know it.’’ The PGNS was aiming for where it thought the target was; the fact that it had no way of knowing it was off course meant that it would land slightly downrange.
‘‘Rog,’’ acknowledged Kranz.
Armstrong confirmed to Houston, ‘‘RCS is good. No flags. DPS pressure is good. Two minutes.’’
‘‘Altitude’s a little high,’’ warned Aldrin. They were about 47,000 feet.
Having re-established tracking by the Manned Space Flight Network following a brief hiatus, Greene said, ‘‘Flight, FIDO. We’ve reinitialised our filters, and we do have an altitude difference.’’
‘‘Rog,’’ acknowledged Kranz.
Since the post-DOI ranging test, the rendezvous radar had been in Auto Track mode.
“Want to get rid of this radar?” suggested Armstrong.
“Yeah,” agreed Aldrin.
“Slew,” Aldrin confirmed.
This item on the checklist was a carry-over from the Apollo 10 mission, on which, because the plan had been to abort at the PDI point, the rendezvous radar had been set to continuously update the computer with the position of the CSM. At this point in Eagle’s descent, however, this data was not only unnecessary, it would soon prove to be a distraction to the computer.
Aldrin noticed a fluctuation in the alternating current voltage. The concern was that the landing radar would need a stable AC power supply. However, there was no fluctuation in the telemetry and the problem was concluded to be an issue with the onboard meter.
“You’re still looking good at 3 minutes,’’ Duke advised.
“Control,” Kranz called. “Let me know when he starts his yaw manoeuvre.’’
“Roger,” acknowledged Carlton.
“How’s the MSFN looking now, FIDO?’’ Kranz asked.
“We’re Go,’’ Greene replied.
“How about you, Guidance?’’
“It’s holding at about 18 feet per second,’’ Bales replied, referring to the radial velocity residual. “We’re going to make it, I think.’’
“Rog,’’ Kranz acknowledged.
On making his final downrange position check, Armstrong observed that they flew over Maskelyne-W fully 2 seconds early. At their current horizontal speed of 1 nautical mile per second, this meant that they were significantly ‘long’. Because the landmark checks at 3 minutes and 1 minute in advance of PDI had been on time, he was puzzled. At PDI, his attention had been inside, checking the performance of the engine, and he had not noticed precisely where they were at that moment. With the vehicle yawed 10 degrees to improve the line of sight of the high-gain antenna, it was difficult to estimate the crossrange error. The target ellipse was 11 nautical miles long and 3 nautical miles wide, with its major axis oriented along the direction of flight. Although they would land beyond the centre of the ellipse, he was certain they were ‘in the ball park’. ‘‘We went by the 3-minute point early,’’ he told Aldrin.
Aldrin was continuing to check their trajectory. One minute earlier they were slightly high, but the guidance system was steering towards the nominal trajectory. ‘‘The rate of descent is looking real good. Altitude is right about on.’’
Armstrong told Houston of their overshoot. ‘‘Our position checks show us to be a little ‘long’.’’
‘‘He thinks he’s a little bit ‘long’,’’ Duke informed Kranz.
‘‘We confirm that,’’ Bales pointed out.
‘‘Rog,’’ Kranz acknowledged. Knowing the western end of the ellipse was rougher terrain than the target, Kranz mused that Armstrong might have difficulty in finding a spot on which to land, and this, in turn, alerted Kranz to the likelihood that the hovering phase of the descent might prove to be protracted.
Having begun the powered descent ‘windows down’ for landmark checking, Eagle now had to rotate around the thrust axis in order that the landing radar at the rear of the underside of the vehicle would face the surface. “Now watch that signal strength, because it’s going to drop,” Armstrong warned Aldrin as he initiated the yaw. With the steerable antenna in Slew mode, Aldrin would have to manually adjust it as the vehicle turned.
“Okay all flight controllers, I’m going to go around the horn,’’ Kranz called as the 4-minute mark approached.
“We’re yawing, Flight,’’ Carlton informed Kranz, as requested.
“Boy, I tell you, this is hard to do,’’ Armstrong observed.
“Keep it going,’’ urged Aldrin.
Owing to the fact that Armstrong had neglected the checklist item to select a rapid rate of yaw, the manoeuvre began sluggishly and was erratic. Realising his error, he correctly set the Rate Switch and restarted the manoeuvre at the planned rate of 5 degrees per second. The torque from the disturbed propellants sloshing in the tanks not only made the yaw ragged, but also induced rates in the other axes, which caused much more thruster activity than he had expected. Despite Aldrin’s attempt to keep the steerable antenna pointing at Earth during the turn, communications became intermittent. Kranz told his team to make their recommendations based on their most recent data, but when telemetry was restored before he could begin his poll he allowed them another few seconds.
Finally, Kranz took his poll, “Retro?’’
“Go!’’ replied Deiterich.
“Go!’’ replied Greene.
“Go!’’ replied Bales.
“Go!’’ replied Carlton.
“Go!’’ replied Puddy.
“Go!’’ replied Willoughby.
“Go!’’ replied Aaron.
“Go!’’ replied Zieglschmid.
“CapCom we’re Go to continue PDI,’’ Kranz announced.
Duke relayed the advisory, “Eagle, Houston, you are Go to continue powered descent.’’
“Roger,’’ Aldrin acknowledged.
Eagle was now at 40,000 feet.
“Everybody, let’s hang tight and look for the landing radar,’’ said Kranz. The static cleared up. “Okay we’ve got data back.’’
