Question: Why were the Apollo space craft's preparations started early?
About the article "Apollo 11", section "Landing".

Candidate passages:
Mission highlights.
Launch and outbound trip.
The launch of Apollo 16 was delayed one month from March 17 to April 16.
This was the first launch delay in the Apollo program due to a technical problem.
During the delay, the space suits, a spacecraft separation mechanism and batteries in the Lunar Module (LM) were modified and tested.
There were concerns that the explosive mechanism designed to separate the docking ring from the Command Module (CM) would not create enough pressure to completely sever the ring.
This, along with a dexterity issue in Young's space suit and fluctuations in the capacity of the Lunar Module batteries, required investigation and trouble-shooting.
In January 1972, three months before the planned April launch date, a fuel tank in the Command Module was accidentally damaged during a routine test.
The rocket was returned to the Vertical Assembly Building (VAB) and the fuel tank replaced, and the rocket returned to the launch pad in February in time for the scheduled launch.
The official mission countdown began on Monday, April 10, 1972, at 8:30 AM, six days before the launch.
At this point the Saturn V rocket's three stages were powered up and drinking water was pumped into the spacecraft.
As the countdown began, the crew of Apollo 16 was participating in final training exercises in anticipation of a launch on April 16.
The astronauts underwent their final preflight physical examination on April 11.
On April 15, liquid hydrogen and liquid oxygen propellants were pumped into the spacecraft, while the astronauts rested in anticipation of their launch the next day.
The Apollo 16 mission launched from the Kennedy Space Center in Florida at 12:54 PM EST on April 16, 1972.
The launch was nominal; the crew experienced vibration similar to that of previous crews.
The first and second stages of the Saturn V rocket performed nominally; the spacecraft entered orbit around Earth just under 12 minutes after lift-off.
After reaching orbit, the crew spent time adapting to the zero-gravity environment and preparing the spacecraft for Trans Lunar Injection (TLI), the burn of the third-stage rocket that would propel them to the Moon.
In Earth orbit, the crew faced minor technical issues, including a potential problem with the environmental control system and the S-IVB third stage's attitude control system, but eventually resolved or compensated for them as they prepared to depart towards the Moon.
After two orbits, the rocket's third stage reignited for just over five minutes, propelling the craft towards the Moon at about .
Six minutes after the burn of the S-IVB, the Command/Service Module, containing the crew, separated from the rocket and traveled for before turning around and retrieving the Lunar Module from inside the expended rocket stage.
The maneuver, known as transposition, went smoothly and the LM was extracted from the S-IVB.
Following transposition and docking, the crew noticed the exterior surface of the Lunar Module was giving off particles from a spot where the LM's skin appeared torn or shredded; at one point, Duke estimated they were seeing about five to ten particles per second.
The crew entered the Lunar Module through the docking tunnel connecting it with the Command Module to inspect its systems, at which time they did not spot any major issues.
Once on course towards the Moon, the crew put the spacecraft into a rotisserie "barbecue" mode in which the craft rotated along its long axis three times per hour to ensure even heat distribution about the spacecraft from the Sun.
After further preparing the craft for the voyage, the crew began the first sleep period of the mission just under 15 hours after launch.
By the time Mission Control issued the wake-up call to the crew for flight day two, the spacecraft was about away from the Earth, traveling at about .
As it was not due to arrive in lunar orbit until flight day four, flight days two and three were largely preparatory days, consisting of spacecraft maintenance and scientific research.
On day two, the crew performed an electrophoresis experiment, also performed on Apollo 14, in which they attempted to prove the higher purity of particle migrations in the zero-gravity environment.
The remainder of day two included a two-second mid-course correction burn performed by the Command/Service Module's Service Propulsion System engine to tweak the spacecraft's trajectory.
Later in the day, the astronauts entered the Lunar Module for the second time in the mission to further inspect the landing craft's systems.
The crew reported they had observed additional paint peeling from a portion of the LM's outer aluminum skin.
Despite this, the crew discovered that the spacecraft's systems were performing nominally.
Following the LM inspection, the crew reviewed checklists and procedures for the following days in anticipation of their arrival and the Lunar Orbit Insertion burn.
