Now Armstrong and Aldrin were traveling feet first, face down, in order to visually verify their altitude. This was the only landing to come in face down this way, a safety feature that the astronauts had suggested during training.2
Then came the first indication they'd be facing something other than a perfect landing. A few minutes after the PDI burn, Armstrong recognized a landmark out the window, a crater called Maskelyne W. But it came across his window two or three seconds early. ''Okay, we went by the three-minute point early. We're long.'' Aldrin had his head inside the cockpit, looking at the AGS and PNGS. Armstrong, again, to Houston: ''Our position checks down range show us to be a little long.'' Flight controllers confirmed Armstrong's reading.
Why was Eagle flying long? They were going just a little too fast. A number of problems could have pushed them ahead. The lunar gravity map was incomplete and could not yet account for the strange concentrations of mass, or ''mascons'' that peppered the moon and would subtly pull the spacecraft and alter the trajectory. Also, when the LM separated from the command module, somehow it picked up a little extra velocity. Flight Director Gene Kranz believed it came from a little extra air pressure caught in the docking tunnel, or perhaps it resulted from the LM's maneuvers around the CSM.3 Considering they had just come a quarter of a million miles, we can marvel they would even be aware of such a discrepancy, let alone be concerned about it. But the landing site had been precisely placed, and this extra velocity would cause them to miss it by several miles. Armstrong would miss the landmarks he had carefully memorized.
Still, everything else was going well. On the ground's intercom loop, Gene Kranz queried his team: ''Go to continue powered descent?'' The engineers yelled their affirmations with nervous enthusiasm: ''FIDO: Go.'' ''Guidance: Go.'' ''EECOM: Go.'' ''Surgeon: Go.'' To save time on the voice loop, Kranz just acknowledged ''Rog.''
Now Armstrong yawed over—using his right-hand stick, he rotated the spacecraft around the axis of the descent engine, bringing the two astronauts' eyes to face the sky. They could no longer see the moon they were about to touch;now they were looking up at the earth, their backs to the lunar surface. Another problem: the rotation was moving too slowly, taking longer than planned. Armstrong realized the switch that determined the rotation rate was in the wrong position. He switched it and the LM quickly yawed around.4
The landing radar was on the other side of the LM from the windows, and the yaw over pointed it down toward the surface. The slow rotation meant the radar took longer than expected to lock on the ground. But it did lock on, at 37,000 feet, and thus resolved one of the major uncertainties in the program.
Aldrin keyed in Verb 16 Noun 68, to display the DELTAH, asking, in effect, ''how different was the altitude reading from the radar from the one calculated inside the computer?'' The computer had pulled its position literally from outer space, from its inertial positions and ground updates, whereas the landing radar was providing ground truth—the first time these ethereal calculations were checked against hard moon rocks. The state estimator in the computer ate this sort of data for lunch. As long as the DELTAH was not too large, the computer could incorporate it and adjust its solution. The display read ''-2900'' to Aldrin. The two solutions were less than three thousand feet apart, within acceptable limits according to the paper chart.
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