The landing radar utilised four microwave beams to measure altitude in terms of echo-location and the rate of change of altitude by the Doppler effect. It was not expected to be very accurate above 35,000 feet. If it failed to function, a mission rule mandated an abort. However, in the event of difficulty bringing the radar on-line Kranz intended to permit the descent to continue to enable the problem to be investigated and, if it persisted, order the abort at 10,000 feet. He had selected this altitude because, in the absence of the radar, the spacecraft’s navigation was based on Manned Space Flight Network tracking, which was calculated against a mean lunar surface measured with respect to the radius of the Moon at the landing site, which might as much as 10,000 feet in error; if the spacecraft were to pass below this altitude without radar it might well hit the surface. As Eagle yawed, the radar on its base began to get ‘returns’ from the surface.
“Radar, Flight,’’ called Bales. ‘‘It looks good.’’
‘‘Rog,’’ Kranz acknowledged.
Because the yaw manoeuvre had run late, by the time it was complete Eagle was somewhat lower than intended at radar lock-on.
‘‘Lock-on,’’ Aldrin told Armstrong.
‘‘Have we got a lock-on?’’
‘‘Yes,’’ Aldrin confirmed. When the radar began to supply continuous data, a light on the control panel went out. ‘‘Altitude light’s out.’’
When the altimetry became available, the PGNS was showing them to be at an altitude of 33,500 feet. The radar said they were somewhat lower. Aldrin reported this to Houston. ‘‘Delta-H is minus 2,900 feet.’’
The computer began by considering the orbital data from the Manned Space Flight Network to be accurate, and the radar altimetry to be suspect. But if the radar was functioning properly, its data would be more accurate. If the radar data differed significantly from the computer’s navigation, the computer was to try to converge towards a compromise altitude. If the computer thought it was at 32,000 feet and the radar read 28,000 feet the computer could not simply accept this and revise its aim for the landing site, because the radar would be tracing the topography of the surface and would fluctuate. Instead, the computer would split the difference and use 30,000 feet, and iterate until it had properly ‘corrected’ its altitude, at which time it would recalculate its descent trajectory for the target. If they had found themselves in excess of 10,000 feet higher than the PGNS estimated, this would have required an abort, because if they had continued they would have run out fuel before reaching the surface.
With Eagle pitched at 77 degrees at this point in the descent, not quite on its back, its forward windows faced Earth which, because the spacecraft was east of the lunar meridian, was to the west of the zenith. Glancing out, Aldrin saw it as a half-disk of blue and white. ‘‘We have got the Earth right out our front window,’’ he observed. As the descent continued, and Eagle progressively changed its pitch angle to face its direction of motion, transitioning to a hover, the home planet would drift out of the top of the windows.
Aldrin asked the computer to calculate and show the delta-H. As a precaution against loss of communication at this juncture, he had a chart with which to judge for himself whether the radar data was valid. Armstrong sought confirmation that Houston was also monitoring this, ‘‘Houston, are you looking at our delta-H?’’
“That’s affirmative,” replied Duke.
“Looks good, Flight,’’ Bales called on the flight director’s loop.
“Is he accepting it, Guidance?’’ Kranz asked.
“Standby,” replied Bales.
As Bales studied the radar data, the yellow Master Alarm in Eagle started to flash, a tone sounded in Armstrong’s and Aldrin’s headsets and the DSKY lit the yellow ‘PROG’ light.
Armstrong keyed his PTT and, with tension evident in his voice, announced, ‘‘Program alarm.’’
Aldrin queried the computer, which flashed ‘12-02’.
‘‘It’s a 12-02,’’ Armstrong elaborated for Houston.
‘‘12-02,’’ confirmed Aldrin.
Armstrong and Aldrin turned their heads in their ‘bubble’ helmet to glance at each other; neither man had seen this alarm during simulations.
‘‘What is it?’’ Armstrong asked Aldrin.
As the computer specialist, Aldrin knew in general terms what a program alarm meant, but had no way of deciding whether this was a hardware or a programming issue. ‘‘It’s in core,’’ he mused.
Although Armstrong knew that their telemetry would enable Houston to show the status of the hardware, he was also aware that if the situation were to turn sour he might have to abort without Houston’s input.
Already psyched up by the task at hand, the alarm further boosted everyone’s adrenaline. ‘‘When I heard Neil say ‘12-02’ for the first time,’’ reflected Duke, ‘‘I tell you, my heart hit the floor.’’ The alarm caused consternation on Management Row. Gilruth, Phillips and Low sought insight from Kraft, but while he knew that some program alarms mandated an abort he was by no means an expert, and was unable to offer an explanation in this case.
Paules was the first to react, ‘‘12, 12-02 alarm.’’ After a pause, ‘‘Yeah, it’s the same thing we had.’’ He was referring to the simulation in which Koos had caught them out – although in that case it had been a 12-01. Bales switched his attention from the radar and conferred with Jack Garman, an expert in the computer, on his support team. Garman, now fully familiar with all the program alarms, said, ‘‘It’s executive overflow – if it doesn’t occur again we’ll be fine.’’
‘‘Flight, Retro,’’ called Deiterich while Bales and Garman were conferring.
‘‘Go, Retro,’’ prompted Kranz.
‘‘Throttle down, 6 plus 25,’’ announced Deiterich, drawing Kranz’s attention to the time (measured in minutes and seconds since the start of the powered descent) at which Eagle was to throttle down.
‘‘6 plus 25,’’ acknowledged Kranz, annotating his console log.
