Background

Apollo 8 was a Type “C prime” mission, a CSM piloted flight demonstration in lunar orbit instead of Earth orbit like Apollo 7. It was the first mission to take humans to the vicinity of the Moon, a bold step forward in the development of a lunar landing capability.

The mission was originally designated SA-503, an unpiloted Earth orbital mission to be launched in May 1968 with boilerplate payload BP-30 instead of an operational spacecraft. The success of Apollo 6 (AS-502), however, led to the decision on 27 April that AS-503 would be a piloted mission with a CSM and LM instead of BP-30.

The change to a piloted flight required that the S-II stage be returned to the Mississippi Test Facility for “man-rating.” Additional tests for a piloted flight continued at KSC. The Mississippi tests were successfully completed on 30 May 1968 and the stage returned to the Kennedy Space Center on 27 June.

After two months of testing, which started 11 June 1968, it was determined that the LM would not be ready for the projected early December launch. Therefore, the decision was made on 19 August that a 19,900-pound LM test article would be installed in the spacecraft/launch vehicle adapter for mass loading purposes, replacing the LM. It was also on this date that the crew was instructed to train for a mission to the Moon, officially designated “Apollo 8.”

The possibility of conducting a lunar mission was first discussed with the crew on 10 August, and the results of Apollo 7, to be launched in October, would determine whether the mission would be lunar orbital, circumlunar, or Earth orbital. All training immediately focused on the lunar orbital mission, the most difficult of the three, and ground support preparations were accelerated. The first simulation exercise was conducted on 9 September, and the space vehicle was transferred to the launch site on 9 October.

Following the successful completion of Apollo 7 on 22 October, the official decision to conduct a lunar orbit mission was made 12 November, just five weeks before the scheduled launch. The decision was made after a thorough evaluation of spacecraft performance during Apollo 7’s ten days in Earth orbit and an assessment of the risks involved in a lunar orbit mission. These risks included the total dependency upon the service propulsion engine for propelling the spacecraft from lunar orbit, and a lunar orbit return time of three days, compared to an Earth orbit return of just 30 minutes to three hours. Also considered was the value of the flight in furthering the goal of landing a human on the Moon before the end of 1969. The principal gains from a lunar mission would include experience in deep space navigation, communications, and tracking; greater knowledge of spacecraft thermal response to deep space; and crew operational experience—all directly applicable to lunar landing missions.

Apollo 8 was the first piloted mission launched with the three-stage Saturn V vehicle; the two previous Saturn V flights had been unpiloted. The spacecraft was a Block II CSM, and the spacecraft/launch vehicle adapter was the first to incorporate a mechanism to jettison the panels that would cover the LM on future missions.

The primary objectives of Apollo 8 were:

• to demonstrate the combined performance of the crew, space vehicle, and mission support team during a piloted Saturn V mission with the CSM; and
• to demonstrate the performance of nominal and selected backup lunar orbit rendezvous procedures.

Selected in the astronaut group of 1962, Borman had been command pilot of Gemini 7. Born 14 March 1928 in Gary, Indiana, he was 40 years old at the time of the Apollo 8 mission. Borman received a B.S. from the U.S. Military Academy in 1950 and an M.S. in Aeronautical Engineering in 1957 from the California Institute of Technology. His backup for the mission was Neil Alden Armstrong.

Lovell had been pilot for the Gemini 7 mission and command pilot for Gemini 12. Born 25 March 1928 in Cleveland, Ohio, he was 40 years old at the time of the Apollo 8 mission. Lovell received a B.S. in 1952 from the U.S. Naval Academy, and was selected as an astronaut in 1962. His backup was Colonel Edwin Eugene “Buzz” Aldrin, Jr. (USAF).

Anders was making his first spaceflight. Born 17 October 1933 in Hong Kong, he was 35 years old at the time of the Apollo 8 mission. Anders received a B.S. in Electrical Engineering in 1955 from the U.S. Naval Academy and an M.S. in Nuclear Engineering in 1962 from the U.S. Air Force Institute of Technology, and was selected as an astronaut in 1963. His backup was Fred Wallace Haise, Jr.

The capsule communicators (CAPCOMs) for the mission were Lt. Col. Michael Collins (USAF), Lt. Commander Thomas Kenneth “Ken” Mattingly II (USN), Major Gerald Paul Carr (USMC), Armstrong, Aldrin, Vance DeVoe Brand, and Haise. The support crew were Brand, Mattingly, and Carr. The flight directors were Clifford E. Charlesworth (first shift), Glynn S. Lunney (second shift), and Milton L. Windler (third shift).

