I. OS and AOS: out of sight

Apollo missions w’ere intensively monitored from Earth. Indeed, because the flight controllers had deep technical visibility into the spacecraft’s systems through telemetry, and huge computing and personnel resources on hand in case of problems, they became accustomed to nursing its crews and machines over the days of the coast to the Moon. It was then a bit of a wrench when some of the most critical events in an Apollo flight, particularly the entry into and departure from lunar orbit, had to occur with a 3,500-kilomctre-diamcter lump of rock obscuring the view/

In future years, operations around the Moon might be supported by a telecoms satellite that will enable communications between Earth and crews that operate around the far side. In the time of Apollo, there w’as no such luxury, and contact depended on line of sight from the Moon to one of the three main ground stations distributed around Earth. But the engineers w’ere not to be denied. On board each spacecraft was a multitrack tape recorder, the data storage equipment (DSE). whose function was to digitally record a suite of measurements from around the spacecraft, particularly the SPS engine, and replay them to mission control on a separate radio channel w’hen communications were restored.

As the Moon pulled the spacecraft around its far side, communications were instantly and completely cut off at the moment an Apollo disappeared behind the limb. NASA referred to this event as loss of signal (LOS) and it occurred with alarming predictability by virtue of the deep understanding the trajectory experts had of an Apollo’s flight path. The first time it occurred w’as during the Apollo 8 mission, and Frank Borman found the accuracy of Houston’s predictions awe­inspiring. At the precise time that he had been told communications would disappear, they did.

“Ceeze!" he said to his crewmates, there being no one else to hear. “That was great, w’asn’t it?’’ Then he mused: “I wonder if they’ve turned it off.’’

Bill Anders laughingly replied: “Chris [Kraft, the boss in Houston] probably said, No matter w’hat happens, turn it off’ Bill’s humorous suggestion was that, in order not to worry the crew’ if the predictions had not been as accurate as they had hoped,

Kraft would have ordered the people at the transmitting station to turn off the radio signal at just the right moment. Borman wondered, however. When next they spoke to Capcom Gerry Carr, he reported: “Houston, for your information, we lost radio contact at the exact second you predicted.”

Carr confirmed that that was what had happened.

Borman probed further. “Are you sure you didn’t turn off the transmitters at that time?”

“Honest Injun, we didn’t,” was Carr’s joking reply.

The thing about LOS and its counterpart, acquisition of signal (AOS), was that they were both highly predictable events. AOS, in particular, had the useful property of being entirely dependent on what occurred around the far side by way of engine burns. Thus, on Apollo 14, for example, the precise time that the spacecraft would disappear behind the Moon’s leading limb had been calculated to the second, as usual. Additionally, mission control knew that if a problem had prevented the LOI burn from occurring, the spacecraft would not be slowed in its path and would reappear around the eastern limb only 25 minutes 17 seconds later, set on its hybrid free-return course towards Earth. On the other hand, if the LOI burn was executed as planned, the spacecraft, having been slowed, would stay out of radio contact for 32 minutes 29 seconds. Any deviation in the burn from that detailed on the PAD would show itself by the deviation of AOS from the predicted time.