In simulations Armstrong had been primed to abort, but now he was primed to push on. Nevertheless, he was concerned by the lack of a response from Houston, ‘‘Give us a reading on the 12-02 program alarm.’’
‘‘We’re Go on that, Flight,’’ Bales finally announced, having established that, despite the alarm, the guidance system was performing its assigned tasks. But as he would reflect later, ‘‘In the Control Center any more than 3 seconds to reach a decision during powered descent is too long; and this took us about 10 to 15 seconds.”
Duke replied to Armstrong, “Roger. We gotcha. We’re Go on that alarm.”
“If it doesn’t recur, we’ll be Go,’’ Bales added.
“Rog,” acknowledged Kranz, noting the confidence in Bales’s voice. “Did you get the throttle down, CapCom?’’ Having missed it, Duke passed this information up to the spacecraft.
Eagle’s altitude was now down to 27,000 feet. Bales, returning his attention to the landing radar, announced, “He’s taking in the delta-H now.’’
“Rog,” acknowledged Kranz.
“Flight, FIDO,’’ called Greene. “We’re converging on delta-H.’’
“Rog,” acknowledged Kranz.
“Flight, Control,’’ called Carlton. “We’re on velocity.’’
“Rog,’’ acknowledged Kranz.
Having received a Go on the 12-02, indicating that the computer was healthy, Aldrin again queried delta-H, and the alarm recurred. “Same alarm,’’ he called, “It appears to come up when we have a 16/68 up.’’ Keying Verb 16 with Noun 68 told the computer to display the altitude and velocity, the range to the landing site, and the time remaining in the braking phase (in essence, the time to the pitch-over manoeuvre). Aldrin was speculating that his checking of the delta-H convergence might be prompting the executive overflow. Aldrin was correct, but the true issue was that the rendezvous radar was needlessly interrupting the computer, leaving it little time to devote to computations in addition to its navigational tasks.
“Roger. Copy,’’ acknowledged Duke.
This time Bales responded promptly, “It’s okay.’’ In the hope of relieving the load on the computer, he offered, “We’ll monitor his delta-H, Flight.’’
“Rog,’’ acknowledged Kranz.
Bales agreed with Aldrin’s line of thought. “I think that’s why he’s getting it.’’ “Okay,’’ said Kranz.
“Eagle, Houston,’’ called Duke. “We’ll monitor your delta-H.’’
“Delta-H is beautiful,’’ Bales observed.
“Delta-H is looking good to us,’’ Duke relayed.
“All flight controllers, hang tight,’’ Kranz prompted, “We should be throttling down shortly.’’
At 6 minutes 25 seconds into the powered descent, the computer throttled down the DPS engine from 100 per cent to 55 per cent.
“Throttle down on time,’’ announced Armstrong.
“Confirm throttle down,’’ Carlton noted.
“Rog, confirmed,’’ replied Kranz.
“Roger,’’ Duke responded to Armstrong. “We copy throttle down.’’
“You can feel it in here when it throttles down better than the simulator,’’ said Aldrin, tongue-in-cheek.
“Rog,’’ acknowledged Duke.
The fact that the computer throttled down the engine on time indicated that it was unaware it was coming in ‘long’, as otherwise it would have delayed the transition in
order to compensate and thereby re-establish its aim for the target.
“AGS and PGNS look real close,” noted Aldrin.
“Flight, Control,” called Carlton. “Everything looks good.”
“Rog,” acknowledged Kranz.
“Flight, FIDO,” called Greene. “We’re looking real good.”
“Rog, FIDO, good,’’ replied Kranz.
The spacecraft’s altitude was now down to 21,000 feet, and it had slowed to 1,200 feet per second.
“At 7 minutes, you’re looking great to us, Eagle,’’ Duke called.
“TELCOM,” Kranz prompted, “how’re you looking?’’
“It looks good, Flight,’’ replied Puddy.
“Rog,” acknowledged Kranz.
With the pitch angle now down to 60 degrees and the rate of change increasing, Aldrin announced, “I’m still on Slew, so you may tend to lose the high-gain as we gradually pitch over.’’ Then he had second thoughts, “Let me try Auto again now, and see what happens.’’
“Roger,’’ Duke acknowledged.
“We’re going to try the steerable again, Don,’’ Kranz warned TELCOM. “Copy, Flight,’’ replied Puddy.
“It looks like it’s holding,’’ reported Aldrin. With a clear line-of-sight and the steerable dish locked on, communications improved markedly.
“Roger,’’ acknowledged Duke. “We’ve got good data.’’
“Are we on the steerable, Don?’’ Kranz asked.
“That’s affirmative, Flight,’’ replied Puddy. “And it’s holding in there pretty good.’’
“Rog,’’ acknowledged Kranz. His concern over telemetry drop-outs abated. It seemed that he would not, after all, face the decision as to whether communications had degraded to the point of requiring an abort.
The spacecraft’s altitude was now down to 16,300 feet, and it had slowed to 760 feet per second.
“Okay, everybody hang tight,’’ Kranz said. “Seven and a half minutes.’’
“Flight, Guidance,’’ called Bales. “His landing radar’s fixed to velocity; it’s beautiful.’’
“Flight, Control. Descent 2 fuel,’’ Carlton announced. Having closely studied the redundant propellant gauging systems, he recommended monitoring the ‘low level’ sensor in gauging system number 2.
‘‘Descent 2 fuel crit,’’ said Kranz.
‘‘Descent 2 fuel, On,’’ corrected Carlton. ‘‘I didn’t want to say ‘critical’.’’
