V
Of course, the computer on the Lem was nowhere so vast. The brain capacity of computers designed in the same period was roughly proportional to their volume, and this computer, identical to the one on the Command Module, was two feet long, one foot high, and six inches wide. It weighed seventy pounds and consumed seventy watts and had a vocabulary of 38,916 sixteen-bit words, a way of stating that the Guidance Computer could take information in the form of ones and zeros in any variety up to sixteen units in a row. That made one word among 38,916 words. Hordes of navigation data and engine data had thus been laid in for reference; also, a series of programs which had been designed to keep the Lem on its trajectory by issuing precise engine and thruster commands. Another of its functions was to keep the Inertial Measurement Unit or gyroscope in constant alignment with the stars. It was capable to a small degree of correcting malfunctions in its own system, and it could also compute navigation information and display it to the astronauts, or answer their questions, through a combination of Display and Keyboard called a DSKY which had lighted numbers in a small frosted window to show answers, offer information, or blink out alarms. In turn, the astronauts communicated with the computer through keys in the DSKY which they punched like adding machine tabs to begin computer actions. The keys went from the number 0 to 9, there was a key for plus and minus, and other keys marked VERB, NOUN, CLEAR, PRO, KEY REL, ENTER and RSET.
The Guidance Computer was the major part of the Primary Guidance and Navigation Section (whose acronym was PGNS) and so was sometimes called Pings to distinguish it from AGS, the Abort Guidance Section, a smaller computer and inertial system ready to take over if Pings failed. Ags could hardly fly the mission, it was there for backup in case the Guidance Computer failed and abort was necessary to return the Lem to the Command Module. So the success of the landing on the moon would depend on Pings’ speed in processing the variety of information it would be absorbing, and its ability to issue commands as the Lem proceeded down. Coming into the maw of its calculations would be questions the astronauts put in on the DSKY, and programs they ordered it to initiate; coming in from other sources and sensors would be all the evidence of altitude change, velocity change, and fuel consumption. Since the rendezvous radar would be sending and receiving signals from the separated Command Module, so too would rendezvous radar be offering data to Pings on the change of distance and the relative position of the two ships. In addition, the landing radar would also be entering its evidence in the Guidance Computer, and to increase confusion the landing radar was bound to disagree with Pings, for it would be bouncing signals off the literal moon ground, rather than estimating like Pings where the ground ought to be on the basis of past observations and calculations. Therefore the computer would be having to make last-minute adjustments at a great rate. But here is Aldrin on the matter:
Suppose the computer tells us we’re at thirty-two thousand. We give that to the radar and the radar comes back and says the computer is a liar; we’re at twenty-eight thousand. The computer goes into a sulk and says it will split the difference; call it thirty thousand. The radar takes that and says no you don’t, and besides you’re down to twenty-seven now. We can keep narrowing the difference during the automatic part of the descent, until we get down to about five hundred feet, and at some point around about there we have to take over manually. That computer isn’t going to dodge a boulder for you.
The Pings obviously would have enough to do. If it had almost thirty-nine thousand words, most of them were already fixed. Only two thousand could be applied to a transient problem. In the welter of functions which might descend upon it, beleaguered as a short-order cook when he has lost a series of orders at the rush hour, the Pings had the power to give priority to particular functions, set other functions aside, or even cease calculations for a moment if an error were made, or a discrepancy could not be accounted for, or information was simply coming in faster than it could be processed. The Pings was presumably an effective computer for its size, but weight was crucial, and so it was squeezed in its tasks. It bore the same relation to the computers at Mission Control that a dollar bill bears to a thousand dollar bill. As limited, relatively, were the services it could purchase. Of course the descent from beginning to end could have been computed on the ground from the tracking of the huge antennae in the Manned Space Flight Network, and in fact it was, but to be dependent on Mission Control for the landing was too dangerous: the telemetered data could always fail. Static, or any other blocking of the signal, could cut out crucial instants of data. Often did. Besides, the ground was a quarter of a million miles away, and information therefore took more than a second to arrive; if it took even an instant to process and send back, three seconds could elapse in the interval, a catastrophe in a situation where the controls on the Lem were rapidly going wrong. So the limited seventy-pound Guidance Computer with its 38,916 words was going to be the chief pilot for the first part of the trip down, its indispensable function to calculate the ratio between the fuel which still remained and the miles yet to descend. Without a functioning computer those rates were next to impossible for a pilot to estimate, since the Lem would be traveling at several thousand miles an hour when it began its descent and settling at only several feet a second by the end. And it must be remembered that one could not fly the Lem down, which is to say one did not have the fuel to take it through circles, allow it to descend, then rise again, then take a long turn. There would be some maneuvering available at the end, but for the majority of the descent, few were the adjustments to be made with the trajectory. The Lem was still riding on the effects of the momentum which had first brought it to the moon, riding like a ball thrown into a canyon. One might nudge it a degree or two from side to side, one might brake its fall a little more or less, but essentially its momentum was its capital and it was traveling on a braking ballistic curve toward the moon ground—it was not going to wheel and circle and soar and turn and negotiate a descent—not in a lunar vacuum. So little changes of attitude or braking thrust were large in importance, and a computer could estimate their effect on fuel far more rapidly than a man. Yet, given its limited capacity and variety of tasks, it was obvious that one burden of preparation at NASA was to make certain Pings never got into the condition of a quarterback to whom everybody is talking in the huddle when the time-outs are used up and the clock is running into the end of the game.