The Apollo 8 launch vehicle was a Saturn V, designated SA-503. The mission also carried the designation Eastern Test Range #170. The CSM combination was designated CSM-103. The lunar module test article was designated LTA-B.

Because this was a lunar mission, it was necessary for the vehicle to be launched within a particular daily launch “window”, or time period, within a monthly launch window. Part of the constraints were dictated by the desire to pass over selected lunar sites with lighting conditions similar to those planned for the later landing missions. Lunar orbit inclination, inclination of the free return trajectory, and spacecraft propellant reserves were other primary factors considered in the mission planning.

The first monthly window was in December 1968, with launch dates of 20 to 27 December, and January 1969 as a backup. It was decided to make the first attempt on 21 December to have the total available daily window during daylight. Targeting for this day would allow the flight to pass over a future lunar landing site at latitude 2.63° and longitude 34.03° with a sun elevation angle of 6.74°. The window for 21 December lasted from 12:50:22 to 17:31:40 GMT, with liftoff scheduled for 12:51:00 GMT.

Launch Preparations

The countdown for Apollo 8 began at 00:00 GMT on 16 December 1968. The terminal countdown sequence (T-28 hours) began at 01:51 GMT on 19 December. At that time, space vehicle operations were functionally ahead of the clock. Later in the count, it was discovered that the onboard liquid oxygen supply for the spacecraft environmental control system and fuel cell systems was contaminated with nitrogen. Preparations were made to replace the liquid oxygen, the reservicing operations were completed, and the tanks were pressurized at T-10 hours.

During the planned six-hour hold period at T-9 hours, virtually all of the countdown tasks, delayed by the liquid oxygen detanking and retanking operations, were brought back in line. When the count was picked up again at T-9 hours, space vehicle operations were essentially on schedule. At T-8 hours, S-IVB liquid oxygen loading operations began. The cryogenic loading operations were completed at 08:29 GMT on December 21, eight minutes into the one-hour scheduled hold. The count was picked up at T-3 hours 30 minutes at 09:21 GMT, and the crew entered the spacecraft at T-2 hours 53 minutes.

A cold front passed through the launch area the afternoon before launch and became a stationary front about launch time, laying through the Miami area. At launch time, surface winds were from the north but changed to westerly at 4,900 feet and remained generally from the west above that region. Cirrus clouds covered 40 percent of the sky (cloud base not recorded), visibility was 10 statute miles, the temperature was 59.0° F, relative humidity was 88 percent, dew point was 56 percent, barometric pressure was 14.804 lb/in2 and winds were 18.7 ft/sec at 348° from true north measured by the anemometer on the light pole 60.0 feet above ground at the launch site.

Ascent Phase

Apollo 8 was launched from Launch Complex 39, Pad A, at the Kennedy Space Center, Florida. Liftoff occurred at a Range Zero time of 12:51:00 GMT (07:51:00 a.m. EST) on 21 December 1968, well within the planned launch window.

The ascent phase was nominal. Moments after liftoff, the vehicle rolled from a launch pad azimuth of 90° to a flight azimuth of 72.124° east of north. The S-IC engine shut down at 000:02:33.82, followed by S-IC/S-II separation, and S-II engine ignition. The S-II engine shut down at 000:08:44.04 followed by separation from the S-IVB, which ignited at 000:08:48.29. The first S-IVB engine cutoff occurred at 000:11:24.98, with deviations from the planned trajectory of only +1.44 ft/sec in velocity and only -0.01 n mi in altitude.

The S-IC stage impacted at 000:09:00.410 in the Atlantic Ocean at latitude 30.2040° north and longitude 74.1090° west, 353.462 n mi from the launch site. The S-II stage impacted at 000:19:25.106 in the Atlantic Ocean at latitude 31.8338° north and longitude 37.2774° west, 2,245.913 n mi from the launch site.

Four recoverable film camera capsules were carried aboard the S-IC stage. Two were located in the forward interstage looking forward to view S-IC/S-II separation and S-II engine start. The other two were mounted on top of the S-IC stage LOX tank and contained pulse cameras which viewed aft into the LOX tank through fiber optics bundles. One of the LOX tank capsules was recovered by helicopter at 00:19:30 at latitude 30.22° north and longitude 73.97° west. Despite film damage caused by sea water and dye marker which had leaked into the camera compartment, the film provided usable data. It was not known if the other three capsules were ejected. There were also two television cameras on the S-IC to view propulsion and control system components. Both provided good quality data.