‘‘Rog,’’ acknowledged Kranz.
Duke relayed the advisory, taking care not to be ambiguous, ‘‘Eagle, Houston. Set Descent 2 fuel to Monitor.’’
‘‘Roger, 2,’’ acknowledged Armstrong.
‘‘Flight, FIDO,’’ called Greene. ‘‘It’s looking real good.’’
Pat Collins, listening to her squawk box, nervously clenched her fist.
Eagle’s altitude was now down to 13,500 feet. Having elected not to use 16/68 to
avoid further 12-02 program alarms, Aldrin asked for the time remaining in the braking phase, “Could you give us an estimated pitch-over time, please, Houston?” “Stand by,” said Duke. “You’re looking great at 8 minutes.”
“Thirty seconds to P64,’’ called Bales, responding to Aldrin’s request.
“Eagle, you’ve got 30 seconds to P64,’’ relayed Duke. The P64 program would switch to the visual approach phase of the descent.
“Have we still got landing radar, Guidance?’’ Kranz asked.
“Affirm,” replied Bales.
“Okay. Has it converged?’’ Kranz asked.
“It’s beautiful,’’ replied Bales.
“Has it converged?’’ Kranz repeated.
“Yes!” Bales replied.
“Flight, FIDO,’’ called Greene. “We look real good.’’
“Rog,” acknowledged Kranz.
“Eagle, Houston,’’ called Duke. “Coming up 8 plus 30. You’re looking great.’’ Having reached a point known as the ‘high gate’ at an altitude of 7,500 feet, Eagle’s computer initiated P64, which rapidly reduced the pitch angle from 55 degrees down to 45 degrees. Thus far, most of the thrust had been devoted to slowing the horizontal velocity. As the pitch was further reduced, more of the thrust would be directed downwards. During the pitch-over, the radar on the base of Eagle swung from its ‘Descent’ position to ‘Hover’, where it would remain, and the horizon rapidly swung up into the bottom of the windows, giving Armstrong his first view of where the computer was heading, which at this altitude was a point some 3.5 nautical miles dead ahead, just on this side of the horizon.
‘‘P64,’’ called Aldrin.
‘‘We copy,’’ Duke acknowledged.
‘‘Okay, they’ve got 64,’’ Kranz announced over the flight director’s loop. ‘‘All flight controllers, 20 seconds to Go/No-Go for landing.’’
‘‘Eagle, you’re looking great,’’ Duke confirmed. ‘‘Coming up on 9 minutes.’’
The spacecraft was down to 5,200 feet and descending at 100 feet per second, which was as planned. Armstrong tested his hand controller in pitch and yaw, and then resumed ‘hands off. ‘‘Manual attitude control is good.’’
‘‘Roger, copy,’’ acknowledged Duke.
As Eagle descended through 4,000 feet, Kranz went around the horn, ‘‘All flight controllers, Go/No-Go for landing. Retro?’’
‘‘Go!’’ called Deiterich.
‘‘Go!’’ called Greene.
‘‘Go!’’ called Bales.
‘‘Go!’’ called Carlton.
‘‘Go!’’ called Puddy.
“Go!” called Willoughby.
“Go!” called Aaron.
“Go!” called Zieglschmid.
“CapCom we’re Go for landing.”
“Eagle, Houston. You’re Go for landing.’’
On hearing this, Jan Armstrong sat up on her heels at the foot of her bed. Pat Collins exclaimed, “Oh God, I can’t stand it.’’
“Roger. Understand, Go for landing,’’ acknowledged Aldrin. “3,000 feet.’’ But then, “Program alarm.’’ He keyed the DSKY for the code, “12-01.”
“Roger,” acknowledged Duke, “12-01 alarm.’’
“Same type,’’ responded Bales immediately. “We’re Go, Flight.’’
“We’re Go. Same type,’’ relayed Duke, the tension evident in his voice. “We’re Go.’’
Armstrong had wanted to look for landmarks to determine how ‘long’ they were, but this alarm distracted him, and when he next looked out they were so low that he could not see any of the landmarks he had memorised, ‘‘So,’’ he later reflected, ‘‘all those pictures Tom Stafford took on Apollo 10 to enable me to pick out where I was going and know precisely where I was, were to no avail.’’
‘‘2,000 feet,’’ called Aldrin.
Pat Collins nervously began to bite her lip.
As Aldrin had explained prior to launch, ‘‘During the landing, there is a fairly even division of labour. Neil will be looking more and more outside, his hand on the ‘stick’. He is not able to look much at the instruments. This is where we must work as a finely tuned team, to ensure that he gets the information he requires to transfer whatever he sees into something meaningful. I’ll relay this information. And at the same time I’ll be looking at the various systems to make sure they’re operating the way they should. However, here I am looking at five or six gauges, and, by telemetry, we’ve got teams of people looking at each gauge on Earth, so, really, I’m confirming what a lot of people are getting.’’
Left to itself, the computer would continue the descent until it either landed or crashed in the attempt, most likely as a result of unfavourable terrain. To find out where the computer was heading, Armstrong asked Aldrin for an angle for his Landing Point Designator, ‘‘Give me an LPD.’’
Aldrin interrogated the computer, ‘‘47 degrees.’’
The panes of Armstrong’s two-layer window were annotated with a scale. The angle was measured downward, relative to directly ‘ahead’. Positioning his head to align the scales, he sighted beyond the 47-degree mark to the position, a little more than 1 nautical mile away, where the computer was taking them. ‘‘That’s not a bad – looking area,’’ he observed to Aldrin.
