The Block II design arose from the need to add docking and crew transfer capability to the CM. Reduction of the CM control weight (from 9,500 to 9,100 kilograms (21,000 to 20,000 pounds)) and deficiencies in several major subsystems added to the scope of the redesign. Additional Details: Apollo changes for Block II.
Eight days later, at its static facility in Santa Susana, California, North American first fired the S-II, intermediate stage of the Saturn V. The event was chronicled as the "second major Saturn V milestone" during April. Additional Details: First clustered firing of Saturn V's first stage.
One anomaly, an auxiliary propulsion system leak on the S-IVB stage, produced an unpredictable thrust and prevented a final S-IVB targeting maneuver after separation from the CSM. Tracking of the S-IVB ended at 4:04 p.m. EST April 17, when the instrument unit's signal was lost. The stage hit the lunar surface at 4:02 p.m. April 19, 260 kilometers northeast of the target point. The impact was detected by the seismometers left on the moon by the Apollo 12, 14, and 15 missions.
Spacecraft operations were near normal during the coast to the moon. Unexplained light-colored particles from the LM were investigated and identified as shredded thermal paint. Other activities during the translunar coast included a cislunar navigation exercise, ultraviolet photography of the earth and moon, an electrophoresis demonstration, and an investigation of the visual light-flash phenomenon noted on previous flights. Astronaut Duke counted 70 white, instantaneous light flashes that left no after-glow.
Apollo 16 entered a lunar orbit of 314 by 107.7 kilometers at 3:22 p.m. April 19. After separation of LM-11 Orion from CSM 112 Casper, a CSM active rendezvous kept the two vehicles close together while an anomaly discovered on the service propulsion system was evaluated. Tests and analyses showed the redundant system to be still safe and usable if required. The vehicles were again separated and the mission continued on a revised timeline because of the 5 3/4-hour delay.
The lunar module landed with Duke and Young in the moon's Descartes region, about 230 meters northwest of the planned target area at 9:23 p.m. EST April 20. A sleep period was scheduled before EVA.
The first extravehicular activity began at 11:59 a.m. April 21, after the eight-hour rest period. Television coverage of surface activity was delayed until the lunar roving vehicle systems were activated, because the steerable antenna on the lunar module could not be used. The lunar surface experiments packages were deployed, but accidental breaking of the electronics cable rendered the heat flow experiment inoperable. After completing activities at the experiments site, the crew drove the lunar roving vehicle west to Flag Crater, where they performed the planned tasks. The inbound traverse route was just slightly south of the outbound route, and the next stop was Spook Crater. The crew then returned via the experiment station to the lunar module and deployed the solar wind composition experiment. The duration of the extravehicular activity was 7 hours 11 minutes. The distance traveled by the lunar roving vehicle was 4.2 kilometers. The crew collected 20 kilograms of samples.
The second extravehicular traverse, which began at 11:33 a.m. April 22, was south-southeast to a mare-sampling area near the Cinco Craters on Stone Mountain. The crew then drove in a northwesterly direction, making stops near Stubby and Wreck Craters. The last leg of the traverse was north to the experiments station and the lunar module. The second extravehicular activity lasted 7 hours 23 minutes. The distance traveled by the lunar roving vehicle was 11.1 kilometers.
Four stations were deleted from the third extravehicular traverse, which began 30 minutes early at 10:27 a.m. April 23 to allow extra time. The first stop was North Ray Crater, where "House Rock" on the rim of the crater was sampled. The crew then drove southeast to "Shadow Rock." The return route to the LM retraced the outbound route. The third extravehicular activity lasted 5 hours 40 minutes, and the lunar roving vehicle traveled 11.4 kilometers.
Lunar surface activities outside the LM totaled 20 hours 15 minutes for the mission. The total distance traveled in the lunar roving vehicle was 26.7 kilometers. The crew remained on the lunar surface 71 hours 14 minutes and collected 96.6 kilograms of lunar samples.
While the lunar module crew was on the surface, Mattingly, orbiting the moon in the CSM, was obtaining photographs, measuring physical properties of the moon and deep space, and making visual observations. Essentially the same complement of instruments was used to gather data as was used on the Apollo 15 mission, but different areas of the lunar surface were flown over and more comprehensive deep space measurements were made, providing scientific data that could be used to validate findings from Apollo 15 as well as add to the total store of knowledge of the moon and its atmosphere, the solar system, and galactic space.
The LM lifted off from the moon at 8:26 p.m. EST April 23, rendezvoused with the CSM, and docked with it in orbit. Young and Duke transferred to the CSM with samples, film, and equipment, and the LM was jettisoned the next day. LM attitude control was lost at jettison; therefore a deorbit maneuver was not possible and the LM remained in lunar orbit, with an estimated orbital lifetime of about one year.
The particles and fields subsatellite was launched into lunar orbit and normal system operation was noted. However, the spacecraft orbital shaping maneuver was not performed before ejection and the subsatellite was placed in a non-optimum orbit that resulted in a much shorter lifetime than the planned year. Loss of all subsatellite tracking and telemetry data on the 425th revolution (May 29) indicated that the subsatellite had hit the lunar surface.
The mass spectrometer deployment boom stalled during a retract cycle and was jettisoned before transearth injection. The second plane-change maneuver and some orbital science photography were deleted so that transearth injection could be performed about 24 hours earlier than originally planned.
Activities during the transearth coast phase of the mission included photography for a contamination study for the Skylab program and completion of the visual light-flash-phenomenon investigation that had been partially accomplished during translunar coast. A 1-hour 24-minute transearth extravehicular activity was conducted by command module pilot Mattingly to retrieve the film cassettes from the scientific instrument module cameras, inspect the equipment, and expose a microbial-response experiment to the space environment. Two midcourse corrections were made on the return flight to achieve the desired entry interface conditions.