Command Module Pilot Mattingly reported a "gimbal lock" warning light, indicating the craft was not reporting an attitude.
Mattingly alleviated this by realigning the guidance system using the Sun and Moon.
At the end of day two, Apollo 16 was about away from Earth.
At the beginning of day three, the spacecraft was about away from the Earth.
The velocity of the craft steadily decreased, as it had not yet reached the lunar sphere of gravitational influence.
The early part of day three was largely housekeeping, spacecraft maintenance and exchanging status reports with Mission Control in Houston.
The crew performed the Apollo light flash experiment, or ALFMED, to investigate "light flashes" that were seen by the astronauts when the spacecraft was dark, regardless of whether or not their eyes were open, on Apollo lunar flights.
This was thought to be caused by the penetration of the eye by cosmic ray particles.
During the second half of the day, Young and Duke again entered the Lunar Module to power it up and check its systems, and perform housekeeping tasks in preparation for lunar landing.
The systems were found to be functioning as expected.
Following this, the crew donned their space suits and rehearsed procedures that would be used on landing day.
Just before the end of flight day three at 59 hours, 19 minutes, 45 seconds after liftoff, while from the Earth and from the Moon, the spacecraft's velocity began increasing as it accelerated towards the Moon after entering the lunar sphere of influence.
After waking up on flight day four, the crew began preparations for the maneuver that would brake the spacecraft into orbit around the Moon, or lunar orbit insertion.
At a distance of from the Moon, the Scientific Instrument Module (SIM) bay cover was jettisoned.
At just over 74 hours into the mission, the spacecraft passed behind the Moon, losing direct contact with Mission Control.
While over the far side of the Moon, the Command/Service Module's Service Propulsion System engine burned for 6 minutes and 15 seconds, braking the spacecraft into an orbit around the Moon with a low point (pericynthion) of 58.3 and a high point (apocynthion) of 170.4 nautical miles (108.0 and 315.6 km, respectively).
After entering lunar orbit, the crew began preparations 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 next day. 
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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 out from the Command/Service Module's Scientific Instruments Bay was stuck in a semi-deployed position.
It was decided that Young and Duke would visually inspect the boom after undocking from the CSM in the LM.
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 in the LM "Orion" from Mattingly in the Command/Service Module "Casper" 96 hours, 13 minutes, 13 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 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, a malfunction occurred in the engine's backup system.
According to mission rules, "Orion" would have then re-docked with "Casper", in case Mission Control decided to abort the landing and use the Lunar Module's engines for the return trip to Earth.
After several hours of analysis, however, mission controllers determined that the malfunction could be worked around and Young and Duke could proceed with the landing.
As a result of this, 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 .
At 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.
After landing, Young and Duke began powering down some of the LM's systems to conserve battery power.
Upon completing their initial adjustments, the pair configured "Orion" for their three-day stay on the lunar surface, removed their space suits and took initial geological observations of the immediate landing site.
They then settled down for their first meal on the surface.
After eating, they configured the cabin for their first sleep period on the Moon.
The landing delay caused by the malfunction in the Command/Service Module's main engine necessitated significant modifications to the mission schedule.
Apollo 16 would spend one less day in lunar orbit after surface exploration had been completed to afford the crew contingency time to compensate for any further problems and to conserve expendables.
In order to improve Young's and Duke's sleep schedule, the third and final moonwalk of the mission was trimmed from seven hours to five.
The next morning, flight day five, Young and Duke ate breakfast and began preparations for the first extra-vehicular activity (EVA), or moonwalk.
After the pair donned and pressurized their space suits and depressurized the Lunar Module cabin, Young climbed out onto the "porch" of the LM, a small platform above the ladder.
Duke handed Young a jettison bag full of trash to dispose of on the surface.
Young then lowered the equipment transfer bag (ETB), containing equipment for use during the EVA, to the surface.
Young descended the ladder and, upon setting foot on the lunar surface, became the ninth human to walk on the Moon.
Upon stepping onto the surface, Young expressed his sentiments about being there: "There you are: Mysterious and Unknown Descartes.