Duke continued his advisories, ‘‘Eagle, looking great. You’re Go.’’
As Eagle descended through 1,400 feet, the computer issued another program alarm. ‘‘12-02,’’ called Aldrin.
‘‘Roger,’’ acknowledged Duke. ‘‘12-02.’’
“How are you doing, Control?” Kranz asked.
“We look good here, Flight,” replied Carlton.
“How about you, TELCOM?”
“Go!” replied Puddy.
“Guidance, are you happy?”
“Go!” Bales replied.
“Go!” Greene replied.
To veteran reporters such as Reginald Turnill of the BBC, who had made the effort to learn something of the systems, this determination to push on regardless of the alarms began to look as if it would end with a crash.
“What’s the LPD?” Armstrong asked.
“35 degrees,’’ replied Aldrin. “750 feet, coming down at 23 [feet per second].’’
Pat Collins now began to bite her finger.
“33 degrees,’’ Aldrin called. “700 feet, 21 down.’’
With Aldrin acting as his eyes inside, Armstrong directed his attention outside. The computer was heading for a crater the size of a football field, surrounded by a field of ejecta excavated by the impact. He later reflected, “I was surprised by the size of the boulders, some of which were the size of small automobiles.’’ The crater was 600 feet in diameter. “Pretty rocky area,’’ Armstrong observed to Aldrin.
“600 feet, down at 19,’’ Aldrin recited.
On the nominal descent, Armstrong was not to take control until Eagle was down to about 150 feet. However, in view of where it was heading, he could not let the computer continue to fly ‘blind’. He considered trying to set down short of the crater or even among its ejecta in order to be able to inspect the boulders for the scientists, but ruled this out as being too risky and instead decided to follow his piloting instincts, and ‘extend’. He selected the semi-automatic flight mode that would enable him to control attitude and horizontal velocity, while the computer – allowing for his commands – operated the throttle. At an altitude of 500 feet, at a point known as ‘low gate’ in the descent profile, he intervened. He cut the pitch angle from its current 20 degrees to about 5 degrees, thereby standing the vehicle essentially ‘upright’ to direct nearly all its thrust downwards in order to maintain the horizontal velocity of 60 feet per second and reduce the rate of descent from 19 feet per second to 9 feet per second. He then selected Attitude Hold, and let Eagle fly a shallow trajectory over the field of ejecta just north of the crater, while he looked for a clearer area further downrange.
‘‘Attitude Hold!’’ called Carlton, on noting the mode change in the telemetry.
‘‘Roger, Att-Hold,’’ acknowledged Kranz.
At this point, as Duke recalled: ‘‘We were down to the last couple of minutes. Deke Slayton is sitting next to me. We’re both glued to the screen on my console, and I’m just talking and talking and telling them all this stuff, and Deke punches me in the side and says ‘Charlie, shut up and let them land’.’’
‘‘I think I’d better be quiet, Flight,’’ Duke said.
‘‘Rog,’’ acknowledged Kranz.
Because Armstrong had overridden the computer, Aldrin deleted the LPD angle from his cycle, and instead began to report their forward velocity: “400 feet, down at 9, 58 [feet per second] forward.”
“The only call-outs now will be fuel,” Kranz directed. Carlton, monitoring the propellant gauging system, would make the calls for Duke to relay. As the tension mounted, the flight controllers unconsciously grasped the handles of their display units; these were nicknamed ‘comfort handles’.
“350 feet, down at 4,’’ called Aldrin.
‘‘P66,’’ announced Carlton, reporting that the computer had switched from the approach phase to the landing phase.
‘‘330, 6-1/2 down,’’ called Aldrin. ‘‘We’re pegged on horizontal velocity.’’ At this point, there was a burst of static on the downlink.
Although Armstrong had not explained why he had intervened, it was evident from the fact that Eagle was passing downrange on an almost horizontal trajectory at high speed that he was taking evasive action. Kranz recognised that the locus of decision-making had transferred to Eagle. The vehicle was not yet into the ‘dead man’s box’, but soon would be. The remainder of the descent would be up to Armstrong. Kranz also knew that as long as Armstrong thought he had a fair chance of making a landing he would press on. But, as Stafford had noted after the low pass by Apollo 10, the western end of the ellipse appeared to be much rougher than the aiming point.
On flying clear of the boulders around the big crater, Armstrong pitched Eagle back again in order to rapidly slow the horizontal velocity which, as a result of his evasive action, was now excessive for their altitude. On spotting a line of boulders up ahead, he neatly ‘side stepped’ off to the left – just as he had done when flying the LLTV, firstly by tilting Eagle in the direction he wished to go in order to use a component of the thrust to set up the requisite lateral velocity then, just before reaching where he wished to be, tilting in the opposite direction in order to cancel this translation, resuming the original orientation directly above his selected position. Although in such manoeuvres Eagle had the familiar sluggish response of the LLTV, he was delighted to find the LM easier to fly. To buy time, he began to use the toggle switch on the hand controller designed to adjust the rate of descent in increments of 1 foot per second; having been sceptical of this feature, Armstrong was delighted to find it very effective.
‘‘Okay, how’s the fuel?’’ asked Armstrong, as he continued to manoeuvre at an altitude of 300 feet.
‘‘8 per cent,’’ Aldrin replied.
Now well clear of the ejecta, Armstrong began to ease down.
‘‘Okay, this looks like a good area here,’’ Armstrong informed Aldrin.
