Small- and medium-class, PI-led Astrophysics missions, as well as Astrophysics missions of opportunity, are selected under the Astrophysics Explorers program, and are managed by the Astrophysics Division. Explorers are opportunities for focused scientific investigations, and fill the scientific gaps between the larger missions. For example, the Nuclear Spectroscopic Telescope Array (NuSTAR) is conducting a census of black holes for the Physics of the Cosmos program and studying the birth of elements for the Cosmic Origins program. In April 2013, the Transiting Exoplanet Survey Satellite (TESS) was selected to move into formulation. TESS will identify terrestrial planets in the habitable zones of nearby stars. The Neutron Star Interior Composition Explorer (NICER) Mission of Opportunity was also selected in April 2013. It will be mounted on the space station and measure the variability of cosmic X-ray sources to explore the exotic states of matter within neutron stars and reveal their interior and surface compositions.