The maximum wind conditions encountered during ascent were 114.1 ft/sec at 284° from true north at 49,900 feet (high dynamic pressure region). Component wind shears were of low magnitude at all altitudes. The maximum was a pitch plane shear of 0.0103 sec-1 at 52,500 feet.

At 000:11:34.98, the spacecraft entered Earth orbit, defined as S-IVB cutoff plus 10 seconds to account for engine tail-off and other transient effects. At insertion, conditions were: apogee and perigee 99.99 by 99.57 n mi, inclination 32.509°, period 89.19 minutes, and velocity 25,567.06 ft/sec. The apogee and perigee were based upon a spherical Earth with a radius of 3,443.934 n mi. The velocity increase relative to Earth from the lunar encounter was 0.79 n mi/sec.

The international designation for the spacecraft upon achieving orbit was 1968-118A and the S-IVB was designated 1968-118B.

Earth Orbit Phase

At 000:42:05, the optics cover was jettisoned and the crew performed star checks over the Carnarvon, Australia, tracking station to verify platform alignment. During the second revolution, at 001:56:00, all spacecraft systems were approved for translunar injection.

Because of the risks involved, the mission had been structured with three commit points: launch, Earth parking orbit, and translunar coast preceding the point where the CSM was to brake into lunar orbit. Had any problems been detected at these points, the plan was to shift to alternate missions, which provided for maximum crew safety and maximum scientific and engineering benefit. Had there been reason for not committing to the third point, the CSM would have continued on its “free-return” trajectory, looping behind the Moon and returning directly to Earth.

After inflight systems checks, it was determined that liquid oxygen venting through the J-2 engine had increased the apogee by 6.4 n mi, a condition that was only 0.7 n mi greater than predicted.

The 317.72-second translunar injection maneuver (second S-IVB firing) was performed at 002:50:37.79. The S-IVB engine shut down at 002:55:55.51 and translunar injection occurred ten seconds later, at a velocity of 35,505.41 ft/sec, after 1.5 Earth revolutions lasting 2 hours 44 minutes 30.53 seconds.

Translunar Phase

The spacecraft was separated from the S-IVB at 003:20:59.3 by a small maneuver of the service module reaction control system, and the high-gain antenna was deployed (later used for the first time at 006:33:04). After spacecraft turnaround, the crew observed and photographed the S-IVB and practiced station-keeping. At 003:40:01, a 1.1 ft/sec maneuver was performed using the service module reaction control system to increase the distance between the spacecraft and the S-IVB. The distance did not increase as rapidly as desired, and a second, 7.7 ft/sec maneuver was performed at 004:45:01.

One objective of the mission was to place the S-IVB into solar orbit. The “slingshot” maneuver required to accomplish this objective included a continuous LH2 vent, a LOX dump, and an auxiliary propulsion system ullage burn. At 004:55:56.02, the LH2 vent valve was opened, and the remaining liquid oxygen and the auxiliary propulsion system propellant in the S-IVB were used to change the trajectory of the S-IVB stage. The liquid oxygen was expelled through the J-2 engine starting at 005:07:55.82 and ended five minutes later.

The auxiliary propulsion motors were fired from 005:25:55.85 to depletion at 005:38:34.00. The resulting velocity increment targeted the S-IVB to go past the trailing edge of the Moon. The lunar radius of closest approach of the S-IVB to the Moon was 1,682 n mi at 069:58:55.2. The point of closest approach to the lunar surface was 682 n mi at latitude 19.2 north by longitude 88.0 east. The orbital parameters after passing from the lunar sphere of influence resulted in a solar orbit with an aphelion and perihelion of 79.770 million by 74.490 million n mi, a semi-major axis of 77,130 million n mi, an inclination of 23.47°, and a period of 340.80 days.

The translunar injection maneuver was so accurate that only one small midcourse correction would have been sufficient to achieve the desired lunar orbit insertion altitude of 65 n mi. However, the second of the 2 maneuvers that separated the spacecraft from the S-IVB altered the trajectory so that a 2.4-second midcourse correction of 20.4 ft/sec at 010:59:59.2 was required to achieve the desired trajectory.[1] For this midcourse correction, the service propulsion system was used to reduce the altitude of closest approach to the Moon from 458.1 to 66.3 n mi. An additional 11.9-second midcourse correction of only 1.4 ft/sec was performed at 060:59:55.9 to refine the lunar insertion conditions further.