Highland plains.
Apollo 16 is gonna change your image.
I'm sure glad they got ol' Brer Rabbit, here, back in the briar patch where he belongs."
Duke soon descended the ladder and joined Young on the surface, becoming the tenth and youngest human to walk on the Moon, at age 36.
After setting foot on the lunar surface, Duke expressed his excitement, commenting: "Fantastic!
Oh, that first foot on the lunar surface is super, Tony!"
The pair's first task of the moonwalk was to unload the Lunar Roving Vehicle, the Far Ultraviolet Camera/Spectrograph (UVC), and other equipment, from the Lunar Module.
This was done without problems.
On first driving the lunar rover, Young discovered that the rear steering was not working.
He alerted Mission Control to the problem before setting up the television camera and planting the flag of the United States with Duke.
The day's next task was to deploy the Apollo Lunar Surface Experiments Package (ALSEP); while they were parking the lunar rover, on which the TV camera was mounted, to observe the deployment, the rear steering began functioning without explanation.
While deploying a heat-flow experiment that had burned up with the Lunar Module "Aquarius" on Apollo 13 and had been attempted without success on Apollo 15, a cable was inadvertently snapped after getting caught around Young's foot.
After ALSEP deployment, they collected samples in the vicinity.
About four hours after the beginning of EVA-1, they mounted the lunar rover and drove to the first geologic stop, Plum Crater, a crater on the rim of Flag Crater, a crater across.
There, at a distance of from the LM, they sampled material from the vicinity of Flag Crater, which scientists believed penetrated through the upper regolith layer to the underlying Cayley Formation.
It was there that Young retrieved, at the request of Mission Control, the largest rock returned by an Apollo mission, a breccia nicknamed Big Muley after mission geology principal investigator William R. Muehlberger.
The next stop of the day was Buster Crater, about from the LM.
There, Duke took pictures of Stone Mountain and South Ray Crater while Young deployed a magnetic field experiment.
At that point, scientists began to reconsider their pre-mission hypothesis that Descartes had been the setting of ancient volcanic activity, as the two astronauts had yet to find any volcanic material.
Following their stop at Buster, Young did a demonstration drive of the lunar rover while Duke filmed with a 16 mm movie camera.
After completing more tasks at the ALSEP, they returned to the LM to close out the moonwalk.
They reentered the LM 7 hours, 6 minutes, and 56 seconds after the start of the EVA.
Once inside, they pressurized the LM cabin, went through a half-hour briefing with scientists in Mission Control, and configured the cabin for the sleep period.
Shortly after waking up on the morning of flight day six three and a half minutes early, they discussed with Mission Control in Houston the day's timeline of events.
The second lunar excursion's primary objective was to visit Stone Mountain to climb up the slope of about 20 degrees to reach a cluster of five craters known as "Cinco Craters."
After preparations for the day's moonwalk were completed, the astronauts climbed out of the Lunar Module.
After departing the immediate landing site in the lunar rover, they arrived at the day's first destination, the Cinco Craters, from the LM.
At above the valley floor, the pair were at the highest elevation above the LM of any Apollo mission.
After marveling at the view from the side of Stone Mountain, which Duke described as "spectacular," the astronauts gathered samples in the vicinity.
After spending 54 minutes on the slope, they climbed aboard the lunar rover en route to the day's second stop, station five, a crater across.
There, they hoped to find Descartes material that had not been contaminated by ejecta from South Ray Crater, a large crater south of the landing site.
The samples they collected there, although their origin is still not certain, are, according to geologist Don Wilhelms, "a reasonable bet to be Descartes."
The next stop, station six, was a blocky crater, where the astronauts believed they could sample the Cayley Formation as evidenced by the firmer soil found there.
Bypassing station seven to save time, they arrived at station eight on the lower flank of Stone Mountain, where they sampled material on a ray from South Ray Crater for about an hour.
There, they collected black and white breccias and smaller, crystalline rocks rich in plagioclase.
At station nine, an area known as the "Vacant Lot," which was believed to be free of ejecta from South Ray, they spent about 40 minutes gathering samples.
