Lunar orbit insertion: why 110 kilometres?

As Apollo w7as being planned, detailed studies were made of how: best to perform lunar orbit rendezvous – the technique that brought two spacecraft together around the Moon, one of which had come up from the surface. As this w as then considered to be difficult and dangerous, much effort was devoted to understanding and then

Пои not to crash into the Moon 229

optimising all the variables that affected the operation. Initially, it was assumed that when the LM lifted off the Moon it would rendezvous with a CSM in circular orbit at 148 kilometres. As planners continued to refine the trade-offs between operational concerns and vehicle capabilities, they came to the conclusion that the CSM should be in an orbit 1K.) kilometres above the Moon. This lower orbit was chosen to reflect the limited propellant budget of the LM’s ascent stage. Planners decided that it was acceptable to expect the ascent stage to get off the Moon within the resulting short launch windows.

This decision meant that every Moon-bound Apollo mission was targeted to make a pass around the far side with a minimum altitude of 110 kilometres. This point of closest approach was called pericynthion a term from celestial mechanics meaning the lowest point in a lunar orbit made by a craft arriving from another body (the highest point being apocynthion) The words are derived from Cynthia, an alternative name for the Greek Moon goddess Artemis. It was around pericynthion that the LOl burn was made. These two terms are rather unwieldy and refer to the particular case of an orbit achieved by a craft from outside the Moon’s vicinity. It is more common to use a shorter pair of terms, perihme and apohme. which arc more general in their use and mean more or less the same thing.