for the Descent Orbit Insertion (DOI) maneuver to further modify the spacecraft's orbital trajectory. The maneuver was successful, decreasing the craft's pericynthion to . The remainder of flight day four was spent making observations and preparing for activation of the lunar module, undocking, and landing the following day. Lunar surface The crew continued preparing for lunar module activation and undocking shortly after waking up to begin flight day five. The boom that extended the mass spectrometer in the SIM bay was stuck, semi-deployed. It was decided that Young and Duke would visually inspect the boom after undocking the LM from the CSM. They entered the LM for activation and checkout of the spacecraft's systems. Despite entering the LM 40 minutes ahead of schedule, they completed preparations only 10 minutes early due to numerous delays in the process. With the preparations finished, they undocked 96 hours, 13 minutes, 31 seconds into the mission. For the rest of the two crafts' passes over the near side of the Moon, Mattingly prepared to shift Casper to a higher, near-circular orbit, while Young and Duke prepared Orion for the descent to the lunar surface. At this point, during tests of the CSM's steerable rocket engine in preparation for the burn to modify the craft's orbit, Mattingly detected oscillations in the SPS engine's backup gimbal system. According to mission rules, under such circumstances, Orion was to re-dock with Casper, in case Mission Control decided to abort the landing and use the lunar module's engines for the return trip to Earth. Instead, the two craft kept station, maintaining positions close to each other. After several hours of analysis, mission controllers determined that the malfunction could be worked around, and Young and Duke could proceed with the landing. Powered descent to the lunar surface began about six hours behind schedule. Because of the delay, Young and Duke began their descent to the surface at an altitude higher than that of any previous mission, at . After descending to an altitude of about , Young was able to view the landing site in its entirety. Throttle-down of the LM's landing engine occurred on time, and the spacecraft tilted forward to its landing orientation at an altitude of . The LM landed north and west of the planned landing site at 104 hours, 29 minutes, and 35 seconds into the mission, at 2:23:35 UTC on April 21 (8:23:35 pm