During the translunar coast, the crew made systems checks and navigation sightings, and tested the spacecraft high-gain antenna, a four-dish unified S-band antenna that swung out from the service module after separation from the S-IVB.

Apollo 8 was the first piloted U.S. mission in which the crew members experienced symptoms of a mild motion sickness, identical to incipient mild seasickness. Soon after leaving their couches, all three experienced nausea as a result of rapid body movements. The duration of symptoms varied between 2 and 24 hours but did not interfere with operational effectiveness. After waking from a fitful rest period at 016:00:00, the commander experienced a headache, nausea, vomiting, and diarrhea. These symptoms were diagnosed inflight as a possible viral gastroenteritis, an epidemic of which noted in the Cape Kennedy area prior to the mission. During the post mission medical debriefing, the commander reported that the symptoms may have been a side effect of a sleeping tablet he had taken at 011:00:00, which had produced similar symptoms during pre-mission testing of the drug (Seconal™).

Two of the six live television transmissions were also made during translunar flight. The first was a 23-minute 37-second transmission at 031:10:36. The wide-angle lens was used to obtain excellent pictures of the inside of the spacecraft and Lovell preparing a meal; however the telephoto lens passed too much light and pictures of Earth were very poor. A procedure for taping certain filters from the still camera to the television camera improved later transmissions. A 25-minute 38-second transmission at 55:02:45 provided scenes of Earth’s western hemisphere.

At 055:38:40 the crew were notified that they had become the first humans to travel to a place where the pull of Earth’s gravity was less than that of another body. The spacecraft was 176,250 n mi from Earth, 33,800 n mi from the Moon, and their velocity had slowed to 3,261 ft/sec. Gradually, as it moved farther into the Moon’s gravitational field, the spacecraft picked up speed.

Ignition for lunar orbit insertion was performed with the service propulsion system at 069:08:20.4, at an altitude of 75.6 n mi above the Moon. The 246.9-second burn resulted in an orbit of 168.5 by 60.0 n mi and a velocity of 5,458 ft/sec. The translunar coast had lasted 66 hours 16 minutes 21.79 seconds.

Lunar Orbit Phase

As the spacecraft passed behind the Moon for the first time, and communications were interrupted, the Apollo 8 crew became the first humans to see the far side of the Moon. After four hours of navigation checks, ground-based determination of the orbital parameters, and a 12-minute television transmission of the lunar surface at 071:40:52, a 9.6-second lunar orbit circularization maneuver was performed at 073:35:06.6, which resulted in an orbit of 60.7 by 59.7 n mi.

The next 12 hours of crew activity in lunar orbit involved photography of both the near and far sides of the Moon and landing-area sightings. The principal photographic objectives were to obtain vertical and oblique overlapping (stereo strip) photographs during at least two revolutions, photographs of specified targets of opportunity, and photographs through the spacecraft sextant of a potential landing site. The purpose of the overlapping photography was to determine elevation and geographical position of lunar far side features. The targets of opportunity were areas recommended for photography if time and circumstances permitted. They were selected to provide either detailed coverage of specific features or broad coverage of areas not adequately covered by satellite photography. Most were proposed to improve knowledge of areas on the Earthfacing hemisphere. The sextant photography was included to provide image comparisons for landmark evaluation and navigation training purposes. A secondary objective was to photograph one of the certified Apollo landing sites.

The Apollo 8 photography afforded the first opportunity to analyze the intensity and spectral distribution of lunar surface illumination free from the atmospheric modulation present in Earth telescopic photography and without the electronic processing losses present in satellite photography.

The crew completed photographic exercises in an excellent manner. Over 800 70 mm still photographs were obtained. Of these, 600 were good-quality reproductions of lunar surface features, and the remainder were of the S-IVB during separation and venting, and long-distance Earth and lunar photography.

Over 700 feet of 16 mm film were also exposed during the S-IVB separation, lunar landmark photography through the sextant, lunar surface sequence photography, and documentation of intravehicular activity.

The still photography contributed significantly to knowledge of the lunar environment. In addition, many valuable observations were made by the crew. Their initial comments during the lunar orbit phase included descriptions of the color of the lunar surface as “black-and-white,” “absolutely no color” or “whitish gray, like dirty beach sand.” As expected, the crew could recognize surface features in shadow zones and extremely bright areas of the lunar surface, but these features were not well delineated in the photographs.

