Space-Available Act of 2012 - Authorizes the Secretary of Defense to establish a program to provide transportation on Department of Defense (DOD) aircraft on a space-available basis for: (1) active duty and reserve members holding a valid Uniformed Services Identification and Privilege Card; (2) retired members who, but for not attaining age 60, would be eligible for military retired pay; (3) an unremarried widow or widower of an active or reserve member; and (3) certain dependents of members described above. Allows the Secretary to establish an order of priority based on considerations of military needs and readiness.