Twenty-five minutes after departing station nine, they arrived at the final stop of the day, halfway between the ALSEP site and the LM.
There, they dug a double core and conducted several penetrometer tests along a line stretching east of the ALSEP.
At the request of Young and Duke, the moonwalk was extended by ten minutes.
After returning to the LM to wrap up the second lunar excursion, they climbed back inside the landing craft's cabin, sealing and pressurizing the interior after 7 hours, 23 minutes, and 26 seconds of EVA time, breaking a record that had been set on Apollo 15.
After eating a meal and proceeding with a debriefing on the day's activities with Mission Control, they reconfigured the LM cabin and prepared for the sleep period.
Flight day seven was their third and final day on the lunar surface, returning to orbit to rejoin Mattingly in the Command/Service Module following the day's moonwalk.
During the third and final lunar excursion, they were to explore North Ray Crater, the largest of any of the craters any Apollo expedition had visited.
After exiting "Orion", the pair drove the lunar rover away from the LM before adjusting their heading to travel to North Ray Crater.
The drive was smoother than that of the previous day, as the craters were more shallow and boulders were less abundant north of the immediate landing site.
Boulders gradually became larger and more abundant as they approached North Ray in the lunar rover.
Upon arriving at the rim of North Ray Crater, they were away from the LM.
After their arrival, the duo took photographs of the wide and deep crater.
They visited a large boulder, taller than a four-story building, which became known as 'House Rock'.
Samples obtained from this boulder delivered the final blow to the pre-mission volcanic hypothesis, proving it incorrect.
House Rock had numerous bullet hole-like marks where micrometeoroids from space had impacted the rock.
About 1 hour and 22 minutes after arriving, they departed for station 13, a large boulder field about from North Ray.
On the way, they set a lunar speed record, traveling at an estimated downhill.
They arrived at a high boulder, which they called 'Shadow Rock'.
Here, they sampled permanently shadowed soil.
During this time, Mattingly was preparing the Command/Service Module in anticipation their return approximately six hours later.
After three hours and six minutes, they returned to the LM, where they completed several experiments and offloaded the rover.
A short distance from the LM, Duke placed a photograph of his family and a United States Air Force commemorative medallion on the surface.
Young drove the rover to a point about east of the LM, known as the 'VIP site,' so its television camera, controlled remotely by Mission Control, could observe Apollo 16's liftoff from the Moon.
They then reentered the LM after a 5 hour and 40 minute final excursion.
After pressurizing the LM cabin, the crew began preparing to return to lunar orbit. 
Unmanned: Several subscale test flights AS-201 AS-202 Apollo 4 Apollo 6 Manned: Apollo 7 Apollo 8 Apollo 9 Apollo 10 Apollo 11 Apollo 12 Apollo 13 Apollo 14 Apollo 15 Apollo 16 Apollo 17 Skylab 2 Skylab 3 Skylab 4 Apollo–Soyuz Test Project Early Space Shuttle Flights  
The Soyuz 16 mission was the final rehearsal and first manned mission in a program which culminated in the Apollo-Soyuz (ASTP) mission seven months later.
The Soviet Union and the United States of America, Cold War rivals, had signed several arms control treaties in the 1960s and 1970s, and had entered into a period of detente by the early 1970s.
In 1972, a treaty was signed to participate in a joint manned space flight as a symbol of this detente.
Early concepts for a joint flight included docking a Soyuz craft to the American Skylab space station, or an Apollo vehicle docking with a Salyut space station.
Once the Americans abandoned their Skylab station in 1974, the Apollo-Salyut concept seemed to be the logical choice, but since the Soviets had started to develop a universal docking adapter for the mission and feared having to publicly reveal details of their military-focused Salyut missions, the two powers opted to link a Soyuz spacecraft with an Apollo spacecraft.
Three test flights of an unmanned version of the ASTP spacecraft were flown: Cosmos 638, launched 3 April 1974; Cosmos 652, launched 15 May 1974; and Cosmos 672, launched 12 August 1974.
These three flights, and Soyuz 16, were all launched with an improved version of a Soyuz booster.