To aid their calculation of the spacecraft's entry trajectory, mission planners adopted an arbitrary height of 400,000 feet or 121.92 kilometres, at which the returning command module was deemed to have left space and begun re-entry. This was entry interface. The Retro flight controller's task was to shape their approach trajectory to ensure that when they reached this altitude, the flight path would form an angle to the horizontal of 6.5 degrees, with a leeway of about + 1 degree to help to cope with weather or unfavourable trajectory conditions.
If the targeted perigee was too low, the entry angle would be steeper than ideal, increasing the heat impulse the heatshield had to deal with and increasing the deceleration forces that the crew would have to endure. It would also tend to shorten the entry flight path, perhaps to the extent to which the CM's flight characteristics could not compensate, and would force a landing short of the planned point. In the extreme case, it would be lethal, either by excessive g-load or by incineration.
A higher than ideal perigee, and therefore shallower angle, would result in a longer entry path and lower g-forces, but this came with the danger that the spacecraft might fail to shed enough energy to enable it to be captured, would pass the perigee point, and would re-emerge from the atmosphere and coast out into space on a long elliptical orbit. Since the command module on its own had no means of propulsion and very limited supplies of power and oxygen, failure to be captured by Earth's atmosphere at the first attempt would be fatal for the crew.
Entry interface, while being handy for the trajectory analysts, was an entirely arbitrary point that had little to do with the real atmosphere and its properties. It was therefore of little use in the conduct of the re-entry itself because it did not take into account the variations that the outer atmosphere would present to the spacecraft. Some reference to a real, physical event was required to indicate that reentry had truly commenced, thereby aiding its coordination and timing. NASA chose the moment when the tenuous gases of the upper atmosphere were exerting enough drag on the spacecraft to create a deceleration equivalent to 0.05 g. When the spacecraft's accelerometers detected this event, they signalled to the relevant instrumentation that re-entry was underway.
Most aspects of the entry were measured with respect to this 0.05-g event. For the sake of calculation prior to entry, just as for entry interface, it was taken to occur at an altitude of 90.66 kilometres. When it was actually reached, two important things began to happen: the computer began to fly the re-entry, and the entry monitor system (EMS; of which more later) began to monitor the progress of the flight path.
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