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I always assumed that after the Apollo Command Module separated from the Service Module, the SM was effectively inert and was simply left to re-enter of its own accord. However, from the Apollo 11 Flight Journal:

194:54:40 Aldrin: Houston, we got the Service Module going by. A little high and a little bit to the right.

194:54:49 Evans: Roger. Thank you.

194:54:53 Collins: And it's rotating just like it should be. Thrusters are firing.

It sounds like it's doing some sort of deliberate, powered maneuver for a controlled re-entry. What was that maneuver, and was it being initiated by the SM, ground control, or someone else entirely?

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The SM left to its own has no onboard computer and no control. On jettison it would fire its roll thrusters for five seconds and its -X thrusters until fuel depletion (up through Apollo 11) or for 25 seconds (Apollo 12 and later). This was accomplished by very simple timer circuits. The sole purpose of these burns was to obtain a minimum separation between SM and CM so there would be no contact on reentry.

My understanding is the mechanism is so simple that an inadvertent separation would trigger the circuits, no matter what flight mode it was in at the time.

The reentry is uncontrolled.

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