The National Aeronautics and Space Administration (NASA) Science Mission Directorate (SMD) is releasing an Astrophysics Explorer 2014 Mission of Opportunity (MO) Program Element Appendix (PEA) (NNH12ZDA006O-APEXMO2) as Amendment 9 to the NASA Second Stand Alone Missions of Opportunity Notice (SALMON-2) Announcement of Opportunity (NNH12ZDA006O). The Astrophysics Explorers Program conducts Principal Investigator (PI)-led space science investigations relevant to SMD's astrophysics programs. Astrophysics Explorer investigations must address NASA's strategic objective to discover how the universe works, explore how it began and evolved, and search for life on planets around other stars. This solicitation will be open from September 17, 2014, through December 18, 2014. Upon the release date, the full text of the PEA will be available electronically at http://nspires.nasaprs.com/ . A preproposal conference will be held in early October; see http://explorers.larc.nasa.gov/APSMEX for details. NASA will release simultaneously the Astrophysics Explorer 2014 Small Explorer Announcement of Opportunity (NNH14ZDA013O) and a solicitation for Astrophysics Explorer U.S. Participating Investigators (APEX USPI) through the Research Opportunities in Space and Earth Sciences 2014 (ROSES-14) NASA Research Announcement (NNH14ZDA001N). Participation is open to all categories of organizations or institutions, U.S. or non-U.S., including educational, industrial, and not-for-profit institutions, Federally Funded Research and Development Centers (FFRDCs), University Affiliated Research Centers (UARCs), NASA Centers, the Jet Propulsion Laboratory (JPL), and other Government agencies. NASA expects to select one or two Astrophysics Explorer Missions of Opportunity to proceed into Phase B and subsequent mission phases. If multiple selectable missions are proposed with combined costs within the available funding, NASA may select more than one proposed investigation. This PEA incorporates a number of changes relative to the previous Explorers Mission of Opportunity SALMON-2 PEA and to the Draft 2014 Astrophysics Explorer Mission of Opportunity PEA. All proposers must read the SALMON-2 AO and this PEA carefully, and all proposals must comply with the requirements, constraints, and guidelines contained within the SALMON-2 AO and this PEA. Comments and questions may be addressed in writing or by E-mail to: Dr. Wilton T. Sanders, Astrophysics Explorers Program Scientist, Astrophysics Division, Science Mission Directorate, NASA Headquarters, Washington, DC 20546-0001; E-mail: wilton.t.sanders@nasa.gov (subject line to read "Explorer MO PEA"); Phone: 202-358-1319; FAX: 202-358-3062. Responses to all inquiries will be answered by E-mail and also posted to the Frequently Asked Questions (FAQ) location of the Explorer Program Acquisition website at http://explorers.larc.nasa.gov/APSMEX/ ; anonymity of persons/institutions who submit questions will be preserved. This notice constitutes a NASA Research Announcement as contemplated in FAR 6.102(d)(2).