Aldrin stole a glance outside and saw Eagle’s shadow on the ground ahead. He was surprised since, being at an altitude of about 260 feet with the Sun low in the east, he had expected the shadow to be too far west to be readily visible; but there it was, distinctly showing the structure of the vehicle. ‘‘I got the shadow out there,’’ he reported. Unfortunately, as a result of manoeuvring, Eagle was yawed slightly left, and the central pillar in front of the instrument panel blocked Armstrong’s view of the shadow.
“250, down at 2-1/2, 19 forward,” recited Aldrin.
“Okay, Bob. I’ll be standing by for your call-outs shortly,” Kranz prompted.
“Altitude/velocity light,’’ noted Aldrin. This warning light indicated that the radar data had degraded. The logic was that the light illuminated when the output from the radar was unusable by the computer – it was lit prior to lock-on, went out with lock-on, and thereafter would come on to alert Aldrin to the fact that the radar had lost track of the surface. Because it had been deemed impractical to try to land by ‘seat of the pants’ flying, as there would not be the visual references to give a sense of altitude and rate of descent, the mission rules stated that if the radar were to fail they would have to abort. But they continued expectantly, and after 20 seconds the radar locked on again. Then Aldrin resumed his calls, ‘‘3-1/2 down, 220 feet, 13 forward.’’
As the downlink was lost to static, Jan Armstrong slipped her arm around son Ricky’s shoulder. Joan Aldrin was standing in silence by the wall, grasping a door, her eyes moist, praying that Eagle would not crash. In Mission Control, Kranz had decided that he would not call an abort unless he was certain it was essential. As regards the mission rule that he had introduced requiring there to be telemetry for the powered descent to continue, he recalled, ‘‘Once we were close, I intended to let the crew go if everything appeared okay to them – I considered a low-altitude fire-in-the- hole abort riskier than landing without telemetry. I looked at a fire-in-the-hole abort the same way that I looked at a parachute when I was flying jets; that is, you use a parachute only when you’ve run out of options.’’ Armstrong would later say that an abort involving (1) shutting down the DPS, (2) firing the pyrotechnics to sever all the structural and electrical connections between the stages, and (3) igniting the APS ‘in the hole’ in rapid succession, in close proximity to the lunar surface, ‘‘was not something in which I had a great deal of confidence”. If the process were not to occur cleanly, it would jeopardise the ascent stage’s departure. It had been done only on the unmanned test of LM-1 in 1968. In fact, this aversion to abort-staging had led to the mission rule that if a problem were to develop after the 5-minute point in the powered descent that did not mandate an in-flight abort, then every effort would be made to land in order to lift off several minutes later. However, if the DPS were to cut off once Eagle was within 200 feet of the surface, it would be doomed as it fell in the weak lunar gravity because by the time the abort-staging sequence was concluded, the APS would not be able to impart a positive rate of climb before the ascent stage struck the surface. Eagle was almost at this critical altitude.
‘‘11 forward. Coming down nicely,’’ said Aldrin.
‘‘I’m going right over a crater,’’ Armstrong pointed out. As he was not using his PTT, Houston did not hear this remark.8
‘‘200 feet, 4-1/2 down, 5-1/2 down.’’
‘‘I’ve got to get farther over here,’’ Armstrong said, as he resumed manoeuvring.
The large rock-strewn crater towards which the computer had been heading was named ‘West’, and the 75-foot-diameter crater over which they passed at this point would later be named variously ‘Little West’ or ‘East’ Crater.
“160 feet, 6-1/2 down,” continued Aldrin.
There were ‘level sensors’ in each pair of propellant tanks, and Carlton had recommended that they use set 2. When either the fuel or oxidiser sensor in these tanks became exposed, it would illuminate the ‘Descent Quantity’ light on Eagle’s control panel and generate the ‘low level’ signal in the telemetry. The signal meant there was now only 5.6 per cent of the initial propellant load remaining, which, in hovering flight with the throttle at about 32 per cent, meant the engine would cut off in 96 seconds. With 20 seconds reserved for the preliminary action of an abort during which the DPS would be throttled up to cancel the rate of descent and impart a positive rate of climb prior to abort-staging, the low-level signal meant that in 76 seconds Armstrong would be required either to abort or forgo the option of aborting and commit himself to touching down within the next 20 seconds. Borrowing pilots’ slang, this decision point was known as the ‘bingo’ call.
‘‘Low level,’’ called Carlton over the otherwise silent flight director’s loop. He started his stopwatch.
‘‘Low level,’’ echoed Kranz. This call ‘‘really grabbed my attention’’ he would later reflect, ‘‘mainly because in training runs we’d generally landed by this time’’.
‘‘5-1/2 down, 9 forward,’’ recited Aldrin. ‘‘You’re looking good.’’ After a burst of static, he was heard to say ‘‘120 feet.’’
Armstrong again slowed the rate of descent in order to manoeuvre to a flatter spot. Slope could be judged visually while hovering because, with the Sun low to the rear, a bright patch was probably sloping up because it was well illuminated, whereas a dark patch was probably sloping down and poorly illuminated. He had to find an evenly lit location that was free of rocks. The presence of rocks could be inferred from the shadows that they cast. As he recalled, ‘‘I changed my mind several times, looking for a parking place. Something would look good, and then as we got closer it really wasn’t so good. Finally, we found an area ringed on one side by fairly good sized craters and on the other side by a boulder field; it wasn’t particularly big, a couple of hundred square feet – about the size of a big house lot.’’
