The original concept for Apollo 10 called for the spacecraft to enter lunar orbit and for LM-4 to undock, enter a slightly different orbit, return and redock as a test of operating in lunar orbit. In December 1968, however, the mission planning and analysis division of Mission Control successfully argued the case for putting the descent propulsion system through a realistic rehearsal in which the perilune would be lower. This would test the ability of the landing radar to lock onto the surface, with the illumination on the low passes exactly as it would be on the landing mission in order to document the primary site and identify landmarks on the approach route. Howard W. 'Bill' Tindall, the assistant division chief, had also suggested that the LM should initiate the powered descent and abort by 'fire-in-the-hole' staging, but this was not pursued. After three outstandingly successful manned missions, consideration was given to assigning Apollo 10 the landing mission. However, because LM-4 was incapable of landing - the software was not ready for either the simulator or the

vehicle, and in any case LM-4 was too heavy to carry sufficient propellant to lift off again - Apollo 10 commander Tom Stafford argued against waiting for LM-5. ''There are too many 'unknowns' up there,'' he insisted. ''We can't get rid of the risk element for the men who will land on the Moon but we can minimise it; our job is to find out everything we can in order that only a small amount of 'unknown' is left.'' The plans, procedures, mission rules, manoeuvres, thermal regime and communications would provide a high-fidelity rehearsal of the landing mission. On 24 March 1969 it was announced that Apollo 10 would conduct this dress rehearsal, and if it achieved its primary objectives then Apollo 11 would attempt to land.

One aspect of the Apollo 10 mission was to assess the operation, tracking and communications of two spacecraft in lunar orbit. Apollo 8 had confirmed that the mascons significantly perturbed the orbit of a spacecraft. By having Apollo 10 fly the profile planned for the landing mission, it would be possible to assess how the guidance and navigation system of the LM coped with these gravitational effects while making the low passes of the descent orbit. Apollo 10 lifted off on schedule on 18 May, and on the fifth day the LM separated in a circular parking orbit at an altitude of 60 nautical miles, entered an elliptical orbit with a 50,000-foot perilune, made two low passes, discarded the descent stage, and made a perfect rendezvous. The first low pass rehearsed an approach to ALS-2 (site 'A3', later 2P-6), and while the aim point itself was acceptable, the western end of the 'landing ellipse' was rougher, and Stafford told Armstrong that if he were to find himself coming in 'long', his best option might be to abort.

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