How Not To Crash Into The Moon Part I

For Apollo, entering lunar orbit was simply a case of burning the SPS engine at perilune and slowing down sufficiently to ensure that the two joined spacecraft did not have enough momentum to leave the Moon's vicinity, but instead entered a close orbit around it. If the burn failed to occur at all, the crew were left in the fail-safe scenario of returning by default to the vicinity of Earth, with only a tweak of their trajectory required to bring them to a safe splashdown. However, in between the two scenarios of 'no burn' and 'a burn of the required duration', there were a range of possible outcomes that depended on exactly how much the engine had managed to slow the spacecraft, and some of these were potentially lethal.

In the scenario of a very short burn, the stack would come around the Moon and begin to head towards Earth, but if it did not have sufficient momentum to leave the Moon's gravity, it would languish in a region above the Moon's near side for some time and return to the Moon's vicinity. However, owing to perturbations from Earth's gravity, and the fact that the Moon was still travelling in its orbit, the stack would pass the Moon's trailing edge and be slung out into the depths of the solar system. Slightly longer burns would shorten the time spent above the lunar near-side, but in these scenarios it was very likely to impact the Moon.

If the burn was long enough, the stack would enter an elliptical lunar orbit with a perilune of about 110 kilometres over the far side, and an apolune over the near side whose altitude depended on the length of the burn. A longer burn resulted in lower altitude at apolune. With all these outcomes stemming from a possible partial failure of the SPS, the flight controllers in mission control kept procedures at hand to measure the extent of the failure and work out how to get the spacecraft home safely. In truth, if the engine started, there was little likelihood of it stopping until commanded. The possibility of it continuing to fire after the required shut-down time - an equally lethal scenario - was more of a concern.

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