‘‘100 feet, 3-1/2 down, 9 forward,’’ recited Aldrin.
At the suggestion of Bill Tindall, the illumination of the Descent Quantity light did not trigger either the caution and warning light or sound an audible tone; it was a normal event after all, not something to risk distracting the crew so near the lunar surface. It was therefore some time before Aldrin noticed the amber lamp, ‘‘Five per cent. Quantity light.’’
Carlton was focused on his stopwatch. ‘‘Coming up on 60,’’ he warned.
‘‘Rog,’’ acknowledged Kranz.
‘‘Okay,’’ continued Aldrin. ‘‘75 feet and it’s looking good.’’
‘‘60!’’ called Carlton.
‘‘60 seconds,’’ echoed Kranz.
Duke, who had been silent for some time, passed this on. Aldrin did not respond, opting instead to maintain his instrument readings for Armstrong.
Jan Armstrong sat forward, one hand over her mouth, her eyes a little brighter than usual. Joan Aldrin, tears in her eyes, was huddled against the frame of a door, one hand resting on a lamp shade, which was shaking.
Since Eagle might easily damage one of its legs (or possibly even tip over) if it were to land with a significant horizontal velocity, once Armstrong was directly over his chosen spot he focused on a point just in front as his visual reference and set about ‘nulling’ his lateral velocity components in preparation for a vertical descent. However, because he had no wish to drift backwards into an obstacle, he retained a very slow forward motion that tests had indicated the legs should be able to resist.
‘‘Light’s on,’’ reported Aldrin. The illuminated altitude/velocity light indicated that the radar data had degraded again, but this time the drop-out lasted only a few seconds. ‘‘60 feet, down 2-1/2,’’ he continued. After a pause, he added, ‘‘2 forward. That’s good.’’ And again, ‘‘40 feet, down 2-1/2.’’
Armstrong cut the throttle in order to descend. The exhaust plume was now in contact with the surface, but because the spacecraft had shed half of its mass since PDI the engine was delivering only about 1,000 pounds of thrust. Nevertheless, it stirred up the fine surface material. ‘‘Picking up some dust,’’ Aldrin reported.
Unable to billow in the absence of an atmosphere, the dust travelled radially outward on ‘flat’ trajectories. The dust moving forward created the illusion that Eagle was drifting backward. Fortunately, the semi-transparent layer of‘ground fog’ was so thin that some of the rocks poked up through it, and Armstrong was able to maintain his visual reference.
‘‘30 feet, 2-1/2 down,’’ called Aldrin. He saw the shadow of Eagle’s right leg, the probe on its foot pad indicating that it was tantalisingly close to the surface. He also noticed that although shadows on the Moon were normally sharply defined, Eagle’s shadow was softened by the dust passing just above the surface. ‘‘Faint shadow.’’ ‘‘And now for 30,’’ called Carlton, monitoring his stopwatch.
‘‘4 forward,’’ Aldrin continued. ‘‘4 forward. Drifting to the right a little.’’
‘‘30!’’ Carlton announced.
‘‘30 seconds,’’ echoed Kranz.
This was relayed by Duke with incredulity evident in his voice, ‘‘30 seconds.’’
In the Mission Operations Control Room, the flight controllers, managers and visitors began to breathe intermittently – some even ceased to breathe.
‘‘20 feet, down a half,’’ Aldrin called. ‘‘Drifting forward just a little bit. That’s good.’’ When one of the three 67-inch-long probes struck the surface it illuminated a blue lamp on the central control panel. Armstrong, his attention outside, did not see this, but Aldrin had the lamp in his peripheral vision. ‘‘Contact light!’’ Carlton had been about to call out 15 seconds as the start of a second-by-second countdown.
The final rate of descent was required not to exceed 3 feet per second, since (as factory testing had indicated) a faster sink rate could shock the legs sufficiently to damage them – possibly so much as to prevent a subsequent liftoff, during which the descent stage was to serve as the platform for the ascent stage. In practice, this meant that the vehicle was not to be allowed to fall in lunar gravity from a height exceeding 10 feet. The contact probe satisfied this requirement. Furthermore, the engine was to be shut down immediately the contact light lit, in order to preclude the possibility of back pressure from the plume in such close proximity to the surface damaging the engine, possibly causing it to explode. However, Armstrong was a second or so late, with the result that instead of falling the final 5 feet, Eagle settled onto the surface very gently at a sink rate of just 1.7 feet per second, with each of its pads pivoting to settle on the uneven surface. Although Armstrong had tried to cancel the lateral velocities and maintain a slight forward creep, it was later determined that Eagle had been drifting to the left at about 2 feet per second and the left leg was first to make contact, indicating that the vehicle had been tilted that way. As Aldrin reflected later, “I would think that it would be natural, looking out the left window and seeing dust moving left, that you’d get the impression of moving to the right and counteract by going to the left.’’ As a result of the final manoeuvring, Eagle landed yawed around at an angle of 13 degrees left. On the uneven surface, its 4.5-degree backward tilt was well within the 10-degree tolerance.
“Shutdown!” announced Armstrong.
Turning their heads in their ‘bubble’ helmets Armstrong and Aldrin grinned at each other. Armstrong later reflected: ‘‘If there was an emotional high point, it was after touchdown when Buzz and I shook hands without saying a word.’’ As Aldrin recalled the event, he was ‘‘surprised, in retrospect, that we even took time to slap each other on the shoulders’’.
