14 CFR 1214.703 - Chain of command.
(a) The Commander is a career NASA astronaut who has been designated to serve as commander on a particular flight, 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 Space Shuttle commander is responsible to the Flight Director, Johnson Space Center, Houston, TX.
(b) The pilot is a career NASA astronaut who has been designated to serve as the pilot on a particular flight and is second in command of the flight. If the commander is unable to carry out the requirements of this subpart, then the pilot shall succeed to the duties and authority of the commander.
(c) Before each flight, the other flight crew members (Mission Specialists) will be designated by the Director of Flight Operations, Johnson Space Center, Houston, TX, in the order in which they will assume the authority of the commander under this subpart in the event that the commander and pilot are both not able to carry out their duties.
(d) The determinations, if any, that a crew member 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 crew member in the chain of command is to succeed to the authority of the commander, will be made by the Director of the Johnson Space Center.