This recognition combined with the photographic information enabled new interpretations of lunar surface features and phenomena. As a result, lunar-surface lighting constraints for the lunar landing missions were widened.

Prior to Apollo 8, the lower limit for lunar lighting was believed to be 6°. The Apollo 8 crew observed surface detail at sun angles in the vicinity of 2° or 3° and stated that these low angles should present no problem for a lunar landing, but landing sites in long shadow areas, however, were to be avoided. At the higher limit, an upper bound of 16° would still provide very good definition of surface features for most of the critical landing phase near touchdown. Between 16° and 20°, lighting was judged acceptable for viewing during final descent. A sun angle above 20° was considered unsatisfactory for a manual landing maneuver.

The crew report of the absence of sharp color boundaries was significant. The lack of visible contrast from an altitude of 60 n mi reduced the probability that a crew would be able to use color to distinguish geologic units while operating near or on the lunar surface.

Just prior to sunrise on one of the early lunar orbit revolutions, the command module pilot observed what was believed to be zodiacal light and solar corona through the telescope. The lunar module pilot observed a cloud or bright area in the sky during lunar darkness on two successive revolutions. The identification, if correct, indicated that one of the Magellanic clouds had been observed.

Long-distance Earth photography of general interest highlighted global weather and terrain features. Lunar photography had not been accomplished during translunar coast because of rigid spacecraft attitude constraints. However, good quality photography of most of the Moon disk was accomplished during transearth coast.

The crew initially followed the lunar orbit mission plan and performed all scheduled tasks. However, because of crew fatigue, the commander made the decision at 084:30 to cancel all activities during the final four hours in lunar orbit to allow the crew to rest. The only activities during this period were a required platform alignment and preparation for transearth injection. A planned 26-minute 43-second television transmission of the Moon and Earth was made at 085:43:03, on Christmas eve. It was during this transmission that the crew read from the Bible the first ten verses of Genesis, and then wished viewers “Good night, good luck, a Merry Christmas, and God bless all of you, all of you on the good Earth.” An estimated one billion people in 64 countries heard or viewed the live reading and greeting; delayed broadcasts reached an additional 30 countries that same day.

Orbit analysis indicated that previously unknown mass concentrations or “mascons” were perturbing the orbit. As a result, the final lunar orbit had an apogee and perigee of 63.6 by 58.6 n mi. The 203.7-second transearth injection maneuver was performed with the service propulsion system at an altitude of 60.2 n mi at 089:19:16.6 after ten revolutions and 20 hours 10 minutes 13.0 seconds in lunar orbit. The velocity at transearth injection was 8,842 ft/sec. During the mission, the spacecraft reached a maximum distance from Earth of 203,752.37 n mi.

Transearth Phase

After emerging from lunar occlusion following transearth injection, Apollo 8 experienced the only significant communications difficulty of the mission. Although two-way phaselock was established at 089:28:47, two-way voice contact and telemetry synchronization were not achieved until 089:33:28 and 089:43:00, respectively. Data indicated that high-gain antenna acquisition may have been attempted while line-of-sight was within the service module reflection region and that the reflections may have caused the antenna to track on a side lobe. In addition, the spacecraft was erroneously configured for high-bit-rate transmission; therefore the command at 089:29:29 that configured the spacecraft for normal voice and subsequent playback of the data storage equipment, selected an S-band signal combination that was not compatible with the received carrier power.

The transearth coast activities included star/horizon navigation sightings using both Moon and Earth horizons. Passive thermal control, using a roll rate of one revolution per hour, was used during most of the translunar and transearth coast phases to maintain nearly stable onboard temperatures. Only one small transearth midcourse correction, a 15.0-second maneuver using the service module reaction control system, was required at 104:00:00, and changed the velocity by 4.8 ft/sec.

Because of a crew procedural error, the onboard state vector and platform alignment were lost at 106:26. Realignment was performed at 106:45.

A special test of the automatic acquisition mode of the high-gain antenna was performed at 110:16:55. Results indicated that the antenna performed as predicted.

The final two television transmissions were made during transearth coast. The fifth was a 9-minute 31-second transmission of the spacecraft interior at 104:24:04. The sixth transmission was for 19 minutes 54 seconds at 127:45:33 and featured views of Earth, particularly of the western hemisphere.

The service module was jettisoned at 146:28:48.0, and the CM entry followed an automatically guided entry profile. No radar tracking data for the service module were available during entry, but photographic coverage information correlated well with the predicted trajectory in altitude, latitude, longitude, and time.

