Young, Duke and Mattingly spent the next three days performing experiments, verifying the functions of the Lunar Module, reviewing mission objectives yet again, and resting. On 19 April, the Moon gave the crew an impressive view out of the Command Module's window and at the precisely planned moment, Mattingly fired the Service Propulsion System engine to slow the spacecraft, allowing the Moon's gravity to capture it as they maneuvered for Lunar Orbit Insertion, or LOI. Four hours later, another SPS burn put the spacecraft into a lower elliptical orbit so that Orion would not have to expend as much fuel during landing. This took them as low as 11 km above the surface of the Moon. Young had been here before on Apollo 10, and he relished pointing out the lunar features to his rookie crewmates.
Landing Orion would not take place until the following day, so the crew took a well-deserved rest from their labors and caught some much-needed sleep. When their wakeup call came from Houston some eight hours later, the crew prepared for the momentous day. Young and Duke suited up and transferred to the Lunar Module. The Command Module and Lunar Module undocked and Young moved Orion away from Casper for Mattingly to perform a circularization burn. The two spacecraft flew in a distant formation as they went around the back of the Moon and out of contact with Houston. However, when Mattingly attempted to gimbal the SPS engine using the backup system, the spacecraft began shaking. Alarmed, Mattingly cancelled the burn and informed Young and Duke. Young told Mattingly that they would hold their formation and wait until they came around from behind the Moon and made radio contact with Houston. All three crewmen were now worried they might have to abort the mission.
When contact was made with CapCom Jim Irwin at Mission Control in Houston, the crew reported the problem with the SPS, saying tersely, "No Circ," for no circularization burn. The red flags went up all over Mission Control. Mattingly was asked to perform the same gimbal commands for both primary and backup systems so they could examine the data. The lunar landing was on hold while mission planners pored over the printouts and conferred with engineers and managers from North American Rockwell, the builders of the Service Module. The hours slipped by, and with them, the hope that Orion would land on the plains at Descartes. Finally, the crew received the welcome news from Mission Control that the primary system for the Service Propulsion System would be used without relying on the backup system, and that they were go for landing. However, the prime time television coverage of portions of the mission, highlighted in a cover story in TV Guide magazine, were lost due to this delay. Mattingly performed his circularization burn using the primary system on the SPS on the back side of the Moon and one-half orbit later, Young and Duke began their powered descent.
"Our trajectory brought us in face up," Duke remembered in an interview with this author. "We couldn't see the Moon until we got to an altitude of about 7,000 feet, and at that point the LM pitched down and the windows were now facing the Moon so we could see our landing site. There were three major craters in our landing area. The surface around our landing site - to the south and the west - turned out to be fairly rough, with rolling terrain that was cratered with scattered boulders. John took over manual control and by 300 feet, he had picked out a landing spot and hovered over the area like a helicopter, trying to zero out the lateral, forward and aft velocities.''
Duke continued to read descent information to Young, barely able to contain his excitement as Young eased Orion slowly to the lunar surface. The Lunar Module's long contact probes touched the surface, illuminating the contact light on the LM's control panel. Young counted "One-potato" and hit the engine stop button. The Lunar Module dropped to the surface. In a matter of seconds, the two astronauts performed the steps they needed to secure the LM and check the spacecraft's health in case they had to make an immediate emergency abort and ascent. Orion landed on the Descartes highlands on 20 April, 1972, at 140 hours, 29 minutes and 38 seconds Ground Elapsed Time (GET).
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