A Mission of Opportunity (MO) is an investigation that forms part of a non-NASA space mission of any size and has a total NASA cost of under $35 million. Missions of Opportunity are conducted on a no-exchange-of-funds basis with the organization sponsoring the mission. NASA solicits proposals for MOs with each Announcement of Opportunity (AO) issued for UnEX (University-class Explorer), SMEX (Small Explorer), and MIDEX (Medium-class Explorer) investigations. Examples include HETE-2, TWINS,and CINDI.