Recovery

The command module reentered Earth’s atmosphere (400,000 feet altitude) at 146:46:12.8 at a velocity of 36,221.1 ft/sec, following a transearth coast of 57 hours 23 minutes 32.5 seconds. The ionization became so bright during entry that the CM interior was bathed in a cold blue light as bright as daylight. At 180,000 feet, as expected, the lift of the CM bounced it to 210,000 feet, where it then resumed its downward course.

The parachute system effected splashdown of the CM in the Pacific Ocean at 15:51:42 GMT (10:51:42 a.m. EST) on 27 December. Mission duration was 147:00:42.0. The impact point was 1.4 n mi from the target point and 2.6 n mi from the recovery ship U.S.S. Yorktown. The splashdown site was estimated to be latitude 8.10° north and longitude 165.00° west. Due to the splashdown impact, the CM assumed an apex-down flotation attitude, but was successfully returned to the normal flotation position 6 minutes and 3 seconds later by the inflatable bag uprighting system.

As planned, helicopters and aircraft hovered over the spacecraft and pararescue personnel were not deployed until local sunrise, 43 minutes after splashdown. At dawn, the crew was retrieved by helicopter and were aboard the recovery ship 88 minutes after splashdown. The spacecraft was recovered 60 minutes later. The estimated CM weight at splashdown was 10,977 pounds and the estimated distance traveled for the mission was 504,006 n mi.

At the time the recovery swimmers were deployed, the weather recorded onboard the Yorktown showed scattered clouds at 2,000 feet and overcast at 9,000 feet, visibility ten n mi, wind speed 19 knots from 70° true north, water temperature 82° F, and waves to six feet from 110° true north.

The CM was offloaded from the Yorktown on December 29 at Ford Island, Hawaii. The Landing Safing Team began the evaluation and deactivation procedures at 21:00 GMT, and completed them on 1 January 1969. The CM was then flown to Long Beach, California, and trucked to the North American Rockwell Space Division facility at Downey, California for postflight analysis. It arrived on 2 January 1969 at 21:00 GMT.

Conclusions

With only minor problems, all Apollo 8 spacecraft systems operated as intended, and all primary mission objectives were successfully accomplished. Crew performance was admirable throughout the mission. Approximately 90 percent of the photographic objectives were accomplished and 60 percent of the additional lunar photographs requested as “targets of opportunity” were also taken, despite fogging of three of the spacecraft windows due to exposure of the window sealant to the space environment and early curtailment of crew activities due to fatigue. Many smaller lunar features, previously undiscovered, were photographed. These features were located principally on the far side of the Moon in areas which had been photographed only at much greater distances by automated spacecraft. In addition, the heat shield system was not adversely affected by exposure to cislunar space or to the lunar environment and performed as expected. The following conclusions were made from an analysis of post-mission data:

1. The CSM systems were operational for a piloted lunar mission.
2. All system parameters and consumable quantities were maintained well within their design operating limits during both cislunar and lunar orbit flight.
3. Passive thermal control, a slow rolling maneuver perpendicular to the Sun line, was a satisfactory means of maintaining critical spacecraft temperatures near the middle of the acceptable response ranges.
4. The navigation techniques developed for translunar and lunar orbit flight were proved to be more than adequate to maintain required lunar orbit insertion and transearth injection guidance accuracies.
5. Non-simultaneous sleep periods adversely affected the normal circadian cycle of each crew member and provided a poor environment for undisturbed rest. Mission activity scheduling for the lunar orbit coast phase also did not provide adequate time for required crew rest periods.
6. Communications and tracking at lunar distances were excellent in all modes. The high-gain antenna, flown for the first time, performed exceptionally well and withstood dynamic structural loads and vibrations which exceeded anticipated operating levels.
7. Crew observations of the lunar surface showed the “washout” effect (surface detail being obscured by backscatter) to be much less severe than anticipated. In addition, smaller surface details were visible in shadow areas at low sun angles, indicating that lighting for lunar landing should be photometrically acceptable.
8. To accommodate the change in Apollo 8 from an Earth orbital to a lunar mission, pre-mission planning, crew training, and ground support reconfigurations were completed in a time period significantly shorter than usual. The required response was particularly demanding on the crew and, although not desirable on a long-term basis, exhibited a capability which had never before been demonstrated.

SOURCE: NASA HISTORY