Who could be more aware of such difficulties than the flight controllers in the Mission Control Room? The functions of a computer were always in as much danger of going awry as society is in danger of some final collapse into crime; experts and chains of experts were forever at work on electronic species of crime detection—they were vigilant to search for sneak circuits in the computer, circuits which if exercised at the same time would interreact in ways no one had foreseen. There were millions of switches and billions of permutations: complex electrical circuits were like patients with modern viruses—after prescribing several medicines, the doctor could never know which, if any, had triggered the cure, and just what in fact the patient had suffered from. As medicine had grown more complex, so the defined edges of all diseases had frayed and shaded into one another. Something like that was forever in danger of happening with computers as their tasks grew more complex and the nature of their response came closer to Webster’s definition of thought: “to form in the mind, to exercise judgment.” As computers developed mysterious malfunctions, computer detectives came into existence with more and more developed instincts for where sneak circuits might occur, where functions could intertwine and paralysis crop up in whole areas of an operation. There was a checklist the astronauts had to go through with every instrument on the Lem, and for the Pings the checklist was detailed. It engaged dozens of switches which produced hundreds of combinations of circuits which in turn offered thousands of possibilities for sneak circuits. For months, Staff Support rooms and every corporation associated with Guidance Control had worked on the anticipation of wher
e and how sneak circuits might occur. No one, however, could be certain—to anticipate every whim of a computer was equal to foreseeing the steps of a virgin whose heart was nymphomaniac—electricity could not run through a multiplicity of circuits without creating a wake of electromagnetisms and interferences whose results were occasionally so bizarre a flight controller could be known to give assent, with unhappy expressions on his face, to the possibility that a computer operator charged with psychic tension on an extraordinary day could also have his effect on the malfunction of the computer. At any rate, the checklist, product of months of work and months of preventive detection, was gone through by the astronauts while circling the moon, and after every item was fulfilled correctly, every switch properly thrown, a sneak circuit was still not avoided. In the no-man’s-land of electrical hegemony, sneak circuits resided at the very edge of thought. “If we had been supersmart,” said the Flight Director Gene Kranz on another day, “we could have picked up the possibility.”
Collins had said, “The most dangerous items are the ones we’ve overlooked.”
Yes, it was not possible to anticipate everything. Not by time, not by cost, not even by space. More than sneak circuits were sitting upon the descent. It was for example impossible to use a real rendezvous radar in the simulator, for how could one maintain a real situation for hours on earth in which two moving vehicles might be anywhere from two feet apart to two thousand miles apart and moving with velocities which varied from inches a second to thousands of miles an hour, no, a project close to the size of the Space Program itself might be necessary to create a real equivalent of rendezvous radar, and yet it was only necessary to make certain that the amount of data coming in by rendezvous radar and landing radar would not overload the Guidance Computer. Therefore, a conceivable set of received signals was written up in electrical code and piped as rendezvous and landing radar data into the computer during these simulations. No overload occurred. Not in the simulations.
Still, there was hardly a sense of security at Mission Control. Decisions would have to be taken on the floor in the course of the descent. Order after order would have to be given to the astronauts on whether to continue with the mission, or abort. Each engineer working a console on the floor of the Mission Operations Control Room was an expert on the limits which could be negotiated by elaborate equipment if things began to go wrong, but the decision to say GO or NO GO might have to be taken in some arena of crisis where the answer would not be clear. So each man on that floor knew he could enter a stricken instant, a cauldron of adrenalin, a failure of nerve which could lay a shadow upon all the hours of his life: an order to abort the mission which later proved to be unnecessary, or an injunction to go ahead which resulted in death would have to leave an isolation of the soul. Suicide could be the neighbor at one’s elbow for many a year. No wonder technicians at NASA so often had hands clammy to the touch.
