The mission rules for Apollo 8, finalized in November 1968, named two people who could act on their own judgment. Rule 1-3: “The Flight Director may, after analysis of the flight, choose to take any necessary action required for the successful completion of the mission.” Rule 1-4: “The Command Pilot may initiate such inflight action as he deems essential for crew safety.”
Federal regulations later did the same for the Space Shuttle. The commander “shall have the absolute authority to take whatever action” is needed to keep order, protect the crew and safeguard the Shuttle.