Apollo 10: Mission Operations and Observations

Apollo 10 undertook verification of the complete spacecraft system in preparation for a lunar landing. NASA had already flown people around the Moon on Apollo 8 and tested a crewed lunar module in Earth orbit on Apollo 9. Apollo 10 brought the command and service modules and lunar module together to the Moon to test navigation, communications, separation, rendezvous and docking at the pace of a landing mission.

Actual descent to the surface, landing, a surface stay and liftoff from the Moon were outside this mission's scope. Astronauts still had to observe terrain and lighting in the planned landing area, test the landing radar and obtain data to improve the lunar gravity model. The mission therefore combined engineering flight tests with orbital observations.

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The Manned Spacecraft Center in Houston handled spacecraft development, astronaut training and flight control; Marshall Space Flight Center handled the Saturn launch vehicle, and Kennedy Space Center launch operations. North American Rockwell built the command and service modules, Grumman the lunar module, and the Massachusetts Institute of Technology managed guidance and navigation technology. All systems had to work together in ground checks and actual flight.

The crew comprised commander Thomas Stafford, command module pilot John Young and lunar module pilot Eugene Cernan. Young would fly the command module alone in lunar orbit while the other two operated the lunar module. This arrangement tested normal rendezvous procedures and required Young to prepare contingency maneuvers if the lunar module encountered a failure.

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At 16:49 on May 18, 1969, a Saturn V launched from Kennedy Space Center, Florida; dates and times here and below are in Universal Time. The combined vehicle first entered an Earth parking orbit. After system checks, the third stage reignited to place it on a lunar-bound trajectory. Young then turned the command and service modules, docked and extracted the lunar module.

A planned course correction during the journey brought flight conditions closer to those of the landing mission scheduled for July. On May 21, the service propulsion engine placed the spacecraft in lunar orbit, followed by another adjustment to a nearly circular orbit of about 60 nautical miles. The crew inspected the lunar module and transferred equipment in preparation for separation.

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On May 22, the two spacecraft separated in lunar orbit. Stafford and Cernan fired the descent engine to enter a low orbit passing near the intended landing area, testing the landing radar and photographing terrain. The mission report recorded the lowest measured surface altitude as 47,400 feet, about 14.4 kilometers. The approach did not continue to a landing.

Around separation of the lunar module's ascent and descent stages, the spacecraft experienced pronounced attitude oscillations. Postflight analysis of telemetry, switch positions and ground checks attributed the problem to a combination of an inadvertent abort-guidance mode change, anomalous yaw-rate gyro output and subsequent control actions. After regaining control, the crew fired the ascent engine, entering orbital conditions simulating the end of a normal ascent from the surface. They then completed rendezvous and redocked on May 23.

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The crew used 70-millimeter still cameras and 16-millimeter motion-picture cameras to record the lunar surface, spacecraft and Earth. Astronauts described color differences between maria and highlands and noted that low Sun angles accentuated shadows and terrain detail. These observations reflected particular lighting and viewing conditions; rock composition could not be determined from visual color alone.

A specialist report published in 1971 brought together crew observations and photographic analysis, discussing terrain, impact craters, photometry and photogrammetry. Some data were not obtained as planned: a strip photographed on the 31st revolution was unsuitable for its intended mapping purpose, and another had a coverage gap caused by changing film magazines. Other corrected photographs were still used to update descent-monitoring charts and simulated training imagery.

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Comparing Apollo 8 and 10, the report found that improved processing and lunar gravity models increased orbit-prediction capability, but errors in different directions did not decrease together. Fitting data from two passes over the Moon's near side helped determine in-plane orbital parameters without similarly improving assessment of the orbital plane. These differences gave later missions specific leads for correction.

On May 24, the service propulsion engine fired again to begin the return. The command module splashed down in the Pacific at about 16:52 on May 26, after approximately 192 hours 3 minutes of flight. A helicopter retrieved the crew, and all three reached the recovery ship Princeton 39 minutes after splashdown. Return, splashdown and crew recovery were completed separately and should not be confused with launch or lunar-orbit insertion dates.

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Apollo 10 connected lunar module radar, propulsion, navigation and rendezvous operations near the Moon into a complete test. It confirmed that many procedures could be performed while revealing problems in attitude control and photographic planning. Postflight evaluation analyzed anomalies individually, and researchers continued processing the photographs, so the results extended beyond a safe return.

The mission also made sixteen color television transmissions totaling nearly six hours, showing viewers on Earth the spacecraft interior and scenes near the Moon. For Apollo 11, the more crucial preparation came from operations in the actual lunar environment, navigation errors and landing-area data. This evidence supported the next decisions but did not replace the descent, landing and surface liftoff that the next mission still had to accomplish.

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