Therefore, the man who would be Flight Director during Eagle’s descent to the moon, Gene Kranz, had taken his flight controller’s crew through a series of planning sessions the month before, a field seminar which could well have been termed the Engineering of Emergency Situations, for Flight Control proceeded to trace out the connotations of every alert and every alarm the computer could show. How quickly, they inquired, would a particular situation deteriorate into disaster after the first alarm was sounded? What might be the symptoms of each deterioration? what were the remedies? what the partial remedies? for which functions could Mission Control substitute for Pings, given those few seconds of lag? which were the functions Mission Control could not support? And then—no small matter—into which consoles, and before whose desks would these alarms terminate? Who would be required to make each final decision on the floor of the room? What would be the feasible criteria to encourage one to go ahead through a minefield of alarms, yes, under which conditions could the torpedoes be damned? It was more intricate to answer than to ask. Not all factors were known. Some variables would have to be estimated, some guesses taken. The Lem would be up at fifty thousand feet when the descent began. From that distance a pilot could hardly fly it to the ground, for he could not compute the fuel consumptions quickly enough, and the Lem was going to burn some large part of eighteen thousand pounds of fuel on the last leg down. So a pilot could probably not take it in from thirty thousand feet, nor from twenty nor ten, probably not even from ten. At NASA the official guideline was two thousand feet. If the computer ceased to function above two thousand feet, the directive was to abort. Kranz had a private estimate of seven thousand feet. Tall, rough, hard-driving, looking like one of those lean hard-running quarterbacks who play in Southwest conferences, he was built for touchdown, he was looking to go all the way: these sessions were geared to find modes to bring Eagle in if necessary on half a computer, and yet bring her down safely so that she did not find herself one hundred feet in the air with no fuel left, no time to abort, and a crash to the moon ground, first lunar explorers dead or, worse, left alive on a craft too damaged ever to ascend again. At Mission Control they must all have felt as if they were breathing through oxygen masks when the hour of descent approached.
VI
“How does it look?” asked the Capcom.
“The Eagle has wings.”
Eagle came back over the hill an intimate distance apart from Columbia. Pads and updates vibrated on the radio waves while the moon passed below. Each ship made little adjustments for trim, two spacecraft passing sixty-odd miles above the craters of the moon.
COLUMBIA: We’re really stabilized, Neil. I haven’t fired a thruster in five minutes.… I think you’ve got a fine-looking flying machine there, Eagle, despite the fact you’re upside down.
ARMSTRONG: Somebody’s upside down.
From the view of the Command Module, the Lem was floating with its legs in the air. Collins took a photograph here. The Lem swims toward us out of some darkness of space, a tropical fish with extraordinary red-gold antennae and a lead-gray skin to its body. The wrapped pads on its feet shine like gold in the sun. “You guys take care,” said Collins. By radio across the hundred feet of gap, Armstrong said, “See you later,” and Columbia gave a burp to its motors and pulled out at a few feet a second until it was almost a quarter of a mile in front.
“Going right down U.S. 1, Mike,” said Armstrong. An earlier flight of astronauts had given this name to the route, and some shade of the loneliness of driving a highway at night must have been in the voice, for they were onstage, the curtain had at last gone up—there was a glare to the footlights. Into that glare they would walk. Only it must have been more like some dream of theater within theater, as if to step before an audience would transport one to still another theater, the stage was dividing, the walls turned out, they were on a new stage, an entrance further within—some sense of their passage into an isolation within the isolation must have come upon them in the trip up from earth, and now alone in the Lem going down Highway One; in another half-revolution they would begin to descend, another stage would appear, the footlights would shine upon the floor of the moon. Was there time in all the rapt transmissions of future data—lunar surface data pads already on the way up—was there time to taste the dimensions of their new anxiety, its pleasures of risk, its throttled fear like the sensuous tremorings of a fall in a dream? Through aisles of quiet fear the psyche would descend.
But nervous they were. Nervous they all were, Eagle, Columbia, Capcom, and the controllers of the flight. The landing was now two hours off, and Collins called down that he had unexplained roll thruster activity. “I may have bumped the hand control.” A little later Capcom was stumbling on instructions. “Give us a mark when you’re at seven miles—I mean seven-tenths of a mile.” Collins started to call “Houston, Apollo,” and quickly corrected to “Houston, Columbia.” They went over the hill, the Loss of Signal occurred. The flight controllers in Mission Operations stood around in little groups. Now Eagle was firing her thrusters on the far side. Her bottom flipped forward lik
e a runner sliding spikes first, she ignited her descent motor and let it burn for a half-minute—29.8 seconds. That braking of her momentum would slow her into a smaller swoop about the moon. Now her orbit would be no longer a rough circle but an ellipse fifty-seven miles by eight and a half miles above the surface, and when she swung down to the lowest reach of her orbit, some fifty thousand feet from the Sea of Tranquility beneath, a decision would be made; GO or NO GO. Either she would fire her motors again to slow her speed still further and thereby begin that descent where her motors continued to burn until her legs touched the ground, or if something seemed amiss she would pass, and motors not fired, would fling out into another orbit fifty-seven odd miles by eight and a half, perhaps to descend next time around.