For the first 26 seconds, the engine fired at only 1,280 pounds, 10 percent of potential thrust. With commander and LM pilot looking on like mother hens, PGNS checked engine performance and gimballed the descent engine to fire through the ship’s center of gravity. Then the first rocket engine that could be throttled in space kicked into high gear.
“At first you don’t feel much of anything,” says Cernan. “But 26 seconds later, when the descent engine went to full throttle, it was like a booming growl and the vibration felt like big wheels churning beneath your feet.”
The LM began to bleed off the forward momentum that had kept it in lunar orbit. Within seconds the astronauts were below orbital velocity. If there was an emergency now, if the PGNS went crazy or the descent engine failed, their crewmate in the Apollo command module could not descend to save them, as he could have if they were orbiting above an altitude of 40,000 feet. The LM crew would now have to work it out for themselves.
Back in Houston, controllers monitored the descent. Had tracking data indicated that the LM was veering away from the planned landing site, they would have transmitted new coordinates for the astronauts to load into the computer. Even without Houston’s input, PGNS could compare the LM’s position to the mission flight plan programmed into its memory and issue corrective throttle and steering commands if needed.
During P-63, the majority of the rocket firing came from the descent engine. But with sloshing fuel making up more than half the entire weight of the LM, things had a tendency to get out of sorts. To fine-tune the LM’s attitude, PGNS called upon 16 small 100-pound-thrust motors mounted on the ascent stage in clusters of four. Called the Reaction Control System, or RCS, the little thrusters, when used in various combinations, rotated the LM about any axis and performed small translational (left-right/up-down/forward-aft) adjustments in any direction. When the RCS fired, the astronauts couldn’t miss it. “The skin of the LM was so thin,” says Apollo 10 commander Tom Stafford, “and the thrusters were right there in front of you. If you want to simulate flying a lunar module, take a washtub, put it over your head, and have a kid bang on it with a hammer.”
While the RCS was hammering away at the LM and its occupants, the LM’s landing radar was calling out to the moon. Four minutes and 55 seconds into the PDI burn, the moon answered. Microwave beams pulsing out of the Challenger’s landing radar provided the first direct contact between Apollo 17 and the lunar surface. With no appreciable atmosphere, LM crews could not rely on air pressure readings to provide altitude and airspeed information. Landing radar was so vital that NASA issued a mission rule: If you don’t get radar lock-on by 10,000 feet, abort. The LM astronauts would have aborted by separating from the descent stage, firing the ascent stage engine, and climbing to an orbit in which they would be able to dock with the command module.
Five of the six Apollo moon landing missions did not have a problem getting good radar data at an altitude of over 35,000 feet. But on Apollo 14, the radar had not yet kicked on and mission commander Alan Shepard was not happy. “They called up and said, ‘Your landing radar is not working,’ ” said Shepard in a 1998 interview. “We said, ‘Thank you very much, we’re aware of that.’ And then a little bit further on they said, ‘You know what the ground rule is, if you’re at [10,000] feet.’ Well, yeah, we knew that. Finally, some bright young man [in mission control] said, ‘Hey, your landing radar is working, but it’s locked to infinity. Have them pull the switch, reset it, and see if it works.’ So we pulled the circuit breaker, put it back in, and sure enough the landing radar came on.”
If there was no abort, the astronauts were ready for the approach phase, which was handled by computer program P-64 and initiated at an altitude of 7,515 feet above the moon’s surface. Traveling at a horizontal velocity of 506 feet per second and a vertical velocity of 145 feet per second, the astronauts were now ready to take their first real gander at their landing site while continuing to reduce forward and vertical velocities to near zero. They had to quickly locate important landmarks like large distinctive craters, specific mountain ranges, and rilles—cracks in the moon’s crust.
On Apollo 15, after astronauts David Scott and Jim Irwin began P-64, they found themselves heading for the wrong location. “As we pitched over and I looked out, there were very few shadows as far as craters go,” said Scott in a 1971 crew debriefing. “I measured my east-west displacement by my relative motion to the rille, and I could see we were in fairly good shape, relative to the rille, but we were south.”
Landing in the correct location came in a close second to landing safely at all. If a moon crew was forced to land an appreciable distance from its intended target, a mission’s entire scientific objective could be compromised. The mission that ended up farthest from the target was Apollo 11, at a whopping 4.2 miles. But on that mission, which was the first moon landing, planting the flag and grabbing any moonrock were plenty good enough. On every subsequent Apollo mission, however, the landing point made all the difference.