14 CFR 1214.703 - Chain of command.
(a) The NASA Commander is a trained NASA astronaut who has been designated to serve as commander on a NASA mission and who shall have the authority described in § 1214.702 of this part. Under normal flight conditions (other than emergencies or when otherwise designated) the NASA Commander is responsible to the Mission Flight Director.
(b) Before each flight, the other flight crewmembers will be designated in the order in which they will assume the authority of the NASA Commander under this subpart in the event that the NASA Commander is not able to carry out his/her duties.
(c) The determinations, if any, that a crewmember in the chain of command is not able to carry out his or her command duties and is, therefore, to be relieved of command, and that another crewmember in the chain of command is to succeed to the authority of the NASA Commander, will be made by the NASA Administrator or his/her designee.