Armstrong later insisted that the landing was everything he could have wished for, and the fact that it had been achieved with just seconds to spare had made it even more satisfying. In fact, he was not concerned by the narrow fuel margin, because this had always been so when flying the LLTV, which had severely limited flight time. A later analysis would show that when he began to manoeuvre, the fluids in the propellant tanks had sloshed around and because the level sensor in each tank was located on top of a 9-inch-tall rod the ‘low level’ signal had occurred 20 seconds prematurely. In fact, when Carlton’s count reached the 15-second mark, the engine could have sustained 25 seconds of hovering prior to the ‘bingo’ point; the halving of the margin from 20 seconds at the ‘low level’ signal to 10 seconds at actual touchdown presumably being because Armstrong had departed from the nominal trajectory ahead of schedule in order to manoeuvre, thereby consuming propellant at an increased rate. Telemetry showed that Armstrong’s heart rate had been 110 beats per minute at PDI, peaked at 156 during his final manoeuvres, and then rapidly dropped back to about 95.
Aldrin immediately started the post-shutdown checklist. ‘‘Engine Stop. ACA out of detent. Mode Control, both Auto. Descent Engine Command Override, Off; Engine Arm, Off; 413 is in.’’9,10
9 The Attitude Control Assembly (ACA) was the hand controller used to fly the spacecraft. It was spring-loaded to stand in its central detent. The computer not only interpreted a displacement as a request for a manoeuvre but also remembered how the stick was being used. By nudging it out of detent after shutdown, Armstrong was essentially clearing it.
10 The AGS used ‘strap down’ gyroscopes, which had a tendency to drift. Now that Eagle was on the surface, Aldrin loaded a specific value into address ‘413’ of the AGS to tell that system to store its attitude information to ensure that if (1) an emergency liftoff became necessary, and (2) by sheer ill luck the PGNS were to malfunction beforehand, obliging them to use the AGS, then this system, by virtue of having stored its attitude immediately after landing, would be able to correct for any drift in its gyros.
“Flight, we’ve had shutdown,” confirmed Carlton.
“We copy you’re down, Eagle,’’ Duke called.
“Houston, Tranquility Base here,’’ called Armstrong. “The Eagle has landed.’’
Duke had been alerted in order that he would not be caught out by a strange call sign, but he fluffed his reply. “Roger, Twank – Tranquility. We copy you on the ground.’’ A moment later he continued, “You’ve got a bunch of guys about to turn blue. We’re breathing again. Thanks a lot.’’ With that, he slumped back in his chair and grinned at Slayton, who grinned back.
In the viewing gallery people stood to applaud, cheer, and wave small flags.
The powered descent had started at 102:33:07, and Armstrong called shutdown at 102:45:41 after a duration of 12 minutes 34 seconds – about half a minute over nominal. As he updated his console log, Kranz thought, ‘My God, they’ve landed!’
At ‘contact light’ Pat Collins, head resting on her hands, broke into a smile for the first time in more than an hour. With the exception of Joan Aldrin, everyone in her house applauded at ‘engine stop’; she had her head buried against the wall and was still shaking. Although Robert Moon went over to comfort her, she escaped to the solitude of her bedroom. Michael Archer, Joan’s father, took daughter Jan, who was visibly shaken, to join her mother. After gathering her senses, Joan handed out a box of cigars. As she would reflect a few hours later, ‘‘My mind couldn’t take it all in. I blacked out. I couldn’t see anything. All I could see was a match cover on the floor. I wanted to bend down and pick it up, and I couldn’t do it. I just kept looking at that match cover.’’ With the ‘landed’ report, Jan Armstrong delightedly hugged son Ricky. A moment later, her sister Carolyn entered the room, leant against the wall and exclaimed, ‘‘Thank you, God.’’
In New York, Walter Cronkite, who was anchoring the CBS special, Man on the Moon: The Epic Journey of Apollo 11, had also been holding his breath. He removed his spectacles to wipe sweat from his forehead and, finding himself speechless, could only say, ‘‘Phew! Wow!’’ The Neilson ratings organisation later estimated that more than half of American households had had their television sets switched on during the landing. However, since all three networks were providing continuous coverage it was hard to avoid the event! Armstrong’s parents were watching on their donated colour television. A baseball game in Yankee Stadium in New York was paused to permit the landing to be announced, and the audience delivered a rendition of The Star-Spangled Banner. Canon Michael Hamilton of Washington Cathedral noted, ‘‘The older people are getting a bigger bang out of this than the younger ones, who have grown up with astronauts and space; older people remember when it was just a dream.’’ Of all the space program managers, the lunar landing must surely have meant the most to Wernher von Braun. It would not have been possible, however, without the challenge laid down by John F. Kennedy, on whose grave at Arlington National Cemetery a bouquet of flowers was deposited several hours later with the anonymous note ‘Mr President, the Eagle has landed’. In Moscow, senior military officers and a dozen cosmonauts had gathered to monitor the American television coverage, and the landing prompted a round of applause. Alexei Leonov, who had hoped to make the first lunar landing for his country, later explained this praise of the American success as ‘white envy’. On its final news bulletin of the day, Soviet television reported that the landing had succeeded, and that the Czar of the flight would soon step out onto the surface.
After watching Jan Armstrong give a press interview, Joan Aldrin went out to do likewise. A NASA Public Affairs Officer held an umbrella against the rain that had started to fall. Frustrated by banal questions such as ‘‘What are your plans for the moonwalk?’’ she burst out, ‘‘Listen! Aren’t you all excited? They did it! They did it!’’ And with that she turned and strode back into the house.